Many gold deposits in the Laverton greenstone belt, in the northeast of the Eastern Goldfields province of the Yilgarn craton, are located adjacent to, or hosted by, granitoids. This has led to controversy over whether the granitoids provided the auriferous fluids from which the deposits formed or were structural traps controlling the siting of the gold deposits. New regional-scale stable isotope data, combined with robust geochronology on several deposits, resolves this controversy in the Laverton greenstone belt. The median S, C, and O isotope compositions of ore and gangue minerals from nine different gold deposits in the Laverton greenstone belt fall in a very narrow range. The only exceptions to this are the more negative δ 34 S values of ore sulfides at the Jupiter gold deposit, which were most likely caused by preexisting highly oxidized host rocks and the more negative δ 13 C values of ore carbonate at deposits with reduced black shale host rocks. Redox conditions and mineralization temperatures for all gold deposits in the Laverton greenstone belt are broadly similar. Therefore, the lack of variation in the isotopic compositions of ore and gangue minerals is consistent with their deposition from a similar ore fluid. There is no convincing evidence to indicate that more than one ore fluid was involved in deposition of gold deposits within the Laverton greenstone belt, although the data do not uniquely define the source of the ore fluid (e.g., whether it was proximal or distal). SHRIMP U-Pb dating of gold-related monazite and xenotime provides a temporal framework for gold mineralization in the Laverton greenstone belt. Synmineralization phosphates have ages of 2650 ± 7 Ma at Mount Morgans, 2649 ± 11 Ma at Jubilee, 2657 ± 21 Ma at Jupiter, and probably 2653 ± 6 Ma at Granny Smith. The similarity in age of these four deposits, as well as previously published ages for the Wallaby (2650 ± 5 Ma) and Sunrise Dam and/or Cleo deposits (2654 ± 8 Ma), places three major constraints on the source of auriferous fluids in the Laverton greenstone belt. First, the Wallaby and, most likely, the Granny Smith gold deposits are not the same age as adjacent granitoids, ruling out the exposed granitic rocks as a proximal magmatic fluid source. Second, the broadly synchronous timing of gold mineralization on a camp scale provides evidence that the deposits have a similar genesis. Third, the range of ages of the gold deposits is not as great as that of the granitoids postulated to be their source. Magmatic activity that has been invoked as the source of ore fluids by various workers is related to several geochemically distinctive granitoid suites that are diachronous over several tens of millions of years in the Laverton greenstone belt and the wider Eastern Goldfields province. In contrast, the consistent age of gold mineralization in the Laverton greenstone belt supports a single fluid source, as implied by the isotope geochemistry. It is concluded that all studied deposits are orogenic gold deposits with a distal and deep source.
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.
High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 ± 10 Ma and 493 ± 9 Ma. Coexisting zircon gave ages of 500 ± 4 Ma and 484 ± 5 Ma for a metatexite (two age populations) and 475 ± 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 ± 6 Ma and 482 ± 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages.
Abstract The Kunene Complex of Namibia-Angola is one of the largest anorthosite massifs on Earth (up to 18,000 km2), consisting of several distinct anorthosite and leucotroctolite intrusions. The Namibian portion of the Kunene Complex measures ~80 × 50 km, ~4,000 km2, and is dominated by the Zebra Mountain lobe, a ~16-km-thick dome-like mass of interlayered, relatively unaltered dark leucotroctolite with relatively altered, “white,” anorthosite. Past studies and the present work have found evidence for intrusion of two distinct phases of dark leucotroctolite into the white anorthosite, namely a relatively early, deformed, phase dated at 1363 ± 17 Ma (U-Pb in baddelyite), and a relatively later and undeformed phase whose absolute age remains unknown. The Kunene leucotroctolites are among the least evolved troctolites known from anorthosite complexes, with olivine containing 59 to 77 mol % forsterite and up to 1,700 ppm Ni, and plagioclase containing 56 to 69 mol % anorthosite. Our isotope data from the troctolites indicate a relatively small crustal component (δ18O, ~5.3–7.3; δ34S, 0.5–1; and ɛNdT, 0.9–1.8), whereas Nd and oxygen isotope data from the white anorthosites, published by other workers, showed a slightly larger crustal component (e.g., ɛNdT as low as −3; δ18O up to 7.5‰). In the periphery of the Kunene Complex are several, relatively small (<10 km2), mafic-ultramafic intrusions comprising peridotite, pyroxenite, gabbro, troctolite, and anorthosite. Some of these bodies are Ni-Cu-PGE mineralized, including the Ohamaremba troctolite, the Oncocua pyroxenite, and the Ombuku peridotite-gabbronorite. The latter additionally contains a massive chromitite layer. A new U-Pb baddelyite age of 1220 ± 15 Ma for Ohamaremba indicates that the latter postdates the main Kunene Complex by ~140 Ma. The relative enrichment in MgO, Cr, and Ni, and the O, Nd, and S isotope characteristics of Kunene magmatism suggest that the primary magmas were predominantly mantle-derived picrites or basalts. The massif-type anorthosites formed through ascent of feldspathic slurries followed by downward draining of residual liquid. Subsequent magma pulses formed troctolitic sills within the anorthosite plutons and mafic-ultramafic satellite intrusions in the periphery of the anorthosites. The recurring nature of Kunene mafic-ultramafic magmatism results from several successive mantle upwellings. Partial mantle melts ascended through reactivated translithospheric lineaments along the southern margin of the Congo craton.
Paleoproterozoic basins are notoriously difficult to date if they are poor in zircon-rich felsic-volcanic rocks. The detail with which basins are mapped often resolves the problem, as many contain thin zircon-rich ash beds. Dating detrital zircon is useful, but there are always questions of the
A sample of the Crixás-Açu gneiss in Central Brazil contains protolith and metamorphic zircons, and two generations of metamorphic titanite. SHRIMP U-Pb data of these different mineral generations indicate the following temporal sequence: tonalitic magmatism at 2817 ± 9 M.y derived from an older source region (3050 to 2930 M.y zircon cores); Archaean metamorphism at 2772 ± 6 M.y (from zircon) with cooling to the blocking temperature of titanite (at 2711 ± 34 Ma); followed by Palaeoproterozoic metamorphism and weak fabric development at 2011 ± 15 Ma, and a possible Neoproterozoic metamorphism. The field relations and these age data indicate the polymetamorphic history of the area and demonstrate the value of in situ age determinations on well-characterized rocks
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