Genesis of the Baoshujian Cu-polymetal deposit in Anqing-Guichi ore-cluster area: Constraints from petrogeochemistry and geochronology

Abstract

The Middle-Lower Yangtze River Valley Metallogenic Belt（ MLYB） is one of the most important metallogenic belts in China. It has long been a hot area for research. The Baoshujian Cu-polymetal deposit,as a newly discovered skarn-type deposit,is located in Anqing-Guichi ore-cluster area,Middle-Lower Yangtze River Metallogenic Belt. It has not been taken research. This paper focuses on petrogeochemistry and geochronology to explain the geneses of igneous intrusion and related Cu-Au polymetallic deposit.Results of the zircon U-Pb geochronology show that the Baoshujian intrusion was formed at ca. 148. 6 ± 3. 2 Ma,which is one of the earliest intermediate-acid ore-bearing intrusions in MLYB. The further study of this metallogenic intrusion and ore deposit will provide a rare example for magmatic activity and mineralization in Yanshanian Period in MLYB. Sulfides of ores in Baoshujian are mainly composed of galena,sphalerite,chalcopyrite and pyrite respectively. Non-metallic minerals include diopside,garnet,tremolite,quartz,albite,calcite,chlorite and epidote. The regional igneous intrusions are mainly composed of diorite and diorite porphyrite.Chemical affinity of the Baoshujian intrusion has high total K2O + Na2O contents,low Al2O3,TiO2,Fe2OT3,P2O5 contents,with enrichments of large ion lithophile elements（ U,Rb,Sr）,high field strength elements（ Nb,Ta,Ti） and light rare earths,and depletion in heavy rare earths. The intrusion has high La/Yb and Sr/Y ratios with low Yb and Y contents,belonging to adakitic affinity. All REE distribution patterns of zircon show a typical light rare earth depletion and heavy rare earth enrichment. The zircon Ti thermometer indicates that the average temperature of rock formation was 674. 3℃. The Sr-Nd-Pb isotopic characteristics of the whole rock indicate that intrusions originated from partial melting of the subducting oceanic crust,related to the far stress field of the PaleoPacific plate subduction. The δ34S values pyrite in ore range from 7. 6‰ ～ 9. 6‰,indicating that the ore-forming materials are derived from magmatic hydrothermal fluids mixed with strata. Their interaction led to the formation of minerals. The Eu anomaly characteristics and Ce4+/Ce3+ ratios of zircon indicate that the magma forming rock had high oxygen fugacity,which was beneficial to the migration of ore-forming materials such as Cu,Au and Zn,and eventually form the deposit. Conjectured from regional geological background,we suggest that the genesis of the Baoshujian intrusion was formed during the subduction of the Pacific plate,resulting in the enriched Cupolymetal adakitic magmas. The hydrothermal fluids produced by the adakitic magmas through differentiation had metasomatism with the overlying Cambrian limestone in the shallow crust,finally formed this typical Cu-polymetal skarn deposit.