Metasomatic Exchange Research Articles

Mineralogical and Isotopic Characteristics of Sodic-Calcic Alteration in the Highland Valley Copper District, British Columbia, Canada: Implications for Fluid Sources in Porphyry Cu Systems

AbstractThe Highland Valley Copper porphyry Cu (±Mo) district is hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera. Fracture-controlled sodic-calcic alteration is important because it forms a large footprint (34 km2) outside of the porphyry Cu centers. This alteration consists of epidote ± actinolite ± tourmaline veins with halos of K-feldspar–destructive albite (1–20 XAn) ± fine-grained white mica ± epidote. The distribution of sodic-calcic alteration is strongly influenced by near-orthogonal NE- and SE-trending fracture sets and by proximity to granodiorite stocks and porphyry dikes. Multiple stages of sodic-calcic alteration occurred in the district, which both pre- and postdate Cu mineralization at the porphyry centers.The mineral assemblages and chemical composition of alteration minerals suggest that the fluid that caused sodic-calcic alteration in the Guichon Creek batholith was Cl bearing, at near-neutral pH, and oxidized, and had high activities of Na, Ca, and Mg relative to propylitic and fresh-rock assemblages. The metasomatic exchange of K for Na, localized removal of Fe and Cu, and a paucity of secondary quartz suggest that the fluid was thermally prograding in response to magmatic heating. Calculated δ18Ofluid and δDfluid values of mineral pairs in isotopic equilibrium from the sodic-calcic veins and alteration range from 4 to 8‰ and −20 to −9‰, respectively, which contrasts with the whole-rock values for least altered magmatic host rocks (δ18O = 6.4–9.4‰ and δD = −99 to −75‰). The whole-rock values are suggested to reflect residual magma values after D loss by magma degassing, while the range of hydrothermal minerals requires a mixed-fluid origin with a contribution of magmatic water and an external water source. The O-H isotope results favor seawater as the source but could also reflect the ingress of Late Triassic meteoric water. The 87Sr/86Srinital values of strongly Na-Ca–altered rocks range from 0.703416 to 0.703508, which is only slightly higher than the values of fresh and potassic-altered rocks. Modeling of those data suggests the Sr is derived predominantly from a magmatic source, but the system may contain up to 3% seawater Sr. Supporting evidence for a seawater-derived fluid entrained in the porphyry Cu systems comes from boron isotope data. The calculated tourmaline δ11Bfluid values from the sodic-calcic domains reach 18.3‰, which is consistent with a seawater-derived fluid source. Lower tourmaline δ11Bfluid values from the other alteration facies (4–10‰) suggest mixing between magmatic and seawater-derived fluids in and around the porphyry centers. These results imply that seawater-derived fluids can infiltrate batholiths and porphyry systems at deep levels (4–5 km) in the crust. Sodic ± calcic alteration may be more common in rocks peripheral to porphyry Cu systems hosted in island-arc terranes and submarine rocks than currently recognized.

The roles of mechanical mixing and fluid transport in the formation of reaction zones in subduction-related mélange: Evidence from highly siderophile elements

Traverses across amphibole-chlorite schist metamorphic reaction zones at contacts between meta-igneous and ultramafic rocks in high-pressure mélange zones were analyzed for highly siderophile element (HSE) concentrations and 187Os/188Os in two mélange zones (Catalina Schist, Santa Catalina Island, CA, and Attic-Cycladic Complex, Syros, Greece) in order to investigate the relative effects of mechanical mixing and diffusive and advective transport by fluids in subduction-related reaction zones. Results from Catalina Schist traverses show co-varying elevated concentrations of Os, Ir, Ru, Cr, and MgO and inversely correlated initial 187Os/188Os that display irregular patterns across the chlorite-actinolite schist reaction zones. These elements are strongly enriched in peridotite relative to mafic crust, and are relatively fluid immobile. The observations suggest that these components were added via mechanical mixing of mantle peridotitic material with material derived from mafic blocks. In a traverse across a Syros mafic eclogite block with a narrow chlorite schist reaction zone and broader glaucophane-rich reaction zone, the Os, Ir, Ru, and Cr concentrations in the chlorite schist are just barely higher than those in the eclogite block, while the initial 187Os/188Os ratio falls between the eclogite core and adjacent serpentinite. These compositions show a relatively minor metasomatic exchange between eclogite block and serpentinite. A second sample set from Syros was chosen across a contact zone between a meta-tuffite and a serpentinite because previous studies suggest it was not affected by mechanical mixing. This traverse contains a ~1 m wide complex metasomatic reaction zone that is used to study the relative contributions of diffusive and advective transport. The traverse reveals strong mobilization of Os, Ir, Ru and Pt beyond the scale of the outcrop, and minor advective enrichment of Re and Pd. This indicates that all HSE may be mobilized by fluid advection beyond the local scale in some subduction zone mélange zones.

