IntroductionIn June 2019, the population of Bou-gainville, a southwest Pacific island of 9,380 km2, will vote on whether to remain an autonomous region of Papua New Guinea or become the world’s newest nation. Proponents of an independent Bougainville are looking for sources of revenue to kick-start the island’s economic development, includ-ing reopening the mothballed Panguna copper-gold mine (Barrett, 2017). How-ever, significant local opposition to such a step remains (e.g., Davidson, 2018), so the future exploration prospects for the island are uncertain.

Panguna commenced production in 1972, as the world’s largest copper mine, with total metal endowment of 18 billion pounds of copper and 30 million ounces of gold (historic production and remaining JORC-compliant resource; Collier et al., 2011; Bougainville Copper Limited, 2016). The operation closed 17 years later amid a prolonged campaign of sabotage by the Bougainville Revo-lutionary Army, stemming from social, environmental, and economic issues catalyzed by the mine (Denoon, 2000; Barrett, 2017). The Autonomous Bou-gainville Government and Papua New

Guinea Government currently each own 36.4% of Bougainville Copper Lim-ited, the publicly listed administrator of the mine after Rio Tinto divested their stake in the company during 2016.

Bougainville has 17 post-Mio-cene volcanoes, including the active or dormant Bagana, Balbi, and Loloru volcanoes (Blake, 1967). There are more than 60

Advancing Science and Discovery

APRIL 2018 NUMBER 113

NEWSLETTERwww.segweb.org

† E-mail, mwagnew2004@yahoo.com.au

Return to Bougainville—Reassessing the Mineral Potential of a Long-Forgotten IslandMichael Agnew† (SEG M), P.O. Box 1560, Buderim, Queensland 4556, Australia

8 cm/yr13.0 Ma

9 cm/yr0 Ma

7 cm/yr0.5 Ma

Trobriand trough

San Cristobal trench

WoodlarkBasin

North Solomon trench

NEW BRITAIN

New Britain trench

SOUTHBISMARK

MICROPLATE

SOLOMONISLANDS

PAPUANEW

GUINEA

Manus Basin

Ontong JavaPlateau

PACIFICPLATENEW

IRELAND

Melanesian

trench

SOLOMON SEAPLATE

AUSTRALIANPLATE

150° E 155° E 160° E

10° E

5° E

FIGURE 1. Tectonic setting of Bougainville and neighboring islands of Papua New Guinea and the 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 bathymetric contour and show major oceanic crustal features.

SEG 2018

Conference

See p. 31–40 for details

September 22–25, 2018seg2018.org

Metals, Minerals, and SocietyKeystone, Colorado, USA

to page 18 . . .

18 S E G N E W S L E T T E R No 113 • APRIL 2018

volcanic vents and intrusive centers and numerous active or extinct fumarolic, acid sulfate-altered areas (Blake and Miezitis, 1967; Rogerson et al., 1989). Small-scale gold mines operated inter-mittently in the Kupei and Panguna areas from 1930 to 1960 (Blake and Miezitis, 1967). Apart from a geologic survey mapping and sampling program during the 1980s, little or no mineral exploration has occurred outside the Panguna mine lease since 1971. A decades-long moratorium on explora-tion and mining was lifted in October 2015. The Autonomous Bougainville Government’s Mining Act 2015 paves the way for a new generation of min-eral exploration on the island, which remains highly prospective for por-phyry, epithermal, and skarn deposits.

This article provides an overview of the tectonic and structural setting, stratigraphy, magmatic history, and geochemistry of Bougainville. Magmatic fertility indicators and whole-rock and stream sediment geochemical data were combined with the interpreted geology to delineate areas that are prospec-tive for magmatic-related copper and gold deposits. A database of 269 X-ray fluorescence and instrumental neutron activation analyses was compiled from published papers and unpublished Ph.D. theses. Sample coordinates from the Panguna mine grid were trans-formed into latitude/longitude using a registered Google Earth image, color plate 2 from Clark (1990), and the original grid orientation (J. Lew, pers. commun., 2017). Linear topographic features, extracted from an ASTER digital elevation model, were combined with published interpreted structures over the island.

