Ore-forming Processes Research Articles

Research on three-dimensional mineral prospectivity modeling by integrating numerical simulation of the ore-forming process: A case study in the Chating area of Xuancheng, Anhui Province, China

Research on three-dimensional mineral prospectivity modeling by integrating numerical simulation of the ore-forming process: A case study in the Chating area of Xuancheng, Anhui Province, China

The genesis of the Baguamiao orogenic gold deposit linked to multistage ore-forming processes in Triassic tectonic transition of South Qinling, China

The Baguamiao gold deposit represents one of the largest Triassic orogenic gold deposits in the South Qinling metallogenic belt of central China. Here we investigate Ar-Ar dating of sericite and muscovite, fluid inclusion of quartz, the trace element and sulfur isotopic composition of pyrite from different mineralization stage in the Baguamiao gold deposit to gain insights into the ore genesis. The Ar-Ar dating results yield 40Ar/39Ar plateau ages of 217.5 ± 2.1 Ma, 209.2 ± 0.2 Ma and 208.8 ± 0.2 Ma, indicating a two-stage gold mineralization during the tectonic transition from the early metamorphic deformation (at ca. 218 Ma) to the late extension (ca. 209 Ma) in the post-collision environment. Fluid inclusion, trace element and sulfur isotopic analyses reveal that the two stages gold mineralization show different sources. The early mineralization period developed CO2-rich inclusions with medium temperature (317–413 °C) and medium–low salinity (4.62–18.81 wt% NaCleqv.), and is characterized by formation of Py1 and Py2 with depleted Au, Cu, Pb, Zn, Ag, Te and Bi. The sulfur isotope data of Py1 and Py2 are between 3.6 ∼ 10.5 ‰, suggesting that the sulfur source was mainly derived from the surrounding rock (Xinghongpu Group). The late mineralization period developed medium–high temperature (321–453 °C), medium–high salinity (3.3–19.62 wt% NaCleqv.) CO2-rich inclusions and high-temperature and high-salinity daughter crystal inclusions, and is characterized by formation of Py3 and Py4 with obviously higher arsenic and metal contents, such as Au, As, Cu, Pb, Zn, Ag, Cr, Ti, Mn, Sb. The δ34S values of Py3 and Py4 are 8.3 ∼ 18.8 ‰, suggesting a mixed fluid of the Xinghongpu Group and Gudaoling Group with deep magmatic-hydrothermal fluids. We propose that the two periods of gold mineralization in the Baguamiao deposit, in response to the collision assembly of Qinling Orogenic Belt, preserve evidence for diverse metal sources with the early mineralization period sourced from the metamorphism of the upper crust (Xinghongpu Group) in the compressional stage (ca. 218 Ma), and the late mineralization period characterized by the contribution of the Xinghongpu Group, Gudaoling Group and deep magmatic-hydrothermal fluids in a post-collisional extensional setting (ca. 209 Ma). The role of metamorphism and magmatic activity during the Triassic Qinling orogenic processes is considered to be important in the remobilization, transport and enrichment of sulfur, arsenic and gold (and potentially other trace elements) from the earlier formed seafloor exhalation sequence.

Genesis of Gabbroic Hosted Copper Mineralisations in the Albanian Mirdita Zone (Kçira, Thirra)

There is a wide variety of ore deposits in Albania, where 20% of the Cu resources belong to a deposit type of unknown genesis (sulphide-bearing quartz veins in gabbroic rocks). The focus of this paper is on two mineralisations of this type (Kçira and Thirra) in the Mirdita Zone, an ophiolite zone representing the Mesozoic Neotethys Ocean in the Dinarides. Our aim is to understand the ore-forming processes and the genesis of these deposits, which can be used in future exploration projects. According to the petrographical analysis, the host rock suffered propylitic alteration or subgreenschist facies metamorphism. Mineral chemistry of pyrite and epidote suggests a VMS related origin, more precisely, the deeper part of its stockwork feeder zone. The bulk rock geochemical analyses confirms that the mineralisations are classified as mafic-, Cyprus-type VMS deposits. Differences in the geochemical compositions and the corresponding mineralogical observations are attributed to the distinct original geotectonic positions: higher amount of compatible elements (Cr, Ni, V and Cu) occur in Kçira, which formed in a mid-oceanic ridge environment, while incompatible elements (Ag, As, Co and Zn) are more abundant in the Thirra deposit, which formed in a supra-subduction zone setting.

