Geological and Geochemical Characteristics of Gold Mineralization in the Salu Bulo Prospect, Sulawesi, Indonesia

Abstract

The Salu Bulo prospect is one of the gold prospects in the Awak Mas project in the central part of the western province, Sulawesi, Indonesia. The gold mineralization is hosted by the meta-sedimentary rocks intercalated with the meta-volcanic and volcaniclastic rocks of the Latimojong Metamorphic Complex. The ores are approximately three meters thick, consisting of veins, stockwork, and breccias. The veins can be classified into three stages, namely, early, main, and late stages, and gold mineralization is related to the main stage. The mineral assemblage of the matrix of breccia and the veins are both composed of quartz, carbonate (mainly ankerite), and albite. High-grade gold ores in the Salu Bulo prospect are accompanied by intense alteration, such as carbonatization, albitization, silicification, and sulfidation along the main stage veins and breccia. Alteration mineral assemblage includes ankerite AE calcite, quartz, albite, and pyrite along with minor sericite. Pyrite is the most abundant sulfide mineral that is spatially related to native gold and electrum (<2-42 μm in size). It is more abundant as dissemination in the altered host rocks than those in veins. This suggests that water-rock interaction played a role to precipitate pyrite and Au in the Salu Bulo prospect. The Au contents of intensely altered host rocks and ores have positive correlations with Ag, Ni, Mo, and Na. Fluid inclusions in the veins of the main stage and the matrix of breccia are mainly two-phase liquid-rich inclusions with minor two-phase, vapor-rich, and single-phase liquid or vapor inclusions. CO 2 and N 2 gases are detected in the fluid inclusions by Laser Raman microspectrometry. Fluid boiling probably occurred when the fluid was trapped at approximately 120-190 m below the paleo water table. δ 18 O SMOW values of fluid, +5.8 and +7.6‰, calculated from δ 18 O SMOW of quartz from the main stage vein indicate oxygen isotopic exchange with wall rocks during deep circulation. δ 34 S CDT of pyrite narrowly ranges from −2.0 to +3.4‰, suggesting a single source of sulfur. Gold mineralization in the Salu Bulo prospect occurred in an epithermal condition, after the metamorphism of the host rocks. It formed at a relatively shallow depth from fluids with low to moderate salinity (3.0-8.5 wt% NaCl equiv.). The temperature and pressure of ore formation range from 190 to 210 C and 1.2 to 1.9 MPa, respectively.

Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia

“…Aside from these relatively young rocks in the magmatic arc, the Cretaceous accretionary complexes also have potential for gold mineralization, as indicated by recent discoveries of gold prospects and deposits in eastern Indonesia (e.g., Idrus et al, 2017). Discoveries of gold mineralization hosted in the Cretaceous metamorphic terrains such as the Awak Mas gold deposits in South Sulawesi (Ernowo et al, 2019; Hakim et al, 2018; Tuakia et al, 2018), the Bulu gold prospect in Maluku, and the Bombana gold prospect in Southeast Sulawesi (Idrus et al, 2017) have shown that the metamorphic terrains represent an alternative for gold exploration in Indonesia.…”

Elemental Gains and Losses during Hydrothermal Alteration in Awak Mas Gold Deposit, Sulawesi Island, Indonesia: Constraints from Balanced Mineral Reactions

“…The Awak Mas gold deposit is situated in the southern arm of the Western Sulawesi Province, where both metamorphic basement units and arc-related igneous units prevail (Figure 1b). The Latimojong Formation is underlaid by phyllites, slates, basic to intermediate volcanics, limestones, and schists, representing a platform and/or fore arc trough flysch sequence intruded by granite, diorite, monzonite, and/or syenite plugs and stocks [16][17][18][19][20][21][22]. The other lithological formation in this area is the Lamasi Complex, composed of mafic to intermediate rocks.…”