Pure Andradite Research Articles

Skarn Zonation of the Giant Jiama Cu-Mo-Au Deposit in Southern Tibet, SW China

Abstract Jiama is a giant skarn-porphyry deposit in southern Tibet, southwestern China. It is the largest Cu (~7.4 Mt), second largest Au (~208 t), and fourth largest Mo (~0.62 Mt) skarn deposit in China. In addition, Jiama also contains considerable amounts of Zn, Pb, and Ag. The mineralization is related to a Miocene (~15 Ma) monzogranite porphyry that intruded the Upper Jurassic Duodigou Formation limestone (now mainly marble) and the overlying Lower Cretaceous Linbuzong Formation sandstone, siltstone, and shale (now mainly hornfels with minor slate). Porphyry-type orebodies occur in the intrusion and in the surrounding hornfels, whereas ~50- to 100-m-wide skarn-type orebodies wrap around the intrusion along the intrusion-marble contact, and as a ~50- to 150-m-thick stratabound blanket along the hornfels-marble boundary away from the contact. The stratabound skarns extend continuously for up to ~2 km on one side of the intrusion until the hornfels-marble boundary is exposed at the surface. Jiama is well zoned with respect to skarn minerals, metals, and stable isotope compositions, both laterally and vertically. Laterally, the proximal skarn is dominated by garnet, whereas the stratabound skarn contains progressively more wollastonite away from the intrusion center. The garnet composition changes from mixed Al and Fe rich (Ad21-79Gr20-77) in the endoskarn, to Fe rich in the proximal exoskarn (Ad70 to pure andradite), and Al rich (Gr69-88) in the stratabound skarn distal to the intrusion. Pyroxene is Mg rich (mostly Di66-97) in the deposit and is slightly more Fe rich in the distal zones. The garnet/pyroxene ratio remains high throughout the skarn. Vertically in the stratabound skarn, garnet is the dominant skarn mineral closer to the hornfels at shallower positions, whereas closer to the marble at deeper positions, there is more wollastonite, up to a wollastonite-dominant zone at the skarn-marble contact. The color of the garnet near the upper hornfels is dark red-brown, and gradually turns yellowish green downward. In terms of metals, the Mo-rich domains are dominantly within the intrusion, the Cu-Au-rich domains are in exoskarns near the intrusion, and the Zn-Pb-Ag mineralization occurs mostly in the distal zones. The Cu ore mineralogy changes vertically in the stratabound skarns, from chalcopyrite rich in the upper garnet zone, to bornite rich in the lower wollastonite zone. Values of δ34SV-CDT for chalcopyrite generally decrease from >–1‰ within the intrusion to –6‰ ~2 km away from the intrusion. The C and O isotope compositions of the marble samples show generally increasing trends downward in the stratabound part of the skarn, indicating that the fluid channel was closer to the hornfels. The lowest marble δ18OV-SMOW values (mostly <14%) occur close to the intrusion, consistent with the fluids being derived from the magmas. The organic carbon content decreases at shallower parts of the stratabound skarns and closer to the intrusion (from 0.412 to 0.003 wt %), indicating loss of organic carbon nearer to the fluid channel and source. These spatial zonation patterns are due to a combination of several factors, among which the oxidized intrusion-oxidized carbonate wall-rock environment likely played the most critical role. Other factors include the temperature gradient from the intrusion center outward, the compositions of protoliths (intrusion, hornfels and carbonate), and the changing fluid compositions caused by fluid-rock reaction and mineral precipitation during fluid flow. The development of the skarn zonation patterns at Jiama and the understanding of their controlling factors are helpful to mineral exploration in the Jiama area and elsewhere.

LA-ICP-MS U-Pb dating, chemical composition and evolution of oscillatory zoned andradite from the Chengmenshan Cu-Au deposit, Jiangxi province

