Skarn Alteration Research Articles

한국 포천 철(-동) 스카른 광상의 지질, 광화작용 및 생성연대

선캄브리아기 경기육괴의 북서부에 위치한 포천 철(-동)광상은 다양한 이론이 제시되어 논쟁 대상이 되었으며, 성인적 측면에서 변성(-배기형) 퇴적 광상이 주된 생성이론으로 인지되었다. 본 논문에서는 포천 스카른화작용/철광화작용의 핵심 증거를 통하여 공간적으로 인접한 명성산 화강암이 관계화성암의 가능성을 제시하였다. 포천 스카른은 석회암과 백운암의 다양한 탄산염암을 모암으로 하여 Ca계열, Mg계열 및 Na-Ca계열 스카른이 형성되었다. 철광화작용은 주로 Na-Ca계열 스카른대를 따라 배태되고 있으며, 후퇴 스카른 단계에 국부적으로 동 광화작용이 중첩된다. 포천 후퇴 스카른 단계에 정출된 금운모의 Ar-Ar과 K-Ar연대측정 결과는 <TEX>$110.3{\pm}1.0{\sim}108.3{\pm}2.8Ma$</TEX>이며, 스카른 철(-동) 광화작용은 천부 관입암체인 명성산 화강암의 관입시기(112 Ma)와 일치하여 관계화성암으로 추정된다. 주변 탄산염암, 스카른 및 맥상의 탄산염광물간 산소-탄소 동위원소 빈화된 경향성은 개방계 조건(<TEX>$XCO_2=0.1$</TEX>)에서 열수의 탈탄산염화 작용과 침투작용에서 유도되었다. 한편 황화광물(황동석-황철석 혼합물)과 경석고에서 매우 높은 황 동위원소 값은 황근원물질이 주변 탄산염암에 함유되어 있던 황산염광물로부터 공급되었을 가능성을 지시한다. 포천 광상 주변의 전단대에서는 파쇄대가 중첩되어 발달하여 있으며, 백악기 화강암으로부터 공급된 고온성 광화유체가 이러한 약선대를 따라 유입되어 약 <TEX>$500^{\circ}{\sim}400^{\circ}C$</TEX>의 온도범위에서 강력한 근지성 스카른화작용과 함께 광화작용이 유도되었다. The Pocheon iron (-copper) deposit, located at the northwestern part of the Precambrian Gyeonggi massif in South Korea, genetically remains controversial. Previous researchers advocated a metamorphosed (-exhalative) sedimentary origin for iron enrichment. In this study, we present strong evidences for skarnification and Fe mineralization, spatially associated with the Myeongseongsan granite. The Pocheon deposit is composed of diverse carbonate rocks such as dolostone and limestone which are partially overprinted by various hydrothermal skarns such as sodic-calcic, calcic and magnesian skarn. Iron (-copper) mineralization occurs mainly in the sodic-calcic skarn zone, locally superimposed by copper mineralization during retrograde stage of skarn. Age data determined on phlogopites from retrograde skarn stage by Ar-Ar and K-Ar methods range from <TEX>$110.3{\pm}1.0Ma$</TEX> to <TEX>$108.3{\pm}2.8Ma$</TEX>, showing that skarn iron mineralization in the Pocheon is closely related to the shallow-depth Myeongseongsan granite (ca. 112 Ma). Carbon-oxygen isotopic depletions of carbonates in marbles, diverse skarns, and veins can be explained by decarbonation and interaction with an infiltrating hydrothermal fluids in open system (<TEX>$XCO_2=0.1$</TEX>). The results of sulfur isotope analyses indicate that both of sulfide (chalcopyrite-pyrite composite) and anhydrites in skarn have very high sulfur isotope values, suggesting the <TEX>$^{34}S$</TEX> enrichment of the Pocheon sulfide and sulfate sulfur was derived from sulfate in the carbonate protolith. Shear zones with fractures in the Pocheon area channeled the saline, high <TEX>$fO_2$</TEX> hydrothermal fluids, resulting in locally developed intense skarn alteration at temperature range of about <TEX>$500^{\circ}$</TEX> to <TEX>$400^{\circ}C$</TEX>.

Dating iron skarn mineralization using hydrothermal allanite-(La) U–Th–Pb isotopes by laser ablation ICP-MS

