Late-stage Crystallization Research Articles

Trace Element and In Situ O-Nd Isotope Signatures of Apatite from the Bilihe Porphyry Au-only Deposit: Magmatic Constraints on Au Mineralization

Abstract The magmatic controls on the formation of porphyry Au-only deposits are still poorly understood. The Bilihe deposit (> 30 t @ 2.7 g/t Au) located at the northern margin of the North China Craton is such an example, where apatite chemistry from a causative granodiorite intrusion constrains the physicochemical conditions and ore-forming potential of the porphyry-related magmas. Apatite can be subdivided by distinct CL intensities into early CL-bright and late CL-gray/dark sub-types both occurring in fresh (Ap-f), as well as in potassic (Ap-k) and phyllic (Ap-p) altered granodiorite. The mostly euhedral CL-bright apatite shows a homogeneous texture and is enriched in LREE and depleted in Fe, Mg, Mn, Y, and U compared to CL-gray/dark apatite, which develops patchy or oscillatory zoning. The CL-gray/dark apatite exhibits decreasing Fe and Mg contents as well as δ18O values from Ap-f to Ap-k and Ap-p due to consecutive fluid alteration during porphyry evolution, which was generally not observed in CL-bright apatite. We thus conclude that all types of CL-bright apatite but only CL-gray/dark Ap-f inherit the original magma composition. Binary O-Nd isotope mixing simulations suggest that both magmatic CL-bright apatite and CL-gray/dark Ap-f were influenced by crustal sediment assimilation. Our results further indicate that the magma parental to the Bilihe porphyry Au-only deposit was H2O-poor (< 2 wt.%), causing plagioclase fractionation in the absence of hornblende, as evidenced by relatively low Sr/Y (< 0.1) and significantly negative Eu anomalies in magmatic apatite. Pressure estimates by magmatic biotite that record the late crystallization stage yielded 48.9 ± 8.4 MPa, indicating a shallow final emplacement depth of 1.85 ± 0.32 km. The studied apatites formed under volatile-saturated from S deficient magmas at relatively reduced conditions (ΔFMQ + 0.5) compared to porphyry Cu deposits. We thus conclude that these physicochemical parameters together with the low-pressure conditions result in a fertile magmatic-hydrothermal environment for the formation of Au-only porphyry deposits.

Low-temperature phase stability in calc-alkaline granitic systems and significance for cold granites

Granite formation and evolution are influenced by various physical and chemical processes in magmatic systems, resulting in diverse textures and compositions. While the generation of granitic rock through dehydration melting of hydrous minerals, which requires higher temperatures, is widely accepted, the formation and evolution of cold granite remains a topic of considerable debate. Our experiments aim to understand how small changes in granitic magma composition influence late-stage crystallization, providing insights into the chemical and textural evolution of granitic rocks and determining the significance of these conditions in shaping the diversity observed in nature. We conducted crystallization experiments at 1 and 2 kbar, between 680 and 815 °C, using two granitic compositions: a natural I-type granitoid (MA) and a slightly more mafic, synthetic analogue (FC). In these relatively low-pressure and low-temperature experiments, orthopyroxene remains stable at 1 kbar regardless of the starting material. At 2 kbar, its stability is limited to the more mafic sample FC when containing 3.7 wt% water. Hornblende only crystallizes in the more mafic rock FC at 2 kbar across all temperatures tested when water concentration exceeds 4 wt%. Plagioclase spans a broad temperature range (700–815 °C) at both 1 and 2 kbar in the FC material, whereas in the MA material at 2 kbar, it appears only at temperatures ≤725 °C. At 2 kbar few crystals were observed in the MA run at 750 °C, indicating that the liquidus temperature was nearly reached, contrasting with the FC starting material. Additionally, temperature cycling proved more efficient in promoting crystal growth in FC samples. Under low-temperature conditions, only local equilibrium was achieved, highlighting the significant role of late-stage crystallization processes and kinetic limitations in shaping the diversity observed in cold granites. Our experiments demonstrate that even small changes in CaO and FeO content can markedly alter the solidus/liquidus temperature of melts, significantly influence phase stability, melt production, and texture in granitic systems. This study surmises the need to further pursue systematic investigations of the influence of major elements on crystallization sequences in granitic systems, and we compare our findings to previous observations made on the Gentio metagranitoids of the Mineiro belt, Brazil.