Metamorphic mineral assemblages in fossils in the contact metamorphic rocks of the Portrush Sill, Northern Ireland

Ammonites and other metafossils have been identified in hornfelsed Jurassic clay from Portrush, Northern Ireland. Thermal metamorphism due to intrusion of a large sill complex produced zoned high temperature mineral assemblages due to reaction between the calcite and calcite/pyrite shells and the clay matrix. Modelling of hornfels compositions using Theriak – Domino gives a minimum temperature for metamorphism of ~600 °C at 200 bars. The metafossils comprise an inner wollastonite-rich core that also contains pyrrhotite and fayalite in the ammonites (zone 1), surrounded by iron-rich wollastonite, hedenbergite, titanite and feldspar (zone 2). The matrix in contact with the metafossil (zone 3) comprises magnesian hedenbergite, plagioclase and potassium feldspar and matrix distant from the fossils (zone 4) comprises augite, plagioclase, potassium feldspar, fayalite, ilmenite and quartz. These assemblages developed by metasomatic exchange of silica, sourced from the matrix with CaO sourced from the calcite shells. The metamorphic transformations, in particular reaction of calcite with silica-bearing fluids to form wollastonite and transformation of pyrite to pyrrhotite result in large volume changes which may have distorted the shape of the fossils, nevertheless original morphological features, particularly of the ammonites can be detected.

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

Abstract. Coronas, including symplectites, provide vital clues to the presence of arrested reaction and preservation of partial equilibrium in metamorphic and igneous rocks. Compositional zonation across such coronas is common, indicating the persistence of chemical potential gradients and incomplete equilibration. Major controls on corona mineralogy include prevailing pressure (P), temperature (T) and water activity (aH2O) during formation, reaction duration (t) single-stage or sequential corona layer growth; reactant bulk compositions (X) and the extent of metasomatic exchange with the surrounding rock; relative diffusion rates for major components; and/or contemporaneous deformation and strain. High-variance local equilibria in a corona and disequilibrium across the corona as a whole preclude the application of conventional thermobarometry when determining P–T conditions of corona formation, and zonation in phase composition across a corona should not be interpreted as a record of discrete P–T conditions during successive layer growth along the P–T path. Rather, the local equilibria between mineral pairs in corona layers more likely reflect compositional partitioning of the corona domain during steady-state growth at constant P and T. Corona formation in pelitic and mafic rocks requires relatively dry, residual bulk rock compositions. Since most melt is lost along the high-T prograde to peak segment of the P–T path, only a small fraction of melt is generally retained in the residual post-peak assemblage. Reduced melt volumes with cooling limit length scales of diffusion to the extent that diffusion-controlled corona growth occurs. On the prograde path, the low melt (or melt-absent) volumes required for diffusion-controlled corona growth are only commonly realized in mafic igneous rocks, owing to their intrinsic anhydrous bulk composition, and in dry, residual pelitic compositions that have lost melt in an earlier metamorphic event. Experimental work characterizing rate-limiting reaction mechanisms and their petrogenetic signatures in increasingly complex, higher-variance systems has facilitated the refinement of chemical fractionation and partial equilibration diffusion models necessary to more fully understand corona development. Through the application of quantitative physical diffusion models of coronas coupled with phase equilibria modelling utilizing calculated chemical potential gradients, it is possible to model the evolution of a corona through P–T–X–t space by continuous, steady-state and/or sequential, episodic reaction mechanisms. Most coronas in granulites form through a combination of these endmember reaction mechanisms, each characterized by distinct textural and chemical potential signatures with very different petrogenetic implications. An understanding of the inherent petrogenetic limitations of a reaction mechanism model is critical if an appropriate interpretation of P–T evolution is to be inferred from a corona. Since corona modelling employing calculated chemical potential gradients assumes nothing about the sequence in which the layers form and is directly constrained by phase compositional variation within a layer, it allows far more nuanced and robust understanding of corona evolution and its implications for the path of a rock in P–T–X space.

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.

Franciscan geologic history constrained by tectonic/olistostromal high-grade metamafic blocks in the iconic California Mesozoic-Cenozoic accretionary complex

