Reconstructing the Soldiers Cap Group – Kuridala Group basin: Implications for BHT and IOCG mineralisation

Connors, Karen1

1Sustainable Minerals Institute, The University of Queensland, Brisbane, Australia

The vast basin hosting the 1700-1650 Ma Soldiers Cap and Kuridala groups (SCG-KG), eastern Mount Isa Province, NW Queensland, extends >300 km east to include the ca 1700-1610 Ma Etheridge Group, Georgetown Inlier. The present-day extent and thickness represents only part of the original depocentre following inversion, uplift and erosion (1610-1500 Ma). Whilst the importance of extension has long been recognised, pervasive compression, voluminous 1535-1490 Ma granites, and limited seismic integration, has prevented elucidation of the extensional architecture and its influence on inversion and mineral systems. Integrated interpretation of seismic and potential field data, and review of geochronology has provided a new understanding of the extensional architecture, potential age range and thickness of the basin, and the tectonic evolution.

The preserved thickness, extent, age range, erosion estimates, and the crustal architecture provide first-order constraints on basin reconstruction. The SCG-KG basin overlies several crustal-scale boundaries, including the Gidyea Suture where the thinned eastern margin of the poorly reflective, Mount Isa crust is thrust over the thinner Numil crust. The Numil comprises a series of moderate to low-angle fault blocks, many only 5-15 km thick, and typically has pervasive, dipping reflections.

While outcrop mapping suggests the SCG group is 2-5 km thick, seismic data indicates 3-5 sec TWT, implying 12-15 km preserved thickness. Mapping indicates localised isoclinal folding, nappes and structural repetition within some highly-deformed zones. Although regional structural repetition can’t be ruled out, the seismic data suggests many areas are dominated by limited repetition and thickening on inverted, normal faults.

The minimum age for the SCG-KG is generally accepted as 1650 Ma. However, the 1650-1610 Ma units of the Tommy Creek Domain and Etheridge Group are likely to have been widespread across the basin. In addition, zircon populations from drainages along the eastern outcrop margin and SCG show peaks in juvenile mafic magmatism at 1630-1625 Ma, as well as 1667 Ma. Although the upper SCG-KG unit (Toole Creek Volcanics (TCV)) is attributed to thermal relaxation from ca 1670 Ma, coeval felsic and mafic magmatism at 1655 and 1625 Ma, and the large volume (20-30%) of high-Fe mafic sills within the TCV suggest extension continued or was episodic.

Prior to inversion and erosion, the 12-15km SCG-KG basin was thicker as well as wider than the present ~350 km. While the stretching factor and total extension are unknown, the thin low-angle fault blocks of the Numil, are consistent with highly-thinned to hyperextended crust (i.e. 10 km thickness or less), and exhumation of lower crust or mantle may have occurred. The resulting high geothermal gradient has implications for BHT mineralisation, and raises questions regarding controls on metal deposition.

The extensional fault system and preliminary reconstruction provide insights into the extensional evolution and controls on later inversion. The structural framework and its links to the underlying basement blocks and crustal-scale structures that form the first-order conduits of the plumbing system provide insights for both syn-sedimentary BHT mineral systems and later IOCG deposits.


Biography

Karen has had a varied career in mineral and petroleum exploration. She specialises in integrated interpretation of seismic with potential field data to understand 3D crustal architecture, structural inheritance and influence of basement on basin evolution, 3D modelling, and controls on mineral systems.

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