Early Alteration Stage Research Articles

Petrological assessment of drying shrinkage of sedimentary rock used as aggregates for concrete

The drying shrinkage of different sedimentary rocks used as aggregates for concrete, mainly sandstone, is evaluated on the basis of petrological characterization and water vapor sorption. The drying shrinkage of aggregates is dependent on their alteration and metamorphic types, and is especially influenced by the types and contents of their constituent minerals and pore spaces between the crystals. There was a positive correlation between drying shrinkage of aggregates and water vapor desorption. When the volume of pores 4–20 nm in diameter was reduced, drying shrinkage and water vapor sorption were drastically reduced. In the case of sandstone aggregates, drying shrinkage of the aggregate is large in the early stage of alteration while it decreases with the progress of alteration and metamorphism processes. The presence of secondary type-A muscovite indicates a transitional stage between these processes, which is an important petrological feature in evaluating aggregate shrinkage. These results clearly show that differences in geothermal gradient and compression intensity from the time of deposition to the present manifested as the alteration and metamorphic types, with divergent pore distributions. The drying shrinkage and the water vapor sorption of aggregates reflect these petrological characteristics and resultant pore distributions.

Diffusion pathways of Fe2+ and Fe3+ during the formation of ferrian chromite: a µXANES study

The alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity (fO2) and Fe2+ ↔ Fe3+ exchange during fluid–rock interaction. Chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid–rock interaction frequently overprints the record of earlier stages of alteration. To shed light on that process we have studied the distribution of Fe2+ and Fe3+ in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron-based microscopic chemical imaging and spatially resolved X-ray absorption near edge structure spectroscopy. Our results show that early stages of alteration do not involve changes in Cr3+ and Fe2+ contents or in Fe speciation but only depletion in Al3+ and Mg2+ resulting in the formation of porous chromite. With ongoing alteration Fe3+ migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries. Sheared-type chromitites record the maximum rates of fluid–rock interaction because in these chromitite-types the accommodation of deformation on porous chromite allowed higher rates of diffusion of Fe3+ and Fe2+ (a magnetite component with Fe3+/Fetotal = 0.66) into the newly formed neoblasts. In porous chromite-type texture (all the original chromite grains fully transformed to porous chromite) the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains. In contrast, in partly altered-type texture (chromite grains with original cores surrounded by porous chromite), such processes are only restricted to the porous rims, giving rise to zoned chromian spinel-ferrian chromite grains.

Correlation Between Engine Oil Degradation, Tribochemistry, and Tribological Behavior with Focus on ZDDP Deterioration

This work aimed to bridge the gap between engine oil degradation, in particular zinc dialkyl dithiophosphate (ZDDP) deterioration, its effect on tribofilm formation, and its eventual impact on friction and wear. Artificial oil alteration of a commercial engine oil SAE 0W-20 was used to produce a series of altered oils that were subjected to high resolution mass spectrometry in order to identify ZDDP degradation products and their abundances over time. It was found that ZDDP was depleted at an early stage of alteration by the replacement of its sulfur atoms by oxygen atoms and the release of the alkyl side chains. As final products of ZDDP deterioration, sulfuric and phosphoric acid were identified. Selected oils were subjected to tribometrical experiments consisting of an oscillating ball-on-disc contact. ZDDP concentration as well as the type and abundance of ZDDP degradation products in the oils directly influenced tribofilm formation and consequently friction and wear. However, the superior tribological performance of the fresh engine oil could not be regained with any of the artificially altered oils. X-ray photoelectron spectroscopy revealed that sulfur occurred mainly in sulfide state in the tribofilm formed from fresh oil. With increasing degree of oil degradation, the sulfate state became more dominant. By trend, the amounts of sulfur, phosphorus, zinc and calcium declined with increasing degree of oil degradation, indicating that tribofilm formation had become increasingly difficult.

