The discovery since 1990 of four major orebodies in the Mt Isa
Inlier - Century Zn-Pb, Osborne Cu-Au, Cannington Pb-Zn and
Ernest Henry Cu-Au - only serves to enhance the already high
stature of the Inlier in world mining importance. Currently the
known mineral resources of the Inlier represent about 11 per cent
of the world's Pb-Zn resources, five per cent of the world's Ag
resources, and one per cent of the world's Cu resources. More
can be expected to be added to these impressive figures given that
two-thirds of the Inlier remains obscured by younger cover. Individually, projects such as Century and Cannington may
ultimately achieve `world's best' status; Century's proposed
production of 450 000 tpa of Zn metal would rank it close to
number one with Cominco's expanded Red Dog project in
Alaska. Cannington, with projected production of 660 tpa Ag,
could become the world's biggest producer of silver. For
comparison, Mt Isa produced 170 000 t of zinc metal and 335 t
of silver in 1995. The apparent physical diversity of deposits in the Mt Isa Inlier
is matched by a great diversity of opinion as to their origins.
Because Ernest Henry is hosted by `porphyries', it is described
by some as a porphyry copper deposit, despite mineralisation age
possibly post-dating volcanic age by 200 Ma. Cannington Pb-Zn
could be a Broken Hill-type (BHT) exhalitive deposit, or a late
epigenetic replacement `regional skarn'. Mt Isa - Hilton Pb-Zn
deposits appear to have experienced all deformational events and
he syn-diagenetic in origin, or may have experienced little of the
above and be epigenetic replacement deposits formed late in the
major Isan orogeny, some 150 Ma after deposition.