Retrograde Skarn Research Articles

Genesis of the Heiyanshan Tungsten Skarn Deposit in the East Tianshan, NW China: Insights From Geology, Fluid Inclusion, Isotopic Geochemistry and Geochronology

The eastern Tianshan Terrane is a highly prospective zone that contains several porphyry Cu–Mo, VMS Cu–Zn, magmatic Cu–Ni, epithermal and orogenic Au deposits. However, few attention has been paid to tungsten deposits. Of these, the source and evolution of the mineralising fluids related to the skarn W deposits are poorly understood. The Heiyanshan W deposit is hosted by metamorphosed clastic and carbonate beds in the Mesoproterozoic Jianshanzi Formation deposited on a continental margin tectonic setting. The Jianshanzi Formation is intruded by biotite monzogranite that yield weighted206Pb/238U age of 326.9 ± 1.6 Ma, which suggest that the Heiyanshan W deposit was formed in the Carboniferous. The mineralisation is hosted by a prograde hydrothermal altered zone represented by a garnet (–pyroxene) skarn, and retrograde skarn characterised by fine-grained scheelite. The paragenesis of the Heiyanshan mineralisation can be subdivided into prograde skarn stage, retrograde skarn stage, quartz-sulphide stage and quartz-calcite vein stage. The types of fluid inclusions recognised in the various minerals in the deposits are liquid-rich aqueous, vapour-rich aqueous, and daughter mineral-bearing. The homogenisation temperatures of fluid inclusions from the Heiyanshan deposit decrease from 290 ± 28°C in garnet, through 232 ± 31°C in scheelite, to 232 ± 36°C in quartz and 158 ± 15°C in non-mineralised calcite, which is typical of W-bearing skarn deposits worldwide. The δ18Owatervalues from the Heiyanshan deposit range from +4.7 to +6.6‰ in garnet, +1.3 to +1.9‰ in quartz and −6.1 to −4.4‰ in calcite. We have measured δD in fluid inclusions from different minerals, although these bulk analyses are just a mixture of the different FIA’s present in the sample. The δD values of fluid inclusions in garnet, quartz, and calcite are from −121 to −71‰, −84 to −75‰ and −101 to −82‰, respectively, also indicative of deep-sourced magmatic fluids mixed with meteoric water. The decrease in the homogenisation temperatures for the fluid inclusions at the Heiyanshan deposit is accompanied by a drop in salinity indicating that tungsten-bearing minerals precipitated during fluid mixing between magmatic fluids and meteoric water. We conclude that eastern Tianshan Terrane contains two pulse of tungsten metallogenic events of Late Carboniferous and Early Triassic.

Genesis of the Dingjiashan Pb-Zn-Ag deposit, central Fujian region, SE China: Constraints from geological, mineralogical, geochronological and sulfur isotope data

The Dingjiashan Pb-Zn-Ag deposit is predominantly hosted in the Proterozoic marble and partly in the Jurassic tuff and quartz sandstone in the central Fujian region of southeastern China. Four mineralization stages have been identified: (Ⅰ) prograde skarn (garnet + pyroxene + rhodonite), (Ⅱ) retrograde skarn (epidote + actinolite + tremolite + biotite + chlorite + ilvaite + magnetite + hematite), (III) sulfide assemblage (sphalerite + galena + pyrrhotite + pyrite + chalcopyrite), and (Ⅳ) quartz-calcite assemblage. In this contribution, LA-ICP-MS U-Pb analysis of hydrothermal titanite coexisting with quartz, epidote, biotite, magnetite, and sphalerite in the mineralized sample reveals 207Pb-corrected weighted mean 206Pb/238U age of 144 ± 2 Ma (n = 37, MSWD = 0.86, f206 < 0.85). In situ sulfur isotope analysis of ore sulfides show a narrow variation of δ34S (-0.7 to + 5.5‰), which is consistent with a magmatic source. The geological, mineralogical, geochronological and in situ sulfur isotopic data collectively imply that the Dingjiashan is a distal stratabound Pb-Zn-Ag skarn deposit. This finding can aid understanding the genesis of other similar polymetallic deposits in the central Fujian region. Our data demonstrates that hydrothermal titanite is a reliable geochronometer for Pb-Zn-Ag skarn deposits and can aid in future prospecting and exploration for concealed porphyry Mo or Cu - Mo deposits that may be present below many of the Pb-Zn-(Ag) deposits in southeastern China.

Fluid origin and evolution of the Ruanjiawan W-Cu-(Mo) deposit from the Edong District in the Middle-Lower Yangtze River metallogenic belt of China: Constraints from fluid inclusions and H-O-C-S isotopes

Ruanjiawan W-Cu-(Mo) deposit, located in the Edong District of the Middle-Lower Yangtze River metallogenic belt, is one of the largest W deposits (40348 tons WO3 at an average grade of 0.32 wt%) in this ore district. The orebodies mainly occur in the contact zone between the granitoid and Ordovician carbonate strata. A detailed field geology and petrographic observation indicates four stages of ore formation: (1) prograde skarn stage, (2) retrograde skarn stage, (3) quartz-sulfide stage, and (4) quartz-carbonate stage. The δ18O values of the ore fluids vary from 3.0‰ to 11.2‰, and the δD values vary from −88‰ to −105‰, suggesting a dominant magmatic water origin, with a small amount of meteoric water mixing during the late stages. The δ13C values of fluid inclusions vary from −15.0‰ to −7.1‰, also indicating a major magmatic carbon source but with some mixing of organic carbon component in the sedimentary strata that probably leached by circulated meteoric water. The sulfides show a large δ34S variation from −4.8‰ to +16.3‰, and the sulfur may have two different sources, with a majority magmatic source (with δ34S around 0), and some marine sulphate source (with high positive δ34S value) in sedimentary strata in the region leached by meteoric water. In the prograde skarn stage, garnet shows a fluid inclusion homogenization temperature above 517 °C. The estimated salinity of the vapour-liquid two-phase inclusions ranges from 15.7 to 21.4 wt% NaCl equiv., and the salinity of the daughter mineral-bearing inclusions ranges from 39.8 to 57.1 wt% NaCl equiv. In the retrograde skarn stage, the fluid inclusions in epidote show a homogenization temperature of 385 °C to 463 °C, and the salinity estimated from the vapour-liquid two-phase inclusions ranges from 9.7 to 16.4 wt% NaCl equiv. The fluid inclusions in coarse-grained garnet from the retrograde skarn stage show similar homogenization temperature of 339 °C to 398 °C, and similar salinity of vapour-liquid two-phase inclusions of 7.4 to 15.3 wt% NaCl equiv., however, the salinity estimated from the daughter mineral-bearing inclusions ranges from 35.2 to 42.0 wt% NaCl equiv. In the quartz-sulfide stage, fluid inclusions in quartz show a homogenization temperature of 274 °C to 344 °C. The salinity varies from 6.5 to 14.0 wt% NaCl equiv. for the vapour-liquid two-phase inclusions, from 5.1 to 8.0 wt% NaCl equiv. for the CO2-bearing three phase inclusions, and from 35.3 to 39.8 wt% NaCl equiv. for the daughter-mineral-bearing inclusions. In the quartz-carbonate stage, the fluid inclusions have homogenization temperatures from 137 °C to 261 °C, and the salinity for the vapour-liquid two-phase inclusions shows a range from 3.9 to 1.0 wt% NaCl equiv. Fluid boiling and water-rock reaction in the retrograde skarn stage lead to the first precipitation of scheelite. The precipitation of scheelite, chalcopyrite and molybdenite in the quartz-sulfide stage was mostly triggered by the mixing of meteoric water and magmatic fluids, and fluid immiscibility also plays an important role. According to petrographic evidence and all available geological and geochemical data, we conclude that the Ruanjiawan deposit is an oxidized skarn-type W-Cu-(Mo) deposit.

