Characterising the basement and mineral potential beneath the Murray Basin.
A major new work program on the Delamerian basement to the Cenozoic Murray Basin aims to provide industry with new data and new constraints on the geological framework and mineral prospectivity of this greenfield region. The program is part of the MinEx CRC National Drilling Initiative (NDI), a collaborative geoscience program between the Geological Survey of South Australia (GSSA), other state and territory geological surveys, Geoscience Australia, CSIRO and universities which aims to increase exploration success in frontier regions of Australia (see previous MESA Journal article in April 2019).
The Delamerian Orogen hosts three of South Australia’s recently active, active and emerging mines – the Angus, Kanmantoo, and Bird-in-Hand deposits respectively – but for the last two decades has received only limited research and data acquisition.
Geological background
Figure 1 Spatial extent of the Delamerian Orogen (Raymond et al. 2018), shown over the 1:1 million scale surface geology of Australia map (Raymond et al. 2012).
The Cambrian–Ordovician Delamerian Orogen (514–490 Ma) marks a major shift in the tectonic setting of the Australian continent from a Neoproterozoic passive margin during rifting of the supercontinent Rodinia to an active subduction margin along the eastern margin of the supercontinent Gondwana in the Paleozoic (Rosenbaum 2018). The effects of Delamerian deformation in South Australia are best observed in the Adelaide Fold Belt (Fig. 1), where the Neoproterozoic Adelaide Rift Complex and Cambrian succession of the Kanmantoo Trough were inverted in a west-verging fold thrust belt (Flöttmann et al. 1994). Metamorphism reached highest grade (amphibolite facies) in the southeastern part of the Adelaide Fold Belt (Alias et al. 2002; Offler and Fleming 1968; Sandiford et al. 1990), where deformation was accompanied by bimodal magmatism (Foden et al. 2002). Delamerian deformation also affected the Curnamona Province, the Warburton and Officer basins, and reworked the eastern and northern margins of the Gawler Craton (Preiss 1995).
In the eastern part of the Delamerian Orogen in New South Wales and Victoria, remnants of the volcanic arcs active during the Delamerian Orogen are preserved (Fig. 1) including: the c. 510 Ma Mount Wright Volcanics in the Koonenberry Belt (Greenfield et al. 2011; Johnson, Phillips and Allen 2016); equivalent sequences extending beneath cover in the Loch Lily Kars Belt to the southeast of the Curnamona Province (Baatar et al. 2019; Sharp et al. 2006); and the c. 520–500 Ma Mount Stavely Volcanic Complex in Victoria. However, the nature of the Delamerian basement between the Adelaide Fold Belt and the preserved remnants of these volcanic arcs is poorly understood. This is primarily because the region is covered by up to 600 m of Murray Basin fluviolacustrine–aeolian to marine sediments (Brown and Stephenson 1989; Geoscience Australia 2017). In the vicinity of the Renmark Trough, an additional 2,500 m of sedimentary cover lies between the Murray Basin and Delamerian basement, comprising successions of the Mesozoic Berri Basin, the late Paleozoic Nadda Basin and the middle Paleozoic Darling Basin (Alley 1995; Rogers 1995).
Current understanding
Figure 2 Existing basement-intersecting drillholes and exploration tenure over the South Australian portion of the Murray Basin.
Only a limited number of basement-intersecting drillholes exist in the Murray Basin region with which to characterise the underlying basement (Fig. 2) and these drastically decrease in density eastwards towards the South Australian border. These drillholes reveal that basement is dominated by variably deformed mafic to intermediate volcanic and volcaniclastic rocks, lesser felsic volcanic rocks, and felsic and mafic intrusives presumed to be contemporaneous with the Delamerian Orogeny, as well as metasedimentary and metavolcanic rocks correlated with the Moralana Supergroup (Kanmantoo and Normanville groups) (Clough and Rankin 1991). Geochemistry and geochronology of these igneous rocks has indicated that the mafic to intermediate volcanics predominantly have a back-arc geochemical affinity; but that some igneous rocks have a subduction-related geochemical signature, indicating evidence of either a primary subduction origin or derivation from a subduction-modified lithosphere (Foden 2019; Rankin, Clough and Gatehouse 1991). This suggests that the Delamerian basement to the Murray Basin in South Australia is likely to mark the transition from a back-arc to continental volcanic arc setting, with potential for a range of deposit styles including volcanic-hosted massive sulfide, epithermal, Carlin-style and orogenic gold, Cu–Au–Mo porphyry, base metal sedimentary exhalative and Ni–Cu – platinum group element magmatic sulfides.
