Download Petrology and Geochemistry of the Nipissing Gabbro

Petrology and Geochemistry of the
Nipissing Gabbro: Exploration Strategies
for Nickel, Copper, and Platinum Group
Elements in a Large Igneous Province
Ontario Geological Survey
Study 58
1996
Petrology and Geochemistry of the
Nipissing Gabbro: Exploration Strategies
for Nickel, Copper, and Platinum Group
Elements in a Large Igneous Province
Ontario Geological Survey
Study 58
by P.C. Lightfoot and A.J. Naldrett
1996
ÓQueen’s Printer for Ontario, 1996
ISSN 0704-2590
ISBN 0-778-4804-X
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Canadian Cataloguing in Publication Data
Lightfoot, Peter C. (Peter Charles)
Petrology and Geochemistry of the Nipissing gabbro: exploration strategies for nickel, copper, and platinum
group elements in a large igneous province
(Ontario Geological Survey report, ISSN 0704-2590; 58)
Includes bibliographical references.
ISBN 0-7778-4804-X
1. Gabbro---Ontario---Nipissing Region. I. Naldrett, A.J. II. Ontario Ministry of Northern Development and
Mines III. Ontario Geological Survey. IV. Series.
QE462.G3L53 1995
552’.3 C95-964107-6
Every possible effort is made to ensure the accuracy of the information contained in this report, but the Ministry of
Northern Development and Mines does not assume any liability for errors that may occur. Source references are
included in the report and users may wish to verify critical information.
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Cette publication est disponible en anglais seulement.
Parts of this report may be quoted if credit is given. It is recommended that reference be made in the following
form:
Lightfoot, P.C. and Naldrett, A.J., 1996. Petrology and geochemistry of the Nipissing Gabbro: Exploration
strategies for nickel, copper, and platinum group elements in a large igneous province; Ontario Geological
Survey, Study 58, 81p.
Critical Reader: J.A. Fyon
Editor: T. Ayalew
ii
Contents
Objective and Approach of the Present Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction and General Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
6
6
Empirical Metallogeny of the Nipissing Gabbro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sampling and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effects of the Textural Variations in the Gabbro on the Analytical Data . . . . . . . . . . . . . . . . . . . . . . . .
Geochemical Evidence for the Emplacement and In-situ Differentiation of Nipissing Intrusions . . . .
Duration of Nipissing Magmatic Activity and Associated Compositional Variation . . . . . . . . . . . . . . .
Compositional Variation in the Parental Nipissing Magma Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Empirical Observations Related to Mineral Potential, Land Use Planning and Exploration . . . . . . . .
Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
11
11
12
24
26
37
48
48
Appendix 1: Sampling, Analysis, Geology, Petrography and Mineralogy of the Nipissing Intrusions . . . . .
1.1 Sampling and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Age and Distribution: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 Differentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.2 Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.3 Emplacement sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.4 Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.1 Geology of the Nipissing sills and dykes around Lake Temagami . . . . . . . . . . . . . . . . . . . . . .
1.4 Geology of the Kerns Sill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Petrography and Mineralogy of the Nipissing Gabbro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6 Metamorphism of the Nipissing Gabbro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
49
49
49
49
49
50
50
50
56
65
Appendix 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
Assimilation and Fractional Crystallisation in the Kerns Intrusion - a Case Study
of the Physical Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
66
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
Conversion Factors for Measurements in Ontario Geological Survey Publications . . . . . . . . . . . . . . . . . . . .
80
FIGURES
1a.
Distribution of Nipissing gabbro across the Southern Province . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
1b.
Sketch diagram showing the relationship between the petrology of the undulatory sills and the
associated mineralisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1c.
Typical sequence of lithologies seen in a well-differentiated intrusion of Nipissing gabbro
showing the relationship between silicate rocks and sulphide mineralization . . . . . . . . . . . . . . . . . . . . . . .
6
1d.
Location of samples and results from U-Pb geochronology work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.
Regional tectonic setting of the Nipissing gabbro and the location of the 2.2 Ga Preissac
Dyke Swarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3a.
Ni versus forsterite relationships in olivines from the Cross Lake Sill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3b.
PGE distribution patterns for the Rathbun Lake showing (after Lightfoot et al., 1993 and
Rowell and Edgar, 1986) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.
Geological relationships between Nipissing intrusions, mineralisation, and geophysical
anomalies, Paleoproterozoic mineralisation, and the Sudbury Igneous Complex . . . . . . . . . . . . . . . . . . . . . 13
5a.
Comparison of primitive mantle normalised spidergrams on vari-textured gabbronorite
patches and gabbronorite host; Emerald Lake gabbro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5b.
Comparison of primitive mantle normalised spidergrams on vari-textured gabbronorite patches
and gabbronorite host; Basswood Lake Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
iii
6.
Location of individual intrusive bodies and detailed study areas referenced in this report . . . . . . . . . . . . . . 20
7a.
Chemostratigraphy of the High Rock Intrusion, Lake Temagami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7b.
Primitive mantle normalised spidergrams of the High Rock Intrusion samples, Lake Temagami. . . . . . . . . 21
7c.
Primitive mantle normalised spidergrams of the High Rock Intrusion samples . . . . . . . . . . . . . . . . . . . . . . . 22
8.
Geochemical stratigraphy of the Miller Lake Intrusion, Gowganda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9a.
Primitive-mantle normalised trace element spidergrams for representative samples
from the Kerns Intrusion, northwest of New Liskeard Chilled basal quartz diabase,
quartz diabase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9b.
Primitive-mantle normalised trace element spidergrams for representative samples
from the Kerns Intrusion, northwest of New Liskeard Hypersthene gabbro. . . . . . . . . . . . . . . . . . . . . . . . . . 24
9c.
Primitive-mantle normalised trace element spidergrams for representative samples
from the Kerns Intrusion, northwest of New Liskeard Vari-- textured gabbro. . . . . . . . . . . . . . . . . . . . . . . . . 25
9d.
Primitive-mantle normalised trace element spidergrams for representative samples
from the Kerns Intrusion, northwest of New Liskeard Granophyric gabbro. . . . . . . . . . . . . . . . . . . . . . . . . . 26
9e.
Primitive-mantle normalised trace element spidergrams for representative samples
from the Kerns Intrusion, northwest of New Liskeard Aplites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9f.
Primitive-mantle normalised trace element spidergrams for representative samples
from the Kerns Intrusion, northwest of New Liskeard Hornfelsed sediment rafts within
the granophyric gabbro, and roof sediments. See text and Appendix 1 and Lightfoot
et al. (1987) for detailed sample locations and descriptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.
Geochemical variations in samples from the Kerns Intrusion, where elemental and oxide
abundances are plotted against Zr concentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.
a) Variation in Th versus Nb, b) Variation in Cu versus Zr in Nipissing gabbros, c) Variation
in Cu/Zr versus SiO for all Nipissing gabbro samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
12.
versus 147Sm/144Nd in samples from the Kerns Intrusion and local
a) Variation in
country rocks. b) Relationship of the array of the Kerns Intrusion to isochron lines based on
U - Pb geochronology for magmas with a range in initial 143Nd/144Nd isotopic composition . . . . . . . . . . . . 37
13.
A model for the evolution of the Kerns Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
14a.
Primitive mantle normalised spiderdiagrams. Representative samples from the Narrows
Island gabbronorite dyke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
14b.
Primitive mantle normalised spiderdiagrams. Representative samples from the Sand Point
gabbronite dyke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
14c.
Primitive mantle normalised spiderdiagrams. Representative samples from the Sand Point
and Narrows Island dykes with the overlying undulatory sills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
15a.
Primitive mantle-normalised compositions of aplites from the Obabika Intrusion, Lake Temagami
Gowganda Formation sediments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
15b.
Primitive mantle-normalised compositions of aplites from the Obabika Intrusion, Lake Temagami
Aplitic granitoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
16a.
Primitive mantle normalised compositions of differentiated granodiorites and quartz
diorites of the Obabika Intrusion that contain disseminated sulphide. Chilled diabase
and quartz diabase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
16b.
Primitive mantle normalised compositions of differentiated granodiorites and quartz
diorites of the Obabika Intrusion that contain disseminated sulphide. Quartz diorite
from the roof of the intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
17a.
Primitive mantle normalised spidergrams demonstrating the similarities in geochemical
signatures of samples collected from sites retaining three different palaeomagnetic remanence
directions in Nipissing Intrusions. N1 samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
17b.
Primitive mantle normalised spidergrams demonstrating the similarities in geochemical
signatures of samples collected from sites retaining three different palaeomagnetic remanence
directions in Nipissing Intrusions. N2 samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2
143Nd/144Nd
iv
17c.
Primitive mantle normalised spidergrams demonstrating the similarities in geochemical
signatures of samples collected from sites retaining three different palaeomagnetic remanence
directions in Nipissing Intrusions. N3 samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
17d.
Primitive mantle normalised spidergrams demonstrating the similarities in geochemical
signatures of samples collected from sites retaining three different palaeomagnetic remanence
directions in Nipissing Intrusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
A1.1 Geological map of the Nipissing Gabbro exposed around Lake Temagami . . . . . . . . . . . . . . . . . . . . . . . . . . 51
A1.2 Geological map of the Narrows Island dyke, Lake Temagami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
A1.3 Geological map of the Sand Point dyke, Lake Temagami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
A1.4 Detailed geological map of the south shore of Obabika Inlet, Lake Temagami . . . . . . . . . . . . . . . . . . . . . . . 54
A1.5 Detailed geological map of the Kerns Intrusion, Kerns Township . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
A1.6 Detailed geological map of Kerns Rock, Kerns Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
A1.7 Sketch map showing relationships between lithologies at the roof of the Kerns Intrusion . . . . . . . . . . . . . . 59
A1.8a. Sketch map showing the locations of samples referred to in MRD 19. Englehart Intrusion
(based on regional compilation maps) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
A1.8b. Sketch map showing the locations of samples referred to in MRD 19. Bruce Mines
(after Lightfoot et al., 1993) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
A1.8c. Sketch map showing the locations of samples referred to in MRD 19. Basswood Lake Intrusion
(after Lightfoot et al., 1993) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
A1.8d. Sketch map showing the locations of samples referred to in MRD 19. Wanapitei Intrusion
(after Lightfoot et al., 1993) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
A1.8e. Sketch map showing the locations of samples referred to in MRD 19. Cobalt Region Intrusion
(after Lightfoot et al., 1993) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
A2.1 Modelling of the crystallisation and assimilation history of the Kerns Intrusion . . . . . . . . . . . . . . . . . . . . . . 68
A2.2 a) Variation in Th/Yb versus La/Yb in the Kerns Intrusion b) Variation in U/Yb versus Th/Yb . . . . . . . . . . 69
A2.3 Effects of assimilation and fractionation compared to mixing on schematic showing
bi- variate plots of incompatible elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
A2.4 a) Modelling of assimilation coupled to fractionation on Th versus Zr. b) Modelling of
assimilation coupled to fractionation on La versus Zr, c) Modelling of assimilation coupled
to fractionation on U versus Zr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
A2.5 Model for the evolution of a Nipissing intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
PHOTOS
1.
Sulphide globules in gabbro from the Wanapitei Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.1
Hornfelsed sediment fragments (Gowganda formation) in quartz diorite at the roof
of the Obabika Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
1.2.
Hornfelsed sediment inclusion (Gowganda Formation) in granodiorite in the roof zone
of the Obabika Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
1.3.
Vein of aplitic granitoid originating in a domain of Gowganda Formation sedimentary
rock inclusions in the roof granodiorite of the Kerns Intrusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
1.4.
Banding in the spotted hornfelsed sediment rafts in the Nipissing granophyric gabbro . . . . . . . . . . . . . . . . . 63
1.5.
A breccia consisting of fragmented Lorrain Formation sediments within an aplite at the
roof of the Kerns Intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
1.6
Chilled Nipissing diabase at contact of High Rock intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
TABLES
1.
Summary of empirical characteristics of the metallogenetic associations of the Nipissing
gabbro intrusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
v
2.
Summary of grade of mineralisation at Kukagami Lake and the Rathbun Lake Showing . . . . . . . . . . . . . . . 15
3.
Analytical data for in-house standard reference materials UTB-1 (University of Toronto
basalt standard) and WHIN SILL (Open University diabase standard) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.
Miscellaneous Release Data (MRD) 19 (Digital data on diskette): Analytical data for
Nipissing intrusions (available separately)
5.
Nd isotope data for samples of Nipissing gabbro. Analyses were performed at the University
of Toronto using a clean laboratory and thermal ionisation mass spectrometer as documented
in the text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6.
Compositional averages of samples from relatively undifferentiated parts of twenty one
different Nipissing gabbro intrusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
7.
Compositional averages for evolved aplitic granitoids of the Kerns and Obabika intrusions . . . . . . . . . . .
vi
47
Abstract
Close spatial associations between magmatic and hydrothermal mineralisation and Nipissing gabbro intrusions have
been recognised, and there is a variation in style and type of mineralisation across the Nipissing magmatic province (Card
and Pattison, 1973). In the Nipissing gabbro of the central portion of the Southern Province in Ontario, mineralisation is
dominantly in the form of magmatic and/or hydrothermal Cu-Ni-platinum group element (PGE) sulphides which occur
disseminated within the intrusions or as massive pods beneath the intrusions.
A number of empirical observations can be made regarding the metallogeny of the Nipissing gabbro. The goal of this
study is to more thoroughly document magmatic components of the Nipissing gabbro found in mineralised, weekly mineralised, and apparently unmineralised intrusions and to focus attention on the metallogeny. A number of important
points are highlighted, and new data are presented for Nipissing gabbros from the Lake Temagami Region, Ontario:
1. Magmatic Ni, Cu, and PGE mineralisation is spatially associated with intrusions which lie on a trend between
Whitefish Falls, Sudbury, and River Valley. The mineralisation is associated with a significant regional gravity and
aeromagnetic high along the trend of which and a number of mineralised Paleoproterozoic intrusions were emplaced, and the Sudbury Igneous Complex was formed.
2. Mineralised Nipissing gabbro intrusions with significant quantities of disseminated sulphide are located southwest
of Sudbury between Whitefish Falls and Emerald Lake - the Casson Lake Complex, southwest of Sudbury (the
“”Sudbury Gabbro” intrusions of Nairn, Lorne, Denison, Waters, Hyman and Drury Townships), east of Sudbury
(the Wanapetei Intrusion), and continuing to the east-northeast in Kelly and Janes Townships. The sulphides occur as
fine disseminations of magmatic pyrrhotite (50-75%) with lesser chalcopyrite and pentlandite. Some of the sulphides exhibit magmatic blebby textures with pyrrhotite-rich lower segments and chalcopyrite-rich upper segments
suggesting in-situ differentiation of the immiscible sulphide liquid during cooling and crystallisation. Massive sulphides rich in copper and platinum group elements (PGE) are known as basal concentrations associated with the
Wanapetei Intrusion. Despite this being a small occurrence, the sulphides carry 1-15 weight % Cu, 2.5- 6.3ppm Pt,
17-53ppm Pd, and 1-6ppm Au. In general, the disseminated sulphides (<5% modal sulphide) tend to be focussed in
the interior of the sills (100- 300m above the base) with coarse gabbronorites and hypersthene-rich gabbros; these
rocks carry 200-1100ppb Pt and 50-4000ppb Pd in mineralised intrusions in Janes Township.
3. The silicate host of the sulphides tends to be relatively undifferentiated, although there is some petrographic indication that there may be cyclical trends in composition (Conrod, 1989). Intrusions which are heavily contaminated in
their roof zones such as the Kerns Intrusion northwest of New Liskeard and the Basswood Lake Intrusion north of
Thessalon appear to be relatively unmineralised, and the low Cu/Yb and Cu/Zr of the most contaminated granophyric rocks is attributed to large amounts of assimilation of sediment with very low Cu/Yb and Cu/Zr rather than the
fractionation of an immiscible sulphide liquid.
4. The mineralised intrusions tend to be those which have a basinal shape and are least strongly differentiated into granophyric zones, yet consist of gabbros which contain a higher modal proportion of hypersthene (20-40 modal %)
than unmineralised intrusions (<20 modal %). As a result of the hypersthene content, the mineralised intrusions tend
to be more mafic as reflected in their elevated MgO (10-14 weight %) and low incompatible element concentrations
(e.g. 0.2-0.4 weight % TiO2, <50ppm Zr). The localisation of the disseminated sulphides inside the sills suggests that
either the sulphides were related to late emplacement of magmas, or that the differentiation of the sills triggered
sulphur saturation only after the initial crystallisation of large amounts of hypersthene. Although there are no olivine
cumulates of the type seen in the Noril’sk Intrusions in Russia (Naldrett et al., 1992), there are intrusions with elevated MgO values (approaching 14 weight %), and these rocks are hypersthene-rich cumulates.
5. Existing geochemical data suggest that there is no obvious difference in the magma type of the mineralised intrusions when compared to the unmineralised intrusions. For example, the undifferentiated quartz diabase, gabbros,
and chills have a narrow range in incompatible element ratios such as La/Sm and Th/Y, and similar abundance levels. This suggests that many of the intrusions were derived from the same source, that they did not differentially mix
or become contaminated by crustal reservoirs en route to the surface, and were emplaced at roughly the same degree
of differentiation through the Nipissing event. The available evidence also suggests that the Cu/Zr ratios of gabbros
with non-detectable S are not significantly different when compared to the mineralised intrusions. This suggests that
there has been little differential depletion of intrusions in copper by the segregation of immiscible sulphide in the
intrusions studied. It is also evident that fresh olivine from an unmineralised intrusion are characterised by elevated
nickel (1500-1700ppm) contents in olivines with forsterite contents of Fo60-70 which compares to 1000-1500ppm in
Fo60-65 olivines which have crystallised from magmas which have not equilibrated with sulphide. Importantly, disseminated and massive sulphide is observed associated with some Nipissing intrusions, and an important challenge
awaiting explorationists is to determine whether the olivines of these intrusions are Ni-depleted, and whether any of
the silicates can be identified to show nickel and copper depletion.
6. The homogeneity of the chilled margins and undifferentiated quartz diabase and gabbro intrusions suggests a single
source for these magmas where there has not been significant differential interaction of the magma with a crustal
vii
7.
reservoir other than at the final site of crystallisation. However, the parental magma type is characterised by strong
light rare earth element (LREE) and large ion lithophile element (LILE) enrichment as well as marked negative
Ta+Nb, TiO2, and P2O5 anomalies. These are all features of a magma that has either interacted with a crustal reservoir (by contamination in a continental or arc environment), or they are features of young rocks typically attributed
to derivation from ancient regions of mantle lithosphere which contain recycled continental crust. This geochemical
signature is also one found in many of the Upper Sequence lavas of the Siberian Trap at Noril’sk, but is not quite as
extreme as that found in the highly contaminated Nadezhdinsky lavas which have low Cu, Ni, and PGE abundances
(Lightfoot et al., 1994; Naldrett et al., 1992).
The presence of disseminated magmatic sulphides towards the centre of the undulatory sills suggest that either: 1)
the magmas were initially injected free of sulphide, and that subsequent magmas were injected as sulphur-saturated
magmas, or 2) that the differentiating liquid became sulphur saturated only after it had time to fractionate and/or mix
with a new pulse of more primitive liquid. Further insight may well be gained from recent modelling of the Fox River
Sill, where recent studies of the geochemical variations show that the vertical distribution of Ni, Cu, and PGE can be
linked to the replenishment of a magma chamber by a gabbroic magma (Naldrett et al., in press.). The controlling
effect of gravitational settling appears to have produced only one example of sulphide mineralisation linked to the
base of the intrusion at Rathbun Lake, and some authors consider this deposit to have a hydrothermal origin, or possibly even be related to the Sudbury Structure. Our new data suggest that sulphur saturation was achieved after the
crystallisation of the magma had commenced, and the semi-consolidated hypersthene gabbro cumulates presumably prevented the gravitational settling of the immiscible sulphide liquid to the base of the intrusions. This, in-turn,
expands the range of mineral exploration targets to the interiors of the Nipissing Intrusions rather than just the basal
contacts.
viii
Petrology and Geochemistry of the
Nipissing Gabbro:
Exploration Strategies for Nickel, Copper, and Platinum
Group Elements in a Large Igneous Province
P.C. Lightfoot
Geologist, Mineral Deposits and Feild Services Section, Ontario Geological Survey, 933 Ramsey Lake Road,
Sudbury, Ontario, P3E 6B5
A.J. Naldrett
Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1
Objective and Approach of the Present Study
Recent studies of the Continental Flood Basalts (CFB) at
Noril’sk in Russia suggest that the intrusions which carry
world- class deposits of Ni, Cu, and PGE (>555*106 tonnes
of sulphide grading 2.7 weight % Ni; Naldrett and
Lightfoot, 1993) are the intrusive conduits of the magmas
giving rise to the Permo- Triassic flood basalt sequence
(e.g. Naldrett et al., 1992; Naldrett et al., 1995). Likewise,
recent studies of the Karoo diabase sills of Southern Africa
confirm a comagmatic relationship with the extrusive
Lesotho CFB sequence (Marsh and Eales, 1984; Lightfoot
and Naldrett, 1984), and some of the differentiated
Karoo-aged intrusions host significant showings of Ni, Cu,
and PGE such as those found at Insizwa (e.g. Scholtz,
1936; Lightfoot et al., 1984). These studies together with
data for other CFB suggest that there are important empirical relationships between the development of mineralisation and the formation of large igneous provinces at continental margins. In some cases, geochemical signatures
within the basaltic rocks record evidence of extensive
crustal contamination accompanied by the depletion of the
magmas in Ni, Cu, and PGE (Naldrett et al., 1992;
Brugmann et al., 1993; Lightfoot et al., 1994), and signatures of this type have been actively sought in other CFB
(e.g. in the Keweenawan Midcontinent Rift; Lightfoot et
al., 1991). The main parameters of the Noril’sk empirical
exploration model may be summarised as follows (after
Naldrett and Lightfoot, 1993):
1.
