Secondary Mineral Assemblages Research Articles

Alterations of High-Carbon (Shungite) Rocks by the Lake Onega Waters: Mineralogy and Geochemistry of the Process

Abstract ––Carbonaceous (shungite) rocks have high contents of trace elements, which can get to the environment through natural weathering. Shungite rocks are a group of Precambrian carbonaceous rocks of volcanic and sedimentary genesis in Karelia. In this work we present results of studying the mineral and geochemical compositions of shungite rocks at their outcrops on the shoreline of Lake Onega. The interaction of the Onega waters with shungite rocks led to: (1) the removal of most elements, except for K, Mn, Ba, and Mg, whose contents in the rocks increased; (2) the formation of an assemblage of secondary minerals, such as hematite, jarosite, goethite, chalcocite, anglesite, brookite, and Mn hydroxides. Based on the results obtained, we propose a model of the transformation of high-carbon (shungite) rocks by the Onega waters.

Sulfur and Carbon–Oxygen Isotopic Geochemistry and Fluid Inclusion Characteristics of the Yolindi Cu-Fe Skarn Mineralization, Biga Peninsula, NW Turkey: Implications for the Source and Evolution of Hydrothermal Fluids

The current study sought to investigate the physiochemical conditions and fluid evolution within the Yolindi Cu-Fe skarn mineralization located in the Biga Peninsula, NW Turkey. This was accomplished through a comprehensive investigation of geological and mineralogical data, along with isotopic analyses of sulfur (δ34S), carbon (δ13C), and oxygen (δ18O) of sulfide and calcite minerals, respectively, as well as fluid inclusion data pertaining to various minerals (e.g., andradite, quartz, and calcite). The Yolindi area features a complex geological framework, including the Paleozoic Kalabak Group (which includes the Torasan, Yolindi, and Sazak formations) and the Triassic Karakaya Complex. These formations were subsequently intruded via Early Miocene Şaroluk granitoids and Hallaçlar volcanics. Skarn formation is zoned into endoskarn and exoskarn types (being categorized into proximal, intermediate, and distal zones), with distinct mineral assemblages indicating concentric and contact metamorphic alteration patterns around the western part of Şaroluk granitoid intrusion in contact with the Torasan formation. The ore mineralogy and paragenesis suggest three distinct stages of evolution: an initial phase of prograde metasomatism characterized by the formation of magnetite and pyrite alongside anhydrous calc-silicate minerals; a subsequent phase of retrograde alteration marked by the formation of epidote, actinolite, and scapolite, accompanied by the occurrence of chalcopyrite and specular hematite; and finally, a post-metasomatic stage involving oxidation processes that led to the development of secondary mineral assemblages containing cerussite, covellite, and malachite. Sulfur isotopes (δ34S) of sulfides from endoskarn (from +0.27 to +0.57‰VCDT) to intermediate exoskarn (from −9.44 to −5.46‰VCDT) zones indicate a diverse sulfur source, including magmatic, sedimentary, and possibly organic matter. δ34S values in hydrothermal fluids suggest a magmatic–hydrothermal origin, with endoskarn and proximal zone fluids showing a slight negative signature and intermediate zone fluids indicating a strong influence from organic-rich or metamorphic sulfur reservoirs. Carbon and oxygen isotopic compositions (δ13C and δ18O) of calcite revealed a progression from marine carbonate signatures in marble samples (from +1.89 to +2.23‰VPDB; from +21.61 to +21.73‰VSMOW) to depleted values in prograde (from −6.0 to +0.09‰VPDB; from +6.22 to +18.14‰VSMOW) and retrograde skarns (from −3.8 to −2.25‰VPDB; from +0.94 to +3.62‰VSMOW), reflecting interactions with high-temperature magmatic fluids and meteoric water mixing. The fluid inclusions in prograde minerals generated under the conditions of fluid boiling exhibited high temperatures, reaching up to 412 °C, and salinities up to 26 wt.% NaCl equivalent. Conversely, the fluid inclusions in retrograde minerals, which were generated due to fluid mixing, exhibited lower temperatures (with an average of 318 °C) and salinities with an average of 4.9 wt.% NaCl equivalent. This indicated that the cooler and more diluted fluids mix with meteoric waters and interact with organic materials in the host rocks. This suggests a multifaceted origin involving various sources and processes. Therefore, this study concluded that the skarn mineralization in the Yolindi area resulted from complex interactions between magmatic, metamorphic, and meteoric fluids, reflecting a dynamic ore-forming environment with implications for the regional metallogeny of Cu-Fe skarn deposits.