Trace elements and U–Th–Pb isotopes of hydrothermal allanite from the Beiminghe iron skarn deposit (eastern North China Craton) were simultaneously analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to demonstrate the suitability and robustness of the U–Th–Pb system in this phase as a geochronometer of ore-forming processes. Allanite grains from the Beiminghe deposit, typically exhibit optical and chemical zoning and are intergrown or texturally equilibrated with prograde and/or retrograde skarn assemblages mainly consisting of tremolite, diopside, magnetite, and garnet. In addition, they contain abundant aqueous two-phase or daughter mineral-bearing three-phase aqueous fluid inclusions, confirming their hydrothermal origin. Most grains have high La/Ce ratios (1.19–1.74) and thus are classified as allanite-(La). A notable feature of the hydrothermal allanite-(La) is the lack of common Pb, with values comparable with those of the zircon standards 91500 and GJ-1 used in this study. LA-ICPMS spot analyses reveal remarkable variation in Ca, REEs (rare earth elements), Fe, Al, and Mg within a single grain, consistent with the substitution of REE3++Mg+Fe2+ for Ca+Al+Fe3+ in the mineral structure. Traverses of zoned grains show that the rims have Th/U ratios ranging from 0.73 to 11.55, significantly higher than the core areas (0.07–1.81). Fifty-nine spot analyses (beam size of 32μm or 60μm) on 5 allanite-(La) grains yield reproducible 206Pb/238U ages of 134±7 to 139±6Ma, with a weighted mean 206Pb*/238U age (206Pbexcess-corrected 206Pb/238U age) of 136±1Ma (2σ). Of these, 34 analyses on allanite rims have a weighted mean 208Pb/232Th age of 139±2Ma (2σ) that is within error of the 206Pb*/238U age. The dating results showed that an increase of beam size from 32 to 60μm significantly reduces the analytical uncertainties (2σ). Petrographic and textural data have demonstrated that the allanite-(La) grains formed during skarn alteration and notably have close paragenetic relationships with retrograde skarn assemblages and iron oxide minerals; their U–Th–Pb ages therefore are interpreted as the timing of iron skarn mineralization at Beiminghe. Thirteen zircon grains from the ore-related diorite intrusion have a weighted mean 206Pb/238U age of 136±2Ma (2σ), consistent with and thus confirming the reliability of the allanite-(La) U–Th–Pb ages when interpreted as the formation age of the Beiminghe iron deposit. Our results suggest that hydrothermal allanite with low Th and U is a promising U–Th–Pb geochronometer and can provide reliable constraints on the timing of hydrothermal mineralization without requiring a matrix-matched external standard. The present dating results also indicate that the U–Pb, rather than Th–Pb, isotope system is preferable for U–Th–Pb dating of low Th/U allanite.

Ore genesis of the unusual Talate Pb–Zn(–Fe) skarn‐type deposit, Altay, NW China: constraints from geology, geochemistry and geochronology

The Talate ore field is located in the Abagong polymetallic metallogenic belt of the Altay Orogen, NW China. Lenticular ore bodies occur in the Kangbutiebao Formation, a package of intermediate‐felsic marine volcanic rocks and terrigenous clastic sedimentary‐carbonate rocks. Skarn alteration (mainly garnet) is present in both ore and wall rocks, especially the carbonate rocks. The mineral assemblages and cross‐cutting relationships of veins allow the alteration and mineralization process to be divided into four stages. From early to late, these are the early skarn (E‐skarn), the late skarn with quartz–magnetite veins (QM), the quartz–sulphide (QS) and the quartz–carbonate (QC) assemblages. Quartz crystals are important gangue minerals in the latter three stages, in which four distinct compositions of fluid inclusions are identified based on petrography, microthermometry and laser Raman microspectroscopy, namely aqueous inclusions (W‐type), pure CO2 inclusions (PC‐type), CO2‐rich inclusions (C‐type) and daughter mineral‐bearing inclusions (S‐type). Microthermometric data and laser Raman analyses show that the quartz crystals from the QM stage contain all four inclusion types, with the W‐type being predominant. Homogenization temperatures range between 271 and 426 °C. The salinities of the W‐ and C‐type fluid inclusions range from 0.5 to 22.4 wt.% NaCl eqv., whereas the S‐type fluid inclusions in the QM stage range from 31 to 41 wt.% NaCl eqv. Daughter minerals in the fluid inclusions include halite, sylvite, pyrite and calcite. Quartz from the QS stage (main mineralization stage) contains the W‐, C‐ and PC‐type inclusions, which are homogenized at temperatures of 204–269 °C, with salinities of 0.2–15.6 wt.% NaCl eqv. Only W‐type fluid inclusions have been identified in the QC stage. These yielded homogenizing temperatures of 175–211 °C and salinities of 1.1–9.9 wt.% NaCl eqv. The C‐type fluid inclusions of the main (QS) mineralization stage yield trapping pressures of 107–171 MPa, corresponding to a depth of 4–6 km. The sulphur isotope values (−1.7‰ and −6.6‰) imply that the QS stage may not be directly associated with the early skarn (−7.4‰) and quartz–magnetite stages (−4.8‰ and −5.0‰), though the QS stage is probably dominated by magmatic‐hydrothermal fluids. 40Ar/39Ar isotope plateau ages of 227.6 and 214.1 Ma for biotite separated from the QM and QS stages are significantly younger than the host Kangbutiebao Formation (ca. 410 Ma). The Talate Pb–Zn(–Fe) deposit is interpreted to be an unusual skarn‐type system formed in a continental collision orogeny. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Petrophysical constraints for inversion models of the Eastern Gawler Craton IOCG Province

