Ministry of Lands and Mineral Resources Fiji Mineral Resources Department GOLD IN FIJI

Gold is a yellow, dense, soft metal. Pure gold has specific gravity (S.G.) 19.3, hardness 2.5-3.0, and melting point 1063ø C. It crystallises in the isometric system; cubes and octahedra are the most common crystalline forms.

Gold usually occurs in the native state as metal. The number of naturally occurring compounds of gold is small. The most common combinations are gold with silver and tellurium. The compounds from these combinations form important ores of the metal. Gold ores of this type are found in well-known places like Kalgoorlie (Western Australia), Colorado (USA) and Vatukoula (Fiji).

Native gold always contains some alloyed silver, whereas other metals are rarely prominent. The degree of purity of gold bullion is expressed as the fineness, measured as parts of gold per thousand. The fineness of vein gold ranges from 500 in electrum (an alloy of gold with high silver content) to about 800 or 850. Alluvial gold varies in fineness from about 500 to 999. The greater fineness of alluvial gold is due to the silver content in the outer layer of the grains or nuggets having been removed through dissolution by water.

The purity of man-made alloys of gold is expresed in carats - pure gold is 24 carats. 18 carat gold contains 18/24ths gold or 75% gold. Red, yellow and green gold so often found in jewellery are alloys with copper and silver in varying amounts - purity is often 9-14 carats. White gold is an alloy of nickel, zinc and copper with gold; occasionally silver and palladium are substituted for the zinc.

Uses of gold

Because of colour, durability, malleability and occurrence in the native state, gold was one of the first metals to attract the attention of man. Highly valued by the earliest civilisations, golden ornaments of various styles and beauty survive to the present day.

Gold has long been the basis for international monetary exchange. Its density allows large quantities to be stored in a small space; one tonne of gold occupies only a 37-cm cube.

Gold is used extensively in jewellery, for ornamental and decorative purposes. Properties of high conductivity and chemical inertness (chemical non-reactivity) are the basis for use in telecommunications, solid-state electronic devices and laboratory equipment. The ability of gold to reflect electromagnetic waves promotes its use as a protective coating for space-craft equipment.

Geology of gold mineralisation

Primary gold occurrences in Fiji are generally classified as being of three types - 'epithermal', 'porphyry copper' or 'massive sulphide'. In porphyry-copper and massive sulphide orebodies other elements such as copper, lead and zinc are the main targets and gold is recovered only as a by-product. Such occurrences can be very large, e.g. Waisoi porphyry-copper deposit at Namosi, but the gold grade is very low. Exploration companies have used the epithermal model extensively particularly during the 1980s.

Epithermal systems are common in Fiji and many contain at least some gold but the biggest problem is to find sufficient gold to justify a mining operation. Epithermal gold deposits form near the earth's surface by numerous complex chemical reactions between rocks and ascending pressurised hot water, containing gold, silver and base metals. Evidence indicates that some epithermal systems are active for between 100 000 years and 1 million years.

Volcanic centres are responsible for heating large quantities of water which rise as superheated mixtures of metal-rich volcanic fluids and groundwater through the volcanic rocks - particularly in the neck of a volcano (Fig.1). As the fluid rises, pressure decreases and the superheated fluid reaches a point where it boils, steam is given off and the metals precipitate. This is referred to as the "Boiling Zone" and is the main target of exploration (Fig.1). Such gold varies from forming on the then surface or down to 1000 m below. The temperature of formation is mostly in the range 170-250øC but varies between 50øC and 300øC. Many deposits also contain up to several percent of lead, zinc and copper - precipitated at the same time as gold.

Where the waters and fluids are channeled through to the surface, then the well-known features of hot springs, fumaroles and hot mud pools are formed.

Large volumes of hot circulating fluid are involved, not only in dissolving and precipitating metals, but in strongly altering the volcanic rocks through which the fluids pass. Alteration is often intense and the primary volcanic rock is changed to clay and silica, and is often zoned due to the different effects as the pressure of the fluid is reduced and the temperature cools. The broad alteration zones have characteristic features and provide a very useful guide to the possible presence of a "blind" or hidden deposit at depth.

