黔东松桃南华系大塘坡组锰矿层物源:来自Sr同位素的证据

锰矿床的物质来源是锰矿床研究的难点问题之一.辨别黔东松桃地区南华系大塘坡组锰矿沉积的物质来源有助于加深对锰矿成矿过程的理解.对黔东松桃地区南华系大塘坡组锰矿沉积的Sr同位素研究显示，15个锰矿石、锰质页岩及炭质页岩样品87Sr/86Sr同位素比值变化范围为0.705 727~0.732 536，其中炭质页岩样品具有最高的Sr同位素比值0.732 536，含锰岩系样品87Sr/86Sr同位素比值平均值为0.711 128.样品中87Sr/86Sr比值随着Al含量的上升，分别出现87Sr/86Sr比值上升与下降的两个分异趋势.87Sr/86Sr比值随Mn含量的上升总体呈现下降的趋势，但该趋势无显著相关性，残差分析显示这主要是由于样品中87Sr/86Sr比值随着Mn含量上升出现收敛性波动造成.上述现象是由于陆源碎屑成分和海底热液成分混合输入造成.通过与大塘坡组同时代(约660 Ma)古海水Sr同位素组成，世界范围内不同时代锰矿沉积以及现代红海沉积物的Sr同位素结果对比，发现黔东松桃地区南华系锰矿层中Sr同位素比值分布范围较宽，部分锰矿样品87Sr/86Sr比值低于古海水87Sr/86Sr比值，与典型大洋成因的锰矿层或铁锰结核具有不同的Sr同位素特征.联系黔东南华系大塘坡组锰矿层形成时期的特殊地质背景，认为锰质积累过程与沉淀过程为不同阶段产物——锰质的积累过程在Sturtian冰期盆地缺氧水体中完成，可能主要以海底热液喷溢系统完成；而锰矿的沉淀过程则是在间冰期伊始古海洋化学条件动荡的水体中完成. 

Provenance of Nanhuan Datangpo Formation Manganese Mn Deposit in Songtao Area,East Guizhou Province: Evidence from Sr Isotope

Provenance of manganese deposit is a key problem in the manganese metallogenic study. For a better understanding on the metallogenic process of the Nanhuan manganese deposit in Songtao area, East Guizhou Province, we need to gain a clear idea of the provenance of this manganese deposit. The Sr isotope study on the Mn deposit in Lower Nanhuan Datangpo Formation in Songtao, East Guizhou Province indicates that 87Sr/86Sr ratios in 15 samples vary in 0.705 727-0.732 536. Studied samples include Mn ore, Mn shale and black shale in the Mn-bearing layer and the highest 87Sr/86Sr ratio appears in the black shale sample. With Al (%) increasing in samples, 87Sr/86Sr ratios show two changing trends: the rising trend indicates terrigenous materials input and the falling trend indicates the deep hydrothermal sources input. As to 87Sr/86Sr ratios via Mn (%), there is no significant correlation coefficient, but 87Sr/86Sr ratios decrease when Mn (%) increase generally, according to residual analysis, we find that this is caused by the convergence fluctuation of 87Sr/86Sr ratios against the Mn(%) changes. The broad variation of 87Sr/86Sr ratios in Mn layer is caused by the dual inputs from terrigenous materials and hydrothermal sources. Compared with the contemporary (about 660 Ma)Sr isotope ratio in paleoseawater and the Sr isotope data from different ages Mn deposit world wide, we find that the broad variation of 87Sr/86Sr ratios in the Datangpo Formation. Mn layer does not conform to the feature of hydrogenetic Mn deposit. Considered with the specific geological background, we believe that the accumulation and precipitation of manganese are separated processes:the accumulation of Mn had finished in the anoxic basin during the Sturtian glaciation, whereas the Mn deposit precipitated in an unstable paleoceanographic environment at the beginning of interglaciation.