The 50 million tonne Mount Morgan gold-copper deposit (4.99 g/t Au,
0.72 per cent Cu) was an irregular pipe-like body of quartz and pyrite,
with subordinate chalcopyrite, magnetite and gold. Mineralisation was
hosted by the Middle Devonian Mine Corridor Volcanics which occur as a
roof pendant within the Late Devonian Mount Morgan Tonalite. On three
sides, the deposit was enclosed by a series of normal, concave, listric
faults and surrounded by outward-dipping Mine Corridor Volcanics. The
tonalite which bounded the western side of the deposit transgresses the
domal structure with no apparent interference. A pipe of intense,
texturally-destructive, quartz-sericite-chlorite pyrite underlay and
partly surrounded the deposit.
The volcanic setting of the deposit, its shape, metal content and
associated footwall alteration are consistent with a subsea-floor
replacement origin for the gold-copper mineralisation. Mineralogical
studies have confirmed the deposit has been recrystallised and annealed
by a thermal metamorphic event, most likely the intrusion of the Mount
Morgan Tonalite. Pyrrhotite Pipe mineralisation replaces annealed
massive pyrite, suggesting that it formed during retrogressive
metasomatism associated with tonalite emplacement. Telluride minerals
have a close association with chalcopyrite, and no evidence for a late
telluride paragenetic stage was observed. Telluride mineralisation is
inferred to be genetically related to the massive sulphide mineralisation,
and a separate magmatogenic origin for this mineralisation is discounted.