Prograde Skarn Research Articles

In Situ U—Pb Dating and Trace Element Analysis of Garnet in the Tongshanling Cu Polymetallic Deposit, South China

The grossular–andradite garnet is an ideal mineral for indicating the formation age of skarn, which also pretends to constrain skarn processes because of its higher REE (rare earth elements) content. The Tongshanling deposit is a medium-sized reduced skarn Cu–W–Pb–Zn deposit associated with a highly evolved I-type granodiorite intrusion in the Nanling metallogenic belt, South China. Different mineral assemblages, microscopic characteristics, and BSE images distinguish two kinds of garnets in the prograde and retrograde skarn stages. The garnet grains from the prograde skarn stage have a U–Pb isochron age of 165.4 ± 3.8 Ma (MSWD = 0.7) and that from the retrograde skarn stage have a U–Pb isochron age of 159.5 ± 1.7 Ma (MSWD = 1.8), implying that the thermal metamorphism and hydrothermal metasomatism mainly occurred in the middle Upper Jurassic. The total amount of rare earth elements (∑REE+Y) in the garnet gradually decreased and the REE patterns shifted from enriched HREE with Eu negative anomaly to HREE-depleted with Eu positive anomaly. The decreasing U content and increasing Eu anomaly in the retrograde skarn stage indicate a redox environment change from oxidation to reduction. However, garnet from different elevations within the same stage (+90 m, +5 m, −35 m, −200 m, and −400 m) exhibit similar REE patterns, despite weak cooling and significant depressurization processes confirmed by fluid inclusion microthermography. As a result, the REE content and patterns are dominated by the REE species of parent fluids, which are changed over time by symbiotic REE-enriched mineral precipitation and the redox environment, while being slightly affected by the fluid pressure. Grossular garnets, rich in U and REEs, and found in reduced skarn deposits, can constrain chronology and reveal the spatio-temporal zonal characteristics.

Genesis of the Brazin Iron Skarn Deposit, NW Iran: Analysis of formation conditions of calc-silicates and the evolution of fluids responsible for the massive magnetite precipitation

The Brazin iron skarn occurs along the contact zone between Late Cretaceous flysch-type sequences and an Oligocene-Miocene granodiorite. The prograde skarn stage is identified by the formation of garnet and clinopyroxene, whereas the retrograde stage is dominated by epidote, scapolite, calcite, and quartz. Metal minerals include magnetite + pyrite + chalcopyrite ± pyrrhotite. Garnet with andradite-grossularite-almandine solid-solution is ubiquitous throughout the skarn zones. Two principal types of garnet were identified in the skarn zones: (I) weakly-altered euhedral coarse-grained isotropic garnet exhibits minor variation in the overall composition (And25.95–39.41 Grs54.95–62.25 Alm + Sps + Py5.39–13.04); (II) strongly-altered anhedral to subhedral isotropic to anisotropic garnet shows a wide variation in the overall range of composition (And25.91–55.25 Grs41.37–61.15 Alm + Sps + Py1.48–12.85) in which FeOt increases, whereas Al2O3 decreases rimward.Clinopyroxene with a salitic-ferrosalitic composition is homogenous, has low to relatively high Fe/(Fe + Mg) ratios, and is poor in TiO2, MnO, and Na2O. It displays a decrease in the diopside content with increasing distance from the causative intrusion, whereas the ferrosalite content is enriched near the marble and hornfels fronts.The fluid system evolves from high temperatures (>580 °C), saline fluids (>53 wt% NaCl equivalent) in the late crystallization stage under lithostatic pressure of ∼750 bar at a depth of ∼3 km. Fluid in the later retrograde stage has lower temperatures (<430 °C) and salinities (<40 wt% NaCl equivalent). Mixing and dilution between the initial hydrothermal and external fluids might cause a decrease in fluid temperature and salinity causing the formation of the magnetite.

Post-collisional magmatic-hydrothermal mineralization in the São Gabriel Terrane (southern Brazil): Insights from mineral chemistry, stable isotopes, and sulfide trace-elements of the Caçapava do Sul skarns

Prograde and retrograde magnesian skarns associated with the Caçapava do Sul Granitic Complex intrusion (central Dom Feliciano Belt, southern Brazil) host Cu–Fe (Mo–Au) mineralization. Vein and contact prograde skarns contain disseminated pyrite, pyrrhotite, molybdenite, and chalcopyrite formed from the interaction of mafic and felsic intrusions and dolomitic marbles at ca. 578 Ma. The prograde skarn mineralization is mainly localized in fold hinges and pinch-and-swell structures. Retrograde skarns host massive chalcopyrite and pyrite with chlorite and calcite associated with brittle structures at ca. 557 Ma. In this contribution, we provide mineral chemistry analyses of magmatic and skarn assemblages, calcite stable isotope analyses, and trace-element characterization of pyrite and chalcopyrite to assess ore-forming processes involved in the formation of the skarns. Amphibole-plagioclase geothermobarometry in granodiorite and monzonite apophyses of the Caçapava do Sul Granitic Complex yielded crystallization pressures between 4 and 5 kbar and temperatures from 680 to 750 °C. Biotite and amphibole composition of granodiorite, monzonite, and garnet-leucogranite apophyses suggest a trend of decreasing oxygen fugacity from early to late intrusions. Calcite-dolomite solvus thermometry in prograde skarns indicates temperatures between 580 and 630 °C whereas chlorite thermometry in retrograde skarn yields lower temperatures between 280 and 310 °C. Calcite from the retrograde skarns shows values of approximately −5.5‰ for δ13CVPDB and ∼9.0‰ for δ18OSMOW, indicating a major contribution of magmatic fluids during the retrograde stage whereas previous studies indicate a mixed igneous-sedimentary character of the calcite in prograde skarns. Our findings propose a multiphase magmatic-hydrothermal origin for the Cu–Fe (Mo–Au) skarn mineralization hosted by the Passo Feio Metamorphic Complex. The prograde and retrograde magnesian skarns of Caçapava do Sul were formed in the same age interval (578 - 557 Ma) as many other magmatic-hydrothermal base and precious metals mineralization in the São Gabriel Terrane. Hence, we suggest that the studied skarns belong to a multi-intrusive Cu-porphyry system in the São Gabriel Terrane formed during the post-collisional stage of the Dom Feliciano Belt.

