Initial exploration results of the Collins epithermal Au‐base metal prospect, Aceh, Indonesia

Abstract

Interpretation of various exploration data, in particular geochemical prospecting, offers a powerful and rapid assessment of grass‐root projects in a green‐field terrain. Here, we present an example of the Collins epithermal prospect in Aceh Province, Indonesia. In this area, the Au+ base‐metal‐bearing sheeted quartz veins (individually mostly 2–4 cm wide), which are controlled by a 250 m wide by 800 m long NNE‐trending structural corridor within Paleogene sandstone and volcanic rocks, are the product of two main stages of deposition. Stage I formed veins with a sliver of cryptocrystalline quartz wall zone followed by an inner zone of comb quartz with interstitial rhombic adularia that terminates in open space. Stage I or main‐stage sulfide mineralization consisting of early galena + sphalerite and later chalcopyrite occurs with the quartz + adularia. Small amounts of galena also occur in the wall zone. Stage II mineralization brecciated Stage I veins and overprinted them with silicification characterized by vuggy texture. Mineralization associated with this episode consists of earlier chalcopyrite + sphalerite + tennantite–tetrahedrite and later, vug‐filling Au–Ag alloy (Ag0.37–0.41Au0.62–0.59). The above mineralized veins are successively flanked by silicic selvages, an illite + chlorite + pyrite ± kaolinite zone and a chlorite + epidote + carbonate + pyrite zone. Local supergene alteration induced replacement of galena by plumbogummite and anglesite and chalcopyrite by covellite. Data from fluid inclusion microthermometry in quartz indicated that the inner zone of Stage I veins formed from fluids with a 2.3 wt% salinity (0.5–3.3 wt% NaCl equivalent), at 174°C (155–211°C). Combining these physico‐chemical parameters with the mineral assemblage, the mineralization occurred under a reduced environment. Rock and soil assays indicate that elevated Au concentrations (up to 16.5 ppm over 1 m) occur along northeast‐trending zones and show a strong correlation with Pb, while Cu (up to 2.58% over 1 m), Zn, As, Sb, and Mo anomalies lie mostly at the periphery. The high‐grade mineralized veins correlate with moderate to high resistivity and chargeability zones, and the pseudosections of such geophysical signals are interpreted as reflecting coalesced or enlarged veins at depth, or inclined veins in other localities. The intermediate sulfidation affinity for Collins points to potential mineralization at depth as well as preservation of Au‐rich and sulfide‐poor zones in the less eroded areas.