Trace element and C-S-Fe geochemistry of Early Cambrian black shales and associated polymetallic Ni-Mo sulfide and vanadium mineralization, South China: Implications for paleoceanic redox variation
Trace element and C-S-Fe geochemistry of Early Cambrian black shales and associated polymetallic Ni-Mo sulfide and vanadium mineralization, South China: Implications for paleoceanic redox variation
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DOI：
10.1016/j.oregeorev.2021.104210
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发表时间：
2021-08
影响因子：
3.3
通讯作者：
Lingang Xu;J. Mao
中科院分区：
文献类型：
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作者：
Lingang Xu;J. Mao
中国南方扬子地台东南缘沿着广泛出露早寒武世黑色页岩，长达1600多km。该层序的基底部分有大量矿化，包括多金属镍钼硫化物、钒、磷块岩、重晶石以及可燃页岩（石煤）。特别是，多金属镍钼硫化物矿石层在地层上相当，但在地理上与钒矿化分开。在湖南张家界地区，东北段多金属镍钼硫化物矿化品位逐渐降低，西南段逐渐过渡为钒矿化。在地层对比的基础上，对比了镍钼多金属硫化物矿、钒矿及其寄主黑色页岩的微量元素组成和碳硫铁体系，发现镍钼多金属硫化物矿和钒矿具有与现代海水相似的稀土元素PAAS标准化配分模式，均表现为Ce负、Y正异常。多金属硫化物矿石的Y/Ho比值呈海水状升高（平均值为52.7 ± 6.8），钒矿石的平均Y/Ho比值为40.2 ± 3.8，反映了陆源物质的影响。虽然多金属镍钼硫化矿和钒矿都是在缺氧条件下形成的，但缺氧程度是可变的，即，多金属硫化矿为富氧条件，钒矿为富氧和亚氧条件。该模型得到了更高的氧化还原敏感性元素（例如，铀和钴）的多金属镍钼硫化矿相比，钒矿石。S/Fe>1.15的镍钼多金属硫化矿中的过量硫表明其形成于铁限制的富氧条件下。与此相反，钒矿石形成在缺氧到亚氧条件下，S/Fe比接近或低于1.15。主机黑色页岩显示TOC和氧化还原敏感性元素的正相关性，这表明他们形成的亚氧条件下。我们推断，镍钼多金属硫化物矿床形成于一个缺乏沉积物的半限制性富氧海盆中。相比之下，钒矿石形成的贫氧和间歇性的亚氧贫氧条件下，在斜坡的盆地与陆地输入增强大陆风化。反复的海侵-海退事件导致形成钒矿石夹层与更柔和的缺氧的亚氧黑色页岩。研究结果表明，钒矿化的勘探重点应放在多金属镍-钼硫化物矿化的东南向横向地层等价物上。
Early Cambrian black shales are widely exposed along the southeastern margin of the Yangtze Platform in South China for a length of over 1600 km. The basal part of this sequence host a large spectrum of mineralization, including polymetallic Ni-Mo sulfide, vanadium, phosphorite, barite, as well as combustible shale (stone coal). In particular, the polymetallic Ni-Mo sulfide ore layer is stratigraphically equivalent but geographically separated from the V mineralization. In the Zhangjiajie region in Hunan province, the tenor of the polymetallic Ni-Mo sulfide mineralization in the northeastern segment decreases gradually and grades into vanadium mineralization in the southwestern segment. Based on the stratigraphic correlation, we compared the trace element composition and carbon–sulfur-iron system of the polymetallic Ni-Mo sulfide ore, vanadium ore, and their host black shales.The polymetallic Ni-Mo sulfide and V ores display similar PAAS-normalized REE distribution patterns with negative Ce and positive Y anomalies, similar to present-day seawater. The polymetallic sulfide ores show elevated seawater-like Y/Ho ratios (mean: 52.7 ± 6.8) and the V ores with an average Y/Ho ratio of 40.2 ± 3.8 reflect the influence of terrigenous material. Although both the polymetallic Ni-Mo sulfide ore and the V ore formed under anoxic conditions, the degree of oxygen deficiency was variable, i.e., euxinic condition for the polymetallic sulfide ore but euxinic and suboxic conditions for the vanadium ore. This model is supported by the more elevated redox-sensitive elements (e.g., U and Co) in the polymetallic Ni-Mo sulfide ores compared to the V ores. Excess sulfur of the polymetallic Ni-Mo sulfide ores with S/Fe ratios>1.15 suggests that they formed under iron-limited euxinic conditions. In contrast, the V ores formed under euxinic to suboxic conditions with S/Fe ratios close to or lower than 1.15. The host black shales display positive correlations for TOC and redox-sensitive elements, suggesting that they formed under suboxic conditions. We infer that the polymetallic Ni-Mo sulfide ores formed in a sediment-starved semi-restricted euxinic marine basin. In contrast, the V ores formed under both euxinic and intermittently suboxic to euxinic conditions at the slope of the basin with terrestrial input from enhanced continental weathering. Repeated transgressive–regressive episodes led to the formation of V ores interlayered with suboxic black shale with more subdued oxygen deficiency. The results of this study suggest that exploration for V mineralization should focus on the southeastward lateral stratigraphic equivalents of the polymetallic Ni-Mo sulfide mineralization.