The Coolgardie Goldfied,Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane |
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| Authors: | J T Knight D I Groves J R Ridley |
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| Institution: | (1) Key Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, 6009 Nedlands, WA, Australia |
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| Abstract: | Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have
been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase
of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of
the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein
sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures
hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west
compressional far-field stress régime, but with local heterogeneous stress orientations. This occurred after an earlier period
of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock
alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation
temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts
with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase
+ calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance
from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of
alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits
that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for
Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist
to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may
play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels. |
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