The relationship between the Hyde‐Macraes Shear Zone, deformation episodes, and gold mineralisation potential in eastern Otago, New Zealand

Abstract

The relationship between the mineralised HydeMacraes Shear Zone (HMSZ), an extensive, low-angle, semibrittle structure in the Otago Schist Belt within the Haast Schist, New Zealand, and folding events affecting metasediments is documented. The shear zone postdates F 3 folds, although it is subparallel to S 2 , a shallow, north-dipping penetrative cleavage. Evidence of the first shortening phase is only rarely preserved because initial fabrics are transposed. D 2 resulted in a strong mineral elongation (L 2 ), a penetrative cleavage, and isoclinal folds. D 3 produced generally asymmetrical northwest-plunging folds in S 2 at all scales but of variable intensity. In regions where intense D 3 shortening has occurred, L 2 is apparently rotated into parallelism with F 3 axes. A fourth event resulted in micro-and mesofolding about a north-plunging axis; after this, D 5 produced a northeast-trending microcrenulation. The HMSZ nucleated on the lower limb of an inclined macroscopic F 3 fold, post-F 3 but pre-D 4 veins. The shear zone follows pre-existing weaknesses but transects areas exhibiting variable intensity of D 3 shortening. This is evident by different degrees of rotation of L 2 towards F 3 fold axes, resulting in changes in the shear direction and potential for mineralisation. Gold mineralised sites along the HMSZ are apparently localised adjacent to the lower limbs and near the hinges of upthrusted F 3 folds and near the boundary of dominantly D 2 and D 3 folded areas, where pre-existing weaknesses were reactivated.

Low‐angle shear zones in Central Otago, New Zealand—their regional extent and economic significance

“…For the purpose of this article, a macroscopic shear zone is defined as one with strike length >5 km. F 3 vergence boundaries are documented across the Rise and Winsor-Low-angle shears, Otago Shine Shear Zone (RSSZ) area 1 (Paterson 1986); the HMSZ (Teagle et al 1990;Winsor 1991); and in the Ophir region, area 4 (Means 1966). The only other known gently dipping macroshear zone crops out in the Cromwell Gorge (area 2), as a diffuse zone separating coarsely layered quartzofeldspathic and finely layered micaceous schist (Turnbull 1988).…”

“…11), and Pataeroa areas ( Fig. 12) and across the HMSZ (Winsor 1991). They display both thrust and normal movement and are taken to represent a post-thrusting conjugate system.…”

“…The Hyde-Macraes Shear Zone (Fig. 1) is a prominent macroshear, containing gold mineralisation at particular sites and exhibiting a complex relationship to F3 b folds (Winsor 1991). The main reason for this variable fold association is the different degree of D 3 strain evident in the upthrust plate.…”