Low-temperature Hydrothermal Deposits Research Articles

Geological Characteristics and Preliminary Genesis Exploration of the Sakay Gold Deposit in Vientiane, Laos

The Sakay gold deposit in Vientiane, Laos, is located in the Indochina landmass of the southeastern segment of the Tethys orogenic belt, specifically within the Vientiane-Pakse micro-landmass and the Vientiane-Pakse metallogenic belt. This area is regionally significant for the concentration of minerals such as gold, copper, and tin. The host rocks of the deposit are intermediate volcanic lavas and volcanic tuffs, occurring in near-east-west brittle shear structural fractures through hydrothermal filling and metasomatism. The ore exhibits granular texture, subhedral texture, porphyritic texture, and oriented polycrystalline texture, with structural features such as disseminated, vein-like, and cataclastic breccia. The main ore minerals are pyrite, sphalerite, galena, and chalcopyrite, while the gangue minerals are primarily quartz, calcite, and dolomite. Gold is mainly present as included gold or fissure gold within the crystal lattices and microfractures of minerals such as pyrite and sphalerite. Based on mineral assemblages and generation timing, the mineralization can be divided into three stages: arsenopyrite-pyrite-dolomite-quartz (I), sphalerite-galena-chalcopyrite-calcite (II), and siderite (III), with the latter stages often overlaying the former, showing evident cross-cutting and metasomatic phenomena. The surrounding rocks in the mining area are altered, mainly showing silicification, carbonatization, limonitization, sericitization, and chloritization. Preliminary studies suggest that this deposit is a low-temperature hydrothermal gold deposit within a brittle shear zone.

Mineral Chemistry and Iron Isotope Characteristics of Magnetites in Pertek Fe-Skarn Deposit (Türkiye)

This study investigates the mineral chemistry and iron isotope composition of the Pertek Fe-skarn deposit in the Eastern Taurides, Turkey, to elucidate skarn formation and ore genesis through chemical and isotopic parameters. The deposit consists of substantial and dispersed magnetite ores formed by the intrusion of a dioritic suite into marbles. Mineral assemblages, including hematite, goethite, andradite garnet, hedenbergite pyroxene, calcite, and quartz, exhibit compositional variations at different depths within the ore body. Magnetite is commonly associated with hematite, goethite, garnet, pyroxene, calcite, and quartz. Extensive LA–ICP–MS analysis of magnetite chemistry reveals elevated trace element concentrations of titanium (Ti), aluminum (Al), vanadium (V), and magnesium (Mg), distinguishing Pertek magnetite from low-temperature hydrothermal deposits. The enrichment of Ti (>300 ppm) and V (>200 ppm), along with the presence of Al and Mg, suggests formation from high-temperature hydrothermal fluids exceeding 300 °C. Discriminant diagrams, such as Al+Mn versus Ti+V, classify Pertek magnetite within the skarn deposit domain, affirming its medium- to high-temperature hydrothermal origin (200–500 °C), characteristic of skarn-type deposits. Magnetite thermometry calculations yield an average formation temperature of 414.53 °C. Geochemical classification diagrams, including Ni/(Cr+Mn) versus Ti+V and TiO2-Al2O3-MgO+MnO, further support the skarn-type genesis of the deposit, distinguishing Pertek magnetite from other iron oxide deposits. The Fe-skarn ore samples display low total REE concentrations, variable Eu anomalies, enrichment in LREEs, and depletion in HREEs, consistent with fluid–rock interactions in a magmatic–hydrothermal system. The δ56Fe values of magnetite range from 0.272‰ to 0.361‰, while the calculated δ56Fe_aq values (0.479‰ to 0.568‰) suggest a magmatic–hydrothermal origin. The δ57Fe values (0.419‰ to 0.530‰) and the calculated 103lnβ value of 0.006397 indicate re-equilibration of the magmatic–hydrothermal fluid during ore formation.

