Paragenetic Sequence Research Articles

Mineral authigenesis within chemosynthetic microbial mats: Coated grain formation and phosphogenesis at a Cretaceous hydrocarbon seep, New Zealand

AbstractMarine hydrocarbon seeps are sites of chemosynthetic microbial activity and authigenic carbonate formation. Seep limestones are typified by a range of geochemical signatures of microbial hydrocarbon oxidation, but only few seep deposits reveal mesofabrics that can be regarded as evidence of microbial activity. A Cretaceous methane‐seep limestone from Waipiro Bay, New Zealand, exhibits a fabric composed of cryptocrystalline carbonate fluorapatite between carbonate coated grains and spheroidal calcite. To understand the unusual Waipiro deposit, a paragenetic sequence has been derived for coated grains, spheroidal calcite and carbonate fluorapatite using thin section petrography, scanning electron microscopy, Fourier‐transform infrared spectroscopy and stable isotope geochemistry. The formation of 13C‐depleted coated grains (δ13C values as low as −15.8‰ Vienna‐Pee Dee Belemnite) and spheroidal calcite (δ13C values as low as −21.3‰) was favoured by hydrocarbon oxidation. Fibrous banded and botryoidal cement, a typical early diagenetic phase of hydrocarbon‐seep deposits, features δ13C values as low as −22.9‰. Coated grains and spheroidal calcite grew by displacive growth in a gel‐like medium, probably a microbial mat. Phosphorus is a mobile element and marine pore waters are typically undersaturated with respect to carbonate fluorapatite. Specific conditions are consequently required to retain sufficient concentrations to precipitate carbonate fluorapatite. Possible sources of phosphorus for the formation of the 13C‐depleted Waipiro carbonate fluorapatite (mean δ13C value of −15.4‰) include (a) the oxidation of sedimentary organic matter by organoclastic sulphate reduction, (b) the degradation of the microbial mat itself and (c) the active release of polyphosphate by sulphide‐oxidizing bacteria. This study suggests that the formation of the Cretaceous Waipiro seep limestone involved an interplay of biogeochemical processes including sulphate‐driven anaerobic oxidation of methane, organoclastic sulphate reduction and possibly nitrate‐dependent sulphide oxidation. It further demonstrates that coated grains resembling oncoids are not restricted to shallow water, photosynthesis‐dependent ecosystems.

The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits

Carbonate-bearing fluorapatite rocks occur at over 30 globally distributed carbonatite complexes and represent a substantial potential supply of phosphorus for the fertiliser industry. However, the process(es) involved in forming carbonate-bearing fluorapatite at some carbonatites remain equivocal, with both hydrothermal and weathering mechanisms inferred. In this contribution, we compare the paragenesis and trace element contents of carbonate-bearing fluorapatite rocks from the Kovdor, Sokli, Bukusu, Catalão I and Glenover carbonatites in order to further understand their origin, as well as to comment upon the concentration of elements that may be deleterious to fertiliser production. The paragenesis of apatite from each deposit is broadly equivalent, comprising residual magmatic grains overgrown by several different stages of carbonate-bearing fluorapatite. The first forms epitactic overgrowths on residual magmatic grains, followed by the formation of massive apatite which, in turn, is cross-cut by late euhedral and colloform apatite generations. Compositionally, the paragenetic sequence corresponds to a substantial decrease in the concentration of rare earth elements (REE), Sr, Na and Th, with an increase in U and Cd. The carbonate-bearing fluorapatite exhibits a negative Ce anomaly, attributed to oxic conditions in a surficial environment and, in combination with the textural and compositional commonality, supports a weathering origin for these rocks. Carbonate-bearing fluorapatite has Th contents which are several orders of magnitude lower than magmatic apatite grains, potentially making such apatite a more environmentally attractive feedstock for the fertiliser industry. Uranium and cadmium contents are higher in carbonate-bearing fluorapatite than magmatic carbonatite apatite, but are much lower than most marine phosphorites.

Bimineralic Middle Triassic ooids from Hydra Island: Diagenetic pathways and implications for ancient seawater geochemistry

AbstractThis study reports on the occurrence of formerly bimineralic, ancient, calcitic coated grains, including half‐moon ooids and micritic to microsparitic ooids in the Middle Anisian (Middle Triassic) grain flow from Hydra Island (Greece), and discusses their importance for palaeoseawater geochemical interpretations. The bi‐partite fabric in half‐moon ooids resulted from early pre‐compaction rapid dissolution of both aragonitic cortices and the more‐soluble components of the high‐Mg calcite core in the primary ooid, as well as the collapse of less‐dissolved remnants to the bottom of the oomouldic cavity which was later filled with calcite cement. Zonal, micritic to microsparitic ooids formed through intracortical dissolution of primary aragonite lamellae which were later replaced by (micro)sparry calcite. At the same time, recrystallization of original high‐Mg calcite layers to micrite occurred. The stratigraphic occurrence of the investigated bimineralic ooids conforms to the global distribution of carbonate ooids through the Middle Triassic period. The cementation history of the grain flow hosting coated grains is well documented by the paragenetic sequence established in half‐moon ooids which predicts that calcite precipitated under burial (shallow to deep) conditions. The main control on precipitation of bimineralic ooids was the Mg/Ca ratio of seawater, with lesser influences from seawater temperature, salinity and ρCO2 which promoted the degree of carbonate saturation, leading to higher precipitation rates for both aragonite and high‐Mg calcite. The studied bimineralic ooids represent a change in the global carbonate factory during the Middle Triassic in response to secular variations in the chemistry of palaeoseawater.

