Fluid-rock Interaction Of The Early Cambrian Black Shale In Yangshui Anticline,Guizhou Province:Low-temperature Metallogenic Implications

The southwest Yangtze block is characterized by large-scale low-temperature mineralization?LLTM?.The Yangshui anticline in the southwest Yangtze block is located at the junction of three ore-concentrated regions of the LLTM,i.e.,the Pb-Zn-Ag ore-concentrated region transboundary of Sichuan,Yunnan and Guizhou provinces,the Au-Sb-As-Hg ore-concentrated region in Youjiang basin,and the Sb-Au ore-concentrated region in central Hunan Basin.The Yangshui anticline strikes NEN with a series of related faults and is characterized by short spindle shape with nearly vertical axial plane and the well-outcrop lower Cambrian black shale?Niutitang Formation?in the east limb.The Lower Cambrian Niutitang Formation black shale has very rich low-temperature ore-forming elements,especially for that of Au?As?Ag and Sb after analyzing each wall rock in the South China Block.By understanding the content of ore-forming elements in each wall rock and the ability of hydrotherm to leach the metallogenic elements,it is greatly important for reflecting the contribution of each strata to the source of the low-temperature ore-forming materials in the South China Block.Based on this theory,we have carried out a series of experimentsaboutfluid-rock interaction on the black shale and meanwhile studied the occurrence of ore-forming elements by EPMA.We conducted the experiments on the basis of pre-existing geochemical data with high-pressure resistant reactors to simulate the leaching process of black shale in the closed system.The experiments have been conducted in 12 control groups with a single temperature of 150?,concentration of 5%,durations of 15 days or 30 days,fluid-rock mass ratio?W/R?of about 13:1 or 26:1.The main medias in hydrotherm include NaCl,NaHCO₃ and Na₂S that respectively representthe Cl^-,HCO₃^-and HS^-(S^2-+H⁺=HS^-),which are the most common anion ligands in hydrotherm and capable to complex with certain cations of ore-forming elements.The main results are as follows:1.The results of EPMA analysis show:the enrichment coefficient of Au Fine-grained pyrite and Soot-like pyrite is the highest,reaching to 0.05%?wt.%?,while it is the lowest in the Strawberry-like pyrite,accounting for 0.013%,but it is still far higher than that in whole rock,and so is other elements.It is implied that the black rock is the main rock including Au,As,Ag and Sb.2.As for Au,the hydrotherm containing Cl^-are more efficient than that in the hydrotherm containing HS^-and HCO₃^-,and the leaching efficiency of Au is negligible in the hydrotherm containing HCO₃^-compared with the other kinds of hydrotherm.Similar to Au,Ag also shows the highest leaching rate in the hydrotherm containing Cl^-.Possibly because different elements have different chemical properties,the leaching effect of As and Sb is not obvious.