Unique mercury isotopic signature of mercury-bearing hydrothermal systems in South China and its geological and environmental implications

Abstract The giant South China low-temperature metallogenic domain (LTMD) hosts many low-temperature hydrothermal systems (e.g., the Youjiang Au-As-Sb-Hg ore deposits, Xiangzhong Sb-Au ore deposits, and Xiangqian Hg ore deposits), which provide ~80% of the global Hg metal production. Here, we study the Hg isotopic composition of the Xiangqian Hg deposits and re-evaluate available data on the Youjiang Au-As-Sb-Hg deposits and Xiangzhong Sb-Au deposits. Cinnabar samples from eight Xiangqian Hg deposits display negative to near-zero Δ199Hg values, suggesting that their Hg is from two sources: (1) the asthenospheric mantle with near-zero Δ199Hg values, and (2) the Precambrian basement with negative Δ199Hg values. Such mixing is also observed in the Youjiang Au-As-Sb-Hg deposits and Xiangzhong Sb-Au deposits. Combining the ages of these deposits, we infer that plate subduction and collision during Caledonian, Indosinian, and Yanshanian tectonic-thermal events triggered lithospheric extension, asthenospheric upwelling, and basement remobilization within the South China craton, leading to the generation of magmatic-hydrothermal fluids and metal-rich crustal fluids to form low-temperature hydrothermal systems in the LTMD. This work demonstrates that Hg-bearing low-temperature hydrothermal deposits in China have distinct δ202Hg-Δ199Hg patterns from those in Europe and North and South America, implying that Hg isotopes can be used for tracing the sources of Hg in the global market.

Dynamics of Cu isotope fractionation during the reactions of pyrite with Cu(I)-bearing hydrothermal fluids

Dynamics of Cu isotope fractionation during the reactions of pyrite with Cu(I)-bearing hydrothermal fluids

Ultrapotassic plutons as a source of uranium of vein-type U-deposits (Moldanubian Zone, Bohemian Massif): insights from SIMS uraninite U–Pb dating and trace element geochemistry

The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif.

Chemical composition of early mediaeval arsenic sulphide glass’ beads as an indicator of the origin of the raw material

Chemical composition of early mediaeval arsenic sulphide glass’ beads as an indicator of the origin of the raw material

3D Mineral Prospectivity Mapping from 3D Geological Models Using Return–Risk Analysis and Machine Learning on Imbalance Data

Three-dimensional Mineral Prospectivity Mapping (3DMPM) is an innovative approach to mineral exploration that combines multiple geological data sources to create a three-dimensional (3D) model of a mineral deposit. It provides an accurate representation of the subsurface that can be used to identify areas with mineral potential. These 3D geological models are the typical data source for 3D prospective modeling. Geological data sets from multiple sources are used to construct 3D geological models. Since in practice there is a significant imbalance in the ratio of mineralized to non-mineralized classes, the classification results will be biased in favor of the more observed classes. Borderline-SMOTE (BLSMOTE) is an oversampling technique used to solve the problem of unbalanced datasets and works by generating synthetic data points along the boundary line between the minority and majority classes. This helps to create a more balanced dataset without introducing too much noise. Non-mineralized samples can be generated by randomly selecting non-mineralized locations, which means that uncertainties are generated. In this paper, we take the shallow-forming low-temperature hydrothermal deposit Guizhou Lannigou gold deposit as an example to extract the ore-controlling elements and establish a 3D geological model. A total of 50 training samples are generated using the sampling method described above, and 50 mineralization prospects are generated using Random Forests. A return–risk analysis was used to explore the uncertainties associated with synthetic positive samples and randomly selected negative samples, and to determine the final mineral potential values. Based on the evaluation metrics G-mean and F-value, the model using BLSMOTE outperforms the model without the synthetic algorithm and the models using SMOTE and KMeansSMOTE. The optimal model BLSMOTE18 has an AUC of 0.9288. The methodology also performs superiorly with different levels of class imbalance datasets. Excluding the predictions where the results highly overlap with known deposits, five target zones were circled for the targets using a P-A plot, all of which have obvious metallogenic geological features. Among them, Target1 and Target2 have good potential for mineralization, which is of great significance for future mineral exploration work.

