Grandite Garnet Research Articles

Mineral chemistry and garnet U-Pb dating in the Bizmişen iron skarn deposit, Erzincan, East-Central Türkiye

In Bizmişen area, Middle Eocene (46.3–42.0 Ma) plutonic rocks (quartz diorite) were intruded into Triassic-Cretaceous limestones and Upper Cretaceous ophiolites and caused to skarn zones containing prograde (garnet, clinopyroxene) and retrograde (amphibole, epidote, calcite, magnetite, phlogopite, serpentinite and chlorite) minerals. Due to contact relationships of quartz diorites with both serpentinized ultramafic rocks and limestone blocks, the mineralogical associations represent calcic (grossular/andradite, amphibole, chlorite and epidote) as well as magnesian (diopside, serpentine, phlogopite, talc) skarns. During the skarnization process, garnet, diopside, epidote, scapolite and amphibole minerals developed in the endoskarn zone within quartz-diorite, and diopside, tremolite, serpentine, phlogopite, talc, Mg-chlorite and calcite minerals developed in the exoskarn zones in ophiolites (serpantinite and listwaenite) and limestones. Skarn and magmatic clinopyroxenes in serpentinized ultramafic hosted exoskarn zone have a compositional range of Wo45-53En35-53Fs0–15 and Wo24-51En38-76Fs0–11, and classified as diopside and augite, respectively. The end members of garnets are Adr46–54Grs43–50Prp1–2Sps1.1–5 in amphibole-epidote-calcite-garnet skarn in endoskarn zone, whereas Adr40–93Grs4–57Prp0.2–2.3Sps0.9–2 in epidote-calcite-garnet skarn in exoskarn zone, and classified as andradite-rich grandite garnets. FeO, MnO and partially MgO contents gradually increase from the edges to the center in zoned garnet grains. Amphibole compositions represent to calcic (Ca >1 atoms per formula unit) group (pargasite) and indicate the high oxygen fugacity conditions which caused the increase in the rate of magnetite crystallization. Al2O3 and FeO contents of epidotes range 23.68–27.63 and 9.10–14.45 wt%, respectively, correspond to composition of epidote and clinozoisite. The iron oxide contents (FeO, wt%) of magnetites change from 91.72 to 98.36 correspond to structural Fe3+1.88–1.97 and Fe2+0.91–1.0 values. The contents of Ti, divalent and trivalent metal cations correspond to ulvospinel-magnetite compositional line at the magnetite end-member. The mineral chemistry of skarn minerals shows similarities to those of most iron skarn deposits. Garnet U-Pb ages (46.95–45.63 Ma) are consistent with the cooling ages of the Bizmişen pluton. The prograde stage of skarn should be occurred syn-intrusion, whereas retrograde stage should be developed later than youngest garnet age (45.6 Ma) and before the oldest late stage argillic alteration (37.5 Ma).

Investigation of the Ablation Behaviour of Andradite‐Grossular Garnets and Rutile with Implications for U‐Pb Geochronology

U‐Pb dating of andradite‐grossular garnet (grandite) and rutile by LA‐ICP‐MS can be used to constrain various metamorphic, metasomatic and igneous geological processes. In this study, we examine and compare the impact of different analytical conditions (fluence, pulse width, laser beam size and ablation frequency) on the ablation crater morphology, ablation rates, down‐hole fractionation and U‐Pb ages of grandite and rutile samples of different compositions. The shapes of grandite ablation craters suggest the mineral ablates by classical evaporation with significant melting that cannot be eliminated even at fluences just above the ablation threshold. Grandite garnets with higher andradite proportions have faster ablation rates. The overall low U contents of grandite require using large laser beam sizes to obtain acceptable precision of U‐Pb ages. At such conditions and crater depths < 10 μm, fluences of 2.1 and 3.5 J cm‐2, laser pulse width of 5 ns and 20 ns, and ablation frequencies between 3.5 and 6.5 Hz, obtain similar and reproducible ages when the proportion of grossular is < 35%. Rutile ablation crater morphology shows evidence of melt splashing and thermal stress cracking. They have significant crater bottom features, which increase in relief with lower fluences and a higher number of laser shots, indicating the features are probably energy‐related and making higher fluences, such as 5 J cm‐2, necessary for uniform ablation when using 193 nm excimer lasers. The slow ablation rate at low fluences and then steep increase at around 2.0 J cm‐2 suggests a transition in the ablation mechanism from exfoliation to classical vaporisation. Crater bottom features and other ablation behaviour vary between samples, which could be related to their difference in colour. Although the down‐hole fractionation patterns of the samples are similar at 5 J cm‐2, the U‐Pb ages of some samples vary significantly with different analytical conditions and/or measurement sessions, particularly when using laser beam sizes of 30 μm, suggesting differences in mass bias and more variable ablation behaviour. A laser beam size of at least 60 μm is recommended for reproducible U‐Pb dating of rutile.

