Gladstone Hill, a
volcanic-hosted low-sulfidation epithermal prospect in the Coromandel Peninsula, is less than 1 km from the
world-class Martha Hill Au-Ag deposit. Gold-silver-bearing quartz veins occur in
Late Miocene andesitic volcanic rocks of the Waipupu Formation that are
extensively altered. The occurrence, spatial distribution and temporal
relationships of hydrothermal minerals have been determined along two cross
sections that transect the Amaranth, Gladstone, Cowshed and Moonlight veins. Quartz
and pyrite are ubiquitous along these sections. Adularia and chlorite are
widespread, but both are scarce in wall rocks adjacent to the Moonlight and
Cowshed veins. By contrast, rare hydrothermal albite and epidote are restricted
to wall rocks surrounding the Amaranth vein. Illite, which replaces adularia, is
widespread and grades laterally eastwards into a zone of interstratified
illite-smectite that envelops the Moonlight and Cowshed veins. During late-stage
hydrothermal activity, rare calcite formed at depth and uncommon kaolinite and
alunite formed at shallow levels. The overall clay mineral zonation with illite
grading eastward into interstratified illite-smectite reflects paleothermal
gradients of >220C to -170C towards the margin. The alteration assemblage
of quartz, adularia, chlorite, albite, epidote, illite, interstratified
illite-smectite and pyrite reflects formation from a near-neutral to weakly
alkaline pH chloride water that contained low concentrations of CO2,
Late calcite formed from very local descending steam-heated CO2rich
water, whereas the rare assemblage of kaolinite and alunite formed from shallow
steam-heated acid-sulfate water. Analysis of the available data indicates that
the Martha Hill deposit and the Gladstone Hill area formed within a single large
geothermal system. We infer that the Martha Hill deposit formed in the main
upflow zone, whereas the Gladstone Hill area may have formed adjacent to this
upflow zone.