Origin and re-Os geochronology of vein bitumen in the Nanpanjiang basin, SW China: Implication for the ore-forming age of Carlin-type gold deposits

Origin and re-Os geochronology of vein bitumen in the Nanpanjiang basin, SW China: Implication for the ore-forming age of Carlin-type gold deposits

Re-Os dating, and Pb-H-O isotope characteristics, of the Abra Cu-Ag-Pb-Au polymetallic deposit in Western Australia

The Abra deposit, a large lead-silver-copper–gold polymetallic deposit in Western Australia, is located at the eastern of the metallogenic belt of the Jillawarra basin in the Bangemall basin. The 4th to the 6th rock section of the Irrigully Group of Edmund Series are the principal ore-host strata, composed mainly of sandstone and fine sandstone. The orebody in Abra can be classified into two types as upper layer-like lead-silver and lower veins or netvein copper–gold. The metal minerals are mainly galena, chalcopyrite, and pyrite, while the gangue minerals are mainly quartz, dolomite, and barite. Both Re-Os isotopic age of the pyrite (1329.5 ± 98 Ma) with the initial (187Os/188Os) = 5.0 ± 3.8 and Pb isotopic compositions (206Pb/204Pb = 15.914–15.967, 207Pb/204Pb = 15.425–15.454, 208Pb/204Pb = 35.584–35.667) suggests that the metal minerals were sourced from the wall-rocks. δDV-SMOW values of quartz range from -35‰ to -17‰ whereas δ18OV-SMOW value range from 12‰ to 16‰ which indicates that the ore-forming fluids of Abra were medium–low temperature and medium–low salinity, and were mainly metamorphic water and secondary atmospheric precipitation. When the medium–low temperature ore-forming fluids are mixed with oxidizing reducing fluids carrying a large number of metal substances, a large number of ore-forming substances will be precipitated when the physical and chemical conditions change, thus it can be considered that the Abra deposit is a medium–low temperature hydrothermal polymetallic deposit.

Trace element geochemistry and O-S-Pb-He-Ar isotopic systematics of the Lishan Pb-Zn-Cu hydrothermal deposit, NE Hunan, South China

Trace element geochemistry and O-S-Pb-He-Ar isotopic systematics of the Lishan Pb-Zn-Cu hydrothermal deposit, NE Hunan, South China

Origin of Qinxi Silver Polymetallic Deposit in Southeast Coast, China: Evidences from H–O–S–Pb Isotopes and Mineral Rb–Sr Geochronology

The Qinxi silver deposit is located in eastern Fujian Province, South China. It is hosted in the late Jurassic Nanyuan volcanic unit and controlled spatially by structural alteration zone. The origins of the deposit are understood poorly. In this study, systematic field geology, precise sulfide geochronology, and H–O–S–Pb isotope analyses were carried out. The symbiotic minerals and ore assemblages are interpreted as evidence that Qinxi ore is a typical middle to low temperature hydrothermal deposit. Galena and sphalerite from the ore yield a Rb–Sr isochronal age of 130 ± 2.0 Ma (MSWD = 1.7), and single galena samples yield a Rb–Sr isochronal age of 130 ± 7.1 Ma (MSWD = 2) and are evidence that the ore was formed in the early Cretaceous period. The silver-bearing quartz vein has O:H and 18O:16O ratios intermediately between magmatic and meteoric water (δ18 Owater-smow from −2.50‰ to −0.1‰ and δDv-smow from −73.6‰ to −60.7‰). The δ34 S values of metal sulfide are close to the primitive mantle sulfur values (δ34S = −1.7 to 7.1‰, avg. = 0.92‰). The Pb isotopic compositions from the ore are similar to those of the hosted volcanic unit, with a value between the upper crust and mantle isotope composition (206Pb/204Pb = 18.421~18.605, 207Pb/204Pb = 15.580~15.772, 208Pb/204Pb = 38.591~39.183). Lead–zinc sulfides have mature crustal-derived Sr isotopic features ((87Sr/86Sr)i = 0.7103). Thus, the metallogenic source materials mainly came from the crust. Moreover, regional geological studies suggest that the Qinxi ore deposit was likely formed in an extensional terrane.

