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Charnockites (orthopyroxene-bearing granitoids) constitute one of the major components in many Precambrian high-grade metamorphic terranes of the world. They are broadly classified into two types based on the nature of occurrence as massive charnockite that forms large (magmatic) batholiths, and incipient charnockite which occurs as patches, veins and lenses of mesoscopic scale developed through local dehydration reactions within felsic (both igneous and sedimentary) protoliths. Here we present petrological data on representative charnockites of different ages and tectonic settings from various parts of the world and evaluate their formation conditions based on phase equilibrium modeling in the system NCKFMASHTO. Massive charnockites ranging from Neoarchean to Paleoproterozoic age from the Salem and Nagercoil Blocks (southern India), Napier Complex (Antarctica), and the Limpopo Complex (Zimbabwe) record P-T conditions that are broadly equivalent to the regional P-T conditions of high-grade metamorphism recorded in these terranes, ranging from normal granultes to ultrahigh-temperature metamorphic rocks. Typical unmetamorphosed magmatic charnockite in a late Paleoproterozoic post-collisional extensional setting and carrying K-feldspar phenocrysts from the North China Craton yield a wide P-T stability range. Incipient charnockite from southern India records P-T conditions that are slightly lower than the peak P-T regimes estimated for the region. Regardless of the discrepancies in P-T conditions estimated from the phase equilibria approach, the data confirm that these rocks, in all the cases considered in this study, were generated under low aH2O conditions. The available fluid inclusion data from both massive and incipient charnockites confirm that the water activity was buffered to low levels, possibly through the presence of CO2-rich fluids, to stabilize the index mineral orthopyroxene in these rocks. Charnockites formed in different plate tectonic settings; in most cases as arc magmas within subduction-collision settings and some cases, post-collisional extension during different periods in Earth history, and serve as potential archives of melt and fluid processes in the continental crust.

In this study, we report a high concentration of cobalt (Co) in the rocks of Ajabgarh Group of Delhi Supergroup from Nasibpur and the surrounding areas of Southwest Haryana, India, which forms a part of the North Delhi Fold Belt (NDFB). Metasedimentary and magmatic phases of the rocks contained high cobalt content ranging from 166 to 3657 ppm. The maximum concentration of cobalt (2371-3657 ppm) was observed in quartzite samples from the Nasibpur area. Cobalt enrichment in these rocks can be attributed to magmatic-hydrothermal and metamorphic fluids in relation to geological features such as shear and foliation zones, which provide a high fluid/rock ratio. Overall, the applications of cobalt are numerous and crucial. The present study warrants further extensive exploration efforts in order to assess the abundance of this valuable metal, as the global cobalt market is increasing in response to a low-carbon economy. [ABSTRACT FROM AUTHOR]

The solubility of uranium (U) in hydrothermal fluid is thought to be controlled by oxidation. In general, uranium is mainly transported as U(VI) in oxidized fluid, but precipitated as U(IV) in reduced fluid. However, many geological observations indicate that metamorphic fluids, which are buffered by metamorphic rocks with oxidized protoliths such as oxidized pelite or altered marine basalt, are not enriched in U. To explore the reason of the low solubility of U in metamorphic fluids, we simulated the hydrous speciation and solubility of U in fluids that are in equilibrium with rocks. The simulations were conducted at pressure–temperature (P-T) conditions of greenschist and amphibolite facies metamorphism. The results show that U is mainly dissolved as U(IV), instead of U(VI), in metamorphic fluids. The solubility of U remains at a low level of ~10−12 molal, and is not significantly influenced by metamorphic temperature, pressure, and fluid salinity. This result is consistent with geological observations and, thus, can explain the low-U nature of natural metamorphic fluids. The simulation also shows high solubility of U(VI) (1.3 × 10−7 molal) in oxidized pelite-buffered fluids at low temperature (<250 °C), consistent with the geological fact that U can be mobilized by low-temperature geofluids. [ABSTRACT FROM AUTHOR]

[Display omitted] • VHMS belong to an ensimatic Tonnian arc and cannot be considered as a gold-rich VHMS. • Gold is introduced in a late metamorphic brittle stage, part of the AN Shield gold province. • Gold is exploitable mainly in gossans. Gold in the Ariab district (Red Sea Hills, Sudan) is mined in gossans developed over volcanogenic massive sulphide (VHMS) deposits formed in the context of a Tonian ensimatic arc of the Haiya terrane, in the Neoproterozoic Arabo-Nubian Shield. All the primary gold occurrences of the Ariab belt, whatever their style (VHMS-, quartz lode- or shear-zone-hosted), appear, however, to have been syn -to late-kinematic, i.e., formed in the context of the final Pan-African collision leading to the Arabian-Nubian Shield (D3 event), and therefore are post-VHMS. Gold is thus found associated with either galena or bismuth telluride in late syn- to post-metamorphic sets of (micro)cracks, mostly within earlier pyrite or arsenopyrite. Gold deposition as native gold or electrum occurred when an intense fluid circulation of metamorphic CO 2 -H 2 O rich fluids underwent significant decompression from lithostatic to hydrostatic pressures at temperatures decreasing from 400 to 430 °C at around 8 km depth down to 300 °C. Although no clear evidence of volatile unmixing was observed, the decompression, together with a temperature decrease due to dilution by aqueous rich fluids, favoured gold deposition. The late-D3 gold event in the Ariab belt has all the makings of an orogenic gold system, notably similar to the Late Carboniferous gold event in the West European Variscan belt. Thus, it is possible to propose that, at the end of the collision events, fluids released from the newly formed terrane root were channelled towards the upper crust through the major D3 shear zones such as the Oko shear zone. The fluids outpouring from these major drains were then conveyed through fault plays and damaged zones, up to specific chemical traps within massive sulphide ore bodies or quartz veins systems. Thus, the Ariab gold district could be classified as a representative of the class of orogenic gold deposits. [ABSTRACT FROM AUTHOR]

The interaction of fluids with rock-forming minerals plays an important role in the chemical evolution of mafic and ultramafic rocks in the lower crust and upper mantle. Recent work highlights the importance of salt-rich fluids in element transport in settings such as the mantle wedge above subduction zones and high-grade granulite facies metamorphism. Forsterite (Mg 2 SiO 4), enstatite (MgSiO 3) and diopside (CaMgSi 2 O 6) are key rock-forming minerals in these settings in the system CaO-MgO-SiO 2. We determined experimentally the solubilities of diopside, enstatite and forsterite in H 2 O-NaCl fluids at a range of pressures and temperatures. Forsterite solubility was determined at 1 GPa, 800 and 900 °C, in pure H 2 O and in H 2 O-NaCl solutions. Forsterite dissolved congruently at nearly all conditions. Its solubility in pure H 2 O is low, but increases greatly with rising NaCl concentration in the fluid. Enstatite solubility was investigated in H 2 O-NaCl solutions at 1 GPa, 800 and 900 °C. Enstatite dissolved incongruently to yield forsterite at all conditions. Addition of excess silica led to suppression of forsterite and showed that fluids in equilibrium with enstatite with or without forsterite are strongly enriched in Si relative to Mg, though Mg solubility is significant at high salinity. Diopside solubility was determined in pure H 2 O at 650–900 °C and 0.7–1.5 GPa, and in H 2 O-NaCl solutions at 800 °C and 1 GPa, with NaCl concentrations approaching halite saturation. Diopside dissolves incongruently yielding residual forsterite at all conditions investigated. The solubility of diopside in pure H 2 O increases with increasing pressure, temperature and salinity. Diopside dissolution in H 2 O-NaCl solutions displays a dependence on fluid salinity similar to that of forsterite and wollastonite. The results of forsterite solubility experiments in H 2 O-NaCl solutions were used to calculate the compositions of fluid coexisting with enstatite or diopside where forsterite was present. The concentration of solutes coexisting with enstatite decreases with rising NaCl, similar to quartz. In contrast, bulk solutes coexisting with diopside increase with NaCl, similar to wollastonite and forsterite. These patterns imply complexing among rock-forming components and fluid components, that Ca-chloride species are substantially more stable than Mg-chloride species, and that hydrous Na-silicate complexes are important components of deep H 2 O-NaCl fluids. The results show that salt-bearing brines have substantial metasomatic power and may exert significant control on the chemical evolution of lower crustal and upper mantle mafic and ultramafic rocks. [ABSTRACT FROM AUTHOR]