![Figure][1] Subduction generated a NW-trending Andean arc along the Californian margin beginning at ~175 Ma. Coeval high-pressure (HP) transformation of oceanic crust in an east-dipping, subduction zone probably accompanied plate convergence, but recovered Franciscan eclogite and garnet blueschist blocks chiefly possess younger recrystallization ages of ~165–150 Ma. These Jurassic HP metamafic rocks were sequestered in a low-temperature environment well into Cretaceous time. Fragments of actinolitic rinds partially surround many such high-grade blocks. Only slightly younger than the HP metamorphism, these rinds reflect metasomatic exchange between metabasalt and serpentinized harzburgite along the dynamic oceanic crust-mantle hanging wall during storage of the mafic rocks at moderate depth. High-grade tectonic blocks later were brought toward the surface in circulating, low-density, sheared mud-matrix melange and/or in buoyant serpentinite bodies. Most exotic HP metamafic blocks occur in melanges of the Franciscan Central Belt, reflecting tectonic insertion within the subduction zone—not near-surface additions to the clastic section. However, rare, high-grade clasts in feebly recrystallized Franciscan conglomerates suggest erosion and sedimentary deposition for some HP blocks. The addition of dense metabasaltic olistoliths to the mid- and Upper Cretaceous section requires that these HP material were carried surfaceward first as tectonic fragments, perhaps immersed in low-density serpentinite or mud-matrix melange, then eroded and transported into the trench. HP rocks are conspicuously lacking in coeval Great Valley strata. Whatever the origin of particular high-grade rocks, widespread post-depositional shearing has largely obliterated their original natures, but all dense metamafic blocks of Jurassic recrystallization age must have been supplied to the Cretaceous Franciscan accretionary complex by entrainment in a low-density, circulating muddy matrix or serpentinite body. The vast majority of exotic HP blocks resides in Central Belt melanges, and appears to be tectonic rather than olistostromal in origin. [1]: pending:yes

The pegmatitic rocks of El-Hudi and Um Hibal areas, Southeastern Desert, Egypt: genesis and petrological characteristics

In an effort to understand the genesis of the pegmatitic rocks of El-Hudi and Um Hibal areas, detailed field, petrographic, and geochemical investigations were conducted. These investigations revealed evidence of two distinct modes of emplacement of these rocks. El-Hudi pegmatites are emplaced concordantly in a belt of folded gneisses and migmatites that mostly fit well with a metasedimentary protolith, which are later cataclased or even mylonitized. El-Hudi pegmatites can be classified into (1) plagioclase or sodic and (2) k-feldspar or potassic pegmatites. Their mineral assemblage, the variable forms of albite bodies in perthitized k-feldspars, and the myrmekite and graphic intergrowths are conclusive evidence indicating metasomatic and later stages of replacement. Geochemically, the peraluminous character of these pegmatites, compressional setting, partial melting parentage, and S-type affinity confirm their metasedimentary source and support a linkage with their host gneisses. The following stages explained the most accepted scenario for the development of these pegmatites “partial melting from metasedimentary protoliths, minimal fractionation and differentiation, injection into foliation, fractures and/or tectonic planes of the host gneisses, and invading the gneiss and migmatite hosts reflecting metasomatic exchanges, segregated earlier or contemporaneous with the genesis of garnet-bearing granites.” In Um Hibal pegmatites, their vein- and miarolitic-type emplacements with the characteristic chilled margin of the later, the recorded hydrothermal nature of the well-preserved silica gels, and the well-crystallized accessory zircons revealed their magmatogenic origin. The similar behavior in depletion or enrichment of the rare earth element contents of Um Hibal pegmatites and their host granites indicate that they are genetically related. They have a mild to peralkaline affinity, emplaced in an extensional regime and shallow depth environment, of within plate and A1-type granite characters. However, the pegmatites seem to be fractionally crystallized from the residual melt of the host granites.

Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ18O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ18O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ18O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ18O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ18O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ18O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ18O garnet rims, and later as temperatures decrease, with rind formation.

Chemical controls on fault behavior: Weakening of serpentinite sheared against quartz‐bearing rocks and its significance for fault creep in the San Andreas system

AbstractThe serpentinized ultramafic rocks found in many plate‐tectonic settings commonly are juxtaposed against crustal rocks along faults, and the chemical contrast between the rock types potentially could influence the mechanical behavior of such faults. To investigate this possibility, we conducted triaxial experiments under hydrothermal conditions (200–350°C), shearing serpentinite gouge between forcing blocks of granite or quartzite. In an ultramafic chemical environment, the coefficient of friction, μ, of lizardite and antigorite serpentinite is 0.5–0.6, and μ increases with increasing temperature over the tested range. However, when either lizardite or antigorite serpentinite is sheared against granite or quartzite, strength is reduced to μ ~ 0.3, with the greatest strength reductions at the highest temperatures (temperature weakening) and slowest shearing rates (velocity strengthening). The weakening is attributed to a solution‐transfer process that is promoted by the enhanced solubility of serpentine in pore fluids whose chemistry has been modified by interaction with the quartzose wall rocks. The operation of this process will promote aseismic slip (creep) along serpentinite‐bearing crustal faults at otherwise seismogenic depths. During short‐term experiments, serpentine minerals reprecipitate in low‐stress areas, whereas in longer experiments, new Mg‐rich phyllosilicates crystallize in response to metasomatic exchanges across the serpentinite‐crustal rock contact. Long‐term shear of serpentinite against crustal rocks will cause the metasomatic mineral assemblages, which may include extremely weak minerals such as saponite or talc, to play an increasingly important role in the mechanical behavior of the fault. Our results may explain the distribution of creep on faults in the San Andreas system.