The superlarge Tyrnyauz skarn W-Mo and stockwork Mo(-W) to Au(-Mo, W, Bi, Te) deposit in the Northern Caucasus, Russia: Geology, geochemistry, mineralization, and fluid inclusion characteristics

The Tyrnyauz deposit (>850 Kt WO3, >220 Kt Mo, ~60 t Au, with additional ~140 t Au at a satellite deposit) is the largest W skarn deposit in Russia and the FSU. It comprises large mineralized (molydoscheelite) skarn zones of the intermediate-redox type, followed by overprinting and much wider systems of post-skarn quartz-garnet-pyroxene, quartz-garnet, quartz-wollastonite, quartz-feldspar and quartz veins with molybdenite and minor scheelite. Younger quartz-molybdenite (±scheelite) stockworks in zones of propylitic (quartz-amphibole-chlorite) alteration are typically more distal, and so are the complex W-Au-Bi-As-Te-Sb quartz-sulfide stockworks occurring in zones of phyllic (carbonate-phyllic) alteration. The mineralization appears to be associated with at least two separate igneous suites, possibly a Late Paleozoic low-K “trondhjemite” plutonic suite (quartz diorite, tonalite-granodiorite, and plagiogranite) followed by the formation of mineralized skarns, and a Neogene moderate- to high-K, plutonic to subvolcanic suite (leucocratic granite, biotite granite, rhyolite to rhyolite-dacite, vitroandesite, to trachyandesite-trachybasalt). The Neogene intrusions exhibit a reverse (toward less silicic rocks) overall differentiation trend, cut skarns, and alternate with the post-skarn hydrothermal stages.Prograde calcic skarn was formed from a carbonic-free aqueous, low-salinity (~3 wt% NaCleq.), high-pressure (~1.3 kbar), hot fluid, which could be sourced from crystallizing magma. As the magma degassing progressed, a Na-K-Ca-chloride, high salinity (~50 wt% NaCleq.), but still high-pressure (~1.4 kbar) fluid formed mineralized (with molybdoscheelite) retrograde skarn. After the Neogene leucocratic granite emplacement, the molybdenite (±scheelite) stockworks were formed from a boiling aqueous-carbonic fluid at decreasing (from 420 to 390 °C to 370–350 °C) temperatures and much lower (~0.5 kbar) pressures. A short (?) episode of the late quartz-pyroxene-fluorite-calcite veins with scheelite occurred after the rhyolite emplacement, and a corresponding influx of boiling high-temperature (600–650 °C), Na-K-Ca-chloride, high-salinity (>50 wt% NaCleq.), methane-rich fluid interrupted the generally low- to moderate-salinity carbonic-aqueous fluid supply. A moderate- to low-salinity (22 to 4–9 wt% NaCleq.), aqueous Na-K-chloride fluids formed the late Cu-Zn sulfide to W-Au-Bi-As-Te-Sb mineralization. The superposition of different fertile igneous suites and related mineralization styles may have been the key reason for the giant metal endowment.

The Glafirinskoe and related skarn Cu-Au-W-Mo deposits in the Northern Altai, SW Siberia, Russia: Geology, igneous geochemistry, zircon U-Pb geochronology, mineralization, and fluid inclusion characteristics

The Glafirinskoe and related skarn Cu-Au (+Mo, W) deposits represent an Early Paleozoic (Late Cambrian to very early Ordovician) mineralization in the Altaid Belt, also known as the Central Asian Orogenic Belt. They are associated with multiphase plutons of high-K calc-alkaline to shoshonitic rocks that are related to a low-degree partial melting of metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal ?) magma chamber and then by magma fractionation and emplacement at shallower crustal levels. Isotopic U-Pb zircon dating of the igneous rocks yields early Ordovician (ca. 480–478 Ma and 483 Ma) age. These rocks were emplaced in a continental magmatic arc after the Late Cambrian collision, in a post-collisional setting involving the stall and break-off of the subducting slab during the Late Cambrian to Early Ordovician.The deposits comprise prograde and retrograde skarns that are characterized by abundant andradite-rich garnet, which defines their oxidized type. Wider zones of potassic (quartz-K-feldspar) and calc-potassic (quartz-K-feldspar-calcite-garnet) alteration occur in the igneous rocks, with their further overprint by propylitic (with abundant scapolite) and phyllic (carbonate-phyllic) alteration. All these alteration assemblages bear abundant chalcopyrite and minor bornite. Phyllic alteration assemblages also contain various As, Sb, Pb, Zn, Co, Ni sulfides and sulfosalts, tellurides, Bi minerals, and native Au. Scheelite and/or molybdenite are locally present in some mineralized zones. The Cu-Au-Mo(-W) mineralization occurs as skarn orebodies that are spatially associated with porphyry-style alteration assemblages reflecting the high exploration potential for additional, but concealed, skarn-porphyry Cu-Au-Mo deposits in the vicinity.The retrograde skarn and potassic (calc-potassic) alteration assemblages were formed from high temperature (570–580 °C), high salinity, aqueous chloride magmatic-hydrothermal fluid, with its boiling and phase separation indicating hydrostatic conditions at a relatively deep (~6 km) skarn-porphyry environment. Propylitic alteration assemblages were formed at lower temperatures (~380–420 °C) from a Ca-Na-chloride, low salinity (~8 wt% NaCl-eq.) magmatic-hydrothermal fluid. Fluid inclusions in minerals from phyllic alteration assemblages indicate the influx of a CO2-rich fluid at about 270–290 °C and 1400 ± 50 bars and subsequent mixing with more saline (20 wt% NaCl-eq.) and cooler (~240 °C) hydrothermal fluids.