Microanalytical investigation of K-rich fenites from the Catalão II alkaline‑carbonatite complex in Central Brazil: Implications for ore-forming processes within the world's largest niobium province

Alkaline‑carbonatite complexes are the main source of critical raw materials such as niobium (Nb) and rare earth elements (REE), which concentrate through a combination of magmatic and carbohydrothermal processes. These systems typically occur in close spatial connection with altered country rocks resulting from metasomatic alteration by exsolved K-Na-rich fluids, a process known as fenitization. Thus, the association between Nb-REE-rich carbonatites and fenites provides a unique opportunity for the investigation of carbohydrothermal alteration processes leading to critical metal enrichment. In this study, we focused on K-rich fenites associated with a shallow dike swarm system in the Boa Vista niobium deposit, the second-largest global producer of Nb, which is hosted within the Catalão II alkaline‑carbonatite complex in Central Brazil. We used a combination of micro-analytical techniques including EBSD, EMPA, and μ-EDXRF to unravel the complex micro-textural features of the fenites. Our data suggest that alkaline fluids exsolved vigorously from the carbonatite melts upon dike emplacement and pervasively metasomatized the country rocks, forming a melanocratic proximal fenite (phlogopitite), and distal fine-grained orthoclase-phlogopite-calcite fenites. Fluid alteration mobilized soluble cations (K, Fe, Mg, Ba, and Sr) plus S, CO2 and OH−, whereas Nb and REE were retained in the carbonatite dikes. The consistent dike-orthogonal orientation of metasomatic phlogopite and orthoclase grains in the proximal fenite, determined by EBSD, suggests that newly formed metasomatic minerals precipitated from the fenitizing fluids along the flow path. These orientations differ from the isotropic textures defined by primary (magmatic) minerals like pyrochlore, tetraferriphlogopite, dolomite, and calcite. However, and despite the microstructural differences between magmatic and carbohydrothermal phlogopite, their mineral chemistry is similar, suggesting that fenites are almost synchronous to the emplacement of carbonatites. These data indicate that fenites provide a rich archive of alkali-rich fluid infiltration around Nb-REE-rich carbonatite intrusions.

The Solubility of Antimony (Sb) in Liquid Hydrocarbons and Its Implication for the Ore-Forming Process of Orogenic Antimony-Gold Deposits in Southern Tibet

Orogenic antimony-gold deposits contribute significantly to the global antimony resource base. China’s orogenic antimony-gold deposits are primarily in southern Tibet. Investigations indicate that antimony combines and migrates with sulfur hydroxides in ore-forming fluids. Previous research on fluid inclusions in orogenic antimony-gold ores with sedimentary rock accommodation revealed the presence of organic inclusions, including liquid hydrocarbons, alongside common components such as CO2, H2O, CH4, and NaCl. However, the impact of liquid hydrocarbons on antimony migration and mineralization is still debatable. To investigate the transportability of antimony by liquid hydrocarbons in orogenic antimony ores, we selected n-dodecanethiol and n-dodecane as the subjects. We measured the solubility and occurrence form of antimony in these compounds at various temperatures and durations. The results indicate that after 5 and 10 days of reaction at 100 °C, the antimony concentrations in the n-dodecanethiol and n-dodecane groups were 67.44 ± 7.62 ppm, 75.15 ± 16.74 ppm, 1.40 ± 1.02 ppm, and 3.02 ± 3.09 ppm, respectively. At 150 °C for 5 and 10 days, the respective concentrations were 50.58 ± 5.39 ppm, 77.26 ± 45.20 ppm, 2.66 ± 3.08 ppm, and 2.41 ± 2.03 ppm. At 200 °C for 5 and 10 days, the corresponding concentrations were 339.76 ± 71.94 ppm, 218.97 ± 25.03 ppm and 6.53 ± 7.17 ppm, 2.27 ± 0.82 ppm (n = 3). The measured solubility of antimony in the n-dodecanethiol group increased gradually with rising temperature. The solubility of antimony in the n-dodecane group was low and notably inferior to that observed in the n-dodecanethiol group. X-ray photoelectron spectroscopy (XPS) analysis demonstrated a distinct thiol (R-SH) peak at 163.31 eV and compound peaks of antimony reacting with thiols at 162.06 and 160.87 eV. This suggests that antimony predominantly forms complexes with thiols for migration. Our findings suggest that specific liquid hydrocarbon components, predominantly thiols, can interact with antimony at metallogenic temperatures and persist in ore-forming fluids, facilitating migration and mineral enrichment. Earlier experimental studies on gold and crude oil have indicated that liquid hydrocarbons also play an essential role in the transportation and enrichment of gold during the formation of gold deposits, thus indicating that liquid hydrocarbons possess the considerable potential to act as an ore-forming fluid during orogenic antimony-gold deposit formation in southern Tibet.

