Fiji lies at the midpoint of the opposing TongaKermadec and New Hebrides convergence zones, separated from these actual convergence zones by two extensional backarc basins, the North Fiji Basin to the west and the Lau Basin to the east, and a series of transform faults including the Fiji Fracture Zone and the MatthewHunter Ridge. Most reconstructions of the past configuration of this part of the Pacific indicate, however, that Fiji was not so long ago an integral part of the Pacific "Rim of Fire", the complex plate boundary between the Pacific and the IndoAustralia Plates. This boundary is well recognised as the locus of several major world-class porphyry coppergold and epithermal gold systems.
Before the Late Miocene, roughly 8 million years (Ma) ago, the Pacific crust was subducted from the east along this plate boundary, with Fiji forming part of an extended Outer Melanesian island arc system, the Vityaz arc, that incorporated the Solomon Islands, New Hebrides, Fiji and Tonga island arcs. Remnants of this subduction zone are preserved as part of the Vityaz Trench, whilst EoceneMiocene cores of the ancient arc system form part of the geological basement in Tonga ('Eua), Fiji (Viti Levu) and Vanuatu.
Subduction along the Vityaz arctrench system was partially blocked through the arrival of a thick sequence of oceanic crust (Ontong Java Plateau) at the portions of the trench along the Solomon Islands and northern New Hebrides. Subduction was effectively immobilized and later reversed in the areas to the north and west of the Fiji arc. Shortly after this reversal, back-arc spreading began to the west of Fiji, forming the North Fiji Basin, with clockwise rotation of the New Hebrides Arc to the southwest, away from Fiji, and anticlockwise rotation of the Fiji Platform.
Further breakup of the arc occurred in more recent times (about 5.5 Ma) with the initiation of intraarc extension behind the Tonga Trench. This caused the opening of the Lau Basin, separating the remnant Lau Ridge from the active Tofua Arc in Tonga.
View Geological Map of Fiji (31Kb)
Much of the younger (Late Miocene to Pliocene) structural and volcanic features of the Fiji Platform can be related to transformation of the older arc to its present-day configuration through creation of the North Fiji and Lau Basins. This period also saw major changes in volcanism throughout the group, with initial eruption of voluminous shoshonitic volcanics in northern Viti Levu (5.5 to 3.0 Ma) followed by later alkalic volcanism more akin to oceanic basalts.
In terms of crustal development, the geological evolution of Fiji can be viewed as having had four main stages:
(a) Early arc stage (3512 Ma);
(b) Mature arc stage (127 Ma);
(c) Early arc rifting stage (7-3 Ma); and
(d) Late arc rifting stage (3 Ma onwards).
These stages reflect the growth of Fiji as an island arc, its maturity and final arc breakup, with the early periods (up to 7 Ma) dominated by subduction-related geology, and geochemical signature changing to a later regime more related to extension, with relatively diminished amounts of subduction component.
Fiji's location along the boundary between the Pacific and IndoAustralia Plates implies that it has major potential for porphyry coppergold and epithermal gold mineralization. This plate boundary hosts major epithermal gold deposits and copper-gold porphyry systems in northern and eastern PNG at Lihir (Ladolam Deposit), Porgera, Ok Tedi and Misima, in the Solomon Islands, at Gold Ridge, and at Emperor and Mt Kasi in Fiji. Several similar epithermal gold deposits have been found on the Coromandel Peninsula, in New Zealand's North Island. Bougainville (North Solomons Arc) and Namosi (Fiji) are two major porphyry-copper deposits.
Mineralisation styles in Fiji can be broadly correlated to the various phases of arc evolution.
Early arc stage
In the Fiji archipelago, early Tertiary rocks are found only on Viti Levu and in the Mamanuca, Narokorokoyawa and Yasawa islands, where there is a succession of volcanic rocks and their sedimentary derivatives with minor intercalations of carbonate rock. The succession ranges in age from Late Eocene to Oligocene. This stage is characterised by a geochemically primitive lowK tholeiitic series of volcanics trending towards slightly more evolved types including low to mediumK calcalkaline types within Wainimala Group rocks exposed in the Yasawa and Mamanuca islands. Mineralisation is restricted in the most part to the Wainimala Group and comprises:
(a) important massive sulphide occurrences at ColoiSuva, Wainaleka and the Wainivesi district of southern, southeastern and eastern Viti Levu; and
(b) manganese mineralisation in SW Viti Levu hosted within volcanics and associated sediments, particularly in the Sigatoka and Nadi Basins.
