THE SOLOMON ISLANDS AND VANUATU

PLACER GOLD POTENTIAL OF FIJI THE SOLOMON ISLANDS AND VANUATU Jackson Lum SOPAC Secretariat May1995 SOPAC Technical Report 214 This study was funde...
Author: Colin Wiggins
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PLACER GOLD POTENTIAL OF FIJI THE SOLOMON ISLANDS AND VANUATU Jackson Lum SOPAC Secretariat

May1995

SOPAC Technical Report 214

This study was funded by the Government of Canada

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TABLE OF CONTENTS

Page SUMMARY........................................................................................................................................5 ACKNOWLEDGEMENTS................................................................................................................6

OBJECTIVE .....................................................................................................................................7 INTRODUCTION..............................................................................................................................7

FIJI

...........................................................................................................................................8 Colo-i-Suva .............................................................................................................................10 Kingston Mine..........................................................................................................................12 Mistry Mine ..............................................................................................................................15 Nasivi Delta .............................................................................................................................18 Navua ......................................................................................................................................21 Vuda Prospect ........................................................................................................................22 Nadi Bay..................................................................................................................................26 Waimanu River .......................................................................................................................31 Wainadoi Mine.........................................................................................................................33 Yanawai Goldfield....................................................................................................................37

SOLOMON ISLANDS Chovohio River........................................................................................................................40 Matepono River .......................................................................................................................44

VANUATU.......................................................................................................................................47 Big Bay....................................................................................................................................48 Pealorai Prospect ...................................................................................................................50

CONCLUSION...............................................................................................................................54

BIBLIOGRAPHY.............................................................................................................................55

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LIST OF FIGURES

Figure 1

Placer Potential of Fiji Islands............................................................................................9

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Colo-i-Suva ......................................................................................................................11

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Kingston Mine ..................................................................................................................14

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Mistry Mine .......................................................................................................................17

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Nasivi Delta......................................................................................................................20

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Navua River .....................................................................................................................23

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Vuda Mine ........................................................................................................................25

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Nadi Bay...........................................................................................................................28

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Waimanu River and Wainadoi Mine ................................................................................33

10 Working plan of Wainadoi Mine .......................................................................................36 11 Mount Kasi.......................................................................................................................39 12 Placer potential of the Solomon Islands ..........................................................................41 13 Chovohio River ................................................................................................................42 14 Annual gold production of the Solomon Islands ..............................................................43 15 Matepono River................................................................................................................45 16 Placer potential of Vanuatu..............................................................................................47 17 Big Bay.............................................................................................................................49 18 Peolorai Prospect............................................................................................................52

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SUMMARY

This report summarises all known placer occurrences in the Solomon Islands, Vanuatu and Fiji. A historical background, geology and production figures if any, for each occurrence are documented.

Despite the fact that epithermal gold and porphyry copper/gold deposits occur in these countries, many of them are large by world standards, the placer gold potential of these islands has yet to be fully tested. The implications for large placer gold discoveries are real if exploration for placer gold is vigorous. Included in the report are details of several deposits which are currently economically viable at today’s gold price of US$378 per oz.

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ACKNOWLEDGEMENTS

The Author acknowledges the assistance of the following Directors who freely provided information;

Director of Mineral Development, Mineral Resources Department, Fiji, Mr Abdul Rahiman; Director of Water and Mineral Resources Division, Solomon Islands, Mr Donn Tolia; and Director of Department of Geology, Mines and Water Resources, Vanuatu, Mr Stanallison Temakon.

This project was funded by the Government of Canada.

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OBJECTIVE

There is currently no compilation of prospects for placer gold. The primary objective of this compilation is to interest the private mining sector in the potential of placer gold in the region. The study was carried at the request of some member countries of SOPAC, namely Fiji, Solomon Islands and Vanuatu. The report presents the results of Task 94. RG. 1.1.

INTRODUCTION

The South Pacific Islands contain several of the world’s largest epithermal gold and gold porphyry systems, many of which occur near or on the coast. Undoubtedly, erosion has removed large quantities of gold to rivers, and to nearshore environments, therefore, it is not unreasonable to postulate that large gold placers associated with these onshore deposits occur as typical beach placers and offshore placers in the region. As an example, Clark (1990) estimated that the Bougainville Mine in Papua New Guinea has shed over 200 t of gold, worth some US$3 billion at today’s gold price, into the surrounding streams and offshore areas through erosion.

Although most of the South Pacific's gold deposits were discovered by following 'colours' in the respective rivers and coastline, there is so far no major discovery of a large economic placer occurrence. Why? Simply, there has been relatively little to no exploration for placer minerals conducted in nearshore areas of the region. This may be a result of many reasons. Firstly, the current mining legislations of the region, namely the Solomon Islands, Vanuatu and Fiji, do not allow for nearshore or offshore mineral exploration. Under the present Mining Acts of these nations, boundary lines of mining and exploration licences are granted by governments to the high-water mark. However, there have been numerous cases where governments have provided prospecting licences to mining companies beyond the high water mark. Also, governments have requested the assistance of regional institutes to conduct exploration. Examples of these are presented in this study. Secondly, there is tendency for exploration geologists to be biased towards the search for the 'mother lode' and ignore the placer gold. The difficulty of evaluation of placer gold deposits encourages exploration geologists to shy away from placer occurrences. Compared to hardrock ore estimation, the risk of misinterpretation is higher from incorrect estimation of physical features of the ground, such as degree of compaction, sediment grading from silt to boulders, and tenacity of clay. Also, the sources of error in sampling a placer deposit are so much greater. Placer

