Clay Alteration Research Articles

Geophysical characterization of volcanic layering

Seismic methods, electrical resistivity, and outcrop observations provide an integrated subsurface view of young basaltic lavas and their overlying weathered products. The study site is a sea cliff in the Kohala province in the northwestern part of the Big Island of Hawai'i, which provides an ideal laboratory for combined seismic reflectivity, shear-wave velocity, electrical resistivity, and outcrop observations. Our primary observation is the division of the shallow subsurface into weathered (saprolite) and underlying unweathered (hard basalt) zones. Significant heterogeneity is evinced by strong reflectivity changes within and between the two zones and the structural variability of the basal surface separating the weathered zone from the unweathered zone (reflector undulations, fault offsets, and prominent diffractions). Seismic reflectivity contrasts arise from variations in rigidity and porosity (density) corresponding to seaward-dipping lava flow units. The degree of clay alteration is interpreted to govern subsurface resistivity variations along with the influence of fresh groundwater and the incursion of seawater saturating the porosity. Our study provides a generalized model for visualizing weathered versus unweathered basaltic lavas in the shallow subsurface.

Exploration Implications of Multiple Formation Environments of Advanced Argillic Minerals

Abstract Advanced argillic minerals, as defined, include alunite and anhydrite, aluminosilicates (kaolinite, halloysite, dickite, pyrophyllite, andalusite, zunyite, and topaz), and diaspore. One or more of these minerals form in five distinctly different geologic environments of hydrolytic alteration, with pH 4–5 to <1, most at depths <500 m. (1) Where an intrusion-related hydrothermal system, typical of that associated with porphyry Cu ± Au deposits, evolves to white-mica stability, continued ascent and cooling of the white-mica–stable liquid results in pyrophyllite (± diaspore) becoming stable near the base of the lithocap. (2) A well-understood hypogene environment of formation is vapor condensation near volcanic vents, where magmatic SO2 and HCl condense into local groundwater to produce H2SO4 and HCl-rich solutions with a pH of 1–1.5. Close to isochemical dissolution of the host rock occurs because of the high solubility of Al and Fe hydroxides at pH <2, except for the SiO2 component, which remains as a siliceous residue because of the relatively low solubility of SiO2. This residual quartz, commonly with a vuggy texture, is largely barren of metals because of the low metal content in high-temperature but low-pressure volcanic vapor. Rock dissolution causes the pH of the acidic solution to increase, such that alunite and kaolinite (or dickite or pyrophyllite at higher temperatures) become stable, forming a halo to the residual quartz. This initially barren residual quartz, which forms a lithocap horizon where permeable lithologic units are intersected by the feeder structure, may become mineralized if a subsequent white-mica–stable liquid ascends to this level and precipitates copper and gold. (3) Boiling of a hydrothermal liquid generates vapor with CO2 and H2S. Where the vapor condenses above the water table, atmospheric O2 in the vadose (unsaturated) zone causes oxidation of H2S to sulfuric acid, forming a steam-heated acid-sulfate solution with pH of 2–3. In this environment, kaolinite and alunite form in horizons above the water table at <100°C. Silica derived within the vadose zone will precipitate as amorphous silica at the water table, as the condensate follows the hydraulic gradient, causing opal replacement above and at the aquifer. (4) By contrast, where condensation of this vapor occurs below the water table, the CO2 in solution forms carbonic acid (H2CO3), leading to a pH of 4–5. This marginal carapace of condensate, with temperatures up to 150°–170°C, commonly acts as a diluent of the ascending parental NaCl liquid. This steam-heated liquid forms intermediate argillic alteration of clays, kaolinite, and Fe-Mn carbonates; this kaolinite, which can be present at depths of several hundreds of meters, can potentially be mistaken as having been caused by a steam-heated acid-sulfate or supergene overprint. (5) The final setting is supergene, caused by posthydrothermal weathering and oxidation of mainly pyrite, locally creating pH <1 liquid because of high concentrations of H2SO4 within the vadose zone and forming kaolinite, alunite, and Fe oxyhydroxides. This genetic framework of formation environments of advanced (and intermediate) argillic alteration provides the basis to interpret alteration mineralogy, in combination with alteration textures and morphology plus zonation, including the overprint of one alteration style on another. This framework can be used to help focus exploration for and assessment of hydrothermal ore deposits, including epithermal, porphyry, and volcanic-hosted massive sulfide.