Mature arc stage
The mature stage of arc development is dominated by plutonic rocks, the Colo Plutonic Suite comprising primarily lowK tholeiitic gabbros, tonalites and trondjhemites. The later part of the Late Miocene was a period of intense orogenic activity in Fiji during which folding and faulting of the Wainimala and Tuva Group rocks occurred. The cores of some folds contain elongate plutonic bodies assigned to the Colo Plutonic Suite. Mineralisation within this stage is widespread and comprises:
(a) numerous vein systems carrying base and precious metals occurring close to, and probably related to the Colo plutonics, particularly in SW Viti Levu (Momi and Kubuna areas);
(b) disseminated mineralisation within the roof zones of the various Colo plutonic stocks and the various Wainimala host rocks, pyrite and minor basemetal mineralisation being widespread throughout the WainimalaColo volcanoplutonic belt; and
(c) skarntype mineralisation associated with various plutonic stocks, consisting of small but relatively highgrade deposits distributed throughout the plutonic belt.
Early rifting stage
The initial part of this stage is marked by eruption of voluminous tholeiitic to calcalkaline rocks, particularly in SE Viti Levu (Namosi Andesite), and the later part of the stage by the extensive development of varied volcanism on Vanua Levu. In northern Viti Levu the later parts of this stage, c 5.53.0 Ma, saw extensive eruptions of shoshonitic volcanics (Ba and Koroimavua Volcanic Groups). Mineralisation is similarly extensive and varied in nature and intimately related to the various volcanointrusive centres. They include the following types:
(a) major porphyry-type copper deposits associated with the Namosi Andesite at Namosi the Waisoi, Wainabama and Waivaka deposits together with their associated skarns and peripheral epithermal vein systems;
(b) massive sulphide mineralisation associated with acid volcanics of the Udu Volcanic Group at Nukudamu, Wainikoro, Mouta and Coqeloa in NE Vanua Levu;
(c) epithermal gold deposits within the BaKoroimavua volcanic centres of northern Viti Levu at Vatukoula, Vuda, upper Sabeto Valley, Tuvatu and Rakiraki; and
(d) important epithermal gold mineralisation related to tholeiitic volcanism of the Natewa Group on Vanua Levu, particularly in the Yanawai District (Mt Kasi), and at Koroinasolo, Waimotu, Dakuniba, and Savudrodro.
Late rifting stage
Volcanism of this stage had an oceanic character, with eruption of ocean-island basalts on Taveuni and in SW Vanua Levu (Bua) and parts of Lomaiviti and Lau. HighK calcalkaline andesites were erupted on Kadavu. Mineralisation is minor and is dominated by residual and placer type deposits. Deposits are represented by:
(a) epithermal veintype deposits within the highK andesites on Kadavu;
(b) surficial residual bauxite deposits developed on erosion surfaces at Drasa (Lautoka) and Wainunu (Bua); and
(c) placertype deposits at Waimanu (gold) and in the Sigatoka sand dunes and the Ba Delta (magnetite).
Mines and Advanced Prospects: Goldsilver
Tavua Goldfield The Tavua Goldfield is situated in northern Viti Levu within and along the margins of the caldera of the Tavua Volcano. Payable quantities of gold were discovered in Lololevu Creek, at the site of the present mine complex in 1932.
The rocks of Tavua Volcano were derived from a potassiumrich magma of the shoshonite association with an apparently relatively simple evolution from an absarokite (olivine-basalt) parent magma to shoshonite, banakite (trachyandesite) and monzonite derivatives.
Mineralisation at Vatukoula is located within a large fractured block created where prominent northweststriking shears intersect the northstriking caldera fault zone. The major lodes cover an area of 2 km2, and are mostly within 600 m of the surface. Lodes occur in three main structural settings:
i) steeply dipping northweststriking shears;
ii) flatdipping (1040 ) fractures (flatmakes); and
iii) shatter blocks between shears.
