Hydrothermal Fluids Research Articles

Mineralogy and Geochemistry of Jasperoid Veins in Neoproterozoic Metavolcanics: Evidence of Silicification, Pyritization and Hematization

The Wadi Ranga sulfidic jasperoids in the Southern Eastern Desert (SED) of Egypt are hosted within the Neoproterozoic Shadli metavolcanics as an important juvenile crustal section of the Arabian Nubian Shield (ANS). This study deals with remote sensing and geochemical data to understand the mechanism and source of pyritization, silicification, and hematization in the host metavolcanics and to shed light on the genesis of their jasperoids. The host rocks are mainly dacitic to rhyolitic metatuffs, which are proximal to volcanic vents. They show peraluminous calc-alkaline affinity. These felsic metatuffs also exhibit a nearly flat REE pattern with slight LREE enrichment (La/Yb = 1.19–1.25) that has a nearly negative Eu anomaly (Eu/Eu* = 0.708–0.776), while their spider patterns display enrichment in Ba, K, and Pb and depletion in Nb, Ta, P, and Ti, reflecting the role of slab-derived fluid metasomatism during their formation in the island arc setting. The ratios of La/Yb (1.19–1.25) and La/Gd (1.0–1.17) of the studied felsic metatuffs are similar to those of the primitive mantle, suggesting their generation from fractionated melts that were derived from a depleted mantle source. Their Nb and Ti negative anomalies, along with the positive anomalies at Pb, K, Rb, and Ba, are attributed to the influence of fluids/melt derived from the subducted slab. The Wadi Ranga jasperoids are mainly composed of SiO2 (89.73–90.35 wt.%) and show wide ranges of Fe2O3t (2.73–6.63 wt.%) attributed to the significant amount of pyrite (up to 10 vol.%), hematite, goethite, and magnetite. They are also rich in some base metals (Cu + Pb + Zn = 58.32–240.68 ppm), leading to sulfidic jasperoids. Pyrite crystals with a minor concentration of Ag (up to 0.32 wt.%) are partially to completely converted to secondary hematite and goethite, giving the red ochre and forming hematization. Euhedral cubic pyrite is of magmatic origin and was formed in the early stages and accumulated in jasperoid by epigenetic Si-rich magmatic-derived hydrothermal fluids; pyritization is considered a magmatic–hydrothermal stage, followed by silicification and then hematization as post-magmatic stages. The euhedral apatite crystals in jasperoid are used to estimate the saturation temperature of their crystallization from the melt at about 850 °C. The chondrite (C1)-normalized REE pattern of the jasperoids shows slightly U–shaped patterns with a slightly negative Eu anomaly (Eu/Eu* = 0.43–0.98) due to slab-derived fluid metasomatism during their origin; these jasperoids are also rich in LILEs (e.g., K, Pb, and Sr) and depleted in HFSEs (e.g., Nb and Ta), reflecting their hydrothermal origin in the island arc tectonic setting. The source of silica in the studied jasperoids is likely derived from the felsic dyke and a nearby volcanic vent, where the resultant Si-rich fluids may circulate along the NW–SE, NE–SW, and E–W major faults and shear zones in the surrounding metavolcanics to leach Fe, S, and Si to form hydrothermal jasperoid lenses and veins.

Impacts of temperature and fluid seepage on organic matter composition in sediments of an active hydrothermal basin

Marine sediments are one of the largest organic carbon (OC) sinks on Earth. Yet, major knowledge gaps remain in our understanding of sedimentary OC cycling, particularly regarding temperature-induced alteration processes of OC from different sources. Here, we investigate OC-rich sediments of Guaymas Basin (Gulf of California) across two hydrothermal areas, one with only conductive geothermal heating and the other additionally experiencing seepage of hydrocarbon-rich fluids from deeper layers. We use Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) to investigate diagenetic OC changes and show that cold control sites in both hydrothermal areas are dominated by similar contributions of lipid-derived compounds, nitrogenous (likely protein-derived) compounds, carbohydrates, and polyaromatic hydrocarbons (PAHs). These OC compound groups are largely derived from phytoplankton detritus from overlying water. Conductively heated sediments, which reach in situ temperatures of ∼ 80 °C, have similar general OC compositions and contents to these cold sites, but show evidence of diagenetic modifications of individual carbohydrate groups in deeper layers. By contrast, strong decreases in carbohydrate and nitrogenous compound abundances at the seep sites indicate that these compound groups are not only modified but also selectively degraded at the higher temperatures (>80 °C) of these sites. Increases in pyrolysis products of PAHs, prist-1-ene, and alkanes with depth, moreover, show that import of OC by deep hydrothermal fluids contributes significantly to sedimentary OC pools mainly in deeper layers of these sites. Our study provides the first comprehensive analysis of major OC compound groups in Guaymas Basin sediment and indicates that the supply of OC by hydrothermal fluid flow only has minor impacts on particulate organic matter compositions at the seafloor, even at active seep sites. We furthermore show that temperatures up to ∼ 80 °C already result in thermochemical modifications of organic matter (OM) that are potentially linked to the onset of kerogen formation. The sequence and time scales of chemical modifications and activations in relation to temperature are an important subject for future investigations.

