Structural controls on gold mineralisation in the Hyde-Macraes Shear Zone, East Otago.
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Abstract:
The Hyde-Macraes Shear-zone is a structural feature, of approximately 25km length, within the Haast Schist Terrane, cropping out about 60km to the north of Dunedin. Two prospects, Nunn's at the north-western end and Golden Bar at the south-eastern end of the shear-zone, have been investigated and mapped in detail. The structure is oriented (s/d) ~070/l5°NW and ~140/20°NE at these prospects respectively.
A potentially economic gold and scheelite mineralisation zone of between 5 and l00m thickness is present within the shear-zone, bounded at the top by a thrust fault that shows little relief or variation in orientation over large distances. The lower bounding fault is less planar. Where the shear-zone is thickened movement has taken place along a number of sub-parallel low-angle faults, connected by hinterland-dipping moderate to high angle imbricate faults.
From observations of the structures within the shear-zone, such as asymmetric folds, the flexure of schistosity planes and mesoscopic displays of anastomosing fault planes, the evidence for the thrusting nature of the Hyde-Macraes Shear-zone is convincing. The macroscopic structure falls within the Hinterland-dipping Duplex sub-class of the Boyer and Elliot (1980) classification of thrust systems.
Though duplexes are essentially compressional features, extensional sites are developed by the ramping and imbrication of the intra-shear schist along the internal inclined faults. Dilational jogs, formed at the bends and flats of these ramp faults provide optimal sites for the concentration of hydrothermal mineralisation.
There are three major manifestations of mineralisation; fault related implosion breccias, caused by incremental opening at dilational jogs; grey quartz ± schist breccia veins formed at the bends and flats of ramps, and pull-apart fractures and tension gashes in which large volumes of quartz have been deposited at sites with the orientation expected for extension within a zone subjected to simple shear. High-angle, sub-vertical veins are common through-out the deposit, generally thin and cross-cutting other features, and are interpreted as being of later stage in origin. Elements (Au, W, As) previously precipitated in one of the major mineralisation sites, commonly have been remobilised within these veins.
The direction of thrusting has been calculated using the orientation and vergence of mesoscopic asymmetric folds observed within the shear-zone. Estimates from within each prospect are broadly consistent and indicate that movement has been at a high angle to the strike of the shear-zone. The mode direction of movement is to the south-east (155°) at Nunn's and to the south-west (240°) at Golden Bar-Golden Ridge. If it is assumed that the Nunn's and Golden Bar prospects are part of the same macroscopic structure and were formed synchronously, a rotation of the Nunn's prospect of 60-70° about a near vertical axis is required in order to bring both the strike and the direction of movement into parallelism. Such a rotation could only be obtained by a pivoting motion on a low-angle fault plane, however there is no evidence that such a feature exists between Nunn's and Golden Bar.
Phase relations within the system Gold-Arsenic-Iron-Hydrogen-Sulphur-Oxygen have been calculated thermodynamically and the influence of Temperature, Pressure, Total Dissolved Sulphur (TDS), pH and the activity of Oyxgen on gold precipitation investigated. It is proposed the gold is deposited when a solution containing both arsenic and gold encounters a reducing horizon (graphite - carbonaceous material). A small change to more reducing conditions causes a large drop in the solubility of arsenic, precipitating arsenopyrite and thus removing large amounts of sulphur from solution. The decrease in the log(TDS) results in the decrease in solubility of the Au(HS)2-complex and the precipitation of gold.
The conditions of formation of mineralisation in the Hyde-Macraes Shear-zone have not been well established. From observations of the varying brittle-ductile nature of deformation in the intra-shear schist and the use of the arsenopyrite geothermometer it is suggested that the mineralisation was deposited from metamorphic fluids at a depth of 7±1km and a temperature of 310±30°C.
