| 摘要 | 氧化还原敏感元素硒的地球化学循环非常复杂, 它的循环主要受氧化还原势、酸碱度、温度、压力等条件的控制。长期以来，关于硒的主要研究集中在环境生物地球化学领域。对于硒的地质地球化学行为的研究，学者们认为硒不可能独立形成矿床,它主要以伴生元素的形式赋存于一些热液矿床（如，拉尔玛硒-金热液矿床等）。 二十世纪末, 我国学者开始关注硒的矿床地球化学行为, 发现了目前唯一的渔塘坝沉积型的独立硒矿床。另外，下寒武统底部黑色岩系（如，遵义牛蹄塘组）处于地球发展演化的关键时期且富集了众多的有用金属元素, 因此它引起了全世界广大地质学家的普遍关注和重视。这些黑色岩系的共同特点是含有大量的有机质和丰富的金属元素(PGE，Cu，Ni，Mo，Au，U，V，Mn，Fe，Co，Bi，Cr，Se等)。但遵义牛蹄塘组中Ni-Mo多金属层的成因一直争论不休。
本文以拉尔玛硒-金热液矿床、渔塘坝独立硒矿床、遵义黄家湾含Ni-Mo-Se多金属层剖面为研究对象，主要通过硒稳定同位素的测试和不同化学形态的分析结合其他的地球化学参数（如，C-S-Fe体系，氧化还原敏感元素），示踪富硒地层的沉积环境，研究不同地质体系中硒稳定同位素的组成，进而完善硒同位素的理论体系，探讨硒富集过程中的形态迁移途径，并为矿床的形成提供更完善的地球化学证据。通过一系列的研究工作，我们取得了以下几点认识：
(1) 通过不同的消解方法准确测定了低硒,高硒样品的总硒含量,相对标准偏差小于10%，而且不同的消解方法测得同样的结果, 满足了地质样品中微量元素的测试要求。由于不同消解方式存在自身的优缺点,我们建议根据不同的样品类型,不同的研究目的选取合适的消解方法。
(2) 通过巯基棉吸附装置达到了纯化富集样品硒的目的, 硒的回收率一般大于90%, 满足同位素测试的需要。应用自制的氢化物发生器与Nu-MC-ICP-MS联用,实现了在线气体进样测试硒稳定同位素的目的。 采用标准样品匹配测试方法校正仪器测试过程中的质量分馏。 硒稳定同位素的测试精度为2δ=0.30‰, 标准NIST SRM 3149采用与样品同样的处理方法,没有发现前处理过程的同位素分馏。
(3) 传统地球化学参数对沉积环境的指示意义。通过氧化还原敏感微量元素及其与TOC的关系和C-S-Fe体系的分析，对渔塘坝独立硒矿床的多元素富集及沉积环境, 遵义黑色岩系沉积环境进行了详细的解释。TS-TFe的关系表明样品中有过量的硫存在，可能为有机硫或为其他亲硫元素提供有利的沉淀条件。C-S-Fe体系及微量元素指数V/(V + Ni) 说明渔塘坝矿区的硅质岩和页岩的沉积环境为缺氧到静海环境， Ni/Co、V/Cr几乎不能作为该区缺氧环境的指示参数, 后者可能由于Cr的外源输入（碎屑、热液等）所致。 C-S-Fe体系及微量元素指数V/(V + Ni)体系指示遵义黑色岩系的沉积环境为微含氧到缺氧的条件, 但对于Ni-Mo矿而言, V/(V + Ni)体系指示其为含氧条件沉积，这可能与Ni-Mo多金属层的成因有关。但铁的硫化度可以指示Ni-Mo多金属层极端的缺氧还原环境。
(4) 硒的形态对古氧化还原条件和矿物质来源的示踪意义。黑色岩系的硒形态分布不同于海洋沉积物，体现了成岩作用对硒形态分布的改变。黑色岩系中硒的主要形态为有机结合态和硫化物/硒化物结合态。硫化物/硒化物结合态比例与铁的硫化程度（DOS）之间明显的相关关系说明在海洋环境中硒主要通过氧化还原反应富集在富有机质的沉积物与沉积岩中。这种相关关系与岩石类型没有关系，这使得将硫化物、硒化物结合的Se（-II）比例作为一个氧化还原条件示踪剂更加可行。DOS与硫化物/硒化物结合态硒的关系，及Se（IV）与Se（-II）的关系均说明遵义牛蹄塘组的K-斑脱岩形成于碱性的氧化环境，Ni-Mo多金属层沉积于微酸性的极端还原环境，而渔塘坝矿床形成于微碱性的还原缺氧环境。生物的同化作用与异化作用之间本身存在互补关系，但这种互补关系却存在不同的转化趋势。不同的转化趋势可能主要受氧化还原反应和酸碱度的控制，其他的地质作用也可能起着重要作用（如，硅酸盐、Fe（II）的含量，硫化物、有机质含量，风化程度等）。同时我们发现相同的氧化还原条件下可能存在不同的富集途径。Ni-Mo多金属层中极少的有机结合态硒暗示海洋同生沉积主要被生物异化还原控制，而沉积物从海洋富集硒的途径主要为直接由Se（VI）和Se（IV）到Se（-II）的还原途径，从高价态到元素态的还原途径可能偶有发生但强度很小。
(5) 硒稳定同位素对矿床成因及物质来源的示踪意义。热液或表生环境中,硒的再次活化迁移对硒同位素的大范围分馏是很重要的。较少的硫化物结合态硒和较大范围的硒同位素组成说明渔塘坝矿床形成时经历了多次氧化还原过程。干酪根硒同位素组成与全岩相似, 而且样品富集轻同位素暗示该矿床硒来的富集主要通过海洋硒的生物吸收同化异化还原与多次无机氧化还原实现。对于拉尔玛金硒矿床而言，未蚀变岩体的硫化物结合态硒富集轻同位素，而后期低温流体的蚀变作用导致蚀变岩体中硫化物结合态硒富集重同位素。干酪根的硒同位素组成暗示热液输入的硒为该矿床硒的主要来源，硒从热液直接进入干酪根的机制是一致的。随着硫含量的增加岩石和矿体中的硒逐渐富集重同位素, 说明海水对矿床富集的贡献是很有限的。遵义黑色岩系中硒同位素组成与Se（-II）之间的关系, 单质硒的缺乏，硒的富集与同位素的关系说明，热液为主要的硒来源，而早寒武世海水中的硒主要通过Se（VI）和Se（IV）到Se（-II）的直接生物还原实现。三个区域的S/Se比值综合说明硫与硒的共同沉降过程中不存在硒的同位素分馏。不同化学形态的分布与不同形态中硒稳定同位素的结合可能会更好的解释硒的全球地球化学循环。
(6) 根据目前硒的同位素分馏体系及所测得的数据,我们初步建立了不同时代、不同成因黑色岩系中硒稳定同位素的分馏模式。 |
