Geochemical Studies On Yanshan Magmatic Rock And Fe(Cu-Au)Deposit In The Southeastern Of NCC

Xu Huai area is located in the southeast of the North China Craton,which it is also located in the west of Tan-Lu fault zone.The fault and fold structures are development,and the related deposits are widely distributed.The deposit is mainly distributed on arc direction along the NE fault distribution,within the development zone of iron,copper,gold,etc.,have proven more than 20 deposits.All types of deposits in Xu-Huai area have related to Yanshanian intrusive rocks,which is recognized as adakite with the adakitic characteristics like high Sr low Y,etc.,This thesis study on the classic Fe-Cu-Au deposit(Liguo deposit)and the adakitic Mesozoic intrusive rocks(Liguo rocks)nearby Xu-Huai area of eastern China region with systemic geochemical analysis,including field observation and sampling,mineral flakes microscope,the whole rock major and trace elements composition analysis,LA-ICP-MS zircon U-Pb dating,apatite EPMA,LA-ICP-MS trace elements composition analysis and S isotopic composition analysis to investigate the petrogenesis,genesis of mineral deposit and geological significance.The intrusive rocks samples are taken from the ore district related with regional Fe-Cu-Au deposits.The lithology results show that Liguo rock is porphyry diorite,belong to calc-alkaline-high-K calc-alkaline series,with the feature of peraluminous.Most samples have high SiO2,high Na low K,with K2O/Na2O is 0.46?0.89,and high Mg#(average 62)features.The primitive mantle normalized spider diagrams indicate that the diorite porphyry are characterized by enrichments in large ion lithophile elements(Rb,Ba,Sr)and depletions in high field strength elements(HFSE;Nb,Ta,P,Zr,Ti),The chondrite normalized REE patterns for the diorite porphyry samples are right type,exhibiting a high degree of fractionation between LREEs and HREEs,with ratios of LREE/HREE and LaN/YbN ranging from 6.47 to 9.55 and from 5.78 to 12.51,respectively.There have no obvious negative Eu anomaly-positive Eu anomalies,with Eu/Eu*=0.79?1.08.Geochemical characteristics of samples are same with typical adakites,combining with the high SiO2,indicating High silicon adakites(HSA).Liguo high silica adakites have high MgOcontents,Mg#,Sr contents and Sr/Y ratios,low(La/Yb)N,decoupling Sr/Y and(La/Yb)N,exhibiting characteristics of slab melt,combining with the feature of Sr-Nd further support the fact that they are derived from partial melting of subducted oceanic crust minor overlying sediments.Zircon U-Pb geochronology shows the result of age of Liguo rock as:16LG01,131.4 ± 1.5 Ma(MSWD=0.35,n=12),16LG02,130.8 ± 1.8 Ma(MSWD=0.63,n=13),and coincide with the massive magmatism in eastern China at the late Early Cretaceous.The analysis of the trace elements of apatite shows that it was formed by magmatic differentiation,the feature of REE indicates that the dioritic magma was metasomatized by mantle-derived fluids.The high ?Eu(average 0.45)also means the process of evolution of magma is under relatively open structure environment and high oxygen fugacity condition.The sulfur isotope compositions of pyrite and chalcopyrite exhibite the typical distribution of tower,indicating relatively homogenous sulfur source with the forming of ore minerals.The low 834Sv-CDT(1.1‰)show that the sulfur of ore minerals in the deposit should be from magmatic hydrothermal system.Their Eu/Eu*values and high Ce(IV)/Ce(?)ratios indicate that the oxygen fugacities of the magma are high,thus favor for Cu-Au mineralization.Petrogenesis and mineralization mechanism of Yanshan magmatic rock and related Fe-Cu-Au deposits could be explained by ridge subductions.The oblique subduction of the paleo-Pacific plate along the East China in Mesozoic may support the point,which may be associated with subduction of oceanic ridge.At 130Ma,ocean subducted to Xu-Huai area.Magma formed by partial melting of oceanic crust should have relatively high copper content,forming adakitic magmatic rocks rich in iron,copper and gold.Meanwhile,Subducted oceanic slab with high oxygen fugacity can additional sulfur in the form of sulfate during partial melting,liberating more chalcophile elements such as Cu and Au,be further enriched for mineralization.