Geology and mineralization characteristics of epithermal gold, suwan prospect, changwat Phitsanulok

The Suwan prospect is located about 6 km northwest of the Chatree gold mine in Phitsanulok province, central Thailand. Gold-silver mineralization occurs as veins and stockworks hosted in volcaniclastic and volcanogenic-sedimentary rocks of Late Permian- Early Triassic age which is classified as a low sulfidation epithermal deposit base on mineralization texture, alteration pattern and sulfide mineral assemblages. From top to bottom, the hosted volcanic succession can be divided into 3 units, namely 1) Felsic volcanic unit (Unit 1), 2) Volcanogenic-sedimentary unit (Unit 2), and 3) Porphyritic andesite unit (Unit 3). Unit 1 consists predominantly of quartz-rich fiamme breccia, lithic-rich fiamme breccia and feldspar-phyric rhyolite breccia. The volcanogenic-sedimentary unit (Unit 2) consists of fine to coarse-grained sandstone, sandy-matrix polymictic breccia, polymictic intermediate-felsic breccia, mudstone and limestone lenses. Unit 3 comprises plagioclase-phyric andesite, plagioclase-hornblende-phyric andesite and monomictic andesitic breccia. At least 3 stages of mineralization have been identified namely, 1) pre-gold stage; quartz-pyrite vein (stage 1), 2) main gold stage; quartz-carbonate-sulfides-electrum vein (stage 2), and 3) post-gold stage; quartz-carbonate vein (stage 3). In the main gold stage (stage 2), pyrite is a major sulfide mineral with minor amount of sphalerite, chalcopyrite and galena. These sulfide minerals are closely associated with quartz, calcite and major gangue minerals of stage 2. Gold mainly occurs as inclusions in pyrite and EPMA analysis confirms that it forms as electrum. On the basis of petrographic observation and X-Ray Diffraction Analyzes (XRD), the hydrothermal alteration at the Suwan prospect can be divided into four zones. From proximal to distal to the ore zone, they are 1) Silicic zone (quartz-adularia), 2) Argillic zone (adularia-quartz-illite-smectite-kaolinite), 3) Propylitic zone (chlorite-calcite) and 4) Clay minerals (illite-smectite-chlorite-kaolinite). Based on geological information such as mineralogy, vein textures, and hydrothermal alteration, the Suwan prospect could be classified as low sulfidation epithermal gold-silver deposit similar to the well known Chatree deposit.