Structural and lithological controls on mineralisation in the Hyde-Macraes shear zone

Petrie, Brigitte Sarah.

The Hyde-Macraes Shear Zone is a complex, NW-trending, late Mesozoic low angle thrust system in East Otago which is host to an actively mined mesothermal gold-scheelite deposit. The area of study is the southernmost active pit (as at March 2001) - Frasers Stage I Pit - where gold and sulphides are dominantly disseminated through argillitic schist in contrast to the quartz-vein hosted mineralisation style typical of the deposits 5 - 6 km NW along strike. Compositionallayered, light grey, blocky (Massive) schist and grey, fissile (Layered) schist in the Hanging Wall and Intrashear Schist are geochemically distinct from Torlesse-derived Otago Schist argillites or sandstones, being enriched in mafic elements (Mg, Cr and Ni), trace elements (Ti and Se with an island arc affinity) and relatively depleted in silica. Massive Schist is distinct from Layered Schist in that it has a coarse grained quartzofeldspathic conglomeratic component. All other geochemicai differences between the Hanging Wall and Intrashear Schist are attributable to hydrothermal alteration associated with development of the shear zone, suggesting that they were initially part of the same sequence of rocks. The only difference between Hanging Wall and Intrashear Schist, therefore, is a significantly higher proportion of conglomeratic (Massive) schist in the Hanging Wall. The Cr- and Ni- anomaly in both the Hanging Wall and Intrashear Schist reflects the presence of Cr-bearing minerals in the primary rock type and enrichment during secondary hydrothermal alteration. The primary rock type has influenced the rheological response of the rock during peak regional metamorphism (D1/D2) and transposition of late synmetamorphic folds (F3). The attitude, wavelength and amplitude of F3 folds is a function of the transposition of the D3 stress field, which in turn, is a function of the homogeneity, rheology and anisotropy of different rock types. Two 'end member' synmetamorphic fold styles are characterised - steeply inclined, gentle, metre-scale F3• folds (in Massive Schist) and shallowly inclined, tight cm-scale F3b folds (in Layered Schist) - both of which have been redeformed and/or mineralised during brittle/ductile top-to-the-southwest thrust movement on the proto- Main Hanging Wall Shear under brittle/ductile (D4) deformation conditions. Schist-hosted mineralisation is associated with D4 deformation of these late synmetamorphic folds. Disseminated, Pyritic Whole Rock Mineralised Schist is coherent, pervasively hydrothermally altered and dynamically recrystallised schist with an abundance of large, euhedral pyrite grains. Gold and sulphides (dominantly arsenopyrite) have replaced quartz during progressive simple shear and inhomogeneous flattening of shallowly inclined F3b folds (leading to fmmation of incipient sheath folds). Gold and sulphides, dominantly arsenopyrite, are encapsulated in microcrystalline quartz along the refolded synmetamorphic cleavage of F3b folds (modified by the shear zone) and along the selvedges of mm-scale, mineralised quartz veinlets. Gold is cryptically held in microcrystalline quartz and cannot be liberated through cyanide-leaching. Microsheared/Microveined Schist is blocky, variably disrupted schist with thin, fissile pelitic zones. Particulate gold and composite pyrite/arsenopyrite±sphalerite±chalcopyrite±galena grains occur within micron scale fractures and graphitic shears associated with mobilisation of arsenic and introduction of tungsten and antimony. The homogeneity and competency contrast at a 1-10m scale between rocks overlying and underlying a 'zone of · focussed strain' have been instrumental in determining the orientation and nature of the Main Hanging Wall Shear. A block of relatively homogeneous Massive Schist ~10 metres thick acts as an impermeable cap to the Main Hanging Wall Shear, helping to maintain the lithostatic fluid pressure condition required for shear reactivation. If the overlying rocks are too permeable (ie; Layered), strain associated with top-to-the-southwest thrust movement is partitioned onto a lower fault which develops under a ~10 metre block of Massive Schist below the top bounding structure. Schist-hosted mineralisation is associated with the transfer of focussed strain in an intervening thin (1-10 metres thick) horizon of Layered Schist between two blocks of Massive Schist 10- 20 metres thick. In contrast, quartz vein-hosted mineralisation (> 10 cm thick) is associated with shear reactivation of the Main Hanging Wall Shear under supralithostatic fluid pressures. Overpressured fluids will only be sustained where the schists overlying and underlying the Main Hanging Shear/Intrashear Fault are blocks of massive, homogeneous schist ->10 m thick. Structurally, the Main Hanging Wall Shear propagates along the hinge plane of a mesoscopic (F3.) fold so that Hanging Wall and Intrashear Schist formed part of the same synmetamorphic fold (and rock unit) prior to displacement on the Main Hanging Wall Shear.

1 v. (various pagings) : ill. (Some col., some folded), col. maps ; 30 cm.

2001Petrie