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 $\le$120 °C.

KEY POINTS

1. 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.

2. 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.

3. Manganese mineralisation was deposited by neutral chloride waters, probably $\le$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.

4. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units, and forming 3–4 m-thick bedded deposits.

Keywords:

• alkalic volcanism
• Ambitle Island
• New Ireland Basin
• Papua New Guinea
• stratabound manganese mineralisation
• Tabar–Lihir–Tanga–Feni alkalic volcanic arc
• volcanology

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Acknowledgements

The geological mapping reported in this paper was completed with the late Wanu Tamu. I am grateful to Steven Mirang and his wife who provided accommodation at their Nanmikit haus during our work. Michael Koibua is thanked for his camp management and keeping on top of daily matters. I am also grateful to the Ward 10 and Ward 11 communities from the Nanum Valley, Natong, Pikan and Fatkarson for their enduring hospitality and friendliness during fieldwork that has spanned 33 years. Special thanks to Vincent and Darius Tarere and their families for hospitality while we were at Nanum Bay, Jude Tomainder (Ward 10 Chairman), Patrick Sok’lou, Joanne Kolo’ke, Charles Maras, Clement Tomais (Ward 11 Chairman), Anton and Teresia Tengmil of Nisnis, John Vingsal, Maisie Micah of Kavieng, our field crews and many others. Craig Powell, pilot with Niugini Helicopters, is thanked for drop off and pickup from Ambitle Island. I am grateful to Mayur Resources Limited for access to data and permission to publish. Tim Crossley, Executive Director, and Tom Charlton, Chief Geologist, in particular, ensured the successful completion of fieldwork. The author would like to thank reviewers Tim McConachy and Richard Moore for their comments and suggestions that improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author.

Data availability statement

Details of analytical methods, analyses of manganiferous rocks and concentrations of yttrium and rare-earth elements in manganiferous rocks that support the findings of this study are openly available in figshare at http://doi.org/10.6084/m9.figshare.13604906.