Albitization related U and Th mobilization under reducing conditions

Hydrothermally altered quartzofeldspathic gneisses, granites and pegmatites of the West Garo Hills, Shillong Plateau (eastern India) preserve alteration assemblages of monazite, xenotime and zircon comprising thorite and coffinite in the altered domains. Albitization of muscovite and K-feldspars, sericitization of K-feldspar and albite, alteration of biotite to hydrobiotite, and crystallization of tourmaline is suggestive of the involvement of acidic fluids. The microtextural relations and the mineral assemblages indicate prevalence of low-temperature (T < 300 °C) conditions during hydrothermal alteration. The light δ11B values of tourmaline (–15.1 to –13.5 ‰) suggests derivation/interaction of the fluids from/with metapelitic rock and/or S-type granite. Low ‘inferred’ Fe3+/Fe2+ ratio in tourmaline (based on high Al content in Y-site, 0.16–0.56 apfu, avg. 0.36 apfu), estimated excess charge (0.22–0.47, avg. 0.35 apfu) and lack of any Fe–Al relation support a reduced nature of the hydrothermal fluid. Mass balance calculations reveal that Th and U required for the formation of thorite and coffinite, respectively were likely derived from monazite and xenotime, and zircon. Significantly lower Cl─ contents in hydrobiotite (avg. 0.03 wt%) compared to the unaltered biotite (avg. 0.13 wt%) suggests release of Cl─ to the hydrothermal fluid during alteration. Textural evidences of albitization together with Cl─ release from biotite suggest increased Cl─ concentrations in the fluid during hydrothermal alteration. Based on solubility calculations for various U and Th species, we propose that high Cl─ content in the fluid aided mobilization of Th and U as ThCl40 and UCl40/UO2Cl20 complexes from accessory radioactive phases under reducing conditions. These results suggest that significant mobility of U and Th can be achieved in acidic high salinity fluid even under reducing conditions. Thermodynamic calculations for the solubility of Th- and U-chloride complexes and stability of monazite/xenotime in a range of pH and temperature suggest that thorite and coffinite precipitation was the result of an increase in pH.

Mineralogy and geochemistry of altered Emeishan basaltic volcaniclastics with respect to their critical element mineralization

Research on the mineralogical and geochemical characteristics of altered end-Guadalupian basaltic volcanoclastic rocks can increase the understanding of the alteration history of the overlying stratabound Nb(Ta)-Zr(Hf)-REY-Ga mineralization in the Late Permian coal-bearing sequences. In this paper, based on petrographic observations in addition to XRD, XRF, and ICP-MS analyses, we present and discuss in detail the relationships between primary and secondary minerals and outline the distribution of major and trace element chemistry in the middle/Late Permian basaltic volcaniclastics from the ELIP's zone, western Guizhou, southwest China. The primary clastic suite consists of plagioclase-group minerals, clinopyroxenes, feldspathoids, spinels, and basaltic glasses. Fragments of mafic and, less commonly, felsic and alkaline volcanic rocks are minor components of the studied samples. The alteration products are represented by various chlorite-group minerals (including abundant chamosite), quartz, calcite, albite, analcime, barite, and pyrite, along with relatively minor amounts of titanite, sanidine, magnetite, rutile, and copiapite. The mineralogical and geochemical characteristics of the studied volcaniclastics provide strong evidence correlating them with the high-Ti basalt group, widely distributed within the inner Emeishan large igneous province (ELIP) and coeval rift zone of the middle and outer ELIP. After deposition, the volcaniclastics reacted with complex solutions including heated meteoric waters and were periodically infiltrated by seawater and ascending hydrothermal fluids. As a result, the primary volcanic rocks partly lost alkalis, titanium, silica, and most of the trace elements. These elements, especially the incompatible elements, were probably enriched in the overlying tuffaceous and coal-bearing sequences in the middle and outer ELIP.

Integrated geochemical and microbiological assessments of Astroni lakes reveals Campi Flegrei unrest signatures

Astroni volcano in the Campi Flegrei caldera (southern Italy) is a 2 km wide, densely vegetated tuff ring and hosting several volcanic structures, including tuff cones, scoriae cones, lava domes, and three small lakes. Geochemical data of waters and dissolved gases from the lakes, coupled with microbiological analyses on lake water and sediments, were used to shed light on the possible relationship between the lakes and the hydrothermal fluid circulation system as suggested by previous geophysical surveys. Water chemistry was dominated by solutes, mainly Na+ and HCO3−, deriving from fluids and CO2-rich gases typically found in discharges located at the periphery of hydrothermal-volcanic systems. Lago Grande (LG) lake showed an anoxic hypolimnion with abundant non-atmospheric dissolved gases, consisting of biogenic CH4 and CO2, the latter having a twofold origin, biogenic and hydrothermal. The occurrence of anaerobic methanotrophs coupled with the lack of hydrogenotrophic methanogenic archaea along the whole vertical profile of LG suggested that CH4 was mostly produced from degradation of abundant terrestrial organic matter within the deep lake sediments, and then consumed during its diffusion through the lake. Notwithstanding, the output rate of CH4 from LG surface was anomalously high relative to those commonly measured in lakes. Carbon dioxide from the hydrothermal source and produced by CH4 oxidation was partially fixed in the lake via the acetyl-CoA pathway. Accordingly, the CO2 fluxes from the LG surface were relatively low, in the range of those measured in volcanic lakes dominated by biogenic CO2. The dependence of the chemistry of the Astroni lakes on inputs from the Campi Flegrei hydrothermal system, besides on biogeochemical processes, offers a possible explanation for the anomalous increase of the LG water level occurred in the last years, which was not consistent with the recorded local rainfall but likely caused by an increasing hydraulic pressure related to the enhanced hydrothermal activity recorded at Campi Flegrei in the last decades. According to this hypothesis, the future evolution of the current volcanic unrest may govern the fate of the lake water level with important implications for the functioning of the precious Astroni ecosystem.

