Epithermal Au Research Articles

Lead and noble gas isotopic constraints on the origin of Te-bearing adularia-sericite epithermal Au-Ag deposits in a calc-alkaline magmatic arc, NE China

Abstract Tellurium (Te)-bearing adularia-sericite epithermal Au-Ag deposits are widely distributed in calc-alkaline magmatic arcs and are an important current and future source of precious and critical metals. The source of ore-forming fluids in these deposits remains unclear due to the lack of in situ isotopic evidence on Au-, Ag-, and Te-bearing minerals. To advance the understanding of the source and evolution of Te and precious metals, herein, we combine in situ Pb isotope analysis with He, Ne, and Ar isotope and microthermometric analysis of fluid inclusions in ore and gangue minerals from two Te-rich and two Te-poor epithermal Au-Ag deposits that occur in an Early Cretaceous magmatic arc in the North Heilongjiang Belt, northeastern China. Ore minerals (hessite, petzite, calaverite, altaite, pyrite, chalcopyrite, and galena) from Te-rich Au-Ag deposits, including Sandaowanzi and Yongxin, have the least radiogenic Pb isotope compositions (206Pb/204Pb from 18.1 to 18.3) and the lowest μ1 values (the 238U/204Pb ratio of the lead source down to 9.14) of the deposits studied. For these Te-rich deposits, noble gas isotope data show that fluid inclusions in ore minerals contain a large proportion of mantle He (up to 25%), whereas barren early-stage minerals do not (<1%). The Pb, noble gas isotope, and fluid inclusion microthermometric results suggest that Te-rich ore-forming fluids were likely discharged from mafic magmas into convecting meteoric flow systems at shallow levels (<2 km). In contrast to the Te-rich deposits, ore minerals from the Te-poor Dong’an Au-Ag deposit have radiogenic Pb isotope compositions (206Pb/204Pb from 18.8 to 18.9) and the highest μ1 values (up to 10.54). Fluid inclusions in ore minerals contain a small proportion of mantle He (1% to 5%). The results suggest that metals and ore-forming fluids in these deposits were discharged from either more crustally contaminated intermediate-felsic magmas or leached from upper crustal rocks by convecting meteoric flow systems. Although the Te-poor Tuanjiegou Au-Ag deposit has a non-radiogenic Pb isotope composition consistent with a mafic magma source, Te is much less abundant (electrum [>95%] is the major gold- and silver-bearing mineral) than Au. The main exploration implication of these results is that unexplored volcano-plutonic centers in the northeast Xing’an Block with less radiogenic Pb isotope compositions (206Pb/204Pb < 18.3) and containing fluids with a high proportion of mantle He are more likely to generate Te-rich epithermal Au-Ag deposits than other volcano-plutonic centers in NE China.

Effectiveness of LOF, iForest and OCSVM in detecting anomalies in stream sediment geochemical data

This paper compares three unsupervised machine-learning algorithms – local outlier factor (LOF), Isolation Forest (iForest) and one-class support vector machine (OCSVM) – for anomaly detection in a multivariate geochemical dataset in northeastern Iran. This area contains several Au, Cu and Pb–Zn mineral occurrences. The methodology incorporates single-element geochemistry, multivariate data analysis and application of the three unsupervised machine-learning algorithms. Principal component analysis unveiled diverse elemental associations for the first seven principal components (PCs): PC1 shows a Co–Cr–Ni–V–Sn association indicating a lithological influence; PC2 shows a Au–Bi–Cu–W association suggesting epithermal Au mineralization; PC3 shows variability in Zn–V–Co–Sb–Cu–Cr; PC4 shows a Au–Cu–Ba–Sr–Ag association indicating Au and polymetallic mineralization; PC5 reflects Zn–Ag–Ni–Pb related to hydrothermal mineralization; and PC6 and PC7 show element associations suggesting epithermal and intrusive-related polymetallic mineralization. It was found that OCSVM performed slightly better than LOF and iForest in detecting anomalies associated with known Cu occurrences, and it successfully delineated dispersion from all known Au occurrences. LOF outperformed iForest and OCSVM in identifying all four Pb–Zn occurrences, and the three methods substantially limited the areas of the anomaly class. The analysis showed that LOF produced a less cluttered anomaly map compared to the isolated patterns in the iForest map. LOF was accurate in identifying anomalies associated with Au–Pb mineralization, while iForest detected anomalies associated with Pb–Zn–Cu occurrences and neighbouring Pb–Zn occurrence. OCSVM performed similarly in the northern and western areas but displayed unique discrepancies in the SE and west by detecting anomalies associated with two Cu occurences and a Pb–Cu occurrence. This study examined the influence of contamination fraction on detection of geochemical anomalies, revealing a noteworthy rise in the count of mineral occurrences delineated by anomalies when the contamination fraction increases from 5 to 10%. However, even with a 35% contamination fraction, some Cu occurrences remained outside the anomaly category, indicating potentially overlooked geochemical signals from mineral occurrences due to sampling schemes.

