Morphological And Geochemical Studies Of The Cassiterite In The Taoxikeng Tin Deposit, Jiangxi Province

Taoxikeng tin polymetallic deposit is a magmatic-hydrothermal tungsten-tin polymetallic deposit in South China, which is14.5km southwest to Chongyi County, Ganzhou City, Jiangxi Province. There is a granite body called Taoxikeng granite body, lying400m below the Taoxikeng tin deposit. This granite body is the ore-forming parent rock of the Taoxikeng tin deposit. The ore-bearing quartz veins of the Taoxikeng generally locate at the external contact zone of the granite body, showing multi-stage characters.This thesis presents a detailed study on the morphology and geochemistry of cassiterite crystals from the ore-bearing quartz veins of the Taoxikeng tin deposit. An inovative method for quantitative representation of cassiterite morphology was provided, including three crystallographic factors:the prism index (/pr), which illustrates the relative size of{110} prismatic form and{100} prismatic form; the pyramid index (Ipy), which reveals the relative size of{111} pyramidal form and{101} pyramidal form; and the extension index (Iei), which illustrates the relative size of width as a function of prism length. The cassiterite crystals in the Taoxikeng tin deposit can be divided into three types of morphology according to their different crystallographic forms:Type A cassiterite is composed of{111} pyramidal form+{110} prismatic form, type B cassiterite is composed of{111} pyramidal form+{110} prismatic form+{100} prismatic form, and type C cassiterite is composed of{111} pyramidal form+{101} pyramidal form+{110} prismatic form+{100} prismatic form. Type A cassiterite has both Ipr and Ipy equal to1; type B cassiterite has Ipy equal to1, and Ipr ranging from0.5to1; type C cassiterite has both Ipr and Ipy ranging from0to1. Based on previous studies, it appeared that both type A and type B cassiterite should be formed at a higher temperature than type C cassiterite, i.e. both Ipr and Ipy of cassiterite declined as the crystallizing temperature decreases.For the first time, the internal structure within the cassiterite grains was observed by the cathodoluminescence (CL) imaging. We have revealed that cassiterite in the Taoxikeng tin deposit has two crystallization stages.. The early stage of cassiterite commonly shows a relatively weak CL brightness (black to dark grey) with the oscillatory and sector zoning, and the margins of the phase were highly corroded. Their crystallographic characters show that they belong to either A-type or B-type cassiterite. The late stage of cassiterite shows a relatively strong CL brightness (light grey to white) without either oscillatory or sector zoning. It occurrs either as overgrowth on the early stage of cassiterite, or as veinlet within the early stage ones. The crystallographic characters of the late stage of cassiterite shows that they belong to type C.The electron microprobe analysis shows that cassiterite in the Taoxikeng tin deposit is high in FeO and Ta2C"5, indicating that it should be formed at a high temperature. It also shows high HfC2contents with low Zr/Hf ratio, suggesting ore-forming fluid should be derived from a highly differentiated granitic magma. The early stage of cassiterite contains higher FeO and lower In2O3than the late stage ones, which shows that the early stage of cassiterite should be formed under both higher temperature and pressure than the late stage of cassiterite. The two stages of cassiterite crystallization are considered to be related to the two stages of magmatic-hydrothermal activities in the Taoxikeng tin deposit, respectively. The Fe content of the prismatic form and{321} pyramidal form were found higher than that of{111} pyramidal form and{101} pyramidal form, which may be a main cause for the appearance of sector zoning and{321} pyramidal form.