Most of the gold and silver occurs in tellurides and there are also significant quantities of gold in pyrite.
From the commencement of mining in 1933 to June 1994, the goldfield produced about 4.69 million ounces from 15.6 Mt of ore. In June 1994, the proven and probable underground ore reserves at the mine were estimated at 1.81 Mt at 7.07 g/t Au (411 000 ounces). Total underground geological ore resources (measured, indicated and inferred) were estimated at 7.45 Mt at 7.37 g/t Au (1 765 000 ounces). In addition, a previously built tailings dam contains an additional 257 000 ounces of gold with a proven and probable reserve classification. Hence the gold contained in known reserves and resources totals over two millions ounces. It should be noted that recent exploration successes in 1996 have led the company to substantially expand their ore-resource estimates by an additional 119 000 ounces.
Yanawai District Mt. Kasi Major mineralisation occurs within a northwesterly trending structural feature termed the Kasi Corridor. Within the zone there are strongly altered, usually silicified rocks, and in addition to the mine area, several other major prospects have been located.
All the recent writers on Mt Kasi have considered it to be epithermal, and because of the abundance of sulphide minerals (up to 20% in silicified rock) it has been classed as being a highsulphidation (enargitegold) type.
Following extensive diamond drilling in 1992, the latest revised hardrock resource estimate is 1 240 000 t at 3.0 g/t Au, using a cutoff grade of 1 g/t Au. The measured, indicated and inferred eluvial resources total 838 000 t at 1.9 g/t Au, using a cutoff grade of 0.5 g/t Au. The best grades are at shallow level, weathering has produced a significant eluvial gold resource comprising an undifferentiated combination of slumped ore, colluvium and weathered bedrock.
Mining commenced early in 1996, initially to mine and treat the eluvial resources followed by open-cut mining of the hardrock resource. The first gold bar was poured on 15 April 1996. Mt Kasi Ltd, the developer, a wholly owned subsidiary of Pacific Islands Gold NL (PIGNL), holds a mining lease for the site, as well as two prospecting licences for adjacent areas.
Upper Sabeto Valley Tuvatu Prospect Tuvatu is one of several gold prospects known from the Sabeto area of northwestern Viti Levu.
Advanced exploration including extensive drilling at Tuvatu has located numerous auriferous veins and flatmake structures associated with late stage epithermal mineralising events within augitebiotite monzonite of the Navilawa Stock. Initial studies indicated oxide proven resources of 11 350 t at 9.9 g/t; oxide indicated and proven resources of 25 600 t averaging 8.2 g/t. Extensive exploration and drilling activity at Tuvatu is continuing and is managed by Emperor Mines Limited under an option agreement with Geopacific Ltd. EML also manages exploration at the nearby Vuda Prospect under a similar arrangement with a local company.
Advanced Prospects: Porphyry Copper
Namosi District The mineralised area of the Namosi district lies about 30 km NW of Suva, centred upon the upper reaches of the Waidina River and tributaries, particularly within the Waisoi and Waivaka drainage areas.
Mineralisation in the area has been known since the early 1930s but a detailed search for porphyry-copper deposits only commenced in 1968. Between 1968 and the early 1980s, over 50 000 m of drilling was completed and feasibility studies indicated a combined estimate of 590 Mt of ore at 0.47% copper at the two main prospects at Waisoi. The project was unfortunately declared uneconomic. Placer Pacific Ltd commenced reinvestigation of the prospect in 1991.
The project is still at a preliminary stage and further geological investigations need to be undertaken to complete the exploration stage and to define the mineable reserves and possible surrounding deposits. The Waisoi mineralisation is considered mammoth by world standards, although of low grade. Estimates suggest that the geological resource in the Waisoi East and Waisoi West deposits is close to 950 million tonnes at 0.43% Cu, 0.14 g/t Au, with a cutoff grade of 0.3% copper equivalent, while ore reserves have been calculated at approximately 1.1 billion tonnes at 0.4% Cu and 0.13% Au.
For more information about any of these projects please contact the companies directly or the Director of Mines at MRD; the addresses are listed in the section entitled Key Sector Contacts.