Mineral Chemistry and U-Pb Garnet Geochronology of Strongly Reduced Tungsten Skarns at the Pampa de Olaen Mining district, Córdoba, Argentina

Los Guindos scheelite (±Zn, Bi, Sn, Ag) skarn presents mineral assemblages and a mineral chemistry similar to other worldwide strongly reduced W skarn deposits. Its reduced nature is defined based on the predominance of subcalcic garnets, Mo-free scheelite and absence of magnetite. Both the prograde and retrograde stages are evident at Los Guindos scheelite skarn. The prograde skarn is characterized by three zones: A zone I of garnet + helvine (Gr57Sps24Ad19Alm8 - Sps50Alm24Gr22Ad3; Grt + Hlv); a zone II of clinopyroxene + garnet (Di67Hd24Jo9 + Gr66Sps19Ad12Alm3; Cpx + Grt) and a zone III of garnet + vesuvianite (Gr73Ad22Sps3Alm2 - Gr58Sps22Ad10Alm9; Grt + Ves). Retrograde skarn is mainly represented by epidote - actinolite and minor F-rich actinolite (0.663 apfu of F) – potassium feldspar - chlorite (chamosite/clinochlore: ∼ 50/50) – muscovite – calcite - quartz. A hydrothermal stage developed in temporal continuity with retrograde skarn formed variable infilling associations of the following species: epidote – actinolite – scheelite – fluorite – calcite – quartz – sphalerite and chlorite. Scheelite mineralization process was triggered by an increase of Ca released during retrograde skarn replacements and was deposited during the following hydrothermal infilling stage. Other than sphalerite, minor bismuthinite and tetradymite, andorite, lillianite, gustavite, matildite and kësterite occur as hydrothermal associations after scheelite deposition. Scheelite-free reaction skarn preceding scheelite skarns was observed. Geobarometric calculations in this reaction skarn suggests an initial confining pressure of 2.5 kbar for the Los Guindos scheelite skarns. This pressure matches the estimated emplacement pressure of the Devonian-Carboniferous Achala batholith reported by previous authors. Geochemical correlation analyses suggest that this magmatism may have contributed mineralizing fluids channeled through regional structures and lithological contacts, causing infiltration metasomatism that originated scheelite (±Zn, Bi, Sn, Ag) mineralization in Cambrian and Ordovician country rocks. U-Pb analyses (LA-ICP-MS) of garnet in the Los Guindos scheelite skarn gave an age of 361 ± 11 Ma representing the age of the prograde stage of scheelite skarns and it should be framed within the Devonian-Carboniferous Metallogenic Epoch.

Evolution of the magmatic–hydrothermal system and formation of the giant Zhuxi W–Cu deposit in South China

The Zhuxi deposit is a recently discovered W–Cu deposit located in the Jiangnan porphyry–skarn W belt in South China. The deposit has a resource of 3.44 million tonnes of WO3, making it the largest on Earth, however its origin and the evolution of its magmatic–hydrothermal system remain unclear, largely because alteration–mineralization types in this giant deposit have been less well-studied, apart from a study of the calcic skarn orebodies. The different types of mineralization can be classified into magnesian skarn, calcic skarn, and scheelite–quartz–muscovite (SQM) vein types. Field investigations and mineralogical analyses show that the magnesian skarn hosted by dolomitic limestone is characterized by garnet of the grossular–pyralspite (pyrope, almandine, and spessartine) series, diopside, serpentine, and Mg-rich chlorite. The calcic skarn hosted by limestone is characterized by garnet of the grossular–andradite series, hedenbergite, wollastonite, epidote, and Fe-rich chlorite. The SQM veins host high-grade W–Cu mineralization and have overprinted the magnesian and calcic skarn orebodies. Scheelite is intergrown with hydrous silicates in the retrograde skarn, or occurs with quartz, chalcopyrite, sulfide minerals, fluorite, and muscovite in the SQM veins.Fluid inclusion investigations of the gangue and ore minerals revealed the evolution of the ore-forming fluids, which involved: (1) melt and coexisting high–moderate-salinity, high-temperature, high-pressure (>450 °C and >1.68 kbar), methane-bearing aqueous fluids that were trapped in prograde skarn minerals; (2) moderate–low-salinity, moderate-temperature, moderate-pressure (~210–300 °C and ~0.64 kbar), methane-rich aqueous fluids that formed the retrograde skarn-type W orebodies; (3) low-salinity, moderate–low-temperature, moderate-pressure (~150–240 °C and ~0.56 kbar), methane-rich aqueous fluids that formed the quartz–sulfide Cu(–W) orebodies in skarn; (4) moderate–low-salinity, moderate-temperature, low-pressure (~150–250 °C and ~0.34 kbar) alkanes-dominated aqueous fluids in the SQM vein stage, which led to the formation of high-grade W–Cu orebodies. The S–Pb isotopic compositions of the sulfides suggest that the ore-forming materials were mainly derived from magma generated by crustal anatexis, with minor addition of a mantle component. The H–O isotopic compositions of quartz and scheelite indicate that the ore-forming fluids originated mainly from magmatic water with later addition of meteoric water. The C–O isotopic compositions of calcite indicate that the ore-forming fluid was originally derived from granitic magma, and then mixed with reduced fluid exsolved from local carbonate strata. Depressurization and resultant fluid boiling were key to precipitation of W in the retrograde skarn stage. Mixing of residual fluid with meteoric water led to a decrease in fluid salinity and Cu(–W) mineralization in the quartz–sulfide stage in skarn. The high-grade W–Cu mineralization in the SQM veins formed by multiple mechanisms, including fracturing, and fluid immiscibility, boiling, and mixing.