Cation ordering, twinning, and pseudo-symmetry in silicate garnet: The study of a birefringent garnet with orthorhombic structure

AbstractThe crystal structure of a birefringent garnet (~Adr53Grs47) that occurs as a late-stage rim on andradite from Stanley Butte, Graham County, Arizona is analyzed and refined using single-crystal XRD. The structure has an orthorhombic I 2/a 1 2/d (unconventional setting for Fddd) space group symmetry, with unit-cell parameters of a = b = 11.966(3) Å, c = 11.964(3) Å, α = β = 90°, γ = 90.29(2)°, V = 1713.0(7) Å3, Z = 8. The orthorhombic garnet displays very high birefringence (δ ~0.021) produced by the strong Fe-Al ordering in the octahedral sites, with Fe occupancies of 0.804 and 0.221 in Y1 and Y2 sites, respectively. Diffraction peaks (such as 101 and 103) violating the Ia3d symmetry of cubic garnet are obvious even in powder XRD pattern. The homogenization temperatures of the fluid inclusions suggest that the low-crystallization temperature is responsible for the ordered orthorhombic structure. The strong ordering state of the structure and the sharp boundaries in the chemical zoning in the crystal (between ~Adr53Grs47 and ~Adr100) indicate the orthorhombic intermediate grandite garnet is a thermodynamically stable phase at low temperature, separated by wide miscibility gaps from the pure end-members (grossular and andradite) with cubic structures. Most of the previously reported triclinic garnet structures are likely artifacts produced by pseudo-merohedral twinning of less-ordered orthorhombic structure, as indicated by the characteristic pairing pattern of different Y-sites with the same occupancies.

Precise U[sbnd]Pb dating of grandite garnets by LA-ICP-MS: Assessing ablation behaviors under matrix-matched and non-matrix-matched conditions and applications to various skarn deposits

LA-ICP-MS UPb dating of garnet has become one of the important geochronometric methods in recent years. Both the matrix-matched and non-matrix-matched external standards were used for dating. However, under which conditions reliable ages can be obtained and the difference between the matrix-matched and non-matrix-matched methods have not been well constrained. In this contribution, a series of ablation experiments (including spot analysis and line scanning) were conducted on different grandite garnets using various ablation parameters (spot size, repetition rate and fluence) under both the matrix-matched (garnet as external standard) and non-matrix-matched (zircon as external standard) conditions, with the aim to obtain the optimal conditions for precise LA-ICP-MS garnet UPb dating. The results show that UPb fractionation associated with downhole depth, intrinsic properties of U (more refractory) and Pb (more volatile) and radiation damage (alpha dose) of crystal lattice occurs in zircon and garnet. The fractionation can be minimized or even eliminated by optimizing the ablation parameters (e.g., large spot size and low repetition rate) or the analytical modes (e.g., line scanning). This makes it feasible to use zircon as external standard for garnet UPb dating. Line scanning is most favorable for garnet UPb dating at broad ablation parameters regardless of matrix-matched or non-matrix-matched. In spot analysis mode, all the garnets can be precisely dated using a large spot size (e.g., ≥90 μm) coupled with moderate-low repetition rate (e.g., 2–5 J/cm2) and fluence (e.g., 2–5 J/cm2) under matrix-matched condition. Under non-matrix-matched condition, all the garnets can also be precisely dated when a larger spot size (e.g., ≥120 μm) coupled with moderate repetition rate (e.g., 5 Hz) and fluence (e.g., 5 J/cm2) is used. Too high or too low repetition rates (e.g., <2 Hz and > 10 Hz) and fluences (e.g., <2 J/cm2 and > 10 J/cm2) were found to be unsuitable for garnet UPb dating. Low-U and high-U samples are best treated differently due to their different UPb fractionation coefficients, especially at the high fluence and repetition rate conditions. Equally, U-concentration matched garnet standards are required to improve the accuracy and precision of the UPb datingNatural grandite garnets selected from Fe, W and CuMo skarn deposits can be precisely dated using the optimized methods. These garnets have compositions varying widely between andradite and grossular with variable U (1–100 ppm) and common Pb (f206 of 0–0.7) concentrations, indicating that the methods are reliable and thus suitable for wide applications.

