Paul A. Morris

Warakurna large igneous province: A new Mesoproterozoic large igneous province in west-central Australia

Coeval mafic igneous rocks emplaced rapidly over;1.5 3 10 6 km 2 in western and central Australia represent the erosional rem- nants of a late Mesoproterozoic large igneous province, named here the Warakurna large igneous province. SHRIMP U-Pb dating of rocks separated by as much as 1500 km indicates that the main episode of magmatism occurred between 1078 and ca. 1070 Ma. The Warakurna large igneous province includes layered mafic- ultramafic intrusions and mafic to felsic volcanic rocks and dikes in central Australia, a 1000-km-long mafic sill province in Western Australia, and several swarms of mafic dikes. The large areal ex- tent and short duration imply emplacement above a mantle-plume head. Despite their wide separation, the mafic rocks have similar mid-oceanic-ridge basalt-normalized trace element patterns and rare earth element characteristics. West-directed paleocurrents, westward-radiating dike swarms, and the occurrence of high-Mg rocks indicate that the center of the plume head was located be- neath central Australia. Other late Mesoproterozoic large igneous provinces, in the Laurentia and Kalahari cratons, appear to be significantly older than the Warakurna large igneous province in Australia and thus are unlikely to be related to the same mantle- plume head.

Geochemical mapping of the deeply weathered western Yilgarn Craton of Western Australia, using laterite geochemistry

Multi-element analysis of ferruginous nodules and pisoliths from lateritic residuum, derived lag and ferruginous gravel, selected from locally derived colluvium (laterite) sampled at an approximate 9 km interval (triangular grid) over the western Yilgarn Craton, shows regional geochemical trends, major lithologies and dispersion halos around significant bedrock mineralization. The sample density (one sample per 60–100 km2, depending on sample availability) and extent of the coverage (c. 400 000 km2, including large unsampled areas in drainage) mean that the data are potentially valuable for both exploration and environmental purposes. More than 3100 samples have been analysed for 53 elements by XRF, ICP-AES and ICP-MS, with selected samples also analysed for PGE. Elevated Au abundances in the NE of the survey area not only cluster around known gold deposits but extend beyond them, indicating the likelihood of more widespread mineralization in these areas. A chalcophile element index illustrates potential for Au and base metal mineralization in the westernmost part of the Yilgarn Craton, whereas a pegmatophile index shows a regional NW trend parallel to regional structures. Abundant chromium in granite-dominated areas might indicate mafic-ultramafic remnants (some with anomalous Au) beyond known greenstone belts. A newly discovered regional Hg anomaly trends NW for more than 500 km. Anomalous As, Bi, Mo and Sb along the southern margin of the craton may be related to Au mineralization. The spatial and geochemical consistency of the dataset means that it is well suited to multivariate statistics.

Proterozoic mineralization identified by integrated regional regolith geochemistry, geophysics and bedrock mapping in Western Australia

The advantages of integrating datasets in the search for mineralization in areas characterized by subdued relief, extensive areas of transported regolith, and a semi-arid climate, are shown using direct observations (e.g. bedrock and regolith mapping, regolith geochemistry) and remotely-sensed data (e.g. airborne magnetics, gravity, Landsat TM) over a 48 000 km2 area in central Western Australia. In this area, Archaean granite and greenstones are unconformably overlain by a 5 km-thick sequence of Paleoproterozoic (c. < 1840 Ma) and Mesoproterozoic (c. 1200 Ma) sedimentary rocks, both of which are intruded by c. 1070–1400 Ma dolerites. Known mineralization comprises small lode gold deposits hosted by Archaean greenstones (e.g. <0.25 Mt @ ∼4 gt−1) and MVT-type mineralization in Paleoproterozoic stromatolitic carbonate rocks. Outcrop is sparse, with transported regolith covering about 85% of the area. Regolith sampling and subsequent multi-element analysis has been carried out on a 4 × 4 km sampling grid over the whole area, and simultaneous measurement of gravity at each regolith sampling site has been carried out in the eastern two-thirds of the area. Integration of these data with existing gravity data, airborne magnetics, Landsat TM, and detailed bedrock mapping of selected exposures has identified seven potential areas of mineralization, including structurally controlled gold and base metal deposits associated with regional deformation, stratiform MnO and base metal mineralization, and magmatic sulfide mineralization associated with the intrusion of thick mafic sill complexes. Although regional regolith chemistry is capable of identifying all seven areas of mineralization, detailed bedrock mapping and geophysics are essential to understand the style of mineralization and to put the mineralization into a tectonic framework. The preferred tectonic model comprises the deposition of siliciclastic and chemical sedimentary rocks on a passive continental margin, followed by regional deformation, and emplacement of high-level mafic sills and dykes.

