Metallogenic Processes Research Articles

In-situ trace element and sulfur isotope compositions of Multi-Stage sulfides in the Buzhu orogenic gold (Antimony) Deposit, southern Tibet: Implications for the metallogenic process

The Buzhu gold (antimony) deposit, located within the gold-antimony polymetallic belt of southern Tibet in the Tethys Himalayan belt, is a recently discovered medium-sized gold polymetallic deposit. The orebody consists primarily of gold-bearing quartz sulfide veins. Currently, the metallogenic mechanism of the deposit remains largely unexplored. Utilizing results from the mineralogical observation of principal ore minerals, LA-ICP-MS trace element analysis, and in-situ sulfur isotope analysis of pyrite, investigations reconstruct the evolution process of pyrite-arsenopyrite and constrain the metallogenic process of the Buzhu gold (antimony) deposit. It has been discovered that the gold-bearing sulfides in the Buzhu gold (antimony) deposit are primarily pyrite and arsenopyrite. Pyrite can be categorized into six generations: framboidal pyrite Py0, dissolution cavity pyrite Py1a (some exhibiting oscillatory zoning), disseminated pyrite Py1b, euhedral pyrite Py2a, irregular pyrite Py2b, and Py3. Arsenopyrite, on the other hand, can be classified into three generations: early arsenopyrite Apy0, disseminated arsenopyrite Apy1, and euhedral Apy2. The analysis of δ34S in pyrite throughout the ore-forming process indicates sulfur derives from multiple sources, with Py0 indicative of a biological sulfur source. The evolutionary process of pyrite, supported by trace element analysis, suggests that the remaining pyrite derives from sedimentary fluid transformation, with sulfur primarily sourced from metamorphic sediments. Elemental spectrum analysis and BSE observations indicate that the Buzhu gold (antimony) deposit primarily undergoes two distinct stages of mineralization: the initial stage is the gold-antimony mineralization phase, during which antimony precipitates in the fissures of Py1a as stibnite, while gold is primarily found in Py1-Apy1 as invisible gold, with the highest gold content in Apy1 (28.20–60.13 ppm, average 40.51 ppm). The subsequent stage of mineralization is characterized by gold mineralization, with gold primarily residing in the internal fissures of Apy2 as micron silver gold ore. The evolution characteristics of gold-bearing sulphide and the solubility curve of Au indicate that gold mainly derives from gold-bearing unsaturated fluid, rather than from early framboidal pyrite Py0 (0.52–1.06 ppm, average 0.79 ppm). Only a minor fraction of gold (originating from early sediments) in Py0 was liberated and integrated into subsequent pyrites. The content of antimony in Py0 significantly reduces after dissolution, suggesting that antimony predominantly originates from Py0 (206–440 ppm, average 335 ppm). Throughout the genesis of the Buzhu gold (antimony) deposit, the gold constituent underwent a sequence of pre-enrichment, re-enrichment, liberation, and precipitation. The processes of coupled dissolution-reprecipitation (CDR) and boiling fostered the reactivation and reenrichment of gold. The recrystallization into cubic pyrite led to the expulsion of gold from the pyrite lattice, and fluid oxidation resulting from multistage fluid boiling, coupled with a decrease in sulfur fugacity, led to gold precipitation.

Genesis of the Longfengchang polymetallic sulfide deposit in the southwest Fujian depression, southeast China, with a comparative study of the “Makeng-Type” iron deposit

The Southwest Fujian Depression Belt is a prominent metallogenic zone for skarn-type iron polymetallic deposits in China, with the Longfengchang (LFC) sulfur polymetallic deposit representing a medium-scale, sulfide-dominated deposit in this region. This study conducted a detailed analysis of the LFC deposit, focusing on its mineralogy, mineral composition, and in-situ sulfur isotopes, alongside a comparative study with the “Makeng-type” deposit. The study aims to elucidate the genesis of the LFC deposit, its relationship with the “Makeng-type” deposit, and the factors underlying differences in dominant economic minerals and resource scale. The LFC deposit is hosted within the skarn above the fault contact zone between the Lindi Formation sandstone and the Chuanshan–Qixia Formation carbonate, with mineralization stages classified as skarn-magnetite, quartz-sulfide, and carbonate. LFC garnets are primarily composed of CaO, TFeO, and SiO2, with minor Al2O3 and trace amounts of MgO and MnO, classifying them as distal exoskarn andradite. The presence of Mn3+ substituting for Fe3+ in garnet suggests that the ore-forming fluid during the garnet skarn stage was likely oxidizing and weakly acidic. LFC pyrites exhibit Co/Ni ratios primarily ranging from 1 to 10, decreasing from Py1 to Py3. In-situ sulfur isotope δ34S values range from −1.48 to 3.51 ‰, centering around 0 ‰, and increase from Py1 to Py3, suggesting a magmatic-hydrothermal origin and a cooling metallogenic process. Thus, the LFC deposit is classified as a magmatic-hydrothermal skarn-type deposit, consistent with the genesis of “Makeng-type” deposits. The absence of the Jinshe Formation, and mantle-derived magma contribution, and less developed “Si-Ca” interface may explain the smaller scale and different mineralization type in the LFC deposit compared to the “Makeng-type” deposit. The key prospecting area for large iron-sulfur polymetallic deposits in the Southwest Fujian Depression Belt should feature a nappe structural window, well-preserved Jinshe Formation, developed “Si-Ca” interface, Yanshanian high-K calc-alkaline to shoshonitic intrusions, and coeval mantle-derived magma.

