| Summary: | Deposits of the Reefton Goldfield, West Coast, New Zealand, are hosted on N-NE striking, upright folds of the Greenland Group. High-grade quartz lodes are hosted on axial planar shears, typically have short strike lengths, and form shoots that pitch steeply on the hosting shears. Two deposits within the goldfield are distinct. The Globe-Progress Deposit is hosted on an E-W striking discordant shear in which a brittle zone of disseminated mineralisation encompasses high-grade quartz lodes. The Blackwater Deposit has a combined strike length of 1 km, striking parallel to the hosting folds. This study of Greenland Group deformation within the Reefton Goldfield was undertaken in order to further determine structures that control the orientation, position and extent of the ore bodies. Detailed field mapping and microstructural analysis have demonstrated that the Greenland Group was affected by a multideformational sequence. The N-NE trending folds are termed F2 because the related, axial-planar cleavage (S2 ) deforms a micaceous, peak-metamorphic foliation (S1). Deposits of the Reefton Goldfield are hosted on shears axial planar to the F2 folds, with mineralisation occurring during subsequent deformations (D3-D5). D3 deformation occurred with a NW-SE shortening direction relative to the N-NE trending F2 folds. During D3, steeply dipping S2 parallel shears were reactivated as sinistral strike-slip faults. Concomitant with faulting, F3 folding produced asymmetric folds with moderate to steeply plunging axes. The interaction of F3 folding and sinistral faulting produced dilational zones on the long limbs of the folds. The Blackwater Deposit is interpreted to have formed during D 3, with mineralisation confined to the shear zone during retrogressive metamorphic conditions. Deformation during D4 resulted from N-NE directed shortening, near parallel to the trend of the F2 folds. This event produced F4 kink folds that were accommodated by layer parallel slip on S2 parallel shears. Many deposits of the Reefton Goldfield are interpreted to be saddle reefs, mineralisation during D 4 being confined to the dilational hinges of F4 kink folds. The semi-brittle nature of D 4 allowed the formation of stockwork veining and disseminated mineralisation within the F 4 hinge zones. Continued N-NE directed shortening caused F4 kink folds to lock-up, producing discordant E-W striking thrust faults (D5). The Globe-Progress deposit is interpreted to have formed during D5, high-grade ore shoots forming at dilatant intersections between the Globe-Progress Shear and F2 axial planes. Brittle D5 deformation produced a zone of disseminated mineralisation around the high-grade quartz lodes. A comparison between the Reefton Goldfield and deposits of the Lachlan Fold Belt of Victoria, Australia highlights the episodic nature of gold mineralisation. Mineralisation occurred sporadically in Victoria over a period of 70 Ma. Reefton's deposits formed during an evolving deformational sequence during a mid-Devonian period of mineralisation. The Greenland Group deformational sequence occurred with declining pressure and temperature, resulting in an increase in brittle styles of deformation and permitting alteration and disseminated styles of mineralisation. Stibnite, occurring in deposits formed late in the deformational sequence, is absent in early deposits. Mineralisation at Reefton may have occurred near the bottom of a vertical range in the upper crust that is favourable to stibnite mineralisation (c. 5 km). Detailed field mapping throughout the goldfield will further refine the relationships between the F2 folds, later deformations and the localisation of ore bodies. Additional mapping may also help constrain the position, orientation and extent of nearby ore bodies.
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