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 Ta2 O5, indicating that it should be formed at a higher temperature. It also shows high HfO2 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 In2O3 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 rnagmatic—hydrothermal activities in the Taoxikeng tin deposit, respectively.%本文对淘锡坑锡矿床的含矿石英脉中的锡石晶体开展了晶体形态学和地球化学的系统研究.按晶面组合特征,淘锡坑锡矿床中的锡石可分为三种类型:由{111}锥面+{110}柱面共2个单形构成的A型锡石、由{111}锥面+{110}柱面+{100}柱面共3个单形构成的B型锡石和由{111}锥面+{101}锥面+{110}柱面+{100}柱面共4个单形构成的C型锡石.综合各种锡石晶型与结晶条件的资料分析,本文推测A型、B型锡石的结晶温度高于C型锡石的.本文首次利用电子探针分析仪对锡石进行阴极发光(CL)成像观测,发现淘锡坑锡矿床中的锡石经历了两期结晶作用:早期锡石以CL荧光亮度较小(黑色—暗灰色)为特征,边缘有明显的溶蚀痕迹,常见典型的韵律振荡构造和砂钟构造,其晶面组合特征显示了早期锡石属于A型或B型锡石；晚期锡石以CL荧光亮度较大(白色—浅灰色)为特征,往往沿着早期锡石的溶蚀边界面充填生长,一般不显示韵律振荡构造和砂钟构造,其晶面组合特征显示了晚期锡石属于C型锡石.电子探针分析显示,淘锡坑锡矿床中锡石的FeO、Ta2O5含量较高,指示其形成于高温热液环境；HfO2含量较高,Zr/Hf比值较低,说明成矿热液来源于高度分异的花岗质岩浆.两期锡石的FeO含量和In2O3含量的对比结果显示,早期锡石相对富FeO而贫In2O3,反映了早期锡石的结晶温度和压力均高于晚期锡石的,故两期锡石的结晶作用分别与淘锡坑矿床的两期岩浆热液成矿作用相关.
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