Crystallization of a hydrous magma ocean in the shallow lower mantle

The solidification of a deep magma ocean occurred early in Earth's history. Although the initial amount of H2O in Earth's magma ocean is predicted to be low (e.g., <3000 ppm), as an incompatible element it becomes highly enriched (e.g. >10 wt%) in the final few percent of crystallization. In order to understand how a hydrous magma ocean would crystallize at the top of the lower mantle, we determined liquidus phase relations in the MgO-FeO-CaO-Al2O3-SiO2-H2O system at 24 GPa. We find that the bridgmanite (brg) + stishovite (st) + melt and bridgmanite (brg) + ferropericlase (fp) + melt cotectic boundary curves trend to Mg-rich melt compositions with decreasing temperature and extend to very high H2O contents (∼80 mol% H2O). The brg+st+melt curve is a subtraction curve at < ∼18 mol% H2O and a reaction curve at higher H2O contents, whereas the brg+fp+melt is a subtraction curve throughout its length. The density of melts along the two cotectics leads to neutral buoyancy with respect to shallow lower mantle and transition zone minerals at H2O contents up to ∼25 mol%. A transient melt-rich layer can form at the top of the lower mantle during late-stage crystallization in a mushy magma ocean when melt percolation dominates. When crystallization exceeds ∼98%, hydrous melts (>25 mol% H2O) become buoyant and can percolate into and hydrate the mantle transition zone.

Mineral and crystal chemical study of pseudo-C2/mnon-metamict chevkinite-(Ce): An investigation into the intracrystalline distribution of LREE, HREE, and octahedral cations in samples from the Azores and Pakistan

AbstractThe cation distribution among the A, B, C, and D sites of 13 crystals of chevkinite-(Ce) by X-ray single-crystal diffraction and electron microprobe analyses is reported in this article. The general formula of chevkinite-(Ce) is A4BC2D2O8(Si2O7)2, where A = REE, Ca, Th, Sr; B = Fe, Mn, Mg; C = Fe, Ti, Nb, Zr, Mg, Al; and D = Ti. Ten crystals come from quartz-bearing syenite samples of Agua de Pau (Azores Islands), and three are euhedral chevkinite crystals of a quartz-bearing pegmatite from Tangir Valley, northwestern Pakistan. The real space group for these samples is P21/a, but the average structure in space group C2/m was used for structural refinement and further interpretation. Cation distribution was determined by minimization of the function fΣNσ2 that represents the sum of the squared numbers of standard deviation by which each expected quantity differs from the calculated one. Both X-ray diffraction and electron microprobe data were considered in minimization. Our results show that the two non-equivalent A sites are occupied dominantly by REE and Ca, distributed mainly as a function of their ionic size. Lanthanum enters only the 10-coordinated A2 site, whereas heavier and smaller REE enter only the 8-coordinated A1. Furthermore, comparison between bond valence sum and formal charge revealed highly stretched bonds for the A1 polyhedron, suggesting that bond lengths are too long for HREE. These findings can help better characterize the well-known ability of chevkinite to fractionate LREE from HREE. The Azores chevkinite is particularly enriched in Nb relative to its host rock. Our results show that Ti occupancy in the D sites can be as low as ca. 50% due to the presence of significant amounts of Nb, besides Fe and Zr. For two Azores crystals showing patchy zoning, a complex history of late-stage crystallization is inferred, with multiple events of Ca and REE enrichments. One of these crystals showed the best structure refinement, implying that late-magmatic to deuteric alteration stages did not affect its crystallinity. The CaO content of Azores chevkinites practically spans the entire chevkinite field in the chevkinite/perrierite CaO vs. FeO discrimination diagram. In addition, we identified possible structural stability limits that can be linked to the occupancy of the B site at high CaO, and of the C sites at high-FeO contents. The Pakistan chevkinites were also analyzed by Mössbauer spectroscopy. Only one Fe3+ component was identified, which occupies the C sites. Among the two ferrous doublets, the component with the highest isomeric shift value is Fe2+ entering the most distorted crystallographic site B, while the other, representative of a less distorted octahedral site, corresponds to Fe2+ entering the D sites.

The Influence of Hydroxyethylmethyl Cellulose on the Heat Released during the Hydration of Modified Gypsum Materials

This article presents the results of experimental research of modified gypsum materials. The primary aim of the research was to assess the influence of the hydroxyethylmethyl cellulose (HEMC) polymer on the heat of hydration of gypsum. In the study, the polymer was used as an additive in weight ranges from 1% to 3% in relation to pure gypsum. The water to gypsum ratio was at the level of 0.75. The heat of hydration of building materials is crucial, because it is one of many aspects that directly affects the binding time, therefore defining the potential applications of gypsum in the construction industry. It was shown that the addition of the polymer influences the value of the heat of hydration, and also extends the crystallization process. Based on the Avrami equation, two areas of the early and late stage crystallization of gypsum were observed. The obtained values of the kinetic parameter n of this equation indicate a different mechanism of crystal growth. With an increased content of the polymer, there was a decrease in the K parameter, which confirms that HEMC interferes with gypsum nucleation and crystallization.