EMPEROR (Vatukoula) is currently Fiji's only operating mine and the ore is of the epithermal type. The distribution of the orebodies shows a variation on the general theoretical theme outlined in Figure 1. Orebodies are within steep shears, faults, flat-lying structures ('flatmakes' with dips of less than 45ø) and shatter zones - all related to the collapse downwards of the volcanic rocks and formation of the Tavua Caldera. The central vent of the volcanic caldera is infilled with younger volcaniclastic and sedimentary rocks and is mostly barren of mineralisation. Mineralisation is instead along the rim of the caldera. Gold is within tellurides, pyrite and arsenopyrite - only very small amounts occur as free gold.. Fluid temperatures at the time of gold deposition were 200-250øC. Mineralisation occurred about 4 million years ago. Proven recoverable reserves are quoted at 1.2 Mt grading 6.4 g/t.

Numerous other prospects have received considerable attention and many have been sites of historical gold production, but none have sufficient known grade and tonnage to be considered economic at this stage. Such sites include Mount Kasi, Vuda, Faddy's Prospect, Mistry Mine and Kingston Mine. At Faddy's, the total measured and indicated resource at a 2 g/t cut-off grade is 920 000 tonnes at 4.9 g/t.

Secondary Gold Deposits

The processes of weathering and erosion break up outcropping gold-bearing veins and structures, liberating grains of gold into the soil. The gold is then concentrated either in situ from the deeply weathered primary deposit (i.e. eluvial gold) or is transported downslope, downstream and reworked into a new occurrence (alluvial gold). Eluvial gold is known at Mount Kasi whereas alluvial gold has been reported from the Rewa Delta, Nasivi River mouth, Vuda River mouth and foreshore area, Waimanu alluvial deposits and Yanawai River mouth and coastal flats. All of the alluvial occurrences are considered uneconomic, though only the Waimanu deposit is well documented.

Mining at Vatukoula

Emperor Gold Mining Co. Ltd took complete control of mining operations at Vatukoula in 1956. In 1983 a joint-venture partnership was formed between Emperor and Western Mining Corporation of Australia.

This deal allowed Western Mining a 20% ownership of the current Vatukoula workings (SML54). A new orebody was found immediately south of Vatukoula and this was opened in 1986 (SML 55) with Emperor and Western Mining each having equal ownership in the Tavua Basin Mining Joint Venture. Western Mining is the manager for both operations.

The Emperor Mine now has four main shafts and a decline giving access to the underground workings and ore is also mined from the surface by open-cut methods. Smith Shaft (about 700 m deep) is the main rock-hoisting shaft and the lowest operating level is known as the 16 level at 564 m below the surface. Cayzer Shaft and Borthwick give access down to the 14 level at a depth of 502 m. The three shafts are connected by tunnels at several levels down to the 14 level. A new orebody was discovered to the south of the main workings in 1983/84 (Prince William Flatmake) and a tunnel was driven on the 13 level (471 m) from Borthwick Shaft to gain access to it at the same time that a new shaft (Philip Shaft) was sunk from the surface. The shaft and tunnel were connected and mining of the orebody has been in progress for over two years. A powerful fan at the top of Cayzer Shaft draws fresh air through all the mine workings removing fumes from blasting operations and from diesel-powered machinery.

Whereas in the past the underground mining methods made use of small railway trains to convey the gold-bearing rock (ore) to the shaft for hoisting, more-efficient methods have been developed using rubber-tyred vehicles. Access into the mine for these vehicles is in the form of the decline (a steep spiral roadway from the surface) and this has so far reached the 8 level (318 m). The rubber-tyred vehicles have also been lowered down the shafts to operate at lower levels.

Mining of the orebodies underground involves the development of drives and crosscuts (tunnels) along ore zones at levels which are about 40 m apart and then mining out (stoping) the ore from any one level to the level above, leaving sufficient pillars to prevent the rock from collapsing. The ore is mined essentially by drilling holes in it and blasting it out with explosives. The blasted ore is then loaded into rail or rubber-tyred wagons and carted to Smith Shaft or the decline for hoisting to the surface.

