Geological And Geochemical Characteristics And Genesis Of Meishan Iron Deposit In Ningwu Basin

^Located in the north margin of Yangtze plate and west of the Tan-Lu fault zone,Ningwu basin is the succession volcanic rift basin in middle-lower Yangtze RiverValley metallogenic belt. Meishan iron deposit is a typical large iron ore deposit inNingwu basin with the characteristics of high grade and large reserves. Its ore genesisremains controversial, the main metallogenic point of view including the post-volcanic gas, liquid metasomatic deposit（Zhang et al.，1977）pulp hydrothermaldeposits（Chen，1981；Yuan et al.，1997）skarn-like deposit（Mao et al.，2012）the research on geological, geochemical characteristics and genesis of this deposithave the double value both of research and production.On the basis of previous studies, this paper conducted a detailed fieldgeological survey and mineral petrography, geochemistry, isotope geochemistry onMeishan iron deposit, researched on its geological, geochemical characteristics andore-forming process, and achieved new understandings as follows:The main ore body of the deposit contained a large number of skarn mineralswith skarn-like metallogenic characteristics；its edge net-vein disseminated ores havethe hydrothermal feature. This deposit experienced a skarn-like stage, net-veindisseminated magnetite stage, quartz sulfide stage and carbonate stage. In which thestage of forming the massive and spotted structure magnetite ore skarn-like stage isthe main mineralization stage.The rare earth elements of magnetite in this deposit showing a Eu negativeanomaly, compared with the previous measure of rare earth elements characteristics ingabbro-diorite porphyry indicating that Meishan iron deposit magnetite andgabbro-diorite porphyry are homologous as the product of the same volcanic cycle.Theδ34S value of pyrite in the deposit range from8.9‰to-14.3‰, with anaverage of11.9‰; theδ³⁴S of pyrite in the deposit range from18.5‰to23.3‰,with an average of21.43‰. The sulfur isotope geochemistry studies have shown thatsulfide and δ³⁴S of sulfates in the deposit are both from the Triassic gypsolyte, sulfurcompounds formed by the reduction of SO₄^2-,and it may include the possibility of asmall amount of sulfur from magmatic hydrothermal. Theδ¹³CV-PDBvalue of siderite in the deposit varies form0.1‰to0.9‰,withan average of0.5‰; δ18OV-SMOWvalue range from15.3‰to17.7‰, with anaverage of16.8‰. Theδ13CV-PDBvalue of calcite in the deposit varies form-5.3‰to-1.6‰, with an average of-1.12‰; δ¹⁸OV-SMOWvalue range from16.7‰to18.9‰, with an average of17.86‰. The carbon and oxygen isotope geochemistry studieshave shown that the carbon in formation of siderite and calcite in the late stagemineralization is mainly the marine carbonate source， and experienced thehydrothermal water-rock reactions，is the superimposition of late hydrothermalformation and a small portion of which may come from the hydrothermal.In summary and by comparing Meishan iron ore deposit with typical skarndeposits we can come to the conclusion that the formation of main ore body inMeishan iron ore deposits and general skarn-type deposits are quite different andshould be classified as skarn-like hydrothermal metasomatic mineralization.