Underplating of an Archean basement approximately 2100 to 1900 Ma is believed to have resulted in local extension of the pre-existing Archean continental crust. Repeated rift and sag phases of extension were responsible for a rapidly subsiding basin in which huge thicknesses of sandstones, greywackes, siltstones, shales and interlayered volcanics were deposited, giving rise to the Warramunga Group rocks.
The offshore extrusion of rhyolites, tuffs and pyroclastic material provided a ready source of volcanic glass, quartz and feldspar, which were reworked by the action of turbidity currents, active during basin formation. Chemical breakdown of volcanic glass held in suspension, by sea water, resulted in the formation of montmorillonites, palygorskites, illites and kaolinite that together with quartz and minor feldspars were deposited as repetitive fining upwards sequences of sediments in the deeper and more anoxic portions of the basin. The large interlayer spacing and high cation exchange capacity of montmorillonite allowed the fixing of certain ore elements and magnesium onto and/or into the clay structure. Diagenesis of these volcaniclastic sediments yielded the assemblage sericite+chlorite±quartz:±]Jyrite±magnetite+trace ore minerals. The variations in this assemblage by the action of turbidity currents was responsible for the deposition of the metasiltstones and metagreywackes outcropping in the Orlando study area.
Deposition of the Warramunga Group sediments was halted by onset of the Barramundi Orogeny approximately 1860 to 1870 Ma. The Barramundi Orogeny was characterised by substantial horizontal shortening, which manifest as a north-south compression in the Tennant Creek region. In the Orlando area this compression resulted in a number of ductile, brittle-ductile and brittle deformation phases which formed open to tight upright folds which were over-printed by east-west trending shear zones, formed during continued compression.
These shear zones acted as major fluid pathways, which were exploited by iron bearing fluids precipitating foliated and massive magnetite+hematite±quartz ironstone bodies during ductile and brittle-ductile deformation. Hydrothermal fluids were active during shearing and resulted in intense chloritisation of the sheared country rock. Subsequent brittle deformation resulted in micro fracturing of the ironstone bodies which allowed ingress of ore bearing hydrothermal fluids which reacted with the ironstone bodie$ forming a redox reaction precipitating ore minerals.
Au-Cu mineralisation in the Orlando East deposit occurs as quartz vein networks in highly sheared metasiltstones. Here the lack of an appreciable ironstone associated magnetic signature represents a style of mineralisation not previously explored in the Tennant Creek region.