Sulfate sources constrained by sulfur and oxygen isotopic compositions in the upper reaches of the Xijiang River,China

While it is critical to accurately understand the sources and transformation of sulfate based on time-series analysis, there are limited studies on temporal variation of sulfate in rivers and on rock weathering by sulfuric acids.We conducted a monthly sampling campaign in the Beipan, Nanpan, and Hongshui Rivers over the course of one hydrological year. This study examined seasonal variations in riverine sulfate impacted by the monsoon climate in the upper reaches of the Xijiang River basin. In general, the SO_4^(2-) contents in these rivers dropped from relatively high levels to low values during the high-flow season, in response to increasing discharge. The sulfate was generally enriched in heavy isotopes during the low-flow season compared to the high-flow season. The calculated results indicate that the riverine sulfate was mainly derived from sulfide oxidation, but that evaporite dissolution could be an important source during the low-flow season, based on isotopic evidence. Mine drainage is likely an important source of sulfate to these rivers during the high-flow season due to contributions from fast surface flow, which responds to frequent heavy rain in monsoonal climate regions. Arelatively high proportion of HCO_3^- was found to be derived from rock weathering by sulfuric acid during the high-flow season when compared to that observed during the low-flow season. The results suggest that approximately one quarter of the HCO_3^- in the Hongshui River originated from carbonate weathering by sulfuric acid derived from the oxidation of sulfide. Such information on the specific dual isotopic characteristics of riverine sulfate throughout a hydrological year can provide unique evidence for understanding the temporal variability of sulfate concentrations and weathering processes in rivers.

Coupled trace element and SIMS sulfur isotope geochemistry of sedimentary pyrite:Implications on pyrite growth of Caixiashan Pb-Zn deposit

Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb-Zn deposit.However,the genesis of colloform pyrite in Pb-Zn deposits,its growth controls and their geological implication are insufficiently understood.Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers.These colloform pyrite occur as residual phases of core-rim aggregates,the cores are made up of very fine-grained anhedral pyrite particles,with some rims being made up of fine-grained and poorlycrystallized pyrite,while the other rims were featured with euhedral cubic pyrite.which are cemented by fine-grained calcite and/or dolomite with minor quartz.Sulfur isotope analysis shows that some wellpreserved rims have negative δ^34 S values(-28.12‰to-0.49‰),whereas most of the cores and rims have positive δ^34 S values(>0 to+44.28‰;peak at+14.91‰).Integrating with the methane and sulfate were observed in previous fluid inclusion study,we suggest that the 34 S depleted rims were initially formed by bacteria sulfate reduction(BSR),whereas the positive δ^34 S values were resulted from the sulfate reduction driven by anaerobic methane oxidation(AOM).The well-developed authigenic pyrite and calcite may also support the reaction of AOM.Combined with petrographic observations,trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite:Pb and Zn were present as mineral inclusion and likely precipitated before Fe,as supported by the time-resolved Pb-Zn signal spikes in most of the analyzed pyrite grains.Other metals,such as Hg,Co and Ni,may have migrated as chloride complexes and entered the pyrite lattice.Arsenic and Sb,generally influenced by complex-forming reactions rather than substitution ones,could also enter the pyrite lattice,or slightly predate the precipitation of colloform pyrite as mineral inclusions,which are controlled by their hydrolysis constant in the ore fluids.The colloform pyrite may have grown inward from the rims.The successive BSR reaction process would enrich H^32/2S in the overlying water column but reduce the metal content,the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes.The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column.The higher AOM reaction rate and the higher metal supply(not only Fe.but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.