• Cl and SO 4 -rich metamorphic scapolite imply saline and oxidised metamorphic fluids. • Cl-rich rims on scapolite imply that fluid salinity increased during metamorphism. • Increase in salinity likely due to phase separation of a complex NaCl-H 2 O-CO 2 fluid. • Metaevaporitic terranes maintain fertility through prolonged orogenesis. • Metamorphic fluids from evaporites can generate a range of mineralisation styles. Detailed petrologic and chemical investigation of mid-amphibolite facies calcareous, scapolite-rich metasedimentary rocks from the Mount Isa region in northern Australia is used to explore changing fluid chemistry with prograde metamorphism. The presence of widespread scapolite with Cl- and variably SO 4 -rich compositions in upper amphibolite facies rocks makes it unavoidable that the regional metamorphic fluids were locally highly saline and oxidised, and that high salinities persisted throughout metamorphism. Electron microprobe analyses and chemical maps of individual scapolite grains show zoning in Cl and S, likely to reflect buffering of the metamorphic fluid by scapolite during progressive metamorphism. The zoning in Cl and S demonstrates that scapolite has the potential to record changes in fluid chemistry during metamorphism. The variation in scapolite composition between samples, in combination with whole rock geochemistry, shows that different layers within this heterogenous rock package generated fluids of different chemistries. Interaction between scapolite-bearing rocks and externally-derived magmatic or metamorphic fluids that are out of equilibrium drives scapolite breakdown, releasing Cl to the fluid. In the Mount Isa region, metamorphic fluid production was enhanced by periods of magmatism, which promoted development of a regionally extensive and unusually saline fluid system that was active at multiple stages over a 250 million-year period. The highly saline and oxidised fluids formed through interaction with scapolite are well suited to transporting a broad range of metals, and may explain the diverse range of syn-orogenic mineral deposits in the Mount Isa Inlier. Metamorphic belts with large volumes of evaporitic material are ideal for generating a broad spectrum of syn-orogenic hydrothermal ore deposit types - including Fe oxide Cu-Au, Fe sulphide Cu-Au, Mo-Re and U-REE, but lacking the Au-only deposits found in typical orogenic belts. Unlike regions hosting traditional orogenic gold deposits, belts containing evaporitic sequences can preserve Cl-rich minerals such as scapolite in the metamorphosed source region, allowing them to remain active as ore forming systems through relatively high-grade metamorphism and multiple stages of tectonism. Periods of supercontinent breakup, such as the Mesoproterozoic, may have resulted in the formation of large, intracontinental basins well suited to the development of widespread evaporitic sequences. This, in combination with overprinting orogenesis and high temperature magmatism, may have provided the ingredients for widespread ore deposit formation at a global scale. [ABSTRACT FROM AUTHOR]

The term orogenic gold deposits has been widely accepted, but there has been continuing debate on their genesis. Early syn-sedimentary or syn-volcanic models and hydrothermal meteoric-fluid models are now invalid. Magmatic-hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships. The most plausible models involve metamorphic fluids, but the source of these fluids is equivocal. Intra-basin sources within deeper segments of the hosting supracrustal successions, the underlying continental crust, subducted oceanic lithosphere with its overlying sediment wedge, and metasomatized lithosphere are all potential sources. Several features of Precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic-fluid source. These include the presence of hypozonal deposits in amphibolite-facies domains, their anomalous multiple sulfur isotopic compositions, and problems of derivation of gold-related elements from devolatilization of dominant basalts in the sequences. The Phanerozoic deposits are largely described as hosted in greenschist-facies domains, consistent with supracrustal devolatilization models. A notable exception is the Jiaodong gold deposits of China, where ca. 120-Ma gold deposits are hosted in Precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization. Other deposits in China are comparable to those in the Massif Central and elsewhere in France, in that they are hosted in amphibolite-facies domains or clearly post-date regional metamorphic events imposed on hosting supracrustal sequences. If all orogenic gold deposits have a common genesis, the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package, or the associated metasomatized mantle wedge, with CO2 released during decarbonation and S- and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. Although this model satisfies all geological, geochronological, isotopic, and geochemical constraints, and is consistent with limited computer-based modeling of fluid release from subduction zones, the precise mechanisms of fluid flux are model-driven and remain uncertain. From an exploration viewpoint, the model re-emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction-related orogenic belts and importance of continental-scale lithosphere-tapping fault and shear zones to focus large volumes of auriferous fluid. It confirms the importance of the consistent spacing between world-class deposits, broadly equivalent to the depth of the Moho, as derived from empirical observations. [ABSTRACT FROM AUTHOR]

The greenschist-amphibolite facies transition zone within metabasites is of great importance due to the common occurrence of this transition within greenstone belts, and the proposition that fluids released across the transition may be involved in orogenic gold deposit formation. In this work, the nature of devolatilization reactions occurring across the greenschist-amphibolite facies transition zone is assessed from petrological examination and thermodynamic modelling of an exceptionally exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan). The sequence at Flin Flon comprises an intact metamorphic sequence of ~10 km length spanning the greenschist and lower amphibolite facies, the latter of which can be subdivided into three zones (from S to N): the hornblende-actinolite zone, the hornblende-actinolite-oligoclase zone, and the hornblende-oligoclase zone, demarcated by the hornblende-in, oligoclase-in and actinolite-out isograds. The crossing of the hornblende-in isograd is associated with small amounts of hornblende growth, relatively little change in the proportions of chlorite, and negligible fluid loss. By contrast, significant changes in the modal mineralogy occur across the 1500 m wide hornblende-actinolite-oligoclase zone, with the breakdown of approximately 75% of the chlorite and the loss of approximately 1-2 wt% H 2 O. There is no textural or modal evidence for significant loss or gain of carbonate or sulphide minerals going through this interval. Petrological estimates of devolatilization across the greenschist-amphibolite facies transition zone are compared with predictions from thermodynamic modelling. Although there are differences between the two, both indicate that the fluid loss is strongly tied to variation in bulk composition, the latter predominantly the result of pre-metamorphic alteration processes. High-Mg and high-Ca basalts, representative of pillow rim and core material, undergo an average of 1.8 wt% (modelling estimate: 2.2 wt%) and 1.1 wt% H 2 O loss (modelling estimate: 1.8 wt%), respectively, across the hornblende-actinolite-oligoclase zone. T-XCO 2 modelling predicts that the XCO 2 content of fluid produced from the hornblende- and oligoclase-producing reactions is low (<0.08) and is not buffered to higher XCO 2 compositions, consistent with the lack of carbonate breakdown documented within the Flin Flon sequence over this interval. Fluid buffering path modelling predicts significant volumes of fluid release with higher XCO 2 contents (>0.2) above the greenschist-amphibolite facies transition zone within samples containing high carbonate contents (>5%). However, in contrast to the modelling, the majority of chlorite (>75%) breaks down at the main greenschist-amphibolite facies transition zone in the Flin Flon sequence, and most samples have low carbonate contents (1.5% average), limiting the volumes of fluids with higher XCO 2 compositions that can be generated at higher grades. Thus, whilst the greenschist-amphibolite facies transition zone at Flin Flon is the site of significant devolatilization (1-2 wt%) over a small interval of P-T space, is does not appear to have been accompanied by significant carbonate and sulphide breakdown, and the fluids generated across this interval were CO 2 -poor. If the Flin Flon sequence is representative of other metamorphosed greenstone belts, it may be that metabasites metamorphosed across the greenschist-amphibolite facies transition zone do not, in general, release the CO 2 – and gold hydrosulphide-bearing fluids characteristic of orogenic gold deposits. • 1-2 wt% fluid loss across transition zone, mainly resulting from chlorite breakdown. • Devolatilization focused over narrow interval (~1500 m) above oligoclase-in isograd. • No evidence for carbonate or sulphide breakdown over devolatilization interval. • Fluids released interpreted to have a low XCO 2 composition. • Fluid produced at Flin Flon unlikely to be fluid source for orogenic gold deposits. [ABSTRACT FROM AUTHOR]

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension. [ABSTRACT FROM AUTHOR]

We found that the particle RA-QD02-0115 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains the iron carbide haxonite (Fe 21.9-22.7 Co 0.2-0.3 Ni 0.2-0.8 )C 6 and several Fe,Ni alloys, including multi-domain tetrataenite and spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 Itokawa. The calculated solubility of carbon in Fe,Ni metal indicates that the carbide formed at temperatures larger than 600 °C during thermal metamorphism of the LL-chondritic mineral assemblage. Haxonite formed metastably with respect to graphite and cohenite, probably due to its high degree of lattice match with neighboring taenite, a low cooling rate at peak metamorphic temperatures, and the hindered nucleation of graphite. Thermodynamic equilibrium calculations indicate that the fluid present was dry (H 2 O-poor) and dominated by methane. The reactive fluid most plausibly had an atomic H/C ratio of 4–5 and was derived from the reduction of macromolecular, insoluble organic matter (IOM) that initially co-accreted with water ice. The initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed IOM. Metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high-temperature stage of metamorphism. An exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as: 330 CH 0.8 O 0.2(IOM)  + 500 H 2 O (ice/g)  + 681 Fe (in alloy)  + 566 FeSiO 3(in Opx)  → 300 CH 4(g)  + 32 H 2(g)  + 5 Fe 23 C 6(in Hx)  + 566 Fe 2 SiO 4(in Ol) (Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species). The best estimate of the fluid/rock ratio in the region of the LL parent body where RA-QD02-0115 formed is about 3 × 10 −3 and corresponds to an initial ice/rock ratio of about 7 × 10 −3 (both by mass). [ABSTRACT FROM AUTHOR]