Ore genesis and hydrothermal evolution of the Shaxi porphyry Cu–Au deposit, Anhui province, Eastern China: evidence from isotopes (S–Sr–H–O), pyrite, and fluid inclusions

The Shaxi porphyry Cu–Au deposit is located in the northern part of the Luzong Basin on the outer edge of the southern Middle-Lower Yangtze River metallogenic belt, China. It is typical of the porphyry Cu–Au deposits in the metallogenic belt, with more than 1 Mt Cu at 0.55 wt% and 45 tons Au at 0.47 g/t. Hydrothermal alteration consists of a core of potassic alteration overprinted by a phyllic alteration zone. Propylitic alteration occurs peripheral to the potassic zone, but can also be recognized in the core of the deposit, where vein density is low. The main mineralization occurs as sulfide veinlets in the potassic zone where it is superposed by phyllic alteration. The paragenetic sequence show that chalcopyrite began to precipitate late in stage 1, associated with potassic alteration, forming quartz-sulfide veins with potassium feldspar halos, and reached a peak in stage 3, forming chalcopyrite-pyrite ± quartz ± anhydrite ± chlorite ± illite veins that accompanied phyllic alteration. Sulfur, hydrogen-oxygen and strontium isotopes, fluid inclusions, and pyrite trace element data indicate that the stage 1 assemblage associated with potassic alteration were deposited from a magmatic brine. With the change from lithostatic to hydrostatic pressure late in stage 1, the brine generated stage 1 g quartz-pyrite-chalcopyrite ± anhydrite ± bornite ± molybdenite ± potassium feldspar veins with minor copper and gold. Mixing with minor external fluids (meteoric water or gypsum-saturated groundwater) in stage 2 triggered the peripheral propylitic alteration. As the proportion of external fluid increased with the onset of hydrostatic conditions, it changed the redox state and composition of the hydrothermal system, resulting in extensive stage 3 Cu and Au precipitation with phyllic alteration. Argillic alteration associated with stage 4 veins was generated by external fluids mixed with minor hydrothermal fluid, at shallow depths.

Mesogenetic diagenesis of the Ordovician limestone in Yubei area, Tarim Basin, NW China

The Lower Ordovician succession, consisting of the Yingshan and Penglaiba formations, encompasses significant petroleum reservoirs in Yubei area, southwest Tarim Basin. Mesogenetic dissolution and dolomitization strongly contributed to the increase in rock porosities here. Wells YB 7, YB 5, and YB 1-2X drilled in eastern Yubei area met the deposits of Yingshan and Penglaiba formations, providing abundant data for the mesogenetic diagenesis analysis. Among them, Well YB 5 encountered the thickest Penglaiba Formation and was chosen for detailed analysis. Eight diagenetic elements (i.e., micrite encrust, radial fibrous calcite cement, stylolites, poikilotopic calcite cement, dolomite, fracture, bladed quartz, and equant quartz) are identified and ascribed to seven paragenetic events. Their Paragenetic sequence evidences Yingshan and Penglaiba Formations experienced severe deep burial diagenesis. The presence of green luminescent fluorite, non-luminescent and zoned quartz, and violet to orange luminescent saddle dolomite with sweeping extinctions, together with the positive Eu2+ anomalies shown in the rare earth element (REE) patterns all help to ascribe the buried dissolution and dolomitization to the incorporation of hydrothermal fluids. The inconsistency between the REE patterns of the Yingshan and Penglaiba formations and that of the organic-rich source rocks of Lianglitage and Bachu Formations exclude the assumption that hydrocarbon influx caused the mesogenetic dissolution. High-angle fractures, generated by the Middle Caledonian–Early Hercynian tectonic movements, acted as conduits for hydrothermal fluid migration and promoted the mesogenetic dissolution to form dolomite reservoirs. The coincidence that samples with positive Eu2+ anomalies all fall in the basalt province, while samples lacking positive Eu2+ anomalies from Tahe fall out, backs up that the main source of the hydrothermal fluid promoting the mesogenetic dissolution was probably derived from the Permian magma. Strata possessing similar geochemical and petrological characteristics as Yingshan and Penglaiba formations in eastern Yubei area, and experiencing subsequent tectonic and magmatic activities could be potential exploration targets in further exploration practice in Tarim Basin and in the other basins in the world.

