膏盐层在长江中下游陶村玢岩铁矿中的响应：来自钙同位素的制约

A large number of iron deposits occur in the Ningwu-Luzong volcanic basins in the Middle-Lower Yangtze River. Many researchers have investigated diagenetic and metallogenic ages, geochemical characteristics and genesis of deposits, and they have established the famous “Porphyrite iron ore” metallogenic model, which has the unique diopside (actinolite)-apatite-magnetite assemblage. At present, the sulfur isotope is the main mean to study the relationship between the Triassic evaporite and the porphyrite iron mineralization. However, due to the complexity of geological processes and the multiplicity of sulfur isotopes, the understanding of the contribution of evaporites in the porphyrite iron ore is not comprehensive. Therefore, it is necessary to introduce a new analytical method to study it. Calcium is one of the main elements of evaporites and calcium-bearing minerals in the porphyrite iron deposit. Calcium isotopes have a large fraction scale in the supergene process, which is a good indicator to distinguish the supergene process from the magmatic process. In this project, we will determine calcium isotopic compositions of the Triassic evaporite and calcium-bearing minerals in the different metallogenic stages or symbiotic equilibrium of calcium-bearing mineral pairs in the same metallogenic stages of Taocun iron deposit in the Middle-Lower Yangtze River, combined with in situ sulfur isotope analysis of sulfides and gypsum, to obtain the calcium isotope information and fractionation behaviors in the metallogenic process, constrain the contribution of evaporites in the metallogenic process, and provide the discriminant index for the participation of evaporites in the deposit and the degree of the contribution. It is of great significance to reveal its metallogenic mechanism and expand the application of calcium isotopes in ore deposits.

长江中下游宁芜-庐枞火山岩盆地内产出大量铁矿床。前人对其进行了成岩成矿时代、地球化学特征及矿床成因等工作，提出了著名的“玢岩铁矿”成矿模式，并形成独特的透辉石(阳起石)-磷灰石-磁铁矿组合。目前硫同位素是研究三叠纪膏盐层与铁矿化关系的主要手段，但是由于地质过程的复杂性和硫同位素的多解性，导致对以上关系的认识不全面，因此有必要引入新的分析手段对其进行研究。钙是膏盐层和玢岩铁矿含钙矿物中的主体元素，其同位素组成在表生过程中分馏尺度大，是区分表生和岩浆过程的良好指示剂。本项目拟对长江中下游三叠纪膏盐层和陶村玢岩铁矿不同成矿阶段含钙矿物以及同一成矿阶段共生平衡的含钙矿物对进行钙同位素分析，结合硫化物和石膏原位硫同位素，以期获得成矿过程中的钙同位素信息及分馏行为，限定膏盐层在成矿过程中的贡献，为膏盐层是否参与成矿、贡献大小等提供判别指标，对揭示其成矿机制和扩大钙同位素在矿床中的应用具有重要意义。

长江中下游宁芜-庐枞火山岩盆地内产出大量玢岩铁矿床，其中三叠纪膏盐层是否参与铁矿化一直存在争议。S同位素是研究三叠纪膏盐层与铁矿化关系的主要手段，但是由于地质过程的复杂性和S同位素的多解性，导致对以上关系的认识不全面。Ca是膏盐层和玢岩铁矿含钙矿物中的主体元素，其同位素组成在表生过程中分馏尺度大，是区分表生和岩浆过程的良好指示剂。我们对长江中下游三叠纪膏盐层和陶村玢岩铁矿中的闪长玢岩、不同成矿阶段的蚀变围岩、浸染状矿石及脉状矿石的全岩及磷灰石开展Ca同位素分析，以期获得成矿过程中的Ca同位素信息，限定膏盐层在成矿过程中的贡献，这对揭示其成矿机制和扩大钙同位素在矿床中的应用具有重要意义。前人通过古沉积相、古生物群落等古环境特征在长江中下游识别出一套三叠系碳酸盐岩-硫酸盐岩海相地层。我们对安徽含山县陶厂镇出露的三叠系膏盐层进行方解石和硬石膏原位U-Pb定年，显示其年龄为281.6 ~ 287.0 Ma，为早二叠世而非前人认知的三叠世。同时分析其Ca同位素组成，为δ44/40CaSRM915a= 0.83 ~ 1.35‰，落于前人报道的全球二叠纪海相蒸发岩的Ca同位素范围内。陶村铁矿全岩Ca同位素结果显示：闪长玢岩为δ44/40CaSRM915a= 0.91 ~ 1.08‰，钠长石化围岩为δ44/40CaSRM915a= 0.70 ~ 0.82‰，浸染状矿石为δ44/40CaSRM915a= 0.85±0.04‰，绿帘石化围岩为δ44/40CaSRM915a= 0.85 ~ 0.92‰，碳酸盐-硫酸盐化围岩为δ44/40CaSRM915a= 0.62 ~ 0.70‰。从以上结果可以看出，逐渐矿化的闪长玢岩显示膏盐层参与的重Ca同位素特征。从早期钠化蚀变、含矿蚀变（矿石）、绿帘石化蚀变到晚期碳酸岩化、硫化蚀变，也显示成矿阶段蚀变富集重Ca同位素，这一重Ca同位素端元很可能为膏盐层的贡献。即膏盐层参与了玢岩铁矿的成矿过程。相对于全岩，磷灰石的Ca同位素变化范围有限（δ44/40CaSRM915a= 0.96 ~ 1.10‰）。这可能是因为Ca是磷灰石的主体元素，其Ca同位素可能主要受限于矿物本身对于Ca同位素的吸纳能力，从而导致对熔/流体中Ca同位素变化不够灵敏。以上研究对于玢岩铁矿的成矿机制和Ca同位素在矿床学中的开展应用具有重要的参考价值。