Calcic Skarn Research Articles

Fluid inclusions and H–O isotopes of the super-large Nanyangtian tungsten deposit in southeastern Yunnan, southwestern China

The Nanyangtian deposit is a super-large reduced skarn tungsten deposit located in the Laojunshan WSn polymetallic ore province in southeastern Yunnan (SW China). The deposit is represented by three flat-lying mineralized zones formed vertically by the replacement of limestone or minor calcareous schist. The tungsten orebodies are mainly stratiform, lenticular, and vein types, hosted in the Paleoproterozoic Nanyangtian Formation. Three mineral formation stages have been identified based on the mineral assemblages and vein crosscutting relationships (pre-, syn-, and post-ore). The Nanyangtian is a calcic skarn deposit, dominated by a grossular-diopside-tremolite-actinolite assemblage. Scheelite, pyrrhotite, pyrite, and chalcopyrite are the main ore minerals. Detailed petrographic observations show three types of fluid inclusions (FIs) in various hydrothermal minerals: liquid-rich two-phase (type-I), gas-rich two-phase (type-II), daughter mineral-bearing three-phase (type-III). Their homogenization temperatures (193–298 °C) and salinities (1.2–9.3 wt% NaCl eq.) indicate that the Nanyangtian ore-forming fluids were of medium to low temperature and low salinity compared to the statistical data from representative skarn tungsten deposits in South China. Laser Raman microprobe analysis of the FIs shows that the inclusions are dominated by H2O with minor CH4 and N2. The δD values (relative to Vienna-Standard Mean Ocean Water, VSMOW) of fluid inclusions and calculated δ18Owater values (relative to VSMOW) of the fluids in equilibrium with hydrothermal minerals are −105 to −69 ‰ and − 1.9 to 7.6 ‰, respectively. These oxygen–hydrogen isotopic compositions indicate that the ore-forming fluids were magmatic-derived, which may have metasomatized the Nanyangtian Formation carbonaceous calcareous rocks along interlayer structures to form the prograde skarn minerals. Then the fluids mixed with meteoric water migrated along faults to form the retrograde skarn and tungsten ore. Mixing of magmatic fluid with meteoric water is likely the main factor for the ore precipitation at Nanyangtian.

The Geochemical Characteristics of Trace Elements in the Magnetite and Fe Isotope Geochemistry of the Makeng Iron Deposit in Southwest Fujian and Their Significance in Ore Genesis

The Makeng iron deposit in southwest Fujian is a significant iron polymetallic deposit containing various types of iron ore, including garnet magnetite, diopside magnetite, and quartz magnetite. The metallogenetic type of the deposit has been a subject of debate, particularly in relation to the genesis of magnetite and the source of iron. In situ microanalysis of trace elements in magnetite from different ores shows relatively low levels of V, Ti, Cu, and Zn, with higher concentrations of Ca and Si, indicating the characteristics of a skarn type deposit. The δ57Fe values of the magnetite range from −0.091‰ to 0.317‰. Combining these data, whole-rock iron isotope analyses, including Juzhou and Dayang granites, diabase, and the Lower Carboniferous Lindi Formation sandstone, suggest that Fe in the magnetite primarily originates from granitic pluton, with potential contributions from diabase and the Lower Carboniferous Lindi Formation sandstone. Combined with field work, these results indicate that Makeng iron deposit is a skarn-type magnetite deposit associated with Yanshanian granitic intrusions. Therefore, the initial ore-forming fluid is postulated to be a high-temperature magmatic hydrothermal fluid with high oxygen fugacity. This fluid infiltrates spaces such as interlayer fracture zones between the Upper Carboniferous Jingshe Formation–Middle Permian Qixia Formation carbonate rocks and the Lower Carboniferous Lindi Formation sandstone, resulting in diverse magnetite ores due to metasomatism. The mineralization process of the Makeng iron deposit is basically the same, as it is composed of typical skarn deposits. Magnetite was mainly formed during calcic skarn formation stage, and this process persisted until the initial phase of the retrograde alteration of skarns. In contrast, sulfide minerals, including molybdenite, sphalerite, and galena, precipitated during the quartz–sulfide stage.

