Approximately 42 t of mercury were produced from Puhipuhi
from the late nineteenth century until the end of World War II, with no
significant gold or silver production. Sinter, eruption breccias and veins,
mined for mercury at Puhipuhi and essentially barren of gold, are inferred in
many geological models to represent the surficial levels of adularia-sericite
style low sulfidation gold-silver systems. These systems may contain economic
gold-silver mineralisation at depth. Conceptual geological models have been
employed at Puhipuhi in the search for buried gold mineralisation. Homestake
Mining utilised a McLaughlin model for exploration from 1982 to 1987, targeting
mainly the vicinity of the sinter deposits. From 1987 to 1989 BHP sought deeper
structurally controlled gold mineralisation within the basement greywackes,
utilising models derived from studies of Round Mountain, USA and Hishikari,
Japan. Macraes Mining Company Limited have accessed the previous work and
applied a conceptual geological which invokes a north-south structural control
for the emplacement of high-level felsic intrusions at depth. Dextral movement
on these structures associated with oblique convergence facilitated the
formation of a pull-apart basin in which lacustrine sediments overlie basement
greywackes and are in tum overlain by Pliocene basalts. Continued activation of
the NS structures created a series of inferred NE-trending dilational
structures, which are targeted as fluid upflow centres for the surficial
outflows. Upflows occur as eruption breccias (e.g., Plumduff) and outflow as
silica sinters (eg Mt Mitchell, Williams) which are also partly obscured by the
basalt cover. Argillic alteration and mercury mineralisation occur at elevated
crustal levels (eg Puhipuhi mine). Modelling from structure and alteration
zonations have targeted a fluid upflow in the vicinity of Bush Hill, which was
not accessed by previous workers. This target was subsequently supported by
surface geology and multi-element soil geochemistry.