Deep gold prospectivity modeling in the Jiaojia gold belt, Jiaodong Peninsula, eastern China using machine learning of geometric and geodynamic variables

Gold mineralization in the Jiaojia gold belt was formed in a structurally-dominant hydrothermal mineral system showing a close spatial association with the Jiaojia detachment fault. This study delves into the Jiaojia gold belt from the perspective of coupled spatial association and ore-forming processes by employing spatial analysis of three-dimensional (3D) models, 3D ore-forming numerical modeling, and 3D prospectivity modeling using machine learning techniques (random forest (RF) and multilayer perceptron (MLP)). The overarching goal is to gain insight into the structural-hydrothermal gold system and pinpoint potential areas of deep-seated gold deposits for future exploration endeavors. The spatial analysis of ore-controlling faults uncovers a close correlation between gold enrichment and specific fault geometrical attributes, including a dip angle ranging from 20° to 40°, minimal variations in dip angle (less than 5°), and convex topographical features. These attributes likely stem from the influence of fault morphology on the flow and pooling of fluids. In conjunction with this, 3D ore-forming numerical modeling of structural deformation and fluid flow reveals that gold mineralization is intertwined with moderate volumetric strain and shear strain of rock and fluid divergence. This interaction seems particularly pronounced in areas characterized by channel-like or gentle features. Consequently, it is plausible that gold distribution in the Jiaojia region is the outcome of a comprehensive coupling process involving strain localization, rock deformation, fluid flow, heat transfer and/or interaction. The deep gold prospectivity models of RF and MLP for the Jiaojia district jointly using the predictive variables of fault geometry features and ore-forming simulation data (volume strain, shear strain, temperature variation, and fluid flux) exhibit higher AUC (area under the curve) values compared to models employing individual predictor variable datasets. This improvement underscores their enhanced predictive capability. The prospectivity results thus were used for identifying gold potential within the Jiaojia region, where five promising gold targets at depth were ultimately determined.

Multi-isotopic composition (Li and B isotopes) and hydrochemistry characterization of the Lakko Co Li-rich salt lake in Tibet, China: Origin and hydrological processes

A comprehensive comprehension of the origin and ore-forming process of lithium brine deposits is imperative prior to engaging in lithium resource exploration, development and utilization. Nevertheless, the provenance of lithium in salt lakes has always been controversial. The lithium and boron (Li and B) isotopes, which serve as discerning geochemical tracers, can facilitate the monitoring of geological processes and sources. In this study, we investigated the hydrochemistry, Li and B isotope characteristics of surface brine and recharge river samples collected from the Lakko Co on the Qinghai-Tibet Plateau in China. The dissolved Li concentrations range from 0.11 to 590.8 mg/L, δ7Li and δ11B values exhibit a range of 0.75∼8.11 ‰ and -18.29∼-4.01 ‰, respectively. Concentrations of elements such as Rb, and Cu are relatively high and exhibit a strong correlation with Li. Ions composition and content ratios indicate the deep origin of Li and B. The analysis of δ7Li and δ11B suggests that, apart from hot spring, weathering of surrounding rocks also contributes to the presence of Li and B. Besides, variations in δ11B values within the lake brine analysis may be attributed to the deposition of water Magnesite at the lake bottom. The findings suggest that the recharge water system and hot springs serve as the primary source of Li and B in Lakko Co. Additionally, the interaction between water and rock in the deep crust as well as chemical weathering in the shallow environment, significantly contribute to formation of Li-rich fluids. The transportation process is characterized by the precipitation of secondary mineral, resulting in an increase of δ7Li and δ11B. Furthermore, the presence of water Magnesite in the lake brine leads to fluctuations in δ11B. This investigation of the lake contributes to a more comprehensive understanding of the origins and mechanisms involved in the transportation and storage of lithium in lithium-rich salt lakes.

Hidden Markov model for spatial analysis of three-dimensional mineralization distribution: Insights into magma flow and mineral exploration targets in the Jinchuan Ni-Cu-(PGE) sulfide deposit, China

The spatial distribution of mineralization includes significant clues about the magma/fluid flow process for ore formation. However, given the complex nature of the ore-forming process and its weak signature in the mineralization data, revealing the magma/fluid flow process in their mineralization footprint presents a considerable challenge. In this paper, we present a hidden Markov model (HMM) for the analysis of three-dimensional (3D) spatial distribution of mineralization. The HMM assumes the magma flow is a Markov process that is “hidden” in the “observable” mineralization features. Under this setting, the HMM quantifies the states of magma flow and links them as a whole in a probabilistic manner. This allows for the comprehensive combination of mineralization information, facilitating holistic inference of magma flow hidden in the mineralization signal. Based on the maximum likelihood principle, the magma trajectories and the associated injection points are inferred from the HMM. The proposed model is applied to spatial analysis of the mineralization in the world-class Jinchuan Ni-Cu-(PGE) magmatic sulfide deposit in NW China. Based on the 3D mineralization data interpolated from drill assays, the HMM infers magma trajectories and the associated injection points at Jinchuan, which indicate the Jinchuan deposit was formed by a multiple magma conduit system that comprises four independent injection points of magma ingress. According to the identified magma trajectories and injection points, four sites beneath the injection points and along the extension of magma trajectories are considered promising for future mineral exploration. With the insights gained from this case study, the proposed HMM can serve as an effective tool in the exploration information system to unveil magma/fluid flow history and guide exploration efforts toward concealed, deep-seated mineralization.