Genesis of the Brazin Iron Skarn Deposit, NW Iran: Analysis of formation conditions of calc-silicates and the evolution of fluids responsible for the massive magnetite precipitation

The Brazin iron skarn occurs along the contact zone between Late Cretaceous flysch-type sequences and an Oligocene-Miocene granodiorite. The prograde skarn stage is identified by the formation of garnet and clinopyroxene, whereas the retrograde stage is dominated by epidote, scapolite, calcite, and quartz. Metal minerals include magnetite + pyrite + chalcopyrite ± pyrrhotite. Garnet with andradite-grossularite-almandine solid-solution is ubiquitous throughout the skarn zones. Two principal types of garnet were identified in the skarn zones: (I) weakly-altered euhedral coarse-grained isotropic garnet exhibits minor variation in the overall composition (And25.95–39.41 Grs54.95–62.25 Alm + Sps + Py5.39–13.04); (II) strongly-altered anhedral to subhedral isotropic to anisotropic garnet shows a wide variation in the overall range of composition (And25.91–55.25 Grs41.37–61.15 Alm + Sps + Py1.48–12.85) in which FeOt increases, whereas Al2O3 decreases rimward.Clinopyroxene with a salitic-ferrosalitic composition is homogenous, has low to relatively high Fe/(Fe + Mg) ratios, and is poor in TiO2, MnO, and Na2O. It displays a decrease in the diopside content with increasing distance from the causative intrusion, whereas the ferrosalite content is enriched near the marble and hornfels fronts.The fluid system evolves from high temperatures (>580 °C), saline fluids (>53 wt% NaCl equivalent) in the late crystallization stage under lithostatic pressure of ∼750 bar at a depth of ∼3 km. Fluid in the later retrograde stage has lower temperatures (<430 °C) and salinities (<40 wt% NaCl equivalent). Mixing and dilution between the initial hydrothermal and external fluids might cause a decrease in fluid temperature and salinity causing the formation of the magnetite.

Multiple fabrics in the Tavares pluton (NE Brazil): Insights on the interplay between emplacement mechanism, internal magmatic processes, and tectonic deformation

Internal dynamic processes and contemporaneous deformation may produce multiple magmatic fabrics and structures during the crystallization of granitic plutons. Here we report the case of the Tavares pluton (Borborema Province, northeastern Brazil), a ∼160 km2 NE-SW elongate pluton bounded in the north and southeast by dextral and sinistral shear zones, respectively. A remarkable feature of this pluton is the occurrence of numerous elliptical flow-related structures delineated by mafic schlieren resulting from the interaction of dioritic and granitic melts. We analyzed airborne geophysical data, conducted a field and anisotropy of magnetic susceptibility (AMS) study at the pluton scale, and detailed the magnetic fabric of two elliptical structures. The pluton has a well-developed magmatic foliation defined by the shape preferred orientation of K-feldspar megacrysts and planar concentrations of mafic minerals. The foliation trajectories are concentric and inwardly dipping, indicating a lopolith shape for the pluton. NE-trending magmatic shear zones and faults attest to sinistral shearing before complete crystallization. The dominant AMS fabric at pluton scale, represented by NE-to NNE-trending magnetic foliations with steep dips associated with lineations of moderate to shallow plunges, is consistent with the sinistral strain regime. In contrast, the magnetic foliation and lineation in the two elliptical structures dip moderately to the east, with the magnetic foliation commonly transecting the orientation of the mafic schlieren. We relate this latter fabric to regional contraction, characterized by top-to-the-west tectonic transport in the country rocks, during pluton construction. We propose that (i) tectonic overpressure due to regional compression assisted progressive pluton inflation by floor subsidence, generating the concentric magmatic fabric, and (ii) a small component of noncoaxial shear during bulk contraction led to sinistral shearing in the late stages of crystallization. We found no evidence for shear zone-controlled pluton emplacement. Instead, (i) the SE-bounding sinistral shear zone is restricted to the pluton vicinity, suggesting nucleation due to thermal softening of country rocks by the pluton, and (ii) magmatic foliations and lineations that could be related to the N-bounding dextral shear zone are absent. Also, the available geochronological data suggest that emplacement of the Tavares pluton predated by several tens of millions of years the intrusion of well-documented syn-transcurrence plutons in the Borborema Province. Therefore, the spatial association of the Tavares pluton with strike-slip shear zones is not strong enough evidence to assume a shear zone control on pluton emplacement.