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

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

Geochemical evolution of rodingites during subduction: insights from Cerro del Almirez (southern Spain)

Metarodingites from Cerro del Almirez (Betic Cordillera, southern Spain) experienced multiple mineralogical and compositional changes during their evolution, from rodingitization on the seafloor to dehydration of the hosting subducted antigorite serpentinites at ~ 1.7 GPa and 650 °C. Basaltic and doleritic protoliths of Grandite-metarodingites lost alkaline metals (Na, K, Cs and Rb), Si, Ba, Pb and Sr, and gained Ca through intense seafloor rodingitization by highly reducing fluids reacting with serpentinites. Rare earth elements in whole-rocks, conversely, preserved their igneous abundances during rodingitization and partitioned into the new metamorphic assemblage dominated by granditic garnet, chlorite and diopside. At the ocean floor and during subduction, the strong gradients of chemical potentials of Mg, Ca and Al at the serpentinite-rodingite interface triggered the transformation of metarodingites to metasomatic chlorite-blackwalls and of host serpentinites to chlorite (± olivine)-diopside rims.Grandite-metarodingites interacted during subduction with fluids released by the brucite breakdown in serpentinites at ~ 1.5 GPa, leading to oxidation of iron, precipitation of new andraditic garnets with high MREE/HREE ratios, and loss of Re. Strong fluxing of fluids produced by the antigorite breakdown in serpentinites at ~ 1.7 GPa promoted the transformation of Grandite- to Epidote-metarodingites. This transformation caused the release of Ca and Mn into serpentinite-derived fluids and the incorporation of Si, Sr, Pb, Ba ± Eu into metarodingites, especially in epidote. Pyralspite-metarodingites formed at peak metamorphic conditions (~ 1.8 GPa and 680 °C) by, among other reactions, epidote breakdown that liberated Ca, Sr and Pb into fluids. Finally, all metarodingite types underwent variable degrees of retrograde amphibolitization induced by external fluids derived from metasedimentary rocks, causing Ca depletion and enrichment in Mg, Ba and alkaline elements (Na, K, Cs and Rb).These geochemical variations reflect the exchange of major and trace elements between different lithologies (serpentinites, metarodingites and metasedimentary rocks) in a subducting slab or slab-mantle interface. Metarodingites may influence the redox state and trace element signature of slab fluids, especially through iron oxidation, fractionation of Sr, Pb and Ba into epidote, and release of Ca and Re in fluids that may induce metasomatism in the sub-arc mantle. Recycling of residual metarodingites in the convective asthenosphere may produce Ca-rich reservoirs in the deep mantle.

