The
Morobe Goldfield inPapua New
Guinea has produced
in excess of 120 tonnes of gold from predominantly alluvial sources. Hardrock
gold has been mined historically at Wau (23 t) and Edie Creek (4
t) and geological resources have been established at Hidden Valley (78 t),
Kerimenge (36 t) and Hamata (30 t).Numerous prospects have also been identified
within the goldfield.
Primary
gold mineralisation is related to intrusion of dacitic to andesitic Edie
Porphyry (2.4 to 3.8 Ma), often with associated diatremes, which have intruded
along major regional structures. A full spectrum of deposit styles from proximal
(mesothermal) to distal (epithermal) to the porphyry source is recognised. The
transition between the proximal and distal styles is recognised both between
deposits and within an individual deposit. The deposit styles should be termed
porphyry related gold mineralisation rather than given the generic epithermal
and mesothermal labels.

It is
proposed that magmatic fluids which evolved from parent melts to the high level
Edie porphyry stocks mixed with convecting meteoric fluids and were channelled
along major structure and diatreme contacts. Proximal to the intrusives, gold
mineralisation is associated with deposition of
pyrite-hematite-quartz+/-magnetite-gold (Hamata) and
pyritebasemetals-quartz-gold (Wau). The mineralising fluids were hot, and
periodically saline due to pulses of magmatic fluid. Gold is non-refractory and
occurs as 20 to 30 micron blebs within the pyrite.

More
distal from the intrusives or under late cooler conditions, mixing of cool
dilute meteoric and magmatic fluids resulted in gold deposition associated with
arsenic rich pyrite-and arsenopyrite-mineralisation (Kerimenge, Hamata and Edie
Creek). Gold is refractory and occurs within the sulfides either as sub-micron
inclusion or within the lattice.

CO2 and H2S gases which evolved
off the upwelling hydrothermal fluids as a result of boiling and pressure
reductions, condensed within superficial groundwater and formed cool oxygenated
bicarbonate fluids. These fluids percolated back down the structures depositing
manganocarbonate in the upper parts of the hydrothermal system. At deeper levels
in the system mixing of the descending bicarbonate fluid with upwelling
mineralised fluids deposited manganocarbonate-quartz-sulfides nonrefractory gold
(Hidden
Valley, Wau and Edie
Creek).

At Kerimenge the cool bicarbonate fluids have also
quenched a late stage magmatic fluid rising along high permeability structures.
The quenching has resulted in deposition of non-refractory gold mineralisation
associated with hessite (Ag2Te)-tennantite-chalcopyrite deposition.

Within the Morobe Goldfield, a transition from deeper
level pyrite-quartz+/-hematite-magnetite to intermediate level quartz sulfide
and quartz-carbonate-sulfide deposition to upper level, massive manganocarbonate
deposition is recognized. Gold mineralisation can occur within all levels up the
system.