The Archean amphibolite facies Coolgardie Goldfield, Yilgarn craton, western Australia: Nature, controls, and gold field-scale patterns of hydrothermal wall-rock alteration

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 synte ctonic Calooli monzogranite. Greenstones at Coolgardie preserve a broad met amorphic gradient, with peak metamorphic temperature varying from 480 degre es +/- 50 degrees C at the center of the gold field to 545 degrees +/- 50 d egrees C adjacent to the western granitoid-greenstone contact, at an approx imate 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 iso tope chemistry of vein minerals at Coolgardie are correlated with plan-view distance from the Calooli monzogranite. Evidence supporting synpeak metamo rphic gold mineralization in the Coolgardie Goldfield includes the equilibr ium textural relationships between gold sulfides, and high-temperature sili cate gangue; the occurrence of undeformed auriferous quartz veins, envelope d by gamet-hornblende-plagioclase-calcite alteration, which crosscut peak-m etamorphic fabrics; and the siting of variably deformed gold ores in synpea k-metamorphic structures. Conditions of gold mineralization at deposits les s than 1 to 2 km from the Calooli monzogranite are determined from geotherm ometry and barometry to be 510 degrees +/- 50 degrees C to 590 degrees +/- 25 degrees C at 3 to 4 kbar, whereas those at greater distances from the mo nzogranite are 490 degrees to 525 degrees +/- 50 degrees C at 3 to 4 kbar. Alteration assemblages in mafic host rocks can be divided into garnet-beari ng (garnet-hornblende-plagioclase-calcite) and garnet-absent (biotite-amphi bole-plagioclase-calcite). The presence or absence of garnet is mainly cont rolled by the Mg number of the host mafic rocks. Ore in deposits with garne t-bearing alteration is enriched in Ag, Na, Pb, S, and W, but only weakly e nriched or depleted in K2O and other large ion lithophile elements, CO2, As , Mo, Sb, and Te, whereas deposits with biotite-amphibole-plagioclase-calci te alteration are strongly enriched in Ag, As, S, Sb, W, CO2, and large ion lithophile elements. Sulfide-oxide assemblages are regionally zoned from p yrite-ilmenite in deposits in granitoids and adjacent to the granitoid-gree nstone contact, through pyrrhotite-ilmenite +/- pyrite in garnet-bearing al teration 1 to 2 km from the contact, to arsenopyrite-pyrrhotite-ilmenite as semblages in biotite-bearing alteration 22 km from the greenstone-granitoid contact. This variation is potentially related to gradients in fluid f(O2) away from the granitoids. Isotopic compositions of oxygen in quartz (delta(18)O = 10.8-12.4 parts per thousand), scheelite (delta(18)O = 4.0-4.1 parts per thousand), and oxygen and carbon in calcite (delta(18)O = 8.9-13.2 parts per thousand, delta(13) C = -0.5 to 5.3 parts per thousand) are generally more positive in deposits with garnet-bearing alteration than in those with biotite-bearing alterati on (delta(18)O(quartz) = 6.6-11.8 parts per thousand, delta(18)O(scheelite) = 2.3-4.6 parts per thousand, delta(18)O(calcite) = 8.7-11.4 parts per tho usand, delta(13)C(calcite) = -4.3 to -8.4 parts per thousand), whereas both alteration styles have delta D-biotite and delta D-amphibole values in the range -65 to -86 per mil. These differences are interpreted to reflect int eraction of isotopically topically heavy ore fluids with relatively deplete d greenstone host rocks during fluid migration through structurally control led 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 te mperature, to host-rock composition, and to changes in fluid composition re sulting from reaction with greenstone-belt rocks. The correlation between t he calculated temperature of alteration and distance from the western grani toid-greenstone contact suggests that the Calooli monzogranite played some genetic role in determining the nature of hydrothermal alteration across Co olgardie.