Silver-rich telluride mineralization at Mount Charlotte and Au–Ag zonation in the giant Golden Mile deposit, Kalgoorlie, Western Australia

The gold deposits at Kalgoorlie in the 2.7 Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, occur adjacent to the D2 Golden Mile Fault over a strike of 8 km within a district-scale zone marked by porphyry dykes and chloritic alteration. The late Golden Pike Fault separates the older (D2) shear-zone system of the Golden Mile (1500 t Au) in the southeast from the younger (D4) quartz-vein stockworks at Mt Charlotte (126 t Au) in the northwest. Both deposits occur in the Golden Mile Dolerite sill, and display inner sericite-ankerite alteration and early-stage gold-pyrite mineralization replacing the wall rocks. Late-stage tellurides account for 20% of the total gold in the first but for less than 1% in the second deposit. In the Golden Mile, the main telluride assemblage is coloradoite + native gold (898-972 fine) + calaverite + petzite ± krennerite. Telluride-rich ore (>30 g/t Au) is characterized by Au/Ag = 2.54 and As/Sb = 2.6-30, the latter ratio caused by arsenical pyrite. Golden Mile-type D2 lodes occur northwest of the Golden Pike Fault but the Hidden Secret orebody, the only telluride bonanza mined (10,815 t at 44 g/t Au), was unusually rich in silver (Au/Ag = 0.12-0.35) due to abundant hessite. We describe another array of silver-rich D2 shear zones, which are part of the Golden Mile Fault exposed on the Mt Charlotte mine 22 level. They are filled with crack-seal and pinch-and-swell quartz-carbonate veins, and are surrounded by early-stage pyrite + pyrrhotite disseminated in a sericite-ankerite zone more than 6 m wide. Gold grade (0.5-0.8 g/t) varies little across the zone but Au/Ag (0.37-2.40) and As/Sb (1.54-13.9) increase away from the veins. Late-stage telluride mineralization (23 g/t Au) sampled in one vein has much lower Au/Ag (0.13) and As/Sb (0.48), and comprises scheelite, pyrite, native gold (830-854 fine), hessite, and minor pyrrhotite, altaite, bournonite, and boulangerite. Assuming 250-300°C, gold-hessite compositions indicate a fluid log fTe2 of -11.5 to -10, values well below the stability of calaverite. The absence of calaverite and dominance of hessite in D2 lodes of the Mt Charlotte area points to a km-scale mineral and Au/Ag zonation along the Golden Mile master fault, which is attributed to a lateral decrease in peak tellurium fugacity of the late-stage hydrothermal fluid. The As/Sb ratio may be similarly zoned to lower values at the periphery. The D4 gold-quartz veins constituting the Mt Charlotte orebodies represent a younger hydrothermal system, which did not contribute to metal zonation in the older one.