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Vein, stockwork, skarn, and carbonate-replacement Sn deposits commonly contain little or no Cu, but examples rich in Cu are also well known in many Sn belts worldwide. The origin of Sn-Cu deposits in association with fractionated and reduced, ilmenite-series granites of mostly metasedimentary crustal parentage is enigmatic because Cu, in contrast to Sn, normally accompanies oxidized, magnetite-series intrusions ultimately of mantle origin. Decompression mantle melting in back-arc or post-collisional settings generates mafic magmas, which are commonly invoked as the triggers for partial melting of thick overlying metasedimentary crust to form the fractionated, peraluminous, ilmenite-series magmas required to generate any associated Sn mineralization. Therefore, it is reasonable to suppose that oxidized fluids exsolved from these mafic magmas can contribute Cu to the synchronous and coexisting silicic chambers, thereby adding Cu to reduced Sn-bearing fluids and any resultant Sn deposits. Copper could also be released from Cu-bearing immiscible magmatic sulfides, formed during early crystallization of the reduced silicic magmas, as a result of either the introduction of these oxidized fluids and/or magma degassing. These putative mechanisms are restricted to thick metasedimentary crust, whereas in dominantly metaigneous crust the resultant magnetite-series magmas generate Cu deposits. This fundamental crustal influence on Sn versus Cu metallogeny of back-arc or post-collisional settings is responsible for both along- and across-strike transitions from the Central Andean Sn belt to regions displaying partly coeval Cu ± Au mineralization. [ABSTRACT FROM AUTHOR]

Baogutu is a large gold deposit (4 Mt @ 5.5 g/t Au) in West Junggar, NW China. We conducted a Re‐Os geochronological study on seven pyrite samples from auriferous quartz‐pyrite veins, which yielded a Re‐Os isochron age of 312 ± 11 Ma (2σ). This age defines the timing of gold mineralization, and overlaps the emplacement age of diorite to granodiorite porphyry stocks (319–310 Ma) in the Baogutu district. The Os initial ratio of 0.46 ± 0.21 (2σ) suggests a mix of components from both mantle and continental crust in the ore‐forming system, which is similar to the proposed magma source of porphyry stocks. The δ34S values of sulphide minerals vary in a narrow range from +0.1 to −3.3‰, indicating that the sulphur is likely of igneous origin. Our data suggest that the Baogutu gold mineralization is probably genetically linked to the regional diorite to granodiorite magmatism. The new data provide important constraints for an evolving genetic model of the Baogutu gold deposit. [ABSTRACT FROM AUTHOR]

Gold precipitation in hydrothermal systems is traditionally attributed to supersaturation of gold due to decreasing gold complex stability triggered by changes in physicochemical conditions of the ore fluid. However, ultrahigh-grade gold veins in orogenic (shear zone related) gold deposits can contain kilograms per tonne of gold or more, in marked contrast to the typically very low gold concentrations (tens of parts per billion) in fluid. The gold mineral assemblage is commonly restricted to native gold and/or Au-(Ag)-tellurides and occurs in micro-fractures of sheared quartz veins. Textural and compositional characterization of such assemblages, coupled with hydrothermal diamond anvil cell experiments and heating-freezing experiments, provides evidence for an alternative ultrahigh-grade gold enrichment mechanism via growth of polymetallic melt droplets induced by quartz fracturing. We propose that polymetallic melt droplets of Au-Ag-Te-Bi-rich composition form through adsorption-reduction of metal complexes on fractured quartz surfaces, where surface silanol groups and hydrogen serve as reductants. The melt droplets subsequently grow by catalyzing reduction of metal complexes and absorbing metals from fluids percolating in the fractured quartz network. The mobile and reactive polymetallic melt droplets can repeatedly react with the fluid on protracted quartz fracturing and efficiently continue to scavenge gold from multiple pulses of gold-undersaturated ore fluids. [ABSTRACT FROM AUTHOR]

