Au Deposits Research Articles

Genetic link between the Kangjiawan gold deposit and late Jurassic granitic magmatism, South China: Constraints from in situ calcite U-Pb dating, sulfur isotope and trace elements of pyrite

(Meta)sedimentary rock-hosted gold (Au) deposits are globally important Au resources, of which ore genesis and mineral exploration have been attracted considerable attention. However, the precise dating of Au mineralization of those Au deposits remains challenging due to the absence of suitable dating minerals, thus impeding a comprehensive understanding of their genesis. The Kangjiawan Au deposit, situated in the southern Hunan Province, South China, is one of representative (meta)sedimentary rock-hosted Au deposits. Due to the lack of mineralization age, the genesis of Kangjiawan Au deposit is still a matter of debate. Here we report the data of in situ calcite U-Pb dating, in situ trace elements and sulfur isotope analysis of pyrite, to further constrain the mineralization timing and the sources of ore-forming materials of the Kangjiawan Au deposit. Based on the detailed field investigation and petrographic observations, three mineralization stages have been identified: pyrite (Py1) + quartz (Stage I), gold-bearing pyrite (Py2) + polymetallic sulfides + quartz + calcite (Stage II), and gold-bearing pyrite (Py3) + calcite (Stage III). The results of mineralogical and trace elemental analyses indicate that Au are mainly hosted in Py2 (average 14.19 ppm) and Py3 (average 4.80 ppm), primarily as the form of “invisible gold”. In situ LA-ICP-MS U-Pb dating on calcite intergrowth with the gold-bearing pyrite (Py3) yields a Tera-Wasserburg lower intercept age of 151.7 ± 5.4 Ma. This age is well consistent with the published zircon U-Pb age (158.3 ± 1.2 Ma) of the nearby Shuikoushan granodiorite, the molybdenite Re-Os age (157.8 ± 1.4 Ma) of the Laoyachao skarn Pb-Zn deposit in the Shuikoushan ore field, and the garnet U-Pb age (159.1 ± 1.9 Ma) of the skarn Pb-Zn ore in depth of the Kangjiawan Au deposit. These suggested that the Au mineralization of the Kangjiawan deposit is coeval with the emplacement of the Shuikoushan granodiorite and Laoyachao Pb-Zn mineralization, which are important parts of the large-scale Middle-Late Jurassic Cu-Au-Pb-Zn Metallogenic event of the Qin-Hang Metallogenic Belt, South China. In addition, trace elemental contents, such as relative low Co, Ni, and Se (i.e., <0.01 to 100 ppm), and a narrow δ34S range of −2.94 ‰ ∼ +3.03 ‰ of pyrite, indicate that ore-forming materials were mainly derived from granitic magmas. Integrated with previous studies, it is suggested that the Au mineralization in the Kangjiawan deposit is genetically associated with the late Jurassic granitic magmatism and is probably as the distal products of the deep-seated concealed granitic intrusions and related proximal skarn Pb-Zn ore.

New insights into the metallogenic genesis of the Xiadian Au deposit, Jiaodong Peninsula, Eastern China: Constraints from integrated rutile in-situ geochemical analysis and machine learning discrimination

The genesis of the Mesozoic Au deposit within the Jiaodong Peninsula is debated, with vague connections to magmatism, deep-source metamorphic devolatilization, or the involvement of mantle materials over the long term. This study presents an integrated investigation involving textures, in-situ geochemical analysis, and machine learning discrimination of the rutile of the Xiadian Au deposit at the northwest Jiaodong Peninsula, aiming to provide metallogenic insights. Rutile is distinguished into three types: Type 1 rutile formed in the pre-ore stage is fine-grained and replaces and coexists with relict titanite, with euhedral crystal and being homogeneous and dark BSE response. Type 2 and type 3 rutile exhibit subhedral and anhedral crystals and are associated with quartz-sericite-pyrite alteration. The former has low W contents and is dark BSE response, while the latter is brighter in BSE and exhibits pronounced fragmentation and erosion. Chemical trends indicate the elemental substitution mechanism within rutiles from early to later stages: (Nb, Ta)5+ + (V, Fe, Al, Cr)3+ ↔ 2Ti4+ to 2(V, Fe, Al, Cr)3+ + W6+ ↔ 3Ti4+. Elevated concentrations of trivalent iron ions due to elevated oxygen fugacity of the ore-forming fluids are more likely to have contributed to this change, rather than the increment of W content. This may induce the destabilization of gold-sulfur complexes and contribute to gold mineralization. Machine learning models, including Extreme Randomized Tree (ET), Support Vector Machine, and Naive Bayes classifiers, were trained using a collected dataset containing 6598 rutile geochemistry data. These models consistently classify Xiadian rutile as the hydrothermal origin and having affinity to orogenic Au deposit. The SHapley Additive exPlanation (SHAP) framework was used to explain the discrimination criteria for the best-performing ET models. It revealed that elements such as Y, Ta, Mo, Sn, W, and Nb significantly contribute to classifying Xiadian rutile as affiliated with orogenic Au deposit. Further mass balance calculations with titanite, the predominant precursor Ti-bearing mineral of rutile, suggest that the W content in rutile is controlled by ore-forming fluids, likely having a deep source. In contrast, elements like Y, Mo, Cr, Nb, and Ta are influenced by leaching or re-equilibration with host rocks. Collectively, deep geodynamic processes likely provide major Au mineralization-related materials and fluids, while elemental leaching or re-equilibration with shallow crust rocks near fluid pathways influences the geochemical characteristics of the Xiadian gold system.