Sulfur Isotope Composition of Evaporites in Different Strata in the Tarim Basin

Evaporite series is both an important target area looking for high-quality large potash deposits,but also one of the main types of hydrocarbon cap.Outcrops and drilling results indicate that five sets of evaporites were developed

Diverse transformations of sulfur in seabird-affected sediments revealed by microbial and stable isotope analyses

Microbial communities,sulfur isotope of sulfides(δ^(34)S_(AVS)and δ^(34)S_(CRS)),and sulfur and oxygen isotopes of sulfate(δ^(34)S_(SO_(4))and δ^(18)O_(SO_(4)))in sediments were analyzed to reveal the biogeochemical transformations of sulfur in a seabird-affected lake Y2 and a se abird-free YO from Fildes Peninsula,Antarctic Peninsula.The microbial communities in Y2 were mainly associated with penguin activities,while those in YO were limited by nutrients.The much enriched δ^(34)S_(SO_(4))recorded at depth of 30,41,and 52 cm in Y2indicates very strong sulfate reduction therein.The sulfur-degrading bacteria Pseudomonas in 0-23 cm of Y2 was 3.5 time s as abundant as that of sulfur oxidizing bacteria(SOB),indicating remarkable remineralization of organic sulfur.The abundant SOB and ^(34)S-depleted sulfate indicate considerable sulfur oxidation in 34-56-cm layer in Y2.In YO sediments,the highest abundance of Desulfotalea and the most enriched δ^(34)S_(SO_(4))(35.2‰)and δ^(34)S_(CRS)(2.5‰)indicate the strongest sulfate reduction in 28-cm layer.High abundance of Pseudomonas indicates active remineralization of organic sulfur in 3-5-cm layer in YO.The medium δ^(34)S_(SO_(4))and considerable abundance of SOB and sulfate-reducing bacteria(SRB)indicate concurrence of sulfur oxidation and sulfate reduction in other layers in YO.Therefore,a high level of organic matter input from penguin populations supported the diverse microbial community and transformations of sulfur in aquatic ecosystems in Antarctica.

Isotopic Compositions of Sulfur in the Jinshachang Lead–Zinc Deposit, Yunnan, China, and its Implication on the Formation of Sulfur-Bearing Minerals

The Jinshachang lead-zinc deposit is mainly hosted in the Upper Neoproterozoic carbonate rocks of the Dengying Group and located in the Sichuan-Yunnan-Guizhou （SYG） Pb-Zn-Ag multi- metal mineralization area in China. Sulfides minerals including sphalerite, galena and pyrite postdate or coprecipitate with gangue mainly consisting of fluorite, quartz, and barite, making this deposit distinct from most lead-zinc deposits in the SYG. This deposit is controlled by tectonic structures, and most mineralization is located along or near faults zones. Emeishan basalts near the ore district might have contributed to the formation of orebodies. The j34S values of sphalerite, galena, pyrite and barite were estimated to be 3.6‰-13.4‰, 3.7‰-9.0‰, -6.4‰ to 29.2‰ and 32.1‰34.7‰, respectively. In view of the similar δ34S values of barite and sulfates being from the Cambrian strata, the sulfur of barite was likely derived from the Cambrian strata. The homogenization temperatures （T ≈ 134--383℃） of fluid inclusions were not suitable for reducing bacteria, therefore, the bacterial sulfate reduction could not have been an efficient path to generate reduced sulfur in this district. Although thermochemical sulfate reduction process had contributed to the production of reduced sulfur, it was not the main mechanism. Considering other aspects, it can be suggested that sulfur of sulfides should have been derived from magmatic activities. The δ34S values of sphalerite were found to be higher than those of coexisting galena. The equilibrium temperatures calculated by using the sulfur isotopic composition of mineral pairs matched well with the homogenization temperature of fluid inclusions, suggesting that the sulfur isotopic composition in ore-forming fluids had reached a partial equilibrium.

Rock chemical weathering by sulfuric acid: pathway, method and prospect

To investigate the role of sulfuric acid-based carbonate weathering in global CO_2 sequestration of climate changes, we systematically discussed the pathway of sulfuric acid in rock chemical weathering and its feedback mechanism for global warming. We showed the methods used to determine the accurate amount of sulfate flux,accounting for the sulfuric acid resulted from sulfide oxidation. Finally, we pointed out some prospects for further detailed work on the exact calculation of the sulfate fluxes for the CO_2 net-release.