Helvine Mineralization in Devonian-Carboniferous Skarns of the Oriental Pampean Ranges, Córdoba, Argentina: Mineralogy and Nature of Ore-Forming Fluids

AbstractHelvine [Be3Mn2+4(SiO4)3S] occurs in less than one meter-sized spotty concentrations in some calcic skarns of Córdoba province, Argentina. The local geology, mineral chemistry, paragenetic mineral associations, and the evolution of helvine mineralizing fluids were studied from two selected localities (i.e., the Chingolo scheelite mine and Casa la Plata). Helvine from the Chingolo scheelite mine occurs in idiomorphic crystals up to 15 cm long, partially intergrown with spessartine-rich garnet, and in part or totally included in vug-filling spar calcite in the prograde garnet-vesuvianite skarn zone developed between replaced aplite-pegmatite dikes and calcic phlogopite-bearing marbles. Helvine from Casa la Plata occurs abundantly in vein-like, fluorite-rich garnet-vesuvianite skarn associations, where tetrahedrons up to 2 cm long occur preferentially included in fluorite in an illite-sericite-chlorite strongly replaced schist. The composition of helvine from both localities does not differ from other worldwide known compositions. Associated garnet from both localities is enriched in molar subcalcic garnet, largely as spessartine. Clinochlore is a conspicuous phase that occurs both as late infills and/or replacing phases in both the Chingolo scheelite mine and Casa la Plata. Primary fluid inclusions from helvine of both localities suggest that helvine deposited from moderate to high temperature and salinity aqueous fluids of likely magmatic origin. In both localities a late influx of gas-rich, CO2-bearing, moderate temperature, and moderate to low salinity fluid was trapped as secondary fluid inclusions. At the Chingolo scheelite mine, CO2-bearing fluids likely originated from decarbonation along the skarnification reaction front distanced from the prograde zone at lower temperature. In both studied areas, the latest trapped secondary fluids were of lower temperature and lower salinity. Pressure-corrected homogenization temperatures between 1.8 and 2.0 kbar sustain trapping temperatures for primary fluid inclusions within the 510–610 °C range for both localities. In the Chingolo scheelite mine, helvine formed at the contact zone between Ordovician aplite-pegmatites and Cambrian marbles that differentially reacted with infiltrated distal metasomatic-hydrothermal Be-bearing fluids fractionated from evolved granitic facies and/or pegmatites of the neighboring Devonian to Carboniferous Achala Batholith. Calculated δ18OH2O in equilibrium with garnet at ∼575 °C yielded δ18OH2O = 11.1 or 12.2‰. These heavy δ18OH2O values may derive from a magmatic fluid source possibly enriched in 18O from marbles or any other metasedimentary country rock during skarnification. This interpretation is supported by δ18OH2O values (8.3 to 10.1‰) and heavy δD values (−21.6 to −20.6‰) from retrograde epidote of other neighboring scheelite mines related to the Chingolo scheelite mine. The presence of beryl partially replaced by bertrandite + K-feldspar, non-paragenetically associated with helvine, confirms that early prograde crystallization conditions switched from a high temperature, subaluminous environment to a lower temperature, aluminous and increasingly acidic environment toward the retrograde skarn stage. At Casa la Plata, Cambrian schists and marble-amphibolite were also skarnified after the circulation of Be-bearing fluids derived from an epizonal Carboniferous granite (Capilla del Monte pluton) and its pegmatite dike swarm, which share parental links with the Achalian magmatism. The major Be supply for both studied localities should be attributed to Devonian and Carboniferous postorogenic to extensional peraluminous A-type granitic magmatism, which is the major source of beryllium not only for the Pampean Ranges but for the whole country.

Ladik ve Esirağıl Alanı (Konya, Orta Anadolu) Yöresindeki Geç Silüriyen Erken Karbonifer Yaşlı Metakarbonatların Metamafik Dayklarla Kontak Metamorfizması

In NNW Konya, mafic rocks form as dyke swarm or isolated linear dykes, which are cut by a series of normal faults that developed possibly by emplacement of the dyke-forming magma. Some anhydrous oxidized calcic skarn zones have ever been found out in Silurian-Early Carboniferous metacarbonates next to the Triassic metamafic dykes faulted, and in roof pendants within the intrusion. The metamafic rocks are suggested to have a low temperature and a small amount (or even absence) of the heat of crystallization, which causes the development of a restricted skarn zone. Mineralogical studies show that the skarn contains garnet, magnetite/hematite, chlorite, and sericite. The magnetite/hematite minerals form mostly as large euhedral crystals, up to 0.5 cm in length, which may be resorbed by chlorite and felsic minerals or rimmed by goethite. The garnet typically forms as spectacular euhedral large crystals, up to ~1.5 mm in size. The existence of chlorite and sericite, and the lack of hornblende, biotite and wollastonite in the region indicate that contact metamorphism took place under low pressure, relatively low temperatures (albite-epidote hornfels) and/or high CO2 conditions in the area.