Mineral assemblages from chromitites of the Alapaevsk dunite-harzburgite massif (Middle Urals)

Research subject. Minerals and mineral assemblages of noble elements in chromitites of the Alapaevsk massif. Aim. A systematic mineralogical study of high-chromium (Cr) and alumina-rich (Al) chromitites with the development of a sequence scheme for mineral formation, including platinum-group minerals (PGMs) and gold alloys. Materials and Methods. Samples of high-Cr and Al chromitites from chromite deposits in various parts of the Alapaevsk massif. Scanning electron microscopy (Tescan VEGAII XMU and JSM-6390LV Jeol with EDX INCA Energy 450 X-Max 80 spectrometers) and electron microprobe analysis (Cameca SX 100 with five wave spectrometers) were used. Results. A diagram showing the sequence of mineral formation in chromitites was designed; primary and secondary mineral assemblages were distinguished, with the latter assemblage being divided into early and late mineral associations. Primary assemblages of high-Cr (Cr2O3 > 50 wt %) and Al (Cr2O3 < 50 wt %) ores are represented by similar minerals, including chromespinel, clinopyroxene, and olivine, with characteristically distinct compositions of these minerals in each type of ores. Minerals of the primary assemblage are synchronous with chrome-spinel and are represented by pentlandite, Cu-bearing pentlandite, chalcopyrite, pyrrhotite, bornite, as well as PGMs (laurite RuS2, erlichmanite OsS2, native osmium) and Cu-rich gold. Minerals of the secondary early association occur in the form of polyphase inclusions within chrome-spinel. Polyphase inclusions are composed of Cr-bearing chlorite, amphibole, garnet, sulfides (millerite, heazlewoodite) and minerals of native elements, including (Ni, Cr)-bearing copper, nickel-bearing copper, (Cu, Fe, Cr)-bearing nickel, awaruite. Noble metal minerals from the secondary early association were found only in Al chromitites and are represented by laurite, Pt- and Pd arsenides and stibnides, Ru-bearing pentlandite, and high-grade native gold. The secondary late mineral association consists of native copper and awaruite, which are intergrown with serpentine in high-Cr ore. The temperature conditions for the formation of secondary assemblages were estimated using a chlorite geothermometer. The formation temperatures of the studied chlorites from chromitites fall within the range of 250–284°C. Conclusions. Noble metal minerals from secondary associations were formed at temperatures below 350°C together with garnet, amphibole, chlorite, and nickel sulfides. Grains of primary Os–Ir–Ru alloys during epigenetic processes underwent sulfurization with the formation of a fine-grained porous mixture of native and sulfide (sometimes with As) phases, and replacement by Ru-pentlandite. The presence of awaruite and native Cu and Ni in both primary and secondary assemblages of chromotites indicates the reducing conditions for the formation of noble metal minerals. The limited occurrence of high-Cr ores, along with the manifested processes of sulfurization for primary grains of Os–Ir–Ru alloys, led to the scarcity of detrital PGM in the area of the Alapaevsk massif.

Metasomatic alteration of coarse‐grained igneous calcium‐aluminum‐rich inclusions from CK3 carbonaceous chondrites

AbstractWe report on the primary and secondary mineralogies of three coarse‐grained igneous calcium‐aluminum‐rich inclusions (CAIs) (Compact Type A [CTA], Type B [B], and forsterite‐bearing type B [FoB]) from the Northwest Africa (NWA) 5343 (CK3.7) and NWA 4964 (CK3.8) carbonaceous chondrites, compare them with the mineralogy of igneous CAIs from the Allende (CV3.6) chondrite, and discuss the nature of the alteration processes that affected the CK and CV CAIs. The primary mineralogy and mineral chemistry of the CK3 CAIs studied are similar to those from Allende; however, primary melilite and anorthite are nearly completely absent. Although the secondary minerals identified in CK CAIs (Al‐diopside, andradite, Cl‐apatite, clintonite, forsterite, ferroan olivine, Fe,Ni‐sulfides, grossular, ilmenite, magnetite, plagioclase, spinel, titanite, and wadalite) occur also in the Allende CAIs, there are several important differences: (i) In addition to melilite and anorthite, which are nearly completely replaced by secondary minerals, the alteration of CK CAIs also affected high‐Ti pyroxenes (fassaite and grossmanite) characterized by high Ti3+/Ti4+ ratio and spinel. These pyroxenes are corroded and crosscut by veins of Fe‐ and Ti‐bearing grossular, Fe‐bearing Al,Ti‐diopside, titanite, and ilmenite. Spinel is corroded by Fe‐bearing Al‐diopside and grossular. (ii) The secondary mineral assemblages of grossular + monticellite and grossular + wollastonite, commonly observed in the Allende CAIs, are absent; the Fe‐bearing grossular + Fe‐bearing Al‐diopside ± Fe,Mg‐spinel, Fe‐bearing grossular + Fe,Mg‐olivine ± Fe,Mg‐spinel, and Ca,Na‐plagioclase + Fe‐bearing Al‐diopside + Fe‐bearing grossular assemblages are present instead. These mineral assemblages are often crosscut by veins of Fe‐bearing Al‐diopside, Fe,Mg‐olivine, Fe,Mg‐spinel, and Ca,Na‐plagioclase. The coarse‐grained secondary grossular and Al‐diopside often show multilayered chemical zoning with distinct compositional boundaries between the layers; the abundances of Fe and Ti typically increase toward the grain edges. (iv) Sodium‐rich secondary minerals, nepheline and sodalite, commonly observed in the peripheral portions of the Allende CAIs, are absent; Ca,Na‐plagioclase is present instead. We conclude that coarse‐grained igneous CAIs from CK3.7–3.8 s and Allende experienced an open‐system multistage metasomatic alteration in the presence of an aqueous solution–infiltration metasomatism. This process resulted in localized mobilization of all major rock‐forming elements: Si, Ca, Al, Ti, Mg, Fe, Mn, Na, K, and Cl. The metasomatic alteration of CK CAIs is more advanced and occurred under higher temperature and higher oxygen fugacity than that of the Allende CAIs.

