Dissolution-reprecipitation Reactions Research Articles

Trace-element remobilisation from W–Sn–U–Pb zoned hematite: Nanoscale insights into a mineral geochronometer behaviour during interaction with fluids

AbstractPreferential removal of W relative to other trace elements from zoned, W–Sn–U–Pb-bearing hematite coupled with disturbance of U–Pb isotope systematics is attributed to pseudomorphic replacement via coupled dissolution reprecipitation reaction (CDRR). This hematite has been studied down to the nanoscale to understand the mechanisms leading to compositional and U/Pb isotope heterogeneity at the grain scale. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) imaging of foils extracted in situ from three locations across the W-rich to W-depleted domains show lattice-scale defects and crystal structure modifications adjacent to twin planes. Secondary sets of twins and associated splays are common, but wider (up to ~100 nm) inclusion trails occur only at the boundary between the W-rich and W-depleted domains. STEM energy-dispersive X-ray mapping reveals W- and Pb-enrichment along 2–3 nm-wide features defining the twin planes; W-bearing nanoparticles occur along the splays. Tungsten and Pb are both present, albeit at low concentrations, within Na–K–Cl-bearing inclusions along the trails. HAADF STEM imaging of hematite reveals modifications relative to ideal crystal structure. A two-fold hematite superstructure (a = b = c = 10.85 Å; α = β = γ = 55.28°) involving oxygen vacancies was constructed and assessed by STEM simulations with a good match to data. This model can account for significant W release during interaction with fluids percolating through twin planes and secondary structures as CDRR progresses from the zoned domain, otherwise apparently undisturbed at the micrometre scale. Lead remobilisation is confirmed here at the nanoscale and is responsible for a disturbance of U/Pb ratios in hematite affected by CDRR. Twin planes can provide pathways for fluid percolation and metal entrapment during post-crystallisation overprinting. The presence of complex twinning can therefore predict potential disturbances of isotope systems in hematite that will affect its performance as a robust geochronometer.

Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition

Abstract Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0—coupled substitution of Al3+ charge-balanced by interstitial Na+—which are typical of cristobalite. However, new electron probe microanalysis (EPMA) element maps show individual crystals to have impurity concentrations that systematically decline from crystal cores-to-rims, and atom probe tomography reveals localized segregation of impurities to dislocations. Impurity concentrations are inversely correlated with degrees of crystallinity [observed by electron backscatter diffraction (EBSD), hyperspectral cathodoluminescence, laser Raman, and transmission electron microscopy (TEM)], such that crystal cores are poorly crystalline and rims are highly ordered tetragonal α-cristobalite. The VPC-plagioclase interfaces show evidence that dissolution-reprecipitation reactions between acidic gases and plagioclase crystals yield precursory amorphous SiO2 coatings that are suitable substrates for initial deposition of impure cristobalite. Successive layers of cubic β-cristobalite are deposited with impurity concentrations that decline as Al-bearing gases rapidly become unstable in the vapor cooling within pores. Final cooling to ambient temperature causes a displacive transformation from β→α cristobalite, but with locally expanded unit cells where impurities are abundant. We interpret this mechanism of VPC deposition to be a natural proxy for dopant-modulated Chemical Vapor Deposition, where halogen-rich acidic gases uptake silica, react with plagioclase surfaces to form suitable substrates and then deposit SiO2 as impure cristobalite. Our results have implications for volcanic hazards, as it has been established that the toxicity of crystalline silica is positively correlated with its purity. Furthermore, we note that VPC commonly goes unreported, but has been observed in silicic lavas of virtually all compositions and eruptive settings. We therefore suggest that despite being metastable at Earth's surface, cristobalite may be the most widely occurring SiO2 polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies.