Pyrite in-situ S–Fe isotope constraints on the ore-forming sources and mineralization processes of gold and polymetallic deposits in the Liaodong Peninsula, North China Craton

Plenty of gold and polymetallic deposits are widespread in the eastern part of the North China Craton. They are associated with mafic to felsic dikes and hosted by Precambrian basement or Mesozoic granitoids. However, the sources of gold and other metals in the ore-forming fluids remain controversial. Here we present in-situ S and Fe isotopes and trace element contents of pyrites from various gold and Pb–Zn–(Ag) deposits in the Liaodong Peninsula, China. Pyrites from quartz vein-type gold deposit hosted by Mesozoic granites in the Wulong deposit have relatively homogeneous magmatic-like S isotopes (δ34S values of 0.9 ‰ to 2.5 ‰) and Co/Ni ratios, indicating derivation of sulfur and, by inference, of ore fluids/materials most likely from Mesozoic magmas. In contrast, pyrites from gold and Pb–Zn–(Ag) deposits in the Qingchengzi orefield hosted by Precambrian basement have high and variable δ34S values (9.7 ‰ to 12.7 ‰ for the Wandigou altered rock-type gold deposit and 4.7 ‰ to 8.5 ‰ for the Xiquegou Pb–Zn deposit and the Zhenzigou Pb–Zn–Ag deposit), identical to those of host rocks, indicating the important contributions of gold and other metals from wall rocks to the ore deposits. Pyrites from the various deposits have variable δ56Fe values of 0.08 ‰ to 0.63 ‰ for the Pb–Zn–Ag deposit, –0.58 ‰ to 1.23 ‰ for the altered rock-type gold deposit, and –0.68 ‰ to 0.77 ‰ for the quartz vein-type gold deposit, indicating distinct mineralization processes. Rapid precipitation of pyrites (with negative δ56Fe values) in the alteration rock and subsequent deposition of pyrites (with positive δ56Fe values) from the residual fluids in an Fe-open hydrothermal system during intensive ore-forming fluid-wall rock interaction account for the Pb–Zn–Ag mineralization, while weak fluid-rock interaction and pyrites precipitation from a Fe-closed hydrothermal system contribute to gold mineralization. Our observations provide a robust S–Fe isotope evidence for the contribution of various sources and metallogenic processes for distinct gold and polymetallic deposits.

Hidden deposit exploration using the tectono-geochemistry method in the western Xicheng ore field, China

Geochemical anomalies involve complex geological and geochemical processes. Integrating metallogenic processes into the interpretation of geochemical data can promote mineral exploration. In this study, 3080 subsamples were collected from the western Xicheng ore field and combined into 1312 composite samples using the tectono-geochemistry method. Nineteen elements were analyzed for each composite sample. Factor analysis based on the CLR-transformed data yielded four factors, including the Ag–Sb–Hg–Pb–Au–(B–Ba) association of F1, Zn–Cd–Pb association of F2, Bi–Sn–(Au–As) association of F3, and W–Sn–(Cu) association of F4. Thresholds of each factor were obtained using the concentration–number (C–N) fractal model. Six targets were delineated based on the factor anomaly maps, and one Pb–Zn and two Au deposits were discovered in Targets I and II, respectively. These discoveries and the good spatial correspondence between known deposits and anomalies provide compelling evidence for the effectiveness of the tectono-geochemistry method in the study area. More importantly, a model for the genetic relationship between geochemical anomalies and metallogenesis was constructed. The late tectonic-magmatic-hydrothermal transformation dominated the geochemical pattern in the study area. The degree of interaction between the Au-rich magmatic-hydrothermal fluids and SEDEX-style Pb–Zn mineralizations yielded leakage halos with various elemental assemblages. In addition, W–Sn anomalies may serve as auxiliary exploration indicators for Au mineralizations.

