Regional sulfate–hematite–sulfide zoning in the auriferous Mariana anticline, Quadrilátero Ferrífero of Minas Gerais, Brazil

The distribution of mineral deposits, characterised as barite deposits, hematite-rich auriferous deposits and auriferous tourmaline–sulfide deposits, displays a regional sulfate–hematite–sulfide zoning along the thrust-delineated limbs of the Mariana anticline, in the south-eastern part of the Quadrilátero Ferrífero of Minas Gerais, Brazil. Cross-cut relationships of barite veins and sulfide lodes indicate that sulfidation occurred in a late-tectonic context, which is here attributed to the collapse of the ～0.6-Ga Brasiliano thrust front. Reconnaissance S-isotopic data from barite and pyrite (Antônio Pereira barite deposit and its adjacent gold deposit, respectively), and arsenopyrite (Passagem de Mariana gold deposit), suggest a new interpretation for the hydrothermal fluid overprint in the Mariana anticline. The Antônio Pereira barite has Δ³³S values that are near zero, constraining the sulfate source to rocks younger than 2.45 Ga. The barite-δ³⁴S values are between +19.6 and +20.8?‰. The Passagem arsenopyrite and tourmaline have Co/Ni ratios that define a positive linear trend with the Antônio Pereira pyrite. The latter has homogenous δ³⁴S values, between +8.8 and +8.9?‰, which are compatible with thermochemical reduction of aqueous sulfate with the S-isotopic composition of the Antônio Pereira barite.     

Proto-Adamastor ocean crust (920 Ma) described in Brasiliano Orogen from coetaneous zircon and tourmaline

Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0–0.9 Ga up to 0.75 Ga, and evolved into world Adamastor Ocean at 0.75–0.60 Ga. Mesoproterozoic oceanic crust is poorly preserved on continents, only indirect evidence registered in Brasiliano Orogen. We report first evidence of ophiolite originated in proto-Adamastor. We use multi-technique U-Pb-Hf zircon and δ¹¹B tourmaline isotopic and elemental compositions. The host tourmalinite is enclosed in metaserpentinite, both belonging to the Bossoroca ophiolite. Zircon is 920 Ma-old, ?_{Hf(920 Ma)} = +12, Hf_TDM = 1.0 Ga and has ‘oceanic’ composition (e.g., U/Yb < 0.1). Tourmaline is dravite with δ¹¹B = +1.8‰ (Tur 1), 0‰ (Tur 2), ?8.5‰ (Tur 3). These characteristics are a novel contribution to Rodinia and associated world ocean, because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen.     

Tourmaline growth in pelitic schist and quartzite: A textural,chemical and B-isotopic study from the Gangpur Schist Belt,eastern India

Tourmaline is the principal repository of boron in crustal rocks and therefore useful for tracing B-cycling during prograde dehydration and retrogression of supracrustal rocks. Here, we use the major-trace element, and B isotope composition of tourmaline from schists, quartzites, and tourmaline-quartz veins of the Gangpur Schist Belt in eastern India to constrain the source of boron and the physicochemical evolution of B-rich fluids during prograde dehydration metamorphism. Tourmaline growth and re-equilibration in rocks of the Gangpur Schist Belt was a multi-stage process involving several fluid sources. The δ¹¹B varies between ?6‰ and ?18‰, indicating a dominantly continental source for boron. Tourmaline in schists, quartzites, and tourmaline-quartz veins grew over a wide range of P-T conditions and record multiple episodes of metamorphic dehydration between ca. 1.6 Ga and ca. 0.95Ga. The tourmaline in tourmaline-quartz veins and quartzites has lighter B-isotope composition, typical of continental detritus, while those in the schists and quartzites record pelite-dehydration signature with values decreasing gradually from ca. ?12‰ in the cores to ca. ?17‰ in the rims. Heavier isotopic compositions (δ¹¹B of ca. ?6‰) measured in some grains in the pelites and quartzites indicate boron contribution from meta?carbonate sources. The mixing of a heavier B-rich metacarbonate-derived fluid with pelite-derived metamorphic fluids could explain the lower B-isotope values in such tourmaline. The study also attempts to constrain the controls on the intake of trace elements in tourmaline. The results suggest that the partitioning of Mn, Y, V, Co and Ti in tourmaline is affected by the growth of porphyroblast phases such as garnet, staurolite, and biotite, while Li, Sr, Zn and Sn reflect the signature of the metamorphic fluid.     

