Gold mineralisation at the Kerimenge-Lemenge prospects is a
classical example of myltiple, overprinting events, associated with
intrusive-related hydrothermal systems. Repeated emplacement
of high level dacitic porphyry bodies (Edie Porphyry; 2.4 - 3.8
Ma) was accompanied by a phreatomagmatic eruption which
formed a diatreme-maar complex. Later hydrothermal fluids
utilised permeable fault zones and diatreme contacts.
Mineralisation took place during two main events. Stage I consists of early quartz - illite/sericite - carbonate phase
with associated pyrite-arsenopyrite mineralisation. This phase is
zoned from quartz - sericite-pyrite/arsenopyrite in the south at
depth, to dominantly manganocarbonate - illitic clay - arsenopyrite- pyrite_s significant base metals in the north and at shallow levels.
The majority of known gold (18.1 Mt @ 2.0 g/t Au) at Kerimenge
is associated with Stage I activity. The gold occurs primary within
the lattice of the sulphides and is metallurgically refractory.
Fluid inclusion studies indicate early boiling and progressive
cooling and dilution of hydrothermal fluids during the evolution
of the Stage I event. Near paleosurface condensation of gases,
which evolved off the boiling system at depth, produced abundant
low pH bicarbonate fluids. During the waning phases of Stage I
activity, these bicarbonate condensate fluids descended down
permeable structures and formed banded manganocarbonate veins.
High grade non-refractory gold mineralisation during Stage II event
is restricted to favourable structures at relatively shallow levels
and in the northern regions of the prospect area. Gold in this
event exhibits a distinctive Au - Ag - Te -Cu geochemical
signature, and is associated with hessite - tennantite - chalcopyrite
mineralisation within quartz veining. Alteration and fluid
inclusion studies indicate that this style of mineralisation is a
result of the quenching of pulses of hot, late magmatic mineralised
fluids by the cold condensate waters.