Abstract

Gold mineralization at the Pani prospect in north Sulawesi, Indonesia, is related to a Miocene or younger rhyodacitic volcanic center. The center, which is 3.5 km in diameter, consists of porphyritic lavas, intrusions, breccias, and pyroclastics. It overlies and partly intrudes horn-blende and hornblende-biotite granodiorite and Eocene (?) basaltic volcanics.Mineralization is spatially related to the Baganite dome, which is centered on Gunung Baganite. Low-grade (0.8 ppm) disseminated gold mineralization, associated with pyrite and minor base metals, is hosted by the dome. Silver mineralization (acanthite) occurs in minor quartz-hematite veins in overlying silicified pyroclastics. The most important gold mineralization occurs on the flank of the dome at the Pani ridge, less than 1 km from Gunung Baganite. It is associated with minor base metal sulfides, quartz, and adularia encrustations, found in vuggy fractures and centimeter-wide breccia zones, in weakly silicified rhyodacites, and in adjacent wall rocks. Quartz veins on the Pani ridge are virtually absent. The assemblage chloritealbite-anatase-pyrite + or - quartz-adularia is ubiquitous in flow-banded and massive rhyodacites in the prospect area, whereas carbonates occur within the enclosing pyroclastics. Strong silicification occurs in the upper part of the Baganite dome and extends up to 130 m into the overlying volcanics. Adularia veining increases with depth as does secondary K feldspar in the groundmass. The Mg number of chlorite decreases from 60 in the pyroclastics above the Baganite dome to 25 in the rhyodacite at a 300-m depth, and there is significant intrasample variation.Temperatures of chlorite formation in the Baganite dome, calculated from electron microprobe analyses, vary from about 125 degrees to 275 degrees C and are in good agreement with the homogenization temperatures of 150 degrees to 300 degrees C from secondary fluid inclusions in quartz phenocrysts. The salinities of the majority of the fluid inclusions range from near zero up to about 4 equiv wt percent NaCl, but some salinities are as high as 40 equiv wt percent. Vapor-rich inclusions also occur. The redox states of the fluids associated with the Baganite dome, calculated from chlorite compositions, varied significantly, from about -2 to -8 log f (sub O 2 ) units below the hematite-magnetite buffer. Hydrogen sulfide levels in the fluids decreased from a maximum of 30 ppm around 250 degrees C, with decreasing temperature and oxidation state, and pH values were close to neutrality.Gold was apparently transported at 1-ppb levels at the highest temperature in the most oxidized fluids, of around 4 equiv wt percent NaCl salinity, which probably rose through the root zones and the lower contacts of the Baganite dome. Gold precipitated as the fluids cooled by boiling (indicated by vapor-rich inclusions, adularia, and Mg-rich chlorites) and also by mixing with low-salinity, reduced fluids inferred to have circulated around the flanks of the dome. The latter mechanism was probably the most efficient and suggests that there may be potential on the margins of volcanic domes for large-tonnage, low-grade gold deposits.

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