Known epithermal deposits in the Coromandel region
belong to the low-sulphidation type, represented by three subtypes:
(1) andesite-hosted epithermal Au-Ag,
(2) rhyolite-hosted epithermal Au-Ag, and
(3) andesite-hosted polymetallic veins.
Paleo-depths for a range ofdeposits are estimated using
various features including: paleo-surface indicators (e.g. sinters), hydrostatic
pressures estimated from homogenisation temperature measurements on fluid
inclusions, and alteration mineralogy. Paleo-depth estimates to the base of the
ore zone for the rhyolite-hosted Coromandel deposits are in the range 200 to 500
m. Andesite-hosted Au-Ag deposits have more variable paleo-depths, and are
deeper (up to 950 m) in those deposits with base metals at depth. The majority
of Coromandel Au-Ag deposits have limited depth extents (<200 m) of economic>
grade mineralisation, although Waihi and Karangahake are notable exceptions with
depth extents of 575 m and 700 m respectively. Limited depth extents of economic
ore are a feature of epithermal systems worldwide and are commonly attributed to
gold deposition from boiling fluids.
Under hydrostatic conditions in epithermal/geothermal
systems, boiling in an up-flowing hydrothermal fluid is controlled by the
boiling point for depth (BPD) curve. From fluid inclusion studies of many
low-sulphidation epithermal systems, deposition of electrum in economic
concentrations appears to be confined to a temperature window of 260 to 180C.
This means that for dilute, gas-poor fluids that follow the BPD curve, gold
deposition is limited to a vertical depth zone of about 400 m, with its upper
boundary at about 100 m below the ground surface.
The greater depth extent at Waihi and Karangahake, can
be explained by mixing of/a deep, hot (300C) fluid with cooler (c. 200C)
heated ground waters. This has the effect of lowering the 260C isotherm by up
to 300 m, thereby increasing the depth range of economic grade gold deposition
within the 260 to 180Cwindow of electrum deposition.