Manganese mineralization constrained by redox conditions in the Cryogenian Nanhua Basin, South China and its implications for nitrogen and carbon cycling

The Nanhua Basin of South China recorded complete Cryogenian stratigraphic sequence from the Sturtian Glaciation (~717–660 Ma) to the Marinoan Glaciation (~654–635 Ma). The interglacial Datangpo Fm in the Nanhua Basin is divided into two members, and the first member consists of the Mn-carbonate unit and the overlying black shale unit, containing a series of large and superlarge manganese deposits. The metallogenic process of manganese deposits is not clear, and the Mn-carbonates formed through the precursor of Mn-oxide/oxyhydroxide reduction or directly precipitated from an anoxic water column. Moreover, the redox conditions in the deep Nanhua Basin during the precipitation of manganese deposits are also controversial. In this study, the high-resolution nitrogen contents (TN), isotope compositions, carbon isotope compositions of organic and inorganic matter from the first member of the Datangpo Fm are analyzed. The δ15N values of the Mn-carbonate unit (+1.53‰ to +5.26‰, mean +3.36‰) are higher than those of the overlying black shale unit (−3.74‰ to +3.54‰, mean +0.89‰). The Mn contents show a negative relationship with TN but a positive relationship with δ15N in the Mn-carbonate unit, implying that the formation of Mn-carbonates is related to redox variations. The relatively higher δ15N values in the Mn-carbonate unit indicated oxic conditions, and NH4+can be released and partially oxidized during the mineralization of organic matter, resulting in the residual 15N-enriched NH4+ being transferred into clay minerals. Meanwhile, the lower δ15N values in the black shale unit indicated anoxic conditions, which recorded primary N isotope signals. The Mn-carbonate unit is characterized by negative δ13Ccarb values (−11.17‰ to −5.22‰, mean −8.30‰), which show a positive relationship with δ13Corg, but a negative relationship with Mn contents, implying that the negative δ13Ccarb excursions were related to the organic matter degradation during Mn-carbonate formation. The findings of this study indicated that the metallogenesis of manganese deposits in the Cryogenian Nanhua Basin was constrained mainly by the oxic interval in the deep basin. The nitrogen and carbon cycling process can provide new insights into geochemical cycling after the Sturtian Glaciation.

Copper isotope fractionation during magmatic evolution in a convergent tectonic setting: Constraints from sulfide Cu-S isotopes and whole-rock PGE of the Xiarihamu Ni-Cu sulfide deposit

Nickel and cobalt are potentially critical metals in many countries because of their great significance for national security and economic development, and they are mainly derived from magmatic Ni-Cu‑platinum-group element (PGE) sulfide ore deposits hosted in mafic-ultramafic intrusions. Although Cu isotopes have been used to trace the metallogenic processes in Ni-Cu-(PGE) sulfide deposits, the Cu isotopic fractionation mechanism during magma generation and evolution in convergent tectonic settings is still debated. The Xiarihamu magmatic NiCu sulfide deposit, located in the East Kunlun orogenic belt, northern Tibetan Plateau, is the world's largest known magmatic NiCu sulfide deposit in an orogenic setting. Here we report the whole-rock element and sulfide CuS isotope compositions of samples from the Xiarihamu deposit. The δ65Cu and δ34S values of the sulfide grains from the massive, heavily disseminated, and disseminated sulfide ores range from 0.19 ‰ to 0.79 ‰, and from 4.2 ‰ to 9.4 ‰, respectively. Most of the samples have δ65Cu values higher than normal mantle values, except for two samples with δ65Cu values within the mantle composition range. The whole rock S/Se ratios range from 3200 to 22,500, and the Cu/Pd ratios range from 330 to 820,000, which are also mainly higher than the corresponding mantle values. Modeling calculations of the δ65Cu, S/Se, and Cu/Pd values reveal that the sulfide liquid to silicate melt mass ratio (R factor) is not the main reason for the observed δ65Cu variations in the Xiarihamu deposit. The variations in the Cu/Pd ratios of the whole-rock samples, and the sulfide δ65Cu and δ34S values with depth indicate that sulfide segregation and crustal contamination can reasonably jointly produce the Cu isotope variations in the Xiarihamu deposit, and the variations of samples from different parts of the deposit are caused by variations in these controlling factors. Therefore, Cu isotope fractionation in convergent tectonic settings is mainly caused by magma evolution. The complicated controlling factors of the Cu and S isotopes indicate that the correlation between δ65Cu and δ34S values may not be helpful in evaluating the metallogenic potential of Ni-Cu-(PGE) sulfide deposits. Cu isotopes can be used to ascertain the migration path of magmas.

Deterministic modelling for driving factors of mineralization in Shanggong gold deposit (China)

1. Three-dimensional (3D) deterministic modeling is crucial for analyzing the controlling factors of mineralization. The Shanggong gold deposit, situated in the southern margin of North China Craton, represents a magmatic hydrothermal deposit. To construct the 3D deterministic model, boreholes, cross-sections, and geochemistry assays are integrated based on the geological characteristics of the Shanggong gold deposit. This paper summarizes its contents as follows: 1) GoCAD software can be utilized to build a 3D geology model encompassing faults and ore bodies; 2) Geostatistics and discrete smooth interpolation (DSI) can be employed to create a 3D attribute model involving grade and Au/Pb ratio. The findings reveal that: 1) High-value zones are associated with NE and NNE trending cross faults; 2) Deterministic modeling, such as grade model and Au/Pb ratio model, effectively elucidates the metallogenic process mechanism.

