Download The Bassett Lake mafic intrusion is situated on and immediately

- 150 The Bassett Lake Mafic Intrusion: Preliminary Report
by I. Graham1, B.R. Watters1 and L. Hulbert2
Graham, I., Watters B. R. , and Hulbert L. (1986): The Bassett Lake mafic intrusion: preliminary report; in Sunmary
of Investigations 1986 , Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report"lJ6- 4.
The Bassett Lake mafic intrusion is situated on and
immediately south of Bassett Lake in the
southernmost part of the Central Metavolcanic Belt
of the La Range Domain. The Bassett Lake area
mainly comprises a suite of plutonic
mafic-ultramafic rocks bounded by a predominately
intermediate to mafic volcanic country rock
assemblage to the south, a compositionally variable
granodioritic body to the northwest and a diorite
intrusion to the east.
Present Project
Detailed mapping and sampling of the mafic
intrusion has been carried out and will form the
basis for mineralogical and geochemical studies.
Samples of drill core from the Triangle Lake area in
the central part of the body will constitute an
important component of these studies.
Major aims of the study will be to determine the
nature of the intrusion, its origin and relationships
to other rock units in the La Range Domain, and the
potential for mineralization involving elements of
the platinum group, vanadium, titanium and
chromium. A variety of analytical techniques,
including electron probe microanalysis, will be
employed to investigate the mineralogy, mineral
zoning, chemical characteristics and phase relations
of oxide and sulphide minerals.
Previous Work
The earliest reference to work done in the area is
that of Mclarty (l 936) who mapped the area as part
of a regional project. An airborne magnetic survey
over the "basic and ultrabasic" rocks in the area for
the Saskatchewan Department of Mineral Resources
in 1953 indicated magnetic highs in the Triangle
Lake area (Fig. 1).
surveys were flown by the Great Plains
Development Co. of Canada Ltd., and the Don
Fisher Syndicate undertook a ground
electromagnetic survey. Regional mapping by
Padgham ( 1960) included a report on the "Bassett
Lake Gabbro", in which he referred to chloritized
and epidotized hornblende gabbro and the
development of some anorthosite. He also noted
chalcopyrite, pentlandite, pyrrhotite and pyrite
mineralization.
Richards (1964) descr ibed pyrrhotite and pyrite
mineralization in a variety of gabbroic rocks from
five diamond - drill hole cores from the Bassett Lake
body. Reynolds (l 967) subsequently mapped the
area and reported a significant development of
pyroxenites in a predominantly dioritic body, the
surrounding rocks including granulites, granitic
gneisses and volcanics.
Recent work by Harper ( 1982) included an
investigation of both drill core and surface geology
in the Triangle Lake area. The study was primarily
an economic evaluation, and particular attention
was given to the magnetite-rich rocks and their
potential as an iron ore. Layered gabbro, olivine
gabbro, minor peridotite and anorthosite are
described by Harper ( 1982).
Intrusive Mafic Rocks
The most common rock types in the Bassett Lake
Intrusion are variably uralitized and hornblendic
medium-grained to very coarse-grained gabbros.
These are commonly foliated though locally massive
or display only a weak fabric.
Trenching of the sulphide showings in 1957 and 1961
covered the claim area held by A. Studer. Beck
( 1959) reported pyrite, pyrrhotite and chalcopyrite
from a pit blasted at the main showing. In 1965,
further airborne electromagnetic and magnetic
On weathered surface the rock appears very dark,
except where 'leucogabbro' is developed. The
leucogabbro has well-developed plagioclase laths
typically altered to aggregates of 'saussurite' as a
result of deuteric processes and/or incipient
metamorphism. Alteration of the plagioclase also
occurs in the more mafic gabbros, although these
rocks commonly display clearer and in places visibly
zoned plagioclase crystals. Such zoning is due to
inclusions of pyroxene, spinel and iron oxide.
l Department of Geology, University of Regina
2 Economic Geology and Mineralogy Division, Geological
Survey of Canada
Project funded under the Canada component of the
Canada-Saskatchewan Subsidiary Agreement on Mineral
Development 1984-89, with some logistic support from the
Saskatchewan Geological Survey.
The coarse-grained gabbroic rocks contain
hornblende, occurring as large poikilitic plates.