Cassiterite U-Pb dating and micro-XRF analysis constraint on the formation of Xinlu Sn-Zn deposit, South China

This study presents rock slab and thin section reflection mapping, element mapping by Micro-XRF (μ-XRF) analysis, and in situ cassiterite U-Pb laser ablation-inductively coupled plasma mass spectrometer (LA-ICP-MS) ages of the Xinlu Sn-Zn deposit, Nanling Sn-W belt, South China. The new results provided constraints on the age and processes of Sn mineralization and thus further discussed the role of the Paleo-Pacific plate in the formation of Sn-W mineralization in the Nanling Sn-W belt. Based on mineralogy and μ-XRF analysis, four mineralization episodes are distinguished in the Baimianshan segment, Xinlu deposit, including 1) prograde skarn stage characterized by the mineral assemblages of garnet, pyroxene, marble, hornfels; 2) retrograde skarn-sulfide composed of massive pyrrhotite, sphalerite with parts of epidote, cassiterite; 3) quartz sulfide vein stage consisting of arsenopyrite, pyrite, quartz, and 4) barren calcite crosscutting the former episodes. The time of retrograde skarn-sulfide mineralization is dated by LA-ICP-MS cassiterite U-Pb at 168.7 ± 1.3 Ma. These U-Pb ages, combined with previous geochronological studies, support one Sn mineralization event associated with the 159.5–168.7 Ma magmatism in the Guposhan-Huashan district. The Xinlu Sn-Zn mineralization and the Middle-Late Jurassic Sn-W mineralization in the Nanling belt, South China is genetically related to asthenospheric upwelling and crust-mantle interaction caused by the subduction of the Paleo-Pacific plate. Our results demonstrate that thin section reflection and μ-XRF element mapping are powerful tools for determining paragenesis in ore deposits and are highly effective to guide the selection of analytical positions for cassiterite U-Pb dating. This method may also be applicable to in-situ geochronology of other minerals.

Chemical changes during endoskarn and porphyry‐style alteration and Cu—Fe exoskarn mineralization in the Tonglushan system, eastern China

AbstractElement mobility and chemical mass transfer are evaluated in the formation of Cu—Fe exoskarn deposits and endoskarn and minor porphyry‐style alteration in the Tonglushan quartz monzodiorite (QMD) system, eastern China. Endoskarn formation involved the migration of Ca into the QMD from the exoskarnification of carbonates (now marble) xenoliths and wall rocks, addition of Fe and Mn by magmatic‐hydrothermal fluids emanating from the interior of the QMD, and removal of alkali elements due to the replacement of feldspars and mica by prograde skarn minerals. Zirconium, Hf, U, and rare earth elements (REE) were added by hydrothermal fluids which were able to carry these often poorly mobile high field strength elements (HFSE) due to elevated F activity. Additions of Al were likely from Na‐rich fluids that also caused sodic alteration. Several factors favored mineralization within the exoskarns rather than endoskarns and QMD. The endoskarns were relatively oxidizing, as evidenced by a significant addition of Fe3+, which caused Cu to remain in magmatic‐hydrothermal fluids until they entered and precipitated sulphides in the more reducing environment of the exoskarns. Fluid migration from the QMD through the endoskarns and into the exoskarns was favored due to decarbonation of wall rock carbonates and related upwards migration of CO2 to produce a self‐sustaining chimney effect, which drew further fluids towards the carbonates to form, alter and mineralize the exoskarns. The higher porosity and permeability of the endoskarns compared with the QMD further promoted the lateral flow of Cu‐bearing fluids towards the exoskarns and limited porphyry‐style alteration and mineralization within the QMD. This proposed mechanism is only likely to be relevant for porphyry‐type systems developed predominantly within carbonate host rocks. Its significance for exploration models is that relatively poorly mineralized porphyry stocks in this setting may be associated with more substantive exoskarn deposits on their margins.

New insights into the multiple magmatic fluid generations from scheelite in the Bastielieke W-polymetallic deposit, Chinese Altay (NW, China)

Scheelite is the main ore mineral in skarn-type tungsten (W-Mo, W-Sn, and W-Cu) deposits, and is also a good proxy for ore-fluid evolution and mineralization. The Bastielieke deposit is the first medium-size W-polymetallic skarn deposit discovered in the Chinese (Xinjiang, NW China) Altay. Scheelite grains at Bastielieke are distributed in biotite granite, skarn and quartz-fluorite veins. They exhibit different textures, and can be divided into four types and six subtypes, including those in granite (Schm), prograde skarn (SchI), retrograde-altered rocks (SchII), and those in late-stage quartz-fluorite veins (SchIII).SchIa and SchIb were formed in the early and late prograde stage, respectively. SchI displays homogeneous texture, enrichments of light rare earth elements (LREEs) relative to heavy REEs (HREEs), and significantly negative Eu anomalies. SchIa has higher Sr-Mo contents and LREE/HREE than SchIb. SchII shows patchy texture by overgrowth and dissolution-reprecipitation, and can be subdivided into dark (SchIIa) and light (SchIIb) zone based in CL imaging. All SchII grains are LREE-enriched with negative Eu anomalies and relatively low LREE/HREE ratios. SchIIb has much higher W-Mo-Nb-Sr contents than SchIIa, which is ascribed to late-stage hydrothermal modifications. Schm and SchIII display homogeneous texture and similar MREE-enriched patterns, as well as very low Mo-W-Sr and different Nb contents.The texture and compositional variations in Bastielieke scheelites reveal that two magmatic fluids derived from different granitic magma reservoirs were involved in the mineralization. The earlier ore-fluid is relativly oxidized and has low HREE contents, forming the early prograde skarn-stage mineralization. Episodic influxes of later F-rich granitic magmatic fluids may have modified the earlier scheelite compositions, leading to multistage W enrichment and varying intragrain compositions.