山西长治辛安泉域岩溶水硫酸盐污染来源分析

辛安泉域核心区域工农业相对发达,人类活动强烈,逐渐出现岩溶地下水位下降、泉流量减小和水质污染等水环境问题。硫酸盐污染是泉域岩溶水的主要污染物之一,通过科学手段进行污染来源分析和识别,可为泉域保护提供决策依据。硫氧同位素是地下水领域应用较广的一种环境同位素方法,不仅技术成熟且识别精度高。本次研究将硫氧同位素方法与水化学方法结合,在辛安泉域采取岩溶泉水、岩溶地下水、孔隙水、裂隙水和河水五类水样,测试硫同位素δ^(34)S、氧同位素δ^(18)O和硫酸根离子SO_(4)^(2-)含量,以分析辛安泉域岩溶水硫酸盐污染来源。结果显示,辛安泉域岩溶水水质的硫酸盐大部分符合地下水III类标准,硫酸盐含量较高的区域主要集中在西部滞流缓流区与襄垣和长治子系统的分界处、长治子系统和平顺-壶关子系统南部等地区。西部滞流缓流区硫酸盐主要来源于石膏、硬石膏的溶蚀,人类活动影响较小。泉域南部岩溶水中硫酸盐主要来源是煤矿开采的影响,大气沉降也是重要来源之一。

Variations of sulfur and carbon isotopes in seawater during the Doushantuo stage in late Neoproterozoic

Successive analyses of sulfur and carbon isotopic compositions of carbonates strata in the Doushantuo Formation in the Yangtze area were accomplished through a method of extracting trace sulfate from carbonates. Sulfur and carbon isotopic compositions of coeval seawater were estimated from the samples that show the least diagenetic alteration. A high-resolution age curve of sulfur isotopes in seawater sulfates was obtained in the Doushantuo stage, which reflects the trend of variation in seawater sulfur isotopes after the Neoproterozoic snowball Earth event. Similar characteristics of variation in carbon isotopes were observed in the coeval carbonates. A large positive d 34S excursion over +20?occurs in ancient seawater sulfates in the early Doushantuo stage. Simultaneously, the d 13C values in ancient seawater carbonates exhibit a positive excursion up to 10. The maximum d 34S and d 13C values are +46.4 and +6.9? respectively. In the middle Doushantuo stage, the range of variation in d 34S values of seawater is relatively narrow, but d 13C values are quite high. Then, d 34S values of seawater become oscillating, and the same occurs in d 13C values. Negative excursions in d 34S and d 13C values occur simultaneously at the end of the Doushantuo stage, and the minimum d 34S and d 13C values dropped down to -10.1 and -5.7? respectively. The characteristics of variations in the sulfur and carbon isotopes of ancient seawater imply strong changes in oceanic environment that became beneficial to inhabitation and propagation of organism. The organic productivity and burial rate of organic carbon once reached a quite high level during the Doushantuo stage. However, the state of environment became unstable after the global glaciation. The global climate and environment possibly were fluctuating and reiterating. The negative excursions in d 34S and d 13C values occurring at the end of the Doushantuo stage may represent a global event, which might be related to oxidation of deep seawater.

Chemical, carbon and sulfur isotopic compositions constrain the origin of Upper Carboniferous-Lower Triassic gases in eastern Sichuan Basin, SW China

Methane dominated gas is one of the cleanest energy resources;however, there is no direct method to determine its source rock. Natural gases produced from the eastern Sichuan Basin together with seismic data were studied for their sources and secondary alteration by thermochemical sulfate reduction(TSR). Our results demonstrate that Upper Permian to Lower Triassic(P_(3)ch-T_(1)f) gases in the surrounding of the Kaijiang-Liangping area show volatile organic sulfur compounds(VOSCs) δ^(34)S values close to those of the associated H_(2)S, and may have been altered by methane-dominated TSR, resulting in positive shift in methane δ^(13)C_(1)values with increasing TSR extents. Other(or group 2) gases produced from the P_(3)ch-T_(1)f reservoirs from the southern area and the Upper Carboniferous to Middle Permian(C_(2)h-P_(2)q) from the eastern Sichuan Basin are not significantly changed by TSR, show similar δ^(34)S values between the kerogens and some VOSCs, and may have been derived from the Lower Silurian and Middle Permian source rocks. This study demonstrates a case for the first time showing the δ^(34)S values of VOSCs can be used as a tool for direct correlation between non-TSR altered gas and source rocks. Methane-dominated gas pools can be found using gas and source rock geochemistry combined with seismic data.