Mineral chemistry, petrogenesis and evolution of the Ghorveh-Seranjic skarn, Northern Sanandaj Sirjan Zone, Iran

The Ghorveh-Seranjic (GS) skarn is located in the northern part of the Sanandaj-Sirjan zone, NW Iran, which is part of Alpine-Himalaya orogenic belt. The GS metamorphic complex is the oldest unit in the GS area composed of marble, dolomitic marble, greenschist, and amphibolite of Early Jurassic age. The complex is intruded by NW-SE trending Late Jurassic peraluminous granitoids, which caused contact metamorphism and resulted in the development of skarn, hornfels and crystallization of marble. The skarn is showing distinct textural, mineralogical and geochemical zonation. At least four stages of skarn development have been recognized; stage I, clinopyroxene+garnet±vesuvianite±quartz±calcite±scheelite± pyrrhotite; stage II, garnet+clinopyroxene +vesuvianite+scheelite±apatite+calcite±pyrrhotite; stage III, amphibole+vesuvianite+epidote+chlorite±quartz±calcite±pyrrhotite±pyrite±chalcopyrite and stage IV, quartz+calcite±amphibole±epidote±chlorite±pyrite±chalcopyrite. Scheelite occurs in stages 1 and 2 together with garnet and clinopyroxene, and its abundance slightly increases with vesuvianite growth. In general, mineral chemistry of the GS skarn shows enrichment in Ca, Al and Mg. Two types of garnet, clinopyroxene and vesuvianite are identified in the prograde stage within the GS skarn. Variable Mg:Mn:Fe proportions in clinopyroxene of the early prograde stage suggest formation from a relatively homogeneous F-rich volatile phase. Mineralogical documentation of the GS skarn indicates that presence of the F-rich volatile phase affected zoning patterns and mineral abundances. Addition of fluorine increases the solubility of Al in the hydrothermal fluid by forming strong Al-F complexes, causing an increase in vesuvianite instead of clinopyroxene during the late prograde substage, resulting in high garnet/clinopyroxene ratios. The presence of granditic (grossular-andradite) and subcalcic (grossular-almandine-spessartine) garnet during the skarn evolution suggests variable Fe/Mn and Fe2+/Fe3+ ratios during the prograde stage of the skarn formation. Subcalcic garnet formed in a relatively reduced environment compared to the granditic garnet. Paragenetic reconstructions indicate that clinopyroxene, garnet and scheelite grew together during the prograde stage. These minerals were stable and coexisted at temperatures between 580 °C and 400 °C and at a logfO2 = −18 to −28.

Andradite skarn garnet records of exceptionally low δ18O values within an Early Cretaceous hydrothermal system, Sierra Nevada, CA

Skarn garnets in the Mineral King roof pendant of the south–central Sierra Nevada within Sequoia National Park, California, USA reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the Early Cretaceous Sierra Nevada Batholith. We focus on andradite garnet associated with Pb–Zn mineralization in the White Chief Mine. Laser fluorination oxygen isotope analyses of δ18O of garnet (δ18O(Grt)) from sites along the skarn show a large range of values (− 8.8 to + 4.6‰ VSMOW). Ion microprobe (SIMS) analyses elucidate that individual andradite crystals are strongly zoned in δ18O(Grt) (up to 7‰ of variation). Total rare-earth element concentrations (∑REE) across individual garnets show progressive depletion of skarn-forming fluids in these elements during garnet growth. These findings support a skarn model of earliest red high-δ18O grandite garnet consistent with a magmatic-dominated equilibrium fluid (δ18O(H2O) as high as ≈ + 8‰). Later, green andradite crystallized in equilibrium with a low-δ18O fluid indicating a significant influx of meteoric fluid (δ18O(H2O) ≈ − 6 to − 5‰), following a hiatus in garnet growth, associated with late-stage Pb–Zn mineralization. Latest orange overprint rims have higher δ18O values (δ18O(H2O) ≈ 0–2‰), and depleted total REEs, suggesting influx of high-δ18O, trace-element depleted fluid derived from regional metamorphism of the carbonate host. Remarkably, low δ18O(Grt) values in the White Chief canyon skarn require a significant proportion of meteoric fluid available during > 400 °C andradite-forming metasomatism. Fluid flow was channelized at the pluton–wallrock contact, evidenced by the narrow extent of skarn.