Textures and formation of microporous gold in the Dongping gold deposit, Hebei Province, China

Microporous gold is common in many Te-Au deposits. Experimental formation and textures of microporous gold have recently been studied. However, few studies have worked on naturally occurring microporous gold. At the Dongping gold deposit, which is one of the most studied Au-Ag-Te deposits in China natural microporous gold is widely distributed in both primary and oxidized ores. This provides a good opportunity to study the textures and the formation of microporous gold in nature. This gold is pseudomorphous in addition to being porous, indicating that it formed via interface coupled dissolution-reprecipitation (ICDR) reactions. Four textures that were not described previously in experiments were observed in samples from this deposit. (1) A hackly interface between the microporous gold and parent calaverite. Te content shows a decreasing trend with increasing distance from the interface. This texture may result from diffusion of excess surface energy as the ICDR reaction moves towards equilibrium. (2) A less-porous band exists in the microporous gold. The band has fewer pores, smaller crystal sizes and similar chemical composition with the surrounding microporous gold. It likely formed due to the fluctuation of the fluid physicochemical conditions (including decreasing temperature, αO2(aq) and increasing pH) that changes the dissolution rate of calaverite. (3) Microporous gold precipitated between quartz and calaverite. As well, gaps developed between gold and remnant calaverite. This texture is derived from catalytic effects of a heterogeneous nucleation or recrystallization process. It is helpful for extraction and aqueous processing of Au-Te ores, and accounts for the phenomenon that quartz can accumulate gold in some hydrothermal gold deposits. (4) Petzite coexists with Ag-bearing microporous gold, indicating that petzite may also decompose to microporous gold under hydrothermal conditions.Calaverite could be dissolved without altered pyrite at high-temperature (>200 °C) conditions, but under weathering conditions, pyrite would be altered first. In primary ores, microporous gold coexists with pyrite and baryte, indicating that it was formed during the late-stage mineralization. In oxidized ores, microporous gold coexists with limonite and baryte, indicating it may have generated during late-stage mineralization and then altered by weathering. Hydrothermal fluids derive from late-stage mineralization may play the main role in the formation of microporous gold compared to weathering processes. This observation also leads us to reappraise the importance of hydrothermal fluids in the weathering dominated ICDR geological processes.

Gold Remobilization: Insights from Gold Deposits in the Archean Swayze Greenstone Belt, Abitibi Subprovince, Canada

Abstract Recognizing if and how Au is remobilized, in solid, melt, or fluid state, is critical for understanding the origin of high-grade ore zones in Au deposits. When evidence for Au remobilization can be demonstrated, then primary versus secondary processes can be distinguished, resulting in a more complete understanding of Au deposit formation. To address this, samples from two Au deposits, Jerome and Kenty, in the Archean Swayze greenstone belt of northern Ontario, Canada, together with archived samples from 39 high-grade Au deposits from the Abitibi greenstone belt across Ontario and Quebec, were geochemically characterized using integrated scanning electron microscopy-energy dispersive spectroscopy and electron microprobe imaging and analyses in addition to laser ablation-inductively coupled plasma-mass spectrometry elemental mapping. These data provided the basis to develop a model for Au remobilization and upgrading of Au that is widely applicable to orogenic gold settings. Data for the Jerome deposit indicate that Au uptake into early pyrite was not due to pulsing of different fluids, but instead was predominantly controlled by S availability, whereby the oscillatory/sector zoning in pyrite resulted from the substitution of As into S sites during rapid growth due to local chemical disequilibrium. In addition, Au-bearing pyrite from both the Jerome and Kenty deposits records textures, such as porosity development coincident with the presence of native gold and accessory sulfide phases, that are strongly suggestive of coupled dissolution-reprecipitation (CDR) reactions that liberated Au and associated elements from earlier auriferous (100–5,000 ppm Au) pyrite. During the remobilization process, Au and Ag were decoupled, which resulted in (1) a change in Au/Ag ratios of 0.5 to 5 in early pyrite to ≈9 in the new native gold (900 Au fineness) and (2) incorporation of Ag into cogenetic secondary mineral phases (e.g., chalcopyrite, tetrahedrite, and galena). Evidence for an association of low-melting point chalcophile elements (LMCE; Hg, Te, Sb, and Bi) with Au at the Jerome, Kenty, and many (>50%) of the 39 historic deposits sampled, along with native gold filling structurally favorable sites in vein quartz in all samples, indicates a fluid might not have been the only factor contributing to remobilization. This systematic Au-LMCE association strongly supports a model whereby Au is released by CDR reactions and is then remobilized by fluid-mediated, LMCE-rich melts that began to form at 335°C and/or by local, nanoparticle (nanomelt?) transport during deformation and metamorphism. Conclusions drawn from this study have implications for Au deposits globally and can account for the common presence of coarse-grained, commonly crystalline, native gold filling fractures in quartz and the paragenetically late-stage origin of gold in veins. They can also better explain the inability of Au in solution remobilization models to account for locally high gold grades, given the relatively low solubility of Au in hydrothermal fluids.

