APPLICATION OF CHEMISTRY TO CONSTRAIN TYPE OF PROTOLITHS AND METAMORPHISM CONDITIONS OF THE HOST ROCKS OF FLUORITE MINE AT KHAO CHONG INSI, KANCHANABURI PROVINCE, THAILAND

Abstract

Khao Chong Insi is a small mountain range in Hui Krajao District, Kanchanaburi Province that lies in north-south trending. Even though the mountain is very narrow with the width of 7 km and length of 34 km, but it has a very high potential of mineral resources including pure quartz for solar cell industry, feldspar for ceramic industry, limestone for construction aggregate and fluorite for chemical industry. At least 5 mines have been operated on the Khao Insi and some of them are still being run currently. These mines last longer than 30 years especially the fluorite mine.  Here we apply some publish methods to find out the source of basement rocks which play as a role of host-rock for fluorite deposit in the area. Basement rocks found in the area which is a host for fluorite are mainly gneiss intercalated with schist, quartzite, calcsilicate and marble. Eleven representative metamorphic samples were analyzed using wave length dispersive X-ray fluorescence (WD-XRF) and two samples that contain appropriate mineral constituents were selected for P-T calculation based on their mineral chemistry. As a result, the protolith (or original rock) of these representative samples are mainly pelitic rocks (shale or mudstone) and quartzo-felsparthic rock (i.e. granite or sandstone), however, one sample has been calcareous rocks. This could be concluded that these host-rock of fluorite deposit have been metamorphosed from both igneous and sedimentary rocks before the metamorphism event. Thermodynamic principles waere applied to calculate the pressure and temperature conditions of metamorphism in this area. The result shows that most metamorphic rocks have experience the conditions of around 4-6 kbar and 630-720 ⁰C at the depth of around 25-29 km. Consequently, these basement rocks were exhumed to the current position by a fault which is a branch of Sri Sawat Fault zones. Denudation process finally caused and exposure of these basement rocks at the current surface.