| 其他摘要 | 新元古代晚期两次全球性冰期事件在我国华南地区广泛分布，在冰期之间沉积了一套保存良好冰期记录的沉积地层，并沉积了具有重要经济价值的锰矿资源和黄铁矿资源。大塘坡组地层在浅水相和过渡相主要以一套锰碳酸盐岩、黑色页岩和粉砂岩为主，在深海盆地相以变质砂岩-变质粉砂岩-硅质板岩建造为主。华南大塘坡期保存完好的沉积地层为古海洋环境的反演提供了优良的地质背景，本文对华南大塘坡期典型的大降坪黄铁矿矿床和大塘坡式含锰建造中黄铁矿进行矿相学、岩石学、元素地球化学及同位素地球化学的研究，取得以下主要成果：1) 首次对大降坪超大型硫铁矿矿床的黄铁矿进行了精细的分类。 Pym1：块状矿中的微细粒（20-40μm）他形致密黄铁矿集合体，代表了硫化物丘中快速沉淀的原生黄铁矿，具有最高的S/Fe比（2.12），微区分析厘定的S同位素平均值1.3‰（n=19）。Pym2：块状矿中的中粗晶他形到自形碎裂黄铁矿集合体，普遍伴生微细粒方铅矿，主要产于裂隙中，少量包裹于黄铁矿中，几乎未见闪锌矿伴生，代表了Pym1在热水活动晚期遭受低温热水改造或带状精炼的产物；S/Fe比值为2.04，厘定高正异常端元的平均值为20.2‰(n=25)。Pym3：块状矿中的中粗晶他形到自形碎裂黄铁矿集合体，与Pym2不同的是脉石矿物中有较为丰富的夕卡岩型矿物如白云母，代表了Pym2在后期遭受矽卡岩化改造的产物；S/Fe比值为2.08，微区分析的S同位素平均值为-10.5‰，厘定的端元平均值为-12.1‰（n=14）。Pyb1：条带矿中保存的微细粒（2-10μm）自形分散浸染状黄铁矿，局部观察到伴生闪锌矿条带，代表了热水羽原始沉积形成的硫化物，微区分析厘定的S同位素平均值为-4.2‰（n=17）。Pyb2：三号矿区条带矿中的细晶到中粗晶半自形到自形浸染状条带状黄铁矿，伴生磁黄铁矿、硫锰矿、方铅矿、闪锌矿及少量的砷锑镍矿，脉石矿物为透闪石、透辉石、方解石等，S/Fe比值为2.05，微区分析厘定的S同位素平均值为-30.0‰（n=41)。2）大塘坡期大降坪超大型硫铁矿具有极端分馏的硫同位素值。挑选的单矿物硫同位素特征为：条带状黄铁矿-7.6~-27.8‰，平均为-18.2‰；块状黄铁矿分为Ⅲ组：a. -22‰~-6‰，平均为-12.1‰；b. -4‰~8‰，平均为2.6‰；c. 8‰~26‰，平均为16.2‰。通过微区硫同位素分析厘定的原生热水沉积黄铁矿硫同位素位于零值附近，分别为Pym1（δ34S值-12.6‰~+5.9‰，平均为-3.6‰）和Pyb1（δ34S值-8.8‰~+0.2‰，平均为-4.2‰）；受晚期海底低温热液改造的块状黄铁矿Pym2表现为正值（δ34S值-0.5‰~22.9‰，平均为14.2‰），遭受后期辉绿岩脉影响的块状黄铁矿Pym3表现为中等负值(δ34S值-14.0‰~-7.2‰，平均为-10.5‰），遭受后期辉绿岩强烈改造的条带状黄铁矿Pyb2表现为较低的负值（δ34S值-32.9‰~-28.5‰，平均为-30.0‰）。这些硫同位素特征表明原始热水沉积的黄铁矿经历了两期改造，晚期海底低温热液利用海水或早期形成的硫酸盐通过热化学还原降解形成具较高硫同位素值的中粗晶黄铁矿，后期辉绿岩脉侵入对地层有机硫的热还原降解形成极负值的硫同位素。3) 完善了大塘坡期超大型硫铁矿矿床的热水沉积模式。大降坪黄铁矿矿床的形成类似于现代海底热水沉积成矿作用，块状黄铁矿矿体相当于海底喷流沉积形成的硫化物丘，冷海水沿洋壳裂隙下渗并被加热后的海水淋滤并溶解硫，之后海水沿断裂、裂隙上升喷出，与冷的正常的海水混合，由于热水 ( 200～400℃) 与周围海水温度 ( 2℃)及成分的巨大差异，热液快速沉淀，形成粒度细小的硫化物和硫酸盐，并在热液喷口附近热液通道浅部沉淀形成硫化物丘。喷口处高温热液向上迁移，与周围冷海水混合后形成热水羽，热水羽携带大量的Fe、Pb、Zn等金属元素在远离喷口端形成品位较低的条带状矿体。当热水体系温度逐渐降低，后期的低温热液对早期形成的硫化物丘进行了一系列改造，主要包括：低温热液溶解早期形成的石膏等硫酸盐，进而对早期微细晶黄铁矿改造形成具有明显S同位素正异常的粗晶黄铁矿；对早期的铅锌硫化物的溶解和再分配，即锌的带状精炼过程，形成铅锌的明显分异现象，在大降坪表现为块状矿中普遍含有裂隙型微细方铅矿，但几乎没有伴生闪锌矿，分异出的闪锌矿局部在条带矿中富集。4) 总结了大塘坡期不同沉积环境中黄铁矿硫同位素的空间展布特征，异常高的硫同位素值主要发育在扬子地台边缘的浅水盆地相（一系列大塘坡式成锰盆地），在过渡相（盆架山）依然保持硫同位素的高正特征，在深海盆地相如广西三江一带趋于零值附近；在华夏古陆向华南裂谷过渡的浅水盆地相，原生热水沉积成因的黄铁矿硫同位素趋于零值附近。; The two global glacial events (Sturtian and Marinoan) in the late-Neoproterozoic are widely distributed in south China, and a glacial sedimentary stratigraphy is deposited during the interglacial period, at the same time, manganese resources and pyrite resources with significant economic value are formed. The Datangpo Formation is dominated by a set of manganese carbonatite, black shale and siltstone in the shallow water and the transitional phase, and it is mainly composed of the metamorphic sandstone-metamorphic siltstone-siliceous slate in the deep basin. The well-preserved sedimentary strata in the South China have provided an excellent geological background for the back analysis of the paleoceanic environment. In this study, We focused on the petrology, mineralogy, elemental geochemistry and isotope geochemistry of the typical Dajiangping pyrite deposits and the pyrite