Alluvial gold mining in New Zealand and in particular on the West Coast
underwent a renaissance during the early 1980's. This was based on two main
factors:
a) the sharp rise in the gold price
b) the advent of the hydraulic excavator with which to mine the auriferous
gravels.
The scale of operations
varies considerably. A small operation typically consisting of one ortwo
people using an excavator to feed either a land based or pontoon mounted
(floating) gold recovery unit, which will typically feature a rotary screen with
or without a tailings stacker with the screened fines< 10 - 15 mm) fed
over riflle tables. The largest screening plants of this type are operated by
DML Resources in their Arahura and Mikonui mining operations where the plants
have the capacity to screen up to 250 m3 of gravel per hour. These plants
usually employ jigs and centrifugal concentrators for gold recovery. Unique
amongst current operations is the Grey River Dredge, a bucket ladder dredge
capable of processing 700 m3 per hour similar to those that
operated earlier this century. Gold concentration is by a series of jigs. The
number of plants operating in recent times peaked at around 130 in 1987
(coinciding with a nominal average gold price of $751) and since that time has
steadily declined to 80 units in 1993 and about 45 - 50 units today. This cycle
of rapid expansion and then steady decline has been repeated on the West Coast
since the original gold discoveries in the mid nineteenth
century.