Large Fe-Mn oncoids from Early Jurassic Ammonitico Rosso Facies (eastern Sakarya Zone, NE Turkey): new insight into palaeogeographic conditions of Tethys Ocean

ABSTRACT Oncoids, significant sedimentary structures within the Jurassic strata of the Mediterranean Region, serve as valuable indicators of paleoenvironmental conditions during their formation. One of the important examples of oncoids has been discovered in the Pliensbachian (Early Jurassic) Ammonitico Rosso Facies (ARF) of the eastern Sakarya Zone (Eastern Pontides, NE Turkey). This study presents the first comprehensive sedimentological and petrographic (textural and compositional characteristics) examination, as well as geochemical analyses (trace and rare earth elements) of these Fe-Mn oncoids. The oncoids consist of nuclei coated with irregular laminae and exhibit a range of sizes, from 10 to over 45 mm, displaying various colours such as brown, reddish, and metallic-looking. They mostly have a discoidal shape, although some are spherical. The nuclei consist of bioclastic wackestones, containing remnants of ammonoid shells. Some oncoids, especially the discoidal forms, have multiple nuclei. The cortex of the oncoid predominantly consists of wrinkled bands displaying micritic laminae, with an abundance of filamentous bodies and the local presence of encrusting microfossils. In the cortex of the oncoids, iron predominates over manganese, and there are compositional variations within the cortex, especially enriched in Si, Fe, and Ca. They predominantly fall within the region associated with hydrothermal Fe and Mn sediments, as indicated by discrimination diagrams involving Ni, Co, Zn, and plots of Ce/Ce* vs Y/HoN and Nd contents, suggesting a hydrothermal origin. Furthermore, their rare earth elements (REE) chemistry displays distinct Y/Ho, Sm/Yb, Ce/Ce, and Nd values, indicating the presence of seawater mixing with hydrothermal fluids. However, they show relatively low Sm/Yb, Y/Ho, Eu/Sm, Nd/YbN, and La/Yb values, suggesting a limited contribution from hydrothermal sources to the surrounding seawater. Nevertheless, the cortex of the oncoids exhibits a complex and variable mineralogy that changes over short distances, implying the dynamic nature of the depositional environment, characterized by fluctuating hydrological regimes, varying oxygen levels, elemental supply, and the saturation of specific elements in seawater. The increased presence of specific trace elements, including Fe, and REE during this period, is likely linked to hydrothermal fluid input into marine environments, coinciding with intensified syn-sedimentary tectonic activity, where ongoing extensional tectonic movements occasionally influenced paleoenvironmental conditions. In these conditions, the formation of oncoids is influenced by various sedimentary factors, including the availability of metal sources, specific paleoenvironmental conditions, and the presence of microbial organisms, thereby enriching our understanding of palaeogeography during the Early Jurassic period. Additionally, the studied oncoids, with their comparable stratigraphic position and petrographic characteristics to those in both the eastern and western parts of the Tethyan basin, underscore their significance in palaeogeography and stratigraphy.

Archean atmospheric sulfur stocked in the Cretaceous gold deposits

The giant Cretaceous Jiaodong gold province is the world's third largest gold mining district, while the source of the ore-forming fluids and AuS remains controversial. The elemental sulfur is a key carrier for gold and the isotopic composition is an ideal tracer of the sulfur source, which makes it crucial for understanding gold deposit genesis and guiding ore exploration. Here, we show the Au-bearing pyrite in the gold deposits from Jiaodong bears a non-zero Δ33S signature from −0.07 ‰ to 0.31 ‰ showing the mass-independent fractionation (S-MIF). This clearly indicates involvement of Archean atmospheric sulfur during the Cretaceous gold mineralization. The MIF-bearing sulfur was likely remobilized from the Archean supracrustal rocks in the basement of the Jiaodong region via fluid-rock interaction while the hydrothermal ore-forming fluids passed through the crustal-scale fault zones. The Phanerozoic gold deposits from Jiaodong have relatively minor S-MIF signals compared with the Archean orogenic gold deposits due to more material (S) cycling and dilution. This new discovery of S-MIF signal in the Phanerozoic ore deposits implies that the recycling of crustal sulfur may be a more common process and play an important role in forming hydrothermal gold deposits.

