Mineralisation in the Gunung Limbung area, Bogor, Indonesia.

Abstract

Quartz veins containing sulphides of Cu, Pb, and Zn and trace Au and Ag occur in a dioritic intrusion at Gunung Limbung in West Java (Indonesia). Mineralogical, geochemical and fluid inclusion studies have been used to characterise the deposit and propose models of ore genesis. The ore at Gunung Limbung is hosted in an (early) monzodiorite stock which was previously enriched in certain 'ore' elements. The mineralisation resulted from the filling of fissures and fractures by mineralised hydrothermal solutions forming a vein type deposit, and was formed in several stages. These are related closely to the subduction-related tectonic movements and magmatism (volcano-plutonism ) of West Java which resulted in compressional and tensional forces. During and after mineralisation these tectonic movements caused brecciation of the ore.The formation of ore resulted in alteration within the wall rock around the mineralisation zones, which zonally shows changes toward the ore zone from: propylitisation (at the outer margin) to sericitic-silicic-potassic alteration, and advanced argillic alteration at the ore zone.The origins of the ore metals are still uncertain, but they could have been derived from either the country rock or from the (later) quartz diorite intrusion.The temperature of the mineralisation by the fluids was 260° to 350°. Two fluids were involved, one with a low salinity (0.2%-5% wt NaCl eq.) with a high Ca/Na ratio and one with a higher salinity (10%-15% wt NaCl eq.) with a low Ca/Na ratio. The hydrothermal solutions which were involved in the transportation and formation of ore were probably derived from meteoric/connate water mixed with magmatic fluids derived from the quartz dioritic intrusions.The type of alteration, the characteristics of the hydrothermal solution (geochemically and geothermometrically determined) and the high concentration of certain elements (including Pb, Zn and Ba) within the host rock, are important criteria to be used as a tool in further exploration in the area.The area is situated on the extension of the 'mineralisation belt' which run from north Sumatra into western Java and so has clear potential for the discovery of further deposits.