Mineralogy of the scheelite-bearing ores of Monte Tamara, SW Sardinia: insights for the evolution of a Late Variscan W–Sn skarn system

AbstractSouthwestern Sardinia, Italy, hosts several skarn, W–Sn–Mo greisen and hydrothermal deposits related to a 289±1 Ma Late Variscan granite suite. Among them, the most representative scheelite-bearing skarns belong to the San Pietro and Sinibidraxiu localities, in the Monte Tamara area, Sulcis region. The San Pietro deposit is a typical calc-silicate skarn whereas Sinibidraxiu is a sharply bounded orebody hosted in a marble unit. Optical petrographic observations and compositional data of major and trace elements were obtained for samples from both localities. San Pietro data suggests evolution from an oxidising prograde skarn stage (andradite–diopside, hematite and scheelite), to progressively more reducing conditions from the early retrograde (magnetite–cassiterite) to the late sulfide stage (arsenopyrite, stannite, molybdenite, Bi sulfosalts and Zn–Cu–Pb–Fe sulfides); Sinibidraxiu has diffuse carbonate–quartz intergrowths pseudomorphic over an early mineral assemblage with fibrous habit, followed by abundant ore mineral precipitation under reducing conditions (scheelite, arsenopyrite and Pb–Zn–Cu–Fe sulfides). Geothermometers indicate a comprehensive temperature range of 460–270°C for the sulfide stages of both deposits. The differences between the two deposits might be controlled by the distance from the source intrusion coupled with the different reactivity of the host rocks. The San Pietro mineralogy represents a more proximal skarn, contrasting with more distal mineralogical and chemical features characterising the Sinibidraxiu orebody (lack of Mo–Sn–Bi phases; LREE–MREE–HREE signature of scheelite). This investigation contributes for the first time to the identification of a W–Sn skarn system in SW Sardinia, thereby suggesting the Monte Tamara area and its surroundings as favourable for further exploration.

A genetic model of the giant Sangdong W–Mo skarn deposit in the Taebaeksan Basin, South Korea

The Sangdong deposit is a giant W–Mo skarn deposit located in the southern Taebaeksan Basin, South Korea. It consists of stratabound orebodies hosted by intercalated limestones of the Myobong Formation and massive limestones of the Pungchon Formation, part of the Cambrian–Ordovician Joseon Supergroup. The Cretaceous Sangdong granite is associated with the W–Mo mineralization. This highly evolved calc-alkaline S-type granite intruded the Precambrian basement and is currently concealed ∼1 km below the surface. F-rich ore-forming fluids exsolved from the Sangdong granite and infiltrated the intercalated limestone of the Myobong Formation via a NE–SW-striking fault system within the Taebaeksan Basin, where the magmatic and meteoritic fluids caused multiple stages of skarn formation, ranging from the early prograde skarns to the final vein stages. Regarding the spatial distribution of the Sangdong deposit, a prograde skarn appears in the outer part of the orebody while a retrograde skarn is observed in the central part of the orebody. This spatial distribution of the multiple-stage skarn is produced by the effect of multiple hydrothermal injections in the Sangdong deposit. Slates of the Myobong Formation prevented the escape of ore-forming fluids toward the surface, and channelized the fluid flow within the intercalated limestone layers. The skarns comprise prograde skarn stages I and II, retrograde skarn stages I and II, and vein stages. Prograde stage I is represented by wollastonite–garnet–clinopyroxene zones with the lowest scheelite contents at the margins of the orebody. The pyroxene–garnet assemblage of prograde stage II replaces the wollastonite–garnet–clinopyroxene assemblage. An amphibole–biotite assemblage (retrograde stage I) replaces the pyroxene–garnet assemblage, and is replaced by a quartz–mica assemblage (retrograde stage II) in the center of the orebody. The vein stage is represented by scheelite and/or wolframite-bearing quartz veins (W vein) and molybdenite-bearing quartz veins (Mo vein) that crosscut the earlier skarns. The final vein stage is also subdivided into two stages: early scheelite–wolframite quartz veins and late molybdenite–quartz veins, with both the vein stages crosscutting the prograde and retrograde skarns. The scheelite contents are the lowest in the marginal wollastonite–garnet–pyroxene zone and the highest in the central retrograde quartz–mica zone and veins. The mineral assemblages and clinopyroxene and garnet compositions within the prograde skarns indicate the mineralizing environment evolved from oxidizing to partly reducing conditions during prograde stage I. The carbon and oxygen isotope compositions indicate the prograde stage skarn mineralization occurred at about 400 °C, within an open system with a fluid XCO2 of 0.1. The growth of fluorite and muscovite during retrograde stage II and that of wolframite during the vein stage, as well as the negative Eu anomalies recorded by the skarns, reflect an ore-forming fluid with high F contents, which enhanced the capacity of hydrothermal fluids to transport and deposit ore metals during skarn formation and greisenization at the Sangdong deposit.