Mercury isotopes display both mass-dependent and mass-independent fractionation and allow the tracing of pathways and storage of surface-derived Hg in the lithosphere. While the subduction-related orogenic recycling of Hg from marine reservoirs into hydrothermal systems in continental arc settings has been documented recently, the source of Hg in intracontinental hydrothermal systems remains unclear. We measured Hg isotopes in two intracontinental anorogenic/postorogenic Late Mesozoic hydrothermal gold deposits in the South China craton and the Central Asian orogenic belt of northern China, respectively. The ore and sulfide samples from the studied systems have positive δ 202 Hg (0.70 ± 0.39‰, 1SD, n = 49) and negative Δ 199 Hg values (−0.12 ± 0.05‰, 1SD, n = 49). These values are different from their country rocks and regional geological environment (volcanic arc granites, marine sedimentary rocks) which have positive Δ 199 Hg values, but similar to that of their Precambrian supracrustal basement rocks of largely non-marine continental materials. We conclude that Hg in the intracontinental hydrothermal systems was leached from basement rocks by upper crustal basinal fluid circulation driven by regional heat flow, likely due to lithospheric thinning and upwelling of the asthenosphere in the Late Mesozoic. The intracontinental hydrothermal systems and their continental sources with positive δ 202 Hg and negative Δ 199 Hg values are complementary to volcanic-arc and marine sedimentary rocks with opposite δ 202 Hg - Δ 199 Hg compositions. The distinct Hg isotopic features of hydrothermal systems in different tectonic settings, in particular the indelible Δ 199 Hg signature, allow the tracing of large-scale material cycling in the Earth. • Hydrothermal systems in arc and intracontinental settings show opposite Δ 199 Hg. • Intracontinental systems have negative Δ 199 Hg reflecting a terrestrial Hg source. • Mercury isotope can be a tracer of Hg cycling in hydrothermal systems. [ABSTRACT FROM AUTHOR]

Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in tire province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilátero Ferrffero of Minas Gerais, the ~2.0 Ga Transamazonian event and the ~0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceição Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaço, along the platiniferous Au-Pd belt of Minas Gerais. [ABSTRACT FROM AUTHOR]

The Dahu Au-Mo deposit is located in the Xiaoqinling region and belongs to the Qinling-Dabie orogen of central China, The quartz vein-style Dahu Au-Mo deposit (311 Au, average grade: 4.7 g/t; 30,0001 Mo, average grade: 0.13 %) formed in four stages, i.e., an early quartz-K-feldspar stage (I), a pyrite-molybdenite stage (II), a sulfide-telluride-sulfosalt-gold stage (III), and a late carbonate-barite stage (IV). Abundant tellurides and sul-fosalts occur in stage III: altaite, tellurobismuthite, buckhornite, hessite, petzite, calaverite, aikinite, kupcikite, lindstromite, salzburgite, wittichenite, and tetradymite. The system is characterized by carbono-aqueous fluids of low to moderate salinity and high oxygen fugacity. Total homogenization temperatures of the H2O-CO2 fluid inclusions in stage I and stage III quartz range from 230° to 440°C and 198° to 320°C, respectively. The variation in CO2/H2O ratios and microthermometric data of the H2O-CO2 fluid inclusions results from a combination of fluid unmixing and sequential trapping of fluid inclusions, but also from postentrapment reequilibration. Abundant minerals are present as solid phases in H2O-CO2 fluid inclusions in stage I quartz: Cu1.65S, covellite, chalcopyrite, bornite, molybdenite, pyrite, colusite, anhydrite, and celestine. δ18Oquartz values of stage I quartz range from 10.2 to 12.0%c, with calculated δ18Ofluid values ranging from 5.6 to 7.4‰. The δD values of fluid inclusion waters (thermal decrepitation) in stage I quartz range from -117 to -54‰. The low and scattered 5D values reflect a mixture of ore-forming fluids trapped by the H2O-CO2 fluid inclusions at deep crustal levels and postmineralization fluids, possibly meteoric waters, trapped by secondaiy aqueous fluid inclusions at higher crustal levels during uplift of the Xiaoqinling core complex. Four molybdenite samples yield an Re-Os isochron age of 206.4 ± 3.9 Ma (MSWD = 0.88), suggesting ore formation in the Late Triassic. Metal signature, mineral association, fluid characteristics, and isotope data of the Dahu deposit, together with the tectonomagmatic setting, imply that the ore-forming fluids were derived from an unexposed and oxidized magmatic system, which likely stems from partial melting of enriched lithospheric mantle beneath the North China craton in a postcollisional extensional setting. [ABSTRACT FROM AUTHOR]