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1‐ to 15‐m‐thick beds within the ≈2·0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680‐m‐thick formation is composed of a stromatolite–dolomite–‘red bed’ sequence formed in a complex combination of shallow‐marine and non‐marine, evaporitic environments. Dolomite‐collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by δ¹³C values from +7·1‰ to +11·6‰ (V‐PDB) and δ¹⁸O ranging from 17·4‰ to 26·3‰ (V‐SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high δ¹³C values ranging from +9·0‰ to +11·6‰ and δ¹⁸O values of 20·0–25·7‰. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water‐derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high δ¹³C values reflect a combined contribution from both global and local carbon reservoirs. A ¹³C‐rich global carbon reservoir (δ¹³C at around +5‰) is related to the perturbation of the carbon cycle at 2·0 Ga, whereas the local enhancement in ¹³C (up to +12‰) is associated with evaporative and restricted environments with high bioproductivity.     

Geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in Central Africa

A simple geochemical balance of lateritization processes governing the development of several tens of meters of weathering profiles overlain by ferricretes is estimated on the basis of detailed mineralogical and geochemical data. The lateritic weathering mantle of the “Haut–Mbomou” area in Central Africa is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite, a mottled clay layer, a soft nodular layer, a soft ferricrete, and a ferricrete in which kaolinite, gibbsite, goethite, and hematite occur in various quantities. Incongruent dissolution of kaolinite leads to the formation of gibbsite in the upper saprolite, whereas the hematite does not clearly replace the kaolinite according to an epigene process in the upper ferruginous layers of the profiles. Instead, that kaolinite is also transformed into gibbsite according to an incongruent dissolution under hydrated and reducing conditions induced by a relatively humid climatic pattern. The respective relations of the silica, iron, and aluminum balances and the Al substitution rate of the hematite on the one hand, and of RHG [RHG = 100 (hematite/hematite + goethite)] and the kaolinite on the other hand, to the consumption or the release of protons H⁺ permit differentiation of aggrading ferruginization and degradation processes operating in the different lateritic weathering profiles. The Al substitution rate of the Fe–oxyhydroxides varies according to the nature of lateritization processes, e.g., saprolitic weathering and aggrading ferruginization vs. degradation. The observations and results indicate that the ferruginization process of the weathering materials of parent rocks is not a simple ongoing process as often thought. This suggests that the actual lateritic weathering mantle of the Haut–Mbomou area may result from different stages of weathering and erosion during climatic changes.     

Geological,mineralogical, and oxygen isotope studies of the Chandmani Uul iron oxide–copper–gold deposit in Dornogobi Province,Southeastern Mongolia

The Chandmani Uul deposit is located in Dornogovi province, Southeastern Mongolia. Iron oxide ores are hosted in the andesitic rocks of the Shar Zeeg Formation of Neoproterozoic to Lower‐Cambrian age. Middle‐ to Upper‐Cambrian bodies of granitic rocks have intruded into the host rocks in the western and southern regions of the deposit. The wall rocks around the iron oxide ore bodies were hydrothermally altered to form potassic, epidote, and sericite–chlorite alteration zones, and calcite and quartz veinlets are ubiquitous in the late stage. Since granitic rocks also underwent potassic alteration, the activity of the granitic rocks must have a genetic relation to the ore deposit. The ore mineral assemblage is dominated by iron oxides such as mushketovite, euhedral magnetite with concentric and/or oscillatory zoning textures, and cauliflower magnetite. Lesser amounts of chalcopyrite and pyrite accompany the iron oxides. Among all these products, mushketovite is dominant and is distributed throughout the deposit. Meanwhile, euhedral magnetite appears in limited amounts at relatively shallow levels in the deposit. By contrast, cauliflower magnetite appears locally in the deeper parts of the deposit, and is associated with green‐colored garnet and calcite. Sulfide minerals are ubiquitously associated with these iron oxides. The oxygen isotope (δ¹⁸O) values of all types of magnetite, quartz, and epidote were found to be ?5.9 to ?2.8‰, 10.5 to 14.9‰, and 3.6 to 6.6‰, respectively. The δ¹⁸O values of quartz–magnetite pairs suggest an equilibrium isotopic temperature near 300°C. The calculated values of δ¹⁸O for the water responsible for magnetite ranged from 2 to 10‰. All the data obtained in this study suggest that the iron oxide deposit at the Chandmani Uul is a typical iron oxide–copper–gold deposit, and that this deposit was formed at an intermediate depth with potassic and sericite–chlorite alteration zones under the oxidized conditions of a hematite‐stable environment. The δ¹⁸O range estimated implies that the ore‐forming fluid was supplied by a crystallizing granodioritic magma exsolving fluids at depth with a significant contribution of meteoric water.     

