Coupled Dissolution Reprecipitation Research Articles

In situ trace elements and sulfur isotopes of sulfides in the Dabaiyang Te-Au deposit, Hebei Province, China: Implications for Au remobilization from pyrite

Coupled dissolution–reprecipitation (CDR) reactions and low-melting point chalcophile elements (LMCE) melt scavenging are effective mechanisms for gold remobilization and are important for the formation of Te-Au deposits. The Dabaiyang deposit is a typical Te-Au deposit in the Zhangjiakou district and contains tellurides, including altaite, hessite, petzite, tetradymite and tellurobismuthite. Two generations of pyrite have been recognized in the deposit: early pyrite without fractures or alteration (Py1) and later pyrite with more porosity, inclusions and fractures (Py2), and Py2 could be separated into Py2a that containing many fractures without mineral inclusions and Py2b that containing microfractures, micropores and mineral inclusions. Pyrite containing large amounts of mineral inclusions and free of microfractures (Py-MI) has also been recognized, but timing relationships between Py-MI and Py1 (or Py2) is hard to establish. The Pyrite textures indicate that Py2a underwent brittle deformation and that Py2b underwent CDR reactions and Te-melt scavenging. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses show that Py2a contains trace elements similar to Py1, while Py2b contains higher Au, Ag, Te and other trace elements, indicating that brittle fracturing has no effect on element mobilization and reprecipitation, and porous pyrite has Au-Ag-Te concentrations. High Co/Ni ratios and Te contents and low Se contents show that the Dabaiyang deposit has a genetic relationship with magmatic activity. In situ δ34S values of sulfides range from −15.77‰ to −6.09‰ (mostly from −13.89 to −10.06‰), which is consistent with bulk sulfide δ34S values. The negative δ34S values more likely resulted from sulfur isotope fractionation in high fO2 hydrothermal fluids. CDR reactions liberated Au, Te, Ag and other trace elements from pyrite and formed porous textures. Then, Te-rich melts scavenged Au from fluid and wallrocks during migration. Finally, the melt precipitated as mineral inclusions in pores or fractures in pyrite. CDR reactions and LMCE melt scavenging played an important role in upgrading the ores and the occurrence of visible native gold in the Dabaiyang gold deposit, as well as other Te-Au deposits in the Zhangjiakou district.

Mineral paragenesis of the Anfangba gold deposit, western Qinling Orogen, China: Implication for coupled dissolution-reprecipitation reactions and the liquid bismuth collector model

The newly discovered Anfangba gold deposit is hosted in the Middle Triassic pumpellyite-actinolite facies metamorphosed turbidites in the western Qinling Orogen, Central China. It is characterized by a pyrite-quartz vein type, a medium-high temperature, and the development of visible gold and abundant bismuthides. It is the first reported Au-Bi-associated deposit in this area and remains poorly understood. In this study, the mineral paragenesis has been studied to further understand ore formation, including sulfides (pyrite, chalcopyrite, pyrrhotite, bornite, bismuthinite, tetrahedrite and arsenopyrite), Cu-Bi(Sb) sulfosalts (emplectite, wittichenite, and skinnerite), sulfoselenide (unnamed sulpho-tellurian nevskite), tetradymite group minerals (joséite-A, joséite-B and tetradymite), native bismuth and electrum. The physicochemical conditions of the ore-forming process can be estimated based on the mineral paragenetic sequence. The calculated logαH2S and logfO2 of the main mineralization stage are higher than −2.25 and lower than −37.96, respectively. EPMA mapping indicated that ore-forming fluid with low sulfur and oxygen fugacities may have played an important role in provoking the activity of the semimetals such as Bi, Sb, Se and Te. This may have result in the formation of accessory minerals, including native bismuth, skinnerite and an unnamed sulpho-tellurian nevskite via coupled dissolution-reprecipitation (CDR) reactions. Mineralography, EPMA and LA-ICP-MS analysis have revealed a close relationship between Au mineralization and Bi-blebs. It suggests that the “liquid bismuth collector model” contributed to the Au enrichment of the Anfangba deposit. As the coexistence of native bismuth and bismuthinite is commonly seen in different types of hydrothermal gold deposits, the presence of Bi-melts and the influence of the “liquid bismuth collector model” in these gold deposits may have been underestimated.