基于硫氧同位素的菏泽市主要河流硫酸盐来源解析

为探明菏泽市河流中硫酸盐污染的主要来源,以洙赵新河流域和东鱼河流域为例,利用硫酸盐(SO_(4)^(2-))浓度、硫氧同位素(δ^(34)S_(SO_(4))和δ^(18)O_(SO_(4)))研究菏泽市主要河流SO_(4)^(2-)的时空分布特征和来源。结果表明:(1)研究区河流中(δ^(34)S_(SO_(4))和δ^(18)O_(SO_(4)))值的变化总体表现为枯水期>平水期>丰水期,且洙赵新河流域δ^(34)S_(SO_(4))值和δ^(18)O_(SO_(4))值变化范围更大,来源更加复杂;(2)丰水期研究区SO_(4)^(2-)主要来源为土壤携带以及污废水排放,贡献率均在30%以上;(3)平水期和枯水期洙赵新河流域SO_(4)^(2-)主要来源为生活污水与工矿企业生产废水,平均贡献率均在50%以上,东鱼河流域主要来源为硫化物氧化,平均贡献率超过30%。

硫同位素对地表水硫酸盐来源示踪的研究进展

硫酸盐是地表水中重要的化学组成之一,并且可以提供丰富的地质过程和人类活动的综合信息,对地表水中硫酸盐来源的精确识别具有极为重要的意义。由于不同硫酸盐具有特定的硫同位素组成,硫同位素已被应用于识别地表水硫酸盐来源。总结了地表水中不同硫酸盐来源δ~(34)S_(SO4)范围,综述了国内外利用硫稳定同位素对地表水中硫酸盐污染源示踪的研究进展。讨论了仅仅使用δ~(34)S_(SO4)来进行溯源在硫酸盐污染示踪方面的局限性,并对未来硫酸盐污染溯源技术进行了展望。

扬子地台灯影组碳酸盐岩中的硫和碳同位素记录

扬子地区灯影组的海相碳酸盐岩地层不仅记录了当时海水的碳同位素变化,也保存了海水的硫同位素记录,能够通过测定所提取的微量硫酸盐的硫同位素组成来获得。灯影组碳酸盐岩中微量硫酸盐的δ^(34)S值大部分在+20.0‰～+38.7‰之间变化,碳酸盐岩的δ^(13)C值变化在+0.5‰～+5.0‰之间。除灯影组顶、底界线处外,δ^(34)S和δ^(13)C值总体上变化幅度较小,大体上呈逐渐降低的变化趋势。灯影组碳酸盐岩中连续的硫、碳同位素记录分别反映了同期海水中溶解硫酸盐和碳酸盐的硫、碳同位素的变化特征。灯影组微量硫酸盐和碳酸盐岩的同位素特征,意味着灯影期海洋中具有高的生物产率和有机碳埋藏速率;除了顶底界线处,具有相对稳定的古气候条件和古海洋环境。灯影期海水的δ^(34)S值和δ^(13)C值同时呈逐渐降低的变化趋势,可能是由海洋深部水体逐渐氧化所致。

东濮盐湖古近系硫酸盐硫同位素组成及地质意义

对东濮凹陷古近系同一层段内和不同层段间硫酸盐硫同位素的研究认为,由于不同时期气候、沉积环境、构造沉降等的变化,东濮凹陷盐湖环境中发育的古近系蒸发岩中的硫同位素具有旋回性,与层序发育的旋回性吻合很好;将东濮凹陷古近系蒸发岩的δ34S值(30‰)与海相蒸发岩的δ34S值相比,结合前人对东濮凹陷古近系蒸发岩锶同位素的研究成果,认为东濮凹陷的湖盆为封闭的陆相盐湖,当时的沉积环境对硫酸盐是封闭的。

