Nb–Ta mineralization in Ti-oxide minerals from the Bagolyhegy Metarhyolite Formation (Bükk Mountains, NE Hungary)

The foliated low-grade metamorphic rocks of the Triassic Bagolyhegy Metarhyolite Formation, mainly of pyroclastic origin, host post-metamorphic quartz-albite veins containing abundant tourmaline and occasionally rutile/ilmenite. The study of the Ti-oxide-mineralized veins with SEM-EDX revealed an unusual mineral assemblage comprising fine-grained Nb–Ta-bearing oxides (columbite-tantalite series, fluorcalciomicrolite and other Nb–Ti–Y–Fe-REE-oxide minerals) intergrown with Nb-rich polymorphs of TiO2 (anatase, rutile), ilmenite and zircon enriched with hafnium. This high field strength elements (HFSE)-bearing paragenesis is unexpected in this lithology, and was not described from any formation in the Paleozoic-Mesozoic rock suite of the Bükk Mountains (NE Hungary) before. The host metavolcanics are significantly depleted in all HFSE compared to the typical concentrations in felsic volcanics and the mineralized quartz-albite veins have even lower Ti–Nb–Ta concentration than the host rock, so the mineralization does not mean any enrichment. From proximal outcrops of the Triassic Szentistvánhegy Metavolcanics, potassic metasomatized lenses with albite-quartz vein fillings containing rutile/ilmenite are known. We studied them for comparison, but they only contain REE mineralization (allanite-monazite-xenotime); the Nb–Ta-content of Ti-oxide minerals is undetectably low. LA-ICP-MS measurements for U–Pb dating of Hf-rich zircon of the Nb–Ta-rich mineral assemblage gave 71.5 ± 5.9 Ma as lower intercept age while dating of allanite of the REE mineralized quartz-albite veins gave 113 ± 11 Ma as lower intercept age. The REE-bearing vein fillings formed during a separate mineralization phase in the Early Cretaceous, while the Nb–Ta mineralization was formed by post-metamorphic alkaline fluids in the Late Cretaceous., controlled by fault zones and fractures.

Eclogite-like rocks from Southern Pirin Mountains – preliminary results about petrology and time of formation

North-northeast of the village of Ilinden (Southern Pirin Mnt.) three eclogite boudins were separated on the geological map in scale 1:50 000 (Sarov, 2010). The rocks belong to the Slasten lithotectonic unit. The mineral assemblage and mineral chemistry do not allow these rocks to be classified as eclogites. They can be considered as eclogite-like ones, formed by postmagmatic-metasomatic alteration of the host rocks. Based on LA-ICP-MS sphene U-Pb dating, eclogite-like rocks yield a Late Jurassic age (160±19 Ma).

Stable isotope (δD, δ18O) data on the structural water of two sericite samples from the Cu-Au high-sulphidation deposit Chelopech, Bulgaria – a tentative study

The epithermal high-sulphidation Cu-Au Chelopech deposit is characterized by a well-developed and well-traceable hydrothermal footprint manifested in the volcanic host rocks. The economic ore mineralization is embedded in the strong silicification, included among the advanced argillic zone of alteration, smoothly transitioning to quartz-sericite alteration that evolves into widespread propylitics. The quartz-sericite alteration zone is accessible for exploration only in underground mining galleries and exploration drillings. The main mineral assemblage in this zone is quartz, sericite, pyrite, minor rutile/anatase and relics of apatite and feldspar. According to XRD data from the studied samples, sericite was defined as illite and muscovite/sericite 2M1 polytype. The abundance of heavy stable isotopes (D, 18O) in the structural water of two sericite samples is the object of this study. A special attention was paid to the separation of extraneous waters from the structural one by thermal fractionation. The extracted structural water was converted to hydrogen and carbon dioxide before the isotopic measurements. The obtained results, put into a δD vs. δ18O plot, indicate that sericite structural water is “heavier” than meteoric water, within uncertainty limits.

