Secondary Mineral Assemblages Research Articles

Thermobarometric constraints on burial and exhumation of 2-billion-year-old eclogites and their metapelitic hosts

One of the first appearances of eclogite-facies mineral assemblages in the geological record occurs in the c. 2000 Ma Palaeoproterozoic Usagaran Belt in central Tanzania, where the extended margin of the Tanzanian Craton is interpreted to have been subducted. Mafic rocks are interpreted to have contained the mineral assemblage garnet + omphacite + rutile + quartz ± amphibole. This high-pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene + plagioclase + hornblende + ilmenite ± orthopyroxene. Mineral equilibria forward modelling indicates that the eclogite-facies assemblages reached minimum peak pressure–temperature (P–T) conditions of ~17 kbar and ~700 °C. Inclusions in garnet document a prograde P–T history consistent with burial through upper amphibolite-facies conditions and possible partial melting. Petrological and compositional evidence from garnet suggests that following peak metamorphism, the eclogite-facies rocks were heated while stalled at approximate peak pressures. Temperature estimates derived from Zr concentrations in interpreted texturally retrograde rutile support a near-isothermal post-peak P–T evolution for the eclogite-facies rocks – an evolution that terminates at retrograde P–T conditions of approximately 7.6–8.2 kbar and 680–790 °C. The relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies (the Isimani Suite). The metapelitic gneisses contain garnet + kyanite + biotite + staurolite + hornblende + plagioclase + muscovite + rutile + quartz and preserve minimal evidence of a high-pressure history, conceivably due to post-peak mineralogical recrystallisation. P–T modelling, inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses — similarly to the relict eclogites — experienced burial to minimum peak pressures of approximately 16.5–17 kbar. Compositional zoning patterns in eclogitic garnet suggest the Isimani system was buried, reached peak metamorphic conditions, and was subsequently exhumed within a timeframe of up to 20 Myr. A tectonic regime involving crustal thickening and subduction, followed by extensional exhumation of the entire Isimani Suite is our preferred model for the development of the c. 2000 Ma Usagaran Belt.

A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels.

Underground repository in crystalline bedrock is a widely accepted solution for long-term disposal of spent nuclear fuels. During future deglaciations, meltwater will intrude via bedrock fractures to the depths of future repositories where O2 left in the meltwater could corrode metal canisters and enhance the migration of redox-sensitive radionuclides. Since glacial meltwater is poor in reduced phases, the quantity and (bio)accessibility of minerogenic Fe(II) in bedrock fractures determine to what extent O2 in future meltwater can be consumed. Here, we determined Fe valence and mineralogy in secondary mineral assemblages sampled throughout the upper kilometer of fractured crystalline bedrock at two sites on the Baltic Shield, using X-ray absorption and Mössbauer spectroscopic techniques that were found to deliver matching results. The data point to extensive O2-consuming capacity of the bedrock fractures, because Fe(II)-rich phyllosilicates were abundant and secondary pyrite was dispersed deep into the bedrock with no overall increase in Fe(II) concentrations and Fe(II)/Fe(III) proportions with depth. The results imply that repeated Pleistocene deglaciations did not cause a measurable decrease in the Fe(II) pool. In surficial fractures, largely opened during glacial unloading, ferrihydrite and illite have formed abundantly via oxidative transformation of Fe(II)-rich phyllosilicates and recently exposed primary biotite/hornblende.

The igneous petrogenesis and rare metal potential of the peralkaline volcanic complex of the southern Peak Range, Central Queensland, Australia

The Oligocene Peak Range Volcanics (PRV) of central Queensland have intruded through, and erupted onto, Permian sedimentary rocks of the Bowen Basin above what is likely to be a lithospheric-scale suture zone. The southern PRV consists of domes and flows of augite (bearing) trachyte, hornblende trachyte, hornblende rhyolite, arfvedsonite rhyolite and aegirine rhyolite. The aegirine rhyolite bodies represent the most evolved rock types, with peralkaline index (molar Na + K/A) values of >1.3 and extreme enrichment in trace elements, including the rare metals Zr, Hf, Nb, Ta, and REE. Clarrys Dome, one of three aegirine rhyolite domes, features a late stage magmatic agpaitic assemblage of dalyite (K2ZrSi6O15), a eudialyte-like mineral and aegirine, which is variably replaced and overprinted by secondary REE carbonates and Zr silicates.Major and trace element geochemistry are used to show that the suite of PRV rock types represent a fractionation sequence from the least evolved augite trachytes to the most evolved aegirine rhyolites. This rock type range can be modelled by extended alkali-feldspar-dominated fractional crystallisation of an alkali basalt parental melt at a shallow crustal levels. Initial εNd values across the range of rock types are between +3 to +4, which is consistent with a common mantle source. Thus, the extreme trace element enrichment and high peralkalinity of the aegirine rhyolites is the product of the extended fractionation of a melt formed by low-degree partial melting of a mildly depleted mantle source. The secondary ore mineral assemblage and REE redistribution observed in the core of Clarrys Dome is interpreted to be due to a combination of hydrothermal alteration by fluids derived from subjacent devolatilising magma bodies, and late-stage weathering.The extreme enrichment of rare metals in the aegirine rhyolite bodies in the southern PRV support their potential to represent low grade, but large tonnage (100 s of Mt) resources of rare metals. Clear evidence of hydrothermal alteration across the region also raised the potential that higher grade rare metal mineralisation produced by hydrothermal activity may exist in the region.