Hydrothermal fluid source constrained by Co/Ni ratios in coexisting arsenopyrite and tourmaline: the auriferous lode of Passagem, Quadrilátero Ferrífero of Minas Gerais, Brazil

The auriferous lode of Passagem de Mariana is characterised by abundant tourmaline, which is intergrown with arsenopyrite. Spot measurements using laser ablation–inductively coupled plasma–mass spectrometry show that Co and Ni are the most abundant trace elements in the arsenopyrite (45–538?ppm Co, 246–828?ppm Ni), with Co/Ni ratios consistently <1. The coexisting tourmaline also has Co/Ni <1, with Co and Ni contents that are ~2 orders of magnitude lower than those in the arsenopyrite. The Co/Ni ratios of tourmaline and arsenopyrite are tightly distributed along a positive linear trend, the angular coefficient of which represents the Co/Ni of the hydrothermal fluid from which these minerals precipitated. The fluid Co/Ni ratio is close to the average Co/Ni value for the upper continental crust. In conjunction with the abundance of lode tourmaline and its B-isotope data (from the literature), the Co/Ni ratios of tourmaline and arsenopyrite fingerprint a continental evaporitic source of B.     

Development and Re‐Evaluation of Tourmaline Reference Materials for In Situ Measurement of Boron δ Values by Secondary Ion Mass Spectrometry

Six tourmaline samples were investigated as potential reference materials (RMs) for boron isotope measurement by secondary ion mass spectrometry (SIMS). The tourmaline samples are chemically homogeneous and cover a compositional range of tourmaline supergroup minerals (primarily Fe, Mg and Li end‐members). Additionally, they have homogeneous boron delta values with intermediate precision values during SIMS analyses of less than 0.6‰ (2s). These samples were compared with four established tourmaline RMs, that is, schorl IAEA‐B‐4 and three Harvard tourmalines (schorl HS#112566, dravite HS#108796 and elbaite HS#98144). They were re‐evaluated for their major element and boron delta values using the same measurement procedure as the new tourmaline samples investigated. A discrepancy of about 1.5‰ in δ¹¹B was found between the previously published reference values for established RMs and the values determined in this study. Significant instrumental mass fractionation (IMF) of up to 8‰ in δ¹¹B was observed for schorl–dravite–elbaite solid solutions during SIMS analysis. Using the new reference values determined in this study, the IMF of the ten tourmaline samples can be modelled by a linear combination of the chemical parameters FeO + MnO, SiO₂ and F. The new tourmaline RMs, together with the four established RMs, extend the boron isotope analysis of tourmaline towards the Mg‐ and Al‐rich compositional range. Consequently, the in situ boron isotope ratio of many natural tourmalines can now be determined with an uncertainty of less than 0.8‰ (2s).     

Xenotime–hematite aggregates on opaline filaments: evidence for biomineralization in weathered siliciclastic rocks, Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil

Dissolution cavities in weathered pebbly quartzite of the ~2.5-Ga Moeda Formation at Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil, are decorated with suspended filaments of opaline silica. The filaments sustain xenotime–hematite aggregates in the open space. Xenotime occurs as inclusions in buds and botryoidal aggregates of hematite. The filamentous structures consist of strand-forming buds, hypha-like extensions, and thin strands that compose mat-like arrangements. They resemble microbial filaments that were replaced by opaline silica and fossilized. The occurrence of spherical hematite as protuberances on hematite-free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers. Phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare-earth elements from groundwater to the iron-accreting polymers. These would have resulted in botryoidal aggregates of hematite with xenotime inclusions. The presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology.     

