Characteristics of epithermal gold-base metals prospect in the Anggai block, northern Obi Island, Indonesia

Abstract

The Obi island, located in the North Moluccas Au-Ag-Cu province, hosts several mineral prospects, one of which is gold-base metals in the Anggai block. Several companies have historically explored the area since the end of the 1900s. The ore bodies are white to smoky and brownish-white, banded, laminated, colloform, crustiform, and massive quartz vein structures or textures, trending north northwest to south southeast. The ore bodies consist of gold and base metals hosted in the Bacan Formation and are enveloped by dominantly argillic alteration and the addition of silicic alteration. In this study, we provide several analyses, namely petrography, measurements using Portable Infrared Mineral Analyser (PIMA), Atomic Absorption Spectrometry (AAS), and fluid inclusion analysis, to understand geological and geochemical characteristics related to formation of gold-base metals in the Anggai block. Under the microscope, gold (i.e., electrum) is freely observed and closely occurs with argentite. While galena, chalcopyrite, sphalerite, covellite, chalcocite, and pyrite are mainly observed together. The correlation coefficient between Au and other metals shows a strong positive correlation with Zn (r = 0.810188) and a weak positive correlation with Ag (r = 0.459857). Ag shows only a weak positive correlation to Au, while the other elements show a strong positive correlation. In addition, Cu, Pb, and Zn show a strong positive correlation with each other. The micro-thermometry analysis of fluid inclusions shows two modes of homogenization temperature (Th) and salinity: 180-200 °C and 240-280 °C, and 1.4-1.6 and 1.9-2.2 wt% NaCl eq., respectively, suggesting overprinting evidence to enrich the ore bodies with occurrences of gold and base metals in the quartz veins. Gold-base metals were probably carried by the sulfide complex, which later precipitated during the cooling of the hydrothermal fluid.