A new mode of mineral replacement reactions involving the synergy between fluid-induced solid-state diffusion and dissolution-reprecipitation: A case study of the replacement of bornite by copper sulfides

Mineral replacement reactions are one of the most important phenomena controlling the geochemical cycle of elements on Earth. In the early years, solid-state diffusion was proposed as the main mechanism for mineral replacement reactions, but over the past 20 years the importance of the coupled dissolution-reprecipitation (CDR) mechanism has been recognized. In the presence of a fluid phase and at low temperatures (e.g., <300 °C), CDR is the predominant mineral replacement process compared to relatively slow solid-state diffusion. However, in the present case study, we show that the rate of solid-state diffusion is comparable to the rate of CDR processes during the replacement of bornite (Cu5FeS4) by copper sulfides at 160–200 °C. The experiments initially produced chalcopyrite lamellae homogeneously distributed in the entire bornite grain, and each lamella was enveloped by digenite. The lamellae were formed by removing Fe3+ from bornite via solid-state diffusion, since there was no evidence for fluid entering the bornite grains during lamellae formation. An interesting discovery is that solid-state exsolution of chalcopyrite lamellae was induced by the bulk hydrothermal fluids surrounding the mineral grains, because in the absence of fluids under otherwise identical conditions, no exsolution occurred, and because the exsolution rate and lamellae size were sensitive to the composition of hydrothermal fluids. We hypothesize that this fluid-induced solid-state diffusion (FI-SSD) mechanism is made possible by the similar topology of the crystal structure of these phases. The solid-state diffusion of Fe3+ within bornite and across the resultant chalcopyrite and digenite phase boundaries is facilitated by the near-identical S framework. Parallel to and after lamellae exsolution, CDR reactions proceeded from the surface to the interior of the grains or along fractures, replacing chalcopyrite by digenite, and digenite by covellite and/or chalcocite, depending on experimental conditions. The synergy between FI-SSD and CDR resulted in complex reaction pathways for reactions in five acidic hydrothermal fluids with or without added Cu2+, Cu+, Cl−, SO42−, and SO32−. The outcomes of these experiments imply that (1) under conditions where cation diffusion rates are of the same order of magnitude as dissolution and precipitation rates, hydrothermal fluids can induce and control solid-state diffusion processes, e.g., exsolution; (2) mineral replacement can be a result of the synergy between FI-SSD and CDR mechanisms; this happens at low temperatures (≤200 °C) in chalcogenide systems, but could affect silicate and oxide systems at amphibolite to granulite to eclogitic metamorphic grade; and (3) the synergy between FI-SSD and CDR mechanisms can lead to complex reaction pathways that cannot be easily predicted empirically.

Geochemical and spectroscopic investigation of apatite in the Siilinjärvi carbonatite complex: Keys to understanding apatite forming processes and assessing potential for rare earth elements

The Siilinjärvi phosphate deposit (Finland) is hosted by an Archean carbonatite complex. The main body is composed of glimmerite, carbonatite and combinations thereof. It is surrounded by a well-developed fenitization zone. Almost all the rocks pertaining to the glimmerite-carbonatite series are considered for exploitation of phosphate. New petrological and in-situ geochemical as well as spectroscopic data obtained by cathodoluminescence, Raman and laser-induced breakdown spectroscopy make it possible to constrain the genesis and evolution of apatite through time. Apatite in the glimmerite-carbonatite series formed by igneous processes. An increase in rare earth elements (REE) content during apatite deposition can be explained by re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in these elements. This late carbonatite emplacement has been known as a major contributor to the overall P and REE endowment of the system and is likely connected to fenitization and alkali-rich fluids. These fluids - enriched in REE - would have interacted with apatite in the fenite, resulting in an increase in REE content through coupled dissolution–reprecipitation processes. Finally, a marked decrease in LREE is observed in apatite hosted by fenite. It highlights the alteration of apatite by a REE-poor fluid during a late-magmatic/hydrothermal stage. Regarding the potential for REE exploitation, geochemical data combined with an estimation of the reserves indicate a sub-economic potential of REE to be exploited as by-products of phosphate mining. Spectroscopic analyses further provide helpful data for exploration, by determining the P and REE distribution and the enrichment in carbonatite and within apatite.

