湖北金山店矽卡岩型铁矿田硫同位素特征及其地质意义

Sulfur isotopic character and geological implications for the Jinshandian Fe skarn ore field,Hubei Province

矿集区或矿田尺度硫同位素的空间分布特征研究不仅具有重要的理论意义,而且对找矿实践具有重要的指示作用。本文对金山店矿田范围内的矽卡岩型铁矿床开展了系统的硫同位素研究工作,发现该矿田内矽卡岩型铁矿床中的黄铁矿(+15. 1‰^+25. 6‰)、硬石膏(+24. 9‰^+31. 5‰)和石膏(+27. 5‰^+30. 4‰)均具有富集重硫同位素的特征,明显不同于岩浆热液矿床中这些矿物的δ^(34)S值组成特征,且硬石膏和石膏的δ^(34)S值与三叠系嘉陵江组地层中沉积石膏的δ34S值较为接近,暗示金山店矿田内矽卡岩型铁矿中的硫可能主要来自于含膏盐地层。含膏盐地层广泛参与了金山店矿田中矽卡岩型铁矿的成矿作用,对成矿作用产生了重要的影响:大量硫酸根(SO_4^(2-))的还原过程可以将成矿体系中的Fe^(2+)氧化成Fe^(3+),导致大量磁铁矿的形成;含膏盐地层与流体作用形成大量的盐溶角砾岩,有利于加速成矿流体与围岩之间的水岩作用,并提供容矿空间。系统的对比研究发现,大冶地区的矽卡岩型铁矿和矽卡岩型铁铜矿的赋矿地层、热液硬石膏/石膏规模和硫同位素值组成均存在明显的差异,暗示这些矿床的成矿围岩存在显著的差异。鄂东矿集区尺度的硫同位素等值线所揭示的空间变化规律具有重要的找矿指示作用:在天青石矿区(如狮子立山)或附近可能具有寻找大冶式矽卡岩型铁矿床或铁铜矿床的潜力,而在硬石膏/石膏发育的矽卡岩型铁矿区(如金山店铁矿田和程潮铁矿)或附近则具有寻找狮子立山式热液天青石矿床的潜力。

Investigation on sulfur isotopic distribution of an ore district or an ore field not only is of great theoretical significances,but also provides critical implications for its mineral deposit exploration. In this paper,we present sulfur isotope studies on sulfurbearing minerals of Fe skarn deposits from the Jinshandian ore field. Pyrite（ ＋ 15. 1‰ ～ ＋ 25. 6‰）,anhydrite（ ＋ 24. 9‰ ～＋ 31. 5‰） and gypsum（ ＋ 27. 5‰ ～＋ 30. 4‰） of Fe skarn deposits within the Jinshandian ore field all show heavy sulfur isotope values. They are significantly different from the restricted distribution of that in magmatic hydrothermal deposits,meanwhile δ34 S values of anhydrite and gypsum are comparable to that of sedimentary gypsum in the Triassic Jialingjiang Formation,indicating that sulfur in the Jinshandian ore field system was mostly derived from evaporites and the hydrothermal system of Fe skarn deposits of this ore field probably experienced significant incursion of evaporites. The involvement of evaporites has played an important role during the oreforming process. Reduction of large amounts of sulfate（ SO42-） in these ore-forming systems would be accompanied by oxidation of Fe2＋ to Fe3＋,which has a fundamental control on the formation of magnetite crystals,resulting in the formation of large amounts of magnetite. In addition,the production of large amounts of breccia could not only play a role as conductive to water-rock interaction between the ore-forming fluid and host rocks,but also provide space to host the ores. A comparison of the ore host rocks,reserves of the hydrothermal anhydrite/gypsum and distribution of δ34 S value of Fe skarn and Fe-Cu skarn deposits of the Edong district shows that the ore related sedimentary rocks of these two subtypes of the Daye-type iron deposits were significantly different. The spatial distribution of the sulfur isotopic values of the Edong district is an important prospecting indication and guide in this area： the Fe skarn and Fe-Cu skarn deposits would be found just in or near the hydrothermal celestite deposits,for example the Shizilishan deposit,whereas the hydrothermal celestite deposits would be found in or near the anhydrite and gypsum bearing Fe skarn deposits,such as the Jinshandian ore field and the Chengchao deposit.