Advancing Science and Discovery NEWSLETTER www.segweb.org APRIL 2018 NUMBER 113 See p. 31–40SEG for 2018details Return to Bougainville—Reassessing the Conference Mineral Potential of a Long-Forgotten Island Michael Agnew† (SEG M), P.O. Box 1560, Buderim, Queensland 4556, Australia Introduction Guinea Government currently each Bougainville has 17 post-Mio- own 36.4% of Bougainville Copper Lim- cene volcanoes, including the active In June 2019, the population of Bou- ited, the publicly listed administrator of or dormant Bagana, Balbi, and Loloru gainville, a southwest Pacific island the mine after Rio Tinto divested their volcanoes (Blake, 1967). 2 to page . of 9,380 km , will vote on whether stake in the company during 2016. There are more than 60 18 to remain an autonomous region of Papua New Guinea or become the 150° E 155° E 160° E world’s newest nation. Proponents of an PACIFIC NEW M PLATE IRELAND e Manus Basin la independent Bougainville are looking n e s 7 cm/yr for sources of revenue to kick-start the ia 0.5 Ma n Ontong Java tr e Plateau island’s economic development, includ- n 8 cm/yr c 13.0 Ma SOUTH h ing reopening the mothballed Panguna BISMARK copper-gold mine (Barrett, 2017). How- MICROPLATE ever, significant local opposition to such 5° E a step remains (e.g., Davidson, 2018), so the future exploration prospects for the NEWBRITAIN island are uncertain. h nc tre in N Panguna commenced production in ita ort Br h S ew olo 1972, as the world’s largest copper mine, N mo SOLOMON SEA n t ren with total metal endowment of 18 PLATE ch billion pounds of copper and 30 million Trob rian d tro ounces of gold (historic production and ugh remaining JORC-compliant resource; SOLOMON Collier et al., 2011; Bougainville Copper ISLANDS Woodlark Limited, 2016). The operation closed 17 Basin S PAPUA an years later amid a prolonged campaign Cr NEW isto ba GUINEA l t of sabotage by the Bougainville Revo- ren 10° E 9 cm/yr ch lutionary Army, stemming from social, 0 Ma AUSTRALIAN environmental, and economic issues PLATE catalyzed by the mine (Denoon, 2000; Barrett, 2017). The Autonomous Bou- FIGURE 1. Tectonic setting of Bougainville and neighboring islands of Papua New Guinea and the gainville Government and Papua New Solomon Islands. Metal endowment of major deposits was calculated from resource and/or pro- duction data. Modified from figure 2 of Holm et al. (2016). Shaded areas occur above the 2,000-m † E-mail, [email protected] bathymetric contour and show major oceanic crustal features. Metals, Minerals, and Society Keystone, Colorado, USA September 22–25, 2018 seg2018.org 18 SEG NEWSLETTER No 113 • APRIL 2018 . from page 1 Return to Bougainville—Reassessing the Mineral Potential of a Long-Forgotten Island (continued) volcanic vents and intrusive centers and 2016). However, from 45 Ma, S-directed Crown Prince Range is the Bougainville numerous active or extinct fumarolic, subduction of the Pacific plate has Group of Quaternary andesitic volcanic acid sulfate-altered areas (Blake and occurred along the SW-dipping Melane- centers, including the oldest Emperor Miezitis, 1967; Rogerson et al., 1989). sian trench. Subduction progressively Range, Numa Numa, Tore, Billy Mitch- Small-scale gold mines operated inter- locked from the southeast to northwest ell, Taroka, and Takuan volcanics, and mittently in the Kupei and Panguna when the Ontong Java plateau arrived the younger Bagana, Balbi, and Loloru areas from 1930 to 1960 (Blake and at the Melanesian arc from 22 Ma volcanics. These units, comprising Miezitis, 1967). Apart from a geologic (Peterson et al., 1999; Sheppard and andesitic lavas, agglomerate, and diorite survey mapping and sampling program Cranfield, 2012; Lindley, 2016). Cessa- stocks and dikes, are exposed as vari- during the 1980s, little or no mineral tion of submarine andesitic volcanism ably dissected stratovolcanoes with lava exploration has occurred outside the was followed by growth of platform flows, resurgent domes, and crater lakes. Panguna mine lease since 1971. A carbonates around the emerging There are four major Miocene to decades-long moratorium on explora- volcanic edifices during the Miocene Quaternary intrusive complexes on tion and mining was lifted in October (Sheppard and Cranfield, 2012). There Bougainville and numerous smaller 2015. The Autonomous Bougainville was a second cycle of magmatism and stocks and dikes as feeders to the over- Government’s Mining Act 2015 paves volcanic activity, commencing during lying volcanic sequence (Fig. 2; Table the way for a new generation of min- the late Miocene due to N- and NE-di- 1). The Isinai Monzonite (7.43–4.82 Ma) eral exploration on the island, which rected subduction of the Solomon Sea consists of quartz monzonite, monzon- remains highly prospective