The Evolving Chronology of Moon Formation

Defining the age of the Moon has proven to be an elusive task because it requires reliably dating lunar samples using radiometric isotopic systems that record fractionation of parent and daughter elements during events that are petrologically associated with planet formation. Crystallization of the magma ocean is the only event that unambiguously meets this criterion because it probably occurred within tens of millions of years of Moon formation. There are three dateable crystallization products of the magma ocean: mafic mantle cumulates, felsic crustal cumulates, and late-stage crystallization products known as urKREEP (uniform residuum K, rare earth elements, and P). Although ages for these materials in the literature span 200 million years, there is a preponderance of reliable ages around 4.35 billion years recorded in all three lunar rock types. This age is also observed in many secondary crustal rocks, indicating that they were produced contemporaneously (within uncertainty of the ages), possibly during crystallization and overturn of the magma ocean. ▪The duration of planet formation is key information in understanding the mechanisms by which the terrestrial planets formed.▪Ages of the oldest lunar rocks range widely, reflecting either the duration of Moon formation or disturbed ages caused by impact metamorphism.▪Ages determined for compositionally distinct crust and mantle materials produced by lunar magma ocean differentiation cluster near 4.35 Gyr.▪The repeated occurrence of 4.35 Gyr ages implies that Moon formation occurred late in Solar System history, likely by giant impact into Earth.

Sulfur in apatite from the Nakhla meteorite record a late-stage oxidation event

Estimates of the oxygen fugacity (fO2) recorded by the Martian nakhlite meteorites from direct observations of the main igneous phenocryst assemblages range from values similar to that recorded by the quartz-fayalite-magnetite oxygen buffer to ∼two orders of magnitude lower. Inferences of changes in fO2 during the late stages of crystallization, volcanic degassing, and emplacement of the nakhlite cumulate pile have been made based on variable sulfide and apatite chemistry. We present S-XANES measurements of the oxidation state of sulfur in apatite and associated mesostasis glass in Nakhla to place direct constraints on the magnitude of changes in fO2 experienced by the Nakhla portion of the nakhlite cumulate pile during apatite crystallization. Nakhla apatites range from containing dominantly S2− to containing dominantly S6+. This, together with correlations between S2−, Cl, and FeO in the mesostasis glass near these apatites, suggest that our measurements capture directly the oxidation of the interstitial late-stage Nakhla magmas as the result of Cl-saturation and degassing. As the result of this degassing, at least part of the nakhlite cumulate pile experienced an increase in fO2 of ∼1.5–2.5 orders of magnitude during apatite crystallization and final mesostasis cooling. Based on these measurements, the sulfur oxidation states of apatites in the other nakhlite meteorites are predicted to range from exclusively S2−-bearing to exclusively S6+-bearing.

Remote sensing techniques and geochemical constraints on the formation of the Wadi El-Hima mineralized granites, Egypt: new insights into the genesis and accumulation of garnets

The Wadi El-Hima Neoproterozoic I- and A-type granites in the Southern Eastern Desert of Egypt are rich in garnets (up to 30 vol%) and are cut by NW–SE strike-slip faults, as confirmed from structure lineament extraction maps. These mineralized granites and garnet mineralization zones can be successfully discriminated using remote sensing techniques. Spectral angle mapper and matched filtering techniques are highly effective for mapping garnet-rich zones and show that the highest garnet concentrations occur along the intrusive contact zone of NW–SE striking faults. El-Hima granites have high SiO2 (73.5–75.1 wt%), Al2O3 (13.4–15.3 wt%) and total alkali (6.7–8.7 wt%) contents, suggesting that they were sourced from peraluminous (A/CNK > 1) parental magmas. Garnet-bearing trondhjemites are metasomatic in origin and formed after I-type tonalite-granodiorites, which originated in a volcanic arc tectonic setting. Garnet-rich syenogranites and alkali-feldspar granites are both post-collisional A-type granites: the syenogranites formed from peraluminous magmas generated by partial melting of lower crustal tonalite and metasedimentary protoliths during lithospheric delamination, and the alkali-feldspar granites crystallized from highly fractionated, felsic and alkali-rich peraluminous magmas in the upper crust. Garnets in El-Hima mineralized granites occur in three forms: (1) subhedral disseminated crystals, (2) vein-type crystals, and (3) aggregated subhedral crystals, reflecting different mechanisms of accumulation. All are dominantly almandine in composition (Alm76Sps10 Prp7Grs6Adr1) and have high average concentrations of heavy rare earth elements (HREE) (ΣHREE = 1636 ppm), Y = (3394 ppm), Zn (325 ppm), Li (39.17 ppm) and Ga (34.94 ppm). Garnet REE patterns show strong negative Eu anomalies with HREE enriched relative to LREE, indicating a magmatic origin. These magmatic garnets are late-stage crystallization products of Al-rich hydrous magmas, and formed at low temperature (680–730 °C) and pressure (2.1–2.93 kbar) conditions in the upper continental crust. Peculiar garnet concentrations in syenogranites near and along contact zones with alkali feldspar granites are related to peraluminous parent hydrous magma compositions. These garnets formed by in situ crystallization from A-type granite melts, alongside accumulation of residual garnets left behind after partial melting of the host garnet-rich granites along the intrusive contact. Magmatic-fluid flow along the NW–SE striking fault of Najd system enhanced garnet accumulation in melts, which formed clots and veins of garnet.