Mineral deposit formation in Phanerozoic sedimentary basins of north-east Africa: the contribution of weathering

The intra- and epicontinental basins in north-east Africa (Egypt, Sudan) bear ample evidence of weathering processes repeatedly having contributed to the formation of mineral deposits throughout the Phanerozoic.The relict primary weathering mantle of Pan-African basement rocks consists of kaolinitic saprolite, laterite (in places bauxitic) and iron oxide crust. On the continent, the reaccumulation of eroded weathering-derived clay minerals (mainly kaolinite) occurred predominantly in fluvio-lacustrine environments, and floodplain and coastal plain deposits. Iron oxides, delivered from ferricretes, accumulated as oolitic ironstones in continental and marine sediments. Elements leached from weathering profiles accumulated in continental basins forming silcrete and alunite or in the marine environment contributing to the formation of attapulgite/saprolite and phosphorites.The Early Paleozoic Tawiga bauxitic laterite of northern Sudan gives a unique testimony of high latitude lateritic weathering under global greenhouse conditions. It formed in close spatial and temporal vicinity to the Late Ordovician glaciation in north Africa. The record of weathering products is essentially complete for the Late Cretaceous/Early Tertiary. From the continental sources in the south to the marine sinks in the north, an almost complete line of lateritic and laterite-derived deposits of bauxitic kaolin, kaolin, iron oxides and phosphates is well documented.     

Ržanovo metamorphosed lateritic Fe-Ni deposit, Republic of Macedonia

The R?anovo deposit is a unique example of metamorphosed lateritic Fe-Ni deposits in the wellknown Vardar ophiolitic belt of the Balkans, where Fe-Ni mineralization is a product of the Early Cretaceous lateritic weathering mantle after Jurassic ultramafic rocks subsequently redeposited into the Cretaceous shallow-water marine basin. As a result, a layer of Fe-Ni ore 30–50 m thick with an average Ni grade of ～1% was formed at the R?anovo deposit; this layer is traced for more than 4 km along the strike and 500 m down the dip. At the end of the Late Cretaceous, intense Alpine tectonic faulting and folding resulted in overturning of the initially horizontal ore layer, which is now nearly vertical. The most abundant fissile hematite ore contains 0.70–1.27% Ni, whereas the economically most important massive hematite ore with 0.93–1.49% Ni occurs locally. The major Ni-bearing minerals are magnetite, hematite, chromite, sulfides, talc, chlorite, amphibole, and stilpnomelane.     

Using oxygen isotope chemistry to track hydrothermal processes and fluid sources in itabirite-hosted iron ore deposits in the Quadrilátero Ferrífero,Minas Gerais,Brazil

The Quadrilátero Ferrífero, Brazil, is presently the largest accumulation of single itabirite-hosted iron ore bodies worldwide. Detailed petrography of selected hypogene high-grade iron ore bodies at, e.g. the Águas Claras, Conceição, Pau Branco and Pico deposits revealed different iron oxide generations, from oldest to youngest: magnetite → martite (hematite pseudomorph after magnetite) → granoblastic (recrystallised) → microplaty (fine-grained, <100 μm) → specular (coarse-grained, >100 μm) hematite. Laser-fluorination oxygen isotope analyses of selected iron ore species showed that the δ¹⁸O composition of ore-hosted martite ranges between ?4.4 and 0.9?‰ and is up to 11?‰ depleted in ¹⁸O relative to hematite of the host itabirite. During the modification of iron ore and the formation of new iron oxide generations (e.g. microplaty and specular hematite), an increase of up to 8?‰ in δ¹⁸O values is recorded. Calculated δ¹⁸O values of hydrothermal fluids in equilibrium with the iron oxide species indicate: (1) the involvement of isotopically light fluids (e.g. meteoric water or brines) during the upgrade from itabirite-hosted hematite to high-grade iron ore-hosted martite and (2) a minor positive shift in δ¹⁸O_fluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated δ¹⁸O values and/or the infiltration of small volumes of isotopically heavy (metamorphic and/or magmatic) fluids into the iron ore system. The circulation of large fluid volumes that cause the systematic decrease of ¹⁸O/¹⁶O ratios from itabirite to high-grade iron ore requires the presence of, e.g. extensive faults and/or large-scale folds.     