Fluid-rock interactions recorded by bulk-rock and mica compositions in the Xiuwacu granite-related quartz-vein W–Mo deposit, western Yunnan Province, China

The Xiuwacu deposit is a typical granite-related quartz-scheelite-molybdenite vein type deposit hosted within sandstones and sandy slates in the southern Yidun Terrane, western Yunnan Province, China. The Xiuwacu complex comprises the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry (MGP) and aplite granite. Extensive and intensive alteration including potassic, propylitic, phyllic, silicic and argillic alteration developed within the Late Cretaceous monzogranite porphyry. Greisenization and pegmatization occurred in the roof of the Xiuwacu complex, suggesting large-scale fluid-rock interactions. However, the effects of fluid-rock interactions on the elemental exchange and ore-forming processes at Xiuwacu still remained unclear. This study firstly reported whole-rock major and trace elements of representative altered rocks and in situ major and trace elements of micas from fresh and altered rocks, and quantitively constrained the mobilization of elements by normalized isocon diagrams. The altered MGP are characterized by higher loss on ignition (LOI) values (>2), lower light rare earth elements (LREEs) and variable large ion lithophile elements (LILEs) contents, with higher Cs, K, Rb and Ba contents and lower Sr content than fresh MGP. The classification diagram based on mica composition shows that magmatic biotites from the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry belong to Fe biotites. Hydrothermal micas from pegmatoid and greisen belong to Li-phengites and zinnwaldites, and hydrothermal micas from quartz-molybdenite vein are classified as zinnwaldites. Li-phengites and zinnwaldites have relatively lower F contents, higher W, Sn, Li, Cs contents than biotites, intergrown with calcite and ankerite, suggesting CO2-rich, F-poor and rare metal-rich nature of hydrothermal fluids. Based on a compilation of fluid inclusions, H–O–S-Sr isotopes, scheelite geochemistry from previously published literatures and compositions of altered rocks and micas from this study, the tungsten ore-forming processes at Xiuwacu could be concluded as follows: (1) the initial moderately-oxidized, F-rich fluids exsolved from parental magmas of MGP form fluorite-quartz assemblages (pegmatoid; 400–300 °C); (2) the F-poor, W-rich, moderately acid fluids cause the breakdown of Ca-rich plagioclases, which consumes H+ and provides Ca2+ for the precipitation of early-stage scheelites (greisen; approximately 300 °C); the evolved nearly neutral and medium- to low-temperature fluids (300–150 °C) drive the fluid-rock interactions to supply Ca2+ for the deposition of main-stage scheelites. The deposition of Mo was not determined by the fluid-rock interactions, but by the ore-forming fluids transformed from oxidized to reduced nature.

Computational simulation of the hydrothermal ore-forming system associated with the Luziyuan Pb-Zn Deposit, Yunnan, China: An example of pore-fluid convection controlled mineralization

The giant Luziyuan Pb-Zn deposit is a typical hydrothermal ore-forming deposit in the southern Baoshan block of SE Tibet. Although extensive studies were conducted to understand the ore-forming system associated with the Luziyuan Pb-Zn deposit through using the traditional geoscience methods, the ore-forming process involved in this deposit has not been justified in a strictly scientific manner to date. In this paper, the hydrothermal ore-forming process of the Luziyuan Pb-Zn deposit is computationally simulated through using the dual length-scale approach associated with the finite element method (FEM). In the dual length-scale approach, the FEM is first used to simulate the regional model of a large length-scale in the geological region where the Luziyuan Pb-Zn deposit is located. Afterward, the FEM is then used to simulate the deposit model of the Luziyuan Pb-Zn deposit. Particularly, the boundary conditions of the deposit model are consistently determined from using the computational simulation results of the regional model. The related numerical results demonstrated that: (1) the pore-fluid convection provides a continuous source of ore-forming fluid and ore-forming material for the Luziyuan Pb-Zn deposit; (2) the convective pore-fluid flow is the main dynamic mechanism, which controls the temperature, pore-fluid velocity and chemical species distribution in the Luziyuan Pb-Zn deposit; (3) the localized structure is the main controlling factor for the localized pore-fluid flow pattern, which further controls the formation and location of the orebody in the Luziyuan Pb-Zn deposit; (4) the dual length-scale approach associated with the FEM is very useful for simulating the hydrothermal ore-forming process of the Luziyuan Pb-Zn deposit.