Characteristics and Deep Mineralization Prediction of the Langmuri Copper–Nickel Sulfide Deposit in the Eastern Kunlun Orogenic Belt, China

The discovery of a Cu-Ni sulfide deposit in Langmuri of the Eastern Kunlun Orogenic Belt holds significant geological implications. This study, based on the examination of the metallogenic geological body, metallogenic structure, and metallogenic process characteristics, suggests that the deposit is a magmatic Cu-Ni sulfide deposit formed in the collision of orogenic and post-extension processes of the Late Ordovician. The early mineralization of the deposit was primarily derived from the differentiation of sulfides in the mafic–ultramafic rock (450–439 Ma) of the Late Ordovician, while the late-stage mineralization underwent significant superimposed modification by the magmatic–hydrothermal activity of crustal-contaminated biotite granite (415 Ma). In addition, this article analyzes the measurements of the geochemical studies of sediments, and the magnetic and gravity measurements carried out in the area, focusing on the geochemical and geophysical anomaly characteristics in the study area, and selects favorable exploration areas, which have been confirmed to have multiple mineral bodies. By integrating comprehensive gravity, magnetic, induced polarization, and audio-frequency magnetotelluric profile measurements, this study analyzes delineated mineralized zones and the deep extensions of surface mineral bodies to assess deep mineralization potential and identify deep ore-finding targets. It suggests that diverse and scattered mafic–ultramafic complexes in the Langmuri mining area have a large-scale distribution of ore-bearing rocks in the deep. Through the analysis and inverse of the geophysical data, a deep mineralization predictive model was established in the basic–ultrabasic rock mass. The study presents prospects for the delineation of the deep-seated mineralization in the Langmuri deposit.

Cobalt enrichment and metallogenic mechanism of the Galinge skarn iron deposit in the Eastern Kunlun metallogenic belt, western China

Skarn iron deposits, as representative examples of Co-rich magmatic-hydrothermal deposits, are attracting increasing attention due to rising cobalt demand worldwide. However, the specific Co enrichment mechanisms and metallogenic processes in skarn deposits remain elusive. This study presents high-precision in situ U–Pb geochronological data for garnet from skarns, major and trace element analyses of sulfarsenides and sulfides, and X-ray mapping in the Galinge deposit. The Galinge skarn iron deposit formed at 237.1 ± 0.3 Ma (garnet U–Pb dating), during which time the region was in the post-collisional stage. During this period, the crust was thickened, causing delamination of the lithospheric mantle, which further led to the upwelling of asthenospheric materials and partial melting of the lower crust. As the resultant mixed magma ascended, it reacted with carbonate strata to form skarn deposits. Cobalt shows two occurrence modes in the Galinge deposit: independent cobalt minerals such as skutterudite and cobaltite, and isomorphic substitution of Co with other metals in ore minerals (arsenopyrite, pyrrhotite, sphalerite, pyrite, magnetite, and chalcopyrite). Our results revealed that arsenopyrite is the most cobalt-enriched ore mineral in the Galinge deposit, with an average Co content of 34,077 ppm. Other minerals generally contain insignificant Co contents of less than 500 ppm, including sphalerite (471 ppm) > pyrite (194 ppm) > pyrrhotite (145 ppm) > alabandite (∼100 ppm) > magnetite (7 ppm) > chalcopyrite (2 ppm). In arsenopyrite, cobalt and nickel replace iron in accordance with the inverse correlation between the concentrations of cobalt and nickel (wt%) and that of iron. In pyrite and chalcopyrite, a portion of the isomorphic cobalt substitutes for Fe or Cu. The weak correlation between Co (ppm) and Cu or Fe (wt%) indicates that only isomorphic cobalt is carried out by substituting Fe or Cu. The negative correlation between Co + Fe and Zn or Mn suggests that cobalt and iron replace Zn or Mn in sphalerite and alabandite. No evidence of element substitution was observed in pyrrhotite. Our study highlights that during the early mineralization stage, cobalt in the magmatic-hydrothermal fluids migrated in the form of CoCl42-. Subsequently, under the influence of an increase in pH, CoCl42- reacts with H3AsO30 to form CoAs3 (skutterudite). With the continuous precipitation of arsenides and sulfarsenides, the As/S (reduced) ratio decreased, leading to the CoAs3 (skutterudite) changing to CoAsS (cobaltite).