Tectonic Setting of BougainvilleBougainville and the Solomon Islands lie on a ridge of oceanic crust, bound by the San Cristobal and North Solo-mon trenches (Fig. 1). The Solomon Sea and South Bismark microplates and the Ontong Java oceanic plateau are situated to the west and east, respec-tively. Bougainville coincides with a major flexure, characterized by the intersection of WNW- and NW-trend-ing arc segments. The San Cristobal trench currently accommodates N-di-rected convergence and subduction of the Australian plate (Fig. 1; Holm et al.,

2016). However, from 45 Ma, S-directed subduction of the Pacific plate has occurred along the SW-dipping Melane-sian trench. Subduction progressively locked from the southeast to northwest when the Ontong Java plateau arrived at the Melanesian arc from 22 Ma (Peterson et al., 1999; Sheppard and Cranfield, 2012; Lindley, 2016). Cessa-tion of submarine andesitic volcanism was followed by growth of platform carbonates around the emerging volcanic edifices during the Miocene (Sheppard and Cranfield, 2012). There was a second cycle of magmatism and volcanic activity, commencing during the late Miocene due to N- and NE-di-rected subduction of the Solomon Sea plate at the New Britain and San Cris-tobal trenches, respectively. The precise timing of events has been a matter of speculation. However, recent calcula-tions of the noise-reduced Euler vectors (polar wander path) indicate that there were significant changes to the clock-wise rotation and kinematics of the Pacific plate during the period 10 to 5 Ma (Stotz et al., 2017), consistent with collision of the Ontong Java plateau with the Melanesian arc and conse-quent reversal in subduction polarity.

StratigraphyThe stratigraphic nomenclature of Bou-gainville, outlined in Blake and Miezitis (1967), was updated by Rogerson et al. (1989) following additional mapping, the incorporation of offshore seismic data, and a review of available fossil, C, and K-Ar dates. The Kieta Volcanics were divided into the Atamo Volcanics, Toniva Formation, and Arawa Conglom-erate. The oldest unit, the Atamo Vol-canics (Fig. 2), is a package of pillowed and nonpillowed basalt-andesite lavas and minor volcaniclastic sandstone, conglomerate, and dolerite to diorite intrusions, exposed in the Crown Prince Range (Rogerson et al., 1989). The younger Keriaka Limestone platform is a shallowly WSW dipping massive to diffusely bedded organic limestone, originally deposited 20 to 30 m deep, and is 1,300 m thick (Blake and Miezi-tis, 1967). The Keriaka Limestone is unconformably overlain and onlapped by the Toniva Formation (Fig. 2), a mid-dle to late Miocene unit of interbedded volcaniclastic sandstone, mudstone, and basalt-andesite lavas. Surrounding the

Crown Prince Range is the Bougainville Group of Quaternary andesitic volcanic centers, including the oldest Emperor Range, Numa Numa, Tore, Billy Mitch-ell, Taroka, and Takuan volcanics, and the younger Bagana, Balbi, and Loloru volcanics. These units, comprising andesitic lavas, agglomerate, and diorite stocks and dikes, are exposed as vari-ably dissected stratovolcanoes with lava flows, resurgent domes, and crater lakes.

There are four major Miocene to Quaternary intrusive complexes on Bougainville and numerous smaller stocks and dikes as feeders to the over-lying volcanic sequence (Fig. 2; Table 1). The Isinai Monzonite (7.43–4.82 Ma) consists of quartz monzonite, monzon-ite, and microsyenite (Rogerson et al., 1989). The Kupei Intrusive Complex (4.9–1.3 Ma), hosting the Panguna cop-per-gold porphyry deposit, comprises diorite, quartz diorite, granodiorite, and quartz-feldspar porphyry (Ford, 1976; Clark, 1990). Within the Emperor Range Volcanics are the Puspa (2.11 Ma) and Melilup (1.72–0.676 Ma) intrusions. Both comprise biotite + hornblende or biotite + pyroxene leucomonzonite. Elsewhere in the Crown Prince and Duero ranges are numerous small, Plio-cene to Quaternary diorite to microdio-rite stocks within the Toniva Formation and Atamo Volcanics, including at Jaba River, Malabita Hill, Umum River, Kangu Hill, and Karatu (Ford, 1976; Rogerson et al., 1989). The Isinai and Kupei complexes have intruded middle to late Miocene andesitic volcanic rocks of the Toniva Formation and a sequence of coarse-grained fragmental rocks of Pliocene age, referred to as the “Arawa Conglomerate” (Fig. 2).

