Evidence for a multi-stage history of uplift and metasomatism is preserved in a suite of highly c... more Evidence for a multi-stage history of uplift and metasomatism is preserved in a suite of highly chromian, garnet-rich peridotites found as xenoliths in Newlands and Bobbejaan kimberlites, South Africa. A high proportion of the garnets and chromites in these rocks plot in the diamond facies fields on Cr2O3-CaO and Cr2O3-MgO wt. % plots respectively. Petrographic evidence suggests that the earliest known mineralogy is a garnet-olivine rich rock with granuloblastic texture. A down-P-T event caused exsolution of spinel and pyroxenes in garnet and these inclusion textures have subsequently been partially annealed in most samples. Harzburgitic samples have garnets with inclusions of serpentine ± chromite and in lherzolitic samples, cpx is also present. Most of the garnets have strongly developed, diffusion-controlled, major element zonation patterns which are a result of: (1) External reequilibration where diffusion is towards the matrix. (2) Internal re-equilibration where diffusion is t...
Newlands and Bobbejaan kimberlites, South Africa, contain a suite of highly chromian, garnet-rich... more Newlands and Bobbejaan kimberlites, South Africa, contain a suite of highly chromian, garnet-rich peridotites amongst their xenolith population. A high proportion of garnets and chromites in these rocks plot in the diamond facies fields on Cr2O3-CaO (Gurney, 1984; Grutter et al., 2004) and Cr2O3-MgO wt. % (Fipke et al., 1995) plots respectively. Many samples have garnets with inclusions of serpentine ± chromite (+ clinopyroxene in lherzolitic samples) whose arrangement is sometimes indicative of exsolution or annealed exsolution textures. Modal reconstructions reveal that the original crystals were garnets of higher chromium content. A particularly strong chromite-clinopyroxene association is seen in the matrix and included within garnet in most lherzolitic samples; a feature not reported in other garnet lherzolites. Most of the garnets have strongly developed zonation patterns which are a result of diffusion towards the matrix (external zonation) followed by zonation towards inclus...
Following the recognition that Archean siliceous high-magnesian basalts of the Yilgarn Craton wer... more Following the recognition that Archean siliceous high-magnesian basalts of the Yilgarn Craton were komatiitic magmas that had undergone crustal assimilation, the idea of Archean boninites fell out of general favour. Studies of boninitic rocks in the Abitibi belt, initially by Kerrich et al. (EPSL, 1998), firmly established, however, that the igneous suite was present in Archean terranes. The recent identification of low-metamorphic grade boninites in the Murchison Domain, which preserve distinctive boninites textures as well as chemical compositions, has brought the Yilgarn Craton boninite story full-circle. A survey of the comparatively sparse geochemical data for volcanic rocks across the Murchison indicates that mafic-intermediate rocks (SiO 2 ~ 48 – 55 wt%; MgO ~ 14 – 6 wt %) with low TiO 2 (~ 0.3 – 0.7 wt.%), high Al 2 O 3 /TiO 2 (30 – 60) and low Zr/Y (1.4 – 3.0) are widespread. Where REE data are available, the rocks are commonly characterized by depletion of the MREE versus ...
Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma c... more Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma chambers. These magmas are originally sourced from the mantle, likely via plume activity, but may interact with the overriding lithosphere during ascent and emplacement in the crust. The magma injections lead to the establishment of different layers and zones with complex macroscopic, microscopic and cryptic compositional layering through magmatic differentiation and associated cumulate formation, sometimes accompanied by crustal assimilation. These complex mineralogical and petrological processes obscure the nature of the mantle sources of LMI, and typically have limited the degree to which parental liquids can be fully characterised. Here, we present Lu–Hf isotope data for samples from distinct layers of the Upper Zone of the Windimurra Igneous Complex (WIC), an immense late-Archean LMI in the West Australian Yilgarn Craton. Lu–Hf isotope systematics of whole rocks are well correlated (...
Evidence for a multi-stage history of uplift and metasomatism is preserved in a suite of highly c... more Evidence for a multi-stage history of uplift and metasomatism is preserved in a suite of highly chromian, garnet-rich peridotites found as xenoliths in Newlands and Bobbejaan kimberlites, South Africa. A high proportion of the garnets and chromites in these rocks plot in the diamond facies fields on Cr2O3-CaO and Cr2O3-MgO wt. % plots respectively. Petrographic evidence suggests that the earliest known mineralogy is a garnet-olivine rich rock with granuloblastic texture. A down-P-T event caused exsolution of spinel and pyroxenes in garnet and these inclusion textures have subsequently been partially annealed in most samples. Harzburgitic samples have garnets with inclusions of serpentine ± chromite and in lherzolitic samples, cpx is also present. Most of the garnets have strongly developed, diffusion-controlled, major element zonation patterns which are a result of: (1) External reequilibration where diffusion is towards the matrix. (2) Internal re-equilibration where diffusion is t...