A multidisciplinary approach to investigate weathering processes affecting gneissic rocks (Calabria, southern Italy)

The paper focuses on the weathering processes affecting gneissic rocks of the western Sila Grande Massif (Calabria, Italy) through the development of an interdisciplinary research based on field studies and investigations, minero-petrographical analyses and geochemical modeling. Both physical and chemical weathering affect gneissic rocks of the study area, where the combination of tectonic and past climatic conditions played an important key role in the development of complex and deep weathering profiles. Field surveys and cut slope analyses highlight articulated and complex geometric relationships between various classes of weathering (i.e., out-of-sequence weathering horizons giving rise to a partial, or even complete inversion of the “normal” weathering profile). The weathering profile has turned out particularly intense, as classes IV, V and VI are widespread. Among the class VI, the colluvial soils are prominent in comparison whit the residual soils. Borehole logs, that confirm the intensity and complexity of weathering profiles in deep, allow to estimate the presence of weathered rocks to be at least 70 m in thicknesses.The main mineralogical modifications linked to weathering processes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary mineralogical phases (clay minerals and Fe-oxides). Neoformed clay minerals (such as sericite, illite and mixed layer phases) and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Results of XRD analyses and geochemical modelling provide a good indication on the secondary mineral assemblage due to the increase of the weathering intensity (form class III to class VI) that reflects the different contributions of chemical elements provided by dissolution of silicate minerals into the surrounding groundwater system. The chemical composition of the studied rock samples and the oxides variations suggest a removal of some alkali (Na and K) and alkaline earth (Ca and Mg) into solution as a consequence of weathering reactions. The chemical analysis and the weathering indices (CIA, PIA, CIW and CIW’) show a marked alteration process, with ferromagnesian minerals and feldspars probably dissolved and leached into the surrounding groundwater system. The CO2-controlled dissolution of plagioclase appears to be the most important reaction during chemical weathering. The progressive dissolution results then dominated by biotite, followed by a minor amounts K-feldspar, chlorite and garnet; whereas the sillimanite shows a neglectable amounts. The secondary solid phases observed during the geochemical modeling (illite, followed by vermiculite, ferrihydrite and saponite) are similar to those found in this natural system. The proposed approach could be used to characterize weathered crystalline rocks and related weathering profiles in similar geological setting, and the obtained results represent a key point for the evaluation of the control exerted by weathering on landscape evolution under current environmental settings in terms of sediment generation, soil erosion rates, and mass movements, and for the mechanical characterization of weathering profiles for engineering geological purposes.

Mineralogical and geochemical studies of secondary mineral assemblages related to deterioration of building materials

The study was aimed at mineralogical and geochemical characteristics of secondary phases related to deterioration of historic building materials. The investigations, carried out in the Holy Trinity Basilica in Krakow, Poland, focused on the southern facade of the 17th-century Myszkowskis Chapel, built of the Miocene Pinczow limestone. Lower part of the facade is covered with a cement render, and the exposed foundations are made of Jurassic limestone and Cretaceous sandstone, both of local origin from the Krakow region and neighbouring Carpathians, in the form of irregular blocks bound with a cement mortar. The wall surface exhibits clear signs of damage; from dark grey soiling and scaling to efflorescences. Sampled materials, deteriorated, altered crusts and efflorescences were analysed with optical and scanning electron microscopy, X-ray diffraction, and Raman microspectroscopy methods. The secondary minerals distinguished include abundant gypsum CaSO 4 ·2H 2 O, less common thenardite Na 2 SO 4 (and/or mirabilite Na 2 SO 4 ·10H 2 O), aphthitalite (K, Na) 3 Na(SO 4 ) 2 , darapskite Na 3 (SO 4 )(NO 3 )·H 2 O, ettringite Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26H 2 O, monosulphite Ca 4 Al 2 O 6 SO 3 ·11H 2 O, as well as scarce nitre KNO 3 , nitratine NaNO3 and halite NaCl. Gypsum usually forms surface crusts and fills the pores inside some materials. The efflorescences, sampled from the exposed foundations, consisted of thenardite and/or mirabilite, aphthitalite and darapskite, whereas ettringite and monosulphite were connected with cement renders. Traces of nitre, nitratine and halite were detected at various elements of the chapel facade and foundations. The origin of the salts is related to composition and physicochemical properties of the building materials, as well as to anthropogenic factors.