Fluid–rock interaction in the basement granitoids: A plausible answer to recurring seismicity

The Koyna–Warna Seismogenic Region in the western part of the Indian Subcontinent has been recognized as one of the most significant sites of Reservoir–Triggered–Seismicity (RTS) during the last five decades. The basement granitoids, overlain by the porous and vesicular Deccan Trap basalt, contain numerous interconnecting fractures which act as the ascending and descending pathways of fluid flow. As a result of this fluid flow along fractures, the host rock has been subjected to significant chemical alteration along with the subsequent formation of some new minerals at the expense of a few other pre–existing mineral phases. Mesoscopic observations followed by Optical microscopy in the core samples of the basement rocks upto 1.5 km depth retrieved from the borehole KBH1 near Rasati (about 4.7 km from the Koyna Dam) have revealed the presence of chlorite and the precipitation of calcite, whereas the bulk mineralogical XRD has reaffirmed the presence of chlorite, calcite along with illite at a certain depth. This entire secondary mineral assemblage resembles the propylitic kind of hydrothermal alteration at temperatures < 350°C under acid–to–neutral solution conditions and also indicates water channelization up to the deeper level in the basement granitoids (>1.5 km). In addition, the presence of the hydrophilic clay minerals along fault and fracture zones may be responsible for triggering the seismicity in the Koyna Seismogenic Region as their absorption of water reduces the shear strength of faults and their low frictional strength accelerates the fault weakening process causing the generation of slip surfaces. Thus, in addition to several seismotectonic features, fault geometry and existing stress pattern, the clay mineralisation along the pre–existing faults and fractures of the basement rocks may also be a factor behind the recurring seismicity in this region.

METASOMATIC ORIGIN OF LAMELLAR-LIKE INCLUSIONS IN CLINOPYROXENES FROM MANTLE XENOLITHS OF THE OBNAZHENNAYA PIPE (KUOIKA FIELD, YAKUTIAN DIAMONDIFEROUS PROVINCE)

The article reports new data on previously undescribed lamellar-like (linearly oriented) structures in clinopyroxenes from mantle xenoliths of garnet pyroxenites (Obnazhennaya pipe, Northern Siberia). The origin and mineral diversity of lamellar structures in xenoliths from the Obnazhennaya pipe are supposed to be related to a breakdown of solid solutions during a decrease of P–T parameters. Our study shows that metasomatic recrystallization of initial clinopyroxene with linear exsolution structures leads to the formation of new secondary mineral assemblages. The secondary clinopyroxene reproduces (inherits) framework with linearly oriented inclusions, which are morphologically similar to the primary exsolution structures. Thus, the pyroxenite xenoliths from the Obnazhennaya pipe contain two generations of mineral associations with lamellar structures: primary and secondary. The primary association is composed of diopside, containing lamellae of enstatite, pyrope, spinel and rutile. The secondary (metasomatic) clinopyroxene contains half as much Al2O3 (3.4 wt %) and Na2O (1.7 wt %) and is enriched in MgO and CaO compared to the primary clinopyroxene. Nevertheless, the chemical composition of the secondary pyroxene still corresponds to diopside end-member. The metasomatic association comprises aligned pargasite (Al-rich amphibole) instead of orthopyroxene (enstatite) lamellae, and the secondary rutile is enriched in chromium (up to 3.1 wt % Cr2O3) in contrast to rutile from the primary mineral assemblage.

