Ore Mineralogical Characteristics of Epithermal Gold Deposit in Buladu area, Gorontalo, Northern Sulawesi, Indonesia

The study area is situated in Buladu area, Sumalata district, Gorontalo province, Indonesia. Gorontalo located in the K-shape Sulawesi Island, where this unique shape is tectonically resulted from interactions of the Pacific plate, Indo-Australian plate and Eurasian plate. Lithotectonically, Gorontalo is included in the West and North Sulawesi Plutono-Volcanic Arc which is Miocene to Pliocene in age. The North Sulawesi Volcanic Arc is a region that rich in many kinds of prospects and deposits of precious- and base metals. This paper describes a recent study on epithermal gold mineralization that has been operated by local artisanal miners in the study area, based on hand specimen and microscopic observation. In addition, XRD analysis was also conducted to determine some minerals that were hard to indentify using the routine microscope. \ud Litologically, the study area is arranged by andesitic-basaltic volcanics of the Pliocene Wobudu Breccia which consists of volcanic breccia, agglomerate, tuff, lapilli tuff, and lava. Petrographic study reveals that host rock of the mineralization is porphyritic andesite which shows flow and intergranular structures. The rock composed mainly by phenocrysts of plagioclase with less K-feldspar and pyroxene which are distributed in microcrystalline groundmass. All the rock samples studied have hydrothermally altered which dominated by clay, chlorite, carbonate, and quartz. The alteration is pervasive where clay and quartz totally replace groundmass and phenocrysts, whereas chlorite and pyrophyllite were selectively altered the plagioclase phenocrysts. Clay mineral specieses identified by XRD include kaolinite and pyrophyllite. Under the microscope, calcite was observed in veinlets and some together with sericite and clay replace groundmass. \ud The mineralization mostly occurred in disseminated style, both in highly-altered host rock or in quartz gangue. The quartz-gangue shows vuggy, crystalline, comb, and less bladed, crustiform and colloform (bottroydal) banding textures. Hypogene ore mineral assemblages identified under the microscope include pyrite, chalcopyrite, sphalerite, magnetite, marcasite, gold, galena, tennantite-tetrahedrite, enargite, bornite, and native copper. Supergene minerals covellite, chalcocite and malachite were also recognized. Identification of enargite, tetrahedrite and gold was also supported by XRD analysis. Under the microscope, fine grains euhedral gold were observed as inclusions sphalerite. Native copper was also identified as inclusions in silicate minerals and some surrounded by covellite at the fringe. The hypogene minerals showed vary of textures include intergrowth, replacement and inclusion along the crystallographic direction. The occurrences of bornite, enargite and tennantite-tetrahedrite suggested an assemblage of intermediate- to high sulfidation epithermal