Secondary amphibole ("uralite") observed in some
thin sections is predominately actinolite, with very
minor nonpleochroic tremolite. Carlsbad twinning
of the plagioclase is recognized in some hand
specimens. A thin section from a core sample of
gabbro from the Triangle Lake area shows
- 151 -
Legend
fa~§~~~ Glacial cover
Intermediate intrusive
Metagabbro and metanorite
Magnetite gabbro/norite
Magnetite gabbro (magnetic intensity
greater than 10,000 gauss)
-.., """" Shear zone
,·-·-·-
-
Conductor
Diamond-drill hole sites
Main trench sites
Foliation
Granodiorite and granitoid rock
Topographic contours (10 m interval)
Mafic and Intermediate metavolcanic rock
10,000 gauss magnetic contour
Felsic metavolcanic rock
Inferred lithological contacts
Figure 1 - Preliminary geological map of the Bassett Lake mafic intrusion.
- 152 development of augite (with possible pigeonite
exsolution lamellae), zoned calcic labradorite and
possibly late-magmatic hornblende.
Norite has been identified in the field and in thin
section from the Triangle Lake area and may
represent the "mottled type metagabbros" referred
to by Harper (1982). These rocks are commonly
weakly and more rarely strongly magnetic. In hand
specimen they show oikocrystic development of
hypersthene. The orthopyroxene crystals are
medium grained, idiomorphic and bronze toned, and
are responsible for the highly reflective, mottled
outcrop surfaces common in the Triangle Lake
area. Orthopyroxene observed in noritic horizons
along the north shores of Bassett Lake is somewhat
finer grained and 'sugary' textured with poorly
formed crystals. Both ortho- and clinopyroxenes
show evidence of high temperature alteration by
hydration to amphibole. These rocks also contain
plagioclase, commonly zoned and in the range
calcic andesine to sodic bytownite, small flakes of
hematized dark brown biotite, various amounts of
opaque minerals and accessory apatite. Opaque
minerals have not yet been fully identified but
include magnetite, ilmenite, possibly chromite and
trace sulphides such as pyrrhotite, pyrite and
chalcopyrite.
Areas underlain by norite and gabbro commonly
have smooth, rolling and densely wooded hills.
Outcrop is not abundant except along the shores of
Bassett Lake and on steep hillsides.
Magnetite-rich gabbro and norite are concentrated
in the Triangle Lake area and along the western
flank of the body. These melanocratic rocks contain
up to 30 percent opaque minerals, the major
component of which is probably magnetite.
Although most of the noritic rocks contain opaque
minerals, 'normal' norite is regarded here as that
containing less than about l 5 percent opaques in the
mode. The textures are granular and somewhat
anhedral, except where individual crystals are
enclosed by magnetite and where subhedral crystal
development occurs.
Very highly magnetic ultramafic rocks (Fig. l),
typically exhibit a surface magnetic intensity in
excess of 10,000 gauss (Great Plains Development
Co. of Canada Ltd., surface EM survey). One thin
section of such an ultramafic rock (from diamonddrill core in the Triangle Lake area) contains
approximately 60 percent opaque minerals, 20
percent olivine (including alteration products such
as iddingsite and minor antigorite), augite and
minor oligoclase or andesine. Reynolds (1967)
mapped 'pyroxenites' in the Bassett Lake intrusion,
but no rocks of this type were identified during the
recent field study.
Anorthosite occurs in most parts of the Bassett
Lake body where layering is well developed. This
rock type has a well-developed granular cumulate
texture and is more leucrocratic than the more
common leuco- or anorthositic gabbro. It contains
as little as 5 percent mafic minerals (usually
hornblende), as well as saussuritized plagioclase
feldspar. The anorthite content of the feldspars
cannot be determined because of the alteration.
"Magnetite anorthosite" containing approximately
l O to 15 percent magnetite, occurs at a locality
near Triangle Lake Creek.
Well-developed layering is present mainly in the
northern and western parts of the body, and occurs
on a scale varying from 1 and 2 cm to over 2 m.
The layering is commonly compositional, but
textural layering is also developed. To the west of
Bassett Lake, in the area of very high magnetic
intensities (Fig. l ), rhythmic layering on a scale of
several centimetres is very well developed. The
layers alternate in composition between an
intensely magnetic "magnetite gabbro" and
completely nonmagnetic leucogabbro.
