Sampling and analyses
of deep waters from Wairakei and Rotokawa geothermal systems indicate a wide
range of precious metal concentrations (<0.1 to 23 ppb au; 6 to 2400 ppb ag),>
suggesting that eastern andesitic TVZ geothermal systems (eg Rotokawa) have
higher aqueous gold and silver than western basaltic geothermal systems (eg
Wairakei); this interpretation is consistent with known occurrences of high
grade precipitates in wells from eastern systems. The fact that the deep waters
are undersaturated in gold further suggests that the aqueous gold concentrations
of deep waters are limited by metal availability. The flux of gold for the two
systems ranges from ~1 to 50 kg Au/yr, and the flux of silver ranges from 65 to
6900 kg Ag/yr. At Rotokawa, it only takes ~600 years to flux one million oz of
Au and >100 million oz of Ag. The supply of hydrothermal gold and silver is
obviously important during genesis of an epithermal deposit, but ore formation
also depends on the existence of a suitable trap that efficiently extracts
metal from solution in a small volume of rock. Such traps seem to have been
poorly developed until geothermal drilling in the TVZ.