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deposits are considered insignificant and as tell-tale signs of a bigger and better deposit nearby they are soon forgotten when the search for an epithermal deposit or porphyry gold deposit begins. Further, the environmental sensitivity of coastal areas keeps larger mining companies away from exploring these areas. Thirdly, many have considered the potential for marine placer mineralisation in island countries to be poor because of the limited nearshore ground and lack of wave energy in comparison to continental shelves. This is a fallacy, with the contrary being more likely. Island nations of the Pacific are surrounded by fringing reefs and tend to be closed systems, thus enhancing gold entrapment and increasing the chances for an economic gold discovery, compared to the open system in continental shelves, where gold can be dispersed to open waters. Frequent storms and strong waves provide sufficient energy for placer gold concentration in tropic waters. Because storm-generated waves are high-energy waves, and they break close to the beach, they are very effective in forming beach placers as observed in many Pacific Island nations. The common occurrence of magnetite on beaches is an obvious example.

Although the search for and assessment of placer deposits pose many potential sources of error and uncertainties, placer exploration is the least costly of marine mining ventures and has the most promise for early return for the dollar, especially for the island nations.

Following are descriptions of potential placer gold sources and occurrences in the Solomon Islands, Vanuatu and Fiji. The relative number of prospects described within each country reflects the availability of data rather than the prospectivity of the country.

FIJI

There is at least 10 known areas where placer gold has been identified in Fiji (see Figure 1). These are described below. Currently, apart for the SOPAC surveys at Nadi Bay, initiated and requested by the Fiji Government, there is no exploration for detrital gold. All Fiji‘s gold production is from underground mining of an epithermal gold system at Emperor and Nasomo Mines at Vatukoula on Fiji’s largest island, Viti Levu. Open pit mining was conducted intermittently during the early 1980s at Emperor Gold Mine.

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Figure 1. Placer potential of Fiji Islands.

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Colo-i-Suva Background The Colo-i Suva area was first pegged for precious metal exploration in 1935 by Mr P.T. Cahill. During the later part of the year payable gold was discovered in the headwaters of the Savura Creek. Eight shafts to depths of up to 21m and adits and trenches, the latter to lengths of 75 m, was dug by Mr Cahill and Mount Morgan Development Co. Ltd Joint Venture (MRD Exploration Report 287/---/394-1). In spite of the considerable amount of work undertaken, the companies abandoned the prospect because of poor overall gold values.

From 1957 to 1961 the Geological Survey identified the mineralisation at Colo-i-Suva as a copperzinc system rather than a gold system (Ibbotson 1959,1960). During 1968 to 1974, Emperor Gold Mining Co. Ltd took out a prospecting licence over the area and conducted detailed geochemical and geophysical investigations culminating in 14 drill holes. The company concluded that mineralisation is too limited to justify mine development. Exploration in the Colo-i-Suva area has been carried out by Barringer Resources (1971-1972), Getty Oil Development and Phelps Dodge Venture (1973), Anglo American Corporation (1976), Newmont Pty Ltd (1983-1985), Nationwide Resources (1986-1990), and Paragon Resources (1987).

Geology The geology of Colo-i-Suva is presented by Ibbotson (1960) and by the various exploration companies who explored the area. Three major stratigraphic groups are present at Colo-i-Suva. These are the Ba Volcanic Group (Nakobalevu Basalt), the Savua Volcanic Group (Vago Volcanics) and the Wainimala Group.

Most of the rocks exposed at the mine area are assigned to the Savura Volcanic Group. They are generally dacitic rocks belonging to the Vago Volcanics. The Savura Volcanic Group is overlain by augite- and olivine-bearing basalts belonging to the Nakobalevu Basalt.

Intense hydrothermal argillic alteration and silicification occurs in the vicinity of the mine. Silicification is well developed at the mine site and argillic alteration is mainly confined to the dacite. Sulphides, principally pyrite and sphalerite, form stringers and are disseminated

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throughout the silicified host rocks and concentrated in shears which trend north to northwest (Colley 1976). Chalcopyrite, bornite, covellite and galena also occur in shears and in small lenses and pods scattered throughout the highly argillised host-rocks.

The Colo-i-Suva prospect has been interpreted to be a Cu-Zn volcanogenic occurrence with probable affinities with a Kuroko-type deposit (Colley 1976). Estimated reserves of 200 000 t with grades of 2 % Cu, 8 % Zn, 31 g/t Ag were estimated by Anglo American Pty Ltd in 1980 (see Figure 2).

Figure 2. Colo-i-Suva.

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Production No gold is reported to have been extracted in the Colo-i-Suva area.

Location Colo-i-Suva is located on the east flanks of Nakobalevu at the head of Savura Creek, around 18° 03.5'S, 178°25.7'E. VIT 20, 028. Access to the area is by a four-wheel-drive vehicle turning off at the Colo-i-Suva village from Princes Road.

Kingston Mine Background The history of Kingston Mine area can be traced back to about 1905 before the first Mining Ordinance was introduced by the Legislature of Fiji (White 1940). Records indicate that two prospectors discovered bornite cropping out on the banks of the Sabeto River.