Project focus
While the cover succession of the Murray Basin continues to be the target of exploration for heavy mineral sands, exploration for base and precious metals within the Delamerian basement has until now been largely focused along the western margin of the Murray Basin, where cover is shallowest and the nature of the underlying basement is more readily extrapolated from adjacent outcrop of the Adelaide Fold Belt and Padthaway Ridge. The objective of the GSSA’s Delamerian NDI project is to encourage exploration interest in the basement to the Murray Basin, particularly further east into the basin where little exploration has previously occurred, by providing industry with new data and new constraints on the geological setting and mineral prospectivity.
The Delamerian NDI Program is a four-year work program (2019–2023), with the major component being a stratigraphic/mineral systems drilling program scheduled to occur in 2021–22. The focus area, the untenured ground within the Murray Basin, has been placed under Section 15 of the Mining Act 1971 to ensure that no one company will receive a precompetitive advantage from the MinEx work program (Fig. 2). This Section 15 will be released in stages to industry for competitive application via the exploration release area process.
The current phase of the project is focused towards logging and analysis of existing basement-penetrating drillholes in the Murray Basin region and integrating this information with potential field data to make a preliminary interpretation of the distribution and structural framework of basement units. This work includes the collecting of new geochemical and geochronological data and HyLogger scanning of key drillholes, as well as an audit of the legacy exploration data not currently stored in the GSSA database (accessible via SARIG) and a focused program to capture this data. In addition, reconnaissance structural and lithostratigraphic mapping along a number of key transects within the eastern Mount Lofty Ranges and surrounds will inform on the context of units encountered in drillholes beneath sedimentary cover, as well as provide structurally constrained samples for further analysis.
This preliminary basement interpretation will drive the selection of drillhole targets, which will be finalised by July 2020. It is anticipated that the drilling will primarily comprise a series of orientated diamond holes, given that sample retrieval for geochemical, geochronological and spectral analysis, structural measurements, rock textures and lithological contact relationships will be key to determining the geological framework in this poorly characterised region. The drilling program may also include use of the coiled tubing rig initially developed by DET CRC and now under further development by the MinEx CRC.
Collaborative MinEx CRC research
Figure 3 MinEx CRC researchers Naina (PhD student, right) and Dr Marnie Forster (left) of the Australian National University together with Dr Geoff Fraser (centre) of Geoscience Australia investigating roadside exposures of the Kanmantoo Group as part of the reconnaissance mapping and geochronology project. (Photo 417929)
The Delamerian NDI program includes a number of proposed research projects on the geological framework and mineral prospectivity of the Delamerian Orogen that will be carried out by MinEx CRC researchers from the University of Adelaide, University of South Australia, Australian National University, Curtin University and CSIRO (Fig. 3). These projects will focus on both the Adelaide Fold Belt and Delamerian basement beneath the Murray Basin and include:
- characterising the metamorphic petrology, geochronology and thermochronological constraints on the Delamerian Orogeny and post-orogenic history
- sedimentary provenance and chemostratigraphy of the Kanmantoo and Normanville groups
- isotopic constraints on metal sources and ore-forming processes and resistate mineral chemistry of existing Delamerian mineral deposits and prospects
- tectonic setting and metal fertility of Delamerian igneous rocks
- potential field and electromagnetic modelling and a magnetotelluric profile across the Anabama region, northern Murray Basin.
