The Research Of The Lead-zinc Mineralization Of Volcaniclastic Rocks In The YiDun Island Arc (Dege-Xiangcheng Main Arc), The Late Triassic

Yidun arc is formed by the subduction westward of Ganzi-Litang oceanic in theLate Triassic, as the subduction of oceanic crust, the intense volcanic magmatismoccurred in the Yidun island arc, accompanying the formation of a variety types ofmetal deposits. In the Dege-xiangcheng main arc zone, formed a large number oflead-zinc deposits (ore spot), including the typical and ultra-large type deposit of theVolcanic Hosted Massive Sulfide(VHMS)-Gacun deposit. The geological explorationwork of the recent years has showed that the DeGe-xiangcheng main arc band of Yidunisland Arc hosts a large number of Lead-zinc polymetallic deposits,which are hosted inthe volcaniclastic rock and volcanic-sedimentary rocks that formed in the late Triassicepoch, but these deposits don’t all belong to the VMS type deposit.In order to find outthe genetic mechanism and the geological characteristics of the Lead-zinc polymetallicdeposits which have the similar mineralization environments, and to discuss theintrinsic link between the various lead-zinc mineralization, then puts forwardreasonable advice for the ore searching of Lead-zinc polymetallic deposits in the area,we choose the typical deposits(GaCun,GaYiQiong,Edi,RuJinGou) to carry out adetailed study of the ore deposit geology,these deposits belong to the middle andnorthern part of Dege-xiangchengmain arc zone.Through field investigation; indoor microscope observation;test and analysis ofmajor elements, ore-forming elements of country rock and ore body, the trace elementanalysis of galena,chalcopyrite, pyrite, calcite and other single-mineral, sulfur and leadisotope testing and analysis of sulfide ore, zircon U-Pb age test of subvolcanic rock foreach deposit. Through the research this article have got the idea of the objective andin-depth understanding of the mineralization process each deposit:①Gacun deposit isa typical deposit of the Volcanic Hosted Massive Sulfide(VHMS)，formed in back-arcrift basin which with the deep-sea environment,the mineralizationgo with intermittenteruption of volcanic.when the ore-forming hydrothermal fluids erupt out volcanicconduit to deposit and form the stratiform ore body;while the filling metasomatism of ore-forming hydrothermal fluids occurred under the caldera and form thevein-stockwork ore body.The long time magmatic activities and deep-sea environmentlaid the foundation of the formation of superlarge deposit of gachun: long timemagmatic activities provide mineral material for the formation of deposits,theore-forming medium,and the power (energy) needed for the formation of deposits;while the deep-sea environment effectively prevent the proliferation of the ore-formingmaterials in a wide range.In addition, the Gacun deposit shows the phenomenon of thezonation of Cu(selectively enriched in the upper layered ore body), this phenomenon isformed in rift basins in deep-sea environment, Cu gathers in the vapor phase fluidselectively in the ore-forming hydrothermal fluid during the process of the energyenriching, and the vapor phase hydrothermal ore-forming fluid gathers in the forefrontof the hydrothermal system and erupts out of the volcanic pipe firstly,formed thelayered orebodies which enriched Cu elements significantly.②GaYiQiong deposit isnot the typical VMS-type deposit, which considered in previous studies. It’s formed bythe exhalative hydrothermal mineralization firstly and superimposed with thesubvolcanic hydrothermal filling mineralization. The two mineralization occurrs in thesame volcanism activities at the different stages, and the subvolcanic hydrothermalfilling mineralization is the main mineralization stage.③Throughthe careful study ofthe geological characteristics of the EDi deposit, find the deposit formed by two stages,firstly, the volcanic eruption happens in the shallow marine environment and forms thesource bed. Secondly, the deep hydrothermal fluids enriched the metallogenic materialsin the source bed and occured the filling and replacement mineralization.