Thaumasite in calcic skarns from the Zvezdel-Pcheloyad Pb-Zn deposit, Eastern Rhodopes, Bulgaria

Thaumasite was found in zoned calcic skarns hosted by the monzonitic rocks of the Zvezdel pluton, Eastern Rhodopes. Associated minerals are wollastonite, clinopyroxene, grossular-andradite garnets, Ti-rich garnets, plagioclase, calcite, quartz, epidote, prehnite, and chlorite. Titanite, apatite and magnetite are present as accessories. The mineral has been characterized using optical microscopy, PXRD, SEM/EDS, EPMA, and FTIR. Thaumasite forms sheaf-like and fan-shaped aggregates, which consist of acicular crystals elongated along the c axis. The IR spectrum of thaumasite is in accordance with the chemical composition and contains absorption bands in the range 3500–3291 cm–1 (O–H stretching vibrations); 1683, 1650 cm–1 (bending vibrations of H2O); 1389 cm–1 (stretching vibrations of CO32–); 1100, 1069 cm–1 (asymmetric stretching vibrations of SO42–); 882 cm–1 (symmetric bending vibration of CO32–); 768, 742, 680 cm–1 (Si–O stretching vibrations of Si(OH)6 octahedra); 670 cm–1 (bending vibrations of SO42–).

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.

Geology, mineralization, igneous geochemistry, and zircon U-Pb geochronology of the early Paleozoic shoshonite-related Julia skarn deposit, SW Siberia, Russia: Toward a diversity of Cu-Au-Mo skarn to porphyry mineralization in the Altai-Sayan orogenic system

The small but high-grade (average 1.29% to 2.04% Cu and 0.09–0.12% Mo for different orebodies, with also significant Au contents) Julia skarn Cu-Au-Mo deposit is situated some 30 km northeast from the Sorskoe porphyry Mo-Cu deposit in the Kuznetsk Alatau, a Paleozoic terrane in the northernmost part of the Altai-Sayan orogenic system, part of the Central Asian Orogenic Belt. The deposit comprises prograde calcic skarns and later retrograde skarn characterized by abundant andradite-rich garnet defining their oxidized type. These alteration assemblages bear abundant Cu sulfides (chalcopyrite and bornite), which are associated with magnetite and are partially overprinted by weak propylitic and stronger phyllic (carbonate-phyllic) alteration assemblages. Molybdenite is present in these assemblages, and various sulfides and sulfosalts, tellurides, and Bi minerals, as well as native Au are associated with phyllic alteration.The deposit is related to a magnetite-series, I-type, multiphase monzonite-syenite(-quartz syenite)-quartz monzonite-monzogranite igneous complex. Isotopic U-Pb zircon data indicate a Late Cambrian age (∼505 to 485 Ma) of the igneous rocks, whereas their geochemical signatures correspond to high-K calc-alkaline to shoshonitic intrusions emplaced in a post-collisional environment, with possible relationships to a subducted slab break-off. Magmatic evolution included a generation of shoshonitic magma by a low-degree partial melting of the 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. Although the overall magmatic evolution of the parental intrusions was long-lasting (ca. 20 m.y.), the mineralization is associated with a relatively rapid (ca. 5 m.y.) emplacement of more differentiated granitoid intrusions as well as late mafic dikes.The deposit represents an early Paleozoic (Late Cambrian) Cu-Au(-Mo) mineralization in the Altai-Sayan orogenic system and the broader Central Asian Orogenic Belt, thus confirming the Late Cambrian (to Early Ordovician) Cu-Au-Mo metallogenic event in the region. This, together with the broad regional occurrence of the Cambrian-Early Ordovician igneous complexes and related porphyry/skarn Cu-Au(-Mo) mineralization, emphasizes the early Paleozoic porphyries/skarns as a major regional event prior to much better known late Paleozoic porphyries/skarns in the Central Asian Orogenic Belt. There is a potential of the early Paleozoic terranes to host much more significant Cu-Au(-Mo) mineralization including that of skarn, porphyry and associated types.

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.

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.

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.