The Eastern Gawler Craton IOCG province , South Australia, hosts a variety of deposits associated with Hiltaba Suite magmatism. Adjacent to the southern margin of the Carrapateena haematite breccia deposit lies the Emmie Bluff and Punt Hill regions which host skarn mineralisation in the palaeoproterozoic Wallaroo Group beneath 1 km of cover. Within those two smaller regions are two distinct iron and copper skarn styles of mineralisation. Exploration methods under deep cover rely upon geophysical signatures from airborne and ground surveys to select drill targets, define the cover sequences and basement. This process can be improved using petrophysical properties to constrain 3D inversion models. A wide range of petrophysical properties have been collected over the area to relate the known geology with geophysical surveys. Magnetic susceptibility and specific gravityhave been captured to characterise altered units. The Emmie Bluff and Punt Hill regions has been the subject of 3D inversion modelling projects undertaken by the Geological Survey of South Australia which required geological constraints due the distinct styles of alteration and mineralisation. Within the Wallaroo Group, the both gravity and magnetic sources within Emmie Bluff are controlled by iron oxide alteration, whereas in the neighbouring Punt Hill region the density characteristics are dominated by skarn alteration and the magnetic source appears independent of the altered stratigraphy. This work presents characterisation of mineralisaed stratigraphy used for 3D inversions of the region. The data is available for download from the SARIG website.

Sr–Nd–Pb isotope systematics of magnetite: Implications for the genesis of Makeng Fe deposit, southern China

The Makeng Fe(−Mo) deposit is the largest Fe deposit in the southwestern Fujian depression belt in southern China. Although it has been extensively studied, there are different views on the genesis of the deposit. In the present study, Sr–Nd–Pb isotope data were used to determine the source of fluids and metals in the deposit. The initial 87Sr/86Sr and εNd (133Ma) ratios of magnetite samples from Makeng ore vary from 0.71067 to 0.71267 and from −11.2 to −8.7, respectively. Ratios for granites vary from 0.70878 to 0.71349 and from −11.2 to −8.7, respectively. The magnetite samples yield narrow ranges for 206Pb/204Pb (18.405–18.926), 207Pb/204Pb (15.657–15.710), and 208Pb/204Pb (38.598–38.809). The granites show ranges from 18.787 to 19.154, 15.670 to 15.692 and 39.158 to 39.413 for the same Pb isotopic ratios, respectively. The isotopic data of magnetite and granites shows that the Sr–Nd–Pb isotopic characteristics of magnetite samples are similar to the granites. According to the Sr–Nd–Pb isotopic characteristics, O–H isotopic systematics and rare earth elements characteristics, the initial mineralizing fluids and metals probably originated from granite magmas involving materials from Hercynian diabases or intruded country rocks. Field and laboratory observations suggest that skarn alteration is widespread in the Makeng ore district, indicating that the Makeng Fe deposit is a skarn-type iron deposit.

Application of geographically weighted regression to identify spatially non-stationary relationships between Fe mineralization and its controlling factors in eastern Tianshan, China

Marine volcanic-sedimentary Fe mineral deposit is one of the most productive mineralization types in eastern Tianshan mineral district, Uyghur Autonomous Region, China. Previous researches have revealed that Fe deposits in this area are mainly hosted by the Lower Carboniferous Yamansu bimodal volcanic rocks and characterized by extensive skarn alteration. Due to variable effects of controlling factors across the study area, Fe mineralization occurs at limited locations with favorable geological environment. In order to understand ore genesis of different Fe deposits and to assist in Fe mineral exploration in this area, it is essential to investigate the spatial relationships between Fe mineralization and its controlling factors across the study area. In this paper, Fe mineralization and its controlling factors are characterized and delineated on the basis of spatial distributions of geochemical signatures obtained by principal component analysis. The spatially non-stationary relationships between these spatial distributions of geochemical signatures are further analyzed by a geographically weighted regression (GWR) method. The comparison of applications of the ordinary regression and GWR shows that GWR is superior in depicting localized spatial relationships between Fe mineralization and its controlling factors. The GWR results demonstrate that influences of controlling factors on Fe mineralization are spatially varied and imply that geological environments favorable to Fe mineralization are diverse at different locations. Based on currently achieved results, suggestions to support future Fe mineral exploration at different subareas are proposed at the end of this paper.