Newlands and Bobbejaan kimberlites, South Africa, contain a suite of highly chromian, garnet-rich... more Newlands and Bobbejaan kimberlites, South Africa, contain a suite of highly chromian, garnet-rich peridotites amongst their xenolith population. A high proportion of garnets and chromites in these rocks plot in the diamond facies fields on Cr2O3-CaO (Gurney, 1984; Grutter et al., 2004) and Cr2O3-MgO wt. % (Fipke et al., 1995) plots respectively. Many samples have garnets with inclusions of serpentine ± chromite (+ clinopyroxene in lherzolitic samples) whose arrangement is sometimes indicative of exsolution or annealed exsolution textures. Modal reconstructions reveal that the original crystals were garnets of higher chromium content. A particularly strong chromite-clinopyroxene association is seen in the matrix and included within garnet in most lherzolitic samples; a feature not reported in other garnet lherzolites. Most of the garnets have strongly developed zonation patterns which are a result of diffusion towards the matrix (external zonation) followed by zonation towards inclus...
Following the recognition that Archean siliceous high-magnesian basalts of the Yilgarn Craton wer... more Following the recognition that Archean siliceous high-magnesian basalts of the Yilgarn Craton were komatiitic magmas that had undergone crustal assimilation, the idea of Archean boninites fell out of general favour. Studies of boninitic rocks in the Abitibi belt, initially by Kerrich et al. (EPSL, 1998), firmly established, however, that the igneous suite was present in Archean terranes. The recent identification of low-metamorphic grade boninites in the Murchison Domain, which preserve distinctive boninites textures as well as chemical compositions, has brought the Yilgarn Craton boninite story full-circle. A survey of the comparatively sparse geochemical data for volcanic rocks across the Murchison indicates that mafic-intermediate rocks (SiO 2 ~ 48 – 55 wt%; MgO ~ 14 – 6 wt %) with low TiO 2 (~ 0.3 – 0.7 wt.%), high Al 2 O 3 /TiO 2 (30 – 60) and low Zr/Y (1.4 – 3.0) are widespread. Where REE data are available, the rocks are commonly characterized by depletion of the MREE versus ...
Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma c... more Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma chambers. These magmas are originally sourced from the mantle, likely via plume activity, but may interact with the overriding lithosphere during ascent and emplacement in the crust. The magma injections lead to the establishment of different layers and zones with complex macroscopic, microscopic and cryptic compositional layering through magmatic differentiation and associated cumulate formation, sometimes accompanied by crustal assimilation. These complex mineralogical and petrological processes obscure the nature of the mantle sources of LMI, and typically have limited the degree to which parental liquids can be fully characterised. Here, we present Lu–Hf isotope data for samples from distinct layers of the Upper Zone of the Windimurra Igneous Complex (WIC), an immense late-Archean LMI in the West Australian Yilgarn Craton. Lu–Hf isotope systematics of whole rocks are well correlated (...
ABSTRACT The Archean Yilgarn Craton of Western Australia represents a world-class metallogenic pr... more ABSTRACT The Archean Yilgarn Craton of Western Australia represents a world-class metallogenic province, hosting considerable resources of Au, Ni-sulfides and iron ore. Despite close geological similarities with the volcanic-hosted massive sulfide (VHMS)-rich Superior Province of Canada, there is a strong disparity in the number of discovered VHMS deposits between the two areas. This paper brings together recently published U-Pb zircon geochronology and stratigraphic constraints from across the Yilgarn Craton, with a large number of existing whole rock geochemical datasets (881 samples from ∼125 localities). Recognized VHMS occurrences are placed in a detailed tectonic and stratigraphic framework. Temporal and geochemical associations to mineralization are discussed. Areas of VHMS mineralization in the Yilgarn Craton are preferentially associated with areas of thinned, juvenile crust as revealed through regional (Nd, Pb) isotope variations. The characteristics identified here for prospective host sequences are: largely bimodal volcanic complexes, synvolcanic faults, a spatial and temporal association to HFSE-enriched synvolcanic intrusions, and the following geochemical signatures of felsic rocks. VHMS-bearing felsic rocks in the Youanmi Terrane and Eastern Goldfields are characterized by low Zr/Y, La/YbCN and Th/Yb ratios, high Sc/TiO2, Sc/V, HFSE and HREE contents, and flat HREE profiles similar to those of Abitibi greenstone belt, Canada, and the Pilbara Craton of Western Australia. Chondrite-normalized REE profiles for felsic rocks overlying 2.82-2.80 Ga plume-related basalts and large igneous complexes of the Youanmi Terrane are flat. Other VHMS-bearing felsic rocks are characterized by slight LREE enrichment (La/SmCN <3) and flattish HREE profiles. In the Youanmi Terrane four distinct periods of economic mineralization can be recognized: (i) >2.9 Ga (e.g. Golden Grove, Ravensthorpe), associated with early bimodal-mafic greenstone belts subjected to extension; (ii) c. 2815 Ma (e.g. Austin, Just Desserts, Youanmi), following a major plume event and coeval with the emplacement of large igneous complexes across the northern Youanmi Terrane at shallow levels in the crust (Meeline and Boodanoo suites); (iii) 2760-2745 Ma (e.g. Hollandaire, Mt. Mulcahy) in areas of rift-related magmatism during the deposition of the Greensleeves Formation; (iv) c. 2725 Ma, in the Gum Creek greenstone belt associated with a second major plume event, broadly coeval with the emplacement of high-level sills of the Yalgowra Suite. In the Eastern Goldfields Superterrane, VHMS mineralization formed between c. 2700 and 2680 Ma (e.g. Teutonic Bore camp, Anaconda, Nimbus, Erayinia). All episodes of VHMS mineralization in the Yilgarn show strong temporal and spatial associations to suites of HFSE-enriched granitic intrusions (e.g. Eelya suite, Mt. Kenneth suite, Kookynie supersuite).
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