The role of magmatic versus metasomatic processes in Ta-Nb mineralization at the Jiabusi Deposit, Inner Mongolia: Evidence from textural and chemical features of rare metal minerals and mica

The exact role magmatic versus metasomatic processes play in Ta-Nb mineralization is a question that has been debatable for several decades. The Jiabusi deposit is chosen to address this question, for its ample endowments of a variety of Ta-Nb oxides and mica in the various rock types that could provide insights into the whole evolutional history of ore-forming processes during magmatic-hydrothermal evolution. Textural and chemical evidence suggests that Fe-mica in the Fe-mica granite and pegmatitic portion is magmatic, indicating that the co-precipitated columbite-group minerals (CGM) and pyrochlore-supergroup minerals (PSM) are also magmatic. In the lepidolite granite and aplite, on the other hand, two stages of mineralization occured, represented by unzoned CGM that are in equilibrium with lepidolite, snowball quartz, and fluorite that all have a metasomatic origin and subsequent CGM/PSM overgrowths that co-precipitated with metasomatic Cs-lepidolite and saccharoidal albite. These CGM overgrowths have 1–2 time(s) higher Ta/(Nb + Ta) values than the preceded CGM, generating a clear compositional gap between them. Both stages of CGM and CGM/PSM precipitated from a hydrosilicate liquid (HSL), the partial migration of which results in the disequilibrium textures observed. The lepidolite granite is largely a metasomatic rock, the protolith of which has been obscured by intense metasomatism governed by the HSL. This HSL, however, is mainly constrained to the lepidolite granite but a few migrated into the Fe-mica granite. This is the main reason for the different mineralizing styles and intensities between the two rock types.

Role of faults and tectonic blocks in the Daugiztau gold deposit, Uzbekistan

This article delves into the comprehensive study of geodynamics concerning ore formation within the local area of gold ore mineralization in the central KyzylKum province. The primary focus lies in understanding the influential role of faults and tectonic blocks in the distribution of gold mineralization at the Daugyztau ore field. Geodynamics, as a significant factor in ore formation processes, is driven by external forces and prominently characterized by the dynamic activity of faults and blocks.The central KyzylKum province is renowned for its rich gold deposits, and exploring the underlying geodynamic processes is crucial for deciphering the mechanisms behind the formation and distribution of these valuable mineral resources. The study aims to shed light on the relationship between geodynamic phenomena and gold mineralization within the Daugyztau ore field.One of the key aspects under investigation is the role of faults, which are fractures or discontinuities in the Earth's crust, in controlling the migration and concentration of gold-bearing fluids. These faults act as pathways and conduits for the transportation of hydrothermal fluids, facilitating the deposition of gold and other associated minerals. By analyzing the spatial distribution and characteristics of faults in the area, researchers can gain insights into the geological processes responsible for gold mineralization.

Textural and chemical variations of sphalerite in the Giant Shuangjianzishan Ag-Pb-Zn deposit, South Great Xing’an Range, China: Implications for ore-forming processes

The Shuangjianzishan vein-type Ag-Pb-Zn deposit (145 Mt of ore grading 128.5 g/t Ag, 1.8 wt% Pb and 0.5 wt% Zn) in the South Great Xing’an Range (NE China) is hosted in NW- and NE-trending veins crosscutting the Lower Permian Dashizhai Formation, which is intruded by a granite porphyry. Although Ag-Pb-Zn sulfide mineralization has been extensively studied, the detailed ore-forming processes remain unclear. We have identified four stages of sulfide assemblages containing three generations of sphalerite: (1) Quartz arsenopyrite sphalerite (Sp1) stage; (2) Galena Ag-minerals sphalerite (Sp2) stage; (3) Ag-minerals pyrrhotite sphalerite (Sp3) stage; and (4) Pyrite calcite stage. Stages 2 and 3 are economically important. The textures of Sp1, Sp2, and Sp3 directly record the ore-forming processes, and the mineralization of silver shows an evolution trend from simple sulfides to complex sulfosalts. LA-ICP-MS trace element analyses showed that the Sp1 contains low content of silver and other trace elements (Fe, Mn, Sn, Cu, and Pb), indicating an anoxic to an euxinic environment. Stage 2 contributed largely to the silver endowment, with relatively high silver content in sphalerite or Ag-bearing mineral inclusions in Sp2a, Sp2b, and galena. Most of the silver minerals were deposited during stage 3, with sulfide and sulfosalt assemblages (e.g., pyrargyrite + canfieldite + tetrahedrite with minor pyrrhotite) replacing Sp2b. Selenium is significantly enriched in Sp3 and selenium-rich canfieldite from Stage 3. Sulfur isotopes determined by in situ LA-multicollector (MC)-ICP-MS analyses of sphalerite vary significantly from −9.6 to 2.2 ‰ (δ34SSp1 = -4.1 ∼ 1.1 ‰; δ34SSp2 = -4.2 ∼ 2.2 ‰; δ34SSp3 = -9.6 ∼ -2 ‰). Sulfur was possibly derived from the magmas forming the granite porphyry (δ34S = 1.4 ‰) and the magmatic sulfur reservoir was oxidized by mixing with the Dashizhai stratum sulfur (δ34S: −4.9 to −6.5 ‰). This resulted in lowering of the fluid δ34S value during the deposition of Sp3. The textural, chemical and isotopic data indicate distinct ore-forming episodes at Shuangjianzishan. We suggest that the deposition of abundant Ag-bearing minerals was induced by a new volume of Ag-rich ore-forming fluids and triggered by extensive decompression and cooling during Stage 3. The sulfur isotope and chemical geothermometers suggest the involvement of relatively high-temperature (>330 °C) hydrothermal fluid during Stage 2. Unlike a single cooling mineralization process, the superposition of relatively high-temperature mineralizing fluids may be an important factor in the large-scale Ag-Pb-Zn mineralization at Shuangjianzishan.

