Mineral Assemblages Research Articles

Carbonate fluorapatite coatings on phillipsite represent a significant sink of phosphorus in abyssal plains of the western Pacific Ocean

As an essential micronutrient, phosphorus plays a key role in oceanic biogeochemistry, with its cycling intimately connected to the global carbon cycle and climate change. Authigenic carbonate fluorapatite (CFA) has been suggested to represent a significant phosphorus sink in the deep ocean, but its formation mechanisms in oceanic low-productivity settings remain poorly constrained. Applying X-ray absorption near edge structure, transmission electron microscopy, and laser ablation inductively coupled plasma mass spectrometer analyses, we report a unique mineral assemblage where CFA crystals coat phillipsite in abyssal sediments of the East Mariana Basin and the Philippine Sea. This finding suggests that phillipsite provides an ideal microenvironment for CFA formation due to its ability to release calcium cations and its strong adsorption capacity for phosphate anions. The feasibility of such a mechanism was confirmed by long-term (10 mo) laboratory experiments. Furthermore, the CFA forming in the sediment of abyssal plains is found to be enriched in rare earth elements (REE). The previously unrecognized mechanism of CFA formation may therefore not only contribute to the cycling of phosphorus in oceanic low-productivity environments but may also represent a significant sink of REE.

Assessing Thermal Gradients Across Archean Stratigraphy Using Raman Spectra of Carbonaceous Material Thermometry and Mineral Chemistry in the Western Dharwar Craton, India

ABSTRACTThis study investigates the metamorphic evolution of the Chitradurga Schist Belt (CSB) in the Western Dharwar Craton, India, emphasising its relationship with tectonic processes. Due to the limited availability of ideal mineral assemblages for calculating metamorphic temperatures, we selected metasedimentary rocks containing carbonaceous material (CM) from each stratigraphic unit in the CSB to understand the tectono‐metamorphic evolution. Raman spectra of carbonaceous material (RSCM) thermometry was integrated with mineral chemical analyses to elucidate the regional metamorphic conditions. These findings were then coupled with the microstructural evolution and deformation history of the CSB to clarify the tectonic evolution of the terrane. Our findings reveal a distinct metamorphic gradient, with the Bababudan Group exhibiting amphibolite‐facies metamorphism at temperatures exceeding 500°C. Other stratigraphic units in the study area recorded greenschist‐facies metamorphism at temperatures below 450°C. Detailed examinations of metamorphic mineral assemblages align with RSCM temperature estimates; hornblende is a major constituent in the Bababudan Group and is replaced by actinolite and chlorite during D2 or D3 deformation. Hydrous minerals such as muscovite and chlorite are distributed across all stratigraphic units, appearing along S2 or S3 foliation. The metamorphism in the Bababudan Group is likely linked to the early stages of collisional events/metamorphism of pre‐rift sequences. In contrast, the pervasive hydration and lower‐grade metamorphism are associated with the later stages of hinterland‐thrust belt formation. This study highlights the significant influence of plate tectonic processes on regional‐scale metamorphism and deformation in the Meso‐Neoarchean Dharwar Craton.

Climatic and tectonic controls on ferroan dolomite formation – insights on Early Miocene Mediterranean anoxia (il-Blata, Malta)

Records from the Miocene il-Blata section in Malta offer insights into the Central Mediterranean's depositional environments following an Early Miocene restriction of the Mesopotamian Seaway (∼20 Ma). Inorganic and organic stable carbon isotope values suggest relatively steady depositional environments, whereas authigenic Fe-dolomite abundances exhibiting substantial δ 18 O and δ 13 C variations at a sequence-scale indicate dynamic sedimentation conditions leading to differential diagenesis. Petrographically, the dolomitic levels exhibit matrix-selective dolomitisation, occasional silicification and phosphatisation, and textural indicators pointing to subsurface microbial influence which collectively point to complex shallow burial diagenesis. In addition, the presence of framboidal pyrite and gypsum infilling foraminiferal chambers along with the absence of large planktonic foraminifera, suggests the development of paleoenvironmental stress imposed by a density-stratified water column affecting pore waters. Towards the top of the studied succession, a shift from organic to siliceous deposits reflects water column perturbations possibly linked to changes in oceanic circulation associated with a temporary re-opening of the Mesopotamian Seaway. This study not only underscores hydrochemical controls exerted by North African terrigenous fluxes over the Mediterranean, but also highlights the intricate interplay between shifting depositional environments and shallow burial diagenetic processes in shaping the geochemical and textural fabrics of authigenic mineral assemblages.