On the iron isotope heterogeneity of lithospheric mantle xenoliths: implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth

With the aim to better understand the cause of the iron isotope heterogeneity of mantle-derived bulk peridotites, we compared the petrological, geochemical and iron isotope composition of four xenolith suites from different geodynamic settings; sub-arc mantle (Patagonia); subcontinental lithospheric mantle (Cameroon), oceanic mantle (Kerguelen) and cratonic mantle (South Africa). Although correlations were not easy to obtain and remain scattered because these rocks record successive geological events, those found between δ57Fe, Mg#, some major and trace element contents of rocks and minerals highlight the processes responsible for the Fe isotope heterogeneity. While partial melting processes only account for moderate Fe isotope variations in the mantle ( 0.9 ‰). The kinetic nature of rapid metasomatic exchanges between low viscosity melts/fluids and their wall-rocks peridotite in the mantle is the likely explanation for this large range. There are a variety of responses of Fe isotope signatures depending on the nature of the metasomatic processes, allowing for a more detailed study of metasomatism in the mantle with Fe isotopes. The current database on the iron isotope composition of peridotite xenoliths and mafic eruptive rocks highlights that most basalts have their main source deeper than the lithospheric mantle. Finally, it is concluded that due to a complex geological history, Fe isotope compositions of mantle xenoliths are too scattered to define a mean isotopic composition with enough accuracy to assess whether the bulk silicate Earth has a mean δ57Fe that is chondritic, or if it is ~0.1 ‰ above chondrites as initially proposed.

Alteration of plagioclase to nepheline in the Khariar alkaline complex, SE India: Constraints on metasomatic replacement reaction mechanisms

At the Khariar alkaline complex in southeastern India, some tholeiitic amphibolites inter-layered with syenite and nepheline syenite show microtextural evidence of fenitization. Typical metasomatic textures include the replacement of plagioclase by vein nepheline and albitic-plagioclase or by symplectitic nepheline+albite+Na–Al-rich clinopyroxene. Whole rock mass balance calculations indicate that the fenitized variants gained alkalis (Na+K), some large ion lithophile elements (Sr, Rb and Ba) and high field strength elements like Zr and Hf during the metasomatic exchange. The plagioclase replacement textures can be explained by a fluid-mediated coupled dissolution–reprecipitation process whereby the infiltration of alkali-bearing fluid along grain boundaries and deformation-induced intragranular fractures led to the dissolution of the host plagioclase along an inward moving reaction interface. Na+ ions were introduced and Ca2+ ions removed by the fluid phase. The Al and Si released from the breakdown of the anorthite component of the plagioclase combined with Na+ from the fluid to precipitate nepheline+albitic-plagioclase or Na–Al-rich clinopyroxene. Reaction enhanced cracking during metasomatism due to local volume changes in the alteration zone produced a hierarchical system of fractures. Mass-balance calculations using the volume proportion of phases in the plagioclase replacement zones show that SiO2 and Al2O3 were broadly conserved during the replacement; there was significant loss of CaO (ca. 73%) and huge gains of Na2O and K2O. These results indicate that alkali and alkaline elements (Na, K and Ca) were exchanged freely between the fluid phase and the reaction zone whereas Si and Al released during plagioclase dissolution were consumed in the replacement zone itself to form nepheline and albite-rich plagioclase or Na–Al-rich clinopyroxene. The trace element and Sr–Nd isotope signature of the rocks indicate that the fenitizing fluids were derived from the nepheline syenites.

Petrology, Geochemistry, and Stable Isotope Studies of the Chehelkureh Cu-Zn-Pb Deposit, Zahedan, Iran