In 1906, a shaft was sunk to 15 m and a drive of 10 m was made at the bottom of the shaft (Colley 1976). White (1940) reported that two shipments of ore were made, the initial shipment being 5 to 7 tons assaying 33% Cu and 85 g Au and 130 g silver per ton. The second shipment contained an unknown tonnage of ore averaging 40.6% Cu and 176 g of Au and 130 g of silver per ton.

Up until 1934, little exploration was undertaken in the area probably due to the depression years.

Between 1934 and 1964, the Mines Department and the Geological Survey conducted sporadic exploration and mapping programmes to promote the area to industry.

In 1952, one ton of selected ore containing 20.3 % Cu and 460.3 g of silver and 98.9 g of gold per ton was extracted (Skiba 1954).

In the late 1960s, exploration was conducted by local prospectors namely the Ah Koy Syndicate and AMAD N.L. Also during the period, Crawford Marine Specialists Inc. conducted exploration

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for alluvial gold deposits in the Nadi Bay into which the Sabeto River and its tributary streams drain directly. The mining company reported anomalous gold values in the Bay (Collins and Lum 1993). Gold values greater than 1360 mg/m 3 were recorded.

In 1970, Barringer Fiji Ltd conducted intensive exploration in the Kingston area. Diamond drilling was conducted on geochemical and geophysical anomalies defined in previous exploration. The drilling programme outlined the presence of disseminated mineralisation over an elongate area of 300 m x 900 m.

The area since has been prospected by Layton and Associates, Aquitaine Fidji, Amoco Minerals, and Cluff Oil.

Currently, Pan Continental and Venture Ltd are in joint partners conducting exploration in the Kingston area.

Geology The geology of the Kingston area is reported in Skiba (1954), Rao (1983) and in various mining company reports (see Figure 3).

The Kingston Mine area is located within an eroded volcanic centre consisting of a core of monzonite surrounded by an envelope of micromonzonite-latite and hornfelsed extrusives (Colley 1976). The extrusives are mainly shoshonitic flows and breccias belonging to the Koroimavua Volcanic Group. Away from the intrusive complex, unaltered Koroimavua breccias dominate the sequence. To the northwest these rocks underlie basaltic flows and breccias of the Ba Volcanic Group, but to the south, along the Sabeto River, they overlie pillow lavas, polymict breccias and tuffs which belong to the Wainimala Group (Colley 1976).

There is a pyritic halo peripheral to the copper mineralisation, and within this halo there is widespread propylitic alteration where chlorite, epidote, calcite, zeolite, magnetite, sericite and quartz are common.

Chalcopyrite is finely disseminated through the host-rocks and occurs along fracture planes and in quartz-calcite stringers. Minor amounts of bornite are associated with the chalcopyrite, and old

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Figure 3. Kingston Mine.

assay results suggest that the mine workings were developed along a vein containing rich amounts of bornite and precious metals.

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Production There is no metal production recorded at Kingston Mine except for bulk sampling of selected ore which was probably sold for its copper, gold and silver content.

Location Kingston Mine is located on the east bank of Sabeto River 500 m downstream from Navilawa, on the bend below Nasala Creek, at 17° 41.6'S, 177° 35.5'E. VIT 4, L27.

Mistry Mine Background A prospecting licence for the Mistry Mine area was first issued to Mr John Percy Bayly for precious metals and minerals in 1935. Exploration continued in the area by several prospectors but it was not until 1947 that gold was produced, by Mistry Mining Company. From 1947 to 1958 some 23.3 kg of gold, 6.4 kg of silver and 20.3 t of lead was extracted from 1720 t of ore (Lum 1989). Production came from open pit-mining and by simple milling of surface material. Due to a lack of ore the company abandoned the mine.

During 1958 to 1960, the Fiji Geological Survey conducted investigations in an attempt to find additional ore (Houtz 1958, 1960) but was not successful.

During 1969-1971, Crawford Marine Specialists Inc. conducted work at Momi Bay and recorded anomalous gold values in sediments.

In 1974, the Mistry Mine area was explored again, by Emperor Gold Mining Company Ltd. The company conducted geochemical prospecting, geological mapping and drilling, without much success; the company relinquished the area.

In 1985, Hallcroft Pty Ltd was granted a special prospecting licence to conduct an extensive drilling programme. After the programme, the company announced a discovery of a potentially large epithermal gold deposit of about 2 million tonnes of ore at 2 g/t Au (Anon. 1987). Currently, Hallcroft and Climax hold the tenement over the area and continue to explore the area for epithermal gold.

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Geology The Mistry Mine area is in rocks of the Wainimala Group. The Wainimala Group within the area consists of tuffs, fine volcaniclastic sediments and flows and breccias of basic and acid composition (see Figure 4). The rock units are folded generally along an south-east shallow plunging axis.

Gold mineralisation is associated with a breccia zone that strikes 070° and dips 45° to the north, and extends over a width of approximately 40-50 m with a strike in excess of 300 m. Pyrite occurs as dissemination and as veins. Free gold particles are found near surface.

Production (Rodda, in prep.). 1947

63.64 oz gold, 11.52 oz silver

1948

no production

1949

23 oz gold [23.39 oz gold valued at £314, 5 oz silver [4.52 oz silver valued at £1, from 53 tons of ore (Taylor 1950).