Genesis of the Kangshan Au-polymetallic deposit, Xiong’ershan District, North China Craton: Constraints from fluid inclusions and C-H-O-S-Pb isotopes

Genesis of the Kangshan Au-polymetallic deposit, Xiong’ershan District, North China Craton: Constraints from fluid inclusions and C-H-O-S-Pb isotopes

Metallogeny of critical metals in the SouthwesternYangtze Block

The Yangtze Block is one of the major geological units in China. Numerous and diverse mineralization are extensively distributed in this block, making the Yangtze Block be one of the well-known providers of mineral-resources worldwide. In the southwestern Yangtze Block, since Proterozoic, the diverse mineralization involves magmatic, magmatic-hydrothermal, low-temperature hydrothermal and weathering-sedimentary processes were extensively developed, of which critical metal mineralization (e.g., In, Ge, Ga, Cd, Re, W, Sn, Li, Nb, REE, and PGE) is generally present as by-products in some basic or precious deposits or as independent deposits. Previous studies have revealed ten types of critical metal mineralization in the region, specifically including the REE-rich phosphorite, PGE-Re-rich black shales, carbonate clay-type Li-Ga-REE, basalt weathering-type Nb-Ga-Zr-REE, In-rich cassiterite sulfide, REE-rich IOCG, Tl-rich low-temperature hydrothermal, Ge-rich Pb-Zn, Cu-Ni-PGE sulfides and carbonatite-related REE deposits. In the past decade, numerous studies have advanced our understanding of the mineralization styles, timing and ore genesis of these types of critical-metal mineralization, and are summarized in this paper. It was indicated that the Cu-Ni-PGE sulfide deposits are genetically related to the late Permian E’meishan mantle plume, whereas the carbonatite-related REE deposits are Cenozoic in age and have formed during post-collision between the Indian and Asian continents. Moreover, the REE-rich IOCG deposits were newly identified, constituting a Proterozoic IOCG metallogenic province that was first documented in China, and enrichments of REEs were suggested to be related to the chemical attributes of the late Cu mineralizing fluids; the In-rich sulfide deposits, distributed in a small area of the SW Yangtze Block, were suggested to be genetically related to late Yanshanian granites; the low-temperature hydrothermal mineralization, characterized mainly by the Pb-Zn and Au-As-Sb-Hg-Tl deposits that define one of the two world-largest low-temperature metallogenic provinces, was mainly formed at the Indosinian and Yanshanian periods during which granitic activities are relatively weak; numerous marine-facies sedimentary rocks particularly including the black shales and carbonate rocks are associated with the development of multi-episodes of sedimentary and/or weathering-sedimentary deposits that are uniquely enriched in Li, Nb, Zr, Ga. Re, REE and PGE. We also proposed some issues that are important but currently poorly addressed, including aspects of the evaluation of REE resource potential and mechanisms of REE concentrations in IOCG deposits, and key factors controlling the extreme enrichment of Ge, Ga, Cd and Tl in the Mesozoic low-temperature metallogenic province, with focuses on the background values of these elements in the region. Moreover, available studies show that In is extremely enriched in four giant cassiterite-rich, granite-related sulfide deposits distributed in the border area of Yunnan and Guangxi, making this area a unique In metallogenic province in the world. However, it is currently not clear why In is enriched in cassiterite-rich sulfide deposits and why In is enriched in the sphalerite as a function of “indium window” effect, which should be constrained through a combined study involving crust-mantle circulation, crystallization fractionation of magma, geochemical behaviors of metals and mineralogy. We also proposed that the weathering-sedimentary deposits, such as the clay-type Li-Ga-REE and basalt weathering-type Nb-Ga-Zr-REE ones, are likely brand-new types with huge potential of critical metal resources, and emphasized that in addition to ore genesis studies, researches on the distributions of metals and potential utilization of resources should also be paid more attention. We also highlight that current prospecting techniques and methods that are mostly used for exploration of some common basic or precious deposits are likely not applicable for the exploration of critical metal deposits. As such, new techniques and methods involving the integration of prospecting information, predication and locating of ore bodies and evaluation of resources should also be developed for critical metal deposits.