Crystal Chemistry of an Erythrite-Köttigite Solid Solution (Co3–xZnx) (AsO4)2·8H2O

A wide compositional range, covering about 90% of an expected erythrite-köttigite substitutional solid solution with extreme compositions of (Co2.84Mg0.14Zn0.02) (AsO4)2·8H2O and (Zn2.74Co0.27) (AsO4)2·8H2O, was revealed in a suite of samples from a polymetallic ore deposit in Miedzianka, SW Poland. Members of the solid solution series were examined by means of Electron Probe Microanalysis (EPMA), Scanning Electron Microscopy (SEM)/Energy-Dispersive Spectrometer (EDS), X-ray single-crystal and powder diffraction, and Raman spectroscopy. Metal cations were randomly distributed between two special octahedral sites in the erythrite–köttigite structure. In response to Co ↔ Zn substitutions, small but significant changes in bond distances (particularly in [AsO4] tetrahedra), rotation, and distortion of co-ordination polyhedra were observed. Two sub-series of dominant cationic substitutions (Co-Mg-Ni and Co-Fe-Zn) were noted within the arsenate series of vivianite-group minerals linked by erythrite. The paragenetic sequence erythrite → Zn-rich erythrite → Co-rich köttigite → köttigite reflects the evolution of the solution’s pH towards increased acidity and a relative increase in the concentration of Zn ions following precipitation of erythrite.

The formation of magnetite ores of the Glubochenskoe deposit, Turgai iron belt, Russia: new structural, mineralogical, geochemical, and isotopic constraints

The magnetite ore bodies of the Paleozoic Glubochenskoe iron deposit (315.7 Mt at ~ 30.15% Fe) are located in the northern part of the Valerianovka arc zone (“Turgai belt”) within the Transuralian Megazone, Russia. They occur in calcareous-volcaniclastic rocks, exhibit layered textures, and contain primary seafloor hematite ores. The sequence of mineral formation reflects the diagenetic to metamorphic evolution of the iron ores: (i) finely dispersed hematite-1; (ii) tabular hematite-2 crystals; (iii) pseudomorphic magnetite-1 after hematite-2; (iv) zoned magnetite-2 crystals with relict hematite-2 (or magnetite-1); (v) thin oscillatory zoned magnetite-3 crystals; and (vi) magnetite-4 porphyroblasts. A gangue assemblage of Fe-rich and Fe–Mg chlorite, illite, quartz, albite, carbonates, rutile, and apatite with rare monazite, xenotime, and zircon occurs in ore and calcareous-volcaniclastic layers. The gangue clasts (volcanic glass, Ca–Mg and Ti minerals, and altered volcanic rocks) are replaced by hematite and further by magnetite. Low siderite δ13C values from layered magnetite ores (− 8.0 to − 19.5‰ PDB) indicate the presence of primary organic matter in calcareous-volcaniclastic rocks. Siderite δ18O values (6.5 to 17.4‰ SMOW) are evidence of isotopic exchange between minerals and fluids during metamorphism. Negative δ34S values for pyrite (down to − 4.5‰) likely indicate derivation of sulfur from organic matter in clastic sedimentary rocks. LA-ICP-MS analysis of zoned magnetite demonstrates highly variable Si, Al, Mg, Na, K, Ca, Ti, Mn, Rb, Y, Zr, Sr, U, and P contents, related to inclusions of gangue minerals. Elevated homogenous V and Ga contents and low contents of Sc, Co, Ni, Ge, As, Mo, Sn, and W (average < 5 ppm) indicate their incorporation into the structure of magnetite. All element contents (except V and Ga) are significantly higher in the inner zone of magnetite (magnetite-1) compared to the outer zone of magnetite (magnetite-2). The calcareous-volcaniclastic material, which was altered during submarine weathering and leaching, is proposed to be a major source of iron to form oxides. Altogether, the paragenetic sequence, and the mineralogical and geochemical data suggest the Glubochenskoe deposit can be characterized as a volcanic-sedimentary type of banded iron formation.