The Chengmenshan Cu-Au deposit is a typical porphyry-skarn deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRMB), developing abundant oscillatory zoned andradite. However, the physiochemical conditions, hydrothermal evolution and age of skarn formation is unknown. This paper presents BSE image, EMPA point analyses, LA-ICP-MS point analyses and mapping, U-Pb isotope data for andradite from the Chengmenshan Cu-Au deposit. From core to rim, three generations of andradite could be identified (Grt-1, Grt-2, Grt-3). Grt-1 (Adr73-88) exhibit compositional variations as oscillatory zoning. Grt-2 (Adr93-100) is almost pure andradite, with smooth surface. Grt-3 (And71-93) occur as halo surrounding earlier andradite, with broken surface. Incorporation of REE and U into all generations of andradite mainly follows coupled YAG-type substitution mechanism and the coupled substitution of [U4+]VIII + 2[Fe3+, Al3+]Ⅳ – [Ca2+] – 2[Si4+]Ⅳ in the dodecahedral position, respectively. Their associated intrusions, mineral assemblages and consistent positive Eu anomalies indicate an acidic, oxidizing and low salinity forming conditions. From core to rim, the contents of REE and HFSE are decreasing, and the REE patterns evolve from flatted, through LREE-enriched, HREE-depleted, to REE-depleted. The chemical composition and evolution of oscillatory zoned andradite suggest that the fluids in Chengmenshan deposit experienced the sequences of pore fluid-mixed, through magmatic-hydrothermal dominated, to meteoric water influx. In this process, the temperature, pH and fluid compositions varied. U contents are closely correlated with REE contents, especially LREE contents. The andradite samples yield Tera-Wasserburg lower intercept 206Pb/238U ages of 140.1 ± 2.6 Ma (MSWD = 1.8), representing the age of Cu-Au mineralization. These results indicate the evolutionary characteristics of hydrotheraml fluids and precisely date the Cu-Au skarn formation, which may serve as a useful indicator in the exploration of similar skarn deposits and point selection for U-Pb dating.

Skarn formation and Cu[sbnd]Ag mineralization in the McKenzie Gulch area, northern New Brunswick, Canada: Implication for the applications of mineral chemistry in exploration for porphyry copper and skarn deposits

The McKenzie Gulch (MG) copper–silver skarn occurrences are associated with the Middle Devonian intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork of veinlets, disseminations, patchy, and locally as replacement of calc-silicate skarns in argillaceous limestone. This skarn is interpreted to have developed in three stages: the earliest contact metamorphic stage resulting in hornfels developed from calcareous mudstone and very fine-grained clastic sedimentary rocks (Stage I); this was followed by metasomatic replacement resulting in prograde anhydrous skarn containing grossular-andradite (grandite), diopsidic pyroxene, and wollastonite (Stage II); increasing fluid/rock interaction results in retrograde skarn dominated by epidote, calcite, green amphibole, chlorite, sulfide minerals, titanite, andradite and hedenbergite commonly along veins and veinlets (Stage III). Subordinate sphalerite and pyrite occur in late veins cross-cutting both porphyry dykes, skarns and hornfels zone.Prograde garnets in the study area belong to the grossular-andradite solid solution that ranges from Adr16Gr82 to almost pure andradite - Adr99Gr1, whereas other garnet end-members constitute less than 5% collectively. Pyroxenes form a diopside-hedenbergite solid solution with composition between Di31Hd60 and Di93Hd7, while other pyroxenes constitute less than 10% collectively. These garnets and pyroxenes exhibit chemical zonation patterns generally characterized by a rim of Fe enrichment relative to cores of grains, which may suggest possible change in salinity (or other factor that promotes Fe transport). This means grossular and diopsidic cores are rimmed by more andradite and hedenbergitic compositions for garnet and pyroxene, respectively. In general, there is a tendency for increasing Fe component in both garnet and pyroxene from the prograde to retrograde stages.Epidote formed during the retrograde stage exhibits oscillatory zoning similar to retrograde andradite and indicates Fe enrichment increasing from core to rim. Based on the major element composition, textural and optical characteristics of garnets, we conclude that grandites (Al-rich) formed under low water/rock ratios, in equilibrium with metasomatic fluids whose composition was locally buffered by the host rocks, whereas andradite (Fe-rich) resulted from relatively high water/rock ratios that were in equilibrium with a magmatic-derived fluid. The results of this study indicate that there is a relationship between the composition of pyroxenes and garnets of the skarn alteration facies and the dominant metal of the mineralized skarns. These facies plot in the compositional field of Cu-dominated skarns. As a result, this relationship may be applied in exploration for skarn and porphyry copper mineralization.