The presence of mineralised picritic gabbroic
intrusions which have acted as open-system magma
conduits.
2.
The presence of highly contaminated, mantle derived,
magmas either as intrusions or within the stratigraphy
of the comagmatic lavas. Working cut-off values
established at Noril’sk are: SiO2=52-56 weight %,
La/Sm>3, 87Sr/86Sro>0.706.
3.
The strong depletion of magmas in Ni, Cu, and PGE.
At Noril’sk, lavas with <50 ppm Ni, <50ppm Cu, and
<1ppb Pt+Pd are considered very depleted (Naldrett
and Lightfoot, 1993).
4.
A source of S, which is spatially linked to the location
of the magma conduit system, is assimilated by the
magma. At Noril’sk this was achieved in the Talnakh
Intrusion which appears to have been an open system
conduit through which magma migrated, and interacted with crustal sulphur contained in evaporite-rich
sediments.
5.
Deep mantle-penetrating faults which permitted the
unhindered migration of mantle-derived magmas.
6.
The presence of picritic lavas and intrusions.
Not all of these criteria are evident in the Nipissing
gabbro, but we do describe a number of features which are
encouraging from the perspective of mineral exploration
potential.
The Nipissing intrusions are located within the
Huronian sedimentary sequence at the margin of the
Superior Province, and the focus of the 2.2Ga Preissac
dyke swarm centers on the Nipissing Province (Figure 1).
The intrusions consist dominantly of gabbros with lesser
diabase and granophyre, and are collectively termed
Nipissing Gabbro Intrusions of the Nipissing Magmatic
Province. The Nipissing intrusions are known to host or be
associated with small but significant showings of Ni, Cu,
and PGE which have been actively explored. Importantly,
many of the mineralised Nipissing intrusions are located in
the Sudbury Region close to the giant Sudbury Ni, Cu, and
PGE deposits. Mineralised Nipissing gabbro is known to
extend from southwest of Sudbury near Whitefish Falls
(Card, 1976) and through the Wanapetei-Kelly-Janes
townships (Dressler, 1979; 1982). Moreover, the trend of
these mineralised intrusions passes through the Sudbury
Structure, along the line of the Wanapetei gravity anomaly,
and centers on a regional northeast-southwest aeromagnetic and gravity anomaly (Figure 2). Along this gravity
anomaly are a number of Paleoproterozoic intrusions
which host Ni, Cu, and PGE showings such as the East Bull
Lake Gabbro-Anorthosite Intrusion, the ShakespeareDunlop Gabbro- Anorthosite Intrusion, the Wanapetei
gabbronorite intrusion, and the River Valley Anorthosite
(e.g. Peck et al., 1993) (see Figure 2).
The recognition of the importance of gabbroic intrusions as the roots of large igneous provinces which often
host significant quantities of economic mineralisation has
been important in reevaluating the mineral potential
of the Nipissing intrusions. The presence of mineralised
Nipissing intrusions associated with a trend of geologically anomalous mineralisation covering a period of
almost 0.8Ga and associated geophysical anomalies has
led to a more vigorous re-interpretation of existing data
and provision of new data for the Nipissing gabbro which
have a bearing on mineral exploration.
In this study we report new data for Nipissing intrusions from across the Nipissing magmatic province, and
compare and contrast their chemical compositions. We
address the following questions:
1.
Were Nipissing intrusions formed from a single batch
of magma or multiple pulses of magma and where did
the magma enter the undulating sill?
2.
What role did crustal contamination play in the
genesis of the Nipissing magmas, and did the
contamination play any significant part in triggering
the segregation of immiscible sulphides as suggested
by Irvine (1975)?
3.
What processes gave rise to the Nipissing parental
magma, and are there any geochemical data to suggest
that mineralisation is associated with particular
Nipissing intrusive magma types?
3
OGS Study 58
Figure 1a. Distribution of Nipissing gabbro across the Southern Province, northern Ontario (modified from Card and Pattison, 1973).
4.
What spatial and temporal relationships exist
between Nipissing intrusions in the context of
existing palaeomagnetic and geochronological
studies, and do these relationships have any value in
predicting mineralisation?
5.
What are the best empirical data relevant to the
construction of exploration models for deposits of Ni,
Cu, and PGE-enriched sulphides in the Nipissing
magmatic province?
6.
Did gabbroic magmatism occur at a passive or active
continental margin in the context of plume contributions from an active mantle plume versus lithospheric
contributions from above a passive rift caused by
shallower plate tectonics (e.g. Hawkesworth and
Gallagher, 1994)?
To achieve these goals we use data for samples collected
from a number of intrusions located across the magmatic
province: 1) We have studied two intrusions (Basswood
Lake Intrusion at Thessalon and the Kerns Intrusion at
New Liskeard; Figure 3) in a great degree of detail to
understand the differentiation history and the mechanism
of contamination, and some of the results are published
elsewhere (Lightfoot et al., 1987). 2) We have sampled
material from the less well differentiated intrusions across
the Nipissing Province (see Figure 3) to characterise the
4
parental magma compositions using undifferentiated gabbronorite and chilled diabase, and we have focussed attention on the Temagami Region where a large number of
undifferentiated sills exist in an area currently under the
Temagami Land Caution. We contrast these data with
analysis by Conrod (1988, 1989) of the Gowganda,
Cobalt, and Sudbury areas. 3) We have attempted to characterise aplitic granitoids related to the Nipissing intrusions in order to determine their relationship with the country rocks and their genesis; we have focussed attention on
an intrusion of aplite from the Obabika Intrusion (Obabika
Inlet on Lake Temagami), and on aplites from the Kerns
Intrusion (New Liskeard). 4) We compare and contrast the
chemical compositions of gabbro and diabase sampled
from locations where the palaeomagnetic remanence
direction has been characterised by others (e.g. Buchan
et al., 1989). We also use these data and U-Pb geochronology to determine whether the difference in emplacement age is real or whether the palaeomagnetic remanence
direction is correlated to any change in parental magma
composition. 5) We have sampled gabbronorites from
mineralised and barren intrusions in order to determine
whether mineralised intrusions are geochemically different when compared to unmineralised intrusions. We present empirical observations regarding the characteristics of
mineralised and unmineralised intrusions.
Figure 1b. Sketch diagram showing the relationship between the petrology of the undulatory sills and the associated mineralisation (after Lightfoot et al., 1993).
Petrology and Geochemistry of the Nipissing Gabbro
5
OGS Study 58
acquisition of geochemical data under contract
38ST23233-7-0068. The following staff at the Geological
Survey of Canada are thanked for their assistance with this
project: Dr. J. M. Duke, Dr. K. Buchan, Dr. K. Card,
and Dr. R.O. Eckstrand. Analytical data were acquired at
the Open University, U.K., the University of Toronto,
and the Geoscience Laboratories. Dr. R.G.V. Hancock
and Dr. M.P. Gorton, University of Toronto, are thanked for
assistance with Instrumental Neutron Activation Analysis.
Dr. S. Noble, Jack Satterly Geochronology Laboratory,
Royal Ontario Museum, is thanked for assistance with
U-Pb geochronology and Sm-Nd systematics, and
Dr. T. Krogh is thanked for access to the geochronology
laboratory at the Royal Ontario Museum. For hospitality in
the field, Peter and Linda Phalen of Loon Lodge are
thanked, and field assistance from Grant Phalen is
acknowledged. We thank Norm Evensen, University of
Toronto, for assistance with Sm-Nd isotope studies and
Debbie Conrod for numerous discussions on Nipissing
petrogenesis.
The former staff of the Resident Geologist’s Office in
Cobalt are thanked for assistance, and L. Owsiacki and
P. Anderson are thanked for pointing out suitable locations for sampling in Kerns Township. This manuscript has
benefited from reviews by Dr. A. Fyon, Dr. W.T. Jolly and
Dr. F. Dudas. We also acknowledge the assistance of
G. O’Reilly, B. Wright, F. Racicot, J. Rauhala, G. Salo,
and D. Brunne. Diagrams were prepared by Steve Josey,
Ontario Geological Survey.
INTRODUCTION AND GENERAL
GEOLOGY
Figure 1c. Typical sequence of lithologies seen in a well-differentiated
intrusion of Nipissing gabbro showing the relationship between silicate
rocks and sulphide mineralization.
ACKNOWLEDGEMENTS
A study of the Nipissing Gabbro was initiated at the
University of Toronto. The Ontario Geological Survey
supported this work through an OGRF grant which allowed
D. Conrod to complete her MSc thesis. This work benefited from the encouragement of B. Dressler. Additional
work was completed by D. Conrod whilst she worked at the
Ontario Geological Survey, and access to information in
a published report and a report in preparation are
acknowledged.
The authors acknowledge support from Energy Mines
and Resources, Canada, for the field studies and the
6
The 2.22Ga Nipissing gabbro of northern Ontario comprise a suite of dominantly tholeiitic to calc-alkaline rocks
ranging from chilled diabase through quartz diabase, gabbronorite, gabbro, vari-textured gabbro, pegmatitic gabbro, quartz diorite, granodiorite, granophyre, and aplitic
granitoids. The intrusions extend from Sault Ste. Marie
through the Sudbury Region, to the Cobalt and Gowganda
regions (see Figure 1a) and are grouped as a large igneous
province termed the Nipissing Gabbro Province in this report. The intrusions range in thickness from a few hundred
meters to over a thousand meters, and outcrop at the present erosional level as open ring structures, ring dykes, cone
sheets, dykes and undulatory sills (Hriskevich, 1952, 1968;
Card and Pattison, 1973). The intrusions are dominantly
located in the Huronian Supergroup, but are also localised
along the Archean- Proterozoic unconformity.
Precise U-Pb geochronology on magmatic baddeleyite from Nipissing gabbro has yielded crystallisation ages
of 2219±3.6Ma from the Gowganda area (Corfu and
Andrews, 1986), 2212±2Ma from the Sudbury area
(Conrod, 1989), 2217±4Ma and 2210±3.8Ma from the Cobalt area (Noble and Lightfoot, 1992) (see Figure 1d). Palaeomagnetic data (Buchan et al., 1989) are interpreted to
reflect at least three discrete phases of magma emplacement separated by at least 50Ma. However, U-Pb geochronology on samples collected from sites giving two different remanence directions separated by 50Ma (N1 and N2
Petrology and Geochemistry of the Nipissing Gabbro
Figure 1d. Location of samples and results from U - Pb geochronology work (Corfu and Andrews, 1986; Noble and Lightfoot, 1992; Conrod, 1989).
Figure 2. Regional tectonic setting of the Nipissing gabbro and the location of the 2.2 Ga Preissac Dyke Swarm (after Card et al., 1994 and Osmani,
1991).
7
OGS Study 58
of Buchan et al., 1989) yield magmatic baddeleyite ages
overlapping by less than 7.8Ma (Noble and Lightfoot,
1992). One of the intrusions dated by Noble and Lightfoot
(1992) at 2210Ma has associated quartz-carbonate veins
with silver and cobalt mineralisation.
The Nipissing gabbros northeast of Sudbury in the
Cobalt Plate area (see Figure 1a) are relatively undeformed
and unmetamorphosed gabbros. Intrusions which have
traditionally been termed “Sudbury Gabbro” consist largely of amphibolites (e.g. Ginn, 1965; Card, 1965, 1968), and
grouped with the Nipissing between Sudbury and Blind
River, have undergone some deformation and regional
amphibolite facies metamorphism perhaps related to the
Paleoproterozoic Penokean Orogeny between 1900 and
1850Ma (Card, 1978). West of Blind River the Nipissing
gabbros have undergone some deformation, but less extreme regional metamorphism. The intrusions southwest
of Sudbury are elongated parallel to the main structural
fabric of the Murray Fault System, and the emplacement of
these intrusions may have been genetically linked to
faulting accompanying the deposition of the Huronian
sedimentary rocks (Buchan and Card, 1985).
The Nipissing Intrusions have traditionally been
described as undulatory sheets consisting of a series of basins and arches connected by limbs (Hriskevich, 1968) (see
Figure 1b). The basinal portions consist of quartz diabase
overlain by hypersthene gabbro, and an overlying vari-textured gabbro with pegmatoidal patches. The arches consist
of vari-textured gabbro overlain by quartz diorite, granodiorite, granophyre and aplitic granitoids. In detail, many
of the undulatory intrusions are relatively undifferentiated,
8
consisting of a basal quartz diabase overlain by gabbro,
and extreme differentiation into hypersthene gabbro and
granophyric gabbro is the exception rather than the rule
(see Figure 1c).
The intrusions are dominantly tholeiitic, but the
evolved rocks trend towards calc-alkaline compositions,
and recent geochemical studies suggest that the petrological variations within the intrusions are controlled by fractional crystallisation of the magma with or without assimilation of overlying country rock after emplacement (e.g.
Lightfoot et al., 1987; Conrod, 1988, 1989; Lightfoot et al.,
1993). The chilled quartz diabase and least differentiated
gabbro samples from a number of different Nipissing intrusions are consistent with the emplacement of a compositionally uniform low-Mg parental magma (termed the
Nipissing magma type) across a wide tract of the Southern
Province (Lightfoot et al., 1993). The geochemical characteristics of this parental magma include moderate MgO
(8-9 weight %), elevated SiO2 (50.0-51.5 weight %),
strong light rare earth element (LREE) and large ion lithophile element (LILE) enrichment with La/Sm = 2.5-3.5
and Th/Nb = 0.7-0.9, and marked negative anomalies for
TiO2, P2O5, and Nb+Ta (Lightfoot et al., 1993). These are
all features of Phanerozoic basalts from continental settings typified by the Continental Flood Basalts (CFB), and
there is currently a vigorous debate as to whether these
characteristics are features of the source regions of the
magmas or whether they were imparted to the magmas by
continental crustal contamination as the magmas evolved
in deep crustal reservoirs and migrated from the mantle to
the crust.
Empirical Metallogeny of the Nipissing Gabbro
Significant close spatial associations between mineralisation and Nipissing intrusions have been recognised (Card
and Pattison, 1973; Innes and Colvine, 1984), and there is a
variation in style and type of mineralisation across the
Nipissing Province. In the east, mineralisation is dominantly Ag, Co and Ni as native metals, arsenides, and sulfarsenides associated with quartz- carbonate veins which
cut the sediments and the Nipissing intrusions (Jambor,
1971), and may be Neoproterozoic in age. In the central
portion of the Nipissing Province, mineralisation is dominantly Cu-Ni-platinum group element (PGE) sulphides
which occur disseminated within the intrusions or as massive pods beneath the intrusions (Rowell, 1984; Rowell
and Edgar, 1986; Lightfoot et al., 1991; Lightfoot et al.,
1993). In the western part of the Province, the mineralisation consists of Cu-sulphides as fine disseminations or in
quartz-carbonate veins which cut vertically through the
Nipissing gabbro.
A number of empirical observations have been made
in previous studies, and one goal of this study is to focus
attention on this aspect of the study. For this reason, these
features are introduced at this early stage, and then
returned to in the summary of metallogenetic impact.
Table 1 summarises some important observations about
the geology, petrology, mineralogy, and geochemistry of
the Nipissing Gabbro which relate to mineral potential. A
number of important points set the scene for this study of
the Temagami Region, and these are summarised below:
1. Magmatic Ni, Cu, and PGE mineralisation is spatially
associated with intrusions which lie on a trend
between Whitefish Falls, Sudbury, and River Valley
(Figure 4 and see Table 1).
2. Mineralised Nipissing gabbro intrusions have associated disseminated sulphide (see Table 1). The sulphides occur as fine disseminations of magmatic pyrrhotite (50-75%) with lesser chalcopyrite and pentlandite. Some of the sulphides exhibit magmatic blebby
textures (Appendix 1; Lightfoot et al., 1991; Lightfoot
et al., 1984; Naldrett et al., 1992) with pyrrhotite- rich
lower segments and chalcopyrite-rich upper segments
suggesting in-situ differentiation of the immiscible
sulphide liquid during cooling and crystallisation
(Photo 1) Massive sulphides rich in Cu and PGE are
known as basal concentrations associated with the
Wanapetei gabbronorite intrusion, and despite this
being a small occurrence, the sulphides carry 1-15
weight % Cu, 2.5-6.3ppm Pt, 17- 53ppm Pd, and
1-6ppm Au (Lightfoot et al., 1991; 1993). The disseminated sulphides (<5% modal sulphide) tend to be
localised in the interior of the sills (100-300m above
the base) within coarse grained gabbronorites and
hypersthene-rich gabbros; these rocks carry
200-1100ppb Pt and 50- 4000ppb Pd in mineralised
intrusions in Janes Township (Lightfoot et al., 1991;
1993). Table 2 summarises the grade of these massive
and disseminated sulphides.
3.
4.
5.
6.
The silicate host of the sulphides tends to be relatively
undifferentiated, although there is some petrographic
indication that there may be cyclical trends in mineral
composition and whole-rock chemical composition
(e.g. Conrod, 1989; Finn and Edgar, 1986). Intrusions
which are heavily contaminated in their roof zones
(Lightfoot et al., 1989; 1993) such as the Kerns
Intrusion, northwest of New Liskeard, and the
Basswood Lake Intrusion, north of Thessalon, appear
to be relatively unmineralised. The low Cu/Yb and
Cu/Zr of the most contaminated granophyric rocks in
these specific intrusions is attributed to large amounts
of assimilation of sediment with very low Cu/Yb and
Cu/Zr rather than the fractionation of an immiscible
sulphide liquid (Lightfoot et al., 1994; Hawkesworth
et al., 1995).
The mineralised intrusions tend to be those which are
least strongly differentiated into granophyric zones,
yet consist of gabbronorites which contain a higher
modal proportion of hypersthene (20-40 modal %)
than unmineralised intrusions (<20 modal %). As a result of the hypersthene content, the mineralised intrusions tend to be more mafic as reflected in their elevated MgO (10-14 weight %) and low incompatible
element concentrations (e.g. 0.2-0.4 weight % TiO2,
<50ppm Zr) (Lightfoot et al., 1991, 1993).
Existing geochemical data suggest that there is no obvious difference in the magma type of the mineralised
intrusions when compared to the unmineralised intrusions. For example, Lightfoot et al. (1993) demonstrated that the undifferentiated quartz diabase, gabbros, and chills have a narrow range in incompatible
element ratios such as La/Sm and Th/Y, and similar
abundance levels.
The chemical homogeneity of the chilled margins and
undifferentiated quartz diabase and gabbro intrusions
suggests: a) a single source for these magmas, and b)
there has not been significant differential interaction
of the magma with a crustal reservoir other than at the
final site of crystallisation (Lightfoot et al., 1989;
1993). However, the parental magma type is characterised by strong light rare earth element (LREE) and
large ion lithophile element (LILE) enrichment as
well as marked negative anomalies for Ta+Nb, TiO2,
and P2O5. These are all features of a magma that has
either interacted with a crustal reservoir, or they are
features of young rocks typically attributed to derivation from ancient regions of mantle lithosphere which
contain recycled continental crust (e.g. Lightfoot et
al., 1993a, b; Hergt et al., 1991). This geochemical
signature is also one found in many of the Upper Sequence lavas of the Siberian Trap at Noril’sk, but is not
quite as extreme as that found in the highly contaminated Nadezhdinsky lavas which have low Cu, Ni, and
PGE abundances (Lightfoot et al., 1990; 1993; 1994;
Naldrett et al., 1992; Brugmann et al., 1993).
9
OGS Study 58
Table 1. Summary of empirical characteristics of the metallogenetic associations of the Nipissing gabbro intrusions, Ontario. Main points from this
study, Lightfoot et al. (1991), Conrod (1988, 1989), Lightfoot et al., (1993), and Card and Pattison, (1973).
Criteria for Metallogenetic
Evaluation of Nipissing Gabbros
Observations Regarding Nipissing Gabbro
Regional setting of mineralised
sectors of large igneous provinces
Possible roots of a large igenous province; similarity to the diabase sills of the Karoo Province
(Lightfoot, 1982; Marsh and Ealses, 1984) and Tasmanian Province (Hergt et al., 1989); similarity to
diabase-- gabbro sills in the epicontinental rocks of the Siberian Trap (Naldrett et al., 1992). Noril’sk is a
classic example where the gabbrodolerite and picritic sills have controlled mineralisation.
Structural association of
mineralised rocks and
mantle-- penetrating faults
Many dykes and sills associated with regional east-- west tectonic systems such as Murray fault systems.
Feeder dykes at Narrows and Sand Point (Temagami) are east-- west oriented. The importance of deep
mantle-- penetrating faults is highlighted at Noril’sk, where the Noril’sk-- Kharaelakh Fault controls
mineralisation. It is in known whether the major east-- west and north-- south fault systems associated with
the Nipissing were mantle-- penetrating, or conduits, but the association of gabbroic rocks with these
faults suggests that this is a possibility.
Basinal association of some
mineralised gabbros
Focus of Nipissing activity in basinal sediments of Huronian with equivalent amounts of activity
focussed in the Cobalt Embayment and east-- west along the extension of the Huronian geosynclinal
package. The basinal setting is quite different to Noril’sk in so far as the Noril’sk deposits were formed in
an epicontinental setting where a thick evaporite-- carbonate sequence was developed. The Nipissing
example is more like the Karoo where large nickeliferous intrusions such as Insizwa are known (Scholtz,
1936; Lightfoot et al., 1984).