Neoproterozoic ophiolite exotic blocks in the Outer Western Carpathians, southern Poland: a record of the fast ocean-floor cooling and alterations

Ophiolitic blocks, represented by metagabbro and serpentinite containing relict pyroxene, olivine, baddeleyite, zirconolite, Ni-pyrite and pyrrhotite, were found as exotic blocks in an olistostrome in the Magura Nappe, Outer Western Carpathians. The geochemical and isotopic features of the blocks suggest they represent mantle-derived rocks, with within-plate geochemistry signatures, modified by subduction, with lithospheric mantle input. A U-Pb apatite cooling age (614 ±3 Ma) is within age uncertainty of a published U-Pb zircon magmatic crystallization age (~614 Ma) implying rapid post-crystallization cooling. Pervasive alteration with replacement of primary minerals by low-temperature assemblages is observed in all rock fragments and is interpreted as contemporaneous with shearing. The secondary mineral assemblages and temperature modelling allow the interpretation that the pervasive ocean-floor metamorphism is the alteration in these meta-mafic rocks. These ophiolitic fragments can be linked to the Neoproterozoic break up of Rodinia/Pannotia.

Heterogeneity of the Noachian Crust of Mars Using CRISM Multispectral Mapping Data

AbstractWe used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) multispectral mapping data to assess heterogeneity of primary and secondary mineral compositions of Noachian‐aged regions of Martian crust. Multispectral data corroborate interpretations from CRISM targeted observations and Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activité global mapping of large horizontal differences in vertical crustal structures, and degree and grade of alteration to secondary minerals. CRISM multispectral data analysis conducted at a combination of high spatial resolution and coverage not available in other data sets also reveals previously unrecognized exposures. At one extreme, basaltic crustal material is minimally altered, mostly to smectite clay; at the other, Al‐enriched alteration products are present, alteration is widespread, and superposed surface materials rich in salts and precipitates, suggesting multiple episodes of alteration. The revealed vertical structure of primary mineralogy is consistent with that inferred in previous studies. Controls on the extent and metamorphic grade of secondary mineral assemblages are proximity to heat sources including large impact basins and inferred magmatic bodies.

Potassium isotopic fractionation during multistage alteration of oceanic crust in the southern Mariana Trench

To further constrain the migration trajectory of K isotopes during oceanic crust alteration, we present detailed mineral, elemental, and K isotopic data of oceanic crust samples from the southern Mariana Trench. These samples are divided into four groups with various colors and alteration intensities, in which secondary mineral assemblages indicate a progressive alteration of oceanic crust at low temperatures (<100 °C). Group I samples are fresh basalts with chemical compositions identical to the mid-ocean ridge basalts (MORBs). Their higher δ41K values (0.08‰ to 0.11‰) than MORBs (−0.44 ± 0.17‰) might point to a 41K-enriched magmatic source. Group II samples are slightly altered basalts at the initial stage of low-temperature alteration and have lower δ41K values (−0.31‰ to 0.05‰) than Group I, which are interpreted as a result of chemical exchange with seawater via equilibrium and kinetic isotopic fractionations. Group III samples are celadonite phase-dominated rocks at the second stage. They have higher K2O contents (3.53–8.39 wt%) and lower δ41K values (−0.71‰ to −0.45‰) than Group II, reflecting the preferential incorporation of isotopically light K. In contrast, the clay mineral phase at the late stage dominates Group IV samples with lower K2O contents (1.79–4.20 wt%) and higher δ41K values (−0.22‰ to −0.07‰) than Group III, indicating a loss of isotopically light K during the transformation of the high-K celadonite phase to the low-K clay mineral phase. Overall, systemic variations in secondary minerals, K2O contents, and δ41K values from Group I to Group IV samples suggest a multistage K isotopic fractionation and shed new light on a key role of the high-K celadonite phase in K isotopic behavior during oceanic crust alteration.