of the Datangpo-type manganese deposits, and achieved the following main results.1) The pyrite of giant Dajiangping is detailed classified for the first time. Pym1: micro fine pyrite particles (20-40μm), allotriomorphic granular and the dense massive pyrite represents the primary pyrite precipitated rapidly in the sulfide mound, with the highest S / Fe ratio (2.12), and sulfur isotope as an average value of 1.3 ‰ (n = 19). Pym2: the medium-coarse crystalline allotriomorphic granular to the automorphic granular dense massive pyrite, generally associated with micro fine particles of galena, which mainly distributed in the cracks of pyrite, and minor wrapped in pyrite, without sphalerite. The S / Fe ratio is 2.04, and the average value of the sulfure isotope positive anomalies is 20.2 ‰ (n = 25), which probably is the result of low temperature hydrothermal transformation or band refining from Pym1. Pym3: the medium-coarse crystalline allotriomorphic granular to the automorphic granular dense massive pyrite, and the gangue minerals are more abundant skarn-type minerals than Pym2, such as muscovite, represented the Pym2 in the late stage. The S / Fe ratio is 2.08, the S isotope analysed by Nano-SIMS is -10.5 ‰, and the average value of the end element is -12.1 ‰ (n = 14). Pyb1: micro fine particles (2-10μm), the automorphic disseminated granular preserved in the band pyrite locally observed sphalerite strip, represented the sulfide formed by the original deposition of hydrothermal plume. The sulfure isotope average value is -4.2 ‰ (n = 17). Pyb2: fine-grained to medium-grained, the hypidiotopic to the automorphic granular disseminated and band pyrite, associated pyrrhotite, galena, sphalerite and a small amount of arite. The S / Fe ratio is 2.05, and the sulfure isotope average value is -30.0 ‰ (n = 41).2) Datangpo period giant pyrite deposits in Dajiangping shown sulfur isotope value of extreme fractionation. The sulfur isotopic value of the selected pyrite have the following characteristics: Striped pyrite -7.6 ~ -27.8 ‰, with an average value of -18.2 ‰. The massive pyrite is divided into three groups: a.22 ‰ ~ -6 ‰, an average value of -12.1 ‰; b. -4 ‰ ~ 8 ‰, an average value of 2.6 ‰; c. 8 ‰ ~ 26 ‰, an average value of 16.2 ‰. The sulfur isotopes of the primary pyrite are determined to be near the zero value by means of the Nano-SIMS sulfur isotope. Pym1 (δ34S value: -12.6 ‰ ~ +5.9 ‰, mean -3.6 ‰) and Pyb1 (δ34S value -8.8 ‰~+0.2 ‰, mean -4.2 ‰); Pym2 was affected by late-stage low-temperature hydrothermal fluids with positive sulfure isotopes (δ34S value -0.5 ‰ ~ 22.9 ‰, with an average value of 14.2 ‰), and Pym3 was influenced the later stage diabase vein with medium sulfure isotopes (δ34S value -14.0 ‰ ~ -7.2 ‰, with an average value of -10.5 ‰), Pyb2 was influenced the later stage diabase vein with lower negative sulfure