Mineralogy and metamorphism conditions of manganese ore at the South Faizulino deposit, the southern Urals, Russia

The mineralogy of slightly metamorphosed manganese ore at the South Faizulino hydrothermalsedimentary deposit in the southern Urals has been studied; 32 minerals were identified. Quartz, hausmannite, rhodochrosite, tephroite, ribbeite, pyroxmangite, and caryopilite are major minerals; calcite, kutnahorite, alleghanyite, spessartine, rhodonite, clinochlore, and parsettensite are second in abundance. This mineralic composition was formed in the process of gradual burial of ore beneath the sequence of Middle Devonian-Lower Carboniferous rocks. The highest parameters of metamorphism are T ≈ 250°C and P ≈ 2.5 kbar. The relationships between minerals and their assemblages made it possible to reconstruct the succession of ore transformation with gradually increasing temperature and pressure. Manganese accumulated in the initial sediments as oxides and a gel-like Mn-Si phase. Rhodochrosite and neotocite were formed at the diagenetic stage. In the course of a further increase in temperature and pressure, neotocite was replaced with caryopilite; ribbeite, tephroite, pyroxmangite, and other silicates crystallized afterwards. In addition to the PT parameters, the formation of various metamorphic mineral assemblages was controlled by the Mn/(Mn + Si) ratio in ore and X _CO2 in pore solution. The latter parameter was determined by the occurrence of organic matter in the ore-bearing rocks. Ore veinlets as products of local hydrothermal redistribution of Mn, Si, and CO₂ were formed during tectonic deformations in the Middle Carboniferous and Permian.     

Manganiferous phyllosilicate assemblages: occurrences,compositions and phase relations in metamorphosed Mn deposits

Mn-rich members of the pyrosmalite-family [(Mn, Fe)₈Si₆O₁₅(OH, Cl)₁₀], friedelite and schallerite have been identified as rock-forming minerals together with caryopilite, in several metamorphosed carbonate Mn-deposits. The phase assemblages and mineral compositions are described for eight of these localities each of which represents a distinct geologic situation. Friedelite is always Cl-bearing and occurs both as a prograde phase in low-grade metamorphic rocks (Pyrenees, Haute-Maurienne) and as a secondary phase formed by retrogressive replacement of primary anhydrous phases in higher-grade rocks. Schallerite, an Asbearing relative of friedelite, occurs in the greenschist metamorphic deposit of the Ködnitztal (Austria) together with other As-minerals. In these deposits, caryopilite is typically formed during retrograde metamorphism by alteration of, generally anhydrous, Mn-silicates. Based upon these occurrences, a qualitative petrogenetic grid for the system MnO-SiO₂-CO₂-H₂O with the phases friedelite, caryopilite, pyroxmangite/rhodonite, tephroite, rhodochrosite, quartz, CO₂, and H₂O is proposed. The phase relations imply that Cl- (or As-) free friedelite is not stable in hydrous systems with respect to caryopilite. From the mineral assemblages containing hydrous Mn silicates, waterrich fluids are inferred during the retrograde metamorphic evolution of the investigated deposits. Chemical data for Mn-rich chlorites, which are basically members of the clinochlore-pennantite series which coexist with the pyrosmalite minerals, show the absence of intermediate Mn/Mg ratios. This supports the existence of a miscibility gap as previously hypothesized by other authors.     

Mineralogy of metamorphosed manganese deposits of the South Urals

A mineralogical investigation of metamorphosed manganese rocks was carried out at ore deposits related to the Devonian volcanic complexes of the Magnitogorsk paleovolcanic belt of the South Urals. The mineralogical appearance of these rocks is determined by three consecutively formed groups of mineral assemblages: (1) assemblages occupying the main volume of orebodies and formed during low-grade regional metamorphism (T = 200−250°C, P = 2–3 kbar); (2) assemblages of segregated and metasomatic veinlets that fill the systems of late tectonic fractures; and (3) assemblages of near-surface supergene minerals. Sixty-one minerals have been identified in orebodies and crosscutting hydrothermal veinlets. The major minerals are quartz, hematite, hausmannite, braunite, tephroite, andradite, epidote, rhodonite, caryopilite, calcite, and rhodochrosite. The mineral assemblages of metamorphosed manganese rocks (metamanganolites) are characterized. Chemical compositions of braunite, epidote-group minerals, piemontite, pyroxenes, rhodonite, pyroxmangite, and winchite are considered. The bibliography on geology and mineralogy of the South Ural manganese deposits is given.     

