西藏那阿钨矿床中石榴子石地球化学特征及成矿意义OA北大核心CSTPCD

The Na'a deposit in Chayu County,Xizang,Southwest China,is located in the most eastern section of the Gangdis metallogenic belt,which is a typical skarn tungsten ore deposit,contains multiple generations of garnet in skarn and sulfide-carbonate altered rocks.Garnet is a representative mineral in skarn deposit.The systematic research on garnet fills the gap of previous research and provides new data.The composition characteristics and ring band changes of garnet are often used to indicate the physical and chemical properties,migration and evolution,and the metallogenic process of the fluid.According to the alteration mineral combination and interpenetration relationship in field observation and indoor identification,garnet can be divided into three types from early to late:BR type(brown red),DB type(dark brown),and B type(brown)garnet.The garnet is mainly composed of grossularite,followed by andradite.Its formation is characterized by brown-red to dark brown to brown,which is accompanied by the decrease of grossularite component and the increase of andradite and spessarite components.All three types of garnet developed typical octagon and ring band characteristics under the microscope,with the characteristics of high calcium,low manganese and low magnesium components,showing the typical hydrothermal erosion into the cause.The high grossularite content of BR and DB indicates the relative reduction of the forming environment,whereas the increasing andradite content of type B indicates the trend of fluid evolution to oxidation environment.The REE con-tent and Fe3+/(Fe3++Al3+)ratio are indicating the relative reduction of the overall crystallization conditions.The garnet all showed the characteristics of depleted light rare earth,enriched heavy rare earth content,negative Eu content and gradually increasing content,indicating that the metallogenic hydrothermal fluid in the skarn stage is in relatively neutral conditions.The mass-like substitution mechanism of rare earth elements is mainly[REE3+]Ⅷ+[Z3+]Ⅳ→[X2+]Ⅷ+[Si4+]Ⅳ及[REE3+]Ⅷ+[Y2+]Ⅵ→[X2+]Ⅷ+[Y3+]Ⅵ By comparing the contents of garnet W and Sn in different types of polymetallic skarn deposits,the results showed that the content of garnet W and Sn in W mineralized deposits was significantly higher than that of non-W mineralized deposits,indicating that the W and Sn content of garnet had certain indicator significance for mineralization.Furthermore,elements U and Eu can dynami-cally reflect the redox conditions for mineral growth,the W-U and W-δEu relationship characteristics of garnet can be used as an indicator of different types of skarn W deposits.