Formation mechanism of carbonate cement in tight sandstone reservoirs in the depression zone of foreland basin and its impact on reservoir heterogeneity: The upper Triassic Xujiahe Formation in Western Sichuan foreland basin, China

The carbonate cements in tight sandstone reservoirs are diverse, complex, and important determinants of reservoir quality. This study provides a comprehensive analysis of the characteristics and formation mechanism of carbonate cement in the tight sandstone reservoirs of the Xu3 Member within the Xujiahe Formation in the depression zone of the Western Sichuan foreland basin. Core samples were examined on thin sections using cathodoluminescence and scanning electron microscopy, fluid inclusion analysis, in situ carbon and oxygen stable isotopes, and rare earth element analysis. The study reveals that the Xu3 Member primarily comprises sublitharenite and litharenite, which has three stages of carbonate cementation within the reservoirs. On average, the reservoirs exhibit a porosity of 3.82% and a permeability of 0.1mD. In the eodiagenetic stage, calcite and dolomite (δ13CPDB = 0‰ to −4.07‰; δ18OPDB = −5.64‰ to −12.69‰) formed, filling both intergranular pores and intragranular dissolution pores. This process is driven by the expulsion of organic acids through compaction from the clay and coal-bearing sections during shallow burial. The carbon sources originate from external organic acids and internal carbonate rock fragments, which combine with Ca2+ and Mg2+ ions released from the dissolution of carbonate rock fragments. During the mesodiagenetic stage A, the predominant formation of ferroan calcite and ankerite (δ13CPDB = −5.98‰ to −7.64‰; δ18OPDB = −14.47‰ to −19.87‰) occurred, filling intergranular residual pores and secondary intragranular dissolution pores. This phenomenon is caused by the expulsion of organic acids formed by the compaction of matured organic matter at greater burial depths. The carbon sources primarily originate from external organic acids and combine with Ca2+, Mg2+, and Fe2+ ions in the sandstone, alongside accompanying organic acids. During the mesodiagenetic stage B, coarse-macro crystalline calcites (δ13CPDB = −1.06‰ to −1.64‰; δ18OPDB = −17.95‰ to −20.36‰) with significant positive Eu anomaly-filled fractures were formed. These calcites are associated with hydrothermal fluids in the deep subsurface, precipitating directly in the fractures due to the presence of hot hydrothermal fluids rich in Ca2+ and CO32−. The Xu3 Member displays two sandstone superimposed types. In the mudstone overlain by sandstone type, carbonate cement is mainly present in the middle of the cycle, and reservoir quality is relatively high at both the bottom and top of the cycle. In the sandstone overlain by sandstone type, carbonate cement is predominantly found in the middle and top sections of the cycle, with higher-quality reservoirs primarily forming at the bottom of the cycle. It resulted in significant heterogeneity observed in the tight sandstone reservoirs of the depression zone in the Western Sichuan foreland basin.

Genesis of the sediment-hosted Qukuleke Au deposit in the East Kunlun Orogenic Belt, NW China: Constraints from geology, apatite U–Pb age, and C–O–S–Pb isotopes

Genetic classification of sediment-hosted Au deposits is often disputed in many orogenic belts around the world. The marine sediment-hosted Qukuleke Au deposit (>5 t Au) is an example of this, which has been attributed to orogenic or intrusion-related type. This paper addresses its metallogeny through detailed geologic and petrographic studies, together with apatite U–Pb dating and C–O–S–Pb isotopes.The hydrothermal paragenesis at Qukuleke is divided into three stages (I to III): Stage I (pre-ore) sheeted quartz veins; Stage II (main ore) pervasive sericite ± carbonate ± apatite ± pyrite ± arsenopyrite alteration associated with quartz–calcite ± stibnite veins; Stage III (post-ore) barren calcite veins. LA-ICP–MS hydrothermal apatite U–Pb dating on Stage II disseminated Au ore yielded 208.1 ± 6.5 Ma, coeval with the Late Triassic post-collisional intrusions from the periphery of Qukuleke deposit. Carbon–oxygen isotopes of Au–Sb ore-related hydrothermal calcite show that the hydrothermal fluids were derived from the marine carbonate ore host with considerable magmatic input. Lead–sulfur isotopes of the Au–Sb ore-related are consistent with those of coeval intrusions around the deposit, but different from those of the local sedimentary strata. Our age and geochemical data suggest the presence of a magma source that supplied the hydrothermal fluids, sulfur and metals for the mineralization. Combining the low sulfide content and the Au–As–W–Sb–Mo assemblage characteristics, the Qukuleke is best classified as a sediment-hosted intrusion-related Au deposit. Considering also the discovery of the large coeval Qukukekedong Au deposit nearby, we propose that Late Triassic intrusion-related Au system is a high-potential exploration target in the western EKOB.

