西昆仑晚古生代恰尔隆弧后盆地演化与锰成矿作用

The evolution and manganese mineralization of the Late Paleozoic Qiaerlong back-arc basin in the Western Kunlun Mountains, China

晚古生代,康西瓦洋壳指向西昆仑地块俯冲,在恰尔隆盆地内沉积了下石炭统—上二叠统的一套以海进-海退为特征的旋回序列。新发现的主乌鲁克碳酸锰矿床位于恰尔隆盆地北部,含锰层位为下石炭统他龙群中段细碎屑岩夹碳酸盐岩,锰矿层为黑色泥质碳质页岩夹铁锰质微晶灰岩,显示明显韵律。锰矿石的Mn品位为8.00%~16.59%,Mn/Fe比值为0.36~1.46,铁锰分异不明显。矿石矿物主要为锰碳酸盐矿物,具有典型生物结构。碳酸锰矿石的正Eu异常(δEu=3.46±2.00)显示成矿物质主要为海底热水来源。碳酸锰矿石以及围岩的^(87)Sr/^(86)Sr偏向于大陆地壳组成(0.711925±0.000601),表明成矿物质主要来自西昆仑古老陆壳基底。锰矿石中不明显的Ce异常(δCe=0.96±0.14)和低的(Mo/U)_(auth)(自生矿物的Mo/U摩尔浓度比值,5.36±4.41)显示其主要沉积于局部快速氧化环境;而明显的Mo富集(平均Mo_(auth)=99.14±97.04)和纹层状自形黄铁矿表明围岩主要形成于还原环境。碳酸锰矿石的有机碳同位素组成(δ^(13)C_(org-PDB)=-24.65‰±2.12‰)、无机碳同位素的组成和变化趋势(δ^(13)C_(carb-PDB)=-12.67‰±3.86‰)以及总有机碳含量(TOC)与锰品位成负相关,表明锰可能最初以(氢)氧化物形式沉淀,而后在成岩埋藏过程中被有机质还原。总之,在早石炭世的恰尔隆盆地内,海侵过程带来丰富的自由氧和有机质,沉积了一套黑色泥质碳质页岩夹铁锰质微晶灰岩。同生断层活动带来富Mn^(2+)热水;Mn^(2+)在短暂的氧化条件下被氧化为Mn^(3+)/Mn^(4+)的(氢)氧化物。由于含氧量不足,铁和锰未完全分离。随着沉积作用进行,这些(氢)氧化物被表层沉积物覆盖而埋藏。随后在缺氧孔隙水中,Mn^(3+)/Mn^(4+)的(氢)氧化物被有机质还原为Mn^(2+)碳酸盐矿物。晚石炭世,海平面上升,Mn^(2+)的氧化作用逐渐减弱,使得上覆地层缺乏含锰层位。

In the Western Kunlun Mountains,due to the subduction of the Kangxiwa oceanic crust towards the Western Kunlun Block in the Late Paleozoic,a series of Lower Carboniferous to Upper Permian sedimentary sequences characterized with a single transgression-regression cycle were deposited in the Qiaerlong Basin.The newly discovered Zhuwuluke Mn ore deposit is located in the northern part of the Qiaerlong Basin.Its orebodies are hosted in fine clastic rocks with carbonates in the middle part of the Lower Carboniferous Talong Group.The ore-hosting layer,consisting of black argillaceous carbonaceous shale interlayered with ferric manganese microcrystalline limestone,has obvious rhythmic layering structure.The ores have Mn grades varying from 8.00%to 16.59%and Mn/Fe ratios varying from 0.36 to 1.46,with no obvious differentiation between iron and manganese.Manganese carbonate minerals including Ca-rhodochrosite,Fe-rhodochrosite,Mn-siderite,and kutnahorite,are main ore minerals with typical oolitic and biological textures.The positive Eu anomalies (δEu=3.46±2.00) of manganese carbonate ores indicate that the ore-forming materials were mainly sourced from the hydrothermal fluid.The^(87)Sr/^(86)Sr ratios of the manganese carbonate ores and wall rocks are relatively close to the value of continental crust (0.711925±0.000601),indicating that the ore-forming materials could be taken from the ancient Western Kunlun basement by the hydrothermal fluid.The indistinctive Ce anomalies (δCe=0.96±0.14) and low (Mo/U)_(auth)ratios (Mo/U molar concentration ratios of authigenic minerals,with the average of 5.36±4.41) suggest that the manganese carbonate ores were mainly formed in a local water column with rapid oxidation environment.However,the obvious enrichment of Mo (average Mo_(auth value of)99.14±97.04) and the existence of laminated euhedral pyrite aggregates in wall rocks suggest that the wall rocks were mainly formed under a reduction condition.The C isotope compositions of organic carbons (δ^(13)C_(org-PDB)=-24.65‰±2.12‰) and inorganic carbons (δ^(13)C_(carb-PDB)=-12.67‰±3.86‰) and their variation trends,and the negative relationship between the total organic carbon contents (TOC) and Mn grades of manganese carbonate ores suggest that the initial Mn could be precipitated in the form of Mn-oxyhydroxides which were then reduced by organic matters in the diagenetic burial process.Thus,we have suggested that the transgression during the Early Carboniferous period had brought abundant organic matters into the Qiaerlong Basin and then resulted in the sedimentation of a suite of black argillaceous carbonaceous shale intercalated with ferromanganese-bearing microcrystalline limestone in the Qiaerlong basin.The Mn^(2+)-enriched hydrothermal fluid was brought into the basin through the active syngenetic fault structures.Then the Mn^(2+)was oxidized to Mn-oxyhydroxides(Mn^(3+)/Mn^(4+)) under the transient oxidizing condition.The relatively high contents of Fe in samples including manganese carbonate minerals and some wall rocks could be attributed to the insufficiency of oxygen.The Mn-oxyhydroxides were covered and then buried by surficial sediments at that time with the development of the sedimentary process,and then were reduced by organic matters in anoxic pore water to have formed manganese carbonate minerals.Whereafter,in the Late Carboniferous,due to the rise of the sea-level,the oxidation of Mn^(2+)was gradually weakened.This resulted in the absence of the Mn-enriched layer in the overlying strata.