Fiji - A Young Geological Environment

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Fiji:
A Young Geological Environment

In geological terms, Fiji's islands are relative newcomers to the world scene, current knowledge indicating that the oldest rocks present date from between 40 and 36.5 Ma. Many of the islands are covered with volcanic rocks of Cenozoic age, the most recent having been deposited during eruptions that occurred within the last 20,000 years. Together with the associated plutonic activity, these volcanic rocks offer major potential for epithermal and porphyry-style metal deposits. Platform and reef limestones were deposited during various stages of island arc development, and sedimentary basins offshore the main islands offer potential for oil and gas.

Fiji is situated at the edge of the Indo-Australian plate, beneath which the Pacific plate was subducted during the geological period from the Early Eocene to the Late Miocene. Subsequently, the direction of subduction was reversed, with Pacific plate subduction recommencing beneath the Tonga Ridge since the Pliocene. There have thus been four distinct periods of island arc development in this part of the western Pacific: an early arc stage from 35 to 12 Ma, a mature arc stage (12-7 Ma), an early arc rifting stage (7-3 Ma) and late arc rifting from 3 Ma onward.

These stages reflect the growth of Fiji as an island arc, its maturity and final arc breakup. The early phases (up to 7 Ma) were dominated by subduction-related geology while later events were more closely associated with extension. Mineralisation can thus be correlated in general terms with these various phases.

Early Tertiary (Late Eocene to Oligocene) rocks occur only on Vitu Levu and on a few other islands in the group. They comprise a succession of volcanic rocks and their sedimentary derivatives with minor carbonate intercalculations. The Wainimala Group, which occurs in southern and eastern Vitu Levu, and on Yasawa and Mamanuca Islands, hosts important massive sulphide occurrences at Colo-i-Suva and Wainaleka, and in the Wainivesi district. There is also manganese mineralisation of this age in southwestern Viti Levu.

The major event during the mature arc stage was the emplacement of the Colo Plutonic Suite, comprising mainly low-potassium tholeiitic gabbros, tonalities and trondjhemites. The later part of the Late Miocene was a period of intense orogenic activity, with folding and faulting of earlier rocks such as the Wainimala and Tuva Groups; the cores of some folds contain elongate plutonic bodies assigned to the Colo Plutonic Suite.

Widespread mineralisation occurred during this stage of development, with three principal types having been recognised. The first, vein systems carrying base and precious metals, occur close to Colo plutonics, and are identified mainly in southwestern Viti Levu. There is also disseminated mineralisation in the roof zones of the various Colo plutonic stocks and in the host rocks of the Wainimala Group. The third type, skarn mineralisation also associated with stocks, has given rise to small, high-grade deposits that are found throughout the plutonic belt.

The initial period of the early rifting stage was marked by extensive volcanic activity, depositing tholeiitic to calc-alkaline rocks over areas of southeastern Viti Levu. Later activity resulted in the extensive development of varied volcanism on Vanua Levu, while on northern Viti Levu, shoshonitic volcanics formed rocks of the Ba and Koroimavua Volcanic Groups.

Extensive and varied mineralisation is also associated with this stage, intimately related to the various volcano-intrusive centres. It includes the major porphyry-type deposits, skarns and epithermal vein systems of the Namosi district, massive sulphide mineralisation contained in the Udu Volcanic Group, epithermal gold deposits within the Ba-Koroimavua volcanic centres of northern Viti Levu, and epithermal gold mineralisation related to tholeiitic volcanism of the Natewa Group on Vanua Levu. Significant deposits related to this stage of arc evolution include the Tavua goldfield in northern Viti Levu, which contains the Emperor mine at Vatukoula, and the Mt Kasi deposits in south-central Vanua Levu.

Outline Geology Legend

There is only minor mineralisation associated with the final, late-rifting stage, which featured oceanic-type volcanism. Epithermal veins occur on Kadavu Island, south of Viti Levu, some residual bauxite deposits were formed, and placer gold accumulated in various locations.


	Fiji's volcanic heritage is clearly visible throughout the main islands. A series of collapsed calderas, recognised as being prime targetsfor gold and other metals, lies across northern and western Viti Levu

Offshore, grabens formed during tectonic movement at various stages, offering suitable locations for considerable thicknesses of sedimentation. The Bligh Water Basin, north of Viti Levu, contains over 1,300 m of Late Oligocene to Early Miocene sediments, and the Bau Water Basin, to the east of the island, at least 750 m. Subsequent deposition of Late Miocene to Recent sediment increased these thicknesses by around 2,800 m, presenting prospective locations for oil and gas development given the abundant sources of plant material available at the time of deposition.

Communications Centre
Its location in the mid-western Pacific provides Fiji with an enviable position as a centre of regional communications. Nadi international airport, in the west of Viti Levu, is used by many airlines on trans-Pacific routes, with direct flights to Los Angeles, Vancouver, Sydney and Auckland. The national airline, Air Pacific, links Fiji with New Zealand, Australia, Japan, Korea and the US, as well as with other island countries in the western Pacific region.

Fiji's position has also been attractive to shipping companies for over a century, and today Suva, Lautoka, Levuka and Savusavu are ports of entry for the country, handling its imports and exports. Sea and air routes also link the major islands, and provide access to other parts of the archipelago.

Infrastructure development is a priority for the Government, which has committed to a long-term programme aimed at improving the islands' roads, bridges and water supply systems. The road network extends to nearly 5,000 km. Tar-sealed roads link Suva with other major towns on Viti Levu.

The country has a modern telephone system, and cellular services are now in place on the major islands. Higher education is centred on the multi-national University of the South Pacific in Suva, with technology courses being offered by the Fiji Institute of Technology and the Fiji College of Advanced Education, amongst others. The Government aims to phase in compulsory primary schooling for all by 2000, and is also bringing in a World Bank-assisted programme to upgrade and increase vocational training at all levels.

In consequence, Fiji offers a well-educated work force, which currently totals around 280,000. Where specific skills may be unavailable, as in middle and senior management for exploration and mining projects, companies are welcome to use appropriately accredited overseas staff.

English is the country's official language, while Fijian and Hindi are widely used in education, the media and general communication.

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