Epithermal gold-silver mineralization and alteration of B-prospect, Phichit province

Abstract

The study area (B prospect) is located about 3 km south-east of Chatree deposit. The rocks in the prospect area can be grouped into 4 units namely, 1) Polymictic breccia unit (Unit 1), 2) Volcanogenic sedimentary unit (Unit 2), 3) Fiamme breccia unit (Unit 3) and 4) Andesitic polymictic breccia unit (Unit 4). The coherent rocks occur as dykes, it can be categorized into 3 types namely, 1) Hornblende-plagioclase phyric andesite, 2) Plagioclase phyric andesite and 3) Hornblende phyric andesite. Gold mineralization occurs as quartz ± carbonate – sulfides – electrum veins/veinlets and stock works hosted in volcanics and volcaniclastic rocks of Late Permian-Early Triassic. Mineralizations consist of 3 stages namely, 1) Quartz -pyrite veins, 2) Quartz ± carbonate – sulfides – electrum veins and 3) Quartz ± carbonate veins. The main gold stage (Stage 2) is characterized by typical textures of low sulfidation epithermal deposit which consists of mainly quartz and minor calcite gangue. Pyrite is a major sulfide with minor sphalerite. Gold identifies both as inclusion in sulfide particularly pyrite and free grain associates with quartz and calcite. EPMA analysis confirms that gold occurs as electrum with fineness ranging from 595 to 632. Petrographic and XRD study suggested that the hydrothermal alteration at B prospect can be divided into 3 types: 1) Quartz - adularia, 2) Adularia – sericite – illite – quartz ± calcite and 3) Sericite – illite ± chlorite ± calcite ± adularia ± quartz. The mineralization at B prospect can be distinguished from Chatree deposit by its simple mineral assemblages. And it can be classified as a low-sulfidation epithermal deposit.