Estimates of chlorine isotope fractionation factors using density functional theory: Applications to ore-forming systems

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in 37Cl compared to chloride ions, but the change in δ37Cl of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (37Cl) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the δ37Cl value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, δ37Cl values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid–vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in δ37Cl in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the δ37Cl values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

Silver isotope analysis and systematics of native gold from the Rajapalot Co-enriched gold deposit, Finnish Lapland

Silver is probably the closest isotopic proxy to track monoisotopic gold and has been shown to have great potential to yield new information on the origin and enrichment processes of gold. This study describes the development of a tailored analytical protocol for accurate analysis of Ag isotopes and provides the first Ag isotope data for the Paleoproterozoic Rajapalot Au-Co deposit, Finnish Lapland. Six native Au samples yield ε109Ag values (relative to NIST SRM 978a) from −6.8 to +2.1 and are within the range of Ag isotopic compositions reported for native Au samples. The mean of the analyzed Au samples is ε109Ag = −3.8 ± 1.7 (2SD) with most of the samples with negative ε109Ag values (−6.7 to −2.0); one sample has a positive ε109Ag value of +2.1 ± 0.5. Silver isotope fractionation in the Rajapalot Au deposit was likely associated with physicochemical processes related to deposition and/or re-mobilization of the ore rather than with source region inheritance. It is suggested that redox reactions involving Ag+ ↔ Ag0 phase change primarily account for the isotopic differences within the deposit. Our results also suggest that the Rajapalot Au-Co deposit was formed via multistage ore-forming processes and/or that the primary ore was re-mobilized, which caused isotope fractionation along fluid pathways. Silver isotope variation within a deposit may mark a fractional crystallization trend with the lightest isotopic composition representing the earliest precipitate. Hence, Ag isotopes show potential as an isotopic vectoring tool in search of Au-enriched domains.

Combining numerical modeling and machine learning to predict mineral prospectivity: A case study from the Fankou Pb–Zn deposit, southern China

Predictive modeling of mineral prospectivity, which integrates diverse evidence of ore-forming processes based on an ore deposit genetic model constitutes a significant task in mineral exploration targeting. Descriptive deposit models suffer from overwhelming dependence on the explicitly mappable characteristics of a given deposit used to simplify the complex mineral system, but this hinders mineral prospectivity modeling. This study proposes a hybrid approach that combines numerical modeling with machine learning to predict mineral prospectivity. This approach first uses the numerical modeling method to provide more insights and better characterize a complex mineral system. These comprise implicit footprints such as singular stress deformation, heat transmission and fluid flux inherent in the transient geodynamics of ore genesis. The approach then applies a machine learning algorithm as a data-fusion tool to integrate the simulated evidence to minimize uncertainty in the predictive modeling of mineral prospectivity, which fundamentally results from an incomplete understanding of the targeted mineral system. Finally, we use the Fankou Mississippi Valley type Pb–Zn deposit as an example to illustrate the application of the proposed hybrid method. The prospectivity modeling result is critically compared with that of the spatial modeling based on descriptive deposit modeling by means of measures of accuracy, area under the receiver operating characteristic curve, and normalized density of prediction–area plot. It is shown that the overall performance of the prospectivity map predicted using a hybrid method is significantly improved. As such, this study demonstrates that a hybrid framework combining numerical modeling with machine learning provides a useful and novel way of predicting mineral prospectivity. The proposed hybrid method embodies a pioneering paradigm that integrates physics and data, assimilating ore deposit genesis models and reconnaissance data for the prediction of mineral prospectivity in support of exploration targeting. This approach can be implemented to enhance exploration information systems through the predictive modeling of mineral prospectivity in a unified computing and decision-making environment.

In-situ molybdenite and rutile geochemistry of the Nannihu porphyry Mo-W deposit, East Qinling, China: A perspective on ore-forming processes

