Low-temperature Hydrothermal Deposits Research Articles

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

AbstractThe 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

In the early mediaeval cemetery in Bodzia (Poland) a unique large collection of 85 small red beads in a rich burial of an adult woman were found. Analysis of the chemical composition and structure of the selected artefacts was made by use of electron microprobe (EMPA), X-ray fluorescence analysis (XRF) and Raman spectroscopy (RMS). These archaeometric tests showed that they are made of amorphous arsenic sulphide glass (ASG). This glass was obtained by melting natural arsenic sulphide (orpiment). The artefacts were formed at a temperature of about 350 °C. Increased content of Sb, Pb, Cu, Te and Se may indicate that the arsenic sulphide used in the production of ASG co-occurred with stibnite and other sulphides and sulphosalts of copper and lead. This mineral assemblage is typical of low-temperature hydrothermal polymetallic deposits from Maramuresh region in Romania.

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

The Nanpanjiang basin has the second-largest concentration of Carlin-type gold deposits in the world. Although significant progress has been made during the past two decades, the age of gold mineralization has not been well constrained. Field observation shows that these gold deposits are spatially associated with hydrocarbon reservoirs. At the Lannigou gold deposit, solid bitumens exist as films within structural fractures and cut through the orebodies, indicating that these bitumens formed after gold mineralization. In this work, we systematically studied bitumen reflectance (Rb), trace element, carbon isotope, and Re-Os isotopic compositions. These results show that these bitumens have high Rb values ranging from 2.40 to 2.71, indicating that they are high-maturity pyrobitumen. Trace element analysis reveals that bitumens in the deposit have high V/Ni, V/(V + Ni), and Zr/Sc ratios, but low Ni/Co, La/Sc ratios, and exhibit relatively flat rare-earth element (REE) patterns with no obvious Ce or Eu anomalies, comparable to the corresponding elemental ratios and REE patterns of Permian source rocks in the region. These bitumens also have similar δ13C values (-24.4 ‰ to –23.8 ‰, mean = -24.1 ‰) to the Permian source rocks (–22.4 ‰ to 24.7 ‰, mean = –23.6 ‰), indicating that they originate from the Permian source rocks. Five bitumen samples cut through the orebody yield a well-defined isochron age of 173 ± 6 Ma, indicating that the Lannigou gold deposit formed before this time. Our new data, combined with previously published geochronological data, suggest that the majority of Carlin-type gold deposits in the Nanpanjiang basin were mineralized at late Triassic to early Jurassic. The Re-Os data of this study also imply that the bitumen Re-Os chronometer has a large potential to constrain the timing of metal mineralization in low-temperature hydrothermal 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

