I have recently been asked to assess the historic significance of a one time
"gold extraction works' located on the Hauraki goldfield, North Island New
Zealand. The plant involved was one of three post 1900 "works" of its type
in N.Z.  The companies involved dredged sludge (pulped rock) discharged into
rivers (then quite lawfully) from batteries (stamp mills) upstream.  The
sludge was then conveyed by barges to the "extraction works" where it was
submitted to cyanide treatment and further gold recovered.  The three plants
in question were moderately successful.
 
While the reprocessing of tailings dumps on land is/was a reasonably common
event on goldfields, to my knowledge (at this point anyway), the recovery
and reprocessing of mill sludge discharged into waterways (no EIA's to worry
about in those days!) was not a common practice.  Indeed it was possibly
quite innovative, with firstly the realisation that lots of fine gold was
going out with the waste (particularly prior to the advent of the cyanide
process), and secondly by using the combination of dredging and the cyanide
process, the sludge could be recovered, reprocessed and at least a portion
of the formerly "lost gold" recovered.
 
Obviously, these plants have some historical significance in the context of
the history of goldmining in N.Z., but I would like to see how they stack up
from a more global perspective.  I would appreciate any information about
similar operations elsewhere in the world, that is, the recovery and
reprocessing of mill wastes discharged into waterways.
 
Thanks in anticipation
 
Neville Ritchie
Regional Archaeologist
Dept of Conservation
Private Bag 3072
Hamilton
NEW ZEALAND