Magnetite geochemistry of giant alkalic-type epithermal gold deposits: Insights into the magmatic evolution of mineralized alkaline systems

Magnetite, a common mineral in alkaline magmatic complexes hosting giant epithermal Au deposits, is examined to obtain insights into the magmatic evolution and ore fertility of alkaline systems. Textural and compositional analyses of magmatic magnetite from Cripple Creek, USA, and Lihir Island, Papua New Guinea, reveal distinct magma sources and evolution paths. High HFSE concentrations and high La/Yb ratios in magnetite from Cripple Creek suggest relatively low-degree partial melting of lithospheric and asthenospheric mantle sources, whereas magnetite from Lihir Island indicates higher degree partial melting from subduction-modified mantle. Chromium concentrations track magma evolution, while V systematics record decreasing oxygen fugacity with magma differentiation. Sulfide saturation is observed at Cripple Creek, potentially affecting ore fertility. Magnetite itself bears the capability to scavenge Au from melts as indicated by Au concentrations of up to 40 ng/g. This might point towards elevated Au concentrations in the parental magmas, although experimental data on Au partitioning is lacking. Magnetite compositions overlap across different mineralization styles, challenging its reliability as an exploration tool but analysis of noble metals in magnetite and its inclusion cargo could offer promising alternative approaches. Overall, magnetite serves as a valuable tool for investigating ore deposit petrogenesis and holds potential for exploration.

Origin of the Dongga Au deposit in the giant Xiongcun porphyry Cu–Au district, Tibet, China: Constraints from multiple isotopes (Re, Os, He, Ar, H, O, S, Pb) and fluid inclusions

The Dongga Au deposit (9.55 t Au; average grade = 6.9 g/t [up to 61.6 g/t]) is located in the Xiongcun giant porphyry Cu–Au district of the Gangdese porphyry Cu belt, Tibet. Its mineralization age, ore-forming processes, and relationship to adjacent porphyry mineralization remain unclear. To address this, we conducted a geological, pyrite Re–Os dating, He–Ar–H–O–S–Pb isotopic, and fluid inclusion study of the Dongga deposit. Pyrite samples from ore-bearing chlorite–sulfide veins yielded a weighted-mean Re–Os age of 178.4 ± 2.6 Ma (MSWD=0.01), implying the deposit formed during the Early Jurassic. Fluid inclusion analyses yielded homogenization temperatures of 237–360 °C for quartz–sulfide veins and 125–201 °C for late quartz veins, with corresponding salinities of 2.7–43.8 and 0.9–9.9 wt% NaCleq, respectively. Fluid inclusion analyses of pyrite samples yielded 3He/4He and 40Ar/36Ar values of 0.50–1.08 Ra and 325.1–559.4, respectively, and δD and δ18Ofluid values of –86.2 ‰ to –72.1 ‰ and –5.6 ‰ to 3.9 ‰, respectively. The He–Ar–H–O isotopic data suggest the mineralizing fluids were derived mainly from a crustal source with a small mantle contribution, and also contained a mixture of magmatic and meteoric waters. The S (δ34S = –1.57 ‰ to –0.55 ‰) and Pb (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are 18.17–18.41 ‰, 15.55–15.63 ‰ and 38.15–38.37 ‰, respectively) isotopic compositions of pyrite suggest the ore-forming metals were derived from a magma source in the mantle containing minor amounts of subducted sediment. Based on the geology, and isotopic and fluid inclusion data, we infer that mixing between magmatic and meteoric waters was the main trigger of Au precipitation. In addition, based on the geochronological, spatial, and genetic relationships with the adjacent No.2 deposit in the Xiongcun ore district, we propose that the Dongga Au deposit is a sub-epithermal deposit, which represents the transition between porphyry and epithermal deposits. The two deposits constitute a porphyry system and record the continuous evolution of hydrothermal fluids transported outwards in a porphyry system.

Characterization of Co-bearing pyrite from the SEDEX Tuolugou Co(Au) deposit, northern Qinghai-Tibet Plateau, China

The Tuolugou sedimentary-exhalative (SEDEX) Co-Au deposit is located in the eastern Kunlun metallogenic belt of the northern Qinghai-Tibet Plateau in China. Pyrite is the prevalent Co-hosting sulfide mineral that occurs in the Tuolugou deposit. In this study, we investigated mineral characteristics of Co-bearing pyrite and effect of Co on crystal structure of the pyrite to obtain a better understanding of Co mineralization. Three pyrite generations were identified, including Co-barren pyrite (Py-1, formed during the early mineralization stage I, with Co content of 1-3844 ppm), Co-rich pyrite (Py-2, formed during the sedimentary-exhalative stage II, Co content 15–36522 ppm), and Co-rich pyrite (Py-3, formed during metamorphic stage III, with Co contents of 448–10505 ppm in Py-3A, and 1371–14904 ppm in Py-3B). As Co enters the pyrite structure, it is discovered that the octahedrons in pyrite is twisted and tilted, the lattice parameters (a0) rise, and the sulfur atom positions alter. The Co-rich Py-2 shows a similar REE and trace element distribution as those in quartz albitite, which indicates that Co mineralization is related to SEDEX genesis. Subsequent metamorphism resulted in additional Co enrichment. This detailed study of Co-bearing pyrite provides insights on the details of isomorphism Co in pyrite similar to the situation of this deposit as medium–low Co grade.

