1.2 Ga Mafic dyke near York, southwestern Yilgarn Craton, Western Australia
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Abstract:
A SHRIMP 207 Pb/ 206 Pb zircon age of 1204 ± 10 Ma is reported for an east–west‐trending dolerite dyke from near York in the southwestern Yilgarn Craton. This age is identical within analytical uncertainty to previously reported ages of ca 1210 Ma for dykes from the central Wheatbelt and the Western and Eastern Goldfields. The consistency of the dyke ages and the wide areal extent of the dykes suggests that emplacement occurred as a single magmatic pulse at ca 1210 Ma throughout the southwestern Yilgarn Craton. The similarities between the age of the dykes and the ages of late events in the Albany–Fraser Orogeny, and the approximate parallelism of the east–west‐trending dykes to the margin of the orogen, raises the possibility that these events are related.Keywords:
Yilgarn Craton
Orogeny
Geochronology
Baddeleyite
Reconstructing the tempo and emplacement mechanisms of large igneous provinces (LIPs) and establishing potential links to environmental change and biological crises requires detailed and targeted high-precision geochronology. Contact metamorphism during LIP intrusive magmatism can release large volumes of thermogenic gas, so determining the timing of these events relative to global climate change is crucial. The most reliable age information comes from U-Pb geochronology; however, LIP mafic igneous rocks do not commonly crystallize U-bearing minerals, such as zircon or baddeleyite. Recent work has shown that U-rich minerals can crystallize in fractionated melt pockets in intrusive components of LIPs after contamination of the melt by sedimentary rocks at emplacement level. Zircon and baddeleyite from these pockets make high-precision U-Pb geochronology of LIPs possible, but these unique mechanisms add other complexities.
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The origin of sulfur and gold in Archean orogenic gold systems should provide significant
insights into the dynamics of fluid movement in the crust of the early Earth, but is poorly
constrained and highly debated. Our natural laboratory to address this knowledge gap is the
metal-endowed Yilgarn Craton (Western Australia), where we measured the multiple sulfur
isotope signatures of representative sulfide-bearing auriferous samples from 24 Archean
orogenic gold deposits varying in size and geological setting. Utilizing chemically conservative
mass-independent fractionated sulfur (MIF-S) isotope signatures, we fingerprinted a
major source of sulfur in these deposits. Contrary to previous studies, our data show that
they display MIF-S isotope anomalies, with Δ 33 S values ranging from –1.18‰ to +2.04‰;
most of the studied deposits show a sulfur signature that is consistent with a crustally derived
source. Unlike smaller deposits, which may form with sulfur derived from a single sedimentary
sulfur source, therefore providing a coherent Archean atmospheric signal in their Δ 33 S-Δ 36 S
slope (~0.9–1.5), the formation of giant deposits may require sourcing from a wider range
of sulfur reservoirs and using different processes, as reflected in their apparently random
Δ 33 S-Δ 36 S slopes.
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This study investigates the microstructures, geochemistry, and hydrothermal evolution of gold-bearing pyrite and arsenopyrite from six orogenic gold deposits in the Archean Eastern Goldfields Province, Western Australia. Scanning electron microscope (SEM), electron microprobe (EMP) and laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) analyses show that the gold-bearing minerals possess a number of similar textural features, including the occurrence of invisible gold within initial phases of growth, and later-stage visible gold associated with alteration rims. The alteration rims are characterized by a higher-than-average atomic mass (mainly owing to arsenic enrichment) and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite. These observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite. Textural observations and LA-ICP-MS data suggest that some invisible gold was remobilized from early-formed pyrite and arsenopyrite to form visible gold during development of these alteration rims. Gold may also have been added by hydrothermal fluids during a later stage of mineralization. In situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures. Based on sulfide stability relations, our data indicate that development of arsenopyrite alteration rims associated with late-stage visible gold formation was related to an increase in temperature (maximum increase from 310° to 415°C) and up to of sic orders of magnitude increase in sulfur fugacity, whereas changes in oxygen fugacity were less important. LA-ICP-MS analyses show that the relative and absolute variations in selected trace element (Au, Ag, Sb, Bi, Ba, Te, Pb, Co, and Mo) concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite. In general, trace elements within pyrite and arsenopyrite have a relatively uniform distribution, whereas later-stage alteration rims have more variable trace element distributions. Although the observed textures are typical of prograde metamorphic coronae, we suggest that they are the consequence of variations in fluid conditions and chemistry, and that mineralization occurred in response to syn- and/or postpeak metamorphic fluid infiltration.