Impact glass applied as a standard for long-term performance assessment of Na–Ba borosilicate glass forms in geological environment

High-level nuclear waste is usually immobilized into high silica glass matrix for long-term and safe disposal in the geological repository. Performance of glass forms is uncertain for the lack of standardization against a natural analogue. Thus, alteration studies were performed on the impact glass as it is surviving in a natural environment since its inception (570 ka). Further, these results were compared to laboratory induced alteration in the impact glass and Na and Ba borosilicate glass from the literature. Alteration experiments were carried out in the de-ionized water under accelerated conditions at 100 °C temperature and 17.24 bar pressure in a variable time framework. Owing to rapid dissolution of synthetic glass, Al3+, Ca2+, K+, Mg2+, Na2+ and Si4+ were released into the solution and finally secondary silicates (SS) were formed in the residue. Comparing to impact glass, a three-fold higher alteration rate is noticed in the Na and Ba borosilicate glass forms. X-ray diffraction (XRD) and scanning electron microscopic (SEM) - energy dispersive X-ray spectrometric (EDS) analyses revealed formation of secondary mineral assemblages (analcime, aragonite, illite, montmorillonite, saponite and stilbite) for both naturally and experimentally altered impact glass types. Moreover, nontronite phase was formed as a major post-alteration product in Na and Ba borosilicate glass forms. The neo-formed SS were compared to naturally derived SS from the impact glass in three time domains of (1) 570 to 5.41 ka, (2) 5.41 to 0.53 ka and (3) 0.53 to Recent Period as determined by high resolution 14C Accelerator Mass Spectroscopy (AMS). Naturally altered impact glass presents paragenetic order of saponite > calcite > montmorillonite > montmorillonite/chlorite mixed layer > siderite > chabazite > aragonite > analcime. As compared to the impact glass, synthetic glass structure is very weak and friable towards alteration. Former attended as a more reliable standard glass.

Geology and fluid geochemistry of the Na-metasomatism U deposits in the Longshoushan uranium metallogenic belt, NW China: Constraints on the ore-forming process

The Longshoushan belt in Gansu Province is a very important uranium metallogenic belt in Northwest China and contains a series of different types (magmatic, Na-metasomatism, supergenetic) of uranium deposits and occurrences. Of all these mineralization types, the Na-metasomatism type is the most predominant with great potential, represented by two Na-metasomatism uranium deposits (Xinshuijing and Jiling). Different from the fluorite-, illite-, quartz-dominated alteration minerals in other hydrothermal uranium mineralization system, the Na-metasomatism type uranium deposits are characterized by the unique albite-dominated (up to 90 vol%) alteration mineral assemblage, which is referred to albitite in many literatures. All the uranium orebodies in both the Xinshuijing and Jiling deposits are hosted within the metasomatic albitites, and controlled by the subsidiary faults of the regional Malugou Fault. The mineralization process in both deposits can be divided into four stages, namely: the Na-metasomatism stage, U mineralization stage, post-mineralization stage and supergene mineralization stage. The Na-metasomatism stage is predominantly composed of albite (with hematite staining), with minor chlorite and anatase, while the U mineralization stage is characterized by the occurrence of uranium minerals (mostly pitchblende), intergrown with some tiny albite, calcite, chlorite, pyrite, apatite, REE-minerals and sometimes hydrothermal zircons. The mineral assemblages of the Na-metasomatism stage and U mineralization stage are too intimately related to each other and might be the products of the evolution from the same ore-forming fluids. The post-mineralization stage is mainly the calcite-quartz veinlets or occur as aggregate clusters, while the supergene mineralization stage includes mineral assemblage of secondary uranium mineralization-clay minerals (e.g. kaolinite)-limonite. Mineralogical studies and comparison of major and trace elements assay results between fresh biotite granite (predominate host rocks), albitite and uranium ores suggest that the ore-forming fluids resulting in the formation of the Na-metasomatism alteration are oxidized and rich in Na, Sc, V, Cr, Co, Ni, U, Th, while the ore-forming fluids of the U mineralization stage are relatively reduced and rich in heavy rare earth elements, U, Th, PO34-. Fluid inclusion and calcite C-O isotope studies indicate that the ore-forming fluids are meteoric water-dominated, which circulated at depth (~8 km, by fluid inclusion geobarometer) and scavenged uranium from the host biotite granites and metamorphic rocks of the Longshoushan Group. The ore precipitation is mainly attributed to physicochemical changes (fO2, temperature, fluid composition) from fluid-rock interaction and further temperature decrease of the ore-forming fluids, while fluid boiling and fluid mixing processes are limited. The proposed ore-forming process is not only important for the understanding of the ore genesis for the Na-metasomatism mineralization systems in Longshoushan, but also highlights the significance of albite alteration, which could be used as an important indicator during further uranium exploration.

