Geology, hydrothermal alteration and mineralization of the Carlin-type gold deposit at South Ratatotok, Southeast Minahasa Regency, North Sulawesi Province, Indonesia

Abstract

South Ratatotok in North Sulawesi, Indonesia is one of the regions that was once a gold-producing region in North Sulawesi. Mesel gold deposit in this region is approved as Carlin type gold deposit with a total of resources of 12.25 Mt @5.21 g/t Au. Near the Mesel deposit, gold mineralization is currently discovered prospects such as Leon, Bulex Hill, Heind's Find and Monkey Forest. This study is aimed to elucidate the types and distribution of hydrothermal alteration and ore mineralization encountered in these four prospect areas. Research methods including field work (including geological and hydrothermal alteration mapping, rocks and ore sampling) and various laboratory works consisting of petrographic analysis, ore microscopy, X-Ray Diffraction (XRD) and fire assay were performed. As a result, the study area is composed of several lithological units including limestone, volcanic breccia, porphyritic andesitic intrusion units as well as colluvial deposit. The wall rocks have been suffered by various hydrothermal alteration as follows: silicification in limestone, carbonatization in volcanic breccia, argillic alteration in volcanic breccia and propylitic alteration developed in porphyritic andesitic intrusion, minor decarbonatization associated with limestone and minor oxidation in volcanic breccia. Gold mineralization is present as a replacement type in very fine size of sulphides and gold is closely associated with silicified limestone. Ore chemistry by fire assay analysis indicates that gold and silver contents vary from 0.012 g/t to 2.41 g/t Au, and <0.5 g/t to 2.1 g/t Ag, respectively. The increase of gold content is geochemically followed by high values of Ag, As, Sb, Cu, Pb, Zn and by low value of Hg because of oxidation. Supergene process was also the key factor in the enrichment of gold where the oxidized ore has relatively higher gold grade than in non-oxidized ore.