Scheelite U-Pb geochronology and trace element geochemistry fingerprint W mineralization in the giant Zhuxi W deposit, South China

Abstract Skarn-type tungsten deposits are widely distributed all over the world and contribute more than 70% of the world’s W supply. The temporal relation between the W mineralization and causative intrusions and the evolution of ore-forming fluids are matters of ongoing debate. In this study, we combine in situ LA-ICP-MS U-Pb dating and trace element compositions of scheelite from Zhuxi, the world’s largest W deposit, and compare them with literature data to address the above issues. Three primary ore stages exist at Zhuxi: prograde skarn, retrograde skarn, and quartz-sulfide stages. Most scheelite occurs in the retrograde skarn stage and is further subdivided into three generations: Sch A, B, and C. The obtained LA-ICP-MS U-Pb ages for three scheelite generations in the Zhuxi deposit are 154.0 ± 2.8, 150.3 ± 3.5, and 150.4 ± 6.3 Ma, respectively, indicating that the entire W mineralization is closely related to the emplacement of the nearby Late Jurassic granites (~154 to 150 Ma, zircon U-Pb ages). In situ LA-ICP-MS trace element results demonstrate that Sch A shows the highest Mo content (mean = 1002 ppm), where those for Sch B and Sch C are 109 and 45 ppm, respectively. These, combined with the gradually increasing trend of Ce contents and δCe values, indicate a shift from oxidizing to reducing conditions for the ore-forming fluid. All three scheelite generations yield significant positive δEu anomalies, which are considered to be unrelated to the redox state, but caused by the addition of Eu (e.g., feldspar dissolution). The high-Y/Ho ratio of scheelite and a good correlation between Y/Ho ratio and δEu (R2 = 0.96) suggest that intense fluid-rock interactions between ore fluids and the Shuangqiaoshan Group metasedimentary rocks as well as earlier-formed skarns drove fluid evolution. This study demonstrates that scheelite U-Pb geochronology is a useful technique when identifying the temporal link between hydrothermal W mineralization and the causative intrusion. Our results also highlight that the reactions of the ore fluids with wall rocks and earlier-formed skarns significantly modify the primary fluid compositions.

Fluid origin and evolution of the Pusangguo Cu–Pb–Zn skarn deposit in Tibet: Constraints from fluid inclusions and isotope compositions

he Pusangguo Cu–Pb–Zn skarn deposit is a representative Co-bearing deposit in the Gangdese metallogenic belt. The mineralization mainly has a close relationship with the biotite granodiorite. Here we identified four paragenetic stages during the ore formation: prograde skarn stage (stage 1) dominated by garnet and less diopside, retrograde stage (stage 2) which is characterized by epidote, chlorite and actinolite, quartz polymetallic sulfide stage (stage 3) featured by chalcopyrite, sphalerite, galena with minor cobaltite and aikinite, and quartz calcite stage (stage 4). In this study, fluid inclusion, H–O, He–Ar, S and Pb isotope compositions were determined to constrain the sources and evolution of the ore-forming fluids, metal deposition mechanisms and metallogenetic model for the Pusangguo deposit. The He–Ar isotopic compositions (3He/4He = 0.39–0.55 Ra, 40Ar/36Ar = 299–366) show that the ore-forming fluids predominantly derived from crust source with minor mantle input. The H–O isotopic analysis results (δ18OH2O = −13.3 to + 4.9 ‰, δD = −168 to −79.5 ‰) suggest that the ore-forming fluids were sourced from a magmatic origin that mixed with some meteoric water. The S–Pb isotopic results suggest that the ore-forming materials were mostly derived from the upper crust with a minor amount of mantle materials and the biotite granodiorite magma contributed most of the lead and possibly other metals. Fluid inclusion petrography and microthermometry results show that the fluid inclusion assemblages developed in the different mineralization stages of Pusangguo deposit changed from type II (vapor-rich two-phase fluid inclusions) + type III (daughter mineral-bearing three-phase fluid inclusions) to type I (liquid-rich two-phase fluid inclusions) + type II + type III + type IV and eventually evolved into type II. Correspondingly, the ore-forming fluids mainly belonged to H2O-NaCl solutions and changed from high temperature (413–532 °C), moderate–high salinity (15.5–53.1 wt% NaCl equiv.) fluid system in stage 1 to a lower-temperature (176–234 °C) and lower-salinity (0.7–4.2 wt% NaCl equiv.) meteoric water during stage 4. Fluid boiling and mixing were likely the vital factors controlling the metal precipitation.

Geochemistry and geochronology of multi-generation garnet: New insights on the genesis and fluid evolution of prograde skarn formation

Origin of garnet in skarn (magmatic vs. hydrothermal) and the prograde skarn fluid evolution are still controversial. Two generations of garnet (Grt1, Grt2) were identified at the Tongshankou deposit: Grt1 is anisotropic with oscillatory zoning and resorbed boundary, whilst Grt2 grew around Grt1 and formed oscillatory rims. In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept 206Pb/238U age of 142.4 ± 2.8 Ma (n = 57; MSWD = 1.16) and 142.3 ± 9.6 Ma (n = 60; MSWD = 1.06), respectively, coeval with the ore formation and ore-related granodiorite emplacement. Positive Eu anomaly, non-CHARAC Y/Ho value and low TiO2 content, together with the mineral assemblages indicate that both Grt1 and Grt2 have a hydrothermal origin. The existence of melt and melt-fluid inclusions in Grt1, together with similar LREE-enriched patterns to the granodiorite, further indicate that Grt1 may have formed in the magmatic-hydrothermal transition. Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO2. The inner gray Grt2 rims (Grt2A) is HREE-enriched with low U contents, indicating that fluid II has nearly neutral pH and high fO2. The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims (Grt2B) show that the evolved magmatic fluid II had mixed with an external fluid, characterized by being mildly acidic pH and with high fO2. Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou, and the mixing of meteoric water may have been prominent in the prograde skarn stage.