The multiphase Kupei intrusion includes, from oldest to youngest, the Panguna Andesite (intrusive phases), Kaverong Quartz Diorite, Biuro Gran-odiorite, and Nautango Andesite (Ford, 1976; Clark, 1990). Leucocratic feldspar porphyry bodies intruded the first three units. Rogerson et al. (1989) argued that the informal names used at the mine, “Biotite Diorite” and “Biotite Granodi-orite” (e.g., Collier et al., 2011), were potassic-altered phases of the Kaverong Quartz Diorite. The lowest copper and gold grades are in the Biotite Granodio-rite and late-mineral Biuro Granodiorite. Hydrothermal biotite from the Panguna deposit yielded a K-Ar age of 3.42 ± 0.25 Ma (Page and McDougall, 1972).

Return to Bougainville—Reassessing the Mineral Potential of a Long-Forgotten Island (continued). . . from page 1

No 113 • APRIL 2018 S E G N E W S L E T T E R 19

StructuresAlthough previous workers found little field evidence for major faulting, three dominant structural orientations have been recognized (Figs. 2, 3; Blake and Miezitis, 1967; Rogerson et al., 1989; this study):

1. West-northwest (270°–300°), defined by the long axis of Eocene to Mio-cene units exposed in the Crown Prince and Deuro ranges,

2. North-northwest (335°–340°), defined by the long axis of Buka Island, and

3. Northwest (~320°), represented by the axis of Pleistocene to Holocene

volcanic vents, including Taroka, Takuan, and Balbi volcanoes.

In addition, ENE-, NE-, and NNE-trending faults occur offshore and throughout the island, within Eocene to Miocene and Pliocene to Holocene units. Intersections of these structures have produced rhombic and triangu-lar fault blocks, onshore and offshore of Bougainville (Fig. 3). The Crown Prince and Deuro ranges are charac-terized by numerous WNW-trending, arc-parallel faults with apparent sinis-tral offsets. The Miocene intrusive complexes appear compartmentalized

between WNW- and NNE-trending structures (Figs. 2, 3). The Panguna porphyry copper-gold deposit occurs at the intersection between one of these WNW-trending arc-parallel faults and potentially an NNE-trending arc-trans-verse fault. In the northern part of Bougainville, the Melilup and Puspa intrusive complexes lie along NE- to NNE-trending arc-transverse structures.

The volcanosedimentary rocks on Bougainville have undergone minor folding and tilting. Oligocene to Mio-cene and Pleistocene platform carbon-ate units dip less than 5° (Blake and Miezitis, to page 20 . . .

sand, alluvium, colluviumHolocene

NumaNumaVolcano

BillyMitchellVolcano

Reini Volcano

Pangunamine

TakuanVolcanoTaroka

Volcano

LoloruVolcano

ToreVolcano

BalbiVolcano

BaganaVolcano

ISINAI

PUSPA

MELILUP

KUPEI

Bougainville Group

diorite, monzonite, syenite, granodiorite

Toniva Formation

Keriaka Limestone

Buka Formation

Sohano Limestone

Stratigraphy / lithology

Pleistocene

L. Oligocene - E. Miocene

Pliocene - Holocene

M. - L. Miocene

Miocene - Pleistocene

L. Eocene - Miocene

Arawa ConglomerateL. Miocene - Pliocene

Pleistocene or older

Pleistocene - HoloceneHolocene

Volcanic centres

BOUGAINVILLE

ISLAND

BOUGAINVILLE BASIN

Atamo VolcanicsL. Eocene - E. Oligocene

KUNAIHILL

UMUMRIVER

KURUAJABARIVER

PINEIRIVER

ARAWA

KARATU

KANGU HILL

MALABITA HILL

ATAMO

6°S

155°E

0 50km

Miocene - Pliocenearc trend

Pleistocene - Holocenearc trend

N

Kupei mine

SOLOMONISLANDS

PAPUANEW

GUINEA

INDONESIA

AUSTRALIA

FIGURE 2. Geology map of Bougainville Island showing simplified stratigraphy, structures, and the location of volcanic and intrusive centers. Intrusions are highlighted with capital letters. Compilation of features from figure 2.2 of Rogerson et al. (1989) and the geology map in Blake and Miezitis (1967). Additional structures and linear features were interpreted from the ASTER global digital elevation model (see Fig. 3).

