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• C-O isotopes suggest that both granite and marble contributed to ore-forming material. • H-O isotopes suggest a primarily magmatic-sourced ore fluids with later meteoric waters input. • S isotopes suggest a mixed sulfur source from granite and seawater sulfate. • The Luziyuan Zn-Pb-Fe(-Cu) mineralization likely occurred in a distal skarn system related to a concealed pluton. The superlarge Luziyuan Zn-Pb-Fe(-Cu) polymetallic deposit is located in the southern Baoshan Block of western Yunnan (SW China). The deposit contains over 4.2 million tonnes (Mt) of contained Zn + Pb metal reserves at 5.09% Zn and 2.40% Pb, and 301 Mt of Fe ore resources at 30.02% TFe. The stratiform or vein-type orebodies are hosted in the Upper Cambrian Shahechang Formation. Similar to many major Zn skarn deposits worldwide, the Luziyuan deposit shows a distinct metal zonation pattern with depth, which correlates with the different calc-silicate mineral assemblages: From Zn-Pb (sphalerite + galena) ores in marble, through Zn-Pb-Fe-Cu (sphalerite + galena + magnetite ± pyrite + chalcopyrite) ores in pyroxenoid (± clinopyroxene)-bearing skarn, to Fe-Cu (magnetite ± pyrite + chalcopyrite) ores in garnet (± clinopyroxene ± pyroxenoid)-bearing skarn. The syn-ore calcite has δ13C PDB and δ18O SMOW values of −1.0 to +0.9‰ and +10.3 to +12.7‰, respectively, suggestive of isotopic affinities to both granite and marble. δD H 2 O values of the fluid inclusions in the pre-ore rhodonite and the syn-ore quartz range from −81.7 to −68.8‰ and −76.6 to −56.3‰, respectively; whilst the calculated δ18O H 2 O values of the rhodonite, syn-ore calcite, and quartz are of +5.4 to +6.1‰, +1.4 to +3.8‰, and −5.3 to −2.0‰, respectively. This indicates that the ore-forming fluids were sourced from both magmatic and meteoric waters. δ34S values of the syn-ore sulfides (+8.9 to +12.0‰) are slightly higher than those of typical crustal-derived granites (δ34S = −4.0 to +9.0‰), but are significantly lower than those of Cambrian seawater sulfates. This indicates that the source of S is correlated with both granite and seawater sulfates. We propose that the Luziyuan Zn-Pb-Fe(-Cu) polymetallic mineralization occurred in a distal skarn system, and was genetically linked to concealed Early Cretaceous granite intrusions. [ABSTRACT FROM AUTHOR]

The chemistry of garnet can provide clues to the formation of skarn deposits. The chemical analyses of garnets from the Astamal Fe-LREE distal skarn deposit were completed using an electron probe micro-analyzer. The three types of garnet were identified in the Astamal skarn are: (I) euhedral coarse-grained isotropic garnets (10–30 mm across), which are strongly altered to epidote, calcite and quartz in their rim and core, with intense pervasive retrograde alteration and little variation in the overall composition (Adr 94.3–84.4 Grs 8.5–2.7 Alm 1.9–0.2 ) (garnet I); (II) anhedral to subhedral brecciated isotropic garnets (5–10 mm across) with minor alteration, a narrow compositional range along the growth lines (Adr 82–65.4 Grs 21.9–11.7 Alm 11.1–2.4 ) and relatively high Cu (up to 1997 ppm) and Ni (up to 1283 ppm) (garnet II); and (III) subhedral coarser grained garnets (> 30 mm across) with moderate alteration, weak diffusion and irregular zoning of discrete grossular-almandine-rich domains (Adr 84.2–48.8 Grs 32.4–7.6 Alm 19.9–3.5 ) (garnet III). In the third type, the almandine content increases with increasing grossular/andradite ratio and increasing substitutions of Al for Fe 3 + . Almost all three garnet types have been replaced by fine-grained, dark-brown allanite that is typically disseminated and has the same relief as andradite. The Cu content increases while Ni content decreases slightly towards the rim of garnet II and garnet III. Copper in garnet II is positively correlated with increasing almandine content and decreasing andradite content, indicating that the almandine structure, containing relatively more Fe 2 + , is more suitable than andradite and grossular to host divalent cations such as Cu 2 + . Nickel in garnet II is positively correlated with increasing andradite content, total Fe, and decreasing almandine content. This is because Ni 2 + substitutes for Fe 3 + in the Y (octahedral) position. There are unusual discrete grossular-almandine rich domains within andraditic garnet III, indicating the low diffusivity of Ca compared to Fe at high temperatures. [ABSTRACT FROM AUTHOR]