鄂尔多斯白垩系地下水盆地硫酸盐的水文地球化学特征及来源

鄂尔多斯白垩系地下水盆地蕴藏有丰富的地下水资源,地下水赋存于白垩系砂岩中。为了查明地下水微咸水的成因,对40件地下水样品的水化学组成、SO42-的区域分布特征和28件硫同位素组成进行了分析,并对34件岩石样品进行了溶滤试验研究。结果表明:SO42-是地下水中主要阴离子组分,具有东部低西部高,北部低南部高的区域分带特征;硫酸盐是白垩系地层中主要的易溶盐组分,并以石膏和芒硝形式存在;地下水中SO42-主要来源于地层中石膏和芒硝,其次是地层中的硫化物,少量来源于有机硫。

湖北江陵凹陷古新统沙市组四段硫酸盐硫同位素组成及其地质意义

江汉盆地江陵凹陷古新统沙市组四段硬石膏和钙芒硝中硫同位素组成为25.2‰～32.6‰,远远高于同时期海水硫同位素(20‰)。结合研究区古新统沙四段蒸发岩87Sr/86Sr研究,借助Mg、Ca、Sr、Ba等元素及其Mg/Ca、Sr/Ba,认为江陵凹陷沙四段时期湖盆为封闭的陆相盐湖。根据典型暖相盐类矿物(原生钙芒硝)的广泛发育和指示气候干旱程度的Fe2O3/FeO特征,可以推断研究区在沙市组四段沉积晚期的古气候属于暖旱型气候。在温暖干旱的气候环境下,硫酸盐沉积物供给率较低,因此当时的沉积环境对硫酸盐是封闭的。通过Fe、Mn、Al、Mg、Ca、Ni、V、Co、Cu等主微量元素以及Fe/Mn、(Fe+Al)/(Ca+Mg)、w(V+Ni+Mn)、V/(V+Ni)、Ni/Co、Cu/Zn的研究表明,研究区沙市组四段环境为湖盆浪基面之下的缺氧还原环境,为常年性较深水分层湖泊。上述缺氧沉积环境下存在大量的厌氧细菌,表生(≤50℃)条件下,厌氧细菌使硫酸盐SO24-还原成H2S,后者与金属离子结合形成硫化物或直接从体系中逸出,再加上当时的内陆环境对硫酸盐是相对封闭的,这种硫循环过程是造成研究区硬石膏和钙芒硝硫同位素值较高的原因,也是研究区最重要的硫同位素分馏过程。

硫和氧同位素示踪黄河及支流河水硫酸盐来源

为了准确识别河水硫酸盐受自然风化和人为活动影响的过程,做好地表水资源管理,选择黄河小浪底水库以下干流和支流河水为主要研究对象,分期采集河水样品,采用硫酸盐硫和氧同位素,结合水化学组成及潜在硫酸盐来源硫和氧同位素范围,判定黄河及支流河水硫酸盐的来源及混入比例。结果表明:①研究区黄河河水硫酸盐主要来源于第四纪黄土中易溶硫酸盐,干流河水SO24-含量均值为2.23 mmol/L,δ34SSO4和δ18OSO4均值分别为+8.9‰和+10.4‰;②研究区沁河丰水期河水硫酸盐24%来源于大气降水,61%来源于土壤硫酸盐溶解,15%来自于石膏溶解;平水期河水硫酸盐39%来源于大气降水,36%来源于土壤硫酸盐溶解,25%来源于石膏溶解。沁河河水SO24-含量均值为2.44 mmol/L,δ34SSO4和δ18OSO4均值分别为+9.8‰和+9.7‰;③研究区洛河河水硫酸盐受生活污水影响较大,伊河河水硫酸盐受到土壤硫酸盐溶解和化学肥料溶解的共同影响,伊洛河河水SO24-含量均值为1.27 mmol/L,δ34SSO4和δ18OSO4均值分别为+10.4‰和+6.5‰。蒸发盐类矿物溶解以及土壤硫酸盐溶解等自然风化过程是控制区域河水硫酸盐来源的重要过程,人为活动对伊洛河河水硫酸盐的贡献不容忽视。