20 S E G N E W S L E T T E R No 113 • APRIL 2018

1967). Bedding is difficult to recognize within the thick volcanosedimentary units. However, a gentle anticlinal fold axis occurs along the crest of the Crown Prince Range.

Whole-Rock GeochemistryWhole-rock analyses from least-al-tered volcanic and intrusive rocks were chosen using the selection criteria of Loucks (2014): volatile-free major ele-ment oxides sum to 97.5 to 101.5 wt %, and loss on ignition or H2O + CO2 + S amount to less than 3.5 wt %.

On a K2O versus SiO2 diagram (Fig. 4A), volcanic rocks from Bougainville plot predominantly as medium- to high-K, calc-alkaline andesite and basaltic andesite. Samples from the Emperor Range, Balbi, and Tore volca-nics in northern Bougainville are mostly high-K andesite, whereas those from central and southern parts of the island are medium-K andesite and basaltic andesite—a spatial trend previously recognized by Rogerson et al. (1989). Six samples from Eocene to Miocene units, including the Toniva Formation and Arawa Conglomerate, are basalt. One sample each from the Toniva

Formation, Numa Numa Volcanics, and Emperor Range Volcanics plots as absa-rokite or shoshonite.

Both alkaline and calc-alkaline series intrusive rocks occur on Bougainville. Samples from the Kupei Intrusive Complex, adjacent Jaba River Diorite, and Isinai Monzonite plot mostly as medium- to high-K diorite, quartz diorite, and granite on total alkali versus SiO2 and K2O versus SiO2 diagrams (Fig. 4B, C). The Puspa and Melilup intrusive complexes, hosted in the Emperor Range Volcanics of northern Bougainville, comprise syenite, syeno-diorite, and alkali granite, plotting in the shoshonite field of a K2O versus SiO2 diagram. Samples from the Pinei River, Malabita Hill, Arawa, and Kangu Hill areas were classified as diorite and quartz diorite.

The samples have trace elements typical of an island-arc setting. Volcanic and intrusive rocks are enriched in K, Rb, Pb, and Sr and depleted in Nd and Ti compared to normal mid-ocean ridge basalt (N-MORB) compositions (Fig. 4D), indicating they were derived from enriched mantle in a subduction zone (e.g., Wilson, 1989; Richards, 2003). Within and between individual units,

the more evolved samples are enriched in light rare earth elements compared to N-MORB in the order dacite > andesite > basaltic andesite, consistent with nor-mal fractionation trends.

Copper and Gold Fertility IndicatorsWhole-rock fertility indicators such as Sr/Y ratios and normalized Yb concen-trations have been applied to intrusions (e.g., Baldwin and Pearce, 1982; Loucks, 2014) and cogenetic volcanic rocks (e.g., Behnsen et al., 2017) to predict if the causative magmas were hydrous and therefore prospective for copper-gold mineralization (Richards, 2003; Muller and Groves, 2016). Volatile-rich and oxidized magmas are known to produce copper and gold deposits. High Sr/Y ratios, >35 (Loucks, 2014), and depleted Yb and Y concentrations (Richards, 2011) indicate fractional crystallization of hornblende from a hydrous magma at the expense of plagioclase. Strong positive Eu anomalies may also result from crystallization and accumulation of plagioclase (Bea, 2015). Although the presence of hornblende could not be confirmed in original samples, Sr/Y and

Return to Bougainville—Reassessing the Mineral Potential of a Long-Forgotten Island (continued). . . from page 19

TABLE 1. Features of Major Intrusions and Selected Volcanic Units in Bougainville

Cu max Zn maxUnit/location K/Ar age Lithology Composition Sr/Y (Eu/Eu*)/Yb ppm ppm