Insights into the skarn Cu mineralization processes of an intracontinental setting: A case study of the Xiaohekou deposit, southern Qinling Orogen, China

Traditional metallogenic models posit that porphyry–skarn Cu systems form in subduction and collision zones. However, in eastern China, many important Late Mesozoic porphyry–skarn deposits have formed in intracontinental settings related to lithospheric break-up during the celebrated tectonothermal, Yanshanian event. The Xiaohekou Cu skarn deposit in the southern Qinling Orogen presents an ideal case study to describe such ore deposit geology, using mineral and fluid inclusion analysis, in addition to sulfide and granitoid SPb isotope studies. A detailed metallogenic process model for the intracontinental skarn Cu deposit is proposed.The authors recognize four main paragenetic stages: (I) anhydrous skarn, (II) hydrous skarn–Fe oxide, (III) quartz–polymetallic sulfide, and (IV) carbonate–quartz. Stage I is characterized by high temperatures (346–547 °C), high to moderate salinities (6.74–49.2 wt% NaCleqv), together with acidic and oxidizing H2O-NaCl fluids. From early to late anhydrous skarn stage, un-zoned grossular (Adr24–31Gr68–74), a grossular–andradite alternation series with wide zones (Adr26–68Gr31–72) and diopside (Di73–91Hd8–24)–pure andradite with dense oscillation zones (Adr68–100Gr0–30) crystallized sequentially, representing a gradual shift toward higher fO2 conditions. Diopside-hosted fluid inclusions (FIs) contain halite-bearing three phase (H-type), liquid-rich two-phase (L-type), and vapor-rich two-phase (V-type) inclusions, suggesting such fluids were sourced from a silicate melt and stage I & II boiling of magmatic-hydrothermal fluids at lithostatic depths of 1.1–3.3 km.. The FIs in actinolite (stage II) are characterized by homogenization temperatures (Th) of 273–502 °C and salinities of 4.34–42.8 wt% NaCleqv. Cooling and dilution of the ore-forming hydrothermal fluid, as well as the high fO2 conditions were responsible for the hydrous skarn alteration in addition to magnetite and specularite deposition. Stage III constitutes the principal Cu mineralizing event. Quartz from this stage contains distinctive liquid-rich inclusions, with Th of 143–423 °C and salinity of 2.07–17.8 wt% NaCleqv. Their Th values include two populations (210–270 °C and 303–359 °C) suggesting a commingling of hot magmatic-hydrothermal fluid and meteoric water accompanied by a significant fO2 decrease and pH increase was a principal factor behind chalcopyrite-pyrite-pyrrhotite deposition. The fluids in stage IV (81–218 °C and 0.18–14.1 wt% NaCleqv) are likely be the chilled equivalent of fluid from the Cu mineralization stage which equilibrated with marble. SPb isotopes reveal that the metals were derived from the granitic rocks and produced by hybrid (mantle+crust)-derived magmas. We propose hydrothermal fluids exsolved from the cooling granitic magma contributed the majority ore-forming materials. Whereas ore-forming fluids clearly interacted with country rocks affecting ore precipitation, skarn development and deposit geometry and emplacement. Devonian host rocks donated little if any of either sulfur or base metals to the mineralization process.

Garnet major and trace element evidence of the alteration and mineralizing processes associated with genesis of the Qiaomaishan skarn deposit, Xuancheng ore district, eastern China

The Qiaomaishan deposit is a skarn-type Cu–W deposit located within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The mineralization within the deposit is predominantly located along the contact between a porphyritic granodiorite intrusion and surrounding Carboniferous limestone units. The deposit is associated with a well-developed and zoned skarn system that includes both endoskarn and exoskarn zones. This study presents new garnet major, trace, and rare earth element (REE) geochemical data for these skarn zones, discusses the factors (e.g., crystal chemistry, oxygen fugacity and water/rock interaction) that control the incorporation of trace elements into garnet, and constrains the formation of skarn within the Qiaomaishan deposit. Grossular (Adr21–70Grs29–78; type 1) garnet is generally located within the endoskarn and is light REE (LREE) enriched but heavy REE (HREE) depleted with weakly negative Eu anomalies and higher total REE contents than the exoskarn garnet in the study area. Type 1a grossular garnet is further split from type 1b grossular garnet by the presence of positive Pr and Nd anomalies and high LREE concentrations. Exoskarn garnet has pure andradite compositions and is split into type 2 and 3 garnets, where type 2 andradite garnet has variably positive Eu anomalies and variable total REE contents that are indicative of formation under transitional or oscillating conditions (e.g., pH, oxygen fugacity). In comparison, the type 3 andradite garnet has low total REE contents and positive Eu anomalies. Endoskarn garnet Al2O3 and total REE contents positively correlate, indicating that REE is incorporated into this garnet through coupled YAG-type substitution. The differences between the total REE contents of the type 2 exoskarn and endoskarn garnets may reflect host rock buffering as well as variations in the crystal chemistry of the two garnets present within the different parts of the skarn system (grossular and andradite). In comparison, the differences in the total REE contents of the two types of exoskarn garnet may reflect variations in water–rock ratios and crystal growth rates. The different Eu anomalies and U contents present within the different types of garnets record variations in the pH and oxygen fugacity of the hydrothermal fluid over time. These changes may have promoted the crystallization of magnetite and scheelite and the formation of the tungsten mineralization within the deposit. This study demonstrates the usefulness of garnet in tracking variations in mineralizing conditions and processes and confirms the validity of these minerals as vectors or indicator minerals that can be used in combination with traditional exploration approaches to determine whether a system is mineralized or not during the early stages of exploration.

Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

In Val d’Ala (Piedmont, Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and offers new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d’Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution.

Origin and Evolution of Oscillatory Zoned Garnet From Kasva Skarn, Northeast Tafresh, Iran

Within the central Urumieh-Dokhtar Magmatic Belt of northern Iran, the Kasva skarn deposit (KSD) formed by metasomatic alteration of Eocene intercalated carbonate and volcaniclastic sediments in response to Oligo-Miocene intrusion by granitoid porphyries. The KSD contains abundant oscillatory-zoned garnet crystals, which are characterized by isotropic cores of nearly pure andradite that are rimmed by anisotropic grossular–andradite (grandite). The Fe-rich andraditic cores are enriched in U and LREE, with positive Eu anomalies, whereas the Al-rich granditic rims are enriched in Ti, Nb, Zr, Hf, and HREE, without Eu anomalies. Variation in optical and chemical features in Fe- and Al-rich garnet are controlled by external factors such as (1) infiltration of compositionally distinct fluids, (2) incorporation of LREE and U at the {X} site in association with substitution of Fe^(3+) for Al^(3+) at [Y] within the crystal structure of andraditic garnet, and (3) substitution of Ti and HFSE for Al in granditic garnet.

LA-ICP-MS trace element geochemistry of garnets: Constraints on hydrothermal fluid evolution and genesis of the Xinqiao Cu–S–Fe–Au deposit, eastern China

The Xinqiao Cu–S–Fe–Au deposit in the Tongling ore district, an important part of the Middle-Lower Yangtze River Valley metallogenic belt in eastern China, is located along the northern margin of the Yangtze Block. The deposit consists of two major mineralization types: stratiform and skarn (the genesis of the former is disputed). The more important stratiform orebody (90% of the total Cu, S and Fe reserves) has abundant garnet as the main gangue mineral, similar to the skarn orebody hosted in the intrusive contact.Garnets from the Xinqiao stratiform orebody range from pure andradite (And100) to And50Gro46, and can be divided into the early (Grt1) and late (Grt2) generation. Both generations contain low MnO (0.19–0.89%), wide range of Y/Ho (39.0–40.7 for Grt1, 29.8–67.9 for Grt2), and do not contain melt or fluid-melt inclusions, indicative of a magmatic-hydrothermal replacement origin. Grt1 and the Grt2 cores are grandite, whilst the Grt2 rims are mainly andradite with some grandite zones. Grandite is enriched in heavy rare earth elements (HREEs), and displays low ΣLREE/ΣHREE, LaN/YbN and U concentrations with negative Eu anomalies, whereas andradite is HREE-depleted, and displays high ΣLREE/ΣHREE, LaN/YbN ratios and U concentrations with positive Eu anomalies. The distinct trace element features suggest that the hydrothermal fluids for Grt1 and the Grt2 cores were of nearly neutral (pH), relatively oxidized and HREE-enriched, whereas those for the Grt2 rims had experienced episodic inflections between a mildly acidic, relatively reduced and HREE-depleted fluid and a nearly neutral, relatively oxidized and HREE-enriched fluid. Meanwhile, they also reveal that Grt1 and the Grt2 cores may have formed by diffusive metasomatism in a closed system, whereas the Grt2 rims may have formed by infiltration metasomatism in an open fracture system, possibly related to the unconformity that hosts the Xinqiao stratiform orebody. We propose that the Xinqiao stratiform orebody may have formed from Early Cretaceous magmatic-hydrothermal fluids associated with the Jitou stock, similar to the skarn-type orebodies in the district.