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.

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.

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.

Epithermal Ag–Au mineralization at Galim-Legalgorou, Cameroon Volcanic Line: insights from alteration mineralogy and mineral chemistry of electrum and sphalerite

The Galim-Legalgorou area, located along the north-western branch of the Cameroon Volcanic Line (CVL), is known for artisanal gold mining and primary gold exploration, but the style and origin of the mineralization has not been fully described in the scientific literature. The close spatial association of the mineralization with Tertiary volcanic rocks raises the question of whether or not this is an epithermal system in a region where all previously studied gold occurrences have been classified as orogenic. In this contribution, we use ore mineral and alteration assemblage and mineral microchemistry to constrain the origin and evolution of the mineralization. Gold is alloyed mainly with Ag (max. 53.1 wt%) and show fineness values that vary 379 and 721 with an average of 455. A cumulative percentile plot of Ag concentrations in the cores of all electrum particles, and the observed step-change from Au-rich cores to more Ag-rich mantles in zoned electrum particles, suggest that the mineralization was deposited by two distinct hydrothermal fluid influxes. The observed ore mineral association-electrum, Se/Tl-acanthite, pyrite, Fe-rich sphalerite, ± galena; the measured FeS contents of sphalerite (average 23.1 mol%), and the presence of colloform banded chalcedony and hydrothermal breccia are all consistent with features of low sulfidation epithermal deposits. This study, therefore, demonstrates for the first time, the presence of significant low sulfidation epithermal Ag–Au mineralization on the CVL, an intracontinental rift region only previously known for orogenic gold mineralization.Graphical

Oligocene to Miocene magmatic and hydrothermal evolution in the Maricunga belt (Región de Copiapó, Chile): A new look to a long known metallogenic belt

The Maricunga metallogenic belt, herein defined to encompass the region from 25° S to 28° S along the Cordillera Frontal of northern Chile, is known for hosting a series of gold-copper and lesser copper-molybdenum porphyries and gold-silver epithermal systems that are hosted in a segment of the late Oligocene-Miocene Andean calc-alkaline magmatic arc. Based on an up-to-date geochronological database, four episodes of magmatism and related hydrothermal deposits are proposed to characterize intensity and style of volcanism and hypabyssal intrusive rocks over time. The late Oligocene episode (26-21 Ma) is characterized by stratovolcanoes, dome complexes, pyroclastic sequences, and hypabyssal intrusives of andesitic to dacitic composition that host gold-rich porphyry systems such as Caspiche, Maricunga, and La Pepa. The early Miocene episode (21–17 Ma) has similar rock types but is volumetrically reduced, hosting epithermal and porphyry deposits like La Coipa and Caserones, respectively. A reinvigoration of volcanism occurred during the third stage between 17 and 11 Ma, giving rise to large volcanic complexes and intra-to extra-caldera ignimbritic deposits that host several gold-rich porphyry deposits such as Lobo-Marte, Luciano, and Cerro Casale; as well as epithermal deposits such as Fenix. Some of the 17 to 11 Ma and younger igneous rock belong to the high-K calc-alkaline series, showing a higher alkali content compared to previous periods. The last magmatic episode, from 11 to 5 Ma, coincides with the shallowing of the subduction angle and is characterized by an eastward migration of volcanism, and more heterogeneous rocks including trachytic compositions. During this period mineralization was emplaced at the Salares Norte epithermal deposit and El Volcan porphyry Au–Cu deposit.Porphyry and epithermal systems formed throughout the evolution of the Maricunga belt. Overprint of advanced argillic alteration on porphyry style alteration is widespread albeit commonly devoid of significant addition of epithermal Au–Ag mineralization. The Au-rich nature of the majority of the porphyry systems is attributed to the shallow crustal depths of emplacement compared to Cu- and Mo-rich porphyry systems. Conversely, high-sulfidation epithermal Au–Ag deposits such as La Coipa and Nueva Esperanza are not known to be telescoped on coeval porphyry gold deposits.The location of the mineralized centers was favored by crustal weaknesses expressed by intersection of oblique- and parallel-to arc structures, the former inherited from the construction of the continent before to the formation of the Andean range. The arc oblique structures also segment the Maricunga belt into domains with unique magmatic and hydrothermal styles.