IVA iron meteorites as late-stage crystallization products affected by multiple collisional events

Although IVA irons have O- and Cr-isotopic compositions resembling those of equilibrated LL chondrites, the bulk composition of refractory elements (e.g., Re, Ir, Pt) in the IVA core appears to be significantly lower than LL. These compositional discrepancies suggest known IVA irons may be missing early crystallized samples. We hypothesize the bulk composition of the IVA core is LL-like, but current collections do not include early fractional-crystallization IVA products. Our fractional-crystallization modeling of element vs. Au trends suggests that extant IVA irons are products of > 40% crystallization of the core, assuming an initial 2.9 wt.% S content. The model-derived bulk (Ni-normalized) composition of the IVA core is depleted relative to LL in most moderate volatiles: S (82% depletion), Ge (99.9% depletion), Ga (95% depletion), As (50% depletion); however, Au is enriched by 10%. Because moderate volatiles with depletions > 80% relative to LL have 50%-condensation temperatures < 1,020 K, it seems likely these depletions reflect post-accretion impact-induced volatilization of the IVA asteroid. The mean Ni-normalized compositions of analyzed IVA irons yield a lesser depletion of As (30%) and greater enrichment of Au (48%) relative to LL. The IVA asteroid may have experienced a complex parent-body thermal and collisional history: (1) differentiation, (2) impact-induced mantle stripping, devolatilization, and fractional condensation, (3) rapid crystallization of the core from the outside inwards, (4) shattering of the core after ∼75% crystallization, (5) quenching of thinly insulated samples (e.g., Fuzzy Creek), (6) formation of amorphous free silica in several IVA irons after impact-induced vaporization of portions of the overlying silicate mantle, followed by fractional condensation, (7) loss of portions of the core representing the first 40% of crystallization, (8) reaccretion of some core fragments, facilitating relatively slow cooling of a few IVA irons (e.g., Duchesne, Duel Hill (1854), Chinautla), and (9) collisional resetting of the Re-Os clock 4456 ± 25 Ma ago.

The formation of three-grain junctions during solidification. Part II: theory

We provide a simple geometric theory of crystal growth which predicts the shape and final dihedral angle of three-grain junctions of an augite crystal with two plagioclase grains. The predicted dihedral angle Delta depends on the initial impingement angle psi formed by the plagioclase grains, and also on the relative growth rates of the augite and the plagioclase, and shows reasonable agreement with data obtained from natural samples. We show that the two augite-plagioclase grain boundaries will normally curve towards each other, which is consistent with the first two types of junction described in the companion paper. However, the third type, the eagle’s beak, is formed by the meeting of grain boundaries which curve in the same direction. Although it is possible to account for this type of junction by invoking the localised dissolution of one of the plagioclase grains, this is unlikely to occur. A more plausible explanation involves the late impingement of the two plagioclase grains, consistent with the observation that eagles’ beaks are common in gabbros and strongly orthocumulate troctolites, in which the plagioclase framework has not been established by the time augite is growing in substantial quantities. An observed flattening of the curve of Delta values at high values of psi can be explained by taking into account the importance of interfacial energy in late-stage crystallisation.

Ferrotorryweiserite, Rh5Fe10S16, a New Mineral Species from the Sisim Placer Zone, Eastern Sayans, Russia, and the Torryweiserite–Ferrotorryweiserite Series