Manganocummingtonite from the mesoproterozoic,Sausar fold belt,central India

Manganocummingtonite occurs with spessartine, quartz and pyrolusite in the Chikmara area, Sausar fold belt, central India. Its composition is [Ca_0.3–0.35(Mg_3.3–3.5Mn_1.6–1.8Fe²⁺ _1.4–1.5)(Si_{7.931–7.997}Al^iv _{0.003–0.069})O₂₂(OH_1.5–2.0F_0.0–0.5)] being fairly rich in Ca, which is indicative of metamorphic temperature in the amphibolite facies. The garnet contains 77.5% spessartine, 13% almandine and minor andradite, grossular and pyrope components. Unusually, there is no carbonate, pyroxene, pyroxmangite, rhodonite, magnetite or hematite. The available Al in the rock stabilized garnet and this mineral incorporated minor Fe³⁺ present in the rock as andradite component. The manganocummingtonite-garnet pairs developed at ～600°C during amphibolite facies metamorphism in low $X_{CO_2 } $ system, stabilized with $X_{Mn/(Mn + Fe^{2 + } + Mg)} $ = 0.25 to 0.28 in the amphibole and 0.85 in the garnet and formed under unusually low fO ₂ conditions for the Sausar region, near channelized fluids which deposited quartz may have controlled the fO ₂ .     

The application of P–T–X(CO2) modelling in constraining metamorphism and hydrothermal alteration at the Damang gold deposit,Ghana

Orogenic gold mineralization at the Damang deposit, Ghana, is associated with hydrothermal alteration haloes around gold‐bearing quartz veins, produced by the infiltration of a H₂O–CO₂–K₂O–H₂S fluid following regional metamorphism. Alteration assemblages are controlled by the protoliths with sedimentary rocks developing a typical assemblage of muscovite, ankerite and pyrite, while intrusive dolerite bodies contain biotite, ankerite and pyrrhotite, accompanied by the destruction of hornblende. Mineral equilibria modelling was undertaken with the computer program thermocalc , in subsets of the model system MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O–TiO₂–Fe₂O₃, to constrain conditions of regional metamorphism and the subsequent gold mineralization event. Metapelites with well‐developed amphibolite facies assemblages reliably constrain peak regional metamorphism at ~595 °C and 5.5 kbar. Observed hydrothermal alteration assemblages associated with gold mineralization in a wide compositional range of lithologies are typically calculated to be stable within P–T–X(CO₂) arrays that trend towards lower temperatures and pressures with increasing equilibrium fluid X(CO₂). These independent P–T–X(CO₂) arrays converge and the region of overlap at ~375–425 °C and 1–2 kbar is taken to represent the conditions of alteration approaching equilibrium with a common infiltrating fluid with an X(CO₂) of ~0.7. Fluid‐rock interaction calculations with M–X(CO₂) diagrams indicate that the observed alteration assemblages are consistent with the addition of a single fluid phase requiring minimum fluid/rock ratios on the order of 1.     