Coupling between mineral replacement reactions and co-precipitation of trace elements: An example from the giant Olympic Dam deposit

The mechanisms of scavenging of minor elements in reactive, fluid-dominated systems control the distribution, nature, concentration, and reactivity of economically valuable or deleterious elements in many environments, ranging from hydrothermal ore deposits to sedimentary basins. Here we show that in a high-grade Cu-U ore from the super-giant Olympic Dam ore deposit, remobilized U was scavenged during the fluid-driven replacement of chalcopyrite (CuFeS2) by bornite (Cu5FeS4). Hydrothermal experiments show that the pseudomorphic replacement of chalcopyrite by bornite proceeded via an interfaced coupled dissolution reprecipitation (ICDR) reaction mechanism, with further bornite growing via overgrowth. Addition of U(VI) to the experimental fluid resulted in scavenging of U in the form of nano- to micro-particles of uraninite. Uraninite precipitation occurred concurrently with replacement, resulting in abundant uraninite decorating the initial surface of the parent chalcopyrite grain, as well as the coarse porosity that developed within overgrowth bornite.Our results highlight the importance of disequilibrium or local equilibrium in controlling the endowment of U in complex ores. Under far from equilibrium, fluid-dominated conditions, e.g., during seismic or eruptive events, fast fluid movements associated with brecciation and/or faulting can lead to fast mineral reactions. The resulting ultra-local conditions (e.g., surface of reacting mineral; reaction-induced porosity) play an important role in controlling the precipitation of minor elements, U in this case, and interpretation of precipitation mechanisms as reflecting the evolution of bulk fluid may be erroneous.

Nanoscale processes of trace element mobility in metamorphosed zircon

Several examples of zircon grains from high- to ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) metapelites exhibit a characteristic, yet atypical, core–rim interface domain < 5-μm wide observed in cathodoluminescence (CL) imaging. The interface domain is located immediately against the magmatic core and is comprised of an irregular, 0–2-μm wide, CL-dark domain that is rimmed by a complex, 0–5-μm wide, CL-bright domain with cuspate margins. The outer margin of the interface domain is rimmed by intermediate-CL zircon with low contrast zoning. To characterize the nature of the interface domain and to identify mechanisms of trace element mobility in metamorphosed zircon, we analyzed several specimens prepared from zircon from the Rhodope Metamorphic Complex (eastern Greece) and the Goshen Dome (western Massachusetts, USA) via atom probe tomography (APT). The data reveal three types of geochemical anomalies, each with a unique morphology. (1) Toroidal clusters with high concentrations of Pb (+ Y, Al) are found exclusively within the core of the Rhodope grain. These clusters are interpreted as decorated dislocation loops that formed during metamorphism and annealing of radiation damage to the lattice. Geochronological and geochemical data support this interpretation. (2) Complex, cross-cutting planar and linear features with anomalous concentrations of Y + P + Yb or U are spatially restricted to the core–rim interface domain; these features do not correlate with inherited geochemical variation (oscillatory zoning) or deformation-induced microstructures. Instead, the planar features likely formed in response to an interface-coupled dissolution–reprecipitation reaction that propagated into the crystal during metamorphism. The observed cross-cutting relationships are the product of either multiple events or complexity of the process that originally formed the domains. (3) Ellipsoidal features with high concentrations of Y + P + Yb (+ Al) are found exclusively within the high-Y + P + Yb planar features. These features are interpreted as the product of spinodal decomposition that occurred during exhumation as the zircon passed the solvus where local equilibria favored nm-scale exsolution to minimize the Gibbs free energy. The presence of multiple types of geochemical features in these examples indicates that trace element mobility in zircon is driven by multiple processes over the course of orogenesis. Given that these atypical domains are apparently restricted to zircon metamorphosed at UHT and (U)HP conditions, their presence may represent a marker of metamorphism at very high-grade conditions.

