The Onemana prospect is located on the eastern Coromandel Peninsula, North
Island, New Zealand. Systematic exploration involving surface geological
mapping, geophysics, rock and soil geochemistry, reverse-circulation and diamond
drilling has identified a number of important features of a fossil hydrothermal
system at Onemana. This relatively intact gold mineralised system is hosted
within a sequence of rhyolitic volcanics and volcaniclastics, overlain by lake
sediments, sinters and hydrothermal eruption breccia. The system lies within a
graben/half graben bounded by major NE-trending faults which form the
approximate boundaries to hydrothermal alteration.
Rock and soil geochemistry indicate a broad zone of anomalous gold, silver,
arsenic, mercury and antimony. Overlapping elevated rock and soil gold anomalies
indicate strongest mineralisation occurs over two zones of hydrothermal
brecciation named 'Breccia Knob' and 'Western Ridge'.
Geophysical data obtained from Gradient Array Resistivity and Ground
Magnetics have been used to produce anomaly maps. The terrain corrected
resistivity plot shows a number of well defined linear resistivity anomalies
which are believed to represent large silicified and quartz veined bodies at
relatively shallow depths. The strongest of these anomalies correspond with
'Breccia Knob' and 'Western Ridge'.
Reverse-circulation and diamond drilling targeting these resistivity and
geochemical anomalies has confirmed wide intervals of quartz veining,
silicification and gold mineralisation. Petrologic data based on drill samples
indicates the existence of a laterally extensive gold bearing epithermal system.
Hydrothermal alteration is thought to have been caused by strongly convecting,
two-phase (boiling), near-neutral chloride fluids which persisted at high
temperatures (170 - 220C) to the paleosurface.
A geological model is postulated of a large, complex, hydrothermal system
developed in a shallow lake or marginal lake environment created by rifting and
graben development within a recently extinct rhyolitic volcanic centre.
Geothermal activity alternated between relatively quiescent periods of
unrestricted fluid upflow and sinter development interspersed with violent
hydrothermal eruptions caused by near surface silica capping and overpressuring
above a major fluid conduit centred under 'Breccia Knob'.
Opportunities for precious metal mineralisation are evident from
drilling to date. Upflow of near-neutral chloride fluids to shallow depths
favours the presence of strongly mineralised stockworks at relatively shallow
depths. Periodic hydrothermal eruptions would have caused fracturing and rapid
depressurisation of the hydrothermal system, strongly favouring the
precipitation of gold from solution both near surface and within feeder-vein
systems at depth.