Submarine silica-rich hydrothermal activity during the earliest Cambrian in the Tarim Basin,Northwest China

Siliceous rocks were widely deposited in many continents during the Ediacaran–Cambrian (E–C) transition. Based on detailed field investigations in the Aksu area of the Tarim Basin in Northwest China, this study presents evidence of a submarine silica-rich hydrothermal system preserved in the E–C boundary successions. This system consists of a lower stockwork silica-dominant vein swarm zone in the karstified dolostone of the uppermost Ediacaran Qigebulake Formation, which terminates directly under the overlying bedded chert and black shale succession of the lowermost Cambrian Yurtus Formation. The stockwork vein swarms were filled dominantly by a wide spectrum of silica precipitates (amorphous silica, chalcedony, spherulite, fine to coarse quartz) with subordinate pyrite, Fe-(oxyhydr)oxide, and barite. The host dolostones that were dissected by the vein swarms also suffered extensive silicification and recrystallization. The vertical stacking relationship of silica-dominant vein swarms and overlying bedded chert suggests they were formed by an identical low-temperature, silica-rich diffusive submarine hydrothermal system in the earliest Cambrian. This suggestion is further supported by fluid inclusion microthermometry (T_h 40–200°C) of the quartz-barite vein fills. In this case, silica-rich hydrothermal fluids were channelled and precipitated partially along the stockwork veins in the antecedent karstified dolostone and vented mostly into seawater, promoting widespread deposition of bedded chert on the seafloor of Tarim Basin in the earliest Cambrian. This study provides a useful clue and analogue to understand the widespread silica deposition and coeval vast oceanic and geochemical changes during the E–C transition in the Tarim Basin and elsewhere.     

中扬子地区晚震旦世-早寒武世转折期台-盆演化及烃源岩形成机理

在中国南方中扬子地区早寒武世早期,沉积了一套黑色碳质页岩、碳硅质页岩、黑色粉砂质页岩,分布范围广,厚度大,有机碳含量高,属于腐泥型为主的有机质类型,为中国南方海相区域主力烃源岩之一.为了认清该套烃源岩的形成环境,通过对岩石学特征、沉积相的变化、台-盆转化的研究并结合一些地球化学指标分析了该地区在转折期古环境发生的变化,初步探讨了下寒武统这套主力优质烃源岩的形成背景,提出了一个热水活动-上升洋流-缺氧事件复合模式,即为在地壳拉张背景下,热液活动与上升洋流共同作用造成生物产率提高,水体缺氧,使得有机质大量埋藏并得以保存,构成了在中扬子克拉通上和东南边缘重要的下寒武统优质烃源岩.     

Seawater contribution to polymetallic Ni–Mo–PGE–Au mineralization in Early Cambrian black shales of South China: Evidence from Mo isotope,PGE, trace element,and REE geochemistry

The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ^98/95Mo values of 0.94 to 1.38‰ (avg. 1.13 ± 0.14‰, 1σ, n = 11), and the host black shale and phosphorite have slightly more variable δ^98/95Mo values of 0.81‰ to 1.70‰ (n = 14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 10⁷ compared to present-day seawater. PAAS-normalized REE + Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts.     

东太平洋北部胡安·德富卡洋中脊Mothra热液场硫化物烟囱体成矿物质来源:铅和硫同位素组成

对采自Mothra热液场Faulty Towers硫化物烟囱体群(47°57.447,N,129°06.568W)的一个硫化物烟囱体进行了铅和硫同位素组成的研究工作.分析结果表明,铅同位素组成的分布范围为:206Pb/204Pb=18.665～18.828; 207Pb/204Pb=15.460～15.607; 208...     

Sulfur and lead isotopic compositions of massive sulfides from deep-sea hydrothermal systems: Implications for ore genesis and fluid circulation