مطالعه کانی شناسی و فرآیندهای اسکارنی شدن در اسکارن مس- آهن آوان، شمالخاور خاروانا، شمال باختر ایران

اسکارن مس- آهن آوان در نتیجه نفوذ باتولیت گرانیتوئیدی قره داغ به‌ داخل سنگهای کربناته ناخالص کرتاسه بالایی تشکیل شده است. اسکارن آوان متشکل از پهنه های مرمر، اگزواسکارن و اندواسکارن بوده و بخش اصلی آن پهنه اگزواسکارن، است. بررسیهای کانی شناسی بیانگر آن است که پهنه اگزواسکارن شامل زیرپهنه های گارنت اسکارن، پیروکسن- گارنت اسکارن و کانه اسکارن است. مهمترین کانی کالک سیلیکاته بی آب در پهنه اگزواسکارن، گارنت و ترکیب آن اوگراندیتی (Ad53-89) است. برخی بلورهای گارنت، منطقه‌بندی دارد و ترکیب آنها به طرف حاشیه‌ها به آندرادیت خالص (Ad99) میل می‌کند. کلینوپیروکسن موجود در زیرپهنه پیروکسن- گارنت اسکارن از نوع دیوپسید (Di75-96) است. زیرپهنه کانه اسکارن از کانه‌های مگنتیت، هماتیت، پیریت، کالکوپیریت، بورنیت، مالاکیت و گوتیت تشکیل شده است. وجود میزان محدود ولاستونیت در مجموعه کانی شناسی اسکارن آوان، هم‌رشدی بلورهای گارنت و کلینوپیروکسن و نبود هاله واکنشی بین این دو کانی و نبود بافتهای جانشینی نشان‌دهنده آن است که این کانیها به‌صورت هم‌زمان در محدوده دمایی C°600- 430 و فوگاسیته اکسیژن بیشتر از 10-26 تشکیل شده‌اند.

Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F–Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800–850 °C, >15 kbar). Zircons show four main age groupings: 1.0–0.5 Ga, 50–45, 35–20, and 16–13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50–45 Ma zircons match the crystallization age and juvenile character (eHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35–20 Ma Miocene ages are derived from zircons with similar Hf–O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73–8.97 ‰) values, and extremely low eHf i (−12.7 to −9.4) values. Such evolved Hf–O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10–12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35–20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.

Garnets in porphyry–skarn systems: A LA–ICP–MS, fluid inclusion, and stable isotope study of garnets from the Hongniu–Hongshan copper deposit, Zhongdian area, NW Yunnan Province, China

The Late Cretaceous Hongniu–Hongshan porphyry–skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr14Grs86 to almost pure andradite (Adr98Grs2) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their 18Ofluid (5.4–6.9‰) and low Dfluid (−142‰ to −100‰) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr22–57Grs78–43) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12–39wt.% NaCl eq. and have liquid–vapor homogenization temperatures (Th) of 272–331°C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr75–98Grs25–2) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high ΣREE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33–80wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465–591°C) than liquid–vapor homogenization temperatures (319–473°C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6km and reacted with carbonates at depths shallower than 2.0km. (3) Al-rich garnet veins (Adr14–60Grs86–40) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids.

Geochemical Characteristics, Origin, and Evolution of Ore‐Forming Fluids of the Khut Copper Skarn Deposit, West of Yazd in Central Iran

AbstractThe Khut copper skarn deposit is located at about 50 km northwest of Taft City in Yazd province in the middle part of the Urumieh‐Dokhtar magmatic arc. Intrusion of granitoid of Oligocene–Miocene age into carbonate rocks of the Triassic Nayband Formation led to the formation of marble and a calcic skarn. The marble contains high grade Cu mineralization that occurs mainly as open space filling and replacement. Cu‐rich sulfide samples from the mineralized marble are also anomalous in Au, Zn, and Pb. In contrast, the calcic skarn is only weakly anomalous in Cu and W. The calcic skarn is divided into garnet skarn and garnet–pyroxene skarn zones. Paragenetic relationships and microthermometric data from fluid inclusions in garnet and calcite indicate that the compositional evolution of skarn minerals occurred in three main stages as follows. (i) The early prograde stage, which is characterized by Mg‐rich hedenbergite (Hd53.7Di42.3–Hd86.1Di9.5) with Al‐bearing andradite (69.8–99.5 mol% andradite). The temperature in the early prograde skarn varies from 400 to 500°C at 500 bar. (ii) The late prograde stage is manifested by almost pure andradite (96.2–98.4 mol% andradite). Based on the fluid inclusion data from garnet, fluid temperature and salinity in this stage is estimated to vary from 267 to 361°C and from 10.1 to 21.1 wt% NaCl equivalent, respectively. Pyrrhotite precipitation started during this stage. (iii) The retrograde stage occurs in an exoskarn, which consists of an assemblage of ferro‐actinolite, quartz, calcite, epidote, chlorite, sphalerite, pyrite, and chalcopyrite that partially replaces earlier mineral assemblages under hydrostatic conditions during fracturing of the early skarn. Fluids in calcite yielded lower temperatures (T < 260°C) and fluid salinity declined to ∼8 wt% NaCl equivalent. The last stage mineralization in the deposit is supergene weathering/alteration represented by the formation of iron hydroxide, Cu‐carbonate, clay minerals, and calcite. Sulfur isotope data of chalcopyrite (δ34S of +1.4 to +5.2‰) show an igneous sulfur source. Mineralogy and mineral compositions of the prograde assemblage of the Khut skarn are consistent with deposition under intermediately oxidized and slightly lower fS2 conditions at shallow crustal levels compared with those of other typical Fe‐bearing Cu–Au skarn systems.

