Geophysical anomalies and quartz deformation of the Warburton West structure, central Australia

This paper reports geophysical anomalies and intra-crystalline quartz lamellae in drill cores from the Warburton West Basin overlapping the border of South Australia and the Northern Territory. The pre-Upper Carboniferous ~450×300km-large Warburton Basin, north-eastern South Australia, is marked by distinct eastern and western magnetic, gravity and low-velocity seismic tomography anomalies. Quartz grains from arenite core samples contain intra-crystalline lamellae in carbonate-quartz veins and in clastic grains, similar to those reported earlier from arenites, volcanic rocks and granites from the Warburton East Basin. Universal Stage measurements of quartz lamellae in both sub-basins define Miller-Bravais indices of {10-12} and {10-13}. In-situ quartz lamellae occur only in pre-Late Carboniferous rocks whereas lamellae-bearing clastic quartz grains occur in both pre-Late Carboniferous and post-Late Carboniferous rocks - the latter likely redeposited from the pre-Late Carboniferous basement. Quartz lamellae in clastic quartz grains are mostly curved and bent either due to tectonic deformation or to re-deformation of impact-generated planar features during crustal rebound or/and post-impact tectonic deformation. Seismic tomography low-velocity anomalies in both Warburton West Basin and Warburton East Basin suggest fracturing of the crust to depths of more than 20km. Geophysical modelling of the Cooper Basin, which overlies the eastern Warburton East Basin, suggests existence of a body of high-density (~2.9-3.0gr/cm) and high magnetic susceptibility (SI~0.012-0.037) at a depth of ~6-10km at the centre of the anomalies. In the Warburton West Basin a large magnetic body of SI=0.030 is modelled below ~10km, with a large positive gravity anomaly offset to the north of the magnetic anomaly. In both the Warburton East and Warburton West the deep crustal fracturing suggested by the low velocity seismic tomography complicates interpretations of the gravity data. Universal Stage measurements of quartz lamellae suggest presence of both planar deformation features of shock metamorphic derivation and deformed planar lamella. The latter may be attributed either to re-deformation of impact-generated lamella, impact rebound deformation or/and post impact tectonic deformation. The magnetic anomalies in the Warburton East and West sub-basins are interpreted in terms of (1) presence of deep seated central mafic bodies; (2) deep crustal fracturing and (3) removal of Devonian and Carboniferous strata associated with rebound of a central uplift consequent on large asteroid impact. Further tests of the Warburton structures require deep crustal seismic transects

Carbon dioxide bearing melt inclusions within a gold-mineralized felsic granite

Disseminated gold mineralization at Timbarra forms flat-lying ore bodies constrained beneath a fine-grained carapace within the core of a zoned 249-252 Ma I-type granite. Emplacement at mesozonal lev¬els (~7km) resulted in the development of a late stage volatile bearing (carbon dioxide-rich, chlorine-poor) magmatic fluid that was constrained by lithostatic conditions beneath a fine-grained 'quenched' carapace. The magmatic-hydrothennal fluids that ponded below the carapace formed unidirectional solidification textures (USTs), pegmatite lenses, miarolitic cavities, interconnected miarolitic cavities (IMTs) and vein-dykes. These variably gold-mineralized magmatic-hydrothennal transition textures contain carbon dioxide-bearing melt in¬clusions and low-salinity carbon dioxide-rich aqueous inclusions, whereas moderate to high-salinity aqueous inclusions are completely absent. The detection of carbon dioxide within melt inclusions using laser Raman spectroscopy is a significant observation supporting a magmatic origin for carbon dioxide in intrusion-related gold deposits

Geophysical anomalies and quartz deformation of the Warburton West structure, central Australia