MINERALOGY AND ORIGIN OF COPPERGOLD BEARING SKARN WITHIN THE BATU HIJAU PORPHYRY DEPOSIT, SUMBAWA ISLAND, INDONESIA

The aim of this study ist o emphasize on the origin of copper-gold bearing skarn mineralization at the Batu Hijau deposit which is located at the southwestern corner of Sumbawa Island, Indonesia. Although most skarn are derived from limestones, nolimestone is known in the Batu Hijau deposit. Ca-rich andesitic volcaniclastic host rocks favor skarn alteration within the Batu Hijau deposit. The t ype of skarn can be classified as calcic-exoskarn, and locally controlled by faults and fractures. Two major stages consisting of four sub-stages of skarn forming processes can be divided by the mineral assemblages of skarn as prograde and retrograde stages. The prograde skarn consists of clinopyroxene and garnet ± magnetite formed at the trapping temperature of 440°-480 °C with 34-38 wt% NaCl eq. while retrograde skarn alteration is dominated by Fe-rich minerals such as amphibole and epidote formed at the trapping temperature down to 340°-360°C with 4-8 wt% NaCl eq. Opaque minerals include chalcopyrite, pyrite, sphalertite, and minor galena and bismuth- telluride. Gold was precipitated in the retrograde stage associated with bismuthtelluride minerals. The sulfur isotope data of skarn ranges from +0.1 to +1.7‰ (sulfide), and porphyry systems range from 0.04 to1.4‰ and 10‰ to 15‰ (sulfide and sulfate respectively). According to the fluid inclusion and sulfur isotope data, the origin of skarn and porphyry system can be suggested to be that the magmatic origin. Furthermore, the sulfur isotope data of the deposit evidently shows that a porphyry- related skarn mineralization exhibiting transition from one style to the next can be relatively rapid. The result of this research has indicated that the range of porphyry-related deposits, skarn and porphyry systems can form during a single prolonged hydrothermal event.

Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit, Eastern Tianshan Mountains, NW China: Constraints on the metallogenesis

The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4±0.94 and 323.47±0.95Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO2 (42.90–46.61wt.%), P2O5 (0.08–0.12wt.%), MnO (0.35–0.97wt.%) and TiO2 (0.74–0.82wt.%), and relatively large variations in CaO (6.93–15.13wt.%), Al2O3 (14.71–19.93wt.%), total Fe2O3 (8.14–12.66wt.%) and MgO (4.96–8.52wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type.

Petrology, geochemistry, and evolution of the iron skarns along the northern contact of the Eğrigöz Plutonic Complex, Western Anatolia, Turkey

The Çatak and Küreci skarn districts are located approximately 10 km NW of Emet (Kütahya) in Western Turkey. The skarn and associated ore formations mainly occur at the contact between intrusive rocks of the Eğrigöz Plutonic Complex (EPC) and calcareous pelitic schists with limestone lenses of the Sarıcasu Formation and meta-carbonate rocks of the Arıkaya Formation. The major, trace, and rare earth element analysis of the igneous rocks indicate that they are high level, subalkaline, calc-alkaline, peraluminous to metaluminous I-type intrusions, generated in a continental arc setting. Three distinct skarn-type mineralization, differing in their host rocks and distance from the intrusive body, were chosen to establish the ore-forming conditions in different episodes of skarn formation. The Küreci iron mineralization is hosted in a skarn zone with well-developed zoning from unaltered granodiorite and endoskarn, andradite-diopside exoskarn, to diopside-wollastonite exoskarn towards a marble reaction front. In Sakari, the iron mineralization and associated skarn have formed due to successive fracturing and infiltration processes. From early contact metamorphic rocks to late prograde skarn at the Sakari prospect, the composition of clinopyroxene ranges from (Di_{50-70} Hd_{28-53} Jo_{1-2}) to (Di_{19-73} Hd_{26-77} Jo_{2-6}) and the composition of garnet ranges from (Ad_{95-99} Gr_{1-5}) to (Ad_{40-61} Gr_{36-58}), respectively. The presence of anisotropic grossular garnet with high Fe^{2+}/Fe^{3+} in crosscutting pyrrhotite-pyrite-bearing veinlets coupled with hedenbergitic pyroxene (Mg-poor clinopyroxene with higher Fe^{2+}/Fe^{3+}) is consistent with reducing conditions during the later stage of prograde skarn alteration. The Çatak iron skarn is characteristic, with its high sulphide content due to the presence of pyrrhotite, pyrite, and arsenopyrite, and low proportion of garnet to pyroxene. The sulphur isotope (\delta^{34}S) compositions in the pyrrhotite-dominant skarn zones range between +0.84 to -2.23&permil. We interpret the bulk of the sulphur in the system as of igneous derivation and there has not been any significant sulphur contribution from a crustal source. Fluid inclusion measurements conducted on skarn minerals of the proximal zone and distal zone+vein skarn revealed high homogenization temperatures (371 to >600°C) and varying salinity values (10.5 to >70 wt% NaCl). The fluid inclusion data indicate that there are at least three fluids associated with the genesis of the proximal skarn where the high garnet/pyroxene ratios are found. Fluid inclusions that represent the early stages both in garnet and pyroxene plot in 'Primary Magmatic Fluid' and 'Metamorphic Fluids' fields. A magmatic fluid, presumably located at deeper parts of the system, mixed with a metamorphic fluid during its