The Chehelkureh base metal deposit in southeast Iran is in an abandoned historic copper district that is fault controlled and hosted in a sequence of slightly Eocene interbedded graywackes, siltstones, and shales. Several stocks and dikes of Oligo-Miocene granodiorite to quartz monzodiorite and granite composition, oriented parallel to the dominant northwest-southeast fault system, intruded into the sedimentary sequence. The sedimentary rocks were metamorphosed to the hornfels facies in some outcrops. The intrusions are spatially and temporally related to mineralized faults. The deposit consists of numerous irregular lenses and veins located along faults that cover an area 1,500 m long by 80 to 280 m wide. There are two periods of primary Cu-Zn-Pb mineralization that crosscut each other. The first stage includes quartz, calcite, dolomite, ankerite, siderite, ilmenite, rutile, molybdenite, pyrrhotite, arsenopyrite, pyrite, and chalcopyrite. The second stage consists of quartz, dolomite, ankerite, siderite, chalcopyrite, sphalerite, pyrite, galena, selenian galena, marcasite, nevskite, and paraguanajuatite. The gangue minerals are dominated by quartz and various carbonates, locally associated with chlorite. Hypogene alteration consists of silicification, carbonatization (ankerite, magnesite, siderite, and dolomite), chloritization, kaolinitization, sulfidation, and, less commonly, sericitization. The intrusive rocks at Chehelkureh are calc-alkaline and have chemical features typical of I-type granitoids. Based on Nb-Y and Ta-Yb discrimination diagrams, the geotectonic environment of Chehelkureh granitoids is an intracontinental volcanic arc (Nb/Y ~ 0.4), which is the same setting as that in the multiphase granitoid batholiths of Zahedan, located southeast of Chehelkureh. Granitoids of the Chehelkureh area have moderate REE (rare-earth element) contents (∑REE = 110–174 ppm; average 153 ppm), moderate light REE/heavy REE ((La/Lu)cn = 7–8), and strong negative Eu anomalies (Eu/Eu* ~ 0.2), but no Eu anomaly is evident in monzodiorites. Spider diagram patterns for samples of igneous rocks show Nb and Ta, Sr, and Ti-V negative anomalies, with high amounts of Cu, U, and Th. Fluid inclusion studies of quartz intergrown with sulfides in quartz veins show three main types of fluid inclusions: type 1 inclusions consist of liquid + vapor + daughter crystal (solid); type 2 inclusions are composed of liquid + minor vapor, with vapor/liquid ratios of 0.1 to 0.4; and type 3 inclusions are vapor rich with liquid/vapor ratios up to 0.05 (liquid phase is minor or absent). Homogenization temperatures (Th) for type 2 inclusions vary between 330° and 480°C, whereas their salinities range from 5 to 15 wt % NaCl equiv. The δ 18Owater values of quartz in sulfide-bearing veins are 8.8 to 11‰, with an average of 10‰. Most of the δ 18Owater values of quartz veins are in the range of typical magmatic, metamorphic, and connate waters, so the source of the ore fluids cannot be distinguished based on oxygen isotopes alone. The δ 18Owater and δ Dwater values calculated from chlorite range from 5.6 to 10.6‰ and −31 to −23‰, respectively. These data lie in the metamorphic water box, in the formation water box, or about midway between the magmatic water field and typical seawater. For kaolinite at ~200°C, the calculated δ 18Owater varies from 4.2 to 10.7‰ and the δ Dwater values range from −88 to −49‰. The carbon isotope values (Pee Dee Belemnite) of carbonates vary between −5.7 and −0.9‰, whereas the δ 18O values (Standard Mean Ocean Water) are between 12.4 and 14.9‰. The coupled oxygen and carbon isotope values shift from magmatic values and can be related to metasedimentary decarbonation reactions and metasomatic exchange of oxygen with magmatic fluid. The range of δ 34S values of sulfide minerals is small (2.0–4.2‰, with a mean value of 3‰), consistent with a magmatic origin for sulfur. Isotope thermometry, based on quartz-carbonate pairs, yields an average of 450°C, which is consistent with fluid inclusion temperatures. Isothermal f O2-pH diagrams constructed at 450°C and 1 kbar suggest that the ore formed in the H2S-predominant field with f O2 and pH ranging from 10−29 to 10−24 and 5.0 to 6.2, respectively. The petrogenetic model proposed for the Chehelkureh deposit suggests that during the Oligocene a quartz-monzodiorite stock intruded the Eocene turbidites and provided a magmatic component to the ore-forming system, but also acted as the heat source for hydrothermal convection cells, although metamorphic and connate waters may also have contributed to the ore-forming fluids. In this model, magma is the main source of sulfur, base metals, and carbon, although Eocene turbidites also contributed some metals and carbon.

New insights into the formation of fayalitic olivine from Allende dark inclusions

Abstract– Although considerable progress has been made in unraveling the origin(s) of fayalitic olivines in dark inclusions (DIs), many questions remain still unresolved and/or controversial. We combine a chemical and petrographic study of the Allende dark inclusion 4884‐2B (AMNH, New York) and ATEM studies of a fragment of the dark inclusion Allende AF (NHM, Vienna) and discuss an alternative way in which fayalitic olivines could have formed. Allende dark inclusion 4884‐2B contains a few aggregates with variable proportions of transparent and feathery olivine. Two such objects (aggregates A and B) are the focus of this study as they preserve glasses that can help in deciphering the nature of the processes involved during olivine growth and subsequent olivine transformation. The petrographic and chemical characteristics of aggregates A and B indicate that the forsteritic stack olivines may be pseudomorphs of clear olivine crystals. The ATEM studies in All‐AF suggest that fayalitic olivines may be the result of secondary processes (e.g., metasomatic exchange reactions) operating in the solar nebula. Transformation may have occurred through the mediation of a dry gas phase involving nonvolatile major elements, such as Mg and Fe (e.g., Dohmen et al. 1998). This mechanism could reveal olivine growth patterns (e.g., stacked platelets due to a rapid growth regime) and may have contributed to the development of their fibrous aspect while preserving the shape (i.e., volume) of the crystals. This highly selective process did completely or partially transform ferromagnesian minerals, but affected the fine‐grained mesostasis only slightly.

