The
placer gold industry has experienced a roller-coaster history incorporating four
major boom eras. The first, 'hand-methods' boom arose in the mid-19th Century
from application of an old idea in new places around the globe.
The subsequent hydraulic mining, bucket dredging, and 'hydraulic
excavator-portable plant' booms each arose from application of new ideas
in old places around the globe, ie in places where gold had been
found during earlier booms. The new ideas that heralded the second, third and fourth booms were all significant
advances in excavation technology. These enabled the industry to target the
known deposits that were too deep, too wet, too small, or too low-grade for
recovery by earlier boom methods. Relatively few new deposits or new deposit
styles arose from the 'history book' exploration approach, so the number of
known deposits to which each of the boom methods were applicable was limited.
Consequently, the first three booms were each followed by significant periods of
industry decline, and the fourth boom is currently waning inNew
Zealand. Further decline is not inevitable and
can be avoided by changing to a knowledge-based exploration approach similar to
that adopted by the hard rock gold industry in the 1970s. Such an approach could
easily herald the next placer gold boom.

Most of the relatively coarse, easily won alluvial gold in proximal
and medial reaches of the active placer systems is gone. However, a substantial
alluvial gold resource remains in the distal reaches of active systems, and in
ancient placers formed onshore and on the continental shelf during previous
sedimentary cycles. The resource in these deposits probably exceeds the total
New Zealand placer production to date. Successful
development of these placers requires application of robust geologic models for
the formation and distribution of the placers through time, and advanced
exploration technology that has not been used before in New Zealand.
Nevertheless, the knowledge and technology required to explore and mine these
deposits is available now. Many of these deposits contain a large proportion of
problematic gold types that are difficult to recover with the commonly used
gravity technology without sustaining significant losses. Despite advances in
recovery technology in last few years, too much emphasis is placed on the
incorrect assumption that gold size is the primary control on recovery
efficiency. Other gold particle parameters, especially particle shape, exert a
much greater control on whether gold will be saved or lost to the tails, and
must be considered in the design of recovery technology and configuration of
recovery circuits.