Morphological and Geochemical Studies of the Cassiterite in Taoxikeng Tin Deposit,Southern Jiangxi,China

This paper presents a detailed study on morphology and geochemistry of cassiterite from the ore-bearing quartz veins of the Taoxikeng tin deposit,southern Jiangxi.The cassiterite in the Taoxikeng tin deposit can be divided into three types of morphology according to their different crystallographic forms:A-type cassiterite is composed of {111} pyramidal form + {110} prismatic form,B-type cassiterite is composed of {111} pyramidal form + {110} prismatic form + {100} prismatic form,and C-type cassiterite is composed of {111} pyramidal form + {101} pyramidal form + {110} prismatic form + {100} prismatic form.Based on previous studies,it is considered that A-type and B-type cassiterites should be formed at a higher temperature than C-type cassiterite.For the first time,the internal structure within the cassiterite grains was observed by the cathodoluminescence(CL) imaging.We have revealed that there are two stages of crystallization of cassiterite in the Taoxikeng tin deposit.The early stage of cassiterite commonly shows a relatively weak CL brightness(black to dark grey) with the oscillatory zoning and the sector zoning,and the margin of the phase were highly corroded.The crystallographic type of the phase is responsible for A-type or B-type cassiterite.The late stage of cassiterite shows a relatively strong CL brightness(light grey to white ) without the oscillatory zoning and the sector zoning.It occurrs either as overgrowth on the early stage of cassiterite,or as veinlet within the early stage of cassiterite.The crystallographic type of the late stage of cassiterite is responsible for C-type cassiterite.The electron microprobe analysis data shows that the cassiterite in the Taoxikeng tin deposit contains high levels of FeO and Ta_2O_5,indicating that it should be formed at a higher temperature.It also shows high HfO_2 contents with low Zr/Hf ratio,suggesting that the ore-forming fluid should be derived from a highly differentiated granitic magma.The early stage of cassiterite contains higher FeO and lower In_2O_3 than the late stage of cassiterite,indicating that the early stage of cassiterite should be formed under a condition of 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.