Timing and Duration of High-Temperature Gold Mineralization and Spatially Associated Granitoid Magmatism at Chalice, Yilgarn Craton, Western Australia
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Abstract:
The Chalice gold deposit, in the Eastern Goldfields province, Yilgarn craton, Western Australia, is located in a middle to upper amphibolite facies metamorphic domain and is hosted by a mafic-ultramafic rock sequence that has been locally intruded by four generations of monzogranite dikes. Two stages of gold mineralization, identified on the basis of crosscutting relationships, formed under broadly synpeak to postpeak metamorphic conditions. Main stage gold mineralization (95% of the resource) comprises foliation-parallel, quartz-albite-diopside-titanite-garnet-gold veins and wall-rock replacement, both within locally developed asymmetric folds in mafic amphibolite. Second stage gold mineralization (5% of the resource) is temporally associated with a second-generation monzogranite dike that crosscuts the folds and hence is younger than the Main event. It is represented by disseminated gold in the dike as well as by foliation-discordant quartz-gold, quartz-diopside-gold, actinolite-gold, and molybdenite-tellurobismuthite-gold veins. Gold is in textural equilibrium with the hydrothermal alteration assemblages in both mineralization stages and also with primary igneous phases (quartz and feldspar) in the monzogranite dike in the Second stage ore.
Magmatic zircons and titanite from second and fourth generation monzogranite dikes and a monzogranite stock, as well as hydrothermal titanite and molybdenite in equilibrium with gold from hydrothermal alteration assemblages, allow dating of the important magmatic and hydrothermal events using the U-Pb and Re-Os isotope systems. Two gold events are identified. Main stage mineralization is coincident with asymmetric fold development at 2644 ± 8 Ma (SHRIMP U-Pb on titanite), and Second stage mineralization (2621 ± 10 Ma; Re-Os on molybdenite) is coeval, within the error of the isotopic ages, with the intrusion of the gold-mineralized second-generation monzogranite dike (2626 ± 9 Ma; SHRIMP U-Pb on zircon). Ages of hydrothermal titanite in monzogranite dikes (2631 ± 10 Ma, 2624 ± 7 Ma, 2623 ± 5 Ma, and 2619 ± 6 Ma; SHRIMP U-Pb ages) indicate contemporaneous hydrothermal alteration, gold mineralization, and evolving magmatism during the Second stage event. A fourth generation, flat-lying pegmatite, which truncates all mine rock units, constrains the minimum age of mine-scale gold-bearing alteration and magmatism to 2622 ± 13 Ma.
The geologically constrained geochronologic data suggest that the Chalice gold deposit is a product of two independent gold events separated by up to 20 m.y., within an extended period of granitoid magmatism also extending over ~20 m.y. It demonstrates, for the first time in the extensively gold-mineralized Yilgarn craton, a clear interdependence and interplay between ongoing granitoid magmatism, deformation, hydrothermal alteration, and gold mineralization in amphibolite-hosted deposits. However, although integrated field and geochemical research establish a clear chronology of events, the controversy of contribution from magmatic and/or metamorphic fluids for the ore remains unresolved, because these events are broadly coeval within the resolution of the isotopic techniques.Keywords:
Titanite
Yilgarn Craton
Dike
Geochronology
Arsenopyrite
Titanite
Ilmenite
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Yilgarn Craton
Titanite
Geochronology
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Yilgarn Craton
Arsenopyrite
Banded iron formation
Greenstone belt
Paragenesis
Greenschist
Ore genesis
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Titanite
Ilmenite
Greenschist
Amphibole
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Ilmenite
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Titanite
Protolith
Geochronology