Textures and geochemistry of magnetite: Indications for genesis of the Late Paleozoic Laoshankou Fe-Cu-Au deposit, NW China

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and electron probe X-ray microanalysis (EPMA) have been employed to determine major and trace elemental concentrations in magnetite from different ores in the Laoshankou deposit, which is the most important Fe-Cu-Au deposit in the northern margin of East Junggar. Two magnetite-associated paragenetic sequences have been defined in Laoshankou, including amphibole-epidote-magnetite alteration/mineralization (stage II) and pyrite-chalcopyrite mineralization (stage III-B). On the basis of the magnetite textures and chemical compositions, we have identified six generations of magnetite from three types of ores/rocks, including the high-Ti inclusion-rich M-1-A and inclusion-free M-1-B in massive magnetite-epidote-amphibole ores (type H1) from stage II; the low-Ti, high-Si, inclusion-rich M-1-C in massive magnetite-sulfide ores (type H2) from stage II; the ubiquitous low-Ti M-2 and pristine low-Ti, low-Si M-3 in both H1 and H2 types; and M-4 within massive chalcopyrite-magnetite ores (type H3) from stage III-B. The discriminations of V versus Ti, Ca + Mn + Al versus Ti + V, and Fe versus V/Ti, as well as mineral assemblages and trace element concentrations of Cr (<1000 ppm) and V (<300 ppm), collectively imply that all the generations of magnetite are hydrothermal in origin, and more likely similar to those in iron-oxide copper-gold deposits. The diverse textural and chemical features among these generations of magnetite indicate that they have formed from distinct process. The earliest M-1-A and M-1-B in type H1 ore and M-1-C in type H2 ore were replaced by M-2 along fractures and/or grain boundaries of the M-1-A/B/C, forming a number of mineral inclusions via dissolution and reprecipitation. Following this, M-3 veins generally crosscut all of these above types. Finally, high-Co, Ni, and REE-rich fluid formed the chalcopyrite-associated M-4 with dark-light zoning in the type H3 ore from pyrite-chalcopyrite mineralization. These elevated lithophile (REE) and chalcophile (Co, Ni) elements within the chalcopyrite (sulfide)-associated M-4 further suggest that non-magmatic fluid dominated in the fluid system, with intensive fluid-rock interactions and low-temperature, Cl, and Cu-rich external fluid accounting for the Cu-Au mineralization.

Diagenetic evolution of the Eocene ramp carbonates (a paradigm from the Nile Valley, Egypt): Petrographical and geochemical attributes

Carbonate rocks of east Nile Valley in Egypt have been investigated as a paradigm for revealing the diagenetic evolution of the Eocene ramp carbonates. The integration of petrographical, elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes (i.e. eogenesis, mesogenesis and telogenesis) and are related to four diagenetic environments (i.e. marine-phreatic, meteoric-phreatic, burial and uplifting environments). The paragenetic sequence of the studied carbonates commences with the eodiagenetic episode, which includes the marine- and meteoric-phreatic diagenetic environments. The marine-phreatic diagenetic environment comprises micritization, glauconitization, pyritization, marine cementation (fibrous calcite and syntaxial overgrowth rim cements), early dolomitization (formation of fine-crystalline matrix-replacive dolomites) and early stages of mechanical compaction before significant burial. The meteoric-phreatic diagenetic environment includes the neomorphism, dissolution, meteoric cementation (earlier phases of sparry calcite cements) and precipitation of megaquartz in the void spaces of the peritidal carbonates. During the mesogenesis episode, the studied carbonates were exposed to burial diagenesis. The shallow burial diagenesis resulted in the creation of point-tangenetial contacts between the allochemical constituents and the precipitation of a medium-coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores. The deep burial diagenetic processes embody the chemical compaction (condensed-fitted interpeneteration and pressure-dissolution fabrics), fracturing and burial cementation. The telodiagenesis represents the last episode in the diagenetic history of the studied carbonates. It witnessed the fracturing, meteoric-vadose cementation and hematitization processes.

Mineralogy and geochemistry of carbonate cement in sandstone and implications for mineralization of the Qianjiadian sandstone-hosted uranium deposit, southern Songliao Basin, China

Morphology, concentration, paragenetic sequence, fluid inclusion, and trace element concentrations of carbonate cements in sandstone from the Qianjiadian sandstone-hosted uranium deposit, southern Songliao Basin, are illustrated in order to reveal the properties of ore-forming fluids and the effects of hydrothermal fluids on uranium mineralization. The carbonate cements contain dolomite, siderite, ankerite and calcite. Dolomite with a crystalline granular texture has the greatest average concentration, forming up to 3.3% of the yellow sandstone, and this dolomite has orange red cathodoluminescence. Siderite forms up to 2.7% of the primary grey sandstone, and everywhere forms grains with an idiomorphic shape. Ankerite forms up to 6.6% of the mineralized grey sandstone and commonly have a mosaic granular structure. Calcite characterized by mosaic granular texture reaches the maximum level of average concentration of 10.2% in the non-mineralized grey sandstone, and its cathodoluminescence is orange. Studies of mineral paragenetic sequence indicate that the dolomite, siderite, ankerite and calcite are chronologically generated. Average homogenization temperatures of the fluid inclusions in the ankerite and calcite are 160 °C and 136 °C, respectively, and their average salinities are 9.9 wt% and 7.7 wt%, respectively. The uranium content of the ankerite is up to 11.8 ppm, which is greater than that of the other carbonate cement. This study indicates that the ankerite is associated with uranium mineralization, and the Qianjiadian sandstone-hosted uranium deposit could be mobilized by low-temperature, high-salinity hydrothermal fluids, which can lead to formation of brannerite and coffinite.

