Bulletin of the Geological Society of Malaysia, Volume 43, Dec. 1999, pp. 307 – 319
1Geological Research and Development Centre, Jalan Diponegoro No. 57, Bandung 40122, Indonesia
2Curtin University of Technology, Australia
Abstract: The Cikotok gold-silver deposits are located about 250 km west of Bandung and 200 km southwest of Jakarta. The rocks hosting the precious metal mineralization are Oligo-Miocene felsic volcanics known locally as the “Old Andesite Formation”. They form part of the Bayah Dome Complex, the uplift of which is believed to have caused the fracturing which allowed infiltration of mineralized quartz veins during the Pliocene. The host rocks are calc-alkaline, island arc volcanics consisting of basalt, basaltic andesite, andesite and dacite. Geochemical data provide evidence of strong crystal fractionation, with most rocks exhibiting a moderate negative europium anomaly. The parental magmas were derived by variable amounts of partial melting of a LREE mantle source.
The Cikotok ore deposit is typified by a high silver content which occurs as fine-grained argentite and possible as electrum. Gold is submicroscopic and was not identified during SEM analysis of the ore assemblage. It is, however, believed to be associated with chalcopyrite, sphalerite and galena which postdate an earlier phase of pyrite mineralization. The ore minerals occur in large sulphide-bearing quartz veins and in hydrothermal breccias.
Doubly-polished thin sections, approximately 50 pm thick, were prepared from vein quartz collected at various levels in the Cikotok mine. Ten doubly-polished thin sections were prepared but only three were suitable for fluid inclusion study; the others lacked large enough inclusions. The three samples contained measurable two phase fluid inclusions and were analysed to determine the homogenization temperature. Samples were collected from Level 200 (YN303), Level 300 (YN294) and Level 400 (YN290). The depth difference between each level is approximately 30 m. Microthermometric studies were performed using a Fluid Inc. adapted USGS gas flow heating/freezing system mounted on a Leitz Laborlux microscope that is fitted with a Sony CCTV system.
Measurements of primary inclusions in quartz veins from the Cikotok mine suggest that the homogenisation temperature and salinity of the fluid inclusions (1.18 eq wt% NaCl) are in agreement with the temperature of the deposition of gold and metal-bearing fluids.
There are two types of fluid inclusions: 1) H2O-rich with vapour (two phases) and 2) single fluid phase (HP gas and CO2 gas). The freezing point measurements (-0.1°C to -3.7°C) yield salinity estimates of 0.1-3.15 eq wt% NaCl. The fluid inclusions with a salinity < 3.2 wt% NaCl should have a meteoric origin. The homogenisation temperatures for the fluid inclusions lie between 184°C and 306°C, with a mean 245°C. Trapping pressure of 40 bars was obtained from the P-T plot of Shepherd et al., (1985) by using the homogenisation temperature of 245°C. This is equivalent to a litho static load, and therefore depth, of 210 metres. These are typical of the low salinity epithermal gold deposits common within active hydrothermal systems and within epithermal gold deposits. These results are similar to those of Silitoe (1977), who concluded that epithermal deposits which often contain precious metals were deposited from low temperature (200°C-300°C), low salinity (<2 eq wt% NaCl) meteoric water.