Hydrothermal Alteration Mineralogy Associated with Gold Mineralization in Buladu Area, Gorontalo, Northern Sulawesi, Indonesia

Geologically, the Buladu area is part of the North Sulawesi magmatic arc, located in the morphology of the mountains along the coastline of North Gorontalo district, Indonesia. Multiple subductions of North Sulawesi in the northern arm of Sulawesi and east Sangihe subduction in the east has triggered an active magmatism and volcanism. Formation and occurrences deposits of precious- and base metals have been found in the North Sulawesi Volcanic Arc. This paper describes a recent study on hydrothermal alteration mineralogy associated with one of the prospects in the region, namely Buladu gold prospect that for years has been operated by local artisanal miners. The study is focused on hand specimen and microscopic observation as well as XRD analysis. Lithologically, the study area is arranged by sandstone are members of Dolokapa volcano-sedimentary formation, above unconformity andesitic-basaltic volcanic breccia are members of the Pliocene Wobudu Formation and is intruded by granodiorite rocks. The petrographic study reveals that host rock of the mineralization is flow-volcanic breccia which shows porphyritic and flows structures. Resulting assemblages of secondary minerals are mineral associations characteristic of the magmatic-hydrothermal system the following imperative alteration zones could be recognized an outer zone of chloritic alteration and the inner zone of argillic alteration. The alteration showed characteristics of pervasive to selective alteration, where clay and quartz totally replaced groundmass, whereas chlorite and clay minerals were selectively altered plagioclase phenocrysts. Clay mineral species identified by XRD include chlorite and illite. These mineralogical features indicate that the hydrothermal alteration is an argillic type, which is spatially distal to the mineralization. Mafic mineral in andesite lava partly altered into mineral assemblage chlorite-epidote-quartz-carbonate show propylitic type, which is the outermost zone of mineralization.