Geochemistry of multi-stage scheelite in the skarn: Constraints on ore-forming processes of the Machangqing Cu-Mo polymetallic deposit in Yunnan Province, southwest China

The Machangqing Cu-Mo (-Au) deposit lies in the middle of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt, and skarn is one of the main mineralization styles in the deposit. Scheelite from the Machangqing skarn occurs as discrete grains present in the early to late skarnization stages of the deposit, which can be divided into three types based on their occurrence: scheelite I in the prograde skarn, scheelite II (including the early scheelite II-a and the late scheelite II-b) in the retrograde alteration and scheelite III in the quartz-sulfide stages. In the CL images, scheelite grains of all three types have core- (mantle-) rim textures displaying CL-dark cores and CL-bright rims. The REE content could act as quenchers reducing CL intensity of the Machangqing scheelite. The Mo content of these scheelite types ranges from 0.5% to 13.6%, and Scheelite II-b is significantly enriched in MoO3 compared to other scheelite. Scheelite I and scheelite II-b exhibit right-dipped REE patterns, whereas scheelite II-a and scheelite III exhibit slightly middle rare MREE enriched patterns and flat REE patterns, respectively. The Eu anomaly differs between different types of scheelite, most of which exhibit variable negative Eu anomalies. Our study reveals that the substitution mechanism of REE for Ca in the Machangqing scheelite was probably 3Ca2+ =2REE3+ + □Ca and 2Ca2+ = REE3+ + Na+. The ore-forming fluids may be magma-derived. Mo contents of the Machangqing scheelite are generally controlled by the temperature and fO2 of the ore-forming fluids. The Mo variations in scheelite from different stages indicate a temperature decrease from the prograde to the early retrograde skarn stages, followed by fO2 increasing from the early retrograde to the late retrograde skarn stages, then decreasing to the quartz-sulfide stage. The Eu anomaly of scheelite was probably inherited from the ore-forming fluids and not dependant on the fO2 of hydrothermal fluids in the Machangqing deposit.

The Hera orebody: A complex distal (Au–Zn–Pb–Ag–Cu) skarn in the Cobar Basin of central New South Wales, Australia

AbstractThe Hera Au–Pb–Zn–Ag deposit in the southeastern Cobar Basin of central New South Wales preserves calc‐silicate veins and remnant sandstone/carbonate‐hosted skarn within a reduced anchizonal Siluro‐Devonian turbidite sequence. The skarn orebody distribution is controlled by a long‐lived, basin margin fault system, that has intersected a sedimentary horizon dominated by siliciclastic turbidite, with lesser gritstone and thick sandstone intervals, and rare carbonate‐bearing stratigraphy. Foliation (S1) envelopes the orebody and is crosscut by a series of late‐stage east–west and north–south trending faults. Skarn at Hera displays mineralogical zonation along strike, from southern spessartine–grossular–biotite–actinolite‐rich associations, to central diopside‐rich–zoisite–actinolite/tremolite–grossular‐bearing associations, through to the northern most tremolite–anorthite‐rich (garnet‐absent) association in remnant carbonate‐bearing lithologies and sandstone horizons; the northern lodes also display zonation down dip to garnet present associations. High‐T, prograde skarn assemblages rich in pyroxene and garnet are pervasively replaced by actinolite/tremolite–biotite‐rich retrograde skarn which coincides with the main pulse of sulfide mineralization. The dominant sulfides are high‐Fe–Mn sphalerite–galena–non‐magnetic high‐Fe pyrrhotite–chalcopyrite; pyrite, arsenopyrite; scheelite (low Mo) is locally abundant. The distribution of metals in part mimics the changing gangue mineralogy, with Au concentrated in the southern and lower northern lode systems and broadly inverse concentrations for Ag–Pb–Zn. Stable isotope data (O–H–S) from skarn amphiboles and associated sulfides are consistent with magmatic (or metamorphic) water and sulfur input during the retrograde skarn phase, while hydrosilicates and sulfides from the wall rocks display comparatively elevated δD and mixed δ34S consistent with progressive mixing or dilution of original magmatic (or metamorphic) waters within the Hera deposit by unexchanged waters typical of low latitude (tropical) meteoritic waters. High precision titanite (U–Pb) and biotite (Ar–Ar) geochronology reveals a manifold orebody commencing with high‐T skarn and retrograde Pb–Zn‐rich skarn formation at ≥403 Ma, Au–low‐Fe sphalerite mineralization at 403.4 ± 1.1 Ma, foliation development remobilization or new mineralization at 390 ± 0.2 Ma followed by thrusting, orebody dismemberment at 384.8 ± 1.1 Ma and remobilization or new mineralization at 381.0 ± 2.2 Ma. The polymetallic nature of the Hera orebody is a result of multiple mineralization events during extension and compression and involving both magmatic and likely formational metal sources.

Metallogeny of the Ergu Fe-Zn polymetallic deposit, central Lesser Xing’an Range, NE China: Evidence from skarn mineralogy, fluid inclusions and H-O-S-Pb isotopes