Mineral compositions and fluid evolution of the Tonglushan skarn Cu–Fe deposit, SE Hubei, east-central China

The Tonglushan Cu–Fe deposit (1.12 Mt at 1.61% Cu, 5.68 Mt at 41% Fe) is located in the westernmost district of the Middle–Lower Yangtze River metallogenic belt. As a typical polymetal skarn metallogenic region, it consists of 13 skarn orebodies, mainly hosted in the contact zone between the Tonglushan quartz-diorite pluton (140 Ma) and Lower Triassic marine carbonate rocks of the Daye Formation. Four stages of mineralization and alterations can be identified: i.e. prograde skarn formation, retrograde hydrothermal alteration, quartz-sulphide followed by carbonate vein formation. Electron microprobe analysis (EMPA) indicates garnets vary from grossular (Ad20.2–41.6Gr49.7–74.1) to pure andradite (Ad47.4–70.7Gr23.9–45.9) in composition, and pyroxenes are represented by diopsides. Fluid inclusions identify three major types of fluids involved during formation of the deposit within the H2O–NaCl system, i.e. liquid-rich inclusions (Type I), halite-bearing inclusions (Type II), and vapour-rich inclusions (Type III). Measurements of fluid inclusions reveal that the prograde skarn minerals formed at high temperatures (>550°C) in equilibrium with high-saline fluids (>66.57 wt.% NaCl equivalent). Oxygen and hydrogen stable isotopes of fluid inclusions from garnets and pyroxenes indicate that ore-formation fluids are mainly of magmatic-hydrothermal origin (δ18O = 6.68‰ to 9.67‰, δD = –67‰ to –92‰), whereas some meteoric water was incorporated into fluids of the retrograde alteration stage judging from compositions of epidote (δ18O = 2.26‰ to 3.74‰, δD= –31‰ to –73‰). Continuing depressurization and cooling to 405–567°C may have resulted in both a decrease in salinity (to 48.43–55.36 wt.% NaCl equivalent) and the deposition of abundant magnetite. During the quartz-sulphide stage, boiling produced sulphide assemblage precipitated from primary magmatic-hydrothermal fluids (δ18O = 4.98‰, δD = –66‰, δ34S values of sulphides: 0.71–3.8‰) with an extensive range of salinities (4.96–50.75 wt.% NaCl equivalent), temperatures (240–350°C), and pressures (11.6–22.2 MPa). Carbonate veins formed at relatively low temperatures (174–284°C) from fluids of low salinity (1.57–4.03 wt.% NaCl equivalent), possibly reflecting the mixing of early magmatic fluids with abundant meteoric water. Boiling and fluid mixing played important roles for Cu precipitation in the Tonglushan deposit.

THE PROFILE OF TRACE ELEMENTS, INCLUDING THE REE, IN GEM-QUALITY GREEN ANDRADITE FROM CLASSIC LOCALITIES

Crystals of gem-quality andradite (variety “demantoid”) from Val Malenco, Italy, have been analyzed for major, rare-earth and other trace elements (Sc, Ti, V, Cr, Co, Ni, Zn, Sr, Y, Zr), together with samples from Aosta Valley, Italy, and from the classic deposits of the world (Russia, Iran, Pakistan, Namibia, Madagascar). They are all of gem quality and vary from bright green to yellow-green in color. The samples are homogeneous within the limit of the analytical error and cover a restricted range of composition from almost pure andradite (Adr ≥ 98 mol.%) to members of andradite–uvarovite (Adr 81–96 Uv 3–18 ) or andradite–grossular (Adr 92–94 Grs 6–8 ) solid solution, with the sole exception of the sample from Namibia, showing a composition varying from pure andradite to almost pure grossular (Adr 11 Grs 89 ). All the samples have a low abundance of most trace elements, except for Cr, which ranges from a few ppm to more than 5 wt% Cr 2 O 3 . The substitution of Fe 3+ and Cr 3+ for Al at the [ Y ] site significantly controls the geometry of the structural sites and the incorporation of the REE. In particular, (i) the samples showing a composition close to pure andradite exhibit LREE-enriched and HREE-depleted patterns with a strong positive Eu anomaly, whereas (ii) the uvarovite-enriched samples show flatter patterns with a small positive Eu anomaly, and (iii) grossular-rich samples are LREE-depleted with no Eu anomaly or a negative one. However, such a compositional variation may also arise from differences in the bulk composition of the host rocks and from changes in the physicochemical conditions during growth.