Lithogeochemistry of the Collahuasi porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster, northern Chile: Pearce element ratio vectors to ore

Abstract Lithogeochemical vectors to ore-bodies are obscured by the variety of lithological units and the alteration events that have affected them. In the magmatic-hydrothermal context, Pearce element ratio (PER) analysis can be a cost-effective lithogeochemical technique that identifies and eliminates non-hydrothermal sources of variation, thus permitting the definition of material transfer related only to hydrothermal alteration. Dedicated PER diagrams for mafic and felsic lithologies in the world-class porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster of the Collahuasi district, I Region, northern Chile, effectively model background variability and discriminate between fresh or propylitically altered rocks and those with a hydrolytic alteration overprint. Furthermore, PER diagrams allow for the definition of an alteration index (AI) that quantifies the degree of metasomatic exchange during hydrolytic alteration of a particular rock. Plots of different PER values against the AI show that potassium enrichment during hydrolytic alteration was followed by potassium, calcium and sodium depletion as a result of the destruction of feldspars during sericitic and later argillic alteration. Although PER plots record no mass transfer processes involving major elements during propylitic alteration, the integrated AI for both mafic and felsic units spatially defines the major alteration centres within the district.

On the genetic nature of isomorphism: Mechanism of component selection during crystal growth

The concept of mixed-crystal formation, which has developed since the early 1980s, suggests the leading role of the metasomatic exchange between the crystal and solution. Compositions of solid and liquid phases shift to meet one another up to achieving thermodynamic equilibrium. This mechanism provides the regulation of the crystal composition during the process of isomorphism. The principles of the proposed assembly mechanism of selecting isomorphic components in the proportions dictated by a given combination of conditions (composition of the solution, supercooling or superheating, and temperature) are set forth as follows. The particle assemblages characterize a certain statistical distribution by the composition precipitate on the surface. Only those assemblages, which satisfy the aforementioned conditions, survive, whereas the other assemblages are dissolved, initiating the salting-out of the former assemblages. The validity of the proposed model is corroborated by the development of the assembly structure of autoepitaxial (K,Rb)HC8H4O4 nuclei on a KHC8H4O4 crystal observed with a Park CP atomic force microscope. The nuclei are widely scattered in composition, degree of faceting, and size, demonstrating their different stability and growth rate. Another example is the distribution of spontaneously precipitated mixed K(Br,Cl) crystals by isomorphic composition and size. The small and large fractions are enriched in KBr in comparison with medium-size fractions, while the volumetric proportions of the fractions reveal a bimodality with the complete absence of fractions 30–50 μm. The traditionally discussed statistical selection of particular ions, which is probably second in importance; however, it may become crucial when the amount of one of the isomorphic components is rather small. While oversaturation is fairly high, crystallization without selection may be expected.

Ga/Mg ratio as a new geochemical tool to differentiate magmatic from metamorphic blue sapphires

Using ICP-MS–LA analyses, we demonstrate that the use of the Ga/Mg ratio, in conjunction with the Fe concentration, is an efficient tool in discriminating between “metamorphic” and “magmatic” blue sapphires. Magmatic blue sapphires found in alkali basalts (e.g. southeastern Asia, China, Africa) are commonly medium-rich to rich in Fe (with average contents between 2000 and 11000 ppm), high in Ga (> 140 ppm), and low in Mg (generally < 20 ppm) with high Ga/Mg ratios (> 10). Conversely, metamorphic blue sapphires found in basalts (e.g. Pailin pastel) and in metamorphic terrains (e.g. Mogok, Sri Lanka, Ilakaka) are characterized by low average iron contents (< 3000 ppm), low Ga contents (< 75 ppm), and high Mg values (> 60 ppm) with low average Ga/Mg ratios (< 10). Basaltic magmatic sapphires have Fe, Ga and Mg contents similar to those obtained for primary magmatic sapphires found in the Garba Tula syenite. This suggests that these both sets of sapphires have a possible common “syenitic” origin, as previously proposed from other criteria. In addition, plumasite-related sapphires and metamorphic sapphires also exhibit similar composition in trace elements. Based on results from the present study, we suggest that fluid circulations during a metamorphic stage produced metasomatic exchanges between mafic and acidic rocks (plumasite model), thus explaining the high Mg contents and converging Ga/Mg ratios observed in metamorphic sapphires.