for por- plate at the New Britain and San Cris- ite, and microsyenite (Rogerson et al., phyry, epithermal, and skarn deposits. tobal trenches, respectively. The precise 1989). The Kupei Intrusive Complex This article provides an overview timing of events has been a matter of (4.9–1.3 Ma), hosting the Panguna cop- of the tectonic and structural setting, speculation. However, recent calcula- per-gold porphyry deposit, comprises stratigraphy, magmatic history, and tions of the noise-reduced Euler vectors diorite, quartz diorite, granodiorite, and geochemistry of Bougainville. Magmatic (polar wander path) indicate that there quartz-feldspar porphyry (Ford, 1976; fertility indicators and whole-rock and were significant changes to the clock- Clark, 1990). Within the Emperor Range stream sediment geochemical data were wise rotation and kinematics of the Volcanics are the Puspa (2.11 Ma) and combined with the interpreted geology Pacific plate during the period 10 to 5 Melilup (1.72–0.676 Ma) intrusions. to delineate areas that are prospec- Ma (Stotz et al., 2017), consistent with Both comprise biotite + hornblende or tive for magmatic-related copper and collision of the Ontong Java plateau biotite + pyroxene leucomonzonite. gold deposits. A database of 269 X-ray with the Melanesian arc and conse- Elsewhere in the Crown Prince and fluorescence and instrumental neutron quent reversal in subduction polarity. Duero ranges are numerous small, Plio- activation analyses was compiled from cene to Quaternary diorite to microdio- published papers and unpublished rite stocks within the Toniva Formation Ph.D. theses. Sample coordinates from Stratigraphy and Atamo Volcanics, including at the Panguna mine grid were trans- The stratigraphic nomenclature of Bou- Jaba River, Malabita Hill, Umum River, formed into latitude/longitude using gainville, outlined in Blake and Miezitis Kangu Hill, and Karatu (Ford, 1976; a registered Google Earth image, color (1967), was updated by Rogerson et al. Rogerson et al., 1989). The Isinai and plate 2 from Clark (1990), and the (1989) following additional mapping, Kupei complexes have intruded middle original grid orientation (J. Lew, pers. the incorporation of offshore seismic to late Miocene andesitic volcanic rocks commun., 2017). Linear topographic data, and a review of available fossil, of the Toniva Formation and a sequence features, extracted from an ASTER C, and K-Ar dates. The Kieta Volcanics of coarse-grained fragmental rocks of digital elevation model, were combined were divided into the Atamo Volcanics, Pliocene age, referred to as the “Arawa with published interpreted structures Toniva Formation, and Arawa Conglom- Conglomerate” (Fig. 2). over the island. erate. The oldest unit, the Atamo Vol- The multiphase Kupei intrusion canics (Fig. 2), is a package of pillowed includes, from oldest to youngest, the and nonpillowed basalt-andesite lavas Panguna Andesite (intrusive phases), Tectonic Setting of Bougainville and minor volcaniclastic sandstone, Kaverong Quartz Diorite, Biuro Gran- Bougainville and the Solomon Islands conglomerate, and dolerite to diorite odiorite, and Nautango Andesite (Ford, lie on a ridge of oceanic crust, bound intrusions, exposed in the Crown Prince 1976; Clark, 1990). Leucocratic feldspar by the San Cristobal and North Solo- Range (Rogerson et al., 1989). The porphyry bodies intruded the first three mon trenches (Fig. 1). The Solomon younger Keriaka Limestone platform units. Rogerson et al. (1989) argued that Sea and South Bismark microplates and is a shallowly WSW dipping massive the informal names used at the mine, the Ontong Java oceanic plateau are to diffusely bedded organic limestone, “Biotite Diorite” and “Biotite Granodi- situated to the west and east, respec- originally deposited 20 to 30 m deep, orite” (e.g., Collier et al., 2011), were tively. Bougainville coincides with a and is 1,300 m thick (Blake and Miezi- potassic-altered phases of the Kaverong major flexure, characterized by the tis, 1967). The Keriaka Limestone is Quartz Diorite. The lowest copper and intersection of WNW- and NW-trend- unconformably overlain and onlapped gold grades are in the Biotite Granodio- ing arc segments. The San Cristobal by the Toniva Formation (Fig. 2), a mid- rite and late-mineral Biuro Granodiorite. trench currently accommodates N-di- dle to late Miocene unit of interbedded Hydrothermal biotite from the Panguna rected convergence and subduction of volcaniclastic sandstone, mudstone, and deposit yielded a K-Ar age of 3.42 ± 0.25 the Australian plate (Fig. 1; Holm et al., basalt-andesite lavas. Surrounding the Ma (Page and McDougall, 1972). No 113 • APRIL 2018 SEG NEWSLETTER 19 155°E Volcanic centres UMUM Holocene RIVER KUNAI HILL Pleistocene - Holocene Pleistocene