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A STUDY OF MINERAL AND ROCK FRAGMENTS AND SHOCK FEATURES IN THE LATE GRANITE BRECCIA AT THE FRASER MINE, NORTH RANGE, SUDBURY STRUCTURE. K. A. McCormick1,
R. S. James1, J. S. Fedorowich2, D. H. Rousell1, A. M. McDonald1, and H. L. Gibson1, 1Department of Earth
Sciences, Laurentian University, Sudbury, ON, P3E 2C6, Canada, 2Falconbridge Exploration Ltd., P.O. Box 40,
Falconbridge, ON, P0M 1S0, Canada.
The Late Granite Breccia (Footwall Breccia) is a
heterolithic breccia, post-Sudbury Breccia in age,
that is present in irregularly-shaped, discontinuous
units, ranging from 0 to ~150 m thick, within the
Sudbury Structure [1–3]. It is exposed along the
north, west, and east margins of the Sudbury Structure, where it occurs between the footwall Levack
Gneiss Complex and the Sudbury Igneous Complex
(SIC). The Late Granite Breccia (LGBX) is an important rock unit at Fraser Mine because it is the
host for most of the Ni-Cu ores. This breccia is the
focus of a detailed Falconbridge-funded study, the
object of which is to identify mineralogical and geochemical criteria that indicate proximity to ore
zones.
Late Granite Breccia is composed of rock and
mineral fragments that vary from less than 1 mm to
rock fragments tens of meters in size. The mineral
fragments are primarily quartz and feldspar and the
bulk chemistry of the breccia matrix varies from
dioritic (or tonalitic) to granitic, roughly becoming
more granitic toward the contact with the Archean
rocks [3]. Rock fragments (derived primarily from
the local footwall) consist of felsic, intermediate, and
mafic gneisses of the Levack Gneiss complex and
fragments from mafic dikes which intrude the Levack complex. Sudbury Breccia (pseudotachylite
breccia) fragments are generally rare, but can be
locally abundant. Ultramafic rock fragments are locally present (primarily pyroxenite, but also peridotite). Possible sources for the ultramafic fragments
include footwall mafic to ultramafic bodies such as
those at Strathcona and Fraser Mines, or the “root
system” of the Sudbury Igneous Complex. Some pyroxenite fragments may represent migmititic segments of the Levack Gneiss Complex. The percentages of various clast types in the LGBX, determined
from one of the drill holes in this study, are: 67%
gneiss, 9% diabase and gabbro (local dikes), 6.5%
pyroxenites, 10% amphibolites, 1.5% Sudbury
Breccia, and 6% other.
We are also studying the various shock features
preserved in quartz, feldspar, pyroxene, biotite, and
epidote in the matrix and in rock fragments of the
LGBX. The shock features we find include: as many
as three directions of hexagonally or rhombohedrally
aligned inclusions in quartz; fractures in feldspar
and epidote; bent plagioclase twin lamellae; possible
deformation lamellae in feldspar; finely spaced exsolution lamellae in pyroxene; shock decomposition of
pyroxene and biotite; and kinked and oxidized biotite. The feldspar and quartz fragments in the
LGBX are commonly polygonal in form
(inequigranular to seriate) near the footwall. Remnant aligned inclusions in quartz are present in some
of the individual polygons. Upward from the footwall, quartz typically becomes porphyrocrystic, and
encloses lath-like feldspar. Recrystallization of these
phases could have been induced by the overall elevated temperatures of the shocked rocks, the thermal
metamorphic effect of the cooling SIC above these
breccias, and/or possibly a regional metamorphic
event.
References: [1] Mitchell G. P. and Mutch
A. D. (1957) Sixth Can. Inst. Mining Metal., 350–
363. [2] Coats C. J. A. and Snajdr. P. (1984) OGS
Spec. Vol. 1, 327–346. [3] Lakomy R. (1990) Meteoritics, 25, 195–207.