The retrograde evolution of F-rich skarns: Clues from major and trace element chemistry of garnet, scheelite, and vesuvianite from the Belka Pahar wollastonite deposit, India

Skarn settings host many ore deposits and are ideal for studying the mobility of elements in hydrothermal fluids. Belka Pahar, in the Sirohi district of Rajasthan (NW India), is the largest wollastonite deposit in India (estimated wollastonite reserve of 56 million tonnes). No detailed textural and geochemical study has been conducted on these skarn rocks. In this study, we use petrographic studies, and major-trace element chemistry of skarn minerals from Belka Pahar to deduce the sequence of mineral transformations and to characterize the changes in the composition of the fluid with the progress of fluid-rock interaction during skarn formation. Two major rock types from the exoskarn have been considered for detailed study: prograde skarn rocks comprising pyroxene, garnet, wollastonite, quartz, and calcite, and retrograde skarn rocks containing garnet, scheelite, vesuvianite, fluorite, and sphalerite. Two types of garnet are recognized in the rocks-texturally earlier prograde garnet (Grt 1 ) and texturally late retrograde oscillatory-zoned Grt 2 , which is further sub-divided into three generations Grt 2a , Grt 2b and Grt 2c . The Grt 1 (And 27–47 ) and Grt 2a (And 50–60 ) have high Y-concentrations, and high HREE/LREE ratio, which is in stark contrast to the late Grt 2b (And 40–50 ) and Grt 2c (And 27–40 ), which is characterized by high F, low Y-concentrations, and HREE-depleted REE patterns. The chemistry of different generations of garnet suggests a shift from diffusive to more advective metasomatic conditions and increasing water/rock ratio, accompanied by a shift in pH of the fluid from neutral to acidic to weakly acidic. The crystallization of F-rich garnet, vesuvianite, and fluorite indicates an increase in F-concentration in the later-stage fluid. An increase of W concentration in Grt 2b and co-precipitation of scheelite and fluorite also suggests its possible link with scheelite crystallization. The high grossular component of Grt 2b and Grt 2c suggests transport of Al in the hydrothermal fluid as F-complexes. The hydrothermal fluid composition extrapolated from scheelite chemistry agrees with the LREE enriched patterns exhibited by all skarn minerals. Principal Component Analysis done on a compiled dataset of skarn garnet shows that its trace element composition can be a useful proxy of fluid composition and can be used as an indicator of associated ore type. • Garnet composition is a useful proxy of fluid chemistry in the skarn system. • High F concentration in fluid enhanced Al mobilization, forming grossular-rich garnet. • Scheelite precipitation is linked to interaction of F-rich fluid with calc-silicate rocks. • PCA shows control of fluid composition on garnet chemistry. • Garnet trace element chemistry is a robust proxy of ore-genetic association.

Metallogenic model of the Fengyan stratabound Zn-Pb deposit in Fujian of southeastern China: Constraints from fluid inclusions, C[sbnd]O[sbnd]S-Pb isotopes, and pyrite chemistry

The Fengyan stratabound Zn-Pb deposit (total resources: 24.7 Mt of ore @ 4.09 % Zn and 1.92 % Pb) is hosted by Neoproterozoic marble and metavolcanic rocks in the Meixian ore field of southeastern China. Hitherto, the genesis of this deposit has been a matter of debate, including a Neoproterozoic volcanogenic massive sulfide genesis (VMS) or an Early Cretaceous magmatic-hydrothermal origin. Based on fluid inclusion data from various hydrothermal stages, stable isotope data of hydrothermal minerals, and in-situ trace element analyses of poly-stage pyrite (Py1-3), the deposit can be classified as a distal skarn-type. A complex paragenesis reflects four hydrothermal stages: stage I (pyroxene + garnet + Py1), stage II (epidote + hematite + magnetite + Py2), stage III (quartz + sphalerite + galena + Py3a + Py3b), and stage IV (calcite). The prograde skarn comprises andradite-rich garnet (on average Ad57Gr34Py9) and pyroxene (on average Hd58Di10Jo32) with high Mn contents. Fluid inclusion data reveal salinity and temperature ranges for the prograde and retrograde skarn fluids as 11.5 to 14.6 wt% NaCl equiv. at 278 to 338℃ and 7.5 to 11.2 wt% NaCl equiv. at 235 to 275℃, respectively. The entrapment pressure is calculated as 130 bars. The sulfide ore minerals of stage III were precipitated at lower temperatures (186 to 259℃) from aqueous fluids with lower salinities (<6.6 wt% NaCl equiv.). Late-stage calcite was deposited from a more dilute (<2.2 wt% NaCl equiv.) and cooler (<200℃) fluid with extremely depleted δ18Ofluid values of −10.1 to −2.0 ‰ (V-SMOW), consistent with a meteoric origin. A mean δ34SV-CDT of 1.7 ± 0.9 ‰ of sulfide minerals points to a magmatic sulfur source. Lead isotopic compositions of sphalerite separates (206Pb/204Pb = 18.295–18.381, 207Pb/204Pb = 15.644–15.662, and 208Pb/204Pb = 38.494–38.724) are similar to those of the Yanshanian granitic bodies in the region but different from those of the ore-hosting marble and metavolcanic rocks. Calcite in the late stage has δ13CV-PDB values of −5.4 to −1.5 ‰, consistent with a predominant magmatic origin. Variations of Co, Ni, Se, and As in pyrite of different generations are interpreted to reflect changes in temperature during hydrothermal evolution. The Co/Ni ratios in all generations of pyrite, mainly between 1 and 10, indicate a magmatic-hydrothermal source. Overall, our new data indicate progressive mixing of a granite-derived magmatic fluid with meteoric waters, as is expected in a distal skarn deposit.