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

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

Lithospheric influence on metallogenesis in the East Kunlun Orogen: Insights from isotopic and geochemical mapping

This study examines the influence of lithospheric architecture on regional metallogenesis, using geological and geochemical data from the East Kunlun Orogen (EKO). We explore how specific lithospheric components and structures affect the formation of orogenic gold and various polymetallic mineral deposits. The study reveals that metallogenic processes in the EKO initially targeted sources within the metasomatized-enriched subcontinental lithospheric mantle (SCLM) and reactivated lower crust during the Paleozoic era.Lithospheric architecture, shaped by crust-mantle interactions occurred in the Early Mesozoic led to variations in the sizes of orogenic gold deposits between the eastern and western EKO. In the west, smaller gold deposits are linked to moderate mantle underplating and slightly enriched SCLM. Gold-bearing fluids released by mantle devolatilization mixed with metamorphism fluid in collision stage, contributing to the formation of these deposits. In contrast, the eastern EKO hosts larger gold deposits, resulting from significantly enriched SCLM. Extensive lithospheric thinning process in post-collision stage lead to the mixing of gold-bearing mantle-derived and magmatic-hydrothermal fluids. Fertilized magma differentiation potentially increases gold concentrations in this region.Additionally, Triassic-era melting of the lower crust led to diverse mineralization types in porphyry-skarn-epithermal deposit systems. Key findings include the role of juvenile fertile lower crustal materials in forming copper polymetallic deposits, the importance of metamorphic basement reworking for molybdenum and tungsten deposits, and the influence of lead and zinc leaching from old sedimentary rocks on the formation of silver‑lead‑zinc deposits. This study broadens our understanding of the geological factors influencing metallogenesis and provides a framework for future exploration and research in regions with similar lithospheric architectures.

Genesis of the Baiyun Gold Deposit in Northeast Hubei Province, China: Insights from In Situ Trace Elements and S-Fe Isotopes of Sulfide

The Baiyun gold deposit is a medium-sized deposit in northeastern Hubei around the southern margin of the Tongbai-Dabie metallogenic belt. However, its genesis has not been determined. The metallogenic process of the Baiyun gold deposit can be divided into three stages: quartz + feldspar, quartz + native gold + electrum + polymetallic sulfides, and quartz + pyrite + calcite + iron dolomite + illite. In this study, LA-ICP-MS was used for in situ trace element and isotope analyses in the main and late ore stage hydrothermal sulfides to evaluate the genesis and evolution of ore-forming fluids. Gold is positively correlated with Ag, Cu, Pb, Zn, and Te and the Co/Ni ratio is greater than 1. The S isotope values of Py1 and Py2 are −0.23–3.04‰ and 1.27–6.09‰, respectively. As mineralization progressed, S isotope values increased. In situ S isotope values of the two types of galena symbiotic with pyrite in the main metallogenic stage are 2.97–3.47‰. In situ Fe isotopic values of pyrite are −0.05–0.82‰; values in the two stages are similar without significant fractionation. We inferred that the Baiyun gold deposit formed via magmatic mineralization related to the subduction of the Pacific Plate during the Yanshanian.

Mineralogical and geochemical studies on the Qianjiadian deposit, Songliao Basin, NE China: Insights into multiple metallogenic processes in the sandstone-type uranium deposit

Sandstone-type uranium deposits usually exhibit a range of metallogenic mechanisms. To gain insights into the nature, genetic relationships, and mineralization mechanisms of uranium and associated minerals, representative samples from the Qianjiadian uranium deposit in the Songliao Basin were investigated through detailed mineralogical, elemental geochemical, and in-situ sulfur isotopic analyses. The results provided invaluable information on the various stages of uranium mineralization. During the early diagenetic stage, uranium mineralization occurred in the local organic matter (OM)-enriched sites (TOC = 0.70 ∼ 2.05 %) in the Yaojia Formation. The paragenetic relationship revealed that uranium was firstly precipitated as pitchblende along with the biogenetic framboidal pyrite (δ34S values ranging from −44.1 ‰ to −1.8 ‰) through bacterial sulfate reduction (BSR) process. Simultaneously, organic acids generated through biodegradation of OM caused an acidic fluid condition and subsequent dissolution of feldspars, resulting in the production of kaolinite intergrowth with pitchblende. During the late diagenetic stage, the ore-forming fluid turned alkaline, leading to the conversion of pitchblende into coffinite, with elevated elements such as Si, P, Ca, Zr, and Y in the altered phase. In addition to the contribution of OM and sulfate-reducing bacteria (SRB), pyrite was confirmed as a reducing agent manifested by its replacement by uranium minerals and the Fe-oxide produced along the boundaries. The diagenetic uranium enrichment is mainly hosted in the gray sandstone that experienced reduced alteration of deep hydrocarbons, resulting in pervasive kaolinization and promotion of reduction potential. A significant amount of uranium was fixed by adsorption, as evidenced by the rare observation of independent uranium minerals, but the higher whole-rock U content (U = 133.5 ∼ 394.0 ppm) and the detection of U signal in micro-XRF analysis and EPMA data (U content in representative kaolinite ranges from bld to 0.74 wt%). Overall, the formation of sandstone-type uranium deposits is a long-term and multi-stage process. The early enriched uranium during synsedimentary-diagenetic stages is non-negligible and can even result in an appreciable amount of mineralization, further serving as an important source for the subsequent secondary enrichment.