Latest Miocene (Kapitean/Messinian) glauconite and the central Chatham Rise greensand: an enigmatic, highly condensed, relict/palimpsest deposit on the modern seafloor

ABSTRACT A unique 10–140 cm thick veneer of greensand and glauconite-rich deposits (10–80 wt% glauconite) presently sits atop central Chatham Rise (CR) in water depths of 200-500 m. The glauconite is dominated by chemically mature, polished ovoidal and lobate pellets, with a 10–12 Å phyllosilicate structure and K-Ar ages of 7–5 Ma, indicating that the glauconite pellets are predominantly allogenic (i.e. derived/reworked) grains, and not of in situ authigenic origin. Glauconite possibly evolved from seafloor alteration of detrital and/or volcanogenic smectitic clays, likely concentrated within organic-rich faecal pellets under unique paleoceanographic conditions (nutrient cycling, upwelling, carbon isotope gradients) within the latest Miocene Subtropical Front. The distribution of glauconite abundance suggests a ‘glauconite factory’ existed at this time about Reserve Bank on western central CR. The pellets were dispersed eastwards along the crest by intensified bottom currents within the dynamic Subtropical Frontal zone, perhaps during the numerous glacial periods of lowered sea level throughout the Plio-Quaternary. The bioturbated greensand veneer, and its mix of other reworked Neogene and Quaternary skeletal, phosphatic and siliciclastic components, rest unconformably on mainly Early Oligocene chalks, forming a highly condensed, relict/palimpsest deposit that alone embodies the last up to 30 My of sedimentation history atop central CR.

Interpretation of Subsurface Structure Based on the Magnetic Data at Semurup Geothermal Area Kerinci

The interpretation of subsurface structures in Semurup geothermal area has been carried out using the geomagnetic method. Data were collected in an area of 1500 m × 1400 m consisting of 160 points. The magnetic anomaly value obtained was derived from the total magnetic induction value that has been corrected by IGRF and diurnal variation, then transformed by reduction to equator and upward continuation to remove noise and separate local and regional anomalies. The results of data processing showed the total magnetic field values in the study area ranged from -1730.4 nT to 1909.0 nT. Magnetic anomalies in this study area are dominated by negative values that may be caused by demagnetised rocks (a result of hydrothermal alteration). The results of 2D modeling, it has 5 rock layers that can be classified into 3 main parts of the geothermal system The first and second layers are caprock with a depth of up to 850 meters consisting of sedimentary rock, clay, and sandstone. The third layer is indicated as a reservoir with a depth from 850 to 1450 m and is dominated by sandstone and clay alteration Hot rock in the fourth and fifth layers is dominated by basalt igneous rock and the presence of dacitic lava intrusion from the northeast of the study area at depths below 1450 m, and the Siulak fault as a outflow zone for geothermal fluid. The presence of the caprock, reservoir, hot rock, and fault zones indicates that the Semurup area has geothermal potential and is suitable for further exploration.

Contribution of the Paragenetic Sequence of Clay Minerals to Re-Examination of the Alteration Zoning in the Krafla Geothermal System

This paper revisits the clay mineralogy of the “smectite” alteration zone in the Krafla geo-thermal field via the study of an exploratory well in which temperatures range from 40 °C to 215 °C. The clay alteration consists of several mineral assemblages superimposed in time and space, resulting from different stages of water-rock interaction. Trioctahedral clay minerals (chlorite, corrensite and smectite) are observed throughout the studied section. These minerals can form in nearly closed systems as replacements of groundmass minerals/glass after interactions with resident and nearly stagnant fluids not far from chemical equilibrium (neutral to basic pH conditions) or from direct precipitation from geothermal fluids. They are locally superimposed by Al clay phases (smectite, illite/smectite and kaolinite), which result from intense leaching of the host rocks due to their interaction with low pH fluids under strong W/R ratios. The absence of mineralogical zoning is explained by the fact that hydrothermal alteration is strongly dependent on very recent hydrodynamics. The current fluid circulation generates trioctahedral clays at depth that cannot be distinguished from pervasive earlier alteration. The only easily detectable signature of current activity and the most relevant signature for geothermal exploration is the presence of Al dioctahedral phases since it indicates leaching and intense hydrothermal activity.