As the important strategic mineral resource, molybdenum is widely utilised by various industries of national economy. Most of the Chinese Mo deposits are classified as porphyry deposits, such as the Nannihu porphyry Mo-W deposit that is one of the biggest Mo deposits in China. Previous studies mostly focused on the geochemistry, geochronology, and isotopic geochemistry of the Nannihu deposit, but its element immigration and occurrence as well as the ore-forming source remain unclear, which limits the development of ore prospecting. Here we document in-situ major and trace element geochemistry of different stages of molybdenite and rutile from the Nannihu deposit, with major aims to determine ore-forming element immigration and occurrence and to reveal ore-forming processes. Based on mineral assemblages and micro-texture, three types of rutile were identified in the Nannihu deposit. The type-1 rutile has stable Y/Ho ratios 16.74–20.82 and similar upper crust rare earth elements (REEs) patterns, which suggest magmatic and crustal origin. The type-2 and 3 rutiles show wide variation Y/Ho ratios (10.69–49.55 for type-2 and 17.85–43.64 for type-3) and belong to hydrothermal rutiles. In addition, two types of molybdenite are also presented, with the Mo1-type for the major ore-forming stage and the Mo2-type for the late ore-forming stage. The fluctuation of elements in the time resolved LA-ICP-MS spectra curve diagram of the Mo1- and Mo2-type molybdenites indicate that the Te, Bi, Pb, Fe and Mn elements are hosted as mineral inclusions in molybdenite lattices. However, the Re and Se elements in these molybdenites display flat signatures and have negative correlations with Mo and S elements, which imply that the Mo and S elements are substituted and occur as isomorphous constituents in molybdenite lattices. The low Re contents (1.60–12.90 ppm) of molybdenites also suggest crustal-mantle mixing metallic sources for the Nannihu deposit. The increasing Fe (4245 to 23943 ppm) and W (1631 to 5922 ppm) contents in hydrothermal type-2 and type-3 rutiles as well as the increasing W contents (113 to 243 ppm) in Mo1- and Mo2-type molybdenites both indicate that the Nannihu ore-forming fluids were under the temperature and oxygen fugacity (fO2) of reduction conditions. The Mo1-type molybdenite is enriched in light REEs and depleted in high field strength elements (HFSEs), and coupled with the late ore-forming stage of F-rich fluorite, which indicate the ore-forming fluid evolution from Cl- to F-rich. Integrating with previous studies, the mineralising fluids of the Nannihu deposit are suggested to be featured by high Cl−, CO2 and metallic contents. The Mo and W ions in mineralising fluids immigrate as Cl-complexes at the early stage of mineralisation, the Mo4+ is then replaced by W4+ in molybdenite lattices at the later stage, and the high temperature and fO2 of the early ore-forming fluids promote the migration of Mo and W elements during ore-forming processes. Eventually, the fluids boiling, and the reduction conditions in temperature and fO2 jointly result in the final precipitation of Mo and W to generate the giant Nannihu deposit under extensional tectonics.

Geochemistry and geochronology of Beiya giant Au polymetallic deposit: Implications for multistage ore-fluid pulses in a skarn system

Garnet is one of the most typical minerals in skarn deposits, and its texture and composition is a proxy for the changing physicochemical conditions. Thus, mineralogical and geochemical characterization of different garnet types can provide important information on the nature and evolution of ore-forming fluids. The Beiya gold-polymetallic deposit is China’s largest skarn-type Au deposit with Au reserves of >380 t. Although several studies have addressed the skarn formation mechanism and nature of fluid evolution, a systematic study on the changes in the composition and texture of garnet from wall rocks to ore-related intrusion has not yet been attempted. In this work, we carry out a systematic investigation along a horizonal profile in the open pit of the Beiya deposit. Our results show that the garnet in the profile belongs to grossular-andradite solid solution series with all grains showing well-developed fine oscillatory zoning. The garnets are of two types: the first type (Grt I) formed within the endoskarn, is reddish to red with euhedral-subhedral megacryst structure (>10 mm) and has altered surfaces. The second type (Grt II) occur in the proximal exoskarn, is red to brown–red, euhedral-subhedral, with medium- to fine- grained structure (0.3–4 mm) and displaying oscillatory zoning. Unlike Grt I, hydrous minerals occur in the pores among Grt II garnets, such as calcite, quartz, epidote and chlorite. Also, calcite-chlorite vein, quartz vein and quartz-chalcopyrite-pyrite vein cuts the Grt II garnets. Grt I is dominantly grossular (Adr14.88-43.58Grs55.25-82.87), depleted in the large ion lithophile elements (LILE), relatively enriched in high field strength elements (HFSEs) and enriched in heavy rare earth elements (HREEs) with negative Eu anomalies. Grt II is mostly andradite (Adr62.18-98.91Grs0-36.22), depleted in the LILE and HFSE, enriched in light rare earth elements (LREEs) with slightly positive Eu anomalies. Here we selected Grt II for in-situ LA-ICP-MS U-Pb dating, since Grt I contains negligible uranium. The results yielded a lower intercept 206Pb/238U age of 35.3 ± 1.8 Ma (MSWD = 0.68, n = 23). The calculated oxygen isotopic compositions of hydrothermal fluids in equilibrium with two types of garnets range from 6.46 ‰ to 8.56 ‰ (average 7.89 ‰), which are within the range of values for magmatic fluids. We infer that the physicochemical conditions of the hydrothermal fluid changed from relatively reduced, nearly neutral, and low water/rock ratios to slightly oxidized, slightly acidic, and high water/rock ratios. The chlorine content in the fluid increased during garnet growth. The fluctuating compositions of garnets in the skarn suggest multistage ore-fluid activities and the fluid evolution provide insights into the ore-forming process.