Studies of sulfur and lead isotopic compositions in hydrothermal deposits are an important tool to determine the source and processes of both sulfur and lead, and to understand the origin of hydrothermal ore deposits. Here, the sulfur and lead isotopic compositions of sulfide minerals have been studied for different hydrothermal fields in the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and North Fiji Basin (NFB). The sulfur isotopic compositions of the studied sulfide samples are variable (δ³⁴S 0.0 to 9.6‰, avg. δ³⁴S 4.7‰; n = 60), being close to the associated igneous rocks (~ 0‰ for, e.g., basalt, serpentinized peridotite), which may reflect the S in the sulfide samples is derived mainly from the associated igneous rocks, and a relatively small proportion (< 36%) of seawater sulfur incorporated into these sulfides during mixing between seawater (δ³⁴S 21‰) and hydrothermal fluid. In contrast for a mixed origin for the source of S, the majority of the lead isotopic compositions (²⁰⁶Pb/²⁰⁴Pb 17.541 ± 0.004 to 19.268 ± 0.001, ²⁰⁷Pb/²⁰⁴Pb 15.451 ± 0.001 to 15.684 ± 0.001, ²⁰⁸Pb/²⁰⁴Pb 37.557 ± 0.008 to 38.988 ± 0.002, n = 21) of the sulfides possess a basaltic Pb isotopic composition, suggesting that the lead in the massive sulfide is mainly leached from local basaltic rocks that host the sub-seafloor hydrothermal systems in sediment-free mid-ocean ridges and mature back-arc basins. Furthermore, sulfide minerals in the super-fast and fast spreading mid-ocean ridges (MORs) exhibit less spread in their the δ³⁴S values compared to sulfides from super-slow, and slow spreading MORs, which is most easily explained as a lesser degree of fluid-rock interaction and hydrothermal fluid-seawater mixing during hydrothermal ore-forming process. Additionally, the S and Pb isotope compositions of sulfides are controlled by the fluid processes for forming seafloor massive sulfide deposits. We demonstrate that the variable sulfur and lead isotopic compositions exhibit a relationship with the sulfur and lead sources, fluid–rock interaction, and fluid–seawater mixing.     

Petrology and geochemistry of chert on the marginal zone of Yangtze Platform,western Hunan,South China,during the Ediacaran–Cambrian transition

The Ediacaran–Cambrian transition was one of the most critical intervals in Earth history. During this interval, widespread chert was precipitated, commonly as a stratal wedge in carbonates, along the southern margin of the Yangtze Platform, South China. The chert wedge passes into a full chert succession further basinward to the south‐east. Four lithotypes of chert are identified across the marginal zone in western Hunan: mounded, vein, brecciated and bedded chert. The mounded chert is characterized by irregular to digitiform internal fabrics, generally with abundant original vesicles and/or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals. The host chert (or matrix) of these mounds is dominated by amorphous cryptocrystalline silica, commonly disseminated with pyrite. The vein chert, with minor quartz locally, generally cross‐cuts the overlying dolostone and chert horizons and terminates under the mounded and/or bedded chert bodies. The brecciated chert commonly occurs as splayed ‘intrusions’ or funnel‐shaped wedges and cross‐cuts the topmost dolostones. The bedded chert, the most common type, generally is thin to medium‐bedded and laminated locally; it is composed of amorphous silica with minor amounts of black lumps. Microthermometry of fluid inclusions from vein and void‐lining minerals (mainly quartz and barite) revealed homogenization temperatures from 120 to 180°C for the trapped primary fluids. Compositionally, these chert deposits generally are pure, with SiO₂ > 92 wt%, and only minor Fe₂O₃ and Al₂O₃ contents, most of which show positive Europium anomalies in rare earth element patterns, especially for the mounded chert. All these data suggest that the marginal zone chert deposits resulted from a low‐temperature, silica‐rich hydrothermal system, in which the mounded chert was precipitated around the releasing vents, i.e. as silica chimneys. The vein and splayed brecciated chert, however, was formed along the syndepositional fault/fracture conduits that linked downward, while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor. These petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation; rapid tectono‐depositional and oceanic changes during the Ediacaran–Cambrian transition in South China.     

Origin of natural sulfur-metal chimney in the Tangyin hydrothermal field,Okinawa Trough: constraints from rare earth element and sulfur isotopic compositions