Liancheng is an important Cu–Mo deposit hosted by red-beds in the Mesozoic to Cenozoic Lanping Basin located in the Sanjiang Tethyan Orogen of SW China. The deposit is fault controlled where it has been affected and enriched by four hydrothermal events indicated by four distinct sets of veins and their cross-cutting relationships. A detailed Cu–S–H–O isotopic study is reported here on the mineralization focusing on the source of the metals and fluid, and on the mineralizing processes. The first two hydrothermal events are characterised by CO 2 and fluid-rich inclusions with homogenisation temperatures of 124–446 °C and salinities ranging from 0.4 to 24.6 wt% NaCl equiv. The next two hydrothermal events are characterised by liquid–rich fluid inclusions that homogenise at 145–256 °C, with salinities ranging from 1.4 to 22.9 wt% NaCl equiv. The H–O isotope values obtain from the fluid inclusions indicate that the first three hydrothermal events relate to mineralizing fluids derived from a metamorphic source, but the last event includes a mixed metamorphic and meteoric source. Chalcopyrite, tennantite, and bornite from the deposit have δ 34 S values between  −11.5 and 3‰. The δ 65 Cu values for quartz-Cu sulfide mineralization associated with the second hydrothermal event range from −0.31 to 0.2‰, but carbonate-Cu sulfide mineralization associated with the third hydrothermal event ranges from −1.01 to −1.08‰. The combination of the Cu and S isotopic values and field observations indicate that the controlling factors include structures, the temperature of regional metamorphism, and the pH of the mineralizing fluid. The zation is, therefore, characteristic of an orogenic-type deposit. [ABSTRACT FROM AUTHOR]

Orogenic style gold mineralizations in the Archean Hattu Schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary and volcanogenic rocks, as well as felsic intrusives. The gold mineralizations occur as dissemination in altered wall rocks and within hydrothermal quartz veins. Hydrothermal tourmalines are often associated with the gold mineralizations occurring in the quartz veins, as alteration minerals in contacts of the metasedimentary host rocks and quartz veins, but also within mineral assemblages of the metasedimentary and magmatic rocks. We characterize and compare the major, trace and rare earth element chemistry of these tourmalines in order to evaluate their suitability as a petrogenetic tool for tracing the fluid sources of the hydrothermal system. By comparing the chemistry of tourmalines from gold mineralization with those from metamorphic and magmatic host rocks, we test whether tourmaline composition can be used to identify the source rocks of the gold transporting hydrothermal fluids. All analyzed tourmalines belong to the alkali super group and plot along the schorl-dravite join. The hydrothermal tourmalines have Li, Sr and V concentrations comparable to metamorphic tourmalines and clearly distinct from magmatic tourmalines. Because the concentrations of Ni, Pb, Cr, Mn, Ga, Zn and Sn show a wide overlap between magmatic, metamorphic and hydrothermal tourmalines, these elements do not permit to discriminate between different source rocks. The co-variations of many elements in hydrothermal tourmalines, when plotted against Li, show more similarity to metamorphic tourmalines and the whole-rock compositions of metasedimentary and metavolcanic host rocks than to their magmatic counterparts. This indicates that the major and trace element composition of hydrothermal tourmalines in the Hattu Schist Belt is predominantly controlled by the host rocks and local fluid-rock interactions, and does not reflect the distal fluid sources. The effect of local fluid-rock interaction is also manifested by the REE patterns of tourmalines. The magmatic tourmalines have distinct LREE enriched patterns resembling the whole-rock REE patterns of granitic intrusives, while the metamorphic and hydrothermal tourmalines have flat or weakly fractionated patterns similar to metavolcanic and metasedimentary host rocks. Taken together, the tourmaline data suggest that hydrothermal tourmalines associated to gold mineralizing fluids are most likely genetically related to metamorphic rock sources without important contributions of magmatic fluids, and that local fluid-rock interaction exerted a major control on tourmaline chemistry. [ABSTRACT FROM AUTHOR]

Monazite (CePO4) and xenotime (YPO4) are important hosts for REE and thus can be used to monitor REE mass transfer in various settings. In this investigation, the solubilities of synthetic monazite and xenotime were measured in KCl-H2O fluids at 800 °C and 1.0 GPa, using the piston-cylinder apparatus. The experimental results indicate an increase in monazite and xenotime solubility in aqueous fluids with moderate KCl mole fractions (XKCl) in agreement with previous investigations of the solubility of these phases in NaCl-H2O. Under all conditions, monazite and xenotime dissolve congruently. The solubility of synthetic monazite increases from 8 ppm in pure H2O to 335 ppm at XKCl = 0.506. The solubility of synthetic xenotime rises from 46 ppm in pure H2O to 126 ppm at XKCl = 0.348, above which it is constant or declines slightly. Monazite and xenotime solubilities are considerably lower in KCl-H2O than in NaCl-H2O at the same salt concentration. Best-fit equations for the solubilities of the two phases are: cmz = -464 X²KCl + 891 XKCl + 8 and cxt = -563 X²KCl + 432 XKCl + 46 where mz and xt stand for monazite and xenotime, and XKCl = nKCl/(nKCl + nH2O) where n is moles. The change in solubilities with KCl implies that Ce dissolves as an anhydrous chloride complex (CeCl3), whereas Y forms a mixed Cl-OH solute [YCl(OH)2]. The data also imply that H2O-NaCl fluids and H2O-KCl fluids close to neutral pH can transport substantial amounts of REE and Y, thus obviating the need to invoke low-pH solutions in high-grade environments where they are highly unlikely to occur. [ABSTRACT FROM AUTHOR]

Apatite is an important host for LREE, F, and Cl and thus can be used to monitor elemental mass transfer in high P-T settings. In this investigation, the solubilities of synthetic fluor- and chlorapatite and natural Durango fluorapatite were measured in H 2 O, KCl-H 2 O, and NaCl-H 2 O fluids at 800 °C and 1.0 GPa, using the piston-cylinder apparatus. The solubility of apatite is reported in parts/million by weight (ppm), and is calculated via the formula m Ap = [(∆ m Ap / H 2 O + KCl or NaCl + ∆ m Ap ) ∗ 10 6 ], where ∆ m Ap = m Ap-in − m Ap-out . The experimental results indicate a strong increase in apatite solubility in aqueous fluids with moderate KCl and NaCl mole fractions (X KCl and X NaCl ). Under all conditions, synthetic fluorapatite and synthetic chlorapatite dissolve congruently. The solubility of synthetic fluorapatite increases from 19 ppm in pure H 2 O to 2123 ppm at X KCl = 0.52. The solubility of synthetic chlorapatite rises from 37 ppm in pure H 2 O to 2590 ppm at X KCl = 0.50. At X KCl > 0.5 the solubility of synthetic fluorapatite decreases to 1691 (X KCl = 0.72) and the solubility of synthetic chlorapatite decreases to 2284 ppm (X KCl = 0.65). In contrast, natural Durango fluorapatite dissolves incongruently at X KCl < 0.2 to monazite + fluid and congruently at X KCl > 0.2. The solubility of natural Durango fluorapatite increases from 70 ppm in pure H 2 O to 2073 ppm at X KCl = 0.50 respectively. In contrast to the KCl-H 2 O system, apatite solubilities are considerably higher in the NaCl-H 2 O system. The solubility of synthetic fluorapatite increases to 4909 ppm at X NaCl = 0.51 and the solubility of synthetic chlorapatite rises to 5144 ppm at X NaCl = 0.50. Compared to fluorite, calcite and anhydrite, apatite solubility is significantly lower in H 2 O, NaCl-H 2 O, and KCl-H 2 O. After completion of each experiment, the pH was measured and a comprehensive step-by step procedure (limiting reactant concept) was used to model the observed quench pH in the system apatite-KCl-H 2 O. [ABSTRACT FROM AUTHOR]

The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912 and the Barrovian metamorphic zones continue to be used as a framework for describing regional metamorphism. Pressure, temperature and protolith composition are widely recognized as primary controls on index mineral distribution. Today, metamorphic fluid flow is also recognized as an important driver of metamorphic reactions. The aim of this study is to establish if and how metamorphic fluids control index mineral distribution during Barrovian metamorphism. We use samples from Barrow's type locality in Glen Esk, SE Scottish Highlands, to study possible relationships between veining and index mineral distribution. In addition to petrographic and textural observations, we use whole-rock compositions, mineral compositions and oxygen isotope analyses. At low grade, in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. At higher grade, in the garnet and staurolite zones, index mineral abundance is shown to be higher adjacent to veins. These trends coincide with other mineralogical, chemical and isotopic changes in the vein-proximal rock, indicative of fluid-rock interaction. Kyanite distribution is homogeneous in the kyanite zone. However, we show that this too relates to extensive fluid-rock interaction. Garnet-, staurolite and kyanite-bearing selvedges are common in the sillimanite zone. However, sillimanite distribution is unrelated to these selvedges, which supports models arguing that sillimanite formed during a separate metamorphic event. We infer fluid flow from high grade to low grade because the fluid was out of isotopic equilibrium with the lower grade rocks, but in equilibrium with the higher grade rocks. We conclude that fluid flow played a major role in the stabilization and distribution of Barrovian index minerals in Glen Esk, and that the importance of fluid flow was greater at higher metamorphic grades. [ABSTRACT FROM AUTHOR]

