Geochemical Characteristics Of Evaporites And Its Relation To Metallogenesis In Lanping Basin

The Lanping basin is the most important basic metal-rich area in C hina,and best known as the Jinding lead-zinc mine.It is also the new type of Pb-Zn-Ag-Cu polymetallic deposit that is considered to be the latest terrigenous superlarge sedimentary rock-hosted ore deposit in the age.Research o n its metallogenic theory,much attention by the outside world.A large number of evaporites are developed in the basin,and Pb-Zn-Ag-Cu polymetallic deposits are associated with each other.The first is the study of the origin of evaporites in the basin;the second is the exploration of the provenance of Pb-Zn-Ag-Cu polymetallic deposits in the basin;the third is the analysis of the geochemical characteristics of evaporites and the Pb-Zn-Ag-Cu polymetallic deposits comparative analysis of geochemical characteristics and explore the relationship between them.After research results show that:(1)The evaporites in the basin underwent the stable chemical deposition of the marine sulfate in the Triassic Sanhendong Formation,and then underwent compaction and overlying tectonic movement of the overlying salt so that they formed the bottom of the salt,the salt Mounds,salt punctures,salt beads,salt domes,salt anticlines and other salt structures.Gypsum salt rock is mainly developed with black gypsum,gypsum pattern,limestone breccia plaster,gray green,purple gypsum plaster salt rock salt and so on.They are mainly located in the vicinity of the fault zone,and the salt structure is around the Pb-Zn-Ag-Cu polymetallic ore body.The Jinding and Baiyang mines are the most typical ones.The ore bodies are located on the lens of the gypsum salt rock and alternate with lead-zinc ore.Cu-Ag polymetallic ore in Baiyang discovers a large number of purple,gray-green paste.hough no direct discovery of relations between evaporite and base metal deposits in Baiyangping,Wudichang,Fulongchang.But previous studies on the characteristics of sulfide isotopes in ore bodies show that evaporites provide sulfur sources for ore-forming materials.(2)The trace elements in gypsum and limestone(including limestone)in the basin are characterized by enrichment of U,Sr and depleted Nb,Zr elements,and the accumulation of Th,U,Nd,Loss of Pb,Nb,Sr and Zr.The sandstone is mainly characterized by enrichment of Ba,Sr and Sm ele ments and depletion of Nb and Zr elements,but distinctly different from gypsum and limestone(including limestone).Combined with the eigenvalue of trace elements,the average Sr / Ba values of trace elements in gypsum and limestone(including limestone)are 901.69 and 311.28,respectively,which are all greater than 1,formed in seawater brackish water environment,(0.94 and 0.14),both of which were less than 1.The average values of Sr / C u in gypsum and limestone(including limestone)were 700.96 and 730.47,respectively,Dry and hot climatic conditions,while the mean values of gypsum and sandstone are 13.67 and 8.42,which are formed in humid-dry climate.The average V / Cr values of gypsum and limestone(including limestone)are 4.13 and 5.36,respectively Is greater than 2.It is formed under anaerobic-anaerobic conditions,while the mean values of gypsum and sandstone are 1.86 and 1.75,both less than 2.The formation of gypsum and limestone(with limestone)V /(V + Cr)were 0.70 and 0.82,respectively,which were both higher than 0.6 and formed in hypoxic environment,while the average values of gypsum and sandstone were 0.65 and 0.64.Although they belonged to anoxic environments,there were no gypsum and limestone Limestone)to a high degree and the value is relatively close,but the formation of oxygen levels in the same water body The N i / Co average values of gypsum and limestone(including limestone)are 12.60 and 12.17,respectively,which are both greater than 7 and formed in the anoxic-anoxic environment,whereas the average values of gypsum and sandstone are 2.45 and 1.7,both less than 5,formed in the oxidizing environment;gypsum and limestone(including limestone)U / Th average 10.63 and 9.34,respectively,were greater than 1.25,formed in anoxic environments,and the average paste and sandstone was 0.23 and 0.25,both less than 0.75 and formed in an oxidizing environment.The average values of ?U for gypsum and limestone(including limestone)are 1.70 and 1.77,respectively,which are both larger than 1,which are normal seawater depositional environments,whereas the average value of gypsum and sandstone is 0.82 and 0.86,both less than 1,for the deposition of oxygen-deficient environment.These geochemical characteristics and indicators reveal that the gypsum and limestone(including limestone)in the basin are formed in the dry and hot marine salty water anoxic environment,while the gypsum and sandstone are formed in the warm and wet freshwater-brackish water phase for oxidation Environment,so that the gypsum and limestone(including limestone)with the same sedimentary environment,the characteristics of similar genesis,so gypsum rock should form the Triassic Triassic cave group.(3)The standard curve of rare earth chondrite meteorite in evaporite shows that the distribution pattern of rare earth elements in gypsum and limestone is consistent with the pattern of right-angled "V" with normal Ce and negative Eu anomalies.The distribution curves are similar.Although the pattern features of Eu and Ce and right-angled "V" are similar to those of gypsum and limestone(including limestone),the pattern of the model curves is very neat and shows the unity of provenance.Limestone(including gypsum rock)shows extremely complex features and the source of the material has a multi-source.(4)The S isotopes of the evaporites in the basin mainly consist of heavy sulfur(+9.6‰~17‰),and the Sr isotopes are mainly concentrated in the range of 0.707695~0.708629,indicating that the salt-forming fluid of evaporites in the basin comes from the Triassic ancient The appearance of light sulfur in the ocean and evaporites is the result of the reduction by the action of organic matter and organisms.The sulfur isotopes of S isotopes occur in Pb-Zn-Ag-Cu polymetallic deposits in the basin,which is consistent with the characteristics of S isotopes in the evaporites,indicating that the thick evaporites in the basin are the main sulfur sources.(5)Compared with the carbon and oxygen isotopes of the ore-forming fluids of the Pb-Zn-Ag-Cu polymetallic deposit in the basin and the limestone,the mean carbon and oxygen contents of limestone are-11.56‰ and-15.02 ‰,respectively;The average values of carbon and oxygen isotopes were-9.81‰ and 22.05‰,respectively.The average carbon and oxygen isotopes of Baiyangping were-1.23‰ and 15.45‰,respectively.It is revealed that the source of ore-forming fluid has the characteristics of multi-source fluid of atmospheric precipitation,metamorphic water and deep fluid in the basin.(6)Pb isotopes and S isotopes of sulfide ores and surrounding rocks in the Pb-Zn-Ag-Cu polymetallic deposits in the basin: The Pb isotopes of the ore are mainly characterized by mixed lead in the crustal orogenic rocks and sedimentary rock s;Pb and Zn deposits in Baiyang Factory are mainly light sulfur with a large amount of organic matter.Fulong,Wuji and Baiyangping are mainly heavy-sulfur,while pyrite is enriched in light sulfur.Compared with the S isotopic characteristics of evaporit es,it is indicated that the S sources of metal sulphides in the basin are dominated by organic matter,bioreduced sulphate and deep-seated fluids.In summary,the giant evaporites developed in the basin of Lanping Basin were formed the anaerobic hypoxia-reduction of the dry seawater in the Triassic,and experienced complicated tectonic movements to form various salt structures.These evaporites provide abundant sulfur sources for the Pb-Zn-Ag-Cu polymetallic deposits and provide a large amount of metal cations such as Pb-Zn-Ag-Cu for complicated orogeny and deep faults.Combined with these sulfide ions,a huge base metal mine in the Lanping basin of Yunnan Province has been formed.