Open-cut mining consists of excavating a pit on the surface from which ore can be taken. Waste rock (overburden) which lies on top of the ore has to be removed first and is piled on a waste tip. The ore can then be loaded and carted to the ore treatment plant to join the ore from underground. As for underground mining, hard rock is broken by drilling and blasting. Loaders and excavators then load the broken waste and ore into separate off-highway trucks of up to 50 tonnes capacity. The pit walls are cut in benches to prevent loose rocks from rolling to the bottom.

In the treatment plant the ore is first crushed and ground to a fine sand so that the mineral particles can be separated from waste rock. A process known as flotation is used to separate the mineral particles from the waste. The waste is pumped to a settling pond (tailings dam) in the form of a slurry where it settles out so that clear water can run off the top into a nearby river. The mineral concentrate is dried and roasted in order to convert it into a form which can be dissolved by acids containing cyanide and the gold is then removed from solution by chemical processes. The gold is finally smelted in a furnace to produce gold bullion bars which contain about 79% gold, 20% silver and some impurities. The bullion is exported for refining in Australia.

1989 annual production was 4222 kg of gold from 606 000 t of ore. Production from Vatukoula began in 1933 and by 1989, 132 265 kg of gold had been recovered from 14.18 million tonnes of ore.

The importance of gold to Fiji

The Emperor Mine is currently Fiji's only producing gold mine but the mining settlement is a multi-racial community which has grown to about 10 000 people. The mine provides direct employment for about 1400 people with several hundred more employed by other contracting companies. Benefits are not only to the workforce directly employed and their families but also to those indirectly employed in the adjacent rural area and Tavua town.

Gold and silver production from Vatukoula is outlined in Table 1. Gold makes a significant contribution to the national economy being the third largest contributor to export earnings (in 1941 and 1945 it was in fact the largest). At 13% of total exports it is a major source of foreign exchange for Fiji.

TABLE 1 : PRODUCTION OF GOLD AND SILVER

Click for (Updated Gold Statistics)

Local and international companies continue to explore in Fiji - risking venture capital in the hope of finding gold mineralisation of sufficient grade and tonnage to be economically mined. The risks are substantial and so are the amounts of dollars spent on exploration - since 1985 exploration companies spent between $2 million and $8 million dollars per year, mostly on gold exploration.

Further reading

• Anderson, W. B.; Antonio, M.; Davis, B.; Jones, G.F.P.; Setterfield, T. N.; Tua, P. 1987. The Emperor epithermal gold deposit, Vatukoula, Fiji. In AIMM 1987. Proceedings : Pacific Rim Congress 87. The Australasian Institute of Mining and Metallurgy, Parkville, Vic. Pages 9-12.
• Anonymous, 1988. Epithermal gold. International Mining, February 1988: 7-12.
• Colley, H. 1976. Mineral deposits of Fiji (metallic deposits). Fiji Mineral Resources Division Memoir 1.
• Greenbaum, D. 1979. Vuda Mining area ; history of gold prospecting and mining (1935 to 1975) and implications for further exploration. Fiji Mineral Resources Division Report 11.
• Rodda, P. 1984. Fifty years of gold production in Fiji. In Rahiman, A. 1984. Mineral Resources Department: annual report for the year 1982. Parliament of Fiji, Parliamentary Paper 13 of 1984. Pages 73-81.
• Taylor, G. 1987. Breccia formation and it's relation to gold mineralisation at Mount Kasi, Fiji. In AIMM 1987. Proceedings: Pacific Rim Congress 87. The Australasian Institute of Mining and Metallurgy, Parkville, Vic. Pages 597-601.

MRD Information Notes 3 ISSN 1016 - 2135

Suliana Niurou,Don Flint

Director: A.Rahiman March 1990

ISSN 1016-2135 MINERAL RESOURCES DEPARTMENT

MRD Information Notes 3 FIJI GOLD