The chemical evolution of fluids in Alpine fissure veins (open cavities with large free-standing crystals) has been studied by combination of fluid inclusion petrography, microthermometry, LA- ICPMS microanalysis, and thermodynamic modeling. The quartz vein systems cover a metamorphic cross section through the Central Alps (Switzerland), ranging from subgreenschist- to amphibolite-facies conditions. Fluid compositions change from aqueous inclusions in subgreenschist- and greenschist-facies rocks to aqueous-carbonic inclusions in amphibolite-facies rocks. The fluid composition is constant for each vein, across several fluid inclusion generations that record the growth history of the quartz crystals. Chemical solute geothermometry, fluid inclusion isochores, and constraints from fluid-mineral equilibria modeling were used to reconstruct the pressure-temperature conditions of the Alpine fissure veins and to compare them with the metamorphic path of their host rocks. The data demonstrate that fluids in the Aar massif were trapped close to the metamorphic peak whereas the fluids in the Penninic nappes record early cooling, consistent with retrograde alteration. The good agreement between the fluid-mineral equilibria modeling and observed fluid compositions and host-rock mineralogy suggests that the fluid inclusions were entrapped under rock-buffered conditions. The molar Cl/Br ratios of the fluid inclusions are below the seawater value and would require unrealistically high degrees of evaporation and subsequent dilution if they were derived from seawater. The halogen data may thus be better explained by interaction between metamorphic fluids and organic matter or graphite in metasedimentary rocks. The volatile content ( CO2, sulfur) in the fluid inclusions increases systematically as function of the metamorphic grade, suggesting that the fluids have been produced by prograde devolatilization reactions. Only the fluids in the highest grade rocks were partly modified by retrograde fluid-rock interactions, and all major element compositions reflect equilibration with the local host rocks during the earliest stages of postmetamorphic uplift. [ABSTRACT FROM AUTHOR]

The Wulasigou Cu-Pb-Zn deposit, located 15km northwest of Altay city in Xinjiang, is one of many Cu-Pb-Zn polymetallic deposits in the Devonian Kelan volcanic-sedimentary basin in southern Altaids. Two mineralizing periods can be distinguished: the marine volcanic sedimentary Pb-Zn mineralization period, and the metamorphic hydrothermal Cu mineralization period, which is further divided into an early bedded foliated quartz vein stage (Q1) and a late sulfide-quartz vein stage (Q2) crosscutting the foliation. Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit: H2O-CO2 inclusions, carbonic fluid inclusions, aqueous fluid inclusions, and daughter mineral-bearing fluid inclusions. Microthermometric studies show that solid CO2 melting temperatures ( Tm,co2) of H2O-CO2 inclusions in Q1 are from −62.3°C to −58.5°C, clathrate melting temperatures ( Tm,clath) are from 0.5°C to 7.5°C, partial homogenization temperatures ( Th,co2) vary from 3.3°C to 25.9°C (to liquid), and the total homogenization temperatures ( Th,tot) vary from 285°C to 378°C, with the salinities being 4.9%-15.1% NaCl eqv. and the CO2-phase densities being 0.50-0.86 g/cm3. H2O-CO2 inclusions in Q2 have Tm,c02 from −61.9°C to −56.9°C, Tm,clath from 1.3°C to 9.5°C, Th,co2 from 3.4°C to 28.7°C (to liquid), and Th,tot from 242°C to 388°C, with the salinities being 1.0%-15.5% NaCl eqv. and the CO2-phase densities being 0.48-0.89 g/cm3. The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and 180-370 MPa, respectively. The δ34S values of pyrite from the volcanic sedimentary period vary from 2.3‰ to 2.8‰ (CDT), and those from the sulfide-quartz veins fall in a narrow range of −1.9‰ to 2.6‰ (CDT). The δD values of fluid inclusions in Q2 range from −121.0‰ to −100.8‰ (SMOW), and the δ18OH2O values calculated from δ18O of quartz range from −0.2‰ to 8.3‰ (SMOW). The δD- δ18OH2O data are close to the magmatic and metamorphic fields. The fluid inclusion and stable isotope data documented in this study indicate that the vein-type copper mineralization in the Wulasigou Pb-Zn-Cu deposit took place in an orogenic-metamorphic enviroment. [ABSTRACT FROM AUTHOR]

ABSTRACT An integrated study of U-Pb ages and trace elements was carried out for titanite and zircon from ultrahigh-pressure (UHP) metagranites in the Sulu orogen, east-central China. The results provide constraints on the composition of metamorphic fluids during the exhumation of deeply subducted continental crust. Titanite has two domain types based on REE patterns and trace element variations, Ttn-I and Ttn-II respectively. These two domains show indistinguishable U-Pb ages of 232 ± 14 to 220 ± 8 Ma, in general agreement with anatectic zircon U-Pb ages of 223 ± 4 to 219 ± 2 Ma for the partial melting event during early exhumation. The Ttn-I domains have significantly higher REE, Th, Ta and Sr, and higher Th/U ratios than the Ttn-II domains, indicating that the two domains have grown from metamorphic fluids with different compositions. For the Ttn-I domains, Zr-in-titanite thermometry yields high temperatures of 773-851 °C at 2.5 GPa, and petrographic observations reveal the presence of melt pseudomorphs. Thus, they are interpreted to have grown from hydrous melts in the early exhumation stage. In contrast, the Ttn-II domains were texturally equilibrated with amphibolite facies minerals such as biotite and plagioclase and contain inclusions of plagioclase and quartz. The Zr-in-titanite thermometry yields lower temperatures of 627-685 °C at 1.0 GPa. In combination with their REE patterns, they are interpreted to have grown from aqueous solutions at amphibolite facies metamorphic conditions during further exhumation. The differences in Th and Sr contents are prominent between the Ttn-I and Ttn-II domains, signifying the compositional difference between the hydrous melts and aqueous solutions. Therefore, the polygenetic titanite in the UHP metamorphic rocks provides insights into the geochemical property of metamorphic fluids during the continental subduction-zone processes. [ABSTRACT FROM AUTHOR]

Although the term orogenic gold deposit has been widely accepted for all gold-only lode-gold deposits, with the exception of Carlin-type deposits and rare intrusion-related gold systems, there has been continuing debate on their genesis. Early syngenetic models and hydrothermal models dominated by meteoric fluids are now clearly unacceptable. Magmatic-hydrothermal models fail to explain the genesis of orogenic gold deposits because of the lack of consistent spatially – associated granitic intrusions and inconsistent temporal relationships. The most plausible, and widely accepted, models involve metamorphic fluids, but the source of these fluids is hotly debated. Sources within deeper segments of the supracrustal successions hosting the deposits, the underlying continental crust, and subducted oceanic lithosphere and its overlying sediment wedge all have their proponents. The orogenic gold deposits of the giant Jiaodong gold province of China, in the delaminated North China Craton, contain ca. 120 Ma gold deposits in Precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization. The only realistic source of fluid and gold is a subducted oceanic slab with its overlying sulfide-rich sedimentary package, or the associated mantle wedge. This could be viewed as an exception to a general metamorphic model where orogenic gold has been derived during greenschist- to amphibolite-facies metamorphism of supracrustal rocks: basaltic rocks in the Precambrian and sedimentary rocks in the Phanerozoic. Alternatively, if a holistic view is taken, Jiaodong can be considered the key orogenic gold province for a unified model in which gold is derived from late-orogenic metamorphic devolatilization of stalled subduction slabs and oceanic sediments throughout Earth history. The latter model satisfies all geological, geochronological, isotopic and geochemical constraints but the precise mechanisms of auriferous fluid release, like many other subduction-related processes, are model-driven and remain uncertain. [ABSTRACT FROM AUTHOR]

The Lamego orogenic gold deposit (440,742 oz gold measured reserves and 2.4 million t measured resources, with an average grade of 5.71 g/t Au and a cut-off grade of 2.15 g/t Au; AngloGold Ashanti Córrego do Sítio Mineração S/A (AGA) personal communication, 2014) is located in the 5 km-long trend that includes the world-class Cuiabá deposit. It is hosted in the Neoarchean metavolcano–sedimentary rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil. Mineralization is associated mainly with metachert–banded iron formation (BIF) and carbonaceous phyllites in the reclined Lamego fold, in which the Cabeça de Pedra orebody represents the hinge zone. Mineralization is concentrated in silicification zones and their quartz veins, as well as in sulfide minerals, product of BIF sulfidation. Hydrothermal alteration varies according to host rock, with abundant sulfide–carbonate in BIF, and sericite–chlorite in carbonaceous phyllite. Quartz vein classification according to structural relationships and host rocks identified three vein systems. The V1 system, mainly composed of smoky quartz (Qtz I) and pyrite, is extensional, crosscuts the bedding plane S 0 of BIF, and is parallel to the fold axis. The V2 system, of the same composition, is represented by veins that are parallel to the S 1–2 foliation and S 0 . This system is also characterized by silicification zones in the BIF–carbonaceous phyllite contact that has its maximum expression in the hinge zone of folds. The V3 system has milky quartz (Qtz II) veins, which result from the recrystallization of smoky quartz, located mainly in shear zones and faults; these veins form structures en echelon and vein arrays. The most common ore minerals are pyrite, As-pyrite and arsenopyrite. Fluid inclusion-FI trapped in all quartz veins present composition in the H 2 O–CO 2 ± CH 4 –NaCl system. Fluid evolution can be interpreted in two stages: i) aqueous–carbonic fluid trapped in Qtz I, of low salinity (~ 2% equiv. wt.% NaCl), and ii) carbonic–aqueous fluid, of moderate salinity (average 9 eq. wt.% NaCl) hosted in Qtz II. Both stages are characterized by decrepitation temperatures in the range of 200 to > 300 °C, and suggest a fluid of metamorphic origin. Applying an arsenopyrite geothermometer, the calculated formation temperature for the Cabeça de Pedra orebody is 300 to 375 °C. The vertical intersection of the isochors allows a minimum pressure calculation of 2.6 kbar. The composition of individual FIs of this orebody, obtained by LA-ICP-MS analyses, compared with results of FIs for the Carvoaria Velha deposit, Córrego do Sítio lineament, highlights a standard composition typical of metamorphic fluids with Na > K > Ca > Mg, which increase or decrease in concentration as a function of salinity in both deposits. Trace elements vary according to fluid–rock reactions, and are directly related to the host rock composition. The comparison of data sets of the two deposits shows that the Cabeça de Pedra FIs have a higher enrichment in Zn, while Cu, As and Sb are richer in Carvoaria Velha, suggesting influence of the host rock geochemistry. The suggested mechanisms for gold precipitation at the Cabeça de Pedra orebody, Lamego gold deposit are: i) hydrolysis of the carbonaceous matter of phyllite and BIF, affecting f O 2 , destabilizing sulfur complexes and enhancing gold precipitation; ii) replacement of BIF iron carbonates by sulfides; and iii) continuous pressure changes that lead to silica precipitation and free gold. Other than playing the long-recognized role of the carbonaceous phyllites as a fluid barrier, the data highlight their importance as a source of metals. [ABSTRACT FROM AUTHOR]

