92 The Lawlers Anticline and the Mt Margaret Anticline are located ...

Polarity of D2-D3 deformationAs argued earlier, the long-lived D1 extensional event(s) established the NNW-oriented architectureof the Eastern Yilgarn Craton. Slab roll-back from the east may have favoured the development ofan asymmetric system with a down to the ENE sense of shear. The polarity of the D2 contractioncan be inferred from the numerous PT-dihedra calculations made in this study. In this study, likemany before, D2 contraction was oriented ENE-WSW, and had a likely dextral transpressionalcouple across it. Much of the movement that was dextral strike-slip occurred along N-S trendingstructures.The fundamental grain viewed in the seismic (Figs. 59, 70, 71) is down to the east (ENE), a‘rebirth’ of the D1 extension(?). However, in detail the lineations plunge to the eastern hemispherearound the domes (e.g., at Lawlers Fig. 66a). It has been at Lawlers (and by Williams and Whitaker,1993 at Leonora) that the granites rose upwards in the crust while under extension and thegreenstones were shed off these rising bodies. The regional far-field ENE-directed extension fromthe rolling-back slab to the east was enhanced locally in the domal regions by magmatism. The factthat greenstones slide off to the north, east, and south (and all points in between), and that thegranites were locally ‘extruded’ to the NNW, W, and SSE, imply that the orogen was not fixed atthe sides. Davis and Maidens (2003) also described a late D2 extension in the Laverton area(described as D2e), and related this to collapse of the D2 orogen along its axis. In a slightlydifferent model, SRK (2000) suggested that the polarity of extension was NNW-SSE, building onthe early work of Hammond and Nesbit (1992; 1993). In these models, the NNW-trendingarchitecture formed the transfer faults (localising Late Basins in narrow troughs in these transfers),while the main detachments were arcuate about the broad granite domes. In both cases, SRK (2000)and Davis and Maidens (2003) mapped the local effects of a larger system.The SRK model (Fig. 68) is appealing in many ways, but does not explain the temporalrelationships of the Late Basins, nor the fact that the primary architecture in a multitude ofgeophysical, geochronological, geochemical and geological datasets show that the fundamentalcompartments strike NNW (during D1 and D3). These all data point to an ENE-directed extensiondirection, and may reveal something about the orientation of the orogen’s margin (it was likelyNNW-trending in today’s reference frame).Implications for GoldIn terms of the spatial association of proximity of major gold deposits and Late Basins, it could bethat the basins are a consequence of a process that is linked to both. This common process isextension which established deep-connected fluid pathways onto detachments at depth, and theirdomal focussing architecture. The fact that magmatism (a hot fluid) also favours the domes createdby the extension may also point to a common process (or architecture) for magmatism and goldmineralisation. In this study gold localisation has been observed in both extensional andcompressional structures. However the majority of gold deposits are localised in compressionalstructures indicating that compression is an important aspect in focusing fluid. The relationship ofinferred and known gold mineralisation to mapped structures is presented in Appendix 3.1.1 &3.1.10. Gold occurs in D2 through to D5 structures with the majority of the gold localised in: D2,D3 and D5 structures. Gold related to the D4 deformation appears to be confined to the Lavertonregion. There may be an over estimate of gold mineralisation at D2 and D5 time since at some sitesit was not possible to distinguish between these events. When Au was localised in ductile shearzones it was always localised in the central high strain parts of the shear zone thereby indicatingthat in ductile shear zones permeability and fluid flow is a function of continued deformation.98