The Source Discrimination Of The Stream Sediments In The Zijinshan Orefield

Stream sediments represent the natural mixture of crustal materials in the corresponding catchment area,and record the important information about the formation and evolution of continental crust,and are the main medium for regional geochemical prospecting,material source tracing,and watershed geological event inversion.The age and isotopic composition determination of detrital accessory minerals in these stream sediments samples is an important method to study the formation and evolution of the crust in a specific area.Regional geochemical mapping in stream sediments shows that regional geochemical anomalies(or geochemical blocks)often have close spatial relationship with metallogenic provinces or ore districts,but the genetic relationship discussion between stream sediment anomalies and regional high background rocks or ore bodies is relatively weak.The Zijinshan is an important Orefield in our country,In this study,stream sediments were collected in and around the Zijinshan Orefield,and systematically carried out element geochemistry,detrital zircon U-Pb geochronology and Hf isotope compositions,electron probe and LA-ICP-MS analysis of Fe oxides,In order to ascertain the relationship between the formation age and provenance of the stream sediments and ore-bearing rocks in the Orefield,and to explore the enrichment mechanism of ore-forming elements in stream sediments.Most detrital zircon particles in stream sediments have obvious magma crystal zoning,and the Th/U ratio is greater than 0.3,most of them are magmatic origin zircons;a few zircons have a ratio Th/U<0.1,the zircons are flat and patchy,they are metamorphic zircons;and some zircons have a Th/U ratio between 0.1 and 0.3,they are round and irregular,indicating that these zircons have been transported for a certain distance.The detrital zircon U-Pb age and Hf isotopic composition of the sediments closest to the mining area can reflect the information of the magmatic rocks in the Orefield.However,as the stream sediments are farther from the Orefield,the detrital zircon U-Pb age signals of the magmatic rocks in the Orefield are rapidly weakened.The research results show that the total amount of rare earth elements in the stream sediments is reduced,the differentiation of light and heavy rare earth elements is weakened,the content of heavy rare earth elements is reduced,and the Eu negative anomaly is significant.The ore-forming elements and their associated elements are most significantly enriched in stream sediments near the Orefield.The degree of enrichment of ore-forming elements in stream sediments and the detrital zircon U-Pb age signals of the magmatic rocks in the Orefield are rapidly weakened as the stream sediments become farther from Orefield.This signal weakening may be affected by many factors,such as lithological resistance to weathering,vegetation coverage,micro-topography conditions,hydrodynamic conditions,etc.The content of ore-forming elements of iron oxides in stream sediments is extremely uneven,and the contents of ore-forming elements of most survey points are higher than the contents of corresponding elements in stream sediments.Cu,Mo and other ore-forming elements have a significant correlation with Al₂O₃.Therefore,iron oxides and clay minerals are the main carrier minerals for the migration of ore-forming elements.