小秦岭成矿省大湖、灵湖和金渠金(钼)矿床地质特征和硫同位素组成异同及其控制因素

Similarities and differences in geological characteristics andsulfur isotope composition and their controlling factors of the Dahu,Linghu and Jinqu Au(Mo)deposits in the Xiaoqinling gold province

小秦岭金(钼)矿省发育大量脉状金(钼)矿床,具有重要经济价值。本次研究选择大湖、灵湖和金渠金(钼)矿床,通过系统的构造-蚀变-矿化研究和金相关黄铁矿原位硫同位素分析,拟查明小秦岭金和钼矿化基础地质异同和硫同位素组成及其控制因素。构造-蚀变-矿化研究表明上述金(钼)矿脉赋矿围岩均为前寒武太华群变质岩,控矿构造主要为近EW向(局部为NW向)剪切带,金和钼矿体发育在同一剪切带不同部位或局部叠加。钼矿脉通常发育钾化和硅化蚀变和辉钼矿和黄铁矿等矿石矿物组合,而金矿脉以绢英岩化蚀变为主,主要发育黄铁矿、多金属硫化物和碲化物等矿石矿物。原位S同位素结果显示大湖矿床钼矿脉中黄铁矿δ^(34)S值为-3.3‰~+3.3‰,而大湖、灵湖和金渠金矿脉中黄铁矿δ^(34)S值为-7.8‰~+8.9‰。大湖和灵湖近EW金矿脉中黄铁矿δ^(34)S值分别为-2.8‰~+0.4‰和-7.8‰~+8.2‰,与大湖金矿脉相比,灵湖金矿脉倾向较缓且发育大量围岩角砾表明上述硫同位素组成可能由差异水力破裂和水岩反应控制。金渠金矿脉近EW和NW向矿脉中黄铁矿δ^(34)S值范围分别为-3.9‰~-3.5‰和0.7‰~8.9‰,其中陡倾矿脉黄铁矿δ^(34)S值较负(-3.9‰~+3.7‰),而缓倾矿脉δ^(34)S值较正(+4.1‰~+8.9‰),这些黄铁矿硫同位素组成差异可归因于不同构造背景控制的差异氧逸度等物理化学条件。本次研究结合前人年代学、流体包裹体等数据指示除了源区属性和成矿路径外,成矿末端的水力破裂和水岩反应等过程在控制小秦岭金和钼成矿异同方面同样扮演重要角色。

There are a large numbers of lode Au(Mo)deposits in the Xiaoqinling gold(molybdenum)province with great economic values.In this study,we selected the Dahu,Linghu and Jinqu Au(Mo)deposits to identify their Au-Mo similarities and differences in the basic geology,sulfur isotopic compositions,and the controlling factors based on systematic structural-alteration-mineralization observations and in situ sulfur isotope studies of gold-related pyrite.The studies on structural-alteration-mineralization of the Dahu,Linghu and Jinqu deposits indicate that the Au and Mo veins are hosted in the metamorphic basement of the Archean Taihua Group,mainly controlled by the nearly EW-trending shear zones(locally NW-trending).These ore veins occur in different parts of the same shear zones,or sometimes superimposed with each other at certain area.The Mo veins are related to K-feldspar and quartz alteration,with an ore mineral assemblage mainly associated with molybdenite and pyrite.In contrast,the Au veins are commonly related to sericite quartz alteration with an ore mineral assemblage of pyrite,polymetallic sulfides and telluride.In situ S isotope studies show pyrite from the Mo veins haveδ^(34)S values of-3.3‰~+3.3‰,while those from the Au veins of-7.8‰~+8.9‰.Pyrites in the Dahu and Linghu Au veins hosed in EW-trending shear zones haveδ^(34)S values of-2.8‰~+0.4‰and-7.8‰~+8.2‰,respectively,which suggests that the sulfur isotope compositions may be controlled by differential hydrofracturing and fluid-rock interaction ratios.This is also correspondent to the phenomenon that more gentle-dip breccias in the Linghu gold vein than that in the Dahu vein.At Jinqu,pyrite in the EW-and NW-trending veins haveδ^(34)S values of-3.9‰~-3.5‰and 0.7‰~8.9‰,respectively.Among them,the steeply-dipping Au veins have negative pyriteδ^(34)S values of-3.9‰~+3.7‰,whereas the gentlydipping veins have positiveδ^(34)S values of+4.1‰~+8.9‰.These significant differences in sulfur isotopic compositions of pyrite can be attributed to the controls of different structural settings on different physicochemical conditions,such as the oxygen fugacity.Combined with previous geochronology and fluid composition,this study indicate that the ore-controlling mechanisms(hydrofracturing and fluid-rock interaction),other than source properties and mineralization routes,played an important role on the similarities and differences of Au(Mo)mineralization in Xiaoqinling.