McFarlane, Helen1, Blaikie, Teagan1
1CSIRO Mineral Resources, Perth, Australia
Stradling the Northern Territory-Western Australia border approximately 600 km to the northwest of Alice Springs, the Tanami Region comprises regionally expansive, polydeformed metasedimentary and volcanic rocks (Tanami and Ware groups: ca. 1885–1816 Ma). Recording a protracted and complex tectonic history, the sequences are extensively intruded by 1825–1790 Ma granites and dolerite dykes. To the southeast, the adjoining Aileron Province is comprised largely of metasedimentary and magmatic rocks of a comparable age. The region is highly prospective for gold, and preserves several deposits including the world class Callie deposit. Geological studies of the region are made challenging by extensive Mesoproterozoic to Cenozoic sedimentary cover and Early Cambrian basalt flows, with most geological information derived from sporadic outcrop and drill core. Geophysical data is therefore critical in this region, and is relied upon to understand the structural architecture and extent of potential gold bearing metasedimentary and volcanic rocks under cover.
Newly acquired and legacy government and industry aeromagnetic data available across the Tanami Region and northwest Aileron Province were reprocessed and interpreted for this work to produce new seamless solid geological and structural maps. This new interpretation attempts to resolve inconsistent geological correlations in the Northern Territory and develop a new cohesive structural framework for the entire region.
Deformation is typically best resolved in the magnetic stratigraphy of the Tanami Group which preserve several styles of fold interreference patterns. The earliest deformation event (D1) is characterised by isoclinal folding and low angle thrust faulting. D1 structures are extensively overprinted by subsequent deformation events; however, evidence suggests it is associated with an episode of SW-directed tectonic vergence. D2 deformation is characterised by the refolding of D1 structures by tight to isoclinal NNE- to NE-striking F2 folds, associated with WNW-ESE to NW-SE shortening. Both D1 and D2 are attributed to the ca. 1830 Ma Tanami Event which also involved regional greenschist to amphibolite facies metamorphism and early gold mineralisation. This was followed by an episode of sedimentation and volcanism, preserved as the Ware Group. The oldest recognised deformational structures in the Ware Group are attributed to NE-SW shortening during D3 as evidenced by NW- to NNW-striking chevron folds. The overprinting relationship of D3 on D1 structures generated a Type-2 fold interference pattern in the magnetic stratigraphy of the Tanami Group. The localised development of tight, E-W striking chevron folds is attributed to D4 N-S shortening and is associated with comparatively weaker deformation compared to earlier events. The final folding event (D5) generated long wavelength, open, NE-striking F5 folds during NW-SE shortening. Deformation events D3–D5 are interpreted as polyphase deformation during the 1810-1790 Ma Stafford Event and was coeval with widespread felsic magmatism. The transition to brittle-ductile deformation (D6) is associated with the development of the regionally prominent WNW-ESE to NW-SE striking dextral and sinistral shear zones, associated with the major period of gold mineralisation in the region.
Helen McFarlane is a structural geologist at CSIRO, with a diverse background, spanning Palaeoproterozoic tectonics in West Africa to MVT mineralisation in the Andes. Her current role examines structural controls in Au-, Cu- Mn- and multi-commodity mineral systems around Australia.