Chlorite alteration in pegmatite from the Madan Pb-Zn deposits: mineral association, chemical composition and temperature of formation

Chlorites are common constituent of the secondary mineral assemblage formed as alteration products after aluminosilicate host rocks in the Pb-Zn deposits in Madan district. In concordant pegmatite body from the Petrovitsa deposit, they are formed after mica flakes. Such transformation often results in abundant rutile grains. The dominant chlorite compositions fall in the clinochlore-chamosite series. Minor and trace elements incorporation of Li, B, V, Co, Ni, Zn, Ga, Rb, Sr, Cs, and Ba is detected. Calculated To of formation ranges within 298–306 °C.

Basalt - fluid interactions at subcritical and supercritical conditions: An experimental study

<p><b>To investigate the interaction between fluids and basalt at subcritical, near-supercritical, and supercritical hydrothermal conditions (350-400˚C/500 bar), eight experiments have been conducted. These used a continuous-flow, high temperature and pressure hydrothermal apparatus. The basalt was reacted with three fluids: distilled water; geothermal brine; and natural seawater. Two further experiments used only seawater as a control to determine its behaviour without the influence of basalt.</b></p> <p> With distilled water, the fluid chemistry results show elevated SiO2, K, Cl, SO4, and H2S in solution for the first 12 days of both experiments. This is due to volcanic glass dissolution. After glass was removed, fluid composition was controlled by the remaining rock minerals. At 400˚C, the secondary mineral assemblage at the bottom of the Reactor (fluid entry point) is composed of grossular, wollastonite, anorthite, and chlorite. These results show the effectiveness of distilled water, which lacks any alkali cations, at removing Na and K rapidly from the rock. At the top of the Reactor (fluid exit point) the secondary minerals are anorthite and celadonite. At 350˚C, the secondary mineral assemblage at the bottom is anorthite and chlorite, while celadonite is the dominant secondary mineral at the top. In both experiments, celadonite replaces solely olivine. The formation of celadonite through reaction with distilled water shows that it can be formed by the interaction of deuteric water and basalt without addition of other components.</p> <p> The geothermal brine contains high concentrations of SiO2, K, SO4, Na, Cl and has an acidic pH. At 400˚C, fluid chemistry displays elevated SiO2 concentrations for approximately two weeks due to glass dissolution. At 350˚C, SiO2 concentration is initially high after temperature increase, but decreases gradually over the remainder of the experiment. At 400˚C, the secondary mineral assemblage at the bottom of the Reactor is composed of anhydrite and biotite, while at the top of the Reactor, smectite is the only secondary mineral. At 350˚C, anhydrite and smectite are found at the bottom, while only smectite is found at the top. The lack of biotite at 350˚C suggests this mineral’s precipitation kinetics are too slow to outcompete chlorite precipitation.</p> <p> The seawater-only experiments were conducted as controls to determine its behaviour during heat-up and provide the input solution composition for the seawater-basalt experiments. Both seawater-only experiments (377˚C and 342˚C) show the precipitation of anhydrite, caminite and brucite due to their retrograde solubilities. The effluent solutions are greatly depleted in Ca, Mg and SO4.</p> <p> In the seawater-basalt experiments at near-supercritical (400˚C) and subcritical conditions (350˚C), elevated SiO2 concentrations due to glass dissolution are not observed. This is attributed to rapid secondary mineral precipitation. Fluid chemistry and mass balance calculations show almost complete removal of SO4, and in particular, Mg, from the seawater while Ca shows a considerable loss from the rock. Three mineralization fronts were identified: (1) glass dissolution; (2) chloritization; and (3) anhydrite precipitation. In both experiments, there is a switch from chloritization to smectitization. This is accompanied by a decrease in Mg/Fe ratio in smectite. This mineral was also found at the top of both experiments, but its composition was more reflective of the rock.</p> <p>In terms of reactivity, the order of phases from most to least reactive is glass – olivine – clinopyroxene – plagioclase – Fe-Ti oxide. For the aluminosilicate phases this is attributed their respective Al contents. The seawater-basalt experiments also emphasise the fast rate of reaction at which Mg is fixed by the rock, which is conjectured to take less than a few hours.</p> <p>Considering all experiments, the distilled water results show a rock control on fluid chemistry while in the remaining basalt experiments, the chemistry is largely controlled by the fluid.</p> <p>Temperatures calculated using standard Na/K geothermometer did not estimate, in most cases, values close to the experimental temperature. This is due to the inability of the rock to sufficiently adjust the Na/K ratio given the secondary mineral assemblages that form.</p> <p> </p>