1950

29 oz gold valued at £390, 4 oz silver valued at £1, from 86 tons of ore (Taylor 1951)

1951

27 oz gold gold valued at £370, 15 oz silver valued at £4, from 111 tons of ore (Taylor 1952)

1952

89 oz gold valued at £1221 (silver not stated) from 489 tons ore (amalgamation and cyanidation) (Skiba 1953); 1952 112.66 oz gold, 26.47 oz silver, from 489 tons of ore

1953

78.00 oz gold valued at £755 (silver not stated) from 221 tons ore (Skiba 1954, same figures repeated by Fleischman 1955 and Lloyd 1956); 1953 110.15 oz gold, 38.16 oz silver, from 221 tons of ore

1954

39 oz of gold (valued at £485) and 8 oz of silver (£2) from 95 tons of ore (Fleischman 1955); 8.2 oz gold, 2.69 oz silver, from 95 tons of ore

1955

329.77 oz gold (£4532), 72.45 oz silver (£72), 5 tons of lead concentrate, from 224 tons ore; 417 oz gold sold; (Fleischman 1956 and Lloyd 1956)

1956

193 oz gold (£2981) and 12 oz silver (£12) from 274 tons of ore (table in Fleischman 1957);

1956

43.31 oz gold, 21.96 oz silver, from 274 tons of ore

1957

no gold produced

1958

Kennedy (1959) and Lloyd (1959) give production as 74 oz of gold and 33 oz of silver, valued at £1030, from 173 tons of ore;

1958

74.22 oz gold, 43.18 oz silver from 173 tons of ore milled.

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Figure 4. Mistry Mine.

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Location Mistry Mine is located at 2 km east of Nabila village at 17° 52.3'S, 177° 17.1'E. Vit 9, LZ8.

Nasivi Delta Background Gold was first discovered in the Nasivi River in Tavua in 1872 (Flint and Htay, 1993) but it was not until late 1932 that payable gold was reported in Lololevu, a tributary of Nasivi River (within the Vatukoula Gold Mining area), by Mr W. Borthwick. In 1935, several mines developed around the new discovery. For the purpose of the study, the complex history of Vatukoula Mining area will not be discussed.

During 1967 to 1969, Crawford Marine Specialists Inc. conducted exploration for detrital minerals both onshore and offshore at the Nasivi Delta. The company carried out geological and geophysical investigations, pitting and churn drilling to depths of 50 m. Seventy-five percent of the pit samples returned gold, with an average of 0.316 g/t and a high of 14.04 g/t (MRD Exploration Report No. 1025-2). Drill hole samples yielded gold values ranging from 0.03 g/t to 3.73 g/t gold. Crawford's geophysical investigation outlined a large buried V-shaped basin (MRD Exploration Report 1025-2).

In 1973, Manganex Ltd applied for the area following the relinquishment of the ground by Crawford. Mineral exploration by the company was aimed principally for magnetite with gold being a secondary target. The company undertook a programme consisting of bulk sampling and drilling sludge holes. Bulk samples were 13 tons and 15 tons in weight. The 13-ton sample was tabled and the resulting 114 kg concentrate was sent to Australia for analysis. The sample contained 0.015/yd3 of gold. The 15-ton sample was also tabled and 36 grab samples analysed. The results returned an average of 0.4 g/yd3 gold (MRD Exploration Report 1073).

In 1976, Anglo Pacific drilled four holes using a Jacro rig. The deepest hole reached was 21 m, and none of the holes reached bedrock. The drill cores were sampled at one-metre intervals. Only one sample returned an anomalous gold value, of 0.2 g/t (MRD Exploration Report 1103-1).

During 1978 to 1979, Preussag Fiji Ltd undertook a programme of channel sampling to substantiate the analytical results obtained by Crawford Marine Ltd. The exploration programme

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consisted of channel sampling and bulk sampling. Concentrate panned samples gave values from 0.6 to 2.7 g/t, special selected samples ranged from 0.01 to 0.1 g/t, raw samples ranged from 0.05 to 0.5 g/t, light fractions ranged from 0.02 to 0.15 g/t and bulk samples from 0.015 to 0.07 g/t gold (MRD Exploration Report 1153-1-2).

In 1981, Anglo Pacific conducted exploration again in the Nasivi Delta. Five holes were drilled to a total depth of 87 m using a Conrad PP150 Banka mechanical drill rig. The company concluded that the gold is present in the delta but well below economic levels (MRD Exploration Report 1195-1).

Since 1981, no exploration work for detrital minerals has been conducted in the Nasivi Delta.

Geology Detailed geological descriptions of the Tavua area are given by Blatchford (1953), Cohen (1962), Ibbotson (1967) and more recently by Setterfield et al (1991) (see Figure 5). The Emperor Mine is located on the western rim of the Tavua Caldera. The caldera is formed in the centre of a shoshonitic volcano developed in a submarine to subaerial environment. Rock types comprise lava flows which are dominantly absarokite in composition, with debris flows and conglomerates on the caldera flanks. Within the caldera, avalanche deposits are overlain by lacustrine sediments, shoshonite lavas and hydrovolcanic/pyroclastic lapilli tuffs (Setterfield et al 1991).

Gold mineralisation is epithermal and largely structurally controlled. Gold occurs in quartz-gold telluride veins within three structural environments – flatmakes, steep shears, and in shatter zones. Mineralisation varies in width from millimetres to tens of centimetres. The major gangue minerals are quartz and carbonate. Gold-silver tellurides, native tellurium, native gold and goldbearing pyrite are the ore minerals. Rare native silver also occurs.

