Metallogenic Setting And Metallogenesis Of Lead And Zinc In Tuotuohe Region, Qinghai-Tibet Plateau

The Tuotuohe region is located in the Qiangtang massif, in the hinterland of the Qinghai–Tibet Plateau.The Qiangtang massif, an integral part of the Tethys structural domain, underwent a prolonged andcomplicated course of development, especially in terms of its history of ocean–continent conversion. Thereare two distinct phases in its development: the first involves the evolution of Tethys, and the second thecollision between the Indian and Asian continents, which caused the formation of the Qinghai–TibetPlateau.In this paper, we present new zircon U–Pb ages, whole-rock major and trace element analyses, andzircon Hf isotopic data for the Early Permian–Late Triassic and Cenozoic magmatic rocks in the Tuotuoheregion of the western segment of the Jinshajiang suture.Zircons from the Early Permian–Late Triassic magmatic rocks of the Tuotuohe region areeuhedral–subhedral in shape and display fine-scale oscillatory zoning as well as high Th/U ratios (0.4–4.6),indicating a magmatic origin. The zircon U–Pb ages obtained using LA–ICP–MS are281±1Ma,258±1Ma,244±1Ma, and216±1Ma, which indicate magmatism in the Early Permian–Late Triassic. A dioritefrom Bashihubei (BSHN）has SiO2=57.18–59.97wt%, Al2O3=15.70–16.53wt%, and total alkalis (Na2O+K2O）=4.46–6.34wt%, showing the rock belongs chemically to the calc-alkaline and metaluminousseries. A gabbro from Bashibadaoban (BSBDB）belongs to the alkaline series, and is poor in SiO2(45.46–54.03wt%）but rich inAl2O3(16.19–17.39wt%）and total alkalis (Na2O+K2O=5.48–6.26wt%).The BSHN diorite and the BSBDB gabbro–diorite both display an enrichment in LREEs and LILEs anddepletion in HFSEs, and they have no obvious Eu anomaly; they have relatively low MgO contents(2.54–4.93wt%), Mg#values of43to52, and low Cr and Ni contents (8.07–33.6ppm and4.41–14.2ppm,respectively), indicating they differentiated from primitive mantle magmas. They have low Nb/U, Ta/U,and Ce/Pb ratios (1.3–9.6,0.2–0.8, and0.1–18.1, respectively), and their initial Hf isotopic ratios rangefrom+9.6to+16.9(BSHN diorite）and+6.5to+12.6(BSBDB gabbro–diorite), suggesting their primarymagmas were derived mainly from the partial melting of a mantle wedge that had been metasomatized bysubduction fluids. Taking all the new data together, we conclude that the western and eastern segment of the Jinshajiang suture regions underwent identical processes of evolution in the Early Permian–LateTriassic: oceanic crust subduction before the Early Permian, continental collision during the Early–MiddleTriassic, and post-collisional extension from the Late Triassic.Zircons from the Cenozoic magmatic rocks of the Tuotuohe region, with high Th/U ratios, have agesobtained using LA–ICP–MS of39.6Ma,37.6Ma,34.9Ma, and32Ma, which indicate magmatism in theEocene–Oligocene. Rock geochemical analysis results show that they formed under high pressureenvironment. Combined Cenozoic tectonic setting of the study area, we believe that they are closely relatedto the lower crustal thickening. The Nariliya (NRNY）Trachyandesite-trachyte was a product of thedelaminated lower crustal melting and suffered by mantle. Asthenosphere upwelling and lithospheredelamination were the main causes of the NRNY orthophyre, Zhalaxiageyong (ZLXGY）crumbs tuff, andZhamuqu orthophyre magma activity. K-rich source rocks of the lower crust are the main cause of thesealkali-rich rocks. These data indicate that the Tuotuohe region has been in continuous thickening of thestate in40~32Ma and the main magmatic activity was lower crust recycling.Based on the field work and laboratory study, we are approaching a new understanding of the genesistypes and characteristics of deposits in studying the area:(1）The Duocaima, Kongjie, Chuduoqu, Quemocuo and Labaozhalong lead and zinc deposits wereepithermal vein-type. These lead-zinc deposits were controlled by steeply dipping faults instead layerstructure controlled by the previous. The results of Pb isotopes indicated that major metallogenetic matterscame from the Cenozoic igneous, and not from the formations. Sulfur, hydrogen and oxygen mainly camefrom magma, showing a strong and deep magma source property. The ore-forming fluid is low temperature,low salinity and low density, and atmospheric water added in later. Steeply dipping fracture, thermal dome(alkali-rich magmatic）and lithology (carbonates）were the main ore-control factors. Comprehensive studiessuggest that the type of the deposits was epithermal vein-type deposits, rather than MVT lead-zinc.