112 OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS) Possibly they are indications that the Palaeozoic craton extends northward as a substrate beneath the terranes of the collisional zone Mesozoic amphibolite facies metamorphics near the north coast (Prince Alexander Range) and Oligocene metamorphics associated with ultramafic rocks just beyond the border on the north coast (near Jayapura) also are ‘out of sequence’ and may represent material ramped up from a subjacent older stratum The geology of the eastern part of the collisional zone, from Lae south-eastward, is much simpler and essentially comprises two rock units: an inner core of metamorphic rocks, flanked on the north-east side by a major ophiolite complex The protolith of the metamorphics was felsic and psammitic sediments and some submarine basalts, partly of Cretaceous age, and these were metamorphosed in the Paleocene The ophiolite comprises a 4- to 8-km thickness of tectonite harzburgite, overlain by lenses of cumulate-textured ultramafic rocks, which are in turn overlain by km of gabbro and km of basalt Paleocene and Eocene volcanic rocks that overlie the ophiolite may be arc related Ocean floor basalts that form the eastern end of the island are Late Cretaceous and Middle Eocene and may be related to the ophiolite Miocene and younger, partly volcanogenic, dominantly siliciclastic sediments of the North New Guinea Basin unconformably overlie basement in the western part of the collision zone, and Miocene and younger volcanics and sediments of the Cape Vogel Basin unconformably overlie basement in the eastern part of the collision zone North-Eastern Province The north-eastern province, constructed by volcanic arc activity, embraces the islands of the Bismarck Archipelago and the mainland mountain ranges of the Huon Peninsula The oldest rocks are Eocene and Oligocene volcanics with some dioritic intrusions These are partly overlain by Miocene limestone, which is in turn partly overlain by volcanic rocks of the present-day (Plio-Quaternary) volcanic cycle Bougainville, in the Solomon Islands chain (see later), has similar geology The land areas are separated by several small ocean basins: the Bismarck Sea, Solomon Sea, and Coral Sea (Figure 3) There are active spreading ridges and transforms in the eastern Bismarck Sea (Manus Basin) and southern Solomon Sea (Woodlark Basin) and active trenches on the inner side of New Britain and the Solomon Islands, on the outer side of the Solomon Islands and extending westward to Manus, and off the New Guinea coast west of Wewak A trench on the south side of the Solomon Sea, at the Trobriand Trough, is masked by thick sediment Volcanic activity is associated with the New Britain Trench but not with the trench west of Wewak (New Guinea Trench) The Coral Sea opened by seafloor spreading in the Paleocene (65–55 Ma), the Solomon Sea in the Oligocene (40–30 Ma), and the eastern Bismarck Sea and Woodlark Basin in the Plio-Quaternary (since Ma) Economic Minerals The main mineral deposits of Papua New Guinea are porphyry copper–gold deposits, disseminated and vein gold deposits, and alluvial gold Other less common deposits include lateritic nickel–cobalt ore over ultramafic rocks, minor manganese ore, and the small copper–gold massive sulphide orebodies that have been mined near Port Moresby The Panguna mine (now closed) on Bougainville Island and the Ok Tedi mine in Western Province are classic porphyry copper–gold deposits In both locations, copper and gold occur in Plio-Quaternary dioritic to granitic rocks and in the adjacent country rocks Similar mineralization is known at Frieda River (West Sepik), Yandera (north-west of Goroka), and Wafi, 50 km south-west of Lae Disseminated and vein gold deposits are mined at Porgera; Ladolam on Lihir Island; Tolukuma, north of Port Moresby; and, until recently, Misima Island Production is planned from Irumafimpa near Kainantu and from Hidden Valley, near Wau Production from these mines, together with Ok Tedi, is sufficient to place Papua New Guinea in the top five of goldproducing nations, worldwide The Panguna mine also was a major producer of gold At Porgera, gold mineralization occurs in intrusive dioritic rocks and in the adjacent, baked sedimentary rocks At Ladolam, the gold is in the floor of the caldera of an eroded volcano, with hot springs nearby At Tolukuma, Irumafimpa, and Hidden Valley, the gold is in quartz veins Alluvial gold is mined on a small scale in many parts of Papua New Guinea, notably near Amanab and Maprik in the Sepik, in the Western and Eastern Highlands, around Wau, and in Milne Bay Province From 1930 to 1965, with a break for the Second World War, alluvial gold was mined by dredges in the Bulolo valley A total of about 65 tonnes of gold was produced Within the last few years, a rich alluvial field was found at Mount Kare, south-west of Porgera Where earlier gold rushes had been almost exclusively managed by overseas miners, Mount Kare was the first entirely Papua New Guinean gold rush As many as 8000 miners were on the field at any one time and an estimated 15 tonnes of gold was taken out in years The only other fields to have produced more than a tonne of gold are the Lakekamu and Sepik alluvial fields, and Misima and Muyua (Woodlark), both of which were mixed alluvial and lode mining Lateritic soils over ultramafic rocks south of