Qia’erdunbasixi Fe–Cu Deposit in Sawur, Xinjiang: A Case Study of Skarn Deposit Hosted by Volcanic Rock

The Sawur Cu–Au belt, northern Xinjiang, China, is the eastward extension of the Zarma–Sawur Cu–Au belt in Kazakhstan, where Late Paleozoic volcanic rocks and intrusions are highly developed. The Qia’erdunbasixi Fe–Cu deposit in Sawur is a recently discovered deposit and is still under exploration. The intrusive rocks are syenite and diorite, and the wall rocks consist of andesite and minor basalt, lamprophyre, and tuff. The U–Pb SHRIMP zircon age of the Qia’erdunbasixi syenite intruding into the volcanic rocks is 345 ± 2.2 Ma (MSWD = 1.3), presenting as the lower limit of skarn Fe mineralization. The intrusives belong to the calc–alkaline to high-K calc-alkaline series with large ion lithophile element (LILE) enrichment, high LREE/HREE fractionation, and high field strength element (HFSE) depletion. The initial 87Sr/86Sr ratios of the Qia’erdunbasixi syenite range from 0.70403 to 0.70420, and the εNd(t) values are from +5.5 to +6.8, which are the typical characteristics of island arc igneous rocks. Diorites having similar REE features with syenite should share the same magma source. Magnetite and copper mineralization develop mostly along the contact zones of syenite and diorite, respectively. Fe mineralization develops along the contact zone of syenite, with typical skarn zonation. The metallogenesis event can be divided into the prograde skarn stage (diopside–augite–andradite–magnetite–calcite–quartz), retrograde skarn stage (epidote–chlorite–actinolite–K-feldspar–calcite–magnetite–quartz), and quartz–sulfide stage (quartz–magnetite–K-feldspar–calcite–sercite–chlorite–actinolite–prehnite–chalcopyrite–pyrite). The early–mid-stage magnetite with certain amounts of Ti and V was crystallized from magma, while the late-stage magnetite has the typical characteristics of hydrothermal calcic skarn magnetite. The temperature of mineralization is between 350 and 450°C based on mineral assemblages and phase diagrams. Copper mineralization is concentrated along the outer contact zone of the diorite. Paragenesis sequences of the four stages of mineralization could be identified for copper mineralization: 1) albite–quartz; 2) chalcopyrite–pyrite–gold–seriate–quartz; 3) chalcopyrite–pyrite–epidotic–reunite; and 4) sphalerite–galena–quartz–calcite. Qia’erdunbasixi is a composite deposit with skarn-type Fe mineralization and mesothermal Cu mineralization and has a genetic relationship with magmatism in an island arc setting.

Skarn mineralogy and in-situ LA–ICP–MS U–Pb geochronology of wolframite for the Caojiaba tungsten deposit, southern China: Implications for a reduced tungsten skarn deposit

The Caojiaba tungsten deposit (19.03 Mt@ 0.37 wt% WO3) is hosted in skarn within clastic and carbonate rocks in the Xiangzhong metallogenic province (XZMP), southern China. The deposit is characterized by an early prograde skarn overprinted by a retrograde assemblage, and then by quartz–scheelite–sulfide veins. The Caojiaba skarn is characterized by grossular garnet (Grs65–85Adr7.1–31) and hedenbergitic pyroxene (Hd63–86Di11–33), similar to the well–studied reduced tungsten skarns worldwide. In this paper, we present in-situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analysis of U–Pb isotopes and trace elements of wolframite grains coexisting with scheelite in the quartz–scheelite–sulfide veins. Uranium is correlated positively with Nb5+, tetravalent (Ti4+, Sn4+, Zr4+, Hf4+), and trivalent (Sc3+, V3+, Y3+, REE3+) cations in all wolframite samples, suggesting that the incorporation of U into wolframite is controlled by the coupled substitution mechanisms. Two wolframite samples (CJB–43 and CJB–44) yielded 206Pb/238U ages of 212.4 ± 1.0 Ma (2σ, n = 50, MSWD = 0.92) and 211.8 ± 1.1 Ma (2σ, n = 43, MSWD = 1.4), overlapping with the late Triassic granitic rocks (225.6–204.5 Ma) in the XZMP. Mineralogical and geochronological evidence collectively indicate that Caojiaba is a typical example of the late Triassic reduced tungsten skarn deposits in the XZMP. This study demonstrates the potential of U–Pb dating of wolframite and highlights its importance for directly dating hydrothermal ore–forming processes.

Tungsten mineralization process and oxygen fugacity evolution in the Weijia W deposit, South China: Constraints from the microstructures, geochemistry, and oxygen isotopes of scheelite, garnet, and calcite

Scheelite is a major (W)-bearing mineral enriched in rare earth elements (REEs) and is widely used to investigate the mineralization process, fluid source and redox environment of W deposits. In the Weijia skarn-type W deposit in South China, scheelite was formed during the greisen, retrograde skarn, oxidation, and sulfide-quartz vein stages. Various characteristics of cathodoluminescence (CL) images of scheelite grains (e.g., light to dark color, core-rim texture and oscillatory zonation) have recorded their detailed crystallization process. Scheelite grains formed in the greisen and retrograde skarn stages have similar LREE patterns, and the gradual fractionation in HREEs may be a result of the precipitation of garnet (And38-70Gr29-52Pr1-3) in the prograde skarn stage. In addition, the similar REE patterns of scheelite from the greisen stage and the granite porphyry pluton suggest a genetic relationship. The oxygen isotopic compositions of scheelite and quartz are all plotted near the edge of the magmatic fluid field, confirming their derivation from the same magmatic hydrothermal fluid. Elemental mapping and spot analyses using an electron probe microanalyzer indicated that extensive isomorphic substitution of W by molybdenum (Mo) occurs in scheelite, and the darker color in the CL images corresponds to a higher Mo content and lower W content. The Mo content of scheelite gradually increased from the greisen stage (mean = 0.46%), through the retrograde stage (mean = 2.89%), to the oxidation stage (mean = 19.8%), and then dropped sharply in the sulfide-quartz vein stage (mean = 0.20%), indicating a change in oxygen fugacity. Moreover, scheelite formed in the sulfide-quartz vein stage has an oscillating Mo content, thus implying a slight or periodical change in oxygen fugacity during the same mineralization stage.