Spatial−temporal variations in Mesozoic crustal thickness along the northeast Asian continental margin: Response to the subduction history of the Paleo-Pacific Plate

The initial timing and history of subduction of the Paleo-Pacific Plate beneath Eurasia are controversial. The crustal thickness variations at a convergent margin typically reflect the convergent process between the two plates. This study used a recently proposed machine-learning model to estimate the crustal thickness variations along the northeast Asian continental margin during the Mesozoic. The northeast Asian continental margin, particularly the eastern North China Craton, had its thickest crust during the Early Triassic and underwent crustal thinning during the Middle−Late Triassic. The former reflects the subduction and collision between the South China Block and North China Craton, and the latter occurred in a post-orogenic extensional setting. From the Early to Middle Jurassic, sustained crustal thickening occurred along the northeast Asian continental margin, which coincided with initial subduction of the Paleo-Pacific Plate beneath Eurasia. From the Early to Late Cretaceous, the northeast Asian continental margin generally underwent crustal thinning, but crustal thickening events occurred at 130 Ma, 110 Ma, and 90 Ma, which is consistent with rollback of the subducted Paleo-Pacific Plate beneath Eurasia. The relationship between crustal thickness and mineralization suggests that thicker crust favored the formation of porphyry-type Cu-Mo deposits, whereas thinner crust in an extensional setting favored the formation of epithermal Au deposits related to magmatism.

Coprecipitation of amorphous silica and gold nanoparticles contributes to gold hyperenrichment

Hyperenrichment of Au in orogenic ores occurs overwhelmingly within quartz veins, but the formation pathway of quartz veins in orogenic systems remains enigmatic. We conducted hydrothermal experiments simulating coprecipitation of Au and amorphous silica and subsequent recrystallization to test whether this is a viable mechanism to generate Au nuggets within quartz veins. Within minutes, coprecipitation of amorphous silica and Au nanoparticles occurred, representing an effective Au deposition mechanism. Within one week, amorphous silica had recrystallized to quartz, causing the coarsening of Au particles and their relocation to quartz grain boundaries and fractures. The experimental textures are similar to those observed in high-grade zones of orogenic gold deposits. In addition to trapping Au, amorphous silica may increase competency contrasts that facilitate short-term fracture reactivation during earthquake aftershock periods or swarms, allowing further Au input from fresh fluids. These findings demonstrate that amorphous silica precipitation may be an important transient stage in orogenic gold deposit formation, with significant implications for metal accumulation in quartz veins.

Cobalt remobilization during tectonic–hydrothermal overprinting: A case from the Tuolugou Co(–Au) deposit in East Kunlun Orogenic Belt, China

Stratiform Cu-Co deposits hosted in (meta-) sedimentary and volcaniclastic rocks contribute two-thirds of the global Co supply. However, the mechanisms driving Co remobilization during tectonic-hydrothermal overprinting events remain poorly understood. As a dominant Co repository, pyrite can provide valuable insights into mineralization history due to its refractory nature. Here, we present the first micro-analytical investigation of pyrite from the Tuolugou Co(–Au) deposit in East Kunlun Orogenic Belt. Ore textures indicate that Co mineralization is attributed to two distinct stages: primary enrichment and subsequent overprinting processes. The former is characterized by pyrite (PyI with up to 3.92 wt% Co) remnants exhibiting oscillatory zones, while the latter is marked by deformation of pyrite (PyII) ores. Three overprinting episodes, namely Da, Db, and Dc, are further distinguished according to varying extents of deformation and mineral assemblages. The Da is characterized by synkinematic PyIIa (up to 4.18 wt% Co) augens, which were most likely formed via pressure–solution mechanism. The Db is represented by polygonal PyIIb (up to 4.75 wt% Co) and siegenite with 120° triple–point junctions. The random orientation and rare intragrain misorientation of cobaltiferous sulfides at Db suggest their formation by recrystallization without plastic strains. The Dc resulted in the formation of Co ± As–rich PyIIc (up to 5.36 wt% Co) along with minor occurrence of cobaltite–gersdorffite, pyrrhotite (up to 1.25 wt% Co), and marcasite (up to 1.28 wt% Co). The PyIIc replacing PyI underscores the role of fluid-coupled dissolution and precipitation during fluid–rock interactions in the formation of these cobaltiferous sulfide assemblages at Dc. The presence of PyIIc infilling within the PyI fissures, in conjunction with the transition from siegenite to cobaltite–gersdorffite, suggests a shift towards low fO2 and low temperature conditions under relatively brittle setting. There is a discernible disparity in sulfur isotope compositions between the two mineralization stages. The PyI exhibits a narrow range of δ34SV–CDT values from –2.14 to 1.35 ‰, indicating a magmatic sulfur origin. In contrast, PyII displays a wide δ34SV–CDT range of –5.22 to 6.16 ‰, suggesting an involvement of strata sulfur during the overprinting stage. Monazite U-Pb dating has effectively constrained two mineralization events. The ca. 453 Ma age implies a potential correlation of the primary Co mineralization with the Late Ordovician to Early Silurian exhalative-syngenetic processes in a Proto–Tethyan-related post–collisional extension setting. Conversely, the ca. 190 Ma age indicates that Co remobilization was instigated by the early Mesozoic ductile–brittle deformation associated with the Paleo-Tethyan orogenesis. Finally, geochemical modeling was performed by simulating the interaction of a Co-rich acidic, reducing fluid with typical quartz-albitite rocks. The simulations demonstrate the appearance of independent Co minerals such as Co-pentlandite during the reaction progression, greatly supporting ore upgrading via tectonic-hydrothermal overprinting.