Geochemical bio-signatures in Martian analogue basaltic environments using laboratory experiments and thermochemical modelling

The identification of geochemical bio-signatures is important for assessing whether life existed on early Mars. In this paper, experimental microbiology and thermochemical modelling were combined to identify potential inorganic bio-signatures for life detection on early Mars. An analogue mixed microbial community from an analogue terrestrial fluvio-lacustrine environment similar to an ancient lacustrine system at Gale Crater was used to study microbial dissolution of a basalt regolith simulant and the formation of bio-signatures over a short time frame (1°month) at 14°C, 2 bar. Microbial growth influenced element dissolution (Mg, Fe, Mn, Ca and K) and the formation of morphologies and Fe-Si amorphous layers on mineral surfaces. Thermochemical models were performed at 14°C, 2 bar; the results were compared with experimental data to predict bio-signatures that would occur over geological timescales. The pH was varied to simulate abiotic and biotic experimental conditions. Model results suggest that, at water to rock ratios of 100 to 38, a less complex secondary mineral assemblage forms during biotic dissolution compared to abiotic weathering. Carbonates, quartz, pyrite and hydroxyapatite form under biotic conditions, whereas in the abiotic system magnetite and phyllosilicates would also precipitate. These results could be used to distinguish between abiotic and biotic basalt weathering processes, aiding the interpretation of data from Mars exploration missions.

Supergene Turquoise and Associated Phosphate Minerals of the Porphyry-Lode System at Butte, Montana, USA

ABSTRACTWeathering processes superimposed onto exhumed hydrothermal ore deposits in western North America have developed secondary mineral assemblages that inform the near-surface evolution of these systems within the context of Basin and Range extension. The occurrence of the secondary phosphate mineral turquoise [CuAl6(PO4)4(OH)8·4H2O] in the weathering profile of Laramide porphyry copper deposits is widely documented, although previous studies on the composition and distribution of turquoise are largely restricted to the archaeological literature. In this study, we use the world-class Butte porphyry-epithermal system to study the occurrence, paragenesis, and mineral chemistry of turquoise and related phosphates in the weathering profile of the deposit. Field observations, mineral textures by optical microscopy, electron microprobe analyses, and geochemical modeling show that blue, Al-endmember turquoise formed exclusively at or immediately below the pre-mining water table, within the chalcocite enrichment blanket and in the absence of significant FeIII. At higher structural levels above the pre-mining water table, green FeIII-bearing turquoise [Cu(Al,FeIII)6(PO4)4(OH)8·4H2O] is commonly intergrown with jarosite and/or tinticite at the micron scale. We show that jarosite, tinticite, and FeIII-bearing turquoise formed at the expense of pre-existing, Al-endmember turquoise after extensional faulting caused a relative lowering of the water table in the eastern fault block of the deposit. The low solubility of the FeIII-phosphates tinticite and strengite in the leach cap environment suggests that it is unlikely that these minerals formed directly from oxidized meteoric water. Electron probe microanalysis documented the presence of As and F in blue Al-endmember turquoise and Cl in green FeIII-bearing turquoise. The presence of Cl in FeIII-bearing turquoise may represent an evaporitic signal associated with basin development as the Butte district was exhumed. The paragenesis of Cu- and Fe-phosphate minerals provides another tool with which to interrogate spatial and temporal relationships in the near-surface evolution of porphyry copper systems.

The Fate of Pyroxenes in Mafic Xenoliths from the Kinnaur Kailash Granite, Sutlej Valley, NW Himalaya: Effect of Retrograde Hydration and Insights on the Rare Occurrence of High-Grade Metamorphic Rocks in the Himalayan Orogen

Abstract Pseudosection modelling of post-peak mineral assemblages in metamorphosed mafic xenoliths from the Kinnaur Kailash Granite, Northwest Himalaya, has provided new insights into pre-Himalayan metamorphism and constrained the P–T conditions of hydration. P–T and P–MH2O pseudosections constructed in the NCFMASH system show that retrograde hydration occurred during destabilisation of orthopyroxene and clinopyroxene in mafic granulites after decompression to 0.3 GPa pressure, and as a result, water-containing phases such as cummingtonite and hornblende formed. Results from two of the studied xenoliths indicate that the secondary mineral assemblage is controlled not only by P and T, but also by the amount of water available for hydration. In contrast, another example indicates the development of cummingtonite from hornblende via dehydration reaction after decompression to conditions appropriate for cummingtonite stability. The study shows that the Higher Himalayan Crystalline Sequence (HHCS), from which the xenoliths have been derived, must have been subjected to pre-Himalayan high-grade metamorphism, although it is not evident because of pervasive overprinting by Himalayan metamorphism. The present-day metamorphic pattern of the HHCS is expectedly the result of both pre-Himalayan and Himalayan metamorphism, and careful petrological investigation is required to distinguish their signatures so that a better understanding of the tectonothermal evolution of the Himalayas can come into picture. Isotopic dating of the xenoliths is required to establish the timings of the pre-Himalayan metamorphism and decompression.