About 85% of all historically mined tin of about 27 million tonnes Sn is from a few tin ore provinces within larger granite belts. These are, in decreasing importance, Southeast Asia (Indonesia, Malaysia, Thailand, Myanmar), South China, the Central Andes (Bolivia, southern Peru) and Cornwall, UK. Primary tin ore deposits are part of magmatic-hydrothermal systems invariably related to late granite phases (tin granites, pegmatites, tin porphyries), and may become dispersed by exogenic processes and then eventually form placer deposits within a few km from their primary source, due to the density of cassiterite, its hardness and chemical stability. Alluvial placer deposits were usually the starting point for tin mining, and have provided at least half of all tin mined. The small-volume and late granite phases in spatial, temporal and chemical relationship to tin ore deposits are highly fractionated. Systematic element distribution patterns in these granite phases and their associated much larger multiphase granite systems suggest fractional crystallization as the main petrogenetic process controlling magmatic evolution and magmatic tin enrichment. Oxidation state controls the bulk tin distribution coefficient, with low oxidation state favoring incompatible behavior of divalent tin. Low oxidation state is also mineralogically expressed by accessory ilmenite (FeO TiO 2) as opposed to accessory magnetite (FeO Fe 2 O 3) in more oxidized melt systems. This difference in the accessory mineralogy and hence metallogenic potential (tin-bearing ilmenite-series versus barren magnetite-series granites), can be easily detected in the field by a hand-held magnetic susceptibility meter. The hydrothermal system is a continuation of the magmatic evolution trend and necessary consequence of the crystallization of a hydrous melt. The exsolved highly saline aqueous fluid phase, enriched in boron and/or fluorine plus a wide metal spectrum, can be accomodated and stored by the intergranular space in crystallized melt portions, or accumulate in larger physical domains, accompanied by focused release of mechanical energy (brecciation, vein formation), dependent on emplacement depth (pressure). The hydrothermal mobility of tin is largely as Sn2+-chloride complexes; the precipitation of tin as cassiterite involves oxidation. Tin typically characterizes the inner high-temperature part of much larger km-sized zoned magmatic-hydrothermal systems with the chemical signature Sn-W-Cu-As-Bi in the inner part (greisen, vein/stockwork/breccia systems, skarn) and a broader halo with vein- or replacement-style Pb-Zn-Ag-Sb-Au-U mineralization of lower temperature. This zoning pattern may also occur telescoped on each other. Active continental margins are the favorable site for both copper (−gold) and tin (−tungsten) systems. However, the narrowly segmented metal endowment and the episodic nature of ore formation suggest additional controls. These are the build-up of a subduction-derived metal and fluid inventory in the lower continental crust by flat-slab subduction (very little magmatism) for copper‑gold in the main arc, followed by large-scale intracrustal melting during mantle upwelling in the back arc for tin (chemically reduced reservoir rocks) and/or tungsten mineralization (less sensitive to oxidation state). [Display omitted] • Tin granites display advanced degree of fractionation • Tin granites are of ilmenite series (reduced), irrespective of S-, I-, or A-type affinity • Hydrothermal tin solubility is optimal under reducing and highly saline conditions • Hydrothermal tin ore formation requires oxidation, fluid mixing, cooling [ABSTRACT FROM AUTHOR]

