贵州东部及邻区南华纪锰矿中黄铁矿硫同位素高异常及地质意义

Geological implications of ultra-high δ^(34)S values of pyrite in manganese deposits of Nanhua Period in eastern Guizhou and adjacent areas, China

贵州东部及邻区南华纪锰矿层中的黄铁矿产出形态多样,为了研究它们的成因标志,通过野外和镜下观察发现黄铁矿主要呈浸染状、条带状和结核状产出,并以草莓状和自形-半自形粒状结构为主。硫同位素测试结果显示含锰岩系中黄铁矿具有异常高δ34S值的特征,介于+37.9‰^+62.6‰之间(平均52.7‰),呈塔式分布,峰值在+46‰^+59‰之间,沿同一矿体剖面从下到上逐渐减小,不同矿床间差异大。结果表明沉积盆地的封闭性和冰川事件使海水硫酸盐浓度降低并富集重硫同位素,随后形成的硫酸盐最低带进一步减小了硫同位素的分馏,使黄铁矿的δ34S达到异常高值。随着沉积盆地逐渐变开放,矿层由下而上黄铁矿的δ34S值逐渐减小,而不同矿床黄铁矿的δ34S值差异则是因为沉积盆地封闭性的差异。因此,封闭的局限沉积盆地是锰矿床形成的重要地质背景条件,而渗流热卤水或火山锰源可能是矿床的主要锰质来源。

Occurrence and sulphur isotopic compositions of pyrite in the manganese deposits of Nanhua Period in eastern Guizhou and adjacent areas, China were investigated, and it was found that： the structures of pyrite are mainly disseminated, banded, nodular, and the textures are mainly framboidal and idiomorphic-hypidiomorphic; the δ^34S values of pyrite are ultra-high, ranging from ＋37.9‰ to ＋62.6‰ with an average of 52.7‰; the δ^34S values is distributed in tower-shape, and the peak ranges from ＋46‰ to ＋59‰; the δ^34S values descreases from bottom up along the ore body; the δ^34S values differ significantly for different deposits. This study shows that the closed sedimentary basins are the main cause for the ultra-high δ^34S values of pyrite. The sulfate concentrations in the closed basins were found to be low and enriched in δ^34S, especially after undergoing the glacial event. Then the sulfate-minimum zone （SMZ） was formed in the basins after the glacial event, which further decreases the sulfur isotope fractionation between sulfide and sulfate. Therefore, the pyrite formed in the manganese deposits show ultra-high δ^34S values. As the basin begins to open, the δ^34S values for pyrite decrease from bottom up along the ore body, and the difference in the sealing properties of basins would make the δ^34S values for pyrite different. Base on this study, we suggest that the manganese deposits were formed in the closed basins and mainly originated from the vadose hot brine or volcanic hydrothermal activities.