The alteration, mineralogy and geochronology (SHRIMP U–Pb and 40Ar/39Ar) of copper-bearing Anjerd skarn, north of the Shayvar Mountain, NW Iran

A Cu-bearing skarn zone occurs north of the Shayvar Mountain in northwestern Iran. Skarn-type metasomatic alteration and mineralization occur along the contact between Upper Cretaceous impure carbonates and a Miocene Cu-bearing granitic stock. Both endoskarn and exoskarn developed in the rocks. Exoskarn is the principal skarn zone and is enclosed by a skarnoid–hornfelsic zone. Skarn formation occured during stages: (1) prograde, (2) middle stage and (3) late stage. In the prograde stage, there were two main processes: (a) metamorphic–bimetasomatic and (b) prograde metasomatic. The metamorphic process began immediately after intrusion of the pluton into the enclosing impure carbonates. The prograde metasomatic stage commenced with segregation and evolution of a fluid phase in the pluton and movement into fractures and micro-fractures in the skarnoid–hornfelsic rocks developed in a metamorphic zone. The introduction of considerable amounts of Fe, Si and Mg led to the development of voluminous medium- to coarse-grained anhydrous calc-silicates. During the middle stage, the previously formed skarn zones were affected by intense multiple hydrofracturing in the Cu-bearing stock. In addition to Fe, Si and Mg, substantial amounts of Cu, Pb and Zn, along with volatile components such as H2S and CO2 were added to the skarn system. Consequently, substantial amounts of hydrous calc-silicates (epidote, tremolite–actinolite), sulfides (pyrite, chalcopyrite and molybdenite), oxides (magnetite, hematite) and carbonates (calcite) replaced the anhydrous calc-silicates. The retrograde stage was synchronous with the incursion of relatively low-temperature, more oxidized fluids into skarn system, resulting in partial alteration of the early-formed calc-silicates and development of a series of very fine-grained aggregates of chlorite, clay, hematite and calcite. Zircon grains from the endoskarn zone provide constraints on the timing of solidification of the granite stock (9.91 ± 0.31 Ma) that caused mineralization in the Anjerd area. One sample of primary hornblende from the monzogranitic Shayvar batholith has an 40Ar/39Ar age of 26.54 ± 0.65 Ma and indicates that intrusion of the Miocene stock and associated Cu skarn formation occurred a considerable time after intrusion of the batholith.

Galuskinite, Ca7(SiO4)3(CO3), a new skarn mineral from the Birkhin gabbro massif, Eastern Siberia, Russia

AbstractIn addition to spurrite, Ca5(SiO4)2(CO3), and tilleyite, Ca5(Si2O7)(CO3)2, galuskinite, C is the third mineral in the CaO—SiO2—CO2ternary system. Galuskinite, monoclinic, space groupP21/c (a =18.79,b =6.72,c= 10.47 Å, β = 90.79°,V =1322 Å3,Z =4), occurs in thin veins which cut calcio-olivine, γ-Ca2SiO4, skarn with larnite, β-Ca2SiO4, relics. Pavlovskyite, Ca8(SiO4)2(Si3O10), and dellaite, Ca6(Si2O7)(SiO4)(OH)2, form a margin between the veins and the calcio-olivine skarn. The sanidinite facies high-temperature skarn formed ∼500 Ma ago when gabbroid rocks of the Birkhin complex (Baikal area, Eastern Siberia, Russia) intruded and contact-metamorphosed limestone xenoliths. Galuskinite is a retrograde product of skarn alteration and has neither been described from cement clinker production processes nor from studies of the CaO—SiO2—CO2system. The crystal structure of galuskinite, refined from single crystal X-ray data toRi =3.1%, has a modular character. One may define a polysomatic series with spurrite and larnite as endmembers and galuskinite as a 1:1 polysome built from regular alternating spurrite and larnite modules. Differences between the X-ray powder patterns of galuskinite and spurrite are most obvious in the low 8 region. Galuskinite is named after the Russian mineralogists Irina O. Galuskina and Evgeny V. Galuskin, Faculty of Earth Sciences. University of Silesia, Poland, for their outstanding contributions to skarn mineralogy.

Mineralization and fluid evolution of the Jiyuan polymetallic Cu–Ag–Pb–Zn–Au deposit, eastern Tianshan, NW China

