扬子东南缘“湘潭式”锰矿的地球化学特征及成矿机制

Genesis of Cryogenian Xiangtan-type manganese deposits in Hunan Province,China:Constraints from geochemical evidence

湖南“湘潭式”锰矿是华南扬子地块东南缘南华纪沉积型锰矿的重要组成部分。含锰岩系赋存于大塘坡组底部碳酸锰层中,矿石类型主要为致密块状、条带状和互层状碳酸锰矿石。菱锰矿呈显微“球粒”状,多具3层圈层构造,类似“薄皮鲕”的特征。锰矿石MnO含量为12.08%~40.02%,平均28.93%,具有较低的Fe/Mn值(0.01~0.32)和较高的P含量(0.058%~0.190%),从工业用途的角度,属低Fe高P型锰矿。“湘潭式”锰矿石微量元素相对PAAS具有明显的Co、Mo、Sr富集和V、Cr、Ni、U等亏损,中稀土元素相对轻、重稀土元素富集,呈“帽式”配分模式。“湘潭式”锰矿与黔东“大塘坡式”锰矿具有相似的微量元素富集亏损特征和稀土元素配分模式,反映了它们可能存在相似的成矿背景。“湘潭式”锰矿石较低的Al/Mn(0.02~0.35)、Al/(Al+Fe+Mn)值(<0.35)和较高的(Fe+Mn)/Ti值(>25),表明锰质主要来源于海底热液活动,但锰矿石Ce正异常及弱的Eu负异常表明其并非典型的海相热水沉积,且Al_(2)O_(3)-SiO_(2)图解显示锰矿石为正常海水沉积。综合认为,“湘潭式”锰矿的锰质来源于非成矿期的海底热液活动。锰矿石Fe、Mn分离显著,U、V亏损和Mo富集的特征表明Mn沉淀时水体环境可能较为氧化。锰矿石具有明显的Ce正异常,且Ce/Ce^(*)值与Mn含量之间表现为显著正相关关系,表明Mn是在氧化环境中,以氧化物或氢氧化物的形式沉淀富集,具有与现代海底水成Fe-Mn结核类似的沉淀成因机制。与“大塘坡式”锰矿床类似,“湘潭式”锰矿具有负的δ^(13)C_(carb)值(−9.9‰~−4.3‰),锰矿石中Mn含量和δ^(13)C_(carb)值具有显著相关性,同时考虑到锰矿石S元素的陆源特征、黄铁矿的缺乏和新元古代海水中较低的SO_(4)^(2−)含量,有机质早期成岩阶段,锰氧化物作为主要电子受体氧化有机质为锰碳酸盐岩沉淀提供了HCO_(3)^(−)。但锰矿石δ^(13)C_(carb)明显高于其有机碳库δ^(13)C_(org)(−34.4‰~−32.58‰),且比地质历史时期重要碳酸锰矿床的δ^(13)C_(carb)值更为集中,说明海水可能提供了更多的碳源,锰碳酸盐岩沉淀可能发生在靠近海水–沉积物界面的位置。

The Xiangtan-type manganese(Mn)ore deposit located at the southeastern margin of the Yangtze Block belongs to sedimentary Mn deposit of the Neoproterozoic Nanhua System(Cryogenian).The Mn-bearing sedimentary sequence is located at the base of the Datangpo Formation,and mainly consists of Mn carbonates with massive,banded,and interbeded structures.Electron probe microanalysis shows that each micritic Mn carbonate grain has oolitic texture with three obvious layers.The ores have 12.08%to 40.02%MnO(average 28.93%).The Fe/Mn ratios range from 0.01 to 0.32,and P contents range from 0.058%to 0.190%.The ores normalized to average Post-Archean Australian shale(PAAS)are enriched in Co,Mo,and Sr,and are depleted in V,Cr,Ni,and U.PAAS-normalized REE distribution patterns of Mn ores show relative MREE enrichment compared with LREE and HREE,and obvious positive Ce anomalies.The two distribution patterns are similar to those of the Datangpo-type Mn ore deposits in southern Guizhou province,implying that the ores may have a similar metallogenic mechanism.The low Al/Mn(0.02 to 0.35),Al/(Al+Fe+Mn)ratios(<0.35)and high(Fe+Mn)/Ti ratios(>25)indicate that the Mn source was very likely to have been submarine hydrothermal.However,positive Ce anomalies and negative Eu anomalies imply that the ores are not typical hydrothermal products,but of the hydrogenetic type based on an Al_(2)O_(3)-SiO_(2) graph.The Xiangtan-type Mn ores show very low Fe/Mn ratios,depletion of U and V,and enrichment of Mo,indicating that the depositional environment may have been oxidic.Positive Ce anomalies show a significant positive correlation with MnO contents,suggesting that Mn has been oxidized into Mn oxides and absorbed relatively high quantities of Ce during precipitation,similar to the genetic mechanism of marine hydrogenous Fe-Mn crusts/nodules.Similar to the Datangpo-type Mn ore deposits,the Xiangtan-type Mn ores exhibit obvious negativeδ^(13)C_(carb)(−9.9‰to−4.3‰)andδ^(13)C_(carb) values and have a strong negative correlation with Mn contents in ores.Considering the terrigenous source of sulfur,lack of pyrite and low SO_(4)^(2−) in Neoproterozoic seawater,organically derived HCO_(3)^(−) may have been produced mainly from the reduction of Mn oxides rather than SO_(4)^(2−).However,the evidence thatδ^(13)C_(carb) values are homogeneous and lighter than those of organic matter(−34.4‰to−32.58‰)indicates that Mn carbonate precipitation may have occurred near the sediment-seawater interface.