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The Archean amphibolite facies Coolgardie Goldfield, Yilgarn Craton, Western Australia; nature, controls, and gold field-scale patterns of hydrothermal wall-rock alteration

Joseph T. Knight, John R. Ridley and David I. Groves
The Archean amphibolite facies Coolgardie Goldfield, Yilgarn Craton, Western Australia; nature, controls, and gold field-scale patterns of hydrothermal wall-rock alteration
Economic Geology and the Bulletin of the Society of Economic Geologists (February 2000) 95 (1): 49-83

Abstract

The late Archean Coolgardie Goldfield at the western margin of the Norseman-Wiluna belt, Yilgarn craton, comprises an arcuate belt of deformed mafic, ultramafic, and sedimentary rocks which is bounded to the west by the syntectonic Calooli monzogranite. Greenstones at Coolgardie preserve a broad metamorphic gradient, with peak metamorphic temperature varying from 480 degrees + or -50 degrees C at the center of the gold field to 545 degrees + or -50 degrees C adjacent to the western granitoid-greenstone contact, at an approximate pressure of 3 to 4 kbar. Gold field-scale variations in the gangue and ore mineralogy of zoned wall-rock alteration assemblages around lodes, the ore geochemistry, and the isotope chemistry of vein minerals at Coolgardie are correlated with plan-view distance from the Calooli monzogranite. Evidence supporting synpeak metamorphic gold mineralization in the Coolgardie Goldfield includes the equilibrium textural relationships between gold, sulfides, and high-temperature silicate gangue; the occurrence of undeformed auriferous quartz veins, enveloped by garnet-hornblende-plagioclase-calcite alteration, which crosscut peak-metamorphic fabrics; and the siting of variably deformed gold ores in synpeak-metamorphic structures. Conditions of gold mineralization at deposits less than 1 to 2 km from the Calooli monzogranite are determined from geothermometry and barometry to be 510 degrees + or -50 degrees C to 590 degrees + or -25 degrees C at 3 to 4 kbar, whereas those at greater distances from the monzogranite are 490 degrees to 525 degrees + or -50 degrees C at 3 to 4 kbar. Alteration assemblages in mafic host rocks can be divided into garnet-bearing (garnet-hornblende-plagioclase-calcite) and garnet-absent (biotite-amphibole-plagioclase-calcite). The presence or absence of garnet is mainly controlled by the Mg number of the host mafic rocks. Ore in deposits with garnet-bearing alteration is enriched in Ag, Na, Pb, S, and W, but only weakly enriched or depleted in K (sub 2) O and other large ion lithophile elements, CO (sub 2) , As, Mo, Sb, and Te, whereas deposits with biotite-amphibole-plagioclase-calcite alteration are strongly enriched in Ag, As, S, Sb, W, CO (sub 2) , and large ion lithophile elements. Sulfide-oxide assemblages are regionally zoned from pyrite-ilmenite in deposits in granitoids and adjacent to the granitoid-greenstone contact, through pyrrhotite-ilmenite+ or -pyrite in garnet-bearing alteration 1 to 2 km from the contact, to arsenopyrite-pyrrhotite-ilmenite assemblages in biotite-bearing alteration >2 km from the greenstone-granitoid contact. This variation is potentially related to gradients in fluid f (sub O2) away from the granitoids. Isotopic compositions of oxygen in quartz (delta (super 18) O = 10.8-12.4 per mil), scheelite (delta (super 18) O = 4.0-4.1 per mil), and oxygen and carbon in calcite (delta (super 18) O = 8.9-13.2 per mil, delta (super 13) C = -0.5 to -5.3 per mil) are generally more positive in deposits with garnet-bearing alteration than in those with biotite-bearing alteration (delta (super 18) O (sub quartz) = 6.6-11.8 per mil, delta (super 18) O (sub scheelite) = 2.3-4.6 per mil, delta (super 18) O (sub calcite) = 8.7-11.4 per mil, delta (super 13) C (sub calcite) = -4.3 to -8.4 per mil), whereas both alteration styles have delta D (sub biotite) and delta D (sub amphibole) values in the range -65 to -86 per mil. These differences are interpreted to reflect interaction of isotopically heavy ore fluids with relatively depleted greenstone host rocks during fluid migration through structurally controlled conduits. The gold field-scale variations in alteration mineralogy and ore chemistry are considered to be related not to initial ore-fluid composition but to temperature, to host-rock composition, and to changes in fluid composition resulting from reaction with greenstone-belt rocks. The correlation between the calculated temperature of alteration and distance from the western granitoid-greenstone contact suggests that the Calooli monzogranite played some genetic role in determining the nature of hydrothermal alteration across Coolgardie.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 95
Serial Issue: 1
Title: The Archean amphibolite facies Coolgardie Goldfield, Yilgarn Craton, Western Australia; nature, controls, and gold field-scale patterns of hydrothermal wall-rock alteration
Affiliation: BHP World Minerals, BHP Minerals Discovery Group, East Perth, West. Aust., Australia
Pages: 49-83
Published: 200002
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 57
Accession Number: 2000-039162
Categories: Economic geology, geology of ore depositsIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 8 tables, sketch maps
S30°00'00" - S30°00'00", E118°00'00" - E118°00'00"
Secondary Affiliation: Macquarie University, AUS, AustraliaUniversity of Western Australia, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200013
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