The Lesser Xing'an Range is an important skarn Fe-Zn metallogenic area in Northeast China. To reveal the metallogenic characteristics and attributes of the skarn deposits and establish geological model for deposit formation and prospecting, we selected the Ergu skarn Fe-Zn polymetallic deposit for comprehensive study, including detailed field observation and petrographic, mineralogy, fluid inclusions (FIs), and H-O-S-Pb isotope analyses. The ore bodies can be divided into magnetite, magnetite-sphalerite, pyrrhotite-pyrite-arsenopyrite, copper polymetallic and galena-sphalerite ore bodies. These ore bodies hosted in the contacts between the granodiorite and carbonate rocks of the lower Cambrian Qianshan Formation. The ore-forming processes involved two metallogenic periods–the skarn period (I) and quartz-sulfide period (II), including six stages. The prograde skarn stage (I1) is characterized by garnet and pyroxene. The retrograde skarn stage (I2) is dominated by epidote, actinolite, chlorite, hornblende and a few magnetite. The oxide stage (I3) is typified by abundant magnetite, with lesser amounts of epidote, quartz, molybdenite and pyrrhotite. The quartz-Fe-Cu sulfide stage (II1) is enriched in pyrite, pyrrhotite, arsenopyrite, chalcopyrite and quartz. The quartz-Pb-Zn sulfide stage (II2) is enriched in galena, sphalerite and quartz. The quartz-carbonate stage (II3) is typified by the formation of quartz, calcite and fluorite.Electron probe microanalysis (EPMA) showed that the garnets are composed of reddish-brown to yellowish-green andradite (And63.7~939Gro1.15~34.6Ura + Pyr + Spe1.55~5.00) and brown grossular (And22.4~48.1Gro46.0~74.9Ura + Pyr + Spe1.82~3.22) (Ura-Uvarovite, And-Andradite, Pyr-Pyrope, Spe-Spessartine, Gro-Grossular). The pyroxene is composed of light green diopside (Di81.2~93.7Hd5.30~17.2Jo1.03~1.98) (Di-Diopside, Hd-Hedenbergite, Jo-Johannsenite); the epidote has a high pistacite component (Ps value) (Fe3+/(Fe3++Al) = 0.09–0.18) and Fe3+/Fe2+ value (1.17–4.67). These skarn mineral assemblages record the transition from weakly acidic to alkaline, and an oxidized condition for the skarnization process.The FIs mainly include daughter mineral-bearing three-phase (S-type), liquid-rich (W1-type) and vapor-rich (W2-type) FIs. Laser Raman analysis suggest that the gas phase composition of FIs are dominated by H2O and CO2, and a few CH4. Based on fluid inclusions (FIs) and H-O isotopes studies, the skarn period was formed at high temperature (341–467 °C), low to high salinity (1.22–40.2 wt%) and originated from magmatic-hydrothermal fluids (δ18OH2O = 2.58‰ to 10.2‰, δDH2O = −70.7‰ to −63.8‰). Fe-Cu sulfide-forming stage (stage II1) shows clear meteoric water input, as constrained from the δ18OH2O (8.25‰ to 11.7‰) and δDH2O (−114‰ to −106‰) values. In this stage, the ore-forming fluids belong to a H2O-NaCl-CO2 ± CH4 system with moderate to high temperatures (247–360 °C) and low salinities (1.22–7.15 wt%). This system lasted until stage II2 with moderate-high temperatures (193–306 °C) and low salinities (1.56–4.17 wt%). Finally, in stage II3, the fluid transitioned into a homogeneous H2O-NaCl system with low to moderate temperature (169–263 °C) and low salinity (1.39–4.48 wt%).The sulfur isotope (δ34S = 2.2–4.6%) and lead isotopic (206Pb/204Pb = 18.0–18.6, 207Pb/204Pb = 15.5–15.6, 208Pb/204Pb = 37.8–38.2, similar to granodiorite) compositions of sulfides indicate that the ore-forming materials were mainly derived from granodioritic magma. Based on the above research results, we propose that the Ergu deposit is a typical Fe-Zn skarn deposit and the main mechanisms of mineral precipitation were fluid boiling and cooling.

Fluid evolution and ore genesis of the Chaobuleng skarn Fe[sbnd]Zn polymetallic deposit, Northeast China: Evidence from fluid inclusions, C[sbnd]O[sbnd]S[sbnd]Pb isotopes, and geochronology

The Chaobuleng skarn deposit, located in the Erlian–East Ujimqin region, Northeast China, is a medium-sized FeZn polymetallic deposit. Iron-polymetallic mineralization occurs mainly as stratiform, lenticular, and vein types hosted in the contact zone between the Chaobuleng syenogranite and the Devonian Taerbagete Formation. The Chaobuleng deposit is a calcic skarn deposit dominated by a grossular-rhodonite-allanite-epidote assemblage. The ore minerals mainly consist of magnetite, pyrite, sphalerite, chalcopyrite, and bismuthinite, with lesser amounts of molybdenite, arsenopyrite, and pyrrhotine. Four mineralization stages have been identified: (I) a prograde skarn stage; (II) a retrograde skarn stage; (III) a quartz-minor calcite-polymetallic sulfide stage; and (IV) a quartz-calcite-minor pyrite stage. Three types of fluid inclusions (FIs) in garnet, quartz and calcite were classified: vapor-rich two-phase FIs (V-type), liquid-rich two-phase FIs (L-type), and daughter mineral-bearing three-phase FIs (S-type). The homogenization temperatures of FIs from stages I, II, III, and IV are 446–547, 411–519, 279–415, and 147–217 °C, respectively, with corresponding salinities of 0.87–59.47, 0.52–51.52, 0.70–13.25, and 3.05–7.85 wt% NaCl equiv., respectively. The results suggest that mineralization in the Chaobuleng deposit occurred at a depth of 1.0–2.8 km, with fluid boiling and mixing with meteoric water likely being responsible for ore precipitation. The CO isotope data suggest the early ore-forming fluids were mainly related to felsic rocks and were accompanied by intense water/rock reactions during the fluid migration. The SPb isotopic compositions indicate that ore materials were derived from the mixing between magma and wall rocks. Zircon UPb dating yields a weighted mean 206Pb/238U age of 135.9 ± 0.8 Ma (MSWD = 1.03), which represents the crystallization time of the syenogranite. Together, these data suggest that the Chaobuleng deposit is a skarn ore system that is spatially, temporally, and genetically related to syenogranite and formed in an extensional environment resulted from the roll back of the subducted Paleo-Pacific Plate.

