Host rock petrology, hydrothermal alteration characteristics & ore mineralogy of porphyry copper-gold deposit, Brambang, Lombok, West Nusa Tenggara Indonesia

Abstract

Several world-class porphyry copper–gold deposits/prospects have been discovered in the eastern Sunda arc. One copper–gold prospect area is located at Brambang, Sekotong District, West Lombok Regency, West Nusa Tenggara Province, Indonesia This study is aimed to understand a preliminary geological framework, which characterizes the Brambang deposit in terms of the host rock, hydrothermal alteration characteristics, and ore mineralogy studies of the deposit. Fieldworks and several laboratory analyses were performed including petrography, ore mineralogy and X-ray diffraction (XRD). Host rock was dominated by volcanic rocks consisting of dacitic volcanic rock, diatreme breccia, and diorite intrusion. The hydrothermal alteration is characterized by argillic alteration (kaolinite-illite-smectite), propylitic (chlorite-epidote-calcite), and potassic (quartz-biotite-actinolite-magnetite), with the presence of sulfide minerals such as pyrite (FeS2), chalcopyrite (CuFeS2), and chalcocite (Cu2S), and the oxide minerals such as magnetite (Fe3O4), and hematite (Fe2O3). The alteration zone stage in the study area begins with forming a quartz-biotite-magnetite (potassic zone) formed at a high temperature of around 300–360 °C and at pH 7–8. This stage is followed by forming a chlorite-epidote-calcite (propylitic zone) outside the potassic zone, at a temperature range of 290–340 °C and pH of 5–6, which indicates a hydrothermal system cooling process. Then, due to the increasing influx of meteoric fluid that enters the fractures formed due to fault activity, a quartz-kaolinite-illite (argillic zone) is formed with a temperature range of 130–210 °C and pH 4–6. The presence of diatreme breccia and stockwork within the mineralization in the Brambang deposit supported the theory related to porphyry copper–gold deposits.