Enhancing deep orebody prediction and localization through the revelation of geochemical primary halo patterns in drill holes

Geochemical primary halos play a crucial role in mineral exploration as they provide valuable insights into the dispersion of ore materials, identification of concealed deposits, and determination of ore deposit dimensions. The Naruo mining area in the Duolong mineral district, northern Tibet, China well-known for its worldclass porphyry-epithermal Cu–Au deposits is currently chosen as the study area. We employ statistical and nonlinear methods to analyze geochemical data from drill holes. Specifically, R-type cluster analysis is utilized to evaluate element affinity and concentration index, while an improved Grigorian zoning index is employed to delineate the vertical zoning sequence of the primary halo. These methodologies significantly enhance our understanding of element enrichment within geological contexts and greatly contribute to mineral resource evaluation. Based on these results, we have constructed a vertical zonation index model of denudation coefficient tailored to the geological characteristics of mineralization within the study area. This model effectively represents the vertical variation of the ore body. Our findings reveal a decrease in this zoning index towards the central region of deep ore bodies which correlates with deep granodiorite porphyry occurrences. Furthermore, we observe increasing concentrations of Cu, Ag, and Au with depth in specific sections of drill holes indicating promising exploration potential. Supplementary short-wave infrared data suggests the presence of a deep hydrothermal center in drill hole RNZK2404 which tentatively infers concealed porphyry bodies. Notably, the reversal observed at 4200 m on RNZK2404's zoning index hints at a possible hidden ore body at great depths. Finally, three-dimensional visualization techniques effectively illustrate spatial patterns for elements thereby paving way for future endeavors in mineral exploration.

Copper migration and enrichment in the mantle wedge: Insights from orogenic peridotites and pyroxenites

The refertilized mantle wedge is an important source of ore-forming metals in subduction-related Cu–Au deposits. However, the source and migration of Cu in the mantle wedge are poorly constrained. Here, we present a combined study of the Cu elemental and isotopic compositions (δ65Cu) as well as Fe3+/∑Fe ratios on a well-characterized suite of the Mg-lherzolites, Fe-rich peridotites and pyroxenites from the Bohemian Massif in a Variscan subduction zone. The Mg-lherzolites represent melting residues moderately affected by metasomatism of slab-derived melts/fluids. The Fe-rich peridotites and pyroxenites are, respectively, results of Mg-lherzolite-melt reaction and crystalline products of evolved melts in the lithospheric mantle.The peridotites and pyroxenites display higher Fe3+/∑Fe ratios (0.14–0.56) than the cratonic peridotites and mid-ocean ridge basalts, indicating that the mantle wedge was oxidized by slab-derived components. The Mg-lherzolites have relatively low Cu contents (7.35–39.6 μg/g) and normal mantle-like δ65Cu values (−0.13 to 0.26 ‰), suggesting an insignificant slab-to-mantle wedge Cu transfer. In contrast, the Fe-rich peridotites and pyroxenites have variable but overall high Cu contents (37.0–513 μg/g) and heavier δ65Cu values (up to 0.83 ‰). These signatures are ascribed to secondary sulfide precipitation from the evolved Cu- and 65Cu-enriched melts, which were produced by reaction of Mg-lherzolites with subduction-related oxidative SiO2‐undersaturated basaltic melts. Such melt-peridotite reaction at high oxygen fugacity can cause the partial oxidative decomposition of primary sulfides in peridotites with the preferential release of 65Cu into the evolved melts. Our results thus demonstrate that oxidative melt-rock reaction can result in the Cu migration, redistribution and its local enrichment in the mantle wedge, which may serve as an important source of Cu for subduction-related porphyry copper deposits.

Quartz texture and the chemical composition fingerprint of ore-forming fluid evolution at the Bilihe porphyry Au deposit, NE China