Gallium (Ga), germanium (Ge), thallium (Tl) and cadmium (Cd) resources in China

Dispersed metals are global strategic resources for the development of advanced technologies and future energy. In recent years, due to their limited resources, uneven geographical distribution, and high supply risk, dispersed metals are critical resources that most industrial countries compete for. The signatures of dispersed metals are of “dispersed”, “companion”, and “fine”, suggesting that these metals are difficult to form independent deposits. Thus, deposits enriched in dispersed metals were formed with special geological processes. In comparison with other metals (e.g., Zn and Cu), studies on the mineralization of dispersed metals are limited, restricting theoretical understanding and breakthroughs in prospecting of these metals. Based on the global reserves and geographical distribution of dispersed metals, we systematically summarized the distribution, major deposit type, occurrence, predominant enrichment process, geological, and geochemical signatures of gallium (Ga), germanium (Ge), thallium (Tl), and cadmium (Cd) in China to investigate the distribution characteristics and forming conditions of these elements, and to promote their prediction accuracy and evaluation level. Studies have shown that Ga is widely distributed in all kinds of rocks and has reached to industrial grade in some ore deposits. According to mineralization, Ga-rich deposits in the world can be roughly divided into weathered-sedimentary, hydrothermal, pegmatitic, and magmatic deposits. The Ga-rich deposits of industrial significance in China mainly include three types: Bauxite-associated, lead-zinc-associated, and coal-associated Ga deposits. In China, Ge deposits can be divided into “coal-type” Ge deposits (e.g., the Lincang and the Wulantuga Ge deposits) and “lead-zinc-type” Ge deposits (mainly located at the Yunnan-Sichuan-Guizhou region). China is rich in Tl resources, i.e., approximately 9000 t, mainly distributed in Yunnan, Guizhou, Anhui, Guangdong, Hubei, Guangxi, and Liaoning provinces. Owing to the relatively sparse research on Tl mineralization, there is no recognized classification of Tl deposit types at present. According to the differences in element combination, occurrence state, and metallogenic conditions, two types of deposits with industrial significance are roughly divided, namely low-temperature hydrothermal and massive sulfide Tl deposits. The global distribution of Cd resources is highly consistent with that of zinc resources, which are dominantly distributed in Australia, China, Peru, Mexico, and the United States. In terms of tectonic background, Cd resources in China are mainly distributed in lead-zinc metallogenic belts, such as the three rivers Tethys metallogenic belt, the low-temperature metallogenic zone at Sichuan-Yunnan-Guizhou tringle area, the Nanling metallogenic belt, and the greater Khingan metallogenic belt. In this review, the following prospects should be strengthened in future: (1) Typical dispersed metal deposits and clarification of the major resource types with industrial significance; (2) the occurrence of dispersed metals in minerals by high-precision and high-spatial-resolution microanalysis techniques; (3) the mechanism of super-normal enrichment of dispersed metals; and (4) new types of dispersed metal deposits. This review will be helpful to better understand the reserves of dispersed metals in China and will enhance the support capabilities of strategic resources.

Geological, geophysical, and geochemical characteristics of the Ban Kiouchep Cu–Pb–Ag deposit and its exploration significance in Northern Laos

Geological, geophysical, and geochemical characteristics of the Ban Kiouchep Cu–Pb–Ag deposit and its exploration significance in Northern Laos

Discovery of the large-scale Huangtian scheelite deposit and implications for the structural control of tungsten mineralization in southeastern Yunnan, south China

Discovery of the large-scale Huangtian scheelite deposit and implications for the structural control of tungsten mineralization in southeastern Yunnan, south China

Geology, Geochemistry and genesis of the Longhua low-temperature hydrothermal Ni-Co arsenide deposit in sedimentary rocks, Guangxi, South China

Geology, Geochemistry and genesis of the Longhua low-temperature hydrothermal Ni-Co arsenide deposit in sedimentary rocks, Guangxi, South China

Ore Genesis of Shanmen Ag Deposit in Siping Area of Southern Jilin Province, NE China: Constraints from Fluid Inclusions and H-O, S, Pb Isotopes