At the Nasivi Delta, unconsolidated sediments in offshore areas generally consist of muds and fine sands. Beneath the main channel, the top 6 m of the sediments is highly unconsolidated muds and oozes derived from onshore areas and the adjacent reefs (MRD Exploration Report

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Figure 5. Nasivi Delta.

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1025-2). Seismic investigations by Crawford in 1969 suggested that the thickness of the sediments above bedrock in the main channel varies from 3 m to 20 m with an average about 11 m.

Water depths in the delta rivers are generally not greater than 3 m at high tide. In the main channel, depths vary from 15 m to 23 m (MRD Exploration Report 1025-2).

Production There has been no recorded gold production from the Nasivi Delta area. However, 10 km inland at the Tavua Goldfield, some 140 tonnes of gold has been produced since mining first started in 1933. At the present operating mine, the Emperor Gold Mining Company Ltd produces approximately 110 000 oz of gold per year.

Location Nasivi Delta lies approximately 17° 33' S and 177° 40' E on the northern coast of Viti Levu near the Town of Tavua. The area is accessible by tar-sealed roads and all-weather roads which aid agricultural projects in the region. Alternatively, the area can be reached by boat from any convenient location along the coast.

Navua Background Fiji's first reported gold discovery was made in the gravels of the Navua River in 1868 by Charles Gurney (Niurou, 1990). Although gold was reported in the Navua River, records indicate that there was no exploration for placer gold deposits.

In 1972, the Navua Delta was explored for magnetite by Manganex Ltd. No gold analysis was undertaken. The company reported limited sand deposits off the Navua Delta with low magnetite grades ranging between 0.4 and 1.8% magnetite.

In 1993, the Mineral Resources Department analysed samples from reverse circulation drill holes in the Navua Delta. Results of the analysis are presented in Flint and Htay (1993). Their results

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confirm the presence of gold, and sieve samples from the reverse circulation drill holes returned gold values of up to 0.06 ppm Au.

Geology The Navua River is an extensive river system which cuts several volcanic and sedimentary groups. These include the Wainimala Group sediments and volcanics, tonalite to gabbro of the Colo Plutonic Suite, and andesite and sedimentary rocks of the Medrausucu Group (see Figure 6).

The Navua River cuts at least four known bodies of mineralisation, namely the Cu-Zn sulphide occurrences at Wainaleka and Kula, and the disseminated Cu-Zn sulphides in Waitotolu and Rama Creek. These sulphide occurrences are known to have associated gold.

Production There has been no gold production recorded from the Navua Delta.

Location The Navua Delta is located at approximately 18° 15' S and 178° 09' E.

Vuda Prospect Background The discovery of gold in the Vuda area was first noted in the alluvial black sands at the mouth of the Vuda River early in the century (Bartholomew, 1959). Intensive exploration along the Vuda River started in 1935 following the gold developments at Mt Kasi and Tavua.

The discovery of payable gold may be attributed to Mr M.D. Richmond, who first discovered workable gold deposits in May of 1935 (White 1937) by panning and following 'colours'. The first mining lease was issued in January of 1938 to the Natalau Gold Prospecting Syndicate. Gold mining began at Natalau the same year. Gold was mined by both open-cut and underground

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Figure 6. Navua River.

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methods. The ore contained free-milling gold which was recovered by almalgation.

Intense exploration continued around the mining area. In 1944, an additional mine, the Vatutabua Mine was developed. A small five ton Gibson Mill was set up but the ore was treated at the Natalau Mine.

In 1950, an Interim Permit to Mine over a 5-acre piece of ground to the west of Natalau known as G-Ridge was granted to M.D. Richmond.

Gold production in the Vuda area continued until 1954. From 1938 to 1954 some 880 oz of gold was produced from the area.

The Vuda Mining Area has since been prospected by numerous mining companies including Kennecott Exploration Ltd, Freeport of Australia, Goldfields Mines Ltd, Austpac Ltd, Aurelia Resources, Nullarbor Holdings and Beta Ltd.

Geology The Vuda Mining Area is located within the Sabeto Volcanics, a Late Miocene-Early Pliocene series of pyroclastic and autoclastic breccias, tuffs and occasional massive flows of the Koroimavua Volcanic Group (Greenbaum 1979) (see Figure 7). The rocks are shoshonitic in composition, containing high potash content. Mineralisation occurs within a roughly triangular area of alteration extending some 6 to 8 km 2. No intrusive rocks are observed within the mineralised area but a highlevel monzonite stock intrudes the Koroimavua rocks to the east at Navilawa at the Kingston Mine area. Interpretations by Dickinson (1968), Lawrence (1978), and Colley (1976) refer to the Vuda area as a collapsed caldera underlain at shallow depth by intrusive rocks. Several hydrothermal alteration assemblages including the propylitic, argillic, phyllic, pyritic and potassic types exist in the area. The widespread occurrence of secondary biotite in the Vuda rocks indicates that highgrade hydrothermal potassic alteration is dominant. Pyrite and pyrrhotite are disseminated throughout the rocks in the altered area. Chalcopyrite occurs locally, often forming coatings on fracture planes in the vicinity of faults. Shear zones are frequently filled with puggy clay and contain a variety of sulphides including pyrite, chalcopyrite, bornite, covellite, sphalerite and galena. Most of the free gold is extracted from these shear zones.

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Figure 7. Vuda Mine.