The newly discovered Jiyuan Cu–Ag–(Pb–Zn–Au) deposit is located in the southern section of the eastern Tianshan orogenic belt, Xinjiang, northwestern China. It is the first documented deposit in the large Aqikekuduke Ag–Cu–Au belt in the eastern Tianshan orogen. Detailed field observations, parageneses, and fluid inclusion studies suggest an epithermal ore genesis for the main Cu–Ag mineralization, accompanied by a complicated hydrothermal alteration history most likely associated with the multi-stage tectonic evolution of the eastern Tianshan. The Jiyuan Cu–Ag ore bodies are located along the EW-striking, south-dipping Aqikekuduke fault and are hosted by Precambrian marble and intercalated siliceous rocks. Early-stage skarn alteration occurred along the contact zone between the marble layers and Early Carboniferous diorite–granodiorite and monzogranite intrusions; the skarns are characterized by diopside–tremolite–andradite–pyrite–(magnetite) assemblages. Local REE-enriched synchysite–rutile–arsenopyrite–(clinochlorite–microcline–albite) assemblages are related to K–Na alteration associated with the monzogranite intrusions and formed under conditions of high temperature (310°C) and high salinity (19.9 wt.% NaCl). Subsequent hydrothermal alteration produced a series of quartz and calcite veins that precipitated from medium- to low-temperature saline fluids. These include early ‘smoky’ quartz veins (190°C; 3.0 wt.% NaCl) that are commonly barren, coarse-grained Cu–Ag mineralized quartz veins (210°C; 2.4 wt.% NaCl), and late-stage unmineralized calcite veins (140°C; 1.1 wt.% NaCl). Tremolite and Ca-rich scapolite veins formed at an interval between early and mineralized quartz veins, indicating a high-temperature, high-salinity (>500°C; 9.5 wt.% NaCl) Ca alteration stage. Fluid mixing may have played an important role during Cu–Ag mineralization and an external low-temperature Ca-rich fluid is inferred to have evolved in the ore-forming system. The Jiyuan auriferous quartz veins possess fluid characteristics distinct from those of the Cu–Ag mineralized quartz veins. CO2-rich fluid inclusions, fluid boiling, and mixing all demonstrate that these auriferous quartz veins acted as hosts for the orogenic-type gold mineralization, a common feature in the Tianshan orogenic belt.

Geochemistry, mineralogy and genesis of the Ayazmant Fe–Cu skarn deposit in Ayvalik, (Balikesir), Turkey

The Ayazmant Fe–Cu skarn deposit is located approximately 20 km SE of Ayvalık or 140 km N of Izmir in western Turkey. The skarn occurs at the contact between metapelites and the metabasites of the Early Triassic Kınık Formation and the porphyritic hypabyssal intrusive rocks of the Late Oligocene Kozak Intrusive Complex. The major, trace, and rare earth-element geochemical analysis of the igneous rocks indicate that they are I-type, subalkaline, calc-alkaline, metaluminous, I-type products of a high-level magma chamber, generated in a continental arc setting. The 40Ar– 39Ar isochron age obtained from biotite of hornfels is 20.3 ± 0.1 Ma, probably reflecting the age of metamorphic–bimetasomatic alteration which commenced shortly after intrusion into impure carbonates. Three stages of skarn formation and ore development are recognized: (1) Early skarn stage (Stage I) consisting mainly of garnet with grossular-rich (Gr 75–79) cores and andradite-rich (Gr 36–38) rims, diopside (Di 94–97), scapolite and magnetite; (2) sulfide-rich skarn (Stage II), dominated by chalcopyrite with magnetite, andraditic garnet (Ad 84– 89), diopside (Di 65– 75) and actinolite; and (3) retrograde alteration (Stage III) dominated by actinolite, epidote, orthoclase, phlogopite and chlorite in which sulfides are the main ore phases. 40Ar– 39Ar age data indicate that potassic alteration, synchronous or postdating magnetite–pyroxene–amphibole skarn, occurred at 20.0 ± 0.1 Ma. The high pyroxene/garnet ratio, plus the presence of scapolite in calc-silicate and associated ore paragenesis characterized by magnetite (± hematite), chalcopyrite and bornite, suggests that the bulk of the Ayazmant skarns were formed under oxidized conditions. Oxygen isotope compositions of pyroxene, magnetite and garnet of prograde skarn alteration indicate a magmatic fluid with δ 18O values between 5.4 and 9.5‰. On the basis of oxygen isotope data from mineral pairs, the early stage of prograde skarn formation is characterized by pyroxene (Di 94–97)-magnetite assemblage formed at an upper temperature limit of 576 °C. The lower temperature limit for magnetite precipitation is estimated below 300 °C, on the basis of magnetite–calcite pairs either as fracture-fillings or massive ore in recrystallized limestone-marble. The sulfide assemblage is dominated by chalcopyrite with subordinate molybdenite, pyrite, cubanite, bornite, pyrrhotite, galena, sphalerite and idaite. Gold–copper mineralization formed adjacent to andradite-dominated skarn which occurs in close proximity to the intrusion contacts. Native gold and electrum are most abundant in sulfides, as fine-grained inclusions; grain size with varying from 5 to 20 µm. Sulfur isotope compositions obtained from pyrrhotite, pyrite, chalcopyrite, sphalerite and galena form a narrow range between − 4.8 and 1.6‰, suggesting the sulfur was probably mantle-derived or leached from magmatic rocks. Geochemical data from Ayazmant shows that Cu is strongly associated with Au, Bi, Te, Se, Cd, Zn, Pb, Ni and Co. The Ayazmant mineralizing system possesses all the ingredients of a skarn system either cogenetic with, or formed prior to a porphyry Cu(Au–Mo) system. The results of this study indicate that the Aegean Region of Turkey has considerable exploration potential for both porphyry-related skarns and porphyry Cu and Au mineralization.