A series of large-scale blocky "breccias" occur
along the eastern contact zone of intrusion; angular
blocks composed of fine- to medium-grained gabbro
are set in a matrix of quartz dioritic to
"leucogabbroic" material. The angular gabbroic
blocks may be the result of disruption of the wall
rocks of the Bassett Lake body during intrusion of
the dioritic magma.
Veins of graphic-textured leucocratic granodiorite
intrude the Bassett Lake body mainly in the north
and west, but also in the northern-central parts of
the intrusion. Variably developed in these regions,
the vein margins range from sharp to di ff use.
Quartz is concentrated towards the centres of the
veins, whilst large hornblende crystals up to 7 cm in
length are developed adjacent to the contacts.
Pockets of predominantly uralitized mafic minerals
occur in gabbroic host rocks adjacent to the
veining, suggesting a genetic relation with the veins.
Geology of the Country Rocks
The intermediate and mafic metavolcanics to the
southwest of the Bassett Lake Intrusion are
composed predominantly of crystal tuffs with lesser
mafic flows and minor volcanogenic sedimentary
units. Rocks along the shores of Thorton Lake have
been intensely sheared, and minor shearing occurs
elsewhere in the volcanics, en echelon to the
main shear zone. Contact relationships between
the volcanic rocks and the gabbroic rocks of the
Bassett Lake body could not be established due to
the absence of suitable outcrops.
Diorite and quartz diorite observed to the east of
the mafic intrusion are medium to coarse grained,
generally hornblendic and foliated. The
predominant diorite contains more hornblende than
the quartz diorite which in places is extremely
leucocratic. Quartz diorite also occurs in
- 153 association with the breccias at the contact zone
between the Bassett Lake gabbro and the diorite.
Granodioritic rocks outcropping to the east of
Bassett Lake are compositionally variable, in places
appearing granitic. A number of rafted "blocks" are
present in the granodiorite, the largest xenolith
recognized being a rhyolitic metavolcanic
(indicated as "acid volcanics" in Fig. l). The
north-trending shear zone in the west, well
developed in the granodiorite, is a major zone of
mylonitization over 50 m wide in places. This shear
zone extends northwards through Beaudry and Fault
Line Lakes (Thomas, pers. comm.) and southward
through Thorton Lake (Padgham, 1960).
support from the Saskatchewan Geological Survey
for some aspects of the field work is gratefully
acknowledged. K. Haynes, D. Thomas and C.
Harper are thanked for their assistance during the
field season.
References
Beck, L.S. (1959): Mineral occurrences in
the Precambrian of northern Saskatchewan
(excluding radioactive minerals); Sask. Dep.
Miner. Resour., Rep. 36, l34p.
Harper, C. T. (1976): Core descriptions of
DOH core holes 1PCH BL US DOH 1/2; Sask.
Energy Mines.
Summary
The Bassett Lake body is intrusive into
predominantly metavolcanic rocks and is bounded to
the east and northwest by later diorite and
granodiorite respectively. The intrusion mainly
comprises gabbro, norite and minor anorthosite.
Magmatic magnetite is variously developed,
resulting in a region of anomalous positive magnetic
intensity. Much of the intrusion has been
deuterically altered and a graphic granodioritic
veining is commonly developed. The central parts
of the body characteristically show primary
magmatic layering.
Ack now ledge men ts
The project is supported by a research contract
with the Geological Survey of Canada. Logistic
_ _ _ _ (1983): Iron ores of
northern Saskatchewan; Sask. Energy Mines
Rep. 220, 102p.
Mclarty, D.M.E. (1936): Lac la Range sheet
(east half), Saskatchewan; Geol. Surv. Can.,
Map 358A.
Padgham, W.A. (1960): The Geology of the
Otter Lake area (west half) Saskatchewan;
Sask. Dep. Miner. Resour., Rep. 41, 34p.
Richards, B.R. ( 1964): Description of DOH
core; Sask. Dep. Miner. Resour.
Reynolds, N.W. (1967): Report on geology of
Claim Block CBS 622, Bassett Lake area,
Saskatchewan; Sask. Dep. Miner. Resour ., 4p.