Abstract
The 10 Moz Macraes gold deposit in southern New Zealand is hosted within a 100-m-thick slice of weakly altered schist and the overlying Hanging-wall shear. This shear marks the base of the >1.5-km-thick, gently dipping, Hyde-Macraes shear zone. The term "Hanging-wall" reflects the position of this shear within the current Macraes mine but not its position within the wider Hyde-Macraes shear zone, which includes numerous other mineralized shears at higher structural levels. Major km-scale zones of more intensely mineralized rock within the Macraes mine are developed along (1) intersections between the Hanging-wall shear and foliation in the underlying schist, particularly where the foliation dips more steeply than the overlying shear; (2) a major frontal ramp in the Hanging-wall shear, especially where this ramp splits into several subhorizontal stacked shears; and (3) lateral ramps in the Hanging-wall shear and coincident minor splays in the underlying schist. All these structural sites have limited up/downdip extents on the Hanging-wall shear, and undiscovered repetitions are therefore likely to be blind. Any comparable zones of more intensely mineralized rock at greater depths than those explored to date are unlikely to be associated with detectable structural signatures in either the directly overlying schist or the prospective shear zones more than 100 to 200 m updip of the mineralized zones themselves. The information required to target further blind mineralized zones can probably only be acquired from wide-spaced reconnaissance drilling and/or geophysical methods appropriate for imaging stacked gently dipping structures, such as seismic reflection.
The direction and amount of displacement across the Hyde-Macraes shear zone during mineralization remain poorly constrained. Geometric relationships between shears, extensional veins, duplex structures, and rare slickenlines suggest shortening directions between south-southeast and west-southwest in different parts of the shear zone, with potential for important strike as well as reverse dip-slip displacement. The distribution of mineralized rocks within the Macraes mine suggests a major frontal ramp locally channeled fluid flow parallel to the strike of the Hanging-wall shear, whereas a lateral ramp perpendicular to the strike of the Hanging-wall shear acted as a key fluid pathway updip across the frontal ramp. Some sharp bends in the strike of the mineralized shears may have initiated as transfer segments that compartmentalized fluid flow at different structural levels during mineralization within the wider Hyde-Macraes shear zone, whereas other bends may be folds that formed during Cretaceous and Cenozoic faulting after mineralization.