膏盐层在矽卡岩型铁矿成矿中的作用

矽卡岩型铁矿是我国富铁矿的重要类型,约占全国富铁矿总储量的60%。虽然膏盐层与矽卡岩型铁矿的关系已引起部分矿床学家的关注,但膏盐层的控矿机制尚不清楚。本文以我国最重要的大冶式和邯邢式矽卡岩型铁矿为例,探讨了膏盐层在矽卡岩型铁矿成矿中的作用。膏盐层富含碳酸盐、石膏和石盐等,不仅可以为成矿提供大量Na^+、Cl^-、CO3^(2+)等矿化剂,使围岩发生钠长石化、方柱石化(氯化)和矽卡岩化等蚀变,使Fe^(2+)以NaFe-Cl等络合物形式搬运,膏盐层还是地壳深处最重要的氧化障,能够将硅酸盐熔体和成矿溶液中的Fe^(2+)氧化成Fe^(3+),富集形成铁矿床,是矽卡岩型铁矿成矿的关键因素。大冶地区的膏盐层属于中三叠统下部的嘉陵江组,邯邢地区的膏盐层属于中奥陶统马家沟组—峰峰组。大冶和邯邢式矽卡岩型铁矿中硫化物的δ^(34)S_(V-CDT)值异常高,计算结果表明矿床中约80%的硫来自膏盐层硫酸盐的还原,还原温度多在500℃以上,但硫化物的沉淀温度相对较低,就位时间稍晚;硫酸盐的δ^(34)S_(V-CDT)值和还原温度越高,硫化物的δ^(34)S_(V-CDT)值越高,原始岩浆硫所占比例越高,硫化物的δ^(34)S_(V-CDT)值越低。当炽热的岩浆与膏盐层(CaSO_4)发生同化混染时,SO_4^(2-)将硅酸盐熔体中的Fe^(2+)氧化成Fe^(3+),Fe^(3+)无法进入硅酸盐矿物晶格,而形成Fe_3O_4/Fe_2O_3进入熔体,铁氧化物在磷、氯化钠、水等挥发分和盐类物质的作用下在岩浆房中与硅酸盐熔体发生不混熔,形成铁矿浆,沿构造有利部位贯入,形成矿浆型铁矿床。在矽卡岩型铁矿床中,矿浆充填型和热液交代型矿体同时存在,二者在空间上具有一定的分带性,有时渐变过渡,矿浆充填型铁矿体通常位于深部靠近成矿岩体的部位,而热液交代型铁矿体位于成矿流体运移的前方。在SO_4^(2-)氧化Fe^(2+)的同时自身被还原为S^(2-),与Fe^(2+)结合形成硫铁矿,分布在铁矿的上部或边部。