For the first time, we present the rare earth element (REE) and sulfur isotopic composition of hydrothermal precipitates recovered from the Tangyin hydrothermal field (THF), Okinawa Trough at a water depth of 1206 m. The natural sulfur samples exhibit the lowest ΣREE concentrations (ΣREE= 0.65×10^–6–4.580×10^–6) followed by metal sulfides (ΣREE=1.71×10^–6–11.63×10^–6). By contrast, the natural sulfur-sediment samples have maximum ΣREE concentrations (ΣREE=11.54×10^–6–33.06×10^–6), significantly lower than those of the volcanic and sediment samples. Nevertheless, the δEu, δCe, (La/Yb)_N, La/Sm, (Gd/Yb)_N and normalized patterns of the natural sulfur and metal sulfide show the most similarity to the sediment. Most hydrothermal precipitate samples are characterized by enrichments of LREE (LREE/HREE=10.09–24.53) and slightly negative Eu anomalies or no anomaly (δEu=0.48–0.99), which are different from the hydrothermal fluid from sediment-free mid-oceanic ridges and back-arc basins, but identical to the sulfides from the Jade hydrothermal field. The lower temperature and more oxidizing conditions produced by the mixing between seawater and hydrothermal fluids further attenuate the leaching ability of hydrothermal fluid, inducing lower REE concentrations for natural sulfur compared with metal sulfide; meanwhile, the negative Eu anomaly is also weakened or almost absent. The sulfur isotopic compositions of the natural sulfur (δ³⁴S=3.20‰–5.01‰, mean 4.23‰) and metal sulfide samples (δ³⁴S=0.82‰–0.89‰, mean 0.85‰) reveal that the sulfur of the chimney is sourced from magmatic degassing.     

Hydrothermalism in the Tyrrhenian Sea: Inorganic and microbial sulfur cycling as revealed by geochemical and multiple sulfur isotope data

The Palinuro volcanic complex and the Panarea hydrothermal field, both located in the Tyrrhenian Sea (Italy), are associated with island arc magmatism and characterized by polymetallic sulfide mineralization. Dissolved sulfide concentrations, pH, and Eh measured in porewaters at both sites reveal a variable hydrothermal influence on porewater chemistry.Multiple sulfur isotopic measurements for disseminated sulfides (CRS: chromium reducible sulfur) extracted from sediments at Palinuro yielded a broad range in δ³⁴S range between ?29.8 and + 10.2‰ and Δ³³S values between + 0.015 and + 0.134‰. In contrast, sediments at Panarea exhibit a much smaller range in δ³⁴S_CRS with less negative values between ?11.3 and ?1.8‰. The sulfur isotope signatures are interpreted to reflect a mixture between hydrothermal and biogenic sulfide, with a more substantial biogenic contribution at Panarea.Multiple sulfur isotope measurements were performed on sulfides and elemental sulfur from drill core material from the Palinuro massive sulfide complex. δ³⁴S and Δ³³S values for pyrite between ?32.8 and ?1.1‰ and between ?0.012 to + 0.042‰, respectively, as well as for elemental sulfur with δ³⁴S and Δ³³S values between ?26.7 and ?2.1‰ and between + 0.035 and + 0.109‰, respectively, point to a microbial origin for much of the sulfide and elemental sulfur studied. Moreover, data suggest a coupling of bacterial sulfate reduction, sulfide oxidation and sulfur disproportionation. In addition, δ³⁴S values for barite between + 25.0 and + 63.6‰ are also in agreement with high microbial turnover of sulfate at Palinuro.Although a magmatic SO₂ contribution towards the formation of the Palinuro massive sulfide complex is very likely, the activity of different sulfur utilizing microorganisms played a fundamental role during its formation. Thus, porewater and multiple sulfur isotope data reveal differences in the hydrothermal activity at Palinuro and Panarea drill sites and underline the importance of microbial communities for the origin of massive sulfide mineralizations in the hydrothermal subsurface.     

华南下寒武统Ni-Mo-Se多金属层S-Se同位素体系

为了进一步理解华南下寒武统Ni-Mo-Se多金属层的物质来源及形成环境,文中分析了遵义中南村和张家界后坪两个Ni-Mo-Se矿层及其围岩的黄铁矿硫同位素和全岩的硒同位素组成。硫同位素组成显示两个Ni-Mo-Se矿层形成时的环境存在区域性差别,中南村矿层形成于间歇开放的海洋环境,而后坪矿层形成于封闭的缺氧(静海)环境。较大的硫同位素范围暗示硫酸盐还原菌控制硫同位素的分馏,而热液流体可能提供了大量金属元素,从而导致矿层富集大量的硫化物和稀有金属。硒同位素组成指示牛蹄塘组底部热液流体的Se可能重新经历了氧化还原循环,而Se的富集过程可能受有机质和粘土矿物吸附或类质同象过程控制。因此,控制多金属富集的因素主要为富集金属的热液流体的参与和缺氧环境下的自生沉积。     