Multivariate Analysis Based on Geochemical, Isotopic, and Mineralogical Compositions of Uranium-Rich Samples

The chemical and isotopic (U, Pb, Sr) signatures for a suite (n = 23) of pristine (>80 wt. % UO2) and altered uraninite samples (>70–80 wt. % UO2) from various locations worldwide have been determined for the purpose of identifying potential fingerprints for nuclear forensic analysis. The characterization of the uraninite samples included determination of major, minor and trace element contents, Sr, Pb, and U isotopic compositions, and secondary mineral assemblages. Due to the multivariate approach adopted in this study, principal component analysis (PCA) has been employed to allow the direct comparison of multiple variable types. The PCA results indicate that the geological origin (sandstone, metamorphite, intrusive, granite and unconformity) of pristine uraninite can be readily identified utilizing various combinations of major and/or trace element concentrations with isotopic compositions.

Coesite inclusion in diamondferous kyanite-bearing eclogite from kimberlite pipe Udachnaya (Siberian Craton)

A find of coesite in a kyanite graphite-diamond-bearing eclogite xenolith from the Udachnaya-Vostochnaya kimberlite pipe is described in this paper. The coesite relics were found in intensely fractured garnet indicating some influence of the kimberlite melt, which is supported by the typical secondary mineral assemblage around this inclusion. These data indicate that shallower diamond-free coesite rocks (2,7 GPa) underwent metamorphism distinct from diamond-bearing coesite eclogites (~4 GPa). The metasomatic alteration of rock interacting with C-O-H fluid during diamond crystallization may be another possible reason for the missing coesite in diamond-bearing xenoliths.

A Find of Coesite in Diamond-Bearing Kyanite Eclogite from the Udachnaya Kimberlite Pipe, Siberian Craton

A find of coesite in a kyanite graphite–diamond-bearing eclogite xenolith from the Udachnaya–Vostochnaya kimberlite pipe is described in this paper. The coesite relics were found in intensely fractured garnet indicating some influence of the kimberlite melt, which is supported by the typical secondary mineral assemblage around this inclusion. These data indicate that shallower diamond-free coesite-grade rocks (2.7 GPa) underwent metamorphism distinct from diamond-bearing coesite eclogites (≥4 GPa). The metasomatic alteration of rock as a result of the C–O–H fluid-rock interaction during diamond crystallization may be another possible reason for the absence of coesite in diamond-bearing xenoliths.

The tectonic evolution of the Dras arc complex along the Indus Suture Zone, western Himalaya: Implications for the Neo-Tethys Ocean geodynamics

In this study we present new whole-rock geochemical data on the Dras arc complex exposed along the Dras–Kargil National Highway, western Ladakh. This complex forms a part of the Indus Suture Zone, which is a major tectonic unit extending along the Himalaya from the Pakistan in the west to the Tibet in the east. The Dras arc complex is dominated by mafic volcanics, with subordinate gabbros at the base exposed near Kargil. The Dras volcanics have augite and plagioclase phenocrysts embedded in a groundmass of plagioclase and pyroxene with porphyritic, intergranular and ophitic textural relationships. Also chlorite and epidote are present as secondary mineral assemblage in these volcanics. Plagioclase and pyroxene are the dominant mineral assemblage in the Kargil gabbro, with minor amounts of oxides and relict olivine. These rock types are characterized by ophitic textures and are devoid of any secondary mineral assemblage when compared to the mafic volcanics, which have suffered greenschist metamorphism. Geochemically, the Dras volcanics and the Kargil gabbros are classified as sub-alkaline tholeiites and show a compositional variation from basalt to basaltic andesite. On a Nb/Yb vs. Th/Yb diagram, they fall within the compositional field of arc-related rocks. NMORB-normalized multi-element patterns shows enrichment in large ion lithophile elements (e.g., Rb, Ba, Th, U, K and Pb) and light rare earth elements (LREE), but a depletion in high field strength elements (e.g., Nb, P, Zr and Ti), which supports a subduction-related origin. Lower Ba/La and La/Yb ratios indicate contribution from fluid fluxing, rather than slab melts to the mantle wedge. The Dras volcanics show similar geochemical trends to volcanic arc rocks from the Nidar and the Spongtang ophiolite complex of the Ladakh Himalaya as well as the Hasanbag and Kata Rush arc rocks from the Iraqi Zagros Suture, having similar age relationships in the context of the Neo-Tethys Ocean.