Abstract Quartz is widely distributed in various magmatic-hydrothermal systems and shows variable textures and trace element contents in multiple generations, enabling quartz to serve as a robust tracer for monitoring hydrothermal fluid evolution. This study demonstrates that integrated high-resolution SEM-CL textures and trace element data of quartz can be used to constrain physicochemical fluid conditions and trace the genesis of quartz in porphyry ore-forming systems. The Bilihe deposit is a gold-only porphyry deposit located in the Central Asian orogenic belt, NE China. Four quartz generations were distinguished following a temporal sequence from early-stage dendritic quartz, unidirectional solidification textured quartz (UST quartz), gray banded vein quartz (BQ), to late-stage white calcite vein quartz (CQ), with the Au precipitation being mostly related to dendritic quartz, UST quartz, and BQ. The well-preserved dendritic quartz with sector-zoned CL intensities and euhedral oscillatory growth zones crystallized rapidly during the late magmatic stage. The relatively low Al contents of dendritic quartz were interpreted to be related to contemporaneous feldspar or mica crystallization, while the high-Ti contents indicate high-crystallization temperatures (~750 °C). The comb-layered UST quartz displays heterogeneous, patchy luminescence with weak zoning, hosts coeval melt and fluid inclusions, and retains the chemical characteristics of magmatic dendritic quartz. High-Ti and low-Al contents of UST quartz suggest a formation at relatively high temperatures (~700 °C) and high-pH conditions. Three sub-types can be defined for hydrothermal BQ (BQ1, BQ2, and BQ3) based on contrasting CL features and trace element contents. The Al contents increase from BQ1 to BQ2 followed by a drop in BQ3, corresponding to an initial decrease and subsequent increase in fluid acidity. Temperature estimates of BQ decrease from BQ1 (635 °C) to BQ3 (575 °C), which may, however, be disturbed by high growth rates and/or high-TiO2 activities. The CQ typically displays a CL-bright core and CL-dark rim with oscillating CL intensities and is characterized by the lowest Ti and highest Al, Li, and Sb contents compared to the other quartz types, which suggests a deposition from more acidic and lower temperature fluids (~250 °C). Trace element patterns indicate that a coupled Si4+ ↔ (Al3+) + (K+) element exchange vector is applicable to dendritic quartz, UST quartz, and BQ. By contrast, charge-compensated cation substitution of Si4+ ↔ (Al3+, Sb3+) + (Li+, Rb+) is favored for CQ. The comparison with compiled trace element data of quartz from other porphyry Au, Cu, and Mo deposits worldwide suggests that Ti, Al, Li, K, and Ge concentrations, as well as Al/Ti and Ge/Ti ratios, have the potential to discriminate the metal fertility of porphyry mineralization.

Sensitivity enhancement of hydrogen-gas sensor by sub nm Au on Pd surface of a wireless quartz resonator

Hydrogen (H2) is an important source of next-generation energy production. The various H2 sensors developed to date cannot easily detect very low concentrations of H2 (<10 ppm) at room temperature within 100 s. In this study, we develop H2 sensors by depositing a 200 nm thick palladium (Pd) film on AT-cut quartz resonators and adding a sub nm gold (Au) layer on the Pd surface. Moderate Au deposition on the Pd surface improves the sensitivity of the sensor by decreasing the activation energy of atomic-hydrogen migration from the surface to the subsurface. The optimal Au thickness that minimizes the activation energy is 0.5 nm. Finally, we show that the approximate detection limit at room temperature is 5 ppm.

Mineralogy of Gold, Tellurides and Sulfides from Lianzigou Gold Deposits in the Xiaoqinling Region, Central China: Implications for Ore-Forming Conditions and Processes

The Lianzigou deposit, which has an Au–Te paragenetic association, is hosted in plagioclase gneiss of the Qincanggou Formation in the Taihua Group in the Xiaoqinling region, central China. This quartz vein-type Au deposit comprises native Au and a variety of tellurides. The latter include calaverite (AuTe2), krennerite (Au3AgTe8), petzite (Au3AgTe2), hessite (Ag2Te), melonite (NiTe2), and altaite (PbTe). Four stages have been recognized in this deposit: stage I consists of K-feldspar and quartz; stage II is of milky quartz veins accompanied by coarse-grained disseminated and lumps of pyrite with weak Au mineralization; stage III is composed mainly of Au, tellurides, and sulfides; and stage IV is characterized by abundant carbonate and quartz. Based on mineral assemblage and thermodynamic data, we estimated the physicochemical conditions of the main metallogenic stages. Based on thermodynamic modelling, the physicochemical conditions of Au–Ag–Te mineral associations were estimated. The Au–Ag–Te minerals from stage III formed mainly under conditions of logƒO2 = −43.15 to −33.31, logƒH2S = ~−9.29, pH &lt; 7, logfTe2 = −10.6 to −9.8 and logαAu+/αAg+ = −7.2 to −6.5. In contrast, the physicochemical conditions of stage II were higher, specifically pH (8.3–8.5) and logƒO2 (−34.90−31.96). In the ore-forming fluids of the Lianzigou deposit, the dominant Au species was Au(HS)2− while the dominant Te species were HTe−(aq) and Te22−(aq). Moreover, the Au–Ag–Te metal associations in the Lianzigou Au deposit were derived from mantle materials related to lithospheric thinning of the eastern North China craton in the Early Cretaceous under an extensional tectonic system.