Ferromanganese carbonate metasediments of the Sob area,Polar Urals: Bedding conditions,composition, and genesis

The paper presents the results of study of ferromanganese carbonate rocks in the Sob area (Polar Urals), which is located between the Rai-Iz massif and the Seida–Labytnangi Railway branch. These rocks represent low-metamorphosed sedimentary rocks confined to the Devonian carbonaceous siliceous and clayey–siliceous shales. In terms of ratio of the major minerals, ferromanganese rocks can be divided into three varieties composed of the following minerals: (1) siderite, rhodochrosite, chamosite, quartz, ± kutnahorite, ± calcite, ± magnetite, ± pyrite, ± clinochlore, ± stilpnomelane; (2) spessartite, rhodochrosite, and quartz, ± hematite, ± chamosite; (3) rhodochrosite, spessartite, pyroxmanite, quartz ± tephroite, ± fridelite, ± clinochlore, ± pyrophanite, ± pyrite. In all varieties, the major concentrators of Mn and Fe are carbonates (rhodochrosite, siderite, kutnahorite, Mn-calcite) and chlorite group minerals (clinochlore, chamosite). The chemical composition of rocks is dominated by Si, Fe, Mn, carbon dioxide, and water (L.O.I.): total SiO₂ + Fe₂O ₃ ^tot + MnO + L.O.I. = 85.6?98.4 wt %. The content of Fe and Mn varies from 9.3 to 55.6 wt % (Fe₂O ₃ ^tot + MnO). The Mn/Fe ratio varies from 0.2 to 55.3. In terms of the aluminum module Al_M = Al/(Al + Mn + Fe), the major portion of studied samples corresponds to metalliferous sediments. The δ¹³C_carb range (–30.4 to–11.9‰ PDB) corresponds to authigenic carbonates formed with carbon dioxide released during the microbial oxidation of organic matter in sediments at the dia- and/or catagenetic stage. Ferromanganese sediments were likely deposited in relatively closed seafloor zones (basin-traps) characterized by periodic stagnation. Fe and Mn could be delivered from various sources: input by diverse hydrothermal solutions, silt waters in the course of diagenesis, river discharges, and others. The diagenetic delivery of metals seems to be most plausible. Mn was concentrated during the stagnation of bottom water in basin-traps. Interruption of stagnation promoted the precipitation of Mn. The presence of organic matter fostered a reductive pattern of postsedimentary transformations of metalliferous sediments. Fe and Mn were accumulated initially in the oxide form. During the diagenesis, manganese and iron oxides reacted with organic matter to make up carbonates. Relative to manganese carbonates, iron carbonates were formed under more reductive settings and higher concentrations of carbon dioxide in the interstitial solution. Crystallization of manganese and iron silicates began already at early stages of lithogenesis and ended during the regional metamorphism of metalliferous sediments.     

The initial garnet‐in reaction involving siderite–rhodochrosite,garnet re‐equilibration and P–T–t paths of graphitic schists in the Black Hills orogen,South Dakota,USA

It is generally thought that garnet in metapelites is produced by continuous reactions involving chlorite or chloritoid. Recent publications have suggested that the equilibrium temperatures of garnet‐in reactions may be significantly overstepped in regionally metamorphosed terranes. The growth of small spessartine–almandine garnet crystals on Mn‐siderite at the garnet isograd in graphitic metapelites in the Proterozoic Black Hills orogen, South Dakota, demonstrates that Mn‐siderite was the principal reactant that produced the initial garnet in the schists. Moreover, the positions of garnet compositions in isobaric, T–(C/H) pseudosections for the schists show that the temperature of the garnet‐in reaction from Mn‐siderite was overstepped minimally at the most. In the Black Hills, garnet was initially produced during regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons, and was subsequently partially or fully re‐equilibrated at more elevated temperatures and pressures during intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet occurs in graphitic schists in garnet, staurolite and sillimanite zones, the latter being a product of contact metamorphism by HPG. During metamorphism, coexisting fluid contained both CO₂ and CH₄. In the garnet zone, garnet crystals contain petrographically distinct cores with inclusions of quartz, graphite and other minerals. Centres of the cores have distinctly elevated Y concentrations that mark the positions of garnet nucleation. The elevated Y is thought to have come from the Mn‐siderite onto which Y was probably absorbed during precipitation in an ocean. In the upper garnet and staurolite zones, the cores were overgrown by inclusion‐poor mantles. Mantles are highly zoned and have more elevated Fe and Mg and lower Mn and Ca than cores. The growth of mantles is attributed to late‐orogenic heating by leucogranite magmas and attendant influx of H₂O that caused consumption of graphite in rock matrices. A portion of the Proterozoic terrane that includes the HPG is surrounded by four large faults. In this ‘HPG block’, garnet is inclusion‐poor and its composition does not preserve its early growth history. This garnet appears to have re‐equilibrated by internal diffusion of its major components and/or recrystallization of an earlier inclusion‐rich garnet. It has equilibrated within the kyanite stability range, and together with remnant kyanite in the high‐strain aureole of the HPG, indicates that the HPG block had a ≥6 kbar history. The HPG block has undergone decompression during emplacement of the HPG. The decompression is evident in occurrences of retrograde andalusite and cordierite in the thermal aureole of the HPG. The data support a polybaric metamorphic history of the Black Hills orogen with different segments of the orogen having their own clockwise P–T–t paths.     