Oxygenation of shallow marine environments and chemical sedimentation in Palaeoproterozoic peritidal settings: Frere Formation,Western Australia

The Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western Australia, is a ca 600 m thick succession of iron formation and fine‐grained, clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling. Lithofacies stacking patterns suggest that deposition occurred during a marine transgression punctuated by higher frequency relative sea‐level fluctuations that produced five parasequences. Decametre‐scale parasequences are defined by flooding surfaces overlain by either laminated magnetite or magnetite‐bearing, hummocky cross‐stratified sandstone that grades upward into interbedded hematite‐rich mudstone and trough cross‐stratified granular iron formation. Each aggradational cycle is interpreted to record progradation of intertidal and tidal channel sediments over shallow subtidal and storm‐generated deposits of the middle shelf. The presence of aeolian deposits, mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind‐blown sediment input. Iron formation in the Frere Formation, in contrast to most other Palaeoproterozoic examples, was deposited almost exclusively in peritidal environments. These other continental margin iron formations also reflect upwelling of anoxic, Fe‐rich sea water, but accumulated in the full spectrum of shelf environments. Dilution by fine‐grained, windblown terrigenous clastic sediment probably prevented the Frere iron formation from forming in deeper settings. Lithofacies associations and interpreted paragenetic pathways of Fe‐rich lithofacies further suggest precipitation in sea water with a prominent oxygen chemocline. Although essentially unmetamorphosed, the complex diagenetic history of the Frere Formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean‐atmosphere evolution. Unlike studies that focus exclusively on their chemistry, an approach that also considers palaeoenvironment and oceanography, as well the effects of post‐depositional fluid flow and alteration, mitigates the potential for incorrectly interpreting geochemical data.     

澳大利亚西部哈默斯利铁成矿省BIF富铁矿的成矿特征与控矿因素

澳大利亚西部哈默斯利铁成矿省含有世界级高品位的赤铁矿体。主要铁矿床包括芒特维尔贝克、汤姆普莱斯山、帕拉伯杜等,它们均产于元古宙早期布罗克曼BIF型含铁建造中。高品住铁矿体的空间分布明显受到元古宙区域隆起和拉张环境下形成的古老正断层系统的控制。该成矿省高品位铁矿层的形成可分为3个阶段：第1阶段为深层阶段,该阶段硅从含铁建造中淋滤出来,留下薄层状富含铁氧化物、碳酸盐岩、硅酸镁和磷灰石的残余物;第2阶段为深部大气水氧化阶段,该阶段含铁建造的磁铁矿-菱镁矿组合被氧化为赤铁矿-铁白云石,并以发育假象赤铁矿为特征;第3阶段为浅层风化作用。通过对成矿特征和成矿模式的总结,认为成矿时代、断层、褶皱等构造特征及流体和表生风化作用是富铁矿床形成的主要控矿因素。     

The snowball Earth hypothesis: testing the limits of global change

The gradual discovery that late Neoproterozoic ice sheets extended to sea level near the equator poses a palaeoenvironmental conundrum. Was the Earth's orbital obliquity > 60° (making the tropics colder than the poles) for 4.0 billion years following the lunar‐forming impact, or did climate cool globally for some reason to the point at which runaway ice‐albedo feedback created a `snowball' Earth? The high‐obliquity hypothesis does not account for major features of the Neoproterozoic glacial record such as the abrupt onsets and terminations of discrete glacial events, their close association with large (> 10‰) negative δ<sup>13</sup>C shifts in seawater proxies, the deposition of strange carbonate layers (`cap carbonates') globally during post‐glacial sea‐level rise, and the return of large sedimentary iron formations, after a 1.1 billion year hiatus, exclusively during glacial events. A snowball event, on the other hand, should begin and end abruptly, particularly at lower latitudes. It should last for millions of years, because outgassing must amass an intense greenhouse in order to overcome the ice albedo. A largely ice‐covered ocean should become anoxic and reduced iron should be widely transported in solution and precipitated as iron formation wherever oxygenic photosynthesis occurred, or upon deglaciation. The intense greenhouse ensures a transient post‐glacial regime of enhanced carbonate and silicate weathering, which should drive a flux of alkalinity that could quantitatively account for the world‐wide occurrence of cap carbonates. The resulting high rates of carbonate sedimentation, coupled with the kinetic isotope effect of transferring the CO₂ burden to the ocean, should drive down the δ¹³C of seawater, as is observed. If cap carbonates are the `smoke' of a snowball Earth, what was the `gun'? In proposing the original Neoproterozoic snowball Earth hypothesis, Joe Kirschvink postulated that an unusual preponderance of land masses in the middle and low latitudes, consistent with palaeomagnetic evidence, set the stage for snowball events by raising the planetary albedo. Others had pointed out that silicate weathering would most likely be enhanced if many continents were in the tropics, resulting in lower atmospheric CO₂ and a colder climate. Negative δ¹³C shifts of 10–20‰ precede glaciation in many regions, giving rise to speculation that the climate was destabilized by a growing dependency on greenhouse methane, stemming ultimately from the same unusual continental distribution. Given the existing palaeomagnetic, geochemical and geological evidence for late Neoproterozoic climatic shocks without parallel in the Phanerozoic, it seems inevitable that the history of life was impacted, perhaps profoundly so.     