Basalt - fluid interactions at subcritical and supercritical conditions: An experimental study

<p><b>To investigate the interaction between fluids and basalt at subcritical, near-supercritical, and supercritical hydrothermal conditions (350-400˚C/500 bar), eight experiments have been conducted. These used a continuous-flow, high temperature and pressure hydrothermal apparatus. The basalt was reacted with three fluids: distilled water; geothermal brine; and natural seawater. Two further experiments used only seawater as a control to determine its behaviour without the influence of basalt.</b></p> <p> With distilled water, the fluid chemistry results show elevated SiO2, K, Cl, SO4, and H2S in solution for the first 12 days of both experiments. This is due to volcanic glass dissolution. After glass was removed, fluid composition was controlled by the remaining rock minerals. At 400˚C, the secondary mineral assemblage at the bottom of the Reactor (fluid entry point) is composed of grossular, wollastonite, anorthite, and chlorite. These results show the effectiveness of distilled water, which lacks any alkali cations, at removing Na and K rapidly from the rock. At the top of the Reactor (fluid exit point) the secondary minerals are anorthite and celadonite. At 350˚C, the secondary mineral assemblage at the bottom is anorthite and chlorite, while celadonite is the dominant secondary mineral at the top. In both experiments, celadonite replaces solely olivine. The formation of celadonite through reaction with distilled water shows that it can be formed by the interaction of deuteric water and basalt without addition of other components.</p> <p> The geothermal brine contains high concentrations of SiO2, K, SO4, Na, Cl and has an acidic pH. At 400˚C, fluid chemistry displays elevated SiO2 concentrations for approximately two weeks due to glass dissolution. At 350˚C, SiO2 concentration is initially high after temperature increase, but decreases gradually over the remainder of the experiment. At 400˚C, the secondary mineral assemblage at the bottom of the Reactor is composed of anhydrite and biotite, while at the top of the Reactor, smectite is the only secondary mineral. At 350˚C, anhydrite and smectite are found at the bottom, while only smectite is found at the top. The lack of biotite at 350˚C suggests this mineral’s precipitation kinetics are too slow to outcompete chlorite precipitation.</p> <p> The seawater-only experiments were conducted as controls to determine its behaviour during heat-up and provide the input solution composition for the seawater-basalt experiments. Both seawater-only experiments (377˚C and 342˚C) show the precipitation of anhydrite, caminite and brucite due to their retrograde solubilities. The effluent solutions are greatly depleted in Ca, Mg and SO4.</p> <p> In the seawater-basalt experiments at near-supercritical (400˚C) and subcritical conditions (350˚C), elevated SiO2 concentrations due to glass dissolution are not observed. This is attributed to rapid secondary mineral precipitation. Fluid chemistry and mass balance calculations show almost complete removal of SO4, and in particular, Mg, from the seawater while Ca shows a considerable loss from the rock. Three mineralization fronts were identified: (1) glass dissolution; (2) chloritization; and (3) anhydrite precipitation. In both experiments, there is a switch from chloritization to smectitization. This is accompanied by a decrease in Mg/Fe ratio in smectite. This mineral was also found at the top of both experiments, but its composition was more reflective of the rock.</p> <p>In terms of reactivity, the order of phases from most to least reactive is glass – olivine – clinopyroxene – plagioclase – Fe-Ti oxide. For the aluminosilicate phases this is attributed their respective Al contents. The seawater-basalt experiments also emphasise the fast rate of reaction at which Mg is fixed by the rock, which is conjectured to take less than a few hours.</p> <p>Considering all experiments, the distilled water results show a rock control on fluid chemistry while in the remaining basalt experiments, the chemistry is largely controlled by the fluid.</p> <p>Temperatures calculated using standard Na/K geothermometer did not estimate, in most cases, values close to the experimental temperature. This is due to the inability of the rock to sufficiently adjust the Na/K ratio given the secondary mineral assemblages that form.</p> <p> </p>