Tectonomagmatic setting of
mineralised gabbros
No strong evidence for a mantle plume during the Nipissing event. Tectonomagmatic setting may
resemble Karoo with lithospheric extension being the main driving force (Hawkesworth and Gallagher,
1994). The absence of a mantle plume as a driving force may explain the absence of high-- Mg Nipissing
rocks, and also account for the extreme uniformity of the magma in composition. The Insizwa Complex
in the Karoo is developed in a basinal setting presumably associated with lithospheric extension, and the
gabbros of this complex are mineralised. However, Insizwa does show well developed picrites, and these
are not evident in the Nipissing.
Geophysical signatures linked to
metallogenic province
Major aeromagnetic and gravity anomalies are present along a trajectory between Englehart, Temagami,
Wanapetei, Sudbury, and Manitoulin Island. This ESE-- WNW trend of anomalies may be related to a
string of deep mafic-- ultramafic complexes. The trend is linked to the presence of the mineralised Early
Proterozoic intrusions (Peck et al., 1993), mineralised rocks of the Temagami Island deposit (Simony,
1964), the mineralised Sudbury Igneous Complex (Pattison, 1979; Lightfoot et al., 1994) and is the
trajectory of MOST mineralised Nipissing gabbro intrusions. This association is presumably not
coincidental, but is the signature of a metallogenic province, where Ni-- Cu-- PGE mineralisation was
developed over a protracted time interval by different processes.
Physical traps for sulphides
Immiscible sulphide liquids which are able to settle under gravitational forces to the base of the
undulatory Nipissing sills may readily accumulate in the basinal portions of the undulatory sills. If this
process has happened, then the only possible example is Rathbun Lake, and many authors consider this
body to be of hydrothermal origin. On the grounds that much of the mineralisation is within hypersthene
gabbros well above the base of the intrusion, there is some indication that the saturation of the magma in
S was achieved only after the crystallisation of the lower parts of at least some of the sills. This suggests
that the physical traps for sulphide mineralisation are more closely linked to the centers of the sills. The
late formation of the sulphides implied by this association would make gravitational accumulation of
sulphides into massive bodies less likely in flat sills, but perhaps more likely in stagnant dyke systems,
inclined sheets, or ring complexes.
Sulphur source
A source for the sulphur in many of the mineralised Nipissing gabbros remains uncertain. Either a
mantle or crustal source appears likely. An important issue is the S content of Huronian sediments, and
the role that they played in the metallogenesis of the Sudbury Region. No S/Se or S-- isotope data for
mineralised Nipissing gabbro exists at this time.
Silica control on sulphur saturation
The Nipissing gabbros are unusual in chemical composition. They have elevated SiO2, high La/Sm, high
Th/Nb, and other Nd-- isotopic compositions that suggest that their chemical composition is controlled by
a crustal material. Contamination has been documented in-- situ in the Kerns Intrusion (Lightfoot and
Naldrett, 1989), but is not linked to metallogenesis because the most fractionated rocks are also the most
contaminated, and their low Cu is due to assimilation of low-- Cu crustal rocks. Rather, the contaminated
nature appears to be an inherent feature of the Nipissing magma, and may be linked to the source and the
recycling of ancient crust into this source (Lightfoot et al., 1993). The similarity in composition to
dolerites in young CFB such as the Karoo and Siberia is an interesting observation, and the fact that the
silica content of the gabbros is high means that it would not take much differentiation or contamination
to achieve sulphur saturation of the Nipissing magma (Irvine, 1979).
10
Petrology and Geochemistry of the Nipissing Gabbro
Table 1. (cont.) Summary of empirical characteristics of the metallogenetic associations of the Nipissing gabbro intrusions, Ontario. Main points
from this study, Lightfoot et al. (1991), Conrod (1988, 1989), Lightfoot et al., (1993), and Card and Pattison, (1973).
Criteria for Metallogenetic
Evaluation of Nipissing Gabbros
Observations Regarding Nipissing Gabbro
Multiple batches of magma or
open system conduits?
Naldrett et al. (1995) suggest that the mineralised intrusions at Noril’sk were open system conduits to
flood basalt magmatism. Flow through a conduit will produce geochemical differences compared to
multiple influx and in-- situ crystallisation of batches of magma. In the Nipissing, Conrod (1989) reports
important data from the Gowganda area where she records evidence of four pulses of magma. On a
simple empirical basis, and extending this observation to other Nipissing sheets, there would appear to
be good evidence for the emplacement of multiple batches of magma into some intrusions. At issue is
whether these intrusions were conduits to high level volcanic edifices. Based on the similar bulk
composition of the different Nipissing intrusions, it appears that we have not yet found any intrusions
which contain significant amounts of cumulates left behind in a horizontal conduit such as that found at
Noril’sk. The search should continue for olivine and hypersthene cumulates, and these intrusions are
likely to be the ones which have fed any volcanic edifice.
Petrology - empirical association
of sulphides and mafic rocks
There is a well documented association of mineralisation with hypersthene-- rich, high-- Mg rocks, with
lower abundances of Tio2 and Zr in the Nipissing gabbro. The typical target values for identification of
these rocks are:
10-- 30 modal percent hypersthene
>9 wt.% MgO
<0.4 wt.% TiO2
<52 ppm Zr
Chemostratigraphy of intrusions
The presence of mineralisation high within the Nipissing gabbros suggests that exploration efforts in
these rocks should play close attention to the chemical stratigraphy of the intrusions. The geochemical
variations may well provide important information which constrain the possible role of gravitational
settling of sulphides, and the extent to which whole-- rock compositions are consistent with the
scavenging of Cu, Ni, and PGE from the magma.
Other miscellaneous observations
The present structural configuration of many sills is not the original configuration. A careful analysis of
the shape of the intrusion with respect to the Huronian stratigraphy is warranted.
We have no evidence at this time for more than one Nipissing magma type. A very important new
understanding would be achieved if picritic rocks were to be recognised in the Nipissing Province.
The similarity of the Nipissing to rocks of the diabase gabbro sills of the Trans-- Hudson orogen makes
other Ontario targets within the Sutton Inlier an increasingly exciting exploration target (Bostcok, 1971;
Lightfoot, 1994)
7.
Metamorphism of country rock and S sources. We
presently have no evidence that high-S shales or evaporites exist within the Huronian sequence, nor that the
metamorphic aureole around mineralised sills is substantially larger than that found around barren sills.
Exploration models which follow that for the Noril’sk
rocks should incorporate a search for crustal sulphur
sources and a search for significant contact
metamorphism of Huronian sediments.
SAMPLING AND ANALYSIS
Full details of sampling and analytical protocols are given
in Appendix 1. Sample locations and descriptions are
given in Miscellaneous Release Data (MRD) 19. Results
for in-house reference materials are given in Table 3.
EFFECTS OF THE TEXTURAL
VARIATIONS IN THE GABBRO ON
THE ANALYTICAL DATA
Wherever possible, fresh samples of rock were used for
analysis, and thin sections showed no evidence of strong
recrystallisation, albitisation or growth of metamict
minerals.
Studies of basalt, diabase and gabbro samples from
young CFB indicate that even very limited degrees of
alteration may be sufficient to mobilise some of the LILE
such as Rb and Ba (e.g. Cox and Hawkesworth, 1985), but
it is generally accepted that the REE, HFSE, Th, U, Ta, Nb,
and Y are relatively immobile under these conditions (e.g.
Hawkesworth and Morrison, 1978; Humphris and
Thompson, 1978). In some intrusions there are good
positive correlations between Rb and immobile elements
such as Zr (e.g. Lightfoot et al., 1987), and these data argue
that even the Rb data records petrogenetic information
rather than the effect of late alteration.
Compositional differences due to variations in grain
size between coarse and fine patches within the vari-textured gabbro were investigated using fresh samples from
vari-textured gabbro patches and the local host of the patch
(<1m from the patches) at two locations. The pairs were
86-155 (fine-grained host) and 85-156 (vari-textured
patch), and 86-160 (fine-grained host) and 86-161 (varitextured patch). Figure 5a-b demonstrated that the primitive mantle normalised (Sun and McDonough, 1989) spidergram patterns of the coarse and fine- grained samples
from each of the locations have essentially similar patterns
with the vari-textured samples showing an overall enrichment in the concentrations of the incompatible elements
compared to the finer grained host gabbronorite. Importantly, samples 86-154 and 86-155 also have similar
143Nd/144Nd and similar Sm/Nd. These data therefore
o
suggest that both the fine-grained host and the coarser11
OGS Study 58
Figure 3a. Ni versus forsterite relationships in olivines from the Cross
Lake Sill (after Conrod, 1988). Data for Insizwa from Lightfoot and
Naldrett (1984) and Lightfoot et al. (1984); data for Moxie Pluton from
Thompson and Naldrett (1984). Field of undepleted olivines from
Simpkin and Smith (1970).
Figure 3b. PGE distribution patterns for the Rathbun Lake showing
(after Lightfoot et al., 1993 and Rowell and Edgar, 1986).
grained vari-textured patches crystallised from the same
parental magma, and that the patches tend to trap the more
differentiated REE, LILE, and HFSE-enriched liquid than
the matrix.
Lightfoot et al. (1993) document geochemical data
for coarse- grained and fine-grained portions of a
vari-textured gabbronorite and a granophyric gabbro from
the Basswood Lake Intrusion (Figure 5c-d). They show
that the coarse- and fine-grained portions of the
vari-textured gabbro have very similar chemical compositions, but that the coarse-grained granophyric gabbro has
elevated REE relative to the finer grained matrix.
Lightfoot et al. (1993) suggest on these grounds that the
process producing the vari-textured gabbros does not produce significant relative fractionation of the incompatible
elements although it appears to have some effect on the
abundance levels of these elements.
12
GEOCHEMICAL EVIDENCE FOR
THE EMPLACEMENT AND IN-SITU
DIFFERENTIATION OF NIPISSING
INTRUSIONS
Petrographic data indicate that many of the Nipissing gabbro intrusions are characterised by well-developed chilled
diabase margins between 50cm and 5m wide, overlain by
10- 20m of quartz gabbro and then by 100-500m of hypersthene gabbro which grades up into 100-500m of hypersthene-poor gabbronorite and vari-textured diabase. In a
few locations such as the Kerns Intrusion, the Basswood
Lake Intrusion, and at Obabika Inlet on Lake Temagami
(Figure 6, the intrusions have localised zones of quartz diorite, granodiorite, granophyre, and aplitic granitoids, and
these were viewed by Bowen (1928) as extreme differentiates of the Nipissing magmas. At issue is whether: 1)any of
these variations point to an upward decline in incompatible
element concentrations away from the base of the
Figure 4. Geological relationships between Nipissing intrusions, mineralisation, and geophysical anomalies, Early Proterozoic mineralisation, and the Sudbury Igneous Complex (after Muir, 1984).
Petrology and Geochemistry of the Nipissing Gabbro
13
OGS Study 58
intrusions which might be termed a “reversed differentiation trend”, or provide evidence for multiple cyclical emplacement of magma into a single differentiating intrusion,
2)there are any compositional breaks within the intrusions
which might provide evidence for the repeated influx of
magma into or through the intrusions, or record evidence
of mixing of different batches of magma, 3)there are any
intrusions that underwent closed-system in-situ Rayleigh
Fractionation, 4)the compositions of the most fractionated
rocks record interaction with the Huronian sediments
through either melting and assimilation, or assimilation
linked to fractional crystallisation of the magma, 5) any of
the aplitic granitoids are direct anatectic melts of the
Huronian sediments.
A complete description of the study areas and rocks
are provided in Appendix 1, Lightfoot et al. (1989, 1991,
1993) and Conrod (1988, 1989, in preparation).
1.
Variation across the least differentiated Nipissing
gabbronorite Intrusions: Samples were selected
across the High Rock Island Sill, Portage Bay, Lake
Temagami (Appendix 1), where there is a good cliff
section exposing the lower part of a relatively undifferentiated gabbronorite sill. Eleven samples were
taken through a vertical distance of 100m along a section approximately perpendicular to the dip of the
lower contact of the intrusion (Figure 7a). Samples of
the fine- grained chilled diabase, basal quartz diabase,
gabbronorite, and hypersthene gabbro were analysed
and trace element data are shown in Figure 7b and c
normalised to the composition of primitive mantle
(Sun and McDonough, 1989). The chilled diabase and
basal quartz diabase and lowermost hypersthene gabbro samples show an upwards decline in La, Th, and
Zr concentration with little systematic change in MgO
or Ni content (see Figure 7a). The uppermost
hypersthene gabbro samples show an upward increase
in La, Th, and Zr which is accompanied by a significant decline in MgO and Ni content (see Figure 7a).
The spidergrams all have approximately the same
shape and marked positive Ba, Sr and Zr anomalies,
and negative Rb, Th, Ta, Eu, and Ti anomalies (see
Figure 7a), but the more basal quartz diabase has
elevated trace element abundances relative to the
gabbroic rocks.)
A similar reverse differentiation trend has been documented by Conrod (1988) in the Cross Lake and Bonanza
Lake Intrusions within the Cobalt and Sudbury areas, and
by Conrod (1989) in the Duncan Lake Intrusion west of
Gowganda (Figure 8). In a study of the Wanapetei Intrusion, east of Sudbury, Finn et al. (1982) suggest that compiled data from five traverses through the intrusion record
Photo 1. Sulphide globules in gabbro from the Wanapitei Intrusion, Sudbury District. The sulphide consists of pyrrhotite at the base of the globule and
chalcopyrite at the top of the globule. This texture is much like that described from the Waterfall Gorge deposit of the Karoo-aged Insizwa Complex
(Lightfoot et al., 1984), and the Noril’sk-Talnakh sulphides of the Siberian intrusions (Naldrett et al., 1992). Traditional wisdom suggests that these
globules are the product of the crystallisation of a bleb of immiscible sulphide in the silicate host. The in-situ fractionation of the sulphides is ascribed to
magmatic differentiation of the sulphide liquid to form a monosulphide solid solution pyrrhotite cumulate with an overlying trapped liquid. Blebs of
this type are assumed to record the bulk composition of the sulphide liquid which may form massive basal concentrations of sulphides in differentiating
intrusions.
14
Petrology and Geochemistry of the Nipissing Gabbro
Table 2. Summary of grade of mineralisation at Kukagami Lake and the Rathbun Lake Showing, Wanapetei-Kukagami Lake areas. Based on
Lightfoot et al. (1991). See Table 1 for additional information.
Rathbun Lake (n=14; Lightfoot et al., 1991):
Kukagami Lake (n=9; Lightfoot et al., 1991):
Concentration
Element
Concentration
S
1-- 18 wt.%
S
<3 wt.%
Cu
0.3-- 12.4 wt.%
Cu
0.1-- 1.1 wt.%
Ni
0.1-- 1.1 wt.%
Ni
0.1-- 0.4 wt.%
Element
Pt
100-- 6500 ppb
Pt
50-- 1200 ppb
Pd
200-- 35000 ppb
Pd
50-- 4200 ppb
Au
500-- 6500 ppb
Au
20-- 600 ppb
variations through a 822m thick gabbronorite intrusion.
They suggest that the intrusion develops five or more multiple reverse differentiation trends through the thickness of
the intrusion. In detail, the proximity of many of the traverses to the lower contact raise the possibility that these
cycles are repetitions of the same magmatic unit which was
sampled along strike within the same intrusion, which
would then suggest that only a single reversed differentiation trend is recorded in this intrusion. However, Conrod
(1989), in a systematic study of the Miller Lake Intrusion
west of Gowganda, shows that four trends of reverse
followed by normal differentiation are recorded in the
whole-rock major element and incompatible element
abundance data (see Figure 8). Conrod (1989) suggests
that the series of samples reflect their true relative stratigraphic position in a single intrusion with no fault- repetition. The data of Conrod (1989) are particularly valuable
as they indicate that the basal increase in Mg-number is
coupled to a decline in Zr, Y, Sr, and Rb, and although there
are no published data for the compatible trace elements
such as Ni, the Mg-number, Zr, Y, Sr, and Rb data correlate
with the increase in modal hypersthene abundance away
from the lower contact which is also a general feature of
the Portage Bay intrusion from this study of the Lake
Temagami Region (see Figure 7a). This suggests that the
basal compositional reversal reflects the emplacement and
crystallisation of magmas which are progressively more
heavily loaded with hypersthene phenocrysts. These data
therefore indicate that Nipissing gabbro intrusions frequently record systematic compositional trends which are
not readily explained by in-situ differentiation of a single
pulse of magma.
Reverse differentiation has been recognised in other
sills such as the Insizwa Complex of the Karoo Province
(Lightfoot et al., 1984), and in the Fongen-Hilligen
Complex in Norway, and are very different to the normal
differentiation trends of the Skaergaard Intrusion of East
Greenland (Wager and Brown, 1968) and the Pallisades
Sill of New Jersey (Walker, 1969). Traditional models require the emplacement of progressively more phenocryst
laden magmas to explain these basal reversals in composition. The data for the Nipissing sills which illustrate reversed differentiation trends are consistent with the emplacement of different batches of magma with variable hypersthene phenocryst abundance. The first magmas to be
emplaced were phenocryst poor and formed the chilled
diabase and quartz diabase. Magmas laden with hypersthene phenocrysts which formed in the conduits or chambers at depth were subsequently emplaced. Above this level there was a reversal towards the emplacement of phenocryst-poor magmas, and this process may have been repeated many times within a single intrusion. The continuous trends in petrology and geochemistry through the
cycles in the Miller Lake Intrusion and the lack of internal
baked or chilled contacts suggest that this event was a continuous one rather than an episodic emplacement of different batches of magmas into discrete sills. Importantly, this
suggests that geochemical variations in the Nipissing gabbro are consistent with the emplacement of multiple
batches of magmas into single magma chambers in a similar way to that proposed for the Talnakh Intrusion of the
Noril’sk Region (Czemanske et al., 1994; Naldrett et al.,
1995). It is not clear based on the available data whether
these sills were open system conduits, feeding higher level
volcanic edifices. Thus, although this makes the Nipissing
sills potential horizontal conduits for higher level flood basalt magmatism, the products of which have long since
been removed by erosion, the geochemical data available
at this time do not constrain whether: 1)these chambers
contain gabbroic units with igneous contacts; 2)the chambers were conduits through which vastly larger volumes of
magma passed than are presently seen, or 3)the intrusions
were the focus of magma mixing events within the chamber. Each of these models may be linked to different styles
and amounts of mineralisation.
2.
In-situ differentiation and crustal contamination
history of Nipissing intrusions: The Kerns Intrusion,
located northwest of New Liskeard, was chosen for
detailed study, and some of the data and conclusions
were published by Lightfoot et al. (1989). Here we report detailed data for this sill as it provides valuable
information on the differentiation and contaminationprocesses responsible for the evolution of the
Nipissing magma after emplacement into the
Huronian sedimentary sequence.
Figure 9a-d show primitive mantle-normalised (Sun
and McDonough, 1989) spidergrams for representative
samples from the different rock types in the Kerns
Intrusion. For a more detailed description of the sill and the
rock types, see Appendix 2, Lightfoot et al. (1989), and
Lightfoot (1995). The samples from the different rock
types all show moderate LREE and LILE enrichment, and
15
OGS Study 58
Table 3. Analytical data for in-house standard reference materials UTB-1 (University of Toronto basalt standard) and WHIN SILL (Open University
diabase standard) acquired in the course of this study, and comparison with expected values given in Lightfoot (1985).
Element
Whin Sill
(1984/85)
Whin Sill
(1986)
Whin Sill
(1987)
Whin Sill
Lightfoot (1985)
Expected
Whin Sill
n (INAA)
6
12
7
7
12
24.4(.8)
58.8(.8)
30.0(3.0)
7.0(0.3)
2.00(0.05)
2.41(0.11)
0.34(0.02)
24.0(1.9)
59.8(3.1)
29.7(1.9)
7.61(0.21)
1.87(0.19)
2.43(0.16)
.35(0.05)
25.9(1.5)
58.4(3.1)
33.3(2.0)
7.56(0.55)
2.30(0.12)
2.60(0.13)
.41(.02)
22.5(2.6)
57.5(3.1)
32.9(3.5)
7.27(2.1)
2.25(n.a.)
2.54(n.a.)
0.39(n.a.)
2.75(0.26)
n.a.
4.92(0.48)
0.45(0.14)
48.3(2.0)
30.4(0.8)
419(48)
2.82(0.11)
1.06(0.9)
4.8(0.3)
0.49(0.11)
n.a.
n.a.
387(59)
2.78(.11)
1.06(.08)
4.5(0.2)
0.52(0.08)
47.8(1.2)
31.0(0.6)
3.12(0.17)
1.31(0.06)
5.02(0.22)
0.80(9.14)
49.6(2.5)
n.a.
n.a.
3.05(n.a.)
1.26(n.a.)
4.93(n.a.)
0.90(n.a.)
47.4(3.1)
n.a.
n.a.
(1984/85)
UTB1
1sd
(1986)
UTB1
1sd
(1987)
UTB1
1sd
50.5
3.06
13.8
15.9
50.6
La
Ce
Nd
Sm
Eu
Yb
Lu
24.7(.5)
60.4(3.8)
28.4(5.2)
7.02(0.26)
2.04(0.10)
2.51(0.14)
0.35(0.02)
Th
Ta
Hf
U
Co
Sc
Ba
Element
SiO2
TiO2
A12O3
Fe2O3
MnO
MgO
CaO
Na2O
K2 O
P2O5
LOI
50.4
306.0
14.6
14.9
0.22
3.6
8.6
2.9
1.17
(0.6)
(0.12)
(0.6)
(0.6)
(0.00)
(0.4)
(0.4)
(0.4)
(0.10)
(0.06)
0.51
0.23
3.6
8.3
3.1
1.22
(0.5)
(0.08)
(0.2)
(0.3)
(0.01)
(0.3)
(0.1)
(0.5)
(0.03)
(0.04)
3.02
13.1
15.0
0.22
4.3
8.5
2.5
-------------------
(Lightfoot, 1985)
UTB1
1sd
49.1
3.00
13.3
14.6
0.22
4.5
8.5
2.85
1.23
0.72
n.a.