Isinai 7.43 ± 0.17–4.83 Quartz monzonite, High-K calc-alkaline 5–40 (5) 0.65–0.88 (4) 234 (5) 103 (5) ± 0.05 Ma syenite to alkalineKupei 4.9–1.3 Ma Quartz diorite, Medium-K calc-alkaline 28–70 (35) 1.17–3.12 (4) 701 (35) 329 (35) granodiorite, andesitePuspa 2.11 ± 0.03 Ma Monzonite, syenite Alkaline 24–43 (3) 0.75–1.16 (2) 64 (3) 137 (3)Melilup 1.72 ± 0.03–1.11 Monzonite Alkaline 31–40 (2) 0.60–1.16 (2) 0 (2) 133 (2) ± 0.03 MaJaba River Bt + cpx + opx Alkaline 34–52 (4) 1.69 (1) 18 (4) 93 (4) phyric dioriteMalabita Hill Diorite and andesite High-K calc-alkaline 22 (1) 1.23 (1) 150 (1) BDL (1)Pinei River 4.63 ± 0.85 Ma Qtz + hbl phyric Medium-K calc-alkaline 22 (1) - 31 (1) 57 microdioriteArawa Andesite/diorite Medium-K calc-alkaline 67 (1) - BDL (1) 65 (1)Kangu Hill Cpx phyric microdiorite High-K calc-alkaline 22 (1) - 168 (1) 63 (1)Balbi Volcanics Basaltic andesite, High-K calc-alkaline 22–38 (28) 0.89–1.23 (6) 147 (28) 91 (28) andesiteBagana Basaltic andesite, Medium-K calc-alkaline 33–54 (44) 1.47–2.81 (11) 81 (44) 327 (44) Volcanics andesiteBilly Mitchell 0.284 ± 0.09– Andesite Medium-K calc-alkaline 35–61 (9) 1.86–1.95 (3) 39 (9) 65 (9) Volcanics 0.228 ± 0.15 MaTarkoa Andesite Medium-K calc-alkaline 35–49 (7) 1.48–2.21 (3) 63 (7) 61 (7) Volcanics

Notes: Radiometric ages were from Rogerson et al. (1989) and Page and McDougall (1972); ranges and/or maximum Sr/Y, (Eu/Eu*)/Yb, copper, and zinc values are shown; number of analyses are in parentheses; whole-rock geochemical data were sourced from the references listed in Figure 4 Abbreviations: bt = biotite, cpx = clinopyroxene, hbl = hornblende, opx = orthopyroxene, qtz = quartz

No 113 • APRIL 2018 S E G N E W S L E T T E R 21

(Eu/Eu*)/Yb ratios were calculated using whole-rock geochemical data from Bougainville and correlated with the reference samples of Loucks (2014). The europium anomaly (Eu/Eu*) was deter-mined by comparing the N-MORB-nor-malized Eu, Sm, and Tb values, using Eu* = (SmN

2 *TbN1/3) (eq. 3a of Lawrence

and Kamber, 2006) rather than the conventional equation, as Gd was rarely included in the historical analyses.

A large proportion of intrusive rocks and some of the volcanic rocks from the Bougainville database had high Sr/Y ratios and plotted in the field for copper-gold–productive intrusions (Figs. 5, 6; Table 1). Samples with the highest Sr/Y were from the Bagana Volcanics, Kaverong Quartz Diorite, Biuro Gran-odiorite, Toniva Formation dacite dome, and Billy Mitchell Volcanics. Samples from the Jaba River Diorite and the Isi-nai, Puspa, and Melilup intrusive com-plexes returned maximum Sr/Y values

of 40 to 52. The highest (Eu/Eu*)/Yb ratios were from the Kaverong Quartz Diorite and Biuro Granodiorite phases of the Kupei Intrusive Complex, Bagana Volcanics, Taroka Volcanics andesite dome, and Toniva Formation dacite dome. Samples from the Jaba River Diorite and Puspa, Isinai, and Melilup intrusive complexes had maximum (Eu/Eu*)/Yb values of 0.88 to 1.69.

Exploration GeochemistryDuring the 1980s, two field seasons undertaken by the Geological Survey of Papua New Guinea yielded rock chip samples and 860 stream sediment samples, mostly from second- and third-order creeks. Surface geochem-ical anomalies were identified using cutoff values of 0.6 ppm Au, 1.3 ppm Ag, 0.8 ppm Te, 44 ppm Cu, and 84.4 ppm Zn (Rogerson et al., 1989). These anomalies (Fig. 6) indicate areas that

are prospective for porphyry, skarn, volcanogenic massive sulfide, and epithermal deposits. The geochemical anomalies encompass the Melilup and Puspa intrusive complexes and large areas of the Emperor Range and Balbi volcanics in northern Bougainville (Fig. 6). There are also anomalies within the Isinai Monzonite and throughout the Atamo Volcanics and Toniva Formation in central Bougainville.

