The Chemistry of Metal Transport and Deposition by Ore-Forming Hydrothermal Fluids

Seward, Terry M.; Williams‐Jones, Anthony E.; Migdisov, Artas

doi:10.1016/b978-0-08-095975-7.01102-5

Cited by 157 publications

(102 citation statements)

References 296 publications

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“…In that sense, it has been documented that aqueous solutions with substantial amounts of hydrocarbons could effectively leach Ti from igneous rocks and re-precipitate titanium bearing minerals in sedimentary [48] and subduction zone [49] environments. The transport of Ti and other elements (Sn, REE, U, and Zr) is also believed to be favoured by fluoride, which increases rutile solubility in high-temperature hydrothermal fluids due to the formation of mixed hydroxofluoro-complexes [50] (and references therein). It has also been shown that both zircon and rutile can crystallize from alkali and Al-Si bearing aqueous fluids [51] in quartz veins emplaced in HP/UHP terrains (in [52]).…”

Section: Origin Of the Cedrolina Chromititementioning

confidence: 99%

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

et al. 2016

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Abstract:The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs (up to 1.5 cm in size), often strongly deformed and fractured, immersed in a matrix of silicates (mainly chlorite and talc). Chromite is characterized by high Cr# (0.80-0.86), high Fe 2+ # (0.70-0.94), and low TiO 2 (av. = 0.18 wt %) consistent with variation trends of spinels from metamorphic rocks. The chromitite contains a large suite of accessory phases, but only irarsite and laurite are believed to be relicts of the original igneous assemblage, whereas most accessory minerals are thought to be related to hydrothermal fluids that emanated from a nearby felsic intrusion, metamorphism and weathering. Rutile is one of the most abundant accessory minerals described, showing an unusually high Cr 2 O 3 content (up to 39,200 ppm of Cr) and commonly forming large anhedral grains (>100 µm) that fill fractures (within chromite nodules and in the matrix) or contain micro-inclusions of chromite. Using a trace element geothermometer, the rutile crystallization temperature is estimated at 550-600 • C (at 0.4-0.6 GPa), which is in agreement with P and T conditions proposed for the regional greenschist to low amphibolite facies metamorphic peak of the area. Textural, morphological, and compositional evidence confirm that rutile did not crystallize at high temperatures simultaneously with the host chromitite, but as a secondary metamorphic mineral. Rutile may have been formed as a metamorphic overgrowth product following deformation and regional metamorphic events, filling fractures and incorporating chromite fragments. High Cr contents in rutile very likely are due to Cr remobilization from Cr-spinel during metamorphism and suggest that Ti was remobilized to form rutile. This would imply that the magmatic composition of chromite had originally higher Ti content, pointing to a stratiform origin. Another possible interpretation is that the Ti-enrichment was caused by external metasomatic fluids which lead to crystallization of rutile. If this was the case, the Cedrolina chromitites could be classified as podiform, possibly representing a sliver of tectonically dismembered Paleoproterozoic upper mantle. However, the strong metamorphic overprint that affected the studied chromitites makes it extremely difficult to establish which of the above processes were active, if not both (and to what extent), and, therefore, the chromitite's original geodynamic setting.

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Section: Origin Of the Cedrolina Chromititementioning

confidence: 99%

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

et al. 2016

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“…This diagram was constructed for a redox state buffered by the Ni-NiO equilibrium and salinities of (i) 1m NaCl, which is typical, for example, of relatively low-temperature (300-400 • C) seafloor hydrothermal systems (Bortnikov et al [30]; Hannington et al [31]) and high-temperature magmatic fluids (800 • C) separated from arc dacite magma of porphyry-generating intrusion (Blundy et al [32]); and (ii) 7m NaCl which is typical of magmatic brine inclusions and products of the separation of single-phase magmatic fluids (Seward et al [1], Aranovich et al [33]). At a low temperature of 200 • C, hydrosulfide complexes predominate at any pH and chlorinity (Seward et al [1], Trigub et al [34]). The maximum solubility is observed in near-neutral solutions in the Au(HS) 2 − predominance region with pH~pK H2S (that is, at similar concentrations of H 2 S • (aq) and HS − , which are necessary for the formation of the Au complex).…”