isotopes value (δ34S value -32.9 ‰ ~ -28.5 ‰, with an average value of -30.0 ‰). These sulfur isotopic characteristics indicate that the original hydrothermal sedimentary pyrite undergone two stages of alteration: Late-stage alteration are characterized by submarine low temperature hydrothermal fluids with the use of seawater or early formation of sulphates by thermochemical reduction to form a higher sulfur isotope value; and later diabase veins invade into stratum with the thermal reduction of organic sulfur to form a very negative sulfur isotope.3) Perfect the hydrothermal deposition model of the superscale pyrite deposit in the Datangpo stage. The formation of the Dajiangping pyrite deposit is similar to the modern submarine hydrothermal depositional mineralization. The massive pyrite is equivalent to the sulfide mound formed by the submarine exhalative sedimentation. The cold sea water is infiltrated along the oceanic crust and is heated and then the heated seawater along the fracture or cracks rise and mixed with cold seawater. Because of the different ingredients and temperature between hydrothermal fluid (200 ~ 400 ℃) and the surrounding seawater (2 ℃), hydrothermal fluid rapidly precipited, small size sulfide and sulfate formed, and formed sulfide mound near the hydrothermal vent. High temperature hydrothermal fluid upward migrate and formed the hydrothermal plume after mixed with cold water. The hydrothermal plume carried a large amount of Fe, Pb, Zn and other metal elements to form a lower grade band pyrite in the distance from the vent. When the temperature of the hydrothermal system is gradually decreased, the later low temperature hydrothermal fluid reconstructed the early sulfide mound, including: low temperature hydrothermal fluid dissolved the early gypsum and other sulphates, and then transformed the early micro-crystal pyrite to form the the pyrite with obviously positive sulfur isotope; the dissolution and redistribution of the early lead-zinc sulfide, that is, the zone refining process of zinc, the formation of lead and zinc obvious differentiation. Massive pyrite generally contains micro fine galena, but almost no associated sphalerite, the division of the sphalerite enriched in the band pyrite.4) Summary the spatial distribution of sulfur isotopes of pyrite in different sedimentary environments. The unusually high sulfur isotope values are mainly developed in the shallow basin of the Yangtze platform (a series of Datangpo type manganese-forming basin). In the transitional phase (Penjiashan) still maintain the high positive characteristics of sulfur isotopes. In the basin phase, such as the Sanjiang area, Guangxi tends to zero; the Cathysia to South China rift transitional shallow basin phase, sulfur isotopes of primary hydrothermal pyrite tend to near zero. |
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