Magnetotelluric imaging of the Ceboruco Volcano, Nayarit, Mexico

Ceboruco Volcano is a Quaternary stratovolcano located in Nayarit near the Trans-Mexican Volcanic Belt western limit. It is one of the most active in the region and the only volcano in Nayarit with historically documented eruptions, the last one between 1870 and 1875. Currently, seismicity and fumarolic activity suggest hydrothermal activity. To study the geoelectrical setting at Ceboruco Volcano and its hydrothermal system, a magnetotelluric exploration campaign was carried out in November 2016, establishing 24 measuring stations at the caldera, and foothills of the volcanic edifice. Three broadband instruments were used in the sampling frequency range of 32,768–128 Hz. Recorded time series were processed to determine the MT transfer functions using a robust algorithm and consider the remote-reference station technique. Dimensionality analysis based on the phase tensor determined a shallow 1D geoelectrical structure at periods shorter than 1 s, while a 3D formation was defined at longer periods. To approximate the Ceboruco Volcano substructure, the 3D inversion of magnetotelluric data was carried out. 3D modeling revealed surficial resistivity regions that were related to young volcanic deposits. A relative conductor (10–30 Ωm) was also associated with the shallow regional aquifer. The two most intriguing results of 3D inversion were one uppermost conductive region (~1 Ωm) that established characteristics, resistivity, position, and thickness related to a clay alteration minerals zone. The other one (2–5 Ωm) corresponds to a conductive area broadening and fading with depth, centered slightly to the west of the summit at Ceboruco Volcano. The preferred interpretation of this geoelectrical feature was an envelope of high-temperature fluids, convection of steam, and brines, and they are interacting with the host-rock and surficial groundwater. This geoelectrical feature extends in depth, suggesting a permeable region at the Trans-Mexican Volcanic Belt basement that may be targeted for in-depth geothermal exploration. The geoelectrical setting identified in this study provides information that contributes to reducing uncertainty about the geological, geophysical, and hydrogeological conceptual models at Ceboruco Volcano geothermal system.

Iron and magnesium isotope geochemistry in hydrothermal uranium ore systems: Insights from the Bong Deposit, Canada

The basement-hosted Bong uranium deposit in the Kiggavik region is associated with a broad hydrothermal alteration halo characterized by an inner illite-rich zone and an outer chlorite-rich zone. The uranium-bearing zones characterized by intense illite alteration yield high δ56Fe and δ26Mg values relative to the host rocks, with average whole-rock values of δ56Fe = 0.58 ± 0.15‰ and δ26Mg = 0.61 ± 0.29‰, and clay-size fraction values of δ56Fe = 0.51 ± 0.20‰ and δ26Mg = 0.82 ± 0.09‰. These isotopic values correlate with a decrease in average Fe2+ concentrations (0.47 versus 2.30 mol% Fe2+ in least-altered hosts).The high δ26Mg values reflect intense illite alteration, which preferentially incorporated heavier magnesium isotopes during alteration of both the ore zones and an upper hematite-altered horizon. Although the clay alteration with high δ56Fe and δ26Mg values coincides with the main uranium-bearing zone, the δ56Fe signatures reflect multiple stages of alteration and mineralization. The high δ56Fe values and lower Fe2+ contents of the uranium-bearing zones support leaching of Fe2+(aq) enriched in lighter iron isotopes. Sequential leach δ56Fe experiments also indicate that formation of fine-grained iron oxides/hydroxides with positive δ56Fe values (+0.23 to +0.60‰) in late chemical fronts enhanced the primary positive δ56Fe signatures recorded in ore zones. Near surface samples with similar isotopic signatures provide clues to buried mineralization and support structural disruption of the systems after primary mineralization. Distal geochemical expressions of this system may include enrichment in iron contents with negative δ56Fe values and/or negative δ26Mg whole-rock or clay-size fractions values, reflecting precipitation of the isotopically light iron and magnesium leached during hydrothermal alteration. Although more costly to process, sequential leach δ26Mg or δ56Fe analyses produced greater variance in the ore zones relative to background values and removed anomalous values from carbonate minerals. Isotopic analyses of the clay-size fraction produced results comparable to the final digestion in the sequential leach process but this method in particular requires greater understanding the mineralogical controls on the isotopic signatures.

Rapid alteration of fractured volcanic conduits beneath Mt Unzen

The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes.