贵州渣拉沟剖面下寒武统黑色硅质岩微量元素富集机制

华南早寒武世发育了一套富有机质黑色硅质岩,其成因尚有较大争议。选择贵州省三都县渣拉沟剖面早寒武世牛蹄塘组底部硅质岩段进行研究,发现该硅质岩具有微量元素富集的特征。其可能的原因包括,静海环境、上升流和热液活动。Ce/Ce*比值显示硅质岩沉积时水体为次氧化条件,不支持静海环境;而Ba、Zn、Cu、Ni、Cd、P与有机碳含量不存在相关关系,也不支持上升流为微量元素富集的主要原因。Al-Fe-Mn三角图、Al2O3/(Fe2O3+Al2O3)、REY配分模式等指标则表明存在热液活动。上述认识与现代上升流、静海环境的微量元素富集系数对比结果相吻合。与华南地区早寒武世初期不同沉积环境硅质岩对比,发现:深海环境并不富集Ba、Zn、Cu、Ni等氧化还原敏感元素,而沿着斜坡相带同沉积断层分布的热液活动,更有可能造成这些微量元素的富集。     

The origin of the Dapingzhang volcanogenic Cu–Pb–Zn ore deposit,Yunnan province,SW China: Constraints from host rock geochemistry and ore Os–Pb–S–C–O–H isotopes

The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100 km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417 ± 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive ε_Nd(t) values (+ 4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ³⁴S values of the sulfides from the deposit vary from − 1.24 to + 4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.310–18.656, ²⁰⁷Pb/²⁰⁴Pb = 15.489–15.643 and ²⁰⁸Pb/²⁰⁴Pb = 37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ¹³C_PDB and δ¹⁸O_SMOW values of the associated hydrothermal calcite crystals vary from − 2.3‰ to + 0.27‰ and from + 14.6 to + 24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ¹³C_PDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ¹⁸O values for the hydrothermal fluids, calculated from the measured values for quartz, are between − 2.1‰ and + 3.5‰. The corresponding δD values for the fluids range from − 59‰ to − 84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.     

南大西洋中脊26°S热液区成矿物质来源探讨

南大西洋中脊的26°S热液区广泛发育多金属硫化物、底泥、枕状熔岩、非活动性烟囱体和活动性烟囱体。为了有效探索硫、铜等成矿物质的来源以及成矿作用过程,分别以玄武岩、烟囱体残片及块状多金属硫化物为研究对象,开展了熔融包裹体、硫同位素和铜同位素研究。结果显示：区内玄武岩新鲜未蚀变且斑晶中产出大量熔融包裹体;熔融包裹体气泡壁附着黄铜矿、黄铁矿及磁铁矿等子矿物,说明在岩浆作用过程中可从熔浆中分离出成矿所需的金属元素和硫,这些成矿元素随着岩浆去气作用进入挥发分中,并随着脱气作用迁移出来。通过对烟囱体残片及块状多金属硫化物中黄铁矿的硫同位素组成进行比对分析,发现26°S热液区内硫化物的硫同位素与大西洋各热液区硫化物的硫同位素变化范围相一致,但δ³⁴S_V-CDT值略低（3.0‰~3.9‰）。低的δ³⁴S_V-CDT值指示硫以岩浆硫源为主,海水硫酸盐还原硫占比低。黄铜矿呈现略微富铜重同位素特征且分馏程度较低,其δ⁶⁵Cu值（0.171‰~0.477‰）趋近于大洋中脊玄武岩的铜同位素值（0）。综合硫同位素及铜同位素特征,表明热液流体经历了岩浆和海水的混合过程,成矿物质主要来自于岩浆热液,成矿作用过程中可能有少量海水混入。     

Was there a conflict at the Neoproterozoic-Cambrian boundary: Evidence from sulfur isotope composition?

Based on evaporite sequences of the Irkutsk amphitheater, it is shown that sulfur in the Vendian-Lower Cambrian sedimentary sulfates displays a very wide scatter of sulfur isotope ratios and enrichment in heavy sulfur up to average values of δ³⁴S ≈ +(27–30)‰. These features are related to sulfate reduction, which is distinctly expressed like other secondary alterations in the studied sections. Average δ³⁴S values reflect the dynamics of isotopic effects rather than the initial sulfur composition of the oceanic water. The Irkutsk amphitheater can be considered natural model of sulfur isotopic variations in ancient evaporites. Existing concepts of the sulfur isotopic composition of Cambrian oceans need to be revised.     

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge,Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.     