(0.4)
(0.03)
(0.3)
(0.1)
(0.01)
(0.3)
(0.1)
(0.03)
(0.05)
(0.03)
1.37
0.51
Method
L.L.D.
49.6
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
GRAV.
0.5
0.02
0.2
0.2
0.01
0.2
0.2
0.01
0.01
0.02
3.09
13.5
15.2
0.21
4.5
8.5
2.83
-----
0.74
0.64
n.a.
Exp.
UTB1
0.74
0.31
n.a.
V
Cr
Ni
Zn
Cu
Ga
Rb
Sr
Y
Zr
Nb
Ba
367.0
123
20
n.a.
30
n.a.
35.2
313
46
205
16.3
593
n - number of analyses
n.a. not available
16
(82.0)
(46)
(8)
(6)
(1.6)
(3)
(1)
(2)
(1.2)
(49)
n.a.
143
42
151
29
23
35
323
47
213
16.6
635
(18)
(5)
(4)
(4)
(3)
(2)
(4)
(1)
(4)
(0.9)
(45)
n.a.
44
22
123
32
n.a.
31
279
54
176
21
522
401.0
101
26
139
34
22
35
311
46
200
15.3
536
(7.0)
(7)
(7)
(5)
(6)
(2)
(1)
(5)
(1)
(4)
(0.6)
(16)
n.a.
n.a.
25
153
31
n.a.
32
312
41
202
n.a.
n.a.
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
WD-- XRF
5
10
5
10
5
5
5
5
5
5
3
10
Petrology and Geochemistry of the Nipissing Gabbro
Table 3. (cont.) Analytical data for in-house standard reference materials UTB-1 (University of Toronto basalt standard) and WHIN SILL (Open
University diabase standard) acquired in the course of this study, and comparison with expected values given in Lightfoot (1985).
Element
La
Ce
Nd
Sm
Eu
Yb
Lu
Th
Ta
Hf
U
Co
Sc
Cs
(1984/85)
UTB1
1sd
(1986)
UTB1
26.4
62.1
33.6
26.2
60.1
33.0
7.98
2.30
4.05
0.64
4.21
(1.6)
(2.8)
(2.0)
(0.18)
(0.22)
(0.42)
(0.06)
(0.06)
(0.06)
(0.96)
(0.18)
(2.0)
(1.3)
0.96
5.06
1.02
49.3
40.0
n.a.
8.10
2.26
3.90
0.58
1sd
(0.6)
(3.8)
(4.0)
(0.30)
(0.12)
(0.20)
(0.04)
(0.12)
(0.12)
(0.5)
(0.20)
(1987)
UTB1
23.5
61.4
30.5
8.28
2.09
3.81
0.59
4.02
4.00
1.03
4.9
.83
4.6
0.98
1.01
47.9
39.5
n.a.
n.a.
n.a.
1sd
(2.0)
(5.4)
(1.2)
(0.41)
(0.15)
(0.40)
(0.10)
(0.15)
(0.05)
(0.2)
(0.11)
(3.2)
(0.9)
(Lightfoot, 1985)
UTB1
1sd
27.4
61.0
36.1
8.0
2.6
Exp.
UTB1
Method
L.L.D.
26.7
6.5
32.0
8.0
2.4
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
INAA
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
4.13
4.00
0.70
4.4
0.62
4.6
1.03
5.1
1.02
4.3
n.a.
n.a.
n.a.
n.a.
n.a.
n(ME-- XRF)
n(TE-- XRF)
n(INAA)
Exp.
n.a.
LOI
L.L.D.
4
4
6
7
7
8
1
1
6
18
52
2
----------
- Expected n - number of analyses
- not available
- loss of ignition
- lower limit of detection
this is most pronounced in the granophyric and granitoid
samples with higher LREE and LILE abundance. The
gabbroic rocks exhibit negative Nb+Ta, P2O5 and TiO2
anomalies, and these anomalies are developed in samples
showing an entire spectrum of LILE and LREE
enrichment, including the least differentiated quartz diabase samples and the chilled margin (see Figure 9b). The
vari-textured diabase samples also exhibit marked negative Sr anomalies (see Figure 9c), and the granophyric
samples exhibit pronounced negative Eu, Sr, Rb, and K2O
anomalies (see Figure 9d), and pronounced negative TiO2
and P2O5 anomalies. Lightfoot et al. (1989) suggest that
the Rb and K2O anomaly reflects the fractional removal of
potassic feldspar, the Sr and Eu-anomaly reflects the removal of feldspar (possibly plagioclase), and the behaviour of P2O5, and TiO2 suggest that apatite and ilmenite,
respectively, were removed during fractionation. In detail,
the chilled diabase and undifferentiated quartz diabase
both have small negative anomalies for TiO2, P2O5, and
Ta+Nb (see Figure 9a), and Lightfoot et al. (1993) suggest
that these are features of the original parental magma. The
more pronounced negative anomalies for TiO2 and P2O5 of
the granophyres would then reflect fractional crystallisation of ilmenite and apatite within the sill, and the removal
of these phenocrysts.
The hanging wall Lorrain Formation of the Huronian
metasedimentary rock sequence is compared to the
composition of the aplitic granitoids and hornfelsed
sedimentary rock rafts from within the roof zone of the
Kerns Intrusion in Figure 9e. Interestingly, the primitive
mantle normalised patterns for these rocks are remarkably
similar suggesting that the metasedimentary rafts are bodies of the local Lorrain Formation hanging wall that were
broken off the roof of the intrusions and incorporated into
the magma as pendants. In detail, the field information
reported in Appendix 1 confirms that these pendants have
undergone considerable baking, and are petrographically
like the overlying Lorrain Formation sedimentary rocks.
The compositional similarity of the aplitic granitoids to the
roof rocks suggests that these aplites are anatectic melts of
the Lorrain Formation sediments, and this is consistent
with the field evidence given in Appendix 1 which records
evidence of large amounts of partial melting of the hanging
wall sedimentary rocks along arkosic layers to produce
fine veins of aplite which cross-cut the Lorrain
stratigraphy.
The coherent patterns of variation documented in
Figure 9a-e are perhaps better illustrated on geochemical
variation diagrams which use a common index of differentiation. Although no single element is an ideal choice, Zr
represents a useful compromise. It is readily determined,
with precision and accuracy of 5% (2s root mean standard
deviation), apparently immobile during alteration and
metamorphism, does not readily enter the crystal structure
17
OGS Study 58
of gabbroic mineral phases such as plagioclase and pyroxene, and is generally enriched in the residual liquid during
differentiation. Zr can be concentrated in sedimentary
materials, and the abundance is influenced to some extent
by crustal contamination, but more typically abundances
of Zr in crustal rocks are no more than 2-3 times that of the
parental Nipissing magma.
Figures 10a-y illustrate variations in selected major
and trace elements versus Zr. For comparison, these plots
also show the compositional average of Nipissing diabase
chilled margins (Lightfoot et al., 1989), and the average of
hanging wall Lorrain Formation sediments from above the
Kerns Intrusion (Table 7), and the compositional average
of aplitic granitoids from Obabika Inlet on Lake
Temagami (Appendix 2). These data reveal several important features relevant to a petrogenetic understanding of
the differentiation-contamination evolution of the
Nipissing Intrusions. The rocks from within the Kerns
Intrusion illustrate tight variations on element versus Zr
plots, and these trends pass through the compositional average of the chilled diabase. The vari-textured gabbronorite and granophyric diabase samples trend to low Mgnumber at high SiO2 and Zr; they are also poor in CaO and
Al2O3 (see Figures 10a-d). Above about 200 ppm Zr, TiO2,
K2O and P2O5falls with increasing Zr (see Figure 10e-g),
but the onset of TiO2 depletion slightly precedes the onset
of K2O and P2O5 depletion. Depletion of K2O in the granophyric gabbro samples is variable, but depletion of TiO2
and P2O5 is continuous with increasing Zr concentration.
The abundances of Sm and Zr increase along a 1:1 array
(see Figure 10i), but La is enriched with respect to the 1:1
La- Zr array, and Yb is depleted with respect to the 1:1 YbZr array (see Figures 10h and j). Figure 10k shows that
there is pronounced deepening of the magnitude of the Euanomaly with increasing Zr, and this is presumably a function of feldspar removal. Th and U behave much like La
(see Figure 10l-m), but Hf and Zr are closely correlated
along a 1:1 array (see Figure 10o) with Zr/Hf close to a
primitive mantle value of 38 (Sun and McDonough, 1989).
Ta is tightly correlated with Zr, but is slightly enriched in
the samples with highest Zr content relative to the 1:1 array
of ideal fractional crystallisation of a magma represented
in composition by the average chilled Nipissing diabase
(see Figure 10n-o). The behaviour of Rb and Ba follows
that of K2O (see Figures 10f, p, and s), and suggest significant removal of potassic feldspar. Sr falls with declining
Eu/Eu* which is consistent with plagioclase fractionation
(seeFigures 10k and q). Finally, the compatible elements
Cr, Ni, Co, V, Sc, and Zn all decline with increasing Zr concentration (see Figure 10t-y) suggesting that they are either
removed by fractionating minerals or have their
Figure 5a. Comparison of primitive mantle normalised spidergrams on vari-textured gabbronorite patches and gabbronorite host; samples 86PCL154
(coarse-grained) and 86PCL155 (fine grained) came from one outcrop of vari-textured gabbro in the Emerald Lake gabbro; samples 86PCL161
(coarse-grained) and 86PCL160 (fine-grained) came from one outcrop in the vari- textured gabbro of the Emerald Lake gabbro.
18
Petrology and Geochemistry of the Nipissing Gabbro
abundances diluted by the addition of crustal material
which has low abundances of these elements.
Many of the variations documented in Figures 9 and
10 are indicative of the entry of fractionating minerals, but
the behaviour of elements such as Th, U, and the La (see
Figures 10h, l, and m), and the compatible elements such as
Cr, Ni, Co, V, Sc, and Zn (see Figures 10t-y) are not readily
explained solely by the fractional removal of silicate
minerals. Importantly, the compositional average of the
Lorrain Formation hanging wall sediments is displaced to
high Th/Zr, U/Zr and La/Zr and is low in Cr, Ni, Co, V, Sc,
and Zn, which provides strong evidence that the varitextured gabbronorites and granophyric gabbro samples
are displaced away from the composition of the fractionating Nipissing magma towards the composition of the country rock. Moreover, the compositions of the aplitic granitoids from the roof of the Kerns Intrusion and the Obabika
Inlet region are compositionally similar to the Lorrain
Formation sediments, and it is reasonable to suspect that
these aplitic rocks are anatectic melts of the sediments, and
therefore likely candidate compositions for crustal melts
generated in-situ above the Nipissing sills, and assimilated
into the mafic magma (c.f. Lightfoot et al., 1989).
In the context of the available data set of all Nipissing
gabbros (see MRD 19), the Kerns Intrusion rocks fall on a
common trend of increasing Th/Nb, decreasing Cu/Zr, and
increasing SiO2 which are consistent with the contamination of the magma by crustal material rather than the fractional segregation of magmatic sulphide. It is not yet clear
whether this relationship also holds in the more strongly
mineralised Nipissing intrusions (Figure 11a-c).
Nd-isotope data were acquired for a number of samples from the Kerns Intrusion (Table 5) in order to test
whether there is an isotopic record of the assimilation of
Lorrain Formation sediments into the Nipissing magma.
Present day Nd-isotopic compositions shown in Figure 12a
range from 0.5110 in the aplites and sediments to 0.5122 in
the chilled basal quartz diabase, and these are accompanied by a systematic change in Sm/Nd ratio. The relationship between 143Nd/144Nd and 147Sm/144Nd for samples
from within the Kerns Intrusion does not define a tight isochron relationship, and regression of the data for the gabbronorites and granophyric rocks define a line with a slope
of 0.016 and an intercept of 0.5094 on 21 analyses (Figure
12b). This corresponds to a model age of 2.42Ga which is
significantly older than the U- Pb magmatic baddeleyite
ages of the vari-textured gabbronorite sample taken from
the Kerns Intrusion and determined by Noble and
Lightfoot (1992). Furthermore, the trend of the data on
the isochron diagram points towards the analysed
Figure 5b. Comparison of primitive mantle normalised spidergrams on vari-textured gabbronorite patches and gabbronorite host; Basswood Lake
Intrusion. Sample 88PCL115 (fine-grained) and 88PCL116 (coarse- grained) came from vari-textured gabbro; 88PCL120 (fine-grained) and
88PCL121 (coarse-grained) came from granophyric gabbro of the Basswood Lake Intrusion. Note the strong enrichment in the incompatible element
abundances of the vari-textured rock compared with the finer-grained gabbroic rock.
19
OGS Study 58
compositions of the Lorrain Formation sediments and the
aplites, which provides further quantitative evidence that
the Lorrain Formation sediments were assimilated by the
Nipissing magma.
Compositional variations within other strongly differentiated Nipissing Intrusions define similar variations to
the Kerns Intrusion. For example, the Basswood Lake
Intrusion described in Lightfoot et al. (1993) shows a range
in La/Zr, Th/Zr, and U/Zr which is consistent with assimilation of crustal sediments. Once again, it is the samples
with highest Zr and SiO2 which exhibit the largest
contribution from the sediments.
Lightfoot and Naldrett (1989; 1993) model the variations in the Kerns and Basswood Lake Intrusions in terms
of assimilation coupled to fractional crystallisation of the
magma. This model follows the algorithm of DePaolo
(1981) and Taylor (1980) as applied to the process first recognised by Bowen (1928). In this model, it is the latent
heat of crystallisation of the Nipissing magma which is
responsible for the production of a commensurate amount
of assimilation of the country rock at the roof of the
intrusion. In this model (Figure 13), the anatectic melts of
the country rocks are assimilated into the magma progressively as the magma differentiates until a compositional
interface is developed in the magma column which prevents further assimilation of the anatectic melt of the country rock, and results in the final crystallisation of an aplitic
granitoid melt at the roof of the intrusion. A more complete
documentation of this model is provided in Appendix 2.
3.
Dykes and sheets of undifferentiated gabbronorite compositional relationships: Two of the undulating
sills in the Temagami Region are fed by sub-vertical
gabbronorite dykes (see Appendix 1 for locations,
geology, petrology, and distribution of samples).
These dykes are termed the “Narrows Island dyke”
and the “Sand Point dyke” (Appendix 1). Sampling
across each of these intrusions revealed some local
variation in major and trace element concentration
which was dominantly controlled by the grain size and
the development of vari-textured gabbronorite
patches which are locally enriched in incompatible
elements. No strong systematic differentiation across
Figure 6. Location of individual intrusive bodies and detailed study areas referenced in this report. Locations of individual intrusions and references
where further information is given: 1) Bruce Mines Intrusion (see Figure 1.8b for sample locations); 2) Basswood Lake Intrusion (see Figure 1.8c and
Lightfoot et al., 1993 for sample locations); 3) Casson Lake Intrusion (see Card, 1976, 1984).; 4) Sudbury Gabbro Intrusions ( see Card, 1986); 5) Black
Lake Intrusion; 6) Wanapitei Intrusion (see Figure 1.8d and Lightfoot et al., 1993 for sample locations); 7) North Janes Intrusion (see Dressler, 1979); 8)
South Janes Intrusion (see Dressler, 1979); 9) Emerald Lake and Temagami Intrusions (see Figures 1.1 through 1.4); 10) High Rock Intrusion (see
Figure 1.1); 11) Slide Rock Intrusion (see Figure 1.1); 12) 20-22 Cobalt intrusions (see Figure 1.8e, Lightfoot et al., 1993, and Conrod, 1988 for sample
locations); 13) Kerns Intrusion (see Figures 1.5 and 1.6, and Lightfoot et al., 1993); 14) Englehart Intrusion (see Figure 1.8a); 15) -19 Gowganda
Intrusions (see Conrod, 1989); and 21) Bonanza Lake Intrusion (see Conrod, 1988).
20
Petrology and Geochemistry of the Nipissing Gabbro
Figure 7a. Chemostratigraphy of the High Rock Intrusion, Lake Temagami.
Figure 7b. Primitive mantle normalised spidergrams of the High Rock Intrusion samples, Lake Temagami.
the dykes was found, and there is no evidence for
variations in hypersthene content across the dykes
which might reflect flowage differentiation during
emplacement. Petrologically and geochemically, the
dykes are almost identical to the overlying gabbroic
intrusions, and this is reflected in the primitivemantle normalised (Sun and McDonough, 1989) diagrams for representative samples (Figures 14a-b). On
these grounds, these undulatory sills and their feeder
zones appear to have been derived from one parental
21
OGS Study 58
Figure 7c. Primitive mantle normalised spidergrams of the High Rock Intrusion samples. Normalisation factors from Sun and McDonough
(1989).
Figure 8. Geochemical stratigraphy of the Miller Lake Intrusion, Gowganda. After Conrod (1989). Note the four breaks in stratigraphy which are
interpreted to correspond to new pulses of gabbroic magma into the chamber (Conrod, 1989).
4.
22
magma type which has undergone a limited amount
of in-situ differentiation to form vari-textured gabbronorite pods in an otherwise medium-grained
gabbronorite.
Origin of aplitic granitoids at Obabika Inlet - anatectic melts of Huronian sediments?: A large body of
aplitic granitoid is located at Red Rock in the
Obabaika Inlet of Lake Temagami (Appendix 1). This
aplite consists of a relatively undifferentiated body in
sharp contact with the Obabaika Intrusion of
Nipissing gabbronorite. The aplite is exposed in two
different areas, and each consists of 500 by 500m wide
Petrology and Geochemistry of the Nipissing Gabbro
units with >100m thickness; these aplitic granitoids
appear to grade into dioritic Nipissing gabbro south of
Obabaika Inlet, and locally these rocks are in sharp
contact with Gowganda Formation shales. The aplitic
granitoids are strongly granophyric and are similar to
the aplites developed in the roof zone of the Kerns Intrusion (Lightfoot et al., 1989). The aplitic granitoids
range from granophyric gabbros through to pure
quartz-feldspar granitoids (Simony, 1964). The large
size of these felsic intrusions and the apparent association with the roof zone of a Nipissing intrusion suggests some genetic link, and we therefore investigated
how the bulk compositions of these aplitic granitoids
relate to the composition of the Nipissing gabbros and
the locally complex quartz diorite-granodiorite-granophyre zone at the roof of the Obabika Intrusion in
the context of existing models for the assimilation of
roof sediments in the Kerns Intrusion.
Figure 15a-b compares and contrasts mantle normalised
spidergrams for the aplites from Obabika Inlet with the
local Gowganda Formation sedimentary rocks. The aplites
are compositionally quite uniform and similar in incompatible element pattern to the Gowganda Formation sedimentary rocks (see averages in Table 7). This suggests that
this large body of aplite was generated by melting of the
Gowganda Formation sedimentary rocks. In this particular
example it is possible that the heat from the Nipissing
intrusion was responsible for this melting event, but that
assimilation of the aplite was not complete.
5. Relationships in the roof zone of the Obabika
Intrusion, Temagami - in-situ assimilation of
Huronian sediments by a Nipissing gabbro sill: The
Obabika Intrusion west of Lake Temagami and south
of the Obabika Inlet is described in Appendix 1. The
intrusions consists of a series of differentiates of
Nipissing magma which appear to have variably interacted with pendants of Gowganda Formation sedimentary rocks which are hosted within the granodioritic and quartz dioritic rocks. Locally, the quartz diorites and granodiorites with sedimentary inclusions
and rafts contain blebby sulphide mineralisation, but
these sulphides appear to be devoid of Cu, Ni, Pt, and
Pd.
The geochemical variations in the chilled diabase, basal
quartz diabase, quartz diorites, granodiorites are relevant
to the relationship of the aplitic rocks and footwall Gowganda Formation sediments to the vari-textured gabbros,
quartz diorites, and granodiorites of the Obabika Intrusion.
Figure 16a-b shows representative primitive mantle
normalised spidergrams for these rocks. Many of the granodiorites and quartz diorites from the roof of the intrusion
are compositionally similar to the aplitic granitoids (c.f.
Figure 15b). These granodiorites and quartz diorites may
be Nipissing magmas which are heavily contaminated.
Figure 9a. Primitive-mantle normalised trace element spidergrams for representative samples from the Kerns Intrusion, northwest of New Liskeard.
Chilled basal quartz diabase, quartz diabase.
23
OGS Study 58
This is certainly supported by the good field evidence for
assimilation of Gowganda Formation sedimentary rocks
described in Appendix 1.
DURATION OF NIPISSING
MAGMATIC ACTIVITY AND
ASSOCIATED COMPOSITIONAL
VARIATION
Buchan and Card (1985) and Buchan et al. (1989) report
remanence directions from undulating sills throughout the
eastern portion of the Nipissing Gabbro Province.
Petrographic studies indicate that fresh diabases and gabbros carry three characteristic remanence directions. The
N1 remnance is usually up to the north, N2 remnance always dips down steeply to the west, and N3 is up to the west
(Buchan et al., 1989). Buchan and Card (1985) demonstrate that N1 and N2 signatures are carried by moderately
fresh lithologies. Nevertheless thermal overprinting could
not be ruled out. Likewise, Buchan et al. (1989) demonstrate that the N3 signature is also present in fresh gabbros
throughout the larger part of a major Nipissing intrusion.
