The lower Carboniferous Luzhai and Baping Formations (ca. 359 Ma) of the South China block, Guangxi Province, comprise an ca. 170-m-thick clastic-carbonate succession capped by Mn ore horizons near the town of Longtou. Excellent exposure of the stratigraphic succession provides an unparalleled opportunity to investigate the origin of carbonate-hosted Mn deposits, which are generally understudied. Lithofacies associations suggest inner and middle shelf clastic rocks accumulated with deposition of carbonates on a mesotrophic middle to outer shelf. In the Longtou region, carbonate deposition during marine transgression culminated with the precipitation of high-grade Mn deposits during maximum flooding. Mn ore horizons are composed of amalgamated alabandite-bearing rhodochrosite, Mn calcite, and braunite laminae. Mn carbonates have been largely interpreted as forming in oxic water columns via reduction of Mn oxides by organic matter. However, paragenetic relationships and δ¹³C values (similar to those of seawater) indicate the Mn carbonates of Longtou were formed during authigenesis by the emplacement of anoxic, Mn-rich water masses on the distal to middle shelf. Such anoxia is interpreted to have shut down the carbonate factory and diminished sedimentation, a prerequisite for the concentration and precipitation of Mn carbonates in pore water. This research supports the notion that areas of the Paleozoic deep ocean were persistently anoxic and periodically tapped by coastal upwelling to produce Mn- and Fe-rich deposits. Application of this emerging ore deposit model to other economically important carbonate-hosted Mn deposits may improve resource exploration.

中文翻译：

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华南区块龙头矿床石炭系锰矿成因新模型

广西省华南地块的下石炭统鹿寨组和八坪组（约 359 Ma）包括约 170 米厚的碎屑-碳酸盐岩层序被龙头镇附近的锰矿层覆盖。地层序列的极好暴露为研究碳酸盐岩锰矿床的起源提供了无与伦比的机会，而这些矿床通常未被充分研究。岩相组合表明内陆架碎屑岩与碳酸盐沉积一起堆积在中营养的中到外陆架上。在龙头地区，海侵期间的碳酸盐沉积以最大洪水期间高品位锰沉积物的沉积而告终。锰矿层由含钠沸石的菱锰矿、锰方解石和布劳恩石层组成。Mn 碳酸盐在很大程度上被解释为通过有机物还原 Mn 氧化物在含氧水柱中形成。然而，共生关系和 δ¹³ C 值（与海水相似）表明龙头的锰碳酸盐是在自生过程中由缺氧、富锰水团在陆架远中段侵位形成的。这种缺氧被解释为关闭了碳酸盐工厂并减少了沉积，这是孔隙水中锰碳酸盐浓缩和沉淀的先决条件。这项研究支持这样的观点，即古生代深海区域持续缺氧，并定期被沿海上升流开采以产生富含锰和铁的矿床。将此新兴矿床模型应用于其他具有重要经济意义的碳酸盐岩锰矿床可能会改善资源勘探。