Apatite as a Record of Magmatic–Hydrothermal Evolution and Metallogenic Processes: The Case of the Hongshan Porphyry–Skarn Cu–Mo Deposit, SW China

Apatite, as a common accessory mineral found in magmatic–hydrothermal deposits, effectively yields geochemical insights that facilitate our understanding of the mineralization process. In this research, multiple generations of magmatic and hydrothermal apatite were observed in the Hongshan porphyry–skarn Cu–Mo deposit in the Yidun Terrane in SW China. The geochemical compositions of the apatite were studied using in situ laser ablation–inductively coupled plasma mass spectrometry and an electron probe microanalysis to understand the magmatic–hydrothermal processes leading to ore formation. The apatite (Ap1a) occurs as subhedral to euhedral inclusions hosted in the phenocrysts of the granite porphyry. The Ap1b occurs later than Ap1a in a fine-grained matrix that intersects the earlier phenocrysts. Increases in F/Cl, F/OH, and F/S and decreases in ΣREE and (La/Yb)N from Ap1a to Ap1b suggest the exsolution of a volatile-rich phase from the magma. The skarn hosts three types of hydrothermal apatite (Ap2a, Ap2b, and Ap3), marking the prograde, retrograde, and quartz–sulfide stages of mineralization, respectively. The elemental behaviors of hydrothermal apatite, including the changes in Cl, Eu, As, and REE, were utilized to reflect evolutions in salinity, pH, oxygen fugacity, and fluid compositions. The composition of Ap2a, which occurs as inclusions within garnet, indicates the presence of an early acidic magmatic fluid with high salinity and oxygen fugacity at the prograde skarn stage. The composition of Ap2b, formed by the coupled dissolution-reprecipitation of Ap2a, indicates the presence of a retrograde fluid that is characterized by lower salinity, higher pH, and a significant decrease in oxygen fugacity compared to the prograde fluid. The Ap3 coexists with quartz and sulfide minerals. Based on studies of Ap3, the fluids in the quartz–sulfide stage exhibit relatively reducing conditions, thereby accelerating the precipitation of copper and iron sulfides. This research highlights the potential of apatite geochemistry for tracing magmatic–hydrothermal evolution processes and identifying mineral exploration targets.

Characterization of deep ore-forming fluid in the Zhaoxian gold deposit within the Jiaodong gold province: insights from quartz vein fluid inclusion, in-situ trace element analysis, and S isotopic composition in pyrite

The Jiaodong gold province, situated in the southeastern margin of the North China Craton, is globally renowned for its substantial gold reserves exceeding 5,000 tonnes. The Zhaoxian gold deposit is part of the significant Jiaojia gold belt within the Jiaodong gold province. Fieldwork has identified four distinct stages of ore formation in this study: an early barren quartz vein stage (Stage 1) containing fine-grained pyrite; a gold-bearing stage (Stage 2) consisting of quartz, pyrite, and native gold; a polymetallic sulfide-rich stage (Stage 3) comprising quartz, polymetallic sulfides, and native gold; and a late-stage (Stage 4) primarily composed of quartz and calcite with minimal pyrite content. We conducted fluid inclusion analysis using microthermometry and Raman spectroscopy techniques to examine the fluid characteristics. In-situ analysis of trace elements in pyrite was performed to investigate the fluid composition and evolution. Additionally, we determined the sulfur isotope composition in pyrite to analyze the source of sulfur. Our findings indicate that the ore-forming fluid in the Zhaoxian gold deposit belongs to a medium-to-low-salinity H₂O-NaCl-CO₂-CH₄ system. Fluctuations in Au and As concentrations observed along with oscillating zones and sulfide inclusions during Stage 2 suggest potential fluid boiling processes occurring during mineralization. High concentrations of Ag, Cu, Zn, Cd, In, Pb, and Bi without oscillating zones during Stage 3 imply precipitation of polymetallic sulfides under stable fluid conditions. The δ34S values observed in the Zhaoxian gold deposit are slightly higher than those found in granitoids from other areas within Jiaodong but similar to those seen in other deposits within the Jiaojia gold belt region. In conclusion, magmatic-hydrothermal ore-forming fluids were involved along with significant fluid-rock interaction during metallogenic processes of the Zhaoxian gold deposit.