Within most sites studied by Buchan and Card (1985)and
Buchan et al. (1989), one of the three remanence directions
is recorded. Three phases of Nipissing magmatism are
proposed by Buchan et al. (1989) as an explanation for the
variation, assuming that the magnetization is primary. The
fact that the three remanence directions are so varied
suggests that either the directions are not primary, or
magmatism occurred over a protracted period of time.
The locations of sites studied for paleomagnetic remanence direction within Nipissing intrusions was provided
to the author by K. Buchan (personal communication,
1986). During this study, the paleomagnetic drill sites
(which were originally recorded accurately on Ontario
Base Maps) were located in the field. Detailed site information for the Kerns and Hudson Townships and the
Englehart area are presented in Appendix 1 (see Figure
1.8). Samples were collected from sites representing each
of the three distinctive paleomagnetic signatures - N1, N2,
and N3. Analytical and locational data for these samples
are presented in MRD 19.
Mantle-normalized trace element variation diagrams
(Thompson et al. 1984) for samples from sites showing the
average and range in analyses corresponding to each of the
three remanence directions are plotted in Figure 17a-c.
Several features are clear from the spidergrams: the most
trace-element depleted patterns are found in samples
recording an N1 remanence; N3 sites show slightly higher
incompatible element levels. N2 sites show the strongest
overall incompatible element enrichment relative to N1
and N3.
Figure 9b. Primitive-mantle normalised trace element spidergrams for representative samples from the Kerns Intrusion, northwest of New Liskeard.
Hypersthene gabbro.
24
Petrology and Geochemistry of the Nipissing Gabbro
In detail, data for samples from the three sites were
plotted on trace-element variation diagrams, using Zr as an
index of trace-element enrichment. Figure 17d shows representative plots of selected elements versus Zr. As with
data for samples collected within single intrusions (see discussion of the Kerns Intrusion), tightly correlated variations are recorded on all of the trace element variation plots
(e.g., Figure 10). Of notable importance is the observation
that N1 and N3 data fall in overlapping fields, and N2 data
falls in a separate field. There is a progressive trend of
overall increase in the abundance of incompatible trace
elements from N1 samples through N3 samples to N2 samples. The increase in Zr is accompanied by progressive increases in LILE/HFSE (e.g: Th/Yb, La/Zr), such that samples with higher overall incompatible trace element concentrations have higher Th/Yb and La/Yb as well as higher
overall LILE and LREE concentrations. Samples with
highest incompatible element concentrations are also depleted in compatible elements (Sr, Ni, Co, Cr, Sc, V), and
have the lowest Mg-numbers.
When compared to the data trends of other Nipissing
intrusions (e.g., Lightfoot and Naldrett, 1989), the trend of
the N1-N3-N2 data is very similar to the normal
differentiation trends of the magma.
These features could result from the emplacement of
variably differentiated magmas corresponding to the N1,
N2, and N3 events, or, more likely, they could reflect the
way in which sampling was performed within a single differentiated magma type. Generally, samples from N2 sites
are more differentiated than samples from N1 and N3 sites,
but there is a broad compositional spectrum associated
with each remanence direction. On a gross scale, if the N1
sites are located in sill basins, whereas N2 and N3 sites record locations on the basin-limb and limb-arch region of a
sill, then the observed features are consistent with the
zonation of lithologies within Nipissing sills. However,
this does not resolve the problem posed by the different remanence directions. To study this problem, U-Pb
geochronological studies were undertaken on magmatic
baddeleyite from vari-textured gabbronorites.
Until recently, precise U-Pb ages could not easily be
acquired from mafic rocks, but more recently, techniques
have been developed to date small crystals of baddeleyite.
Corfu and Andrews (1986) first reported an age of
2219.4(+3.6, -3.5)Ma on the Nipissing diabase from the
Castle Mine in Gowganda based on one baddeleyite and
two rutile analyses. Baddeleyite was the most abundant
non-magnetic mineral suitable for U-Pb dating purposes
and was present in the pegmatoidal facies of the varitextured gabbro.
To investigate the ages of the Kerns and Triangle
Mountain Intrusions, two suites were collected for U-Pb
dating (see Appendix 1 for locations and Table 2 for wholerock analyses). The samples come from opposite sides of
Figure 9c. Primitive-mantle normalised trace element spidergrams for representative samples from the Kerns Intrusion, northwest of New Liskeard.
Vari-textured gabbro.
25
OGS Study 58
the Cross Lake Fault. One (south west) comes from the
center of a quartz diabase, and consists of vari-textured
diabase; the other comes from the north east of the fault
and is a pegmatoidal vari-textured diabase.
The sample from the Kerns sill was dated at
2215±3Ma, whereas the sample from the Triangle
Mountain sill was dated at 2209±4Ma. Full details of the
methods and results are presented elsewhere (Noble and
Lightfoot, 1992).
It can be concluded from these data that the three sills
were emplaced over a relatively short period of time. However, there are resolvable differences in the timing of individual intrusive events. A maximum of 10 million years
elapsed between the intrusion and cooling of the two magmas. If a period of several hundred million years (as would
seem likely assuming that the pole has wandered at a similar rate to that observed over the past 500 million years),
then the slightly different age is apparently at variance with
the marked difference in the paleomagnetic signatures of
the sites either side of the Cross Lake Fault. Buchan et al.
(1989) discuss the implications of the paleomagnetic data
in greater depth, but we note the following constraints on
models for temporal evolution of the Nipissing gabbro:
1.
Nipissing magmatic activity took place over a relatively short time period (less than 10 Ma) in the
Gowganda and Cobalt areas.
2.
3.
During the magmatic event, large volumes of
essentially homogeneous magma were generated and
intruded throughout the entire magmatic province.
If the remanence directions are not primary in origin
(and this is still not fully resolved), then the paleomagnetic data have little direct bearing on the recognition
of multiple phases of magmatic activity.
COMPOSITIONAL VARIATION IN
THE PARENTAL NIPISSING
MAGMA TYPE
As part of a regional study, samples of the chill and basal
quartz diabase and gabbro were collected from the least
differentiated Nipissing gabbros in order to assess their
compositional range. The goal of this was to establish the
degree of variation in the parental magma composition
throughout the magmatic province. The importance of this
exercise rests in the application of geochemical criteria to
identify Nipissing intrusions with unusual geochemical
compositions, and to determine whether the geochemical
compositions of mineralised intrusions are different to
apparently barren intrusions.
Samples were selected of chilled quartz diabase,
quartz diabase, and hypersthene gabbro. Mineralogically,
these rocks consist of approximately similar proportions of
quartz, feldspar, and pyroxene, with the exception of the
Figure 9d. Primitive-mantle normalised trace element spidergrams for representative samples from the Kerns Intrusion, northwest of New Liskeard.
Granophyric gabbro.
26
Petrology and Geochemistry of the Nipissing Gabbro
hypersthene gabbro which has up to 30 modal %
hypersthene as a cumulate mineral phase. Geochemical
variations due to contamination and extreme fractionation
were avoided by exclusion of vari-textured gabbros, felsic
gabbronorites, granophyric diabases, and granitoids. With
the exception of rocks strongly enriched in cumulus hypersthene or carrying 1-5% modal sulphide, the variations in
the compositions of the least differentiated rocks may then
be taken as an approximation to the parental magma composition. In the case of the mineralised hypersthene-rich
rocks, the abundances of the major and trace elements will
reflect the modal cumulate hypersthene content, however
the ratios of the incompatible elements provide a robust indicator of the parental magma composition which can be
compared with the undifferentiated diabase, gabbro, and
chilled margin samples.
Table 6 summarises the average compositions of
selected samples from 20 different intrusions. In some
cases, the average is biased compositionally towards the
hypersthene gabbros (e.g. Kukagami Lake). In the case of
the Beaton Bay Intrusion (Conrod, 1989), Narrows island
Dyke, Sand Point Dyke, Duncan Lake Intrusion (Conrod,
1989), Milner Lake Intrusion (Conrod, 1989), and Portage
Bay Intrusions (Conrod, 1988), the sample suite consists of
more felsic gabbronorites which are believed to be more
differentiated than the typical gabbros as reflected in their
overall higher incompatible element concentrations.
Table 6 also shows representative analysis of good
samples of chilled quartz diabase collected at the margins
of the Nipissing Intrusions. These analysis arguably
provide a good index of the parental magma composition
as they were some of the first magmas to crystallise as they
were rapidly chilled against the footwall sediments. The
value of chilled margins as indicators of parental magma
composition has been questioned, and there is some indication that the chills of large layered complexes are not
representative of the bulk composition of the intrusion, and
are sometimes influenced by the assimilation of footwall
sediments. However, the data that are shown in Table 6 are
valuable in demonstrating a number of points:
1.
CHILLED MARGINS: The chilled margin samples
from the 9 different samples have similar major- and
trace-element compositions (within analytical uncertainty; see Table 1). Despite coming from locations
throughout the Nipissing and Temiskaming Regions
spanning over 250km laterally, no statistically significant variation in the compositions of these chills was
found. If these chills recorded assimilation of footwall
sediments, then some bias in their compositions towards the compositions of the footwall sediments
would be expected, and the amount of displacement in
the composition of the chill away from the parental
magma would be a function of the composition of the
footwall and the amount of assimilation. As no
Figure 9e. Primitive-mantle normalised trace element spidergrams for representative samples from the Kerns Intrusion, northwest of New Liskeard.
Aplites.
27
OGS Study 58
assimilation is evident from the geochemical data, and
as the chilled margins are devoid of inclusions, we argue that the chilled margin data are of paramount
importance in the characterisation of the parental
magma composition. The average composition of
these chills shown in Table 6 reflects the composition
of a uniform magma emplaced into at least seven different intrusions. The characteristics of this parental
magma are unusual as Lightfoot et al. (1993) demonstrated. The Nipissing magma type is silica- rich, with
elevated LREE/HREE and LILE/HFSE ratios, and
pronounced negative Ta+Nb, Ti, and P anomalies.
These are all characteristics of a magma which has interacted with a crustal reservoir. Moreover, as recent
models demonstrate the importance of contamination
(Naldrett et al., 1992; Lightfoot et al., 1994) in the
context of the triggering of sulphur saturation in mafic
magmas (Irvine, 1975), it is particularly good news to
explorationists that so many of the Nipissing magmas
have this contaminated geochemical composition. At
issue is whether this contamination signature is a feature of the source (e.g. Lightfoot et al., 1993), or
whether large quantities of mantle- derived Nipissing
magma interacted with the crust in a large deep-crustal reservoir of the type proposed by Cox (1980) for
flood basalt settings.
2.
QUARTZ DIABASE AND GABBROS: A considerably
larger data base of quartz diabase and gabbro samples
from 21 different intrusions has been assembled in
MRD 19 and summarised in Table 6 as averages for
each of the individual intrusions. These data indicate
that the magma giving rise to all 21 of the studied intrusions was remarkably similar in composition across
the magmatic province. Moreover, the compositions
of these samples is remarkably similar to the compositional average of the chilled margin samples. This
suggests that the magma giving rise to many of the intrusions of the Nipissing have the characteristics of the
chilled margin. Examination of the Ni data and the
limited amount of Cu data for these samples suggest
that the concentrations of these elements in the unmineralised samples are of the order of 80-160ppm, which
is not unusual for a gabbro. We do not see any S-poor
samples with very low Ni and Cu contents, although a
few samples with low Cu (6ppm) have been characterised in the Wanapetei Intrusion by Dressler (1982,
page 63); these values now need to be confirmed in the
context of a search for Ni and Cu-depleted gabbros.
Nd-isotopic variation within the basal quartz diabases and
chills at 2.22Ga is small; epsilon-Nd values of -2 to -4 are
found in four different intrusions (see Table 5 and
Lightfoot and Naldrett, 1989).
Figure 9f. Primitive-mantle normalised trace element spidergrams for representative samples from the Kerns Intrusion, northwest of New Liskeard.
Hornfelsed sediment rafts within the granophyric gabbro, and roof sediments. See text and Appendix 1 and Lightfoot et al. (1987) for detailed sample
locations and descriptions.
28
Petrology and Geochemistry of the Nipissing Gabbro
Figure 10. Geochemical variations in samples from the Kerns Intrusion, where elemental and oxide abundances are plotted against Zr concentration.
For detailed description, see text. a-g major element oxide variations with Zr concentration. h-k Variations in rare earth element (REE) abundances and
magnitude of Eu-anomaly with Zr concentration. l-s Variation in incompatible trace element abundances with Zr concentration. t-y. Variation in compatible element abundance with Zr concentration.
29
OGS Study 58
Figure 10. (cont’d) Geochemical variations in samples from the Kerns Intrusion, where elemental and oxide abundances are plotted against Zr concentration. For detailed description, see text. a-g major element oxide variations with Zr concentration. h-k Variations in rare earth element (REE)
abundances and magnitude of Eu-anomaly with Zr concentration. l-s Variation in incompatible trace element abundances with Zr concentration. t-y.
Variation in compatible element abundance with Zr concentration.
30
Petrology and Geochemistry of the Nipissing Gabbro
Figure 10. (cont’d) For detailed description, see text. a-g major element oxide variations with Zr concentration. h-k Variations in rare earth element
(REE) abundances and magnitude of Eu-anomaly with Zr concentration. l-s Variation in incompatible trace element abundances with Zr concentration.
t-y. Variation in compatible element abundance with Zr concentration.
31
OGS Study 58
Figure 10. (cont’d) Geochemical variations in samples from the Kerns Intrusion, where elemental and oxide abundances are plotted against Zr concentration. For detailed description, see text. a-g major element oxide variations with Zr concentration. h-k Variations in rare earth element (REE)
abundances and magnitude of Eu-anomaly with Zr concentration. l-s Variation in incompatible trace element abundances with Zr concentration. t-y.
Variation in compatible element abundance with Zr concentration.
32
Petrology and Geochemistry of the Nipissing Gabbro
Figure 10. (cont’d) Geochemical variations in samples from the Kerns Intrusion, where elemental and oxide abundances are plotted against Zr concentration. For detailed description, see text. a-g major element oxide variations with Zr concentration. h-k Variations in rare earth element (REE)
abundances and magnitude of Eu-anomaly with Zr concentration. l-s Variation in incompatible trace element abundances with Zr concentration. t-y.
Variation in compatible element abundance with Zr concentration.
33
OGS Study 58
Figure 10. (cont’d) Geochemical variations in samples from the Kerns Intrusion, where elemental and oxide abundances are plotted against Zr concentration. For detailed description, see text. a-g major element oxide variations with Zr concentration. h-k Variations in rare earth element (REE)
abundances and magnitude of Eu-anomaly with Zr concentration. l-s Variation in incompatible trace element abundances with Zr concentration. t-y.
Variation in compatible element abundance with Zr concentration.
34
Petrology and Geochemistry of the Nipissing Gabbro
Huronian sedimentary rocks
(
average, n=6, after Lightfoot & Naldrett, 1989)
Quartz diorite
Aplite
LREE depleted granophyric gabbro
LREE enriched granophyric gabbro
Vari-- textured gabbro
Hypersthene gabbro
Basal quartz gabbro
Chill (average, n=6)
Figure 10. (cont’d) Geochemical variations in samples from the Kerns
Intrusion, where elemental and oxide abundances are plotted against Zr
concentration. For detailed description, see text. a-g major element
oxide variations with Zr concentration. h-k Variations in rare earth
element (REE) abundances and magnitude of Eu-anomaly with Zr
concentration. l-s Variation in incompatible trace element abundances
with Zr concentration. t-y. Variation in compatible element abundance
with Zr concentration.
Figure 11. a) The variation in Th versus Nb falls on a trend between
average chilled diabase and an average of sediments from the roof of the
Kerns Intrusion; in contrast, the fractionation vector points to much
higher Nb.; b) Variation in Cu versus Zr in Nipissing gabbros (all data
grouped by rock type). Except for some vari-textured gabbros, the data
fall below the ideal fractionation array, and this is ascribed to the
assimilation of low-Cu Lorrain or Gowganda Formation sediments by
the Nipissing magma.; and c) Variation in Cu/Zr versus SiO2 for all
Nipissing gabbro samples (data grouped by rock type). This plot
demonstrates that the granophyric gabbros with lowest Cu/Zr have the
highest SiO2 content, and therefore depletion in Cu is ascribed to
assimilation of sediment with low Cu/Zr rather than fractionation of
sulphides which have exceptionally high Cu/Zr.
35
OGS Study 58
Table 5. Nd isotope data for samples of Nipissing gabbro. Analyses were performed at the University of Toronto using a clean laboratory and thermal ionisation mass spectrometer as documented in the text.
Location/Sample
Rock
Type
143Nd/144Nd
Nd
(ppm)
Sm
(ppm)
147Sm/144Nd
143Nd/144Nd
0
Present Day
Kerns Intrusion
87-- 167
86-- 169
86-- 172
86-- 189
86-- 186
86-- 197F
86-- 197C
86-- 198
H
V
V
G
G
S
S
T
0.512127
0.511828
0.511670
0.511520
0.511606
0.511096
0.510906
0.511098
5.43
20.07
39.62
46.18
12.61
47.08
43.66
56.03
1.54
5.06
9.30
10.53
3.04
7.82
7.14
10.26
0.1710
0.1524
0.1419
0.1378
0.1457
0.1004
0.0988
0.1107
0.50963
0.50960
0.50959
0.50951
0.50948
0.50949
0.50946
0.50948
Kerns Intrusion
(Suite 87-- 2)
Whole rock
V
0.51206
9.68
2.63
0.1642
0.50966
Whole rock
V
0.51189
15.41
4.11
0.1612
0.50960
Basswood Lake
Intrusion
84-- 23
84-- 14
84-- 22
84-- 18
84-- 11
84-- 27
84-- 21
V
V
H
H
V
V
V
0.511770
0.511962
0.512051
0.511990
0.511706
0.511508
0.511559
23.30
12.67
5.89
5.63
14.09
35.10
35.10
5.74
3.44
1.65
1.54
3.36
8.12
7.98
0.1489
0.1641
0.1697
0.1652
0.1442
0.1398
0.1374
0.50959
0.50956
0.50957
0.50957
0.50959
0.50946
0.50955
S
A
H
C
0.511109
0.511120
0.512048
0.512040
31.87
30.58
8.75
7.36
5.78
5.74
2.47
2.10
0.1096
0.1134
0.1706
0.1725
0.50951
0.50946
0.50955
0.50952
B
B
0.512028
0.512007
8.57
19.46
2.35
5.36
0.1658
0.1665
0.50960
0.50597
High Rock
Island Intrusion
86-- 136
C
0.512156
8.46
2.40
0.1714
0.50965
Portage Bay
Intrusion, Cobalt
(samples from
Conrod, 1988)
PB744
PB745
PB747
PB743
-----
0.511718
0.511449
0.511834
0.511739
39.40
25.39
11.97
30.45
9.24
5.62
3.13
7.45
0.1418
0.1338
0.1580
0.1479
0.40965
0.50949
0.50952
0.50958
Triangle
Mountain
Intrusion
(Suite 87-- 1)
Obabika
Intrusion
86-- 203
86-- 073
86-- 104
86-- 100
Emerald Lake
Intrusion
86-- 155
86-- 156
H-- Hypersthene gabbro
B-- Basal quartz diabase
C - Chilled contact diabase
S - Huronian sediment
V-- Vari-- textured gabbro
36
G-- Granophyric gabbro
T - Sedimentary rock inclusion
A - Aplitic granitoid
Suites 81-- 1 and 87-- 2 were dated by U - Pb geochronology
(Noble and Lightfoot, 1993).
Petrology and Geochemistry of the Nipissing Gabbro
On a local scale, chemical variations are more complex (see above discussion), but on a regional scale, these
data suggest that the chilled margins and basal quartz diabases which make up over 25% of most intrusions are compositionally similar. More importantly, these data, like other data for many modern CFB, suggest that the compositional evolution of the parental magma had reached a remarkably uniform stage throughout the Province at the
time of emplacement. Whilst it is possible that the polybaric fractionation of plagioclase, pyroxene, and olivine may
have buffered the composition of the magma to produce a
relatively uniform degree of differentiation (e.g., Cox,
1980), it is hard to see how this magma could retain it’s
gross homogeneity, unless it was entirely uncontaminated
by the continental crust as it migrated from the mantle to
the site of intrusion. This in-turn suggests that the source
was very homogeneous. .
If the Nipissing magma was erupted as a uniform magma, then it might be reasonable to suggest that most of the
magmatic activity occurred over a relatively short time
span, where common petrological processes controlled the
composition of the magma. This is consistent with the
U-Pb geochronology which suggests that at least three of
the intrusions were emplaced over a period of <10 million
years.
EMPIRICAL OBSERVATIONS
RELATED TO MINERAL
POTENTIAL, LAND USE
PLANNING AND EXPLORATION
This report presents new data for samples from across the
Nipissing Province, and detailed new information for the
intrusions of the Temagami Region which lie east of the
major Wanapetei gravity and aeromagnetic anomaly and
around the Temagami copper deposit. The study documents new occurrences of disseminated sulphide mineralisation not previously described in the Obabika and Kerns
Intrusions, and shows that these late blebs of sulphide associated with heavily contaminated granodiorites and granophyres are devoid of Ni, Cu, and PGE. However, the study
also highlights a range of empirical observations which
suggest that continued exploration of the Nipissing gabbros, especially along the trend between Whitefish Falls
and Temagami is justified. Table 1 summarises these
Figure 12. a) Variation in 143Nd/144Nd versus 147Sm/144Nd in samples from the Kerns Intrusion and local country rocks. b) Relationship of the array
of the Kerns Intrusion to isochron lines based on U - Pb geochronology for magmas with a range in initial 143Nd/144Nd isotopic composition. See text
and Lightfoot et al. (1989) for further discussion.