Geology, fluid inclusions and S-Pb-C-O isotopes of the Kuokuqueke Fe-Cu skarn deposit in Western Tianshan, China

The late Paleozoic Boluokenu island arc belt, located on the northern margin of Western Tianshan, Xinjiang, hosts a series of epithermal Au, hydrothermal vein-type Pb-Zn and porphyry-skarn Fe-Cu polymetallic deposits and is one of the most important metallogenic belts in China. The Kuokuqueke skarn deposit is located in the middle of the Boluokenu metallogenic belt. Skarn and Fe-Cu orebodies crop out along the contact between the lower Silurian Nilekehe Formation and the Kenaxia-Kekeqiaoke granitic intrusions. Based on field investigations and petrographic observations, four main paragenetic stages of skarn formation and ore deposition are identified: the prograde skarn stage (I), retrograde skarn stage (II), quartz-sulfide stage (III), and quartz-calcite stage (IV). Ore minerals mainly consist of magnetite and hematite, with lesser amounts of pyrite, chalcopyrite, sphalerite and galena. Four types of fluid inclusions were identified in garnet, epidote, quartz, and calcite at different stages, i.e., bi-phase (liquid and vapor) aqueous inclusions (L-dominated homogenized to liquid phase and V-dominated homogenized to vapor phase), tri-phase (liquid, vapor and solid) aqueous inclusions (S-bearing) and liquid-only aqueous inclusions. From the early to late stages, ore-forming fluids show an evolutionary trend from high temperature and medium–high salinity to low temperature and low salinity. The fluid inclusions were trapped at pressures of 400∼600 bars and 100∼300 bars for the prograde and retrograde skarn stage, respectively, corresponding to depths of 1.6∼2.4 km (under lithostatic conditions) and 1∼2 km (under hydrostatic conditions), respectively. The δ34S values of sulfides range from −5.2 to 0.5‰, indicating a magmatic origin of the sulfur in ore-forming fluids. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of the sulfides vary from 17.893 to 18.242, 15.550 to 15.591 and 37.996 to 38.175, respectively, indicating that lead was derived from a mixed crust-mantle source of subduction; the δ13CV-PDB values and δ18OV-SMOW values of calcite range from −0.6 to 2.4 and 8.1 to 13.3, respectively, indicating that carbon in fluids was mainly sourced from the mixture of ore-related granite and host carbonate rocks. Comprehensive studies indicate that the Kuokuqueke deposit is a typical skarn deposit, in which fluid boiling occurred locally in the pre-ore stage (I) and boiling in the main ore stage (II∼III) was the main cause of ore mineral deposition, while meteoric water may have been involved in the post-ore stage (IV).

Genesis of the Talate Pb–Zn (–Fe) deposit in the Altay, Xinjiang, NW China: Evidence from fluid inclusions and stable isotopes

The Talate Pb–Zn (–Fe) deposit, located in the Kelan volcanic-sedimentary Basin, Chinese Altay (part of Central Asian Orogenic Belt, NW China), is hosted in the second unit of the lower member of the Lower Devonian Kangbutiebao Formation. This deposit has undergone two periods, including submarine exhalative-sedimentary period (I), and magmatic hydrothermal (skarn) period (II), and period II can be further divided into three stages: prograde skarn stage (II-1), retrograde skarn stage (II-2), and quartz–sulfide stage (II-3). Electron microprobe analyses data show that garnet from II-1 stage is consists of solid solutions of almandine, spessartine, pyrope, and grossularite, with small fraction of andradite and uvarovite. Chlorite forming temperature ranging from 163 to 169 °C. L-type fluid inclusions in period I sphalerite has two total homogenizing temperature of 259 °C and 286 °C, respectively. The ore-forming fluids in the II-1 and II-2 stage belong to the H2O–NaCl system with moderate to high temperatures (mainly at 206–368 °C and 202–416 °C, respectively) and moderate–low salinities (7.17–17.34 wt% NaCl eqv. and 6.45–19.21 wt% NaCl eqv., respectively). The II-3 stage ore-forming fluids belong to H2O–CO2 (±CH4/N2)–NaCl system, their temperature and salinity varies widely (193–445 °C, 0.42–37.40 wt% NaCl eqv., respectively). The garnet and quartz samples from II-1 and II-3 stages yield δDV-SMOW and δ18Ofluid values (–79.8 to –64.9‰ and 4.14 to 6.17‰, respectively) and (–80.7 to –67.0‰ and –0.15 to 3.05‰). The microthermometry data combined with stable isotope data indicate ore-forming fluids in II-1 and II-3 stages are derived from primary magmatic water, but in II-3 stage is mixed with more meteoric water. In addition to meteoric water, it is supplied by carbon-rich, high-temperature and high-salinity fluid in II-3 stage. The δ34S values of the period I sulfides range from –11.7‰ to 2.4‰. Sulfur source in Talate deposit derived from the deep magmatic hydrothermal fluid. The Talate Pb–Zn (–Fe) deposit belongs to the volcanogenic massive sulfide (VMS) + skarn type, thus a genetic model is proposed to explain the metallogenic process of the Talate deposit.