CopperHigh stream sediment copper values occurred in the pre-Pleistocene rocks of central Bougainville, including the Karatu, Kopani, and Kupei areas sur-rounding the Panguna mine (Figs. 2, 6). Whole-rock geochemical analyses of samples from the Emperor Range Volcanics and Isinai Monzonite yielded up to 241 and 234 ppm Cu, respectively (Fig. 6). In the geologic survey study, a quartz vein sample from the Kupei adit returned 7.25% Cu (Rogerson et al., 1989).

GoldTwo point three percent of stream sed-iment samples returned >0.6 ppm Au. High stream sediment gold values were correlated to areas containing the Arawa Conglomerate and late Miocene to Pliocene intrusions (Figs. 2, 6; Rogerson et al., 1989). These samples delineated areas previously known for copper and gold anomalism. Free gold was associ-ated with magnetite-pyrite skarn, where a diorite had intruded the Keriaka Lime-stone in the Atamo area. Rogerson et al. (1989) reported two samples contain-ing 10 g/t Au from the Puspa Intrusive Complex in northern Bougainville and a creek surrounding the Panguna mine.

ZincMost of the stream sediment samples had high, >300-ppm background zinc concentrations (Rogerson et al., 1989). In this study, volcanic rocks of Bougainville were found to have 60 to 70 ppm Zn (Fig. 6). Samples from the Kupei Intru-sive Complex yielded up to 329 ppm Zn, whereas those from the surrounding Arawa Conglomerate and Toniva Forma-tion contained up to 112 ppm Zn. The Melilup and Puspa intrusive complexes and Isinai Monzonite returned up to 133, 137, and 103 ppm Zn, respectively.

Total alkalisSamples from the Melilup and Puspa intrusive complexes and Emperor Range to page 22 . . .

FIGURE 3. Bougainville stitched digital elevation model (DEM) with logarithmic color stretch, over-lain by intrusive centers and a compilation of interpreted structures from this study, Blake and Miezitis (1967), and Rogerson et al. (1989). DEM data obtained from NASA Land Processes Distrib-uted Active Archive Center (2011).

6°S

PangunaMine

ISINAI

PUSPA

MELILUP

KUPEI

155°E5°S

DEURO

RANGE

JABA RIVER

BOUGAINVILLEISLAND

BUKAISLAND

0 50km

CROWNPRINCE

RANGE

22 S E G N E W S L E T T E R No 113 • APRIL 2018

Volcanics had >7.5 wt % total alkalis, as did three samples from the Isinai Monzonite (Fig. 6). The more primitive volcanic samples, with <5 wt % total alkalis, were from the Atamo Volcanics, Toniva Formation, and Arawa Conglom-erate, immediately surrounding the Panguna mine.

Discussion and ConclusionsThe oldest rocks on Bougainville record the transition from mafic to interme-diate-composition magmas and from submarine to subaerial volcanism during the early Miocene. The Atamo Volcanics, Toniva Formation, and Arawa Conglomerate contain pillow basalt with <5 wt % total alkalis, whereas the younger Bougainville Group comprises

basaltic andesite and andesite. In contrast, the Pliocene to Holocene New Georgia Group in the Solomon Islands includes large volumes of picrite lavas and gabbroic intrusions, reflecting mantle mixing and a greater proportion of primitive mantle in the arc mag-mas, due to subduction of the adjacent Woodlark oceanic crust (e.g., Schuth et al., 2004; Chadwick et al., 2009).

Intrusions on Bougainville Island are controlled by multiple structural orien-tations. Oblique convergence between the Australian and Pacific plates, ongo-ing collision of the Ontong Java plateau with the Melanesian trench, and the involvement of microplates has led to a major arc flexure, whereby the older Miocene to Pliocene intrusions occur along WNW-trending arc-parallel faults

and the younger Pleistocene intrusions and recent volcanic centers appear to be controlled by NNW-, NNE-, and ENE-trending structures. The intersec-tion of these arc-parallel and oblique structures has provided a conduit for the rapid ascent of mantle-derived material to the upper crust, required for mineralization (e.g., Muller and Groves, 2016). Slab tears, crustal thickening, and potential detachment of the Pacific plate at the Melanesian arc resulting from the collision of the Ontong Java plateau have played a role in the gen-eration of long-lived, mantle-derived magma chambers beneath Bougain-ville and their consequent uplift and erosion.

