Sarah Jones (SJGeology) presents 'Granite-up, greenstone-down tectonics in the Eastern Goldfields, WA, and implications for early gold mineralisation' for the October 2021 GSA GOLD.
Abstract: The early tectonic history of the Eastern Goldfields Terrane (EGT) is poorly understood, but in several locations c. 2800 Ma mafic-ultramafic sequences are paraconformably overlain by c. 2720-2670 Ma sequences (e.g., Leonora and Laverton districts), suggesting minimal early deformation. The first significant angular unconformities occur at the base of the c. 2670-2655 Ma late basin sequences and indicate that deformation was contemporaneous with deposition of the late basins. The late basins mark the end of the volcano-sedimentary record in the EGT. They typically grade upwards from polymictic mafic-dominated conglomerates to more siliciclastic compositions with abundant granitic clasts. The clastic sequences record the uplift and exhumation of granite-cored domes, with the eroded detritus deposited into the basins. There is a distinct lack of clasts with internal deformation fabrics (e.g., schist or gneiss). The timing of late basin formation overlaps with the c. 2672-2660 Ma D1 event, which has been variously described as an extensional or compressional event. D1 structures comprise a bedding parallel S1 that is axial planar to F1 folds. The late basins and early D1 fabrics are overprinted by upright north-trending F2 folds and a sub-vertical S2. In the EGT, a period of crustal thickening was achieved by autochthonous processes with deposition of thick greenstone sequences into local basins between c.2720 to 2670 Ma. Partial convective overturn (or granite-up, greenstone-down tectonics) developed from gravitational instability. As the large granite-gneiss bodies rose, solid-state extensional D1 shears developed around the granites with a radial pattern of L1 extension lineations. Recumbent F1 folds formed in the overlying greenstone sequences above the D1 shears and folds verge away from the rising granites bodies. Areas of compression and thrust faults developed within the sinking greenstone sequences. A final phase of subsidence in the central parts of the greenstone belts is associated with the formation of depo-centres (late basins). A partial convective overturn model (granite-up, greenstone-down) allows differential uplift of domal granitic complexes without significantly deforming the overlying mafic-ultramafic sequences and preserves younging directions away from the granites. This model also explains the lack of schist and gneiss clasts in the late basins. The shift from dominantly vertical tectonics to horizontal tectonics at c. 2655-2650 Ma is marked by the onset of east-west D2 compression and the change in granite compositions from high-Ca to low-Ca granites. Geochronological studies indicate that gold deposits in the EGT display a broad range of ages (~2670 to 2635 Ma) and that some have formed during the early D1 event. Structural controls for the early gold deposits differ markedly from orogenic gold deposits formed during later horizontal compression (D2-4 events).
Biography: Sarah moved to Australia in the mid-1990s after studying at Otago University, NZ, to work as an exploration geologist in the Eastern Goldfields. In 2002 she completed a PhD at CODES on a VHMS deposit in Canada and moved back to the Eastern Goldfields to work for GSWA as a regional mapping geologist. Since then, she has worked in industry as a structural geologist. Sarah recently moved back to NZ to run her own consultancy and now resides in Cromwell, Central Otago, with her fur family.