BIF-hosted iron mineral system: A review

The BIF-hosted iron ore system represents the world's largest and highest grade iron ore districts and deposits. BIF, the precursor to low- and high-grade BIF hosted iron ore, consists of Archean and Paleoproterozoic Algoma-type BIF (e.g., Serra Norte iron ore district in the Carajás Mineral Province), Proterozoic Lake Superior-type BIF (e.g., deposits in the Hamersley Province and craton), and Neoproterozoic Rapitan-type BIF (e.g., the Urucum iron ore district).The BIF-hosted iron ore system is structurally controlled, mostly via km-scale normal and strike-slips fault systems, which allow large volumes of ascending and descending hydrothermal fluids to circulate during Archean or Proterozoic deformation or early extensional events. Structures are also (passively) accessed via downward flowing supergene fluids during Cenozoic times.At the depositional site the transformation of BIF to low- and high-grade iron ore is controlled by: (1) structural permeability, (2) hypogene alteration caused by ascending deep fluids (largely magmatic or basinal brines), and descending ancient meteoric water, and (3) supergene enrichment via weathering processes. Hematite- and magnetite-based iron ores include a combination of microplaty hematite–martite, microplaty hematite with little or no goethite, martite–goethite, granoblastic hematite, specular hematite and magnetite, magnetite–martite, magnetite-specular hematite and magnetite–amphibole, respectively. Goethite ores with variable amounts of hematite and magnetite are mainly encountered in the weathering zone.In most large deposits, three major hypogene and one supergene ore stages are observed: (1) silica leaching and formation of magnetite and locally carbonate, (2) oxidation of magnetite to hematite (martitisation), further dissolution of quartz and formation of carbonate, (3) further martitisation, replacement of Fe silicates by hematite, new microplaty hematite and specular hematite formation and dissolution of carbonates, and (4) replacement of magnetite and any remaining carbonate by goethite and magnetite and formation of fibrous quartz and clay minerals.Hypogene alteration of BIF and surrounding country rocks is characterised by: (1) changes in the oxide mineralogy and textures, (2) development of distinct vertical and lateral distal, intermediate and proximal alteration zones defined by distinct oxide–silicate–carbonate assemblages, and (3) mass negative reactions such as de-silicification and de-carbonatisation, which significantly increase the porosity of high-grade iron ore, or lead to volume reduction by textural collapse or layer-compaction. Supergene alteration, up to depths of 200 m, is characterised by leaching of hypogene silica and carbonates, and dissolution precipitation of the iron oxyhydroxides.Carbonates in ore stages 2 and 3 are sourced from external fluids with respect to BIF. In the case of basin-related deposits, carbon is interpreted to be derived from deposits underlying carbonate sequences, whereas in the case of greenstone belt deposits carbonate is interpreted to be of magmatic origin. There is only limited mass balance analyses conducted, but those provide evidence for variable mobilization of Fe and depletion of SiO₂. In the high-grade ore zone a volume reduction of up to 25% is observed.Mass balance calculations for proximal alteration zones in mafic wall rocks relative to least altered examples at Beebyn display enrichment in LOI, F, MgO, Ni, Fe₂O_3total, C, Zn, Cr and P₂O₅ and depletions of CaO, S, K₂O, Rb, Ba, Sr and Na₂O. The Y/Ho and Sm/Yb ratios of mineralised BIF at Windarling and Koolyanobbing reflect distinct carbonate generations derived from substantial fluid–rock reactions between hydrothermal fluids and igneous country rocks, and a chemical carbonate-inheritance preserved in supergene goethite.Hypogene and supergene fluids are paramount for the formation of high-grade BIF-hosted iron ore because of the enormous amount of: (1) warm (100–200 °C) silica-undersaturated alkaline fluids necessary to dissolve quartz in BIF, (2) oxidized fluids that cause the oxidation of magnetite to hematite, (3) weakly acid (with moderate CO₂ content) to alkaline fluids that are necessary to form widespread metasomatic carbonate, (4) carbonate-undersaturated fluids that dissolve the diagenetic and metasomatic carbonates, and (5) oxidized fluids to form hematite species in the hypogene- and supergene-enriched zone and hydroxides in the supergene zone.Four discrete end-member models for Archean and Proterozoic hypogene and supergene-only BIF hosted iron ore are proposed: (1) granite–greenstone belt hosted, strike-slip fault zone controlled Carajás-type model, sourced by early magmatic (± metamorphic) fluids and ancient “warm” meteoric water; (2) sedimentary basin, normal fault zone controlled Hamersley-type model, sourced by early basinal (± evaporitic) brines and ancient “warm” meteoric water. A variation of the latter is the metamorphosed basin model, where BIF (ore) is significantly metamorphosed and deformed during distinct orogenic events (e.g., deposits in the Quadrilátero Ferrífero and Simandou Range). It is during the orogenic event that the upgrade of BIF to medium- and high-grade hypogene iron took place; (3) sedimentary basin hosted, early graben structure controlled Urucum-type model, where glaciomarine BIF and subsequent diagenesis to very low-grade metamorphism is responsible for variable gangue leaching and hematite mineralisation. All of these hypogene iron ore models do not preclude a stage of supergene modification, including iron hydroxide mineralisation, phosphorous, and additional gangue leaching during substantial weathering in ancient or Recent times; and (4) supergene enriched BIF Capanema-type model, which comprises goethitic iron ore deposits with no evidence for deep hypogene roots. A variation of this model is ancient supergene iron ores of the Sishen-type, where blocks of BIF slumped into underlying karstic carbonate units and subsequently experienced Fe upgrade during deep lateritic weathering.     