Geology and in-situ S-Pb isotopes of the Luoboshan carbonate-hosted Pb-Zn deposit in Yunnan Province, SW China

The Luoboshan Pb-Zn deposit (0.2 Mt Pb + Zn metal reserves, @ 0.94 % Pb and 5.33 % Zn) is located in the Lanping-Simao Basin in the southern part of the Sanjiang Tethys metallogenic domain in Yunnan Province, SW China. The deposit comprises three ore bodies hosted in the lower Permian bioclastic and micritic limestone. Ore body I is stratiform (mineralization is small in scale and ores have been mined out), with average grades of 0.02–1.44 % Pb and 1.5–14.88 % Zn. Ore bodies II and III are lens-like, the average grades of them are 0.10–5.07 % Pb and 1.68–12.39 % Zn, and 0.10–1.71 % Pb and 1.14–11.79 % Zn, respectively. Two hydrothermal stages were identified in the Luoboshan deposit: Pyrite-I-sphalerite-galena-calcite (stage I) and Pyrite-II-calcite (stage II). The in-situ δ34S values of pyrite and sphalerite reveals a medium range of −8.80 ‰ to +10.1 ‰. Furthermore, Py-I and sphalerite exhibit low δ34S values from −8.80 ‰ to −3.04 ‰, but Py-II displays high δ34S values (+2.37 ‰ to +10.0 ‰). These results suggest distinct formation mechanism for reduced sulfur during different stages of Pb-Zn mineralization. A reasonable explanation is that a transition from bacterial sulfate reduction (BSR) in the early stage to thermochemical sulfate reduction (TSR) in the late stage as the likely mechanism for the formation of reduced sulfur. In addition, the in-situ Pb isotopes exhibit a narrow range (206Pb/204Pb = 18.660–18.672, 207Pb/204Pb = 15.665–15.677 and 208Pb/204Pb = 38.940–38.980), indicating that the metals originated from either a single source or a uniform mixture of different sources. Comparison of geological charateristics and isotopic geochemical data (S and Pb) indicates that the basin sedimentary strata (wall rocks) is the only and most probable source. Hence, we propose that the formation of the Luoboshan deposit involved biogenic in-situ acid generation of marine sulfates, which subsequently mixed with Pb-and-Zn-rich hydrothermal fluids, initiating TSR and resulting in Pb-Zn mineralization.

Magnetite geochemistry as a proxy for metallogenic processes: A study on sulfide-mineralized mafic–ultramafic intrusions peripheral to the Kunene Complex in Angola and Namibia

AbstractTrace element concentrations in magnetite are dictated by the petrogenetic environment and by the physico-chemical conditions during magmatic, hydrothermal, or sedimentary processes. This makes magnetite chemistry a useful tool in the exploration of ore-forming processes. We describe magnetite compositions from Ni-Cu-(PGE)-sulfide mineralized rocks from seven mafic–ultramafic intrusions peripheral to the Mesoproterozoic AMCG (anorthosite-mangerite-charnockite-granite) suite of the Kunene Complex of Angola and Namibia to investigate metallogenic processes through the geochemical characterization of Fe-oxides, which were analyzed in-situ via Electron Probe Microanalysis (EPMA), and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). We identified magmatic magnetite, segregated from both a silicate liquid and an immiscible sulfide liquid. Elements like Cr, Co and V suggest that the sulfide-related magnetite segregated from a relatively primitive Fe-rich monosulfide solid solution (MSS). Secondary Cr-rich magnetite appears in intrusions with abundant chromite or Cr-spinel. Two types of hydrothermal magnetite were identified, related to the pervasive replacement of sulfides and a late-stage, low-T fluid circulation event. Magnetite replacing sulfides is associated with serpentinized ultramafic rocks and is preferentially observed in the intrusions with the highest base and precious metal tenors. The high concentration of Ni, Co, Cu, Pd, As and Sb in these grains is corroborated by the identification of micron-size PGE mineral inclusions. We infer that serpentinization during hydrothermal fluid circulation was accompanied by desulphurization of sulfides with metal remobilization and reconcentration to generate magnetite carrying Pd microinclusions. We suggest that the highly serpentinized ultramafic rocks in the Kunene Complex region may become a possible target for economic Ni-Cu-(PGE) mineralization.

Kontrastujące style metamorficznych i metasomatycznych złóż kamieni szlachetnych w Cabo Delgado w północnym Mozambiku

From a conceptual point of view, in some ruby and garnet deposits of the Lalamo Supergroup – Cabo Delgado Nappes Complex, Northern Mozambique – the structural control of the primary mineralization seems to be spatially and geochemically related to the genesis of calcsilicate rocks, also with gemstones. Two main locations, with contrasting paragenesis, were studied in detail – Miteda area and Micute to N’Djekwa sector. Structural and paragenetic analysis, departing from high-resolution cartography, fabric microanalysis, SEM microscopy and mineral identification by X-ray diffractometry, were used with a major incidence in outcrops and small-scale mine diggings. Regional Neoproterozoic formations, especially its metacarbonate members, host Mg, B - rich skarns, bearing dravite and B-Mg-calc-silicate gemstones, especially in Micute and N’Djekwa area. Following the composition of the probable lithological precursor, these minerals, consistently, reveal a geochemical Mg - B signature that could have been inherited from an evaporite type ancestor, inter-stratified with carbonate rocks in the protolithic Lalamo sequence. The focus of hydrothermal circulation in multistage shear zones is invoked as the main pathway to interpret the complex mineralization observed in Miteda. In this case, the structure of a corundum + garnet major spot is coherent with a pull-a- part dilation associated with sinistral shear. There is a relation between red color saturation of ruby and the content of Cr in corundum. Cr can be leached from the meta-ultramafic host-rocks as a byproduct of desilication. Calcsilicate metasomatism could trigger Si sequester, producing metamorphic desilicated fluids. The focus of hydrothermal circulation in shear zones is invoked as the main pathway for the entrapment of metamorphic and metasomatic gem deposits. These occur in corridors of anomalous host-lithologies and vein swarms where peraluminous paragenesis are located inside highly modified meta-ultramafic or metacarbonate rocks. Skarn like lithologies result from reaction with a sedimentary carbonate protolith and gem deposition is mainly metasomatic. Veining affecting metaultramafic sequences of amphibolitic to granulitic facies show occasional, metasomatically affected, eclogite. For the contrasting styles of deposits represented in Micute – N’Djekwa and in Miteda, although the corresponding paragenesis are separated in space, their deposition can be situated in a conceptual model characterized by a geochemical interdependency and time proximity.

