Metallogenic Model And Prediction Of The Carbonate-Hosted Pb-Zn Deposits In Western Hunan And Eastern Guizhou Province,South China

The Western Hunan and Eastern Guizhou district is located in the southeast margin of the Yangtze craton and developed with more than 200 Pb-Zn deposits/occurrences.The deposits are distributed in NNE-trending belts and hosted in Sinian,Cambrian and Ordovician strata,especially the lower Cambrian carbonate rocks.After many years of exploration and development,basic geological data is well documented about this area.However,due to the limits of research methods and test techniques,issues on hydrothermal fluid evolution,metal sources,precipitation mechanism and metallogenic model are still in hot debates.In addition,large Pb-Zn deposits only occur in Huayuan ore field,Western Hunan Province.Other areas in this district,developed with medium to small size deposits,are plagued by the shortage of the reserves.Therefore,where to find the large Pb-Zn deposits is another issue urgently needed to be solved.In this dissertation,systematic studies are conducted on numerous Pb-Zn deposits in this district.The author firstly identifies the orebody characteristics,mineral assemblages and hydrothermal alterations;then carries out trace element,isotope and fluid inclusion analyses to discuss the sources of the fluids and metals,as well as the precipitation mechanisms.Combined with the geochronological evidences,the author also discusses the geodynamic background and establishes a metallogenic model.Integrating the model with ore-controlling factors and prospecting indicators,the author establishes a regional prospecting model as well,and conducts comprehensive information metallogenic prediction and resource estimate on the GIS platform,which helps to delineate the prospective areas and further prospect.The Pb-Zn deposits in Western Hunan and Eastern Guizhou district are dominantly hosted in algal limestone or dolomite of the lower Cambrian Qingxudong Formation.The ore bodies are mainly stratiform,stratoid and lentoid,with less amount vein and stockwork.The ore minerals mainly consist of sphalerite,galena and pyrite.And the gangue minerals are composed of calcite,barite,fluorite,dolomite and quartz.Hydrothermal alterations including calcilization,fluoritization,pyritization,baratization,silicificatoin and dolomitization are closely associated with Pb-Zn mineralization.Microthermometry reveals the homogenization temperature and salinity of hydrothermal fluid ranging from 120 ? to 200? and 8% to 20%(Na Cleqv),respectively.The liquid phase of fluid inclusions are mainly composed of Ca2+,Na+,Mg2+,SO42– and Cl–,whereas the gaseous phase consists of H2 O,N2,CO2 with a small amount of CO,CH4 and H2.The ore fluids have ?DSMOW and ?18Ofluid of-60‰ to-15‰ and 0.3‰ to 9.2‰,respectively.The evidences above thus confirm the hydrothermal fluid was mostly derived from the formation water mixed with variable amounts of meteoric water.Compared with the wall rocks,the hydrothermal calcite is characterized by relatively lower ?13CPDB value(range from-4.89 to 1.50‰)and significantly lower ?18OSMOW values(range from 13.37 to 25.09‰),indicating carbon was mainly sourced from the wall rocks and the fluid-wall rocks interaction resulting in the precipitation of the hydrothermal calcite.The sulfides from these Pb-Zn deposits yield a narrow ?34S values(from 22.3 to 36.1‰),suggesting that the sulfur was derived from the sulfate by thermochemical reduction,during which the organic matters also played an important role.The lead isotope composition of sulfides are relatively homogeneous and plotted near the upper crust in the evolutionary tectonic settings of lead isotopes.The sphalerite has 87Sr/86 Sr values of 0.70915 to 0.71025,which resembles the crustal stronrium isotope composition and consistent with that of the underlying strata.Besides,the underlying strata contain high content of Pb and Zn.Thus,the author proposes that the underlying strata supplied the metals for the Pb-Zn mineralization.The Pb-Zn deposits in the Western Hunan and Estern Guizhou district formed in late Silurian to early Devonian,genetically related to the basin fluid migration caused by the Caledonian orogeny.The Pb-Zn deposits in this district resemble the typical MVT Pb-Zn deposit in geological setting,host rocks,deposit geology,hydrothermal fluid and sources,thus can be classified as MVT Pb-Zn deposit.Geological,geochemical and geochronological evidences help the author establish a metallogenic model for the Pb-Zn deposits in Western Hunan and Estern Guizhou district.Deep sourced metalliferous fluid migrated up and mixed with organic-and sulfate-rich formation/meteoric water.The mixture of these two kinds of fluids resulted in the precipitation of galena,sphalerite and other minerals.The Pb-Zn deposits in the study area are jointly controlled by stratum,structures and lithofacies.1)Specific lithology assemblages.The ore bodies are hosted in porous algal limestone and dolomite,which are underlied by permeable sanstone and overlied by impermeable argillaceous dolomite.2)The ore bodies are structurally controlled by regional faults,anticline and secondary faults and fractures.3)The Pb-Zn deposits restrictedly occur in the middle to shallow ramp facies.On the basis of comprehensive analyses of metallogenic model,controlling factors and prospecting indicators,the author also build a geological-geochemical regional prospecting model,which helps to recognize two first-order prediction target areas(A1,A2)and two second-order prediction target areas(B1,B2)areas in Western Hunna and Eastern Guizhou district using characteristic analysis method on GIS platform.Based on the theory of geochemical blocks,three zinc-geochemical blocks are delineated with a total predicted mineral resource of 21.52 million tons zinc(rock thickness 1000 m,mineralization coefficient 0.378%).According the predictive results,the author suggests that the Zhenyuan-Tongren area of eastern Guizhou Province is the most potential area in further exploration.