The role of evaporite layers in the mineralization of endogenetic metal deposits
The evaporite layer is evaporated sedimentary rock rich in gypsum/anhydrite.In addition to SO42-,the evaporite layer is also rich in Cl-,CO32-,Na+,K+,and other kinds of salts with H2O,which exist widely in nature.The evaporite layer can provide volatiles such as oxidizing agent(SO42-),mineralizing agent(Cl-,CO32-,Na+,K+,etc.),reduced sulfur(S2-),and H2O to the metallogenic system to migrate or precipitate ore-forming materials.It is the key factor for the mineralization of iron-oxide apatite ores(IOA),skarn-type iron ores,MVT-type Pb-Zn ores,magmatic Ni-Cu-PGE sulfide ores,and some hydrothermal uranium ores.However,the role of the evaporite layer in the mineralization of endogenetic metal deposits has not been focused on,which restricts the understanding of related metallogenic theories and affects the improvement of prospecting efficiency.The evaporite layer controls the spatial distribution of endogenic metal deposits related to the evaporite layer.The sulfate miner-als such as gypsum are generally developed in these deposits,with abnormally high δ34S values of sulfate and sulfide and the δ18O values of iron oxides such as magnetite.The addition of evaporite layers can greatly increase the oxygen fugacity of the ore-forming system and make the iron oxide formed in advance on the consolidation line of the melt,which is an important condition for the liquid immiscible melting of the magma and the formation of pulp type rich iron ores.The evaporite layer provides oxidants(SO42-)to the magmatic-hydrothermal mineralization system,oxidizing Fe2+to Fe3+and enriching it to form an iron deposit.Meanwhile,SO42-is reduced to S2-,which combines with Fe2+to enhance and form sulfide deposits.Therefore,the evaporite layer,IOA/skarn iron ores,and pyrite ores coexist closely and provide mineral exploration indicators.In the MVT Pb-Zn polymetallic mineraliza-tion system,SO42-in the evaporite layer would be reduced by carbon,organic,and methane to form H2S,which mixes with oxidizing ore-forming solutions rich in Pb2+and Zn2+that rise along the thrust and thrust structures.S2-combines with Pb2+,Zn2+,etc.,to enrich and form Pb-Zn polymetallic deposits.The combination of the evaporite layer,organic matter,and Pb-Zn ores are closely interdependent.
evaporite layerSO42-oxidantreduced sulfur S2-sulfur and oxygen isotopesendogenous metal deposits
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