Genesis and Prospecting Potential of the Da’anhe Skarn Au Deposit in the Central of the Lesser Xing’an Range, NE China: Evidence from Skarn Mineralogy, Fluid Inclusions and H-O Isotopes

Skarn Au deposits exist in the circum-pacific metallogenic belt. Interestingly, the Da’anhe Au deposit is the only independent skarn gold deposit in the Lesser Xing’an Range. To determine the metallogenic mechanism and prospecting potential of the Da’anhe deposit, we performed skarn mineralogy, fluid inclusion (FI) and H-O isotope analyses. The results show the following: (1) The Da’anhe deposit is a calcareous reduced skarn Au deposit that formed between an Early Jurassic gabbroic diorite and the Permian Tumenling Formation marble. Its metallogenic process includes five stages: the early skarn stage (Stage I1), late skarn stage (Stage I2), early quartz-sulfide stage (Stage II1), late quartz-sulfide stage (Stage II2) and quartz-carbonate stage (Stage II3). Gold precipitated in Stage II1 and Stage II2. (2) The initial ore-forming fluid was derived from magmatic water and featured a high temperature and intermediate to high salinity. After boiling and mixing, the fluid eventually changed to a low-temperature and low-salinity reducing fluid dominated by meteoric water. (3) The formation depth of the Au orebodies was 2.27–3.11 km, and the orebodies were later lifted to the surface (<500 m). The potential for finding skarn Au deposits in the study area is limited. (4) The distinctive nature of the ore-related magma (i.e., source, reducing conditions and high water content) was key to the formation of the Da’anhe skarn gold deposit.

Formation mechanism of skarn iron deposit: Evidence from Fe-rich enclave in porphyritic monzonite

Skarn deposits constitute a significant reservoir of high-grade iron (Fe) ores in China, yet the metallogenic processes remain debated. This study focuses on the iron-rich enclaves embedded within porphyritic monzonite, aiming to unravel the metallogenic mechanisms underlying skarn Fe deposits. The host porphyritic monzonite intrudes into porphyritic diorite. Within the iron-rich enclaves three types of magnetite (Mt) are discerned. Mt-I, ranging from 50 to 100 μm in size, predominantly occupies the core of the enclave, enshrouding diopside at its center. Mt-II, with a size range of 20–40 μm, is primarily distributed in the middle of the enclave. The smallest variant, Mt-III, measuring approximately 1 μm, typically manifests at the peripheries of Mt-I and Mt-II. Comparative analysis reveals that Mt-II exhibits higher total rare earth elements (REE), Sr, Ca, alongside lower TiO2 content in contrast to Mt-I. Notably, Mt-II displays a characteristic W-type tetrad effect of REE, indicative of crystallization in a fluid-saturated environment. In contrast, Mt-III, characterized by microcrystalline magnetite, is inferred to have formed under conditions of rapid cooling.Our interpretation posits a two-stage genesis for these enclaves. The initial stage involves the formation of iron-rich magma within a magma chamber situated at depths of 8–10 km, during which Mt-I crystallizes. Subsequently, the second stage unfolds as mantle-derived fluids infiltrate the magma chamber, leading to the formation of Mt-II. The fluid overpressure within the magma chamber triggers a swift ascent of iron-bearing melt-fluid, localized at depths ranging from 1.5 to 2.6 km, resulting in the crystallization of Mt-III. Our results provide valuable insights into the metallogenic processes governing skarn Fe deposits, and the complex geological evolution of these deposits.

Multistage ore formation in the world’s largest REE-Nb-Fe deposit of Bayan Obo, North China Craton: New insights and implications

The Bayan Obo deposit in the northern margin of the North China Craton is a world-class REE-Nb-Fe deposit, the complex mineralization history of which remains unresolved. In this study, we employ SEM and Image J software combined with mineralogical data obtained by in-situ EDS and EPMA analyses to analyze petrographic images rapidly and to investigate the mineralogical assemblages of each type of rocks/ores from the Main Orebody in an attempt to understand the mineralization process and ore-formation mechanism of this deposit. We identify three metallogenic periods and six ore-forming stages based on the field occurrence of ores combined with published geochronological data. The Mesoproterozoic magmatic event comprises two stages, which were distinguished as the coarse-grained dolomite stage (stage 1) and the fine-grained dolomite stage (stage 2). Three stages were recognized in the Mesoproterozoic shear deformation-hydrothermal mineralization period, including the disseminated mineralization stage (stage 3), the banded mineralization stage (stage 4) and the massive mineralization stage (stage 5). The Paleozoic hydrothermal period involved the vein mineralization stage (stage 6). Based on detailed studies of the mineral assemblages and paragenesis, as well as the complex textures of the ores, we propose a model of the multi-stage metallogenic process as follows: (1) The REE minerals and rutile occurring as inclusions wrapped within magmatic idiomorphic dolomite grains indicate that the REE-Nb mineralization might have started during the magmatic period. (2) The Nb minerals, such as fergusonite and ilmenorutile, intergrown with REE minerals, such as monazite and bastnäsite, indicate that the Nb and REE were transported by the same ore-forming magma or fluid and precipitated during the same metallogenic process. (3) From the fine-grained dolomite stage to the massive mineralization stage (from stage 2 to stage 5), the mineral assemblages become more complex, with a gradual increase in the degree of hydrothermal alteration suggesting that more hydrothermal fluid was involved in the ore mineralization. (4) The complex mineralogical assemblages and textural relationships indicate multiple formation mechanisms for the REE-Nb-Fe minerals. We propose that the unusually large volume of REE-Nb-Fe resources in the Bayan Obo deposit was the result of combined magma and hydrothermal fluid activities in the Mesoproterozoic (from stage 1 to stage 5), whereas the fluids only caused reactivation of the ore materials previously deposited without the addition of exogenous materials in the Paleozoic stage (stage 6).