Overpressure release: Fluid inclusion evidence for a new mechanism for the formation of heavy oil

AbstractHeavy oils in sedimentary basins are commonly related to biodegradation and water washing, and thermal degradation of sulfur-rich kerogen at an early hydrocarbon generation stage. However, the potential for overpressure release to form heavy oil has been seldom considered and rarely demonstrated. Paragenetic sequences of diagenetic and oil charge events, pressure-temperature-composition (P-T-x) evolutionary history reconstruction, and molecular geochemical data from a single generation of oil inclusions reveal that heavy shale oil in the PS18–1 well in the Dongpu Depression, Bohai Bay Basin, China, was neither a product of biodegradation nor due to early oil generation during kerogen maturation. Instead, the precipitation and retention of polar compounds of a previously charged, higher-maturity oil from deeper source rocks, induced by intense pressure reduction during basin uplift, represent the most likely mechanism for the formation of the heavy oil. The precipitation of polar compounds during primary and secondary migration due to intense pressure release may be an important mechanism for explaining compositional fractionation effects in the expelled petroleum fluids in source rocks, bitumen, and heavy oil distributions in unconventional shale systems, and deep non-biodegraded heavy oils. This mechanism has wider implications for understanding the hydrocarbon distribution in overpressured basins.

Order of diagenetic events controls evolution of porosity and permeability in carbonates

AbstractThe properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three‐dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non‐fabric selective dissolution, and then a second‐generation of (post‐dissolution) cement. Models are 3 mm3 with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny‐Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny‐Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny‐Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios.

Fracture, Dissolution, and Cementation Events in Ordovician Carbonate Reservoirs, Tarim Basin, NW China

Ordovician carbonate rocks of the Yijianfang Formation in the Tabei Uplift, Tarim Basin, contain deeply buried (&gt;6000 m), highly productive oil and gas reservoirs associated with large cavities (&gt;10 m). Previous workers inferred that large cavities are paleocaves (paleokarst) formed near the surface and subsequently buried. Alternately, caves may have formed by dissolution at depth along faults. Using 227 samples from 16 cores, we document textures and cement compositions bearing on cavity histories with petrographic, high-resolution scanning electron microscopy (SEM), isotopic, and fluid inclusion microthermometric observations. Results show that dissolution occurred at depth and was caused by (1) acidic fluids derived from Middle-Late Silurian and/or Devonian-Permian hydrocarbon generation and maturation, (2) high-temperature fluids, of which some were associated with Late Permian igneous activity, and (3) Mg-rich fluids that accompanied Jurassic-Cretaceous deformation and the formation of partially open fractures and stylobreccias (fault breccias). The relative paragenetic sequence of the structure-related diagenesis suggests seven stages of fracturing, dissolution, and cementation. Mottle fabrics in the Yijianfang Formation contain argillaceous carbonate-rich silt and are bioturbation features formed within the marine environment. Those mottled fabrics differ from clearly karstic features in the overlying Lianglitage Formation, which formed by near-surface dissolution and subsequent infilling of cavities by allochthonous sediment. Mottle fabrics are crosscut by compacted fractures filled with phreatic-vadose marine cements and followed by subsequent generations of cement-filled fractures and vugs indicating that some fractures and vugs became cement filled prior to later dissolution events. Calcite cements in fractures and vugs show progressively depleted values of δ18O documenting cement precipitation within the shallow (~220 m), intermediate (~625 m), and deep (~2000 m) diagenetic environments. Deep (mesogenetic) dissolution associated with fractures is therefore the principal source of the high porosity-permeability in the reservoir, consistent with other pieces of evidence for cavities localized near faults.

Cobalt–nickel–copper arsenide, sulfarsenide and sulfide mineralization in the Bou Azzer window, Anti-Atlas, Morocco: one century of multi-disciplinary and geological investigations, mineral exploration and mining