Ti minerals in ores and skarns of the Aktash magnetite-polymetallic deposit, Western Karamazar, Tajikistan

The article characterizes Ti minerals from different mineral assemblages of the Aktash magnetite-polymetallic deposit (Western Karamazar, Northern Tajikistan). Anatase, ferrous pyrophanite, perovskite, calzirtite, zirconolite and titanite are described in ores of the deposit and most of these minerals are found for the frst time in both the Aktash deposit and the entire Karamazar region. The minerals are identifed using optical and electron microscopy. The presence of anatase, pyrophanite, perovskite and titanite is also confrmed by Raman spectroscopy. Perovskite and calzirtite are found in serpentine-phlogopite-calcite skarns, anatase, pyrophanite, perovskite, zirconolite and titanite are determined in serpentine-garnet-pyroxene skarns, which host galena-sphalerite ores, and pyrophanite and titanite are identifed in sulfde-magnetite ores. Based on mineral assemblages and relationships, it is concluded that perovskite, calzirtite, and zirconolite formed at the beginning of the skarn stage, which is followed by the formation of anatase, pyrophanite, titanite and zircon.

Elemental Substitution and Compositional Evolution of Tourmaline and Dumortierite Formation in Muscovite-schist Rock of Sakoli Area in Bhandara District, Central India

ABSTRACT Paleoproterozoic metapelitic schistose rocks are closely associated with the tourmaline deposit in the Girola area of the Sakoli Group of rocks of the Central India Tectonic Zone (CITZ). In this area, shear zones have effectively played a significant role in the formation and distribution of tourmaline along with hosts of minerals like garnet, sillimanite, dumortierite, etc. Local tourmaline concentrations can be found within quartz mylonites, granite mylonites, phyllonites, etc. wherein they are present as solitary crystal clusters, or discontinuous tourmaline bands or pockets. Mineralogical and chemical changes have been documented within these rocks in Girola Hill. The preliminary study had indicated the presence of tourmaline varieties, namely- schorl, dravite, foitite, elbite, liddicoatite, dumortierite, along with sillimanite, muscovite, sericite, rutile, topaz, and corundum. The presence of apatite, Sr-phosphate, and fluorite was also noticed. Within the assemblage zone, variations in boron metasomatism and potash leaching appear to be regulating variations in mineral assemblages and overall rock chemistry. Magmatic-metasomatic fluids have partially altered the original sedimentary patterns, according to the elemental analysis of the materials. The alteration could be due to interactions of schists with B-, F-carrying fluid. Tourmaline-rich assemblages develop when the fluid’s boron reacts with the nearby schistose rocks, leading to sheet silicate to cease functioning as a trap for Na, K, and Ba. Further metasomatic activity results in the formation of dumortierite from the tourmaline assemblages.

Mineralization Characteristics of the Hokuryu Epithermal Au‐Ag Deposit, Hokkaido, Japan