Early diagenetic evolution based on petrography and stable isotope analysis in the Barra Velha Formation of the Brazilian Pre‐salt

AbstractMineralogy and texture of diagenetic phases in the Aptian Pre‐Salt Barra Velha Formation are described, quantified and compared by facies and structural setting to understand their spatial and temporal distribution, and to develop predictive concepts for their genesis. This study examined data from eight wells from one oil‐field in the Santos Basin. Calcite is the most abundant mineral and occurs with fibro‐radial texture as spherulites and shrubs and sparse microcrystalline mudstone. The δ18O values from calcite spherulites and shrubs suggest water of similar composition and temperature, but they have different δ13C values. Mudstones show lower δ18O, suggesting warmer lake water and/or lower evaporation, whereas δ13C values indicate a variable, but commonly strong influence of biogenic CO2. Dolomite with rhombohedral habit was the first to precipitate, followed by lamellar, saddle and anhedral varieties. Rhombohedral dolomites show a positive δ13C‐δ18O correlation and a similar range in values to spherulites and shrubs, suggesting similar lake water. The lamellar dolomite is related to biofilms, whereas anhedral dolomite is attributed to mixing of pore and meteoric waters. Lamellar and anhedral dolomites have similar isotopic values, precipitating after rhombohedral dolomite in slightly warmer and/or less evaporatively concentrated pore water. Saddle dolomite is related to hydrothermal fluids that percolated the formation during early diagenesis. Silica occurs as replacement of primary calcite and Mg‐clay, but also as a cement and more rarely as a depositional chert. Both dolomite and silica display a complex range of petrographic textures, many of which are facies dependent. This study focusses on the most important phases of the paragenetic sequence that took place pre‐burial and are (1) formation of Mg‐clay, calcite spherulites and shrubs, (2) partial dolomitisation of shrubs and spherulites and precipitation of rhombohedral and lamellar dolomites, (3) precipitation of saddle dolomite, matrix and carbonate dissolution and (4) anhedral dolomite and all textures of precipitated or substituted silica.

Dolomitization history and fluid evolution of end-ediacaran multi-phase dolomites from the near-surface to deep burial depths in the tarim craton, northwestern China

The deeply buried (>8 km) end-Ediacaran Qigebrak Formation dolostones in the Tarim Basin, China, have significant petroleum potential, but have experienced multi-stage dolomitization that has resulted in the occurrence of diverse dolomite phases. The features and origins of the different dolomite types were merely investigated, which hinders an in-depth understanding of the complex dolomitization processes. This paper presents an integrated petrological, geochemical (C–O–Sr isotope and in situ trace element), fluid inclusion microthermometric, and dolomite U–Pb dating study of outcrop and core samples, to investigate the dolomitization mechanisms, and the nature and evolution of the dolomitizing fluids. The results indicate the occurrence of early dolomitization triggered by seepage–reflux of mesosaline seawater, which retained the primary marine geochemical properties of most of the matrix dolomite. Subsequently, during early diagenesis, fibrous dolomite cement was precipitated from marine porewaters, which have similar geochemical characteristics to the matrix dolomites. Following this, during intermediate-depth burial diagenesis in the Ordovician, fine-to medium-crystalline dolomite cements formed at temperatures of 80–117 °C in association with dissolution–reprecipitation of the matrix and early fibrous cements or diagenetic interactions with clay minerals of the lower Cambrian Yurtus Formation shales. During deep burial diagenesis and dolomitization in the Carboniferous, coarse-crystalline dolomite cements formed at temperatures of 104–138 °C from saline brines enriched in middle rare earth elements. Saddle dolomites, formed by hydrothermal dolomitization (162–184 °C) that was closely related to Permian–Triassic volcanic activity, have positive Eu anomalies and depleted δ18O values, that resulted from the mixing of basinal brines with 87Sr-enriched hydrothermal fluids. This study provides a detailed description of multiple dolomitization events in deep time that affected deeply buried dolostones.