37
OGS Study 58
Figure 13. A model for the evolution of the Kerns Intrusion which is applicable to other differentiated Nipissing Intrusions. Stage 1. basic magma is
emplaced within the Lorrain Formation stratigraphy, and chilled along the lower margin. Stage 2. Near equilibrium crystallisation of the basal quartz
diabase and hypersthene diabase produces latent heat which melts the roof sediments. Stage 3. The less dense aplitic magma is separated from the
underlying mafic magma by a double diffusive interface (DDI) which permits heat transfer, but limited chemical transfer. A transition from equilibrium
to fractional crystallisation is accompanied by a reduction in the density of the mafic magma, thereby promoting the convective erosion of the interface.
Assimilation and fractionation are linked at this stage, but the ratio of the amount of assimilation to the amount of fractionation increases. Blocks of
hanging-wall sediment are entrained within the aplitic magma as large rafts. Eventually the interface is almost completely destroyed and the
differentiated Nipissing magma and the anatectic melt of the sediments mix freely. In some cases the aplitic melts are isolated from further mixing with
the Nipissing magma and crystallise as aplitic granitoids which are geochemically similar to the country rocks. See text for further discussion.
Figure 14a. Primitive mantle normalised spiderdiagrams. Representative samples from the Narrows Island gabbronorite dyke.
38
Petrology and Geochemistry of the Nipissing Gabbro
Figure 14b. Primitive mantle normalised spiderdiagrams. Representative samples from the Sand Point gabbronorite dyke.
Figure 14c. Primitive mantle normalised spiderdiagrams. Comparison of the compositions of gabbronorites from the Sand Point and Narrows Island
dykes with the overlying undulatory sills.
39
OGS Study 58
Figure 15a. Primitive mantle-normalised compositions of aplites from the Obabika Intrusion, Lake Temagami. Note the similarity of the pattern to the
Gowganda Formation sediments from the eastern contact. Gowganda Formation sediments.
Figure 15b. Primitive mantle-normalised compositions of aplites from the Obabika Intrusion, Lake Temagami. Note the similarity of the pattern to the
Gowganda Formation sediments from the eastern contact. Aplitic granitoids.
40
Petrology and Geochemistry of the Nipissing Gabbro
Figure 16a. Primitive mantle normalised compositions of differentiated granodiorites and quartz diorites of the Obabika Intrusion that contain
disseminated sulphide. Chilled diabase and quartz diabase.
Figure 16b. Primitive mantle normalised compositions of differentiated granodiorites and quartz diorites of the Obabika Intrusion that contain
disseminated sulphide. Quartz diorite from the roof of the intrusion.
41
OGS Study 58
Figure 17a. Primitive mantle normalised spidergrams demonstrating the similarities in geochemical signatures of samples collected from sites
retaining three different palaeomagnetic remanence directions in Nipissing Intrusions. Sample sites and magnetic remanence from Buchan and Card
(1985), Buchan et al. (1989), and Buchan (personal communication). N1 samples.
Figure 17b. Primitive mantle normalised spidergrams demonstrating the similarities in geochemical signatures of samples collected from sites
retaining three different palaeomagnetic remanence directions in Nipissing Intrusions. Sample sites and magnetic remanence from Buchan and Card
(1985), Buchan et al. (1989), and Buchan (personal communication). N2 samples.
42
Petrology and Geochemistry of the Nipissing Gabbro
Figure 17c. Primitive mantle normalised spidergrams demonstrating the similarities in geochemical signatures of samples collected from sites
retaining three different palaeomagnetic remanence directions in Nipissing Intrusions. Sample sites and magnetic remanence from Buchan and Card
(1985), Buchan et al. (1989), and Buchan (personal communication). N3 samples.
Figure 17d. Primitive mantle normalised spidergrams demonstrating the similarities in geochemical signatures of samples collected from sites
retaining three different palaeomagnetic remanence directions in Nipissing Intrusions. Sample sites and magnetic remanence from Buchan and Card
(1985), Buchan et al. (1989), and Buchan (personal communication).Variations in elemental abundance versus Zr concentration and Th/Yb versus the
magnitude of the Eu-- anomaly for samples from sites recording three different palaeomagnetic remanence directions. See MRD 19 for complete data.
43
OGS Study 58
44
Petrology and Geochemistry of the Nipissing Gabbro
45
OGS Study 58
46
Petrology and Geochemistry of the Nipissing Gabbro
47
OGS Study 58
points, and we now look at the new data in the context of
these observations.
Significant close spatial associations between various
types of mineralisation and Nipissing gabbro intrusions
have been recognised (Card and Pattison, 1973; Innes and
Colvine, 1984), and there is a variation in style and type of
mineralisation across the Nipissing Province. In the east,
mineralisation is dominantly Ag, Co and Ni as native metals, arsenides, and sulfarsenides associated with quartz
carbonate veins which cut the sediments and the Nipissing
intrusions (Jambor, 1971a). In the central portion of the
Nipissing Province, mineralisation is dominantly Cu-Niplatinum group element (PGE) sulphides which occur disseminated within the intrusions or as massive pods beneath
the intrusions (Rowell, 1984; Rowell and Edgar, 1986;
Lightfoot et al., 1991; Lightfoot et al., 1993). In the western part of the Province, the mineralisation consists of
Cu-sulphides as fine disseminations or in quartz-carbonate
veins.
A number of empirical observations have been made
in previous studies, and one goal of this study is to focus
attention on these empirical observations as a basis for further exploration. Table 1 summarises some important
observations about the geology, petrology, mineralogy,
and geochemistry of the Nipissing Gabbro which relate to
mineral potential.
FURTHER WORK
Detailed geochemical studies are presently under way on
three Nipissing bodies which host significant disseminated
mineralisation. These rocks were pointed out to the senior
author by Gerry O’Reilly, Dan Brunne, Frank Racicot,
Gord Salo, Jack Rauhala and Brian Wraight who are
prospectors working or have worked locally in the Sudbury
Region. Specific studies are under way on: The
Casson Lake Intrusion, northwest of Whitefish Falls, the
48
Kelly-Janes Township Intrusion, west of Sudbury, and on
the mineralised “Sudbury Gabbro” intrusions located
southwest and south of the Sudbury Igneous Complex.
These new studies will focus on whole-rock major and
trace element geochemistry and provide new high quality
platinum group element data for both mineralised and unmineralised intrusions in order to better constrain the S saturation history of the Nipissing intrusions. The data are
pertinent to answering a number of questions, for example:
1) Were the Nipissing intrusions open system magma
chambers or did they differentiate in-situ? 2) What was the
S-saturation history of the Nipissing magma type?, and
3) Are there geochemical differences between the
mineralised and unmineralised silicates in the sills and
between portions of the sills? Answers to these questions
potentially will provide more satisfying criteria which can
be used in exploration.
CONCLUSIONS
The Nipissing gabbros of the Southern Province were emplaced over a short time interval of <10 million years
dominantly into the Huronian sedimentary sequence. The
parental magma appears to have been remarkably uniform
in chemical composition, but appears to have undergone
in-situ differentiation and contamination within the intrusions. A small number of intrusions host heavily disseminated sulphide mineralisation rich in Ni, Cu, and PGE.
These intrusions are restricted to the Sudbury Region and
form part of a spatial and temporal Ni, Cu, and PGE metallogenic province (Fyon et al., 1995). The challenge is now
to better define the petrology and geochemistry of these
mineralised intrusions, and compare and contrast their
compositions with the data in this report. The result of this
work should be some satisfying new criteria which can be
used in further evaluation of the mineral potential of the
Nipissing gabbro.
Appendix 1: Sampling, Analysis, Geology, Petrography
and Mineralogy of the Nipissing Intrusions
1.1 SAMPLING AND ANALYSIS
Sampling for geochemical studies concentrated on the
location of fresh unmetamorphosed materials which are
free of alteration veins and jointing in the study areas
shown in Figure 6, and described in Appendix 1. Samples
were selected to be representative of the rock type; typically gabbronorite samples with a grain size of <5mm were
represented by 2kg samples, whereas 2-3kg samples were
obtained from vari- textured gabbro and granophyric
rocks. Sample locations and descriptions are given in
MRD 19, figures in text and Appendix 1 and Conrod (1988,
1989). Samples were cleaned of weathered and altered surfaces in the field, and crushed using steel plates and ground
to -200 mesh in agate mills. Whole-rock major element oxides were acquired by wavelength dispersive x-ray fluorescence (XRF) and energy dispersive XRF (Harvey and
Atkin, 1982; Potts et al., 1984; 1985). Selected trace elements were determined by XRF: Nb, Rb, Sr, Y, Zr, Cu, Ni,
V, and Cr. Rare earth elements, Th, Ta, Hf, U, Sc and Co
data were acquired by instrumental neutron activation
analysis using the SLOWPOKE reactor and counting facilities at the University of Toronto. Quality control was
achieved by stringently monitoring the performance of the
XRF on international reference materials and in-house reference materials. UTB-1, the University of Toronto inhouse basalt standard and WHIN SILL, the in-house Open
University INAA standard were used to monitor INAA
results, and the results are given in Table 3.
The new data acquired in this study were used to compliment published data sets (Lightfoot et al., 1989; 1991;
1993; Conrod, 1988; 1989; in press). Complete analytical
data are given in MRD 19 where the samples from different
locations are grouped according to location.
Nd-isotope data and Sm-Nd data were determined by
conventional thermal ionisation mass spectrometry using a
conventional cation exchange column and HDEHP method modified from Zindler (1980). Samples were analysed
on single rhenium filaments using platinum activated carbon solution (Noble and Lightfoot 1992). Nd isotopic measurements were monitored, and achieved a total Nd blank
of <300pg; La Jolla gave 0.511864±13 on 26 determinations, and BCR-1 gave 0.512605±7 on 3 determinations
with 146Nd/144Nd =0.7219. Analytical methods used in the
U-Pb study and results are given in Noble and Lightfoot
(1992).
1.2 AGE AND DISTRIBUTION:
The 2.15-2.22Ga Nipissing Diabase sills (Corfu and
Andrews, 1985; Conrod, 1989; Noble and Lightfoot, 1992;
Fairbairn et al., 1969; Van Schmus, 1965; Kanasewich and
Farquhar, 1965) outcrop over a wide tract of Central
Ontario between Sault Ste. Marie in the west and Cobalt in
the East, bounded to the northwest by the Abitibi greenstone-granite terrain, and to the southeast by the Grenville
fault-thrust zone (see Figure 1a). Gabbros are largely confined to the 2450Ma Huronian meta- sedimentary supracrustal belt (see Figure 1a), although smaller diabase intrusions cut Archean granites, gneisses, metasediments, and
metavolcanics at the margin of the belt. The intrusions
form arc-like exposures derived by the erosion of undulating sills (see Figure 1b). Intrusive contacts generally follow bedding planes or unconformities, but also traverse
the Huronian stratigraphy and the Huronian/Archean
unconformity.
The pre-Nipissing stratigraphy consist of Archean
granites and greenstones, and Proterozoic sedimentary and
volcanic rocks (e.g., Young, 1982; Frarey and Roscoe,
1970). The oldest rocks consist of greenstones and granites. The greenstones are chloritic and hornblende-rich
schists, representing altered basic volcanic rocks and iron
formations. The schists were folded and intruded by granite batholiths. Subsequent erosion and deposition formed
the Huronian sequence, composed of conglomerates,
quartzites, and slates. A number of volcanic units within
the Huronian stratigraphy have been described by Jolly
(1987a, 1987b). The Nipissing sills were emplaced into the
sediments.
1.2.1 Differentiation
The sills are differentiated and exhibit a large variation in
petrography and mineralogy. Most have extensive and laterally continuous basinal regions which are composed
dominantly of hypersthene diabase and vari-textured diabase, whereas adjacent arch zones are composed of coarse
pegmatoidal vari-textured diabase, granophyric diabase,
and in places, aplite (see Figure 1b). There is a ubiquitous
fine grained basal quartz diabase along the lower contact
which coarsens away from the chilled margin into the hypersthene gabbro. Rock types in the sills are easily distinguishable in the field, and it is only in the roof zones where
relationships are less straightforward.
1.2.2 Thickness
Individual sills range in thickness from a few tens of meters
to over a thousand meters. The original areal extent of individual intrusions is difficult to ascertain because of the
structural complexity and variable thickness of the sills
(Card and Pattison 1973). Many of the intrusions (see
Figure 1a) have elliptical outcrop patterns termed “diabase
basins”, and may be cone sheets.
1.2.3 Emplacement sites
The distribution patterns and orientation of the sills
may reflect control by pre-existing faults and folds (Card
and Pattison, 1973). Sites of emplacement of the sills also
49
OGS Study 58
appear to controlled by the competency contrasts of the
sedimentary rocks. Many of the Nipissing intrusions were
emplaced within the Lorrain and Gowganda formation
sedimentary rocks of the Cobalt Group, and in some cases,
the contact between these units is a preferred plane of emplacement. Lorrain and Gowganda formation conglomerate and arkose beds commonly overlie the sills, and appear
to have acted as barriers to further rise of the diabase
magmas.
1.2.4 Deformation
The degree of metamorphism, alteration and deformation
of individual intrusions generally decreases northwards
from the Murray Fault zone. Within the Cobalt embayment, sills are virtually undeformed; undulatory upper and
lower contacts more likely reflect structural controls at the
time of emplacement than post-intrusive deformation
(Hriskevich, 1952, 1968). Within the Elliot Lake region,
folding about an east-west axes has produced striking arcuate exposure patterns. Folding of the supracrustal belt appears to have occurred at 1.7Ga during the Penokean orogeny (Zolnai et al., 1984). Metamorphic history and location in relation to possible domains of unconsolidated sediments at the time of emplacement are discussed in Jackson
(1995) and Young (1995).
1.3.1 Geology of the Nipissing
sills and dykes around Lake
Temagami
The Nipissing Gabbro is well exposed on the shores of
Lake Temagami (“Lake of Deep Water”), where glaciation
left the contact between the sediments and gabbro well exposed near water level on the lake shores (see Figure 6 and
1.1; locations 4-11; Simony, 1964). Two areas of gabbro,
discussed below, were given detailed attention, and a
further four bodies were studied by reconnaissance.
The normally concordant contact of the diabase with
the hornfelsed Gowganda Formation sedimentary rocks is
exposed along much of the west shore of the Lake, where
vertical cliffs rise from Lake level to a maximum of 200m.
Basal contacts are always low-angle (<15_) dipping
westward beneath the intrusion.
Detailed mapping at the northern margin of the
Obabika Sill (Figure 1.4)+, in an area burnt-out by fire in
1977, revealed more petrological variation than the other
diabases in this region. Homogeneous aplite in a large outcrop at the northwestern edge of the Obabika Sill is apparently related to it. To the southwest, the aplite appears to be
in sharp contact with quartz diorite and granodiorite,
which grades into vari- textured diabase and thence into
gabbro and hypersthene diabase, which is characteristic of
the remainder of the southern portion of the Obabika and
Skunk Lake sills. Within the quartz diorite, granodiorite,
and vari-textured gabbro, there are zones containing hornfels fragments (Photo 1.1), blocks of Gowganda Formation
sediment which are partially melted along the bedding
planes (Photo 1.2), aplitic veins cross-cutting granodiorite
and originating within blocks of Gowganda Formation
sedimentary rock (Photo 1.3), and blocks of aplitic material, as well as associated spotted disseminated sulfide
mineralisation.
Four other sills around Lake Temagami were given
reconnaissance coverage. The High Rock Island Sill
southeast of the lake (see Figure 1.1) exhibits excellent exposure of the basal contact, and a complete sequence of
samples was collected upward from the lower contact
through the basal quartz diabase, hypersthene diabase, and
gabbro. The Slide Rock, Devil Bay, and Mount Furgusson
sills (see Figure 1.1) were all sampled on a reconnaissance
basis; material was collected from the lower chilled
margin of the former two sills.
All of the Temagami intrusions contain occasional
veins of quartz-carbonate at the base. Invariably, these
veins carry some mineralisation (dominantly pyrite and
chalcopyrite). The diabase cut by the veins is highly
altered, containing disseminated sulphide mineralisation
(e.g: the Sand Point Dyke, Mount Furgusson Sill, and the
Skunk Lake Sill - see Figure 1.1).
On the western shore of Lake Temagami there is an extensive undeformed and unmetamorphosed area of gabbro
(see Figure 1.1). Contacts between the gabbro and Huronian sedimentary rocks are generally sharp, and strongly
hornfelsed Gowganda Formation arkose outcrops next to
fine-grained chilled diabase at the base of all of the
intrusions.
1.4 GEOLOGY OF THE KERNS
SILL
Contacts commonly show three-dimensional exposure. Low-angle to horizontal contacts are preserved beneath all the sills studied for this report, except for portions
of two bodies, which are sub- vertical. One of these trends
northwest-southeast from Narrows Island across northeastern McLean Peninsula to a small group of islands in the
southeast (Figure 1.2); this is termed the Narrows Island
Dyke, and is presumably a feeder which joins the base of
the diabase sheet (the Skunk Lake sill) exposed on the
western margin of the lake. The other sub-vertical intrusion trends east- southeast from Cayuga Island east of Sand
Point towards Long Island (Figure 1.3). This appears to be
a vertical dyke which may have fed the Obabika Sill.
The Kerns sill is located in Kerns and Hudson Townships
northwest of New Liskeard (Figure 1.5) between the Cross
Lake Fault and the Lake Timiskaming West Shore Fault.
The sill is apparently undeformed and appears to thicken
away from the Kerns Rock exposure from about 25m
(location shown in Figure 1.5 and enlarged in Figures 1.6
and 1.7) to reach >100m at the southeastern margin. The
upper contact of granophyric diabase and upper quartz diabase against arkosic Lorrain sediments is preserved only
in Kerns Rock and at an isolated exposure to the south
(see Figure 1.5). The southeastern and eastern lower
contact dips west at about 10_. There is no exposure of
the lower contact to the northwest. Footwall rocks
50
Petrology and Geochemistry of the Nipissing Gabbro
Figure 1.1. Geological map of the Nipissing Gabbro exposed around Lake Temagami, based on mapping performed by the Ontario Geological Survey
(Simony, 1964), and additional work by Lightfoot and Naldrett (1987). Sample locations are shown.
51
OGS Study 58
Figure 1.2. Geological map of the Narrows Island dyke, Lake Temagami (based on Simony, 1964, and additional mapping during the course of this
study). Sample locations are shown.
52
Figure 1.3. Geological map of the Sand Point dyke, Lake Temagami (based on Simony, 1964, and additional mapping performed during the course of this study). Sample locations are shown.
Petrology and Geochemistry of the Nipissing Gabbro
53
Figure 1.4. Detailed geological map of the south shore of Obabika Inlet, Lake Temagami, based on new mapping by the authors. Sample locations are shown.
OGS Study 58
54
Petrology and Geochemistry of the Nipissing Gabbro
Photo 1.1. Hornfelsed sediment fragments (Gowganda formation) in quartz diorite at the roof of the Obabika Intrusion, Obabika Inlet of Lake
Temagami, Nipissing District (see Appendix 1 for location). This section of the Obabika Intrusion is characterised by many inclusions of sediment
which exhibit partial melting along their boundaries. Locally, the partial melts coalesce and dykes (10m long by 1.5m wide) of aplitic magma are found
cross-curring the quartz diorite of the sill. The inclusions have sharp contacts and well-defined bedding suggesting that the sediment was well-lithified
at the time of emplacement of the Nipissing intrusion.
Photo 1.2. Hornfelsed sediment inclusion (Gowganda Formation) in granodiorite in the roof zone of the Obabika Intrusion showing partial melting or
injection of aplitic magma along bedding.
55
OGS Study 58
are Gowganda Formation meta-argillites. Archean
Temiskaming sedimentary rocks are present close to the
Cross Lake Fault which bounds the southwestern margin of
the intrusion.
Structurally, the intrusion has the appearance of a limb
of a synform with marginal arch and limb regions exposed
at Kerns Rock, and limb-basinal portions exposed to the
southeast.
The basal chilled margin (Photo 1.6) grades upwards
through basal quartz diabase into hypersthene gabbro.
Vari-textured diabase, generally, but not always, is overlain by granophyric gabbro and occurs at the top of the
intrusion. The Kerns Rock exposure provides unparalleled
three dimensional exposure of the roof zone (see Figures
1.5 and 1.6). Granophyric diabase and aplite outcrops
below and adjacent to pegmatoidal vari-textured diabase.
Within 5m of the Lorrain sedimentary rocks at the roof,
large (5m by 5m by 2m) inliers of coarse spotted
hornfelsed sediment are found entrained within the granophyric diabase and aplite. These appear to be highly metasomatised blocks of Lorrain sediment that retain ghost
bands that are interpreted as original layering (Photo 1.4).
These blocks presumably broke away from the roof of the
intrusion, and floated within the anatectic melts at the roof
of the magma chamber. The adjacent granophyric diabase
contains patches of fragmented Lorrain sediment (Photo
1.5). Below the upper contact is a discontinuous layer of
quartz diabase which coarsens into the vari-textured gabbro. The vari-textured diabase is in sharp contact with the
granophyric gabbro, but no contact between upper quartz
diabase and granophyric gabbro was observed. A zone of
quartz diorite is present along the western side of the outcrop; the relationship of this unit to the roof sediments is
unclear. A limited amount of aplite was found associated
with brecciated patches within the granophyric diabase. In
places brecciated Lorrain sedimentary rocks occur at the
contact, and granophyre or aplite fills the space between
the fragments.