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Mineral redox buffer
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The currently accepted model for the Archean lode gold deposits of the Yilgarn craton postulates that they represent a coherent group of epigenetic deposits, the majority of which formed during a craton-scale, broadly synchronous hydrothermal event late in the tectonothermal evolution of the granite-greenstone terranes at ca. 2640 to 2630 Ma.Felsic rocks from the southern Eastern Goldfields, which host or are cut by gold mineralization, have SHRIMP II U-Pb zircon ages of 2673 + or - 3 Ma at Mount Charlotte, 2669 + or - 17 Ma at Mount Percy, 2663 + or - 3 Ma at Racetrack, and 2657 + or - 8 Ma at Porphyry. All these ages are consistent with gold mineralization at ca. 2640 to 2630 Ma.Intermediate to felsic dikes cut typical syn- to postmetamorphic lode gold mineralization at the Mount McClure and Jundee deposits in the Yandal greenstone belt in the north of the Kurnalpi terrane. The dikes give ages of 2656 + or - 4, 2663 + or - 4, and 2668 + or - 10 Ma from Mount McClure, and 2656 + or - 7 Ma from Jundee, requiring that mineralization and peak regional metamorphism in the belt occurred prior to ca. 2660 Ma. However, both the characteristics of the Jundee and Mount McClure deposits and the relative timing of mineralization with respect to the metamorphic and structural history of the belt are similar to that seen for gold deposits elsewhere in the Yilgarn craton. This implies that mineralization at Jundee and Mount McClure was produced prior to 2660 Ma by similar processes to those seen elsewhere in the Yilgarn at 2640 to 2630 Ma.Peak metamorphism in the western, higher metamorphic grade terranes of the Yilgarn was not reached until ca. 2630 Ma, some 10 to 30 m.y. after peak metamorphism in the Kalgoorlie terrane and more than 30 m.y. after metamorphism in the Yandal belt. In addition, almost all of the published robust ages supporting gold mineralization at ca. 2640 to 2630 Ma are from the west of the craton. Consideration of the new data from the Yandal belt in conjunction with previously published geochronology throws doubt on the hypothesis that lode gold mineralization occurred approximately synchronously across the Yilgarn craton. Rather, it suggests that mineralization, along with regional metamorphism, is earlier by at least 30 m.y. in the northeastern Yilgarn craton.
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Extrapolation of the fold from the partial limb comprising the Joyner's Find Greenstone Belt near Wiluna in Western Australia suggests that an overall thickness of strata >10 km commensurate with the postulated thickness of the Archean oceanic crust was folded as a unit giving the folds a total amplitude of up to 50 km. This suggests that early (before 2 Ga) crustal plate collisions resulted in folding of the thin ocean crust instead of subduction and this process resulted in the formation of the typical Archean cratons. The folded crust would be deeply depressed into the mantle by isostatic compensation with consequent partial melting at depth forming a felsic melt and a dense ultramafic restite. Anatectic replacement of the down-folded ocean crust by granite and extended erosion of the upper parts of the folds resulted in the formation of an Archean granite and greenstone style craton with an ultimate thickness of around 30-35 km. The collision of two cratons folded the intervening mafic crust, including sediment deposited on the ocean floor, to form a new craton that combined the cratons into a continent. Uplift and erosion of the cratons during the late Archean deposited sediments on the thinner margins of the continents forming extensive continental margin basins. The subduction of ocean crust did not occur until the formation of deep sedimentary accumulations on the sea floor beyond the margins of the continents depressed the ocean crust and was able to subduct the oceanic plate beneath the continental margin. The subducting ocean crust caused compression of the sedimentary basin and the formation of modern style orogenic fold belts and volcanic arc systems that were not present during the Archean era.
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