The Thomson Orogen project
The Thomson Orogen is a major structural element of Queensland geology. The vast majority of the Thomson Orogen (approximately 800,000km2) is buried by sedimentary cover of varying thickness throughout Central Queensland and South West Queensland, so its geology is poorly understood.
Current knowledge of the orogen comes from approximately 22,000km2 of outcropping rock occurring predominantly between Greenvale and Anakie in central-eastern Queensland. These areas have significant mineral deposits such as Thalanga (Zn-Cu-Pb-Ag-Au), Charters Towers (Au) and Greenvale (Ni-Co-Sc), so there is potential for greenfields mineral discoveries and geothermal energy resources in the more extensive undercover area. This undercover area is co-incident with a major temperature anomaly.
The Thomson Orogen project is a collaboration between the Geological Survey of Queensland (GSQ), the Geological Survey of New South Wales, Geoscience Australia and several universities.
The objectives are to:
- develop a better understanding of eastern Australia's geological development - in terms of lithological distribution, age ranges, deformation and metamorphic history, and tectonic setting of subsurface elements like the Thomson Orogen
- identify prospective areas for greenfields mineral exploration beneath shallow cover and reduce the risk of undercover greenfields exploration.
A variety of methodologies are used to source data, including logging of the core and cuttings, and geochronological, geochemical and geophysical techniques.
Logging of core and cuttings
Direct knowledge of the undercover Thomson Orogen is restricted to ~1400 drillholes; ~200 of those are cored (Figure 1). Logging of the core and cuttings has been used to produce a basement lithology map (Figure 2), which shows a dominance of metasedimentary rocks and the location of granitic batholiths. It also shows that significant known mineralisation in the Thomson Orogen is restricted to the outcropping rocks. Drill intersection data has been compiled to produce a depth to basement map (Figure 3).
Figure 1: Drill core showing grey slaty rocks of the Thomson Orogen in contact with overlying sedimentary rock of the Eromanga basin
Figure 2: Map showing basement lithology
Figure 3: Map showing depth to basement
Geochronological techniques used in the project include:
- U-Pb (SHRIMP) dating of detrital and magmatic zircon for high temperatures (~900oC)
- (U-Th)/He dating of zircon for low temperatures (~200oC)
- Ar-Ar dating of mica for moderate temperatures (~300oC).
These multiple dating techniques constrain the timing of key events within the Thomson Orogen, from deposition to metamorphism and uplift. Assessment of provenance signatures from U-Pb (SHRIMP) zircon dating also enables regional correlation with comparable regions in Central Australia, South-East Australia and Antarctica. We are currently targeting U-Pb (SHRIMP) zircon dating of mineralising events from outcropping rocks. The U-Pb (SHRIMP) zircon dating program is run in conjunction with Geoscience Australia, as part of the National Geoscience Agreement.
Figure 4: Typical detrital zircon age spectra from the undercover Thomson Orogen (GSQ Eromanga 1)
Geochemical techniques used in the project include:
- major and trace element analyses
- Sm-Nd isotope analyses of whole rocks
- Lu-Hf and O isotope analysis of detrital zircon.
The resulting data is used to identify genetic links between units and potential sources and mechanisms of melt generation. Identifying and correlating units with known mineralising events also enables more opportunities for exploration.
The Thomson Orogen is almost entirely covered by high-resolution gravity, magnetic and radiometric data. We are using this data to interpret rock types and structures throughout the subsurface area (Figure 5), which enables us to extrapolate the interpretation of the adjoining areas across the border from the Geological Survey of New South Wales.
Figure 5: Basement interpretation based on magnetics in the southern Thomson Orogen
The work completed to date has inspired a three-year extended collaborative project between GSQ, Geoscience Australia and the Geological Survey of New South Wales. As part of this collaboration, an airborne electromagnetic survey and selected gravity and magnetotelluric profiles have been undertaken by Geoscience Australia to refine knowledge of basement depth and character, in preparation for a reconnaissance drilling program planned for 2016.
You can view the compilation of drillhole data with depth to basement, basement lithology and a summary of previous work in Queensland Geological Record 2012/06 available from QDEX Reports.
You can view the literature review and synthesis of the geology, mineralisation, geothermal potential and tectonics of the Thomson Orogen and its relation to adjacent terranes in Queensland Geological Record 2013/01 available from QDEX Reports.
We are currently working on integrating new age data, geological observations (from drill core and outcrop in Queensland and New South Wales), geochemistry and geophysics into a working model of the Thomson Orogen and Tasmanides.