Ore-forming process of the Xiaoyuzan gold deposit in the Western Tianshan, Northwest China: Constraints from mineralogy and S isotopes of sulfides

The Xiaoyuzan deposit is a typical intermediate-sulfidation epithermal Au deposit in the Boluokenu metallogenetic belt. The orebodies are hosted by the Lower Carboniferous Dahalajunshan Formation volcanic rocks and controlled by the NW- and NNW-striking faults. The metal minerals present in the ores mainly include pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite. The non-metallic minerals are primarily composed of quartz, calcite, and sericite. Three ore-forming stages are distinguished based on mineral assemblages, wall-rock alteration, and vein crosscutting relationships, including the quartz-pyrite stage (I) with silicification and propylitization, quartz-sulfide stage (II) with phyllic alteration, and quartz-carbonate stage (Ⅲ) with carbonatization. Three different types of pyrite are classified: coarse-grained PyI with cubic from the wall rock, fine-grained PyII with a crystal form of pentagonal dodecahedron in the quartz-sulfide veins, and coarse-grained cubic PyⅢ in the quartz-carbonate veins. The in-situ δ34S value range of sulfides from stage I, stage II and stage Ⅲ are 5.0 ‰ to 5.5 ‰, 4.3 ‰ to 6.5 ‰ and 5.7 ‰ to 6.2 ‰, respectively. The composition of S isotopes indicates that the source of the sulfur is magmatic in origin, with main contribution from the host rock. All the types of pyrite are relatively enriched with Sb, Cu, Ag, Pb, Bi, and As. The composition of pyrite suggests that the Au in the pyrite present as lattice gold (Au+1). The gradual decrease in Co contents from in PyⅠ, PyⅡ to PyⅢ indicates a gradual decrease in temperature during fluid evolution. The contents of trace elements in sphalerite are relatively low, with Fe, Mn, Cd, and Cu being relatively enriched. Using the sphalerite geothermometer (GGIMFis), the calculated temperature falls within the range of 303.0 to 334.3 °C. Pyrite II is characterized by the occurrence of oscillatory zones, suggesting rhythmic changes in fluid physicochemical conditions and compositions. Although coexisting of liquid-rich and vapor-rich aqueous inclusions were locally observed in stage II quartz according to previous studies, the absence of crustiform/colloidal/lattice bladed quartz in the main stage suggests that slight or gentle fluid boiling has occurred. In summary, it is proposed that fluid-rock reactions made great contribution for the precipitation of gold and sulfides in the Xiaoyuzan deposit.

The hidden link between dry ankaramitic melts and giant alkalic-type epithermal Au deposits: The Lihir Island example

The magmatic controls on the formation of giant epithermal Au deposits genetically associated with alkaline magmatic systems are not well understood. The general model, which attributes the generation of alkaline magmas to low-degree partial melting of metasomatized lithospheric mantle in a post-subduction tectonic setting cannot easily explain why some of these magmas are highly fertile for Au mineralization while others are not. This knowledge gap hampers the effective exploration of this economically important deposit type. In this study, we focused on the alkaline magmatism on Lihir Island and the Conical Seamount, Papua New Guinea, that gave birth to the Ladolam deposit and investigated the composition of silicate melt inclusions (SMIs) hosted in primitive volcanic rocks. Our findings offer important and novel insights into the early and deep magmatic processes that controlled the ore fertility of the melts that produced the largest known alkalic-type epithermal Au deposit on Earth. Olivine- and clinopyroxene-hosted primitive SMIs reveal nepheline-normative ankaramitic melt compositions derived from high-degree partial melting of metasomatized clinopyroxene-dominated lithologies, presumably amphibole-bearing clinopyroxenitic cumulates and metasomatically overprinted lithospheric mantle, both produced during earlier active subduction. These ultra-calcic parental melts are relatively low in H2O, S, and Cl, but enriched in Cu and highly oxidized. Due to their low water contents, significant clinopyroxene crystallization over a narrow temperature interval drove melt evolution from ankaramitic to alkaline compositions by strongly increasing incompatible element concentrations. Despite initially low H2O concentrations, evidence for high-temperature degassing is present in the form of low-density single-phase fluid inclusions co-existing with SMIs in clinopyroxene. This degassing event led to a strong decrease of S concentrations in the melt, whereas Cl and chloride-complexing elements remained largely unaffected. We propose that high-temperature volatile saturation as consequence of significant clinopyroxene crystallization is a key process for subsequent epithermal Au mineralization and can explain the efficient separation of Au from Cu. The complex tectonic setting of Lihir Island, marked by a reversal of subduction polarity, a tear in the slab beneath the island, and trans-lithospheric extensional structures, facilitated the generation and ascent of highly oxidized and relatively dry ankaramitic melts that ultimately exsolved a vapor-like, S- and presumably Au-rich fluid phase and might thus represent a prerequisite for alkalic-type epithermal Au mineralization.

Understanding the links between volcanic systems and epithermal ore formation: A case study from Conical Seamount, Papua New Guinea

The Tabar-Lihir-Tanga-Feni (TLTF) island chain in northeastern Papua New Guinea formed by tectonic and alkaline to shoshonitic magmatic activity since the Pliocene. Several volcanic centers are CuAu mineralized including the world-class Ladolam Au deposit and Conical Seamount south of Lihir. The latter has been recognized as a juvenile analogue to the Ladolam deposit located on-shore. Whereas the mineralization at Conical Seamount is reasonably well studied, the specific magmatic processes that promote epithermal mineralization at this seamount but not at others are poorly understood. Here, we present new petrological and geochemical data from Conical Seamount, and compare them with those from the barren (unmineralized) Edison, Tubaf and New World seamounts nearby. We focus on whole rock compositions and major and trace element analysis of melt inclusions and minerals including clinopyroxene, sulfide and magnetite. We combine our observations with modelled constraints on mantle source composition and partial melting as well as magma evolution. A first-stage melting leaves a residual mantle source enriched in Au. Second-stage melting of a previously subduction-metasomatized mantle generally promotes the transfer and concentration of metals and volatiles in the ascending melts. These magmas are unlikely to control ore formation as all seamounts show evidence for similar mantle sources and parental melt composition. However, the presence of a shallow crustal magma chamber is unique to Conical Seamount. It is characterized by frequent melt replenishments and extensive magma fractionation leading to sulfide and magmatic volatile saturation. These specific magma chamber processes lead to the pre-enrichment of the magma in chalcophile elements including Au, while sulfide saturation coeval with magmatic volatile exsolution provide the way for an effective Au transfer from the magmatic to the epithermal system.