Fluid inclusions, stable isotopes (H–O–S), and age constraints on the genesis of the Basitielieke polymetallic tungsten deposit in the Altay, Xinjiang, NW China

The Chinese Altay in Northwest China and adjacent regions in the Russian and Mongolian Altay form a tungsten province within the Central Asian Orogenic Belt. Unlike the Russian and Mongolian Altay, almost no tungsten deposits in the Chinese Altay have been reported. The Basitielieke deposit located in the Altay orogenic belt in Xinjiang is a newly discovered polymetallic tungsten deposit hosted in the skarn in the exocontact zone of a granite pluton. Mineralization can be divided into the skarn period and supergene oxidization period, the skarn period can be further divided into a prograde skarn stage (I), a retrograde skarn stage (II), and a quartz–sulphide stage (III). Scheelite mineralization mainly occurred in stage II, and Cu–Zn–Au formed in stage III. The ore‐forming fluid can be divided into a H2O–NaCl systems, including stage I (211 to 480°C and 2.24 to 14.04 wt% NaCl equiv) and stage II (178 to 336°C and 4.96 to 9.47 wt% NaCl equiv), and a H2O–CO2 (±CH4/N2)–NaCl system in stage III (130 to 434°C and 1.40 to 9.60 wt% NaCl equiv). Hydrogen, oxygen, and sulphur isotope compositions indicate that magmatic water participated in the hydrothermal mineralization system in stage I, and meteoric water increased with the process of mineralization. Sulphur was derived from deep‐seated magmas. The Basitielieke deposit is a reduced skarn polymetallic tungsten deposit. The deposit formed by hydrothermal fluid from the granite interacting with strata during the Early Permian, the fluid–rock interaction and fluid mixing (magmatic water and meteoric water) was the main precipitation mechanism of the Basitielieke polymetallic tungsten deposit.

Constraints of Fe isotopes on the origin of volcanic-hosted Fe-oxide deposits: A case study of the Late Cambrian 7918 Fe deposit, North Altun orogenic belt, Xinjiang, NW China

• The 7918 iron deposit is hydrothermal in origin. • Volcanic rocks hosting orebodies act as a main Fe source in the 7918 deposit. • A three-stage model has been proposed to account for genesis of 7918 iron deposit. The 7918 Fe deposit in the North Altun orogenic belt , NW China, is composed of strata-bound orebodies between Late Cambrian basalts and marble, associated with intensive skarn-type alteration. Basalts close to the orebodies are altered, with gradational contacts to skarns. Variations of major and trace elements and δ 57 Fe values of low-Ti and high-Ti basalts reflect the role of both magmatic evolution and hydrothermal alteration . Negative correlations between δ 57 Fe and TFeO/TiO 2 and between Na 2 O and TFeO/TiO 2 of high-Ti basalts are explained by lighter Fe isotopes leached from basalts during water–rock reaction. Hence, we considered basalts as the main source of Fe. Magnetite precipitation was related to a retrograde skarn stage. Early-precipitated magnetite has lower incompatible trace elements (Lu, Zr, Hf, Nb, Ta, and Yb), higher compatible trace elements (Co and Ni), and lower δ 57 Fe values than late-precipitated magnetite. Hydrothermal fluids equilibrated with early-precipitated magnetite (−0.11‰ to 0.16‰) and late-precipitated magnetite (0.22‰ to 0.27‰) have δ 57 Fe values of −0.38‰ to −0.11‰ and −0.05‰ to 0.00‰, respectively. The isotopically heavier late-phase fluids indicate an increase of oxygen fugacity, in which Fe 2+ was mostly oxidized to Fe 3+ . This is also consistent with reduced V concentrations from early-precipitated to late-precipitated magnetite. Our new data and previous geochronological results suggested that the 7918 Fe deposit records a continuous process of volcanic eruption , hydrothermal alteration, and Fe mineralization , which was related to a deep “magma chamber” formed at a subduction-related setting.

Lithium and chlorine isotopic constraints on fluid sources and evolution at the Luziyuan distal skarn Zn–Pb–Fe–(Cu) deposit, western Yunnan Province, China

The sources and temporal evolution of fluids related to distal skarn mineralization strongly influence the minerals that form, but remain poorly understood. In this study, we use the lithium and chlorine isotopic compositions of fluid-inclusion leachates (FIL) to elucidate the nature and evolution of fluids that formed the Luziyuan distal skarn Zn–Pb–Fe–(Cu) deposit, China. The δ7Li values of prograde skarn stage rhodonite FIL (+5.37‰–+7.31‰) are similar to those of typical magmatic fluid. The δ7Li values (+11.69‰–+12.54‰) of actinolite FIL are higher than those of rhodonite FIL; this is attributed to Rayleigh fractionation within a closed magmatic–hydrothermal system. It is inferred that up to 70% of fluid-borne Li is sequestered by hydrous minerals during retrograde skarn alteration, consistent with conclusions drawn from previous studies of fluid-inclusions. The δ7Li values of galena FIL (+9.78‰–+13.19‰), black sphalerite FIL (+11.62‰), and reddish-brown sphalerite (+10.44‰–+11.66‰) overlap with those of actinolite FIL, indicating that phase separation occurred during Zn–Pb mineralization at the Luziyuan deposit. The low δ7Li values of light-yellow sphalerite FIL (+6.75‰) and calcite FIL (+3.95‰–+6.49‰) record external fluid input, probably meteoric water. The positive δ37Cl values of rhodonite FIL (+1.82‰–+1.83‰) are consistent with those of magmatic fluids. However, the δ37Cl values of actinolite FIL (+0.49‰) are lower than those of rhodonite FIL, indicating that FIL within hydrous minerals might not record fluid sources reliably.

Geology and genesis of the Debao Cu polymetallic skarn deposit, southwestern China

The Debao deposit, with stratiform orebodies in Middle-Upper Cambrian carbonate rocks, is a rare Early Paleozoic skarn copper deposit in the South China Block. Garnet U-Pb dating yielded a lower intercept 206Pb/238U date of 438.8 ± 7.3 Ma, which is coeval with the related granite (439 ~ 441 Ma). Skarns contain garnets with andraditic compositions (Ad53~93Gr4~46Py1~8) and clinopyroxene (Hd5~11Di89~94Jo0~1 and Hd52~85Di13~46Jo1~4) with low Mn/Fe ratios (<0.1), showing typical mineral compositions of Cu skarn deposits. The proximal garnet skarn mainly developed at the bottom and both sides of the orebody, while distal pyroxene skarn developed in its interior and upper parts. The garnets in the bottom of the orebodies contain more andraditic compositions than their upper parts. The prograde skarn minerals have a range of δ18Ofluid values from 8.81 to 9.53‰, and the sulfide minerals of the main ore stage have a range of δ34S values from −2.40 to 2.57‰, both suggesting a magmatic origin of the ores. In the Debao deposit, hydrothermal magnetite from different skarn zones contains substantial amounts of Al, Sn, Ga, Mn, Mg, Ti, Zn, Co, V, and Ni. The content of Al and Ti in the magnetite, which was considered to derive from the wall rock due to fluid-rock interaction, was positively correlated with temperature. Magnetite in the retrograde skarn stage from the proximal garnet skarn zone contains more Ti and V than that from the distal pyroxene skarn zone, indicating a gradual decrease in temperature and increase in oxygen fugacity of the ore-forming fluid in this stage. The magnetite has higher Sn content (mainly hundreds to thousands of ppm) than normal skarn deposits, implying that high oxygen fugacity occurs during the retrograde skarn stage. Subsequently, the formation of magnetite led to a decrease in the oxygen fugacity of the ore-forming fluid. The decrease of oxygen fugacity and temperature provided favorable conditions for subsequent precipitation of Cu sulfides. Minor vein-type Cu mineralization related to late magmatic fluid extraction of sulfur from the sedimentary strata was recognized. All geologic and geochemical data suggested that the deep skarn in the region transformed from Early Paleozoic carbonate has copper mineralization potential.

