The gold
province of
Otago
occupies an area of upwarped
basement, the Otago Schist, comprising predominantly quartzofeldspathic and
volcanogenic metasediments. This assemblage is the product of collisional
tectonic deformation and associated metamorphism, mainly to lower greenschist
facies, during the Rangitata Orogeny (200-100 Ma). Historically this region has
produced 240 tonnes of gold, 4% (10 t.) of which was from the mining of
Cretaceous goldquartz mineralised vein systems. Auriferous vein systems in the
Otago Schist are mainly hosted within NW trending normal fault-fracture arrays,
however NW trending reverse fault-hosted vein systems also occur. We propose
that the Late Cretaceous-mid Tertiary Otago Schist structure represents a
regional anticlinorium containing flexural folds of at least two wavelengths.
The anticlinorium structure has been quantified by determining variations in
angular discordance between schistosity and the Otago Peneplain, a Late
Cretaceous-mid Tertiary erosion surface covering most of the Otago district. A
structural model for the evolution of the post-collisional anticlinorium
requires the cooling surficial zone (carapace) of a retrograding metamorphic
belt to behave as an elastic lid. Erosion of topographic load on the carapace
drove isostatic uplift, which was greatest in the tectonically thickened core of
the orogen. In this way differential uplift produced flexural deformation in the
elastic carapace, expressed as a large scale arching of the orogenic belt.
Flexure created tangential longitudinal strains in the outer-arc, leading to
extension and the formation of mineralised normal faults. These structures are
separated by a neutral surface from shortening and associated mineralised
reverse faults in the inner-arc. Our structural model predicts that the
distribution of auriferous lodes should be localised about zones of maximum
flexural curvature, where the tangential longitudinal strains are highest. As a
result, NW-SE hinge regions separating schistosity dip domains in the Otago gold
province may have new found significance as potential exploration
targets.