2730–2670 Ma rifting triggers sagduction prior to the onset of orogenesis at ca 2650 Ma: implications for gold mineralisation, Eastern Goldfields, Western Australia

Abstract

The dominant structural fabric in the Eastern Goldfields is the steep north- to north-northwest-trending S2 foliation that is axial planar to upright F2 folds, developed during intense horizontal east–west shortening (D2–4 event). These structures consistently overprint D1 structures, which comprise layer-parallel S1 foliation, extensional shears, thrusts and recumbent F1 folds. The D1 event represents a separate and distinct episode of deformation with a markedly different stress regime (dominant vertical σ1), compared with the stress regime during the D2–4 events (horizontal east–west oriented σ1). There is little evidence for pre-2670 Ma ductile deformation in the Eastern Goldfields, as: (1) the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences, with the first significant angular unconformities observed at the base of the post greenstone late basins; (2) a lack of schist or gneiss clasts in the late basins suggests that the surrounding uplifted sequence was largely undeformed; (3) the layer-parallel S1 foliation is observed throughout the 2720–2670 Ma greenstone sequence; and (4) the dominant low north- and south-plunges of F2 folds suggest that the sequence was predominantly flat-lying prior to strong east–west shortening (D2–4 events). The ca 2670–2655 Ma D1 event represents a period of sagduction that occurred immediately after cessation of rifting as a result of gravitational instability from deposition of dense, cold, greenstone sequences onto thinned, light, hot felsic crust. Late basins represent depocentres on the sinking greenstones during sagduction. Marked contrasts in the structural style of gold deposits, metallogeny and fluid sources, typically attributed to progressive deformation during orogenesis, could instead reflect temporally distinct mineralising events during changing tectonic regimes. In the East Yilgarn, an early plumbing system dominated by north-northwest-trending basin-controlling structures was likely established during rifting and focused fluids during multiple hydrothermal circulation events.