A test of the viability of fluid–wall rock interaction mechanisms for changes in opaque phase assemblage in metasedimentary rocks in the Kambalda-St. Ives goldfield, Western Australia

Evans, Katy

doi:10.1007/s00126-009-0260-4

Cited by 22 publications

(13 citation statements)

References 15 publications

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“…Gold deposition in these environments may be related to reduction of the ore fluid by the carbon (e.g., Gize, 1999). However, carbonation and sulfidation of ferric iron-rich minerals, such as magnetite, can produce alteration assemblages with pyrite in equilibrium with hematite and sulfate (Evans, 2010;Evans et al, 2006). Fluid mixing has been invoked by some workers (e.g., Neumayr et al, 2005Neumayr et al, , 2008 as important for orogenic gold formation based on observations of oxidized and reduced mineral phases in deposit halos.…”

Section: Ptx Constraints On Ore Depositionmentioning

confidence: 99%

Geochemistry of Hydrothermal Gold Deposits

Saunders

Hofstra²,

Goldfarb³

et al. 2014

Treatise on Geochemistry

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Section: Ptx Constraints On Ore Depositionmentioning

confidence: 99%

Geochemistry of Hydrothermal Gold Deposits

Saunders

Hofstra²,

Goldfarb³

et al. 2014

Treatise on Geochemistry

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“…CO 2 –H 2 S; Phillips & Groves, 1983), and despite a variety of alteration minerals of different oxidation states, fluid redox conditions may be near‐constant in many gold systems. For example, at Hunt mine in the Kambalda–St Ives goldfield, an alteration progression of distal hydrothermal magnetite, to intermediate pyrrhotite, to proximal pyrite surrounds auriferous quartz veins: using the widespread chlorite, thermodynamic calculations indicate that the alteration assemblages reflect constant redox conditions through this progression despite the various oxidation states of the Fe‐bearing minerals (Neall & Phillips, 1987; see also Evans, 2010).…”

Section: Nature Of Gold Deposits and Provincesmentioning

confidence: 99%

“…Importantly, this equation does not require any external source of oxidant despite it being a hematite‐forming equation, and the ferric iron required for hematite is sourced from the host‐rock magnetite. The various oxidation states of Fe in minerals do not necessitate nor imply fluid mixing (Evans, 2010).…”

Section: Metamorphic Devolatilization Modelmentioning

confidence: 99%

Formation of gold deposits: a metamorphic devolatilization model

Phillips

Powell

2010

Journal Metamorphic Geology

493

177

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A metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many goldfields such as those found in Archean greenstone belts, slate-belts and other goldonly provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist-amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H 2 O and CO 2 but also S and, in turn, Au. Elevated gold in solution is achieved by complexing with reduced S, and by H 2 CO 3 weak acid buffering near the optimal fluid pH for gold solubility (the buffering is more important than being at the point of maximum gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and ⁄ or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre-scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold. Deposition of gold from solution necessitates breakdown of the gold-thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon-bearing host rocks and ⁄ or by sulphidation of wallrocks to generate iron-bearing sulphide and precipitated gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite-bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite ⁄ biotite alteration where the host rock is of suitable bulk composition. The correlation of major gold deposits with rock type, even when the gold is primarily in veins, argues for rock-dominated depositional systems, not fluid-dominated ones. As a consequence, a general role in gold deposition for fluid mixing, temperature decrease and ⁄ or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at gold-only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher-grade metamorphism and deformation, and ⁄ or (palaeo)-weathering may provide many of the most-obvious features of goldfields including their mineralogy, geochemistry, geometry, small-scale timing features, geophysical response and even mesoscopic gold distribution.

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“…Evans et al. , 2006; Evans, 2010), but anhydrite is identified rarely in thin sections of these rocks (see Clout et al. , 1990 for an exception), possibly due to its poor preservation potential and/or a lack of appropriate fluid composition during metamorphism.…”

Section: Application To Sulphide‐bearing Mineral Assemblages In Greenmentioning

confidence: 99%

Internally consistent data for sulphur‐bearing phases and application to the construction of pseudosections for mafic greenschist facies rocks in Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–O–S–H₂O

Evans

Powell

Holland

2010

Journal Metamorphic Geology

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The Holland and Powell internally consistent data set version 5.5 has been augmented to include pyrite, troilite, trov (Fe 0.875 S), anhydrite, H 2 S, elemental S and S 2 gas. Phase changes in troilite and pyrrhotite are modelled with a combination of multiple end-members and a Landau tricritical model. Pyrrhotite is modelled as a solid solution between hypothetical end-member troilite (trot) and Fe 0.875 S (trov); observed activity-composition relationships fit well to a symmetric formalism model with a value for w trot)trov of )3.19 kJ mol )1 . The hypothetical end-member approach is required to compensate for iron distribution irregularities in compositions close to troilite. Mixing in fluids is described with the van Laar asymmetric formalism model with a ij values for H 2 O-H 2 S, H 2 S-CH 4 and H 2 S-CO 2 of 6.5, 4.15 and 0.045 kJ mol )1 respectively. The derived data set is statistically acceptable and replicates the input data and data from experiments that were not included in the initial regression. The new data set is applied to the construction of pseudosections for the bulk composition of mafic greenschist facies rocks from the Golden Mile, Kalgoorlie, Western Australia. The sequence of mineral assemblages is replicated successfully, with observed assemblages predicted to be stable at X(CO 2 ) increasing with increasing degree of hydrothermal alteration. Results are compatible with those of previous work. Assemblages are insensitive to the S bulk content at S contents of less than 1 wt%, which means that volatilization of S-bearing fluids and sulphidation are unlikely to have had major effects on the stable mineral assemblage in less metasomatized rocks. The sequence of sulphide and oxide phases is predicted successfully and there is potential to use these phases qualitatively for geobarometry. Increases in X(CO 2 ) stabilized, in turn, pyrite-magnetite, pyrite-hematite and anhydrite-pyrite. Magnetite-pyrrhotite is predicted at temperatures greater than 410°C. The prediction of a variety of sulphide and oxide phases in a rock of fixed bulk composition as a function of changes in fluid composition and temperature is of particular interest because it has been proposed that such a variation in phase assemblage is produced by the infiltration of multiple fluids with contrasting redox state. The work presented here shows that this need not be the case.

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A test of the viability of fluid–wall rock interaction mechanisms for changes in opaque phase assemblage in metasedimentary rocks in the Kambalda-St. Ives goldfield, Western Australia

Cited by 22 publications

References 15 publications

Geochemistry of Hydrothermal Gold Deposits

Geochemistry of Hydrothermal Gold Deposits

Formation of gold deposits: a metamorphic devolatilization model

Internally consistent data for sulphur‐bearing phases and application to the construction of pseudosections for mafic greenschist facies rocks in Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–O–S–H₂O

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A test of the viability of fluid–wall rock interaction mechanisms for changes in opaque phase assemblage in metasedimentary rocks in the Kambalda-St. Ives goldfield, Western Australia

Cited by 22 publications

References 15 publications

Geochemistry of Hydrothermal Gold Deposits

Geochemistry of Hydrothermal Gold Deposits

Formation of gold deposits: a metamorphic devolatilization model

Internally consistent data for sulphur‐bearing phases and application to the construction of pseudosections for mafic greenschist facies rocks in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–CO2–O–S–H2O

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Internally consistent data for sulphur‐bearing phases and application to the construction of pseudosections for mafic greenschist facies rocks in Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–O–S–H₂O