Gold deposits and hydrothermal eruption breccias associated with a maar volcano at Wau, Papua New Guinea

Abstract

Emplacement of endogenous domes and eruption of pyroclastic flows in the Morobe goldfield took place in the interval from 4 to 2.4 m.y. ago. During the later part of this time interval, following tilting of the ignimbrite pile, a maar was generated at Wau. The subjacent diatreme is believed to extend downward for about 1 km to a regional dip-slip fault zone which hosts deeper level gold mineralization at the nearby Ribroaster Mine. Following initial maar formation, hydrothermal brecciation and gold deposition were controlled by low-angle extensional faults which developed during incipient subsidence of a wedge of poorly supported rock between the regional fault zone and the ring fault delimiting the maar. Epiclastic sedimentation was widespread in the maar. Volatile-depleted magma reached the surface around the perimeter of the maar during active sedimentation and constructed two principal endogenous domes, one dacitic and the other andesitic. The dacitic dome, dated previously at 2.4 m.y., gave rise to an apron of crumble breccia along the northwestern edge of the maar. Accumulation of the intramaar sequence was accompanied and followed by widespread subsidence and slumping which resulted in steep dips and locally overturned beds. The regional drainage system eventually breached the crater rim and resulted in deposition of channel conglomerates and fluviolacustrine beds, all carrying alluvial gold. Radiocarbon dating of wood from this unit showed that it accumulated >42,000 years ago. The most recent events at Wau are believed to be a series of hydrothermal eruptions which constructed a composite breccia apron at Koranga Crater on the southeastern edge of the maar. The most recent eruption is tentatively inferred to have taken place in 1967.--Modified journal abstract.