This paper reports geophysical anomalies and intra-crystalline quartz lamellae in drill cores from the Warburton West Basin overlapping the border of South Australia and the Northern Territory. The pre-Upper Carboniferous ~450 × 300 km-large Warburton Basin, north-eastern South Australia, is marked by distinct eastern and western magnetic, gravity and low-velocity seismic tomography anomalies. Quartz grains from arenite core samples contain intra-crystalline lamellae in carbonate–quartz veins and in clastic grains, similar to those reported earlier from arenites, volcanic rocks and granites from the Warburton East Basin. Universal Stage measurements of quartz lamellae in both sub-basins define Miller–Bravais indices of {10–12} and {10–13}. In-situ quartz lamellae occur only in pre-Late Carboniferous rocks whereas lamellae-bearing clastic quartz grains occur in both pre-Late Carboniferous and post-Late Carboniferous rocks — the latter likely redeposited from the pre-Late Carboniferous basement. Quartz lamellae in clastic quartz grains are mostly curved and bent either due to tectonic deformation or to re-deformation of impact-generated planar features during crustal rebound or/and post-impact tectonic deformation. Seismic tomography low-velocity anomalies in both Warburton West Basin and Warburton East Basin suggest fracturing of the crust to depths of more than 20 km. Geophysical modelling of the Cooper Basin, which overlies the eastern Warburton East Basin, suggests existence of a body of high-density (~2.9–3.0 gr/ cm3 ) and high magnetic susceptibility (SI ~ 0.012–0.037) at a depth of ~6–10 km at the centre of the anomalies. In the Warburton West Basin a large magnetic body of SI= 0.030 is modelled below ~10 km, with a large positive gravity anomaly offset to the north of the magnetic anomaly. In both the Warburton East and Warburton West the deep crustal fracturing suggested by the low velocity seismic tomography complicates interpretations of the gravity data. Universal Stage measurements of quartz lamellae suggest presence of both planar deformation features of shock metamorphic derivation and deformed planar lamella. The latter may be attributed either to redeformation of impact-generated lamella, impact rebound deformation or/and post impact tectonic deformation. The magnetic anomalies in the Warburton East and West sub-basins are interpreted in terms of (1) presence of deep seated central mafic bodies; (2) deep crustal fracturing and (3) removal of Devonian and Carboniferous strata associated with rebound of a central uplift consequent on large asteroid impact. Further tests of the Warburton structures require deep crustal seismic transects

Microanalysis of regional fluids from the eastern succession of the Mount Isa Block, NW Queensland

Fluid plays a key role in the formation of giant iron-oxide-copper-gold (IOCG) deposits according to either fluid mixing or unmixing models (e.g. Barton & Johnson 1996, Pollard 2001). Our previous work shows some evidence for fluid mixing processes in the formation of both regional alteration assemblages and deposits from the Mount Isa Eastern Succession, NW Queensland, Australia (Fu et a1. 2003). In this study, we investigated aqueous inclusions in regional qUaliz veins associated with albitisation using state-of-the-art microanalytical techniques

Fluid bubbles in melt inclusions and pillow-rim glasses: high-temperature precursors to hydrothermal fluid?

Hypotheses for the formation of many types of hydrothermal ore deposits often involve the direct contribution of magma-related fluids (e.g., Cu–Mo–Au porphyries) or their superimposition on barren hydrothermal cells (e.g., volcanic-hosted massive sulfide deposits). However, the chemical and phase compositions of such fluids remain largely unknown. We report preliminary results of a comprehensive study of fluid bubbles trapped inside glassy melt inclusions in primitive olivine phenocrysts and pillow-rim glasses from basaltic magmas from different tectonic environments, including mid-ocean ridges (Macquarie Island, SW Pacific and Mid-Atlantic Ridge 43°N Fracture Zone), ocean islands (Hawaii) and a variety of modern and ancient backarc–island arc settings (eastern Manus Basin, Okinawa and Vanuatu Troughs, Troodos, New Caledonia and Hunter Ridge–Hunter Fracture Zone). Fluid bubbles from all localities, studied using electron microscopy with EDS and laser Raman spectroscopy, are composed of CO2-(±H2O±sulfur)-bearing vapor and contain significant amounts of amorphous (Na–K–Ca–Fe alumino-silicates and dissorded carbon) and crystalline phases. The crystals are represented mainly by carbonates (magnesite, calcite, ankerite, dolomite, siderite, nahcolite and rhodochrosite), sulfates (anhydrite, gypsum, barite and anglesite), and sulfides (pyrite, arsenopyrite, chalcopyrite and marcasite), though other minerals (brukite, apatite, halite, clinoenstatite, kalsilite, nepheline, amphibole and mica) may occur as well. We argue that chemical components (e.g., C, H, S, Cl, Si, Al, Na, K, Fe, Mn, Cr, Ca, Mg, Ba, Pb and Cu) that later formed precipitates in fluid bubbles were originally dissolved in the magmatic fluid, and were not supplied by host glasses or phenocrysts after entrapment. Magma-related fluid rich in dissolved metals and other non-volatile elements may be a potential precursor to ore-forming solutions