Decoupled U-Pb date and chemical zonation of monazite in migmatites: The case for disturbance of isotopic systematics by coupled dissolution-reprecipitation

Monazite is an abundant accessory mineral in metasedimentary rocks and their anatectic products. Trace element analysis combined with U-Pb dating of monazite is widely used to reconstruct P-T-t histories of high-grade metamorphic terranes. This approach relies on interpreting U-Pb dates, which requires understanding the processes that cause isotopic disturbances. We present in-situ laser ablation split-stream (LASS) results of U-Pb dating and trace element spot analyses of single monazite grains from anatectic granite samples from a HT-LP migmatitic turbidite terrane exposed in the Sierra de Quilmes, NW Argentina. This terrane was part of the back-arc and remained close to anatectic temperatures for most of the ∼510–440 Ma Famatinian Orogeny, in the Western Gondwana margin. Sampled outcrops display evidence of remelting of the anatectic granites, marked by leucosomes and confirmed by zircon chronology that define multiple thermal peaks and anatexis events during ∼60 Myr duration of the arc. U-Pb monazite chronology of these granite samples reveals a continuum of dates between ∼500 and 450 Ma recording most of the duration of the Famatinian event and matching the range of zircon growth ages, but without their multiple, well-defined age populations. Despite each sample yielding a reasonable Famatinian monazite mean date, the results reveal that the isotopic system has been perturbed. Data for each rock sample yield similar date spread for both the cores and the well-defined rims of single monazite grains, indicating a disconnect between U-Pb dates and chemical zonation. We interpret these random within-grain date variations as a result of coupled dissolution-reprecipitation reactions between monazite grains and ambient fluids, most likely silicate melts. These reactions occurred during a single orogeny marked by a long-lasting, high-temperature metamorphic history. During coupled dissolution-reprecipitation reactions, ingrown radiogenic Pb was remobilized and/or redistributed within the grain giving rise to unsupported Pb. This process was likely significant here because dissolution of apatite into silicate melts during migmatization stabilized monazite, preventing their dissolution but not preventing their reaction with the melt. Redistribution of radiogenic Pb caused by the coupled dissolution-reprecipitation reactions gave rise to meaningless individual dates spreading along the 50 Myr duration of the thermal event associated with the orogeny. In this case, rather than reacting with fluids from a distinct event, monazite dates were perturbed during the same orogeny.

Intermobility of barium, strontium, and lead in chloride and sulfate leach solutions

Production of radionuclide-free copper concentrates is dependent on understanding and controlling the deportment of daughter radionuclides (RNs) produced from 238U decay, specifically 226Ra, 210Pb, and 210Po. Sulfuric acid leaching is currently employed in the Olympic Dam processing plant (South Australia) to remove U and fluorine from copper concentrates prior to smelting but does not adequately remove the aforementioned RN. Due to chemical similarities between lead and alkaline earth metals (including Ra), two sets of experiments were designed to understand solution interactions between Sr, Ba, and Pb at various conditions. Nanoscale secondary ion mass spectrometry (NanoSIMS) isotopic spatial distribution maps and laser ablation inductively coupled-plasma mass spectrometry transects were performed on laboratory-grown crystals of baryte, celestite, and anglesite which had been exposed to different solutions under different pH and reaction time conditions. Analysis of experimental products reveals three uptake mechanisms: overgrowth of nearly pure SrSO4 and PbSO4 on baryte; incorporation of minor of Pb and Ba into celestite due to diffusion; and extensive replacement of Pb by Sr (and less extensive replacement of Pb by Ba) in anglesite via coupled dissolution-reprecipitation reactions. The presence of H2SO4 either enhanced or inhibited these reactions. Kinetic modelling supports the experimental results, showing potential for extrapolating the (Sr, Ba, Pb)SO4 system to encompass RaSO4. Direct observation of grain-scale element distributions by nanoSIMS aids understanding of the controlling conditions and mechanisms of replacement that may be critical steps for Pb and Ra removal from concentrates by allowing construction of a cationic replacement scenario targeting Pb or Ra, or ideally all insoluble sulfates. Experimental results provide a foundation for further investigation of RN uptake during minerals processing, especially during acid leaching. The new evidence enhances understanding of micro- to nanoscale chemical interactions and not only aids determination of where radionuclides reside during each processing stage but also guides development of flowsheets targeting their removal.

