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Wednesday, August 24, 2011

Gold Occurrences in New Zealand


Map of New Zealand



There are several different types of gold deposit that are found in western New Zealand.  Orogenic gold deposits are found in both fault and shear zones at all crustal levels in quartz veins found within and around brittle ductile transition zones. 

The framework of New Zealand is controlled by two major faults the Wellington fault in North Island that terminates at the Bay of Plenty to the south coast, and the Alpine Fault that traverses South Island. New Zealand was rifted from Australia millions of year ago.  There is a subduction zone to the east that today drives the volcanoes found on the west side of North Island.  It is these volcanoes and earlier volcanism that has formed the gold deposits of the Islands.

Together these two faults represent a suture zone that runs the length of the islands.  It is a well known fact that many metal deposits are found associated with suture zones, and that is true in New Zealand.  Another mineral that is often found in suture zones is nephrite that is commonly called jade.  Jade is found in areas of high pressure-low temperature metamorphism commonly in blue schist grade rocks.

These deposits originated ranging from 3 km to 12 km deep with hot water from 200o to 400o C.  The gold deposits were brought to the surface by earth movements and erosion. The gold deposits are found mostly in regionally metamorphosed Paleozoic sediments that are found including those found at Golden Blocks in the northwest at Nelson, Reefton and Lyell.  Other deposits are found at Preservation Inlet in Fiordland where gold bearing quartz veins are found dipping steeply and are generally striking parallel to the axes of folding in greywacke and argillite slate host rocks.  Most of these veins are found to de discordant to the bedding. 

The lodes in these locations are usually less then a meter wide and 200 meters long.  The Birthday Reef at Blackwater, Reefton is the largest quartz vein found in the Paleozoic rocks.  This vein averages about 60 cm wide and has been mined for a distance of 1070 m that extend to a depth of 830 m.  There is also disseminated gold deposits that are next to the quartz lodes at Globe-Progress as well as others at the Reefton deposits.

There are also deposits of gold found in the Mesozoic rocks in the schist of the Otego and Marlborough as well as the greywacke found in the Southern Alps.  Here the lodes are typically lensoidal in shape that are rarely more then a meter wide and are localized along single or multiple shear zones that are parallel that usually dip steeply and are discordant to the bedding of the country rocks.  One exception is a lode vein found at McRaes where the deposits are found in the Hyde McRaes shear zone.  These gold bearing veins have a strike of more 25 km.

Many of the gold deposits are found in saddle reef deposits similar to those found in eastern Australia where they occur as veins of quartz that approach near horizontal deposits of quartz found parallel to the bedding planes in turbidites ranging in age from Silurian to Devonian in age.  A good example of this kind of deposit can be found at the Hill End Gold Field in New South Wales.  This deposit was developed in regional metamorphism over a considerable length of time.

Gold deposits can be found the length of New Zealand, but are mostly found on the western side of the islands.

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