Chloritization Of Ore-Bearing Granite In Bailashui Tin Deposit, Southern Hunan Province

Bailashui tin deposit named chloritized granite type tin deposit is a new type deposit which was found in recent years in southern Hunan province.The deposit is located in Furong tin ore field which reserves more than 60 million tons of tin.More than 90% tin reserves of this ore field is concentrated in Bailashui tin deposit,which is closely related to the chloritization.The orebody occurs in the strong chloritization granite.This research indicates that Bailashui tin deposit is associated with the evolution of Qitianling multi-stage composite batholith.The granites in different stages of evolution show a range of different geochemical characteristics.The Sn content in the granite and the degree of fractional crystallization are negatively correlated,that may be related to the decreasing oxygen fugacity in the process of magmatic evolution.The study of chlorite shows that, the sequence of rock-forming minerals of the granite replaced by chlorite is:from amphibole and biotite to plagioclase and potassium feldspar,then to quartz.The variations of the chlorite shape were, on the one hand, related to the change of chlorite composition, on the other hand, related to the crystal texture of rock-forming minerals of the granite which were replaced by chlorite.The stronger the chloritization of granite was, the higher the LOI and Zn contents of whole-rock sample were.And also more Cl was lost from granite into hydrothermal liquid.The Si content of chlorite was decreasing with the increasing degree of chloritization.The Al content was released after the alteration of feldspar. The mobilization and redistribution of Al on the thin-section scale caused the Al content of chlorite increased.Fe was brought in by hydrothermal fluid.The amounts of chlorite formed are largely a function of how much Fe has been introduced into the hydrothermal system.The Mg needed for the formation of chlorite were primarily related to the redistribution of Mg which were released by mafic minerals.The Mn of chlorite was mainly brought in by the outer hydrothermal fluid.The changes of chlorite chemical composition characteristics reflect that the alteration fluid was acidic and reductive. Study on fluid inclusion shows that the ore-forming fluid was derived from high temperature and high salinity magmatic fluid during magmatic differentiation.In the process of mineralizing,a lot of low temperature and low salinity meteoric water had been brought in.The mixing of the tin-rich magmatic fluid and the meteoric water (possibly accompanied by the release of pressure)led to the boiling, which resulted in the precipitation of tin.The ore-forming temperature is in the range of 280℃to 420℃.This tin deposit belongs to medium-high temperature hydrothermal deposit.According to the features of granite geochemistry,chlorite and ore-forming fluid,we can got some conclusions:the forming of chloritized granite type tin deposit in Bailashui is related to the enrichment and migration of Sn and Fe introduced by hydrothermal fluids.The hydrothermal fluids have the characteristics of high tempera-ture, rich in volatiles (H2O, CO2), K+, Na+, Cl-,acidic and reductive.When the hydrothermal fluids reacted with the meteoric water and wall rock, the oxygen fugacity, pH and temperature in the system would changed,and then the chloritization and cassiterite precipitation were simultaneously occurred, which led to the forming of chloritized granite type tin deposit.An attention to searching for this type tin deposit in W-Sn-bearing granites in the Nanling Range should be especially paid.