ABSTRACTVeins intersected by a ~600‐m‐long drill core into the Hokuryu epithermal Au‐Ag deposit in the northeastern part of Hokkaido, Japan, were investigated including mineralogy, texture, quartz composition, and fluid inclusion characteristics, with relation to elevation. Host rocks of these veins are flow‐banded rhyolite, pyroclastic breccia, and andesite and are mainly altered by illite and chlorite. The veins display crustiform and massive macroscopic textures. The crustiform veins generally located at higher elevations, contain Ag‐rich and Au‐Ag‐rich bands. The massive veins are mainly present at lower elevations, and they are generally barren of precious metals. The Ag‐rich bands in the crustiform veins contain mineral assemblage of hessite + sphalerite + pyrite + galena. These minerals, together with anhedral to rhombic adularia, exist within the interstices of quartz that display microspherical texture. The Au‐Ag‐rich bands contain mineral assemblage of electrum ± naumannite‐aguilarite associated with quartz with colloform, ghost‐sphere, and ghost‐bladed textures. Bulk compositions of the veins reflect the ore mineralogy observed in which Ag is strongly correlated with Pb, Te, Zn, Cd, and Bi. The barren massive veins are mainly composed of granular quartz. Textural and mineralogical characteristics of Au‐ and Ag‐bearing bands in the crustiform veins indicate amorphous silica and calcite precursors. Together with the coexistence of adularia, these suggest liquid boiling during vein formation. Quartz associated with Au‐ and Ag‐bearing minerals exhibits blue cathodoluminescence (ca. 400 nm wavelength) triggered by high Al impurity probably due to pH and pressure fluctuations or impurity redistribution during recrystallization from metastable amorphous silica. Fluid inclusions hosted in quartz and adularia, which contain negligible amounts of CO2, show a modal homogenization temperature range of 260°C–290°C and a salinity range of 1.2–6.9 wt% NaCl equivalent. The homogenization temperatures follow a boiling point curve and indicate an erosion of up to ~340 m for the deposit. The variable salinities in relation to elevation suggest extreme vapor loss during boiling at depth, episodic influx of high salinity fluids from deeper source, or mixing with high salinity fluids along anastomosing fractures at relatively shallower depths.

Spatial and temporal variability of physicochemical characteristics in Lancang River sediments amid hydropower development

The construction of river dams disrupts river continuity and sediment transport, altering the riverbed between sediment “sources” and “sinks” and changing the sediment characteristics of the river. In this study, 256 sediment samples from 54 major control cross-sections of the Lancang River (LCR) were analyzed to examine the spatial and temporal distribution of clay and non-clay minerals in the sediment and their relationship with the environmental changes caused by the construction 11 hydropower plants. The results indicate that the construction of terrace dams on the LCR interrupted the downstream refinement trend of sediments, which reappeared once the terrace reservoirs stabilized. Additionally, the interception of the dam increased the accumulation of clay mineral in sediments. From 2011 to 2023, the relative abundance of clay mineral in the regulated reaches (RRs) increased by 2.4 %, the quartz content decreased by 26.2 %, and the contents of calcite, mica, kaolinite, and chlorite increased, respectively. Consequently, the main non-clay mineral assemblage in the sediments shifted from quartz+feldspar to quartz+feldspar+mica. Furthermore, the clay mineral content was higher upstream the main dams (15.3 %) than downstream (14.8 %), with the sediment quartz and calcite contents being higher downstream, and mica and chlorite contents being higher upstream. The feldspar content showed a minimal change. However, the mineral composition of the sediments in individual reservoirs varied significantly, owing to the specific environmental conditions of each reservoir. The distribution of sediment clay also differed between the dry and rainy seasons. This study provides a scientific basis for managing reservoir sediment scheduling and regulating the ecological and environmental safety in watersheds.

Origin and evolution of heavy mineral assemblages of the Late Cretaceous Szozdy Delta System (Polish Cretaceous Basin): insights into burial history, weathering processes and parent rocks

Origin and evolution of heavy mineral assemblages of the Late Cretaceous Szozdy Delta System (Polish Cretaceous Basin): insights into burial history, weathering processes and parent rocks

Mineralogical indicators of Holocene climate in sediments of the high-mountain Lake Sagan-Nur (East Sayan Mountains)

