Ore Fluids Research Articles

Chemical and boron isotopic composition of tourmaline from the Yixingzhai gold deposit, North China Craton: Proxies for ore fluids evolution and mineral exploration

Chemical and boron isotopic composition of tourmaline from the Yixingzhai gold deposit, North China Craton: Proxies for ore fluids evolution and mineral exploration

Mass transfer during hydrothermal bleaching alteration of red beds in the Lanping Basin, SW China: Implications for regional Cu(–Co) and Pb-Zn mineralization

The Lanping Basin in SW China hosts many hydrothermal Cu(–Co) and Pb-Zn deposits. Widespread bleaching alterations of red beds in the basin record mass transfer during the hydrothermal fluid migration and may provide useful information about metal sources for the deposits. Based on fluid inclusion study, mineral and elemental mapping of thin sections, and whole-rock major and trace element analyses, this study investigates fluid properties and mass transfer during the hydrothermal bleaching alterations at T1 and T2 two places where Jurassic red siltstones have been bleached in association with sulfides-free hydrothermal quartz(-siderite) veins. Mineral compositions and fluid inclusion microthermometry indicate that both T1 and T2 fluids are CO2-free, weakly acidic to neutral, S-poor, hydrocarbon-bearing reduced fluids, but the former is higher than the latter in the homogenization temperatures (188 – 265 ℃ versus 136 – 184 ℃) and salinities (17 – 36 wt% NaCl eq. versus < 11 wt.%NaCl eq.). These reduced fluids caused the bleaching alterations due to the loss of Fe3+ (in terms of hematite) from the red siltstones. The T1 fluid is comparable to the vein-type Cu-Co and Pb-Zn ore fluids in the Baiyangping district. Enrichment of Cu-Co and depletion of Pb-Zn in the bleached rocks relative to the protolith at T1 suggest that the two pairs of elements are less possibly derived from the same source rocks. Red beds probably provided Pb and Zn, while other rock types, such as mafic volcanic rocks, were likely the sources of Cu and Co. As such, various degrees of contributions from different source rock types during the ore fluid migration may determine the variable (Cu-Co)/(Pb-Zn) ratios observed in the vein-type Cu-Co and Pb-Zn deposits. The T2 fluid is similar to the ore fluids of some vein-type Cu deposits along the western margin of the Lanping Basin. Considering that the T2 hydrothermal veins are free of sulfides and the fluid inclusions from the vein-type Cu deposits in this region either contain CO2 or do not contain CO2, the abundance of reduced sulfur rather than CO2 determines whether the reduced fluids in this area are capable of causing Cu mineralization. Different from T1, Mg and TFe2O3 were introduced into the bleached rocks at T2 site. This possibly resulted from interaction between the migration fluids and widespread mafic volcanic rocks in this region. Enrichment degree of Cu is apparently greater than those of Co, Zn, and Pb in the bleached rocks, which is consistent with the Cu-only mineralization for the vein-type Cu deposits in this area.

Prograde metamorphism provides gold and base metals to orogenic gold deposits in southern Tibet: Insights from thermodynamic modeling

The metamorphic devolatilization model is presently the most widely accepted model for the formation of orogenic gold deposits. According to this model, Au is likely derived from metamorphic rocks that experienced metamorphism from greenschist to amphibolite facies. Nevertheless, in recent years, some researchers have suggested that Au may have originated from different source regions, such as the mantle. As one of China's most significant orogenic gold provinces, the Indus-Yarlung Zangbo suture zone (IYZSZ) in southern Tibet plays a critical role in this discussion. Orogenic gold deposits within the IYZSZ are typically hosted in Triassic metasedimentary rocks called the Langjiexue group. This paper aims to investigate the suitability of metasedimentary rocks of the Langjiexue group as a source region of Au and base metals. Furthermore, we also examine whether the metamorphic devolatilization model is applicable to the IYZSZ. Our study involves reporting the mineral compositions and evolution of metamorphic zones (chlorite to sillimanite zones) in metasedimentary rocks obtained from the IYZSZ and providing detailed petrographic descriptions of these rocks. Thermodynamic modeling based on metamorphic mineral compositions was used to reveal the behaviors of H2O, Au, S, Cu, Pb, and Zn within prograde metamorphism. Our modeling results proved that the breakdown of chlorite and muscovite, both of which are the main mechanisms of Au and base metals release in metasedimentary rocks, occurs under different geothermal gradients. Given the observed metamorphic mineral compositions, it is the breakdown of chlorite that is the predominant reaction responsible for releasing Au and base metals under the real geothermal gradient of lower crustal metasedimentary rocks (∼25°C/km). This gradient is capable of releasing at least 1.5 ppb Au, 5.2 ppm Cu, 4.3 ppm Pb, 5.0 ppm Zn, and 20 ppm S (as part of the bulk rock) into the ore fluids. Through the mass balance calculations, we found that over 4.05 t of Au, 14040 t of Cu, 11600 t of Pb, and 13500 t of Zn can be mobilized from 1km3 of metasedimentary rocks. Therefore, lower crustal metasedimentary rocks can play a significant role in contributing to Au and base metals in the IYZSZ and other metasediment-hosted orogenic gold deposits worldwide.