Multispectral discrimination of spectrally similar hydrothermal minerals in mafic crust: A 5000 km2 ASTER alteration map of the Oman–UAE ophiolite

Multispectral remote sensing of hydrothermal alteration in volcanogenic massive sulfide (VMS) ore systems in mafic crust is relatively uncommon, in part due to the short-wave infrared spectral similarity of several key alteration minerals: epidote, chlorite, actinolite, and serpentine. In this study, we developed regional mosaic generation and classification workflows for Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery to discriminate these minerals over the entire crust of the Semail ophiolite (Oman–UAE). Spectral discrimination was achieved through adaptation of the ASTER (pre-)processing workflow to the specific mapping targets, available datasets, and location of this study. Necessary steps included the pre-selection of ASTER scenes without residual atmospheric water features, mosaic normalization based solely on overlapping target outcrops, correcting cross-mosaic ramp errors, and alteration map classification based on image-derived reference data. The resulting alteration map, validated through comparison with field mapping and sampling, is the most areally extensive continuous survey of hydrothermal alteration yet presented for oceanic crust, providing a renewed framework for research and mineral exploration of Earth's largest ophiolite. Our map confirms that the vast majority of the upper oceanic crust is regionally altered to a spilite type secondary mineral assemblage. Localized areas of epidosite alteration, marking focused hydrothermal flow paths, are confined to the upper oceanic crust, whereas areas of previously unrecognized but intense actinolite alteration are common in both the lower and upper oceanic crust. Our methodological developments expand the standard considerations necessary for regional geological mapping using infrared image mosaics. They further demonstrate the underappreciated capability of multispectral data for mapping spectrally similar rock types. Although the specifics of the method are necessarily optimized for the Oman–UAE ophiolite, re-optimization based on local reference data should allow similar results to be achieved in other well-exposed mafic-hosted VMS districts.

Characterization of groundwater chemistry beneath Gale Crater on early Mars by hydrothermal experiments

Hydrothermal groundwater may have played many roles in hydrogeochemical cycles on early Mars, including being a source of cations and reductants, a sink of CO₂, and a causative fluid of post-depositional diagenesis. However, few laboratory experiments have been investigated hydrothermal reactions within the Martian crust. Here we describe hydrothermal experiments simulating water–rock reactions in the Martian subsurface, with the aim of characterizing the fluid chemistry. Experiments at 120 °C and 200 °C show that the hydrothermal fluids have i) high Si concentrations (~1–10 mmol kg−1), ii) low Fe and Mg concentrations (<~10−3 mmol kg−1), iii) low H₂ concentrations (<10−2 mmol kg−1), and iv) alkaline pH (in-situ pH ~8). Effective drawdown of CO₂ (4–5 mmol kg−1) occurs through the formation of calcite. Concentrations of Si in the fluids are buffered by the dissolution equilibrium of quartz at 200 °C and its metastable phase (cristobalite) at 120 °C. These characteristics would not explain the observed secondary mineral assemblages of Vera Rubin Ridge (VRR), in particular high abundance of Fe (hydro)oxides and low levels of silica and clay minerals, in the Murray formation of Gale Crater. This implies that upwelling hydrothermal groundwater can be precluded as a source of post-depositional diagenetic fluids. Our results under reaction temperatures of ≤200 °C indicate that upwelling hydrothermal groundwater provided only limited amounts of reductants and greenhouse effect gases (Fe2+ and H₂) to the surface of early Mars.

A mineralogical study of glacial flour from Three Sisters, Oregon: An analog for a cold and icy early Mars

Geomorphic and mineralogical data from the martian surface indicate liquid water was abundant on the martian surface and near subsurface ∼3.5 to 4 Gyr ago, but whether early Mars had a warm and wet climate or whether it was cold and icy with punctuated periods of warmth is still unknown. Mineral assemblages of sedimentary rocks on Mars help determine past aqueous conditions and sediment sources. Here, we report on the primary and secondary mineral and amorphous assemblage of glacial flour from Collier Glacier valley on the northern flank of North Sister in Oregon, U.S.A. to identify mineralogical characteristics of mafic sediments altered under cold, wet conditions. Collier glacial flour is dominated by primary igneous minerals (plagioclase is dominant, with lesser amounts of pyroxene and olivine) and comprises 10-40 wt.% X-ray amorphous materials. Crystalline secondary phases (e.g., phyllosilicates, zeolite) are not significant contributors to the authigenic alteration assemblage. High-resolution transmission electron microscopic observations of the <2 μm size fraction of the flour demonstrate that the X-ray amorphous materials are both primary (i.e., volcanic glass) and secondary in nature. The secondary X-ray amorphous materials are enriched in Si, Al, and Fe, and we observe incipient phyllosilicate formation associated with primary and secondary amorphous materials. Our results indicate chemical weathering on a cold and icy early Mars would have produced X-ray amorphous materials, but not crystalline secondary phases. We suggest that the abundant X-ray amorphous materials recognized from orbit and in situ on Mars could have formed under cold and periodically wet conditions similar to those on North Sister today. Furthermore, the lack of volumetrically significant phyllosilicate formation in Collier Glacier flour indicates phyllosilicates on Mars did not form in a cold and wet climate.

