西藏谷露热泉型铯矿床地质地球化学特征与成矿作用

Geologic and geochemical features and ore forming process for hot spring cesium deposit of Gulu Area,Nagqu Region,Tibet,China

谷露热泉型铯矿床位于西藏那曲县谷露地区,达中型规模。在前期野外地质特征与年代学研究的基础上,对其地质、地球化学特征及成矿作用进行了系统的研究。矿石的构造类型有粗大孔隙状构造,角砾状构造,碎裂叶片状构造,细小孔隙状构造,致密块状构造与鲕状构造。结构类型有粒状结构、胶状结构、生物结构、溶蚀结构、长方体状结构、被膜状结构、球状结构、管状结构等。生物结构在硅华形成的各阶段均普遍存在。硅华的主要矿物为蛋白石,但不同的形成阶段蛋白石的种类不同,第2、第3阶段均为CT蛋白石与石英,而在第4与第5阶段仅出现A-蛋白石,没有CT蛋白石。蛋白石矿物中的SiO2最高达97.48%,∑(CaO+Na2O+MgO)最高达到3.2%,Cs2O最高可达3.57%。随着由早到晚的变化,SiO2呈现出明显的降低趋势,铯含量则呈升高的趋势。其中第2～4阶段的铯平均含量已经低于铯的工业标准(500×10-6)。谷露硅华不同阶段的样品在Fe-Mn-Al图解中的位置不同,39件样品中有30件位于生物沉积物区,有9件位于热水沉积物区,硅华的稀土元素含量均比北美页岩的低,Cr、Zr的含量与Th、U的含量均相对一般的热水沉积物低。由早期到晚期,δ30Si呈由高到低、δ18OSMOW由低到高的变化趋势。谷露硅华δ13CPDB与87Sr/86Sr均体现出生物作用的特点,是在深源(至少是下地壳)地质作用下以生物作用为主、热水作用为辅形成的。

Gulu hot spring is located on the west bank of Sangqu river ,Gulu town, Nagqu county of Tibet .The elevation is about 4700-4750m. It is still intensely activing. Consisted mainly by geyserite, the sinter formed a medium scale hot spring type Cs-deposit. On the basis of investigating the characteristics of the field geology and study on chronology, this paper systematically studies the characteristics of geology - geochemistry and the ore forming process.The kinds of structure are megagramed massive,brecciform, cataclasis and foliated, fine-grained pore ,campact massive and oolitic. The kinds of texture are grained, colloform, organic, corrosion, rectangle, coating and tubular etc. Organic texture exists in all ore-forming stages of Gulu Cs-deposit. The main mineral of geyserite is Opal. But different ore-forming stages have different kinds of opal. In stage 2 and 3, there are CT-opal and Quartz.In stage 4 and 5,is A-opal ,but there is no CT-opal.In opal, the highest content of SiO2 is 97.48%, ∑ （CaO ＋ Na2O ＋ MgO） is 3.2% and Cs2O is 3.57%. From early to late stage, the content of SiO2 gradually falls while contents of Cs2O rise. From stage 2 to 4, the contents of Cs are lower than the standard of Chinese pay grade（500×10^-6）.In different stages, sample locations in the figure of Fe-Mn-Al are different.In all 39 samples, 30 samples are located in biological sediment region,9 samples in hydrothermal sediment region.The contents of REE is lower than that of North America shale . The contents Of Cr-Zr and Th-U are relatively lower than that of ordinary hydrothermal sediment. From early to late stage,δ^30Si gradually falls while δ^18SMOW rises. δ^13CPDB and ^87Sr/^86Sr of Gulu geyserite show the organic characteristics. On the basis of deep geological process, Gulu Cs-deposit is formed mainly by organic process.