Genesis of the Lamasu large skarn Cu deposit, northern Xinjiang: Constraints from garnet U–Pb dating, micro‐textural and geochemical analyses

The Lamasu deposit is the first large copper (Cu) deposit with ~0.6 Mt Cu reserve in the Chinese Western Tianshan Orogen (NW China). The Cu orebodies are mainly hosted in the exoskarn of the Proterozoic Kusongmuqieke Group. Now, the metallogenic mechanism of newly discovered skarn Cu orebodies (0.5 Mt Cu) remains poorly constrained. In this study, we conducted LA‐ICP‐MS U–Pb isotope dating, EPMA and LA‐ICP‐MS geochemical analyses on different generations of garnet from Lamasu to elucidate the magmatic hydrothermal evolution and its timing. Garnets from the Lamasu exoskarn can be divided into three types: reddish‐brown coarse‐grained Grt‐I, light brown fine‐grained Grt‐II and yellowish‐green heterogranular Grt‐III, with U–Pb age of 389.1 ± 2.0 Ma, 387.1 ± 1.8 Ma and 387.0 ± 2.3 Ma, respectively. These dates represent the oldest Cu mineralization age in the Chinese Western Tianshan, coeval with the Middle Devonian subduction of the North Tianshan oceanic plate. Grt‐I to Grt‐III particles are mainly andradite with minor grossularite, and they have different REE compositions. The Grt‐I has total REE contents (∑REE) of 101.47 to 262.87 ppm, with steeply right‐inclining REE distribution patterns (LREE/HREE of 3.81 to 68.50) and positive Eu anomaly. The Grt‐II core has ∑REE of 163.49–249.52 ppm, LREE/HREE of 2.00–4.71, and negative Eu anomaly. The Grt‐II rim has ∑REE of 46.34–99.99 ppm, with LREE/HREE of 18.06–177.23, and positive Eu anomaly. The ∑REE of Grt‐III range from 31.71 to 219.02 ppm, with flat REE distribution pattern and positive Eu anomaly, and the LREE/HREE ranges from 2.16 to 9.07. These garnets have similar trace element compositions, featured by LILE‐depletions (e.g., Rb, Ba and Sr) and HFSE enrichments (e.g., Th, U, Nb and Ce). Micro‐texture and geochemical composition of garnets indicate that the Lamasu magmatic hydrothermal system could have changed from an open to a closed environment, and from infiltration metasomatism to diffusive metasomatism, which formed Grt‐I, Grt‐II and Grt‐III successively. These garnets generally formed in a relatively oxidized fluid environment, which inhibited early sulphide precipitation and favoured for Cu enrichment during the ore fluid evolution.

Geology, geochemistry, fluid inclusion microthermometry and depositional environment of the Early Cretaceous Meymeh sub-seafloor replacement sideritic-ankeritic iron deposit, Malayer-Esfahan Metallogenic Belt, Iran

Meymeh sideritic-ankeritic iron deposit in the eastern part of Malayer-Esfahan Metallogenic Belt (MEMB) is formed in the sedimentary-volcanic rocks of the Early Cretaceous sequence. Ore mineralization in the study area based on stratigraphic location and type of host rocks divided in two ore horizons. Lower ore horizon located in the dolomitic-sandy limestone (Kc3) unit, which upper ore horizon is formed in thin-bedded limestone (Km) rocks. Petrographic studies indicates that mineralization comprises three ore facies: stockwork, bedded and massive ore facies. The most important primary minerals are siderite, ankerite, ferroan-dolomite, pyrite, pyrolusite, barite and minor chalcopyrite. The most frequent textures in the ore zones include laminae, replacement, vein-veinlet, massive and banded. Dolomitization and silicification are the main wall rock alteration styles; alteration intensity increases towards the ore zones. Based on relationships between ore minerals and rock forming minerals, ore mineralization (hypogene and supergene) in the Meymeh deposit formed during three main stages: fine-grained Fe-carbonate bands are intricately interlayered with dolomite beds. Sideritic-ankeritic bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a syn-sedimentary to early diagenesis origin. Coarser-grained stage two siderites and ankerites show breccia and vein-veinlet textures, and are considered to have formed by replacement during burial diagenetic sub-seafloor fluid flow. In stage three, siderite and ankerite were converted to secondary iron oxides such as oxide/hydroxide Fe minerals during meteoric water flow through the inverted normal and thrust faults and uplift. The primary two-phase fluid inclusions in the quartz-2 and ankerite-2 minerals that have been investigated from the ore mineralization section of the Meymeh deposit are homogenized at temperatures between 110.3 to 226.9 °C. Salinities of the primary fluid inclusions range from 3.39 wt.% to 14.77 % NaCl eq. This finding suggests that hydrothermal brine fluid mixing with seawater could be the primary mechanism that prompted ore formation. The similarity of REE patterns between siderite and ankerite in different ore facies and host rock carbonates indicates their derivation from the same ore fluids. The Meymeh deposit is considered a typical case of sedimentary hydrothermal diagenetic sideritic-ankeritic mineralization, in which minerals deposited when hydrothermal fluid was released from anoxic to suboxic water columns.

Magmatic fluids responsible for lode gold mineralization in the giant Linglong deposit at Jiaodong, North China Craton: Constraints from Li[sbnd]O isotopes