The Jiangnan Orogenic Belt, as an important polymetallic metallogenic belt in South China, hosts numerous Au-Sb, W, Cu, Co, and Pb-Zn deposits. The newly discovered Lishan Pb-Zn-Cu deposit (> 0.64 Mt at 1.64% Pb, 2.54% Zn, and 0.42% Cu) in the northeastern Hunan Province is mainly accommodated by the Late Jurassic to Early Cretaceous Mufushan pluton, with vein-type Pb-Zn-Cu orebodies controlled by nearly N-S-trending faults. However, ore genesis, mineralization process, and the contribution of magmatism are unclear despite its closely spatial association with the Mufushan pluton. In this contribution, in-situ trace element, O and S isotopic analyses, as well as Pb, He and Ar isotopic analyses were conducted to determine the sources of ore-forming materials and fluids and potential mineralization process in the Lishan Pb-Zn-Cu deposit. Four stages of hydrothermal processes, from early to late, are recognized: (1) quartz; (2) quartz + fluorite + chlorite + chalcopyrite + pyrite; (3) quartz + fluorite + chlorite + sphalerite + galena + chalcopyrite + pyrite; and (4) quartz + calcite + chalcopyrite + pyrite. The trace element compositions reveal several important substitutions, including 4Zn2+ ↔ 2Sb3+ + Pb2+ + □ (vacancy) in sphalerite, and Ag+ + Sb3+ ↔ 2Pb2+ and (Ag, Cu, Tl)+ + (Bi, Sb)3+ ↔ 2Pb2+ in galena. The Lishan sphalerite is characterized by high contents of Mn (44.3–244 ppm), Fe (5379–19037 ppm), Co (30.0–186 ppm), and Ga (3.81–381 ppm) and low contents of Ag (1.04–20.7 ppm) and Sb (0.03–1.88 ppm). The sphalerite geothermometer combined with Ga/In (1.32–9716) ratios suggest moderate to low mineralization temperatures of 174–251 °C. The δ34S values of sphalerite, galena, chalcopyrite, and pyrite vary between −5.1 and −0.4‰, consistent with those of the magmatic sulfur source. The Pb isotopic compositions (206Pb/204Pb = 18.140–18.378, 207Pb/204Pb = 15.666–15.819 and 208Pb/204Pb = 38.574–39.207) of sulfides demonstrate that the ore metals were most likely derived from the Mufushan granitoids, but contaminated by the Lengjiaxi Group rocks. The oxygen isotopic values of quartz and noble gas isotopic compositions of sulfides indicate that the ore-forming fluids were mainly formed by the mixture of crust-derived magmatic hydrothermal fluid and meteoric water. Combined with principal component analysis, the Lishan sphalerite is different from the trace element characteristics of sphalerite in other types of deposits. In combination with the available data, we propose that the Lishan deposit belongs to a moderate to low temperature hydrothermal vein-type deposit.

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

The Kangshan Au-polymetallic deposit, in the Xiong’ershan District of China, lies on the southern margin of the North China Craton. The orebodies are hosted in (E)NE–(W)SW-trending brittle faults formed during the Mesozoic, which crosscut Neoarchean metamorphic rocks and Mesoproterozoic volcanic rocks. The ore-forming process can be divided into three stages: a quartz ± pyrite stage (Stage 1), a quartz-polymetallic sulfide stage (Stage 2), and a quartz-calcite stage (Stage 3). Stage 2 includes two substages: a quartz-pyrite-chalcopyrite-electrum substage (Stage 2-1; the main Au mineralization stage) and a galena-sphalerite-pyrite-ankerite-Ag-bearing tetrahedrite ± chalcopyrite ± electrum substage (Stage 2-2; the main Pb-Zn-Ag mineralization stage). Four types of fluid inclusions have been identified in the Kangshan deposit: three-phase CO2-rich inclusions, two-phase aqueous inclusions, one-phase vapor inclusions and one-phase aqueous inclusions. Inclusion microthermometric, laser Raman, and C-H-O-S-Pb isotope analyses were undertaken to clarify the properties of the ore-forming fluids, sources of these fluids and ore-forming materials, and mineral precipitation mechanisms. Laser Raman spectroscopic analysis indicate that the ore-forming fluids were formed in a NaCl-H2O-CO2 system. Inclusion homogenization temperatures and salinities are 130 °C–359 °C, and 0.53–11.46 wt% NaCl equivalent (eqv.), respectively. δ18Ofluid values calculated from the δ18OSMOW and δD values of the inclusions in auriferous quartz veins are 1.0–9.0‰ and -92‰ to -67‰, respectively. δ13CPDB and δ18OSMOW values of ankerite are -4.89‰ to -3.67‰ and 12.0‰–13.9‰, respectively. Fluid C-H-O isotopic compositions indicate that the ore-forming fluids originated mainly from magmas, with minor addition of meteoric water. δ34S values of the ore sulfides are 4.8‰–7.4‰; 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of the ore sulfides are 17.625–17.889, 15.543–15.620, and 38.389–38.740, respectively. S-Pb isotopic compositions indicate that the ore-forming materials of the deposit originated from granitic magmas and host rocks. The Kangshan Au-polymetallic deposit is a medium–low-temperature hydrothermal deposit. Its mineralization may be related toEarly Cretaceous granite formed in an extensional tectonic setting caused by craton destruction on the southern margin of the North China Craton.