Depositional mode and Provenance of Kopili sandstones from parts of Dima Hasao District, Assam, NE India: Evidence from Grain size distribution and Heavy mineral assemblage

This study presents a record of the depositional mode and provenance of the Kopili sandstones outcropped in and around Umrangso, the Dima Hasao district of Assam, India using proxies such as grain size and heavy mineral study. The Grain size study reveals that the sandstones of the Kopili formation consist predominantly of fine-grained sands mixed with medium sands. The grain size statistical parameters divulge that the sandstones are characterized mostly by moderately sorted sands, coarse skewed to near symmetrical, leptokurtic to extremely leptokurtic nature. The frequency distribution curves reflect mostly bimodal as well as polymodal distribution of sediments. The Linear Discriminant Function analysis indicates shallow marine beach sub environment in agitated water depositional condition for the Kopili sediments. The CM pattern discloses that saltation and suspension are the dominant transporting modes. The Heavy mineral analysis discerns that opaques dominate over transparent heavies. Transparent heavies in decreasing order of abundance are zircon, tourmaline, rutile, clinopyroxene, staurolite, hornblende, garnet, chlorite and epidote. The zircontourmaline- rutile (ZTR) maturity index (average 53.15%) discloses sub-mature to mature nature of the Kopili sandstones. The heavy mineral assemblage corroborates their mixed provenance (silicic igneous and metamorphic) for the Kopili sandstones.

Hadean zircon formed due to hydrated ultramafic protocrust melting

Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (&lt;10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of &lt;750 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth.

Metamorphism of metachert from the Southern Alps, New Zealand. A thermodynamic forward modelling study

