Gold
mineralisation at Porgera is inferred to have been derived from a deep
differentiated magmatic source which is capped by a series of (now tilted)
inward dipping sills and stocks, which comprise the Porgera Intrusive Complex.
An arc normal transfer structure provides a conduit, at tile intersection with
an arc parallel structure, into which basic magma derived from tile deep melting
of mantle material has been emplaced. Two phases of mineralisation display an
association with late stage feldspar porphyry intrusives. Stage I carbonate-base
metal-style gold mineralisation occurs as early quartz-sericite, then massive
sulphide and late zoned Mn-Fe-Mg-Ca carbonate vein/breccias. Magmatic fluids
derived from the deep magma source, are interpreted to have migrated along me
margin of a feeder stock to the sills, and south to Waruwari. Stage II
epithermal Au-Ag quartz-style gold mineralisation occurs as early fluidised
breccias and carbonate veining, followed by zoned
quartz-roscoelite-carbonate-gold vein/breccias, and late carbonate and/or
sulphate open space infill. Magmatic fluids are interpreted to have utilised the
same plumbing system as the late stage feldspar porphyry intrusive, and then
migrated into the Roamane Fault which was dilated by; dextral rotation on
elements of the transfer structure, normal faulting, and refraction around the
competent footwall hornblende diorite. Bonanza gold is localised in a jog in the
Roamane Fault, hangingwall splits and at intersections with cross structures.
Bulk low grade Stage I gold has been deposited by the mixing of magmatic fluids
with cool gas condensate waters, whereas high grade Stage II gold mineralisation
resulted from the rapid quenching of mineralised magmatic fluids by oxygenated
groundwaters.