MINERALOGIC, ALTERATION AND FLUID-INCLUSION STUDIES OF EPITHERMAL GOLD-BEARING VEINS AT THE MT MURO PROSPECT, CENTRAL KALIMANTAN (BORNEO), INDONESIA

The Mt. Muro prospect contains numerous steeply dipping, gold-bearing quartz veins that formed within an epithermal environment during mid-Tertiary calc-alkaline volcanism of Central Kalimantan. The two centers of mineralization studied, Gunung (Mt.) Baruh and Luit, are approximately 10 km apart, and represent the upflow zones of former geothermal systems. Gold mineralization throughout the region is structurally controlled, and grades are unevenly distributed within veins. The Kerikil veins at Gunung Baruh are characteristic of the mineralization style and are one of the most extensively drilled targets. These vein structures trend north-northwest and extend discontinuously over a 1000-m strike along the west flank of Gunung Baruh. Host rock consist of porphyritic andesite flows, a hypabyssal basaltic andesite intrusion, and volcanic breccias. Primary sulfide mineralization within the Kerikil veins occurred in four district stages related to hydrothermal brecciation (stage II), and local occurrences of laminated quartz-sulfide veins (stage IV). Quartz is the predominant gangue mineral throughout, with subordinate adularia, calcite, and rhodochrosite. Electrum and acanthite are fine grained (≤30 μm) and associated with minor disseminated pyrite, sphalerite, galena, chalcopyrite, and covellite. Bleached zones of quartz-illite-pyrite envelop the mineralized veins, but where host rock was less permeable, a weak alteration assemblage of epidote-chlorite-albite ± pyrite ± illite ± calcite formed instead. Fluid inclusion studies indicate that mineralizing solutions were dilute, ≤4.1 eq. wt.% NaCl, < 4.0 wt.% CO2, and ranged between 207 and 253°C. These alteration and fluid inclusion features are characteristic of deeply convecting, near-neutral pH, alkali-chloride fluids found in active geothermal systems. Fluid-inclusion and mineralogic evidence suggest that fluids were boiling under conditions which favored gold deposition; however, most gold mineralization appears restricted spatially and temporally to the timing of hydrothermal brecciation. Mineralization at Luit is similar, although exposed veins here apparently formed at greater depths and higher temperatures (238-262°C); gold grades diminish sharply with depth. During the late collapse of the geothermal system, shallow acid fluids (produced from steam condensate) descended preferentially down vein structures and reacted with adjacent host rocks to produce a late kuolin overprint. These same fluids also partially digested vein carbonate, adularia and rock fragments, forming enhanced secondary permeability within vein structures. This secondary permeability was important in the development of supergene oxidation; the latter locally extends to 100 m below the present surface. Erratic supergene gold enrichment is mostly restricted to depths less than 25 m below the present surface and is closely associated with iron and manganese oxides; however, comparison of Au, Ag, Mn, Fe, and combined Cu+Pb+Zn contents between oxide and sulfide zone samples does not show significant trends. Microprobe analyses of supergene electrum indicate an average composition of 71 wt.% Au and 29 wt.% Ag, which is very similar to the average composition of primary electrum associated with sulfides. These compositions suggest that thiosulfate complexes in groundwaters acted as transporting agents for remobilization of gold during oxidation. Gold and silver adsorption onto amorphous oxides was ultimately important for their reprecipitation; however, where steeply dipping veins transect steep terrain, some gold may have been lost through dispersion into nearby soils and subsequent erosion. © 1990.