Fluid Chemistry of Mid-Ocean Ridge Hydrothermal Vents: A Comparison between Numerical Modeling and Vent Geochemical Data

Seawater-basalt interaction taking place at mid-ocean ridges was studied using numerical modeling to determine the compositional evolution of hydrothermal fluids and associated alteration mineralogy forming within newly emplaced crustal material. Geochemical modeling was carried out in a closed seawater-basalt system at discrete temperature intervals between 2 and 400°C at 500 bars, varying fluid/rock ratios, and secondary mineral assemblages representative of basalt alteration in natural systems. In addition to temperature, the fluid/rock ratio has a fundamental control on the resulting system chemistry. At rock-buffered conditions (low fluid/rock ratios), the mineral-solution equilibrium was characterized by high cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and very low dissolved Mg concentrations due to the precipitation of smectites and chlorite. A complex secondary mineral alteration assemblage dominated by Ca- and Na-bearing minerals including zeolites, calcite, epidote, prehnite, clinozoisite, and albite was predicted to form. The resulting fluid composition was alkaline and reduced relative to ambient seawater, with Eh values ranging between −0.2 and −0.6 V. In contrast, seawater-buffered conditions (high fluid/rock ratios) resulted in lower cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and higher dissolved Mg concentrations comparable to the value of this element in ambient seawater. A more simple mineral assemblage was predicted to form at these conditions with the predominance of Al-Si- and Mg-bearing minerals including kaolinite, quartz, and talc in addition to large amounts of anhydrite. The resulting fluid composition was mildly acidic and oxidized relative to seawater with Eh values ranging between −0.2 and 0 V. These modeling results were compared to a compilation of submarine hydrothermal vent fluid compositions from mid-ocean ridge settings and analogous basalt-dominated environments. The agreement obtained between the simulations and the compiled fluid data indicates that mid-ocean ridge hydrothermal processes can be closely reproduced by mineral-solution equilibria for a broad range of temperatures and fluid/rock ratios.

Hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite, Oman

Hydrothermal circulation is a key process for chemical and isotopic exchange between the solid Earth and oceans, and for the extraction of heat from newly accreted crust at mid-ocean ridges. However, due to a dearth of samples from intact oceanic crust, or continuous sample suites from ophiolites, there remain major shortcomings in our understanding of hydrothermal circulation in the oceanic crust, especially in the lower plutonic crust. In particular, it remains unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channelled within discrete zones such as faults.Here, we describe a hydrothermally-altered fault zone that crops out in the Wadi Gideah in the layered gabbro section of the Samail ophiolite of Oman. A one metre thick normal fault comprising deformed chlorite ± epidote fault rock with disseminated chalcopyrite and pyrite, offsets gently dipping layered olivine gabbros. The chlorite-rich fault rocks surround strongly altered clasts of layered gabbro, 50 to 80 cm in size. Layered gabbros in the hanging wall and the footwall are partially altered and abundantly veined by epidote, prehnite, laumontite and calcite veins. In the wall rocks, igneous plagioclase (An82±2%) is partially altered towards more albitic compositions (An75-81), and chlorite + tremolite partially replaces plagioclase and clinopyroxene. Clinopyroxene is moderately overgrown by Mg-hornblende.Whole rock mass change calculations show that the chlorite-rich fault rocks are enriched in Fe, Mn, H2O + CO2, Co, Cu, Zn, Ba, and U, but have lost significant amounts of Si, Ca, Na, Cr, Ni, Rb, Sr, Cs, light rare earth elements (LREE), Eu, and Pb. Gabbro clasts within the fault zone as well as altered rock from the immediate hanging wall show enrichments in Na, volatiles, Sr, Ba and U and depletions of Si, Ti, Al, Fe, Mn, Mg, Ca, Cu, Zn, Rb, Cs, LREE, and Pb. Chlorite thermometry suggests a formation temperature of 300–350 °C for the fault rock and based on the Si loss and solubility of silica in hydrothermal fluids the intensity of alteration requires a fluid to rock ratio of up to 900:1. Strontium isotope whole rock data of the fault rock yield 87Sr/86Sr ratios of 0.7043–0.7048, which is considerably more radiogenic than fresh layered gabbro from this locality (87Sr/86Sr = 0.7030–0.7033), and similar to black smoker hydrothermal signatures based on epidote, measured from epidote veins in the footwall and elsewhere in the ophiolite (87Sr/86Sr = 0.7043–0.7051). Altered gabbro clasts within the fault zone show similar values with 87Sr/86Sr ratios of ~0.7045–0.7050. In contrast, the hanging and footwall gabbros display values only slightly more radiogenic than fresh layered gabbro. The elevated strontium isotope composition of the fault rock and clasts together with the observed secondary mineral assemblages and calculated mass changes strongly supports the intense interaction with seawater-derived up-welling hydrothermal fluids, active during oceanic spreading. Assuming that such a fault zone is globally representative of faulting in the lower crust, an extrapolation of our results from mass change calculations to elemental fluxes, shows a significant contribution to the global hydrothermal budgets of Si, Ti, Fe, Mn, Mg, Ca, H2O, Cu, Zn, Sr and Cs.

