Spatial And Temporal Distribution And Genesis Of Copper Geochemical Blocks In The Middle-lower Yangtze River Region

The geochemical provinces and even larger geochemical distribution patterns that exist in the natural are actually reflections of geochemical blocks on the earth's surface.The formation of geochemical distribution patterns can be caused by regional high-background stratigraphy,intrusion of rock masses,addition of mantle materials,mineralization and later weathering and dispersion.The Middle-Lower Yangtze River region is an important place containing a huge amount of mineral resources such as copper,iron and gold in China.In this paper,the spatial distribution and genesis of copper geochemical blocks in this area were studied based on the geochemical data of stream sediments obtained in the Regional Geochemistry-National Reconnaissance Project(RGNR)and the geochemical data of rocks obtained in the China Geochemical Baselines(CGB).Also the temporal properties of copper geochemical blocks were studied using the LA-ICP-MS zircon U-Pb dating technique.The following conclusions were achieved:(1)Three factors were extracted by factor analysis of 21 log-transformed trace elements from RGNR.Factor 1(Nb?Zr?Th?La?Pb?Y?Zn?Ag?Hg?Ti?Sn)represents geological processes related to intermediate-acidic magmatic events.Factor 2(Cr?Ni?V?Co?Cu?Ti)represents geological processes associated with basic-ultrabasic magmatic activities,volcanic sedimentation,and Cu and Fe mineralization.Factor 3(As?W?Bi?Sb?Sn)represents medium-high temperature hydrothermal mineralization.The spatial distribution of Au is closely related to gold mineralization.(2)A total of six copper geochemical blocks(Cu1,Cu2,Cu3,Cu4,Cu5 and Cu6)were delineated with composite stream sediment geochemical data in the study area,namely:Daye-Jiujiang copper geochemical block(Cu1),Anqing-Tongling copper geochemical block(Cu2),Maanshan-Nanjing copper geochemical block(Cu3),Dongzhi-Jixi-Ningguo-Kaihua copper geochemical block(Cu4),Xiushui-Wuning copper geochemical block(Cu5)and Dexing copper geochemical block(Cu6).Four age peaks were identified in detrital zircon samples sampled in Cul,Cu2 and Cu6 geochemical blocks,corresponding to Paleoproterozoic,Neoproterozoic,Early Paleozoic,and Mesozoic,respectively.And we conclude that geochemical blocks exist across time,and substances in the range of geochemical blocks came from multiple geological events.Neoproterozoic tectonic-magmatic events had important influence on the formation of copper geochemical blocks.Mesozoic era is another important period for the formation of copper geochemical blocks in the study area.(3)The medians of Cu in stream sediments vary with that of rocks among different tectonic units in the study area.It shows that the content of Cu in stream sediments is constrained by the background rocks.The medians of Cu in stream sediments do not vary with that of rocks among different copper geochemical blocks,and the medians of Cu in rocks do not increase with the enhance of anomalous concentrations within each copper geochemical blocks.It shows that the Cu content in the stream sediments is not strictly controlled by the background rocks within copper geochemical blocks.(4)Copper anomalies in the range of Cul,Cu2,and Cu3 were irrelevant to the strata,which were mainly caused by mineralization and later weathering dispersion.Copper anomalies in the Cu4 range were mainly related to rocks with high Cu concentrations.Copper anomalies in the Cu5 and Cu6 range were caused by rocks with high Cu concentrations and mineralization and later weathering dispersion together.The areas of mantle upwelling control the distribution of copper geochemical blocks associated with large deposits/ore districts.The copper geochemical blocks are strictly controlled by the fault structures in the study area.Comparing the contents of elements in stream sediments and rocks in the range of geochemical blocks can help study the cause of the anomalies and can effectively identify the existence of mineralization.(5)All copper ore districts in the study area are located within the range of copper geochemical blocks.Of the 10 large-scale,25 medium-sized,and 43 small-sized copper deposits discovered in the study area,8 large-scale(80%),18 medium-sized(72%),and 32 small-sized(74%)copper deposits are located in the range of the copper geochemical blocks,respectively.Geochemical blocks delineated with low-density geochemical data of one composite value at each grid cell of 10km by 10km can effectively track the locations of ore deposit clusters or large ore deposits.There are huge copper geochemical blocks in the study area,which can provide large amount of metallogenic materials for the formation of the Middle-Lower Yangtze River metallogenic belt.Geochemical blocks with mineralization element endowment is an important metallogenic condition for the formation of ore districts and large deposits in the study area.(6)There are a large number of Neoproterozoic detrital zircons in the stream sediment samples,indicating that Neoproterozoic tectonic-magmatic events had important influence on the formation of copper geochemical blocks in the study area.The Neoproterozoic basic materials coming from the mantle resulted in a certain degree of enrichment of Cu.Mesozoic era is another important period for Cu accumulation in copper geochemical blocks.The huge amount of Cu was mainly derived from the enriched lithospheric mantle,and some of Cu was derived from crustal material.