Diagenesis of mollusc aragonite and the role of fluid reservoirs

Biogenic carbonate minerals are widely used as archives in paleoenvironmental research, providing substantial information for past depositional and diagenetic regimes. However, nearly all biogenic carbonates undergo post-mortem diagenetic alteration to variable degrees. Diagenetic features are essentially caused by complex fluid-solid interaction including recrystallization and neomorphosis of shell architecture and related geochemical resetting. A common conception is that a given primary shell texture is replaced by a secondary fabric via micro-scale dissolution-reprecipitation reactions that may reach geochemical and/or isotopic equilibrium with the diagenetic fluid. Here we document that the process of petrographic and geochemical alteration of a biogenic carbonate archive progresses stepwise, and the re-equilibration processes can be separated in space and time. More specifically, attached and bound aqueous fluids within the shell organic matter and fluid inclusions likely play a crucial role in the early stages of alteration within a relatively rock-buffered system. Degradation of organics was observed via decreased sulfur concentration and decreased fluorescence. In this early stage of alteration, the conversion of micro-scale aragonite biominerals to metastable diagenetic calcite (meso)crystals is documented by RAMAN spectroscopy, electron backscatter diffraction (EBSD), and a decrease in strontium concentrations. Further stabilization to larger, neomorphosed calcite crystals resulted from re-equilibration with external fluids and was evidenced by EBSD, oxygen isotope data, and further reduction of strontium. Spatial chemical and isotope data were used to decipher the impact of fluid availability and transfer behavior, i.e. carbonate-buffered versus diagenetic-fluid buffered, and isotope and element exchange behavior, and analyzed in context to textural alteration features. These data suggest that during diagenesis, the evolution to more sustainable (diagenetically induced) fabrics is accompanied by individual reaction steps traced by elemental and isotopic signatures. Kinetics, and thus degree of diagenetic alteration of biominerals at a given exposure to physicochemical alteration conditions is initially strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results shown here have significance for those concerned with biogenic carbonate archives and have wider implications for a mechanistic understanding of carbonate diagenesis.

Origin of sulfur and crustal recycling of copper in polymetallic (Cu-Au-Co-Bi-U ± Ag) iron-oxide-dominated systems of the Great Bear Magmatic Zone, NWT, Canada

The Great Bear Magmatic Zone, in northwest Canada, contains numerous polymetallic mineral occurrences, prospects, and deposits of the iron oxide copper-gold deposit (IOCG) family. The mineralization is hosted by the Treasure Lake Group and igneous rocks of the Great Bear arc and was deposited concomitantly with the arc magmatism (ca. 1.88 to 1.87 Ga). In situ δ 34S (n = 48) and δ 65Cu (n = 79) analyses were carried out on ore-related sulfides from a number of these systems. The δ 34S values mainly vary between 0 and +5‰, consistent with derivation of sulfur from the mantle. Lower δ 34S values (−7.7 to +1.4‰) from the Sue-Dianne breccia may indicate SO2 disproportionation of a magmatic hydrothermal fluid. The δ 65Cu values vary between −1.2 and −0.3‰, and are lower than the igneous δ 65Cu range of values (0.0 ± 0.27‰). The S and Cu isotopic data are decoupled, which suggests that Cu (and possibly some S) was dissolved and remobilized from supracrustal rocks during early stages of alteration (e.g., sodic alteration) and then precipitated by lower temperature, more oxidizing fluids (e.g., Ca-Fe-K alteration). A limited fluid inclusion dataset and δ 13C and δ 18O values are also presented. The δ 18Ofluid values are consistent with a magmatic origin or a host-rock equilibrated meteoric water source, whereas the δ 13Cfluid values support a marine carbonate source. Combined, the S and Cu isotopic data indicate that while the emplacement of the Great Bear magmatic bodies may have driven fluid convection and may be the source of fluids and sulfur, metals such as Cu could have been recycled from crustal sources.

GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites

The Paris meteorite is a weakly altered CM chondrite that has been discovered recently (Hewins et al., 2014). Its matrix offers the opportunity to search for well-preserved pristine pre-accretional material, as well as to study the earliest stages of aqueous alteration in the CM parent body. The study of the matrix of Paris has been conducted by analytical transmission electron microscopy on focused ion beam sections extracted from matrix areas showing different degrees of aqueous alteration.The least altered matrix sample consists of amorphous silicate grains, a few hundreds of nm in size, separated from one another by an abundant porosity. The amorphous silicates enclose numerous Fe-sulfide nanograins and their average composition is close to the chondritic composition. They share many similarities with GEMS (glass with embedded metal and sulfides) grains present in chondritic-porous interplanetary dust particles and with primitive type 3.0 carbonaceous chondrites. This first discovery of GEMS-like texture in a CM chondrite suggests that GEMS grains could have been the building blocks of the CM matrices.In more aqueously altered samples, pronounced microstructural heterogeneities were detected at the micrometer scale. The matrix consists mostly of a mixture of amorphous material and Fe-rich, spongy to fine-fibrous, poorly crystalline phyllosilicates. The porosity fraction is significantly reduced and the mixed amorphous-fibrous material frequently forms a continuous groundmass. The close association between these two material types suggests a replacement mechanism due to aqueous alteration. Chemical compositions correlate strongly with the microstructure. The amorphous material has a composition close to the chondritic value while the fine-fibrous phyllosilicate material is Fe-enriched. This Fe enrichment is found to be continuous from weakly to more heavily altered areas, in which the fibrous morphology is coarser and better crystalline. Cronstedtite with intercalated tochilinite is also found, but in pore spaces. This chemical evolution, concomitant with the maturation of the phyllosilicates, demonstrates that the early aqueous fluids that interacted with silicates in the matrix were enriched in Fe. This composition is probably the consequence of the preferential dissolution of metal and iron sulfides during the first stages of alteration. The enrichment of phyllosilicates in Mg seen in more altered CM chondrites is not observed in Paris.