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

The Simao–Indochina Basin is located between the Loei–Luang Prabang–Dien Bien Phu fold belt and the Jinghong–Nan-Uttaradit suture zone, composed of Mesozoic and Cenozoic red beds. It overlies the marine clastic rocks and volcanic sedimentary rocks of a Paleozoic back-arc foreland sequence. The Late Triassic Indosinian orogeny and subsequent superimposition and intracontinental shearing and structural superimposition may have facilitated the formation of various deposits in this basin, including hydrothermal, skarn, and porphyry types of mineralization. The Ban Kiouchep Cu–Pb–Ag deposit is a medium–low-temperature hydrothermal deposit that is largely controlled by faults and is hosted in carbonate rocks and volcanic (-sedimentary) rocks. The formation of this deposit includes a series of geological and metallogenic processes (i) neritic–littoral facies (volcanic-) sedimentation in a Late Paleozoic to Late Triassic back-arc foreland setting, (ii) Indosinian orogenic transformation, (iii) Mesozoic–Cenozoic tectonic-hydrothermal overprinting, and (iv) epigenetic oxidized leaching. The ore minerals are primarily malachite, azurite, and galena, followed by sphalerite, chalcocite, and chalcopyrite, with associated silver minerals. Using induced polarization (IP) geophysical surveys and soil geochemical surveys, large-scale and highly overlapping coincident anomalies and a mineralized zone at a scale of 100–110-m size (ore body I) was discovered in the area. Two anomalies, P1-C1 and P5-C4, with the best metallogenic conditions and the deepest extensions of the known ore bodies, were further selected as engineering verification targets. The two following verifying drill holes revealed Pb–Zn–Ag mineralization in the fault zone; however, no mineralized ore bodies consistent with the anomalies were found. The high background value of the Fe–Cu–Pb–Zn (Ag) elements, strong fracturing, and hydrothermal alteration, as well as later oxidized leaching with lateritization, resulted in in-situ dispersion and local secondary enrichment of the metallogenic elements. These may be the primary causes of the overlapping anomaly distribution along the strata and faults therein. Lateritic Ag mineralization has been also found in the area. The geological, geophysical, and geochemical characteristics and the prospecting are of great importance for further research and prospecting for other related deposits, for example, the VHMS, SEDEX, hydrothermal Cu polymetallic deposits and porphyry, skarn, epithermal, sediment-hosted/orogenic Au deposits, with lateritic Ag (Au) deposits in the Simao–Indochina Basin of Northern Laos.

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

The large-scale Huangtian scheelite deposit represents one of the most important such deposits discovered recently in southeastern Yunnan, China. Results of the geological, metallotectonic, and geochemical investigations indicate that this deposit formed in a magmatic hydrothermal system during the Yanshanian period. Its deposition is controlled by the ore-hosted structure of a secondary fold and by interlayer faults (F0-1 and F0-2), which are linked to the ore-guided structure of a secondary oblique strike-slip fault (F7) associated to the regional Maguan deep fault, and to an EW-trending tectonic zone that represents the main metallogenic tectonic system. The oblique strike-slip structure and the Tianchong granitic porphyries, located along the southern margin of the Laojunshan granite body, are primary ore-controlling structures, which directly controlled the movement of the ore-bearing fluid and its subsequent precipitation within the Huangtian scheelite deposit. The distribution of the orebodies is controlled by a group of metallogenic structural surfaces: the fold and fault systems, as well as a silicon-calcium surface. Silicification, fluoritization, and calcitization are the main hydrothermal alterations observed and exhibited a distinct alteration zonation. The hydrothermal mineralization process can be divided into three stages: the quartz-fluorite-(scheelite-tourmaline) stage, the scheelite-quartz-calcite- (fluorite) stage, and the calcite-pyrite-quartz-scheelite stage. The evolution of the mineralizing fluid proceeded as follows: medium-temperature and low-salinity fluid → medium- to low-temperature and low-salinity fluid → low-temperature and low-salinity fluid. The results concerning oxygen and hydrogen isotopic compositions demonstrate that the ore-forming fluids derived from a magmatic fluid mixed with a small amount of meteoric water. Overall, these features indicate that the Huangtian scheelite deposit represents a medium- to low-temperature magmatic hydrothermal deposit that is obviously controlled by the EW-trending tectonic zone. This study demonstrates the significance of such mineral exploration in similar geological regions.