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Production GOLD AND SILVER PRODUCTION (from Greenbaum, 1979) YEAR 1938 1939 1940 1941 1941 1942 1942 1943 1947 1951 1952 1954

TONNAGE MILLED 280 unknown unknown unknown unknown unknown unknown unknown unknown 43 15 188

GOLD(oz) 42.21 48.56 424.11 131.41 118.65 6.45 69.66 18.09 5.25 9.42 6.64 unknown

SILVER(oz) 12.09 15.73 103.98 21.22 18.35 1.43 12.89 2.61 2.54 2.16 1.75 unknown

AREA Natalau Natalau Natalau Natalau Natalau Natalau Vatutabua Natalau Natalau G-Ridge Natalau Natalau

Location The Vuda Mining Area is located at 17° 41' S,177° 29' E in the upper reaches of the Vuda River, about 8 km south of the city of Lautoka. Access is by feeder road leaving the Queens Road at the Natabua Road or the Vuda road.

Nadi Bay Background There has been interest in the Nadi Bay area since gold was first discovered in the hinterlands of Vuda and Sabeto during the early 1900s, rejuvenated when Crawford Marine Specialists Inc. reported widespread high gold values during its exploration of the bay in 1969.

Nadi Bay has been investigated for minerals, particularly gold and magnetite, by three exploration companies in the last twenty-three years, namely Crawford Marine Specialists Inc. (1969-1971), Manganex Ltd (1972-1974) and Consolidated Gold Fields (Fiji) Ltd (1980-1981). An excellent summary of these companies' results is reported in Htay et al. (1990). Offshore exploration by the three companies consisted of drag-bucket and shallow gravity core sampling and beach-sand augering. A seismic bottom-profile survey was conducted by Crawford Marine Specialists Inc.

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Early offshore sampling by Crawford Marine reported anomalous gold values up to 3740 mg/m 3 (1.87 ppm). The survey reported 20 samples with values > 1360 mg/m 3 over approximately 20 km 2 in the Nadi Bay area. Thirty-one samples out of seventy-eight samples yielded gold values. Seismic sub-bottom profile interpretation indicated sediments up to 15 m thick overlying layered inclined bottom sediments with angular unconformity below. For unknown reasons, the company later discontinued exploration and relinquished the licensed area.

In 1972, Manganex Ltd investigated the Nadi Bay area in search for magnetite with gold as a secondary interest. Manganex reported "no gold was detected in the interface zone, and only one speck of gold was found in the upper part of the sediment". Assay values taken from 25 offshore samples recorded gold values below detection limits. Three samples were taken from holes drilled to 12 m. The Atomic Absorption Spectrometry method was used to analyse for gold.

Ten years later, Consolidated Gold Fields, after examining Crawford Marine's data, acquired the right to prospect for alluvial gold in Nadi Bay. The increase in price of gold, from US$35/oz in 1970 to US$620 in 1980, was the reason for the company's interest in the area. Consolidated Gold Fields conducted beach-sand augering, offshore grab sampling and gravity coring. Beach augering was carried out at 35 stations at the mouths of the four major rivers and also at Momi Bay. The depth of the auger holes was limited by the water table. The gravity coring programme consisted of 79 offshore stations and had a maximum penetration of 2 m. Samples were concentrated by panning and treated with dilute acid to remove shell fragments prior to gold analysis by fire assay.

Both the sand augering and the offshore gravity coring returned poor gold values. The highest gold assays were 0.313 ppm in a sand auger sample from the mouth of the Vuda River, and 1.19 ppm in a gravity core sample taken from a site adjacent to the old Mistry gold mine. In 1993, SOPAC undertook a survey of bathymetric and, seismic surveys and surface sampling in Nadi Bay. The results of the survey clearly defined numerous large buried channels associated with old fluvial systems that were previously hypothesised but never confirmed. The bathymetry of Nadi Bay is illustrated in Figure 8. In November of 1994, SOPAC and the Mineral Resources Department undertook a comprehensive geophysical and drilling programme. The survey acquired fifty-two lines of bathymetry and high resolution seismic profile data identifying target for drilling. A total of eighty three holes and thirty sites were drilled by vibro jet airlift drilling. Drill holes were very encouraging, confirming the presence of buried channels and panning of selected samples during the survey showed several specks of gold associated with the buried channels.

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Samples are presently being analysed and will be reported in detail in a technical report by SOPAC for the Fiji Government.

Geology The geology of the Nadi area has been described by Geological Survey and Mineral Resources Department geologists: Houtz (1959), Batholomew (1960) and by Rao (1983). Various parts of the Nadi and the Sabeto areas have been mapped in detail by mining-company geologists prospecting in the area. These reports are available in the Mineral Resources Department open-file library.

The Nadi Bay alluvial plain extends some 6 km inland from the coast and rises 15 m above mean sea level. It is drained in the northern part by the Vuda River, Sabeto River, and Nasoso Creek; in the central and southern part by the Nadi River and its tributaries; and in the south by Nawaka and Masi tributaries. These rivers and creeks all form deltas. Some rivers and creeks are

Figure 8. Nadi Bay.

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typical ephemeral streams that deeply incise the underlying rocks of the Nadi Sedimentary Group, the Koromaivua Volcanic Group the early Pleistocene Meigunyah beds. Water deepens gradually offshore, typical of delta systems elsewhere.