The Logrosán Sn–(W) ore deposits in the metallogenic Sn–W province of the European Variscan Belt consist of endo- and exogranitic greisen-type and quartz–cassiterite veins associated with a S-type granite. Mineral characterization, fluid inclusion study, isotope geochemistry and Ar–Ar geochronology have been combined in order to reconstruct the conditions for Sn–(W) mineralization. The endo- and exogranitic mineralization must have been developed in a relatively long-lived system (~ 308–303 Ma), during or soon after the emplacement of the Logrosán related-granite (at ca. 308 Ma). The mineralizing fluids are characterized by complex aqueous and volatile (H 2 O–N 2 –CO 2 –CH 4 –NaCl) fluid inclusions. Microthermometry and Raman analyses indicate that fluid composition evolved from N 2 –CH 4 to N 2 -rich, followed by CO 2 -rich fluids, with varying amounts of H 2 O. The presence of N 2 and CH 4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks. A model of host-rock interaction, assimilation, and mixing of metamorphic and magmatic fluids, resulting in change of the redox conditions, is proposed for tin deposition. Later sulfide minerals were precipitated as a result of pressure and temperature release. [ABSTRACT FROM AUTHOR]

Analysis of the δ18C and δ13C values of carbonate rocks from Islay, Scotland reveals structural channelling of metamorphic fluids through the axial region of a major en echelon anticlinal fold system. Metamorphic fluid flow produced axial planar veins with higher vein density in the axial region of the fold. Fluid:rock ratios were more than 30:1 within this axial region, at least four times greater than the regional mean ratio of 7·661·5:1 for carbonate rocks on Islay. This supports the interpretation that metamorphic fluids were channelled through the axial region of the Islay Anticline. Fluid:rock ratios were calculated using a model for coupled δ18C and δ13C exchange with a metamorphic fluid. The metamorphic fluid was calculated to have δ18C and δ13C values of 15·3% and -6·1%, respectively and XCO2 of 0·2. This is in isotopic and chemical equilibrium with chloriteand graphite-bearing metamudstones that are structurally below the folded metacarbonate rocks on Islay. Devolatilization of these metamudstones is therefore a likely source mechanism for this metamorphic fluid. Removal of the effects of metamorphic fluid flow on δ13C values recorded by metacarbonate rocks on Islay allows us to re-evaluate evidence used to reconstruct Neoproterozoic climate. This evidence includes a large negative δ13C excursion reported from the Lossit Limestone Formation. This unit underlies the Port Askaig Formation, which is dominated by diamictites that have been interpreted as glacial tillites. This 'Islay anomaly' has been correlated with other such anomalies worldwide and together with overlying tillites has been cited as evidence of major (worldwide) glaciation events. In this study, we show that the magnitude of this negative δ13C anomaly can partly be explained by exchange with metamorphic fluids. However, we also show that extremely negative δ13C values in the Bonahaven Dolomite Formation, which overlies the Port Askaig Formation and has been interpreted as a 'cap carbonate', cannot be attributed to metamorphic fluid flow. [ABSTRACT FROM AUTHOR]

The crust of Tibetan Plateau may have formed via shortening/thickening or large-scale underthrusting, and subsequently modified via lower crust channel flows and volatile-mediated regional metamorphism. The amplitude and distribution of crustal anisotropy record the history of continental deformation, offering clues to its formation and later modification. In this study, we first investigate the back-azimuth dependence of Ps converted phases using multitaper receiver functions (RFs). We analyze teleseismic data for 35 temporary broadband stations in the ASCENT experiment located in northeastern Tibet. We stack receiver functions after a moving-window moveout correction. Major features of RFs include: 1) Ps arrivals at 8–10 s on the radial components, suggesting a 70–90-km crustal thickness in the study area; 2) two-lobed back-azimuth variation for intra-crustal Ps phases in the upper crust (< 20 km), consistent with tilted symmetry axis anisotropy or dipping interfaces; 3) significant Ps arrivals with four-lobed back-azimuth variation distributed in distinct layers in the middle and lower crust (up to 60 km), corresponding to (sub)horizontal-axis anisotropy; and 4) weak or no evidence of azimuthal anisotropy in the lowermost crust. To study the anisotropy, we compare the observed RF stacks with one-dimensional reflectivity synthetic seismograms in anisotropic media, and fit major features by “trial and error” forward modeling. Crustal anisotropy offers few clues on plateau formation, but strong evidence of ongoing deformation and metamorphism. We infer strong horizontal-axis anisotropy concentrated in the middle and lower crust, which could be explained by vertically aligned sheet silicates, open cracks filled with magma or other fluid, vertical vein structures or by 1–10-km-scale chimney structures that have focused metamorphic fluids. Simple dynamic models encounter difficulty in generating vertically aligned sheet silicates. Instead, we interpret our data to support the hypothesis of vertical metamorphic-fluid domains whose alignment is determined by shear motion within the boundary layers of crustal channel flow. [ABSTRACT FROM AUTHOR]

Symplectitic intergrowths of sodic plagioclase + diopside ± amphibole as replacement of omphacite are commonly found in eclogites. These symplectites are interpreted as the exhumation-related decompression of eclogite into the amphibolite facies. The role of aqueous fluid in symplectite development, which would act as a catalyst and favor open-system reaction, has been suggested but not yet clearly established. In the Vårdalsneset outcrop of the Western Gneiss Region (Norway), eclogites that were not amphibolitized either display a primary eclogitic dry paragenesis (garnet + omphacite + rutile ± quartz) or a paragenesis including phengite. In the last case, omphacite is partly transformed into symplectite. The two groups have been further distinguished from a combined petrological, geochemical, and thermochemical study. Group I samples have a fine-grained unaltered microstructure, with medium Al 2 O 3 (14–16 wt.%), high Fe 2 O 3 (13–16 wt.%) and TiO 2 (1.4–2.4 wt.%). Group II samples have a coarse-grained microstructure and are characterized by the presence of symplectites and phengite. They display higher Al 2 O 3 (17.5–23 wt.%) and lower Fe 2 O 3 (5.5–8 wt.%) and TIO 2 (0.2–0.5 wt.%) contents than Group I . The P – T estimates for samples from both Groups I and II lead to similar conditions for peak eclogite metamorphism: temperatures range from 590 to 720 °C and pressures from 15 to 25 kbar. Perple_X modeling indicates that for Group I eclogites, temperature range is similar to the temperature range of the water saturation curve, whereas for Group II eclogites, due to slightly different chemical composition, the water saturation curve is located at much higher temperatures (770–900 °C), so that OH − remains in residual phengite at peak eclogite temperature. With no residual hydrous phase in the eclogite assemblage, and although some structural water may persist in nominally anhydrous minerals, Group I eclogites were preserved without change during exhumation. In contrast, Group II eclogites retain hydrous minerals, and this hydrous component is available as a kinetic factor for symplectite formation during exhumation. This suggests that symplectite development can be promoted by an internally derived hydrous component, signaled by the presence of a hydrous mineral, and, so, does not necessarily require an influx of external fluid. [ABSTRACT FROM AUTHOR]

Available fluid inclusion data on hydrothermal deposits, deep-seated xenoliths, magmatic and metamorphic rocks were used to generalize the determinations of physicochemical parameters (temperature, pressure, and compositions) of natural fluids. It was established that fluid pressures during formation of hydrothermal deposits often exceeded lithostatic loading (250-270 bar/km) of the overlying rocks. Fluids from deep-seated xenoliths, magmatic and metamorphic objects are considered as the possible sources of high-pressure fluids. Data on temperatures, pressures, and fluid composition are generalized for each object. [ABSTRACT FROM AUTHOR]

