Tethys Tectonic Evolution And Metallogenesis Of Important Mineral Deposits In The Sanjiang Region, SW China

The Sanjiang region in SE Tibet Plateau and NW Yunnan is known to haveformed by amalgamation of Gandwana-derived continental blocks and arc terranes asa result of oceanic subduction in the Paleozoic. During Cenozoic, intense intraplatedeformation, episodic magmatism and diverse mineralization occurred overprintingthe Pre-Cenozoic tectonic framework of the region.The paper report zircon U-Pb dating, Hf isotopic compostion and bulk chemistryof the granite porphyry and quartz porphyry from the Zhenyuan area in the northAilaoshan suture zone. The results show that these granitic rocks having206Pb/238Uweighted mean age of255-248Ma have intruded under the post-collisional extensivesetting after the closure of Ailaoshan Paleo-Tethys ocean. It was summarized that thetransition from subduction-related to collision-related magmatisms along theAilaoshan suture was at~260Ma. The exact location of the Ailaoshan suture betweenthese Simao and Yangtze continental blocks has been contested. The age distributionpattern of detrital and inherited-xenocrystic zircons from the Zhenyuan-Mojiangsuspect terrane is similar to that of detrital zircons from the Simao block but differentfrom that of those from the Jianshui area in the western margin of the Yangtze block.This indicates that the Zhenyuan-Mojiang suspect terrane belongs to the Simao block.The actual location of the suture between the Simao and Yangtze blocks is suggestedto superimpose the Ailaoshan late-Devonian to early-Carboniferous ophiolite beltwithin the suspect terrane.The age distribution pattern of detrital zircons from the continental blocks in theSanjiang region suggested that the South China, Simao (north of the Indochina),Western Qiangtang and Eastern Qiangtang blocks located along the Indian Gondwanamargin at Early Paleozoic, whereas the Lhasa and Tengchong-Baoshan (north of theSibumasu) located along the Australian margin.The Laochang ore deposit was a Pb-Zn-Ag-Cu volcanic-sediments hostedmassive sulfide (VHMS) deposit formed in the Late Mississippian (~320Ma) oceanic island. The deposit geology, sedimentary facies and fossil records evidenced that themassive sulfides accumulated on the sea floor and the ore-forming fluids formedtheir own local anoxic environment in the oxic bottom water conditions. The localanoxic conditions together with fluid inclusions and S-Pb and H-O-C isotopic dataare more supportive to the magmatic degassing model. The hosted volcanic rocks(OIB) are sourced from an enriched mantle, which was mingled with subductedupper continental crust component. This process possibly occurred during theshrinking of the Proto-Tethys ocean in Late Cambrian-Silurian or the possible initialsubduction of Paleo-Tethys oceanic plate in latest of Early Carboniferous. Byconsideration of the Zhenyuan Au deposit, the S and Pb isotopic compositionsobtained in this paper, together with the analysis on the previously publishedH-O-He-Ar isotopic data of the auriferous fluid, support that the ore fluid is mainlymetamorphic fluid with subordinate mantle fluid. The mineralization age wasindicated by the previous Ar-Ar isochrone age~27Ma of phlogopite in themineralized lamprophyre, which implies the South China block subducted westwardat Oligocene. The continental subduction induced the release gold-chargedmetamorphic fluid and facilitated the shearing of the Ailaoshan fault.In the Sanjiang region, the Paleo-Tethys comprised a main ocean and threebranches, Ailaoshan, Jinshajiang and Garzê–Litang. They were bounded byGondwana-derived continental blocks and were consumed by oceanic subductionfrom Earliest Permian to late Triassic. Hereafter, the evolution of the Baoshan andTengchong blocks was largely influenced by eastward oceanic subduction of theMeso-and Neo-Tethys from late-Permian to middle-Cretaceous and fromlate-Cretaceous to~50Ma, respectively. From Late Paleozoic, various continentalblocks separated and northwards migrated from Gondwana occurred in three periodslinked with the successive opening and closure of three intercalated Tethyan oceans,the Palaeo-Tethys (Devonian to Triassic), Meso-Tethys (Earliest Permian to LateCretaceous) and Ceno-Tethys (Late Triassic to Paleogene). The oceanic subductionwas coupled with the birth of a new ocean. This implies that oceanic subduction wasthe main driving force of geodynamics in the Sanjiang Tethys region. The four main phases for the Cenozoic evolution of the Sanjiang region arefirstly proposed:(1) Subduction of Neo-Tethys oceanic plate caused lithosphereshortening, and emplacement of magmatic intrusions in Tengchong block andCu-dominated ore deposits within Lanping basins resulting from the India-Eurasiasoft collision (>45Ma);(2) Intra-plate mafic rocks in the Thengchong block inducedby the breakoff of Neotethys oceanic plate and potassic magmatic rocks along theJinshajiang-Ailaoshan suture by the delamination (45-32Ma);(3) South china platedetachment-underthrusting and Simao-Indochina block extrusion due to thesustained compression from the India-Eurasian continental collision (31-13Ma);(4)Extensions and volcanic eruption induced by the oceanic slab subduction of Indianoceanic plates (12Ma-present).Many important ore deposits in the Sanjiang region are linked to the evolution ofthe Tethys oceans in Paleozoic–Mesozoic and the Indian-Asian continental collisionin Cenozoic. In this paper, the metallogenic system comprising the ore depositdistributings on time and space in the Sanjiang region is constructed based on theregional tectonic evolution. The transformation of tectonic regime plays a decisiverole in the formation of important deposits in the Sanjiang region. The multipleepisodes of metallogenesis located in the different time and tectonic backgroundoccurred in one orefield or ore deposit, causing various types of superposed deposits.These deposits can be categorized into three types, that is:(Type1) superposition ofmagmatic hydrothermal deposit on the previous VMS deposit;(Type2)superposition of magmatic hydrothermal deposit on the previous sedimentary deposit;(Type3) superposition of between multiple episodes of magmatic hydrothermaldeposit.