Houtz (1959) first described the older gravels of the Nadi River. Despite their youth and unconsolidated nature, they weather to material that simulates an older weathered conglomerate. Houtz also described gravels from a former delta of the Nadi River. Batholomew (1960) noted two types of recent alluvial sediments in the Nadi area: (i) old sandy gravels, red clays, silts; and (ii) minor gravel forming small flat-topped terraces up to 40 m above sea level. In 1989, Gibb Australia, an engineering firm, conducted geotechnical investigations in southern Nadi Bay for a new hotel and commercial development. Twelve rotary holes and 375 vibro-core holes were drilled. Rotary holes were drilled to 18 m and vibro cores to 6 m. The results of this work showed that the coastal plain is formed from a mixture of alluvial material and marine sediments 4 to 6 m thick, underlain by a basement of silty brown clays derived from the weathering of a sedimentary rock. The alluvial material consists of silt, sand and gravel-sized rock fragments derived from volcanic and sedimentary rocks. Many of the rocks are weathered; they are generally well sorted with little organic material present. These samples have not been tested for gold.

Marine sediments and beach deposits form the islands Denarau and Buabua, as well as numerous islands at the mouth of the Nadi River and within the mangrove swamp areas. These deposits were formed by reworking of older alluvial deposits by wave action and are usually 2 to 7 m and occasionally up to 15 m in thickness. They consist of unconsolidated fine- and mediumgrained sand with shell fragments.

Highly flocculated, soft, grey-green marine silts on the ocean floor extend from just below low-tide level to considerable distances offshore, and vary in thickness from 3 m to 10 m. Shell fragments are common.

The source of gold is from the Vuda and Sabeto hinterlands. In the Vuda area, gold tails up to 5 cm can be panned in the Vuda River, and gold values up to 20 g/t can be sampled in the Vuda valley from shoshonitic lava and agglomerate of the Koromaivuia Volcanic Group which have been completely altered by fumarolic activity. This gold-bearing locality extends over an area of 6 to 8 km 2. Gold mining took place in the Vuda valley from 1938 to 1943, producing 670 oz of gold and 157 oz of silver. Mining ceased because of World War II restrictions. Since then, the entire Vuda valley has been intensely and vigorously explored by mining companies. Despite the long history of

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exploration and widespread gold mineralisation in the Vuda area, no significant deposit has been defined. Indications are that mineralisation is associated with zones of fracturing or faulting. Dickinson (1968) and Lawrence (1978) refer to the Vuda area as a collapsed caldera underlain at shallow depth by intrusive rocks. Usual last-moment comments by departing frustrated exploration geologists are "all the gold has been eroded away and is now in the bay" – the reason for no discovery after months of hard exploration in the Vuda area.

In the Sabeto area, the Sabeto River runs adjacent to Kingston Mine which operated periodically from 1906 to 1952. Closures of the mine were always due to flooding of the Sabeto River. Vein material from the old workings assays up to 97 ppm gold and 434 ppm silver.

Production No gold has been produced from the alluvials at Nadi Bay but gold has been extracted at Vuda some 5 km away (see Vuda area mine production above).

Location Nadi Bay is located in the west of Viti Levu, Fiji's largest island. Nadi Town lies 2.5 km south of the bay. Nadi International Airport sits on an alluvial plain next to the bay. Water in Nadi Bay is generally calm, being protected by the Mamanuca Group of islands and fringing coral reefs. Four rivers, the Vuda, Sabeto, Nasoso and Nadi Rivers, discharge into Nadi Bay and form extensive deltas. Vegetation in the deltas consists of dense mangrove forestation. Road access to the bay is excellent through a network of sugar-cane and feeder roads leading to residential properties that are rapidly building up along the bay. Access to the delta area of Sabeto is currently more difficult due to destruction of a bridge during Cyclone Oscar in 1983, but travel is possible by punts that can readily be hired in the area.

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Waimanu River Background The Waimanu area was first pegged as a gold prospect in 1909. The area is Fiji's earliest discovered gold prospect. From 1909 to 1940, the area attracted almost continuous interest from prospectors. One prospector, Mr W.G. Johnson, extracted and sold 7 oz of gold by sluicing (Rickard and Monro 1960; Rickard and Rodda 1962).

In 1922, Mr F.G. Renou, a mining surveyor, who was commissioned to prospect the area, reported the existence of a substantial deposit of alluvium at Waimanu. Mr Renou stated ‘millions of yards are available’ and it was the largest and best alluvial deposit he had ever seen.

Several of the early reports are still available at the Mineral Resources Department Library. The reports contain conflicting assessments. Whereas some of these reports regard the Waimanu occurrence as extremely promising, others report it to be discouraging and to be uneconomic, because of the low grade and/or low tonnage.

From 1959 to 1961, the Mines Department and the Mineral Resources Department (then Fiji Geological Survey) carried out re-investigations of the economic potential of the Waimanu alluvial gold (Romanu 1959; Rickard and Monro 1960; Rickard and Rodda 1962). Their work involved a pitting survey to determine the available volume and probable grade of the alluvial gravels. The final report concluded that some 2 million cubic metres of gravel is present at an overall average grade of probably less than 40 mg/m 3 which was (and still would be) uneconomic.

In a couple of areas, particularly in Tom Creek, panning in the alluvial deposits gave good indications of gold on every occasion. A large pan yields 15 to 25 ‘colours’ and in one case as much as 62. Gold occurs usually as flattened flakes or wires, or as irregular grains. They are usually up to 1 mm in maximum dimension and the largest seen was 3 mm long, weighing 17.4 mg.

