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 (Ag 0.37-0.41 Au 0.62-0.59 ). 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 physicochemical 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

Toward a mineral potential map of central Aceh, North Sumatra: contributions from spectrum-area and local singularity fractal analysis of stream sediment geochemistry and its relationships to magmatic-hydrothermal Cu and Au mineralisation

“…As porphyry Cu and epithermal Au deposits are the dominant types in central Aceh and stream sediments with anomalous Pb surrounding the Ise-Tengkereng porphyry Cu group, the following analysis focuses on the geochemical patterns of Cu, As, and Pb. The second element is selected in lieu of Au analysis and because of its strong positive correlation with Au in the Miwah high-sulfidation epithermal Au–Ag deposit (Mulja et al 2020), the Collins intermediate-sulfidation epithermal Au–Ag prospect (Mulja et al 2021) in the northeast, and the Abong jasperoid-hosted Au deposit (Taylor 2012) in the southeast (Figure 1(c)).…”

“…These spatial correlations are interpreted to relate to a series of northerly structures commonly traversing the region. Within central Aceh, they control the Au feeder zones at Miwah (Mulja et al 2020), the Collins Au–base metal veins (Mulja et al 2021), the Paleogene plutons in the SW, and the Geureudong–Telago volcanic centres. Interestingly, a southward extension of these N–S structures coincides with the F2 mega-fracture zone offshore Sumatra (inset map in Figure 2).…”

Porphyry and skarn Cu prospectivity of Sumatra with GIS spatial modelling techniques

“…9 Copper also constitutes a minor phase in carbonate-hosted Mississippi Valley-type (MVT) Pb–Zn deposits, 38 shale-hosted massive sulphide Zn–Pb deposits, 39 Fe skarn showings, 40,41 intrusion-hosted Mo occurrences 42,43 and epithermal deposits. 44,45 Porphyry Cu and Cu-rich polymetallic skarn prospects, hereinafter abbreviated as PCP and CSP, respectively, developed mainly along the main Barisan range and rarely on the Barisan west coastal range or eastern foothills (Figure 1(b)–(d)). An exception to this is the Sable porphyry Cu + Au + Mo prospect that stands only 70–150 m above sea level (asl) within the Meulaboh embayment in Aceh.…”