Formation of emeralds at pegmatite-ultramafic contacts based on fluid inclusions in Kianjavato emerald, Mananjary deposits, Madagascar

AbstractKianjavato emerald (Mananjary deposits, East coast of Madagascar) was formed during metasomatic processes at the contact between pegmatites and hornblendites. The metasomatic exchange was related to a Pan-African tectonometamorphic event.Fluid inclusions in the Kianjavato emerald and quartz were studied by means of microthermometry and Raman probe analysis. Three main types of inclusions were revealed: CO2-rich, CH4-rich and aqueous-rich, with a salinity of ∼2 wt.% NaCl equiv. The inclusions occurred along the same primary and pseudosecondary trails and were considered to be formed simultaneously. Based on fluid-inclusion data, the conditions of emerald growth were 250°C &lt; T &lt; 450°C and P = 1.5 kbar.The fluid inclusion data for Kianjavato emerald were compared to the data for another Madagascar emerald, Ianapera. The latter is of similar age, but its genesis was determined by a shearing event. Our fluid inclusion data suggested that shearing was also important as a mechanism of introducing CO2-rich fluid for the genesis of the Kianjavato emerald.

Prograde Evolution and Geothermal Affinities of a Major Porphyry Copper Deposit: The Cerro Colorado Hypogene Protore, I Region, Northern Chile

The middle Eocene (51.8 ± 0.6 Ma) hypogene protore underlying the supergene orebody of the Cerro Colorado porphyry Cu (-Mo) deposit, I Region, Chile, exhibits features not commonly documented in such hydrothermal systems. Early-stage alteration of Upper Cretaceous plagioclase-phyric andesite generated a sub-horizontal blanket of pervasive, extremely fine grained but texture-preserving biotite (≤35 modal %)-albite (≤40%)-magnetite (≥3%) alteration, 8 km 2 in area but lacking sulfide minerals. At least seventy percent of the chalcopyrite > pyrite stockwork mineralization was emplaced during the subsequent Main-stage alteration, which comprises, with decreasing depth, quartz-albite, sericite-chlorite-clay (smectite), quartz-sericite-clay, and andalusite-diaspore-pyrophyllite assemblages. The deposit is apparently unique among documented central Andean porphyry systems in the association of the highest grade copper mineralization with intermediate argillic alteration. The subsequent Transitional-stage phyllic (i.e., quartz-sericite-pyrite ± tourmaline) alteration was associated with the emplacement of molybdenite-rich breccia bodies. The occurrence of undumortieri-tized tourmaline veinlets cutting andalusite-diaspore assemblages confirms that much of the advanced argillic alteration took place during the Main stage. Early-stage alteration was the product of nonboiling, cool (trapping temperature ≤ca. 380°C), low-salinity (≤8 wt % NaCl equiv) fluids which added substantial K, Na, Mg, Fe 2+ , Cl, F, and water to the host andesite. The initial Main-stage fluids, boiling at a paleodepth of ca. 2.5 to 3.0 km, were up to 160°C hotter (≤544°C) and highly saline (≤52 wt % NaCl equiv). As these fluids rose, they cooled to ≤320°C, were diluted (to ≤37 wt % NaCl equiv), deposited sulfides, and leached K, Na, Ca, Mg, Fe 2+ ,and Cl from the host rocks, yielding diverse, broadly contemporaneous, intermediate and advanced argillic alteration facies. Pressure estimates require that low-density (≤0.3 g/cm 3 ), and thus more acidic, fluids were primarily responsible for the formation of the quartz-sericite-clay and shallow advanced argillic alteration. Subsequent phyllic alteration was similarly caused by boiling fluids which were hot (≤486°C) and saline (≤47 wt %) at depth but cooler (≤334°C), dilute (≤8 wt %), and vapor dominated at shallower levels. Even Terminal-stage pyrite veins formed at temperatures as high as 450°C, albeit from low-salinity (≤8 wt %) fluids. Following a major prograde thermal transition from the Early to the Main stage, each sulfide-depositing alteration episode at Cerro Colorado was generated by a pulse of high-temperature fluid which cooled and diluted as it rose. Such changes in fluid characteristics, temperature, and alteration relationships are well documented in numerous geothermal fields, where potassic alteration generally develops at ca. 270° to 350°C and are likely to occur at an early stage in any hydrothermal system in which magmatic fluids, exsolving at relatively high pressures, ascend into the near-surface environment. The initial alteration at Cerro Colorado plausibly developed under conditions similar to those in nonexplosive geothermal systems, in which hydrothermal fluids cool and disperse laterally at shallow depths. Extensive zones of biotite-rich alteration, generally barren and with a hornfelsic appearance, occur in many andesite-hosted porphyry copper deposits, but few data are available elsewhere for mineralogical, pressure, temperature, or metasomatic exchange relationships in such systems. Cerro Colorado may, however, be representative of a subclass of porphyry copper deposits exhibiting unusually close analogies with geothermal systems.

