Geological and mineralogical studies of the cassiterite-sulfide ore deposit at the Takua Pit Thong Mine, Changwat Ratchaburi

Abstract

The Takua Pit Thong cassiterite-sulfide (magnetite) deposit is one of the rare occurrence of primary complex tin ores ever reported in Thailand. The mineralization was closely related to the Upper Cretaceous Takua Pit Thong granite and took place at or near the contact zone of the granite and marble intercalated with calcsilicate hornfels of presumably the Ordovician Khao Noi Sethi Formation in the Thung Song Group. At least two major types of the Takua Pit Thong granite are recognized, i.e., coarse-grained biotite-muscovite granite and fine-to-medium-grained biotite (±muscovite, tourmaline) granite, especially the latter one as a highly differentiated phase which might have had the same magmatic origin as the Khao Daen granite. The cassiterite-sulfide (magnetite) mineralization occurred in the granite/country rocks contact and in the fracture (sheared) zone within the granitic rocks close to the contact zone. The orebodies in the contact zone are characteristically composed of cassiterite, pyrrhotite with minor to trace amounts of chalcopyrite, pyrite, arsenopyrite, sphalerite, magnetite and fluorite. The important associated alteration is the so called pervasive green biotitization and phengitization. In contrast, the orebodies in the sheared zones are characteristically consisted of cassiterite and magnetite with only trace amounts of sulfide minerals and fluorite. The pervasive green biotitization is the only alteration process occurred in this type of orebodies. Also a generalized mode1 for the mineralization at the Takua Pit Thong mine is proposed. The overall depositional condition of the ore-forming fluid responsible for the mineralization at the Takua Pit Thong mine was likely to be very close to magnetite-pyrite-pyrrhotite buffer within the cassiterite stability field. The mineral assemblages observed in both types of orebodies were probably precipitated from the same hydrothermal fluid by slight change in their physicochemical conditions through the processes of wall rock alteration rather than from the two separate fluids.