Mineralogy and fluid characteristics of the Waschleithe Zn skarn – a distal part of the Schwarzenberg mineral system, Erzgebirge, Germany

The Waschleithe skarn is situated in the northern sector of the Schwarzenberg district in the western Erzgebirge (Germany), a district which hosts several large polymetallic skarn deposits. The Waschleithe is a Zn-dominant skarn comprising abundant dark Mn-rich hedenbergite, minor grandite garnet and magnetite related to the prograde stage as well as epidote, quartz, fluorite, amphibole, chlorite, ilvaite and Mn-rich pyroxenoids related to the retrograde stage of skarn formation. Ore minerals are related to the early retrograde stage and include sphalerite, galena, chalcopyrite, pyrite as well as minor amounts of scheelite and cassiterite. Fluid inclusion homogenization temperatures range from ~385 to 360 °C and from ~320 to 285 °C for the prograde and retrograde skarn stage, respectively. Thus, late-stage cooling is proposed as the major controlling factor for sulfide mineral precipitation. Fluid salinities for both stages are very low (<3% eq. w[NaCl]) and all observed fluid inclusion assemblages show homogeneous liquid–vapor ratios. Formation pressures >25 MPa suggest a minimum depth of formation of approximately 3 km. Oxygen fugacity during prograde skarn formation was initially low and increased towards the late prograde stage. The late-Variscan Eibenstock granite is identified as the most likely source of the skarn-forming fluids. The recognition of the Waschleithe skarn as a distal Zn skarn has major implications for the understanding of the Schwarzenberg district mineral system and provides valuable information for exploration targeting.

Genesis of the Cuonadong tin polymetallic deposit in the Tethyan Himalaya: Evidence from geology, geochronology, fluid inclusions and multiple isotopes

The Cuonadong deposit is located in southern Tibet and represents the first large-scale tin polymetallic deposit in the Himalayan region. Sn-(W) skarn mineralization is spatially related to leucogranites, whereas Be-Nb-Ta mineralization mainly develops in pegmatites. Alteration and veining in this deposit can be divided in to five stages: prograde skarn (stage I), retrograde skarn (stage II), cassiterite-quartz vein (stage III), cassiterite-sulfide vein (stage IV) and fluorite quartz vein (stage V). 40Ar39Ar dating of muscovite from the skarn and phlogopite from the cassiterite-sulfide vein yield isochron ages of 15.4 ± 0.3 Ma and 15.0 ± 0.3 Ma, respectively, whereas UPb dating of cassiterite from the skarn yields a Tera-Wasserburg low-intercept precise age of 14.2 ± 0.2 Ma. Zircon and monazite UPb ages of the causative stanniferous leucogranites are 15.3 ± 0.1 Ma and 14.9 ± 0.2 Ma, respectively. Mineralization ages are consistent with the emplaced ages of the granites within the analytical uncertainties, which indicates that the tin polymetallic mineralization is genetically related to the Miocene leucogranites. Sn-bearing leucogranites have zircon εHf(t) values that vary from −13.3 to −8.5 (-10.7 on average), with an average TDM2 value of 1.56 Ga, which reveals that the leucogranites are derived from partial melting of ancient metasedimentary rocks. According to the zircon trace elements, the Miocene leucogranites are highly differentiated reduced S-type granites that formed at relatively high temperature. The average homogenization temperatures of fluid inclusions in different minerals that formed at stages II, III, IV and V are 351, 315, 240 and 175 °C, respectively, whereas their salinities are 8.3, 4.9, 9.5 and 3.8 wt% NaCl equiv., respectively. The C-H-O-S-Pb isotopes indicate that the ore-forming fluid and materials mainly originated from Miocene leucogranite. Dehydration and partial melting of mica in the Greater Himalayan crystalline complex due to east-west extension at 18–14 Ma developed the stanniferous leucogranite and ore-controlling fault system. Because of the pervasive occurrence of gneiss domes and Miocene Sn-bearing leucogranites similar to Cuonadong, the Himalaya has strong potential to be a new globally important Sn-(W) rare metal metallogenic belt.

The Watershed Tungsten Deposit, Northeast Queensland, Australia: Permian Metamorphic Tungsten Mineralization Overprinting Carboniferous Magmatic Tungsten