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe 3+) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids. Garnets from the Crown Jewel deposit range from Adr 30Grs 70 to almost pure andradite (Adr >99). Fe-rich garnets (Adr >90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE 3+ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr >90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE 3+ following a coupled, YAG-type substitution mechanism ( [ X 2 + ] - 1 VIII [ REE 3 + ] + 1 VIII [ Si 4 + ] - 1 IV [ Z 3 + ] + 1 IV ) , whereas Eu 2+ substitutes for X 2+ cations. Thermodynamic data (e.g., H mixing) in grossular–andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions. Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/ R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/ R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/ R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.

Radiation effects in ferrate garnet

Radiation effects in four synthetic ferrate garnets (A 3B 2(XO 4) 3, Ia3 d, Z = 8) were examined by ion beam irradiations with in situ observation ( T = 298–873 K) using transmission electron microscopy (TEM) at the IVEM Tandem Facility at Argonne National Laboratory. The garnet compositions include: A = Ca, Gd, Th, and Ce; B = Zr, Fe. The critical amorphization temperatures ( T c), the temperature above which the target material cannot be amorphized due to dynamic annealing, were between 820 and 870 K. The amorphization doses at room temperature are between 0.17 and 0.19 dpa (displacement per atom), which is similar to that of silicate- and aluminate-garnets. The small variations in the amorphization dose and T c of the different compositions suggest that radiation effects in ferrate garnets are structurally constrained. Qualitative analyses of the valence states of Ce and Fe in garnet before and after irradiation were completed using electron energy-loss spectroscopy (EELS). The Ce and Fe in the unirradiated garnet were dominantly trivalent and divalent, respectively. The characteristic peaks of Ce 4+, at ∼5 eV higher energy for the M-edges, were present in unirradiated garnet as a minor peak, and the peaks did not disappear after complete amorphization, suggesting that the valence state did not change significantly. EELS analysis was conducted on a nearly pure andradite, Ca 3Fe 2Si 3O 12, which ideally contains only ferric iron. The andradite was amorphized at 0.18 dpa. EELS analysis revealed that some of ferric iron was converted to ferrous iron during the irradiation.

Mineralogy, Geochemistry, and Age Constraints on the Beni Bou Ifrour Skarn Type Magnetite Deposit, Northeastern Morocco

Abstract: The Beni Bou Ifrour deposit of northeastern Morocco is a skarn type magnetite deposit. K‐Ar age determination suggests that the mineralization occurred at 7.040.47 Ma. The spatial relationship between skarn and dikes of microgran‐odiorite derived from the batholith of Wiksane Granodiorite, and the similarity of age (8.020.22 Ma), confirms that the Wiksane Granodiorite is the igneous rock most probably related to mineralization. The skarn is distributed asymmetrically in the limestone, and magnetite ore was developed just below the calc‐silicate skarn as two parallel beds separated by 100 m of barren limestone and schist.The mineralization can be divided into three stages. The early stage is characterized by the formation of calc‐silicate minerals, mainly clinopyroxene (80–70 % diopside) and garnet (early almost pure andradite to the late 60 % andradite). The main stage is characterized by the formation of a large amount of magnetite. Epidote and quartz formed simultaneously with magnetite. Fluid temperatures exceeded 500 C during the early to main stages. Fluid with very high salinity (50–75 wt% NaCl equiv.) was responsible for the formation of the magnetite ore. The oxygen isotope composition, together with the fluid inclusion data, suggests that magmatic fluid was significant for the formation of calc‐silicate skarn minerals and magnetite. Low temperature (‐230C) and low salinity (‐10 % NaCl equiv.) hydrothermal fluids dominated by meteoric water were responsible for the late stage quartz and calcite formation.

