Geology, porphyry CuAu, and epithermal CuAuAg mineralization of the Tombulilato district, North Sulawesi, Indonesia

Abstract

The geology of the Tombulilato district, North Sulawesi, is characterized by an island arc-type volcano-sedimentary pile, > 3400 m thick and of late Miocene(?)-Pleistocene age, which is made up of submarine to subaerial basic to acid volcanic rocks interbedded with marine and continental sedimentary rocks. The sequence is intruded by high-level stocks and dikes, and cut by diatreme breccias of late Pliocene and Pleistocene age, some of which are associated intimately with porphyry CuAu and epithermal CuAuAg mineralization. A main compressive deformation event took place in the Pliocene. Three main mineralization types are present in the district: porphyry CuAu; high-sulfidation epithermal CuAuAg; and low-sulfidation epithermal AuAg. Porphyry CuAu mineralization is present at Cabang Kiri, Sungai Mak, Kayubulan Ridge, and Cabang Kanan. Hypogene CuAu mineralization is associated typically with magnetite-bearing K-silicate assemblages, which are partially to completely obliterated by sericite, illite and chlorite. Copper as chalcopyrite and bornite, and Au show a positive correlation. Multiphase intrusions and alteration-mineralization events are commonplace. Biotite-bearing K-silicate alteration at Cabang Kiri and Kayubulan Ridge yields KAr ages of 2.93±0.06 and 2.36±0.05 Ma, respectively. Cabang Kiri possesses a gold-rich zone (> 1.5 ppm Au) open at depth, and the bulk of the mineralization at Sungai Mak is contained in a supergene chalcocite blanket. High-sulfidation epithermal CuAuAg mineralization is present at Motomboto, which displays an alteration pattern consisting of a core of residual (vuggy) silica, bordered outward by zones of quartz-alunite, quartz-kaolinite, and chlorite. This alteration zoning is interpreted to have formed by magmatic-volatiles charging a meteoric convection cell, with the resulting fluids becoming progressively neutralized outward by fluid-rock interaction. CuAuAg mineralization is contained in the silica core associated with multiphase hydrothermal breccias, which are typically cemented by pyrite, enargite, luzonite, marcasite, and minor sphalerite and galena. Late-stage barite contains trails of secondary liquid + vapor-rich inclusions with T h between 154 and 178°C. Available K-Ar dating on alunite suggests that advanced argillic alteration at Motomboto was active between 1.89 and 0.93 Ma. Low-sulfidation epithermal AuAg mineralization at Kaidundu, and elsewhere in the district, is confined to narrow (to 1 m wide) silica veins. They consist of chalcedony and quartz displaying comb and cockade textures, and abundant open vugs. The veins contain <5 vol.% sulfides (dominantly pyrite), and illitic alteration is restricted to narrow selvages and occluded wallrock fragments. Gold is free in quartz, and also associated with pyrite and tellurides in millimeter-scale bands and disseminations. Crystalline quartz contains primary liquid + vapor-rich fluid inclusions with T h between 166 and 324°C and salinities of up to 0.8 equiv. wt.% NaCl. Preliminary geologic reconstructions suggest that these mineralization types in the Tombulilato area were generated over about 2 M yr (between 2.9 and 0.9 Ma) as part of a district-scale hydrothermal system. Continuous syn-mineralization uplift and erosion, which are interpreted to have removed some 2 km of rock in the last 3 M yr, were responsible for the progressive unroofing of the hydrothermal system and the superposition of epithermal environments over relics of higher temperature, deeper-seated mineralization. In the near-surface environment, intense uplift accompanied formation of a chalcocite blanket at Sungai Mak.