The Soreang prospect is one of a number of gold prospect occurred in the western part of Sunda-Banda magmatic arc that has mostly low-sulfidation deposit types. The presence of gold mineralisation at Soreang is characterized by its association with base metals and occurrence in hydrothermal breccia zone. Previous study proposed that Soreang area may represent a porphyry mineralisation environment. However, there are a number of problems related to the hydrothermal systems involved in ore deposition that required investigation. The characteristics of hydrothermal fluids and the physical and chemical environments of ore transport and deposition are fundamental to establish mineralisation style or deposit model.
Soreang occurs in a Tertiary to Quarternary volcanic environments. Rocks in this area consist mainly of volcanic breccia and porphyritic-andesite and dacite that formed during Late Miocene to Pleistocene. These rocks were identified to harbour gold and sulfide minerals. They mostly altered to sericite/illite, carbonate, chlorite and kaolinite. Based on petrographic and mineralogic observation, four paragenetic stages of mineral formation have been identified. Stage I: sericite/illite, quartz, carbonate and pyrite. Stage II: mineralisation including pyrite, sphalerite, galena, pyrrhotite, chalcopyrite, tetrahedrite, gold and marcasite. Stage III: carbonate (calcite) and quartz. Stage IV: kaolinite. Mineralisation occurs in a narrow veinlets (hairlines) and also as dissemination in groundmass of rocks.
Based on fluid inclusion studies, two types of inclusion were distinguished in calcite and quartz veinlets. They are two-phase liquid-rich inclusions (type II) with vapour/liquid ratios from 10 to 20 %, and single-phase vapour-rich inclusions (type IV) with nearly 100% vapour. The former types are predominant in the sample and thus used for observation homogenisation temperature (Th) and salinity (NaCl wt % equivalent) analyses. Calcite has low salinity (1.6-4.3 % NaCl wt% equiv) with Th ranging from 158° to 267°C, while quartz indicates a wider range, 0.0-24.1 NaCl wt% equiv with Th ranging from 260° to 333°C. From these data can be interpreted that the primary high temperature fluids were derived from magmatic waters and then they mixed with meteoric waters. Oxygen stable isotope determinations indicated that the isotopic composition (δ18OH20) of the mineralising fluid range from 2.1 to 8.1 per-mil (calculated from calcite), which is compatible with mixed magmatic-meteoric waters. Sufur isotope analyses indicate that sulfur was derived from magmatic fluid. Gold was precipitated from the fluids after the temperature decreased.
According to the study results, gold mineralisation at Soreang can be clasified as epithermal-porphyry system or high-sulfidation type. Gold mineralisation occurred synchronously with phyllic alteration and probably also with porphyritic andesiticdacitic rocks emplaced 2.65 to 2.07 Ma (based on illite dating). The structures that controlled gold mineralisation may be the thrust fault trending NE-SW. Because gold deposition occurred as in the Au-Cu porphyry stages, stockwork Au-Cu may occur at depth, beneath phyllic zone, while high-sulfidation epithermal type may occur laterally to the north west. The implication for exploration of the other prospects in the western part of Sunda-Banda magmatic arc particularly in Java and Sumatra is that such a similar occurrence model likely occurs in these regions because their similarity in regional geology.