Unveiling growth histories of multi-generational garnet in a single skarn deposit via newly-developed LA-ICP-MS U[sbnd]Pb dating of grandite

Although garnet UPb dating method has been reported recently, yet the accurate concordia 206Pb/238U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS UPb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS UPb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept 206Pb/238U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia 206Pb/238U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations ([U]avg > 80 ppm) contents. The precision UPb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon UPb ages of the quartz dioritic stock and 40Ar39Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite UPb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia UPb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns.

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.

Oscillatory zoning in skarn garnet: Implications for tungsten ore exploration

The Sangdong and Weondong deposits, Taebaeksan mineral district, South Korea, are tungsten skarn ore deposits. Andradite (Fe-rich) garnet is a characteristic mineral in these skarn deposits, and is characterized by notable trace element contents (W, Mo, Sn, and U) compared to grossular (Al-rich) garnet. We report characteristics of the trace elements including W, in the garnet structure from the vein-skarn system (outermost exoskarn) and identify a relationship of exploration significance between the trace element composition and W-ore. The chemical characteristics of the garnets were determined using Back-Scattered Electron (BSE) imagery, Electron Probe Micro-Analyzer (EPMA) point analyses and X-ray dot-mapping, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) point analyses and mapping. Garnets in the study area are mainly composed of grandite (grossular–andradite) garnets, and exhibit compositional variation as oscillatory zoning. The chondrite-normalized REE patterns of Al-rich garnets are HREE-enriched. In contrast, Fe-rich garnets are LREE-enriched. Tungsten is strongly fractionated into the Fe-rich garnets (average concentration: 618ppm), compared to the Al-rich rims (average concentration: 19ppm). In addition, other trace elements (Mo, Sn, and U) are also closely correlated with W and REE signatures. The Fe-rich garnets are attributed to rapid growth by infiltration metasomatism during disequilibrium, which caused the incorporation of these trace elements into the garnet structure that reflect the composition of the ore-forming fluid. Consequently, these results indicate that andradite is a useful indicator mineral in the exploration of tungsten ore deposits.

Metasomatic changes during periodic fluid flux recorded in grandite garnet from the Weondong W-skarn deposit, South Korea

The Weondong skarn deposit primarily consists of grandite (andradite-grossular) garnets, which exhibit poikilitic/intergrowth textures and compositional zoning in metasomatism. The garnets can be classified into three main types based on their textural characteristics and rare earth element (REE) concentrations: (1) exoskarn garnet near quartz porphyry with a poikilitic texture and oscillatory zoning (core: And31–50, rim: And54–63) (T1 garnet), (2) exoskarn garnet near quartz porphyry with intergrowths (core: And55–83, mantle: And55–59, rim: And77–97) and poikilitic textures (T2 garnet), and (3) vein-hosted garnet with reverse zoning (core: And70–96, rim: And31–36) and epitaxial growth on distinct cores (T3 garnet). The textural features of the garnets are considered to have been caused by periodic fluctuations. The chondrite-normalized REE patterns of the garnet types show that the Al-rich garnets are HREE-enriched and formed at equilibrium with slow growth rates according to the kinetics of the garnet's growth. In contrast, Fe-rich garnets are slightly LREE-enriched and HREE-depleted. In terms of the kinetics of the garnet's growth, the Fe-rich garnets grew rapidly from externally derived fluids during infiltration metasomatism. The enriched W contents in the grandite garnets also indicate the type of growth mechanism, the infiltration process, and a specific skarn environment (such as advective metasomatism). Furthermore, each textural type of garnet was influenced differently by fluid/rock interactions, the magnitude of the fluid flow and the fluid compositions in the skarn system. Consequently, periodic fluid fluctuation in the skarn system affected the morphology and composition of individual garnet grains. Thus, the geochemical features of the grandite garnet from the Weondong polymetallic deposit provide key information to understand the evolution of the skarn deposit and its metasomatic history.

