SummaryAirborne electromagnetic data generated by the AusAEM Survey are shown to map mineral deposit host rocks and regional geological features within the AusAEM Survey area. We have developed new functionality in Geoscience Australia’s sample-by-sample layered earth inversion algorithm, allowing inversion of the magnitude of the combined vector sum of the X- and Z-components of TEMPEST AEM data. This functionality improves the clarity of inverted interpretation products by reducing the degree of along-line incoherency inherent to stitched 1D inversions. The new inversion approach improves the interpretability of sub-horizontal conductors, allowing better mapping of geological features under cover.Examples of geological mapping by the AusAEM survey highlight the utility of wide line spacing, regional AEM surveying to improve geological, mineral systems and groundwater resource understanding in the regions flanking outcropping mineral deposit host rocks in northern Australia.
Introduction The $2.67 m Frome airborne electromagnetic (AEM) survey was flown between May and November 2010 as a joint survey between Geoscience Australia (GA), the Geological Survey of South Australia (PIRSA, now DMITRE) and industry partners. The survey includes 2.5 km and 5 km spaced east-west flight lines across the Murray Basin, Frome Embayment and Strzelecki Desert areas of South Australia (Figure 1) using the Fugro TEMPEST TM AEM system. The survey data were inverted using the Geoscience Australia Layered Earth Inversion (GA LEI; Brodie and Sambridge 2006) and a series of data products including grids and sections were produced. The wide line spacing allows data to be gridded with 500 m cell sizes, providing a regional view of electrical conductivity of the ground, but also provides along-line detail in conductivity sections, allowing detailed interpretations of the near-surface to a depth of up to 400 m where conductivity conditions are ideal. A detailed volume of interpretations and implications from the survey will be published in 2012 (Roach, in prep.). The survey was flown with the intention of reducing risk for mineral explorers by showing areas where uranium systems could be interpreted and where detailed AEM and ground EM surveys could be performed, but the data are also capable of being interpreted for groundwater, structural and landscape evolution studies, as discussed here.
Outcrops of leucitite lavas occur as scattered remnants up to 40 m thick in the El Capitan area, northeast of Cobar in western New South Wales. Two eruption sites have been located for these lavas. Preserved volcanic features indicate that the lavas were erupted on to a relatively low-relief, Early Miocene land surface, flowed along a shallow valley and underwent inflation. Geochemical analyses of the leucitites indicate only limited fractionation. Remnant outcrops of the leucitite lavas represent a very important time marker in the geomorphic history of the Cobar region, preserving evidence of both Early Miocene and post-Early Miocene landscape evolution and weathering conditions. A deep-weathering profile, similar to those common throughout the region and characterised by a ferruginous mottled zone and underlying bleached saprolite, is preserved beneath a dissected flow at one eruption site. Other deposits beneath the leucitite flows include baked soils, silcretes, and quartz-rich gravels and grits. Palaeomagnetic dating of the upper part of the deep-weathering profile indicates an Early to Middle Miocene age for hematite fixation. A new 40Ar/39Ar age on the volcanic plug at this site (17.14 ± 0.20 Ma, 2σ) refines the minimum eruption age for the leucitites and supports an Early Miocene age for ferruginisation of the deep-weathering profile. Topographic inversion of the basal contact of one of the leucitite flows indicates an average minimium erosion rate for the area of 1 m per million years. Weathering profiles on the leucitites are thin and lack significant ferruginisation or chemical leaching, indicating that post-Early Miocene weathering in the region has been very limited. These profiles also contain a significant aeolian component including abundant quartz dust.
The Southern Thomson Project aims to advance understanding of tectonic history and mineral prospectivity of basement rocks of the southern Thomson Orogen beneath extensive Mesozoic and Cenozoic cover. The project area in northwestern New South Wales (NSW) and western Queensland is considered underexplored, with poor definition of structural corridors and mineral systems in the Palaeozoic rocks. This collaborative project between Geoscience Australia, the Geological Survey of New South Wales (GSNSW) and the Geological Survey of Queensland is acquiring and interpreting new geoscience data, including geophysical, geochemical, and isotopic investigations. Geophysical data interpretations and models have been tested through a program of stratigraphic drilling to provide basement cores for studies of petrology, geochronology, geochemistry, petrophysics and mineral systems. Downhole geophysical logging of the drillholes also aims to characterise the cover sequences and define depth to basement.Interpretation of regional aeromagnetic and gravity data and synthesis of existing mineral, water-bore and petroleum drillholes created a geological map of basement lithologies and structures. Stratigraphic drilling has targeted areas with no data to constrain lithologies or their ages, with four key areas selected for drilling in far northwestern NSW. Information on the thickness of cover sequences over resistive basement interpreted from AEM inversion models has proven useful during the selection of drilling locations.Seven stratigraphic drillholes were completed within NSW using a combination of mud rotary and diamond drilling. All holes penetrated surface regolith, as well as sedimentary rocks of the Eromanga Basin, to achieve approximately 50 metres of basement core for analyses. Resulting improvements to geological understanding and development of exploration techniques will be a keystone to progress future investigations in the southern Thomson Orogen.
INTRODUCTION This paper describes regolith-landforms and the local landscape evolution in the immediate vicinity of the Hazeldean Plug (Lambert & White 1965), which is an ankaramite volcanic plug assumed to have erupted at ca. 50 ± 2-3 Ma (Roach 1996), within the Monaro Volcanic Province (MVP), 20 km southwest of Cooma. The local basement consists of Ordovician metasediments (Lewis et al. 1994) and granitoids of the Berridale Batholith (White et al. 1977). These have been weathered and stripped and are overlain by quartzose Late Cretaceous to Early Tertiary palaeochannels and lacustrine deposits which have been preserved by the lava pile. The contemporary landscape includes volcanic plains and rises above relatively low relief, undulating granitic-derived landforms with torfields and a hornfelsed Ordovician sediment metamorphic aureole around the Berridale Batholith immediately to the east.
The characterisation of the thickness and geology of cover sequences significantly improves targeting for mineral exploration in buried terrains. Audio-frequency Magnetotelluric (AMT) data is applicable to characterise cover sequences, where their conductivity (inverse resistivity) can be differentiated. We present a regional study from the under-cover East Tennant region in the Northern Territory (Australia) where we have applied deterministic and probabilistic inversion methods to derive 2D and 1D resistivity models. We integrated these models with information of co-located basement penetrating boreholes (lithological and geophysical logs) to ground-truth and validate the models and to improve geophysical interpretations. In the East Tennant region, borehole lithology and wireline logging demonstrate that the modelled AMT response is largely controlled by the mineralogy of the cover and basement rocks. The bulk conductivity is due primarily to bulk mineralogy and the success of using the AMT models to predict cover thickness is shown to be dependent on whether the bulk mineralogy of cover and basement rocks are sufficiently different to provide a detectable conductivity contrast. Our investigation of a range of geological scenarios that differ in thickness, complexity and geology of the cover and basement rocks suggests that in areas where there is sufficient difference in bulk mineralogy and where the stratigraphy is relatively simple, AMT models predict the cover thickness with high certainty. In more complex scenarios interpretation of AMT models may be more ambiguous and requires integration with other data (e.g. drilling, wireline logging, potential field modelling). Overall, we conclude that the application of the method has been validated and the results compare favourably with borehole stratigraphy logs once geological (i.e. bulk mineralogical) complexity is understood. This demonstrates that the method is capable of identifying major litho-stratigraphic units with resistivity contrasts. Our results have assisted with the planning of regional drilling programs and have helped to reduce the uncertainty and risk associated with intersecting targeted stratigraphic units in covered terrains.
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