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Base and Precious Metal Mineralisation

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In the minds of most explorationists, Fiji is traditionally associated with gold. However, such an attitude is an over-simplification of what is a complex geological endowment, with copper, manganese, bauxite and iron deposits, amongst others, having been identified during exploration over the past 100 years and more.

The geology of the Tavua and Yanawai goldfields, which host the operating Vatukoula and Mt Kasi mines respectively, has been described extensively elsewhere, so this review concentrates on providing an overview of the various types of gold mineralisation that occur, together with selected examples.

Gold occurs in four principal settings: high-level volcanic structures, as at Vatukoula; quartz veins associated either with volcanic host rocks, as at Mt Kasi, or with plutonics; in copper-bearing porphyries, as at Waisoi; and in volcanogenic massive sulphides. Erosion of primary mineralisation has in turn led to the formation of gold-bearing placers in some localities.

	The Korobasabasaga Range, in southern Viti Levu which hosts the Waisoi and Wainabama copper prospects. Royal Oak Mines has recently taken a lease over Waisoi, which has resources of 950 Mt grading 0.43% copper and 0.14 g/t gold.

Polymetallic deposits in Fiji typically carry high precious and base metal grades. A number of massive sulphide, skarn, epithermal vein and porphyry-associated vein deposits have been identified and evaluated, often on several occasions, for both their base and precious metal potential. None, however, has yet proved to hold sufficient tonnage to support mining.

Massive sulphides are usually associated with felsic volcanic centres, occurring as pods of massive ore within pipe- or dyke-like structures. Skarns and polymetallic veins occur more widely, often in association with Colo Plutonic Suite stocks which themselves commonly host disseminated pyrite. Hydrothermal 'smokers', discovered at great depths offshore in the North Fiji Basin, are believed to represent modern counterparts to the small, but often spectacular polymetallic deposits found on both Vanua Levu and Viti Levu.

One massive sulphide deposit that has already formed the basis for mining, Udu in northeast Vanua Levu, was worked between 1961 and 1968. Banno Mining Co.'s production totalled 32,500 t of ore grading 5.9% copper and 6.7% zinc from a number of isolated ore pockets.

	Drilling underground at Emperor's Vatukoula mine, scene of continuous gold production since 1933

Similarly, small-scale production at the Wainivesi mine, eastern Viti Levu, resulted in the recovery of less than 2,000 t of ore from discontinuous polymetallic deposits that had generated high-grade samples. Diamond drilling carried out in the late 1980s returned grades of 2.6 g/t gold, 103 g/t silver, 14.1% zinc and 1% copper from an indicated resource of some 107,000 t.

Fiji's most widely studied disseminated deposit, Waisoi in the Namosi district of south-central Viti Levu, was discovered during a major exploration effort directed at copper porphyries in the 1970s and 1980s. By the mid-1980s, some 590 Mt of mineralisation grading 0.47% copper had been identified in two adjoining areas, a figure that has been increased through Placer Pacific's recent work to 950 Mt at 0.43% copper and 0.14 g/t gold at a 0.3% copper cut-off. More details are contained in Exploration Activity.

© Mining Journal 1998

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