The perigranitic W-Au Salau deposit (Pyrenees, France): polyphase genesis of a late Variscan intrusion related deposit

A field study combined with a laboratory study and 3D modeling have been performed in order to decipher the genesis of the Salau deposit W-Au mineralization (Pyrenees, France), one of the most important for tungsten in Europe. Results show the existence of two superimposed ore types, emplaced ca. 10 km depth and within decreasing temperature conditions: a calcic silicates skarn with rare scheelite and disseminated sulphides followed by a mineralized breccia with massive sulphides (pyrrhotite and chalcopyrite dominant), coarse-grained scheelite and gold, representing the main part of the ore mined in the past. This breccia is localized in ductile-brittle shear-zones which crosscut the granodiorite. U/Pb dating on zircon, apatite and scheelite, previously realized, confirmed this polyphase evolution. These two types of mineralization, linked to the emplacement of two successive intrusions as confirmed by sulphur isotopic analysis, granodioritic then leucogranitic, can be classified as belonging to the Intrusion-Related Gold Deposit type (IRGD). The emplacement of the high-grade gold and scheelite breccia was initiated by the progressive localization of the regional deformation in the Axial Zone of the Pyrenees during the Permian within E-W dextral-reverse faults.

Geology, fluid inclusions, 40Ar/39Ar geochronology and H-O-S-Pb isotopes of the Galale Cu[sbnd]Au deposit, Northwestern Tibet, China

The Galale CuAu skarn deposit is located on the southern margin of the Bangong-Nujiang metallogenic belt, Tibet, and the CuAu mineralization is related to the Late Cretaceous granodiorite. The orebodies occurring as stratiform, lenticular and vein types are hosted within the skarn along the contact between the granodiorite and dolomitic marble or calcitic marble of the Jiega Formation. At least four hydrothermal alteration stages of the deposit have been recognized: prograde magnesian and calcic skarn stage (Stage 1), retrograde skarn stage (Stage 2), hydrosilicate alteration stage (Stage 3), and phyllic alteration stage (Stage 4).Fluid inclusions from skarn rocks and quartz veins are indicative of multiple fluid events. The prograde skarn was formed from a medium-salinity, hot (466–518 °C), high pressure (>1000 bar) homogenous magmatic-hydrothermal fluid, under its possible direct separation from crystallizing magma. The subsequent retrograde skarn was formed from a slightly lower temperatures (370–460 °C) and lower pressure (~800 bar) homogenous magmatic-hydrothermal fluid that could be supplied by crystallizing magma. Sharp drop of pressure (from 800 bar to 300 bar) and significant decrease of temperatures (low to 300 °C) may be related to fracturing and increase of porosity during alteration of skarns, and caused the fluid boiling in hydrosilicate alteration stage. The fluid from the latest phyllic alteration stage is characterized by dominant meteoric water and more lower temperatures (low to 178 °C). Decrease of fluid temperatures and pressures, fluid boiling and changes of redox regime are considered the crucial mechanisms for controlling the precipitation of the ores.The δ34S values of the sulfides range from −4.4 to 6, primarily from −0.9 to 1.7, that suggest a magmatic source. The sulfides yield narrow ranges of 206Pb/204Pb ratios (18.141–18.871) and 207Pb/204Pb ratios (15.588–15.701) and a wide range of 208Pb/204Pb ratios (38.359–39.008), indicating a mixed crust-mantle ore-forming metal source. The 40Ar39Ar plateau age of phlogopite from the chalcopyrite-bearing phlogopite tremolite skarn (Stage III) is 87.5 ± 0.6 Ma, which is consistent with the crystallization age of the Late Cretaceous granodiorite. Consequently, taking the data together suggests that the Galale CuAu deposit is a typical oxidized CuAu skarn deposit related to the Late Cretaceous magmatic-hydrothermal systems.

Analysis of the infiltrative metasomatic relationships controlling skarn mineralization at the Abbas-Abad Fe-Cu Deposit, Isfahan, north Zefreh Fault, Central Iran