Bougainville has both medium-K and high-K, calc-alkaline to alkaline

Return to Bougainville—Reassessing the Mineral Potential of a Long-Forgotten Island (continued). . . from page 21

FIGURE 4. Major and trace element diagrams of least-altered volcanic and intrusive rock samples from Bougainville. A. K2O versus SiO2 diagram of volcanic rocks overlain by the fields of Ewart (1982). B. Wilson (1989) modified total alkali-silica (TAS) diagram of intrusive samples. C. K2O versus SiO2 diagram of intrusive rocks with the subdivisions of Rickwood (1989) and the fields of Peccerillo and Taylor (1976) and Ewart (1982). D. Spidergram of intrusive samples, normalized to the N-MORB values of Sun and McDonough (1989). Gray polygon represents rare earth element pattern of 15 volcanic rocks from the Bougainville Group. Whole-rock geochemical data are from Taylor et al. (1969), Mason (1975), Ford (1976), Bultitude et al. (1978), Mason and McDonald (1978), and Rogerson et al. (1989).

Billy Mitchell Volcanics

Emporer Range Volcanics

Laluai Volcanics

Loloru Volcanics

Numa Numa Volcanics

Reini Volcanics

Takuan Volcanics

Taroka Volcanics

Tore Volcanics

2

4

10

K0

+ N

a0

wt %

22

0

6

12

14

16

50 60 70SiO wt %2

Diorite

Gabbro

Gabbro

Gabbro

Syenite

Syenite

Granite

Alkali granite

Syeno-diorite

Nephelinesyenite

SubalkalineAlkaline

0.1

1

10

100

1000

Cs Rb Ba Th U Nb K La Ce Pb Pr Sr P

Nd

Sm Zr Hf

Eu Ti Tb Dy Y

Ho Er Tm Yb Lu

D.

sam

ple

/ N-M

OR

B

Low-K rhyolite

Rhyolite

Low-K rhyolite

Dacite

High-K dacite

Low-K andesite

Andesite

High-K andesite

Low-KbasalticandesiteLow-K basalt

Basalt

High-K basalt

Absarokite

Shoshonite

High-Kbasalticandesite

Basalticandesite

Medium-K calc-alkaline

High-K calc-alkaline

Alkaline

45 50 55 60 650

1

2

3

70 75SiO wt %2

K0

wt %

2

Granodiorite

Toniva Forma

Isinai Monzonite

Kaverong Quartz Diorite

Biuro Granodiorite

Nautango Andesite

Melilup intrusive complex

Puspa intrusive complex

Bougainville Group

Jaba River Diorite

B.

45 55 65 75

Banakite High-K rhyolite4

5 A.Arawa Conglomerate

Toniva Forma

Kupei andesites

Bagana Volcanics

Balbi Volcanics

45 50 55 60 650

1

2

4

5

3

70 75SiO wt %2

K0

wt %

2

Low K

Medium K

High K

Shoshonitic

6

8

C.

KupeiIntrusiveComplex

No 113 • APRIL 2018 S E G N E W S L E T T E R 23

intrusions capable of hosting porphyry, skarn, and telescoped epithermal deposits. The Isinai, Puspa, and Meli-lup intrusive complexes and the Jaba River Diorite had high K concentrations and total alkalis, Sr/Y ratios >35, and variable but generally high (Eu/Eu*)/Yb ratios, indicating that they resulted from hydrous arc magmas rich in potas-sium. The Isinai, Puspa, and Melilup intrusive complexes yielded rock and stream sediment samples anomalous in Cu, Zn, Au, Ag, and Te. Samples from the Kupei Intrusive Complex hosting the Panguna porphyry copper-gold deposit had the highest Sr/Y and (Eu/Eu*)/Yb ratios and the highest copper and zinc concentrations. In the Atamo, Karatu, and Pinei River areas of cen-tral Bougainville, diorite bodies have intruded the Oligocene to Miocene Keriaka Limestone and are prospective for copper-gold skarn mineralization. One of these intrusions has been partly altered to magnetite and sheds free gold into adjacent streams (Rogerson et al., 1989).