Structural and diagenetic origin of breccias in the carbonate-hosted Polaris Zn–Pb deposit, Nunavut, Canada

The Polaris deposit, located on Little Cornwallis Island in the Canadian Arctic, was a Mississippi Valley-type Zn–Pb deposit hosted by brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Mapping indicates that strike-slip faults on the east side of the Polaris deposit were active during the last stage of the Late Devonian Ellesmerian Orogeny. Polaris is on a jog in the north-oriented, Early Devonian Boothia fault system and was the site of localized extension during south-directed Late Devonian Ellesmerian compression. This structural setting elsewhere in the district may be prospective for Zn–Pb mineralisation.Ore fluids rising in the Late Devonian interacted with the host rock causing dissolution, brecciation and collapse. Carbonate beds are thinned, indicating widespread removal of carbonate material. Five breccia types (crackle, pseudo, cobble, mega and collapse) are present in the vicinity of the deposit. Crackle breccia is preserved around the periphery of the deposit and is indicative of structural dilation or the early stages of mineralisation. Dolomite and pseudobreccias are also preserved around, and extend beyond, the periphery of the deposit. These are considered an early stage of alteration directly related to the mineralising fluids and could act as a vector to the centre of the mineralising system where collapse, mega and cobble breccias occur intimately with massive mineralisation.

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO 2) alteration surrounded by alunitic alteration zones. This paper focuses on a geochemical and stable isotope study of the surficial alteration facies of Vulcano – particularly the horizon characterized by strong silicic alteration – and on deep seated xenoliths ejected during the last eruption of La Fossa volcano (1888–90) that can be considered as representative of fragments of the deep conduit system of La Fossa volcano. Using directly measured temperatures at the sites of sampling, we have calculated fluid composition in isotopic equilibrium with the alteration products. The large range of measured silica δ 18O (12.3 to 29‰) reflects the wide range of formation temperatures (80–240 °C). The fluid compositions calculated for intense silicic alteration vary from − 0.9 to + 6.5‰. These are significantly heavier than local meteoric water (− 6‰), and are consistent with derivation from the condensation of high-temperature fumarolic gases, dominated by magmatic fluids and rich in acid gases (SO 2, H 2S, HCl, HF), into shallow groundwaters of meteoric origin, with dynamically variable ratios of fumarolic steam/meteoric water. The calculated δ 18O and δD of water in equilibrium with alunite also suggest the mixing of magmatic and meteoric waters for the fluids involved in the genesis of advanced argillic alteration facies. The calculated δ 18O of water in equilibrium with hedenbergitic clinopyroxene, found in a veinlet in a metasomatized xenolith is + 8.9‰. This value cannot reasonably result from water–rock interaction between the host volcanic rocks and surface water. Instead, it most likely represents a fluid (brine) exsolved from magma, which was responsible for high temperature metasomatism in the deep conduit system.

Improved assessment of biodegradation extent and prediction of petroleum quality

Biodegradation of crude oil causes volumetrically important compositional changes, which lead to significant deterioration in quality, in particular during the early stages of alteration. To better understand these effects we focussed on a detailed assessment of light to moderate levels of alteration. We investigated a suite of 40 crude oil samples from five different petroleum systems to evaluate the extent of alteration occurring in reservoirs. Based on a comprehensive geochemical characterization, five individual crude oil sequences were defined, where compositional variability is mainly due to microbial activity in the reservoir. In particular, samples from the Gullfaks field (offshore Norway) and from a petroleum system offshore Angola illustrate that conventional molecular biodegradation parameters, such as the Pr/ n-C 17 and Ph/ n-C 18 alkane ratios are not suitable for defining the extent of biodegradation in petroleum reservoirs. Here, we suggest a new molecular biodegradation parameter, the degradative loss, that can be used to quantify depletion in individual crude oil constituents. The approach allows improved assessment of the extent of biodegradation in crude oil samples by means of the mean degradative loss. It is demonstrated that crude oil quality, as assessed from API gravity, can be predicted directly from the molecular composition of crude oils. Our data clearly indicate that the degradation patterns of light hydrocarbons and n-alkanes differ in different petroleum systems. This suggests that microbial communities are different and therefore generate different molecular degradation patterns which have to be evaluated individually for each system.