Abstract The Bou Azzer mining district in the central part of the Anti-Atlas belt, Morocco, is renowned worldwide for its serpentinized Neoproterozoic ophiolitic complex that hosts peculiar cobalt-dominated and nickel-rich Co–Ni–Fe arsenide and sulfarsenide deposits with significant credits of gold and copper. The Bou Azzer central mine and its satellite deposits, which are spatially associated with serpentinite bodies and tectonically controlled, generally form breccias at the contact between serpentinites and more competent rock types (e.g. diorite) but may also comprise anastomosing vein and tension gashes networks rooted in the serpentinites and breaching into the competent rocks. A consensus on a genetic model for Bou Azzer has not been reached given the most controversial and still highly debated topics dealing with the absolute age of mineralization, and the identification of the source(s) of metals and arsenic. Here, within the geological framework of the Bou Azzer window and the geodynamic history in the Anti-Atlas, we summarize the key features of ore mineralogy, textures, paragenetic sequence and district-scale metal zonation. Using knowledge based on the current state of research – until high-precision and robust geochronological data are produced and permit a refinement of the genetic model – the key parameters for the origin of the Bou Azzer ores are: (1) arsenides of Ni and Co, which are followed by Co–Ni-sulfarsenides and Cu-sulfides, initiating a sequence of mineralization with precipitation controlled by the parameters mentioned in (2)–(4); (2) hydrothermal high salinity fluids (&gt;30 wt% total salt eq.) fostering leaching of Co and Ni from debated source rocks and their transport as highly soluble chloride complexes; (3) dilution and cooling of those fluids from &lt;320 to &lt;200°C; (4) variable redox conditions that would control the progressive oxidation of As from the arsenide stage (As 1− ) to the sulfarsenide stage (As 3+ ); (5) unlike Neoproterozoic metalliferous and As-rich black shales, serpentinites fail to convincingly stand as a robust source of As despite being a reasonable source rock for Ni and Co; (6) the controversial U–Pb and Sm–Nd dates of brannerite and carbonate, respectively, although arguing in favour of an Hercynian timing of mineralization, do not preclude the possibility of a primary Pan-African mineralization (&lt;615 Ma) and/or at c. 540 Ma, followed by remobilization of a pre-existing mineralization during Hercynian events.

Genesis of the Yujiadian F-Pb-Zn-Ag deposit, Inner Mongolia, NE China: Constraints from geochemistry, fluid inclusion, zircon geochronology and stable isotopes

The Yujiadian F-Pb-Zn-Ag deposit in the South Great Xing’an Range (SGXR) consists of vein-type fluorite and F-Pb-Zn-Ag mineralization associated with granitoid. The orebodies occur in NS-trending extensional faults. Wallrock alterations are characterized by a series of medium-low temperature hydrothermal alteration, including silification, kaolinization, chloritization, sericitization, pyritic and carbonate alteration. Four stages of primary paragenetic sequences are recognized: polymetallic sulfide stage (stage I), sulfide fluorite sub-stage (stage II-1), silver-fluorite sub-stage (stage II-2), pure fluorite stage (stage III) and late carbonate stage (stage IV). Silver mineralization primarily belongs to the stage II. The 139–135 Ma quartz syenite porphyry is metaluminous and peralkalic, and is classified as A-type granite that formed in a post-orogenic extensional setting. The δ34S (1.8‰) of hydrothermal fluid calculated from coeval minerals (barite and galena) is comparable with the majority of adjacent magmatic hydrothermal deposits in the SGXR, implying a magmatic source. Microthermometric measurements show that ore minerals were deposited at intermediate temperatures (109.7–320.1 °C) and had low salinities (0.18–5.62 wt% NaCl equive). Ore-forming fluid originated from post-magmatic hydrothermal fluid and mixed with heated meteoric water. Successive precipitation of base metals occurred with changing physicochemical conditions and boiling of ore fluid. The main precipitation mechanism for fluorite in the late stage is related to fluid-rock reaction. We propose that the A-type granite related to the fracture-controlled hydrothermal vein-type Yujiadian F-Pb-Zn-Ag deposit forms part of the magmatic response to Paleo-Pacific subduction and associated lithospheric thinning, extension and delamination processes. Furthermore, the deposit represents the terminal and distal member of the regional Mesozoic large-scale mineralization events.

Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

AbstractThe upper Palaeocene–lower Eocene Umm er Radhuma Formation in the subsurface of Qatar is dominated by subtidal carbonate depositional packages overlain by bedded evaporites. In Saudi Arabia and Kuwait, peritidal carbonate depositional sequences with intercalated evaporites and carbonates in Umm er Radhuma have been previously interpreted to have been dolomitized via downward reflux of hypersaline brines. Here, textural, mineralogical and geochemical data from three research cores in Qatar are presented which, in contrast, are more consistent with dolomitization by near‐normal marine fluids. Petrographic relationships support a paragenetic sequence whereby dolomitization occurred prior to the formation of all other diagenetic mineral phases, including chert, pyrite, palygorskite, gypsum, calcite and chalcedony, which suggests that dolomitization occurred very early. The dolomites occur as finely crystalline mimetic dolomites, relatively coarse planar‐e dolomites, and coarser nonplanar dolomites, all of which are near‐stoichiometric (50.3 mol% MgCO3) and well‐ordered (0.73). The dolomite stable isotope values (range −2.5‰ to +1‰; mean δ18O = −0.52‰) and trace element concentrations (Sr = 40 to 150 ppm and Na = 100 to 600 ppm) are compatible with dolomitization by near‐normal seawater or mesohaline fluids. Comparisons between δ18O values from Umm er Radhuma dolomite and the overlying Rus Formation gypsum further suggest that dolomitization did not occur in fluids related to Rus evaporites. This study provides an example of early dolomitization of evaporite‐related carbonates by near‐normal seawater rather than by refluxing hypersaline brines from overlying bedded evaporites. Further, it adds to recent work suggesting that dolomitization by near‐normal marine fluids in evaporite‐associated settings may be more widespread than previously recognized.