Advanced Use of Borehole Acoustic Televiewer (ATV) for Structural Interpretation of Unconformity-Related Uranium Deposits

Abstract The structural controls on unconformity-related uranium deposits of the Athabasca basin, Saskatchewan, Canada, are a matter of debate regarding the role of inherited fault systems and their reactivation. This can be related to the lack of outcrops allowing for direct observations, the strong clay alteration halos wrapping deposits that often obliterate structures, and the poor core recovery related to drilling strongly altered and mineralized intervals, which limits observation of structures and reliable oriented measurements. Borehole imaging technology is an invaluable alternative for obtaining oriented data through challenging drilling intervals. The use of borehole Acoustic Televiewer (ATV) has been integrated in recent exploratory campaigns in the Athabasca basin by Orano Canada. Here, we present the inputs and benefits of the use of the ATV in the exploration of unconformity-related uranium deposits and the structural analysis of oriented data from seven inclined diamond drill holes completed in 2016 during the McClean project (Sue deposits). The main objectives were to precisely identify the structural controls of the basement-hosted mineralization, and to test the tool in a well-known site. This work shows the applicability and added value of using televiewer probes to provide reliable oriented data in zones where there is much less information available. The ATV data structural interpretation supports the concept of mineralization of dilational jogs opening during preexisting shear-related foliation under right-lateral reverse fault reactivation. The ATV provides robust oriented data, allowing for a better understanding of the meaning of flat-lying mineralized structures along the Sue trend.

The Tarim Basin, China, a prospect for plume-related Zr(Hf)-Nb(Ta)-REY-Ga-U mineralization

This study is based on published data from the Tarim and Emeishan Large Igneous Provinces (TLIP and ELIP, respectively). The data are interpreted in a comparative manner, using the well-studied ELIP’s mineralization as a reference. Some additional information on related U and REY-carbonate ores from Far Eastern Russia are also included in this comparison. The results show that the Tarim Basin has both positive and negative indicators for tuffaceous Zr(Hf)–Nb(Ta)–REY–Ga–U ores. Like in ELIP, the source of the mineralization in the Tarim Basin is likely to be the Permian volcaniclastic deposits. The volcaniclastic deposits in the Tarim are not well studied geochemically; however, since the composition of the parent magmatic rocks are similar between TLIP and ELIP, the volcaniclastic material is also thought to be similar. Positive indications are that the TLIP’s deposits are larger (volumetrically), more felsic than the ELIP’s volcaniclastics. In addition, the former are concentrated in an almost closed terrestrial basin (Tarim Basin), whereas the latter are disseminated throughout a wide continental margin. The Paleozoic strata of the Tarim Basin are deeply altered in and around fracture zones, which are widespread. The alteration is caused by acidic volcanic-related fluids that are a key factor in metal mobilization and redistribution. However, in contrast to the ELIP’s area, the Tarim Basin was under influence of semi-arid climate that probably resulted in a weak activity of meteoric waters and poorly developed vegetation during deposition of the Permian volcaniclastic strata, which have a relatively low content of organic matter (<5%, commonly <2%) and contain no coal seam. These may be considered negative indicators for tuffaceous Zr(Hf)–Nb(Ta)–REY–Ga–U ores because organic acids might play a significant role in the leaching, redistribution, and the finishing processes that concentrated Zr, Hf, Nb, Ta, REY, Ga, and U within rocks with clay alteration. Finally, the Permian tuffaceous mudstones located in and around deep faults of the Tarim Basin may be a likely target for geochemical prospecting for Zr(Hf)–Nb(Ta)–REY–Ga–U ores similar to those found in southwestern China. In addition, the fracture zones in the Tarim Basin are likely to contain REY-carbonate and U ores similar to those discovered in Primorye, the Far Eastern Russia.

Permeability Alteration of Low Plastic Clay and Poorly Graded Sand Using Lime and Fly Ash

Work in this paper evaluates the effectiveness of soil treatment with lime and fly ash in terms of response with permeability and void ratio. The laboratory investigations were carried out on low plastic illite clay and poorly graded sand. Three different lime and fly ash contents (1%, 3%, and 5%) were considered for treatment purpose. After mixing soil with additives, a reaction period ranging from 3 to 14 days was allowed for the bond development between lime and fly ash with soil particles. From test results, it is found that plasticity indices decrease for clayey soil by adding both lime and fly ash. The reduction of the coefficient of permeability due to treatment with lime and fly ash is higher for clay than for sand. Permeability of the lime and fly ash treated clayey soil decreases subsequently up to 58–92% and 68–95% during a reaction period of 14 days. For sandy soil, under similar testing conditions, this reduction is 30–84% and 55–95% for lime and fly ash treated soil, respectively. The change in void ratio pattern with additives illustrates that sandy soil shows better formation with lime than fly ash, whereas clayey soil interacts better with fly ash than that of lime. Fly ash is found more effective than lime in decreasing the permeability of clayey and sandy soils (5–30%). The results indicate that the selected additives are effective for stabilizing both the low plastic clay and poorly graded sandy soil to utilize these soils as cladding materials for water bodies.

