Geology and mineralisation of the Kerimenge-Lemenge prospect, Wau district, Papua New Guinea

Mineralisation at the Kerimenge and Lemenge deposits, wau, Papua New Guinea, is an example of a fault controlled, porphyry-related, gold-carbonate-base metal epithermal system. Repeated intrusion of Edie Porphyry (2.4 to 3.8 Ma) was accompanied by a local permeability-generating phreatomagmatic eruption to form a diatreme. Deposition of quartzmanganocarbonate-sulfide-telluride-gold mineralisation was controlled by the Kerimenge Fault within the Kerimenge Sill Porphyry. The gold mineralisation was deposited in two events. In the first event, gold was deposited in four stages from convecting meteoric water. The four stages are: Stage I quartz-pyriteminor gold; Stage II quartz-pyrite-gold, Stage III carbonatequartz-arsenopyrite-gold and Stage IV (mangano)carbonate-minor gold. The mineral paragenetic sequence reflects cooling of the system with Stage I and II veins deposited from a boiling CO2- rich fluid with concomitant partitioning of volatiles including CO2, and H2S into the vapour phase. Condensation of the vapour phase higher and laterally within the system resulted in deposition of Stage IV veins. Stage III veins are regarded as transitional between Stages 1,11 and Stage IV. Quartz-sericite/(illite)-pyrite-rutile+/-carbonate phyllic alteration within the Kerimenge Sill Porphyry forms an selvage to Stage I, II and III veins. Gold is present throughout the four stages of mineralisation but dominantly occurs in Stage II and III veins. Gold occasionally occurs as electrum intergrown within Stage II pyrite but in this study gold is observed to occur primarily within the lattice of Stage II pyrite and Stage III arsenopyrite. This mineralisation is not amenable to conventional cyanide extraction. An uneconomic resource of 55Mt @ 1g/t Au has been delineated within this refractory mineralisation (Hutton et al 1990)