The distribution of gold in the weathering blanket at the Belikombone gold prospect in east Cameroon provides insights into gold mobility in the secondary environment and in tropical terrains worldwide. Both gridline-controlled sampling of topsoil (surface samples) and sampling of various layers in pits are used and the gold assay for each sample determined by NiS fire assay with ICP-AES finish. One hundred and thirty-two (132) surface samples and a total of 206 samples from 19 exploration pits were analyzed. The results from the topsoil samples show an anomaly with the highest Au concentration at 5.9 mgkg−1. The mineralization corridor follows a NE-SW trend. The horizons within the pits range from sap rock at the base, through saprolite, rubble layer rich in relict quartz material to a ferruginous loose layer at the top although some horizons are missing in some pits. All the layers contain gold and the highest concentration in the sap rock horizon is 3.4 mgkg−1 while the rubble layer has a gold high of 6.1 mgkg−1. The individual soil horizons show no systematic gold trends and given the presence of gold in all layers, the patterns point towards supergene dissolution and redistribution of gold. Gold enrichment within the upper horizons in the weathering blanket is attributed to sequestration by Fe oxides of chemically remobilized gold. However, the high gold content within the sap rock and saprolite layers suggests that migration of gold in the particulate form supersedes chemical gold redistribution. Particulate gold obtained by panning samples from the pits varies in shape from euhedral, elongated to irregular. Electron microprobe analysis on the grains record high contents of gold in the rim zones (90.0 to 99.8 wt%). The cores are relatively rich in Ag (12.6 to 14.2 wt%) while the rims are poor in Ag. The low Ag content in the rims is attributed to the preferential leaching of Ag. The soil pH value in this area varies between 3.6 and 7.3. Under such acidic to near neutral conditions, bisulfide and thiosulfate ions can dissolve and transport Au and Ag to be precipitated under surficial conditions creating authigenic Au haloes especially in the saprolite and sap rock layers. Such pH values together with oxidizing Eh conditions explain the solubility of gold in the area. These results are important for geochemical exploration of gold in tropical terrains, and confirm previous studies. • Gold in the weathering profiles an underlying NE-SW hypogene gold quartz vein system • No systematic gold enrichment in the pits pointing to both particulate gold and chemical gold distribution in the soil • Quartz vein fluid inclusion data and galena inclusions in the gold grains suggest mixed metamorphic and basinal brine fluids. • Gold grain with Ag-depleted rims indicates weathering driven leaching of Ag. [ABSTRACT FROM AUTHOR]

We report here 40Ar/39Ar whole-rock and sericite data for host-rock (sericite-quartz altered rock) and gold ore (pyrite-quartz-feldspar-carbonate) from the giant Kumtor gold deposit in the Tien Shan fold and thrust belt of Kyrgyzstan, one of the largest orogenic gold belts on Earth. Plateau ages for whole-rock samples of sericite-quartz altered rock and sericite-bearing gold ore are 285,5 ± 1.2 and 288.4 ± 0.6 Ma. Sericite concentrates gave plateau ages of 284.3 ± 3.0 (host rock) and 285.4 ± 0.2 (ore) Ma. The age of mineralization is slightly younger than a U-Pb zircon age of 296.7 ± 4.2 Ma obtained for the postcollisional Djangart granite, about 80 km southeast of Kumtor, and slightly older than two published U-Pb ages of 268 ± 1 and 280 ± 9 Ma on a postcollisional granite intrusion about 10 km west of Kumtor. These ages also overlap with data from the other major gold deposits of the 2,000-km-long southern Tien Shah fold and thrust belt. The ages define a late Paleozoic event of gold mineralization related to regional-scale fluid flow and granite magmatism controlled by transcrustal shear zones during the postcollisional stage. [ABSTRACT FROM AUTHOR]

Photochemical processes generate mass-independent fractionation (MIF) of mercury (Hg) isotopes in the atmosphere-ocean system, and the subduction of marine sediments or hydrated oceanic crust may recycle the resultant Hg isotope signature into the volcanic-arc environment. This environment typically hosts epithermal gold deposits, which are characterized by a specific Hg-Sb-As metal association. We investigated the Hg isotopic composition of seven volcanic-arc-related epithermal gold deposits in northeast China and revisited the isotopic composition of Hg in hydrothermal ore deposits in circum-Pacific and Mediterranean volcanic arcs. The gold ore samples in northeast China mostly display positive Δ199Hg values (0.11‰ ± 0.07‰, 1σ, n = 48) similar to those observed in the Pacific Rim (0.07‰ ± 0.09‰, 1σ, n = 182) and the Mediterranean Cenozoic volcanic belt (0.09‰ ± 0.08‰, 1σ, n = 9). Because Hg in marine sediments and seawater has positive Δ199Hg, we infer that Hg-bearing epithermal deposits in active continental margin settings receive most Hg from recycled seawater in marine sediments, through the release of Hg by dehydration from the subducting oceanic slab. However, negative to near-zero Δ199Hg values were observed in Hg-bearing deposits in the South China craton (-0.09‰ ± 0.05‰, 1σ, n = 105) and in the intraplate magmatic-hydrothermal Almadén Hg deposit in Spain (-0.02‰ ± 0.06‰, 1σ, n = 26), which are considered to relate to basement and mantle sources, respectively. Hg isotopes have the potential to trace lithospheric Hg cycling. [ABSTRACT FROM AUTHOR]

