In
the Mesel sediment-hosted disseminated gold deposit, the spatial distribution of
hydrothermal alteration is controlled by steeply dipping faults and shallowly
dipping lithologies. Hydrothermal alteration of carbonate rock is characterised
by decalcification, dolomitisation and silicification. Intense argillic
alteration occurs in a laccolithic andesite intrusion adjacent to decalcified
and silicified carbonate units. Sulphide minerals (pyrite, realgar, orpiment,
stibnite and cinnabar) accompany hydrothermal alteration and precious metal
mineralisation. The trace elements Ag, As, Sb and Hg are zoned spatially
throughout the deposit. Mesel contains a mineable gold reserve of 7.8 million
tonnes at 7.3 g/t Au. Gold mineralisation is preferentially localised in
carbonate rocks adjacent to and beneath the andesite intrusion. The ore body
consists of a central, near-vertical, high grade 'feeder' system flanked by
tabular, inward dipping, moderate grade 'ouflow' zones. Mineralisation is
controlled predominantly by two steeply dipping faults with high grade (10 g/t)
gold zones centred about structural intersections and fault splays. Gold is
submicron size and locked in very fine grained (<10 microns), disseminated>
and fracture-controlled arsenical pyrite. The Mesel deposit developed where high
angle faults guided ascending, sulphidising fluids into reactive carbonate
stratigraphy adjacent to and beneath the andesite intrusion and during
subsequent migration of the auriferous fluids along 'outflow' channels. Early
decalcification and dolomitisation enhanced porosity and permeability prior to
silicification and sulphidation. Gold deposition occurred late in the genesis of
the deposit.