Matangkaka manganese deposit, Ambitle Island, Feni Island Group, Papua New Guinea: a Quaternary epithermal stratabound manganese oxide deposit

Abstract

The Matangkaka manganese deposit is an unusual Quaternary epithermal stratabound manganese oxide deposit from Ambitle Island in the Feni Island Group, New Ireland Province, Papua New Guinea. Ambitle is a small volcanic island that has undergone Quaternary explosive and effusive volcanic activity. Oldest volcanic rocks on the island consist of mafic–intermediate lavas and epiclastic rocks of the Pleistocene Ambitle Volcano (Waramung Volcanics). These rocks rest unconformably on Oligocene calcilutites of the New Ireland Basin. The summit of Ambitle Volcano is now modified as a semi-circular topographic rim around Nanum Valley, known as the Nanum Valley Crater. This crater formed during gravity induced failure (or sector collapse) of the southwest flanks of the summit of Ambitle Volcano. Ambitle Crater is a resurgent Pleistocene–Holocene crater located within the summit crater of Ambitle Volcano. Deposits of resurgent volcanism include block and ash deposits, mafic–intermediate lavas, trachyte lavas, tephra and lacustrine sediments (Babamis Volcanics). Two new members are described from the Babamis Volcanics (Nanmikit Trachyte Member and Danmagal Tephra Member). The tephra member is the youngest deposit in the Plio-Pleistocene Tabar–Lihir–Tanga–Feni alkalic volcanic arc. Three new intrusive units are described (Kabang Syenite, Matangkaka Monzonite and Saddle Monzonite). The Saddle hydrothermal system (new name) is an active geothermal system, elongated along the arc-parallel northwest-trending Niffin structural zone. The Matangkaka occurrence is classified as a Mn–Fe–Cu–Mo deposit. Active northwest-trending structures and northeast-trending faults allowed sea water to circulate to depth, leach metals from basement and overlying volcaniclastic rocks and ascend to shallow levels. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units and forming 3–4 m-thick bedded deposits. Circulating neutral chloride waters of the Saddle hydrothermal system played a key role in the deposition of manganese in porous and permeable volcaniclastic units. Mineralising fluids were probably 120 °C. KEY POINTS The Matangkaka manganese deposit is an unusual Quaternary epithermal stratabound manganese oxide deposit on Ambitle Island in the Feni Island Group, Tabar–Lihir–Tanga–Feni alkalic volcanic arc, New Ireland Province, Papua New Guinea. Ambitle Island is a small volcanic island that has undergone Pleistocene–Holocene explosive and effusive volcanic activity. Active fractures have allowed sea water to circulate to depth, leach metals from basement and overlying volcaniclastic rocks and convect to shallow levels. Manganese mineralisation was deposited by neutral chloride waters, probably 120°C. Y/Ho ratios indicate there was a sea water component to mineralising fluids. The manganese occurrence is classified as a Mn–Fe–Cu–Mo deposit. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units, and forming 3–4 m-thick bedded deposits.