Lithological controls on structural evolution of mineralised schist, Macraes gold mine, Otago, New Zealand

Abstract

Gold mineralisation in the Hyde‐Macraes Shear Zone was controlled by interactions between structure and lithology in the host Otago Schist. Mineralisation was initiated in the latter stages of Jurassic metamorphism and uplift of the host schist, in a transition from fold‐dominated deformation in large volumes of rock (>100 m scale), to focused cataclastic thrust deformation in and around specific shears (<10 m scale). At Frasers Pit in the Macraes mine, most gold is hosted in hydrothermally altered schist, rather than in quartz veins. The host schist has a composite metamorphic foliation that has been folded by late metamorphic folds on all scales from centimetres to tens of metres through large volumes of rock (>100 m scale). Folding was accompanied by minor recrystallisation of muscovite and chlorite. Mineralisation was initiated in the latter stages of late metamorphic fold development, and was accompanied by tightening, rotation, and disruption of those folds, with minor mica recrystallisation.

Two lithologic end‐member schist types (centimetre to metre scale) have distinctive associated mineralisation styles. Micaceous schist hosts gold predominantly in hydrothermally altered schist while feldspathic schist hosts gold predominantly in quartz veins. Micaceous schist is characterised by tight, shallowly inclined late syn‐metamorphic folds, which are further flattened and rotated during a ductile deformation event associated with pyrite and arsenopyrite mineralisation. This produced curved and disrupted syn‐metamorphic fold axes with a new fold axial surface cleavage defined by recrystallised micas. Metamorphic and vein quartz are locally microcrystalline and display crystal‐plastic deformation textures. In contrast, feldspathic schist, enriched in albite, is characterised by open, moderately inclined, late syn‐metamorphic folds. These folds are variably tightened and disrupted during a brittle syn‐mineralisation deformation event, with cataclastic microshears developed along fold hinges and sulfides hydrothermally introduced along carbonaceous shears and associated quartz microveinlets.

Both micaceous and feldspathic schists were subsequently deformed by cataclastic shearing accompanying hydrothermal introduction of graphite and more sulfides. The resultant black shears (micrometre to centimetre scale) pervade parts of both end‐member rock types, anastomosing around less deformed pods. Similar structures have developed at the 10 m scale, where massive schist pods dominated by feldspathic schist have more fissile sheared schist anastomosing around them. The fissile sheared schist occurs in packets (10 m scale) of interlayered micaceous and feldspathic schist horizons (1 m scale). Shear deformation and disseminated mineralisation of schist occurred principally in micaceous horizons. In contrast, rare swarms of steeply dipping extensional quartz veins are mainly confined to large (10 m scale) massive schist pods.

Keywords:

• gold
• Otago Schist
• structure
• hydrothermal
• lithology
• rock type
• mineralisation
• mining