Physico-chemical constraints associated with hypogene hydrothermal alteration and supergene oxidation at the Farallón Negro intermediate-sulfidation epithermal Au-Ag deposit, NW Argentina

The Farallón Negro deposit is located in the Farallón Negro Volcanic Complex (FNVC; ∼9.40 to 4.88 Ma) in NW Argentina and includes intermediate-sulfidation, epithermal Au-Ag mineralization in the Alto de la Blenda deposit. Detailed mineralogical and geochemical investigations of the Esperanza-Esperanza Sureste, Cecilia veins, both hosted in the Alto de la Blenda monzonite, and the Laboreo vein, hosted in the volcaniclastic Morada Central andesitic breccia, reveal four hypogene vein stages. Each of these four stages are comprised of a texturally early quartz, followed by a sulfide-sulfosalt and a late Mn-carbonate sub-stage. Stage 1 is characterized by a quartz, base metal and Mn-carbonate sub-stage showing high δ18OVSMOW (δ18OQuartz = 13.7 ± 0.4‰ and δ18OMn-Calcite = 11.6 ± 0.2‰) and low δ13CVPDB values (δ13CMn-Calcite = –4.7 ± 0.2‰). Stage 2 includes most gold and silver minerals in a tennantite-tetrahedrite-sphalerite-pyrite-galena±electrum sub-stage. Fluid inclusions trapped in vein-anhydrite, only associated with stage 2, record the highest temperatures (TEntrapment = 327–335°C) and highest salinity (3.8NaCleq.wt.%) when compared to all other stages. Stage 2 quartz (δ18OQuartz = 9.9 to 11.9‰) and carbonate display lower δ18OVSMOW (δ18OMn-Calcite = 7.9 to 10.3‰), as well as lower δ13CVPDB values (δ13CMn-Calcite = -5.2 to 4.7‰) than all other stages. Stages 3 and 4 consist of barren quartz and carbonate sub-stages that show lower temperatures (TEntrapment = 187–262°C) and higher δ18OVSMOW (δ18OQuartz = 13.4 to 13.9‰ and δ18OMn-Calcite = 10.7 to 11.8‰) and δ13CVPDB values (δ13CMn-Calcite = –3.5 to −3.9‰) when compared to earlier stages.Petrographic, sulfide mineral chemistry, microthermometric, and stable isotope data combined with distinct alteration mineral assemblages allow the reconstruction of a model for the Alto de la Blenda deposit. During the hypogene hydrothermal fluid flow stage, around 7.16 Ma, at ∼2.2 km depth (PHYDROSTATIC = 220 bars), the high temperature alteration mineralogy, low fluid salinities and 18O and 13C isotope compositions indicate cooling of ascending magmatic-hydrothermal fluids. These fluids were derived from a contracted magmatic vapor phase that exchanged with unexposed sedimentary rocks of the Sierras Pampeanas. Hypogene quartz-R3 illite/smectite-chlorite-calcite-pyrite alteration in the andesitic-breccia and quartz-illite-R3 illite/smectite-chlorite-calcite-pyrite alteration in the monzonite are temporally and spatially to the hydrothermal vein minerals deposited between 187 and 335°C. During the post-hydrothermal supergene weathering event, around 2.70 Ma, sulfuric acids generated by meteoric fluids and supergene oxidation of hypogene sulfides, particularly pyrite, descended along normal fault zones and related vein and fracture networks to at least 450 m depth below the surface level, forming the supergene alteration assemblage of dioctahedral smectite-kaolinite-Mn-/Fe-oxides/-hydroxides-gypsum±interstratified chlorite/smectite±interstratified R1 illite/smectite in both wall rock types at temperatures <50°C.The dominant hypogene alteration assemblage, present in the monzonite and volcaniclastic andesitic breccia, may be indicative of deeper-seated, transitional magmatic-hydrothermal systems (>1 km depth), generally controlled by fluid cooling, as is the case at Farallón Negro. Pervasive supergene alteration can conceal these systems but the identified supergene alteration assemblage of dioctahedral smectite-kaolinite-interstratified chlorite/smectite-interstratified R1 illite/smectite that replaces the hypogene alteration assemblage along the permeable fault, vein and fracture zone network may present a useful vectoring tool towards concealed, deeper-seated, epithermal Au-Ag mineralization in arid climates.