| 其他摘要 | The geochemical behavior of redox-sensitive selenium (Se) is extraordinary complex in natural environment, and it mainly dependents on Eh, PH, temperature and pressure. Se was studied for many years in environmental and biological geochemistry. For a long time, scholars peoposed that the formation of independence Se deposit is impossible in nature, and occurrence of Se in some hydrothermal deposition is associated element (eg.Laerma Se-Au hydrothermal deposit, etc.). The end of last century, however, scholars found that there is the Yutangba independence Se deposition in China that is the only sedimentary Se ore deposit. The black rocks series in bottom of Niutitang Formation (eg. Niutitang Formation of Zunyi region) deposited during the critical period of earth evolution. And some metal elements, such as PGE，Cu，Ni，Mo，Au，U，V，Mn，Fe，Co，Bi，Cr，Se, enriched in the strata where contain amount organic material. Up to now, the argument of the genesis of Ni-Mo polymetallic layer was going on for thirty years.
The Laerma Se-Au hydrothermal deposit, the Yutangba independence Se deposition and the section containing Ni-Mo-Se polymetallic layer in Zunyi were selected as objects in our study. The Se isotope and chemical speciation combined with other geochemical factors (eg. C-S-Fe systmatics and redox-sensitive elements) was applied. The goals of the paper include that understanding Se isotopic composition of the different systematics, completing the theoretical system of Se isotope, investigating the transportation pathway of Se speciation during sedimentary processes and providing some stronger geochemical evidences. We achieved some primary conclusions from the study, in following:
(1) The total Se of some geological samples（with the low Se and the high Se）was digested by different methods and the results are consistent. The relative standard deviation is less than 10%, which satisfied with requirement of the determination of selenium in geological samples at or below the g/g level. Due to different characteristics of methods, we suggest that the different methods should be applied based on the sample types and the aims of study.
(2) The thiol cotton fiber (TCF) was utilized to enrich and sublimate Se within samples solution. And the recovery rate of TCF for our samples solution is higher than 90%. The isotopic Standards were used in our experiment, including MERCK and NIST SRM 3149. And NIST SRM 3149 was treated by the same method that was applied to treat other natural samples. Quantitative recovery of Se ensures that samples are not isotopically shifted, and this enables analysis by MC-ICP-MS without a double spike. The manner continuous flow hydride-generation was combined with Nu-MC-ICP-MS before analysis, so that the hydrides by hydride-generation are transported via Teflon tubing directly to the ICP torch. Sample-standard bracketing was used, with 1:1 alternation of sample and standard. This method yielded a precision of ±0.30‰ on 82/76Se, consistent with that of other international laboratory.