The Dairi SEDEX Zn + Pb + Ag deposit (North Sumatra, Indonesia): Insights from mineralogy and sulfur isotope systematics

The Dairi Zn + Pb + Ag deposit is the only giant SEDEX deposit that has been discovered to date in Sumatra. Ore-gangue mineralogy and sulfur isotope studies have been carried out to elucidate the paragenetic sequence, deformation of ore, flow direction of ore forming fluid, and sources of sulfur in the Dairi deposit. Stratiform orebodies hosted by the Julu unit (interbedded dolomitic siltstone and carbonaceous shale; multilayers; accounting for greater than 90% of metal resources) consist of pyrite-rich massive, sphalerite-rich massive, bedded sulfide, galena-rich breccias, and vein ore types, whereas discordant orebodies hosted by the Jehe unit (massive and brecciated arenaceous dolostone) comprise vein and disseminated ore types. Fine-grained, subhedral-euhedral and framboidal pyrite (enriched in Co, Ni, Cu, As, Bi, Ag relative to hydrothermal pyrite) were precipitated in the diagenetic stage, followed by precipitation of pyrite (enriched in Zn, Pb, Mn, Tl relative to diagenetic pyrite), sphalerite, galena, chalcopyrite, pyrrhotite, acanthite, tetrahedrite, tennantite, pyrargyrite, boulangerite, freieslebenite, and diaphorite in the hydrothermal stage derived from the exhaled ore-forming fluid. Metamorphic pyrite (enriched in Co and depleted in Cu, Zn, Mn, Tl, Ag relative to the other pyrites) formed from pyrrhotite (depleted in Co relative to metamorphic pyrite) and soft sulfides were remobilized into pyrite fractures and veins during sub-greenschist metamorphism. Ore-forming fluid ascended through fractures in the Jehe unit (forming the discontinuous orebodies) and then it discharged and flowed from SE to NW on the seafloor (forming the stratiform orebodies). Reduced sulfur for diagenetic pyrite (δ34S = -4.1‰ to + 11.6‰) was supplied by bacteriogenic reduction of Carboniferous seawater sulfate. Hydrothermal sulfur was considered as the source of sulfur for the Jehe-hosted orebodies (δ34S = +3.5‰ to + 8.0‰). The heavier δ34S of sulfides in the Julu-hosted orebodies (δ34S = +6.4‰ to + 28.8‰) suggests large-scale bacteriogenic reduction of Carboniferous seawater sulfate in a restricted, anoxic basin. A bacteriogenic reduced sulfur reservoir, rather than hydrothermal sulfur, may be more plausible to account for the scale of the giant Dairi deposit.

In-situ trace elements on pyrite and arsenopyrite of the Zhengchong gold deposit, Jiangnan Orogen: Insights for the mineralization mechanism

The Zhengchong gold deposit, with a gold reserve of 19 t, is located in the Liling Goldfield, central Jiangnan Orogen, Southern China. The orebodies are structurally bound by NW- and NNE-NE-trending faults, hosted in the Neoproterozoic slates. The ore styles primarily comprise auriferous pyrite-arsenopyrite-quartz vein, disseminated pyrite-arsenopyrite-sericite-quartz mineralized slate, and intense mineralized slate breccia. The gold-bearing sulfide-quartz veins have average Au grades up to 13.5 g/t, generally higher than that of mineralized rock in all orebodies of 0.6–2.0 g/t and intense mineralized breccia within quartz veins of 2.5–4.0 g/t. Three primary and successive paragenetic sequences are identified: (i) Barren quartz – muscovite; (ii) Quartz – polymetallic sulfides – native gold – muscovite – minor chlorite; (iii) Barren quartz – calcite veins. Native gold has only been observed in the fractures of early barren quartz, pyrite and arsenopyrite in NW-trending auriferous veins. According to textural differences, five types of pyrite and four types of arsenopyrite are recognized from NW- and NNE-NE-trending orebodies.Based on LA-ICPMS spot and imaging analyses of different types of pyrites and arsenopyrites, the refractory, invisible gold in pyrites and arsenopyrites range from < 1 to >500 ppm and was incorporated in these sulfides through substitution of As1− for S2− as the Au and As data plotting below the both gold solubility lines for Carlin and orogenic gold deposits. The invisible gold as the form of Au+ concentrates in sulfides. Besides, minor micro-scale gold with Cu-, Pb-, Sb-, Ag-, and Bi-minerals occur in the pores of pyrite and arsenopyrite. The initial ore-bearing fluid with abundant Si, S, Fe, As and minor Au, Co, Ni, Cu, Zn, Sb, Te, Pb, Bi reacting with Neoproterozoic slates to generate low-grade pyrite (Py-I and Py-III) in both-trending orebodies. For NNE-trending sulfide-quartz veins, cyclic fluid pressure dropping resulted fluid phase separation and destabilization of bisulphide-gold. Due to the small fluid flux in narrow NNE-trending faults, Au+ exhausted after the precipitation of high-grade Py-II, Apy-II and Au-rich rim of Py-I with invisible gold. In NW-trending auriferous quartz veins, hydrofracturing under high fluid pressure in thrust shear-faults, generated massive slate breccias. The sharply pressure decrease in dilations along the ore-controlling faults resulted in significant auriferous fluid phase separation and the solubility reducing of trace elements and gold, which in turn led in the deposition of native gold with chalcopyrite, sphalerite and tetrahedrite in fractures of Py-IV and Apy-III.