Influence of different tectonic settings on fracture formation and fluid flow around upper-crustal magmatic intrusion: insights from numerical modelling

Large porphyry Cu and epithermal Au deposits tend to form in distinct tectonic, porphyry and high-sulfidation epithermal deposits in compressional settings, and low-sulfidation epithermal deposits in extensional settings. Given that the analysis of the shallow metallogenic dynamic processes at the upper-crust scale is insufficient, especially the ore-bearing fracture formation and fluid-focusing mechanism around the mineralizing magmatic intrusion under different tectonic backgrounds, we aimed to study how tectonic settings influence fracture formation and fluid hydrodynamics in and around a hot intrusion. We developed a finite element model coupling thermal-hydrological-mechanical processes to simulate the fracture formation, evolution of fluid velocities, and accumulation of water-rock interactions. The model results show that tectonic compression increases the degree of fracturing, hydrothermal fluid velocities, and water-rock interaction within and laterally around the intrusion; tectonic extension enhances fracturing, hydrothermal fluid velocities, and water-rock interaction at shallow depth. These results confirm that tectonic compression may promote the formation of porphyry Cu deposits, while tectonic extension may promote the formation of shallow hydrothermal deposits. Our model explains the effects of tectonic activity on fracture formation and fluid flow around hot magmatic intrusions in upper crust and deepens our understanding of the relationship between tectonic activity and deposit formation there.

Positive Δ199Hg anomalies in Mesozoic porphyry Mo deposits of Northeastern China and their implications to the metallogeny of arc-related hydrothermal systems at convergent margins

Mercury (Hg) isotopes display unique mass-independent isotope fractionation (MIF, reported as Δ199Hg), which is primarily generated via Hg photochemical processes in the land-ocean-atmosphere system. Magmatic and hydrothermal processes do not trigger MIF of Hg isotopes, therefore, Hg-MIF signals detected in deep reservoirs (mantle and crust) are theoretically sourced from recycled surface materials. Here, we observed positive Δ199Hg signals (0.10 ± 0.07‰, SD) in Mesozoic porphyry Mo deposits of NE China. These values correspond to the recently reported Δ199Hg signature of Mesozoic epithermal AuAg systems in NE China (Δ199Hg: 0.11 ± 0.07‰, SD), suggesting that both mineral systems receive large amounts of Hg from subducted marine materials, given that marine Hg has positive Δ199Hg composition. Our study thus provides novel geochemical evidence on the linkage between oceanic plate subduction and metallogeny of various hydrothermal systems at convergent margins. Subduction contributes large amounts of volatiles to arc magmas, which then release metal-rich magmatic-hydrothermal fluids of the porphyry to epithermal ore deposit spectrum.

Specification and formation process of enriched portions in Au veins in an epithermal deposit via clustering and geostatistical approaches

Even though epithermal vein-type deposits are the main sources of Au and Ag worldwide, accurate three-dimensional modeling of vein structures and the extraction of high-grade zones are still difficult because of geologic structure heterogeneities and the scarcity of available data, mostly obtained via drilling. Accordingly, by selecting a low-sulfidation epithermal Au–Ag deposit in the Philippine archipelago, we develop an efficient and reliable workflow for determining the spatial distribution of Au-rich veins in detail and the transport of hydrothermal fluids. The studied deposit is mostly hosted in open fractures and breccias with a relatively large tonnage of low Au+Ag grades and/or a small tonnage of high Au+Ag grades. The whole-rock geochemical data (1877 core samples) and lithological data (56 drilling sites) are processed using hierarchical clustering analysis (HCA) and two geostatistical simulations: turning bands simulation for the Au grade distribution and interpretation of mineralization process and truncated Gaussian simulation for the spatial distribution of clusters with similar Au–Ag grade features as classified by HCA. The general correlation between the Au and Ag grades, with a linear correlation coefficient of ≈ 0.69, is based on the HCA application, and the clusters were likely formed by three-stage mineralization processes. A plausible three-dimensional geologic model is then constructed, delineating in detail hydrothermally mineralized and altered zones with Au enrichment and vein system structures in the study area.