Regolith geochemical mapping as an adjunct to geological mapping and exploration; examples from three contiguous Proterozoic basins in Western Australia

Abstract Regolith and regolith-geochemical mapping of the Proterozoic Padbury, Bryah and Yerrida basins of central Western Australia can be used to define the extent of the underlying bedrock and indicate areas of potential mineralisation. The three basins cover approximately 20,000 km 2 , more than 90% of which is covered by regolith, dominated by colluvium/sheetwash. About 30% of the area lacks well-defined drainage and, as the climate is arid, most sediment is transported during cyclonic flooding. Soils are skeletal, and colluvially modified, except in areas of aeolian sand. Most stream sediment and sheetwash samples retain some components of the bedrock from which they were derived, apart from sediments found in major (but ephemeral) river channels on plains and aeolian sands. Prolonged exposure over many millions of years has resulted in deep chemical weathering; the weathering products are currently subject to mechanical erosion, and transport by flood or wind. Alkali and alkaline earth elements are leached at the fresh rock–saprock interface, and are in part redeposited in valleys, calcrete and saline playas. Though base metals are variably leached, many elements of economic interest are retained in coarse-grained altered lithic fragments and iron-rich debris, whereas rare earth elements, Th, Zn and Zr are concentrated in the fine fractions. In several instances regolith composition can be used to delineate the extent of, or sub-divide bedrock units. Regolith over a black sulphidic shale in the Yerrida Basin shows that unit to have three subfacies. Regolith derived from mafic rocks has higher CaO, Ga, Sc, and V than that over nearby sedimentary rocks, whilst different types of mafic units can be separated using MgO, Cr, Cu, and Ni. Regolith in the Padbury Basin is more Fe-rich and characterised by higher concentrations of MnO, P 2 O 5 , As, Ba, F, Pb and Th, except in one area of lower MnO and F. Variations in the K 2 O, Ba and Sr contents of regolith, relate to different granitic sources to the underlying sedimentary rocks. Statistical treatment of regolith-chemical data suggests areas of mis-identified bedrock and, using normalised additive indices, highlights mineralised areas and points to other prospective areas.

Fine fraction regolith chemistry from the East Wongatha area, Western Australia: tracing bedrock and mineralization through thick cover

The fine fraction (<50 μm; ‘silt – clay’) of regolith from an area dominated by thick (up to 120 m) transported regolith is shown to be a viable sampling medium for detecting bedrock and bedrock-hosted mineralization. A partial digest of the fine fraction of regolith reveals that elements which have migrated vertically from bedrock have become sequestered at a shallow depth in regolith. Nickel, Cr, Li, Mn, V, Co and Rb are useful in tracing the extent of different bedrock types, with Ni, Cr, V and Co particularly useful for identifying more mafic bedrock. Higher concentrations of Au (maximum 29 ppb; median 2 ppb; n = 835) are spatially related to either Archean greenstones or unconformity-related uranium – base metal – gold mineralization. Although some samples with anomalous Au concentrations are carbonate-rich, there is no consistent relationship between elevated Au concentrations and the carbonate content of regolith. Deionized water digestion of the <50-μm fraction of 50 samples correlates with aqua regia data for several elements (particularly Au, Ni), suggesting that some mineralization- or bedrock-related elements in the fine fraction of regolith are either labile or microparticulate.

The effect of sample medium on regolith chemistry over greenstone belts in the northern Eastern Goldfields of Western Australia

Gold and Ni concentrations in regolith samples collected from a 16 km2 grid over Archaean granite–greenstones of the Yilgarn Craton show a strong correlation with lithology and areas of known mineralization, even though most samples are from transported regolith. The influence of sample medium on regolith composition has been assessed by statistical comparison of stream sediment, soil and sheetwash data from four of the largest greenstone belts. Changes in composition according to sample medium can be explained by periodic recharge of stream sediments by fresh or ferruginized bedrock, and different degrees of weathering and sorting during transport, with enrichment or depletion in groups of analytes showing the progressive breakdown of rock-forming minerals. The behaviour of Ti, La, Ce, Cr, Nb and Zr is consistent with the concentration of resistate minerals such as allanite, monazite, chromite, rutile and zircon, due to the combined effects of weathering of less stable phases and mechanical sorting. A statistical assessment shows that comparison of regolith composition between greenstone belts must be made using the same sample medium type. Furthermore, this approach can be used to assess the appropriateness of different sample media in regional geochemical surveys, and to refine analytical programs.