Origin and evolution of W mineralization in the Tongshanling Cu–polymetallic ore field, South China: Constraints from scheelite microstructure, geochemistry, and Nd–O isotope evidence

Multistage, skarn-type W mineralization has recently been discovered in the Tongshanling Cu–polymetallic ore field, South China. This contribution reports new results from cathodoluminescence, trace element, and Nd–O isotopic analyses of scheelite samples from Tongshanling, designed to better constrain the W mineralization process and origin of associated metals and metallogenic fluids. Three generations of scheelite were identified that can be attributed to three paragenetic phases: (i) an early prograde skarn stage dominated by massive garnet and diopside intergrown with fine-grained scheelite, (ii) an intermediate retrograde skarn stage characterized by an assemblage of abundant scheelite, medium-grained chlorite and epidote, and (iii) a late sulfide-quartz vein stage typified by coarse-grained, subhedral scheelite grains that are typically intergrown with chalcopyrite, sphalerite, and galena. Our data show that although WO3 and CaO concentrations are similar in scheelite of all three stages and rare earth element (REE) patterns vary, the ∑REE + Y values of scheelite markedly and progressively decrease from the prograde skarn (mean = 901 ppm) to retrograde skarn (70.1 ppm) to the sulfide-quartz vein (34.1 ppm) stages. This signature is consistent with either of the following scenarios: The coprecipitation of REE-enriched minerals (e.g., garnet and scheelite), a change in fluid oxygen fugacity change and/or minor distortions in crystal texture. The δ18OH2O values of fluids linked to the prograde skarn and sulfide-quartz vein stages (5.02‰–6.19‰) are consistent with a magmatic fluid input. The δ18OH2O values of fluids associated with the retrograde skarn stage (2.85‰–3.45‰), on the other hand, are indicative of fluid mixing, possibly reflecting dilution of magmatic fluids by meteoric waters. Interestingly, scheelite samples of the sulfide-quartz vein stage returned low εNd(t) values (−7.46 to − 7.07) and moderate two-stage Nd model ages (T2DM = 1526 –1557 Ma) that are identical to the whole rock Nd isotope composition of an adjacent Jurassic I-type granodiorite porphyry pluton. Overall, the results of our study indicate a close genetic relationship between this pluton and the W mineralization, a scenario that is at odds with the common association between I-type granodiorites and Cu–Pb–Zn mineralization in South China.

The Kultuma Au–Cu–Fe-Skarn Deposit (Eastern Transbaikalia): Magmatism, Zircon Geochemistry, Mineralogy, Age, Formation Conditions and Isotope Geochemical Data

The Kultuma deposit is among the largest and most representative Au–Cu–Fe–skarn deposits situated in Eastern Transbaikalia. However, its genetic classification is still a controversial issue. The deposit is confined to the similarly named massif of the Shakhtama complex, which is composed mainly of quartz monzodiorite-porphyry and second-phase monzodiorite-porphyry. The magmatic rocks are characterized by a low Fe2O3/FeO ratio, low magnetic susceptibility and belong to meta-aluminous, magnesian high-potassic calc-alkalic reduced granitoids of type I. The results of 40Ar-39Ar and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating showed that the formation of magmatic rocks proceeded during the Late Jurassic time: 161.5–156.8 Ma. Relatively low Ce/Ce*, Eu/Eu* and Dy/Yb ratios in the zircons indicate that the studied magmatic rocks were formed under relatively reduced conditions and initially contained a rather low amount of magmatic water. A mineralogical–geochemical investigation allowed us to outline five main stages (prograde skarn, retrograde skarn, potassic alteration, propylitic (hydrosilicate) alteration and late low-temperature alteration) of mineral formation, each of them being characterized by a definite paragenetic mineral association. The major iron, gold and copper ores were formed at the stage of retrograde skarn and potassic alteration, while the formation of polymetallic ores proceeded at the stage of propylitic alteration. The obtained timing of the formation of retrograde skarn (156.3 Ma) and magmatic rocks of the Shakhtama complex, along with the direct geological observations, suggest their spatial–temporal and genetic relationship. The data obtained on the age of magmatic rocks and ore mineralization are interpreted as indicating the formation of the Kultuma deposit that proceeded at the final stages of collision. Results of the investigation of the isotope composition of S in sulfide minerals point to their substantial enrichment with the heavy sulfur isotope (δ34S from 6.6 to 16‰). The only exclusion with anomalous low δ34S values (from 1.4 to 3.7‰) is pyrrhotite from retrograde skarns of the Ochunogda region. These differences are, first of all, due to the composition of the host rocks. Results of the studies of C and O isotope composition allow us to conclude that one of the main sources of carbon was the host rocks of the Bystrinskaya formation, while the changes in the isotope composition of oxygen are mainly connected with decarbonization processes and the interactions of magmatic fluids, host rocks and meteoric waters. The fluids that are responsible for the formation of the mineral associations of retrograde skarns and the zones of potassic alteration at the Kultuma deposit were reduced, moderately hot (~360–440 °C) and high-pressure (estimated pressure is up to 2.4 kbar). The distinguishing features of the fluids in the zones of potassic alteration at the Ochunogda region are a lower concentration and lower estimated pressure values (~1.7 kbar). The propylitic alteration took place with the participation of reduced lower-temperature (~280–320 °C) and lower-pressure (1–1.2 kbar) fluids saturated with carbon dioxide, which were later on diluted with meteoric waters to become more water-rich and low-temperature (~245–260 °C). The studies showed that the main factors that affected the distribution and specificity of mineralization are magmatic, lithological and structural–tectonic ones. Results of the studies allow us to classify the Kultuma deposit as a Au–Cu–Fe–skarn deposit related to reduced intrusion.