Exploration advancement of the Stewardson Lake uranium project (Athabasca Basin, Canada) using an integrated geological and geochemical methodology and academic–industry collaboration

ABSTRACTKurt Kyser contributed significantly to understanding the role that fluids play in the formation of unconformity-related U deposits in Canada and Australia and also in the exploration for these deposits. Kurt's exploration research was collaborative with industry, and arguably the most productive of the many industry-collaborative relationships Kurt developed was with Uravan Minerals, a junior exploration company. Ten years of collaborative U-deposit and exploration geochemical research with Uravan, including development of innovative multi-media surface sampling and analytical methods, culminated with the Stewardson Lake uranium exploration project in the Proterozoic Athabasca Basin, Canada. Soil clay separates, pine and spruce tree cores and vegetation, and glacially transported sandstone boulders collected at Stewardson Lake record 207Pb/206Pb ratios as low as 0.18, indicative of radioactive decay of U in a Proterozoic-aged U deposit, and elevated pathfinder elements such as Ni and Co in two areas of the Stewardson Lake property named Areas A and B. Four diamond drill holes in Areas A and B tested targets consisting of anomalous surface-media geochemistry coincident with conductive host rocks determined using geophysical methods. The favorable targeting characteristics were not explained by two of the drill holes in Area A, where only narrow intervals of elevated U, radiogenic Pb, and pathfinder elements or structural disruption that can accompany Athabasca U deposits were intersected. Two drill holes in Area B intersected a broad zone of characteristic chlorite + hematite + illite alteration and U concentrations &amp;gt;1 ppm (aqua regia) in the basal host sandstone. One drill hole in Area B intersected a ∼30-meter-thick hydrothermal alteration zone consisting of silicification, chlorite + kaolinite clay alteration, smoky quartz, Ni-Fe sulfides, and disseminated uraninite accompanied by elevated gamma count rates. Fractures in the host sandstone in Area B record the highest pathfinder concentrations and the most radiogenic Pb isotope ratios and were likely a conduit for secondary dispersion of alteration-related components to the surface. The results validate the Uravan/QFIR exploration model developed over a decade of collaborative applied research and confirm that the Stewardson Lake area is highly prospective to host unconformity-related U mineralization. Lastly, clay alteration mineralogy in the Stewardson Lake area is contrasted with that in the eastern Athabasca Basin, and the value of the industry–academic relationship that benefited both Uravan Minerals and Kurt's Queen's Facility for Isotope Research laboratory is discussed.

Geothermal potential of the eastern end of the Gediz basin, western Anatolia, Turkey revealed by three-dimensional inversion of magnetotelluric data

Three-dimensional models of the crustal electrical resistivity were derived from the inversion of magnetotelluric data in the eastern end of the Gediz basin, western Anatolia. In total, 179 MT sites measured in three different areas have been used in modeling. The study aims at providing information about the electrical resistivity distribution in the subsurface and its relations to structural setting and fluid pathways mainly controlling the development of geothermal reservoirs in the upper crust. The electrical resistivity models reveal a surficial low-medium resistivity layer associated with Neogene-Quaternary supradetachment sedimentary succession and a deeper high resistivity layer related to the Central Menderes Massif. The sedimentary cover over the metamorphic basement includes undulations, presenting an asymmetric extensional basin. The thickness of the sedimentary layer varies along strike from west to east and reaches approximately 3 km (max) in the graben. An anomalously conductive zone associated with a clay alteration zone was observed in this layer. The resistivity models also bring out two deep conductive zones along the southern margin of the basin: a conductive zone that could be interpreted as a geothermal reservoir at the easternmost end of the basin, however instead described as a part of a deep low resistivity zone with regional extent and another conductive zone interpreted as a region possessing an interconnected fracture network developed mainly by the extensional tectonic regime and that probably contains geothermal fluids. Moreover, normal faults exposing along the basin margins characterize the geometry of the basin, and particularly the main graben-bounding fault and Gediz detachment fault provide possible fluid pathways in the upper crust.

