Metallogeny Of The Late Mesozoic Porphyry Tin Deposits In South China

Being the most important tin producer in China,South China hosts abundant tin-multiple metallic ore mineral resources,most of which are associated with the Yanshanian granites.Ever since 1980s,many research works have been conducted on the ore geology,geochronology,and metallogeny of the tin-multiple metallic ore deposits in South China,and accumulated plenty of geological and geochemical data concerning the ore-bearing granites,ages of ore formation,and ore-forming processes Researches showed that the late Yanshanian is the primary period of tin mineralization,and all the large to superlarge tin deposits are located in the southwestern part of the Cathysia block.On the contrary,tin deposits of the late Yanshanian in the coastal area of Southeast China are commonly small in size and are much less well-documented As a result,metallogenic study and exploration of the late Yanshanian tin deposits in South China as a whole are greatly hampered by the paucity of the geological and geochemical data on the relatively small-sized tin deposits in the coastal area Porphyry tin deposits constitute only a small part of granite-related tin mineral resources,however,detailed investigation of the petrology of the tin-bearing granites and geology and geochemistry of the ores will shed light on the metallogeny of granite-related hydrothermal tin deposits due to their obvious spatial,temporal,and genetic relationship,and thus,unravel the myth of the unevenly distribution of tin resources in South China.Two of the three typical late Yanshanian porphyry tin deposits(Yanbei,Yangbin,and Yinyan)in the coastal area of Southeast China were investigated in this study.Even though some preliminary investigation have been carried out on the Yanbei and Yangbin porphyry tin deposits,the origin of the tin bearing granite porphyries,ages of the ore formation,and origin of the deposits are still controversy.To determine the ages of the magmatism and ore mineralization,reveal the origin of the tin deposits,establish the metallogenic models,and thus,guide future ore expectation,we carried out systematic field geological survey,and mineralogy,trace element and isotope geochemistry,and geochronology investigation of the granites and ores of the Yanbei and Yangbin deposits.Our SIMS zircon U-Pb dating results confirmed that the tin-bearing granite porphyries of the Yanbei and Yangbin deposits were emplaced during the early Cretaceous.In the Yanbei ore field,the pre-ore granodiorite porphyrite and dacite have zircon U-Pb ages of 138.56±0.83 Ma(MSWD=1.47)and 139.58±0.78 Ma(MSWD=0.32),respectively;while the tin-bearing granite porphyries have ages of 135.52±0.71 Ma(MSWD=2.03)to 136.45±0.69 Ma(MSWD=3.04).Whereas in the Yangbin deposit,the pre-ore granodiorite porphyrite and ignimbrite have zircon U-Pb ages of 187.08±1.40 Ma(MSWD=0.57)and 133±0.67 Ma(MSWD=0.23),respectively,but the tin-bearing granite porphyry has much younger age of 95.05±1.64 Ma(MSWD=5.63).It is noteworthy that inherited zircon grains with ages of ca.2.5 Ga have been detected in the granite porphyry,which may imply the existence of the Neoarchean basement or recycled Archean crust at the depth of the Cathysia block.Tin related granites were most likely derived from crustal melts,however,we argue that the isotope geochemical signatures such as the relatively higher Nd and lowr zircon ?18O valucs of the ore-bearing granites are result of isotope disequilibrium during crust anataxis,rather than input of mantle or juvenile crust which is a conventional interpretation in literature.The tin-bearing granite porphyry in the Yanbei deposit has higher Nd(t)(-2.14 to-3.39)and lower zircon ?18O(6.42‰to 7.71‰)than those of the pre-ore dacite(-8.77 to-8.88,and 8.13%o to 8.99‰,respectively).Similarly,the tin-bearing granite porphyry and topaz-quartz porphyry have Nd(t)and zircon ?18O values in ranges of-4.35 to-9.05 and 3.98 to 9.50‰,respectively,whereas the pre-ore ignimbrite in the Yangbin deposit has Nd(t)of-10.45 to-11.89 and zircon ?18O values of 4.47 to 5.86‰.Shifting of the isotope signatures like the enriched radiogenic Nd isotope is conventionally interpreted as input of mantle or juvenile crust,however,due to the lack of contemporaneous mantle rocks and geological evidence showing mixing or mingling of mantle and crustal melts,we argue that the isotope signatures are results of isotope disequilibrium during crustal anataxis.Major and trace element analyses showed that the tin-bearing granites are peraluminous to highly peraluminous,rich in Si and F,and highly differentiated.Tin-bearing granite porphyries in the Yanbei and Yangbin deposits have Nb/Ta and Zr/Hf in ranges of 6.0-10.7,20.2-20.7 and 3.73-12.6,6.75-14.6,respectively,suggestive of extreme differentiation.The apparent REE tetrad effects of the tin-bearing granite porphyry from the Yangbin deposit indicate extensive fluid exsolution and fluid immiscibility in the late stages of the granitic magma evolution.Fluid exsolution is also supported by the abundant inhomogeneous and daughter crystal bearing multiphase inclusions in quartz phenocryst in the topaz-quartz porphyrite from the Yangbin deposit.The massive late Yanshanian tin mineralization in South China is genetically associated with the highly fractionated granite magmas derived from extensive anataxis of the middle to lower crust triggered by upwelling asthenospheric mantle under extensional tectonic setting.It is believed that the roll back of the subducting paleo-Pacific slab in the early Cretaceous led to the lithospheric extension of the South China,and subsequent crustal thinning,upwelling of asthenospheric mantle,melting of the lithospheric mantle and underplating of basalt magmas,and the resultant anataxis of ore metal rich crustal rocks.It can be inferred that high degree differentiation of the crustal melts led to the formation of the highly fractionated granites which underwent fluid exsolution,fluid immiscibility,and the ore metal rich hydrothermal fluids precipitated the ore minerals like cassiterite to the granite-related hydrothermal tin deposits.