Microstructure of pyrite related to gold deposit, Huai Yuak area, Sukhothai Province, Northern Thailand

Abstract

Huai Yuak area is located at Ban Kaeng Subdistrict, Sisatchanalai District, Sukhothai Province, Northern Thailand. The occurrences of gold are distributed along the Sukhothai Fold Belt from Phrae to Lampang though Sukhothai provinces. The Huai Yuak area is a one of the primary gold deposits in the Sukhothai Fold Belt. The geology of the Huai Yuak area is comprised of siltstone, sandstone, shale and tuff from the Permian to Triassic age. Quartz veins of the deposit were formed in siltstone, sandstone and shale. Based on the cross-cutting relationship and mineral assemblages, the quartz veins can be divided into three stages. Quartz veins of stage I are composed of a large amount of microcrystalline quartz with a minor amount of arsenopyrite and a trace amount of calcite, illite, pyrite, chalcopyrite, galena and electrum. Quartz veins of stage II consist of mainly quartz. Quartz veins of stage III are comprised of a large amount of dolomite and a small amount of quartz with a trace amount of illite and arsenopyrite. Electrum is associated with arsenopyrite, galena, pyrite, chalcopyrite and sphalerite. The electrum was formed as inclusion and filled in fractures of arsenopyrite and pyrite. Based on structure, texture and occurrence, pyrite and arsenopyrite can be classified into four types. Type 1 pyrite and arsenopyrite were disseminated in sandstone, siltstone and shale. Type 1 is characterized by euhedral to subhedral shape with size ranging from 0.1 to 3.0 mm. Type 2 pyrite and arsenopyrite were formed in a fracture as veins in shale and metashale. Type 3 pyrite and arsenopyrite occur in quartz veins. Type 3 is associated with galena, chalcopyrite, sphalerite and electrum. The electrum in Type 3 at Huai Yuak deposit was formed as inclusions and native gold associated with pyrite, arsenopyrite, chalcopyrite, galena, sphalerite and quartz of stage III. The concentration of electrum is related to arsenic content in pyrite. Type 4 of pyrite and arsenopyrite was formed in shale rock. Pyrite and arsenopyrite gains at the Huai Yuak gold deposit have had developed under a different structure and physical condition from uplift during volcanic arc setting.