Close to the upper contact, the Lorrain sedimentary
rocks are pervasively recrystallized; partial melting is
observed along coarse quartz-rich bands, where aplitic and
quartzitic veins cut through the Lorrain Formation
sedimentary rocks originate in these layers.
To the southeast of the Kerns sill, the Diamond sill
shows well developed vari-textured diabase overlying extensive basal quartz diabase and minor hypersthene diabase. To the south of the Cross Lake Fault, the diabase is
exposed as a thick sheet (reaching 300m at Triangle
Mountain; see Figure 1.5), which is weakly differentiated.
This sheet consists of a quartz diabase and a hypersthene
gabbro, but no vari-textured gabbro or granophyric gabbro
is developed. This intrusion is substantially thicker than
the Kerns sill and its lower part presumably represents a
well- developed basinal sequence of one of the major
Cobalt Nipissing gabbro sills.
1.5 PETROGRAPHY AND
MINERALOGY OF THE NIPISSING
GABBRO:
Photo 1.3. Vein of aplitic granitoid originating in a domain of Gowganda
Formation sedimentary rock inclusions in the roof granodiorite of the
Kerns Intrusion.
56
A generic discussion of the petrographic and mineralogical variation within the Nipissing gabbro appears appropriate, given the similarity in lithologies developed within
many of the intrusions. The literature contains a number of
descriptions of petrological and mineralogical variations
in Nipissing gabbro intrusions (e.g: Hriskevich, 1968;
Card and Pattison, 1973; Conrod, 1988; 1989).
Quartz diabase occurs within a few feet of the contacts
(see Figure 1b). The chill zone consists of a dense aphanitic
rock. Phenocrysts of augite and plagioclase are present at
50cm from the contact. The diabase chill consists of plagioclase and pyroxene crystals forming radiating clusters,
with long needle shaped plagioclase crystals separated by
granular interstitial pyroxene. Opaque oxides occur between the plagioclase laths. Amphibole and chlorite occur
as alteration product after pyroxene, and white mica, epidote, and zoisite occur as alteration products after plagioclase. Phenocrysts constitute a very minor portion of
the chill (<5%), of which over 80% is plagioclase and the
remainder are olivine and pyroxene. Plagioclase phenocrysts range in size up to 2mm which contrasts with the
groundmass needle-like laths which average 0.2mm in
Petrology and Geochemistry of the Nipissing Gabbro
Figure 1.5. Detailed geological map of the Kerns Intrusion, Kerns Township (based on Lovell and Frey, 1970) showing the location of sample sites and
the two suite sampled for geochronological investigations.
57
OGS Study 58
Figure 1.6. Detailed geological map of Kerns Rock, Kerns Intrusion, showing the distribution of lithologies and the location of sample sites (after
Lightfoot and Naldrett, 1989).
58
Figure 1.7. Sketch map showing relationships between lithologies at the roof of the Kerns Intrusion (after Lightfoot and Naldrett, 1989).
Petrology and Geochemistry of the Nipissing Gabbro
59
OGS Study 58
Figure 1.8a. Sketch map showing the locations of samples referred to in MRD 19. Englehart Intrusion (based on regional compilation maps).
Figure 1.8b. Sketch map showing the locations of samples referred to in MRD 19. Bruce Mines Intrusion (after Lightfoot et al., 1993).
60
Petrology and Geochemistry of the Nipissing Gabbro
length. Olivine and augite phenocrysts average about
0.5mm, and olivine is always altered to antigorite clouded
with magnetite.
include minor quartz and myrmekite, ilmenite and apatite.
The upper part of the hypersthene gabbro is a transitional
gabbronorite.
The basal or lower chill becomes progressively
coarser- grained into the overlying basal quartz diabase
with an increasing average grain size away from the chill
from 0.1mm to 1mm over 25 to 30m. Plagioclase, augite,
pigeonite, and inverted pigeonite are intergrown in a diabasic texture. Pigeonite occurs both as individual grains
and as rims enclosing augite crystals in crystallographic
continuity. Quartz occurs as discrete grains and together
with K-feldspar in micropegmatitic and myrmekitic intergrowths, interstitial to plagioclase and pyroxene. Skeletal
crystals of ilmenite and needles of apatite occur as accessory phases; the apatite tends to be associated with the myrmekitic patches (Hriskevich, 1968). Both plagioclase and
pyroxenes are partly altered.
The basal quartz diabase grades rapidly into the overlying hypersthene gabbro, which appears to be hypersthene-rich in basinal portions of the intrusions and quartz
rich on the limbs and arches. Plagioclase and augite phenocrysts average 0.5- 1mm. Orthopyroxene occurs as equidimensional subhedral grains reaching 0.5cm in diameter,
which are slightly altered to antigorite in places. The
orthopyroxene often encloses euhedral plagioclase laths
and augite grains. Olivine occurs in hypersthene diabase at
some locations (e.g. Conrod, 1988), but is either absent, or
extensively altered to antigorite in all the hypersthene diabase samples collected in this study. Accessory phases
The vari-textured gabbro, as the name suggests, consists of irregular patches of coarser-grained diabase with
gradational boundaries occurring within finer-grained diabase. The vari-textured gabbro usually occurs above the
hypersthene gabbro, and the contact of the two rock types
is gradational over about one meter. The vari-textured gabbro contains plagioclase and pyroxene which are intergrown in a diabasic texture. Quartz, K-feldspar, ilmenite,
and apatite occur as accessory phases. Plagioclase is often
altered, and both augite and pigeonite show some alteration. Patches of pegmatoidal vari-textured gabbro on the
meter scale are frequently present throughout the varitextured gabbro in most intrusions. More massive areas of
pergmatoidal vari-textured gabbro are found close to the
roof of the Kerns sill.
The granophyric gabbro occurs in variable amounts
and is usually confined to the arch regions. Plagioclase, augite, pigeonite, microcline, K-feldspar, and quartz are the
major constituents of the rock, together with accessory
sphene, ilmenite and apatite. Miarolitic cavities are common, and reach up to 1cm in diameter. The miarolites have
walls covered in quartz and microcline, and are often filled
with carbonates. These cavities suggest that the most
fractionated phases of the Nipissing were extremely rich in
volatile components. Alteration is more intense in the granophyric diabase, where plagioclase is altered to albite,
Figure 1.8c. Sketch map showing the locations of samples referred to in MRD 19. Basswood Lake Intrusion (after Lightfoot et al., 1993).
61
OGS Study 58
Figure 1.8d. Sketch map showing the locations of samples referred to in MRD 19. Wanapitei Intrusion (after Lightfoot et al., 1993).
62
Petrology and Geochemistry of the Nipissing Gabbro
Figure 1.8e. Sketch map showing the locations of samples referred to in MRD 19. Cobalt Region Instrusion (after Lightfoot et al., 1993).
Photo 1.4. Banding in the spotted hornfelsed sediment rafts in the Nipissing granophyric gabbro at Kerns Rock, Kerns Township (Figure 1.6). Incipient
melting of the quartz-rich beds is observed within the raft of sediment as an aplitic phase.
63
OGS Study 58
Photo 1.5. A breccia consisting of fragmented Lorrain Formation sediments within an aplite at the roof of the Kerns Intrusion, Kerns Township. Rock
sample is 30cm across.
Photo 1.6. Chilled Nipissing diabase at contact of High Rock intrusion. Field of view 1.5cm.
64
Petrology and Geochemistry of the Nipissing Gabbro
white mica, chlorite, and carbonate. Clinopyroxene is
often completely altered to amphibole. Granophyric gabbro grades into aplitic granitoids or is in sharp contact with
aplitic granitoids where the proportion of mafic phases is
lower in the aplitic granitoid than the granophyric gabbro.
The granophyric gabbro is associated with a number
of other lithologies, which constitute only a small proportion of the intrusions as a whole. Aplite occurs as small segregations and dyke-like masses within the upper portions
of many intrusions. The aplites contain plagioclase,
microcline, and quartz, and reveal granophyric textures.
Mafic minerals are absent; accessory phases are ilmenite
and apatite. Zoisite, epidote, sphene, and chlorite are secondary minerals. Miarolitic cavities are well developed,
and commonly filled with carbonate. In most intrusions,
aplites represent a very small proportion of the intrusions.
However, in the Obabika sill, a vast quantity of aplite is
preserved at the northwestern margin. The relationship of
the aplites to the rest of the Nipissing magma is presently
unclear. Quartz diorite and granodiorite are preserved
within the upper portions of the Obabika sill; these lithologies are distinguished from the granophyric gabbro by the
larger proportion of quartz and amphibole.
The sedimentary rocks of the footwall and hangingwall rocks are typically greywackes, with alternating beds
of shale and arkose. The arkosic beds are often strongly
re-crystallized near the intrusions, and myrmekitic to
micropegmatitic textures are often present. Within the
Kerns Intrusion, blocks of hornfelsed sediment have been
strongly metasomatised, such that patches of feldspar are
separated by a matrix richer in amphibole.
A more complete discussion of mineralogical and
mineral chemical information is given in Lightfoot et al.
(1986, 1987) and Conrod (1988; 1989). The following
general points are stressed here:
1. There is an increase in the anorthite content of plagioclase away from the chill margin up to the base of the
hypersthene gabbro, followed by a decrease through
the hypersthene gabbro and vari-textured gabbro into
the granophyric gabbro.
2.
Fresh olivine shows an apparent increase in forsterite
content away from the lower contact of the Cross Lake
sill (Cobalt), followed by a decrease up through the
hypersthene gabbro. The increase in forsterite content
is accompanied by an increase in Ni-content.
3.
The orthopyroxenes show an increase in Mg-number
away from the lower contact towards the base of the
hypersthene gabbro, followed by a decline in Mgnumber with increased distance away from the lower
contact.
The upwards trend towards higher anorthite contents of
plagioclase, forsterite contents of olivine, and Mg-number
in orthopyroxene, followed by a reversal about 25-30m
above the contact is common to Nipissing intrusions
(e.g. Finn et al., 1982), as well as most sills in general
(e.g: Lightfoot and Naldrett, 1984).
1.6 METAMORPHISM OF THE
NIPISSING GABBRO
The Nipissing gabbro intrusions have been metamorphosed under regional conditions ranging from lower
greenschist chlorite zone alteration to lower almandine
amphibolite facies (Card and Pattison, 1973). In the
Cobalt-Gowganda and Sault Ste. Marie- Elliot Lake areas,
metamorphic grade is uniformly low, whereas in the
Sudbury area, the grade is much higher (Card and Pattison,
1973).
Metamorphism is most evident at the margins of intrusions. The change of diabase to metadiabase involves the
replacement of pyroxene and calcic plagioclase by amphiboles, sodic plagioclase, epidote, talc, chlorite, and quartz.
For a more detailed account of metamorphic mineralogy,
see Conrod, 1988.
65
Appendix 2
2.1 ASSIMILATION AND
FRACTIONAL CRYSTALLISATION
IN THE KERNS INTRUSION - A
CASE STUDY OF THE PHYSICAL
PROCESS
It has been suggested that crystallisation and assimilation
in basic magma chambers are coupled (DePaolo, 1981;
DePaolo, 1985; Bowen, 1909, 1910, 1928; Taylor, 1980),
where the latent heat of crystallisation of the magma produces a commensurate amount of assimilation of the roof
rock sediments. Although there are some fairly compelling
geochemical arguments for a spatial and temporal coupling of these processes (e.g., Taylor, 1980; DePaolo,
1985), analogue laboratory investigations of simple binary
salt solutions (Campbell and Turner, 1987) suggest that
diffusive interfaces may develop in the magma column
between molten roof rock and underlying basic magma.
This interface may allow heat transfer, but prevent chemical diffusion, resulting in the accumulation of a pond of
less dense crustal melt above the interface. Thermal erosion by convecting basic magma may progressively break
down the interface and mix more fractionated basic magma with crustal melts. The presence of volatiles in the
magma at the roof of the intrusion may also contribute
slightly to the amount of melting. If the interface breaks
down rapidly, then a component of the variation may reflect direct mixing of magmas which have similar densities. In such circumstances, the rate of change of magma
mass due to assimilation relative to that due to fractionation (r) is not necessarily constant, and the algorithm
derived by DePaolo (1981) for assimilation-fractional
crystallisation (AFC) must be solved accordingly.
Lightfoot and Naldrett (1989) explored the role of
crystallisation and assimilation from a geochemical perspective in the Kerns Intrusion, and were able to demonstrate that assimilation and crystallisation were in-part
coupled. In this report, we use new data for the Kerns
Intrusion and expand our discussion of the roles of crystallisation, assimilation, and mixing. Lightfoot and Naldrett
(1989) pointed out that a significant proportion of the Nipissing lithologies were either undifferentiated or slightly
differentiated. They suggested that equilibrium crystallisation may have played an important role during a large
part of the solidification histories of the sills. The slight enrichment of some gabbros in hypersthene was considered
to be the product of small degrees of cumulus enrichment.
However, equilibrium crystallisation fails to reproduce the
styles of variation found in the more differentiated
lithologies.
The effects of Rayleigh Fractional Crystallization
(RFC) may be modelled using the Rayleigh Equation
(e.g., Wood and Fraser, 1976), where the following
constraints are available:
66
1.
Parent magma composition: Data for chilled margins suggest that the Nipissing parental magma was
very uniform in composition (see Lightfoot et al.,
1993), and that variable amounts of fractional crystallisation and crustal contamination at the time of emplacement are not recorded in different intrusions. Although there appears to be some support for three
phases of intrusion recorded in the remanence paleomagnetic signatures (Buchan et al.,1989), these three
remanence components are found in rocks which are
easily linked by differentiation of a single parental
magma and the U-Pb geochronology suggest that the
intrusive activity covered an interval of significantly
less than the 50 million years required by paleomagnetic data. Thus, the paleomagnetic data appear not to
exclude the possibility that a single parental magma
composition may be represented by the chilled margin
differentiated to produce the compositionally diverse
Nipissing rocks.
2.
Partition coefficients: Distribution coefficients are
sensitive to the physical and chemical character of the
magma. Given the tholeiitic affinity, it is suggested
that low-pressure basaltic values are appropriate
(Henderson, 1983, 1984; Irving, 1978; Pearce and
Norry, 1979), although their magnitudes may change
during the evolution of the magma (Henderson, 1983,
1984).
3.
Closed system evolution: Multiple influx of new
batches of magma of different composition appear unlikely in most intrusions, given the uniform composition of the chilled margin samples. Although Finn
(1981), Finn et al. (1982), and Finn and Edgar (1986)
suggest that multiple intrusions of magma are
recorded in the Wanapitei intrusion, this could represent a sampling problem. However, neither loss of
magma to higher crustal levels nor assimilation of
overlying crust can be ruled out as indicated by the
data of Conrod (1989) for the Miller Lake Intrusion at
Gowganda. Petrographic evidence presented in
Appendix 1 suggest that melting of the roof did occur,
but no evidence for large feeders tapping the top of any
of the Nipissing gabbro intrusions was found.
4.
Cumulus processes: There is no evidence for strong
cumulus processes of the type found in major layered
intrusions (e.g: the Bushveld). Rather, the cumulus
enrichment that has occurred is confined to enrichment of the basinal regions in hypersthene, and is
reflected only in a slight depletion of the hypersthene
diabase in Zr (40-65ppm) and other incompatible
elements relative to the chilled margin (Zr=65ppm).
This contrasts with Zr levels close-to or below detection limits for the picritic layers of the larger intrusions
associated with CFB (e.g: Lightfoot and Naldrett,
1984). Re- equilibration appears to be limited by the
very slow diffusion rates found in gabbroic phases.
Petrology and Geochemistry of the Nipissing Gabbro
Rayleigh fractionation models are shown in Figure
2.1, based on Lightfoot and Naldrett (1989) and new data
presented in this study. Although fractionation is clearly of
significance, it is apparently not the sole process responsible for chemical variations (Lightfoot and Naldrett, 1989),
and for this reason, any conclusions drawn are better based
on the simplest of RFC models which identify the nature of
phases, but not their exact proportions.
roof of the intrusion, or within the vari-textured diabase.
However, enrichment in K2O, Ba, and Rb within the varitextured gabbro are likely to reflect, at least in part, contamination prior to K- feldspar entry. Perhaps a more reasonable explanation for the location of the K-feldspar is
within the aplites at the roof of the intrusion, as these rocks
contain slightly higher K2O abundances than the Lorrain
Formation sedimentary rocks.
As discussed by Lightfoot and Naldrett (1989), much
of the differentiation is explained by the removal of pyroxene, plagioclase, olivine, apatite, K-feldspar, and ilmenite. Summarising the variations in Figure 10a-y, the
following observations are important:
The low LREE concentrations of some of the granophyric diabase samples compared to other samples with
similar Zr contents may suggest the late fractionation of a
LREE-rich phase. Henderson (1984) has pointed out that
allanite has a LREE-enriched REE profile, and appears not
to preferentially enriched in Th or U. This phase has been
observed in some Nipissing intrusions (Jambor, 1971), and
has been suggested by Lightfoot and Naldrett (1989) as
one possible explanation for the low LREE contents of several samples of granophyric gabbronorite from the Kerns
Intrusion.
Following Lightfoot and Naldrett (1989) the variation
in LILE/Zr and LREE/Zr ratios are insensitive to the
effects of crystallisation. The crystal phase extract must
contain about 50% augite in order to explain the rapid increase in the LILE/HFSE with Zr content. Such a large
component of augite in the crystal phase extract is inconsistent with the other trace element data and petrographic
evidence. Lightfoot and Naldrett (1989) also note that the
granophyric diabases have more negative epsilon-Nd than
the other diabase lithologies, suggesting that a contribution from the crust is required. To test this hypothesis, the
variations in Th/Yb, La/Yb, and U/Yb are shown in Figure
2.2a-b for samples from Lightfoot and Naldrett (1989) and
this study. These ratios are very sensitive to mixing
processes, but less sensitive to fractionation.
The basal quartz diabase, hypersthene gabbro, varitextured gabbronorite, and granophyric gabbro data define
trends of coupled overall increase in inter-element ratios
away from the chilled margin towards the compositional
field of the hanging wall Lorrain Formation sedimentary
rocks, the rafts of hornfelsed Lorrain sedimentary rock
within the granophyric gabbro, the quartz diorite, and the
aplites. This data may be interpreted to suggest that Lorrain Formation sedimentary rocks have been involved in
Nipissing petrogenesis, and that the progressive increase
in the element ratios attest to stronger contamination as the
magma became more fractionated.
1.
Plagioclase fractionation explains the progressive
decline in Sr with increasing Zr and magnitude of the
Eu-anomaly (see Figures 10k). Hypersthene diabases
tend to be Sr-poor due to enrichment in cumulus hypersthene, whereas the chill and basal quartz diabase
have essentially similar Sr contents. The overall
decline in Sr content is consistent with a progressive
increase in the plagioclase content of the fractionating
phase extract.
2.
The rapid decline in Ni (see Figure 10u) is partially
explained by the removal of pyroxene. However, the
country rocks are also low in Ni, and so some of this
may reflect the assimilation of material from low Ni
country rocks into the Nipissing magma.
3.
Variation in V (see Figure 10w) is largely accounted
for by the proportions of hypersthene to augite.
Vanadium enters hypersthene more readily than augite, and as the hypersthene diabases have higher V
than the basal quartz diabase or chill, some cumulus
enrichment is suggested.
4.
The plot of TiO2 versus Zr shown in Figure 10e
suggest that ilmenite begins to crystallize after 60%
solidification of the magma.
5.
The plot of P2O5 versus Zr, shown in Figure 10g
indicates that apatite enters after 75% crystallisation
of the magma.
6.
The effects of albite, K-feldspar, and quartz within the
crystalline phase extract on the compositional
changes within the vari-textured and granophyric
diabases are not straightforward, as crustal melts contribute SiO2 and LILE to the magma. However,
Figures 10g and p demonstrate that enrichment of the
magma in K2O and Rb is found only within the less
fractionated vari-textured diabase samples, after
which K2O and Rb decline strongly with increasing
Zr. In contrast, Figure 10l demonstrates that Th continues to increase with increasing Zr throughout the
vari-textured gabbronorite and granophyric gabbro. It
is suggested that this may reflect the entry of K-feldspar within the crystal extract, late in the crystallisation of the vari-textured diabase.
Figure 10f show the effect of removing K-feldspar after
75% fractionation of the original magma. The location of
the K- feldspar extracted, may rest in aplitic selvages at the
Importantly, whilst mixing is recorded on elementratio plots, the element-element plots record at least two
processes - mixing between Nipissing magma and crustal
material, and fractional crystallisation. The element
versus Zr plots illustrate that the two processes have acted
in such a manner as to produce coherent variations (see
Figures 10a-y). Clearly this would not be expected if mixing were to be superimposed on a range of magmas showing different relative amounts of fractionation, or by fractionation superimposed on a range of magma compositions
produced by mixing.
For example, variations in Th, U, and La versus Zr
(seeFigures 10h, l, and m) define tight trajectories (with the
67
OGS Study 58
Figure 2.1. Modelling of the crystallisation and assimilation history of the Kerns Intrusion. a) Sr versus Zr and the effect of plagioclase fractionation.;
b) Ni versus Zr and the effect of orthopyroxene fractionation.; c) Magnitude of the Eu- anomaly versus Zr and the effect of the fractionation of gabbroic
minerals.; d) V versus Zr and the fractionation of pyroxenes.; e) TiO2 versus Zr and the fractionation of ilmenite.; f) P2O5 versus Zr and the fractionation of apatite.; g) K2O versus Zr and the fractionation of potassic feldspar.; and h) Rb versus Zr and the fractionation of potassic feldspar.