Project goals
The final output of the Delamerian NDI project will be a revised tectonic and metallogenic framework for the Delamerian basement to the Murray Basin. It will take the form of a regional dataset, published maps and associated publications documenting the spatial distribution of Delamerian rocks beneath cover of the Murray Basin. This will provide improved depth to basement constraints, the Delamerian and post-Delamerian deformational history and tectonic setting of basement rocks, and an assessment of mineral potential. Critical elements of mineral systems will be mapped at a district scale, such as metal and fluid sources, host rocks and reactive lithologies, active structures and potential fluid pathways, and mineralogical and geophysical footprints.
References
Alias G, Sandiford M, Hand M and Worley B 2002. The P-T record of synchronous magmatism, metamorphism and deformation at Petrel Cove, southern Adelaide Fold Belt. Journal of Metamorphic Geology 20:351–363.
Baatar B, Parra-Avila LA , Fiorentini ML, Polito P and Crawford AJ 2019. Porphyry Cu fertility of the Loch Lily-Kars Belt, western New South Wales, Australia. Australian Journal of Earth Sciences.
Brown CM and Stephenson AE 1989. Geology of the Murray Basin, southeastern Australia. Bureau of Mineral Resources Bulletin 235.
Flöttmann T, James P, Rogers J and Johnson T 1994. Early Palaeozoic foreland thrusting and basin reactivation at the Palaeo-Pacific margin of the southeastern Australian Precambrian craton: a reappraisal of the structural evolution of the Southern Adelaide Fold-Thrust Belt. Tectonophysics 234:95–116.
Foden J 2019. The magmatic history of the Delamerian Orogen in South Australia – W Victoria: tectonic implications. Uncover Curnamona 2019, Symposium presentations, Report GS2019/1007. Geological Survey of New South Wales, Sydney.
Foden J, Elburg MA, Turner SP, Sandiford M, O'Callaghan and Mitchell S 2002. Granite production in the Delamerian Orogen, South Australia. Journal of the Geological Society London 159:557–575.
Geoscience Australia 2017. Murray Basin Cenozoic thickness. Geoscience Australia, viewed September 2019.
Greenfield JE, Musgrave RJ, Bruce MC, Gilmore PJ and Mills KJ 2011. The Mount Wright Volcanic arc: a Cambrian subduction system developed on the continental margin of East Gondwana, Koonenberry Belt, eastern Australia. Gondwana Research 19:650–669.
Johnson, EL, Phillips G and Allen CM 2016. Ediacaran-Cambrian basin evolution in the Koonenberry Belt (eastern Australia): implications for the geodynamics of the Delamerian Orogen. Gondwana Research 37:266–284.
Offler R and Fleming PD 1968. A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15(2):245–266.
Raymond OL, Liu S, Gallagher R, Highet LM and Zhang W 2012. Surface geology of Australia 1:1 million scale dataset. 2012 edn. Geoscience Australia, Canberra.
Raymond OL, Totterdell JM, Stewart AJ and Woods MA 2018. Australian geological provinces, dataset. 2018.01 edn. Geoscience Australia, Canberra.
Rosenbaum G 2018. The Tasmanides: Phanerozoic tectonic evolution of eastern Australia. Annual Review Earth and Planetary Science Letters 46:291–325.
Sandiford M, Oliver RL, Mills KJ and Allen RV 1990. A cordierite-staurolite-muscovite association, east of Springton, Mount Lofty Ranges; implications for the metamorphic evolution of the Kanmantoo Group. In JB Jago and PJ Moore eds, The evolution of a Late Precambrian – Early Palaeozoic rift complex: the Adelaide Geosyncline, Special Publication 16. Geological Society of Australia, pp. 483-495.
Sharp TR, Robson DR, Hallett MS, Mills KJ and Stevens BPJ 2006. Loch Lilly-Kars Belt 1:250 000 Geophysical-Geological Interpretation Map. Geological Survey of New South Wales, Maitland.
– Stacey Curtis and Tom Wise, October 2019