Vein and shear-zone hosted gold deposits in orogenic terrains of Archean to Phanerozoic age are formed from dominantly metamorphic fluids. It is debated, however, whether normal crustal source rocks are adequate to generate economic deposits, or whether selectively gold-enriched sources such as mafic to ultramafic lavas, black shales or an input of magmatic fluid make a decisive difference between the formation of high-grade deposits and barren vein systems. As an essential baseline study in this debate, we have analyzed the metal and sulfur content of fluid inclusions in barren veins across a prograde sequence from greenschist to amphibolite facies in the Central Alpine metamorphic belt. Comparison of the analyzed fluid compositions with thermodynamic solubility calculations shows that the fluids record metamorphic dehydration, decarbonation and desulfidation of the continental crust. However, gold with concentrations of 0.003 to 0.03 ppm is increasingly undersaturated in the highest-temperature aqueous–carbonic fluids, which otherwise resemble those forming major gold deposits elsewhere. Our results show that the regional-metamorphic fluids could carry 10–1000 times higher gold concentrations, implying that pre-enriched source rocks or hydrous magmas may play an essential role in generating highly gold-endowed orogenic belts. [ABSTRACT FROM AUTHOR]

The fluid evolution of late-metamorphic quartz vein systems in the fold-and-thrust belt of the Rhenish Massif has been studied by microthermometry and LA-ICPMS microanalysis. The quartz veins are hosted by very low-grade organic-matter-rich metasiltstones and metapelites of the Hunsrück nappe complex. The veins record two major stages of textural evolution, a massive vein filling assemblage with elongate-blocky quartz, chlorite, albite and apatite, and a later open space filling assemblage with euhedral crystals of quartz, ankerite/dolomite and minor calcite and sulfides. The euhedral quartz crystals host well-preserved fluid inclusion assemblages, which made it possible to reconstruct the chemical evolution of the fluid system with time. The fluid inclusions are all aqueous two-phase with low salinity, and three successive fluid generations record a systematic decrease in salinity from 4.7–5.3 to 3.1–3.9 and then to 1.4–1.7wt.% eqv. NaCl. The salinity decrease is paralleled by a decrease in homogenization temperatures from 210–225°C to 148–164°C and 124–139°C. Using LA-ICPMS microanalysis of individual fluid inclusions, reproducible elemental concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Ba, Sr, B, Al, As, Sb, S, Cl and Br could be determined. The concentrations of the alkali metals and of boron, arsenic and antimony are correlated with the fluid salinity. Element concentrations are highest in the early fluid generation and decrease systematically in the latter two generations. The systematic changes in fluid composition (salinity, homogenization temperature and elemental concentrations) are best explained by fluid mixing between a hot metamorphic fluid of moderate salinity (that carried elevated concentrations of B, As and Sb) and a cooler dilute fluid that was of meteoric origin. The fluid evolution therefore signals deep ingression of meteoric water during late-metamorphic exhumation and uplift. The molar Cl/Br ratios of the fluid inclusions lie on a linear array that extends from close to seawater to values that are substantially below seawater. Because evaporitic sources are conspicuously absent in the shallow-water marine sequence of the Rhenish Massif, the elevated Br concentrations (and Cl/Br ratios below seawater) are best explained by fluid–rock interaction and liberation of Br from the organic matter in the metasediments. [ABSTRACT FROM AUTHOR]

In a geochemical and petrological analysis of overprinting episodes of fluid-rock interaction in a well-studied metabasaltic sill in the SW Scottish Highlands, we show that syn-deformational access of metamorphic fluids and consequent fluid-rock interaction is at least in part controlled by preexisting mineralogical variations. Lithological and structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands, caused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the Dalradian Supergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig shows that carbonation at greenschist to epidote-amphibolites facies conditions caused by infiltration of H2O-CO2 fluids was controlled by mineralogical variations, which were present before carbonation occurred. This variability probably reflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculation of precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chlorite and epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism prior to fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidote ± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primary pyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. These processes formed diffuse epidote-rich patches or semi-continuous layers. These might represent precursors of epidote segregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analyses of epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote. Transient permeability enhancement associated with these changes may have permitted higher fluid fluxes and therefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became more layer parallel, limiting propagation of the reaction front into the sill interior. [ABSTRACT FROM AUTHOR]

Abstract: The deep gold mines of the Witwatersrand Basin, South Africa have gained recent attention not only because of investigations of the deep fracture water and associated CH4- and H2-rich gases found there, but because of recent reports of deep microbial communities persisting to depths of almost 3km — an exotic outpost of the Earth''s deep biosphere. While shallower fluids in the basin (to approximately 1km) were found to contain abundant populations of methanogens and sulphate-reducing bacteria, the deepest, oldest, most saline fracture waters in the basin hosted hitherto unrecognised low biomass and low biodiversity chemoautotrophic ecosystems independent from the photosphere. Shallow and deep fluids also show distinct differences in gas and fluid geochemistry. Paleometeoric waters are dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilising the CO2 reduction pathway. In contrast the deepest, most saline fracture waters contain gases that are dominated by high concentrations of H2 gas, and CH4 and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water–rock reaction. The high salinities (up to hundreds of g/L), highly altered δ18O and δ2H signatures, and both 36Cl and measurements of co-occurring nucleogenic noble gases for these fracture waters are consistent with extensive water–rock interaction over geologically long time scales in these high rock/water ratio environments. While the ultimate origin of these fluids has been attributed alternately to saline waters that penetrated the crystalline basement, formation water, or hydrothermal fluids in some cases, their δ18O and δ2H isotopic signatures have typically been so profoundly overprinted by the effects of long-term water–rock interaction that, for the most saline end-members, little evidence of their primary composition remains. The key objective of the present study is to further investigate the origin of these fluids by integrating for the first time detailed neon isotope analyses on the dissolved gases. Helium isotopic analysis confirmed that there is no significant mantle-derived component associated with these fluids and gases. Neon isotope results show distinct differences in neon composition that correspond to the different fluid geochemical end-members previously identified. Typical crustal neon signatures (type A) are identified in the paleometeoric waters populated with abundant methanogens. In contrast, the deep more saline fracture waters contain an enriched nucleogenic neon signature unlike any previously reported in crustal fluids. These samples show the highest 21Ne/22Ne ratios (0.160±0.003) ever reported in groundwater. Fluid inclusions in these rocks yield even higher 21Ne/22Ne ratios between 0.219 and 0.515, consistent with an extrapolated 21Ne/22Ne value of 3.3±0.2 at 20Ne/22Ne=0. We show that this enriched nucleogenic neon end-member represents a fluid component that was produced in the fluorine-depleted Archaean formations and trapped in fluid inclusions ≥2Ga ago. The observation of enriched nucleogenic neon signatures in deep fracture water implies the release of this billion year old neon component from the fluid inclusions and its accumulation in exceptionally isolated fracture water systems. The observed association of this Archean neon signature with H2-hydrocarbon-rich geogases of proposed abiogenic origin dissolved in the same deep groundwater suggests that the fracture systems have also allowed for the accumulation of various products of water–rock reactions throughout geologic times. One of these fracture systems contained the deepest characterised microbial ecosystems on earth — chemolithotrophs eking out an existence at maintenance levels independent from sunlight. Consequently, the enriched nucleogenic neon isotope signature may indicate regions in the Archaean crust where investigations of the deep biosphere might be focused. [Copyright &y& Elsevier]

Geofluids (2010) 10, 58–72 Chloride-rich brines are increasingly recognized as playing an important role in high pressure and temperature metamorphic and magmatic systems. The origins of these saline multicomponent fluids are debated, but experimental evidence suggests that regardless of their origin they must be important agents of rock alteration and mass transfer wherever they occur. Studies of the solubility of quartz in H2O, CO2–H2O and salt–H2O solutions provide a framework for understanding the role of brines in the deep crust and upper mantle. While quartz solubility in the system SiO2–H2O–NaCl–CO2 is maximal at a given high pressure and temperature if the solvent is pure H2O, the decline in quartz solubility with NaCl content (salting-out) is less severe than in CO2–H2O fluids at comparable H2O activities. Moreover, at lower pressures, quartz solubility initially salts in at low salt contents, before reaching a maximum and then declining. The behavior of quartz solubility in salt–H2O solutions has not yet been fully explained and is the subject of active debate. Experimental investigations of the solubility of some other rock-forming oxides and silicates show enhancements due to NaCl addition. As illustrated by the well-studied CaO–Al2O3–SiO2–NaCl–H2O system, enhancements initially increase to maxima, and then decline. This behavior can be explained by formation of a range of hydrated aqueous complexes and clusters with specific NaCl:H2O stoichiometries. In contrast, solubilities of calcium salts, including calcite, fluorite, fluorapatite and anhydrite, rise monotonically with increasing NaCl, implying complexing to form anhydrous ionic solutes and/or ion pairs. The experimental studies offer new insights into fluid-rock interaction in a range of settings, including carbonatite–fenite complexes, granulite-facies metamorphism, porphyry ore deposits and aluminum-silicate vein complexes in high-grade metamorphic terranes. [ABSTRACT FROM AUTHOR]

