The world-class Waihi (>6.6 Moz Au, >41.8 Moz Ag) and adjacent
Favona (>0.5 Moz Au) deposits are within a single volcanic-hosted epithermal
Au-Ag district located in the southern Coromandel Peninsula, New Zealand. Gold
and silver mineralised quartz veins at both deposits are hosted in porphyritic
andesite flows of the Waipupu Formation. Both deposits have veins surrounded by
intense widespread hydrothermal alteration that is characterised by the
assemblage of quartz, pyrite, adularia, illite or interstratified
illite-smectite, plus less common chlorite and albite, which is locally
overprinted by minor calcite or rare kaolinite. Quantitative mass transfer
calculations using Gresen's general metasomatic equation indicate that
hydrothermally altered andesite at both deposits is enriched in silicon,
potassium, rubidium, barium, sulphur, gold, arsenic and antimony, whereas
sodium, strontium and calcium are mostly lost. Iron and magnesium show mixed
gain and loss. Element gain or loss can be directly related to the prevailing
alteration mineralogy. Addition of potassium, rubidium and barium coupled with
the concomitant loss of sodium, strontium and calcium in intensely altered wall
rocks corresponds to the replacement of plagioclase by adularia or illite.
Partial retention of sodium and calcium reflects the presence of hydrothermal
albite and calcite, respectively. Mass balance calculations show distinct
element gain and loss that can be related to the intensity and type of
alteration mineralogy. However, because most rocks are intensely altered and the
progression into less altered peripheral rocks is not observed in this study,
the lateral extent of elements gain or loss is unconstrained with anomalies
extending at least 200 to 300 m away from major veins.