The Jiaodong Peninsula, located in the eastern margin of the North China Craton, contains numerous world-class lode gold deposits that combined have a proven gold reserve of >5000 t. Despite a large amount of research, the source and evolution of hydrothermal fluids responsible for the gold mineralization in this region remains highly debated. The Linglong gold deposit is one of the largest deposits in the Jiaodong Peninsula. Gold lodes at Linglong are predominantly hosted within the Late Jurassic Linglong granite and structurally controlled by NE-trending faults. They are closely associated with large-scale hydrothermal alteration, which is paragenetically characterized by hydrothermal K-feldspar in pre-ore stage I, and sericite-quartz-pyrite in syn-ore stages II and III. The syn-ore alteration mineral assemblage is distributed within and around the gold lodes, whereas the K-feldspar alteration remains mainly distal to the gold lodes. From the unaltered to progressively intensifying K-feldspar-altered Linglong granites, there are decreasing trends for both whole-rock Li concentrations and δ7Li values, which are attributed to Rayleigh fractionation. The two values increase covariantly within the sericite-quartz-pyrite alteration zone towards the mineralization center, suggesting that further fluid-rock interaction and buffering of the ore fluid accounts for the observed Li elemental and isotopic variations. Fluid inclusion assemblages in quartz of stages I-III have homogenization temperatures of 349°-312 °C, 318°-272 °C, and 307°-246 °C, respectively. In-situ oxygen isotopic analysis on stage I quartz reveals δ18OQ values of 10.9–14.7 ‰, corresponding to δ18OW values of 4.3–9.2 ‰ for the equilibrated fluid. The δ18OW values of the equilibrated fluids of stages II and III are 5.6–8.9 ‰ and 5.2–8.8 ‰, respectively. The oxygen isotopic compositions indicate that magmatic fluid is prevailing for the K-feldspar and sericite-quartz-pyrite alterations. A binary mixing model using Li concentrations and δ7Li values further implies that the hydrothermal fluid was mainly sourced from coeval mafic magmas derived from the metasomatized lithospheric mantle. Our study highlights that lithium and in-situ oxygen isotopes can be used effectively to fingerprint the source of hydrothermal fluids and fluid-rock interactions.

Fluid-rock interaction modeling to constrain Au enrichment: Implication for the giant Jiaodong Au mineralization, eastern North China Craton

The devolatilization model of the metasomatized lithospheric mantle without pre-enriched gold has been proposed to account for the giant gold mineralization. An excellent example is the world-class Jiaodong gold province with >5000 tonnes Au resources in the eastern North China Craton. The auriferous fluid transport and gold enrichment during wallrock alterations are two vital processes to determine the giant gold mineralization formation in this province. However, the effects of the fluid-rock interaction with alterations on the auriferous fluid transport and gold enrichment still keep poor understanding, which leads the above model to be imperfect. The giant Jiaojia goldfield in this province recorded a wallrock alteration evolution from K-feldspar alteration to pyrite-sericite-quartz alteration, and some parts of the latter can become gold orebodies when the gold grade is >1 ppm. This study conducts thermodynamic fluid-rock interaction modeling to reveal auriferous fluid transport and coupled relationship between gold enrichment and alteration mineral assemblage based on the alteration-mineralization and ore fluid characteristics of the goldfield. The modeling of fluid-rock interaction with cooling indicates the transformation of Au-Cl complexes to Au-S complexes combined with the total sulfur concentration decrease by pyrite precipitation when cooling from ∼460 °C can trigger the dispersive gold precipitation, which should hinder the gold long-range transport to lower ambient temperature. The high oxygen fugacity at >400 °C can enhance Au-Cl complexes stability, and the low pH can maintain high total sulfur concentration in the auriferous fluid, both of which facilitate the long-range gold transport to a lower-temperature environment. The auriferous fluid would acquire higher pH by the buffering of feldspars or sericite, which was beneficial for the high-efficiency precipitations of pyrite and gold. The ankerite-siderite assemblage without pyrophyllite in the pyrite-sericite-quartz alteration zone indicates that a cumulative fluid to rock mass ratio (f/r) of 3.8–4.8 should be needed for the transformation from K-feldspar alteration to pyrite-sericite-quartz alteration according to the fluid-rock interaction modeling at 300 °C and 2000 bar. In the case of auriferous fluids with ≤200 ppb Au concentration, the single fluid-rock interaction can only elevate the gold grade to ≤0.69–0.87 ppm in the pyrite-sericite-quartz alteration zone at f/r 3.8–4.8. Therefore, the fracture-induced fluid flow coupled with fluid-rock interaction is proposed to the prerequisite to elevate the gold grade to >1 ppm in the pyrite-sericite-quartz alteration zone. The metasomatized lithospheric mantle volume for the required ore fluid and Au in the Jiaodong province is estimated according to the modeling results and alteration-mineralization characteristics, which provides a link between the mantle without abnormal Au enrichment and the alteration-mineralization processes.

Geology, fluid inclusions, and isotope signatures reveal hydrothermal evolution and genesis of the Aqishan Pb-Zn deposit in Eastern Tianshan and implications for exploration of skarn mineralization

The Aqishan deposit (1.97 Mt at 1.05 % Pb + Zn) is located in the Aqishan-Yamansu arc, on the southwest section of the Eastern Tianshan, Northwest China. Pb-Zn mineralization is mainly hosted by garnet skarn and the volcaniclastic rocks of the Yamansu Formation. To decipher the evolution of ore fluids and source(s) of ore-forming components, and constrain the ore genesis, a combination of geological work, fluid inclusions, and C-O-S-Pb isotope systematics is presented. Detailed field and microscopic observations reveal that the skarn alteration and ore formation at Aqishan could be divided into four hydrothermal stages, consisting of prograde skarn (I), retrograde skarn (II), quartz–sulfides (III), and carbonate stage (IV). Sphalerite and galena are the dominant ore minerals generally occurring as disseminations and veins with locally laminated ores in stage III. Fluid inclusion data show that the fluids responsible for prograde skarn are high-temperature (342–518 °C) and high-salinity (7.3–44.2 wt% NaCl equiv.), and recorded fluid phase separation from an intermediate-salinity supercritical fluid, which was exsolved from crystallizing granitic magma. Microthermobarometry results limit the retrograde alteration within the temperature range of 313 to 388 °C and trapping pressures of 98 to 241 bars (1.0–2.5 km), with pressure dropping from lithostatic to hydrostatic regimes associated with a phase separation event. The boiling fluid inclusions in ore stage quartz show that the Pb-Zn mineralization formed at 205–357 °C, with a decrease in temperature facilitating the precipitation of ore-forming metals from hydrothermal system. Carbon and oxygen isotope compositions of calcite indicate a predominantly magmatic source for ore fluids at Aqishan with later meteoric water involved. Bulk and in situ sulfur values of sulfides range from −6.4 to −0.7 ‰, consistent with a dominantly magmatic origin. Sulfides together with the granite porphyry and tuff, show relatively homogeneous Pb isotope compositions, indicating that abundant ore-forming metals were sourced from a magmatic reservoir with contribution from country rocks. Fluid inclusion study and available calcite C-O isotopes suggest that significant decrease in temperature and fluid-rock interactions remarkably contributed to the metal deposition at Aqishan. The skarn alteration with typical alteration assemblages of garnet–diopside is proposed as an effective mineral exploration target of skarn(–like) ores in Eastern Tianshan.