Genesis of the sediment-hosted Qukuleke Au deposit in the East Kunlun Orogenic Belt, NW China: Constraints from geology, apatite U–Pb age, and C–O–S–Pb isotopes

Genetic classification of sediment-hosted Au deposits is often disputed in many orogenic belts around the world. The marine sediment-hosted Qukuleke Au deposit (>5 t Au) is an example of this, which has been attributed to orogenic or intrusion-related type. This paper addresses its metallogeny through detailed geologic and petrographic studies, together with apatite U–Pb dating and C–O–S–Pb isotopes.The hydrothermal paragenesis at Qukuleke is divided into three stages (I to III): Stage I (pre-ore) sheeted quartz veins; Stage II (main ore) pervasive sericite ± carbonate ± apatite ± pyrite ± arsenopyrite alteration associated with quartz–calcite ± stibnite veins; Stage III (post-ore) barren calcite veins. LA-ICP–MS hydrothermal apatite U–Pb dating on Stage II disseminated Au ore yielded 208.1 ± 6.5 Ma, coeval with the Late Triassic post-collisional intrusions from the periphery of Qukuleke deposit. Carbon–oxygen isotopes of Au–Sb ore-related hydrothermal calcite show that the hydrothermal fluids were derived from the marine carbonate ore host with considerable magmatic input. Lead–sulfur isotopes of the Au–Sb ore-related are consistent with those of coeval intrusions around the deposit, but different from those of the local sedimentary strata. Our age and geochemical data suggest the presence of a magma source that supplied the hydrothermal fluids, sulfur and metals for the mineralization. Combining the low sulfide content and the Au–As–W–Sb–Mo assemblage characteristics, the Qukuleke is best classified as a sediment-hosted intrusion-related Au deposit. Considering also the discovery of the large coeval Qukukekedong Au deposit nearby, we propose that Late Triassic intrusion-related Au system is a high-potential exploration target in the western EKOB.

Critical differences between typical arc magmas and giant porphyry Cu ± Au systems: Implications for exploration

Abstract Porphyry Cu, and porphyry Cu-Au deposits, are associated with arc magmatism and their ore-forming systems generally follow the magmatic evolution of typical arcs. However, most arc magmas are barren and giant economic porphyry Cu ± Au deposits are rare. In this study, we model variations in rare earth element concentrations in the evolving arc magmas and giant porphyry Cu ± Au systems to quantify the percentage of the fractionating minerals required to produce the observed changes. We find that, during the andesitic stage of fractionation, ore-forming systems in thick crusts fractionate ~35% more amphibole than an average of thick arc magma systems (the thick-crust reference suite) and that ore-forming systems in thin crusts fractionate twice as much amphibole as their equivalent thin-arc magma reference suite. Thick-crust ore-forming suites also fractionate ~50% less plagioclase, and thin-crust ore systems ~40% less plagioclase, than their associated reference suites during the same andesitic stage of fractionation. Taken together, these observations imply that ore-producing magmas are appreciably wetter than their associated barren reference suites. Our modelling also shows that 80% more amphibole is required to reproduce the andesite stage of fractionation in the thick-crust reference suite than in its thin-crust equivalent, suggesting that magmas produced under thick crusts are wetter than those produced under thin crusts. On the other hand, the chalcophile element contents of the thick- and thin-crust ore-forming systems are similar to and higher than those of the thick- and thin-crust reference suites, respectively. Therefore, we suggest that the high water content plays a critical role in the formation of giant porphyry Cu ore in thick crusts, whereas both high chalcophile contents and high water contents are required to form giant porphyry Cu-Au deposits in thin crusts. The high fraction of amphibole fractionation in giant economic porphyry suites, compared with their relevant reference suites, results in lower Y in the ore-associated suites and this difference increases with fractionation. As a consequence, plots of Y against MgO can be used to identify porphyries that have economic potential and are preferred to Sr/Y plots because they are less affected by the intense alteration associated with giant porphyry Cu ± Au deposits.