Geochemical Characteristics of the Upper Triassic Xujiahe Formation in Sichuan Basin, China and its Significance for Hydrocarbon Accumulation

The alternative development of coal-bearing hydrocarbon source rocks and low-porosity and low-permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near-source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan Basin is dominated by hydrocarbon gases, of which the methane content is80.16%-98.67%. Typically, the C_2~+ content is larger than 5% in main wet gas. The dry gas is mainly distributed in the western and northern regions of the basin. The non-hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of 2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ~(13)C_1 of-43.8‰ to-29.6‰, δ~(13)C_2 of-35.4‰ to-21.5‰, δ~(13)C_3 of-27.6‰ to-19.8‰,and δ~(13)C_4 of-27.7‰ to-18.8‰. δ~(13)C_3δ~(13)C_4 occurs in some natural gas with a low evolution degree; such gas is mainly coal-related gas from humic-type source rocks of the Xujiahe Formation. As for the natural gas, δ~2 H_(CH4) values ranged from-195‰ to-161‰,δ~2 H_(C2H6) values ranged from-154‰ to-120‰, and δ~2 H_(C3H8) values ranged from-151‰ to-108‰. The dry coefficient,δ~(13)C and δ~2 H_(CH4) are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ~(13)C and δ~2 H_(CH_4) values are, indicative of the characteristic of near-source accumulation. The δ~2 H_(C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ~2 H_(C2H6) value is. The Pr/Ph value of the condensate oil ranged from 1.60 to 3.43, illustrating light oxidization-light reduction and partial-oxidization characteristics of the depositional environment of coal-bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon(C_5-C_7) from the Xujiahe Formation presented two characteristics: the first was the relatively high aromatic hydrocarbon content(19%-32.1%), which reveals the characteristic of natural gas with humic substances of high-maturity; the second was the low content of aromatic hydrocarbon(0.4%-9.3%),reflecting water-washing during the accumulation of the natural gas. The reported research outcomes indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.     

Boron,sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt,eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe^3 +–Al^3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ¹¹B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe^3 +–Al^3 + substitution) and δ¹¹B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ¹¹B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ¹¹B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ³⁴S_VCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ³⁴S_VCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ³⁴S_VCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ³⁴S_VCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ⁶⁵Cu_NBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ⁶⁵Cu_NBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ³⁴S_VCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.     