Trace element immobilization by uranyl minerals in granite-hosted uranium ores: Evidences from the Xiazhuang ore field of Guangdong province, China

The oxidative alteration of uraninite and the fate of trace elements (Y, LREE, Zr, and Th) in a granite-hosted uranium ore deposit in north Guangdong province, China, were investigated to understand the geochemical behavior of spent UO2fuel and associated fission products and transuranium elements under oxidizing conditions. In light of the paragenetic relationship of the alteration products, two alteration series of uraninite were identified: one is the silicate series with a mineral paragenesis of uraninite → uranyl oxide hydrates → Si-rich uranyl phase → uranophane (Ca[(UO2)(SiO3OH)]2(H2O)5), and the other is the phosphate series with a mineral paragenesis of uraninite → uranyl oxide hydrates → autunite (Ca[(UO2)(PO4)]2(H2O)6) → yingjiangite ((K2,Ca)(UO2)7(PO4)4(OH)6(H2O)6). In contrast to the wide distribution and abundance of uranophane and the uranyl phosphate minerals, uranyl oxides were only occasionally found in the ore samples, suggesting that the uranyl silicates and phosphates should be the predominant alteration products of UO2under oxidizing conditions, although uranyl oxide hydrates would be the solubility-limiting phase of uranium in the very early stage of alteration. Furthermore, the interlayer cation of the uranyl phases in the Xiazhuang uranium ore field is dominated by Ca2+, indicating that the release of uranium and other radionuclides will be limited mainly by uranophane and autunite during the oxidative alteration of spent UO2fuel in underground repositories where enhanced calcium concentration is expected due to cement/water reactions.Compositionally, the cation atomic ratios in uranyl phases often deviate considerably from their respective stoichiometric values as indicated by the nominal formulae, but the compositional variation does not result in significant structural change as indicated by X-ray diffraction patterns. This observation indicates that the structure of U6+minerals may easily be adjusted to accommodate impurity elements including crystallographically compatible radionuclides. Compared with the primary uraninites, the secondary minerals are slightly enriched in light REE, but significantly depleted in Y3+probably due to its cation-radius mismatch with interlayer Ca2+. The apparent enrichment of Zr4+in uranophane and uranyl phosphates relative to uraninite may result from the coupled substitutions: Zr4+↔ U6+and REE3+↔ Ca2+(K+). Because an adequate charge-balance mechanism and significant distortion of the coordination polyhedra are required for the substitution An4+↔ U6+, this type of substitution may not be common.

Mineralogic and Stable Isotope Studies of Hydrothermal Alteration at the Jinchuan Ni-Cu Deposit, China

The Jinchuan intrusion hosts one of the largest magmatic Ni-Cu sulfide deposits in the world. Net-textured and disseminated pyrrhotite, pentlandite, and chalcopyrite are found in olivine-rich rocks (60–80 vol %) that occur as a northwest-southeast–striking dike. Olivine in the intrusion has been variably converted to mixtures of serpentine and fine-grained magnetite. Interstitial pyroxene and minor plagioclase have been altered to mixtures of chlorite, amphibole, epidote, clinozoisite, Na-rich feldspar, and lesser amounts of calcite. Although the interstitial net texture of sulfide minerals surrounding olivine grains is preserved, up to 30 vol percent of sulfide minerals are replaced by magnetite, serpentine, and chlorite. An external origin for Mg and Si needed for the isovolumetric replacement of sulfides cannot be discounted, but a local origin related to olivine and pyroxene alteration is favored. Serpentine and chlorite may be enriched in Cu and Ni, but the enrichment can account for less than 1 percent of the metals lost during the replacement of sulfide minerals by silicates and oxides. Some of the metals and sulfur that were removed from the net-textured sulfide assemblages have been redeposited on a centimeter scale in fine ribbons and veins within serpentine, chlorite, and amphibole. Sulfur isotope values of the pyrrhotite-pentlandite-chalcopyrite assemblage at Jinchuan fall in the range of −2 to 8 per mil, with over 80 percent of the values between −2 and 2 per mil. Immediate country rocks in the Jinchuan area are Precambrian granitoids, schists, and marbles with sulfur concentrations less than 100 ppm. Although the δ 34 S values of the main sulfide assemblage at Jinchuan are in the range that is characteristic of mantle-derived sulfur, assimilation of sulfur from unexposed country rocks cannot yet be discounted. Secondary pyrite occurs in veins and stringers, and rarely as a replacement of pyrrhotite, and is characterized by δ 34 S values between −7 and −27 per mil. The strongly negative δ 34 S values of secondary pyrite are thought to be a result of oxidation of primary magmatic sulfide assemblages and partial reduction of aqueous sulfate. Oxygen and hydrogen isotope values of silicate minerals suggest that at least two fluids have been involved in the hydrothermal alteration at Jinchuan. The δ 18 O values of plagioclase, amphibole, and serpentine all suggest the involvement of a fluid with a δ 18 O value between ~2 and 5 per mil at 300° to 400°C. The δD values of amphibole and one population of serpentine suggest a δD value for the fluid in the range of −45 to −63 per mil. These values are consistent with the involvement of evolved seawater or a metamorphic fluid in the early stages of alteration. The possible involvement of evolved seawater in hydrothermal alteration is consistent with a rift setting for the emplacement of the Jinchuan intrusion. However, the wide range in serpentine δD values (computed δD of the fluid = −45 to −103‰) suggest either that serpentinization involved a mixture of evolved seawater and/or metamorphic water and meteoric water or, more likely, that serpentine continued to exchange hydrogen isotopes with late-stage meteoric waters after it formed.