Zircon U-Pb dating and stable isotopic compositions for constraining the genesis of the Chagangnuoer magnetite deposit in western Tianshan, NW China

The Chagangnuoer magnetite deposit in western Tianshan (NW China) is hosted by submarine volcanic rocks and marble lens, with a resource of >220 Mt Fe at 38 wt%. Two major orebodies FeI and FeII are current in production. Pervasive garnet skarn, magnetite, actinolite–epidote, and marble zones are developed at Chagangnuoer. Paragenetic sequences are divided into garnet–pyroxene (I), magnetite–pyrite ± chalcopyrite (II), actinolite–epidote–K-feldspar (III), garnet ± tremolite ± scapolite (IV), magnetite ± hematite (V), actinolite–epidote (VI), and sulfide–calcite–quartz (VII) stages. A diorite beneath the FeII and a granodiorite with skarn alteration have zircon U-Pb ages of 329 ± 2 and 317 ± 4 Ma, respectively. These intrusions have high potassic to shoshonitic, metaluminous affinities, and significant Nb, Ta, Ti and Sr anomalies. Stage II magnetite has δ18OV-SMOW compositions of 1.9–3.5‰, higher than Stage V magnetite (0.5–2.1‰), indicating a hydrothermal fluid source of magmatic origin. The δ18O values of garnet, actinolite, epidote and quartz range from 4.1 to 12.2‰. Stage VII calcite has δ13CV-PDB of −2.0 to −0.8‰ and δ18O of 8.9 to 9.9‰, lower than for the marble (average δ13C + 2.3‰, δ18O 12.8‰). δ34SV-CDT compositions for pyrite and chalcopyrite from Stages II and VII vary between 0.8 and 13.1‰. The isotopic compositions imply that minor non-magmatic sources (e.g., marine carbonate, seawater, and meteoric water) have been circulated into hydrothermal fluids producing the skarn alterations and associated iron mineralization. The early-stage skarn alteration and main Fe mineralization relate to the emplacement of diorite stock at depth, whereas the late-stage skarn-magnetite assemblages are intimately associated with the granodiorite. The Chagangnuoer deposit is concluded to be magnetite skarns formed by magmatic-hydrothermal fluids that emanated from the dioritic and granodioritic magmas intruding above a sub-arc mantle wedge and that metasomatized intensely with the Carboniferous andesitic-rhyolitic volcanic rocks and carbonate.

Paragenetic and petrographic study of uranium mineralization along the Midwest Trend, Northeastern Athabasca Basin

The Athabasca Basin in northern Saskatchewan is host to many world-class uranium deposits associated with the unconformity between the Paleoproterozoic sandstone of the basin and the underlying crystalline basement (Jefferson et al., 2007). While the style and tonnage of these deposits vary, the current genetic model for unconformity-related uranium deposits has been a practical tool for exploration in the Athabasca Basin. However, the factors which control the location and formation of these deposits is still not fully understood. A paragenetic and petrographic study of mineralization along the Midwest Trend, located on the northeastern margin of the Athabasca Basin, aims to refine the current model and to address the general problem: What are the factors which control mineralization and non-mineralization? The Midwest Trend will be used as a "modèle réduit" for uranium mineralization, as it displays many features characteristic of unconformity type deposits. The Midwest Trend comprises three mineral leases that encompass two uranium deposits, the Midwest Main and Midwest A (Allen et al., 2017a, b). Mineralization occurs along a NE-trending graphitic structure, and is hosted by the sandstone, at the unconformity, and in much lesser amounts in the underlying basement rocks. Petrographic observations aided by the use of RAMAN spectroscopy and SEM-EDS, have been used to create a paragenetic sequence of mineralization (Fig.1). Future work will focus on fluid inclusion studies using microthermometry, LA-ICP-MS, and mass spectrometry of contained gases.&#x0D;