Rogerson et al. (1989) found, based upon conventional K-Ar dates, that Bougainville intrusions ranged in age from 8.19 to 0.7 Ma and concluded that there was little evidence for discrete

magma pulses or temporal and spatial relationships between the medium- and high-K suites. However, in view of mod-ern high-precision radiometric dating techniques and additional samples, the ages of individual intrusive phases could be resolved more accurately.

This study has highlighted the poten-tial for the younger volcanic stratigra-phy to host porphyry and epithermal deposits. Except for the Emperor Range Volcanics, stream sediment anomalies are not observed within volcanic facies of the Bougainville Group. However, the Balbi, Billy Mitchell, Taroka, and Bagana volcanics had Sr/Y >35, moderate to high (Eu/Eu*)/Yb ratios of up 2.81, and anomalous Cu and Zn concentrations. Ford (1976) concluded that a mod-ern porphyry deposit may be forming beneath the Balbi volcano, citing the following evidence: (1) explosive volca-nism and active fumaroles, (2) structural control due the inter-section of NE-, N-, and

+ +

++

0 50km

6°S155°E

6°S155°E

Panguna

limestone

Eocene to Pleistocene diorite, monzonite, syenite, granodiorite

Eocene to Pliocene basalt, basaltic andesite

Sr/Y (ppm) 50 to 500 40 to 50 35 to 40 0 to 35

Total alkali (wt %) 7.5 to 15 5 to 7.5 2.5 to 5 0 to 2.5

Cu (ppm) 200 to 4,000 100 to 200 50 to 100 1 to 50

Zn (ppm) 200 to 500 100 to 200 50 to 100 1 to 50

N

FIGURE 6. Sr/Y ratios, total alkalis, and copper and zinc concentrations from all 269 whole-rock geochemical analyses. The dashed lines are Au ± Ag ± Cu ± Zn ± Te stream sediment anomalies from figure 5.2 in Rogerson et al. (1989).

to page 24 . . .

FIGURE 5. Sr/Y ratio versus SiO2 of least-altered intrusive and volcanic rock samples from Bougain-ville overlain by the Loucks (2014) fields for barren and Cu-Au–productive intrusions. Red circles show the average composition of oceanic basalt, basaltic andesite, andesite, and dacite, also from Loucks (2014).

0

50

100

150

200

45 50 55 60 65 70 75

Cu+Auproductiveintrusions

barren intrusions

Sr/Y

ppm

SiO wt %2

intrusive rocks

35

volcanic rocks

oceanic basalt, basal�c andesite,andesite & dacite (average)

24 S E G N E W S L E T T E R No 113 • APRIL 2018

E-trending faults, (3) numerous vents of differing age, indicating multiple con-duits linked to a large, multiphase intru-sion at depth, and (4) evolved high-K andesite lavas that are enriched in Rb, Pb, Zr, Th, U, Nb, Mo, and heavy rare earth elements compared to medium-K volcanic rocks.

Geothermal activity and argillic alteration have been observed on the summit and flanks of the active to recently dormant Bagana, Balbi, and Loloru volcanoes. This includes water vapor and sulfur dioxide emissions, deposition of native sulfur, and accumu-lation of acidic hot springs (Blake and Miezitis, 1967; Rogerson et al., 1989). These areas are undergoing acid sulfate alteration and may be associated with precious and base metal epithermal veins at depth.

Bougainville Island has had a com-plex tectonic and magmatic history dominated by two phases of magma-tism, separated by a hiatus caused by the collision of the Ontong Java oceanic plateau with the Melanesian arc and consequent reversal in subduction polar-ity. The world-class Panguna porphyry copper-gold deposit formed during the second phase of magmatism. With the application of modern exploration tech-niques, there is significant potential for the discovery of porphyry, epithermal, and skarn deposits on an island that has received very little exploration expendi-ture during the last 47 years.

AcknowledgmentsI respectfully thank Richard Tosdal for peer-reviewing the paper. I also wish to thank Alan Wilson, Jeremy Richards, and Brian Williams for their thoughtful reviews and valuable contributions.

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