Microthermometric evidence for the formation of Permian VHMS deposits in Tasik Chini area, Central Belt of Peninsular Malaysia

Abstract

The VHMS deposits of Tasik Chini area are located in the Pahang State of Central Belt, Peninsular Malaysia. The deposits occur in a package of Permian volcanics within mixed volcano-sedimentary sequence, and have been exploited from the Bukit Botol and Bukit Ketaya deposits. They have many similarities with Kuroko-style massive sulfide deposits. The host rocks at both deposits are felsic volcanic rocks of rhyolitic to rhyodacitic composition. Mineralization occurs as distinct ore zonation forming a stringer to massive sulfide zone at the footwall followed by barite lenses and exhalite layers (Fe + Mn ± Si ore) at the stratigraphic top. The sulfide phases are characterized by pyrite as the major sulfide mineral, with subordinate chalcopyrite, sphalerite and rare galena; traces of gold, silver- and Sn-bearing minerals occur in the massive sulfide and barite ores. Primary fluid inclusions within quartz and barite occur as two types: (1) type I: two phase, liquid-vapour inclusions without daughter minerals, and (2) type II: three phase, liquid CO2-bearing carbonic inclusions. These three phase inclusions include liquid H2O, liquid CO2 and vapour CO2, with variable CO2 content, and often decrepitate before complete homogenization. Microthermometric measurements of the type I inclusions show that the temperature of homogenization (Th) varies between 180 °C and 310 °C with salinities ranging from 1.0 to 15.0 wt% NaCl equivalent in the quartz samples. Homogenization temperatures (Th) of 190 °C to 300 °C and salinities between 1.0 and 13.0 wt% NaCl equivalent are recorded from the barite samples. Type II inclusions have an initial melting temperature range of −57.1 °C to −57.5 °C, clathrate melting temperature (Tmclathrate) of 7.4 °C to 8.0 °C and CO2 homogenization temperature (ThCO2) between 30.1 °C and 30.3 °C, and salinity ranging from 4.0 to 5.1 wt% NaCl equivalent. Densities, pressure and depth of ore forming fluids range from 0.711 to 0.970 g/cm3, 12 to 93 bars, and ∼1500 m depth of seawater. Laser Raman spectroscopic analysis shows the presence of CO2 in the carbonic inclusions. The fluid inclusion results suggest that the main ore fluid during the formation of the Tasik Chini VHMS deposit is a seawater-dominated fluid, but contribution from a modified seawater or possibly magmatic origin is also indicated by higher salinities relative to seawater (3.2 wt% NaCl) and the presence of CO2. Taking into account the regional geodynamic evolution, it is also concluded that the formation of VHMS deposits in the Tasik Chini area occurred in a back-arc geotectonic setting.