A potentially economic gold and scheelite mineralisation zone of between 5 and l00m thickness is present within the shear-zone, bounded at the top by a thrust fault that shows little relief or variation in orientation over large distances. The lower bounding fault is less planar. Where the shear-zone is thickened movement has taken place along a number of sub-parallel low-angle faults, connected by hinterland-dipping moderate to high angle imbricate faults.
From observations of the structures within the shear-zone, such as asymmetric folds, the flexure of schistosity planes and mesoscopic displays of anastomosing fault planes, the evidence for the thrusting nature of the Hyde-Macraes Shear-zone is convincing. The macroscopic structure falls within the Hinterland-dipping Duplex sub-class of the Boyer and Elliot (1980) classification of thrust systems.
Though duplexes are essentially compressional features, extensional sites are developed by the ramping and imbrication of the intra-shear schist along the internal inclined faults. Dilational jogs, formed at the bends and flats of these ramp faults provide optimal sites for the concentration of hydrothermal mineralisation.
There are three major manifestations of mineralisation; fault related implosion breccias, caused by incremental opening at dilational jogs; grey quartz ± schist breccia veins formed at the bends and flats of ramps, and pull-apart fractures and tension gashes in which large volumes of quartz have been deposited at sites with the orientation expected for extension within a zone subjected to simple shear. High-angle, sub-vertical veins are common through-out the deposit, generally thin and cross-cutting other features, and are interpreted as being of later stage in origin. Elements (Au, W, As) previously precipitated in one of the major mineralisation sites, commonly have been remobilised within these veins.
The direction of thrusting has been calculated using the orientation and vergence of mesoscopic asymmetric folds observed within the shear-zone. Estimates from within each prospect are broadly consistent and indicate that movement has been at a high angle to the strike of the shear-zone. The mode direction of movement is to the south-east (155°) at Nunn's and to the south-west (240°) at Golden Bar-Golden Ridge. If it is assumed that the Nunn's and Golden Bar prospects are part of the same macroscopic structure and were formed synchronously, a rotation of the Nunn's prospect of 60-70° about a near vertical axis is required in order to bring both the strike and the direction of movement into parallelism. Such a rotation could only be obtained by a pivoting motion on a low-angle fault plane, however there is no evidence that such a feature exists between Nunn's and Golden Bar.
Phase relations within the system Gold-Arsenic-Iron-Hydrogen-Sulphur-Oxygen have been calculated thermodynamically and the influence of Temperature, Pressure, Total Dissolved Sulphur (TDS), pH and the activity of Oyxgen on gold precipitation investigated. It is proposed the gold is deposited when a solution containing both arsenic and gold encounters a reducing horizon (graphite - carbonaceous material). A small change to more reducing conditions causes a large drop in the solubility of arsenic, precipitating arsenopyrite and thus removing large amounts of sulphur from solution. The decrease in the log(TDS) results in the decrease in solubility of the Au(HS)2-complex and the precipitation of gold.
The conditions of formation of mineralisation in the Hyde-Macraes Shear-zone have not been well established. From observations of the varying brittle-ductile nature of deformation in the intra-shear schist and the use of the arsenopyrite geothermometer it is suggested that the mineralisation was deposited from metamorphic fluids at a depth of 7±1km and a temperature of 310±30°C.
Thesis description:
x. 212 p., ill. (some col)., maps (in Pocket),; 30 cm.
Department:
OU geology Identifier:
1987Teagle
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Location (WKT, WGS84):
POLYGON ((170.182934818106077 -45.389221881064849,170.157422793757831 -45.311542906865732,170.346070613945045 -45.229154539997069,170.591010982684054 -45.354860871011823,170.56613887902509 -45.442453406551699,170.504897593504268 -45.500075929813775,170.182934818106077 -45.389221881064849))
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Citation
Teagle, DAH, “Structural controls on gold mineralisation in the Hyde-Macraes Shear Zone, East Otago.,” Otago Geology Theses, accessed April 25, 2024, http://theses.otagogeology.org.nz/items/show/216.