THE THOMSON OROGEN PROJECT – A WORK IN PROGRESS Greenfield J 1,2 , Reid W 1,2 , Gilmore P 1,2 , Caritat P. de 1,3 , Lech M 3 , Hill S 1,4 , Hulme K 1,4 , Watkins J 1,2 and Worrall L 1,3 1 Cooperative Research Centre for Landscape Environments and Mineral Exploration 2 Geological Survey of NSW, Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310 3 Geoscience Australia, GPO Box 378, Canberra, ACT 2601 4 Department of Geology and Geophysics, The University of Adelaide, SA 5005 SUMMARY The Thomson Orogen, which underlies sediments of the Great Australian Basin in northwest New South Wales, is the last major greenfields terrain in NSW. The Orogen has potential for magmatic arc and ocean- crust related gold and base metal deposits. However, the basin cover presents a significant impediment to exploration. The Thomson Orogen Project aims to develop an effective means of exploring through this cover. A multi-disciplinary approach to achieving this objective has resulted in new geological, geophysical and geochemical datasets being acquired. These datasets include high resolution magnetics and radiometrics and the results of overbank sediment, lag and plant sampling. A number of interpretative products including regolith maps and a depth-to-basement map have also been generated. BACKGROUND AND AIMS OF THE PROJECT In July 2005, CRC LEME, in collaboration with the New South Wales Department of Primary Industries (NSW DPI) and Geoscience Australia (GA), established the Thomson Orogen project. This three year (2005– 2008) project has the primary objective of improving prospectivity in the Thomson Orogen, a poorly known and poorly explored terrain, by developing an effective means of exploring through the cover. This project forms part of the ‘New Frontiers’ initiative by NSW DPI and includes regional mapping, seismic, drilling, gas geochemistry and airborne geophysics. WHAT AND WHERE IS THE THOMSON OROGEN? The Thomson Orogen is one of the most poorly understood orogenic belts in Australia. It covers a vast area, mostly throughout south-central Queensland, but extends into northwestern NSW (Figure 1). Named by Kirkegaard (1974) after the Thomson River in central Queensland, it is part of the greater Tasmanides of eastern Australia. New geochronology data suggest the Thomson Orogen has undergone a history distinct to the Lachlan Orogen (Draper 2005). Neoproterozoic to Middle Cambrian sedimentation and ~500 Ma deformation recognised in the Thomson Orogen is more akin to the Kanmantoo and Adelaide fold belts. A felsic magmatic/volcanic event at ~470 Ma that is not recognised in the Lachlan Orogen, as well as an abrupt change in structural grain at the contact between the orogens, suggests some differences in their early Palaeozoic histories (Draper 2005). The orogens share a similar post-Middle Devonian history, with deformed orogenic rocks unconformably overlain by epicratonic Late Devonian infrabasins. The Thomson Orogen is in turn overlain by the Permian Cooper Basin and Mesozoic Great Australian Basin (which incorporates the Eromanga Basin). THE NSW THOMSON OROGEN PROJECT AREA The NSW portion of the Orogen incorporates the entire southern margin with boundaries against the Delamerian and Lachlan orogens (Figure 1). The exact boundary of the Orogen is unclear and is interpreted from gravity and magnetic signatures, and thus the project area incorporates a buffer zone that takes in the Thomson Orogen margin and parts of the Lachlan and Delamerian orogens (Figure 1). The area of interest for this project covers eleven 1:250 000 sheets: the entire Urisino, Yantabulla, Enngonia, Bourke, Louth, and White Cliffs sheets; and, parts of the Milparinka, Cobham Lake, Walgett and Angledool sheets. The depth to pre-Mesozoic basement in the project area, based on drilling and seismic data, varies from approximately 1050 m in the northwestern corner to 0 m at basement exposures along the southern margin. Over most of the project area, the interpreted depth to basement is less than 250 m (Packham & Jovenski 2001). However, recent interpolation of drillhole data indicates approximately 60% of the orogen is covered by less than 300 m of cover. Regolith 2006 - Consolidation and Dispersion of Ideas 118