The Abbas-Abad volcano-sedimentary-hosted Fe-Cu skarn deposit, NE Isfahan, is one of the most important skarns in the central Urumieh-Dokhtar magmatic arc. It was formed along the contact with the Dorojin granitoid massif next to the Zefreh Fault. The plutonic rocks are of I-type, volcanic arc affinity with normal-K, metaluminous, calc-alkaline to calcic quartz diorite, tonalite, and granodiorite, similar to many other Fe-type skarn-related granitoids worldwide. The parent magma involved in this skarn system is high temperature and relatively oxidized. Amphibole geobarometery in quartz diorite yielded a crystallization pressure lower than 200 MPa (2 kb, ~7 km) at 724° to 785 °C. Granitoid compositions around the ore deposit are mainly granodiorite. U-Pb zircon dating yields Early Miocene ages of 23.0 ± 1.6 Ma for a quartz dioritic rock and 21.3 ± 1.5 Ma for a tonalitic sample. The injection and cooling of the granodiorite produced a hornblende hornfels aureole with endoskarn.Paragenetic relationships and microprobe data indicate that Abbas-Abad calcic skarn evolution can be subdivided into three stages as follow: (I) Prograde skarn associated andradite-rich garnet (Adr93-98Grs0-4Spe1-2) and pyroxene, (II) Retrograde skarn starting with garnet (Adr53-69Grs28-43Spe2-4), magnetite, and sulfide minerals associated with calcic-alteration, and (III) Post-ore with pyrite, chalcedony, epidote, quartz, calcite, and zeolite veinlets. Textural and compositional studies of garnet and magnetite from the garnet-bearing exoskarn zone reveal the multiple events associated with skarn formation. Garnets are characterized by low TiO2 and relatively high CaO that are indicative of a calcareous wall-rock among Eocene volcaniclastic rocks. They are grouped into garnet-1 (low w/r) and garnet-2 (inverse zoning at high w/r) with notable Cu-contents (up to 743 ppm). Petrographically, magnetite morphology is divided into fine-grained granular, needle-like, and polycrystalline aggregates. Mineral chemistry of needle-like type reveals impure components (Al2O3, CaO, and SiO2). This type formed from dissolution-reprecipitation processes during a stage of reequilibration in the skarn system. Mixing with cooler external fluid (rich in oxygen and poor in Fe2+) is reflected in individual features during infiltration metasomatism during garnet and magnetite growth, such as oscillatory zoning and needle-like textures. Thus, we infer increasing pH (decreasing acidity) and decreasing T related to carbonate neutralization reactions affecting Fe- and Cu-chloride complexing as the main controls on mineralization.The structural studies of the area show that movement of the dextral transtensional Zefreh Fault provide local zones for emplacement of Dorojin granitoid during the Early Miocene. Consequently, the dextral transtensional Zefreh Fault and dextral transpression associated with the Marbin-Rangan Fault uplifted the skarn and host units and Dorojin body under the roughly N-S directed maximum compression direction. Furthermore, the interplay of Zefreh and Marbin-Rangan faults within the N-S regional compressional regime formed an anticlinal structure that exposed the Dorojin body within the core.

Geochemical composition of magnetite from different iron skarn mineralizations in NE Turkey: implication for source of ore-forming fluids

Iron oxide mineralizations in the eastern part of the Pontides (NE Turkey) are hosted in the skarn environments which are the Pontide paleomagmatic arc. These mineralizations always occur in the contacts between granitoid and limestone. Magnetites are hosted in carbonate rocks and generally formed during garnet-magnetite-epidote-quartz phase. Magnetites have high Co (15.8–43.4 ppm for the Kopuz, 11–12.5 ppm for the Egrikar, and 3.8–1266.7 ppm for the Karadag) concentrations suggesting that the sulfide decreased from the early to the late phases in the iron skarn mineralization forming systems. The Co/Ni ratios in the magnetites (0.12 to 35.38 for Karadag, 2.19 to 13.53 for the Kopuz, and 2.39 to 5.21 for the Egrikar) show the hydrothermal effect on the magmatic source in iron skarns in the study area. Thus, variable Co/Ni ratios reflect interactions between the magma with the host rock during successive alteration stages. Magnetites in this study have Sc-Nb-Mg-Ti depletions and Ta enrichment. In this study, high Co, Ti, and V contents in magnetites suggest the high temperature (300–500 °C) and low ƒO2. The V contents of the magnetite in the Karadag increase with the decreasing oxygen fugacity of fluid(s) forming magnetites, whereas the V contents of the magnetites in the Kopuz and Egrikar decrease with increasing oxygen fugacity of fluid(s) forming magnetites. Element patterns and Ni/Cr ratio (< 1) of the magnetite are geochemically similar to those of magnetite in Fe-skarn deposits and partly magmatic accessory magnetite of I-type granites. As a result, the Co, Ni, V, and Ti elements of the magnetite have played an important role in the discriminating and interpreting of skarn mineralizations in the eastern Pontides and support a calcic skarn origin with studies of the mineralization geology.