Synergistic effects of Mg-substitution and particle size of chicken eggshells on hydrothermal synthesis of biphasic calcium phosphate nanocrystals

Magnesium (Mg2+) ion plays important roles in biomineralization of bone, teeth and calcium carbonate skeletons. Herein, chicken eggshells mainly comprising of Mg-calcite nanocrystals (Mg/(Mg + Ca) ∼2.0 mol.%) were used to fabricate biphasic calcium phosphate (BCP), a mixture of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) nanocrystals, through hydrothermal reactions at 200 °C for 24 h. Our results indicated that β-TCP nanocrystals formed through the ion-exchange reactions of Mg-calcite, while HA nanocrystals were mainly produced by dissolution-reprecipitation reactions on the surfaces of eggshell samples in the hydrothermal system. Mg substitution in calcite resulted in formation of β-TCP nanocrystals instead of HA crystals through ion-exchange reactions. BCP samples with different compositions (∼28.6–77.8 wt.% β-TCP) were produced by controlling particle sizes of eggshells for hydrothermal reactions. The larger particles lead to the larger proportion of β-TCP in the BCP composition. Therefore, Mg substitution and particle size had synergetic effects on the hydrothermal synthesis of BCP using chicken eggshells through balance of ion-exchange and dissolution-reprecipitation reactions. Cell culture results showed that the BCP products were non-cytotoxic to MC3T3-E1 cells, which may be used for bone substitute materials in future.

Partial resetting of U–Pb ages during experimental fluid-induced re-equilibration of xenotime

Experimentally metasomatized xenotime was investigated in terms of preservation of U–Pb ages during fluid-induced alterations at 250, 450, 550 and 650 °C. Starting materials for experiments included xenotime (38.4 ± 0.6 Ma), albite, K-feldspar, biotite, muscovite, garnet, SiO2, CaF2, Na2Si2O5 and H2O that roughly replicated metapelitic to granitic composition with high Na, Ca, Si and F activities in the fluid. Despite partial replacement of xenotime by secondary phases including (Y,REE)-rich fluorapatite to Y-rich fluorcalciobritholite in all runs, and an unnamed (Y,HREE)-rich silicate in 250 °C experiment, only two experiments at 550 °C, 800 MPa and 650 °C, 600 MPa resulted in development of faint to moderate patchy zoning in the xenotime. Experimental results indicate higher chemical-stability of xenotime at lower P-T conditions compared to those corresponding to amphibolite facies conditions. The xenotime domains metasomatized via coupled dissolution-reprecipitation reactions are characterized by depletion of U and Pb and younger 206Pb/238U dates of ca. 15–34 Ma, demonstrating preferential removal of Pb and thus the partial resetting of the age record. The U–Pb data for the first time experimentally confirm that xenotime may be affected by re-equilibration induced by fluid or melt with high Na, Ca and F activities via coupled dissolution-reprecipitation reactions that result in removal of radiogenic Pb under temperature conditions of 550–650 °C. These findings have important implications for U–Pb geochronology of igneous (particularly in pegmatites) and metamorphic processes in terms of preservation of U–Pb ages of xenotime that was affected by multiple thermal events in the presence of alkali-rich fluid and/or melt.

Chemical Trapping of CO2 by Clay Minerals at Reservoir Conditions: Two Mechanisms Observed by in Situ High-Pressure and -Temperature Experiments

This paper presents the results of experiments performed in situ at temperature and pressure relevant to reservoir conditions (T = 323 K and Pfluid = 90 bar) to evaluate whether clay minerals can react with supercritical CO2 to produce carbonate phases by ion exchange–precipitation reactions and dissolution–reprecipitation reactions. The results show that both can occur on a time scale of hours under the conditions of our studies. The dissolution–reprecipitation mechanism was examined using Ca-, Cs-, and tetramethylammonium (TMA+) laponite, a synthetic smectite analogous to hectorite that has small particles (basal dimensions of ∼10 × 10 nm2) and a high fraction of edge sites where only two of the usual three bridging oxygen atoms are shared with other tetrahedra in the silicate sheet (Q2 sites), making it an excellent case for examining the role of T–O–T edges. The ion exchange–precipitation mechanism was observed for a Pb-exchanged natural low-Fe smectite (hectorite). Novel X-ray diffraction and NMR and...

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.