Decoupling of Cu and Zn in sediment-hosted base metal deposits: Evidence from LA-ICP-MS analyses of fluid inclusions and trace elements in minerals at Baiyangping, China

Copper and lead–zinc mineralization in basinal rocks worldwide commonly shows distinct spatiotemporal zonation. The Baiyangping deposit in the Lanping Basin, southwest China, is a large sediment-hosted polymetallic deposit that shows clear and pronounced zoning of Cu versus Pb-Zn mineralization. Here, we analyzed trace elements in pyrite and sphalerite and fluid inclusions in sphalerite, celestine, quartz, and dolomite to seek insights into the origins of the ore-forming fluids and the processes that govern the separation of Cu- from Zn-rich ores. We show that while the later Zn-rich mineralization is characterized by fluid inclusions showing typical hallmarks of basinal brine composition (low homogenization temperature < 230 °C, high salinity > 20 wt% NaCl eq., dominated by Na-K-Ca-Mg chlorides), the earlier Cu-rich mineralization shows markedly distinct composition that more closely resembles deeply-circulated water that has equilibrated with crystalline basement rocks (higher homogenization temperature of 210–280 °C, variable but lower salinity of ∼ 5–15 wt% NaCl, dominated by Na-K-Li chlorides). Pyrites formed during the earlier Cu-rich stage show elevated Cu, Co, Sb and Zn, while pyrites formed during the later Pb-Zn stage are more enriched in As, Pb and Mn. We interpret that the earlier-formed Cu-rich ores were deposited by deeply sourced fluids that ascended along basement-penetrating reverse faults, whereas the later Zn-rich ores were formed by subsequent circulation of fluid from within the basin, whose flux was promoted by shallower strike-slip faults. Hence, the two ore types reflect discrete pulses of chemically distinct hydrothermal fluids. We suggest that the evolving structural controls and fluid pathways during prolonged ore formation allowed sequential pulses of fluids that had acquired different metal budgets by equilibration with different sources. We suggest that this type of two-step process may be more common in sediment-hosted base metal deposits than single-fluid cooling sequential order of precipitation.

A supergene-hydrothermal origin of the itabirite-hosted high-grade iron ores in the Mbarga prospect, Mbalam iron ore district, southern Cameroon, Congo Craton

The Mbarga itabirite deposit in the Mbalam iron district on the northwest edge of the Congo Craton (CC) hosts two main types of iron ore enrichments: supergene and specularite ores. This study presents mineralogical, geochemical, and isotopic datasets on these ores to determine their genesis.Ore microscopic studies indicate that the itabirites are of the oxide facies type, with magnetite showing partial to extensive alteration to hematite-martite. The supergene ores consist of hematite + martite + goethite ± gibbsite ± magnetite ± quartz, while the specularite ores are mainly composed of hematite + martite ± quartz. Magnetite microchemistry suggests formation under low-T hydrothermal conditions (~200–300 °C) with high fO2. Geochemical analyses show that the supergene and specularite ores have higher Fe2O3 (88.27 to ~100 wt%) and lower SiO2 (<0.01 to 0.18 wt%) contents than the itabirites (31.95 wt% Fe2O3, 67.16 wt% SiO2). The enrichment of Fe in the supergene ores is attributed to the depletion of major oxides and trace elements due to weathering and supergene enrichment, while the high Fe content in the specularite ores stems from the precipitation of iron-rich, but trace- and rare earth elements (REE)-deficient hydrothermal fluids. The slightly higher Al2O3 content and positive Ce anomalies in the supergene ores suggest the retention of Al-bearing minerals (gibbsite) and reveal highly oxidative conditions during martitization. Stable isotope analyses reveal that the supergene and specularite ores have δ18O values of −2.5 to −0.3 ‰ and − 2.0 to −3.4 ‰, and δ2H values of −75 to −123 ‰ and − 70 to −119 ‰, respectively, suggesting the involvement of isotopically light-evolved meteoric water in their formation. In contrast, the itabirites exhibit heavier δ18O (8.5 to 10.2 ‰) and δ2H (−85 to −91 ‰) values, suggesting formation from mixed magmatic and metamorphic fluid sources. A “polygenic-supergene-hydrothermal” model is suggested for the formation of the Mbarga itabirite-hosted iron ores.

Statement of Retraction: Discovery and significance of ancient cod fossils in hydrothermal fluid deposition areas: a case study of Chang 7-3 from the Triassic Yanchang Formation in the Ordos Basin

Statement of Retraction: Discovery and significance of ancient cod fossils in hydrothermal fluid deposition areas: a case study of Chang 7-3 from the Triassic Yanchang Formation in the Ordos Basin

How Does Nodular Chromite Nucleate and Grow? An Integrated Microstructural and Petrological Approach