Ferrotorryweiserite, Rh5Fe10S16, occurs as small grains (≤20 µm) among droplet-like inclusions (up to 50 μm in diameter) of platinum-group minerals (PGM), in association with oberthürite or Rh-bearing pentlandite, laurite, and a Pt-Pd-Fe alloy (likely isoferroplatinum and Fe-Pd-enriched platinum), hosted by placer grains of Os-Ir alloy (≤0.5 mm) in the River Ko deposit. The latter is a part of the Sisim placer zone, which is likely derived from ultramafic units of the Lysanskiy layered complex, southern Krasnoyarskiy kray, Russia. The mineral is opaque, gray to brownish gray in reflected light, very weakly bireflectant, not pleochroic to weakly pleochroic (grayish to light brown tints), and weakly anisotropic. The calculated density is 5.93 g·cm–3. Mean results (and ranges) of four WDS analyses are: Ir 18.68 (15.55–21.96), Rh 18.34 (16.32–20.32), Pt 0.64 (0.19–1.14), Ru 0.03 (0.00–0.13), Os 0.07 (0.02–0.17), Fe 14.14 (13.63–14.64), Ni 13.63 (12.58–14.66), Cu 4.97 (3.42–6.41), Co 0.09 (0.07–0.11), S 29.06 (28.48–29.44), and total 99.66 wt.%. They correspond to the following formula calculated for a total of 31 atoms per formula unit: (Rh3.16Ir1.72Pt0.06Ru0.01Os0.01)Σ4.95(Fe4.48Ni4.11Cu1.38Co0.03)Σ10.00S16.05. The results of synchrotron micro-Laue diffraction studies indicate that ferrotorryweiserite is trigonal; its probable space group is R3¯m (#166) based on its Ni-analog, torryweiserite. The unit-cell parameters refined from 177 reflections are a = 7.069 (2) Å, c = 34.286 (11) Å, V = 1484 (1) Å3, and Z = 3. The c:a ratio is 4.8502. The strongest eight peaks in the X-ray diffraction pattern derived from results of micro-Laue diffraction study [d in Å(hkil)(I)] are 2.7950 (202¯5) (100); 5.7143 (0006) (60); 1.7671 (224¯0) (44.4); 3.0486 (202¯1) (39.4); 5.7650 (101¯2) (38.6); 2.5956 (202¯7) (37.8); 3.0058 (112¯6) (36.5); and 1.5029 (42¯ 2¯12) (35.3). Ferrotorryweiserite and the associated PGM crystallized from microvolumes of residual melt at late stages of crystallization of grains of Os- and Ir-dominant alloys occurred in lode zones of chromitites of the Lysanskiy layered complex. In a particular case, the residual melt is disposed peripherally around a core containing a disequilibrium association of magnesian olivine (Fo72.9–75.6) and albite (Ab81.6–86.4), with the development of skeletal crystals of titaniferous augite: Wo40.8–43.2En26.5–29.3Fs20.3–22.6Aeg6.9–9.5 (2.82–3.12 wt.% TiO2). Ferrotorryweiserite represents the Fe-dominant analog of torryweiserite. We also report occurrences of ferrotorryweiserite in the Marathon deposit, Coldwell Complex, Ontario, Canada, and infer the existence of the torryweiserite–ferrotorryweiserite solid solution in other deposits and complexes.

First Evidence of the Post-Variscan Magmatic Pulse on the Western Edge of East European Craton: U-Pb Geochronology and Geochemistry of the Dolerite in the Lublin Podlasie Basin, Eastern Poland

The U–Pb measurements of youngest, coherent group of zircons from the Mielnik IG1 dolerite at the Teisseyre-Tornquist margin (TTZ) of East European Craton (EEC) in Poland yielded age of 300 ± 4 Ma. Zircon dated an evolved portion of magma at the late stage crystallization. It is shown that this isolated dyke from the northern margin of the Lublin Podlasie basin (Podlasie Depression) and regional dyke swarms of close ages from the Swedish Scania, Bornholm and Rügen islands, Oslo rift, Norway, and the Great Whine Sill in northeastern England, were coeval. They have been controlled by the same prominent tectonic event. The Mielnik IG1 dolerite is mafic rock with Mg-number between 52 and 50 composed of the clinopyroxene, olivine-pseudomorph, plagioclase, titanite, magnetite mineral assemblage, indicating relatively evolved melt. This hypabyssal rock has been affected by postmagmatic alteration. The subalkaline basalt composition, enrichment in incompatible trace elements, progressive crustal contamination, including abundance of zircon xenocrysts determines individual characteristics of the Mielnik IG1 dolerite. The revised age of dolerite, emplaced in vicinity of TTZ provides more evidences documenting the reach of the Permo-Carboniferous extension and rifting accompanied by magmatic pulses, that were widespread across Europe including the margin of the EEC incorporated that time into the broad foreland of the Variscan orogen.

Magmatic-hydrothermal tin deposits form in response to efficient tin extraction upon magma degassing

Most of the global Sn resources are from granite-related ore deposits, which form in response to cassiterite precipitation from hydrothermal fluids. However, the physical and chemical controls on the efficiency of Sn extraction from upper crustal plutons by exsolving magmatic fluids are still unclear. In this study, we determine the partition coefficient of Sn between aqueous fluids and granitic melts (DSnfluid/melt) at 800 °C, 150 MPa and the fO2 of the Ni-NiO buffer. To obtain equilibrium partition coefficients, a new experimental method has been used relying on local equilibrium between silicate melt and microscopic-sized fluid bubbles. The latter formed synthetic fluid inclusions in the quenched glasses, which in turn were analyzed by laser ablation inductively coupled plasma mass spectrometry along with the enclosing glass. The results show that at constant aluminum saturation index (ASI = 1.05–1.08) of the silicate melt, DSnfluid/melt increases from 1.9 to 35.0 as the total Cl concentration (mCltotal) in fluid increases from 1.0 to 16.6 mol/kg H2O. At a fixed mCltotal = 2 mol/kg H2O, DSnfluid/melt increases from 4.3 to 10.6 as the HCl concentration in the solution increases from 0.15 to 0.79 mol/kg H2O, which in turn is a function of the ASI of the melt (ASI = 1.06–1.29). Numerical modeling suggests that Sn is extracted by magmatic fluids from upper crustal plutons most efficiently at the late stage of crystallization and degassing. At a similar degree of crystallization, granitic magma with lower initial water concentration and higher ASI will separate a fluid phase with higher Sn concentration and thus has higher Sn mineralization potential. Due to the relatively high DSnfluid/melt value, fluids exsolved from highly evolved magmas can sequester enough Sn to form Sn deposits and the sub-solidus remobilization of Sn from granite bodies is not a pre-requisite for ore genesis.

