The
orogenic gold deposits wall poster is one in a series of posters on different
mineral deposit types being produced by GNS Science to raise awareness of the
characteristics of New Zealand
mineral deposits and their prospectivity. Orogenic gold deposits are typically
quartz vein deposits formed in fault and shear systems at crustal levels within
and above the brittle ductile transition zone, at depths of 3 12 km and
temperatures from 200 to 400C. In New Zealand, orogenic gold deposits
are found in regionally metamorphosed sedimentary rocks of Paleozoic and
Mesozoic age. The deposits in Paleozoic rocks include those at Golden Blocks in
Northwest Nelson, Lyell and Reefton on the West Coast, and Preservation Inlet in
Fiordland. Quartz veins are developed along shear zones that dip steeply and
generally strike parallel to the axes of folding in the greywacke and
argillite/slate host rocks, although most are discordant to bedding. The lodes
are typically less than 1 m wide and extend an average 200 m horizontally and
vertically, but the Birthday Reef at Blackwater, Reefton, the largest quartz
vein deposit in Paleozoic rocks, averages 60 cm in width and was mined over a
strike length of 1070 m, and to a depth of 830 m. Disseminated gold occurs
adjacent to the quartz lodes at Globe-Progress and some other Reefton deposits.
The deposits in Mesozoic rocks are hosted in schist of Otago and Marlborough, and schist and greywacke of the Southern Alps. The lodes are typically lensoidal, less
than 1 m wide and localised along single or multiple parallel shear zones that
generally dip steeply, and are discordant to the schist foliation and bedding. A
notable exception is Macraes, where the lodes are developed in the
semi-concordant, low angle Hyde Macraes Shear Zone that has a strike length
greater than 25 km.