Microanalysis and genesis of Cu-Ag minerals in metadiabase dykes at the Jabal Zalm area, Saudi Arabia

Secondary Cu and Cu-Ag sulphides are reported from altered metadiabases in the vicinity of a Neoproterozoic ophiolitic mélange at the Zalm area in Saudi Arabia. The metadiabases occur in the form of NE-trending dykes forming resistant ridges with occasional chilled margins against a serpentinite host. Microscopic investigations revealed the occurrence of a peculiar assemblage of secondary Cu-bearing minerals down to the nanoscale. In most cases, secondary Cu and Cu-Ag sulphides formed at the expense of disseminated chalcopyrite (presumably argentian)± galena as the hypogene ore minerals. The most common textures are exclusively secondary and they are represented by “atoll” structure and rim replacement textures. The identification of ore minerals is based on combined ore microscopy and microchemistry of minerals. An assemblage of anilite-geerite-chalcocite intergrowth is recorded as the Cu sulphides, whereas mckinstryite (Ag1.18-1.19Cu0.80-0.81Fe0.01S) and stromeyerite (AgCu0.99Fe0.01S) are the extremely minute Cu-Ag sulphides that never exceed 5 μm in width. Ore fabrics and back-scattered electron (BSE) images clearly indicate that all phases of sulphides are secondary with some limited cationic substitution. Non-sulphides secondary Cu minerals are much more abundant and they are confined to rock fractures with visible hydrated Cu silicate (chrysocolla rimming hydrated Cu sulphate, as chalcanthite) that is either weakly crystalline or amorphous as indicated by the XRD data.Based on the chemical composition, ore textures and comparison with some world examples, it is believed that the Zalm Cu-Ag sulphides formed under oxidizing condition (pH = 5.8–7.2). As the temperature decreases, residual (stoichiometric) Ag2S is mobile in meteoric water. Precipitation of residual Ag continued down to 93.3 °C to form stromeyerite (i.e. late hydrothermal). During late stages of supergene alteration at the studied Zalm occurrence, loss of CO2 leads to the formation of Cu-bearing carbonates, sulphates and finally silicates at a very characteristic low S2 fugacity at shallow depths or at surface (i.e. supergene).

Visible, near-infrared, and mid-infrared spectral characterization of Hawaiian fumarolic alteration near Kilauea's December 1974 flow: Implications for spectral discrimination of alteration environments on Mars

The December 1974 flow in the SW rift zone at Kilauea Volcano, Hawaii, has been established as a Mars analog due to its physical, chemical, and morphological properties, as well as its interaction with the outgassing plume from the primary Kilauea caldera. We focus on a solfatara site that consists of hydrothermally altered basalt and alteration products deposited in and around a passively degassing volcanic vent situated directly adjacent to the December 1974 flow on its northwest side. Reflectance spectra are acquired in the visible/near-infrared (VNIR) region and emission spectra in the mid-infrared (MIR) range to better understand the spectral properties of hydrothermally altered materials. The VNIR signatures are consistent with silica, Fe-oxides, and sulfates (Ca, Fe). Primarily silica-dominated spectral signatures are observed in the MIR and changes in spectral features between samples appear to be driven by grain size effects in this wavelength range. The nature of the sample coating and the thermal emission signatures exhibit variations that may be correlated with distance from the vent. Chemical analyses indicate that most surfaces are characterized by silica-rich material, Fe-oxides, and sulfates (Ca, Fe). The silica and Fe-oxide-dominated MIR/VNIR spectral signatures exhibited by the hydrothermally altered material in this study are distinct from the sulfate-dominated spectral signatures exhibited by previously studied low-temperature aqueous acid-sulfate weathered basaltic glass. This likely reflects a difference in open vs. closed system weathering, where mobile cations are removed from the altered surfaces in the fumarolic setting. This work provides a unique infrared spectral library that includes martian analog materials that were altered in an active terrestrial solfatara (hydrothermal) setting. Hydrothermal environments are of particular interest as they potentially indicate habitable conditions. Key constraints on the habitability and astrobiological potential of ancient aqueous environments are provided through detection and interpretation of secondary mineral assemblages; thus, spectral detection of fumarolic alteration assemblages observed from this study on Mars would suggest a region that could have hosted a habitable environment.

Hydrothermal Alteration Mineralogy Associated with Gold Mineralization in Buladu Area, Gorontalo, Northern Sulawesi, Indonesia

