Alteration and epithermal mineralization in the Masupa Ria volcanic center, Central Kalimantan, Indonesia

Abstract

The Masupa Ria district, located in central Kalimantan, Indonesia, is associated with extensive areas of residual quartz and quartz-clay alteration that form elevated topographic regions. The alteration zones lie within a larger area of calc-alkaline volcanic rocks that form part of a mid-Tertiary volcanic are trending northeast-southwest through central Borneo. The Masupa Ria volcanic center is dominated by andesitic flows, fragmental rocks, and high level intrusions of calc-alkaline composition. Epithermal precious metal bearing veins occur throughout the district. These vary from narrow discontinuous veinlets to major veins up to ten meters across. The major veins are hosted by steep structures, some of which are regional in extent (strike lengths in excess of 10 km), and the predominant strike directions are 060°, 290° and 330°. Precious metal mineralization is typically associated with pyrite-sphalerite-galena-tetrahedrite bands within complex quartz veins. Economic grades are restricted to zones of a few tens of meters in length. Wall rock alteration is dominated by a quartz-illite assemblage and adularia is rarely observed in any of the mineralized zones in the district. The most economically important mineralized veins are located on the eroded southern margins of the Masupa Ria alteration zone in the Ongkang area. The upper part of the Ongkang vein cross-cuts a zone of intense silicification that forms major cliffs in the lower part of the exposed Masupa Ria alteration zone. Advanced argillic alteration (quartz-pyrophyllite-diaspore-kaolinite±alunite) occurs beneath the silification and is overprinted by mineralized quartz veins and related quartz-illite alteration. These veins have been subsequently overprinted by later advanced argillic alteration. Similar relationships are present in the Menyawang area, but are not present in mineralized occurrences (Kiwi and Tosa-Ucang) to the east of the regional alteration zone. Alteration mineralogy, supported by preliminary fluid inclusion data, from the Ongkang vein area suggest temperatures of formation between 250 and 300°C. This implies a minimum depth of approximately 400 meters, assuming relatively dilute waters, boiling, and hydrostatic conditions. Approximately 300 meters of acid alteration and silicification have been removed by subsequent erosion. Any model for the development of regional alteration and mineralization in the Masupa Ria area must account for the extent of the residual quartz, silicification and advanced argillic alteration in the regional alteration zones, the timing of emplacement of the major mineralized veins post-formation of the regional alteration zones, and the presence of late advanced argillic alteration that overprints some of the veins. Explanations for these relationships require either an early magmatic-dominated acid alteration system followed by a neutral-pH system, or a complex neutral-pH system with considerable variations in the paleodepth of the water table. The superposition of magmatic high sulfidation (low pH) and low sulfidation (neutral-pH) environments has been documented in other systems within volcanic terrains that are similar to Masupa Ria. The late advanced argillic alteration probably formed during the descent of steam-heated waters.