<p>Scattered, scarce occurrences of garnet- and quartz-rich metamorphic rock, probably derived from Mn- and Fe-rich chert, occur within metamorphosed greywacke sequences worldwide. The metamorphism of such garnetiferous metacherts has not previously been investigated using modern thermodynamic forward modelling techniques due to the lack of appropriate, internally-consistent activity-composition (a–x) models for Mn-bearing minerals. The present study applies thermodynamic forward modelling using the recently-proposed a–x models of White et al. (2014) to investigate the metamorphism of garnetiferous metachert samples from the Southern Alps, New Zealand. Pressure-temperature (P–T) pseudosections are used in combination with results from petrography, element composition mapping using micro X-ray fluorescence (µXRF) and scanning electron microscope (SEM) methods, and garnet composition data from analytical transects by electron probe microanalysis (EPMA), to study metachert metamorphism. All the samples are compositionally layered, so the possibility exists that an input bulk rock composition might not match the effective bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection matched the mineral assemblage in the rock, this was taken as an initial indication of a permissible input bulk rock composition. In that case, refined constraints on the P–T conditions were sought by comparing calculated and measured garnet compositions. The studied rocks include samples that are carbonate-bearing, which require consideration of the effects of fluid composition in mixed H₂O–CO₂ fluids, as well as a sample in which the garnet is strongly zoned, texturally-complex, and inferred to be of polymetamorphic origin. The effects of element fractionation by that garnet were investigated by recalculating the P–T pseudosection using a new bulk rock composition with the garnet core content removed. In none of the samples did the calculated and observed composition isopleths for the garnet cores match, suggesting that initial garnet nucleation in these Mn-rich rocks was locally controlled. For most samples in which the calculated and observed mineral assemblages matched, successful estimates of the peak metamorphic conditions were obtained. A garnet chert (A12E) from the mylonite zone of the Alpine Fault at Vine Creek, near Hokitika, gave a tight intersection of composition isopleths, indicating peak metamorphic conditions of 510 °C/5.5 kbar, after recalculation to correct for element fractionation by the strongly-zoned garnet. This tight, modern constraint is within error of previously-reported results from traditional geothermobarometry (420–600 °C/5.9–13 kbar) and Raman spectroscopy of carbonaceous material (RSCM T = 556 °C) from nearby sites. A peak metamorphic estimate of 520–550 °C/7–10 kbar was obtained from a dolomite-bearing sample from the garnet zone near Fox Glacier (J34), in good comparison with published temperatures from Raman spectroscopy of carbonaceous material in nearby metagreywacke samples (526–546 °C). The prograde metamorphic P–T path was probably steep, based on growth of the garnet core at ~475535 °C/5–9 kbar. The successful results for these garnet chert samples show that the new a-x models for Mn-bearing minerals extend the range of rock types that are amenable to pseudosection modelling. Results obtained in this study also serve to highlight several possible concerns: a) garnet nucleation and initial growth in very Mn-rich rocks may be subject to local compositional or kinetic controls; b) bulk rock compositions may not always mimic the effective bulk composition; c) the existing a–x models for Mn-bearing minerals and white micas may need refining; and d) some rocks may simply be ill-suited to thermodynamic forward modelling. Items a) and b) may be indicated by the common observation of a mismatch between predicted and measured garnet composition isopleths for garnet cores, and by a mismatch between garnet composition isopleths and the appropriate mineral assemblage field for sample AMS01, from the mylonite zone, Hari Hari, Southern Alps. For item c) every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals showed garnet stable to very low temperatures below 300 °C. In addition, the P–T pseudosection for an oligoclase-zone metachporphyroblasts of Fe-Ti oxides (magnetitert (Sample J36) from Hari Mare stream, Franz Josef - Fox Glacier, indicated that the white mica margarite should be present instead of plagioclase (oligoclase), for a rock in which oligoclase is present and margarite is absent, a problem previously noted elsewhere. Item d) is exemplified by a very garnet-rich ferruginous metachert sample (J35, garnet zone, headwater region, Moeraki River, South Westland) which proved impossible to model successfully due to its complex mineral growth and deformation history. This sample contained multiple generations of carbonate with differing compositions, amphibole (not incorporated for modelling with the new a–x models for Mn-bearing minerals), large e associated with smaller, possibly later-formed ilmenite), and the garnet bands were offset by late deformation. The garnetiferous metachert samples studied here preserve in their textures and compositions clues to their growth mechanism and metamorphic history. The textures in at least two of the samples are consistent with the diffusion controlled nucleation and growth model for garnet. This research has successfully used state of the art thermodynamic modelling techniques in combination with the latest internally consistent a-x models on Mn-rich metachert, for the first time, extracting P–T conditions of the metamorphism of garnetiferous metachert from the Southern Alps.</p>

Metamorphism of metachert from the Southern Alps, New Zealand. A thermodynamic forward modelling study