La Palma island (Spain) geothermal system revealed by 3D magnetotelluric data inversion

The study of geothermal systems is nowadays a topic of great importance because of the huge amount of energy that could be converted in electricity for human consumption from such sources. Among the various geophysical methods employed to study geothermal reservoirs, the magnetotelluric (MT) method is capable to reveal the internal structures of the subsurface and interpret the geological structures from the electrical resistivity. We present the first 3D resistivity model of La Palma (Canary archipelago, Spain) obtained from a dataset of 44 broadband magnetotelluric soundings distributed around the island. Our results highlight the presence of resistivity anomalies, spatially coinciding with density anomalies present in literature. In the north of the island, a high resistivity anomaly can be interpreted as the signature of an old intrusive body beneath the Taburiente caldera. In the south, a complex resistivity structure around the Cumbre Vieja volcanic ridge could be indicative of presence of an active geothermal system. In particular, low-resistivity anomalies, located in a high-fractured zone, have values compatible with clay alteration caps (illite and illite–smectite). Such a result suggests the presence of hot rocks, or a dike system, heating fluids in the interior of Cumbre Vieja volcanic system.

Mineralogy and Geochemistry of Ultramafic Rocks from Rachoni Magnesite Mine, Gerakini (Chalkidiki, Northern Greece)

The importance of magnesite for the EU economy and industry is very high, making the understanding of their genesis for the exploration for new deposits a priority for the raw materials scientific community. In this direction, the study of the magnesite-hosting ultramafic rocks can be proved very useful. For the present study, ultramafic rock samples were collected from the magnesite ore-hosting ophiolite of the Gerakini mining area (Chalkidiki, Greece) to investigate the consecutive alteration events of the rocks which led to the metallogenesis of the significant magnesite ores of the area. All samples were subjected to a series of analytical methods for the determination of their mineralogical and geochemical characteristics: optical microscopy, XRD, SEM, EMPA, ICP–MS/OES and CIPW normalization. The results of these analyses revealed that the ultramafic rocks of the area have not only all been subjected to serpentinization, but these rocks have also undergone carbonation, silification and clay alteration. The latter events are attributed to the circulation of CO2-rich fluids responsible for the formation of the magnesite ores and locally, the further alteration of the serpentinites into listvenites. The current mineralogy of these rocks was found to be linked to one or more alteration event that took place, thus a significant contribution to the metallo- and petrogenetic history of the Gerakini ophiolite has been made. Furthermore, for the first time in literature, Fe inclusions in olivines from Greece were reported.

Mineralogy, Fluid Inclusion and H‐O‐C‐S Stable Isotopes of Mengqiguer Uranium Deposit in the Southern Yili Basin, Xinjiang: Implication for Ore Formation

AbstractThe Mengqiguer deposit in the southern Yili basin Ili Basin is a large interlayer‐oxidation‐zone type uranium deposit. In this paper, we applied multiple methods including microscopic observation, scanning electron microscope and electronic probe, to analyze the systematical alteration characteristics of the ore‐bearing sandstone layer. Fluid inclusion and stable isotope studies on the ore‐bearing sandstone have also been carried out to discuss the internal relations between fluid activities, epigenetic alteration and the uranium mineralization. Major epigenetic alteration include clay alteration, carbonatization and pyritization, of which biogenetic pyritization is most closely related to the uranium mineralization. This suggests the existence of microorganism during the uranium mineralization process. The mineralization fluids of low temperature, medium density but varied salinities are suggested to be derived from multi‐source, including the meteoric water and organic acidic vapor components from coal‐bearing strata. Uranium mineralization, grain‐dispersed kaolinite, limonite, colloidal pyrite, and the carbonate cements associated with sulfate‐reducing bacteria were formed by meteoric water and vermicular‐shaped kaolinite, autologous pyrite, and the carbonate cementation associated with the dehydroxylation of organic matter was formed by organic acidic. Based on these results, we consider that the uranium mineralization and epigenetic alteration both resulted from the reciprocity of organic–inorganic fluid and fluid–rock during the formation of the interlayer oxidation zone.

Employing 3-D inversion of geomagnetic data to identify demagnetized rock associated with reservoir zone of Blawan-Ijen geothermal prospect area

Blawan-Ijen geothermal prospect area is placed at Bondowoso, East Java. Geologically, the presence of geothermal system in this field is controlled by the huge ancient Kendeng caldera complex structure. The heat source of the geothermal system is predicted to be situated at the base of Kendeng caldera. Geomagnetic method was used for detecting the presence of the heat source. The high temperature zone as the main target of geothermal exploration mostly situated around the heat source location. The geomagnetic method was applied because of its ability to identify the magnetic variation of rocks such as the demagnetized zone which is caused by the high temperature condition in geothermal area especially in the center of geothermal reservoir. Geomagnetic survey was conducted in 2017 and 2018 with the total of 151 stations. The 3-D inversion of geomagnetic data was then carried out to image the subsurface magnetic variation which is representing the complex structure of geothermal system in this area. The subsurface magnetic variation structure revealed by 3-D inversion showed the distribution of demagnetization reservoir rock inside the Kendeng caldera which has low magnetic susceptibility. In the upper part of this layer that is a higher magnetic susceptibility layer which may associated with the clay alteration that is overlaying the geothermal reservoir in this area. Moreover, the very low magnetic susceptibility anomaly can be observed start from -1000 m elevation, which is interpreted as the heat source of the geothermal system in this area.