Composition of impact melt particles and the effects of post‐impact alteration in suevitic rocks at the Yaxcopoil‐1 drill core, Chicxulub crater, Mexico

Abstract Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m‐thick suevitic sequence at the Chicxulub Yax‐1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post‐impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post‐impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene‐rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi‐stage post‐impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, Al2O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid‐rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the post‐impact hydrothermal mass transfer and was more pronounced in glassy melt particles than in well‐crystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax‐1 suevitic rocks show strong potassium metasomatism with hydrothermal K‐feldspar formation and whole rock K20 enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na‐bearing Mg‐rich smectite, among other minerals. This indicates a general evolution from a silica‐undersaturated fluid at relatively high potassium activities at an early stage toward a silica‐oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction.

The Batalha Au–granite system—Tapajós Gold Province, Amazonian craton, Brazil: hydrothermal alteration and regional implications

The Tapajós Gold Province (TGP) is part of the ∼2.10–1.87 Ga Ventuari–Tapajós (or Tapajós–Parima) tectonic province of the central Amazonian craton. The Ventuari–Tapajós Province resulted from an ocean–continent orogeny and includes two calc-alkaline magmatic arcs that are represented by the Cuiú-Cuiú complex (∼2.1–1.90 Ga) and Parauari intrusive suite (∼1.92–1.88 Ga). The Parauari magmatic event was accompanied by sedimentation and intrusion of post-tectonic ∼1.88 Ga calc-alkaline granites and was followed by subalkaline and alkaline A-type magmatism related to the ∼1.87 Ga Uatumã event. The ∼1880 Ma Batalha granite consists of metaluminous to peraluminous, calc-alkaline to sub-alkaline, hornblende-biotite syeno- to monzo-granites, and shows a late- to post-collisional affinity. Intense pervasive and fissural post-magmatic hydrothermal alteration, linked to gold mineralization, affected the Batalha granite. The early alteration stage involved Na metasomatism represented by crystallization of interstitial albite and replacement of plagioclase and K-feldspar by albite. Precipitation of albite caused Na impoverishment and K enrichment in the hydrothermal fluid, leading to microcline and biotite crystallization and pervasive replacement of older feldspars by K-feldspar. Pervasive and fissural propylitization overprints magmatic and early hydrothermal mineral associations, and is characterized mainly by epidote, clinozoisite, chlorite, albite, carbonate and sulphide. Fissural to pervasive sericitization was the last hydrothermal stage and led to crystallization of sericite, quartz and sulphide. Gold mineralization was related to the last two alteration stages, most importantly sericitization. The hydrothermal alterations were related to exsolved magmatic aqueous fluids 500–290 °C and 2.6–1.2 kbar. The Batalha granite shares certain characteristics with both (1) porphyry-related systems (biotite halogen content, large alteration haloes and base metals associated with gold) and (2) intrusion-related gold systems (reduced environment, depth of emplacement and gold mineralization associated with sericitic alteration). Classification of the Batalha gold mineralization is thus not straightforward. However, calc-alkaline to sub-alkaline subvolcanic granites similar to Batalha (i.e., with identical hydrothermal alteration patterns) are associated with Paleoproterozoic quartz–alunite high-sulphidation volcanic-hosted epithermal gold mineralization in the TGP, implying coeval evolution of the hydrothermal systems. Thus, the shallower and more oxidized Batalha granite may have generated a porphyry Cu–Au type deposit. This has new implications for gold exploration in the TGP—apart from the known gold deposits within shear zones there is high potential for low-grade, large-volume gold and base metal deposits.

Permeability, porosity and pore geometry of chemically altered porous silica glass

In this study, we prepared samples of a well‐controlled, internally smooth, synthetic material (i.e., porous silica glass) and submitted them to chemical alteration while measuring changes in porosity and permeability. In the early stages of alteration we observed a large drop in permeability, whereas porosity remained essentially unchanged. In the late stages the permeability decrease became gradual so that even small decreases in permeability were accompanied by substantial decreases in porosity, as has been observed previously in natural sandstones and shales. A detailed microstructure study showed that smooth pore walls were roughened during alteration. Furthermore, the pore space heterogeneity increased and the connectivity decreased. A simple conceptual model suggests that the alterations occur while fluids are flowing.

