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

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 megagrained 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, δ30Si gradually falls while δ18OSMOW rises. δ13CPDB 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.