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.

Experimental fluid-mediated alteration of zircon under lower greenschist facies conditions

ABSTRACT The use of zircon in the dating of geological processes and tectonic events has become a standard approach in many aspects of Earth science research. As a result, understanding how zircon interacts with aqueous fluids during metasomatism has become increasingly important. The alteration of natural zircon is driven primarily by coupled dissolution–reprecipitation or by ion-exchange with an aqueous fluid. In this study, whole and intact, euhedral light-brown zircon crystals (100–250 μm in length; 2 mg) from the Oligocene Fish Canyon Tuff (FCT) were experimentally reacted with an alkali-bearing reactive fluid and a REE + P source (0.5 mg CePO4 or 0.5 mg YPO4). Experiments were conducted in sealed Au metal capsules at 350 °C and 100 MPa for 182 days. During the experiment, the zircon became colorless, indicating annealing of the radiation damage in the crystal. Two-dimensional element maps of the outermost 3 μm of unpolished zircon crystal surfaces were produced using a grind of contiguous 7 μm analytical spots via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The chemical maps indicate that the surface of the zircon crystals from each experiment heterogeneously reacted with the fluid, such that the Ce and Y concentration of chemically modified areas increased (by an order of magnitude) in the CePO4-bearing and YPO4-bearing experiments, respectively, when compared with the chemical maps of unaltered zircon grain surfaces. Helium ion microscopy of polished crystals revealed discontinuous micron-scale altered domains at the crystal margin, consistent with the findings of the unpolished mapping technique. Interestingly, the Th and U concentration of the altered zircon grain surfaces were consistent with the unaltered zircon regardless of the experiment. Incorporation of REEs on the zircon grain surface likely occurred via the coupled substitution REE3+ + P5+ ↔ Zr4+ + Si4+. The results from these experiments imply that the surfaces of minimally metamict zircon can be chemically modified by alkali-bearing fluids via ion exchange under lower greenschist pressures and temperatures over relatively short time periods with respect to the geological time scale.

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 (&amp;gt;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.

The partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism, Glen Roy, Scotland

AbstractGarnet porphyroblasts in sillimanite‐bearing pelitic schists contain complex textural and compositional zoning, with considerable variation both within and between adjacent samples. The sillimanite‐bearing schists locally occur in regional Barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism. Garnet in these Dalradian rocks from the Scottish Highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution–reprecipitation followed by partial dissolution. Individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co‐exists with those containing largely unmodified characteristic bell‐shaped Mn profiles. This highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation. Cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite‐producing reactions. The modification of garnet via a dissolution–reprecipitation process releases Ca into the effective whole‐rock composition, displacing the pressure–temperature positions of subsequent isograd reactions. This represents the first report of internal metasomatism controlling reaction pathways. The behaviour of garnet highlights the importance of kinetic factors, especially deformation and fluids, in controlling reaction progress and how the resulting variability influences subsequent prograde history. The lack of a consistent metamorphic response, within and between adjacent schists, suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions.

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.