贵州铜仁高地“大塘坡式”锰矿的成矿机制——硫、碳同位素制约

“大塘坡式”锰矿在我国华南黔湘渝地区广泛分布,是我国最重要的锰矿资源类型之一。它的形成与新元古代Sturtian雪球事件密切相关,但其具体的成矿机制尚不十分清楚,还存在许多争论。本文对贵州铜仁地区新近发现的高地超大型锰矿和共伴生黑色页岩中的微量硫酸盐和黄铁矿的硫同位素、菱锰矿等碳酸盐岩及有机碳的碳同位素进行了系统研究,对该类型锰矿的成矿环境和沉淀机制进行了探讨。高地锰矿大塘坡组一段含锰黑色页岩和锰矿石中硫酸盐的含量很低,为30.9~20439.7μg/g,平均3322.5μg/g,硫酸盐的δ^34SVCDT为51.5‰~68.1‰,平均60.4‰。冰碛岩上部铁丝坳组含砾杂砂岩中黄铁矿的δ^34SVCDT为26.8‰~59.6‰,平均52.1‰;上覆大塘坡组黑色页岩和锰矿石中黄铁矿的δ^34SVCDT为53.7‰~65.6‰,平均63.3‰,与前人在该区域其它矿区得到的结果一致,与硫酸盐的δ^34SVCDT值差别不大;黑色页岩和锰矿石全岩的δ^34SVCDT为41.4‰~63.9‰,平均55.7‰。同一样品中,硫酸盐的δ^34S均高于全岩的值,但差异不大。铁丝坳组顶部含砾杂砂岩的δ^13Ccarb为–11.3‰~–8.3‰,平均–9.6‰,δ^13Corg为–31.7‰~–30.1‰,平均–30.9‰;大塘坡组一段黑色页岩和锰矿石的δ^13Ccarb为–12.4‰~–4.6‰,平均–8.5‰,δ^13Corg为–34.3‰~–32.6‰,平均–33.6‰,二者在含锰段同步下降,有机碳含量明显升高,说明有机质对锰矿的形成发挥了重要作用。综上提出,大塘坡式锰矿形成于滨浅海相半封闭性的断陷盆地之中,含锰地层中δ^34S异常高的黄铁矿是在氧化还原分层明显的静水环境中,由δ^34S异常高的孔隙水硫酸盐在成岩过程中几乎全部还原形成的,而海水硫酸盐的δ^34S正异常与雪球事件、生物爆发和沉积演化等密切相关。雪球融化之后,在断陷盆地的浅层海水中,生物活动和光合作用强盛,氧浓度高,海水中Mn^2+不断被氧化形成氧化锰并从海水中沉淀出来,而深部还原缺氧富Mn^2+的海水不断越过构造脊进来补充。浅层海水中微生物大量繁殖,死亡后沉降于海底,导致断陷盆地底部有机质大量聚集,氧逸度急剧下降。在沉积成岩过程中氧化锰被沉积物中大量有机质全部还原为Mn^2+,有机质本身被氧化为CO3^2-,二者结合形成菱锰矿。

四川盆地H_2S的硫同位素组成及其成因探讨

四川盆地天然气绝大部分含有硫化氢,部分含量高达15%以上。其中高含硫化氢天然气主要分布在三叠系飞仙关组、雷口坡组和嘉陵江组;震旦系、石炭系、二叠系属于低含硫化氢,上三叠统须家河组和侏罗系属于微含硫化氢或不含硫化氢天然气藏。研究表明,三叠系飞仙关组、雷口坡组和嘉陵江组、震旦系、石炭系储层中发育的膏质岩类为TSR形成硫化氢提供了物质基础;富含有机硫源岩的高温裂解是二叠系低含硫化氢天然气的主要成因。硫同位素组成表明,高含硫化氢天然气的硫同位素比储层硫酸盐硫同位素δ34S亏损7‰～11‰;而低含硫化氢天然气硫同位素分布区间较宽,在0‰～20‰之间,大部分比同期硫酸盐的硫同位素轻15‰左右。四川盆地三叠系膏岩的硫同位素值分布较宽,并呈现阶梯状变化,而硫化氢的硫同位素则呈现出相似的分布规律,表明各气层硫化氢中的硫来自于本层系的硫酸盐,即TSR发生在各自的储集层中;另外四川盆地三叠系TSR发生时各气藏的温度条件相近,即各气藏的硫化氢在大致相同的温度条件下发生;同时也说明TSR过程中硫同位素的分馏过程与硫酸盐本身硫同位素数值的高低无关,而与TSR反应的温度条件和反应程度有关。还建立了运用硫化氢的硫同位素和含量判识硫化氢成因类型的模式。

天然气藏中硫化氢成因研究进展

目前普遍认为天然气藏中硫化氢主要为生物硫酸盐还原(BSR)、硫酸盐热化学还原(TSR)和含硫化合物热裂解等成因。一般认为,天然气中高含硫化氢是硫酸盐热化学还原作用的结果。从气藏的地质和地球化学等方面,可以找到气藏发生硫酸盐热化学还原作用后的许多证据,根据这些证据,可以很好地判断气藏是否发生过硫酸盐热化学还原作用。尽管对天然气藏中硫化氢研究已取得一定的成果,但仍存在许多亟待认识的问题,如硫化氢的成因机理、分布规律、地质-地球化学特征,硫化氢与油气的关系,以及硫化氢形成的主控因素等。总结了国内外关于硫化氢天然气成因研究进展及其存在的问题,以期引起人们对硫化氢天然气的关注。