蛇纹岩化对洋中脊超基性岩热液硫化物成矿的影响:来自青藏高原德尔尼铜矿床的启示

洋中脊超基性岩热液成矿系统通常与洋底核杂岩构造有关,多发育大型矿床,具有巨大的资源前景。然而,受大洋调查取样手段的限制,超基性岩蛇纹岩化对成矿的影响仍需进一步研究。德尔尼铜矿床是地质历史上该类矿床的典型案例,对于理解其成矿模式,以及大洋硫化物勘探具有指导意义。本文选取德尔尼铜矿床块状硫化物样品进行黄铁矿的S同位素分析,结果表明其δ³⁴S值主要分布在-0.4‰～+6.3‰。结合前人研究发现,形成于深部网脉状、条带状矿石中的δ³⁴S值为负值,而经历表层喷流和破碎作用的块状和角砾状矿石中的δ³⁴S值为正值,二者呈对称分布,这主要是由于还原条件下岩浆排气产生的SO_2和H_2S动态平衡并逐渐沉淀S^2-,表明蛇纹岩化提供的还原环境对热液系统演化产生了重要影响。然而,磁黄铁矿和矿床Ni的分布指示成矿物质中超基性岩的贡献较小,主要物质来源是洋中脊深部的基性岩浆,通过热液循环将物质运移至海底并喷流成矿。对比现今超基性岩赋矿的高温热液硫化物矿床,德尔尼铜矿床形成温度更低,代表了超基性岩赋矿热液硫化物中的中温端元,表明在距离拆离面一定距离(约2～4km)的位置也可能形成大型的热液硫化物矿床,这对于现今洋中脊热液硫化物勘探具有一定的指导意义。

Impacts of Serpentinization on Ultramafic Rock-Hosted Hydrothermal System along Mid-Ocean Ridges: Insight from Dur'ngoi Copper Massive Sulfide Deposit,Tibetan Plateau

Modern seafloor ultramafic rock-hosted hydrothermal systems are generally associated with oceanic core complexes(OCCs).Most of them develop large deposits,suggestive of great resource potential for such systems.However,constrained by ocean investigation and sampling methods,the influence of serpentinization on ultramafic rock-hosted hydrothermal systems remains unclear.The Dur'ngoi copper massive sulfide deposit(DCMSD)in North Tibetan Plateau is such a Carboniferous prototype,providing insight into the understanding of its metallogenic process and guidance for modern ocean sulfide exploration.In this paper,sulfur isotope compositions of pyrite and typical minerals are analyzed.δ^34 S values are between?0.4‰to+6.3‰.Combined with previous data,we find that the sulfur isotopic compositions of DCMSD are closely associated with the ore types.δ^34 S values of ore samples formed in surficial conditions(massive,loose,and brecciated ores)are positive,similar to most of the modern massive sulfides along mid-ocean ridges,while those for the ores formed deep(stockworks and striped ores)are negative.Their 34 S values are symmetrically distributed.This phenomenon cannot be explained by seawater-magma equilibrium,biological isotope fractionation,or SO 2 disproportionation reaction.We propose that the dynamic equilibrium of SO 2 and H 2 S(products of magma degassing)and fractional deposition of sulfur under reducing environment may account for the observation.This process manifests the importance of reducing environment,which is generated by serpentinization of ultramafic host rocks,on the evolution of hydrothermal system.However,the distribution of pyrrhotite and Ni of DCMSD ore body implies that the material contribution of ultramafic rocks is limited.Instead,the mid-ocean ridge mafic magmas provide major material sources,which are transported to the seafloor by hydrothermal fluids and exhale to form the ore.Unlike the modern ultramafic-hosted high-temperature hydrothermal systems,DCMSD has lower metallogenic temperature,and represents the middle temperature end member of ultramafic rock-hosted hydrothermal system.The DCMSD case illustrates that large hydrothermal sulfide fields can be formed at a distance of ca.2 to 4 km away from the detachment fault,which may shed lights on the modern mid-ocean ridge hydrothermal sulfide exploration.