Magmatic rare earth element mineralization and secondary fluid-assisted redistribution in the Kamthai carbonatite complex, NW India

Calcite carbonatites from the Kamthai alkaline complex in western India are highly enriched in Sr and the rare earth elements (REE) owing to their formation from highly differentiated carbonatitic magma. Pegmatitic calcite carbonatites preserve rare magmatic intergrowth of rod/pod-like carbocernaite [(Ca, Na)(Sr, REE, Ba)(CO3)2] and REE + Sr-rich calcite. This intergrowth is interpreted to be the result of rapid simultaneous crystallization of the phases from a brine-rich carbonatitic magma. Primary carbocernaite, calcite, and britholite [(LREE, Ca)5(SiO4, PO4)3(OH, F)] sequestered Sr and REE from the brine-rich melt and constitute the major sources of these elements during hydrothermal redistribution. Replacement textures suggest fluid-assisted, post-magmatic hydrothermal REE redistribution through dissolution-reprecipitation process that concentrated the REE into secondary REE-(fluor/hydroxyl)carbonates, cerianite (ideally Ce4+O2), ancylite [(Sr, Ca)LREE(CO3)2(OH)∙(H2O)], and monazite. Hydrothermal alteration of magmatic calcite leached out the LREE, whereas in case of carbocernaite and britholite, an increase in their REE contents is noticed, followed by pseudomorphic replacement by REE-(fluor/hydroxyl)carbonate minerals.The carbonatite-derived hydrothermal fluids initially transported the REE as sulfate and later as hydroxyl‑carbonate complexes. The ligand transition was a consequence of the precipitation of baryte, celestine, and pyrite, which led to a reduction in fluid sulfur content and increase in its pH due to interaction with carbonates. Precipitation of secondary REE minerals was triggered by decrease in ore fluid temperature due to interaction with relatively cooler, oxidized fluid and interaction with carbonates. Carbocernaite, ancylite, cerianite, and Ce-bearing REE-(fluor)carbonates constitute early precipitated secondary REE minerals along with baryte, celestine, and pyrite as by-product phases, whereas both Ce- and La-rich, hydroxyl-dominated members of bastnäsite, parisite, and synchysite, along with monazite were the dominant REE minerals which precipitated during the advanced hydrothermal stage, with siderite as a by-product.

Disseminated gold mineralization under varied redox conditions: Constraints from microscopic observation, geochemistry, and thermodynamic modeling on fluid-rock interactions in lamprophyres, Zhenyuan gold deposit

The controls of fluid redox states on disseminated gold mineralization remain ambiguous. To address this, this study compared the hydrothermal alteration and auriferous pyrite geochemistry in lamprophyres, Zhenyuan gold deposit, which were infiltrated by fluids with varied redox states. Vein-A, vein-B, and vein-C were distinguished and they contain different sulfur-bearing minerals, i.e. pyrite, pyrite and barite, and barite, respectively. This suggests the gradual oxidzing formation conditions from vein-A to vein-C. Alteration halos enveloping vein-A and vein-B develop disseminated pyrite. In the vein-A halos, alteration minerals transit from pyrite to Fe‑carbonates outwards, which was well replicated by the thermodynamic simulation of the successive reaction of H2O-CO2-H2S-Au-bearing fluid with lamprophyres. The precipitation of pyrite grains in vein-B halos is farther from vein-B, suggesting progressive fluid-rock interaction (FRI) first induced the reduction of oxidized S followed by the pyrite precipitation. Additionally, δ34S values of pyrite are more scattered in vein-B halos (−8.0 to 4.7‰) than in vein-B, vein-A, and vein-A halos, implying the sulfate-sulfide disequilibrium. Pyrite in vein-B halos displays higher As concentrations than that in vein-A halos due to the strong redox dependence of As partition from fluids to pyrite. Au and Au/As of halo-hosted pyrite grains decrease outwards, and pyrite in vein-B halos has low Au and Au/As (below 93.93 ppm and 0.005, respectively) compared to that in vein-A halos (below 1597.08 ppm and 0.031, respectively). It is thus suggested that Au concentrations in ore fluids decreased during outwards FRI, and the lack of pyrite grains in proximal alteration zones results in lower gold concentration in pyrite under relatively oxidized conditions. This study reveals that the fluid redox conditions can affect As concentrations and spatial distribution of pyrite in alteration halos, which directly controls invisible gold precipitation.