④The studyof the Geological characteristics of the RuJinGou deposit indicate that it formed at thelate stage of the terrestrial volcanic eruption magmatic acitivities, the ore-forminghydrothermal fluids accompanied by ultra-shallow emplacement of magma, formed thesubvolcanic rocks (hydrothermal) type deposits, in the volcanic subvolcanic rock,subvolcanic agencies and within the contact area of the subvolcanic rock and volcanicrock, formed copper ore body mainly; and in the periphery of volcanic bodies andsecondary fracture zone formed by the main fault within the mining area, formed thelead and zinc-(silver) orebodies mainly.Through integrate the domestic and foreign research results about the VMS typedepositsseriously, obtained thorough understanding of the geological characteristics forthe VMS type deposits; combined the geological characteristics of Gacundeposit withthe chronology study of the volcanics in the Gacunmining area and the intrusive rock ofthe west side area of the Gacun deposit.Acquireda new understanding of the formation mechanism of back-arc rift basins, at the early stage of the Oceanic crust subductionoccurred a small amount of silicon aluminum material partial melting; when theOceanic crust subducts under area of the Gacun deposit, a large amounts of maficmaterial begin to melt and form the huge Mafic magma chamber.When the huge Maficmagma chamber invade, producing thrust outward to the both sides of the crust, theback-arc rift basin began to develop; at the same time, the tremendous heat of the maficmagma chamber will make the crust remelted and form a larger acidic magma chamber.with the crust remelting, the volume increased significantly, which will promot thegrowth and development of the back-arc rift basins further; what’ s more, with thegrowing of the rift, the acidic and basic magma will rise along the space formed bygrowth of the rift basin and occurred the bimodal volcanic rocks magmatic activities inthe rift basin.The general appearing of volcanic breccia in the mining area of EDi deposit andthe lithic tuff identify with microscopic study which belongs to the country rock; whilethe lithophysa rhyolite, crystal tuff, crystal-vitric tuff and crystal-lithic tuff,whichappeared in the RuJinGou mining area. All of evidence mentioned above indicate thatthe country rock of the EDi and RuJinGou deposits are formed by the volcanic action,what’s more, the evidence above all indicate that there is a volcanic basin between theCangTai basin and XiangCheng basin which belongs to the main Arc of YiDun islandarc, which named DeGe-Xiangcheng arc.Through the geological character study of the Gacun deposit, GaYiQiong deposit,Edi deposit and RuJinGou deposit, have identified three mineralization of the lead-zincdeposits which belong to the main Arc of YiDun island arc (DeGe-Xiangcheng arc):①exhalative hydrothermal mineralization,②subvolcanic rocks (hydrothermal)mineralization,③the filling and replacement mineralization of the late hydrothermalfluids. The three mineralization are associated with volcanic magmatic activitieshappened in the Late Triassic closely, the first two occurred at the same time of thevolcano eruption or during the subvolcanic magma intrusion, and can superimpose onthe same deposit (such as RuJinGou deposit). The third mineralization divided into twostages: at the first stage, the volcanic eruption formed the ore source layer; later, thehydrothermal fluids enriched the metallogenic materials and migrated to the suitableore-forming space, occurred the fluid filling and replacement mineralization andformed the hydrothermal vein type ore body.Though the research of the lead-zinc mineralization of the main arc of the YiDunisland arc (DeGe-Xiangcheng arc), summarized the prospecting directions of the lead and zinc deposits in future: in the north section, from the CangTai basin to the ZengKebasin should find the deposits formed by the exhalative hydrothermal mineralization orformed by exhalative hydrothermal deposit and subvolcanic hydrothermal fillingmineralization; While at the area of QuDeng that on the West of Litang, which in themiddle of the DeGe-Xiangcheng arc, should focus on looking for the hydrothermalvein-type deposits that controlled by the fracture zones and lithologic interfaces.At thesame time, in the area of RuJinGou deposit and its south area should be look for thecopper-lead-zinc deposits of subvolcanic rocks (hydrothermal) type.