Characterization of feed coals and coal combustion byproducts from the Wyodak-Anderson coal zone, Powder River Basin, Wyoming

The U.S. Geological Survey (USGS) determined the physical and chemical properties of more than 260 feed coal and coal combustion byproducts from two coal-fired power plants. These plants utilized a low-sulfur (0.23-0.47 wt. % S) and low ash (4.9-6.3 wt. % ash) subbituminous coal from the Wyodak-Anderson coal zone in the Tongue River Member of the Paleocene Fort Union Formation, Powder River Basin, Wyoming. Fifty-three samples of bituminous coal were collected and analyzed from a Kentucky power plant, which used several sources of bituminous coals from the Appalachian and Illinois Basins. Based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses of feed coal samples collected and analyzed from 1996 through the late 2000s, two mineral suites were identified: (1) a primary or detrital suite consisting of quartz (including beta-form grains), biotite, feldspar, and minor zircon; and (2) a secondary authigenic mineral suite containing alumino-phosphates (crandallite and gorceixite), kaolinite, carbonates (calcite and dolomite), quartz, anatase, barite, and pyrite. The detrital mineral suite is interpreted, in part, to be of volcanic origin, whereas the authigenic mineral suite is interpreted, in part, to be the result of the alteration of the volcanic minerals. The mineral suites have contributed to the higher amounts of barium, calcium, magnesium, phosphorus, sodium, strontium, and titanium in the Powder River Basin feed coals in comparison to eastern US coals. XRD analysis indicates that (1) fly ash is mostly aluminate glass, perovskite, lime, gehlenite, quartz, and phosphates with minor amounts of periclase, anhydrite, hematite, and spinel group minerals; and (2) bottom ash is predominantly quartz, plagioclase (albite and anorthite), pyroxene (augite and fassaite), rhodonite, and akermanite, and spinel group minerals. Microprobe and SEM analyses of fly ash samples revealed quartz, zircon, and monazite, euhedral laths of corundum with merrillite, hematite, dendritic spinels/ferrites, wollastonite, and periclase. The abundant calcium and magnesium mineral phases in the fly ash are attributed to the alteration of carbonate, clay, and phosphate minerals in the feed coal during combustion. The calcium- and magnesium-rich and alumino-phosphate mineral phases in the coal combustion byproducts can be attributed to volcanic minerals deposited in peat-forming mires. Dissolution and alteration of these detrital volcanic minerals occurred either in the peat-forming stage or during coalification and diagenesis, resulting in the authigenic mineral suite. The presence of free lime (CaO) in fly ash produced from Wyodak-Anderson coal acts as a self-contained “scrubber” for SO3, where CaO + SO3 form anhydrite either during combustion or in the upper parts of the boiler. Considering the high lime content in the fly ash and the resulting hydration reactions after its contact with water, there is little evidence that major amounts of leachable metals are mobilized in the disposal or utilization of this fly ash.

Characterization of microseepage-induced diagenetic changes in the Upper-Red Formation, Qom region, Iran. Part II: A new insight using reflectance spectroscopic analysis