Age and petrogenesis of ore–forming volcanic–subvolcanic rocks in the Yidonglinchang Au deposit, Lesser Xing’an Range: Implications for late Mesozoic Au mineralization in NE China

The age and petrogenesis of volcanic–subvolcanic rocks associated with Au mineralization of the recently discovered Yidonglinchang Au deposit in the Lesser Xing’an Range provide insights into late Mesozoic Au metallogenesis in NE China. In this paper, we report zircon U–Pb chronological and Hf isotopic, whole–rock geochemical, and Sr–Nd isotopic data of the andesite porphyry and quartz diorite porphyry from the Yidonglinchang Au deposit. Zircon U–Pb dating shows that the andesite porphyry and quartz diorite porphyry associated with Au mineralization were formed at 101 and 99–97 Ma, respectively, suggestive of an early Late Cretaceous Au mineralization event in the Lesser Xing’an Range. Whole–rock geochemical data reveal that they have low SiO2 contents (59.87–61.48 % and 55.91–63.22 %, respectively) and high Mg# values (41.5–55.4 and 43.2–61.3, respectively). In addition, these rocks are enriched in large–ion lithophile elements (e.g., K, Rb, and Ba) and depleted in high–field–strength elements (e.g., Nb, Ta, P, Ti, Zr, and Hf). The rocks have slightly negative to positive zircon εHf(t) values (−2.3 to 5.7) and negative whole–rock εNd(t) values (−3.3 to −2.9), indicating a metasomatized mantle source with some contamination by crustal materials. Combined with regional magmatic and metallogenic characteristics, we conclude that the Au mineralization at Yidonglinchang formed as a result of early Late Cretaceous magmatic–hydrothermal activities. Previously obtained data show that the orogenic Au deposits in the northern NE China formed mainly during the Early Cretaceous, earlier than the late Early Cretaceous to Late Cretaceous epithermal Au deposits in the eastern NE China. The formation of the late Mesozoic Au mineralization in the northern and eastern NE China was likely controlled by distinct tectonic settings. During the late Early to early Late Cretaceous, the rollback of Paleo–Pacific oceanic slab beneath eastern NE China generated an extensional setting that led to large–scale magmatism and associated Au mineralization.

Mineralogical features and formation conditions of the Zafar copper-gold deposit (Lesser Caucasus, Azerbaijan)

&#x0D; &#x0D; &#x0D; The Zafar copper-gold deposit is located in Gadabay ore district, 3.5 km northwest of the Gadabay gold mine and 2 km southwest of the Ugur mine. This newly discovered cop- per-gold deposit contains 6.8 million tonnes of measured resource (0.5% Cu, 0.4 ppm Au and 0.6 % Zn, according to Zafar JORC Mineral Resource Estimate Update Report 2022). SEM analyses were carried out via «JSM-6610LV Scanning Electron Microscope» manufactured by JEOL USA. Petrographic studies were carried out using a ZEISS microscope with ZEN 2.3 imaging software. In this stage of the research we defined mineralogical properties of rocks and gold bearing mineralization type. According to the SEM analysis, gold mineralization formed in the pyrite, chalcopyrite (their derivatives too) stages. Based on the mineralogical study, the main ore minerals consist of pyrite, chalcopyrite, enargite, sphalerite, arsenopyrite, galena, baryte, pyrrhotite and sternbergite (silver mineral) based on polished section results. The main gangue minerals are quartz, baryte and carbonate. Quartz crystals in bands are euhedral to anhedral, vary in size from microcrystalline to coarse grained. The Zafar deposit was formed by two events. The first event is the formation of the «massive pyrite» related to small Later Bajocian sub-volcanic rhyolite-dacite bodies. The second event is assumed to be «copper-pyrite» and «copper-zinc» mineralization and is reported to be formed by the post-magmatic activity of the Gadabay diorite intrusive. This study has demonstrated new ideas about the formation and genesis of the Zafar deposit. The K-rich calc-alkaline composition of various rocks indicates that subduction-related magmatism occurring along Lok-Karabakh belt had a crucial role in forming this deposit. The information given in the article is of practical importance for regional predictive metallogenic constructions, prospecting and evaluation of epithermal Au–Cu deposits.&#x0D; &#x0D; &#x0D;