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

The Longhua Ni–Co deposit, hosted in Cambrian carbonaceous sandstone and siltstone in South China, consists of disseminated niccolite mineralization and high-grade Ni–Co arsenides–quartz–carbonate vein mineralization. The disseminated mineralization is characterized by niccolite hosted in the center of quartz fragments and in argillaceous concretions in Cambrian banded pisolitic carbonaceous silty mudstone, and by parallel niccolite grains in Cambrian pisolitic pelite. These characteristics indicate that the disseminated niccolite originated through redeposition.The Ni–Co vein mineralization forms high-grade ores and is composed mainly of Ni–Co-arsenides. The homogenization temperatures of fluid inclusions in the mineralized vein quartz range from 118 to 219 °C, with corresponding salinities of 1.2 to 8.8 wt% NaCl equiv. The ore-forming fluids yield δDV-SMOW (‰) and δ18OH2O (‰) values of −54 to −66 and −3.0 to −7.7, respectively, indicating that the source was circulating meteoric water. The Longhua Ni–Co vein mineralization gives a niccolite Re–Os isochron age of 462.6 ± 8.5 Ma (MSWD = 1.03).The geological and geochemical characteristics of the Longhua Ni–Co deposit indicate that it formed as follows. (1) Precambrian quartz–niccolite mineralization was eroded and transported to the Longhua sediment trap, forming low-grade disseminated niccolite mineralization or Ni–Co rich source beds during the Cambrian. (2) High-grade Ni–Co arsenide vein mineralization was formed by further enrichment of Ni–Co from the Ni-rich source beds by circulated meteoric water during the Ordovician (Caledonian orogeny).The formation of the Longhua low-temperature high-grade Ni–Co vein ore indicates that Ni can be transported in low-temperature fluids that are rich in arsenic and HCO3− or CO32−. The Longhua Ni–Co deposit might represent a new type of sedimentary hosted low-temperature hydrothermal Ni deposit.

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

Precise geochronology is a crucial tool for understanding the genesis of low-temperature hydrothermal mineral deposits, but datable minerals are not always available; e.g., mineralization within sedimentary rocks, such as Carlin-type or Carlin-like Au deposits. Here, we demonstrate that hydrothermal apatite associated with Au from the large Nibao deposit in the Yunnan–Guizhou–Guangxi region of South China can be dated to yield the age of Au mineralization. Nibao is a Carlin-like deposit hosted in a Permian carbonate-bearing pyroclastic breccia. The auriferous minerals are predominantly zoned pyrite containing As-, Cu-, and Au-rich rims. Euhedral and subhedral apatite forms a mosaic texture and is intergrown with hydrothermal quartz, sericite, and zoned auriferous pyrite, indicating that the apatite is of hydrothermal origin and was coeval with Au mineralization. Unlike sedimentary and igneous apatite, hydrothermal apatite is relatively depleted in LREEs, enriched in MREEs, and slightly depleted in Eu. Hydrothermal apatite also has high Th/U ratios, negligible U concentrations, and very low concentrations of common Pb. This makes the apatite a potential candidate for ThPb dating. Our SIMS ThPb analyses yield a weighted mean 232Th/208Pb age of 141 ± 3 Ma (N = 23, MSWD = 2.2) for apatite coeval with auriferous pyrite, and this is the best estimate of the timing of Au mineralization. Hydrothermal apatite is also present in similar deposits in the Golden Triangle of South China. Therefore, ThPb dating of hydrothermal apatite potentially allows for the accurate dating of low-temperature hydrothermal deposits in China and worldwide.

