青海东昆仑三通沟北锰矿成矿时代与物质来源：来自Re-Os同位素年代学与地球化学的约束

东昆仑三通沟北锰矿床是近年来青海省发现的规模最大的海相沉积型锰矿。该矿床位于东昆南构造带的北缘, 成矿地质背景与原特提斯洋的演化密切相关。但目前该矿床研究程度较低, 尤其是成矿时代存在中-新元古代与奥陶-志留纪的争议, 成矿物质来源存在海底热液为主还是海底热液与陆源风化共同来源的不同认识。为了解决这些问题, 本文在对三通沟北锰矿进行详细野外地质调查和钻孔岩芯编录的基础上, 选择11件锰矿石进行Re-Os同位素分析, 获得了442±15Ma的Re-Os等时线年龄, 说明三通沟北锰矿带形成于晚奥陶世, 不是前人认为的中-新元古代, 这一成矿年龄与区域上的奥陶纪纳赤台群沉积岩的形成时代一致。同时Re-Os同位素分析获得的¹⁸⁷Os/¹⁸⁸Os初始值为0.67±0.02, 该值明显低于同期海水的Os同位素组成(0.72), 初步表明三通沟北锰矿的锰质来源以海底热液为主。矿石在SiO₂-Al₂O₃、Al/(Al+Fe+Mn)-Fe/Ti、Fe-Mn-(Ni+Cu+Co)×10和lgU-lgTh等图解上主要位于热水沉积区, 矿石稀土元素配分型式也具有海底热液来源的特征。可见三通沟北锰矿的成矿物质来源主要与海底热液活动有关。

Ore-forming age and material sources of the North Santonggou manganese deposit in East Kunlun of Qinghai: Constrained by Re-Os isotopic chronology and geochemistry

The North Santonggou manganese deposit in East Kunlun is the largest marine sedimentary manganese deposit discovered in Qinghai Province in recent years. This deposit is located on the northern margin of the southern structural belt of the East Kunlun mountains, and the manganese-forming basin is closely related to the tectonic evolution of the Proto-Tethys ocean. So far, few studies have been conducted on this manganese deposit, and as a result, there remain some controversies, especially the formation age of its ore-bearing sequence (Meso-Neoproterozoic versus Ordovician-Silurian) as well as its manganese sources (dominated by seafloor hydrothermal fluid or jointly contributed by seafloor hydrothermal fluid and continental weathering). In order to solve these problems, on the basis of detailed geological investigation and drill observation, 11 manganese ore samples were selected for Re-Os isotope analysis which yielded a Re-Os isochron age of 442±15Ma. This age suggests that the North Santonggou manganese deposit was formed in the Late Ordovician rather than the Meso-Neoproterozoic as previously thought, and is consistent with the formation age of the sedimentary rocks of the Ordovician Nachitai Group in this region. In addition, the initial 187Os/188Os ratio obtained from Re-Os isotope analysis is 0.67±0.02, which is significantly lower than that of coeval seawater (~0.72). This ratio indicates that the manganese of the North Santonggou manganese deposit was mainly derived from submarine hydrothermal fluid. In the discrimination diagrams of SiO2-Al2O3, Al/(Al+Fe+Mn)-Fe/Ti, Fe-Mn-(Ni+Cu+Co)×10 and lgU-lgTh, all samples fall into submarine hydrothermal-sedimentary area. The distribution patterns of rare earth elements in ores also have the characteristics of submarine hydrothermal sources. Overall, it is indicated that the metal source of the North Santonggou manganese deposit is closely associated with submarine hydrothermal activity.