Alteration Characteristics of the Archean Golden Grove Formation at the Gossan Hill Deposit, Western Australia: Induration as a Focusing Mechanism for Mineralizing Hydrothermal Fluids

The Archean Golden Grove Formation is a 550-m-thick rhyodacitic tuffaceous volcaniclastic succession that hosts the Cu-Zn-rich Gossan Hill volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn craton, Western Australia. The Golden Grove Formation consists of volcanic quartz and altered pumice and shards, which were deposited during successive episodes of subaqueous mass flow. Coherent volcanics are absent from the Golden Grove Formation at Gossan Hill but form the main rock type in the hanging-wall Scuddles Formation. A massive dacite dome that overlies its volcanic feeder dike is inferred to occupy a synvolcanic structure that focused mineralizing fluids during the formation of the Gossan Hill deposit. The Gossan Hill deposit consists of two stratigraphically separate ore zones interconnected by stockwork. The Cu-rich lower ore zone and the Zn-rich upper ore zone occur in the middle and upper parts of the Golden Grove Formation, respectively. Podiform zones of massive magnetite occur in the Cu-rich ore zone, where the formation of magnetite predates massive sulfide. The asymmetry of massive sulfide, massive magnetite, and alteration zones at the Gossan Hill deposit attest to synvolcanic structural control during mineralization. The principal lithofacies of the Golden Grove Formation are sandstone and pebble breccia, which have a regionally extensive quartz, Fe-rich chlorite, and lesser muscovite alteration. At Gossan Hill, this alteration has resulted in near-complete replacement of tuffaceous components, causing substantial chemical modification of the primary lithologies. Quartz-chlorite (±muscovite) alteration is characterized by severe K2O, Na2O, and CaO depletion, with rocks consisting principally of SiO2, FeO, Al2O3, and MgO, along a quartz-chlorite mixing trend. Widespread preservation of pumice and shard volcanic textures within the Golden Grove Formation indicates that quartz-chlorite (±muscovite) alteration occurred soon after, or possibly during, sedimentation. The absence of diagenetic compaction textures further suggests induration of the succession during this early alteration stage, with the tuffaceous succession largely sealed from the texturally destructive effects of subsequent hydrothermal alteration, except where mineralizing fluids were locally channeled along synvolcanic feeder conduits. Local intense hydrothermal alteration zones surround the Gossan Hill deposit and overprint earlier quartz-chlorite (±muscovite) alteration. These local alteration zones have the same extent as the sulfide vein envelope and represent hydrothermal alteration formed during sulfide-magnetite mineralization. Intense Fe-rich chlorite (ankerite-siderite) alteration occurs as a strata-bound envelope around massive magnetite, Cu-rich veins, and massive sulfide in the lower ore zone. This chlorite-rich alteration has strong FeO and MgO enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration. Iron chlorite (ankerite-siderite) alteration grades upward into discordant to strata-bound intense quartz alteration. Intense quartz alteration forms an envelope around Zn-rich veins and massive sulfide in the stockwork and upper ore zone. The trend from Fe chlorite-ankerite-siderite to quartz alteration toward the top of the deposit is consistent with the cooling of hydrothermal mineralizing fluids nearing the sea floor. Rhyodacite and dacite volcanics of the hanging-wall Scuddles Formation have a pervasive muscovite-calcite alteration. Muscovite-calcite alteration led to Na2O depletion and CaO and K2O enrichment associated with burial of the Gossan Hill mineralizing system. We propose that the Gossan Hill sulfide-magnetite VHMS deposit formed during an evolving Archean hydrothermal system that began as part of a regional-scale, low-temperature seawater convection-alteration system. Initially, this system caused extensive replacement of the Golden Grove Formation by quartz and Fe chlorite (±muscovite); a process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability structures. Due to subsequent impermeability of the host-rock succession, later and hotter mineralizing fluids that generated alteration and massive magnetite and sulfide at the Gossan Hill deposit were constrained to, and focused upward along, a synvolcanic feeder structure.

Characteristics of Clay Minerals in Gouges of the Dongrae Fault, Southeastern Korea, and Implications for Fault Activity

AbstractThe Dongrae fault within the Yangsan fault system is considered one of the major faults in the southeastern part of Korea, extending over 150 km. The results of K-Ar radiogenic dating of fault gouges collected from six localities show a relatively wide range in age from 57.5 million years ago (Ma) to 40.3 Ma. Fault gouges are composed of newly formed minerals, including smectite, illite, zeolite, kaolinite, K-rich feldspar, apatite, and pyrite. The occurrence of abundant smectite and illite-lMd with lesser quantities of zeolite suggests that the fault gouges experienced hydrothermal alteration at low temperatures. Smectite is probably unstable relative to other clay minerals, such as illite and zeolite. Considering that filiform mordenite is replacing the smectite, we suggest that mordenite formed by recrystallization involving a solid-state transformation. Under high fluid/rock ratios, smectite seems to have formed in the early stage of alteration. In contrast, zeolite minerals and authigenic K-rich feldspar progressively appeared with time as the fluid/rock ratio decreased with the changing chemistry of the hydrothermal fluids. The composition of clay minerals in the gouge materials probably was controlled by the chemistry and the amount of circulating fluids derived from adjacent granitic rocks.