Dalnegorskite Ca5Mn(Si3O9)2, a New Pyroxenoid of the Bustamite Structure, a Rock-Forming Mineral of Calcic Skarns in the Dalnegorsk Borosilicate Deposit, Primorsky Krai, Russia

Dalnegorskite, a new pyroxenoid with the crystal chemical formula Ca2Ca2MnCa(Si3O9)2 and simplified formula Ca5Mn(Si3O9)2, is a rock-forming mineral in the B-bearing calcic skarns in the Dalnegorsk borosilicate deposit (Dalnegorsk, Primorsky Krai, Russia). It belongs to the bustamite structural type and forms a continuous solid-solution series with isostructural ferrobustamite Ca2Ca2FeCa[Si3O9]2. These pyroxenoids form beige, pinkish white and milky white radiated aggregates typically consisting of split thin acicular to fiber-like individuals and are associated with hedenbergite, datolite, andradite, galena, sphalerite, and pyrrhotite. Dmeas. = 3.02(2), Dcalc. = 3.035 g cm–3. Dalnegorskite is biaxial negative, α = 1.640 (3), β = 1.647 (3), γ = 1.650 (3)°, 2Vmeas. = 75(10)°. The average chemical composition of the holotype (electron microprobe data) is: 0.23 MgO, 40.02 CaO, 5.02 MnO, 3.64 FeO, 50.65 SiO2, total 99.56 wt %. The empirical formula calculated for 18 O atoms is Ca5.03Mn0.51Fe0.36Mg0.04Si6.01O18. The crystal structure of the new mineral was obtained from X-ray diffraction data using the Rietveld method, Rp= 0.0345, Rwp = 0.0444, R1 = 0.0790, and wR2 = 0.0802. Dalnegorskite is triclinic, P-1, a = 7.2588(11), b = 7.8574(15), c = 7.8765(6) A, α = 88.550(15), β = 62.582(15), γ = 76.621(6)°, V = 386.23(11) A3, and Z = 1. Dalnegorskite is distinct from the related mineral wollastonite in the infrared spectrum. The wave numbers of the maxima of strong bands in the Si–O stretching vibration region in the IR spectrum of dalnegorskite are (cm–1): 905, 937, 1025, and 1070. The holotypic specimen of dalnegorskite is kept in the collection of the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, Russia (no. 96201).

Factors controlling the crystal morphology and chemistry of garnet in skarn deposits: A case study from the Cuihongshan polymetallic deposit, Lesser Xing'an Range, NE China

Abstract The grossular-andradite solid solutions in garnet from skarn deposits in relation to hydrothermal processes and physicochemical conditions of ore formation remain controversial. Here we investigate garnet occurring in association with calcic and magnesian skarn rocks in the Cuihongshan polymetallic skarn deposit of NE China. The calcic skarn rocks contain three types of garnets. (1) Prograde type I Al-rich anisotropic garnets display polysynthetic twinning and a compositional range of Grs18–80Adr10–75. This type of garnet shows markedly low rare earth element (REE) contents (3.27–78.26 ppm) and is strongly depleted in light rare earth elements (LREE, 0.57–44.65 ppm) relative to heavy rare earth elements (HREE, 2.31–59.19 ppm). They also display a significantly negative Eu anomaly (Eu/Eu* of 0.03–0.90). (2) Fe-rich retrograde type II garnets are anisotropic with oscillatory zoning and own wide compositional variations (Grs1–47Adr30–95) with flat REE (13.73–377.08 ppm) patterns. (3) Fe-rich retrograde type III isotropic garnets display oscillatory zoning and morphological transition from planar dodecahedral {110} crystal faces to {211} crystal faces in the margin. Types III garnets exhibit relatively narrow compositional variations of Grs0.1–12Adr85–97 with LREE-enrichment (0.80–51.87 ppm), flat HREE patterns (0.15–2.46 ppm) and strong positive Eu anomalies (Eu/Eu* of 0.93–27.07 with almost all &gt;1). The magnesian skarn rocks contain euhedral isotropic type IV Mn-rich garnet veins with a composition of Grs10–23Sps48–62Alm14–29. All calcic garnets contain considerable Sn and W contents. Type II garnet containing intermediate compositions of andradite and grossular shows the highest Sn contents (64.36–2778.92 ppm), albeit the lowest W range (1.11–468.44 ppm). Birefringence of garnet is probably caused by strain from lattice mismatch at a twinning boundary or ion substitution near intermediate compositions of grossular-andradite. The fine-scale, sharp, and straight garnet zones are probably caused by self-organization, but the compositional variations of zones from core to rim are probably caused by external factors. The zoning is likely driven by external factors such as composition of the hydrothermal fluid. REE concentrations are probably influenced by the relative proportion and temperature of the system. Moreover, the LREE-HREE fractionation of garnet can be attributed to relative compositions of grossular-andradite system. The W and Sn concentrations in garnet can be used as indicators for the exploration of W-Sn skarn deposits.

