膏盐层氧化障在玢岩铁矿成矿中的作用

Ningwu and Luzong Mesozoic volcanic basins are important ore concentration areas of porphyrite type iron deposits in China. A large numbers of geological phenomena show that Triassic sulfate evaporite plays an important role in the ore-forming process of porphyrite iron ores. There may be "deep-seated second metallogenic zone" in the volcanic basins. However the ore-control mechanisms of sulfate evaporite are not clear; it is hotly debated whether the magnetite magma type iron ore exists or not. At present, it is generally considered that sulfate evaporite provides the major mineralizing agents (such as the ions of Na+ and Cl-). Actually, sulfate evaporite layers are oxidation barriers for formation of porphyrite iron ores, oxidizing the Fe2+ into Fe3+, in the silicate magma and hydrothermal solution. It is the key link for the forming of magnetite magma differentiated from source magma and iron mineralization from hydrothermal solution, and essential causation for association of magnetite, pyrite and anhydrite ores, which is rarely mentioned in literatures. In order to solve these problems, S, O isotopes and the other geochemical analysis will be carried out, based on the field investigation and micro-observation. We expect to find out new evidences for magnetite magma and the interreaction between silicate magma and sulfate evaporite, to identify ore-controlling effect of sulfate evaporite on the ore forming of iron deposits, to reveal the mechanism and condition of magnetite magma differentiated from source magma, to define the discriminant mark of magnetite magma iron ore, to establish "double-deck mineralization model of porphyrite type mineral deposit" in the volcanic basin in the Middle and Lower Yangtze River valley.

宁芜、庐枞中生代火山盆地是我国玢岩铁矿集中区。大量地质现象表明三叠系膏盐层在玢岩铁矿成矿过程中发挥了重要作用，盆地深部可能存在"第二成矿带"，但膏盐层的控矿机理不清，深部矿化类型不明，是否存在矿浆型铁矿还存在激烈争论。目前普遍认为膏盐层主要为成矿提供Na+、Cl-等矿化剂，但实际上膏盐层还是成矿的重要氧化障，可将硅酸盐熔体和溶液中Fe2+氧化成Fe3+，是形成铁矿浆和溶液中铁沉淀成矿的关键环节，是铁矿-硫铁矿-石膏矿密切共生的根本原因，但这方面研究国内外鲜有报道。本研究拟以姑山、梅山、泥河、大鲍庄矿床为重点，采用野外地质调查、微区分析观测和微区原位S、Sr 及O、C同位素分析技术等，查找膏盐-岩浆相互作用和铁矿浆的新证据，定量评价膏盐硫贡献大小，阐明膏盐层的控矿机理，揭示铁矿浆形成的机制和条件，构建矿浆型铁矿地质-地球化学综合判别标志，建立玢岩铁矿"双层成矿结构"模型，为深部找矿服务。

玢岩铁矿和矽卡岩铁矿是我国富铁矿的重要类型，经济价值大。前人虽然对玢岩和矽卡岩型铁矿开展了大量研究，但膏盐层氧化障的控矿机理不清，是否存在矿浆型铁矿还存在激烈争论。对长江中下游铁铜金等多金属成矿带宁芜和庐枞火山盆地的玢岩铁矿和大冶隆坳过渡带的矽卡岩铁矿开展了系统研究。宁芜、庐枞玢岩铁矿和大冶矽卡岩铁矿空间分布与三叠系周冲村组/嘉陵江组膏盐层关系极为密切，矿床中普遍含有石膏，部分地段富集形成石膏矿。石膏的δ34S值大部分位于20‰左右，与海相硫酸盐的值相似，黄铁矿的δ34S值异常高，指示矿床中硫主要来自三叠纪膏盐层。宁芜姑山矿田黄铁矿的δ34S值最高，梅山矿田次之，凹山矿田最低，矿床成因类型也发生相应变化，矿浆型→矿浆-热液型→热液型。膏盐层不仅为成矿提供了大量Na+、Cl-等矿化剂，使围岩发生钠长石化、方柱石化（氯化）和矽卡岩化等蚀变，使Fe2+以NaFeCl3等络合物形式搬运，膏盐层还是最重要的氧化障，将硅酸盐熔体和成矿溶液中的Fe2+氧化成Fe3+，富集形成铁矿床，是玢岩/矽卡岩铁矿成矿的关键因素。当炽热的岩浆与膏盐层发生同化混染时，SO42-将硅酸盐熔体中的Fe2+氧化成Fe3+， Fe3+无法进入硅酸盐矿物晶格之中，而形成铁氧化物Fe3O4/Fe2O3和贫铁的硅酸盐矿物透辉石/阳起石、透闪石等。铁氧化物在磷、氯、水等盐类物质和挥发分的作用下在岩浆房中与硅酸盐熔体发生液态不混熔，熔离形成铁矿浆。揭示了铁矿浆的形成机理，建立矿浆型铁矿的氧-硫同位素判别标志。在SO42-氧化Fe2+的同时自身被还原为S2-，形成硫铁矿，在铁矿的上部或边部富集形成硫铁矿，建立石膏矿-铁矿-硫铁矿三位一体成矿模型，发展了玢岩-矽卡岩铁矿成矿理论。建立玢岩铁矿“双层成矿结构”模型，在盆地深部岩体与膏盐层的接触部位产出“大冶式”矿浆-矽卡岩型富铁矿床，规模可能超过了赋存于浅部火山-次火山中的狭义“玢岩铁矿”。该成矿模型已被最新深部找矿成果（庐枞小包庄铁矿、宁芜杨庄铁矿）证实，推动实现了玢岩铁矿找矿突破。