Mt Kasi in Fiji is a high
sulphidation style of gold deposit
exposed at a high crustal level.
Individual alteration centres are
localised at the intersection of cross
structures within a through-going
structural corridor, the Mt Kasi Fault
System. A two stage alteration/
mineralisation model suggests that much
of the high sulphidation alteration is
derived from the initial rapidly moving
gas-rich portion of the hydrothermal
fluid. The gradual cooling and neut-
ralisation of hot acid fluids by mixing
with oxygenated ground waters forms the
characteristic zoned alteration assem-
blages which grade from cores of
residual silica through silica-alunite
to peripheral clays. The liquid-
dominated phase of the hydrothermal
fluid brecciates the competent, altered
rocks and deposits sulphide mineralise-
ation within the breccia matrix. High
gold grades occur in the matrix-rich
breccias and decline from the hydro-
thermal fluid upflow to outflow zones.
CSAMT (controlled-source audio
magneto-tellurics) geophysics has been
used to define the geometry of the
alteration centres while strike slip
movement on the major structures
accounts for the dilational structural
environment in which the ore hosting
breccias form.