The Shanmen Ag deposit, located in the southeastern part of the Siping area, Jilin Province, is one of the large-scale Ag deposits in Northeastern (NE) China. Almost all Ag orebodies, Ag-bearing quartz-sulfide veins are strictly controlled by NE-trending faults or brittle fractures and are hosted in the Yanshanian monzonite and quartz diorite. In terms of deposit geology, three mineralization stages are recognized: the pyrite-quartz stage (I), the quartz-Ag-polymetallic sulfide stage (II), and the carbonate-quartz stage (III). The research results of the fluid inclusions in the different stages indicate that the early stage (Stage I) mainly contains three types of fluid inclusions: liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and CO2 aqueous multi-phase (C-type). The fluid belongs to a medium–high temperature and medium–low salinity H2O-NaCl-CO2 system and has boiling characteristics. The middle stage (Stage II) is mainly characterized by liquid-rich two-phase (L-type) and vapor-rich two-phase (V-type) inclusions, in which the mixing of fluids of different nature leads to the escape of CO2. Only liquid-rich two-phase (L-type) inclusions are distinguished in the late stage (Stage III). The fluids of two later stages belong to the medium-low-temperature and low-salinity H2O-NaCl system. Homogenization temperatures from the early to late stages range from 272.2 to 412.5 °C, 124.1 to 313.3 °C, and 128.6 to 224 °C, respectively. Fluid salinities in the early to late stages range from 1.6 to 12.1, 1.4 to 8.9, and 0.4 to 5.8 wt.% NaCl equivalent, respectively. The gradually decreasing trends of homogenization temperatures and salinities and the reduction in the CO2 content indicate that the release of CO2 and the low-temperature environment are important causes of the precipitation of Ag-bearing minerals. The δ18OH2O values of the ore-bearing quartz veins in the different stages range from −3.7 to +8.1‰, and the δD values of fluid inclusions in the quartz range from −113 to −103‰, indicating that the initial ore-forming fluid was mainly derived from magma and that the input of meteoric water gradually increased during the mineralization process. The δ34S values (ranging from −11.4‰ to +1.8‰) and Pb isotope compositions (206Pb/204Pb = 18.143–18.189, 207Pb/204Pb = 15.543–15.599, 208Pb/204Pb = 38.062–38.251) of sulfides suggest that the ore-forming materials have mixed mantle and crustal sources. Therefore, we propose that the release of CO2 and the low-temperature environment are important conditions for silver minerals precipitation, and the mixing of fluids of different nature is the dominant mechanism causing precipitation. The Shanmen Ag deposit can be classified as an intrusion-related medium–low temperature hydrothermal vein-type deposit.

On the biogenicity of Fe-oxyhydroxide filaments in silicified low-temperature hydrothermal deposits: Implications for the identification of Fe-oxidizing bacteria in the rock record.

Microaerophilic Fe(II)-oxidizing bacteria produce biomineralized twisted and branched stalks, which are promising biosignatures of microbial Fe oxidation in ancient jaspers and iron formations. Extracellular Fe stalks retain their morphological characteristics under experimentally elevated temperatures, but the extent to which natural post-depositional processes affect fossil integrity remains to be resolved. We examined siliceous Fe deposits from laminated mounds and chimney structures from an extinct part of the Jan Mayen Vent Fields on the Arctic Mid-Ocean Ridge. Our aims were to determine how early seafloor diagenesis affects morphological and chemical signatures of Fe-oxyhydroxide biomineralization and how extracellular stalks differ from abiogenic features. Optical and scanning electron microscopy in combination with focused ion beam-transmission electron microscopy (FIB-TEM) was used to study the filamentous textures and cross sections of individual stalks. Our results revealed directional, dendritic, and radial arrangements of biogenic twisted stalks and randomly organized networks of hollow tubes. Stalks were encrusted by concentric Fe-oxyhydroxide laminae and silica casings. Element maps produced by energy dispersive X-ray spectroscopy (EDS) in TEM showed variations in the content of Si, P, and S within filaments, demonstrating that successive hydrothermal fluid pulses mediate early diagenetic alteration and modify the chemical composition and surface features of stalks through Fe-oxyhydroxide mineralization. The carbon content of the stalks was generally indistinguishable from background levels, suggesting that organic compounds were either scarce initially or lost due to percolating hydrothermal fluids. Dendrites and thicker abiotic filaments from a nearby chimney were composed of nanometer-sized microcrystalline iron particles and silica and showed Fe growth bands indicative of inorganic precipitation. Our study suggests that the identification of fossil stalks and sheaths of Fe-oxidizing bacteria in hydrothermal paleoenvironments may not rely on the detection of organic carbon and demonstrates that abiogenic filaments differ from stalks and sheaths of Fe-oxidizing bacteria with respect to width distribution, ultrastructure, and textural context.

Hydrothermal apatite SIMS Th[sbnd]Pb dating: Constraints on the timing of low-temperature hydrothermal Au deposits in Nibao, SW China

Hydrothermal apatite SIMS Th[sbnd]Pb dating: Constraints on the timing of low-temperature hydrothermal Au deposits in Nibao, SW China