(3) The implication of traditional geochemical parameters for the paleo-redox of ore deposition: The C-S-Fe systematic and the correlation between redox-sensitive elements and organic carbon were utilized, which indicates that the Yutangba independence deposition and black rocks series formed under anoxic to euxinic environment. The relation of TS-TFe suggests excess sulfur may be organic sulfur or supplication of free H2S. The ratios of V/(V + Ni) also imply that the black shales and cherts deposited under anoxic to euxinic condition in Yutangba area. However, Ni/Co、V/Cr can not be used as the index for anoxic environment. The latter phenomenon results from the disturbing of outer Cr, such as the decrial and hydrothermal inletting. The C-S-Fe systematic and V/(V + Ni) ratios can not reflect the sedimentary processes for Ni-Mo polymetallic layer in the Niutitang Formation of Zunyi due to interference of the hydrothermal fluid. The degree of sulfidation (DOS), however, can infer the extremely anoxic environment of Ni-Mo polymetallic layer.
(4) The implication of Se speciation for the paleo-environment condition and the source of material of ore deposition: The distribution of Se speciation for black series rocks is different from modern sediments, which implies that the diagenetic processes played an important role in changing Se speciation. The base soluble organic associated Se and the sulfide-selenide Se are the major speciation in the black rocks series. For our samples, there is a good correlation between the percentage of Se (-II) and the degree of sulfidation (DOS), which suggests that the enrichment of Se is controlled by redox reactions in sediments and sedimentary rocks enrich-organic matter. And the correlation is independent of the rock types, providing a strong support for the use of Se(-II) to deduce the extent of the reducing reactions in sedimentary processes. The negative correlation between Se(-II) and DOS and the identical correlation between Se(IV) and Se(-II) suggest that there was subtle change in redox condition and pH from the slightly alkaline K-bentonite, black shales and cherts, to the acidic Ni-Mo layer of the Niutitang Formation in Zunyi area, and also imply that the black shaled of Yutangba region formed under slightly alkaline reducing condition. There is the native strong correlation between the assimilatory and the dissmilatory reduction in nature. Interesting, the transformation of the correlation shows the different trends in different environment. And the different trends may be controlled by redox and pH, but other geological processes could not be excluded. In addition, we found that there are different pathways under the same redox environment. The little proportion of base soluble Se extracted by NaOH solution of Ni-Mo polymetallic layer suggests that synsedimentary from seawater under anoxic condition is dominated by the dissimilatory reduction. And the major pathway of Se enrichment for black series rocks from seawater is reduction from Se(VI) and Se(IV) , beyond Se(0), directly to Se(-II) .
(5) The implication of Se isotope for the genesis of ore deposition and the source of material for ore deposition: For the larger Se isotopic fractionation, the secondary mobilization of Se by hydrothermal or supergene alteration is important. The correlation of the Se isotopic ratio between of kerogen and whole rock suggests that the major Se source of the Yutangba region is the adsorption of organic material and mineral particulates and the biotic and abiotic reduction from seawater. And the less percentage of sulfide-diselenide associated Se and the more organic associated Se indicate that the ore deposit has experienced multiply redox processes. For the Laerma Se-Au ore deposit, the lighter Se isotope enrich in the sulfide of unaltered sample, and the heavier Se isotope enrich in the sulfide of altered sample because of the altered function of the low temperature fluid. The Se isotopic composition of kerogen and the correlation between total sulfur and the Se isotopic composition of samples suggest that the hydrothermal fluid played more important than seawater in contributing Se for the ore deposition. For the black series rocks of the Niutitang Formation in Zunyi region, the positive correlation between the Se isotopic composition and the percentage of Se(-II) and the absence of elemental Se(0) indicate that hydrothermal fluid is the major Se source, and that the major mechanism of Se enrichment for black series rocks from seawater is reduction from Se(VI) and Se(IV) , beyond Se(0), directly to Se(-II). The relation between the S/Se ratio and the Se isotopic composition for the three areas suggests that there is no fractionation of Se isotope during the co-precipitation of Se and S. Combination with the Se speciation might explain perfectly the global bio-geochemical cycling of Se.
(6) Based on the data and the fractionation systematic of Se isotope, we point out the primary fractionation model of Se isotope in black rocks series, whose formation age and genesis is different. |
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