Mercury isotopes display unique mass-independent fractionation (MIF), typically expressed as Δ199Hg, which relates to photochemical reactions occurring at Earth's surface. Pronounced Hg-MIF signals have been observed in various hydrothermal systems in both convergent margins and intracontinental settings, highlighting the recycling of Hg from marine or terrestrial reservoirs into shallow continental hydrothermal systems. However, the geochemical fate of Hg in deep continental environments of intracontinental settings remains poorly understood. Iron oxide‑copper‑gold (IOCG) hydrothermal systems typically involve the circulation of fluids from both deep magmatism and continental basins, providing an opportunity to investigate this issue. Here, we present the Hg isotopic compositions of ore minerals from five representative IOCG deposits in the Kangdian region, South China. All the studied ore samples display large mass-dependent fractionation with δ202Hg of −3.23 to 1.06‰, but a relatively narrow range of Δ199Hg of −0.17 to 0.11‰. These isotopic signatures support the hypothesis of binary mixing of Hg from two sources: (1) a magmatic endmember (∼two thirds) derived from the lithospheric mantle with low δ202Hg (−3 to −1‰) and near-zero Δ199Hg, and (2) a crustal source (∼one third) originating from the basement rocks with relatively higher δ202Hg (−1 to 1‰) and negative Δ199Hg (−0.2 to 0‰). Compared to the shallow Pb-Zn-Au-Sb hydrothermal systems which formed in intracontinental basins through the circulation of fluids in the upper crust, the Hg components (and by analogy, the ore metals) in the Kangdian IOCG system are dominated by a lithospheric mantle source. Our study sheds light on deep Hg cycling and confirms the Hg isotope tracer as a tool for revealing metal sources in hydrothermal ore systems. • The first reported Hg isotopic compositions of IOCG hydrothermal systems. • The IOCG system has Hg and ore metals dominantly from lithosphere mantle. • Hg recycling in intracontinental hydrothermal systems is various at depth. [ABSTRACT FROM AUTHOR]

[Display omitted] • Propylitic Chl-W formed through conversion of two biotite layers to one chlorite layer. • Granoblastic Chl-I formed by dissolution-transport-precipitation. • Chl-V as vein infill precipitated directly from the hydrothermal fluid. • Retrograde Chl-S in skarns resulted from dissolution-transport-precipitation. • Fe-, Zn-, and Sn-rich chlorite is a maker mineral for tin mineralization and prospecting at Furong. The altered granite- to skarn-type Furong tin deposit (530,000 t Sn @ 0.8% Sn) lies at the southern margin of the multiphase Qitianling granite batholith, South China. Chlorite alteration is a distinct feature at Furong, and four chlorite variants are identified: (i) chlorite partially or pseudomorphically replacing biotite in weakly chloritized granites (Chl-W), (ii) chlorite aggregates interlocking irregularly with quartz, cassiterite, and magnetite in intensely chloritized granites (tin ores) (Chl-I), (iii) comb-shaped chlorite in quartz veins crosscutting the intensely chloritized granite (Chl-V), and (iv) chlorite replacing garnet, diopside, and vesuvianite in skarns (Chl-S). Compositionally, all the Furong chlorite samples belong to trioctahedral Fe-chlorite (chamosite). Empirical chlorite geothermometry without Fe/(Fe + Mg) correction yielded formation temperatures of 321–422℃ and 397–420℃ for the granoblastic chlorite variants Chl-I and Chl-V, respectively, and lower temperatures of 273–343℃ and 285–366℃ for the pervasive chloritization variants of Chl-W and Chl-S, respectively. Chl-W is the result of layer-by-layer replacement with two biotite layers being converted to one Chl-W layer, and Chl-S formed by a dissolution-transport-precipitation mechanism. Both variants formed in a rock-dominated fluid environment, with their elemental compositions controlled dominantly by the precursor minerals and temperature. In contrast, Chl-I resulted from the dissolution-transport-precipitation process in a fluid-dominated system, and Chl-V precipitated directly from the fluid. During intense fluid-rock interaction, the fluid provided considerable Fe, Zn, and Sn to form granoblastic Chl-I and Chl-V together with cassiterite. Fluid flow was controlled by NNE-trending fault zones which were the preferential sites of mixing of ascending metal-rich magmatic fluid with meteoric water and fluid interaction with the granites. [ABSTRACT FROM AUTHOR]