Open system sulphate reduction in a diagenetic environment – Isotopic analysis of barite (δ34S and δ18O) and pyrite (δ34S) from the Tom and Jason Late Devonian Zn–Pb–Ba deposits, Selwyn Basin, Canada
Open system sulphate reduction in a diagenetic environment – Isotopic analysis of barite (δ34S and δ18O) and pyrite (δ34S) from the Tom and Jason Late Devonian Zn–Pb–Ba deposits, Selwyn Basin, Canada
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DOI：
10.1016/j.gca.2016.02.015
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发表时间：
2016-05
影响因子：
5
通讯作者：
J. Magnall;S. Gleeson;R. Stern;R. Newton;S. Poulton;S. Paradis
中科院分区：
文献类型：
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作者：
J. Magnall;S. Gleeson;R. Stern;R. Newton;S. Poulton;S. Paradis
硫化物矿物中δ 34 S的高正值是页岩块状硫化物矿床的一个共同特征。这通常归因于海水硫酸盐的近定量消耗，对于Selwyn盆地（加拿大）的古生代地层，这被认为是在受限的富氧水柱中细菌硫酸盐还原（BSR）期间发生的。在这项研究中，我们专注于从两个晚泥盆世SHMS矿床（汤姆和杰森，麦克米伦通行证，塞尔温盆地）的硫化物和重晶石矿化的钻孔岩芯样品，以评估这个富氧盆地模型。利用透射、反射光显微镜和背散射电子成像技术，确定了重晶石、黄铁矿和热液贱金属硫化物的共生关系。该岩相框架提供了重晶石和黄铁矿原位同位素微量分析（二次离子质谱;西姆斯）的背景。这些数据通过分析未矿化（贫瘠）硅质泥岩的钻芯样品中的块状岩石黄铁矿（n= 37）的δ 34 S值进行补充，以提供一种评估寄主岩石中硫的质量平衡的方法。已识别出三代重晶石，所有这些都早于热液输入。三代重晶石的δ 34 S和δ 18 O值具有重叠分布（分别为+22.5‰ ~+33.0‰和+16.4‰ ~+18.3‰），与晚泥盆世改造海水的成因一致。富含钡的放射虫测试在硅质泥岩中丰富，提供了原生钡富集与生物活动相关的证据。因此，我们建议，重晶石形成的生产力衍生的钡在沉积物内的再活化，并沉淀在成岩孔隙流体接近沉积物-水界面。两个世代的黄铁矿在结构上与重晶石共生：草莓状黄铁矿（py-I），具有负δ 34 S值（−23‰至−28‰;n= 9），自形黄铁矿（py-II），其δ 34 S值明显更正（+8‰ ~+26‰;我们认为层状黄铁矿和重晶石沿着成岩氧化还原前缘发育，其中同位素关系（δ 34黄铁矿→ δ 34辉锑矿）由甲烷厌氧氧化与硫酸盐还原耦合（AOM-SR）解释。此外，δ 34 Sb值的相对窄的分布与硫酸盐还原的开放系统模型一致，其中还原硫的产生伴随着减少的同位素分馏（ε 34 S = <15‰）与较高的硫酸盐还原率和AOM-SR有关。（黄铁矿，闪锌矿和方铅矿）都晚于这个成岩重晶石-黄铁矿组合，结构和矿物学证据表明，重晶石替代是一个重要的过程中热液矿化。无论是纹理，也没有记录的同位素关系可以产生的工艺操作在一个富氧水柱，这代表了一个重大的偏离传统的模型在麦克米伦通行证的SHMS形成。我们认为，硫化物中的正δ 34 S值（Selwyn盆地和整个地质记录中SHMS系统的共同特征）可能与AOM-SR有关。在麦克米伦山口，开放系统成岩作用期间形成正δ 34 Spyrite值，这对快速硫循环和有效金属圈闭的形成至关重要。
Highly positive δ34S values in sulphide minerals are a common feature of shale hosted massive sulphide deposits (SHMS). Often this is attributed to near quantitative consumption of seawater sulphate, and for Paleozoic strata of the Selwyn Basin (Canada), this is thought to occur during bacterial sulphate reduction (BSR) in a restricted, euxinic water column. In this study, we focus on drill-core samples of sulphide and barite mineralisation from two Late Devonian SHMS deposits (Tom and Jason, Macmillan Pass, Selwyn Basin), to evaluate this euxinic basin model. The paragenetic relationship between barite, pyrite and hydrothermal base metal sulphides has been determined using transmitted and reflected light microscopy, and backscatter electron imaging. This petrographic framework provides the context for in-situ isotopic microanalysis (secondary ion mass spectrometry; SIMS) of barite and pyrite. These data are supplemented by analyses of δ34S values for bulk rock pyrite (n= 37) from drill-core samples of un-mineralised (barren), siliceous mudstone, to provide a means by which to evaluate the mass balance of sulphur in the host rock.Three generations of barite have been identified, all of which pre-date hydrothermal input. Isotopically, the three generations of barite have overlapping distributions of δ34S and δ18O values (+22.5‰ to +33.0‰ and +16.4‰ to +18.3‰, respectively) and are consistent with an origin from modified Late Devonian seawater. Radiolarian tests, enriched in barium, are abundant within the siliceous mudstones, providing evidence that primary barium enrichment was associated with biologic activity. We therefore propose that barite formed following remobilisation of productivity-derived barium within the sediment, and precipitated within diagenetic pore fluids close to the sediment water interface. Two generations of pyrite are texturally associated with barite: framboidal pyrite (py-I), which has negative δ34S values (−23‰ to −28‰;n= 9), and euhedral pyrite (py-II), which has markedly more positive δ34S values (+8‰ to +26‰;n= 86).We argue that stratiform pyrite and barite developed along diagenetic redox fronts, where the isotopic relationships (δ34Spyrite≈ δ34Sbarite) are explained by anaerobic oxidation of methane coupled to sulphate reduction (AOM-SR). Furthermore, the relatively narrow distribution of δ34Sbaritevalues is consistent with an open system model of sulphate reduction, in which reduced sulphur generation occurred with a reduced isotopic fractionation (ε34S = <15‰) linked to higher rates of sulphate reduction and AOM-SR. Importantly, hydrothermal sulphides (pyrite, sphalerite and galena) all post-date this diagenetic barite-pyrite assemblage, and textural and mineralogical evidence indicates barite replacement to be an important process during hydrothermal mineralisation. Neither the textures nor the documented isotopic relationships can be produced by processes operating in a euxinic water column, which represents a major departure from the conventional model for SHMS formation at Macmillan Pass. We suggest that positive δ34S values in sulphides, a common feature of SHMS systems both in the Selwyn Basin and throughout the geologic record, could be linked to AOM-SR. At Macmillan Pass, positive δ34Spyritevalues developed during open system diagenesis, which was critical for rapid sulphur cycling and the development of an effective metal trap.