Geologically, the Buladu area is part of the North Sulawesi magmatic arc, located in the morphology of the mountains along the coastline of North Gorontalo district, Indonesia. Multiple subductions of North Sulawesi in the northern arm of Sulawesi and east Sangihe subduction in the east has triggered an active magmatism and volcanism. Formation and occurrences deposits of precious- and base metals have been found in the North Sulawesi Volcanic Arc. This paper describes a recent study on hydrothermal alteration mineralogy associated with one of the prospects in the region, namely Buladu gold prospect that for years has been operated by local artisanal miners. The study is focused on hand specimen and microscopic observation as well as XRD analysis. Lithologically, the study area is arranged by sandstone are members of Dolokapa volcano-sedimentary formation, above unconformity andesitic-basaltic volcanic breccia are members of the Pliocene Wobudu Formation and is intruded by granodiorite rocks. The petrographic study reveals that host rock of the mineralization is flow-volcanic breccia which shows porphyritic and flows structures. Resulting assemblages of secondary minerals are mineral associations characteristic of the magmatic-hydrothermal system the following imperative alteration zones could be recognized an outer zone of chloritic alteration and the inner zone of argillic alteration. The alteration showed characteristics of pervasive to selective alteration, where clay and quartz totally replaced groundmass, whereas chlorite and clay minerals were selectively altered plagioclase phenocrysts. Clay mineral species identified by XRD include chlorite and illite. These mineralogical features indicate that the hydrothermal alteration is an argillic type, which is spatially distal to the mineralization. Mafic mineral in andesite lava partly altered into mineral assemblage chlorite-epidote-quartz-carbonate show propylitic type, which is the outermost zone of mineralization.

Experimental evaporation of hyperacid brines: Effects on chemical composition and chlorine isotope fractionation

Hyperacid brines from active volcanic lakes are some of the chemically most complex aqueous solutions on Earth. Their compositions provide valuable insights into processes of elemental transfer from a magma body to the surface and interactions with solid rocks and the atmosphere. This paper describes changes in chemical and δ37Cl signatures observed in a 1750 h isothermal evaporation experiment on hyperacid (pH 0.1) sulphate-chloride brine water from the active lake of Kawah Ijen volcano (Indonesia). Although gypsum was the only evaporite mineral identified in the evolving brine, decreasing Si concentrations may ultimately result in amorphous silica precipitation. Geochemical simulations predict the additional formation of elemental sulphur at lower water activities (aH2O ≤ 0.65) that were not reached in the experiment. Absence of other sulphates and halides despite the high load of dissolved elements (initial TDS ca. 100 g/kg) can be attributed to increased solubility of metals, promoted by extensive formation of complexes between the variety of cations and the major anions (HSO4−, Cl−, F−) present. Chlorine deviations from a conservative behaviour point to losses of gaseous hydrogen chloride (HCl(g)) and consequently an increase in Br/Cl ratios.Chlorine isotope fractionation that accompanied the escape of HCl(g) showed a marked change in sign and magnitude in the course of progressive evaporation of the brine. The calculated factor of fractionation between HCl(g) and dissolved Cl for the initial interval (before 500 h) is positive (1000lnαHCl(g)-Cldiss.=+1.55±0.49‰to+3.37±1.11‰), indicating that, at first, the escaping HCl(g) was isotopically heavier than the dissolved Cl remaining in the brine. Conversely, fractionation shifted to the opposite direction in the subsequent interval (1000lnαHCl(g)-Cldiss.=5.67±0.17‰to-5.64±0.08‰), in agreement with values reported in literature. It is proposed that Cl isotopic fractionation in highly acidic brines is controlled by the distribution of dissolved chlorine species, which changes from Cl− to HClo dominance with the progressive pH decline.The Kawah Ijen lake acquired its extreme composition through influx of sulphur and halogen-rich magmatic gas components and extensive rock dissolution. If hyperacid brines with comparable chemical composition existed on Mars, evaporation processes up to the extent reported here (aH2O=0.85), were likely accompanied by losses of gaseous HCl. The resulting changes in Cl isotope compositions, Br/Cl, S/Cl and other ratios in the residual brine might be potentially recorded in assemblages of halogen-bearing secondary evaporation minerals. Also, volcanic-hydrothermal brines as these would extend the stability of liquid water on the Martian surface down to a temperature of −90 °C.

Neoarchean magmatism in the southeastern Amazonian Craton, Brazil: Petrography, geochemistry and tectonic significance of basalts from the Carajás Basin