Controls on the metal tenors of sulfide ores in the Jinchuan Ni–Cu–PGE sulfide deposit, NW China: Implications for the formation of distinct textural types of sulfide ores in magma conduits

The giant Jinchuan Ni–Cu–platinum-group element (PGE) sulfide deposit is a magma conduit system comprising four main intrusive units termed segments III, I, II-W, and II-E. The deposit comprises disseminated, net-textured, massive sulfide, and Cu-rich sulfide ores, with variations in metal tenors occurring among these different ore types and among the segments. Controls on these metal tenor variations must be linked to metallogenic processes, but this remains poorly constrained. To asses this, we systematically compared metal tenors (Ni, Cu, and the PGE) of ores from two perspectives — i) different ore types in each segment and ii) a single ore type across all four segments. Two major styles of metal tenor variations are documented. Style 1: Segments III and I have higher metal tenors than segments II-W and II-E for any given type of ore. Based on sulfide segregation models, this variation is interpreted to be related to variable degrees of early removal of sulfide liquid from the magmas that formed the four segments. Style 2: Disseminated ores in segments III and I have higher metal tenors than the other ore types, whereas all of the ore types in segments II-W and II-E have similar metal tenors. This is interpreted to be the result of the disseminated ores in segments III and I having formed from sulfide liquids that experienced higher R factors than the sulfide liquids that formed the other ore types, whereas all of the ores in segments II-W and II-E formed from sulfide liquids that experienced similar R factors. This difference suggests that the different ore types in segments III and I formed via early percolation and accumulation of sulfide liquids (the net-textured and massive ores) followed by the capture of disseminated, high R factor sulfides (the disseminated ore), whereas the different ore types in segments II-W and II-E likely formed by the physical percolation of originally disseminated sulfide liquid through the inter-crystal pore space. This study demonstrates that both sulfide ore texture and their metal-tenor variations are critical to characterizing ore-forming processes in dynamic magma conduits.

A lightweight convolutional neural network with end-to-end learning for three-dimensional mineral prospectivity modeling: A case study of the Sanhetun Area, Heilongjiang Province, Northeastern China

With the continuous exploitation of surface and shallow mineral resources, the global demand for concealed ore deposit exploration is increasing. However, concealed mineral prospectivity modeling (MPM) requires significant efforts, particularly in terms of methods and technologies for three-dimensional modeling. In this study, we propose a lightweight three-dimensional convolutional neural network (3D CNN) for MPM, which adopts the inception structure of GoogleNet and combines the idea of end-to-end learning. We replace the fully connected layer in the conventional CNN with deconvolution, which can greatly reduce the required parameters during the training process and accelerate the convergence. The proposed method overcomes the disadvantage that other shallow machine learning methods cannot extract spatial neighborhood information, while it can extract cross-correlations among geological factors and generates less parameters by a lightweight network when facing massive data. Additionally, compared with the patch-wise method commonly used in previous studies, we use the pixel-wise method for end-to-end learning, which not only overcomes the drawbacks of random sampling but also considers the influence of each voxel when calculating the loss function. The three-dimensional multi-source geoscientific characteristics obtained from the geophysical inversion and 3D geological models are not discretized in order to promote effective CNN training while facilitating the ore-controlling representation. Comparing the predicted results between the 3D weight of evidence (WofE) and our proposed 3D CNN method for MPM, our proposed method and WofE delineated 100% of the known mineralization in high-favorability areas with voxel numbers of 70% and 95%, respectively. A case study of a structure-controlled hydrothermal gold deposit in the Sanhetun area of Heilongjiang Province demonstrates that the proposed 3D CNN method performs better than WofE in terms of prediction effectiveness and efficiency and effectively reveals the correlation between mineralization and adjacent ore-controlling characteristics. Moreover, the proposed 3D CNN method can simulate non-linear metallogenic processes and mine hidden relationships to reveal complex ore-controlling characteristics. In conclusion, the proposed 3D CNN method can reduce the exploration effort in 3D MPM, thus greatly improving the efficiency of discovering concealed ore deposits.