Potential field modelling of mafic and exhalative rocks in the Girilambone-Tritton mine area, western NSW

Abstract

Aeromagnetic anomalies of regional significance are investigated in the Coolabah 1:100 000 map sheet area (approximately 70 km east of Cobar) where regional geological mapping is being carried out by the Geological Survey of New South Wales. Drill intercepts and sparse outcrops indicate several source lithologies, including mafic schist (volcanic rocks), quartz-magnetite-hematite horizons (exhalative rocks), ultramafic intrusions, serpentinite, and late intermediate to mafic dykes. These occur within very monotonous sequences of turbiditic metasedimentary rocks which dominate the map sheet. The magnetised features are important as they include altered mafic and ultramafic units associated with copper mineralisation in the vicinity of the Girilambone mine pits, and magnetite alteration reported at the Tritton copper mine. There is also potential for nickel or platinum occurrences associated with ultramafic rocks, for metallic mineralisation synchronous with recently-mapped exhalative rocks, and for structurally-controlled gold occurrences. The strong gravity gradient in the southwest of the area forms part of the regional contrast between the Hume Gravity Low and the Parkes Terrace. Analysis of multi-scale edge gradients and tilt filter imagery has helped define the high gravity features which have little or no surface expression, and to locate deeply-sourced structures. Section modelling of magnetic and gravity data (in ModelVision) highlights the dimensions and structures of the mafic, ultramafic, and quartz-magnetite-hematite rocks across the Coolabah map area. The modelling procedure has been multi-stage, with simple starting bodies adjusted after assessment of inversion outcomes. It was guided and constrained by field observations and petrophysical measurements where available. Geological mapping is increasing understanding of stratigraphic controls on local VHMS deposits, and this modelling is helping to develop the 3D concepts of regional stratigraphy and tectonics.