Large igneous provinces in Western Australia: Implications for Ni-Cu and Platinum Group Elements (PGE) mineralization

Large igneous provinces in Western Australia that may have potential for economic Ni-Cu-PGE sulfide ore deposits include the 2.7 Ga Fortescue Group in the Pilbara Craton, the 1.8 Ga Hart-Carson volcanics and sills in the Kimberley Basin, the 1.07 Ga sill complexes and layered intrusions of the Warakurna province in central and Western Australia and the 0.5 Ga Kalkarinji continental flood basalts (in Western and northern Australia).

The geochemistry and petrogenesis of Carnley Volcano, Auckland Islands, SW Pacific

ABSTRACT Intraplate volcanism across Zealandia, South Eastern Australia, the Ross Sea Embayment and Marie Byrd Land in Antarctica define a magmatic province characterised by basalts with elevated 206Pb/204Pb (18.9–22.5), 87Sr/86Sr = ∼0.7035, Light Rare Earth enrichment [(Ce/Yb)n > 10], and convex-up mantle normalised incompatible multi-element patterns, peaking at Nb-Ta, with negative K and Pb anomalies. Trace element abundances and ratios (e.g. Zr/Nb, Y/Zr) resemble Ocean Island Basalts (OIB), distinct from Mid-Ocean Ridge Basalt (MORB), suggesting derivation from OIB-like reservoirs. Our preferred scenario envisages partial melting across the garnet-spinel stability fields involving asthenospheric and lithospheric mantle components. Melts accumulate in a column where the deep (asthenospheric) source is PM and the shallower source a melange of PM and subcontinental lithospheric mantle (DMM+1) enriched by carbonatite. Evolution of primary and near-primary magmas is controlled by olivine + clinopyroxene fractionation. Trachybasalts, trachytes and rhyolites show isotopic evidence for interaction with continental crust.

Combining geochemistry and geochronology of transported regolith to reveal bedrock-hosted mineralization in the arid east Wongatha area of south central Western Australia

Metal anomalies in transported regolith that overlie bedrock-hosted mineralization indicate that a component of mineralization (the exogenic component) can migrate through regolith. In the east Wongatha area of Western Australia, the exogenic component in the fine fraction of sandplain deposits is spatially linked to known and/or inferred bedrock-hosted Au mineralization. In three regolith profiles, the concentration of Au in aqua regia and deionized water Au are correlated, and in two of the profiles Au varies in concentration independent of changes in regolith composition. In one profile, the Au concentration in chemically-mature regolith dominated by quartz sand decreases from 31 ppb at c. 180 cm depth to 7.3 ppb at 15 cm. Optically stimulated luminescence (OSL) ages of stratigraphically-controlled regolith samples ranging from 166.9 ± 46.6 ka to 5.4 ± 1.1 ka show a strong correlation with depth (r2 = 0.99) over a three-metre interval, indicating a sandplain accumulation rate of c. 17 mm/1000 years. The decrease in Au concentration in east Wongatha regolith can be related to the migration rate of the exogenic component and the rate of sandplain accumulation. Supplementary material: Screening and analytical data for the < 50-µm fraction of regolith from three profile sites in the east Wongatha area are available at https://doi.org/10.6084/m9.figshare.c.4074413

Integrating petrogenesis and weathering to understand regolith chemistry: examples from the Palaeoproterozoic Carson Volcanics, north Kimberley, Western Australia

The rugged topography of the Kimberley area of northern Western Australia preserves examples of the complete weathering cycle to produce regolith. With increasing weathering intensity, Palaeoproterozoic mafic volcanic rocks of the Carson Volcanics are progressively depleted in low field strength element (LFSE) and rare earth elements (REE), and enriched in high-field strength elements (HFSE), Th, Cr and chalcophile elements. This reflects the breakdown of the main rock-forming minerals, the concentration of resistate phases such as zircon, chromite, monazite and rutile, and the development of secondary clay minerals, oxides and oxyhydroxides. Some regolith samples at the base of the volcanic succession have high Cr, As and detectable Au and Pd. This regolith chemistry reflects not only the intensity of weathering, but also the less fractionated and weakly mineralized nature of flow units erupted early in the volcanic cycle. Elsewhere in the Carson Volcanics, two regolith samples with unusually high concentrations of Ba, REE and Zn are located close to regional structures, which acted as conduits along which hydrothermal fluids migrated resulting in surface alteration, probably after regolith formation. Both examples show that regolith chemistry should be interpreted in the context of both weathering and petrogenetic processes that affect bedrock composition.