Abstract Nodular chromite ore deposits are found in ophiolites and crop out in structures interpreted as former dykes. Most of them are transposed parallel to the plastic foliation of the host peridotite. Many studies have been conducted in order to decipher the origin and evolution of nodular chromite but the outstanding lack of consensus paves the way for an integrated field, geochemical and microstructural approach to be carried out. We sampled the well-characterized Maqsad chromitite dyke that crops out at the top of the mantle–crust dunitic transition zone in the Oman ophiolite and that was not affected by transposition. The spectacular variations in nodule size and texture and their distribution within the dyke have been perfectly preserved which is a rather unique situation. We selected about 40 nodules representative of the shapes and size variability of this ore deposit. Nodules have been classified in three categories: Type-1 nodules are large skeletal chromite grains associated with amphibole and olivine filling their former porosity; Type-3 nodules have a central nucleus of chromite/silicate surrounded by a mantle of close-packed chromite grains. Their shape is best described as almond-like, and they may reach 3 cm in length; Type-2 includes all of the intermediate nodules shapes and sizes between type-1 and type-3 varieties. Electron Probe MicroAnalyser (EPMA) transects and maps show that mineral chemical variations in type-1 nodules and in the nuclei of type-2 and type-3 nodules record out of equilibrium crystal growth. They exhibit high XCr and relatively low-TiO2 and likely resulted from transient interactions between hydrothermal fluids and basaltic melts. Contrary to type-1 and nuclei, the mantles of type-2 and -3 nodules have lower Cr2O3 contents, decreasing toward the nodule edges. They are richer in TiO2 than type-1 nodules but concentration patterns of this element along edge to edge transects are quite variable. The chromite mantles of type-2 and -3 are best understood as the fractional crystallization products of a parent melt of type-1 nodules in different pFluids and/or redox conditions. The analysis of the distribution of the misorientation axis between the skeletal grains and the adjacent chromite grains in the mantle (N = 222 in 28 nodules) revealed a clustering around a [111] axis across the whole range of misorientation angles. Accordingly, we suggest that the growth of the Maqsad nodules is achieved by accretion of finer euhedral chromite grains onto a skeletal chromite grain by juxtaposition of their flat crystal facets. Combined EDS-EBSD mapping together with EPMA transects revealed that a ~1-mm-thick rim of high XCr, displaying a homothetic shape to its corresponding nuclei, is a common feature in the mantle when the nodule sizes exceed about 1 cm. We interpret this observation as the result of the accretion of grains with higher XCr compositions during the nodule construction. Overall, our new study of the Maqsad nodular chromitite highlights the peculiar and transient conditions needed to give rise to the textural and chemical complexity that is preserved in these enigmatic rocks.

The composition of apatite in the Archean Siilinjärvi glimmerite-carbonatite complex in eastern Finland

We present a geochemical dataset and cathodoluminescence images of apatite from the Neoarchean (2610 Ma) glimmerite-carbonatite rocks from the Siilinjärvi complex, Eastern Finland. The subhedral, tapered prismatic grains are compositionally fluorapatite, with limited substitution of Ca by Sr, Na and low REE and Si contents. The Sr/Y ratios are among the highest in a global apatite comparison, comparable to those from other calcite carbonatites, dolomite carbonatites, and phoscorites. Some grains show evidence of late- or post-magmatic interaction with a carbonatite magma or a hydrothermal fluid, resulting in REE-rich overgrowth rims or recrystallized grains with abundant fluid inclusions. We interpret the high Sr/Y ratios combined with low REE contents and depleted heavy REE+Y to represent crystallization from a mantle-derived carbonatite parental magma. We show that the Siilinjärvi apatite is chemically heterogeneous but with a limited range in compositions. There are noticeable compositional differences on all spatial scales from micrometer to tens of meters, i.e., within a single crystal, between crystals in a sample, between samples and the two individual sampling locations. We conclude that the intra-crystal geochemical variability in apatite is a suitable tracer of the magmatic and post-magmatic evolution of carbonatite complexes.

Fluid origin and evolution of the Taolin Pb-Zn deposit in northeastern Hunan Province, South China: Insights from fluid inclusions and H-O-He-Ar isotopes

Situated in northeastern Hunan Province, the vein-type Pb-Zn orebodies at Taolin are mainly hosted in NE-/NEE-trending faults between Dayunshan-Mufushan pluton and Neoproterozoic metasedimentary rocks of the Lengjiaxi Group. Hydrothermal mineralization can be divided into five stages: (1) coarse-grained quartz stage, (2) quartz-fluorite-base metal stage, (3) quartz-barite-fluorite-base metal stage, (4) pale pink and colorless quartz stage, and (5) fine-grained quartz stage. In this research, fluid inclusions as well as stable (H-O) and noble gas (He-Ar) isotope compositions were performed to uncover the nature, origin, and evolution of the ore-forming fluids, ore precipitation mechanisms, and mineralization process of the Taolin deposit. Four types of fluid inclusions, i.e., liquid-rich two-phase inclusions (LV-type), pure liquid phase inclusions (PL-type), vapor-rich two-phase inclusions (VL-type), and pure vapor phase inclusions (PV-type), were distinguished in sphalerite, quartz, and fluorite. Microthermometric analysis of fluid inclusion assemblages in sphalerite, quartz, and fluorite from different stages indicates that from the Stage 1 to Stage 5, the homogenization temperatures vary between 168 and 211 °C, between 151 and 198 °C, between 131 and 180 °C, between 132 and 164 °C, and between 118 and 138 °C, respectively, whereas the fluid salinities vary from 12.4 to 16.9 wt% NaCl equivalent, from 9.7 to 14.6 wt% NaCl equivalent, from 5.6 to 10.3 wt% NaCl equivalent, from 3.6 to 9.7 wt% NaCl equivalent, and from 0.9 to 3.8 wt% NaCl equivalent, respectively. The H-O isotope data of quartz and the He-Ar isotopic compositions of sulfide crystals suggest that the ore-forming fluids were a mixture of crust-derived magmatic hydrothermal fluid and meteoric water. Fluid mixing and cooling were likely the crucial mechanisms for ore precipitation.