<p>Scattered, scarce occurrences of garnet- and quartz-rich metamorphic rock, probably derived from Mn- and Fe-rich chert, occur within metamorphosed greywacke sequences worldwide. The metamorphism of such garnetiferous metacherts has not previously been investigated using modern thermodynamic forward modelling techniques due to the lack of appropriate, internally-consistent activity-composition (a–x) models for Mn-bearing minerals. The present study applies thermodynamic forward modelling using the recently-proposed a–x models of White et al. (2014) to investigate the metamorphism of garnetiferous metachert samples from the Southern Alps, New Zealand. Pressure-temperature (P–T) pseudosections are used in combination with results from petrography, element composition mapping using micro X-ray fluorescence (µXRF) and scanning electron microscope (SEM) methods, and garnet composition data from analytical transects by electron probe microanalysis (EPMA), to study metachert metamorphism. All the samples are compositionally layered, so the possibility exists that an input bulk rock composition might not match the effective bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection matched the mineral assemblage in the rock, this was taken as an initial indication of a permissible input bulk rock composition. In that case, refined constraints on the P–T conditions were sought by comparing calculated and measured garnet compositions. The studied rocks include samples that are carbonate-bearing, which require consideration of the effects of fluid composition in mixed H₂O–CO₂ fluids, as well as a sample in which the garnet is strongly zoned, texturally-complex, and inferred to be of polymetamorphic origin. The effects of element fractionation by that garnet were investigated by recalculating the P–T pseudosection using a new bulk rock composition with the garnet core content removed. In none of the samples did the calculated and observed composition isopleths for the garnet cores match, suggesting that initial garnet nucleation in these Mn-rich rocks was locally controlled. For most samples in which the calculated and observed mineral assemblages matched, successful estimates of the peak metamorphic conditions were obtained. A garnet chert (A12E) from the mylonite zone of the Alpine Fault at Vine Creek, near Hokitika, gave a tight intersection of composition isopleths, indicating peak metamorphic conditions of 510 °C/5.5 kbar, after recalculation to correct for element fractionation by the strongly-zoned garnet. This tight, modern constraint is within error of previously-reported results from traditional geothermobarometry (420–600 °C/5.9–13 kbar) and Raman spectroscopy of carbonaceous material (RSCM T = 556 °C) from nearby sites. A peak metamorphic estimate of 520–550 °C/7–10 kbar was obtained from a dolomite-bearing sample from the garnet zone near Fox Glacier (J34), in good comparison with published temperatures from Raman spectroscopy of carbonaceous material in nearby metagreywacke samples (526–546 °C). The prograde metamorphic P–T path was probably steep, based on growth of the garnet core at ~475535 °C/5–9 kbar. The successful results for these garnet chert samples show that the new a-x models for Mn-bearing minerals extend the range of rock types that are amenable to pseudosection modelling. Results obtained in this study also serve to highlight several possible concerns: a) garnet nucleation and initial growth in very Mn-rich rocks may be subject to local compositional or kinetic controls; b) bulk rock compositions may not always mimic the effective bulk composition; c) the existing a–x models for Mn-bearing minerals and white micas may need refining; and d) some rocks may simply be ill-suited to thermodynamic forward modelling. Items a) and b) may be indicated by the common observation of a mismatch between predicted and measured garnet composition isopleths for garnet cores, and by a mismatch between garnet composition isopleths and the appropriate mineral assemblage field for sample AMS01, from the mylonite zone, Hari Hari, Southern Alps. For item c) every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals showed garnet stable to very low temperatures below 300 °C. In addition, the P–T pseudosection for an oligoclase-zone metachporphyroblasts of Fe-Ti oxides (magnetitert (Sample J36) from Hari Mare stream, Franz Josef - Fox Glacier, indicated that the white mica margarite should be present instead of plagioclase (oligoclase), for a rock in which oligoclase is present and margarite is absent, a problem previously noted elsewhere. Item d) is exemplified by a very garnet-rich ferruginous metachert sample (J35, garnet zone, headwater region, Moeraki River, South Westland) which proved impossible to model successfully due to its complex mineral growth and deformation history. This sample contained multiple generations of carbonate with differing compositions, amphibole (not incorporated for modelling with the new a–x models for Mn-bearing minerals), large e associated with smaller, possibly later-formed ilmenite), and the garnet bands were offset by late deformation. The garnetiferous metachert samples studied here preserve in their textures and compositions clues to their growth mechanism and metamorphic history. The textures in at least two of the samples are consistent with the diffusion controlled nucleation and growth model for garnet. This research has successfully used state of the art thermodynamic modelling techniques in combination with the latest internally consistent a-x models on Mn-rich metachert, for the first time, extracting P–T conditions of the metamorphism of garnetiferous metachert from the Southern Alps.</p>