Mineral Transformations in Gold–(Silver) Tellurides in the Presence of Fluids: Nature and Experiment

Gold–(silver) telluride minerals constitute a major part of the gold endowment at a number of important deposits across the globe. A brief overview of the chemistry and structure of the main gold and silver telluride minerals is presented, focusing on the relationships between calaverite, krennerite, and sylvanite, which have overlapping compositions. These three minerals are replaced by gold–silver alloys when subjected to the actions of hydrothermal fluids under mild hydrothermal conditions (≤220 °C). An overview of the product textures, reaction mechanisms, and kinetics of the oxidative leaching of tellurium from gold–(silver) tellurides is presented. For calaverite and krennerite, the replacement reactions are relatively simple interface-coupled dissolution-reprecipitation reactions. In these reactions, the telluride minerals dissolve at the reaction interface and gold immediately precipitates and grows as gold filaments; the tellurium is oxidized to Te(IV) and is lost to the bulk solution. The replacement of sylvanite is more complex and involves two competing pathways leading to either a gold spongy alloy or a mixture of calaverite, hessite, and petzite. This work highlights the substantial progress that has been made in recent years towards understanding the mineralization processes of natural gold–(silver) telluride minerals and mustard gold under hydrothermal conditions. The results of these studies have potential implications for the industrial treatment of gold-bearing telluride minerals.

Controls on the Distribution of Invisible and Visible Gold in the Orogenic Gold Deposits of the Yangshan Gold Belt, West Qinling Orogen, China

Six orogenic gold deposits constitute the Yangshan gold belt in the West Qinling Orogen. Gold is mostly invisible in solid solution or in the sulfide lattice, with minor visible gold associated with stibnite and in quartz-calcite veins. Detailed textural and trace-element analysis of sulfides in terms of a newly-erected paragenetic sequence for these deposits, together with previously published data, demonstrate that early magmatic-hydrothermal pyrite in granitic dike host-rocks has much higher Au contents than diagenetic pyrite in metasedimentary host rocks, but lower contents of As, Au, and Cu than ore-stage pyrite. Combined with sulfur isotope data, replacement textures in the gold ores indicate that the auriferous ore-fluids post-dated the granitic dikes and were not magmatic-hydrothermal in origin. There is a strong correlation between the relative activities of S and As and their total abundances in the ore fluid and the siting of gold in the Yangshan gold ores. Mass balance calculations indicate that there is no necessity to invoke remobilization processes to explain the occurrence of gold in the ores. The only exception is the Py1-2 replacement of Py1m, where fluid-mediated coupled dissolution-reprecipitation reactions may have occurred to exchange Au between the two pyrite phases.

Pseudomorphic Rhythmically Banded and Oscillatory Tetrahedrite–Tennantite Aggregates in the Darasun Gold Deposit (Eastern Transbaikalia, Russia): A Result of Coupled Dissolution–Reprecipitation Reactions

The rhythmically banded aggregates of Fe–tennantite with oscillatory zoning resulting from simultaneous variations in the concentrations of Sb and As and Zn and Fe were registered in the Darasun gold deposit (Eastern Transbaikalia). These aggregates were formed upon pseudomorphic replacement of early Zn–tetrahedrite. This process was initiated by the dissolution of Zn–tetrahedrite with the formation of an aggregate of galena, bournonite, and late tetrahedrite–tennantite. It is shown that rhythmically banded aggregates with an oscillatory zoning were formed as a result of coupled dissolution–reprecipitation reactions under the conditions that are far from the mineral–fluid chemical equilibrium and variations in the fluid composition controlled by kinetic factors.

Feldspar mineralogy and rare-earth element (re)mobilization in iron-oxide copper gold systems from South Australia: a nanoscale study