Fluid evolution and metallogenesis of the Zhangjiapingzi gold deposit in mianning, Sichuan province: constraints from fluid inclusion studies

Introduction: The Zhangjiapingzi gold deposit, located at the western margin of the Yangtze Craton and controlled by the Jinhe-Chenghai deep fault, is a newly discovered super-large gold deposit in the Danba-Mianning metallogenic belt. The gold ore bodies are hosted in the Middle Triassic (T2) altered dolomite (Dm), and have two types of mineralization: altered rock type and quartz vein type. Previous studies on this deposit are rare, especially on the ore-fluid characteristics, which limit the understanding on the ore genesis.Methods: This study focused on fluid inclusions in quartz from altered rocks, and used microthermometry and laser Raman spectroscopy to investigate the properties and sources of ore-forming fluids, and to determine the ore genetic type.Results and discussion: The results show that the fluid inclusions are mainly CO2-H2O-NaCl inclusions, with medium temperature (220–300°C), low salinity (&amp;lt;10%), medium-low density (0.79–1.01 g/cm3) and high contents of CO2 and CH4, resembling typical orogenic gold ore fluids. We suggest that the Zhangjiapingzi is best classified as orogenic type, and our findings provide new insights into the fluid origin and metallogenesis of orogenic gold deposits in the Danba-Mianning metallogenic belt.

Physicochemical evolution and mechanism of a skarn system: Insights from the world-class Mazraeh Cu deposit, NW Iran

The Mazraeh Cu skarn deposit, NW Iran, is situated at the lithological contact between an Oligo−Miocene granodiorite and Cretaceous limestones. The causative granodiorite has I-type medium- to high-K calc-alkaline composition that formed in a subduction arc setting. Metasomatism generated exoskarn and endoskarn characterized by a temporal and spatial mineral-chemistry zonation with massive garnet and garnet-pyroxene proximal and epidote-amphibole skarn at the distal zones of the exoskarn. Our data reveal a multi-stage evolution of the hydrothermal system. Accordingly, the prograde stage was formed by barren, oxidized, high-temperature and hypersaline magmatic fluids, which suggests that the parental magma was emplaced at low lithostatic pressure conditions (∼1.5 km depth). By contrast, during the retrograde stage the regime changed from lithostatic to hydrostatic conditions, resulting in a significant decrease in pressure and oxygen fugacity, boiling, cooling, and mixing, respectively. Dilution by meteoric fluids led to additional cooling and lower salinity of the ore fluids. Sulfur isotope compositions (δ34S = −2.8‰−1.3‰) of sulfides indicate a unique magmatic source of the sulfur. The δ18Ofluid and δDfluid values of the prograde stage fluid also indicate a magmatic origin, while their gradual depletions from the prograde to retrograde stage imply increased mixing with meteoric water. We propose that the fluid mixing scenario continued over the entire life cycle of the mineral system in an open hydrostatic regime as reflected in the oxygen isotope values. It is suggested that boiling and fluid mixing played a significant role in the formation of this giant Cu skarn deposit.

The Peninsular Ranges orogenic gold belt: Supporting evidence from the San Pedro Mártir mining area (Baja California, Mexico)

The gold deposits in the northern Sierra San Pedro Mártir region of the Peninsular Ranges batholith (PRB) occur within a middle-Cretaceous suture zone that juxtaposes the Alisitos island arc against the continental margin of North America. Mineralization shares affinities with orogenic-type gold deposits and typically consists of shear zone-related gold-bearing quartz-carbonate veins, with minor amounts of sulfides, white mica and native gold. Host rock types encompass varied synorogenic plutonic rocks and greenschist to amphibolite facies metamorphic rocks. In this paper, we present four 40Ar/39Ar ages of hydrothermal white micas for representative gold deposits: 106.2 ± 0.2 Ma (Valladares), 100.5 ± 0.2 Ma (La Fortuna-San José), 94.2 ± 0.2 Ma (El Socorro), and 92.2 ± 0.2 Ma (Corazón de Oro de Jesús). The first two ages correspond to highly accurate determinations of the timing of the orogenic gold event, whereas we attribute the younger ages to thermal resetting caused by the emplacement of a nearby large La Posta-type pluton (∼97‒90 Ma). Additionally, in order to establish the maximum age of mineralization for the granitoid-hosted deposits, three new U–Pb zircon ages were obtained for plutonic rocks (136.7 ± 0.9 Ma, Corazón de Oro de Jesús quartz diorite orthogneiss; 107.0 ± 0.5 Ma, Valladares tonalite pluton; 105.0 ± 2.0 Ma, Santa Cruz diorite pluton) Furthermore, fluid inclusions were analyzed from six gold deposits. Four different types of ore fluids were distinguished, with likely temperatures of trapping between ∼300° and >600 °C, and salinities between ∼1 and 17 wt.% NaCl equiv. Gold mineralization formed subsequent to the initial collisional phase, occurring simultaneously with regional metamorphism and pluton emplacement. At least some synorogenic granitoid-hosted deposits were formed under a transient dextral transpressive regime, intercalated between the predominant regional orthogonal compression. A connection is established between these gold deposits and others found in historical mining districts along the exhumed axis of the PRB (e.g., El Álamo), and we propose that all form part of the ‘Peninsular Ranges orogenic gold belt’.

The Influence of Fluid-Exsolving Depth on Mineralization Quality: Evidence from Biotite and Zircon Mineralogy and Fluid Inclusions from the 460 Gaodi Porphyry Mo-Cu Deposit, NE China

The recently discovered 460 Gaodi porphyry Mo-Cu deposit is a sub-economic deposit characterized by low Mo-Cu grades, dispersed mineralization, and separated Mo- and Cu-ore bodies. This study aims to elucidate the factors underlying this type of sub-economic mineralization. Electron-microprobe analyses of biotite from ore-related granite porphyry yielded Ti-in-biotite crystallization temperatures of 677–734 °C (an average of 719 °C) and biotite phenocryst crystallization depths of 6.0 to 12.9 km. LA-ICP-MS analyses of zircons from the same sample revealed average zircon Ce4+/Ce3+ ratios of 299.7 and elevated zircon lg(ƒO2) ratios, with an average ΔFMQ of +6.6 ± 1.9. These discoveries suggest that the magma responsible for ore formation boasts a high degree of oxidation, yet also possesses a magma chamber located at a significant depth within the upper crust. This implies an extensive exsolving depth for fluids. Furthermore, our microthermometry analysis of fluid inclusions reveals that a portion of the fluid experiences considerable conductive cooling as it ascends along the conduit, owing to the depth of fluid exsolution. This process results in the ore fluids remaining in the liquid-only region without undergoing boiling, which is conducive to the enrichment of metals. We emphasize the fact that fluid-exsolving depth plays a critical role in determining the metal grades and economic value of a porphyry deposit by regulating the P-T evolution path of the ore fluids

