Petrogenic, Metallogenetic Environment And Its Exploration Significance In Baiyinchang Copper Deposit, Gansu Province

The Baiyinchang(BYC)District is one of the largest VHMS fields in the north Qilian Orogeny,northwestern China.In recent years,as the mineral resources are exhausted,it has great significance to search for new deposits or orebodies in the deep and outer areas.At present,there are several problems affected further scientific research and exploration.Therefore,the author analyzes bimodal volcanic magmatism and geodynamic evolution,and studies the spatial relation and genetic relationship of volcanic magmatic evolution and VHMS deposit.At same times,the author determines microscopic features of different ores and establishes the mineral metasomatic order of metallic mineral in the typical deposit.Besides,this research divided mineral assemblages and analyzes the geochemical data of both the altered and least-altered host rocks with standardized method of the mass change calculation.Then this paper has distinguished the diagenetic environment of volcanic rocks and the metallogenic environment of VHMS deposits,and summarized the diagenesis-mineralization process.At last,based on the above achievements,the paper proposed the prospecting direction for the further prospecting work of silver mine field.The detailed results and conclusions show that:According to the geochemical characteristics of volcanic rocks in the area,the all volcanic rocks belong to the calcium and alkali series.Among them,the geochemical characteristics of Zheyaoshan spilite is similar to island arc basalt,displayed relatively enriched light rare earth element(LREE)and near-flat heavy rare earth element(HREE)patterns,LILE(Rb,Sr,Ba,Th,U and K)and LREE(La,Ce,Nd,Sm,Eu and Gd)were enrich,HFSE(Nb,Ta,Ti)and HREE were depletion.The geochemical characteristics of Tongchanggou spilite are similar to BABB,and display near-flat light rare earth element and near flat heavy rare earth element(HREE)patterns.The geochemical composition of volcanics has transitional characteristics in the different type volcanics,and has dual geochemistry characteristics of both OIB and BABB,it show the transformational characteristics from island arc environment to post-arc basin environment.Combined the regional geological background and previous research results,it is found the volcanic magmatism can be divided into two stages,mainly contained the stage of island arc magmatism in the western District and the stage of back-arc basin volcanic magmatism in the eastern District.It is thus inferred that subduction of an oceanic plate steepened gradually with the acceleration of rift,and during asthenospheric upwelling partial remelting occurred under the dual effects of adiabatic hypotensive and subduction fluids,and this could ultimately lead to the formation of post-arc basin type volcanic magmas in the east of the Baiyinchang District.Determined microscopic features of different ores and establishes the mineral metasomatic order of metallic mineral,compared and analyzed the tract element contents of metal minerals.According the In contents,In/Ga ratio and In/Ge ratio of sphalerite,it can be found the sphalerite form from setting high temperature,which is related to the magmatic hydrothermal activity.The S/Se ratio and Co/Ni ratio in pyrite have relative difference,which is indicated the different type's pyrites were formed from magmatic hydrothermal deposits or sedimentary deposits.Crystallization temperature of sphalerite has decreasing tendency from Zheyaoshan,to Xiaotieshan and Tongchanggou deposits.The main metal minerals have high contents of Au,Ag,Pb in the Xiaotieshan and Tongchanggou deposits,which is related to the component of eastern volcanics.In addition,the later pyrite veinlet of Zheyaoshan deposit may be formed from the filling metasomatism of Late hydrothermal fluid in the volcanic feeder or fissure,it show the possibility of existing Au ore body in deep deposit.The wall-rock alteration zone predominantly occurs as tubular zones in the BYC deposits,which has great variety of alteration and mineral assemblages.The chloritization and sericitization are closely related to the hydrothermal mineralization,and chloritization can be used as an indication of ore-forming fluids and precipitation.At same times,we have applied the standardized method of the mass change calculation to analyze geochemical behavior in the different altered rock,it found the degree of alteration gradually increase from sericite-silicified one,chlorite-dominant zone to mineralized zone.During since plagioclase break-down during the sericite-silicification process,Be?U?Sr ? Na were depleted and K was enriched.In the chlorite-dominant zone,Mg and Fe were enriched along with the formation of chlorite and pyrite.In addition,Ca,Ba and Mn mainly exist in siliceous rocks,barite or gypsum,these sediments were used as a barrier and segregated sea water and the ore-forming hydrothermal fluids,and then promoted the hydrothermal fluids to precipitate.In the sericitization stage of Zheyaoshan deposits,Ag,Cu,As and Bi were enriched,while Zn,Pb,Sb were depleted.But all of these element were enriched in the sericitization stage of Zheyaoshan deposits.Those migration characteristics of trace elements also were reflect by different character of ore-forming hydrothermal fluids from Zheyaoshan and Xiaotieshan deposit.On the basis of the above,this paper analyzes the metallogenic factors and summarized the diagenesis-mineralization process in BYC Distract,It is considered the structural setting of volcanics in eastern and western Distract have transformational characteristics from island arc environment to post-arc basin environment.The volcanics of eastern and western Distract may be formed from different magma systems,which produced different postmagmatic hydrothermal fluids in the late volcanic magmatism.Those hydrothermal fluids have different composition,physical and chemical conditions,which led to the VHMS deposits have different ore type,ore-forming element and alteration.On the basis of metallogenic geological conditions of BYC Distract,based on the spatial location of the volcanic structures,primary geochemical halos,alterations,and metallogenic geologic condition of VHMS deposits in the Distract,determined definite and optimal prospecting target area.There are 5 prospecting target areas were determined which were divided into Class A(2 prospecting target areas),Class B(2 prospecting target areas)and Class C(1 prospecting target areas).