Genesis Of The Qingshuitang Veined Pb-Zn Deposit In Southern Hunan,South China

There are many large-medium hydrothermal veined Pb-Zn deposits developed around Hengyang Basin in southern Hunan,South China.The genesis of the deposits is widely disputed,mainly including magmatic hydrothermal and MVT.The Qingshuitang Pb-Zn deposit is a typical veined deposit in the area,and the lead and zinc resources reserves are about 2 Mt（average grade of Pb+Zn is 7.6 wt.%）.This study,focusing on the Qingshuitang deposit,based on the refined division of mineralization stages,by the in-situ major and trace element geochemical characteristics of sphalerite,quartz,and pyrite developed in each mineralization stage,the in-situ Pb isotope geochemical characteristics of galena,and the fluid inclusion characteristics,traces the source of ore-forming material,and the source,physicochemical condition and evolution of ore-forming fluid,summarizes the mineral precipitation mechanism,then restricts the genesis of the deposit,and establishes preliminarily the metallogenic model.On the basis of the finite element method,the metallogenic dynamics numerical simulation of the deposit is carried out to effectively quantify the metallogenic mechanism and provide useful information for mineral exploration.The main achievements and understandings are as follows:（1）The mineral paragenetic sequence is established and the ore-forming stages are divided:according to the cutting,mineral paragenetic association and metasomatic relationship,the alteration/mineralization paragenetic sequence at the Qingshuitang deposit can be divided into the following four stages:（I）quartz-pyrite-arsenopyrite;（II）quartz-barite-galena-sphalerite;（III）barite-galena-sphalerite;and（IV）calcite-galena-sphalerite.Sphalerite,quartz,pyrite and galena have in each of the mineralization stages.（2）The evolution of physicochemical condition of metallogenic process is traced:the studies of the fluid inclusion and the geochemical characteristics of trace elements of the sphalerite,quartz and pyrite in the Qingshuitang deposit reveal that the ore-forming fluid is derived from the hot brine which has the characteristics of low temperature（102～151℃）,low sulfur fugacity（f S₂）(10^-28.1～10^-20.8atm),medium-low salinity(19.45～2.24%Na Cl _eqv),medium-low density（0.95～1.10 g/cm³）,and medium-low pressure（8.0～19.8 Mpa）.FromⅡstage to IV stage of the mineralization,it shows that the fluids gradually decrease in density,salinity and pressure,overall gradually decrease in temperature,the sulfur fugacity（f S₂）first decreases from stageⅡto stageⅢ,then increases from stageⅢto stageⅣ,and the oxygen fugacity（f O₂）gradually decreases and evolves towards a more reduced environment.The evolution of the fluids in stage II and III is characterized by the gradual decreasing temperature and increasing p H,and the lead-zinc sulfide in these two stages is precipitated under weakly acidic conditions.The fluid temperature and p H has a large fluctuation during the quartz precipitation in the early phase of stageⅣ,which indicates that the meteoric water is added,and the p H of fluid keeps weak acidic from the early to late phase in stageⅣ.From stage III to stage IV,the physicochemical conditions should have a shift,which is speculated to be related to the supplementation of deep hot brine enrichment in Na and K.（3）The mineral precipitation mechanism is revealed:the fluid boiling and the mixing of meteoric water,results the decrease in fluid temperature,and the increase in p H,which is believed to be the main mechanisms for the precipitation of lead-zinc sulfides in stages II and III.In addition,the continuous addition of meteoric water and the supplementation of deep hot brine may lead to the precipitation of the metal sulfides in the early and late mineralization of stageⅣ,respectively.（4）The genesis of the deposit is defined and the metallogenic model is constructed:according to the comparison of metallogenic geological characteristics between the Qingshuitang deposit and the typical MVT Pb-Zn deposits,and combining with the indication of the genesis by the characteristics of trace elements in typical minerals,the deposit is classified as a deposit similar to MVT Pb-Zn that is controlled by cross-cutting faults,and related to hot brines.The ore-forming model can be divided into the following two periods:the formation of ore-forming fluids and the transport and enrichment of ore-forming hydrothermal fluids,respectively.With the sedimentary strata deepened and subjected by the tectonic compression,the mixing of formation water and meteoric water continuously extracts metal elements from the strata,then forms ore-bearing hot brine rich in lead,zinc and organic matter.Under the background of extensional tectonics,the ore-forming fluids migrates along the fault zones driven by tectonic heating to the favorable locations,and mixes with the reduced sulfur fluid.With the physicochemical conditions changed,lead-zinc sulfide had been precipitated.（5）The mineralization of the deposit is quantified effectively through the numerical simulation of metallogenic dynamics:the numerical simulation results show that the coupled model of thermal,pore-fluid flow and simple chemical reaction can better simulate the formation mechanism,ore body location and enrichment position of the Qingshuitang Pb-Zn deposit.The flow of hydrothermal fluid drivened by temperature is the key controlling factor for the mineralization.The deposit is low temperature mineralization which is mainly concentrated in the fault zone,and is more rich in upper part of the fault zone.There is still a certain prospecting potential in the deep without engineering layout.55 figures,17 tables,and 298 references...