Nebular and asteroidal modification of the iron isotope composition of chondritic components

Abstract The Fe-isotope signatures (δ56FeIRMM and δ57FeIRMM) of twelve chondrules, three Ca-, Al-rich inclusions (CAIs) and multiple matrix aliquots from two type 3 chondrites — Allende (CV3) and Chainpur (LL3) were measured by MC-ICP-MS. The mass-fractionation line for these samples is δ57FeIRMM = (1.45 ± 0.05) · δ56FeIRMM − (0.01 ± 0.02) with R2 = 0.9995. Two processes appear to affect the Fe-isotope composition of these chondritic components. Initially, short-lived high-temperature events within the solar nebula drove isotopic signatures towards isotopically heavier compositions. Subsequently, metasomatism shifted isotopic signatures of chondrules and CAIs towards bulk-like compositions. The Fe-isotope composition of Allende matrix (δ56FeIRMM = − 0.07 ± 0.02‰, n = 6) is similar to that of bulk Allende (δ56FeIRMM = − 0.05 ± 0.05‰). The range of Chainpur matrix compositions is − 0.03 ≤ δ56FeIRMM ≤ + 0.02‰. This relatively homogeneous matrix composition appears to be governed by metasomatic exchange of iron, although the possibility that matrix is heterogeneous on a scale smaller than the sample size cannot be ruled out. CAIs from Allende, in which iron has been metasomatically introduced from the matrix, are mainly unfractionated with respect to the bulk composition of Allende. The least altered CAI, which contains minor bulk FeO (0.4 wt.%), is isotopically lighter (δ56FeIRMM = − 0.16‰) with respect to bulk Allende. This signature may reflect condensation of isotopically light iron in the nebula or preferential incorporation of the lighter iron isotopes contained within the matrix. Chondrule Fe-isotope compositions (− 1.33 ≤ δ56FeIRMM ≤ + 0.65‰) are driven by both high-temperature nebular events and subsequent metasomatic exchange. The relationship between compositional and isotopic signatures clearly points to an evaporation origin for type I chondrules, which are isotopically heavier than type II chondrules. Subsequent metasomatic exchange with matrix iron has driven some chondrules (most notably the smaller chondrules) towards a bulk-like Fe-isotopic signature.

A liquid-supported condensation of major minerals in the solar nebula: Evidence from glasses in the Kaba (CV3) chondrite

Glasses, in the Kaba CV3 chondrite, occur as mesostasis in chondrules and aggregates and as inclusions in olivines, both confined or open and connected to the mesostasis. The inclusions in olivine and the glassy mesostasis of aggregates seem to have formed contemporaneously. The confined glass inclusions and open inclusions in olivine were formed during olivine growth and the mesostasis glass during olivine aggregation. All glasses have high trace element contents ( 10 – 20 × CI ) with unfractionated CI-normalized abundances of refractory trace elements. In contrast, V, Mn, Li, and Cr are depleted in all glasses with respect to the refractory trace elements, as is Rb in the glass inclusions in olivine but not in the mesostasis glass. This abundance pattern indicates vapor fractionation and a common condensation origin for both glasses. Glasses of confined glass inclusions in olivine have a Si Al Ca-rich composition with a chondritic Ca/Al ratio. Glasses of open glass inclusions and mesostasis are poor in Ca and enriched in alkalis. However, Ca contents of olivines indicate crystallization from a Ca-rich melt of a composition similar to that of the glass inclusions. In addition, trace element abundances indicate that these glasses (liquids) probably had an original composition similar to that of the inclusion glass. They apparently lost Ca in exchange for alkalis in a metasomatic exchange reaction, presumably with the vapor. There is now growing evidence that liquids can indeed condense from a solar nebula gas, provided the gas/dust ratio is sufficiently low. In these regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition, liquids (the glass precursor) probably played an important role in growing crystals from the vapor by liquid-phase epitaxy. The glasses appear to be the remnants of this thin liquid layer interface that supported the growth of olivine from the vapor following the Vapor–Liquid–Solid process. This liquid will have a refractory composition and will have trace element contents which are in equilibrium with the vapor, and, therefore, will not change much during the time of olivine growth. The composition of the liquid seems to be unconstrained by the phases it is in contact with. Samples of this liquid will be retained as glass inclusions in olivine. The glassy mesostasis could also be a sample of this liquid that got trapped in inter-crystal spaces. The mesostasis glass subsequently behaved as an open system and its Ca was exchanged—presumably with the vapor—for the alkali elements Na, K, and Rb. In contrast, glass inclusions in olivine were protected by the host, could not react, and thus preserved the original composition of this liquid.