The Campiglia Marittima Fe-Cu-Zn-Pb(-Ag) skarn deposit has long been regarded as a reference example of an exoskarn showing a symmetric outward mineralogical zoning of both skarn and ore minerals with respect to an axial mafic porphyry dike. Detailed field and underground mapping, along with three-dimensional reconstruction of the geometries of skarn and magmatic bodies, integrated with new petrographic, mineralogical and geochemical data, argue against this model. The shapes of the skarn bodies and the growth versors of skarn minerals in particular, are ascribed to the focusing of metasomatic fluids in sigmoid-shaped volumes of fractured host marble. After skarn formation, a mafic magma was emplaced, forming dikelets and filling residual pockets in the skarn. Field evidence and geochemical data show that the “hot” mafic magma interacted with the previously formed Zn-Pb(-Ag) skarn, triggering textural reworking and chemical redistribution of Zn-Pb sulfides as well as contributing to a late Fe-Cu mineralization. Campiglia Marittima skarn-ore system behaved at odd: a telescoping process is recorded, yet in a reverse way. [ABSTRACT FROM AUTHOR]

The Astamal Fe-LREE skarn deposit is the largest iron skarn in NW Iran. It is a unique case, as a distal skarn deposit which crops out approximately 600 m from the associated Oligo-Miocene granodioritic pluton. This deposit formed in the south-southwest of the pluton where fractures and faults within the Upper Cretaceous volcano-sedimentary host rocks acted as conduits for the mineralizing fluids. The deposit contains three iron ore bodies: southern, northern and eastern. The main mineral assemblage within the ore zones is characterized by magnetite, pyrrhotite and pyrite, with lesser quantities of chalcopyrite, hematite, goethite and limonite. The skarn minerals predominantly comprise garnet, epidote, actinolite, calcite, quartz, clinopyroxene and chlorite (in order of abundance). Retrograde alteration is strongly developed in the skarn zone where most of the garnet has been pervasively altered to secondary minerals (e.g. epidote, calcite and quartz) both in the rims and the cores whereas the majority of the clinopyroxene has been replaced by a hydrous retrograde mineral assemblage (e.g. tremolite, actinolite and chlorite). Garnets with andradite–grossular compositions are the dominant mineral in the skarn zone, which are generally isotropic with a narrow compositional range along the growth lines (Adr 94.3–64.5 Grs 21.9–2.7 Alm 11.1–0.2 ). These garnets are Fe-rich and have high Fe/(Fe + Al) ratios (between 0.96 and 0.78). Cu and Ni are enriched in the garnets. This suggests that these elements were enriched in the hydrothermal fluids from which the garnet precipitated. This is supported by the presence of chalcopyrite and Ni-bearing massive magnetite in the study area, which also suggests that Cu and Ni were enriched in the late stage ore-bearing hydrothermal fluids. Clinopyroxene with a hedenbergitic composition is generally homogenous and has particularly high Fe/Fe + Mg ratios (between 0.99 and 0.86) and is poor in TiO 2 , MnO and Cr 2 O 3 . High Zn concentrations were also detected in the clinopyroxenes (up to 1044 ppm), despite an absence of significant Zn mineralization (such as sphalerite) in the district. Therefore, it is believed that the proportion of Zn in the hydrothermal fluids decreased significantly from the time of clinopyroxene formation to the period of the sulfide deposition phase. Allanite and LREE-bearing epidotes are the main LREE bearing minerals in this deposit. The epidote is also Fe-rich with high Fe/(Fe + Al) ratios (between 0.32 and 0.44). Due to a lack of replacement texture between both garnet and clinopyroxene and garnet and actinolite (which is formed by alteration of clinopyroxene), it is believed that these two minerals have grown simultaneously and are coexisting minerals. In the Astamal skarn, these minerals can be stable and coexisting at temperatures between 490 and 560 °C and LogƒO 2 = − 16 to − 31. [ABSTRACT FROM AUTHOR]