The Paleoarchean Northern Mundo Novo Greenstone Belt,São Francisco Craton: Geochemistry,U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with ε_Hf(t) =  ?2.47 to ?1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ¹⁸O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have ε_Hf(t) =  ?5.40 to ?0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ¹⁸O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ³³S between ?1.3‰ to +1.4‰) and a Δ³⁶S/Δ³³S slope of ?0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ³⁴S range (0 to +2‰), Δ³³S ~ 0‰, and Δ³⁶S ~ 0‰. A carbonaceous schist shows positive δ³⁴S (2.1‰–3.5‰) and elevated Δ³³S (1.2‰–1.4‰) values, with corresponding negative Δ³⁶S between ?1.2‰ to ?0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ³⁴S, Δ³³S, and Δ³⁶S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.     

桂北泗里口老堡组硅质岩的常量、稀土元素特征及成因指示

桂北泗里口老堡组为一套埃迪卡拉纪—寒武纪过渡时期（大约550～540 Ma）深水盆地沉积的硅质岩。它们的SiO2含量普遍高（平均93.8%）;Al2O3含量为0.17%～4.92%,沿剖面自下而上明显增加,上部超过2%;Al/(Al+Fe+Mn)比值多高于0.42;Fe/Ti比值大都小于16.3;Al2O3/( Al2O3+Fe2O3)比值多高于0.4,剖面上部样品的比值为0.8～0.9;Y/Ho比值为26.4～36.9,中、下部样品较高（多高于32）,上部样品的比值接近地壳值（27）;Eu/Eu*平均值为1.0,正异常不明显。剖面下部样品的∑REE含量低（15.9×10-6～27.1×10-6）,具有与现代海水相近的REE配分,没有正的Eu异常,不同于海底的热液流体和与其有关的碧玉的REE配分;中部样品的∑REE含量为26.2×10-6～49.4×10-6,由于所含陆源碎屑的增加,REE配分变得平坦,但仍有海水REE的某些特征;上部样品的∑REE含量为40.5×10-6～59×10-6,显示与平均页岩相似的平坦的REE配分,但∑REE含量仅为平均页岩的大约1/4～1/3。这些常量和稀土元素特征表明,海底热液和陆源碎屑都不可能成为泗里口老堡组硅质岩的重要物源。埃迪卡拉纪—寒武纪过渡时期华南深水盆地厚层硅质岩沉积反映了这一时期大气高CO2浓度,大量陆源化学风化的硅质流入海洋和大量生物的降解可能是造成这些硅质岩形成的基本原因。     

Extremely negative and inhomogeneous sulfur isotope signatures in Cretaceous Chilean manto-type Cu–(Ag) deposits,Coastal Range of central Chile

Chilean manto-type (CMT) Cu(–Ag) hydrothermal deposits share a characteristic association of volcano-sedimentary Jurassic to Lower Cretaceous host rocks, style of mineralization, ore and associated mineralogy and geochemistry, with ore grades typically > 1%Cu, that make this family of deposits significant and interesting, both academically and economically. Although often stratabound, geological evidence supports an epigenetic origin for these deposits. We present a detailed stable isotope study of La Serena and Melipilla–Naltahua Lower Cretaceous deposits, central Chile, which reveals extremely negative δ³⁴S values, to − 50‰, which are among the lowest values found in any ore deposit. In addition, the range of δ³⁴S values from sulfides in the two areas is very wide: − 38.3 to − 6.9‰ in La Serena, and − 50.4 to − 0.6‰ in Melipilla–Naltahua. These new data significantly extended the reported range of δ³⁴S data for CMT deposits. Co-existing sulfates range from 7.9 to 14.3‰, and are exclusive to La Serena deposit. The wide sulfide isotopic range occurs at deposit and hand specimen scale, and suggests a polygenic sulfur source for these deposits, where bacteriogenic sulfide dominates. While sulfur isotope data for the bulk of Jurassic CMT deposits, northern Chile, suggests a predominant magmatic source in their origin (mean = − 2.7 ± 1.9‰, 1σ), contributions of a magmatic component is only likely to be involved at Melipilla–Naltahua deposit.The δ¹³C values obtained for calcites associated with the mineralization range from − 20.1 to 0.2‰ also suggesting polygenic carbon sources, with the likely strong involvement of degradation of organic matter and leaching of limestone.Two different genetic models, with involvement of hydrocarbon, are proposed for both areas. For Melipilla–Naltahua, a two-step model can be developed as follows: 1) Framboidal pyrite growth, with very low δ³⁴S, formed by bacterial sulfate reduction in an open system, and with diagenetic degradation of oil-related brines, leaving pyrobitumen. 2) Cu-bearing stage, replacing of framboidal pyrite, inheriting depleted sulfur as low as − 50.4‰, together with sulfides directly precipitated from a hydrothermal fluid with δ³⁴S close to 0‰. For La Serena, a single step model fits best, without framboidal pyrite generation. Cu-bearing sulfides were precipitated mainly in veins where Cu plus base metal-bearing hydrothermal fluids mixed with H₂S generated by bacterial sulfate reduction in the host rocks. Isotopic evidence clearly illustrates that bacterial activity, perhaps enhanced by hydrothermal activity, was fed by hydrocarbon brines and sulfate remobilized from continental evaporites. It is possible that variable ecological conditions led to different extents of isotopic fractionation, adding to the typical sulfur isotopic heterogeneity of such bacterial systems. For both areas, the Cu-bearing stage occurred during the peak to waning stages of the very low-grade metamorphism that affected the Lower Cretaceous sequence.     