The Chain of Processes Forming Porphyry Copper Deposits—An Invited Paper

Abstract Porphyry-related mineral deposits are giant geochemical anomalies in the Earth’s crust with orders-of-magnitude differences in the content and proportion of the three main ore metals Cu, Au, and Mo. Deposit formation a few kilometers below surface is the product of a chain of geologic processes operating at different scales in space and time. This paper explores each process in this chain with regard to optimizing the chances of forming these rare anomalies. On the lithosphere scale, deposits with distinct metal ratios occur in provinces that formed during brief times of change in plate motions. Similar metal ratios of several deposits in such provinces compared with global rock reservoirs suggest preceding enrichment of Au or Mo in lithospheric regions giving rise to distinct ore provinces. The largest Cu-dominated deposits and provinces are traditionally explained by selective removal of Au during generation or subsequent evolution of mantle magmas, but the possibility of selective Cu pre-enrichment of lithosphere regions by long-term subduction cannot be dismissed, even though its mechanism remains speculative. Evolution of hydrous basaltic melts to fertile magmas forming porphyry Cu deposits requires fractionation toward more H2O-rich magmas in the lower crust, as shown by their adakite-like trace element composition. The prevailing interpretation that this fractionation leads to significant loss of chalcophile ore metals by saturation and removal of magmatic sulfide might be inverted to a metal enrichment step, if the saturating sulfides are physically entrained with the melt fraction of rapidly ascending magmas. Ascent of fertile magma delivers a large mass of H2O-rich ore fluid to the upper crust, along points of weakness in an overall compressive stress regime, within a limited duration as required by mass and heat balance constraints. Two mechanisms of rapid magma ascent are in debate: (1) wholesale emplacement of highly fractionated and volatile-rich granitic melt into a massive transcrustal channelway, from which fluids are exsolved by decompression starting in the lower crust, or (2) partly fractionated magmas filling a large upper crustal magma chamber, from which fluids are expelled by cooling and crystallization. Transfer of ore-forming components to a hydrothermal ore fluid is optimized if the first saturating fluid is dense and Cl rich. This can be achieved by fluid saturation at high pressure, or after a moderately H2O rich intermediate-composition melt further crystallizes in an upper crustal reservoir before reaching fluid saturation. In either case, metals and S (needed for later hydrothermal sulfide precipitation) are transferred to the fluid together, no matter whether ore components are extracted from the silicate melt or liberated to the ore fluid by decomposition of magmatic sulfides. Production and physical focusing of fluids in a crystallizing upper crustal magma chamber are controlled by the rate of heat loss to surrounding rocks. Fluid focusing, requiring large-scale lateral flow, spontaneously occurs in mushy magma because high water content and intermediate melt/crystal ratio support a network of interconnected tubes at the scale of mineral grains. Calculated cooling times of such fluid-producing magma reservoirs agree with the duration of hydrothermal ore formation measured by high-precision zircon geochronology, and both relate to the size of ore deposits. Ore mineral precipitation requires controlled flow of S- and metal-rich fluids through a vein network, as shown by fluid inclusion studies. The degree of hydrothermal metal enrichment is optimized by the balance between fluid advection and the efficiency of cooling of the magmatic fluid plume by heat loss to convecting meteoric water. The depth of fluid production below surface controls the pressure-temperature (P-T) evolution along the upflow path of magmatic fluids. Different evolution paths controlling density, salinity, and phase state of fluids contribute to selective metal precipitation: porphyry Au deposits can form at shallow subvolcanic levels from extremely saline brine or salt melt; high-grade Au-Cu coprecipitation from coexisting and possibly rehomogenizing brine and vapor is most efficient at a depth of a few kilometers; whereas fluids cooling at greater depth tend to precipitate Cu ± Mo but transport Au selectively to shallower epithermal levels. Exhumation and secondary oxidation and enrichment by groundwater finally determine the economics of a deposit, as well as the global potential of undiscovered metal resources available for future mining.

Gold Mineralization, Hydrothermal Alteration, and Li Isotope Fractionation at Cochenour Orogenic Gold Deposit, Red Lake, Canada

Abstract Lithium isotopes have been used to study subduction zones, continental weathering, and magmatic–hydrothermal transitions, but little is known about their behavior in metasomatic footprints surrounding hydrothermal ore deposits. Although minerals such as biotite, chlorite, and white micas may sometimes be characteristic of Au mineralization, they are not truly diagnostic given that they also usually occur outside the ore zones. Here, whole rock δ7Li compositions are measured in samples from drill core cutting through metasomatized basalts from the Cochenour orogenic Au deposit, Red Lake, Canada, to verify whether Au ore-related biotite, chlorite, white micas, and quartz have specific δ7Li signatures. Laser ablation ICP-MS maps show 1000–1100 ppm Li in biotite, 200–600 ppm in chlorite, and 20–100 ppm in white mica. In the ‘arsenopyrite-bearing replacement-style’ alteration and mineralization, Au is associated with arsenopyrite, pyrrhotite, and chalcopyrite in the ore zone, but is more concentrated in white mica than in sulfides outside the ore zone. In the cross-cutting ‘quartz-actinolite vein-style’ alteration and mineralization, Au is distributed with chalcopyrite and arsenopyrite in the selvages of the veinlets, but is associated with pyrite, calcite, and biotite outside the ore zone. Mass balance calculations suggest that Au ore-related biotite, chlorite, white mica, and quartz have δ7Li values of ∼+4‰, ∼0 to +2‰, ∼−2‰, and ∼+6 to +12‰, respectively. This is explained by Li isotopic fractionation occurring during the retrograde sequence of auriferous biotite-chlorite-white mica alteration and the late auriferous quartz vein event, identified in the present samples as well as in the Red Lake-Campbell mine complex and the Red Lake greenstone belt. In the ‘arsenopyrite-bearing replacement-style’ alteration, the intercorrelation of δ7Li and Au and their correlation over distance show greater amounts of Au in biotite-enriched rocks, where biotite has δ7Li values of ∼+4‰. Such values indicate a mantle source for the early biotite-related auriferous fluid at Red Lake.