鄂西长阳锰矿稳定同位素特征及其地质意义

Stable Isotopes Characteristics of Changyang Manganese Deposit in Western Hubei Province and Its Geological Implications

长阳锰矿位于扬子陆块北缘,赋存于南华系大塘坡组含锰黑色岩系中,含锰矿物主要是菱锰矿。通过对含锰岩系C、O同位素及黄铁矿单矿物S同位素的分析表明:δ13C、δ18O值分布范围和平均值分别为﹣6.2‰~﹣2.1‰、﹣2.9‰和﹣7.9‰~﹣5.3‰、﹣6.8‰,δ34S介于38.41‰~66.85‰,平均值为60.95‰。研究认为:长阳锰矿炭质主要来源于无机碳,在沉积过程中有有机质碳的加入。通过O同位素反演的古温度一般在39.4℃~58.2℃,均值为48.9℃,古温度偏高说明其沉积时水体较深。研究区古盐度Z值大部分大于120,为古盐度较高的海相沉积环境,有部分降水和陆源淡水进入沉积区。长阳锰矿含锰层位中黄铁矿具有异常高的δ34S值,δ34S介于38.41‰~66.85‰,平均值为60.95‰,沉积盆地的封闭性和冰川事件使海水浓度降低并富集重硫同位素,随着海水硫酸盐和硫化物之间的硫同位素分馏的减小,使黄铁矿的δ34S达到异常高值。

The Changyang manganese deposit, western Hubei, is located in the northern margin of the Yangtze block and occurs in the manganese-bearing black rock series of the Datangpo Formation of the Nanhua System. The manganese-bearing minerals are mainly rhodochrosite. The analysis of C, O isotopes of manganese-bearing rock series and S isotopes of pyrites shows that the distribution range and average values of δ13C and δ18O values are -6.2‰^-2.1‰,-2.9‰ and -7.9‰^-5.3‰,-6.8‰, respectively, and that of δ 34S are between 38.41‰ and 66.85‰, with an average value of 60.95‰. The results show that the carbon of Changyang manganese ore mainly comes from inorganic carbon, and organic carbon is added into the deposit in the deposition process. The paleotemperature inverted by O isotope is generally between 39.4 ℃ and 58.2 ℃, with a mean value of 48.9 ℃. The higher paleotemperature indicates that the water body is deeper during the deposition. The Z value of paleosalinity in the study area is mostly more than 120, which is a marine sedimentary environment with high paleosalinity. Some precipitation and terrestrial freshwater enter the sedimentary area. In the manganese-bearing horizon of Changyang manganese deposit, pyrite has an abnormally high value of δ34S, which indicating sealing of sedimentary basins and glacial events reduce seawater concentration and enrich heavy sulfur isotopes. With the decrease of sulfur isotope fractionation between seawater sulfate and sulfide, the value of δ34S of pyrite reaches an abnormally high value.