A Comparative geochemical study of pelitic schists and metamorphosed carbonate rocks from south-central Maine,USA

Whole-rock major element chemical analyses of progressively metamorphosed impure carbonate rocks and pelitic schists, collected from the same metamorphic terrain, reveal similarities and differences in the chemical response of these rock types to the metamorphic event. Relative to a constant aluminum reference frame, both schist and carbonate exhibit no detectable change in their contents of Fe, Mg, Ti, Si, and Ca with change in metamorphic grade. Carbonate rocks become progressively depleted in K and Na with increasing grade of metamorphism, while schists exhibit no statistically significant change in their contents of K and Na. Both rock types become depleted in volatiles (principally CO₂ and H₂O) with increasing grade.Whole-rock chemical data permit two mechanisms for migration of K and Na from the carbonate rocks during metamorphism: (a) diffusion of alkalis from carbonate to adjacent schist; (b) transport of alkalis by through-flowing metamorphic fluid (infiltration). Mineral equilibria in schist and metacarbonate rock from the same outcrops allow calculation of the affinity for cation exchange between the two rock types during metamorphism. Measured affinities indicate that if mass transport of K and Na occurred by diffusion, chemical potential gradients would have driven the alkalis from schist into carbonate rock. Because diffusion cannot produce the observed chemical trends in the metacarbonates, K and Na are believed to have been removed during metamorphism by infiltration.The disparity in chemical behavior between the pelitic schists and metacarbonate rocks may be a result of enhanced fluid flow through the carbonates. The carbonate rocks may have acted as metamorphic aquifers; the greater flow of fluid through them would then have had a correspondingly greater effect on their whole-rock chemistry.     

Porphyroblastic manganaxinite metapelagites with incipient garnet in prehnite–pumpellyite facies, near Meyers Pass, Torlesse Terrane, New Zealand

Moderately manganiferous siliceous pelagites near Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand, have been metamorphosed in the prehnite–pumpellyite facies. A conodont colour index measurement suggests T _max in the range 190–300 °C. Porphyroblastic manganaxinite, manganoan pumpellyite, manganoan chlorite and trace spessartine-rich garnet and sphalerite have formed in an extremely fine-grained quartz–albite–berthierine–phengite–titanite groundmass. Porphyroblastic manganaxinite semischists and schists are distinctive rocks in prehnite–pumpellyite to lower-grade greenschist and blueschist facies of New Zealand and Japan. Mn in the manganoan pumpellyites substitutes for Ca in W sites. Total Fe/(Fe+Mg) ratios in chlorite are dependent on oxidation state, being ≤0.22 in red hematitic hemipelagites, and ≥0.61 in low-f O₂ grey metapelagites. In the low-f O₂ metapelagites, manganoan berthierine with little or no chlorite is inferred in the groundmass and iron-rich chlorite occurs as porphyroblasts and veinlets, whereas in the red rocks, Mg-rich chlorite occurs both in groundmasses and veinlets. Variably high Si in the manganoan chlorites correlates with evidence for contaminant phases. The Mn content of chlorite contributing to garnet growth is dependent on metamorphic grade; incipient spessartine indicates a saturation value of 6–8% MnO in chlorite in low-f O₂ rocks at Meyers Pass. Lower MnO contents are recorded for otherwise analogous rocks with increasing metamorphic grade, but at a given grade coexisting chlorite and garnet are richer in Mn where f O₂ is high. Manganaxinite and manganoan pumpellyite also contributed to reactions forming grossular–spessartine solid solutions. Formation of garnet in siliceous pelagites is dependent on both Mn and Ca content. The spessartine component increases with grade into the greenschist facies. Partial recrystallization of berthierine to chlorite and the growth of porphyroblastic patches of other minerals was facilitated by brittle fracture and access of fluids to an otherwise impermeable matrix; to this extent the very low-grade metamorphism was episodic.     