Origin of Skarns at Migmatization on Ol’khon Island, Lake Baikal, Russia

Migmatites on the western shore of Ol’khon Island host unusual rocks: zoned lenses of hedenbergite–garnet–epidote–anorthite metasomatites coupled with the migmatites. No intrusive granites were found nearby. The skarn-forming process operated at the interface of the granite gneiss and skarn protolith (perhaps, carbonate rocks). The composition of the metasomatites is analogous to that of calcic skarns with high Al2O3, FeO, and CaO concentrations. The compositions and relations of the minerals provide evidence of the successive development of the hedenbergite–anorthite outer zone, dominantly anorthite–garnet main zone, and quartz-enriched inner zone, with all of the zones parallel to contact with the granite gneiss. The granite gneiss itself is also likely of metasomatic nature, as follows from its supraeutectic concentration of potassic feldspar in the leucosome and low crystallization temperatures. A minimum of the Gibbs free energy (calculated with the SELECTOR-C program package) was reached at 8 kbar and temperatures of 600– 625°C. These parameters are lower than the melting temperature of the granite eutectic, and the absence of melt is confirmed by the absence of melt inclusions in minerals of the granite gneisses. This indicate that the driving force of the process was migmatizing silicic–potassic solutions. The P–T parameters of the skarns are close to the foregoing values. The very high Sr and Ca and low Mg concentrations suggest that the protolith of the skarns was calcite marble. The enrichment of the skarns in the granitophile elements suggests that the skarns were produced simultaneously with and in genetic relation to the migmatization processes. The metasomatites were formed before the partial melts were derived, early in the course of the granite-forming processes and provide important information for better understanding the metasomatic process responsible for the exchange of chemical elements between the rocks.

Geology, mineralization, and fluid inclusion characteristics of the Koitash redox-intermediate W–Mo skarn and W–Au stockwork deposit, western Uzbekistan, Tien Shan

The Koitash W–Mo skarn deposit in the Tien Shan Gold Belt contained resources of 60 kt WO3 in altered skarn (averaging 0.56% WO3 and 0.029% Mo) and an additional 42 kt WO3 in separate zones of phyllic alteration (averaging 0.30% WO3, 0.97 g/t Au, 0.67% Cu, and 0.02% Bi). It is related to an Early Permian multiphase granodiorite–granite–leucogranite pluton that is composed of medium- to high-K, transitional metaluminous to peraluminous, weakly oxidized–weakly reduced, ilmenite–titanite series granitoids. The pluton was emplaced in a tectonic domain with inferred ancient continental crust; the latter, coupled with possible ascent of hot asthenospheric material, has likely defined a distinct W–Mo–Au metallogenic assemblage forming in a post-collisional environment. The deposit includes redox-intermediate prograde and retrograde pyroxene–garnet skarns, partially overprinted by propylitic and phyllic alteration assemblages, comprising scheelite, molybdenite, Cu–Bi–Ag–Te sulfides, and sulfosalts, as well as native Bi and Au. Compositional similarities between W–Mo deposits, containing Au–W–Cu–Bi mineralization in late alteration assemblages, and coeval intrusion-related Au-dominant (with minor W, Bi, etc.) deposits in the region support their possible genetic links. Fluid inclusion data indicate the involvement of carbonic-free moderately saline (10.5–15.0 to 8.0–11.7 wt% NaCl-eq.), high-pressure (2000 to 1500 bars), hot (> 550–600 °C) aqueous fluid, which was sourced from crystallizing magma and formed prograde calcic skarn. This fluid was followed by moderately saline, Ca-rich (14–16 wt% NaCl, 19–21 wt% CaCl2), lower pressure (800 bars) fluids toward the retrograde skarn stage, with the deposition of scheelite in association with pyrrhotite and molybdenite, locally with minor fluorite. Propylitic quartz–amphibole–chlorite–oligoclase–calcite, with scheelite, pyrrhotite, locally molybdenite, assemblages formed from boiling high-methane aqueous-carbonic fluids at temperatures of 370–400 °C and pressures of 950–1000 bars. Phyllic quartz–sericite–Fe–carbonate, locally with albite, fluorite, chlorite, scheelite, molybdenite, and other sulfides, also Bi and Au minerals, alteration assemblages initially formed from homogeneous and then from boiling aqueous-carbonic, low-salinity fluids at temperatures of 365–370 to 300–315 °C and pressures of ~ 2000 bars. δ34S increases from retrograde skarn (δ34S = + 2.0 to + 2.3‰) through propylitic (δ34S = + 3.0 to + 3.4‰) to phyllic alteration (δ34S = + 4.2 to + 4.7‰) stages.

