Mineralogical Study of Cassiterite Grains from the Gejiu Tin Deposit

Through using scanning electron microscope, cathodoluminescence imaging, X-ray powder diffraction and electronic probe test, the mineralogy characteristics of various cassiterite crystals from the Gejiu tin deposit have been studies systematically in this paper, and their ore types include massive sulfide type, tourmaline veinlet belt type, tin dolomite, interlayer oxidized ore. The following conclusions have been drawn: ①the cassiterite particles from these four types of ore are progressively smaller. ②The cell parameters become larger from brunet cassiterite to light-colored ones, and their c:a value change smaller. ③There are seven types of monotype combination for cassiterite crystal, and their monotype are developed from {111} cone to {110} cylinder gradually. ④The internal bands of Cassiterite are changed from uniform development to nonuniform ones. The features mentioned above show that the temperatures for the formation of these cassiterites with four types are reducing successively. According to the luminance difference of cassiterite under CL cathodoluminescence, it's found that two forming periods existed in the cassiterite ore of massive sulfide type, tourmaline veinlet belt and tin dolomite. Thus, it's believed that the high temperature hydrothermal fluid activity of the cassiterite deposits with these three types is associated with early cassiterite crystallization in the early stage, but the low temperature hydrothermal fluid activity is associated with late cassiterite crystallization in the late stage. It's thought that the genesis of ore deposit is oriented argillaceous sedimentary origin, such as interlayer oxidized cassiterite deposit was generated in the suspended in a fluid and might be formed by fluid transportation and deposition.