Previous paleodepth studies of low-sulphidation epithermal Au-Ag deposits
in the Coromandel region indicated that economic gold mineralisation was
generally limited to a depth zone of 100 to 400 m below the paleosurface.
Notable exceptions are Waihi and Karangahake, where ore shoots extended to
depths of about 750 and 900 m respectively. These studies have been extended to
include Au-Ag, Hg and Sb epithermal deposits in Northland and the Taupo Volcanic
Zone. The main paleodepth indicators used are sinters and hydrothermal eruption
breccias, hydrostatic pressures estimated from homogenisation temperature
measurements on fluid inclusions, and hydrothermal alteration mineralogy. Also
the occurrence of lacustrine and fluvial sediments is an indicator of the
paleosurface at their time of deposition. Selected paleodepth indicators have
been compiled and mapped in a GIS environment. The distribution of the
paleodepth indicators are used to define regional variations in erosion levels,
and may be useful in predicting areas most likely to preserve the optimum depth
window for gold deposition. In the Coromandel region, regional stream sediment
geochemical data has also been used to broadly define erosion levels in terms of
Zn, Pb and Cu representing deeper base metal zone mineralization and Hg and Sb
representing shallower cinnabar and stibnite mineralisation. The interpretations
of paleodepth and potential reconstruction of paleosurfaces are limited by the
paucity of data, poor age control on the paleodepth indicators and the
epithermal deposits, and marked relief and complex stratigraphy in the volcanic
terranes.