We present the first results of study of bottom sediments taken from the high-mountain freshwater Lake Sagan-Nur of glacial origin which is located on the Oka plateau (East Sayan Mountains). Comprehensive investigations of the mineral composition of Lake Sagan-Nur sediments accumulated during the last ~8600 cal. years were carried out. The research methods include X-ray diffractometry (XRD), infrared spectroscopy, laser grain size analysis, SiO2bio determination, AMS and 210Pb dating. The mineral composition of the bottom sediments is dominated by layered silicates, quartz and plagioclase; and also, the X-ray amorphous component consisting of biogenic silica and organic matter is constantly present. The method of mathematical modeling of XRD profiles was used for correct identification of the layered silicates. It was found that the clay mineral assemblage was stable comprising muscovite, chlorite, illite, mixed-layer illite-smectite and chlorite-smectite, kaolinite. At the same time, the quantitative ratios of these minerals and their structural characteristics were modified significantly in response to paleoenvironmental changes in the region. This feature reveals the potential of a number of phyllosilicates (illite, mixed-layer minerals) as indicators of paleoclimate in sedimentary sections of freshwater basins. The study of sediment composition and, especially, the precision mineralogical-crystallochemical analysis of layered silicates allowed us to reconstruct the paleolandscapes and climatic conditions of sediment accumulation for different Holocene sub-stages.

Cold Deep Over Hot Shallow Crust: A Peculiar Metamorphic Architecture in the Neoarchean Metasedimentary Pontiac Subprovince, Superior Craton (Canada)

ABSTRACTThe Neoarchean Era is a key period in Earth's history as it witnessed a significant pulse of crustal formation corresponding to the assembly of several cratons, potentially coeval with a transition in the global tectonic regime. Neoarchean metasedimentary subprovinces of the Superior Craton, the largest unreworked Archean craton on Earth, were formed shortly before its final assembly and cratonization, thus providing valuable insights into the tectonic style, thermal state and architecture prevailing during this key period. Among these, the Pontiac Subprovince is one of the most studied, yet has a largely debated geodynamic history. In its northern extent, metamorphosed turbiditic sequences display a southward succession of index minerals—biotite, garnet, staurolite, kyanite and sillimanite—that may be indicative of a Barrovian‐like metamorphic gradient. However, the origin and evolution of this apparent gradient and its link to Neoarchean tectonics remain unclear. New mapping of metamorphic isograds and zones, petrological and microstructural analyses, whole rock and mineral chemistry analyses and phase equilibria modelling are integrated to decipher the Pontiac Subprovince tectonothermal evolution. Our analysis indicates that the peak equilibrium assemblages from the garnet, staurolite and sillimanite/melt zones developed early to late relative to the main regional deformation event (D2) and its associated steeply dipping S2 schistosity. Garnet zone rocks recorded a burial‐heating path with peak P–T conditions at 8.1–8.2 kbar and 582°C–585°C, along a low T/depth ratio of ~20°C/km. In contrast, staurolite and sillimanite/melt zone rocks followed isobaric heating and isothermal decompression paths with peak P–T conditions at 5.9–6.1 kbar and 610°C–625°C and 6 kbar and 700°C, respectively, along a moderate T/depth ratio of ~30–33°C/km. Since there is clear D2 structural continuity across the metamorphic zones over ~12 km and metamorphism occurred pre‐ to post‐D2, we interpret that the diverse P–T paths and contrasting T/depth ratios likely represent a spatially heterogeneous thermal structure developed within a single coherent structural block during and shortly after the formation of the subvertical S2 schistosity. Such features are hardly compatible with either modern inverted or continuous Barrovian sequences—known for consistent P–T evolution paths and similar T/depth ratios—or with discontinuous sequences requiring diachronicity. Our findings therefore do not fully reconcile with the existing accretionary/collisional models for the Pontiac Subprovince, given differences in their predicted apparent thermal gradients, metamorphic evolution and structural patterns. Alternatively, our data more closely match predictions for a sagduction‐dominated vertical process, where high heat influx at the base of the crust causes pooling/diapiric ascent of voluminous S‐type plutons, bordered by sagging synclines. This process led to the development of steeply dipping tectonic fabrics and pre‐ to late‐kinematic mineral assemblages along diverse P–T paths at contrasting yet coeval T/depth ratios. Our study provides insights into the evolution of the southeastern Superior Craton and highlights the need to apply an integrated quantitative approach to decipher its thermal structure and constrain the likely geodynamic processes that operated in the Neoarchean.