Geochemistry of major and rare earth elements in garnet of the Kal-e Kafi skarn, Anarak Area, Central Iran: Constraints on processes in a hydrothermal system

Grossular-andradite (grandite) garnets, precipitated from hydrothermal solutions is associated with contact metamorphism in the Kal-e Kafi skarn show complex oscillatory chemical zonation. These skarn garnets preserve the records of the temporal evolution of contact metasomatism. According to microscopic studies and microprobe analysis profiles, the studied garnet has two distinct parts: the intermediate (granditic) composition birefringent core that its andradite content based on microprobe analysis varies between 0.68–0.7. This part is superimposed with more andraditic composition, and the isotropic rim which its andradite content regarding microprobe analysis ranges between 0.83–0.99. Garnets in the studied sample are small (0.5–2 mm in diameter) and show complex oscillatory zoning. Electron microprobe analyses of the oscillatory zoning in grandite garnet of the Kal-e Kafi area showed a fluctuation in chemical composition. The grandite garnets normally display core with intermediate composition with oscillatory Fe-rich zones at the rim. Detailed study of oscillatory zoning in grandite garnet from Kal-e Kafi area suggests that the garnet has developed during early metasomatism involving monzonite to monzodiorite granitoid body intrusion into the Anarak schist- marble interlayers. During this metasomatic event, Al, Fe, and Si in the fluid have reacted with Ca in carbonate rocks to form grandite garnet. The first step of garnet growth has been coeval with intrusion of the Kal-e Kafi granitoid into the Anarak schist- marble interlayers. In this period of garnet growth, change in fluid composition may cause the garnet to stop growing temporarily or keep growing but in a much slower rate allowing Al to precipitate rather than Fe. The next step consists of pervasive infiltration of Fe rich fluids and Fe rich grandite garnets formation as the rim of previously formed more Al rich garnets. Oscillatory zoning in the garnet probably reflects an oscillatory change in the fluid composition which may be internally and/or externally controlled. The rare earth elements study of these garnets revealed enrichment in light REEs (LREE) with a maximum at Pr and Nd and a negative to no Eu anomaly. This pattern is resulted from the uptake of REE out of hydrothermal fluids by growing crystals of calcsilicate minerals principally andradite with amounts of LREE controlled by the difference in ionic radius between Ca++ and REE3+ in garnet x site.

Cation ordering of {110} and {211} sectors in grandite garnet from Mali

Cation ordering of {110} and {211} sectors in grandite garnet from Mali

Detrital anisotropic grandite garnet as an indicator of denudation level of Permian volcanic arc in the provenance of the South Kitakami Belt, Northeast Japan

In this study, the chemical and optical features of detrital garnets from the Middle Permian to Upper Triassic sandstones in the South Kitakami Belt, Northeast Japan, were examined to reveal the tectonic movement in the provenance. The sandstones contain a large amount of detrital grandite garnet grains with a wide range of andradite content. Among them, some grandite garnet grains show optically anisotropic features and rarely oscillatory zoning and sector twinning. The proportion of the detrital anisotropic grandite garnet increases from the Permian to the Middle Triassic and decreases in the Late Triassic. Such grandite garnets with various andradite contents occur in skarn deposits. Isotropic grandite garnets in the early stage of skarn evolution are distributed widely around a pluton, which affects the thermal metamorphism of the surrounding strata. However, anisotropic grandite garnets are formed along veinlets and fractures in the middle to late hydrothermal stage as the pluton cools, and their distribution is limited to a narrower area near the pluton compared to the metamorphic aureoles formed in the early stage. Changes in the chemical and optical features of the detrital garnets indicate a progressive denudation of the plutonic body accompanying skarn deposits in the provenance. The proportion of detrital anisotropic grandite garnet grains among all of the detrital grandite garnet is considered to be a sensitive indicator of the denudation level in a deeper part of the volcanic arc in association with skarn deposits, together with traditional sandstone composition datasets. This newly proposed method should be useful for clarifying the paleogeography during the Permian to Triassic in the East Asian continental margin, associated with uplift and denudation of the Permian volcanic arc, which seems to have been induced by the collision of the North China and South China Blocks.

Hydrothermal alteration in the Reykjanes geothermal system: Insights from Iceland deep drilling program well RN-17

The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K 2O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92–An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48–An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron.