In-situ U–Th–Pb geochronometry with submicron-scale resolution: low-voltage electron-beam dating of complexly zoned polygenetic uraninite microcrystals

Abstract Complexly zoned microcrystals of uraninite were encountered in orthogneiss from the central Tauern Window in Austria (K1 gneiss, Felbertal scheelite mine) and analysed in-situ for U, Th and Pb with state-of-the-art FE-SEM/EDX techniques. A three times finer spatial resolution was achieved using an acceleration voltage of 8 kV, compared to the classic 15–20 kV set-up of U–Th–total Pb electron microprobe dating. The lower voltage allows a spheroid of material with a diameter of only 0.3 µm to be selectively analysed. Careful tests on three uraninite reference materials show that the low-voltage method yields sufficient precision and accuracy for U–Th–total Pb uraninite dating, with errors on individual spot ages in the order of 10–30 Ma. By means of this innovative analysis technique, small-scale age zoning patterns could be resolved and dated in the uraninite microcrystals from the orthogneiss. Based on microstructures observed in backscattered electron images we interpret that an older uraninite generation in the rock, with a late Permian formation age (~260 Ma), was recycled two times through a coupled dissolution–reprecipitation process at ~210 Ma and at ~30 Ma. The younger dissolution–reprecipitation phase at ~30 Ma coincides with the Alpine regional metamorphism (lower amphibolite facies). The two older ages (~210 Ma and ~260 Ma) have been previously recognized in rocks from the Tauern Window by uraninite dating, but it is the first time here that both are recorded in the same rock and even the same uraninite grain. The present study shows that recrystallized accessory uraninite can provide a sensitive geological “hard disk” where several discrete thermal events of an area are stored. In addition, our work attests that the mineral uraninite has an unexpected geochronological robustness, even on the microcrystal scale.

Pseudomorphous Rhythmic-Banded and Oscillatory Aggregates of Tetrahedrite-Tennantite at the Gold Deposit Darasun (Eastern Transbaikalia, Russia): a Result of the Coupled Dissolution-Reprecipitation Replacement Reactions

Pseudomorphous Rhythmic-Banded and Oscillatory Aggregates of Tetrahedrite-Tennantite at the Gold Deposit Darasun (Eastern Transbaikalia, Russia): a Result of the Coupled Dissolution-Reprecipitation Replacement Reactions

The dark side of zircon: textural, age, oxygen isotopic and trace element evidence of fluid saturation in the subvolcanic reservoir of the Island Park-Mount Jackson Rhyolite, Yellowstone (USA)

The Island Park-Mount Jackson series in the Yellowstone volcanic field, Wyoming (USA), is a suite of rhyolitic domes and lavas that erupted between the caldera-forming eruptions of the Mesa Fall Tuff (1.3 Ma) and the Lava Creek Tuff (0.6 Ma). Combined zircon U/Pb geochronology, Raman spectroscopy, oxygen isotopic and trace elemental compositions document storage conditions of these magmas between consecutive supereruptions. Based on comparison with co-erupted melt compositions and textural criteria, four zircon compositional groups are identified that record different stages along a continuous magmatic evolution from trace element-poor rhyolite at high temperatures to extremely fractionated rhyolite where zircon trace elements are highly enriched (e.g., > 1000 ppm U). These latter zircon domains are dark in cathodoluminescence images and show broadened Raman peaks relative to near-endmember zircon, indicating that substitution of non-stoichiometric trace elements into zircon leads to distortion of the crystal lattice. Some of these zircon domains contain inclusions of U-Th-REE-phases, likely originating from coupled dissolution–reprecipitation of metastable trace element-rich zircon in the presence of a fluid phase. Rhyolite-MELTS simulations indicate that at the conditions required to produce the observed enrichment in trace elements, a fluid phase is likely present. These findings illustrate that zircons can be assembled from a variety of co-existing magmatic environments in the same magma reservoir, including near-solidus volatile-rich melts close to the magmatic–hydrothermal transition.