The Controlling Effect of CaCO3 Supersaturation over Zn Carbonate Assemblages: Co-Crystallization in Silica Hydrogel

Weathering products of sphalerite-bearing ores play an important role in controlling the fate of Zn in the environment. In this framework, the relative stability of Zn carbonates is of special relevance for the common case of ore weathering by carbonated groundwater in the presence of calcium carbonates. We investigated the experimental (co)nucleation and growth of Zn and Ca carbonates at 25 °C in finite double diffusion silica hydrogel media with the purpose of deciphering the system’s reactive pathway and unraveling the major governing factors behind the obtained mineral assemblages. The crystallized solids were carefully extracted two months post-nucleation and studied with micro-Raman spectroscopy, micro X-ray diffraction (XRD), scanning electron microscopy, and electron microprobe (EMP) methods. The obtained results indicate that the grown Zn-bearing phases corresponded to smithsonite and/or Zn hydroxyl carbonate, while CaCO3 polymorphs aragonite and calcite were also crystallized. Moreover, the observed mineral textural relationships reflected the interplay between supersaturation with respect to CaCO3/pCO2 and the grown Zn-bearing carbonate. Experiments conducted in more supersaturated conditions with respect to CaCO3 polymorphs (higher pCO2) favored the precipitation of smithsonite, while the opposite was true for the obtained Zn hydroxyl carbonate phase. The gathered Raman, XRD, and EMP data indicate that the latter phase corresponded to a non-stoichiometric, poorly crystalline solid.

The Weideshan Batholith and Felsic Dykes in the Eastern Jiaodong Peninsula: Is There Any Possible Relation to Gold Mineralization?

The genetic link between Weideshan-type rocks and gold mineralization in the Jiaodong Peninsula remains unclear. In this study, we examined the geochemical characteristics, water content, and oxidation states of Weideshan-type rocks and associated felsic dykes to assess the potential of a Weideshan-type batholith in directly contributing the fluids responsible for extensive gold mineralization. The findings reveal that the emplacement timing of Weideshan-type magmatic rocks in the Jiaodong Peninsula is slightly different from the timing of gold mineralization. Additionally, high zircon Eu/Eu* values indicate a relatively high water content within the Weideshan batholith. However, due to limited data on crystallization pressures, it remains equivocal whether water was saturated during the emplacement of the Weideshan batholith. Even if fluid saturation did occur, the magmatic oxidation states of Weideshan-type rocks are notably high (>FMQ + 1.5), which is incompatible with the reduced mineral assemblages typical of Jiaodong gold deposits. Therefore, our study suggests that the genetic link between gold mineralization and Weideshan-period granitic magmatism may be weak.

<sup>40</sup>Ar/<sup>39</sup>Ar dating of hydrothermal processes in large gold deposits of the Kochkar anticlinorium (South Urals, Russia)

The age restrictions for productive mineral associations of the two largest gold deposits of the Southern Urals — Svetlinsk and Kochkar’, which are located in the East-Uralian megazone, are discussed. The 40Ar/39Ar dating method was performed for the first time for potassium-containing hydrothermal minerals (micas and amphiboles) from the ore veins and ore-host alteration, as well as marbles. The age estimates obtained are in the range of 290–276 Ma; the weighted average value for the Svetlinsk deposit is 284 ±2 Ma, for the Kochkar’ field 276 ±2 Ma. It is assumed that the studied mineral assemblages of the Svetlinsk and Kochkar’ deposits were formed in the beginning of the post-collision stage corresponding to the tectonic relaxation regime. Age of hydrothermal mineral formation in the gold fields of the Kochkar anticlinorium are consistent with the time of post-tectonic plutonic activity, which was expressed in the Middle–Southern Urals in large-scale granitization (~ 300 million years ago). The ore-bearing alteration of a femic profile (of the same age for two deposits), which are most likely basificates, are near-synchronous to the granitization.

