An Isotopic(Sr,Nd and Pb) Tracer Study on the Xiaoxinancha Gold-rich Copper Deposit in Yanbian,China:Implication for the Geodynamic Model of Diagenesis and Metallogenesis

An isotopic study was systemically carried out on the granitic complex,diorite-porphyrite, ores and ore minerals of the 103 Ma Xiaoxinancha gold-rich copper deposit in Jilin province to determine the geodynamic model of diagenesis and metallogenesis.Results show that the initial Nd and Sr isotopic compositions of the granitic complex are in the range of 0.70425-0.70505 for（87Sr/86Sr）i, 0.51243-0.51264 for INd,and -1.31 to ＋2.64 forεNd（t）;those of the diorite-porphyrite are in the range from 0.70438-0.70448 for（87Sr/86Sr）,,0.51259-0.51261 for INd,and ＋1.56 to ＋2.09 forεNd（t）.For ores and sulfides,the（87Sr/86Sr）i,7Nd,andεNd（t） values are in the range from 0.70440-0.70805,0.51259- 0.51279 and ＋1.72 to ＋5.56,respectively.The Pb isotopic ratios of the granitic complex range from 18.2992-18.6636 for 206Pb/204Pb,from 15.5343-15.5660 for 207Pb/204Pb,and from 38.1640-38.5657 for 208Pb/204Pb For diorite-porphyrite,the isotopic ratios of 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb are 18.3919,15.5794 and 38.3566,respectively,whereas those of the ores and ore sulfides vary from 18.2275-18.3770 for 206Pb/204Pb,from 15.5555-15.5934 for 207Pb/204Pb and from 38.1318-38.3131 for 208Pb/204Pb.The results indicate that the mineralization was correlated to the formation and evolution of the granitic complex and the diorite-porphyrite.Combining with the reported data in petrologic characteristics,elemental geochemistry and chronology,conclusions can be drawn that the geodynamic settings of diagenesis and metallogenesis of this deposit were consistent with the subduction of the Izanagi oceanic plate during the Early Cretaceous.The diorite-porphyrite was formed by the emplacement of the adakitic magma triggered by partial melting of the enriched mantle,which originated from the derivative continental lithospheric mantle metasomatized by dehydration fluids from the subducting oceanic crust.The granitic complex was produced by fractional crystallization of the mixture between the adakitic magma and the high-K calc-alkaline acidic magma,which were generated by the remelting of the lower crust in the course of intraplate upwelling of the adakitic magma.The ore-bearing fluid reservoir convened in a late stage of the evolution of the mixed magma chamber.

Red Clay Type Gold Deposits in China

Red clay type gold deposits,located in the south of China,are situated not only in orogenic belts,but also in inner cratons,where climate is tropical-subtropical with clear arid and humid.The lateritic weathering crust often can be divided into five zones,including topsoil,siliceous duricrust zone,multi-color zone(or red clay zone in some deposits),pallid zone and saprolite zone from surface to the base rock,several of which are absent in some deposits.The base rocks are composed mainly of carbonate rocks with minor clastic rocks,intermediate-basic volcanic rocks and intermediate-acid and alkalic intrusions.The orebodies are mainly located in the multi-color zone with part of them in the pallid and saprolite zones.The ore sources include orebodies of Carlin-type gold deposits and porphyry gold deposits,as well as gold-rich base rocks.The red clay type gold deposits experienced early-stage endogenic gold mineralization and laterization during the Tertiary and Quaternary.The areas with endogenic gold deposits,especially Carlin-type gold deposits and porphyry gold deposits in karst depressions on the plateau,structual erosional platforms in the middle-lower mountains,and intermountain basins in southern China are well worth studying to trace red clay type gold deposits.