Genesis of the Xiuwenghala Gold Deposit in the Beishan Orogen,Northwest China: Evidence from Geology,Fluid Inclusion,and H–O–S–Pb Isotopes

The Xiuwenghala gold deposit is located in the Beishan Orogen of the southern Central Asian Orogenic Belt. The vein/lenticular gold orebodies are controlled by Northeast‐trending faults and are hosted mainly in the brecciated/altered tuff and rhyolite porphyry of the Lower Carboniferous Baishan Formation. Metallic minerals include mainly pyrite and minor chalcopyrite, arsenopyrite, galena, and sphalerite, whilst nonmetallic minerals include quartz, chalcedony, sericite, chlorite, and calcite. Hydrothermal alterations consist of silicic, sericite, chlorite, and carbonate. Alteration/mineralization processes comprise three stages: pre‐ore silicic alteration (Stage I), syn‐ore quartz‐chalcedony‐polymetallic sulfide mineralization (Stage II), and post‐ore quartz‐calcite veining (Stage III). Fluid inclusions (FIs) in quartz and calcite are dominated by L‐type with minor V‐type and lack any daughter mineral‐bearing or CO₂‐rich/‐bearing inclusions. From Stages I to III, the FIs homogenized at 240–260°C, 220–250°C, and 150–190°C, with corresponding salinities of 2.9–10.9, 3.2–11.1, and 2.9–11.9 wt.% NaCl eqv., respectively. The mineralization depth at Xiuwenghala is estimated to be relatively shallow (<1 km). FI results indicate that the ore‐forming fluids belong to a low to medium‐temperature, low‐salinity, and low‐density NaCl‐H₂O system. The values decrease from Stage I to III (3.7‰, 1.7–2.4‰, and ?1.7 to 0.9‰, respectively), and a similar trend is found for their values (?104 to ?90‰, ?126 to ?86‰, and ?130 to ?106‰, respectively). This indicates that the fluid source gradually evolved from magmatic to meteoric. δ³⁴S values of the hydrothermal pyrites (?3.0 to 0.0‰; avg. ?1.1‰) resemble those of typical magmatic/mantle‐derived sulfides. Pyrite Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.409–18.767, ²⁰⁷Pb/²⁰⁴Pb = 15.600–15.715, ²⁰⁸Pb^/204Pb = 38.173–38.654) are similar to those of the (sub)volcanic ore host, indicating that the origin of ore‐forming material was mainly the upper crustal (sub)volcanic rocks. Integrating evidence from geology, FIs, and H–O–S–Pb isotopes, we suggest that Xiuwenghala is best classified as a low‐sulfidation epithermal gold deposit.     

Deciphering the depositional environment of the laminated Crato fossil beds (Early Cretaceous,Araripe Basin,North‐eastern Brazil)

The laminated limestones of the Early Cretaceous Crato Formation of the Araripe Basin (North‐eastern Brazil) are world‐famous for their exceptionally well‐preserved and taxonomically diverse fossil fauna and flora. Whereas the fossil biota has received considerable attention, only a few studies have focused on the sedimentary characteristics and palaeoenvironmental conditions which prevailed during formation of the Crato Fossil Lagerstätte. The Nova Olinda Member represents the lowermost and thickest unit (up to 10 m) of the Crato Formation and is characterized by a pronounced rhythmically bedded, pale to dark lamination. To obtain information on palaeoenvironmental conditions, sample slabs derived from three local stratigraphic sections within the Araripe Basin were studied using high‐resolution multiproxy techniques including detailed logging, petrography, μ‐XRF scanning and stable isotope geochemistry. Integration of lithological and petrographic evidence indicates that the bulk of the Nova Olinda limestone formed via authigenic precipitation of calcite from within the upper water column, most probably induced and/or mediated by phytoplankton and picoplankton activity. A significant contribution from a benthonic, carbonate‐secreting microbial mat community is not supported by these results. Deposition took place under anoxic and, at least during certain episodes, hypersaline bottom water conditions, as evidenced by the virtually undisturbed lamination pattern, the absence of a benthonic fauna and by the occurrence of halite pseudomorphs. Input of allochthonous, catchment‐derived siliciclastics to the basin during times of laminite formation was strongly reduced. The δ¹⁸O values of authigenic carbonate precipitates (between ?7·1 and ?5·1‰) point to a ¹⁸O‐poor meteoric water source and support a continental freshwater setting for the Nova Olinda Member. The δ¹³C values, which are comparatively rich in ¹³C (between ?0·1 and +1·9‰), are interpreted to reflect reduced throughflow of water in a restricted basin, promoting equilibration with atmospheric CO₂, probably in concert with stagnant conditions and low input of soil‐derived carbon. Integration of lithological and isotopic evidence indicates a shift from closed to semi‐closed conditions towards a more open lake system during the onset of laminite deposition in the Crato Formation.