Comparisons between individual SW
Pacific Rim gold deposits have facilit-
ated proposal of a new classification
of porphyry-related mesothermal gold
deposit types. Carbonate base-metal
gold systems form at relatively high
crustal levels, peripheral to porphyry
intrusions. There is a spatial assoc-
iation with major structures which
focus fluids from degassing porphyries
at depth. Milled matrix intrusive
breccias and extrusive maar volcano/
diatreme breccias form as a result of
pre-mineralisation phreatomagmatic
eruptions. These focus degassing
magmatic fluids by the provision of
fracture ground preparation, typically
within competent host rocks, adjacent
to the breccia bodies. Local dilational
structures control the form of
individual deposits which may contain
bonanza gold grades at sites of fluid
quenching and/or repeated mineral
deposition.
Early quartz veining and
porphyry-related alteration is common
and later gold mineralisation typically
occurs in association with carbonate
veining and Fe and base metal sulphides
as Zn > Pb > Cu. The progressive
cooling by mixing of the upwelling
mineralised magmatic fluid with
descending bicarbonate waters, produces
characteristic zonations in carbonate
compositions. These vary with distance
from the porphyry source, from Ca
through Mg, Mn and distal Fe types.
Gold mineralisation is preferentially
distributed within the Mn/Mg carbon-
ates.