Equations of State for NaCl and CaCl2 Solutions of Arbitrary Concentration at Temperatures 423.15–623.15 K and Pressures up to 5 kbar

Equations of state valid for all possible salt concentrations in solution are obtained for binary fluids H2O—NaCl and H2O—CaCl2. The equations are based on the empirical form of the Gibbs free energy for salt solutions of arbitrary concentration, developed earlier. Based on experimental data on the PVTx properties the obtained earlier thermodynamic description of H2O—NaCl and H2O—CaCl2 solutions at saturated vapor pressure is extended towards higher pressures. For H2O—NaCl pressures are up to 5 kbar. For H2O—CaCl2 pressures cover the range of available experimental data up to 0.7 kbar. The equations allow precise calculation of the whole set of thermodynamic parameters of the discussed salt solutions in the temperature ranges 423.15–573.15 K and 423.15–623.15 K, corresponding to low temperature hydrothermal ore deposits, that could be relevant for thermodynamic modeling of ore-bearing fluids at such deposits.

Geochemical impacts of hydrothermal activity on surface deposits at the Southwest Indian Ridge

Submarine hydrothermal circulation has attracted much scientific interest since seafloor hydrothermal activity was first observed in the 1970s; an area of particular interest is the impact of exported inorganic and organic materials from hydrothermal vent systems into the open ocean. In 2007, the first active hydrothermal vent field, with vent fluid temperatures up to 379 °C, was discovered at the ultraslow spreading Southwest Indian Ridge (SWIR), where active vents are much less abundant than fast spreading ridges, and the effect of hydrothermal extrusion on surface sediments is not fully understood. To explore how geochemical proxy signatures respond to hydrothermal activity, we investigated the distributions of elements, minerals and lipids in surficial normal marine sediments, metalliferous sediments and low-temperature hydrothermal deposits collected from the SWIR. The results showed different effects of hydrothermal activity on the surface deposits. The normal marine sediments were predominantly calcium carbonate characterized by > 42% CaO and > 90% calcite, with a significant autochthonous marine contribution to organic matter (OM) and a predominance of lower molecular weight alkanols and fatty acids; they were uninfluenced by hydrothermal activity but received some terrigenous input represented by abundant high molecular weight n-alkanes with an odd-over-even predominance. The near-field metalliferous sediments and hydrothermal deposits were very different. Some near-field metalliferous sediments were influenced by low-temperature hydrothermal activity, and their distributions of elements and minerals were similar to those of hydrothermal deposits, which were characterized by abundant Fe/Si and opal/nontronite. Other near-field metalliferous sediments were evidently influenced by mixing of high-temperature hydrothermal sulfides typically containing abundant Cu/Zn. With respect to the organic matter assemblages, near-field deposits contained little evidence for thermal maturation of organic matter and all were characterized by a strong microbial signature, including hopanoids, isoprenoidal and non-isoprenoidal dialkyl glycerol ether lipids, and low molecular weight n-alkanes with an even carbon number predominance. The far-field metalliferous sediments, despite the influence of non-buoyant plumes and slightly higher concentrations of hydrothermal-derived metals (e.g., Fe, Cu and Zn), had the same distribution of organic lipids and major mineral composition (> 90% calcite) as did normal marine sediments. Thus, the influence of non-buoyant plume inputs appears to have been minimal possibly due to the dilution of in situ microorganisms by normal marine organisms in sediment and seawater. Furthermore, these characteristics indicate inorganic indices based on abundant metal elements derived from the hydrothermal systems (such as Fe/Cu/Zn content, ∑REE/Fe, the ternary diagram of Fe, Cu×100 and Ca) are more sensitive, serving as better proxies than organic matter assemblages to differentiate the effects of diverse hydrothermal activity on surface deposits.