ABSTRACTNanoscale characterization (TEM on FIB-SEM-prepared foils) was undertaken on feldspars undergoing transformation from early post-magmatic (deuteric) to hydrothermal stages in granites hosting the Olympic Dam Cu-U-Au-Ag deposit, and from the Cu-Au skarn at Hillside within the same iron-oxide copper-gold (IOCG) province, South Australia. These include complex perthitic textures, anomalously Ba-, Fe-, or REE-rich compositions, and REE-flourocarbonate + molybdenite assemblages which pseudomorph pre-existing feldspars. Epitaxial orientations between cryptoperthite (magmatic), patch perthite (dueteric) and replacive albite (hydrothermal) within vein perthite support interface-mediated reactions between pre-existing alkali-feldspars and pervading fluid, irrespective of micro-scale crystal morphology. Such observations are consistent with a coupled dissolution-reprecipitation reaction mechanism, which assists in grain-scale element remobilization via the generation of transient interconnected microporosity. Micro-scale aggregates of hydrothermal hyalophane (Ba-rich K-feldspar), crystallizing within previously albitized areas of andesine, reveal a complex assemblage of calc-silicate, As-bearing fluorapatite and Fe oxides along reaction boundaries in the enclosing albite-sericite assemblage typical of deuteric alteration. Such inclusions are good REE repositories and their presence supports REE remobilization at the grain-scale during early hydrothermal alteration. Iron-metasomatism is recognized by nanoscale maghemite inclusions within ‘red-stained’ orthoclase, as well as by hematite in REE-fluorocarbonates, which reflect broader-scale zonation patterns typical for IOCG systems. Potassium-feldspar from the contact between alkali-granite and skarn at Hillside is characterized by 100–1000 ppm REE, attributable to pervasive nanoscale inclusions of calc-silicates, concentrated along microfractures, or pore-attached. Feldspar replacement by REE-fluorcarbonates at Olympic Dam and nanoscale calc-silicate inclusions in feldspar at Hillside are both strong evidence for the role of feldspars in concentrating REE during intense metasomatism. Differences in mineralogical expression are due to the availability of associated elements. Lattice-scale intergrowths of assemblages indicative of Fe-metasomatism, REE-enrichment and sulfide deposition at Olympic Dam are evidence for a spatial and temporal relationship between these processes.

Modeling the effects of diagenesis on carbonate clumped-isotope values in deep- and shallow-water settings

The measurement of multiply isotopically substituted (‘clumped isotope’) carbonate groups provides a way to reconstruct past mineral formation temperatures. However, dissolution-reprecipitation (i.e., recrystallization) reactions, which commonly occur during sedimentary burial, can alter a sample’s clumped-isotope composition such that it partially or wholly reflects deeper burial temperatures. Here we derive a quantitative model of diagenesis to explore how diagenesis alters carbonate clumped-isotope values. We apply the model to a new dataset from deep-sea sediments taken from Ocean Drilling Project site 807 in the equatorial Pacific. This dataset is used to ground truth the model. We demonstrate that the use of the model with accompanying carbonate clumped-isotope and carbonate δ18O values provides new constraints on both the diagenetic history of deep-sea settings as well as past equatorial sea-surface temperatures. Specifically, the combination of the diagenetic model and data support previous work that indicates equatorial sea-surface temperatures were warmer in the Paleogene as compared to today. We then explore whether the model is applicable to shallow-water settings commonly preserved in the rock record. Using a previously published dataset from the Bahamas, we demonstrate that the model captures the main trends of the data as a function of burial depth and thus appears applicable to a range of depositional settings.

Textures and high field strength elements in hydrothermal magnetite from a skarn system: Implications for coupled dissolution-reprecipitation reactions