Abstract The Watershed tungsten deposit (49.2 Mt avg 0.14% WO3) lies within the Mossman orogen, which comprises deformed Silurian-Ordovician metasedimentary rocks of the Hodgkinson Formation intruded by Carboniferous-Permian granites of the Kennedy Igneous Association. The Hodgkinson Formation in the Watershed area comprises skarn-altered conglomerate, psammite, and slate units that record four deformation events evolving from ductile, isoclinal, colinear folding with transposition (D1–D3) to brittle ductile shear zones (D4). Multiple felsic to intermediate dikes cut across the metasedimentary rocks at Watershed including the following: (1) Carboniferous, monzonite dikes (zircon U/Pb age of 350 ± 7 Ma) emplaced during D1–2; and (2) Permian granite plutons and dikes (zircon U/Pb ages of 291 ± 6, 277 ± 6, and 274 ± 6 Ma) and diorite (zircon U/Pb age of 281 ± 5 Ma) emplaced during D4. Tungsten mineralization is largely restricted to skarn-altered conglomerate, which preserves a peak metamorphic mineralogy formed during ductile deformation and comprises garnet (Grt40–87 Alm0–35Sps1–25Adr0–16), actinolite, quartz, clinopyroxene (Di36–59Hd39–61Jhn1–5), and titanite. A first mineralization event corresponds to the crystallization of disseminated scheelite in monzonite dikes (pre-D3) and adjacent units, with scheelite grains aligned in the S1–2 fabric and affected by D3 folding. This event enriched the Hodgkinson Formation in tungsten. The bulk of the scheelite mineralization formed during a second event and is concentrated in multistaged, shear-related, quartz-oligoclase-bearing veins and vein halos (muscovite 40Ar-39Ar weighted average age of 276 ± 6 Ma), which were emplaced during D4. The multistage veins developed preferentially in competent, skarn-altered conglomerate units and formed synchronous with four retrograde alteration stages. The retrograde skarn minerals include clinozoisite after garnet, quartz, plagioclase, scheelite, and phlogopite with minor sodium-rich amphibole, which formed during retrograde stages 1 and 2, accompanied by later muscovite, calcite, and chlorite formed during retrograde stage 3. Retrograde stage 4 was a late-tectonic, noneconomic sulfide stage. The principal controls on scheelite mineralization at Watershed were the following: (1) early monzonite dikes enriched in scheelite; (2) D4 shear zones that acted as fluid conduits transporting tungsten from source areas to traps; (3) skarn-altered conglomerate lenses that provide a competent host to facilitate vein formation and a source for calcium to form scheelite; and (4) an extensional depositional environment characterized by vein formation and normal faulting, which provide trapping structures for tungsten-bearing fluids, with decompression being a likely control on scheelite deposition. The coexistence of scheelite with oligoclase in monzonite dikes and veins suggests that tungsten was transported as NaHWO40. Exploration in the area should target Carboniferous monzonite, associated with later syn-D4 shear zones cutting skarn-altered conglomerate.

Hydrothermal evolution and ore genesis of the Laozuoshan Au skarn deposit, northeast China: Constrains from mineralogy, fluid inclusion, and O–C–S–Pb isotope geochemistry

The Laozuoshan mine, located in the central Jiamusi block, is the largest gold skarn deposit in Northeast China. It comprises three ore zones (East, Middle, West) with complex stratigraphic and structural controls. The skarn-type orebodies mainly occur in the East and Middle ore zones along the contact zones between Hercynian granite and Precambrian metasedimentary rocks. The earliest stage of alteration is characterized by potassium hornfels. Subsequently, prograde skarn formation occurred, characterized by garnet in the grossular-andradite series and pyroxene in the hedenbergite-diopside series. This was followed by a retrograde skarn overprint of hydrous silicates and ore minerals. Three types of garnet with distinct textures and elemental compositions are recognized, which indicate fluctuating physicochemical conditions in the hydrothermal fluids.The fluids associated to the prograde skarn are characterized by high temperature and high salinity, which underwent fluid boiling that led to precipitation of minor pyrite and arsenopyrite. The main gold and sulfide mineralization is related to the retrograde skarn overprint, where cooling and strong fluid-rock interactions occurred. Gold precipitation is estimated to have occurred at temperatures of 380–470 °C, based on the arsenopyrite geothermometer and fluid inclusion microthermometry. The fluid inclusion and C-O isotope data indicate that the ore-forming fluids were dominated by magmatic water during the prograde skarn stage and the main ore-forming stages. Mixing between meteoric and magmatic fluids occurred during the latest stages of the retrograde skarn formation. Both sulfur and lead isotope compositions of sulfides support a magmatic origin for the ore-forming materials.On the basis of the available geological and geochemical evidence, we propose that the Laozuoshan deposit is a reduced gold skarn deposit, genetically related to the granitic pluton.

The geology, magnetite geochemistry, and oxygen isotopic composition of the Akesayi skarn iron deposit, Western Kunlun Orogenic Belt, Xinjiang, northwest China: Implications for ore genesis

The Akesayi skarn iron (Fe) deposit is hosted by the Huangyangling Group within the Tianshuihai Terrane of the Western Kunlun orogenic belt, Xinjiang, China, and consists of several orebodies that formed along contacts between Miocene syenogranite, diorite porphyry dykes and late Paleozoic carbonate rocks. Manganese-bearing skarn minerals are common within the deposit. The ore-forming process can be divided into five stages: Prograde skarn stage (I), retrograde skarn stage (II), oxide stage (III), quartz-sulfide stage (IV), and carbonate stage (V), with this evolution being typical of Fe skarns. Magnetite, which formed mainly during the retrograde skarn and oxide stages, includes banded (BO), disseminated (DO), and massive (MO) magnetite. Backscattered-electron images indicate that magnetite within the massive ores can be further classified as primary (MO-1) and secondary (MO-2) magnetite.Magnetite within all ore types exhibits a strongly negative correlation between Si and Fe3+ contents, indicating that Si was incorporated into intra-crystalline sites of magnetite by substitution of Si4+ for Fe3+. Positive correlation among Al, Mn, Mg, and Si contents indicate that these elements behaved similarly during magnetite formation. The δ18OV-SMOW values of MO-1 magnetite range from 4.3‰ to 4.7‰, higher than magmatic magnetite (1‰–4‰), while the BO and DO magnetite have lower δ18OV-SMOW values of 1.7‰ to 2.4‰ and 1.8‰ to 2.5‰ respectively. Its anomalous oxygen isotopic composition indicates that most magnetite in the Akesayi Fe deposit precipitated from evolved ore-forming fluids formed by reaction between country rocks and primary magmatic fluids during the early stages of mineralization, with input from meteoric water during the later stages of mineralization. Trace-element patterns indicate that the different types of magnetite share a common source, with variations in the magnetite geochemistry amongst the different ore types of the Akesayi Fe deposit reflecting variations in the extent of fluid-rock interaction, dissolution-reprecipitation, mineral precipitation, temperature, and oxygen fugacity (ƒO2). The different types of magnetite of the Akesayi skarn Fe deposit form distinguishable populations in (Ca + Al + Mn)-(Ti + V) and (Ni/(Cr + Mn))-(Ti + V) discrimination diagrams, with the range of compositions possibly indicating more variable magnetite compositions in skarn deposits than previously thought.