Platinum‐group element and Au geochemistry of an ultramafic intrusion from the Sonakhan greenstone belt, Bastar craton, Central India: Tectono‐magmatic implications

The Sonakhan greenstone belt (SGB), located in the north‐eastern Bastar craton of the Indian shield, comprises mafic‐ultramafic and volcanic‐intrusive sequences in the lower stratigraphic units. We investigate the Platinum‐group element (PGE) relations of Boradih intrusion of the SGB to evaluate its tectono‐magmatic evolution. The chondrite‐normalized PGE patterns of boninitic cumulate rocks exhibit higher abundance of Palladium group PGEs (∑PPGE = 292–496 ppb) relative to the Iridium group PGEs (∑IPGE = 32–52 ppb) along with variable Au concentrations (51.34–718.05 ppb). The PGE concentrations are attributed to a boninitic parental melt, where the IPGEs in the source possibly partitioned into a monosulphide solid solution. The Cu (22–80 ppm), elevated Pt (22–238 ppb), and Pd (31–377 ppb) concentrations indicate Pt and Pd have partitioned into a semi‐metal rich melt during the later stages of crystallization. The geochemical characteristics of the basalts and ultramafic cumulates of the SGB indicate a supra‐subduction zone tectonic setting for its formation. Similar geochemical and litho‐tectonic correlations are also noticed between the SGB of Bastar craton and greenstone belts of the Eastern Dharwar craton of south India. The SGB (V/Yb =146 ± 25) and greenstone belts of Eastern Dharwar craton (V/Yb = 134 ± 52) record similar oxidation conditions of Phanerozoic subduction zones. Accordingly, we propose magmatic as well as tectonic correlations are possible for the Archean‐Palaeoproterozoic Bastar and Eastern Dharwar cratons.

Autometassomatic and hydrotermal processes in the crystallization and recrystallization of calcite, Floresta Azul Alkaline Complex, NE Brazil

The syenitic intrusion of the Floresta Azul Alkaline Complex is characterized by the presence of intense fluid activity in late stages of crystallization, leading to the formation of sodalitites and to the genesis of REE-rich accessory minerals. Calcite is a common mineral in the nepheline syenite of the complex, its formation being derived from CO3 enrichment during fractional crystallization of phonolitic magma. Calcite may be found as white, pink to brown anhedral crystals which occur in the interstices of essential mineralogy and are commonly associated with cancrinite and siderophyllite. Calcite shows high contents of the rhodochrosite, siderite and strontionite molecules and the REE are present in its structure. From scanning electronic microscopy studies, with backscattered electron detector and a dispersive energy spectrometer, it has been possible to identify and characterize a number of textures and minerals related to calcite. The cooling of calcite promotes the formation of a rare REE-carbonate, the carbocernaite. This mineral occurs as exolutions, being distributed disseminated in calcite with irregular crystals and with acicular and oriented crystals. The interaction with hydrothermal fluids resulted in the another unusual REE-carbonate, the ancylite. That results from the recrystallization of calcite, mainly at its borders. A leaching process also occurred at the hydrothermal stage, leading to remobilization of Fe-Mn-Sr which precipitation as siderite, ankerite and strontianite. Therefore, calcite is an extremely important mineral in the syenitic intrusion of the Floresta Azul Complex, being possible to describe the autometasomatic and hydrothermal stages with their genesis, compositional variation, present textures and mineralogical associations.

Early and Late Mineralization of the Intrusive Rocks in Lokpaukwu, Southern Benue Trough, Nigeria.

The intrusive rocks (dolerites) in Lokpaukwu were studies to evaluate the kind of mineralization in them. The dolerites erupted at three regions in the Lokpaukwu area (Eziama, Eluama, Umuchieze). The results of the petrographic studies show that the dolerites in Eziama, Eluama, are characterized by olivine, pyroxene, plagioclase, biotite, hornblende, hematite. The first pattern of mineralization of the dolerites at LokpaUkwu appears to start from the Eziama or Eluama region, the dolerites of these two regions conforms to the mineralization of dolerites that exist in some other parts of the southern Benue trough. Umuchieze dolerites have some peculiarities that differentiate it from other dolerites of the Southern Benue trough. The second pattern of mineralization exist in dolerites of umuchieze which are characterized by quartz, pyroxene, plagioclase, biotite, and hematite. These phenomena indicate that two types of dolerites exist in LokpaUkwu area, the Olivine dolerites and the quartz porphyry dolerites. The interaction of the magma with sedimentary host rocks or hydrothermal influx or late stage of crystallization of magma probably played vital roles in the formation of the quartz porphyry dolerites, whereas the olivine dolerites may have originated from early stage of crystallization of basaltic magma. The basaltic magma that gave rise to these rocks(dolerites) probably has a single source region but erupted at different rifts which gave opportunity for some magmatic evolutionary processes (fractionation, contamination, assimilation) to play major roles. &nbsp