The general major component composition of aqueous fluids at elevated pressure and temperature conditions can be represented by H2O, different non-polar gases like CO2 and different dissolved metal halides like NaCl or CaCl2. At high pressure, the mutual solubility of H2O and silicate melts increases and also silicates may form essential components of aqueous fluids. Given the huge range of P–T–x regimes in crust and mantle, aqueous fluids at elevated pressure and temperature are highly variable in composition and exhibit specific physicochemical properties. This paper reviews principal phase relations in one- and two-component fluid systems, phase relations and properties of binary and ternary fluid systems, properties of pure H2O at elevated P–T conditions, and aqueous fluids in H2O–silicate systems at high pressure and temperature. At metamorphic conditions, even the physicochemical properties of pure water substantially differ from those at ambient conditions. Under typical mid- to lower-crustal metamorphic conditions, the density of pure H2O is , the ion product Kw = 10−7.5 to approximately 10−12.5, the dielectric constant ε = 8–25, and the viscosity η = 0.0001–0.0002 Pa sec compared to , Kw = 10−14, ε = 78 and η = 0.001 Pa sec at ambient conditions. Adding dissolved metal halides and non-polar gases to H2O significantly enlarges the pressure–temperature range, where different aqueous fluids may co-exist and leads to potential two-phase fluid conditions under must mid- to lower-crustal P–T conditions. As a result of the increased mutual solubility between aqueous fluids and silicate melts at high pressure, the differences between fluid and melt vanishes and the distinction between fluid and melt becomes obsolete. Both are completely miscible at pressures above the respective critical curve giving rise to so-called supercritical fluids. These supercritical fluids combine comparably low viscosity with high solute contents and are very effective metasomatising agents within the mantle wedge above subduction zones. [ABSTRACT FROM AUTHOR]

Abstract: The Qolqoleh gold deposit is located in northwestern part of the Sanandaj–Sirjan metamorphic belt, northwestern Iran. Igneous and sedimentary units exposed in the area have undergone greenschist metamorphism. The area was affected by a NE–SW trending shear zone and subsequent deformation. Two different types of mineralization are distinguished in the Qolqoleh gold deposit based on geological–structural conditions indicated by microtextural analysis: ductile and then brittle. Ore-forming processes are divided into three stages: Early (I), Middle (II) and Late (III), which include quartz–pyrite (I), sulfides and gold (II) and carbonate veinlets (III), respectively. The stage I fluids are characterized by δ18O=15.5‰ at 440ºC, and are thought to be deep-sourced metamorphic waters; the stage III fluids, with δ18O=1.6‰, are shallow-sourced meteoric waters; whereas, the stage II fluids, with δ18O=13.1‰, are a mixture of deep-sourced metamorphic and shallow-sourced meteoric fluids. Based on comparisons of the D–O–C isotopic systematics, the ore-forming fluids with characteristic high δ18O and δ13C and low δD originated from metamorphic devolatilization of Cretaceous volcano-sedimentary (felsic to mafic metavolcanic rocks–shale–carbonate–carbonaceous chert) sequences, locally rich in organic matter. During late Cretaceous continental collision of the Afro-Arabian continent and the Iranian microcontinent, a crustal slab consisting of felsic to mafic metavolcanic rocks, carbonate, shale and carbonaceous chert was underthrust northwards beneath the central Iranian microcontinent along the Zagros fault. During further contraction, deformation was localized in reverse oblique-slip structures with vergence toward south; shear zones generally follow contacts between more competent and less competent rock units. Metamorphic devolatilization of this underthrust slab is the source of the ore-forming fluids that generated the Au ore belt, which includes the Qolqoleh gold deposit. [Copyright &y& Elsevier]

Abstract: The solubility of Durango fluorapatite was measured in H2O and H2O–NaCl at 700–900 °C and 0.7–2.0 GPa in a piston-cylinder apparatus. Solubility was determined by weight loss using a double-capsule method. At all conditions, fluorapatite dissolves incongruently to monazite+fluid, but residual monazite crystals were below weighing detection limits of 0.6 μg. The concentration of fluorapatite dissolved in pure H2O is low at all investigated conditions (56±8 to 288±8 ppm by weight), but increases with temperature (T) and pressure (P). The data are well described by the equation log cap °=−3.56+.00241T +9.17 logρ H2O, where cap ° is apatite concentration in H2O (in ppm), T is in Kelvin, and ρ H2O is H2O density in g cm−3. Fluorapatite solubility was also measured in H2O–NaCl fluids at a range of P and T. Results indicate a strong increase in dissolved fluorapatite concentration with rising NaCl mole fraction (X NaCl) at all P and T, and X NaCl to near halite saturation. The data were fit to the equation c ap = cap °+ Ba NaCl 1/2 where c ap is apatite concentration in H2O–NaCl (in ppm), a NaCl is NaCl activity calculated from an ideal mixing model for H2O–NaCl, and B =(4.4+1.1P 4)exp(0.0070T), with P in GPa, and T in Kelvin. The results differ from previous work in that the newly determined solubilities are ≥10 times lower at comparable conditions, increase with P at constant T, and show a strong positive correlation with X NaCl. The discrepancy is attributed to unrecognized growth of new apatite crystals at run conditions in the earlier study. Compared to fluorite, calcite and anhydrite, fluorapatite solubility is significantly lower in H2O and H2O–NaCl; however, the solubility enhancement by NaCl relative to that in pure H2O is similar to anhydrite and much greater than calcite and fluorite. The NaCl enhancement of solubility increases with T but decreases with P. The undetectable weight of monazite requires significant solubility of light rare-earth elements (LREE), likely as chloride complexes, and indicates that metasomatic interaction of brines with apatite-bearing rocks at high metamorphic grades can mobilize substantial LREE. [Copyright &y& Elsevier]

Abstract: The solubility of corundum was measured at 800 °C and 10 kbar in NaCl–H2O–SiO2 fluids over NaCl mole fractions (X NaCl) of 0 to 0.5 and SiO2 concentrations of zero to quartz or albite+melt saturation, depending on X NaCl. Experiments were performed in a piston-cylinder apparatus. Dissolved Al2O3 and SiO2 were determined by weight losses of corundum and quartz crystals, or by bulk compositions of bracketing experiments. Although kyanite and sillimanite are slightly more stable than corundum+quartz at these pressures (P) and temperatures (T), neither appeared in any of the experiments. Results indicate that the enhancement of corundum solubility by NaCl at this P and T is further promoted by the addition of SiO2. At quartz saturation, significant enhancement occurs in initially pure H2O and addition of NaCl yields yet higher Al2O3 concentrations. At 0.03≤ X NaCl ≤0.1, quartz saturation is replaced by albite+silicate melt. The anhydrous melt composition is nearly on the join NaAlSi3O8–SiO2. Al2O3 molality rises rapidly with NaCl concentration to 0.038 at X NaCl =0.1 and then increases more slowly to 0.052 at halite saturation. There is, in addition, a measurable reciprocal enhancement of Si solubility by virtue of dissolved Al at a fixed X NaCl. Quench pH in NaCl-bearing fluids is strongly acidic and Na/Cl<1 in quench solutes, suggesting low pH at high P and T. These observations, combined with previous work indicating Si–Al and Na–Al complexing, lead to the hypothesis that the reciprocal solubility enhancement of Al and Si is due to formation of Na-aluminosilicate complexes. Mass balance consideration suggests that the bulk Si/Al ratio of the group of complexes ranges from 1 to 2 in the NaCl-free system, to >3 at high X NaCl. Our data show that Al2O3 is a moderately soluble component in quartz+aluminosilicate-saturated rocks in the presence of intergranular salt solutions at deep-crustal metamorphic P–T conditions and commonly realized salinities. Al2O3 should not be regarded as a fixed reference component, because it may exhibit substantial mobility. [Copyright &y& Elsevier]

The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea-floor alteration, has a strong influence, via fluid-rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T– XCO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist-amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P– T conditions, the proportion of carbonate minerals and the XCO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low XCO2. Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO2-rich fluid ( XCO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher XCO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at XCO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of XCO2 = 0.2–0.3 into the upper crust. [ABSTRACT FROM AUTHOR]

The Tharsis massive sulphide deposit, one of the major VMS-type deposits in the Iberian pyrite belt (IPB) was severely deformed by the Variscan tectono-thermal events. The question of whether or not these events affected the metal distribution in the deposit has been addressed by simultaneously studying the mineral parageneses (Tharsis stockwork) and the fluid circulation (at local and regional scales). The results are:(1)The early paragenesis in the stockwork (Q1 quartz–pyrite–chlorite–phengite±cobaltite±ankerite) was strongly overprinted by a late post-kinematic mineral deposition, including new quartz veins (Q3 quartz) and base metal sulphides (chalcopyrite, sphalerite, Bi and Te minerals, pyrite and galena) and gold.(2)At a regional scale, fluids accompanying the peak metamorphism conditions (ca. 300 MPa, ca. 300 °C) were of C–O–H–N–NaCl type, CO2-dominated with CH4 and N2, and are considered to be “metamorphic” on the basis of microthermometry and geochemistry. The late- to post-kinematic evolution (“retrograde” stage) was characterised by a pressure drop, down to 40 MPa (lithostatic to hydrostatic transition), and a heat input leading to temperatures ≥430 °C, then decreasing to temperature around 170 °C. Fluids of the “retrograde” type exhibit both dilution of the C–O–H–N–NaCl fluid by a low salinity “meteoric” water and progressive loss of volatile components.(3)Fluids of the retrograde type pervasively percolated through the Tharsis stockwork and were responsible for the strong mineral overprint on the early (deformed) paragenesis. All the measurable fluid inclusions (f.i.) record these late fluids. There are primary fluid inclusions in Q1, but they are systematically imploded due to the external overpressure generated by the Variscan tectonic events. Although base metal distribution in the stockwork is basically the result of the “retrograde” fluid circulation, it remains unclear whether these metals were newly introduced into the stockwork from elsewhere or simply redistributed from the existing primary assemblages in the massive sulphide bodies. [Copyright &y& Elsevier]