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The late Archean Coolgardie Goldfield at the western margin of the Norseman-Wiluna
belt, Yilgarn craton, comprises an arcuate belt of deformed mafic, ultramafic,
and sedimentary rocks which is bounded to the west by the syntectonic Calooli
monzogranite. Greenstones at Coolgardie preserve a broad metamorphic gradient,
with peak metamorphic temperature varying from 480°±
50°C
at the center of the gold field to 545°±
50°C
adjacent to the western granitoid-greenstone contact, at an approximate pressure
of 3 to 4 kbar. Gold field-scale variations in the gangue and ore mineralogy of zoned
wall-rock alteration assemblages around lodes, the ore geochemistry, and the
isotope chemistry of vein minerals at Coolgardie are correlated with plan-view
distance from the Calooli monzogranite. Evidence supporting synpeak metamorphic
gold mineralization in the Coolgardie Goldfield includes the equilibrium
textural relationships between gold, sulfides, and high-temperature silicate
gangue; the occurrence of undeformed auriferous quartz veins, enveloped by
garnet-hornblende-plagioclase-calcite alteration, which crosscut
peak-metamorphic fabrics; and the siting of variably deformed gold ores in
synpeak-metamorphic structures. Conditions of gold mineralization at deposits
less than 1 to 2 km from the Calooli monzogranite are determined from
geothermometry and barometry to be 510°±
50°C
to 590° ±
25°C at 3 to
4 kbar, whereas those at greater distances from the monzogranite are 490°
to 525° ±
50°C at 3 to
4 kbar. Alteration assemblages in mafic host rocks can be divided into garnet-bearing
(garnet-hornblende-plagioclase-calcite) and garnet-absent (biotite-amphibole-plagioclase-calcite).
The presence or absence of garnet is mainly controlled by the Mg number of the
host mafic rocks. Ore in deposits with garnet-bearing alteration is enriched in
Ag, Na, Pb, S, and W, but only weakly enriched or depleted in K 2 O and
other large ion lithophile elements, CO 2 , As, Mo, Sb, and Te, whereas
deposits with biotite-amphibole-plagioclase-calcite alteration are strongly
enriched in Ag, As, S, Sb, W, CO 2 , and large ion lithophile elements.
Sulfide-oxide assemblages are regionally zoned from pyrite-ilmenite in deposits
in granitoids and adjacent to the granitoid-greenstone contact, through
pyrrhotite-ilmenite ±
pyrite in garnet-bearing alteration 1 to 2 km from the contact, to
arsenopyrite-pyrrhotite-ilmenite assemblages in biotite-bearing alteration >2
km from the greenstone-granitoid contact. This variation is potentially related
to gradients in fluid f O2
away from the granitoids. Isotopic compositions of oxygen in quartz ( δ 18 O
= 10.8–12.4‰),
scheelite ( δ 18 O
= 4.0–4.1‰),
and oxygen and carbon in calcite ( δ 18 O
= 8.9–13.2‰,
δ 13 C
= –0.5 to –5.3‰)
are generally more positive in deposits with garnet-bearing alteration than in
those with biotite-bearing alteration ( δ 18 O quartz
= 6.6–11.8‰,
δ 18 O scheelite
= 2.3–4.6‰,
δ 18 O calcite
= 8.7–11.4‰,
δ 13 C calcite
= –4.3 to –8.4‰),
whereas both alteration styles have dD biotite
and dD amphibole
values in the range –65
to –86 per
mil. These differences are interpreted to reflect interaction of isotopically
heavy ore fluids with relatively depleted greenstone host rocks during fluid
migration through structurally controlled conduits. The gold field-scale variations in alteration mineralogy and ore chemistry
are considered to be related not to initial ore-fluid composition but to
temperature, to host-rock composition, and to changes in fluid composition
resulting from reaction with greenstone-belt rocks. The correlation between the
calculated temperature of alteration and distance from the western
granitoid-greenstone contact suggests that the Calooli monzogranite played some
genetic role in determining the nature of hydrothermal alteration across
Coolgardie.
Yilgarn Craton
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Titanite
Omphacite
Coesite
Migmatite
Rutile
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