Study On The Metalgenesis Of The Nariniya Lead-zinc Deposits, Tuotuohe, Qinghai

Nariniya Lead-Zinc deposit is located in Tuotuohe basin of Qiangtang terrane,northern part of Qinghai-Tibet Plateau, which is the only Lead-Zinc deposit found in thePotassium-ultrapotassic volcanic rocks at present. The tonnage of lead has reached21.34Mt, the average grade of lead is1.47%, indicating it is a medium-sized deposit. Whetherthe Nariniya deposit has a relationship with the activity of potassic volcanic magma?Whether the potassic volcanic magma evolution had any contribution on themineralization? And whether the deposit is the same type with the regional sediment-hostLead-Znc deposits? The answers is not only scientific issues of che concerning,but alsopractical problems of the regional prospecting. So to answer these questions, thefollowing works are included:(1) The study of the geochemistry of the potassic volcanicrocks which contains the analysis of the main elements, trace elements, REE and theSr-Nd-Pb isotope of the rocks;(2) The study of the mineral deposit geology and thecharacteristics of the mineralized alteration;(3) The study of the ages of the diagenesisand the mineralization, which are the U-Pb dating of the zircon and the Ar-Ar dating ofthe altered muscovite;(4) The study of the geochemistry of the ore deposit, whichcontains the studies of the fluid inclusions and the O-S-Pb isotope of hydrothermalminerals. Integrating all of these analyses, this paper proposes the preliminary geneticmodel of the deposit.The SiO2content of the fresh host volcanic rocks is62.9~68.6%, the K2O content is5.1~5.8%, indicating the rocks belong to shoshonite series. The rocks relatively enrichedin LREE and LILE, strongly depleted in Nb, Ta, Ti and other elements. The geochemicalcharacteristics similar to the northern Tibet potassium-high potassium rocks which hasbeen reported. The result of the Sr-Nd-Pb isotope indicates the magma is originated inEMII.The diagenetic year of the trachyte is37.96±0.18Ma through the LA-ICP-MS U-Pb zircon dating of the high potassium trachyte; the ore deposit mineralization age ofNariniya is37.95±0.30Ma through the result of the mineralized alteration sericite40Ar/39Ar isotopic geochronology. Both are highly consistent, indicating that thediagenetic and the mineralization are basically in the same period, which is in theIndia-Asia continent late-collisional transitional stage.Nariniya Lead-Zinc deposit hosted in the Cenozoic trachyte of the Chabaoma Fm(high potassic rocks), the orebody occurs in vein and controlled by the fault runningNW-SE. the type of the ore is mainly quartz+sulfide veinlets, the main gangue mineralare quartz, dolomite and sericite, the main ore minerals are galena and pyrite, and thereare also little chalcopyrite, sphalerite. the host rock occurs a strong pyritization,muscovite and sericitization.The study of the fluid inclusions in quartz veins of the mineralization period showsmost of them are gas-liquid two phase inclusions. The homogenization temperature ofthe fluid inclusions is170～290℃, the salinity of the fluid inclusions is3.0~9.0w％NaCleq,indicating the characteristics of low temperature and low salinity. The δ18O value of thesericite in the mineralization period is11.3~11.5, after converting to fluid the δ18O valueis8.2~9.4, indicating the ore-forming fluids are from magmatic water. The δ34S of galenais-0.1~1.8‰(n=5), which indicates the characteristic of magma sulfur. The Pb isotopiccomposition of the galena is similar to the Pb isotopic composition of the host rocks,indicating the mineralization metal is directly from the host rocks, the potassic volcanicrocks.The integrated results indicate the Nariniya Lead-Zinc deposit is associated with theactivities of potassic volcanic magma, which is significantly different with thecarbonated host Lead-Zinc deposits in Tuotuohe area, they are not belong to the samemineralization system. The mineralization model of the deposit can be described as:Since the Cenozoic, the India-Asia has been continuing converging and squeezing in theSN direction, which led to the shortening of the crust. During38Ma, by the upwelling ofdeep asthenosphere of the Tuotuohe area or other mechanisms the EMII melted andproduced the high potassic magma and then erupted on the surface. With the cooling ofthe magma, the saturation of the water in the magma decreased which led to theformation of free water. While carrying the metal and sulfur of the magma, the regionaltectonic activity caused the formation of the NW-SE fracture in the poorly consolidatedvolcanic rocks. The fluids centrally converged to the fracture, at last the metal and sulfurprecipitated and formed the vein sulfide orebody about170~290℃.