Geology of Poboya Deposit; Metal Zonation and Vein Texture Characteristics

Abstract

The Poboya gold deposit is well known as a low-sulfidation epithermal Au-Ag deposit hosted in metamorphic rocks. The Poboya Project consists of two main prospects: River Reef and Hill Reef 1, located in Poboya, Palu City, Central Sulawesi Province. Gold mineralization is characterized by a bulk average Au: Ag ratio of approximately 1:2, although the ratio varies by elevation and ore zone, with low base-metal (Cu-Pb-Zn) grades of <100 ppm and the presence of quartz veins exhibiting bladed, chalcedonic, colloform, and crustiform textures. Gold in Poboya is generally deposited as electrum within quartz and platy calcite with a fine grain size averaging 10 microns. Vertical zoning at River Reef demonstrates a consistent increase in average gold grades for every 50-meter decrease in elevation. The highest grades, averaging 10 g/t Au, are concentrated between 50 and 100 mRL. These high-grade zones strongly correlate with crustiform and colloform quartz textures, suggesting that fluid boiling was the primary mechanism for gold deposition. Furthermore, fluid inclusion analyses of associated crustiform calcite veins yield homogenization temperatures (Th) ranging from 200°C to 250°C.  The characteristics of the Au-Ag metal zoning indicate a regular grade distribution, with the dip direction of mineralization in several ore zones consistent with the dip direction of the structural zone, and contact between schist and gneiss, indicating structural and lithological control of mineralization. The variation in mineralization thickness reflects the pinch-and-swell patterns at the structural openings.

Keyword: Gold, Metamorphic rock, Metal zonation, Vein texture, Poboya