Late Quaternary geology of Ambitle Volcano, Feni Island Group, Papua New Guinea

Abstract

Ambitle Volcano (new name) is the most recently active of four eruptive centres that make up the mainly Pliocene–Pleistocene Tabar–Lihir–Tanga–Feni (TLTF) alkalic volcanic province, located in the New Ireland Basin, Papua New Guinea. Ambitle Volcano is a submarine and subaerial stratovolcano occupying all of Ambitle Island. The volcano rises 2500 m above the surrounding sea floor to sea level and, with a maximum elevation of 479 m above sea level, indicates a structure nearly 3000 m high. Volcanic deposits rest unconformably on Oligocene basement rocks of the New Ireland Basin. The cone of Ambitle Volcano is constructed mainly of lavas and pyroclastic and epiclastic rocks; lavas are commonly vesiculated. These lavas are strongly undersaturated and intermediate in composition (phonolitic tephrite and tephritic phonolite) with alkali basalt, tephrite and basanite and trachybasalt and trachyandesite also present. Syenite porphyry and monzonite stocks intrude the cone-forming mafic–intermediate sequence at Kabang–Matangkaka and in the upper Nanum River. The central part of the Ambitle Volcano is now modified as a prominent semi-circular topographic rim around the Nanum Valley. The Nanum Valley Crater (new name) is the product of large-scale summit failure of the SW flanks of the summit of the Ambitle Volcano. This event is dated no younger than 0.68–0.49 Ma. The Ambitle Crater (new name), the product of Late Quaternary resurgence of volcanism following sector collapse of Ambitle Volcano, is located in the NE portion of the Nanum Valley Crater. The crater is elongated NNE and measures 900 m × 550 m at its widest development. The strong NNE–SSW linearity of the western rim of Ambitle Crater is structurally controlled by the Kabang Fault. Tephra was erupted from the Ambitle Crater at 2300 ± 100 a and is widely dispersed throughout the Nanum Valley Crater and beyond. This is the youngest volcanic event in the TLTF volcanic province. The Niffin graben is a major NW–SE-trending structural corridor that transects Ambitle Island. The structural corridor is parallel to the NW–SE strike of the TLTF volcanic province suggesting it has been an important control on magmatism and volcanism. Presently active geothermal systems are located along Niffin graben structures in the western valleys of the island and in the Nanum Valley Crater. The volcanic rocks of Ambitle Volcano host porphyry Cu–Au style mineralisation and epithermal Ladolam-type Au mineralisation. Extensive exploration including surface sampling and subsurface drilling completed since 1983 on many prospects has not defined an economic resource.

KEY WORDS:

• alkalic volcanism
• Ambitle Island
• Bismarck plate
• gold mineralisation
• Ladolam gold deposit
• New Ireland Basin
• porphyry copper mineralisation
• Solomon Plate

ACKNOWLEDGEMENTS

I wish to thank Steve Saunders for his constructive review. Wally Johnson reviewed an early draft of the manuscript, resulting in many significant improvements. I am particularly grateful to Bob McNeil, who has facilitated my fieldwork. The writer has completed numerous geological sorties on the Feni Islands during the period 1983–2006, and many village communities are acknowledged for their support. During the work described in this paper, I was accompanied by geologist, Wanu Tamu. I am especially grateful to the many Ambitle Islanders who have worked as field assistants, dinghy operators, cooks and provided accommodation, especially the late Lawrence Waller of Nanum Bay, a long-time friend and field assistant (pictured background Figure 6d); Jack Aiom, who lost a finger on the core saw (Figure 6c); and Clement Tomais of Nabung (Figure 6b). Steve Saunders of the Rabaul Volcanological Observatory is thanked for his description of the 2012 Ambitle Island seismic crisis. This paper is written in memory of my late geologist colleague, John Weir, who was working with me in the Nanum Valley at the time of the 16 October 1987 plane crash at Malekolon Airfield, Babase Island, that claimed his life, and the late Lawrence Waller.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author.