Skarn Alteration and Mineralization at Coroccohuayco, Tintaya District, Peru

Coroccohuayco is a porphyry-related copper skarn deposit in the Andahuaylas-Yauri batholith of southeast Peru with little evidence for the presence of a significantly mineralized hydrothermal system at surface. The deposit occupies the hinge zone of a synform, hosted by the Cretaceous Ferrobamba Formation. Hydrothermal alteration of the carbonate protolith produced skarn between the underlying hornfels-altered Mara Formation and an overlying preskarn sill-like diorite intrusion, with the main zone of mineralization generally below 200 m from the surface. The spatial distribution of skarn was dominantly controlled by permeability variations in the carbonate protolith. The deposit consists of prograde garnet-dominant calc-silicate alteration with mineralization of chalcopyrite-bornite ± chalcocite disseminated as grains in calc-silicates. Precious metal mineralization occurs as gold and silver minerals mostly associated with bornite-chalcocite mineralization disseminated within more distal granular garnet alteration. Fluids were derived from the crystallization of several porphyry phases that intruded into an upper crustal level, possibly as a cupola above an underlying batholith. Fluid inclusion data from garnet and pyroxene indicate that the prograde skarn alteration formed between 400° to >600°C from highly saline fluids. Data from quartz indicate that copper mineralization formed between 250° to 400°C, with magnetite deposition over the upper part of this range. Retrograde alteration is generally magnetite (+carbonate-silica) replacement of calc-silicates and is locally associated with copper mineralization. Hydrous retrograde alteration, commonly observed in other copper skarn systems, is of lesser importance at Coroccohuayco. This is likely related to the geochemical effects of the overlying diorite intrusion which could have impeded the influx of oxidized meteoric waters into the system or buffered the composition of late-stage fluids, as well as the oxidized Fe-rich character of the prograde skarn.

Palladium, Platinum and Gold Concentrations in Fengshan Porphyry Cu–Mo Deposit, Hubei Province, China

Abstract:The Fengshan porphyry‐skarn copper–molybdenum (Cu–Mo) deposit is located in the south‐eastern Hubei Province in east China. Cu–Mo mineralization is hosted in the Fengshan granodiorite porphyry stock that intruded the Triassic Daye Formation carbonate rocks in the early Cretaceous (∼140 Ma), as well as the contact zone between granodiorite porphyry stock and carbonate rocks, forming the porphyry‐type and skarn‐type association. The Fengshan granodiorite stock and the immediate country rocks are strongly fractured and intensely altered by hydrothermal fluids. In addition to intense skarn alteration, the prominent alteration types are potassic, phyllic, and propylitic, whereas argillation is less common. Mineralization occurs as veins, stock works, and disseminations, and the main ore minerals are chalcopyrite, pyrite, molybdenite, bornite, and magnetite. The contents of palladium, platinum and gold (Pd, Pt and Au) are determined in nine samples from fresh and mineralized granodiorite and different types of altered rocks. The results show that the Pd content is systematically higher than Pt, which is typical for porphyry ore deposits worldwide. The Pt content ranges from 0.037 to 1.765 ppb, and the Pd content ranges between 0.165 and 17.979 ppb. Pd and Pt are more concentrated in porphyry mineralization than skarn mineralization, and have negative correlations with Au. The reconnaissance study presented here confirms the existence of Pd and Pt in the Fengshan porphyry‐skarn Cu–Mo deposit. When compared with intracontinent and island arc geotectonic settings, the Pd, Pt, and Au contents in the Fengshan porphyry Cu–Mo deposit in the intracontinent is lower than the continental margin types and island are types. A combination of available data indicates that Pd and Pt were derived from oxidized alkaline magmas generated by the partial melting of an enriched mantle source.

Skarn Alteration and Au-Cu Mineralization Associated with Tertiary Granitoids in Northwestern Turkey: Evidence from the Evciler Deposit, Kazdag Massif, Turkey

