Magnetite deposits with distinctive actinolite
apatite-bearing gangue and spatially-extensive sodic-calcic wall rock alteration
occur in both Proterozoic metamorphic terrains and Mesozoic-Tertiary magmatic
arcs. Examples from both settings constitute significant sources of iron ore (egKiruna, Sweden; El Romeral, Chile)
and some host major Cu-Au deposits that have a local association with K-silicate
alteration (eg Cloncurry district, NW Queensland; Copiapo district, Chile). A growing body of evidence
points to a hydrothermal origin for these systems involving high temperature,
low S oxidised fluids. The Cloncurry magnetite-Cu-Au deposits are products of
the local structurally channelised influx of a specialised high salinity ore
fluid into larger-scale alteration systems developed during pluton emplacement
at greater depths than those associated with subvolcanic (porphyry) Cu-Au
deposits. The geological characteristics of deposits in magmatic arcs are
consistent with a similar mode of origin albeit in rather different tectonic
settings.