A study of alteration phases on glass-bonded zeolite and sodaiate using the vapor hydration test

AbstractThis paper focuses on the reaction of glass-bonded zeolite and sodalite ceramic waste forms, naturally occurring sodalite, and their respective alteration phases that formed after reaction in a Vapor Hydration Test (VHT). Scanning electron microscope (SEM) analysis of the glass-bonded zeolite and sodalite samples indicate that NaCI is the major alteration phase present, along with KCI, Na-Al-Si, and Na-K-Ca-Al-Si phases. The order of precipitation of salt crystals from ceramic waste form occurred in the sequence NaCl ⇒ KC1 ⇒ CaCl2 and MgCl2. We also found aluminum silicate phases containing Ca, Mg, K-Na-Ca, K-Mg, Na-K-Mg, K-Ca-Na-Mg, as well as CaCO3, SiO2. The ratio of Si/Al in the alteration phases was noted to increase from approximately 1.5 to 5 during the progressive alteration of the samples. This trend indicates a relative increase in the contribution of silicon from the relatively Si-rich glass regions of the samples over time. Phases enriched in Cs, Ba and rare earth elements (RE) were also observed on the altered samples. Alteration phases initially appeared along the micro-fractures during the early stages of alteration. The number of micro-fractures, density of alteration phase cover, and grain size of alteration phases increased with reaction time and temperature.

The Arba'at Granite, Sudan: a mineralised, Pan-African intrusion enhanced by hydrothermal metasomatism

The granite at Arba'at shows many of the chemical features of a metallogenetically specialised A-type granite, although no economic mineralisation has yet been found there. The specialised character is, however, only strong within the aphyric granites of low density forming the interior of the intrusion. The exterior is composed of sparsely feldspar-phyric biotite monzogranite and there are no sharp contacts between the two types. The petrography reveals that the transition between the igneous mineralogy and the fabric of the monzogranite and the granite interior was due to pervasive metasomatism, which modified all the igneous minerals and textures. An early alteration stage is distinguished, characterised by albitisation and leaching of primary micas, from a later stage of ‘oxidative hydration’, which caused hematisation, clouding of K-feldspar and growth of strongly birefringent white micas. Fluorite is a common minor constituent of the altered rocks. Chemical data show losses of Ca, Fe, Mg and Ti during metasomatism, with consequent increases in Si but no significant additions of alkalis. Of the trace elements there were overall losses of Ba, Sr, P, Zr, Ti, V, Cu, Zn, W and the LREE, and gains in F, Rb, Th, U, Nb, Ta and Y. Among the significant ratios there were large increases in Rb/Sr and Rb/Ba, while chondrite normalised REE profiles developed the ‘gull-wing’ pattern found in many F-rich rich granites, with severe losses of LREE and large, negative, Eu anomalies. Leaching and the redistribution of elements dominated over elemental additions and there was probably a large mass transfer into the country rocks. The metasomatizing agents are likely to have been high temperature, hydrothermal solutions rendered effective by F and Cl. Textural relations between the chlorite and mica suggest that an interval of low temperature alteration preceded the onset of high temperature metasomatism. Intrusion and metasomatism of the Arba'at granites occurred within the limits of the Pan-African thermotectonic episode (550 ± 100 Ma) at a time when the magmatism of the Arabian-Nubian Shield was changing from orogenic and calcalkaline to anorogenic and alkaline.

Characteristics of semi-conformable alteration zones associated with volcanogenic massive sulphide districts

