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

Abstract

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 (1,500 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 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 a 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 f Te2 of −11.5 to −10, values well below the stability of calaverite. The absence of calaverite and the dominance of hessite in the D2 lodes of the Mt Charlotte area point to a kilometer-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.