Theoretical Considerations on the Characteristic Timescales of Hydrogen Generation by Serpentinization Reactions on Enceladus

AbstractThe Cassini spacecraft demonstrated that Saturn's small moon Enceladus may harbor hydrothermal activity. In particular, molecular hydrogen production could result from water‐rock interactions in a tidally heated, water‐filled porous rocky core. The lifetime of such reactions is key to assess the habitability potential of Enceladus and to constrain plausible durations of the active stage in a context where the evolution of the moon is debated. Although it has recently been suggested that the serpentinization timescale does not exceed a few hundred million years, this estimation was based on assumptions regarding silicate dissolution kinetics that are prone to overestimate the actual reactivity of primary silicates. Here, we investigated plausible rate‐limiting mechanisms governing fluid‐rock interactions that could delay the completion of Enceladus' core serpentinization. In particular, we considered the impact of (a) various secondary mineral assemblages on the Gibbs free energy of Fe‐bearing silicate dissolution and associated dissolution rates; (b) rate‐laws alternative to the transition state theory; (c) diffusion in nanoporous secondary assemblages; (d) slow water supply. Overall, our results confirm that serpentinization timescales never exceed 500 Myr, and indicate that fluid flow ultimately sets the tempo for serpentinization. Only unreasonable grain sizes in Enceladus' core (&gt;1 m) or unexpectedly low diffusivity of secondary coatings covering primary silicates would be consistent with serpentinization durations of several billion years. We thus suggest that either the hydrothermal activity has developed recently on Enceladus, or alternative processes (pyrolysis of insoluble organic matter, microbial activity) must be tested to explain the observed H2 flux in Enceladus' plume.

The Wombat Hole Prospect, Benambra, Victoria, Australia: a Cu–Bi–(Te) exoskarn with unusual supergene mineralogy

AbstractThe Wombat Hole Prospect is a small copper–bismuth–(tellurium) exoskarn cropping out in the Morass Creek gorge, near Benambra, in eastern Victoria, Australia. Its main primary sulfide constituent is bornite in a grossular‒vesuvianite matrix. The skarn formed in a megaclast of Lower Silurian limestone from metal-bearing fluids accompanying the high-level emplacement of the Late Silurian–Lower Devonian Silver Flat Porphyry. Though the primary bornite mineralisation has been nearly obliterated by weathering, there are small relict patches containing exsolved grains of wittichenite (Cu3BiS3) and chalcopyrite, as well as inclusions of bismuth tellurides in the tetradymite group, namely sulphotsumoite (Bi3Te2S) and hedleyite (Bi7Te3). Joséite-A (Bi4TeS2), a mineral with a formula Bi3(Te,S)4, several unnamed Cu–Bi‒Te phases and minute grains of native bismuth have also been detected. Pervasive veining by chrysocolla throughout the garnet‒vesuvianite host contains a range of unusual secondary bismuth minerals that have crystallised at various times. These include mrázekite, namibite, pucherite, schumacherite and eulytine. Other secondary minerals present include wulfenite, bismutite, azurite, malachite and a poorly-crystalline bismuth oxide containing several weight percent tellurium. Rare grains of gold (electrum) containing up to 23 wt.% Ag are also present. The assemblage of grossular–vesuvianite with minor diopside is indicative of formation in a low-${\rm X}_{{\rm C}{\rm O}_ 2}$ environment under fluid-buffered conditions. A temperature range between ~650°C and as low as ~150°C can be estimated from the exsolution of wittichenite and chalcopyrite from the bornite. The tetradymite-group inclusions formed first under low values of $f_{{\rm S}_ 2}$ /$f_{{\rm T}{\rm e}_ 2}$, with bornite crystallising as values increased. The primary Cu‒Bi‒Te mineralogy and the unusual secondary mineral assemblage makes the Wombat Hole skarn unique in southeastern Australia. The deposit provides scope for studying the mobility of elements such as Bi and Te over short distances during weathering of hypogene ore minerals.

