湘南地区东湘桥沉积型锰矿床地球化学特征及成矿机理

Geochemical characteristics and metallogenic mechanism of Dongxiangqiao sedimentary manganese deposit in southern Hunan Province

湘南地区广泛发育二叠纪海相沉积型锰矿,为揭示其成矿过程及机理,文章以湖南永州东湘桥锰矿床沉积剖面为研究对象,对其开展主、微量元素和碳-氧同位素分析。结果表明,富锰岩层富Cr、Ni、Co等元素的地球化学特征,其Co/Zn、Mn/Fe、Y/Ho比值和多判别图解共同指示该矿床为热液沉积,表明成矿物质Mn主要来源于海底热液。Mo和U元素富集因子(MoEF和UEF)、U/Th和V/(V+Ni)比值等沉积氧化还原指标分析结果表明,高品位锰矿石层产于氧化向缺氧转变的背景下,而低品位含锰岩层则主要产于氧化或强烈缺氧环境下。富锰岩层的δ^(13)C_(PDB)(-0.2‰~1.9‰)和δ^(18)O_(SOMW)(18.1‰~23.1‰)与海相碳酸盐岩相似,表明成矿物质的C和O主要来源于海水。基于上述结果,笔者认为东湘桥锰矿床成矿过程受海水CO_(3)^(2-)浓度和沉积氧化还原条件的双重控制,其成矿过程可以概况为2个阶段:①深水盆地的热液Mn在还原条件下以可溶Mn^(2+)向近岸CO_(2)-3过饱和的浅水台地迁移;②当沉积环境由氧化向缺氧转变时,锰氧化物在沉积/水界面还原释放Mn^(2+),显著提高水柱中溶解Mn浓度,促进富锰碳酸盐矿物析出,形成高品位锰矿石。

The Permian marine sedimentary manganese deposits are widely developed in the southern Hunan region.In order to reveal the ore-forming processes and mechanisms,this study focuses on the sedimentary profile of the Dongxiangqiao manganese deposit in Yongzhou,Hunan Province.Major and trace elements,as well as carbonoxygen isotopes,were analyzed.The results indicate that the manganese-rich rock layers enrich in elements such as Cr,Ni,Co.The ratios of Co/Zn,Mn/Fe,and Y/Ho,along with interpretations from multiple discriminant diagrams,collectively indicate that the ore deposit is a hydrothermal deposit,suggesting that the ore-forming material,manganese(Mn),primarily derived from seafloor hydrothermal fluid.The analysis results of sedimentary redox indicators,such as the enrichment factor of Mo and U(MoEF and UEF),as well as U/Th and V/(V+Ni)ratios,indicate that the layers of high-grade manganese ores were formed in the context of the transition from oxic to anoxic conditions.In contrast,low-grade manganese-bearing rock layers were mainly formed in oxic or strongly anoxic conditions.The δ^(13)C_(PDB)(-0.2‰~1.9‰)and δ^(18)O_(SOMW)(18.1‰~23.1‰)values of the manganese-rich rock layers,similar to marine carbonate rocks,suggest that the sources of ore-forming materials C and O are primarily derived from seawater.Based on these results,this study proposes that the ore-forming process of the Dongxiangqiao manganese deposit is controlled by both seawater CO_(2)-3 concentration and sedimentary redox conditions.The ore-forming process can be summarized infollowing two stages:①Hydrothermal Mn in deepwater basins migrates towards nearshore CO_(32)^(2-) supersaturated shallow-water platforms under reducing conditions;②When the sedimentary environment transitions from oxic to anoxic conditions,manganese oxides at the sediment/water interface are reduced to release Mn^(2+),significantly increasing the dissolved Mn concentration in the water column,promoting the precipitation of manganese-rich carbonate minerals,and forming high-grade manganese ores.