Early Cambrian Black Shale-Hosted Mo-Ni and V Mineralization on the Rifted Margin of the Yangtze Platform, China: Reconnaissance Chromium Isotope Data and a Refined Metallogenic Model
Early Cambrian Black Shale-Hosted Mo-Ni and V Mineralization on the Rifted Margin of the Yangtze Platform, China: Reconnaissance Chromium Isotope Data and a Refined Metallogenic Model
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中国扬子地台裂谷边缘早寒武世黑色页岩赋存的钼镍和钒矿化：铬同位素勘察数据和精细的成矿模型
DOI：
10.2113/econgeo.111.1.89
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发表时间：
2016-01-01
期刊：
影响因子：
5.8
通讯作者：
Mao, Jingwen
中科院分区：
文献类型：
--
作者：
Lehmann, Bernd;Frei, Robert;Mao, Jingwen
沿着扬子地台东南缘的早寒武世黑色页岩序列的海侵基底单元，分布着广泛的海相沉积岩，其长度超过1,000 km。几厘米厚的富含硫化物的黑色页岩单元具有壮观的金属含量（Mo和Ni在百分含量范围内，PGE + Au约为1 ppm），由亚毫米级层状硫化物和碳质物质以及厘米级卵石状的Mo-S-C化合物、黄铁矿、在碳质和富磷酸盐基质中的富Ni多金属硫化物遵义矿区富Mo-Ni硫化物黑色页岩的自生值（贵州省），以及从三岔区（湖南省），东北400公里，平均为0.96 +/- 0.22千分之（n = 8），而来自两个地区的富V黑色页岩的平均值为1.34 +/-0.46ppm（n = 5）。这些数据表明，显着正分馏值相比，火成岩硅酸盐地球。研究页岩的铬同位素值与最近发现的正分馏三角洲Cr-53/52值在晚新元古代-古生代海相碳酸盐岩和页岩/泥岩，并证明了一个强化的氧化表面铬循环的操作，至少从大约0.75 Gyr前。我们认为，前寒武纪-寒武纪界线附近Cr循环的主要变化是由大气氧含量的显著升高引起的，Cr同位素数据证实了Mo和Os同位素的早期结论，即海水金属源最终由大陆地壳氧化风化提供。特别是，Re-Os和铂族元素的协变趋势显示出明显的海水特征，以及金属富集的程度，这是正常黑色页岩模式的延伸。富含Mo-Ni硫化物的黑色页岩单元可能是由在缺氧条件下沉积的水下硬地的筛选产生的。同样的地层水平还拥有缺氧/亚氧富钒黑色页岩（伊利石中的V）和在缺氧/亚氧环境中的巨大磷块岩矿床、层状重晶石以及10米厚的腐泥藻岩（可燃页岩）。富钼镍硫化物黑色页岩的金属极端富集可以理解为氧化还原循环和富氧条件下的底层水/沉积物界面清除作用的组合（钼，贵金属），和氧化从透光层沉淀下来的有机物的（矿化），在反硝化和硫酸盐还原条件下，对Ni和其他亲生物元素进行硫化物固定。生物活性高（如在具有高营养物供应的沿海上升流环境中）、非常低的碎屑堆积速率（如在受保护的盆地中）和层状氧化-亚氧化-富氧盆地中的低有机物沉积（高矿化速率）是黑色页岩中高级金属富集的要求。斜坡沉积的特殊水深条件，盆内类似滑塌的物质运动和重力淘选作用，既解释了扬子地台早寒武世岩石单元中特殊的矿石结构，又解释了金属进一步富集到矿石品位的过程。富Mo-Ni硫化物沉积物可视为海洋水成矿存款谱的富氧变体，而铁锰结核/结壳则代表海水极端分馏的氧化变体。
The transgressive basal unit of the Early Cambrian black-shale sequence along the rifted margin of the southeastern Yangtze platform hosts a wide spectrum of marine sedimentary rocks distributed over more than a length of 1,000 km. A few-centimeter-thick sulfide-rich black shale units have spectacular metal tenors (Mo and Ni in the percent range, PGE + Au around 1 ppm) and consist of submillimeter-scale laminated sulfide and carbonaceous material and centimeter-sized pebble-like rip-up clasts of Mo-S-C compounds, pyrite, and Ni-rich polymetallic sulfides in carbonaceous and phosphate-rich matrix.The delta Cr-53/52(authigenic) values of Mo-Ni sulfide-rich black shale samples from the Zunyi mining district (Guizhou province), as well as from the Sancha district (Hunan province), 400 km northeast, have a mean of 0.96 +/- 0.22 parts per thousand (n = 8), while V-rich black shale from both districts has a mean of 1.34 +/- 0.46 parts per thousand (n = 5). These data indicate significantly positively fractionated values compared to igneous silicate Earth. The Cr isotope values of the studied shales compare with recent findings of positively fractionated delta Cr-53/52 values in Late Neoproterozoic-Phanerozoic marine carbonates and shale/mudstones and attest for the operation of an intensified oxidative surface Cr cycle from at least around similar to 0.75 Gyr ago. We propose that the major change in Cr cycling around the Precambrian-Cambrian boundary was caused by a significant rise of atmospheric oxygen levels.The Cr isotope data confirm earlier conclusions from Mo and Os isotopes, which indicate a seawater metal source with ultimate metal supply by oxidative weathering of continental crust. In particular, covariation trends of Re-Os and of platinum-group elements display a distinct seawater signature, and a degree of metal enrichment which is an extension of the normal black shale pattern.The Mo-Ni sulfide-rich black shale units are probably produced by winnowing of subaquatic hardground deposited under euxinic conditions. The same stratigraphic level also hosts anoxic/suboxic V-rich black shale (V in illite) and huge phosphorite deposits in oxic/suboxic settings, bedded barite, as well as 10s of meter-thick sapropelic alginite (combustible shale). We show that the extreme metal enrichment of the Mo-Ni sulfide-rich black shale can be understood as a combination of redox cycling and bottom-water/sediment-interface scavenging under euxinic conditions (Mo, noble metals), and of oxidation (remineralization) of organic matter settling from the photic zone, with sulfide fixation of Ni and other biophile elements under denitrifying and sulfate-reducing conditions.High biological activity (as in coastal upwelling settings with high nutrient supply), very low clastic accumulation rate (as in protected basins), and low organic matter deposition (high rate of remineralization) in stratified oxic-suboxic-euxinic basins are requirements for advanced metal enrichment in black shales. A special bathymetric condition of slope deposition with intrabasinal olistostrome-like mass movement and gravitational winnowing explains both the peculiar ore textures and the further metal enrichment to ore grade in the Early Cambrian rock units on the Yangtze platform. The Mo-Ni sulfide-rich sediments can be regarded as the euxinic variant of the marine hydrogenous ore deposit spectrum, where ferromanganese nodules/crusts represent the oxic variant of extreme fractionation from seawater.