Magmatic-dominated fluid evolution in the Jurassic Nambija gold skarn deposits (southeastern Ecuador)

The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean zone of southeastern Ecuador. The skarns consist dominantly of granditic garnet (Ad20–98) with subordinate pyroxene (Di46–92Hd17–42Jo0–19) and epidote and are spatially associated with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer K- and Na-enriched zone in the volcanosedimentary unit. Gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide-poor vugs and milky quartz veins and veinlets in association with hematite. Fluid inclusion data for the main part of the prograde stage indicate the coexistence of high-temperature (500°C to >600°C), high-salinity (up to 65 wt.% eq. NaCl), and moderate- to low-salinity aqueous-carbonic fluids interpreted to have been trapped at pressures around 100–120 MPa, corresponding to about 4-km depth. Lower-temperature (510–300°C) and moderate- to low-salinity (23–2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote of the end of the prograde stage. The microthermometric data (Th from 513°C to 318°C and salinity from 1.0 to 23 wt.% eq. NaCl) and δ18O values between 6.2‰ and 11.5‰ for gold-bearing milky quartz from the retrograde stage suggest that the ore-forming fluid was dominantly magmatic. Pressures during the early retrograde stage were in the range of 50–100 MPa, in line with the evidence for CO2 effervescence and probable local boiling. The dominance of magmatic low-saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system, which could have delayed the ingression of external fluids until relatively low temperatures were reached. The resulting low water-to-rock ratios explain the weak retrograde alteration and the compositional variability of chlorite, essentially controlled by host rock compositions. Gold was precipitated at this stage as a result of cooling and pH increase related to CO2 effervescence, which both result in destabilization of gold-bearing chloride complexes. Significant ingression of external fluids took place after gold deposition only, as recorded by δ18O values of 0.4‰ to 6.2‰ for fluids depositing quartz (below 350°C) in sulfide-rich barren veins. Low-temperature (<300°C) meteoric fluids (δ18Owater between −10.0‰ and −2.0‰) are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities (about 100°C and 25 wt.% eq. NaCl). The latter are similar to low-temperature fluids (202–74.5°C) with δ18O values of −0.5‰ to 3.1‰ and salinities in the range of 21.1 to 17.3 wt.% eq. CaCl2, trapped in calcite of late veins and interpreted as basinal brines. Nambija represents a deep equivalent of the oxidized gold skarn class, the presence of CO2 in the fluids being partly a consequence of the relatively deep setting at about 4-km depth. As in other Au-bearing skarn deposits, not only the prograde stage but also the gold-precipitating retrograde stage is dominated by fluids of magmatic origin.

Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: constraints on the P–T–fluid regime in parts of the Pan-African mobile belt of South India

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite + clinopyroxene (mg# = 71–73) + grandite (16–21 mol% Adr) + quartz ± calcite, Type II: grandite (25–29 mol% Adr ) + clinopyroxene (mg# = 70) + calcite + quartz, and Type III: grandite (36–38 mol% Adr) + clinopyroxene (mg# = 55–65) + epidote + scapolite + calcite + quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite + SiO2 → wollastonite + V, 2. calcite + clinopyroxene + O2 → grandite + SiO2 + V, 3. scapolite + calcite + quartz + clinopyroxene + O2 → grandite + V and 4. epidote + calcite + quartz + clinopyroxene + O2 → grandite + V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2 11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.

Solubility of andradite, Ca3Fe2Si3O12, in a 10 mol% NaCl solution at 800 C and 10 kbar: Implications for the metasomatic origin of grandite garnet in calc-silicate granulites