In-situ trace element and Li-isotope study of zinnwaldite from the Degana tungsten deposit, India: Implications for hydrothermal tungsten mineralization

Zinnwaldite, a Li, Rb-mica is a common magmatic or hydrothermal mineral in many tungsten deposits. In this study, we use the trace element and Li-isotope composition of zinnwaldite from Degana, the largest tungsten deposit of India, to constrain the source and evolution of the ore fluid, the precipitation mechanism of W and the roles of fractional crystallization, fluid exsolution and fluid-rock interaction in the mineralization process. Textural evidence suggests that the mineralization in the Degana rocks involved at least two stages of fluid infiltration. The earlier of the two was multi-pulsed and responsible for the primary tungsten and zinnwaldite mineralization. The second hydrothermal stage caused partial dissolution and reprecipitation of both wolframite as well as zinnwaldite, reflected in the patchy zones that replace both minerals. The ore fluid had high concentration of Li, Rb, Cs, Nb, and Ta as evident from zinnwaldite chemistry. The δ7Li of the ore fluid (+14 to +19 ‰), estimated from the Li-isotope composition of zinnwaldite in mineralized veins (δ7Li = +12 to +17‰), is significantly heavier compared to upper/middle continental crust, ruling out the possibility of a metamorphic fluid source. The high concentration of incompatible elements including Cs, extremely low K/Rb (10–15), is best explained by its growth from fluids exsolved from an enriched, late-fractionated granitic melt. Alteration of the host granite, formation of greisen, large-scale albitization and muscovitization are indicative of fluid-rock interaction, which might have increased the fluid pH, destabilizing fluoride complexes of W, resulting in the precipitation of wolframite. The shift in the chemical and Li-isotope composition from the early mica to late altered ones in the greisen indicates interaction of the hydrothermal fluid with the host rocks. A U-Pb Concordia age of 838 ± 9 Ma retrieved from magmatic domains of zircon from the Degana granite dates it emplacement and represents the maximum age of the W-mineralization.

Fluid Evolution and Ore Genesis of the Songjianghe Au Deposit in Eastern Jilin Province, NE China: Constraints from Fluid Inclusions and H-O-S-Pb Isotope Systematics

The medium-sized Songjianghe Au deposit is located in the southeastern part of the Jiapigou-Haigou gold belt (JHGB) in central eastern Jilin Province, NE China. The gold mineralization is primarily characterized by disseminated-style ores and hosted in the low-/medium-grade metamorphic rocks of the Seluohe Group. The ore bodies are governed by NNW-striking brittle-ductile structures and spatially correlated with silicic and sericitic alterations. Four alteration/mineralization stages have been distinguished: (I) Quartz-pyrrhotite-pyrite, (II) quartz-polymetallic sulfides, (III) quartz-pyrite, and (IV) quartz-calcite. The fluid inclusion (FI) assemblage in quartz from Stage I comprises C1-type, C2-type, C3-type, and VL-type FIs, with total homogenization temperatures (Th-total) of 292.8 to 405.6 °C and salinities of 2.8 to 9.3 wt% NaCl eqv. Quartz from Stage II (main ore stage) developed C2-, C3-, and VL-type FIs, with a Th-total of 278.5 to 338.9 °C and salinities of 2.8 to 8.1 wt% NaCl eqv. Stage III is characterized by coexisting C3- and VL-type FIs in quartz, with a Th-total of 215.9 to 307.3 °C and salinities of 2.4 to 7.2 wt% NaCl eqv. Only VL-type FIs are observed in Stage IV, with a Th-total of 189.5 to 240.4 °C and salinities of 3.7 to 5.7 wt% NaCl eqv. The Laser Raman spectroscopic results demonstrated minor CH4 in the C-type FIs from Stages I and II. The results suggest that ore fluids may have evolved from a medium-high temperature, low-salinity immiscible CO2-NaCl-H2O ± CH4 system to a low temperature, low-salinity homogeneous NaCl-H2O system. Fluid immiscibility caused by the rapid drop in pressure may have been the main trigger for gold-polymetallic sulfide precipitation. The Songjianghe Au deposit may have been formed under 352–448 °C and 850–1380 bar pressure, based on the isochore intersection for Stage II fluid inclusions. The H-O isotopic compositions (Stage I: δ18Ofluid = 5.6 to 5.8‰, δD = −96.2 to −95.7‰; Stage II: δ18Ofluid = 3.7 to 4.2‰, δD = −98.7 to −89.8‰; Stage III: δ18Ofluid = 1.2 to 1.4‰, δD = −103.5 to −101.2‰) indicate that the hydrothermal fluids are dominated by magmatic water in the early stages (Stages I and II) and mixed with meteoric water since Stage III. The pyrite S-Pb isotope data (δ34S: −2.91 to 3.40‰; 206Pb/204Pb: 16.3270 to 16.4874; 207Pb/204Pb: 15.2258 to 15.3489; 208Pb/204Pb: 36.6088 to 36.7174), combined with Pb isotopic compositions of the intrusive rocks and wall rocks (the Seluohe Group) in the ore district, indicate that the ore-forming materials at Songjianghe are predominantly from a magmatic source and may have been affected by the contamination of the Seluohe Group. In accordance with the features of ore geology, ore-forming fluids and metals, and geodynamic setting, the Songjianghe Au deposit belongs to a mesothermal magmatic hydrothermal vein gold deposit, which formed in the intermittent stage of Paleo-Pacific plate subduction during the Late Jurassic.

