西秦岭阳山金矿带硫同位素特征:成矿环境与物质来源约束

阳山金矿带成岩期的黄铁矿主要为立方体-他形,反映出一种较低温度(<200℃)、成岩流体的过饱和度低、快速冷却、氧逸度和硫逸度低、物质供应不足的成岩条件,δ³⁴S值变化范围较大(-4.2‰～12.5‰),反映了硫源自于泥盆系地层,其中灰岩中黄铁矿硫源自于海水中硫酸根离子的还原作用,千枚岩中黄铁矿经历了细菌还原作用。成矿期黄铁矿具有多种晶形,但立方体单晶较少,指示成矿系统处于中-低温(200～300℃)、成矿流体的过饱和度高、缓慢冷却、氧逸度和硫逸度高、物质供应充分的成矿有利条件。成矿早阶段和主阶段硫化物的δ³⁴S值变化范围为-4.2‰～3‰,接近于岩浆硫范围,其中成矿主阶段的黄铁矿以五角十二面体、八面体和立方体形成的聚形更常见,且聚形黄铁矿的硫同位素值变化范围更窄(-2.1‰～1.2‰),更符合岩浆硫来源特征;成矿晚阶段辉锑矿的δ³⁴S值变化范围为-6.6‰～-4.5‰,而与其共生的黄铁矿δ³⁴S值分别是7.6‰和-12.1‰,反映晚阶段除岩浆岩硫源外,浅变质的泥盆系地层也提供了部分硫源。

Sulfur isotope characteristics of the Yangshan gold belt, West Qinling: Constraints on ore-forming environment and material source

The main crystal habit of diagenetic pyrite in the Yangshan gold belt is cube-xenomorphic,indicating a diagenetic environment with low temperature(< 200℃),low degree of supersaturation of diagenetic fluid,fast cooling,low oxygen fugacity and sulfur fugacity,and insufficient supply of material.The δ34 S value varies in a large range(-4.2‰ ～ 12.5‰),indicating that the sulfur came from Devonian stratum,within which sulfur of pyrite in limestone originated from reduction of sulfate ion in sea water and sulfur of pyrite in phyllite experienced bacterial reduction.The ore pyrite has various morphologies,but with few cubes,indicating that the metallogenic system is under conditions of medium-low temperature(200 ～ 300℃),high degree of supersaturation of ore-forming fluids,slow cooling,high oxygen and sulfur fugacity,and sufficient supply of material,which are favorable for metallogenesis.The δ34 S value of sulfides in the early and main ore stages varies from-4.2‰ to 3‰,which is close to the δ34S ranges of magmatic sulfur.Pyrite in the main ore stage has more combination forms of pyritohedron,octahedron and cube.The δ34S value of the combination forms concentrates in-2.1‰ ～ 1.2‰,which is more likely to be of magmatic origin.The δ34S value of stibnite in the late ore stage varies from-6.6‰ to-4.5‰,while the pyrite which coexists with stibnite has δ34S value of 7.6‰ and-12.1‰,showing that besides magmatic sulfur,Devonian stratum provided partial sulfur for the system.