Recently-discovered Bahçecik Au±Ag mineralization in the Eastern Pontides, Gümüşhane-NE Türkiye: geological and geochemical implications on the intermediate sulfidation epithermal deposit

ABSTRACT Intermediate sulfidation epithermal deposits represent significant reservoirs of precious metals, playing a crucial role in global mining operations. The Anatolian Peninsula, predominantly characterized by relatively young (Tertiary) lithologies, hosts numerous occurrences of epithermal base and precious metal mineralization. Within the realm of research in Northeastern Anatolia, several hydrothermal alteration zones have emerged as indicators of concealed epithermal mineralization, with the Bahçecik mineralization standing out as a recent focal point. The Bahçecik mineralization represents an intermediate sulfidation epithermal Au-Ag system, hosted by Late Cretaceous andesitic rocks, as well as Eocene dacitic lava and tuffs. Employing U-Pb LA-ICP-MS zircon dating of the dacitic host rocks, the mineralization is estimated to have occurred during the Lutetian epoch (43.61 ± 0.35 Ma) or later. The genesis of the Bahçecik deposit is believed to be linked to postcollisional regional extensional tectonics, primarily controlling its structural arrangement. Comprising pyrite, chalcopyrite, low-Fe sphalerite, galena, tetrahedrite/tennantite, enargite-luzonite, and gold-silver minerals, the deposit is accompanied by prominent gangue minerals such as Mn- and Ca-carbonates, quartz, and barite. Microprobe measurements have identified a majority of Au-Ag minerals (krennerite, buckhornite, and nagyagite) closely associated with sulphide minerals. Fluid inclusion data reveal that mineralization and fluid transportation occurred at temperatures ranging from 153 to 327°C, with solutions exhibiting salinity levels consistent with those of intermediate sulfidation deposits (1.7–7.3 wt.% NaCl). This study unequivocally classifies the Bahçecik mineralization as an intermediate sulfidation deposit, contrary to the conclusions drawn in previous research. Moreover, our investigation has unveiled the presence of a substantial ore resource, totalling 7.84 million tons, featuring 1.76 g/t Au and 2.24 g/t Ag. As an intermediate-type deposit, it shares characteristics with other subtypes within the epithermal class, bridging the realms of both high and low sulfidation systems.

Protracted metallogenic and magmatic evolution of the Kirazlı epithermal Au-Ag and porphyry Cu deposits, Biga Peninsula, NW Turkey: evidence from zircon U-Pb, muscovite 40Ar/39Ar, and molybdenite Re-Os geochronology

The Kirazlı deposit is located at the center of the Biga Peninsula metallogenic province, in a geological setting characterized by an extensional tectonic environment. A NNW-SSE trending high-sulfidation (HS) orebody with a total reserve of 33.86 Mt @ 0.69 g/t Au and 9.42 g/t Ag lies beneath the Kirazlı Main zone. A porphyry Cu orebody hosted by Eocene intrusive and volcanic rocks has been intersected by drilling within its vicinity. The HS epithermal deposit is hosted by a partly silicified and brecciated Oligocene volcanic and volcaniclastic sequence consisting mainly of basaltic andesite lava flow and lithic/crystal tuff. Lithogeochemistry and zircon U-Pb radiometric ages allow us to distinguish three distinct high-K calc-alkaline magmatic events at ca. 41, 38, and 32 Ma, sourced by metasomatized mantle melts, which have interacted with the crust during their ascent. Porphyry Cu mineralization took place at 36.7 ± 0.4 Ma (muscovite 40Ar/39Ar age) with subsequent re-opening and base metal deposition. Crosscutting quartz-pyrite-molybdenite veins were emplaced at 33.6 ± 0.2 Ma (molybdenite Re-Os age), and followed by the HS epithermal Au-Ag event at ca. 31 Ma, based on a previous study. Our radiometric data indicate that the Kirazlı deposit has recorded a long-lasting Cenozoic magmatic and metallogenic evolution during about 10 Myr. Our study demonstrates that successive, independent, and overprinting, but genetically unrelated, HS epithermal precious metal, hydrothermal Mo, base metal, and porphyry Cu systems have been active at the same location during protracted extensional tectonics of the Biga Peninsula.