Distal skarns form by the metasomatic reactions of a host rock induced by far-traveled hydrothermal fluids. Physical and structural characteristics and geochemical patterns of distal Pb Zn skarn bodies were studied at the Petrovitsa deposit in southern Bulgaria. Skarn bodies formed from the interaction of hydrothermal fluids with reactive host lithologies (marble and gneiss). These fluids were transported along sub-vertical feeder structures and lithological contacts. Epidote skarn developed in gneiss protolith, and pyroxene (johannsenite) skarn developed in marble. Detailed geological mapping, complimented by measurements of the internal structure of the skarn body using pyroxene growth versors, quantifies the propagation direction of the skarn body: 1) away from the major local fluid conduit (feeder structure), and 2) away from lithological contacts between aluminosilicate rock and marble. Such growth suggests that fluid flow was generally orthogonal to the skarn front propagation direction in the pyroxene skarn. Textural, mineralogical and geochemical data from skarn samples reveal multiple growth generations of major skarn calc-silicates epidote and pyroxene. The epidote skarn is characterized by limited spatial distribution and fine-grained epidote/clinozoisite growth associated with massive replacement and sulfide mineralization. The pyroxene skarn consists of acicular clinopyroxene crystals which form spheroidal aggregates with discrete growth banding. These bands are the physical representation of the cyclic fluid pulses which resulted in rhythmic skarn growth marked by geochemical banding. In situ geochemical analyses in the epidote skarn reveal early Al-rich epidote overprinted by Fe-rich epidote associated with higher Mn and Sr contents and irregular compositional banding. Clinopyroxene (Jo 60 – 95) shows general increase in Na, Al, Mn, and REE + Y with distance from the feeder structure and lithologic contacts. These elements correlate with the distance traveled by the hydrothermal fluid from the feeder to the site of skarnification, which we define using a proxy based on the Al content of pyroxene crystals. This reflects an increasing degree of fluid "contamination" by interaction with the aluminosilicate host rocks and functions as a proxy for fluid transport distance. The spatial distribution of trace-elements in pyroxene on an outcrop scale is indicative of discrete pulses of hydrothermal fluid resulting in precipitation of skarn calc-silicates along the increasingly tortuous fluid pathway between the feeder structure and the skarn front, resulting in both the macro- and micro-scale chemical and textural variability of the skarn body. • Quantitative mapping of pyroxene indicates skarn growth away from fluid conduit • Pyroxene and epidote skarn formed synchronously along lithological contacts • Pyroxene composition records water-rock interaction along flow pathway • Growth banding records cyclic open and closed system metasomatic processes [ABSTRACT FROM AUTHOR]

• Deposits occurred in the faults, along the schistosity planes and the contacts. • Mineralizations are related to the calc-alkaline magmatism. • The mineralization was formed under reduced conditions. • EPMA data are indicative of a distal Pb-Zn skarn mineralization. The Biga Peninsula (NW Turkey) is an important sector of the Tethyan Metallogenic Belt. In addition to Au and Ag, this region hosts several Pb, Zn, Cu, Fe, and Mo deposits. Pb-Zn skarn mineralizations in the Biga Peninsula (Biga Pb-Zn skarn) are bounded by carbonate lenses in the metamorphic rocks and localized along the regional faults and at the contact between carbonate units and igneous-metamorphic rocks. The primary ore minerals are galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, magnetite, and hematite. Ore-bearing altered rocks are composed primarily of calc-silicate alteration minerals, including both prograde (grossular-andradite garnets and johannsenite-hedenbergite pyroxenes) and retrograde (epidote, chlorite, quartz, and calcite) assemblages. The garnet composition is a solid solution with <96% andradite and 1.4% to 94.4% grossular content. Some garnet grains exhibit zonings because of fluctuations in the content of Al3+ and Fe3+. Mn/Fe ratios are high, and the outermost grains have a high MnO content, up to 3 wt%. The pyralspite content is <10 wt%. The proportions of johannsenite and hedenbergite in clinopyroxenes range from 0.35 to 94.69 mol% and 2.13 to 77.90 mol%, respectively. Clinopyroxenes have high Mn/Fe values, with Mn content up to 10 wt%. The epidote-group minerals with a high Al3+ and Fe3+ content belong to the epidote-clinozoisite series (Ep 37-21 Czo 78-63 Pie 1-0). Mineral chemistry data indicate that Biga Pb-Zn skarn deposits and distal Pb-Zn skarn deposits throughout the world have similar characteristics. The abundance of graphite in the metamorphic host rocks and the andradite-hedenbergite content of the skarns imply that the studied mineralizations were formed under reducing conditions. The ranges of crystallization depth and pressure values calculated from the chemistry of garnet-clinopyroxene pairs are 1.3–1.6 km and 10–12 MPa, respectively. Because these values are less than those of plutons hosting the Biga Pb-Zn skarn deposits, the mineralizations likely formed in the distal zones of plutons. [ABSTRACT FROM AUTHOR]