There is increasing evidence to suggest that high field strength elements (HFSE) could be mobile to some extent in hydrothermal fluid due to the influence of halogens (e.g., fluorine and chlorine). However, in natural hydrothermal (fluid) systems, “coupled dissolution reprecipitation” (CDR) reactions at fluid-mineral interfaces that have been emphasized in the past decade may play a key role in controlling the final textures and mineral assemblages. The influences of the CDR reactions in hydrothermal systems on HFSE enrichment or depletion at the mineral scale are enigmatic. In this study, we show that enrichment of Nb and Zr can occur in magnetite on the mineral scale formed by hydrothermal fluids at medium-to-lower temperature in a skarn system. Four stages of mineralization and alteration of magnetite have been identified in the Baishiya iron skarn deposit of the East Kunlun Mountains of China. Magnetite formed in stage 1 (S1) developed obvious oscillatory zonation, whereas that formed in stages 2 (S2) and 3 (S3) shows hydrothermal alteration and metasomatic textures, and that in stage 4 (S4) developed euhedral crystals with simple zoning. Systematic variations in the trace element compositions of different magnetite grains analyzed by EMPA and LA-ICP-MS suggest that the magnetite from S1 to S3 may have formed in a metasomatic process at relatively constant temperature, whereas the magnetite from S4 formed by re-equilibrium processes at lower temperature. The magnetite from each stage can be divided into light and dark domains based on backscattered electron images. The dark domains in the magnetite from S1 and S2 have higher Nb/Ta (8.52–27.00) and Zr/Hf (18.22–52.64) ratios and silicon contents than the light domains (0.55–5.66 and 2.54–16.43, respectively). Compared with other magnetite ores, the ores from S1 and S2 are depleted of V and Ni. This depletion may be induced by increased oxygen and co-crystallized sulfide. However, these variations are unlikely to be responsible for the enrichment of Nb and Zr in magnetite at equilibrium conditions. Conversely, the dark domains of the magnetite from S1 and S2 are porous, irregular, and/or oscillatory with quartz inclusions, indicating nonequilibrium conditions. These textural features could be attributed to the CDR reactions that are ubiquitous in skarn systems. The increased silicon concentrations in magnetite due to the CDR reactions could affect the lattice parameters of the magnetite structure, leading to an overall change in the volume of magnetite ores. The reduplicative processes of volume change, dissolution, and porosity formation within magnetite are further imporved due to an incresed oxygen fugacity and co-crystallized sulfide (e.g., decreased temperature or increased sulfur fugacity) at far-from-equilibrium or local equilibrium conditions, resulting in oscillatory magnetite dark domains of S1. Ripening of the transient porosity can trap nanoscale precipitates of columbite and zircon within pores of Si-magnetite, and this precipitation could be attributed to the co-crystallized phlogopite that would incorporate fluorine from the hydrothermal fluid, and subsequently decrease the solubility of Nb and Zr in the skarn system. This scenario highlights that Nb and Zr could be scavenged and enriched into in the reaction fronts (porosity) by controlling the reaction pathway at a local scale that does not reflect the overall fluid-rock interaction history of the mineral assemblage.

Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

AbstractMetamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.

Evidence of local sourcing of sulfur and gold in an Archaean sediment-hosted gold deposit

Determining the source of sulfur in an ore deposit is key to understanding the nature of the ore forming processes. The Neoarchaean Paulsens sediment-hosted gold deposit (∼1Moz @ 7.6g/t) located in the Pilbara Craton of Western Australia exhibits many of the characteristics of Phanerozoic shale hosted gold deposits (e.g. Huijiabao Trend, Northern Carlin Trend and Sukhoi Log), in that 1) black shales are the dominant host rock, 2) gold is hosted in pyrite as both free gold and dissolved gold in the lattice of the pyrite, and 3) multiple generations of pyrite have formed due to a variety of geological processes. In this contribution we utilised Secondary Ion Mass Spectrometer (SIMS) to measure the in-situ quadrupole (32S, 33S, 34S and 36S) sulfur isotope compositions of the different generations of pyrite. Our results indicate that the both diagenetic and hydrothermal pyrite generations display similar and anomalous Δ33S signatures (up to +0.4‰). Further, the Δ33S-Δ36S arrays in the hydrothermal pyrite generations lie on a slope which is similar to that of the diagenetic pyrite. These data support the hypothesis that the sulfur in the ore zones came from the host Hardey Formation black shales.We also performed trace element analyses of syn-sedimentary and early diagenetic pyrite from the Hardey Formaiton using Nano Secondary Ion Mass Spectrometry (NanoSIMS), Electron Probe Microanalysis (EPMA) and Laser Ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS), all of which show that the syn-sedimentary and early diagenetic pyrite contain high concentrations of many trace elements (As, Ni, Co, Cu, Ag, Se, Te, Bi), including up to 1.5ppm Au. These metals contents are also abundant in the several generations of mineralised hydrothermal pyrite and form clear patterns of growth associated with couple dissolution reprecipitation reactions. These findings clearly indicate that the fluid that transported the Au must have also been enriched in the base and precious metals that are contained in the early, syn-diagenesis pyrite.Data from this study clearly support the hypothesis that in some sediment-hosted gold systems, all the sulfur and gold required to form the deposit are sourced from the local sedimentary package. By using the presence of anomalous mass independent sulfur isotope signatures as chemically conservative and indelible tracers, it is possible to fingerprint the source of sulfur in a wide range of mineral systems, thus enhancing predictive exploration strategies at the regional to camp scales.