Late Mesozoic tectonic evolution and thermal history in the northeast Xing'an Block, NE China: Implications for gold mineralization and preservation

Late Triassic–Jurassic orogenic and Early Cretaceous epithermal Au deposits are significant producers of Au, Ag, and potentially critical elements (e.g., Te, Se and Sb) in the North Heilongjiang Belt in the Xing'an Block, NE China. However, the preservation mechanism of these Au deposits associated with tectonic evolution is unclear. Here, we conducted zircon U–Pb, sericite Ar–Ar, zircon and apatite (U–Th)/He dating as well as whole‐rock geochemistry in the Xing'an Block. Zircon U–Pb dating shows the ages of 174.1 ± 2.4 Ma for the granodiorite, 169.9 ± 1.6 Ma for the tonalite, 154.0 ± 2.0 Ma for the monzogranite and ca.122 Ma for the diorite. Sericite Ar–Ar dating presents plateau and isochron ages of 163.2 ± 1.8 Ma and 163.4 ± 2.8 Ma. The average apatite (U–Th)/He (AHe) and zircon (U–Th)/He (ZHe) ages from the Jurassic rocks have the ranges of ca. 146–89 Ma and ca. 150–116 Ma and from the Cretaceous are ca. 109–76 Ma and ca. 117–111 Ma, respectively. The Early‐Middle Jurassic intrusions are featured by the tonalite‐trondhjemite‐granodiorite (TTG) rock type. Late Jurassic monzogranite has the geochemical signature of adakitic rocks. The Early Cretaceous granitoids are classified as volcanic arc granites with a part of the Early Cretaceous granodiorite featured by adakitic. These new results reveal five group ages associated with tectonic evolution, Au mineralization and the preservation of the Au deposits. (i) Magmatic events were related to the subduction of the Mongolia‐Okhotsk Ocean during the Early Jurassic (ca. 177–170 Ma) and then the compressional settings with regional deformation (ca. 167–163 Ma). (ii) Thickening crust and orogenic Au mineralization were related to the closure of the Mongolia‐Okhotsk Ocean (ca. 154–145 Ma). (iii) The transformation was from compressional to extensional settings followed by exhumation during the Early Cretaceous (ca. 145–120 Ma), which is the factor of the exhumation of the orogenic Au deposits formed from the Late Triassic to Late Jurassic. (iv) Epithermal Au–Ag mineralization was associated with an extension related to the subduction of the Palaeo‐Pacific Plate (ca. 120–100 Ma). (v) Thermal history shows one thermal event during the Late Cretaceous (ca. 89–76 Ma). The Au deposits have been preserved since the Late Cretaceous.

Precious-Metal Mineralization and Formation Conditions of the Biche-Kadyr-Oos Epithermal Au-Ag Ore Occurrence (Eastern Sayan, Russia)

The Biche-Kadyr-Oos epithermal Au-Ag ore occurrence is a prospective object in the Ak-Sug porphyry copper ore cluster (Eastern Sayan) in the northern part of the Central Asian orogenic belt (CAOB). The mineralization consists of gold-sulfide-quartz and gold-polysulfide-carbonate-quartz veins with argillic zones in the Lower Cambrian volcanic-sedimentary rocks. The origin of the Au-Ag ore occurrence is still debatable. To determine the origin, we examined the mineralogical and geochemical features, conditions of formation, and fluid sources of the Biche-Kadyr-Oos ore. A mineralogical and geochemical investigation outlines three stages of mineral formation: early argillic stage; gold-sulfide-quartz stage with pyrite, marcasite, pyrrhotite, arsenopyrite, chalcopyrite, less frequently sphalerite, hessite, gold, and electrum; and late gold-polysulfide-carbonate-quartz stage with gold, electrum, Hg-electrum, Se-acanthite, Se-galena, bornite, tennantite, tetrahedrite, hessite, tellurobismuthite, bismuthinite, matildite, jamesonite, ourayite, native Bi, and barite. Fluid inclusion study (thermometry, Raman spectroscopy) in quartz and mineral thermometry (electrum and sphalerite paragenesis) determined that ore veins were formed at P~0.5 kbar from CO2-water Na-K-chloride fluid (4.9–9.6 wt % NaCl eqv) and temperatures from 300 to 200 °C (early gold-sulfide-quartz veins at 300–230 °C, and late gold-polysulfide-carbonate-quartz veins at 290–200 °C) and variations in fO2, fS2, fSe2 and fTe2. The S isotopic composition in sulfides and δ34SH2S values of the fluid are +1.3‰ and +4.7‰, respectively, (T = 300–275 °C) indicating magmatic S in ore formation. The oxygen isotope data indicate that during the formation of veins, the magmatic fluid mixed with meteoric water (δ18Ofluid is from +3.4 to +6.4‰). The isotopic data that were obtained combined with mineralogical and geochemical features and conditions of ore formation indicate the similarity of Biche-Kadyr-Oos ore occurrence with epithermal Au-Ag deposits of intermediate sulfidation (IS) type. The presence of epithermal Au-Ag mineralization of the Biche-Kadyr-Oos IS type in ore cluster of the Ak-Sug Cu-Au-Mo porphyry deposit indicates the existence of a single porphyry-epithermal ore-magmatic system.