The occurrences, characteristics, and implications of chaotic rocks (accretionary rocks) within the Trans-Saharan orogenic belt in Nigeria

The present research paper reveals the occurrences of metachaotic rocks/accretionary sediments (mélange) in Igarra, southwest Nigeria. These metachaotic rocks are associated with schistose, marble, pegmatite, granitoid, and gneissose units. The mineral assemblage of the surrounding rocks shows garnet, biotite, staurolite, quartz, and chlorite assemblage, which suggest that the rocks were metamorphosed to the epidote–amphibolite facies grade. The chaotic rocks were classified into four groups based on major differences in color, grain size distribution, and stratigraphic and structural characteristics of observed blocks and matrices surrounding them. Group I metachaotic rocks are melanocratic in color, and form pockets of coarse lineation. They are highly deformed with mullions, rodding structures, and burrow-like cavities and show quartzite veins. Group II occurs as a leucocratic to melanocratic mixture of meta-siltstone and metapelite with greenish rock fragments trapped within the meta-siltstone layers. This unit has a cover of meta-siltstone layers occurring in pockets as striations forming mullion and rodding structures as well as ripple-like lineation. Group III metachaotic rocks occur as multiple benches of different strata tilted between 70 and 80º, with mild folding and burrow-like cavities at the base and well-preserved sedimentary structures. In Group IV, there are evidences of pre-metamorphic soft sediment deformational structures that are well-preserved in the metachaotic rocks, with discontinuous blocks of different units trapped within mixtures of metapelite and meta-siltstone. In this group, micro-faults and quartzite veins are scattered within the rocks. Meta-siltstone occurs as horizontal and vertical layers and web-like structures showing dendritic patterns. Overall, the field and petrographic characteristics suggest the Igarra metachaotic rocks are tectonic in origin and may have been formed within the trench basin of an outer accretionary complex.

Differences in chemistry and mineral assemblages of Czech and Slovak serpentinite-type rock aggregates for mixed radiation shielding concrete

Differences in chemistry and mineral assemblages of Czech and Slovak serpentinite-type rock aggregates for mixed radiation shielding concrete

Mineral Assemblages of Chromitites of the Alapaevsk Dunite–Harzburgite Massif (Middle Urals)

Mineral Assemblages of Chromitites of the Alapaevsk Dunite–Harzburgite Massif (Middle Urals)

New insights into the timing of the Yellow River flowing into the North China Plain: Evidence from detrital zircon U Pb geochronology and heavy mineral assemblages of sediments

New insights into the timing of the Yellow River flowing into the North China Plain: Evidence from detrital zircon U Pb geochronology and heavy mineral assemblages of sediments

The Petrochemistry and Origin of Precambrian Gneisses around Madaka and Kwana-Bala Area, North Central Nigeria

This paper discusses the petrochemistry and origin of gneisses within basement complex of Nigeria around Madaka and Kwana-Bala area. The lithological units of rocks from Madaka and Kwana-Bala area comprises of gneisses (migmatitic, banded, granitic), kyanite-sillimanite schist, semi-pelitic schist, amphibolites, talcose rock, quartzite, phyllite, granodiorite, fine-medium-grained granites and porphyritic granite. Seven samples of gneisses were analyzed for major element oxides using (ICM-MS). The gneisses are often variable in colour which depending on the nature and proportion of the feldspar and the ferromagnesian minerals. The variation in textures of these rocks from study area is due to varying size and amount of the K-feldspar porphyroblasts couple with varying degree of foliation. In thin section, the characteristics granoblastic textures were observed in migmatitic and granitic gneisses. The mineral compositions of the gneisses rocks are variable, the common minerals assemblage includes: migmatitic gneiss quartz + plagioclase +microcline +biotite + chlorite +garnet, banded gneiss: quartz + plagioclase+ muscovite +biotite+sphene +chlorite+ garnet and granitic gneiss contain quartz +plagioclase+microcline + muscovite+ biotite + chlorite +garnet + opaque minerals. The gneisses of the study area are product of reworked rocks during the Pan-African with a positive DF values, this suggests igneous sources for the protoliths of these rocks in the study area. The chemistry of the gneissic rock also show a variable proportion of Na2O and K2O with alkali-calcic affinity, which suggests, a closed system behaviour during metamorphism on one hand, and possibly a co-magmatic origin of the igneous protolith on the other hand.

