Sulfur sources of sedimentary “buckshot” pyrite in the Auriferous Conglomerates of the Mesoarchean Witwatersrand and Ventersdorp Supergroups, Kaapvaal Craton, South Africa
Sulfur sources of sedimentary “buckshot” pyrite in the Auriferous Conglomerates of the Mesoarchean Witwatersrand and Ventersdorp Supergroups, Kaapvaal Craton, South Africa
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南非卡普瓦尔克拉通中太古代威特沃特斯兰德和文特斯多普超群金质砾岩中沉积“铅弹”黄铁矿的硫源
作者：
B. Guy;Shuhei Ono;J. Gutzmer;Y. Lin;Y. Lin;N. Beukes
大型圆形黄铁矿颗粒（>1 mm），通常称为“铅弹”黄铁矿颗粒，是南非卡普瓦尔省威特沃特斯兰德和文特斯多普超群中含金砾岩（礁）的特征。对生物礁的详细岩相学分析表明，绝大多数的铅弹黄铁矿颗粒是改造的沉积成因，即，黄铁矿颗粒最初形成于沉积环境中的沉积成岩作用。对来自Main、Vaal、Basal、Kalkoenkrans、Beatrix和Ventersdorp接触礁的41个经过改造的沉积黄铁矿颗粒进行了多种硫同位素组成（δ 34 S、Δ 33 S和Δ 36 S）分析，以确定黄铁矿硫的来源。此外，五个后生黄铁矿样品（沉积和石化后形成的黄铁矿）从Middelvlei和Ventersdorp接触礁进行了测量比较。所有41个经过改造的沉积黄铁矿颗粒的δ 34 S、Δ 33 S和Δ 36 S值分别为−6.8至+13.8 ‰、−1.7至+1.7 ‰和−3.9至+0.9 ‰，显示出明显的质量相关和质量无关分馏特征。相比之下，五个后生黄铁矿样品显示出非常有限的δ 34 S，Δ 33 S和Δ 36 S值范围（+0.7至+4.0 ‰，-0.3至+0.0 ‰）。分别为-0.3至+0.1 ‰）。尽管硫同位素分馏的质量无关的清晰的签名，很少有数据点绘制沿着的主要太古代光化学阵列表明一个弱的光解控制的数据集。相反，其他因素命令更大程度的影响，如黄铁矿共生，当时的沉积环境，和非光解硫源。与黄铁矿共生有关，改造的同生沉积黄铁矿颗粒（黄铁矿最初沿着沉积物-水界面沉淀）具有负δ 34 S和Δ 33 S值的特征，表明硫酸盐供应和微生物硫酸盐还原剂的存在是开放系统条件。相反，大多数改造成岩沉积黄铁矿颗粒（黄铁矿最初沉淀在沉积物-水界面以下）显示正δ 34 S和负Δ 33 S值，表明封闭系统条件。表生黄铁矿的Δ 33 S异常可忽略不计，表明硫来自质量相关或同位素均匀的变质/热液流体。对比硫同位素组成也观察到不同的沉积环境，即河流砾岩和海洋改性河流砾岩。河流砾岩中黄铁矿颗粒的特征是δ 34 S值变化范围大（−6.2 ~+4.8 ‰），Δ 33 S值变化小（±0.3 ‰）。这一特征可能代表了一个地壳硫酸盐储层，该储层来源于火山脱气或内陆硫化物矿物的风化。来自海洋改造的河流砾岩的再加工沉积黄铁矿颗粒具有相似的同位素组成，但也产生正的Δ 33 S/δ 34 S阵列，与太古代重晶石的组成重叠，表明海洋硫的引入。这些结果表明，存在多种来源的硫，其中包括大气，地壳和海洋水库。普遍存在的质量依赖地壳硫同位素签名河流砾岩表明，硫酸盐浓度可能要高得多，在陆地环境相比，海洋环境中，这是硫酸盐缺乏。然而，形成陆源沉积黄铁矿的最佳条件可能不是在河流冲刷作用期间，而是在低角度不整合面的洪水早期阶段，即，在退积河流沉积期间，在某些情况下与海侵相结合，紧接着相对海平面下降的最大速率的拐点。
Large rounded pyrite grains (>1 mm), commonly referred to as “buckshot” pyrite grains, are a characteristic feature of the auriferous conglomerates (reefs) in the Witwatersrand and Ventersdorp supergroups, Kaapvaal Craton, South Africa. Detailed petrographic analyses of the reefs indicated that the vast majority of the buckshot pyrite grains are of reworked sedimentary origin, i.e., that the pyrite grains originally formed in the sedimentary environment during sedimentation and diagenesis. Forty-one of these reworked sedimentary pyrite grains from the Main, Vaal, Basal, Kalkoenkrans, Beatrix, and Ventersdorp Contact reefs were analyzed for their multiple sulfur isotope compositions (δ34S, Δ33S, and Δ36S) to determine the source of the pyrite sulfur. In addition, five epigenetic pyrite samples (pyrite formed after sedimentation and lithification) from the Middelvlei and the Ventersdorp Contact reefs were measured for comparison. The δ34S, Δ33S, and Δ36S values of all 41 reworked sedimentary pyrite grains indicate clear signatures of mass-dependent and mass-independent fractionation and range from −6.8 to +13.8 ‰, −1.7 to +1.7 ‰, and −3.9 to +0.9 ‰, respectively. In contrast, the five epigenetic pyrite samples display a very limited range of δ34S, Δ33S, and Δ36S values (+0.7 to +4.0 ‰, −0.3 to +0.0 ‰. and −0.3 to +0.1 ‰, respectively). Despite the clear signatures of mass-independent sulfur isotope fractionation, very few data points plot along the primary Archean photochemical array suggesting a weak photolytic control over the data set. Instead, other factors command a greater degree of influence such as pyrite paragenesis, the prevailing depositional environment, and non-photolytic sulfur sources. In relation to pyrite paragenesis, reworked syngenetic sedimentary pyrite grains (pyrite originally precipitated along the sediment-water interface) are characterized by negative δ34S and Δ33S values, suggesting open system conditions with respect to sulfate supply and the presence of microbial sulfate reducers. On the contrary, most reworked diagenetic sedimentary pyrite grains (pyrite originally precipitated below the sediment-water interface) show positive δ34S and negative Δ33S values, suggesting closed system conditions. Negligible Δ33S anomalies from epigenetic pyrite suggest that the sulfur was sourced from a mass-dependent or isotopically homogenous metamorphic/hydrothermal fluid. Contrasting sulfur isotope compositions were also observed from different depositional environments, namely fluvial conglomerates and marine-modified fluvial conglomerates. The bulk of the pyrite grains from fluvial conglomerates are characterized by a wide range of δ34S values (−6.2 to +4.8 ‰) and small Δ33S values (±0.3 ‰). This signature likely represents a crustal sulfate reservoir derived from either volcanic degassing or from weathering of sulfide minerals in the hinterland. Reworked sedimentary pyrite grains from marine-modified fluvial conglomerates share similar isotope compositions, but also produce a positive Δ33S/δ34S array that overlaps with the composition of Archean barite, suggesting the introduction of marine sulfur. These results demonstrate the presence of multiple sources of sulfur, which include atmospheric, crustal, and marine reservoirs. The prevalence of the mass-dependent crustal sulfur isotope signature in fluvial conglomerates suggests that sulfate concentrations were probably much higher in terrestrial settings in comparison to marine environments, which were sulfate-deficient. However, the optimum conditions for forming terrestrial sedimentary pyrite were probably not during fluvial progradation but rather during the early phases of flooding of low angle unconformities, i.e., during retrogradational fluvial deposition, coupled in some cases with marine transgressions, immediately following inflection points of maximum rate of relative sea level fall.
影响因子：
5.3
作者：
Turchyn, Alexandra V.;Tipper, Edward T.;Bickle, Mike J.
通讯作者：
Bickle, Mike J.