Deep epithermal Au mineralization at Tuanjiegou, northeast China

To better understand the physicochemical features of deeply mineralized intervals of low-sulfidation (LS) Au deposits, the Early Cretaceous Tuanjiegou Au deposit in northeastern China was characterized as it comprises alteration and mineralization styles that reflect deeper portions of the mineralizing system. From shallow to deeper portions of the Tuanjiegou deposit, the mineralization is zoned from sulfide-poor quartz bands through hydrothermal breccia to deep pyrite–marcasite-rich veinlets that occur along hydrofractures within a granodiorite porphyry stock. The zoned alteration halo is centred on the ore zones, and comprises an inner phyllic alteration zone (illite + quartz + muscovite) and an outer intermediate argillic alteration zone (illite ± smectite), with propylitic alteration being limited. Given that apatite fission track ages are extremely sensitive to the ambient temperatures of 60–120 °C, the overlap in the closure dates of magmatic apatite (108.1 ± 3.8 Ma) and zircon (108.2 ± 1.2 Ma) from the granodiorite porphyry is indicative of shallow emplacement of the granodiorite porphyry, likely at depths slightly >0.7 km. A similar paleodepth (900 ± 188 m) was estimated from homogenization temperatures (291 ± 14 °C) of boiling fluids. Preservation of the deep portions of the Tuanjiegou ore system may imply that the productive ore intervals of poorly explored LS epithermal deposits globally are underestimated. Given the evidence for a magmatic origin of the ore-forming fluids at Tuanjiegou, we highlighted that ore-forming fluids are released initially from a water-saturated, oxidizing felsic melt under low-pressure (∼4 km) conditions.

МОРФОЛОГИЧЕСКИЕ ХАРАКТЕРИСТИКИ, ХИМИЧЕСКИЙ СОСТАВ РОССЫПНОГО ЗОЛОТА БАССЕЙНА РЕКИ КЕНГЕВЕЕМ И ПЕРВЫЕ СВЕДЕНИЯ О СОПУТСТВУЮЩЕЙ ПЛАТИНОВОМЕТАЛЛЬНОЙ МИНЕРАЛИЗАЦИИ (МАГАДАНСКАЯ ОБЛАСТЬ)

The paper presents the results of the study of typomorphic characteristics of placer gold of the tributaries of the Kengeveem River, Magadan region. Placer gold was studied by optical and scanning electron microscopy at the Institute of Volcanology and Seismology of the Far East Branch of the Russian Academy of Sciences. As a result, the first information on the morphology, internal structure and chemical composition of placer gold from the southeastern part of the Taigonos Peninsula was obtained. Three types of the gold were determined. The first type is dominant. The grains are chacterized by sub-ore morphology, with weak and medium degrees of roundness. Only Au-Ag is present in the composition of native gold, other macrocomponents were not found. The gold grade is 750–950 ‰. Some gold grains bear signs of hypergene transformation, expressed in the development of high-grade gold rims and intergrain veins. The second type of gold is represented by well-rounded lamellar extractions with a grade of 980–990 ‰. The third type is loose aggregates of secondary high-grade mustard gold. The obtained data allowed us to determine the different drift distances of native gold and to consider the low-sulfide epithermal Au-Ag mineralization and presumably far-distant gabbroids as the primary sources of gold.