Rutile alteration and authigenic growth in metasandstones of the Moeda Formation, Minas Gerais, Brazil – A result of Transamazonian fluid–rock interaction

Microstructures, trace elements and U–Pb ages of rutile grains are reported from two metasandstone samples of the Moeda Formation, which is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Minas Gerais, Brazil. The Moeda metasandstones, deposited between 2.68 and 2.65 Ga, are intercalated with Witwatersrand-like, pyritiferous and auriferous metaconglomerate layers. Rutile from both samples shows unusually high contents of Th (1–142 ppm) and rare-earth elements (ΣREE = 1–132 ppm), with chondrite-normalized middle REE humps, and reveals post-depositional U–Pb ages of ≤2.25 Ga; only three grains yielded significantly older ages of up to 2.72 Ga, pointing to a detrital origin. The combined data set suggests that rutile formation and alteration at 2.25 Ga was caused by distinct fluid-mediated processes, which erased nearly all provenance information. Detrital, rounded rutile grains in sample A were altered by a coupled dissolution–reprecipitation process (type 1), resulting in highly porous grains. These show variable contents of rutile-compatible and -incompatible trace elements: Th (1–48 ppm), Zr (25–796 ppm), U (12–167 ppm), Pb (9–52 ppm), Cr (900–12,878 ppm), W (143–3939 ppm), Nb (1148–6168 ppm), Sb (6–63 ppm), as well as variable ratios of Zr/Hf, Y/Ho and Th/U. Rutile grains in sample B are mostly authigenic (type 2), and formed after dissolution of pre-existing detrital rutile grains and subsequent transport and homogenization of trace elements. This interpretation is supported by the anhedral shape of some grains and the finding of numerous, randomly oriented muscovite laths. The authigenic population has significantly lower, and less variable contents of trace elements compared to type-1 rutile: Th (0.5–16 ppm), Zr (21–67 ppm), U (5.8–24 ppm), Pb (6-18 ppm), Cr (1379–3153 ppm), W (9.4–70 ppm), Nb (921–3058 ppm), Sb (3.3–10 ppm), and distinctively lower ratios of Zr/Hf (7.9) and Th/U (0.2), but variable Y/Ho ratios. Significant contents of Th and REE are explained by co-precipitation with TiO2 aggregates prior to structural ripening. The close association of rutile with muscovite, the rutile MREEN humps and the results of Zr-in-rutile thermometry suggest that rutile formation and alteration in the investigated sandstones was mediated by K(Na)–Si–P-bearing aqueous fluids at temperatures of about 490 °C. The rutile MREEN humps are interpreted to result from the tetrad effect during leaching of spatially associated detrital zircon grains. Uranium–Pb ages of ≤2.25 Ga indicate that the fluid overprint started at the onset of the Transamazonian orogeny, perhaps related to the opening of the Sabará foreland basin.

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.

The Origin of Rapakivi Feldspar by a Fluid-induced Coupled Dissolution–Reprecipitation Process

The Origin of Rapakivi Feldspar by a Fluid-induced Coupled Dissolution–Reprecipitation Process

Complicated secondary textures in zircon record evolution of the host granitic rocks: Studies from Western Tauern Window and Ötztal-Stubai Crystalline Complex (Eastern Alps, Western Austria)

Samples of metamorphosed and deformed granitic rocks were collected from two Alpine complexes with well-constrained metamorphic history: Western Tauern Window and Ötztal-Stubai Crystalline Complex. Zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques, cathodoluminescence (CL) imaging and Raman spectroscopy. The aims were: to describe and interpret complicated secondary textures and microstructures in zircon; based on cross-cutting relationships between secondary microstructures, reconstruct the sequence of processes, affecting zircon crystals; link the evolution of zircon with the history of the host rocks.The results indicate that zircon in the sampled granitic rocks forms growth twins and multi-grain aggregates, which are unusual for this mineral. Moreover, various secondary textures have been found in the sampled zircon, often cross-cutting each other in a single crystal. These include: distorted oscillatory CL zoning with inner zones forming inward-penetrating, CL-bright embayments, which are the evidence of dry recrystallization via annealing/lattice recovery; CL mosaicism with no preservation of growth zoning, but abundant nano- and micro-scale pores and mineral inclusions, which are the evidence of recrystallization by coupled dissolution–reprecipitation and/or leaching; embayed zircon boundaries filled with apatite, monazite, epidote and mylonitic matrix, indicating mineral-fluid reactions resulting in zircon dissolution and fragmentation; overgrowth CL-dark rims, which contain nano-pores and point to transport and precipitation of dissolved zircon matter. We conclude that zircon in our meta-granites is sensitive to metamorphism/deformation events, and was reactive with metamorphic fluids. Additionally, we have found evidence of crystal-plastic deformation in the form of low angle boundaries and bent grain tips, which is a result of shearing and ductile deformation of the host rock. We suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization, prograde metamorphism, peak of amphibolite-facies metamorphism, post-peak cooling and exhumation, formation of ductile shear zones and final cooling to ~250°C.