68
Petrology and Geochemistry of the Nipissing Gabbro
exception of LREE depleted granophyric diabases),
towards higher Th/Zr, U/Zr, and La/Zr when compared to
1:1 enrichment trajectories. Similarly, the HFSE (e.g: Y,
Yb) fall with increasing Zr (see Figure 10j and r). Consider,
for example, the LILE versus Zr plots; Figure 2.2 demonstrates that 1:1 enrichment vectors originating at different
points on a mixing line drawn between hypothetical elements A (e.g: Th) and B (e.g: Zr) are unlikely to fractionate
from the mixing curve between Coand Ca to the data trajectory Co-E. Indeed, to the contrary, a complete scatter of the
data would be expected unless some physical process
coupled assimilation and fractionation. It is suggested here
that assimilation and fractionation worked hand-in-hand to
explain why the least fractionated samples are least contaminated, whilst the largest amount of contamination is
recorded in strongly fractionated samples.
Coupling between assimilation and fractional crystallisation may be modelled using the algorithms of DePaolo
(1981). Models of the type constructed by Lightfoot and
Naldrett (1989) are shown in Figures 2.3 and 2.4. Assumptions regarding crystalline phase extracts were similar to
those used in RFC modelling, except that a phase extract
composed of olivine, plagioclase, augite, and hypersthene
in the proportions 5:60:20:15 was assumed. Additional parameters required in the models were the average composition of roof rocks at the top of the Kerns sill, and the value
of the ratio of the rate of change of magma mass due to assimilation relative to that due to fractionation (r). In
DePaolo’s models, this factor was a constant; here we
varied it within reasonable limits (0.1 to 0.4), which take
into account the maximum amount of latent heat and
superheat available for melting at the roof of the intrusion.
The modelled trajectories fan away from the composition of the parental magma towards progressively higher
LILE/Zr and lower HFSE/Zr as r increases from 0.1 to 0.4
(c.f. Lightfoot and Naldrett, 1989). A cursory examination
of the trajectories for the models suggests that the general
characteristics of some of the data trends are followed fairly well by the AFC lines; i.e: the increase in La/Zr, Th/Zr,
and U/Zr is predicted by the models, the constant Sm/Zr ratio is reproduced, and the fall in Yb/Zr (see Figure 10j) is
predicted. Furthermore, the range of r values (<0.4) is not
unreasonable for the Kerns Intrusion. Poor fits for some of
the granophyric diabase samples may be explained by the
removal of accessory minerals with high LREE/HREE
ratios.
Figure 2.2. a) Variation in Th/Yb versus La/Yb in the Kerns Intrusion sample suite of this study and Lightfoot and Naldrett (1989). and b) Variation in
U/Yb versus Th/Yb (Lightfoot and Naldrett, 1989).
69
OGS Study 58
Looking in more detail at the results of the modelling
in Figure 2.4, Lightfoot and Naldrett (1989) noted that the
observed data trajectories cross-cut the modelled trajectories on each plot. The chill, basal quartz diabase and hypersthene diabase fall close to the origin. The vari-textured
diabase samples fall on a trajectory which is close to r=0.1;
the granophyric diabase field cross-cuts the trajectories
with r=0.2 to r=0.4. This relationship is preserved on all of
the element versus Zr variation diagrams, suggesting that
the observed variation is not entirely consistent with the
conventional AFC model. Whereas it is possible that distribution coefficients change through magmatic evolution,
they are likely to change by similar amounts, and therefore
fractionation of the Th/Zr ratio is unlikely. More realistic is
the possibility that the value of r was not a constant, as suggested by Lightfoot and Naldrett (1989). Assimilation of
Figure 2.3. Effects of assimilation and fractionation compared to mixing on schematic showing bi-variate plots of incompatible elements. Simple mixing of a parental magma (Co) with a crustal component (Ca) would produce a trend between Co and Ca. Fractionation of a mixture (such as x or y) will
produce liquids falling close to 1:1 enrichment vectors (y-y’, x-x’). In fact, the data trajectory (Co- E) would require that the parental magmas become
more fractionated when more crust is added. This requires that assimilation and fractional crystallisation are coupled, at least in part, by some mechanism which permits the most contaminated magmas to exhibit the largest degree of fractionation.
70
Petrology and Geochemistry of the Nipissing Gabbro
Figure 2.4. a) Modelling of assimilation coupled to fractionation on Th versus Zr. b) Modelling of assimilation coupled to fractionation on La versus
Zr, c) Modelling of assimilation coupled to fractionation on U versus Zr.
71
OGS Study 58
melts at the roof of the intrusion may not take place at a rate
similar to that at which latent heat is supplied to the roof of
the intrusion. For example, Lightfoot and Naldrett (1989)
suggest the possibility of a thermal boundary layer which
might act as a double-diffusive interface permitting heat
transfer but no chemical transfer. In such a circumstance,
melted roof rock may pond as less dense melts of crustal
material above the basic magma. Assimilation of these
melts may not occur until a late stage in the differentiation
of the sill, and the break- down may then be progressive
and non linear.
To expand on this suggestion, the effects of varying r
during the evolution of the magma are modelled where r
increases from 0.1 to 0.4 at 250ppm Zr, and from 0.4 to
0.75 at 300ppm Zr. In reality, modelling this process in
three steps is probably an oversimplification, but it serves
to illustrate that the curved form of the data trend could, as
Lightfoot and Naldrett (1989) point out, be generated by
progressively increasing r as fractionation proceeds.
Whether or not an r value of 0.75 is realistic must be placed
in considerable doubt, as the amount of latent heat available during the later stages of crystallisation was probably
minimal. However, Jolly (personal communication) suggested that partial melts produced at the roof of the intrusion may be mixed into the fractionated basic magma,
thereby reducing the AFC mechanism to a simple latestage mixing process. This possibility clearly requires
further investigation.
Geochemical trends indicate that both fractionation
and contamination have played key parts in Nipissing petrogenesis. It is therefore worth examining theoretical and
experimental data relevant to AFC. Several factors come
into play during AFC. Bowen (1928) suggested that the
latent heat of solution of silicate minerals is small, and
therefore a magma would have to be superheated by at
least 300_C in order to assimilate an equivalent mass of
country rock preheated to its melting point. Although superheat provided by volatiles may play an important role,
the realization that the latent heat of crystallisation was
greater than Bowen suggested, led Taylor (1980) and later
DePaolo (1981) to suggest that AFC could be achieved
without the need for substantial superheating of the magma. This became increasingly evident when masses of granophyric rock above major layered intrusions were recognized as possible melts of the roof rock (e.g: the Bushveld
Complex and the Muskox Intrusions, Irvine and Smith,
1969; McBirney, 1979). The latent heat of crystallisation
of the vast thickness of cumulates in these layered intrusions supplied considerable heat to the roof of the intrusion. Volatiles present in the roof zone of the Kerns sill, as
evidence by vesicular granophyric diabase and aplite, may
have slightly enhanced the ability of the magma to heat the
overlying crustal materials to towards their liquidus point,
producing melts.
DePaolo’s (1981) AFC equation is now universally
applied to AFC modelling of volcanic suites, and DePaolo
(1985) also used this equation for modelling magmatic
evolution in a layered intrusion - the Kiglapait. However,
new experimental data (Campbell and Turner, 1987)
72
suggest that coupling both spatially and temporally,
between assimilation and fractionation is rarely completely achieved. Campbell and Turner (1987) report a series of
experiments designed to model melting at the roof of a
basaltic magma chamber, where melts of granitic and granodioritic composition are produced from country rock;
these melts are less dense than the basic magma. Based on
their experiments on simple salt solutions, they demonstrate that such light magmas would tend to pond under the
roof of the intrusion (c.f. McBirney, 1979). Furthermore,
they demonstrate that interaction between the basic magma and the roof-rock melts would depend on the shape of
the intrusion. Steep sides of the intrusion would promote
mixing, whereas a shallow slope (such as that found at the
roof of a sill) would not promote significant amounts of
mixing. The granitic or granodioritic melts would be separated from the underlying basic magma by a double diffusive interface which permits heat transfer at a rate that is an
order of magnitude faster than the rate of chemical diffusion. Thus, there may be little transfer of mass between the
two layers in the absence of any strong disruptive effects
(e.g: convecting of the magma). Consequently, according
to Campbell and Turner’s results, assimilation and fractionation are likely to be decoupled both spatially and
temporally.
Campbell and Turner (1987) note that although the
amount of mass transfer across the double-diffusive interface will normally be small, a number of different
processes can enhance the mass diffusion rate from the
upper to the lower layer:
1.
Turbulence enhances mixing across the interface.
This can occur when the basic magma is vigorously
convecting (i.e: high Rayleigh number), and
thermally erodes the base of the upper layer. This is
likely if the basic magma is thick, non-viscous, and repeatedly injected with new batches of magma (c.f.
Huppert and Sparks, 1980). Such conditions are
generally prevalent in basic layered intrusions and
thick sills (e.g., Insizwa - Lightfoot et al., 1984).
2.
Influx of a less dense batch of magma into the intrusion can produce extensive mixing (c.f. Huppert and
Sparks, 1980), and under such circumstances it is
possible that an interface may never develop
(Campbell and Turner, 1987). In the absence of chilled
material from the different magma influxes, it is not
straightforward to evaluate whether magmas actually
did mix.
3.
Blocks of roof material may fall through the upper layer into the lower layer, dragging down some of the
lighter magma. Additional melting of the blocks will
further contaminate the lower layer (e.g: Huppert and
Sparks, 1980, and field evidence from this study).
4.
During the normal crystallisation of basic intrusions,
evolved liquids with densities similar to those of crustal melts may be generated. The interface between
such liquids can therefore not be maintained by density stratification, and will likely break down leading to
convective overturn at the roof of the intrusion.
Petrology and Geochemistry of the Nipissing Gabbro
5.
Diffusive interfaces are unlikely to remain during the
later stages of crystallisation, as strong convection
within a limited vertical thickness of magma will
likely produce significant convective mixing.
6.
Finally, as Campbell and Turner (1987) point out, the
roofs of magma chambers are rarely flat, and steeper
sides would promote turbulence. If assimilation exploits weaknesses such as faults, bedding planes, or
unconformities in the sediments, thereby producing
an uneven roof, then undulations in the form of
stepping of the upper contact are likely. This has been
demonstrated in the Kerns sill, where the upper contact is vertical in some locations, and horizontal in others. Furthermore, the presence of large blocks of
spotted hornfelsed Lorrain sediment suggest that the
upper margin of the intrusion was migrating away
from the basic magma, as pendants of roof rock were
split away from the roof and fell into the upper zone of
the intrusion.
Both the geochemical trend patterns in the Kerns sill and
the physical constraints on processes which could produce
these variations were used by Lightfoot and Naldrett
(1989) to construct a schematic model for the evolution of
the magma. Figure 13 (based on Lightfoot and Naldrett,
1989) illustrates one possible interpretation of the above
observations in the context of a larger scale convecting intrusion as modified from Conrod (1989) and illustrated in
Figure 2.5. During the first of the four stages envisaged, the
Nipissing magma intruded along a weakness between the
Lorrain and Gowganda Formation sedimentary rocks. The
magma was chilled along both contacts. Cooling of the
magma was accompanied by crystallisation of the basal
quartz diabase and hypersthene gabbro. The variation in
texture, grain size, phase proportions, and whole-rock geochemistry away from the lower contact is attributed to the
more rapid cooling of the magma closer to the contact, and
the progressive enrichment of the magma in hypersthene
within the hypersthene diabase. It is presently unclear why
the An content of the plagioclase and Mg- number of the
pyroxene appear to increase towards the base of the hypersthene diabase unit. Similar trends are found in other major
layered intrusions and sills (e.g: Insizwa - Lightfoot and
Naldrett, 1984), and their origin is also unclear. During
stage II, crystallisation of the hypersthene diabase unit was
accompanied by the release of latent heat, and this was
accompanied by the migration of volatiles towards the roof
of the intrusion. Heating of the roof produced a commensurate amount of melting of the Lorrain formation hanging
wall sediments. The crustal melts were driven under pressure from the arkosic low- melting point beds within the
Lorrain through cracks, and accumulated at the top of the
magma chamber as a pond of less dense aplitic magma.
Some mixing between aplitic magma and basic magma accompanied convective erosion of the double-diffusive
interface, but a large proportion of the aplite was retained
in a physically decoupled sector of the magma chamber
separated from the less dense basic magma by a doublediffusive interface. Continued crystallisation of the Kerns
sill magma resulted in more melting of Lorrain Formation
sediment, and convection continued to break down the
double-diffusive interface; when well over 90% crystallisation of the basic magma had been achieved, the hypersthene diabase phase was completely crystallized, whilst
the interface between fractionated residual magma and
overlying aplite continued to break down. The crystallisation of the vari- textured diabase appears to have occurred
from slightly more contaminated magma than that responsible for the production of the basal quartz and hypersthene
diabases(evidenced by the higher LILE/HFSE ratio), and
yielded additional latent heat (stage III) to the Lorrain. The
final phase of crystallisation took place as the densities of
the aplitic magma and the fractionated Nipissing magma
closely approached one another (stage IV), and this marks
the break-down of the double-diffusive interface, perhaps
followed by complete mixing of the two magmas (except
material caught in pockets at the upper contact). Blocks of
country rock broken away from the roof were strongly metasomatised throughout stages II to IV. The presence of
limited amounts of aplite in the Kerns sill reflect the strong
mixing during the final stages of fractionation.
Aplites within the Obabika sill are very well developed over an area exceeding 5 square kilometers. This contrasts with the occurrence of small pockets of aplite in the
Kerns sill. The presence of widespread aplites at the roof of
the Obabika sill in Nipissing District (see earlier), and the
absence of both vari- textured and granophyric diabases
suggests that the double- diffusive interface between basic
magma and aplitic magma was well established
throughout the entire crystallisation history of the magma.
The location of the zone of crustal melting in the
Kerns sill above what seems to be the limb- and arch-zone
is apparently at odds with the stronger contamination expected above the basinal regions of the intrusion, where
there is a larger proportion of basic (and hence, dense and
hot) magma (see Figure 2.5). There is no field evidence to
suggest that the undulations in the Kerns sill reflect deformation after emplacement. Feeder zones may be hidden
beneath the Kerns sill; although their location is unclear,
they would likely have produced turbulent mixing in the
magma chamber, thus preventing the establishment of a
double- diffusive interface.
Basinal regions represent areas of magma entry
(Lightfoot and Naldrett, 1989), where hot, turbulent magma forms a chamber and may commence partial melting of
the roof sediments. The absence of granophyric diabase
and aplite along the upper margins of many basinal portions of Nipissing intrusions (e.g: Hriskevich, 1968), and
the presence of an upper chill and upper quartz diabase argues that little, if any melting of crustal material or accumulation of fractionated magmas has taken place in these
zones. This is at odds with the expected relationship, which
in turn undermines some of the thermal implications of the
model presented above. Conrod (1988) suggested that less
dense fractionated magmas may migrate to the arch zones
of intrusions, whilst more dense less fractionated magmas
may accumulate in the basinal portions of sills. This process would also concentrate volatiles and heat at the roof
above the arch zones. If the arch zone was stagnant and
undergoing little convection, then ideal condition would
73
Figure 2.5. Model for the evolution of a Nipissing intrusion (after Conrod, 1989).
OGS Study 58
74
Petrology and Geochemistry of the Nipissing Gabbro
exist for the development of a double-diffusive interface
between the aplitic and basic magmas. However, strong
convection above basinal zones would normally produce
extensive melting of the roof; perhaps such melts are
rapidly assimilated into the magma, and the upper quartz
diabase, does not develop until a later stage in the
crystallisation of the magma.
The evidence that there has been significant in-situ
crustal contamination of Nipissing Intrusions is very
strong based on the geochemical data presented above for
the Kerns Intrusion. Moreover, studies of other Nipissing
intrusions have demonstrated that the most differentiated
phases of intrusions are significantly contaminated by local country rocks of the roof zones of the intrusions
(Lightfoot et al., 1993; Conrod, 1988; 1989). An important
issue which arises from recent studies of layered mafic intrusive complexes and sills is whether this contamination
signature is accompanied by anomalous depletion of the
magma in Ni, Cu, and PGE as suggested by recent studies
of the Siberian Trap at Noril’sk (Lightfoot et al., 1994; Naldrett et al., 1992; 1995; Hawkesworth et al., 1995). It has
been established that many contaminated CFB magmas
show strong depletion in Ni, Cu, and the PGE, and many
investigations have used the work of Irvine (1975) to argue
that the addition of crustal silica to magmas triggers the
segregation and fractionation of immiscible sulphide liquids (e.g. Naldrett and McDonald, 1981; Lightfoot et al.,
1984; Naldrett et al., 1992, 1995). At issue is whether the
contamination signature found in the Kerns Intrusion is
accompanied by any evidence for the depletion of the
magma column in Ni and Cu which can be ascribed to the
in-situ segregation of an immiscible sulphide liquid.
The variation in Cu versus Zr concentrations in samples are shown in Figure 11b. This plot demonstrates that
samples with elevated Zr (which we now shown is coupled
to elevated Th/Yb, La/Yb, and SiO2 with low Ni) also have
very low Cu contents. In the context of Siberian Trap lavas
at Noril’sk which are depleted in Ni, Cu, and PGE, the Cu/
Zr ratio of the high-Zr samples are comparable to the most
Cu depleted Nadezhdinsky Formation lavas at Noril’sk
(Lightfoot et al., 1990; 1993; 1994). However, this low Cu/
Zr ratio is accompanied by very high SiO2 (65-80 weight %
SiO2 - see Figure 9b) - much higher than any of the Nadezhdinsky Formation basalts at Noril’sk (56 weight % SiO2
maximum). The elevated SiO2 of the samples from the
Kerns Intrusion is due to both fractionation and
contamination, and some of the most contaminated lavas
in the context of trace element ratios consist of as much as
90% crustal material assimilated into the Nipissing magma. It is known based on the analytical data for Lorrain
Formation sediments from the roof of the Kerns Intrusion
that the country rocks have very low Cu, and this is also a
feature of the aplitic granitoids which appear to be anatectic melts of the country rock. Based on these observations,
the low Cu/Zr ratios of the evolved samples from the Kerns
Instrusion can be explained by contamination of the
Nipissing magma with significant amounts of low Cu/Zr
country rock without any requirement for the segregation
and removal of immiscible sulphide liquids. This is an important observation as it suggests that caution is required
when using Cu/Zr as an index of the amount of sulphide
fractionation from mafic magmas.
75
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79
CONVERSION FACTORS FOR MEASUREMENTS IN ONTARIO
GEOLOGICAL SURVEY PUBLICATIONS
Conversion from SI to Imperial
Conversion from Imperial to SI
SI Unit
Multiplied by
Gives
Imperial Unit
1 mm
1 cm
1m
1m
1 km
0.039 37
0.393 70
3.280 84
0.049 709 7
0.621 371
LENGTH
inches
1 inch
inches
1 inch
feet
1 foot
chains
1 chain
miles (statute)
1 mile (statute)
Multiplied by
25.4
2.54
0.304 8
20.116 8
1.609 344
Gives
mm
cm
m
m
km
1 cm2
1 m2
1 km2
1 ha
0.155 0
10.763 9
0.386 10
2.471 054
AREA
square inches
1 square inch
square feet
1 square foot
square miles
1 square mile
acres
1 acre
6.451 6
0.092 903 04
2.589 988
0.404 685 6
cm2
m2
km2
ha
1 cm3
1 m3
1 m3
0.061 02
35.314 7
1.308 0
VOLUME
cubic inches
1 cubic inch
cubic feet
1 cubic foot
cubic yards
1 cubic yard
16.387 064
0.028 316 85
0.764 555
cm3
m3
m3
CAPACITY
1 pint
1 quart
1 gallon
1L
1L
1L
1.759 755
0.879 877
0.219 969
pints
quarts
gallons
0.568 261
1.136 522
4.546 090
1g
1g
1 kg
1 kg
1t
1 kg
1t
0.035 273 96
0.032 150 75
2.204 62
0.001 102 3
1.102 311
0.000 984 21
0.984 206 5
MASS
ounces (avdp)
1 ounce (avdp)
28.349 523
ounces (troy)
1 ounce (troy)
31.103 476 8
pounds (avdp) 1 pound (avdp)
0.453 592 37
tons (short)
1 ton (short)
907.184 74
tons (short)
1 ton (short)
0.907 184 74
tons (long)
1 ton (long)
1016.046 908 8
tons (long)
1 ton (long)
1.016 046 908 8
1 g/t
0.029 166 6
1 g/t
0.583 333 33
CONCENTRATION
ounce (troy)/
1 ounce (troy)/
ton (short)
ton (short)
pennyweights/ 1 pennyweight/
ton (short)
ton (short)
L
L
L
g
g
kg
kg
t
kg
t
34.285 714 2
g/t
1.714 285 7
g/t
OTHER USEFUL CONVERSION FACTORS
1 ounce (troy) per ton (short)
1 pennyweight per ton (short)
Multiplied by
20.0
0.05
pennyweights per ton (short)
ounces (troy) per ton (short)
Note: Conversion factors which are in bold type are exact. The conversion factors have been taken from or have
been derived fromfactors given in the Metric Practice Guide for the Canadian Mining and Metallurgical Industries,
published by the Mining Association of Canada in co-operation with the Coal Association of Canada.
80
ISSN 0704-2590
ISBN 0-7778-4804-X