Pressure-induced phase transformation and amorphization in Ca 3Fe 2Si 3O 12: a high pressure X-ray diffraction study

Pure andradite (Ca 3Fe 2Si 3O 12) was synthesized at 1150°C and 3 GPa, and then compressed to pressure above 21 GPa, and heated to about 1000°C by a YAG laser in a Diamond Anvil Cell (DAC). After temperature quenching in situ high pressure X-ray diffraction studies were carried out. The results show that andradite has transformed to a cubic perovskite type polymorph with a=3.460(4) Å. Upon decompression, in situ X-ray observations reveal that perovskite phase has transformed to an amorphous phase. The density of the perovskite polymorph of Ca 3Fe 2Si 3O 12 is about 13.6% greater than that of an isochemical composition at 21 GPa, and ferric iron can replace Ca 2+ and Si 4+ in the perovskite structure (2Fe 3+=Si 4++Ca 2+), giving a chemical formula: (Ca,Fe)(Si,Fe)O 3. Combining with the electron hopping mechanism between Fe 2+ and Fe 3+ in semiconductivity materials, this new Fe 3+-rich perovskite polymorph may provide new data to clarify the electrical conductivity of the lower mantle, based on the mineralogical model with magnesiuwustite (Mw) and perovskite (Pv).

Activation of microglia and expression of heat shock protein during kainic acid-induced hippocampal seizure in the rat

The Late Cretaceous Hongniu–Hongshan porphyry–skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr14Grs86 to almost pure andradite (Adr98Grs2) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their 18Ofluid (5.4–6.9‰) and low Dfluid (−142‰ to −100‰) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr22–57Grs78–43) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12–39 wt.% NaCl eq. and have liquid–vapor homogenization temperatures (Th) of 272–331 °C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr75–98Grs25–2) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high ΣREE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33–80 wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465–591 °C) than liquid–vapor homogenization temperatures (319–473 °C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6 km and reacted with carbonates at depths shallower than 2.0 km. (3) Al-rich garnet veins (Adr14–60Grs86–40) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids.

Petrogenesis of the copper-bearing skarn at the Mason Valley Mine, Yerington District, Nevada

Skarn formation at the Mason Valley mine occurred at a depth of 2,000 m on the outer fringe of a contact metasomatic aureole related to a Jurassic granodiorite to quartz monzonite batholith. The skarn is located in Upper Triassic limestone at the contact with a stratigraphically lower tuff unit and is systematically zoned relative to this contact. The general zonal sequence toward marble is: garnet, garnet-pyroxene-sulfides, pyroxene-sulfides, tremolite-magnetite-calcite, talc-magnetite-calcite, and dolomite-calcite. The zones migrated outward with time.Electron microprobe data indicate that garnets and pyroxenes in the barren garnet footwall zone have compositions similar to the districtwide early metasomatic hornfelses and are represented by low-iron diopsides and intermediate grandites. Pure andradite, as overgrowths and in cross-cutting veins, becomes more abundant as the hanging-wall skarn is approached. Both garnets and pyroxenes shift abruptly to higher iron contents in the hanging-wall skarn which formed at the contact between the early garnet zone and dolomitized limestone. Garnets maintain a constant lower limit of 55 mole percent andradite and are zoned to pure andradite. Later garnets, contemporaneous with chalcopyrite deposition, consist of pure andradite. The iron content of pyroxene increases gradually and systematically toward the marble contact, from an average value of 36 mole percent hedenbergite in the inner garnet-pyroxene zone to 56 mole percent hedenbergite in pyroxene vein centers on the marble contact, and then drops abruptly to 15 mole percent hedenbergite in vein envelopes. Two generations of amphibole are represented by: (1) early tremolite (0 to 10 mole percent ferrotremolite) associated with magnetite-calcite in outermost vein envelopes in marble; and (2) actinolite which contains the same Fe/Mg ratio as associated pyroxene and is contemporaneous with chalcopyrite deposition in the pyroxene and garnet-pyroxene zones.The initial silication process, as represented by zoned veins at the marble contact, may be attributed to isothermal metasomatic diffusion of Ca, Mg, and Si, with X (sub CO 2 ) decreasing toward the vein centers. The abrupt appearance of new minerals coincides with the attainment of appropriate chemical potential values through metasomatism, rather than the crossing of isobaric univariant T-X (sub CO 2 ) equilibria. Bulk composition gradients are extreme and are reflected in the rapid increase in iron content of tremolite and diopside over a few centimeters from vein envelope to vein center.Within the main skarn zone, which formed at higher temperature and/or lower X (sub CO 2 ) , bulk composition gradients are less extreme, and phase-composition trends are opposite to those that would be predicted by a simple isothermal diffusion model. The gradual inward decrease in the iron content of salite within skarn zones of relatively constant bulk composition may have been controlled in part by continuous Fe-Mg reactions.Comparison with phase-composition data from similar zoned skarns indicates that variation of phase compositions within zones is a characteristic phenomenon, but that zonal composition trends in some cases are opposite to those established here.