The solubility of andradite garnet was determined at 800 °C and 10 kbar in a solution of 10 mol% NaCl and 90 mol% H 2 O. Experiments were syntheses carried out with high-purity natural wollastonite, reagent hematite or natural specular hematite containing ~1 wt% TiO 2 , and fluid. All experiments were performed with Mn 2 O 3 -Mn 3 O 4 or Fe 2 O 3 -Fe 3 O 4 oxygen buffers in a piston-cylinder apparatus with NaCl pressure media for 1–3 days. Andradite saturation was determined by the presence or absence of garnet in quenched charges. Andradite dissolves incongruently to hematite and fluid with CaSiO 3 molality ( m CS ) of 0.0838 ± 0.0015 for the reagent hematite and both buffers. Slightly higher m CS of 0.0895 ± 0.0005 for the natural hematite and Mn-oxide f O 2 buffer is interpreted as due to incomplete equilibration and/or ~9 mol% Ti in the run-product andradite. Dissolved Fe molality could be determined only approximately, but must be at least ten times lower than m CS . Quenched fluids were very basic (pH 11–12). The solubility of Fe 2 O 3 in andradite-saturated H 2 O-NaCl fluids is lower than that of Al 2 O 3 at grossular saturation at the same pressure ( P ), temperature ( T ), and fluid composition. The results permit a test of a model of CaSiO 3 dissolution in NaCl solutions to three dominant aqueous species: CaCl + , OH , and H 3 NaSiO 4 . Combination of the CaSiO 3 molality at andradite saturation with wollastonite solubility at the same conditions (0.1253 ± 0.0047 molal; Newton and Manning 2006) leads to a Gibbs free energy change of the reaction 3 wollastonite + hematite = andradite at 800 °C and 10 kbar of −32.21 ± 2.45 kJ. The good agreement between this value and that derived from previous studies supports the dissolution model of CaSiO 3 in NaCl solutions. The low solubility of the Fe 2 O 3 component of andradite contrasts with the high solubility of magnetite and of Fe in pelitic and granitic mineral assemblages measured in acidic chloride solutions by previous workers at lower P and T . The results imply that Fe 2 O 3 is conserved during metasomatic processes affecting calc-silicates at high metamorphic grades.

Changes in detrital garnet assemblages related to transpressive uplifting associated with strike–slip faulting: An example from the Cretaceous System in Kii Peninsula, Southwest Japan

The chemical characteristics of detrital garnet have served as relatively reliable and definitive indicators of the major tectonic settings of sedimentary units in many parts of the world. In Southwest Japan a multidisciplinary approach combining sandstone modal, heavy mineral and garnet compositional analyses of the Cretaceous upper continental slope to trench deposits of the Kurosegawa and Shimanto belts combined with a comparison of the detrital garnet features in and around Kii Peninsula revealed the following characteristics. The garnet assemblages show significant changes with age. The Lower Cretaceous Yuasa Formation is characterized by pyrope-rich almandine garnets derived from granulite-facies metamorphic rocks. These are associated with garnet grains that have pyrope–almandine compositions and high grossular contents suggesting an eclogite-origin. Garnet assemblages of the lower Upper Cretaceous Yukawa Formation are dominated by grains with chemical characteristics indicating an origin as low P/ T type metamorphic rocks. There are also spessartine–almandine garnets derived from acidic igneous rocks, some with a high P/ T type metamorphic origin and others with a grandite composition. Garnet assemblages of the upper Upper Cretaceous Miyama Formation are similar to those of the Yukawa Formation, with the exception that grandite garnets are absent. The above compositional changes of garnets in the deposits allow us to characterize the corresponding provenances and by combining these results with stratigraphic ages temporal changes of these provenances can also be determined. The source area for the granulite- facies metamorphic rocks that continuously supplied detritus throughout the Cretaceous might be Precambrian continental basement in East Asia. In early Late Cretaceous time, the detritus was derived from terranes distributed along the Median Tectonic Line, a major strike–slip fault stretching throughout Southwest Japan. The garnet compositions suggest the source area included eclogite in the Kurosegawa belt that forms the basement to the Cretaceous strata, and the low P/ T type metamorphic rocks and granitic rocks in the Paleo-Ryoke belt including the Higo belt. Studies of the Miyama Formation show that at this time widespread acidic volcanism began and this was the source for much of the volcanic sediment in the formation. The provenance change between the Lower and lower Upper Cretaceous Formations is likely to have been caused by uplifting and unroofing associated with Cretaceous transpression expressed by the strike–slip movement along the Median Tectonic Line and other lateral faults in the Chichibu belt.