The effect of whole-rock MnO content on the stability of garnet in pelitic schists during metamorphism

Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H₂O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with X_Mn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (X_Mn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.     

Granulite facies metasomatism: zoned calc-silicate boudins from the Rauer Group,East Antarctica

Calc-silicate boudins from the Rauer Group, East Antarctica, were metamorphosed under granulite facies conditions during late Proterozoic (ca. 1,000 Ma) M3 metamorphism. Boudin cores contain low to moderate a_{CO 2} assemblages including wollastonite, grossularandradite (grandite) garnet, clinopyroxene, scapolite, plagioclase, quartz±calcite. Petrological and stable isotopic evidence suggests that these core assemblages resulted from pre-peak M3 infiltration of water-rich fluids; there is no evidence for a pervasive fluid phase under peak M3 conditions. The boudins are separated from the surrounding Fe-rich pelites and semi-pelites by a series of concentric, high-variance reaction zones developed under peak M3 conditions. Variations in mineral assemblage, mineral composition and whole rock composition across these zones suggest that they formed by diffusional masstransfer, controlled principally by a chemical potential gradient in Ca across the original calc-silicate-paragneiss lithological boundary. As a consequence of the nearcomplete decarbonation of the calc-silicatesbefore the M3 peak, development of the diffusion-controlled reaction zones did not liberate significant CO₂ during granulite facies metamorphism. Similar calcite-poor, low a_{CO 2} calc-silicate horizons in other granulite facies terrains are unlikely to have been important local fluid sources during deep crustal metamorphism.     

P_{CO_2 } -T-Evolution and origin of zoning in melilite during the regressive stage of contact metamorphism in carbonate-bearing rocks

The article describes the thermal metamorphism of siliceous carbonate rocks near the dolerite intrusive body in Eastern Siberia. The mineral associations at the immediate contact with dolerite are the following: wollastonite+rankinite, rankinite+spurrite (+melilite?), spurrite+melilite+merwinite+calcite and merwinite+monticellite+melilite+calcite. The melilite in these associations is usually unzoned; its composition being essentially gehlenitic. During the regressive stage of contact metamorphism new akermanite-rich melilite and calcite were formed by replacement of merwinite and earlier gehlenitic melilite through participation of CO₂. The newly forming melilite grains have sharp compositional zoning. The origin of zoning was connected with the fall of temperature and decrease of the mole fraction of CO₂ in the fluid equilibrated with the minerals.     

Associations of Mn-bearing minerals as indicators of oxygen fugacity during the metamorphism of metalliferous deposits