The Evciler Au-Cu district is located in the Kazdag region of northwestern Anatolia, Turkey. Skarn alteration and Au-Cu mineralization assemblages are related to an elliptical I-type, magnetite series, metaluminous, calc-alkaline body, which has intruded the Kazdag Massif. The skarns have an oxidized mineralogy dominated by garnet (Ad85Gr15-Ad50Gr50), hedenbergitic pyroxene (Di70Hd30-Di25Hd75), epidote, amphibole, and chlorite and contain up to 80 percent sulfides (pyrrhotite >> pyrite + chalcopyrite). The massive pyrrhotite-bearing mineralization replaced prograde skarn and marble. At least two paragenetic stages of skarn formation and ore deposition have been recognized: stage I hedenbergitic pyroxene ± garnet ± scapolite ± quartz ± magnetite, and stage II amphibole ± epidote ± chlorite ± quartz ± calcite and pyrrothite + chalcopyrite ± pyrite. Early skarn formation (stage I) was dominated by anhydrous minerals, hedenbergitic pyroxene, and garnet. The hydrous skarn assemblage (stage II) replaced early-formed skarn assemblages. Clinopyroxene is hedenbergite rich, whereas garnet is andraditic. The mineralogy and character of the Evciler skarn is consistent with deposition under oxidized conditions, formation at shallow crustal levels and similar to those of other Cu-Au, Fe-bearing skarn systems. Stable isotope data indicate that magmatic water was involved in the early stage of skarn formation (δ18OH2O= 6.3–10.9‰) and a mixed magmatic-meteoric water was responsible for the subsequent mineralization (δ18OH2O = −4.6 to +1.0‰, δDH2O= −30.9 to −17.7‰).

Controls on Skarn Mineralization and Alteration at the Cadia Deposits, New South Wales, Australia

Controls on Skarn Mineralization and Alteration at the Cadia Deposits, New South Wales, Australia

Precise Sm–Nd and U–Pb isotopic dating of the supergiant Shizhuyuan polymetallic deposit and its host granite, SE China

The supergiant Shizhuyuan W–Sn–Bi–Mo deposit is hosted by the Qianlishan granite, a small, highly fractionated granitic pluton (∼10 km2) with multiple phases of intrusions within the Early Yanshanian granitoid province of SE China. Strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and Devonian limestone is responsible for the polymetallic mineralizations. SHRIMP U–Pb zircon analysis indicates that the two early phases of the Qianlishan granite formed contemporaneously at 152±2 Ma. Metasomatic minerals (garnet, fluorite and wolframite) separated from the skarn and greisen yield a Sm–Nd isochron age of 149±2 Ma that is interpreted as the formation age of the Shizhuyuan deposit. Therefore, the mineralization of the supergiant Shizhuyuan polymetallic deposit formed contemporaneously with, or very shortly after, the intrusion of the small, highly fractionated Qianlishan granite.

The occurrence and origin of babingtonite in the El Valle-Boina's Cu-Au deposit in Asturias, Spain

Babingtonite has been found at El Valle-Boinas Cu-Au deposit, Spain, this being the first known occurrence of this mineral in skarn in Spain. It is a locally abundant accessory mineral related to the hydrothermal alteration of the calcic skarn. Babingtonite is present in metasomatized aplitic dykes or occurs as a consequence of alteration of Fe-rich end members of both hedenbergite and andradite and always in association with Fe-rich epidote (Ps 32–48 ). Other related minerals are calcite, quartz, hydroxyapophyllite, hydroandradite, prehnite, pumpellyite-(Fe +2 ), Cu-sulphides and electrum. The babingtonite formation is related to the locally Aldepleted aqueous fluids at relative high fO 2 and low XCO 2 .

Fluid inclusion noble gas and halogen evidence on the origin of Cu-Porphyry mineralising fluids

The naturally occurring noble gas isotopes (40Ar, 36Ar, 84Kr and 129Xe) and halogens (Cl, Br, I) have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry. Quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits (PCD): Bingham Canyon, Utah; and Silverbell, Ray, Mission, Pinto Valley and Globe-Miami in Arizona. In addition, analyses of 3He/4He have been obtained from sulphide minerals hosted by the quartz veins at Silverbell, Ray, Pinto Valley and Globe-Miami.The majority of PCD fluids studied have Br/Cl and I/Cl ratios that overlap those of fluids included in mantle diamond, suggesting that the salinity had a juvenile origin. The high I/Cl (121 × 10−6 mole, M) in samples from the propylitic zone of Silverbell is attributed to the presence of sedimentary formation water.3He/4He ratios have R/Ra values in the range 0.3 to 1.72, and provide evidence for the involvement of a crustal component in addition to mantle volatiles. 40Ar/36Ar ratios vary from meteoric values of ∼317 in the propylitic zone of Bingham Canyon, and 323 in the skarn alteration of Mission up to 3225 in the potassic zone of Pinto Valley. Fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end-member comprising meteoric water and air, and a high salinity end-member consisting of a mixed mantle and crustal fluid.The 40Ar/Cl ratio of fluid inclusions at Pinto Valley (∼10−4 M) is similar to values obtained previously for mantle fluids. The 40Ar/Cl value is two orders of magnitude lower at Bingham Canyon, where a depleted 36Ar concentration (0.2 × 10−6 cm3/g) below that of air saturated water (ASW), and a range of highly fractionated noble gas compositions (F84Kr = 13 and F129Xe = 160) indicate that boiling and pulsed fluid flow have occurred.