Mineralization and genesis of the orogenic gold system in the Kalamaili area, East Junggar, Xinjiang, northwestern China

There are many lode gold deposits and occurrences in the Kalamaili area of the East Junggar, northwestern China. However, little is known about the specifics of ore geology and geochemistry of these gold deposits because of very limited exploration and research work in this region. Field geology, fluid inclusions, stable isotopes, trace elements in quartz, and hydrothermal zircon U-Pb dating are combined in this study to constrain the nature and source of ore fluids, the timing of mineralization, and the mechanism of gold precipitation in Kalamaili. The gold deposits are confined to a narrow zone between two regional NW- to NWW-trending shear zones and are structurally controlled by secondary, high-angle faults of the shears. The orebodies occur in the Middle Devonian and Lower Carboniferous, zeolite to lower greenschist facies clastic sedimentary and pyroclastic rocks. Gold mineralization occurs as auriferous quartz-sulfide ± tourmaline veins/veinlets and disseminated ores in the immediate altered wall rocks. The ore mineralogy is relatively simple and dominated by quartz with minor to trace amounts of sulfides (pyrite and arsenopyrite), sericite, calcite, and native gold. Quartz of various generations contains three types of fluid inclusions, including predominant H2O-CO2-NaCl inclusions and subordinate H2O-NaCl inclusions and CO2 ± CH4 ± N2 inclusions. The mineralizing fluid is uniformly characterized by a medium to high homogenization temperature (mostly 240−330 °C), low salinity (typically &amp;lt;6 wt% NaCl equivalent), reduced, and CO2-rich-H2O-NaCl ± CH4 fluid. The hydrogen and oxygen isotope data (δ18OH2O = +5.6‰ to +14.3‰, δDH2O = −99‰ to −62‰) indicate a metamorphic origin for the mineralizing fluid. The majority of sulfide δ34S values range between 0‰ and +10‰, indicative of a largely sedimentary rock reservoir of sulfur in the ore-forming fluids. Geochemically, the auriferous quartz is characterized by low concentrations of most trace elements including Ti, Al, Li, Ge, and Sb. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb isotope dating of hydrothermal zircons from the auriferous quartz vein yielded a weighted mean 206Pb/238U age of ca. 313 Ma. Phase separation of ore fluids and fluid-rock interaction are suggested as key mechanisms for the gold precipitation. Integrated geological and geochemical evidence indicates that formation of the orogenic gold system in Kalamaili is related to the transition from compressional to transcurrent deformation during the Late Carboniferous. Target gold exploration in this region should focus on the northeast side of the Kalamaili fault zone, where there exist suitable structural and stratigraphic trap sites with high fluid flux and potential gold mineralization.

Genesis of the Jinji gold deposit in the Jiangnan Orogen, South China: Constraints from geology, chlorite geochemistry, age and H-O-S-Pb isotopes

The Jinji gold deposit (northeastern Hunan province) is located in the central Jiangnan Orogenic Belt, South China. The auriferous sulfide-calcite-quartz vein orebodies, controlled by near NNW-trending faults, occur in the Lengjiaxi Group metamorphic rocks spatially associated with the Banshanpu granitic pluton. Metallic minerals include mainly pyrite, arsenopyrite, and pyrrhotite, with minor sphalerite, chalcopyrite, galena, and native gold. The main alteration types include K-feldspar, silicic, sericite, carbonate, and chlorite. The gold mineralization comprises three stages: (1) quartz–K-feldspar–chlorite–carbonate–arsenopyrite–pyrite, (2) quartz–carbonate–chlorite–polymetallic, and (3) quartz–carbonate–fluorite–pyrite. Geothermometry of chlorite yielded formation temperatures of 261–332 °C (stage 1) and 262–290 °C (stage 2), whereas the calculated oxygen fugacity of chlorite is −45 to −40 (stage 1) and −42 to −39 (stage 2). The δ18OH2O and δD values are 8.8–10.5 ‰ and −73.3 to −72.8‰ (stage 1), 3.2–4.8 ‰ and −83.6 to −80.7‰ (stage 2), and −1.4 to −0.4 ‰ and −82.2 to −78.6‰ (stage 3), respectively. These results indicate that the primary ore fluids (stages 1 and 2) were derived from a granitic magma and were mixed with meteoric water in stage 3. The gold-bearing sulfides (pyrite, pyrrhotite and galena) have δ34S values of −4.8 to +5.8‰, whilst their 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios are of 35.975–36.204, 15.160–15.231, and 16.100–16.176, respectively. The sulfide S-Pb isotope compositions indicate that the metals were likely derived from the Banshanpu granitic magma, which was produced by partial melting caused by Early Paleozoic crustal thickening. The Jinji deposit exhibits many features of orogenic gold mineralization, such as the ore-controlling structures and mineralization styles (quartz-calcite-sulfide veins). The Jinji deposit is the first reported gold deposit in the central Jiangnan orogen with a genetic link to early Paleozoic granitic magma.

