Ore-Forming Fluids Characteristics and Metallogenesis of the Anjing Hitam Pb-Zn Deposit in Northern Sumatra, Indonesia

Abstract

The Anjing Hitam Pb-Zn deposit in northern Sumatra (Indonesia) is one of the largest Pb-Zn deposits in the region. The stratiform orebodies are mainly hosted in the middle member of the Carboniferous-Permian Kluet Formation of the Tapanuli Group. Mineral paragenesis and crosscutting relationships suggest a two-stage Pb-Zn mineralization: (I) sedimentary and (II) hydrothermal mineralization. Ore-related calcite from both stages I and II contains mainly liquid- and gas-liquid two-phase-type fluid inclusions (FI). For stage I ore-forming fluids, FI homogenization temperatures (Th) are 105 to 199 °C, and the salinities are 9.6 wt.% to 16.6 wt.% NaCleqiv, reflecting low temperature and medium-low salinity; whereas in stage II, the Th (206 to 267 °C) and salinity (19.0 wt.% to 22.5 wt.% NaCleqiv) are considerably higher. Fluid inclusion and C-O isotope characteristics suggest that the stage I ore-forming fluids were mainly derived from a mixture of seawater and magmatic fluids (probably from deep-lying plutons), whereas the stage II ore-forming fluids were likely magmatic-derived with wall rock input. We propose that the Anjing Hitam deposit was a Carboniferous exhalative sedimentary (SEDEX) deposit overprinted by the Pleistocene vein-style magmatic-hydrothermal mineralization.