The Neoarchean Carajás Basin is situated in the northern part of the Carajás Mineral Province. The main rock assemblages of this basin consist of volcano-sedimentary sequences composed predominantly of mafic volcanic rocks, and banded iron formations. The Grão-Pará Group (GGP) is the dominant sequence in the basin. The basalts of Grão-Pará Group (Parauapebas Formation) are investigated using petrography, geochemistry and geochronology to decipher the nature and tectonic setting of the magmatism. These volcanic rocks occur in extensive succession of massive or amygdaloidal lava flows at least 370 m in thickness. The basalts are grayish green amygdaloidal, porphyritic, aphanitic or fine-grained, and hypocrystalline. The primary mineral assemblages consist predominantly of clinopyroxene and plagioclase. On the other hand, the secondary mineral assemblages consists mainly of chlorite, albite, Fe-epidote, quartz and calcite, and are interpreted as product of submarine hydrothermal alteration and/or sub-greenschist facies metamorphism. Geochemical investigations on volcanic rocks of the Parauapebas Formation show 51.12–55.26 wt% SiO2, 0.69–0.92 wt% TiO2, 7.02–12.35 wt% FeO, and MgO ranging from 4.38 to 7.38 wt%. Moreover, the rocks are sub-alkaline, plot in the transitional and calc-alkaline fields, and show either arc-like trace element patterns or similar to those of the upper continental crust. These features include LILE and LREE enrichment and Nb and Ti depletion. SHRIMP zircon U–Pb dating constrains the crystallization ages at 2749 ± 6.5 and 2745 ± 5 Ma (Early Neoarchean) regarding volcanic mafic rocks. Our data suggest that the basaltic rocks were derived from the subcontinental lithospheric mantle affected by upper continental crustal components. Although this volcanism could be originated by the opening of a back-arc continental basin, a rift setting is more plausible on the basis of regional geology. Thus, the Carajás Basin was likely formed in an extensional regime related to a rift setting at ca. 2.75 Ga. We propose that this rift basin is related to post-orogenic setting in the passage from the Mesoarchean to the Neoarchean. The rifting process could be associated to a slab breakoff and subsequent asthenosphere upwelling that provided heat for partial melting of the lithospheric mantle.

Mineralogical, crystallographic and redox features of the earliest stages of fluid alteration in CM chondrites

The CM chondrites represent the largest group of hydrated meteorites and span a wide range of conditions, from less altered (i.e., CM2) down to heavily altered (i.e., CM1). The Paris chondrite is considered the least altered CM and thus enables the earliest stages of aqueous alteration processes to be deciphered. Here, we report results from a nanoscale study of tochilinite/cronstedtite intergrowths (TCIs) in Paris—TCIs being the emblematic secondary mineral assemblages of CM chondrites, formed from the alteration of Fe-Ni metal beads (type-I TCIs) and anhydrous silicates (type-II TCIs). We combined high-resolution transmission electron microscopy, scanning transmission X-ray microscopy and electron diffraction tomography to characterize the crystal structure, crystal chemistry and redox state of TCIs. The data obtained are useful to reconstruct the alteration conditions of Paris and to compare them with those of other meteorites. Our results show that tochilinite in Paris is characterized by a high hydroxide layer content (n=2.1–2.2) regardless of the silicate precursors. When examined alongside other CMs, it appears that the hydroxide layer and iron contents of tochilinites correlate with the degree of alteration experienced by the chondrites. The Fe3+/ΣFe ratios of TCIs are high: 8–15% in tochilinite, 33–60% in cronstedtite and 70–80% in hydroxides. These observations suggest that alteration of CM chondrites took place under oxidizing conditions that could have been induced by significant H2 release during serpentinization. Similar results were recently reported in CR chondrites (Le Guillou et al., 2015), suggesting that the process(es) controlling the redox state of the secondary mineral assemblages were quite similar in the CM and CR parent bodies despite the different alteration conditions.According to our mineralogical and crystallographic survey, the formation of TCIs in Paris occurred at temperatures lower than 100°C, under neutral, slightly alkaline conditions that favored the formation of both tochilinite and cronstedtite. During the course of alteration, the reduction in sulfur activity and/or the decrease of temperature prevented tochilinite crystallization and favoured the formation of cronstedtite and iron hydroxides. We suggest that iron hydroxides probably formed as ferrihydrite and then progressively converted to goethite between 50° and 80°C, a temperature range that is also favorable for cronstedtite formation. The presence of cronstedtite plays a key role in the reconstruction of the alteration history, demonstrating that the alteration of Paris took place by way of serpentinization processes similar to those described on the Earth.

Corrigendum to: Phosphate minerals in granitic pegmatites from the Mount Wills District, north-eastern Victoria

Tin- and tantalum-bearing LCT-type granitic pegmatites occur in a 45 km long belt between Eskdale and Mount Wills in north-eastern Victoria. Near Mount Wills, several compositionally zoned rare-element pegmatites contain complex assemblages of primary and secondary phosphate minerals, many of which are rare and previously unrecorded in Victoria. The phosphate assemblages can be divided into Al-rich and Fe–Mn-rich suites, in addition to ubiquitous fluorapatite. The Al-rich phosphate suite includes montebrasite, scorzalite, bertossaite and brazilianite. The Fe‒Mn phosphate suite includes heterosite, phosphoferrite, wolfeite, alluaudite (sp.), arrojadite (sp.) and jahnsite (sp.), derived from the metasomatic alteration of primary triplite. Further hydrothermal alteration of this assemblage has resulted in a secondary suite of strengite, rockbridgeite, phosphosiderite, whiteite, jahnsite and whitmoreite forming in etch cavities and fractures. A Late Silurian age of 420±4 Ma was obtained from one of the dykes via CHIME radiometric dating of monazite, suggesting a similar age for the adjacent Mount Wills Granite, which has not been reliably dated. This highly fractionated, peraluminous granite is presumed to be the source of the rare-element pegmatites based on their close spatial relationship.