Semi-comformable alteration zones are present within the footwall stratigraphy of base metal horizons hosted within various volcanic environments. In large-scale, subaqueous, continental rifts (Bergslagen and Iberian Pyrite Belt mining base-metal districts), semi-conformable alteration zones may be tens of kilometres in strike length and hundreds of metres thick. In base-metal camps hosted within smaller tectonic depressions (oceanic rifts and cauldrons) the semi-conformable alteration zones are of smaller scale, but still affect much of the volcanic stratigraphy within the boundaries of the down-faulted terrane. The semi-conformable alteration systems consists of vertically stacked zones that superficially resemble regional metamorphic facies. Their formation includes chloritization, spilitization, silicification and epidotization. Chemical gradients present across these zones indicate that their formation is a result of a series of simultaneous and consecutive metasomatic reactions between seawater and the volcanic pile that take place at progressively higher temperatures with depth in the stratigraphy. The isotherms controlling the reactions are parallel to the sub-horizontal upper contact of the underlying synvolcanic intrusion. The boundaries of these reaction zones are therefore semi-conformable with the volcanic stratigraphy. The semi-conformable alteration zones form by seawater-rock reactions that take place along the downwelling segment of a hydrothermal fluid convection system. Low temperature reactions (50° to 140°C) take place in the shallow subseafloor to form a Mg-K enriched zeolite metasomatic facies. Diffuse hydrothermal discharge associated with this early stage of alteration results in the accumulation of regional-scale Fe-rich chemical sediments. Further downwards circulation of a chemically evolving seawater results in Na-Mg enrichment of the rocks at moderate temperatures (140° to 300°C), followed by an Na-enriched greenschist metasomatic facies (300° to 400°C). Reactions at the greenschist-amphibolite metasomatic boundary include intense leaching of base-metals and the silicification of the rocks. The permeability reduction due to silicification isolates the underlying amphibolite metasomatic facies rocks to form a Ca-Fe enriched, base-metal rich reservoir zone. Periodic breaching of the reservoir allows metal-rich fluids to rise to the seafloor to form massive sulphide deposits. The recognition of the chemical gradients across these semi-conformable alteration zones will allow explorationists to more easily target and locate massive sulphide horizons.

Gold skarn mineralization and fluid evolution in the Nickel Plate Deposit, British Columbia

The Nickel Plate orebody in the Hedley district is the largest gold skarn deposit in Canada. During the period 1904 to 1955 it produced almost 49 million metric tons (Mt) of gold representing approximately half of the gold produced from all skarns in British Columbia. The deposit is hosted by thinly bedded calcareous and tuffaceous siltstones and limestones of the Upper Triassic Nicola Group, an accreted oceanic-arc assemblage. These rocks are intruded by numerous sills, dikes, and small stocks of subalkalic, calc-alkaline quartz diorite, diorite, and gabbro. Skarn alteration and related gold mineralization overprints sedimentary and igneous rock and is zoned about the intrusive contacts.The early stages of alteration are marked by the formation of biotite, potassium feldspar, and quartz hornfels in siltstone and by potassic alteration of intrusive rock. The biotite contains <0.55 mole fraction phlogopite (Mg/(Mg + Fe)) and <2 wt percent TiO 2 which are features indicative of the reduced nature of this mineralization compared to that of porphyry Cu systems. This early stage of alteration is temporally continuous into formation of pyroxene skarnoid which consists of intermediate pyroxene (Hd (sub 48-59) ) in siltstone and Mg-rich pyroxene (Hd (sub 5-30) ) and K feldspar in intrusive rocks. The most volumetrically important alteration style, and the primary host to gold mineralization, consists of dark green, fine-grained pyroxene (Hd (sub 60-94) ) and lesser grandite garnet (Ad (sub 14-82) , avg Ad 35 ) which overprints earlier hornfels and skarnoid, and often completely replaces both sedimentary and igneous protoliths. Ore-bearing sulfides and scapolite occur with the hedenbergite skarn and consist of pyrrhotite, arsenopyrite with inclusions of gold, hedleyite, native bismuth, and minor chalcopyrite, sphalerite, and galena. Retrograde alteration in the deposit is limited, with only minor to trace amounts of wollastonite, prehnite, epidote, and chlorite replacing garnet and pyroxene.Fluid inclusion homogenization temperatures indicate that the main body of pyroxene-garnet skarn formed at an average temperature (pressure corrected) between 460 degrees and 480 degrees C with locally high temperatures of 700 degrees to 800 degrees C. Fluid salinities average 18.3 and 9.7 wt percent NaCl equiv for garnet and pyroxene, respectively. Dissolution temperatures of halite daughters in pyroxene and quartz from skarn indicate a maximum fluid salinity of 37.9 wt percent NaCl equiv. Homogenization temperatures in scapolite associated with sulfide mineralization indicate a lower temperature for this mineralization in the range 320 degrees to 400 degrees C.Estimated physiochemical conditions of formation based on the composition and stability fields of major calc-silicate and sulfide minerals indicate that the hedenbergite skarn, pyrrhotite, and arsenopyrite were deposited under similar conditions of sulfur fugacity (log f (sub S 2 ) = -10 to -3) and temperature (400 degrees -600 degrees C), whereas a log f (sub O 2 ) = -25 to -22 is estimated for the hedenbergite skarn.Spatial trends in pyroxene and garnet composition and the distribution of fluid inclusion homogenization temperatures indicate that the eastern contact of the Toronto stock and the locus of diorite dikes, such as the South dike, in the ore zone acted as critical conduits for channeling of hydrothermal fluids. These two separate fluid paths may have resulted in the two different compositional trends found in the pyroxene and garnet from locations in the ore zone versus those more proximal to the Toronto stock.