Ore-fluid source of multiphase gold mineralization at Tuanshanbei in the central Jiangnan Orogen (NE Hunan, South China): Insight from geology, quartz H-O and monazite in-situ Nd isotope compositions

The Jiangnan Orogen is China’s third largest gold province, and has undergone complex orogenic processes and tectonic overprinting, forming multistage magmatism, deformation, metamorphism, and gold mineralization. Our field studies indicate that the Tuanshanbei gold ores (central Jiangnan Orogen) have two generations of auriferous quartz-ankerite-pyrite-arsenopyrite veins (Q2 and Q3), with the latter containing abundant ankerite and base metal sulfides. Both Q2 and Q3 veins are younger than the emplacement of the Tuanshanbei granodiorite vein-ore host, and cut by post-ore diabase dikes. Q2 veins were likely associated with the Early Devonian near N-S-directed shortening, along sub-horizontal EW-/WNW-striking transpressive faults, whereas Q3 veins (hosting ∼ 70% of the total Au resource), were primarily hosted in the Early Triassic moderately-/steeply-dipping NW-striking tensional/transtensional faults and moderately-dipping NE-/NNE-striking transpressive faults, associated with NW-SE-directed shortening. Discrete structural styles and mineral assemblages from Q2 and Q3 suggest superimposing gold mineralization at Tuanshanbei. To determine the source(s) of the ore-forming aqueous-carbonic fluids, we conducted mineralogical and in-situ LA-(MC)-ICP-MS Nd-isotope analyses on two generations of hydrothermal monazite, and H-O isotope analyses on the quartz from Q2 and Q3 veins.The results suggest that the Tuanshanbei ore fluids have δD = −66.7 to −57.6‰ (Q2) and −66.8 to −66.2‰ (Q3), and calculated δ18OH2O = 4.5–9.7‰ (Q2) and 6.1–7.8‰ (Q3). The H-O isotopic data suggest a metamorphic and/or magmatic water source for the ore fluids. Hydrothermal monazite (coexists with native gold and auriferous sulfides) from Q2 and Q3 veins displays subtle heterogeneous BSE response along cracks and grain margin, suggesting alteration there. The 147Sm/144Nd and 143Nd/144Nd ratios of altered parts of Q2 and Q3 monazite grain are significantly perturbed via dissolution–recrystallization by the hydrothermal fluids, whereas their initial Nd isotope compositions seem to be less affected. The unaltered Q2 and Q3 monazite samples have similar initial εNd values (Q2: −8.07 to −7.36, Q3: −10.87 to −9.99) to the Neoproterozoic Cangxiyan Group greenschist–amphibolite-facies metamorphic rocks. Geological and structural evidence suggest both Q2 and Q3 ore-forming events were related to metamorphism, typical of orogenic gold deposits. We interpreted that various parts of the basement were metamorphosed (to near the greenschist-amphibolite-facies boundary) at different times, during which the gold-bearing metamorphic fluids produced migrated into the same structural conduits and deposited the ores there in two ore-forming episodes.

Trace elements and sulfur isotopic compositions of sulfides in the giant Dahongshan Fe-Cu-(Au-Co) deposit, SW China: Implications for fluid evolution and Co enrichment in IOCG systems

Iron-oxide-copper–gold (IOCG) deposits are characterized by Cu-Au polymetallic mineralization and may contain large amounts of critical metals (e.g. Co, REE, and U) as by-products. Sources of ore-forming fluids and metals of the IOCG deposits remain controversial, either a magmatic or basinal source has been advocated. The ∼1660 Ma Dahongshan Fe-Cu-(Au-Co) deposit in the Kangdian region in southwestern China has spatially separated Fe and Fe-Cu orebodies, in which Co-rich pyrite has been identified in different types of ores and host rocks. Here, we integrate in-situ sulfur isotopic compositions and trace element geochemistry of sulfides to characterize the nature of ore fluids and the Co enrichment processes.Paragenetic sequences of sulfides in the Dahongshan deposit include: Pyrite (Py I) associated with magnetite mineralization in Fe ores, which has rims (Py II-1) intergrown with hematite-chalcopyrite assemblage; Pyrite and chalcopyrite (Sulfide II) in the Fe-Cu ores with protolith of arenaceous dolostone (Sulfide II-2) and interbedded argillaceous siltstone and dolostone (Sulfide II-3); and Sulfide III in post-ore reworking veins formed during regional metamorphism. Py I shows a magmatic sulfur isotopic signature (−2.2 to 1.2‰) and relatively high Au-Cu-Zn contents, indicating a predominance of magmatic-hydrothermal component during magnetite formation. By contrast, the combined sulfur isotopes and trace element compositions of Sulfides II in Fe-Cu ores indicate the mixing of two distinct fluid endmembers: Sulfides precipitated from one endmember show magmatic affinity with low δ34S values (as low as −2.1‰), high Se/S ratios (∼11.8 × 10-4), high Co contents (∼10600 ppm) and Co/Ni ratios (∼110), likely from a magmatic-hydrothermal source of mafic magmas affinity; whereas sulfides from the other endmember have overall higher δ34S values (up to 15.8‰), relatively lower Se/S ratios (∼0.09 × 10-4), lower Co contents (∼900 ppm) and Co/Ni ratios (∼13). Together with the published B-O isotopic data, the high-δ34S fluid is attributed to externally-derived basinal brine at Dahongshan. The clear negative correlations between δ34S values and Se/S ratios, Co contents, as well as Co/Ni ratios of Py II highlight that the incursion of exchanged basinal brines may be vital for triggering economic sulfide mineralization in Dahongshan. Furthermore, when assuming a similar predominance of magmatic fluids, Py I associated with magnetite ores is less enriched in Co (with lower Co/Ni ratios) than Py II from Fe-Cu ores, which can be tentatively ascribed to the decreasing Co solubility limits and formation of cattierite (CoS2) solid solution in pyrite as the fluid evolved. Finally, sulfide III in reworking veins show comparable δ34S values and Se/S ratios, but higher Co contents (up to 3.1%) than sulfide II, implying that Co can be further enriched during later remobilization events. This study exemplifies the utility of sulfide as a robust monitor of ore-forming process in IOCG system.