AbstractA combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well-preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2-bearing NaCl-rich solutions, whereas it changed into CO2-dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low-salinity fluids were involved. In situ UV-laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB-like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra-high-pressure (UHP) eclogites in the Dabie-Sulu area. However, the age-corrected initial εNd(t) is - 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism. [ABSTRACT FROM AUTHOR]

Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350 °C, with modal values for individual samples ranging from 163 to 229 °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting show a narrower range of T h from 86 to 170 °C with modes from 116 to 141 °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400 °C and 4.1–6.0 kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades. [ABSTRACT FROM AUTHOR]

Apatite occurs in the zeolite to greenschist facies metamorphic rocks of the Otago Schist, South Island, New Zealand, as both a groundmass constituent and as a hydrothermal phase hosted in metamorphic quartz veins. Groundmass apatite from low-grade rocks, ranging from the zeolite facies to the pumpellyite–actinolite zone, has chloride contents ranging from 0–1.4 wt%, and fluoride contents ranging from 2.2–4.2 wt%, whilst groundmass apatite from the greenschist facies (chlorite to biotite zone) is virtually pure fluorapatite. Vein apatite from all grades is also fluorapatite with little or no chloride. This difference in composition is interpreted as resulting from the preservation of the primary magmatic compositions of detrital Cl-apatite grains, out of equilibrium with the metamorphic fluid, at low grades, whilst higher-grade groundmass apatite and neoformed apatite in quartz veins have compositions in equilibrium with an aqueous metamorphic fluid. The presence of detrital Cl-bearing apatite during the early stages of metamorphism may constitute a significant reservoir of Cl, given the low porosities of compacted sediments undergoing prograde metamorphism. Calculations indicate that the release of Cl from detrital apatite in the Otago Schist, as a result of re-equilibration of apatite with the pore fluid, may have had a significant effect on the salinity of the metamorphic fluid. [ABSTRACT FROM AUTHOR]

Muscovite and biotite dehydration-melting reactions near the peak of metamorphism played a significant role in the reaction and fluid history of the Rangeley Formation in southwestern New Hampshire, USA. Evidence for in situ melting includes: (1) the consistency among theoretical phase equilibria, observed reaction textures, and the inferred P–T conditions; (2) disseminated, centimeter-scale, leucocratic quartz + plagioclase + muscovite pods; (3) diffusion and growth zoning of major and trace elements in garnet that are best explained as the result of high-T muscovite and biotite breakdown; and (4) oxygen isotope evidence that high-T back-reaction of K-feldspar to muscovite near peak metamorphic conditions did not involve an isotopically disequilibrium (externally derived) fluid. Isotopically equilibrated fluids were apparently stored in melt pockets and then reused as the melts crystallized, thereby driving retrogression. Prograde muscovite dehydration-melting reactions further imply P≥4 kbar at T≤ ∼650°C, so that loading occurred before the peak of metamorphism at T ∼725°C. Oxygen isotope compositions of retrograde garnet that grew during cooling between T ∼650°C and T ∼550°C are consistent with closed-system models, indicating that previous back-reaction of K–feldspar to muscovite did not disturb the isotope compositions of the rocks. Late-stage growth of additional retrograde garnet, staurolite, and chlorite at T ∼475°C requires infiltration of externally derived H2O, but this retrograde infiltration did not affect garnet and staurolite isotope compositions, as expected for differing rates of infiltration-driven hydration vs isotope alteration. Late-stage infiltration continued after garnet and staurolite growth ceased, as evidenced by systematic differences in isotope trends near the base of the nappe for minerals with fast oxygen isotope diffusion rates (quartz, muscovite, and biotite) vs minerals with slow diffusion rates (garnet, staurolite, and sillimanite). This infiltration may reflect the dewatering of structurally lower levels after nappe emplacement. If so, then nappe emplacement occurred at T ∼475°C. [ABSTRACT FROM AUTHOR]

Redistribution of elements by fluids produced by the metamorphic devolatilization (MDV) of sedimentary rocks is one of the major processes responsible for the differentiation of the crust. Comparison of compositions of rocks with similar original composition (protolith) with increasing metamorphic grades is the principal method of observation and quantification of the mass transfer caused by the loss of fluids. This work summarises observations from metamorphic complexes covering a wide range of PT conditions and lithologies that have been studied worldwide for the chemical effects of metamorphism. The metamorphic complexes range from coherent sedimentary sequences where the same stratigraphic horizons have been traced across different metamorphic zones, to folded turbidite piles and accretionary prisms where terranes with different peak PT conditions are divided by tectonic boundaries. The geochemical changes caused by the MDV are different in different complexes. Some elements are depleted in almost all complexes, whereas others are affected only in exceptional cases. The major elements, and the majority of lithophile and siderophile elements are not depleted by metamorphic devolatilization. Mercury is the first element to be lost from sediments at conditions of sub-greenschist metamorphism, which may coincide with the loss of Cd. Arsenic depletion is almost universal in metamorphic rocks, and is correlated with the decomposition or recrystallisation of sedimentary pyrite during greenschist facies metamorphism, where it can occur at similar temperatures both in regional metamorphic rocks and in subduction zone metamorphism. Gold, Sb, Ag, and W depletion are documented to coincide with As loss in regional metamorphic complexes. The content of C and S changes in some cases but remain conserved in others. Base metals including Cu, Pb and Zn are typically conserved during metamorphic devolatilization but exceptions have also been reported, where these metals are progressively depleted during upper greenschist and lower amphibolite facies metamorphism. Chlorine can be extracted by the metamorphic devolatilization in some metamorphic sequences, whereas F is conserved. The majority of lithophile elements show conservative behaviour during metamorphic devolatilization including REE, HFSE, and LILE, with the exceptions of Cs, N, U, and B in some complexes, where the depletion in some cases is associated with dehydration and in others possibly with the beginning of anatexis. The loss of hydrous fluids may be limited during high-pressure metamorphism and in sediments with abundant carbonates; under these conditions depletion of trace elements can be insignificant. Metamorphic devolatilization has the potential to be an efficient and powerful mechanism for the concentration of dispersed elements into ore-grade mineralisation. The connection between devolatilization and the origin of orogenic gold deposits is the most established in this regard, and some other deposit types are likely related to specific types of metamorphic fluids. [ABSTRACT FROM AUTHOR]

[Display omitted] • The metamorphic fluids in Ailaoshan orogenic belt belong to medium temperature, low salinity and low density NaCl-CO 2 -H 2 O system with high CO 2 , Ca2+ and Mg2+ content. • The ore forming fluids of orogenic gold deposits belong to lower temperature, medium salinity and medium density NaCl-H 2 O-CO 2 system with low CO 2 content and high K+. • Combined with the distinct distributions of δ34S and δ18O H2O values between the two kinds of fluids, the ore forming fluids in Ailaoshan orogenic belt cannot be evolved from regional metamorphism. The source of ore-forming fluids for orogenic gold deposits is still disputed with models ranging from derivation from metamorphism of continental crust to derivation from metasomatized mantle lithosphere. This dispute is addressed by this study of the Ailaoshan orogenic gold belt, southeastern Tibet, whose evolution involved intense metamorphism on the eastern side of the Ailaoshan shear zone. Both the regional metamorphic barren quartz veins and gold-related quartz veins in this belt are genetically related to movement on the Ailaoshan shear zone and thus formed over the same limited age span. Microthermometry, fluid compositions and stable isotope analyses conducted on both metamorphic fluids extracted from regional metamorphic quartz veins and ore-forming fluids collected from auriferous quartz veins at Zhenyuan and Jinchang gold deposits reveal significant differences. These include: (1) regional metamorphic quartz veins contain three types of fluid inclusions, including CO 2 -rich, aqueous-CO 2 and aqueous types. The CO 2 -bearing inclusion types are more abundant than those in auriferous quartz veins; (2) those fluids from barren metamorphic veins with high CO 2 content (up to 45 mol%) are typified by low-medium temperature (217–418 °C), low salinity (2–8 wt% NaCl) and variable pressure (28–440 MPa), and belong to the NaCl-CO 2 -H 2 O system. In contrast, the ore-forming fluids, which exhibit lower temperature (mainly 170–270 °C), higher salinity (mainly 5–13 wt% NaCl), and lower pressure (20–282 MPa and 14–44 MPa for Zhenyuan and Jinchang, respectively), belong to the NaCl-H 2 O-CO 2 system; (3) the metamorphic fluids contain high Ca2+ and Mg2+ in liquids and CO 2 , N 2 and CO in gases, whereas the ore-forming fluids are enriched in K+ in liquid and H 2 O, CO 2 , CH 4 in gases; (4) the pyrites from regional metamorphic quartz veins have δ34S values ranging from 2.08 to 2.86‰ (mean = 2.50‰), indicating a single sulfur source of metamorphic fluids, thus contrasting with the broad range of δ34S values of ore sulfides. These distinctions negate a genetic relationship between crustal metamorphism and gold mineralization and demonstrate that ore-fluids that deposit orogenic gold deposits, at least in the Ailaoshan belt cannot be derived or evolved from crustally-derived regional metamorphic fluids. This provides a new line of evidence for the derivation of ore-forming fluids from a sub-crustal, metasomatized mantle lithosphere source for at least some orogenic gold deposits. [ABSTRACT FROM AUTHOR]