Stable Isotopic Evidence for the Involvement of Mantle-Derived Fluids in Wynad Gold Mineralization, South India
Stable Isotopic Evidence for the Involvement of Mantle-Derived Fluids in Wynad Gold Mineralization, South India
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印度南部 Wynad 金矿化过程中幔源流体参与的稳定同位素证据
DOI：
10.1086/629791
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发表时间：
1995
期刊：
影响因子：
--
通讯作者：
M. Javoy
中科院分区：
文献类型：
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作者：
M. Santosh;S. Nadeau;M. Javoy
世界主要金矿床常与碳酸盐蚀变晕和富含CO_2的流体包裹体有关。在石英脉系统中富集金的流体的来源和转移机制是有争议的话题。本文研究了印度南部麻粒岩相地体元古界Moyar-Bhavani跨地壳剪切系统中富CO2流体包裹体的C、O和S稳定同位素组成，以及与Wynad金矿田金石英脉伴生的石英、方解石和硫化物矿物。含金石英脉流体包裹体的光学和显微测温研究表明，流体以CO2-H2O为主(pCO2=0.80-0.90g cm-3)，具有高X值(0.7-1.0)和低盐度(约4-5wt%的氯化钠当量)特征。从流体包裹体中破碎提取的CO2的碳同位素组成在-4.1~-5.9之间显示出非常窄的范围。石英脉的氧同位素组成为+9.2~+10.9。来自碳酸盐脉的方解石的$Delta^{13}C$$(-9.1至$9.2)和$Delta^{18}O$$(+7.6至+7.8)也定义了一个明显的统一范围。与金共生的黄铁矿、黄铜矿和磁黄铁矿中的硫同位素显示了+1 4~+4 8的范围。来自Wynad的流体包裹体和碳酸盐的碳同位素组成与已报道的由地幔CO2引起的高级地体中低温碳酸盐蚀变的碳同位素组成相同。与脉体穿越成分可变的寄主岩石有关的同位素值的狭窄分布符合一种模式，即通过底板地幔岩浆的脱气产生CO2，并通过长英质岩浆管道将年轻的CO2转移到地壳更高的水平。我们设想了元古界CO2侵入与该地区早期的碳酸盐岩形成、碳酸盐蚀变和金矿化之间的共同联系。
Major gold deposits of the world are often associated with carbonate alteration halos and $$CO_{2}$$-rich fluid inclusions. The source and transfer mechanisms of the fluids that concentrate gold within quartz vein systems are topics of debate. Here we investigate the C, O, and S stable isotopic composition of $$CO_{2}$$-rich fluid inclusions, and quartz, calcite, and sulphide minerals associated with the gold-quartz veins from the Wynad Gold Field, emplaced within the Proterozoic Moyar-Bhavani transcrustal shear system in the granulite facies terrain of southern India. Optical and microthermometric studies of fluid inclusions in auriferous quartz veins reveal that the dominant fluid is $$CO_{2}-H_{2}O (p CO_{2} = 0.80-0.90 g cm^{-3})$$, with high $$X_{co_{2}}$$ (0.7-1.0) and low salinity (ca. 4-5 wt % NaCl equiv.). The carbon isotopie composition of $$CO_{2}$$ extracted from fluid inclusions by crushing shows a remarkably narrow range of $$\delta^{13}C$$ values, -4.1 to $$-5.9\%_{o}$$. Oxygen isotopie composition of the quartz veins range from +9.2 to $$+ 10.9\%_{o}$$. The $$\delta^{13}C$$ (-9.1 to $$-9.2\%_{o}$$) and $$\delta^{18}O$$ (+ 7.6 to $$+7.8\%_{o}$$) values of calcite from carbonate veins also define a distinctly uniform range. Sulphur isotopie values in pyrite, chalcopyrite, and pyrrhotite associated with gold display a $$\delta^{34}S$$ range of +1.4 to $$+4.8\%_{o}$$. The carbon isotopie composition of fluid inclusions and carbonates from Wynad are identical to those reported for low-temperature carbonate alterations in high-grade terrains induced by mantle-derived $$CO_{2}$$. The narrow spread in isotopie values associated with veins traversing compositionally variable host rocks is consistent with a model involving derivation of $$CO_{2}$$ by degassing of underplated mantle-derived magmas and transfer of juvenile $$CO_{2}$$ to higher crustal levels through felsic magmatic conduits. We envisage a common link between Proterozoic $$CO_{2}$$ influx and incipient charnockite formation, carbonate alteration, and gold mineralization in this terrain.