Alteration Characteristics And Mechanism Of Huangshaping W-Sn-Mo-Cu Polymetallic Deposit In Southern Hunan

Two skarn mineralization systems related to W-Sn mineralization and Cu mineralization are developed in the Huangshaping deposit,which is an ideal object to study the alteration difference between W-Sn and Cu mineralization systems.In this study,detailed tunnel cataloging was carried out on the reaction area between granite porphyry(301~#)and limestone of the Shidengzi Formation,and between quartz porphyry(304~#)and limestone of the Shidengzi Formation at level-176 m at the Huangshaping polymetallic deposit.According to the alteration types and mineral assemblages of the two metallogenic systems,the spatial alteration zoning model was divided in detail.In addition,the typically altered minerals were identified and analyzed by EPMA and LA-ICP-MS in order to discuss the spatial evolution in elements and the changes of physical and chemical conditions of hydrothermal fluids.The achievements and progress of this study are as follows:According to the tunnel cataloging and microscopic thin-section identification,the W-Sn mineralization system can be roughly divided into six zones from porphyry to wall rock:potassium sericitized granite-porphyry zone,garnet skarn zone,diopside-idocrase-wollastonite skarn zone,magnetite-hematite-scheelite-cassiterite-epidote-fluorite-chlorite skarn zone,quartz-chlorite-pyrite alteration zone and marble zone.The Cu mineralization system can be roughly divided into five zones:potassium quartz porphyry zone,garnet-diopside skarn zone,pyrite-sphalerite-galena metal sulfide zone,quartz-pyrite-chalcopyrite-sphalerite-galena alteration zone and marble zone.Based on the analysis of major and trace elements of garnet,magnetite,chlorite and sphalerite,massive andradite was found that formed in the W-Sn mineralization system in the early skarn stage.The zonation structure is developed with narrow inner and wide outer.The right-dip REE patterns and the high contents of W and Sn elements in andradite show that hydrothermal fluid was formed in an alkaline environment with relatively high oxygen fugacity during the early skarn stage.Garnet in the Cu mineralization system has higher grossular endmember contents with polycrystalline structure and fine zonation structure.The“M”-type REE pattern and low contents of W and Sn of garnet indicate that the hydrothermal fluid was formed in a relatively acidic environment with relatively low oxygen fugacity.Magnetite is developed in the late skarn stage of the W-Sn mineralization system,while found in the Cu mineralization system.In the W-Sn mineralization system,magnetite develops zonation structure and growth boundary structure.The low contents of V and Cr but high contents of Sn and Mn indicate the high fugacity of fluid oxygen fugacity and strong of the water、rock reaction.(Ca+Al+Mg)-(Ti+V)diagram shows that the metallogenic temperature range from 300 to 500℃,and the metallogenic temperature gradually decreased with the increase of distance from the granite.In the quartz-sulfide stage,the W-Sn mineralization system formed massive chlorite and minor sphalerite near the limestone,in which chlorite is mainly brunsvigite and diabantite.According to the substitution mechanism of major elements,the fluid is in a neutral acidic and reducing environment,and the formation temperature of chlorite is between 297 and 468℃.Sphalerite has high Fe content and is Marmatite.The major and trace elements show that the metallogenic temperature of sphalerite is between 337 and 375℃,and the sulfur fugacity is relatively high.At this stage,relatively low-temperature Fe-rich,low p H,reducing hydrothermal fluids were added to the Cu mineralization system,which interspersed with metasomatic skarn zones,and formed little chlorite and much sphalerite in the contact zone.Chlorite is mainly thuringite and chamosite.According to the substitution mechanism of major elements,the fluid is in a relatively acidic and reducing environment,and the formation temperature of chlorite is between 201 and 261°C.The major and trace elements of sphalerite show that the metallogenic temperature is between 268 and 303°C with low sulfur fugacity.W-Sn mineralization is mainly developed in the middle of the contact zone of W-Sn mineralization system,fluorite alteration,magnetite mineralization and chlorite alteration are important prospecting indicators for W-Sn mineralization system.Cu mineralization is mainly developed in the outer skarn area of the contact zone of Cu mineralization,pyrite mineralization,chlorite alteration and other alterations are important prospecting indicators of Huangshaping Cu mineralization.