Algama Gold Ore Cluster (Aldan–Stanovoi Shield): Mineralogy, Formation Conditions, Sources of Ore Matter, and Age of Mineralization

Abstract —The paper presents results of research into the conditions of formation of ores in the poorly studied Algama ore cluster (Bodorono deposit and Dyvok ore occurrence) located at the junction of the Aldan Shield and the Stanovoi area. We have established that Bi and Se minerals (bismuthinite, lillianite, native bismuth, tellurobismuthite, tetradymite, hedleyite, pilsenite, and laitakarite) are present in the ores of the Bodorono deposit. Two successive productive stages of mineral formation have been distinguished: Au–polymetallic and Au–Bi–Te. The corresponding minerals are products of the evolution of a hydrothermal system, during which a gradual decrease in fluid temperature (from 300 to 145 °C) and salinity (from 5 to 1.9 wt.% NaCl equiv.) took place. The fineness of native gold gradually increases from early (~840‰) to late (~940‰) stages and changes in passing from simple sulfides to sulfosalts. The evolution of the ore system is accompanied by a change in the composition of the vapor phase of fluid inclusions from CH4–CO2 to CO2 with an impurity of N2 and CH4. The results of 40Ar/39Ar dating of pre-ore metasomatites point to ore-forming processes at the Bodorono deposit ca. 150 ± 1.8 Ma. Analysis of the isotopic composition of lead in galena shows the leading role of the ancient crustal source of ore matter. The calculated isotopic composition of oxygen (δ18OH2O) in ore-bearing quartz varies from 1.0 to 7.3‰, which corresponds to an aqueous fluid of a mixed source. The Dyvok ore occurrence differs from the Bodorono deposit in the mineral composition of ores and the physicochemical parameters of ore formation. Four mineral stages have been established within the ore occurrence: gold–arsenopyrite–pyrite–quartz, pyrite–chalcopyrite–sphalerite, quartz–boulangerite, and telluride. The telluride stage is represented by hessite, altaite, volynskite, merenskyite, melonite, and rucklidgeite. Gold-bearing mineralization formed from a fluid of medium salinity (0.9–9.2 wt.% NaCl equiv.) with a predominance of CO2 and an impurity of CH4 in the vapor phase at moderate temperatures (310–360 °C). The calculated values of δ34S and δ18O varied from 2.2 to 3.0‰ and from 0.6 to 12.0‰, respectively. The 40Ar/39Ar age of gold mineralization is 124.0 ± 1.5 Ma, which corresponds to the stage of tectonomagmatic activity in the Aldan Shield.

Shoshonitic pluton and an associated base-metal deposit in the Tarom–Hashtjin metallogenic zone, NW Iran: implication for tectono-magmatic evolution and metallogenic considerations

Petrological, geochemical and zircon U–Pb and Hf isotope studies were carried out on the Hajseyran pluton, along the Tarom–Hashtjin metallogenic zone, as part of the Alborz–Azerbaijan magmatic belt, which is located in the central part of the Alpine–Himalayan orogenic belt. The Hajseyran pluton, which is made up of syenite, monzonite, quartz-monzonite and granite, yields a zircon U–Pb age of 39.20 ± 0.56 Ma (for a quartz-monzonite sample) with shoshonitic features. Enrichment in large ion lithophile elements, depletion in high-field-strength elements and negative anomalies of Nb, Ta and Ti in Chomalou samples suggest a subduction-related arc magmatic setting. The Chomalou epithermal Pb–Zn–Cu deposit occurs close to the Hajseyran pluton with the quartz–base metal sulfide veins predominantly hosted by Eocene volcanic rocks. The ore-forming processes include chalcopyrite and sphalerite–galena formation, with the latter the major mineralisation in this region. Microthermometric results from the liquid–vapour fluid-inclusion assemblages show that the homogenisation temperatures in the first-stage quartz range from 230 to 310 °C, in the sphalerite and second-stage quartz from 161 to 293 °C and in the late-stage quartz from 175 to 262 °C. The salinities of fluids decrease from early-stage quartz to late-stage quartz. The calculated stable isotope values of ore–fluid components (δ18Owater = +3.28‰ to −3.68‰; δ34SH2S = +5.5‰ to +7.8‰) show that magmatic fluids have played a significant role during mineralisation. The Chomalou deposit is interpreted as a volcanic-hosted intermediate-sulfidation type of epithermal mineralisation formed through shallow-level emplacement of the Hajseyran pluton, which formed via partial melting of phlogopite–amphibole-bearing sub-continental lithospheric mantle. The Neotethyan slab roll-back and concomitant asthenospheric upwelling led to the decompression melting of metasomatised sub-continental lithospheric mantle and the formation of several K-rich plutons and associated epithermal deposits in the Alborz–Azerbaijan magmatic belt.