The paper summarizes experimental and calculation data on the effect of oxygen fugacity on the origin of mineral assemblages in Mn-bearing rocks and demonstrates the possibility of application of these data to the reconstruction of conditions under which metalliferous deposits were metamorphosed. A new variant of the T-log\(f_{O_2 } \) diagram is proposed for the Mn-Si-O system, which differs from previous ones by the location of the lines for the formation (decomposition) of braunite and tephroite. These two minerals are the most universal indicators of oxygen fugacity during the metamorphism of Mn-bearing deposits, because these minerals are widespread in nature and can be formed in diverse environments: braunite at high \(f_{O_2 } \) values in the pore solution, and tephroite at low \(f_{O_2 } \) values. The occurrence of Mn oxides and rhodonite (pyroxmangite) in a rock makes it possible to constrain the oxygen fugacity range. An original T-log\(f_{O_2 } \) diagram is constructed for the Ca-Mn-Si-O system. As follows from this diagram, a Ca admixture expands the stability field of rhodonite toward higher oxygen fugacity values. Johannsenite can be formed in these rocks at even higher \(f_{O_2 } \). The stability of both minerals is constrained in the region of low \(f_{CO_2 } \). The paper reports data on the Fe-Si-O and Mn-Fe-Si-O systems and discusses the possibility of applying the results of experiments in the Mn-Al-Si-O system to the estimation of conditions under which andalusite, spessartine, and galaxite can be formed in Mn-bearing rocks. Data on the mineralogy of numerous Mn deposits metamorphosed under various PTX parameters indicate that the origin of Mn-bearing mineral assemblages depends not so much on the temperature and pressure as on the oxygen fugacity, which is, in turn, controlled primarily by the composition of the pristine sediments (the presence or absence of organic matter in them) and host rocks and depends on the permeability of the rocks to oxygen, the P-T conditions, and the duration of the metamorphic processes.     

Fluid infiltration and regional metamorphism of the Waits River Formation, north-east Vermont, USA

Abstract The Siluro-Devonian Waits River Formation of north-east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite-albite, (2) ankerite-oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite-albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χ_co2[= CO₂/(CO₂+ H₂O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite-oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χ_CO2 decreases to 0.06. Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole-rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χ_CO2 > 0.50. The χ_CO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H₂O-rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H₂O-rich fluid drove mineral reactions during metamorphism. Average time-integrated volume fluxes (cm³ fluid/cm² rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite-oligoclase zone: c. 1 × 10⁴; (2) biotite zone: c. 3 × 10⁴; (3) amphibole zone: c. 10 × 10⁴; and diopside zone: c. 60 × 10⁴. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation. The mapped pattern of time-integrated fluxes indicates that the Strafford-Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χ_CO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H₂O-rich fluids given off by the crystallizing magmas, and CO₂-H₂O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton-driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system.     

Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (X_CO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al₂O₃–SiO₂–H₂O–CO₂, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower X_CO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.     

Thermal metamorphism and H2O-CO2-NaCl immiscibility at Patapedia,Quebec: evidence from fluid inclusions

The methamorphic history of the Patapedia thermal zone, Gaspé, Quebec, is re-evaluated in the light of results obtained from a study of fluid inclusions contained in quartz phenocrysts of felsic dyke rocks. The thermal zone is characterised by calc-silicate bodies that have outwardly telescoping prograde metamorphic isograds and display extensive retrograde metamorphism with associated copper mineralization. Three distinct fluid inclusion types are recognized: a low to moderate salinity, high density aqueous fluid (Type I); a low density CO₂ fluid (Type II); and a high salinity, high density aqueous fluid (Type III). Fluid inclusion Types I and II predominate whereas Type III inclusions form <10% of the fluid inclusion population. All three fluid types are interpreted to have been present during prograde metamorphism. Temperatures and pressures of metamorphism estimated from fluid inclusion microthermometry and isochore calculations are 450°–500° C and 700–1000 bars, respectively. A model is proposed in which the metamorphism at Patapedia was caused by heat transferred from a low to moderate salinity fluid of partly orthomagmatic origin (Type I inclusions). During the early stages, and particularly in the deeper parts of the system, CO₂ produced by metamorphism was completely miscible in the aqueous hydrothermal fluid and locally resulted in high XCO₂ fluids. On cooling and/or migrating to higher levels these latter fluids exsolved high salinity aqueous fluids represented by the Type III inclusions. Most of the metamorphism, however, took place at temperature-pressure conditions consistent with the immiscibility of CO₂ and the hydrothermal fluid and was consequently accompanied by the release of large volumes of CO₂ vapour which is represented by Type II inclusions. The final stage of the history of the Patapedia aureole was marked by retrograde metamorphism and copper mineralization of a calcite-free calc-silicate hornfels in the presence of a low XCO₂ fluid.     