Monazite-(Ce) and Xenotime-(Y) From An Lct, NYF Tertiary Pegmatite Field: Evidence From A Regional Study In the Central Alps (Italy and Switzerland)

Abstract Monazite-(Ce) and xenotime-(Y) from tertiary pegmatites of the central Alps (Italy and Switzerland) were studied by EMPA and SCXRD analytical methods. The REE-phosphate minerals analyzed are from LCT and NYF pegmatite dikes of the Central Alps, which includes three main areas: Vigezzo (LCT and NYF from the Vigezzo valley), Bodengo (NYF from the Leggia valley and LCT from the Cama valley), and Codera (LCT from Codera and Chiavenna valleys and Alpe Rossaccio). SEM-BSE images of monazite-(Ce) show primary growth zoning, sector zoning, and patchy zoning. SEM-BSE images of xenotime-(Y) show faint patchy-type zoning and spongiform texture. Both REE-phosphates evidence secondary mineral replacements that include thorianite, thorite, and uraninite. EMPA compositions show that the monazite-(Ce) is characterized by variable amounts of huttonite (ThSiO 4 ) and cheralite [Ca(Th,U)(PO 4 )] molecules. SCXRD analyses shows that the unit-cell volume of monazite-(Ce) correlates with its chemical composition; the b cell parameter shows an evident relationship with the huttonite molecule. The unit-cell volume of xenotime-(Y) is well correlated to its chemical composition. Xenotime-(Y) related to the NYF family shows a significant increase in yttrium and a lower U + HREE content. The Tertiary pegmatite field of the central Alps is related to the Masino-Bregaglia intrusion, the San Fedelino granite, and younger magmatic pulses generated by the migration of the metamorphic thermal Lepontine Dome towards the west. The presence of poikiloblastic thorite, thorianite, and uraninite in monazite and xenotime can be induced to form from alkali-bearing fluids via coupled dissolution–reprecipitation, under moderate to high-grade and post-peak Alpine metamorphic conditions.

Recrystallization and hydrothermal growth of high U–Th zircon in the Weondong deposit, Korea: Record of post-magmatic alteration

In this study, we investigate zircons with high U and Th contents of 12,000–24,000 and 11,000–40,000ppm, respectively, from leucocratic granite in the Weondong region of South Korea. Hydrothermally epitaxial growth textures are observed in addition to four distinct textures with different backscattered electron (BSE) intensities within single zircon crystals. We describe the internal textures of the zircon crystals and define the chemical characteristics of each textural domain. The zircon crystals show internally recrystallized and externally crystallized textures, supporting the concept of post-magmatic fluid control. After crystallization of the primary zircon by late magmatism, four types of secondary textures were developed. The type-I domain shows patchy forms within the primary domain due to the structural and chemical recrystallization of self-irradiated zircon by a fluid-dominated diffusion reaction process. The type-II domain is characterized by a pure zircon composition, mineral inclusions (mainly thorite), and micropores due to chemical recrystallization by a coupled dissolution–reprecipitation process during interaction with aqueous fluids. The type-III domain is the purest zircon, is interconnected with type-II, and formed by a second coupled dissolution–reprecipitation process. The type-IV domain is the hydrothermal recrystallization/overgrowth texture formed by the direct crystallization process from fluorine-enriched, zircon-saturated aqueous fluid. These results indicate that the zircon crystals were formed sequentially or intermittently through multi-genetic processes by post-magmatic fluids. Furthermore, we conclude that F-enriched post-magmatic fluid migrated large amounts of zirconium, resulting in the typical overgrowth texture of zircon.