| The Dexing Cu deposit in Jiangxi,east China,is a giant porphyry-type deposit(8.32 Mt Cu)that is located in the interior of the South China Craton(SCC),made up of two lithospheric blocks,the Yangtze and Cathysia blocks.The deposit occurs near a Proterozoic suture zone between these blocks,and is associated with Middle Jurassic felsic porphyries that were intruded into Precambrian metamorphic rocks at the southeastern edge of the Yangtze block.The deposit is controlled by a series of lineaments,and it is worked in the Tongchang,Fujiawu,and Zhushahong mines,these three parts of the deposit being related to three high-level centers of coeval porphyries.The results of zircon U-Pb and molybdenite Re-Os dating indicate that the felsic magmatism was of short duration(172?170 Ma),and that the Cu mineralization formed at around 171 Ma,which suggests in turn that the mineralization was coeval with the crystallization of the felsic magma.The Dexing deposit,although it occurs in the interior of a continent,shows broad similarities with deposits in magmatic arc settings,especially with regard to the hydrothermal alteration,the styles of mineralization,the sulfide assemblages,and the ore-forming fluids.The main phases of the mineralized porphyries are porphyritic granodiorites,which usually contain abundant phenocrysts as well as coeval microgranular mafic enclaves(MME).These MMEs have quenched margins and felsic haloes,and they contain quartz ocelli in their matrixes,suggesting that the mafic melt was injected into the granodioritic magma chamber during the formation and evolution of the Cu-bearing felsic magmas.The granodioritic porphyries mainly belong to the high-K calc-alkaline series,and show geochemical affinities with adakites,but they are characterized by relatively high MgO,Cr,Ni,Th,and Th/Ce ratios.The least-altered porphyries have relatively uniform Nd isotopic compositions with ?Nd(t)(T=171 Ma)varying from-0.9 to +0.6,and a relatively wide range of(87Sr/86Sr)i values which range from 0.7045 to 0.7069,overlapping with the Sr-Nd isotopic compositions of the MMEs and the Middle Jurassic basalts and mafic dikes in the SCC.Zircons in these porphyries also have relatively uniform Hf and O isotopic compositions with ?Hf(t)values ranging from 3.4 to 6.8,and ?180 values in the range 4.66‰ to 6.34‰,close to those of depleted mantle.All of these data suggest that the granodioritic porphyries were derived by water-fluxed melting of a thickened juvenile mafic lower crust,and that the melts subsequently underwent mixing with mantle-derived mafic melts in the SCC.It is likely that the contemporaneous mafic melts were underplating the lower-crust,and were in part injected into the lower-crust-derived felsic melts,thus supplying heat and some fluids rich in H20,metals,and S,and playing an important role in generating the hydrous,high fO2,fertile porphyry magmas of Dexing.The magmatic-hydrothermal event at 171 ± 1 Ma in Dexing is contemporaneous with the Middle Jurassic extension in the SCC at which time the Gan-Hang rift zone formed between the Yangtze and Cathysia blocks,and when within-plate magmatism from 180 to 170 Ma formed a suite of OIB-like alkali basalts,tholeiites,and bimodal and A-type granites in the Cathysia block.This event was closely followed by 165?90 Ma arc-like magmatism in southeastern China,which formed a 600-km-wide belt of volcanic-intrusive complexes along the coast.With respect to the tectono-magmatic evolution of the SCC,the emplacement of Cu-bearing porphyries and the associated Cu mineralization occurred in response to the transformation from a tensional regime,related to Middle Jurassic extension,to a transpressional regime,related to the subsequent subduction of the paleo-Pacific oceanic lithosphere.This transformational regime resulted in the large-scale underplating of mafic melts at the bottom of the crust,and facilitated the development of long-lived,highly evolved magma chambers in the shallow crust. |
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