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex,Spain)

At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO₂ contents at nearly constant Mg/Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO₂ ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H₂O–CO₂ fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H₂O and molecular CO₂ are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H₂O–CO₂ fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)‐serpentinite‐hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration‐driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite‐hosted carbonate–silicate rocks beyond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle.     

Short‐duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island,Canada

This contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break‐up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ～50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1–4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T–X(CO₂)^fluid pseudosections that suggest achievement of CO₂‐rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near‐peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high‐grade portions of aureoles is consistent with eventual levelling‐off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion‐wall rock contacts is demonstrated by narrow zones of anomalously low δ¹³C and δ¹⁸O values of carbonate minerals. Although the up to 5‰ decrease of both δ¹³C and δ¹⁸O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in δ¹³C is attributed to fluid‐mediated exchange with organic low‐¹³C carbon that is dispersed through the unmetamorphosed rocks and the decrease in δ¹⁸O is attributed to fluid‐mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high‐temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T–X(CO₂)^fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation reactions to influx of aqueous fluids during cooling.     

Retrograde fluid evolution at the Rampura Agucha Pb-Zn-(Ag) deposit, Rajasthan, India

Fluid inclusions in mineralized graphite-sillimanite-mica schist from the Rampura-Agucha Pb-Zn-(Ag) deposit, Rajasthan, northwest India, have been investigated by microthermometry and Raman microspectrometry. Three different main types of fluid inclusions in quartz can be distinguished: (1) gaseous (CO₂, partially mixed with CH₄-N₂), (2) low salinity aqueous inclusions (0–8 eq. wt% NaCl) and (3) high salinity aqueous inclusions (NaCl ± MgCl₂-CaCl₂). Low density CO₂-rich and low salinity H₂O inclusions are contemporaneous and occur, together with CH₄-N₂ inclusions, in close association with sulfide mineral inclusions. This indicates immiscibility between the gaseous and aqueous phase and participation of these fluids during the deposition or remobilization of the ore, which occurred over a wide P (1220 to 200 bar) and T (450 to 250 °C). Raman spectra of graphite indicate upper greenschist-facies metamorphic conditions, although host rocks have been metamorphosed at upper amphibolite-facies metamorphic conditions. This indicates that graphite re-equilibrated with the CO₂-rich phase during retrograde metamorphism.     

An unusual manganese silicate occurrence at the Hoskins mine,Grenfell district,New South Wales

Stratiform manganese silicate rocks overlie jasper and metabasah in the ?Middle Silurian Hoskins Formation at the Hoskins manganese mine near Grenfell, NSW. Two dominant mineral assemblages occur in the Mn silicate rocks: (1) a “reduced’ assemblage, probably gradational into underlying jasper, containing abundant rhodonite and/or tephroite, plus subordinate carbonates, quartz, hausmannite, spessartine and Ba minerals, and (2) a well‐laminated ‘oxidized’ assemblage rich in red Mn‐rich alkali pyroxene and amphibole, braunite, manganoan pectolite and minor Mn‐rich mica, alkali feldspars, carbonates, quartz and barite. Several Mn silicates implicitly contain trivalent Mn. The Mn silicate rocks are rich in Mn, Ba and Sr, and also contain anomalously high Co, Cu, As and W; oxidized assemblages are alkali‐rich. Bulk compositions and geological setting suggest a submarine volcanic exhalative origin for the precursors of the Mn silicate rocks and jasper. Metamorphism has occurred at upper greenschist facies with original high oxygen fugacity conditions in the exhalative sediments being largely reflected in the resulting assemblages. Although analogues of the reduced Mn silicate rocks are widespread in metamorphosed Mn deposits, equivalents of the oxidized assemblages appear to be particularly uncommon.