An Empirical Model of the Gibbs Free Energy for Solutions of NaCl and CaCl2 of Arbitrary Concentration at Temperatures from 423.15 K to 623.15 K under Vapor Saturation Pressure

An empirical model for the concentration dependence of the Gibbs free energy for solutions of chlorides of alkaline and alkaline earth metals in water is proposed. A simple analytical form of the Gibbs free energy makes it possible to obtain the equations of state for salt solutions that are equally accurate in the entire range of salt concentrations, from dilute solutions to solubility limits. The high accuracy of the thermodynamic description of solutions of high and intermediate concentration is ensured by the presence in the equation for the Gibbs free energy of two terms related to the Margules decomposition of the Gibbs free energy. Our form of the Gibbs free energy also contains a term that reproduces the thermodynamic behavior of solutions of electrolytes, which ensures high accuracy of the proposed model at low salt concentrations in the solution. Using the model, the equations of state for aqueous solutions of NaCl and CaCl2 at water vapor pressure in the temperature ranges of 423.15 K–573.15 K and 423.15 K–623.15 K were obtained, which corresponds to the parameters of ore-bearing solutions participating in the formation of low-temperature hydrothermal ore deposits.

Characteristics of the Ahmadabad hematite/barite deposit, Iran – studies of mineralogy, geochemistry and fluid inclusions

Abstract The Ahmadabad hematite/barite deposit is located to the northeast of the city of Semnan, Iran. Geostructurally, this deposit lies between the Alborz and the Central Iran zones in the Semnan Subzone. Hematite-barite mineralisation occurs in the form of a vein along a local fault within Eocene volcanic host rocks. The Ahmadabad deposit has a simple mineralogy, of which hematite and barite are the main constituents, followed by pyrite and Fe-oxyhydroxides such as limonite and goethite. Based on textural relationships between the above-mentioned principal minerals, it could be deduced that there are three hydrothermal mineralisation stages in which pyrite, hematite and barite with primary open space filling textures formed under different hydrothermal conditions. Subsequently, in the supergene stage, goethite and limonite minerals with secondary replacement textures formed under oxidation surficial conditions. Microthermometric studies on barite samples show that homogenisation temperatures (TH) for primary fluid inclusions range from 142 to 256°C with a temperature peak between 200 and 220°C. Salinities vary from 3.62 to 16.70 NaCl wt% with two different peaks, including one of 6 to 8 NaCl wt% and another of 12 to 14 NaCl wt%. This indicates that two different hydrothermal waters, including basinal and sea waters, could have been involved in barite mineralisation. The geochemistry of the major and trace elements in the samples studied indicate a hydrothermal origin for hematite and barite mineralisation. Moreover, the Fe/Mn ratio (>10) and plots of hematite samples of Ahmadabad ores on Al-Fe-Mn, Fe-Mn-(Ni+Co+ Cu)×10, Fe-Mn-SiX2 and MnO/TiO2 – Fe2O3/TiO2 diagrams indicate that hematite mineralisation in the Ahmadabad deposit occurred under hydrothermal conditions. Furthermore, Ba and Sr enrichment, along with Pb, Zn, Hg, Cu and Sb depletion, in the barite samples of Ahmadabad ores are indicative of a low temperature hydrothermal origin for the deposit. A comparison of the ratios of LaN/YbN, CeN/YbN, TbN/LaN, SmN/NdN and parameters of Ce/Ce* and La/La* anomalies of the hematite, barite, host volcanic rocks and quartz latite samples to each other elucidate two important points: 1) the barite could have originated from volcanic host rocks, 2) the hematite could have originated from a quartz latite lithological unit. The chondrite normalised REE patterns of samples of hematite barite, volcanic host rocks and quartz latite imply that two different hydrothermal fluids could be proposed for hematite and barite mineralisation. The comparison between chondrite normalised REE patterns of Ahmadabad barite with oceanic origin barite and low temperature hydrothermal barite shows close similarities to the low temperature hydrothermal barite deposits.