Supergénna mineralizácia na II. horizonte Východnej šachty hnedouhoľnej bane Handlová (Slovenská republika)

At the II. horizon of the Eastern shaft of Handlová coal mine the occurrence of mineral assemblage of secondary minerals was identified. These accumulations consist of sulphate group minerals confirmed by PXRD and semiquantitative chemical analysis (EDS), namely gypsum, alunogen, pickeringite, melanterite, sideronatrite and epsomite. Sulphates form coatings, crystalline crusts and aggregates, which crystallized on the walls of mine corridors. The dimensions of these accumulations are reaching approximately 1.5 x 1.3 m with thickness up to 5 cm. The refined unit-cell parameters of alunogen (for the triclinic space group P-1) are: a 7.413(1), b 26.944(4), c 6.051(1) Å, α 90.05(1), β 97.65(1), γ 91.82(1)°, V 1197.1(3) Å3, pickeringite (for the monoclinic space group P21/c): a 6.1808(8), b 24.252(2), c 21.220(2) Å, β 100.36(1)°, V 3128.8(6) Å3 and melanterite (for the monoclinic space group P21/c): a 14.022(5), b 6.503(2), c 10.945(4) Å, β 105.83(3)°, V 960.2(7) Å3. Sideronatrite and epsomite were identified in mixture with the most significant diffraction maxima for sideronatrite: 10.252/100, 4.905/37, 3.424/10, 3.067/9 and epsomite: 5.347/18, 5.302/18, 4.489/26, 4.215/100, 4.201/59, 2.880/24, 2.747/23, 2.677/18. Accumulations of minerals were formed on the mine walls. Secondary sulphates are considered as products of the weathering of Fe-rich carbonates, pyrite and other sulphidic minerals which are part of associated pelosiderites embedded in the clays and claystones. Locally, gypsum and hexahydrite were also identified in the form of transparent crystals and white powdery aggregates on the calcite speleothems genetically linked to Mesozoic limestones and gypsum-rich Lower Triassic Werfen Formation. Unit-cell parameters refined from X-ray powder diffraction data are for hexahydrite (for the monoclinic space group C2/c): a 10.107(1), b 7.2114(8), c 24.424(3) Å, β 98.28(1)°, V 1761.7(3) Å3.

Zeolite Minerals from Wat Ocheng, Ta Ang, Ratanakiri Province, Cambodia – Occurrence, Composition, and Paragenesis

ABSTRACT Located in the southwest corner of the Ratanakiri Volcanic Province, the Wat Ocheng basalt is the first known zeolite locality in Cambodia. The basalt is a fine-grained, vesicular to amygdaloidal, subalkaline to transitional alkaline intraplate tholeiite comprised of 30% lath-like plagioclase (average: Ab51An45Or4), 35% interstitial augite (average: Wo44En35Fs21), 25–30% zeolites after plagioclase and volcanic glass, and minor skeletal ulvöspinel. It contains mineralized amygdales ranging in size from 0.5 × 0.5 cm to 5 × 9 cm. Nine zeolite species occur at Wat Ocheng, including analcime, chabazite-Ca, gonnardite, natrolite, phillipsite-Ca, and thomsonite-Ca, along with clays, aragonite, calcite, and pyrite. All the zeolite species are being described from Cambodia for the first time. The zeolite and secondary mineral assemblages observed at Wat Ocheng are similar to those reported from other alkaline basalt localities, including those in neighboring Vietnam, and are a product of alteration of the primary Ca-Na minerals and volcanic glass as a result of burial metamorphism and infiltration of heated meteoric waters. The mineral assemblage is not homogeneous across amygdales within the exposed lava flow, suggesting localized closed systems, likely the result of early precipitation of clay minerals and fine-grained zeolites. Decreased porosity and differences in fluid geochemistry would account for the diversity in the observed assemblages. Four stages of hydrothermal alteration and zeolitization have been defined based on mineral textures and chemistry. Zeolite formation began with fine-grained Ca- (chabazite-Ca and phillipsite-Ca) and Na- (analcime) dominant, high TSi (Si/Si+Al) species in Stage II following deposition of clay minerals in Stage I. Stage III is characterized by increasing Na+K contents and decreasing TSi. Crystallization of coarse-grained chabazite-Ca and phillipsite-Ca with increasing Na+K contents in the rims of the crystals followed the development of natrolite with a later-stage epitaxial overgrowth of thomsonite-Ca. The final stage of mineralization (Stage IV) included late-stage calcite, pyrite, and termination of growth of acicular sprays of thomsonite-Ca. Post-magmatic cooling and circulation of meteoric water and fluids derived from alluvial sediments overlying the basalts were involved in zeolitization. Thermal sources include an underlying basaltic andesite flow as well as regional deep-seated, extensional pull-apart structures, the result of a thinned lithosphere and injection of fertile mantle following the collision of the Eurasian and Indochina plates during the Himalayan Orogeny.

The thermal properties of hydrothermally altered andesites from La Soufrière de Guadeloupe (Eastern Caribbean)

The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.