Genesis And Geodynamical Setting Of The Chaoshan Gold Skarn Deposit Tongling Cu-Au-Mo District, Anhui Province.

The Chaoshan gold skarn deposit is situated in the Shizishan ore field, the center ofTongling district Anhui province, which is the only one gold deposit of the well-known middleand lower Yangtze River. Regional strata host from the Silurian to the Quaternary exceptthe Devonian lower-medium series and the Carboniferous lower series. Thestratigraphy in this district is dominated by the Carboniferous-Triassicmarinecarbonate carbonate rock, in which Permian period-Triassic period limestone,silicalites, shales interbeding half deep sea-thalassianic terrestrialis alternate clasticrocks are main ore-beating strata. It is marked by widespread distribution of Late Mesozoicintrusive rocks are intermidiate -basic to intermediate-acid rocks, which are dominated bypyroxene diorite, quartz diorite, granodiorite and granite related with the most copper-golddeposits. However, many intrusives characteristic complexes composing of the intermediate-acidrocks and the basic rocks. Preindo Chinese epoch basement faults and Yanshanian NE directionsigmoid cover drape folds compound and constitute "solid network" structure trellis controllingthe distribution of the ore fields and ore deposits. There are more than one hundred mineraloccureences including Cu, Au, Mo, S etc. The intact deposit compages are dominated by skarndeposits and stratabound deposits are the dominant type of mineralization, although porphyrydeposits and hydrothermal lode deposits are also developed.The Chaoshan deposit, small scale, is typical reduced gold skam deposit with high oregrade from 6 t 38 g/t Au, average of 18.4 g/t. Triassic Nanlinghu and Fenshuiling Formationlimestone, mables interbeding minor mudstone or chert bandings are import wall rock or andhost of skarn and stratabound mineralization. The utmost important ore deposits/bodiescontrolling structures are almost east-west, south-north, north -north west folds and interbeddedfracture zone and interbedded faults. The ordinate intrusives are granitic diorite and diorite, theminor are proxene dioriteand the later dike rocks. Ore bodies lie in the east contract between theBaimangshan intermidiate-basic complex and the Nanlinghu limestone and its interbeddedcrannies, Main ones controlled by the strutures are commonly lenticular and podiform in shapeand thicken at the congruence of the structures. Ore minerals are pyrrhotite, arsenopyrite andpyrite, with minor amounts of chalcopyrite, galena, sphalerite, tetrahedrite and bismuthinite etc, in chich pyrrhotite is the most Au-bearing mineral. Gangue minerals are calcite, quartz, chlorite andgart etc. Wall alteration develop skarnlization, k-felspathic alteration, biotite, alterated quartzite,carbonate and in place occur epidote, actinolite, chlorite and sericite etc.Baimangshan complex hosting abundant hornblende and proxene accumulatingcrystal xenolits is dominated by pyroxene diorite with minor quartz diorite belonging tothe pre-aluminum, high potassium calc-alkaline rock. Besides,i t is enriched in large ionlithosphere elements (Rb, Ba, K, Sr) and LREE, but strongly depleted in high field strengthelements (Nb, Ta, Ti). All samples have high Sr, Ba/Thå’ŒBa/La, they are 1007-1100, 64.65-74.76 and 13.61-15.56 respectively, on the contrary, they have low Rb/Sr ratio (0.068-0.09) andSm/Nd ratio(0.18-0.20). Rock has high and even Sr-Nd composition (⁸⁷Sr/⁸⁶Sr)_i=0.7065-0.7074,(¹⁴³Nd/¹⁴⁴Nd)_i=0.5121, and the calculatedε_Na(t)=-7.78--8.01. The geochemistric and isotopiccharacteristics are indistinguishable from the cretaceous basalt and basic rocks origining from theenriched geosphere mantle in from the middle and lower reaches of Yangtze River. All thatmentioned above indicates that primary magma of the Baimangshan complex maybe source fromthe enriched geosphere mantle (EMâ… ). The enriched Lithosphere mantle partialy melted as thetemperature descended, and formed the high potassium(K) calc-alkaline basaltic magma, and thenunderplated and melt the lower crust and came into being the crust-mantle admixing(â… ) magma.These magma represented distinct AFC action with the middle-lower crust during the upliftingprocess resulting in the hornblende and proxene crystal xenolits accumulated at the differentdepth. Under the extending constrcture setting, the evolved intermediate-basaltic magma ascendand formed the Baimangshan proxene diorite at the 2.5-3.2 Km under the earth's surface due tocooling.The Ar-Ar dating of hornblende from the Baimangshan proxene diorite indicate thatmagma intruded at the 142-138 Ma approximately, otherwise, the Re-Os dating of pyrrhotite showthat the Chaoshan gold deposit formed at the 141.7Ma. It suggests that magmasim is consistentwith the mineralization and also prove the genesic intimate relationship between the Baimangshanproxene diorite and Chaoshan gold deposit.The H-O-C-S-Si-Pb-Os isotope composition of the ore minerals and Gangue mineralsindicates that the metallogeny matter derive from the Baimangshan proxene diorite, and also showthat the metallogeny of the Chaoshan gold skarn deposit is related with the later Yanshanianmagmatism intimately. The ore-bearing fluid is dominated by the magmatic water in the early, butatmosphere water increases in the later stage. Deposit is the result from the compositiveinteractions between strata, structure and magma-thermal liquid system. The collapse sections andinterbedded crannies of strata are the favorable sites for metallogeny.The Baimangshan complexprovides matter and heat energy. And that, structures assemble all kinds of metallogeny factors,they are the channels for magma emplacement and fluid transfer, and are also important factorcontrolling the rock and ore deposit (ore bodies).Recent high precise isotope chronology studies (such as Zircon SHRIMP U-Pb, Re-Os ofmolybdenite, Ar-Ar of single mineral) reveal that this district developed extensiveintermidiate-basic to intermediate-acid magma at the 143-136 Ma of the end of the later Jurassic. Meanwhile, The most deposits have been well constrained 145-135 Ma and themetallogeny of this district focus on eight-eleven Ma interval, the coherence indicates that themetallgeny is related with the cretaceous magmatism intimately.The magmatism and mineralization ages of Tongling are not only almost coherent with themetallogeny ages (144-135Ma) of Southeastern Hubei province, and also are inosculate with thefirst large scale mineralization stage (145-133 Ma) of the Middle and lowerreches of Yangtze River.The coeval magmatism and mineralization recognized in distinct regions of the Mid-LowerYangtze River indicate that they had the same geodynamic setting.The middle and lower reaches Yangtze River Metallogenic Belt is coincident with at theYangtze River rift, where the crust is 2-3km thinner than its surroundings. This indicates thatsignificant asthenosphere upwelling and lithospheric thinning since the later Mesozoic. This riftalong several lithospheric fault (such as Yangtze River deep fault, Changzhou-Yangxin fault)wasformed by upwelling of hot asthenosphere and partial melting of lithosphere. Large-scalelithospheric extension and magma underplating of the Early Cretaceous time could have been thegeodynamic setting of Chaoshan gold deposit and the middle and lower reaches Yangtze RiverMetallogenic Belt. It provide vast heat, matter, fluids and important structures and is responsiblefor the voluminous magrnatism and large-scale mineralization. The setting is coincident with thefact that the whole east of China has been extending since the later Mesozoic, and it is mayberelated with the ancient Pacific Ocean Plate dived to Eurasia obliquely.