Elasticity of Nacrite: Implications for Subduction Zone Dynamics

AbstractSubduction zones exhibit heterogeneities in composition due to different mineral assemblages transported into the mantle by the descending slabs, thus affecting the seismic properties of the region. These minerals are typically rich in alumina and silica and often contain hydrous phases. Nacrite, Al2Si2O5(OH)4, a mineral consisting of these components, forms in basaltic crust through hydrothermal alteration and is frequently overlooked due to its structural alikeness with its polytypes, making it hard to distinguish by traditional methods. Its occurrence in oceanic sediments and altered basaltic crust significantly impacts the subduction process by facilitating the transport of water into deeper mantle regions. In this study, we investigate the equation of state and elasticity of nacrite using first‐principles calculations based on density functional theory corrected for dispersive forces over its pressure stability range. Anomalous behavior of elastic coefficients are suggestive of a polytypic transformation, evidenced by anomalous softening in the shear modulus and a decrease of approximately 3% in shear wave velocity observed at low pressures ( 2 GPa). Our studies indicate that nacrite exhibits a significantly lower shear wave velocity compared to the surrounding mantle, resulting in very high VP/VS ratios. These findings emphasize the role of nacrite in the subduction zones of Japan and Alaska, particularly in the formation of low‐velocity layers. We propose that nacrite's presence is a significant factor explaining these observations, alongside other hydrous minerals like lawsonite, glaucophane, etc., contributing to the low‐velocity layers in these regions.

U-Pb ages of apatite, sulfur isotopes of sulfides and possible origin of the Dashui hematite-rich, sulfide-deficient gold deposit in the West Qinling orogen, Central China

The Dashui gold (Au) deposit is one of the largest Au deposits in West Qinling orogenic belt and unique in that its metallic minerals are dominated by hematite but sulfides are extremely scarce. The age and genesis of the Dashui Au deposit, however, remain poorly constrained. To resolve these issues, here we present robust apatite U-Pb dating results under the context of detailed textural and compositional studies. The ore mineral assemblages at Dashui consist of fine-grained quartz, hematite, kaolinite, apatite, native Au, and trace amounts of pyrite. The presence of hematite and gold as inclusions fully enclosed in quartz and apatite suggests that the former minerals are of primary origin rather than, as generally considered, a product of supergene oxidation of Au-bearing pyrite. Cathodoluminescence imaging reveals that the ores contain two types of apatite. CL-bright apatite was likely inherited from the magmatic wall rocks, whereas CL-dark apatite precipitated coevally with the ore minerals. In-situ LA-ICP-MS U-Pb isotope analysis of the CL-dark apatite suggests that the Dashui deposit formed at 217.2 ± 4.1 Ma, contemporaneously with the main stage of regional Au mineralization in the West Qinling orogenic belt that formed under the Triassic subduction-collisional tectonic setting. Given than Au mineralization at Dashui is temporarily and spatially associated with intermediate to felsic porphyry intrusions, and that the ore sulfides have S isotopes consistent with a magmatic-hydrothermal origin, the deposit can be broadly classified as a Carlin-like Au deposit. Nevertheless, the hematite-rich rather than pyrite-rich ore assemblages indicate an ore-forming environment that was more oxidized than that of typical Carlin-type deposits.