Section: Effect Of Temperature Ph and Chlorinity On Au Solubilitymentioning

confidence: 99%

“…at~750 • C (Ulrich et al [2]). In these fluids Au can be efficiently transported in the form of the Au(I)-Cl complexes [1]. The complexation of Au in chloride solutions has been extensively studied by potentiometry, solubility experiments, X-ray absorption spectroscopy, and ab initio molecular dynamics (AIMD).…”

Section: Introductionmentioning

confidence: 99%

“…The chlorinity of natural ore-forming fluids varies in a wide range. For example, solutions migrating in seafloor hydrothermal systems at temperatures from 250 to 400 • C are relatively diluted (~3 wt % NaCl eq., Seward et al [1]), whereas the magmatic brines of porphyry systems may contain up to 50-60 wt % NaCl eq. at~750 • C (Ulrich et al [2]).…”

Section: Introductionmentioning

confidence: 99%

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Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

et al. 2018

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Abstract:Gold is transported in high-temperature chloride-bearing hydrothermal fluids in the form of AuCl 2 − . The stability of this complex has been extensively studied, but there is still considerable disagreement between available experimental data on the temperature region 300-500 • C. To solve this problem, we measured the solubility of gold in HCl/NaCl fluids (NaCl concentration varied from 0.1 to 3 mol·(kg·H 2 O) −1 ) at 450 • C and pressures from 500 to 1500 bar (1 bar = 10 5 Pa). The experiments were performed using a batch autoclave method at contrasting redox conditions: in reduced experiments hydrogen was added to the autoclave, and in oxidized experiments the redox state was controlled by the aqueous SO 2 /SO 3 buffer. Hydrogen pressure in the autoclaves was measured after the experiments in the reduced system. The gold solubility constant, Au (cr) + HCl • (aq)

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“…In contrast to silver, the copper hydrocomplex CuOH 0 is also present in the In the suggested model, gold in the fluid (Figure 9a) in the first reservoir (700 • C) is mainly represented by chloride complex AuCl 2 − [29]. Thereafter, when the fluid was transported to the next reservoirs and the temperature decreased, its amount decreased significantly: at 450 • C, the main gold-bearing complex is hydroxocomplex AuOH 0 in the presence of small amounts of Au(HS) 2 − and AuHS 0 [30], and at 300 • C, the role of gold sulfide complexes increased drastically [29]. NaCl-rich fluid determines the predominance of chloride complexes of Ag and Cu in the 1.-3. reservoirs (Figure 9b,c).…”

Section: Fluid Composition and Forms Of Transfer And Deposition Of Aumentioning

confidence: 99%

Physicochemical Model of Formation of Gold-Bearing Magnetite-Chlorite-Carbonate Rocks at the Karabash Ultramafic Massif (Southern Urals, Russia)

et al. 2018

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Abstract:We present a physicochemical model for the formation of magnetite-chlorite-carbonate rocks with copper gold in the Karabash ultramafic massif in the Southern Urals, Russia. The model was constructed based on the formation geotectonics of the Karabash massif, features of spatial distribution of metasomatically altered rocks in their central part, geochemical characteristics and mineral composition of altered ultramafic rocks, data on the pressure and temperature conditions of formation, and composition of the ore-forming fluids. Magnetite-chlorite-carbonate rocks were formed by the hydrothermal filling of the free space, whereas chloritolites were formed by the metasomatism of the serpentinites. As the source of the petrogenic and ore components, we considered rocks (serpentinites, gabbro, and limestones), deep magmatogenic fluids, probably mixed with metamorphogenic fluids released during dehydration and deserpentinization of rocks in the lower crust, and meteoric waters. The model supports the involvement of sodium chloride-carbon dioxide fluids extracting ore components (Au, Ag, and Cu) from deep-seated rocks and characterized by the ratio of ore elements corresponding to Clarke values in ultramafic rocks. The model calculations show that copper gold can also be deposited during serpentinization of deep-seated olivine-rich rocks and ore fluids raised by the tectonic flow to a higher hypsometric level. The results of our research allow predicting copper gold-rich ore occurrences in ultramafic massifs.

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The Chemistry of Metal Transport and Deposition by Ore-Forming Hydrothermal Fluids

Cited by 157 publications

References 296 publications

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

Physicochemical Model of Formation of Gold-Bearing Magnetite-Chlorite-Carbonate Rocks at the Karabash Ultramafic Massif (Southern Urals, Russia)

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