The Cipoeiro gold deposit,Gurupi Belt,Brazil: Geology,chlorite geochemistry,and stable isotope study

The Cipoeiro gold deposit, located in the Gurupi Belt, northern Brazil, is hosted by tonalites of 2148 Ma. The deposit is controlled by splays related to the major strike-slip Tentugal shear zone, and at the deposit scale, the mineralization is confined to ductile–brittle shear zones. Mineralization style comprises thick quartz veins and narrow and discontinuous quartz-carbonate veinlets associated with disseminations in altered host rocks. The postmetamorphic hydrothermal paragenesis is composed of quartz, calcite, chlorite, white mica (phengite), pyrite, and minor albite. Electron microprobe analysis of chlorites reveals a relatively uniform chemical composition at depths of more than 100 m. The chlorites are characterized by (Fe + Mg) ratios between 0.37 and 0.47 and Al^IV ranging between 2.22 and 2.59 a.p.f.u. and are classified as Fe-chlinochlore. Temperatures calculated by applying the Al^IV contents of chlorites yield a relatively narrow interval of 305 ± 15°C. Stable isotope (O, H, C, S) compositions have been determined in silicate, carbonate, and sulfide minerals. The δ¹⁸O and δD values of the mineralizing fluid range from +2.4 to +5.7 and from −43‰ to −20‰, respectively, and are interpreted as having a metamorphic origin. The δ¹³C values of fluid CO₂ are in the range −10.7‰ to −3.9‰, whereas the fluid δ³⁴S is around 0‰. Carbon and sulfur compositions are not diagnostic of their sources, compatible as they are with mantle, magmatic, or average crustal reservoirs. The hydrothermal paragenesis, chlorite–pyrite coexistence, temperature of ore formation, and sulfur isotope evidence indicate relatively reduced fO₂ conditions for the mineralizing fluid. Geologic, chemical, and isotopic characteristics of the Cipoeiro deposit are compatible with the class of orogenic gold deposits.     

Petrography of Late Cambrian to Middle Ordovician radiolarian chert in Kazakhstan with special reference to the emergence of benthic animals in the pelagic realm

Upper Cambrian to Middle Ordovician radiolarian chert successions from Kazakhstan were studied to clarify the history of the emergence of benthic animals in ocean floor sediments. Radiolarian tests and clay were deposited for a period of 30 Myr without experiencing an influx of continent-derived coarse clastic materials. Red, grey and black cherts of the Upper Cambrian to the upper mid-Darriwilian are thinly laminated, and no trace of benthic animal activity is recognized in that time interval. Bioturbation structures and burrow traces in mid-Darriwilian stage red chert in Kazakhstan suggest that benthic animals colonized the location where radiolarian chert formed, but that there was a significant delay in colonization when compared with similar reported occurrences in Australia and Canada.     

西藏扎西康矿集区夏隆岗铅锌矿床地质特征及其成矿物质来源研究:硫化物原位S同位素制约

夏隆岗铅锌矿床是扎西康矿集区内新近发现的铅锌矿床.矿床已发现的铅锌矿体受控于北东向、近南北向断裂构造.通过系统的钻孔编录,结合脉体穿插关系及矿物共生组合特征,可将矿床热液成矿期分为3个阶段.分别为:铁锰碳酸盐—多金属硫化物阶段(S1);石英—方解石—多金属硫化物阶段(S2);石英—硫盐矿物阶段(S3).本次研究系统测试...