The classic microseepage model supports the notion that hydrocarbon pools leak to the surface over time and trigger an array of diagenetic physicochemical and mineralogical changes in the overlying sediments. Mapping the induced alteration mineralogy for microseepage targeting has been an appealing yet ambiguous practice, mostly because of our incomplete understanding of the diversity of changes occurring in the environment. To close this gap, an integrated spectroscopic approach is adopted here to investigate the variability, quantity, and physicochemistry of diagenetic minerals induced by active microseepage systems over the Alborz oil reservoir in the Qom region, Iran. The area includes an effectively sealed carbonate reservoir overlain by thick Oligocene sediments of the Upper Red Formation. A large suite of samples collected from microseepage-affected zones was spectrally characterized in the 350–2500 nm wavelength region and then statistically contrasted against unaffected samples gathered from a nearby control area. The analysis reveals a novel array of mineralogic indicators for microseepage systems. It indicates that smectites (montmorillonite, nontronite), Al-rich mixed-layer illite/smectite, Fe-rich chlorite and traces of palygorskite constitute the clay alterations. This array is associated with the enrichment of K 2 O and loss of Na 2 O and MgO from the system, as indicated by X-ray fluorescence measurements. In the arid climate of the region, the iron-rich red-beds (Fe 2 O 3 > 6%) are only moderately bleached and, thus, the subsequent chemical weathering of the relict irons has triggered goethite (Al-poor), and likely ferrihydrite and maghemite precipitation. Consequently, besides the typical ferric iron bleaching, the modification in the mineralogy of iron oxides/oxyhydroxides (marked by a shift in the absorption minimum at ~ 900 nm) is suggested as an additional indicator of microseepage systems. Fe-chlorites constitute the dominant ferrous mineral present in the environment. The affected strata are also associated with prevalent calcite and gypsum cements, and occasionally jarosite occurrences. Based on the silica contents of the host-rock, the microseepage-induced alterations are classified into two categories, namely ‘felsic’ and ‘mafic’, wherein the latter is typified by the assemblages described here. This study provides new insights into microseepage-induced alterations and highlights the significance of reflectance spectroscopic techniques in facies determination and the alteration footprints mapping of onshore microseepage systems. • Novel mineralogical indicators for microseepage-induced alterations are introduced. • The illite/smectite composition tends to be Al-rich with high crystallinity degree. • The chlorite composition becomes progressively more Fe-rich in the affected zones. • The host-rock geochemistry and weathering overprint affect the diagenetic alterations.

Terrestrial alteration mineral assemblages in the NWA 10416 olivine phyric shergottite

We report on the alteration history of the olivine-phyric, highly depleted (HD) shergottite, Northwest Africa (NWA) 10416, paying particular attention to the origin of the aqueous alteration seen affecting the meteorite’s olivine megacrysts. The rock’s interior displays 1 mm, zoned, altered olivine megacrysts set in a groundmass of clinopyroxene, unzoned olivine, and interstitial plagioclase and maskelynite. Synchrotron micro X-ray diffraction (µ-XRD) and transmission electron microscopy (TEM) show that plagioclase and maskelynite have been partially replaced by kaolinite. The relict olivine megacryst cores display a unique concentric colouration for martian meteorites, having central amber-coloured zones surrounded by a brown mantle zone, with the rims remaining clear and unaltered. This colouration is a result of fluid alteration and partial replacement, with hydration. TEM analysis revealed the ∼200 nm scale banded and largely amorphous nature of the alteration, but with some (∼20%) relict crystalline olivine patches. Although the coloured olivine zones show cation and anion site vacancies compared to stoichiometric olivine, a relict igneous compositional trend is preserved in the megacrysts, from Mg-rich altered cores (Mg# = 76) to unaltered stoichiometric rims (Fo53). Synchrotron Fe-K X-ray absorption near-edge structure (XANES) analysis revealed that the coloured zones of the megacryst have different Fe oxidation values. High ferric contents are present in the brown mantle zones (Fe3+/ΣFe ≤ 0.92) and the amber zones (Fe3+/ΣFe ≤ 0.30), whereas the clear rims are ferrous. This suggests alteration occurred in an oxidising environment and that the sharp contrast in colour of the megacryst (brown to clear) is a record of a relict fluid reaction front.In order to test the terrestrial or extraterrestrial origin of the alteration, olivine material from a shock-melt vein was analysed by TEM. The analysis revealed 0.952 nm curved d-spacing’s from clay alteration undisturbed by any shock effects, strongly suggesting a terrestrial origin. The d-spacing values most likely represent a collapsed saponite or vermiculite, showing that in some places olivine has been replaced by crystalline clay.Oxygen isotope analysis of bulk (Δ17O = 0.309 ± 0.009 (2σ) ‰) and amber-coloured megacryst material (=0.271 ± 0.002 (2σ) ‰), are also consistent with terrestrial alteration. We propose a model in which, during the meteorite’s time in Northwest Africa, low-temperature, likely acidic, groundwater exploited fractures. The fluid altered the olivine megacrysts in a way that was controlled by the pre-existing, igneous compositional zonation, with Mg-rich olivine being more susceptible to alteration in this fluid environment. The plagioclase and maskelynite were also altered to a high degree. After the alteration event it is likely that NWA 10416 had a significant residence time in Northwest Africa, accounting for terrestrial calcite and the dehydration of some clay phases.