膏盐层在矽卡岩型铁矿成矿中的作用

Effect of Sulfate Evaporate Salt Layer over the Formation of Skarn-Type Iron Ores

矽卡岩型铁矿是我国富铁矿的重要类型,约占全国富铁矿总储量的60%。虽然膏盐层与矽卡岩型铁矿的关系已引起部分矿床学家的关注,但膏盐层的控矿机制尚不清楚。本文以我国最重要的大冶式和邯邢式矽卡岩型铁矿为例,探讨了膏盐层在矽卡岩型铁矿成矿中的作用。膏盐层富含碳酸盐、石膏和石盐等,不仅可以为成矿提供大量Na^+、Cl^-、CO3^(2+)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Fe^(2+)以NaFe-Cl等络合物形式搬运,膏盐层还是地壳深处最重要的氧化障,能够将硅酸盐熔体和成矿溶液中的Fe^(2+)氧化成Fe^(3+),富集形成铁矿床,是矽卡岩型铁矿成矿的关键因素。大冶地区的膏盐层属于中三叠统下部的嘉陵江组,邯邢地区的膏盐层属于中奥陶统马家沟组—峰峰组。大冶和邯邢式矽卡岩型铁矿中硫化物的δ^(34)S_(V-CDT)值异常高,计算结果表明矿床中约80%的硫来自膏盐层硫酸盐的还原,还原温度多在500℃以上,但硫化物的沉淀温度相对较低,就位时间稍晚;硫酸盐的δ^(34)S_(V-CDT)值和还原温度越高,硫化物的δ^(34)S_(V-CDT)值越高,原始岩浆硫所占比例越高,硫化物的δ^(34)S_(V-CDT)值越低。当炽热的岩浆与膏盐层(CaSO_4)发生同化混染时,SO_4^(2-)将硅酸盐熔体中的Fe^(2+)氧化成Fe^(3+),Fe^(3+)无法进入硅酸盐矿物晶格,而形成Fe_3O_4/Fe_2O_3进入熔体,铁氧化物在磷、氯化钠、水等挥发分和盐类物质的作用下在岩浆房中与硅酸盐熔体发生不混熔,形成铁矿浆,沿构造有利部位贯入,形成矿浆型铁矿床。在矽卡岩型铁矿床中,矿浆充填型和热液交代型矿体同时存在,二者在空间上具有一定的分带性,有时渐变过渡,矿浆充填型铁矿体通常位于深部靠近成矿岩体的部位,而热液交代型铁矿体位于成矿流体运移的前方。在SO_4^(2-)氧化Fe^(2+)的同时自身被还原为S^(2-),与Fe^(2+)结合形成硫铁矿,分布在铁矿的上部或边部。

The skarn type iron deposit is one of the most important types of rich iron deposits, accounting tor about 60% of total rich iron ore reserves in China. The ore-controlling mechanism of sulfate evaporate salt layers is unclear, however, the relationship between layers and skarn iron deposits have been attracting widespread attention of geoscientists recently. In this paper, the effect of sulfate evaporate salt layers on the skarn type iron mineralization has been discussed, through the case studies of Daye type and Handan type deposits which are the important iron types in China. The sulfate evaporate salt layers are rich in carbonates, gypsum and halite, and they can provide a large number of agents of mineralization for the sodium alteration, scapolitization, skarn alteration, and the Fe2＋ transport as the complex （e. g. Na-Fe-C1）, like Na＋ , C1 , CO32- and so on. On the other hand, the sulfate evaporate salt layer is the most important oxidation barrier in the depth crust, which could oxidize the Fe2＋ into Fea＋ in the silicate magma and hydrothermal solution and enrich the iron to be the iron deposit. It is a key factor of the ore-forming of the skarn type iron deposit. In Daye district, the salt layers belong to the Jialingjiang Group in the lower part of Middle Triassic, and in Handan Xingtai district, the salt layers belong to the Majiagou Group and Fengfeng Group in Middle Ordovieian. The high bv4S values of sulfide in both districts indicate that most of sulfur of the skarn iron deposit is derived from sulfates in the salt layers, and the proportion is approximately 80 ~. The reduction temperature is more than 500~C. The temperature of the sulfide precipitation was lower and the time of the precipitation was relatively later than reduction. The higher the 4S value of sulfate or the reduced temperature is, the higher the of 4S value of sulfide will be. Otherwise the more the proportion of the original magma sulfur is, the lower the of value of sulfide is. While the magma is assimilating salt layers, SO~ 2- oxidize Fe2＋ into Fe3＋ in the silicate melt, which prevent Fee＋ to enter the lattice silicate minerals, with forming Fe304 or Fe2（）a to enter melt. The immiscibility occurs between iron oxide and silicate melt in magma chamber, by the effects of P, NaC1, water, and so on, forming the iron ore magma which eventually precipitated in a favorable tectonic area as the magma type iron deposit. In skarn iron deposits, ore magma filling type and hydrothermal--metasomatic type ore bodies coexist, with a certain zonation or gradual transition in space. The ore magma filling type ore bodies are usually located deeply near the intrusive rocks related mineralization, while the hydrothermal-metasomatic ore bodies are located in front of ore fluid migration. In addition, SO4z itself is reduced into S2- , while it is oxidizing the Fe2＋ , and S2 combine with Fe2＋ to form pyrite which is located in top or side of iron deposits.