(2）According to the high temperature,low salinity, low density characteristics of ore-formingfluid,and the planar alteration and veinlet disseminated mineralization characteristics,we determined thatthe NRNY and ZLXGY Pb-Zn deposits were Cenozoic porphyry type Pb-Zn deposits. Cenozoic Alkali richporphyry and volcanic rock, which originated from thickened lower crust, provided ore-forming materialand heat source. Origin of Cenozoic Alkali rich porphyry rock and volcano provided ore-forming materialand low temperature heat source; heat them and hydrothermal vein type lead zinc ore is the product of thesame material, the same source of metallogenic heat source, different rock conditions and depth. Theporphyry and hydrothermal vein type lead zinc deposits have the same metallogenetic matters and heatsource, and different rock conditions and metallogenic depth.(3)We found a new type of lead-zinc deposit in this work–Bashihubei (BSHN）lead-zinc deposit,called ‘veinlet type located at the top of diorite’. The fluid characteristics of the ore deposit arecharacterized by high temperature, low salinity. And the mineralization type is similar to the porphyrydeposits, but metallogenetic rock is plutonic rocks and mineralization metallogenic greater depth. Themetallogenic epoch of BSHN lead-zinc deposit was281Ma or slightly later, which is metallogenicrepresentative during Late Paleozoic within the study area. (4）Eighty-eight station (BSBDB）and Guocanglema-Zonglongba (GCLM）lead-zinc deposits arelocated in Tuotuohe valley terrace, and the host rock is late Pleistocene gravel. The galena has two kinds ofdeposition form in this kind of deposit: one have the complete crystal form, which was product ofsynsedimentary together with sand and gravel, which is featured by proximity of its origin and rapiddeposition. And the others have complicated form of ’mosaic’ between the contact gap of gravel, which isfeatured by the slightly further distance. Based on a synthetical analysis of metallogenic characteristics, thedeposits are placer lead as the product of plateau fast differential uplift and denudation in the aridenvironment of weathering. The metallogenic materials of which may be derived from the upper orebody ofporphyry Pb-Zn deposits in the north of Tuotuohe River.With a basic analysis on the metallogenic geologic settings, the genetic types and geological featuresof each type of Pb-Zn deposits in Tuotuohe region are studied. The Carbonate formations in the study areaprovide a good environment and ore in space for the formation of hydrothermal vein-type lead-zincmineralization, and the volcanic strata bred to create the conditions for porphyry lead-zinc deposits. Theregional fracture obviously controlled the mineralization of Tuotuohe region, and these faults are not onlythe guided mine construction, but also the host structures. Late Paleozoic-Mesozoic and early late Eocene-Early Oligocene magmatic activities in the region provided the material source, ore-forming fluids andheat for forming the lead and zinc mineralization.This paper summarized the law of the regional lead-zinc mineralization distribution in time and spatial,and the storage conditions and erosion depth were analyzed and summarized; and analyzed the preservationconditions of the Pb-Zn deposits in Tuotuohe region. According to the deposit types, mineralizationcharacteristics and spatial distribution, We divided into four mineralization zones in Tuotuohe region fromnorth to south: Zhamuqu-Zhalaxiageyong Porphyry Cu-Pb-Zn polymetallic ore belt in the north shore ofthe Tuotuohe river, Guochanglema-Bashibadaoban placer Pb ore belt, Nabaozhalong-Kongjie Porphyry andhydrothermal vein type Pb-Zn-Ag polymetallic ore belt with and Chuduoqu-Rilongma Pb-Zn–Ag-W-Snpolymetallic ore belt. Finally, the paper presents further prospecting direction of the study area.