Prolonged Trachyte Storage and Unusual Remobilization at Piton de la Fournaise, La Réunion Island, Indian Ocean: Li, O, Sr, Nd, Pb and Th Isotope Study

Abstract La Réunion Island includes two major volcanic systems. About 0·5 Myr ago, Piton des Neiges volcano declined, while Piton de la Fournaise volcano grew on its flank. Since then the Piton de la Fournaise shield volcano has produced homogeneous lavas with chemical compositions transitional between alkali and tholeiitic basalts. In April 2007, the volcano emitted a very small volume of trachytic pumice during its largest historical eruption. We conducted a comprehensive petrological and geochemical study of the pumice to understand the occurrence of such silicic melt in the feeding system of this highly active basaltic volcano. Isotopes of Sr, Nd, Pb and O, together with trace elements, indicate that the trachyte is genetically related to the La Réunion mantle plume and derives from crystallization of a typical basalt. The trachyte chemistry records a long and complex history of differentiation and outgassing. The extensive depletion of moderately volatile elements (F, Cl, B, Cs, Cu, Li) and less volatile uranium is consistent with exsolution of dense fluids at depths of several kilometres. Lithium isotopes point to closed-system degassing during the very late stages of crystallization. U-series isotopes and radiogenic 208Pb*/206Pb* constrain the age of U loss to between 0·4 and 2·1 Ma. This age is as old as or older than the Piton de la Fournaise shield edifice. The 2007 trachyte could thus be a liquid remnant of an extinct volcano, such as Piton des Neiges or Les Alizés (Piton de la Fournaise proto-volcano). It could also result from partial melting of an old syenite intrusion or remobilization of interstitial melts not fully solidified. Thermal modelling indicates that the sustained heat flux from hot basaltic magmas rising from the mantle can maintain temperatures above 800 °C in the central feeding system, and prevent total solidification of magmas trapped in this hot core.

Magmatic Evolution of Garnet-Bearing Highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes

The widespread W-(Mo)-Sn-Nb-Ta polymetallic mineralization in Southeast (SE) China is genetically associated with Mesozoic highly fractionated granitic rocks. Such rocks have enigmatic mineralogical and geochemical features, making its petrogenesis an intensely debated topic. To better understand the underlying magma evolution processes, petrography, garnet chemistry and whole-rock major and trace element data are reported for Jurassic highly fractionated granitic rocks and associated microgranite and aplitepegmatite dikes from Macao and compared with coeval similar granitic rocks from nearby areas in SE China. Despite the fact that the most evolved rocks in Macao are garnet-bearing aplite-pegmatite dikes, the existence of coeval two-mica and garnet-bearing biotite and muscovite granites displaying more evolved compositions (e.g, lower Zr/Hf ratios) indicates that the differentiation sequence reached higher degrees of fractionation at a regional scale. Although crystal fractionation played an important role, late-stage fluid/melt interactions, involving F-rich fluids, imparted specific geochemical characteristics to Macao and SE China highly fractionated granitic rocks such as the non-CHARAC (CHArge-and-RAdius-Controlled) behavior of trace elements, leading, for example, to non-chondritic Zr/Hf ratios, Rare Earth Elements (REE) tetrad effects and Nb-Ta enrichment and fractionation. Such process contributed to the late-stage crystallization of accessory phases only found in these highly evolved facies. Among the latter, two populations of garnet were identified in MGI (Macao Group I) highly fractionated granitic rocks: small grossular-poor euhedral grains and large grossular-rich skeletal garnet grains with quartz inclusions. The first group was mainly formed through precipitation from highly evolved Mn-rich slightly peraluminous melts under low-pressure and relatively low temperature (∼700 °C) conditions. Assimilation of upper crust metasedimentary materials may have contributed as a source of Mn and Al to the formation of garnet. The second group has a metasomatic origin related to the interaction of magmatic fluids with previously crystallized mineral phases and, possibly, with assimilated metasedimentary enclaves or surrounding metasedimentary strata. The highly fractionated granitic rocks in Macao represent the first stage in the development of granite-related W-(Mo)-Sn-Nb-Ta mineralization associated with coeval more evolved lithotypes in SE China.