The Geological-geochemical Characteristics And Genesis Of The Caixiashan Zn-Pb Deposit,Eastern Tianshan

Caixiashan,the only superlarge Pb-Zn deposit discovered in the eastern Tianshan in recent years,is located in the north part of the middle Tianshan massif of the Tarim plate.The Pb-Zn mineralization geological setting and ore genesis are important and received much attention.In this paper,based on regional geology and mine geology,a detailed study of petrology,petrogeochemistry and geochronology study of the magmatic rocks in the Caixiashan district,together with ore mineralography and geochemical study,we try to understand the ore-forming mechanism of the Caixiashan lead-zinc deposit,and provide a basis for Pb-Zn prospecting of the eastern Tianshan.Ore bodies mainly assume stratiform and lenticular forms,controlled by fracture structure and are hosted in the clastic rocks and carbonatites of the first member of Neoproterozoic Kawabulake group.The ore mineral assemblage comprise galena,sphalerite,pyrrhotite,pyrite and minor chalcopyrite.The ores show fine-grained hypidiomorphic granular texture and vein,stockwork and breccia structure,revealing magmatic hydrothermal metasomatic-filling characteristics.quartz,sericite and calcite are the main alteration mineralsOre sulfides have ?34S vary from-22.9%o to 19.1%?,implying that sulfur might mainly originate from the marine sulfate thermal chemical reduction.There may be the contribution of deeper magma sulfur.The main sulfides show a narrow Pb isotope composition for 206Pb/204Pb(17.04?17.25),207Pb/204Pb(15.42?15.54),and 208Pb/204Pb(36.69?37.071).The ores and maganic rocks show similar lead isotopic characteristics,while the strata are different,indicating that the metal was closely related to magmatic rocks and sourced from both mantle and crust.The ore-forming fluids were sourced from the magmatic fluid,and mixed with the metamorphic water.The diotite and quartz diorite distributed in Caixiashan ore district have the same magma source and show calc-alkaline affinity,belong to Quasi aluminum rock,formed in a continental arc setting.Diorite and quartz diorite coming from the melted enriched mantle caused by subductio(?)zone's fluid metasomatism under the circumstance of the action that the south Tianshan ocean beneath the middle Tianshan massif.LA-ICP-MS zircon ages of diotite and quartz diorite are 348.2±3.7Ma,351.9±3.5Ma respectively.Thus,it is concluded that the island-arc igneous rocks in the Caixiashan district were formed by the northward subduction of the south Tianshan ocean beneath the middle Tianshan massif.Rb-Sr isochron age of intergrown mineral association of sphalerite,galena and pyrrhotite from the Caixiashan Pb-Zn deposit is 337±5.5Ma(MSWD=0.65)show the related lead-zinc mineralization were mainly happened in island-arc setting.To sum up,the mechanism of Caixiashan Pb-Zn deposit can be concluded as follows.The subduction of the south Tianshan ocean beneath the middle Tianshan massif in Carboniferous period induced island-arc magmatism,the magmatic fluid carries the considerable volatile matter and metals from deep,when it travels trough fault-induced channel and encounters with the wall rocks,it extracted metallogenic material from wall rocks.Futher,these magmas replace the early-formed dolomite marble,forming the Caixiashan lead-zinc deposit.This ore deposit is a magmatic hydrothermal deposit.