Metamorphic evolution of the Loma Marcelo skarn within the geotectonic context of the crystalline basement of the Ventania System (Argentina)

This study describes the mineralogical and isotopic changes that carbonate xenoliths experienced under multiple metamorphic events and hydrothermal fluid circulation during the evolution of the Ventania System basement. The high reactivity of carbonate xenoliths allowed the preservation of mineral assemblages corresponding to at least three metamorphic events in the resulting Loma Marcelo skarn. The Ventania System basement is composed of Neoproterozoic granites and ignimbrites, Early Cambrian granites, and Middle Cambrian rhyolites. Carbonate xenoliths were incorporated during the intrusion of a calc-alkaline granite with an LA-ICP-MS U-Pb crystallization age of 621.6 ± 2.2 Ma. The intrusion induced pyroxene–hornfels facies metamorphism in the carbonate xenoliths and the associated metasomatism generated calcic and magnesian skarns depending on the protolith composition. Garnet, clinopyroxene, wollastonite, bytownite, and meionite were formed in the calcic skarn (CaS), whereas forsterite and spinel were formed in the magnesian skarn (MgS). Crystallization of Early Cambrian alkaline granites was accompanied by intense hydrothermal activity that was responsible for low temperature (≤300 °C) F-metasomatism in the skarn, as evidenced by the presence of F-rich vesuvianite (CaS) and chondrodite (MgS), among other minerals. Vesuvianite was formed from calc-silicate mineral assemblages of the previous metamorphic event, whereas chondrodite was formed by replacement of forsterite. The low temperature formation of these typical high-grade minerals could be an evidence of mineral formation under disequilibrium conditions favoured by the high reactivity of hydrothermal fluids. Neopalaeozoic basement mylonitization under greenschist facies metamorphism was accompanied by hydrothermal fluid circulation. This event promoted extreme mobility of chemical elements in the basement rocks and epidotization (CaS) and serpentinization (MgS) in the Loma Marcelo skarn. The elongated and boudinaged shape of the skarn bodies, parallel to the mylonitic foliation, is a consequence of dextral shearing that affected the basement rocks. Additionally, almost pure grossular crystallized post-tectonically in the CaS. Carbonates of the Loma Marcelo skarn are depleted in 13C and 18O (δ13CV-PDB = −2.5/−10.1‰; δ18OV-SMOW = +7.3/+14.0‰) relative to carbonate sedimentary rocks. The δ13C and δ18O variations can be attributed to the interaction between large amounts of hydrothermal fluids (W/R = 30–50) and Neoproterozoic carbonate sedimentary rocks.

Unique sodic–calcic skarn hosted by ultramafic rocks and albitite at the Ulsan skarn deposit, Gyeongsang Basin, South Korea

The Ulsan Fe–W skarn deposit is located within the Cretaceous Gyeongsang Basin in the southeastern part of the Korean Peninsula. The skarn and mineralization formed along contacts between the Paleogene Gadaeri granite, limestone, partially serpentinized ultramafic rocks (dunite and harzburgite), and hornfelsed volcanic and siliciclastic rocks. These various country rock types produced a system with multiple types of skarn. Typically, skarn within limestone occurs as clinopyroxene–magnetite skarn, clinopyroxene–garnet skarn, and garnet skarn, but the unique host rocks (ultramafic rocks + albitite) at Ulsan also produced unusual skarn types such as a sodic–calcic skarn system. Na–Ca skarn formed by chemical interactions between albitized granites and adjacent ultramafic rocks. This Na–Ca skarn contains unusual Na-rich clinopyroxene (acmite), Na-rich amphiboles, Cr-rich clinopyroxene, Cr-rich amphiboles, Sr-bearing plagioclase, and Ba-bearing K-feldspar. Clinopyroxene and amphibole have Na2O concentrations up to 6.5 wt%, and these minerals contain up to 12.1 and 2.5 wt% Cr2O3, respectively. Na was derived from albitite, and the formation of Cr-bearing acmite (kosmochlor) and Na-amphibole (richterite) appears to have been related to the release of Cr into the fluid phase by the breakdown of chromite in ultramafic rocks during prograde skarn formation. K-feldspar K–Ar dating yields an age of 50.3 ± 1.7 Ma for the prograde garnet skarn. The related Gadaeri granite exhibits a high-K calc-alkaline signature and a weakly metaluminous to peraluminous I-type affinity with a total alkalinity (Na2O + K2O) of 8.5–8.8 wt%. The Gadaeri granite shows an enriched REE pattern compared with albitite, as well as negative Eu anomalies, which together with the K/Rb values (200–500) are suggestive of a highly evolved magma from a continental-margin setting.