In 1987, the Mineral Resources Department undertook a brief investigation (D. Greenbaum and J. Lum, 1981) and concluded that indications of the potential value of the Waimanu alluvial gold prospect are not wholly unfavourable, and further detailed work to evaluate the occurrence was considered justified.

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In the mid 1980s there was renewed interest in the Waimanu area by several small exploration companies. Results of these investigations confirm the presence of alluvial gold in the area but did not add new discoveries.

Geology The geology of the area is described in Geological Survey Bulletin 15 (Band 1968) – Sheet 19. The Waimanu river cuts through a sequence of Wainimala Group tuffs and grits, intruded and contact metamorphosed by Colo gabbro. Overlying the Wainimala rocks are marine conglomerates, grits and sand-stones belonging to the Namosi Andesite. In the west of the area are andesitic breccias and conglomerates of volcanic origin. In the area of the gold prospect, the Wainavuru Conglomerate Member (basal Namosi Andesite) overlies the Wainimala sequence. These rocks are polymict conglomerates made up of well-rounded to angular boulders, pebbles and fragments of plutonic and volcanic rocks of Wainimala appearance, set in a sandy matrix. Band notes that this unit also contains sandstones and mudstones. Coral limestone fragments have been dated as Tertiary (Lower/Middle Miocene). According to Band, dip directions suggest that the conglomerate was deposited in a small basin.

Recent gravels up to a few metres thick form terraces, or ‘flats’, along the banks of the Waimanu. It is these flats that have formed the principal alluvial gold targets to date (see Figure 9).

Production There has been no recorded gold production in the Waimanu River.

Location The main Waimanu alluvial deposit is located in the large horseshoe bend on the Waimanu at approximately 18° 05' S and 178° 16'E.

The area can be reached by a foot track which leads north off Queens Road near the Kalokolevu sawmill. A four-hour walk (with a pack) follows the Naikorokoro Creek, passing close to Mt Rama (Joske's Thumb) and then to Waimabue, and finally the Waimanu. Alternatively, a helicopter from Suva can reach the prospect in 15 minutes.

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Figure 9. Waimanu river and Wainadoi Mine.

If mining development were to take place, it is thought that a road to the area could be constructed without too much difficulty.

Wainadoi Mine Background The Wainadoi area was intensively prospected for gold in the 1920s and 1930s, but no records of these activities can be traced in files (Kennedy, 1960).

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The Mineral Resources Department archive files indicate that the first Prospecting Licence at Wainadoi was issued on 6.8.37 to William Everard MacIndoe, who discovered a payable gold lode containing up to 40 g/t in 1935. In 1940, a gold-treatment plant worth some £2000 was installed in the area by a syndicate to which Mr MacIndoe was a party. Some 30 oz of gold and 6 oz of silver was produced (Rodda, in press).

In 1944, Emperor Gold Mining Co. Ltd sampled the area and concluded that the area was too poor to warrant further work.

In 1960, E.M. Kennedy, the Government Inspector of Mines panned gold in the Wainadoi creek and estimated that the Wainadoi gravels contain approximately 1-2 grains per cubic yard (Kennedy, 1960). The volume of gravels along just one portion of the creek where gravel is being extracted by the Public Works Department is estimated to be some 700,000 cubic meter (Green, 1980).

During the mid 1960s geological mapping of the area was conducted by the Mineral Resources Department (Band, 1968). Also during the same period, SPL 1014 covering the Wainadoi Prospect was granted to Central Mining Finance Ltd, a subsidiary of Australian Anglo Ltd. The company initiated a programme in search of porphyry-copper deposits. The company later formed a joint venture with Amax, CRA Preussag. The joint venture defined three porphyry-copper deposits, the Waisoi, Waivaka and Wainabama deposits, north of the Wainadoi Prospect.

During 1987-1988, Western Mining Corporation Ltd conducted systematic exploration for gold over the Wainadoi area. The company carried out extensive soil sampling, geological mapping and rock-chip sampling. Two diamond drill holes were drilled in the area. WMC defined a number of small mineralised structures containing sub-economic gold grades. The company relinquished the area in 1988.

At present the Wainadoi Prospect is contained in a SPL Licence 1367 by Placer Exploration Ltd.

Geology The geology of the area can be found in Band (1968) and in MRD Exploration Reports 1238–1 to -3 by Western Mining Corporation. The following summary is extracted mainly from the MRD Exploration Reports.

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Mineralisation in the Wainadoi area is hosted by a coarse-grained quartz diorite (tonalite) which grades through hornblende gabbro to xenolithic gabbro along its margins. The tonalite is cut by abundant dolerite dykes which usually show chilled margins. This tonalite is faulted against the Wainimala Group rocks on all sides, to the north between Wainadoi Ridge and the Waimanu river. The Wainimala Group is pervasively altered and contains numerous dyke-like intrusive bodies.

The tonalite forms a distinct trapezoidal shape in plan with a NE-trending basal intrusive contact in the south and an E-trending intrusive contact to the north.

Numerous quartz-sulphide-carbonate veins occur scattered through the Wainadoi area. These veins are usually 1 cm to 30 cm in thickness and have strike lenghs ranging from tens of metres to hundreds of metres. At the Wainadoi main adit, the veins occur in an anastamosing network through a zone, 5 m wide, of intensly bleached altered rock comprising quartz, kaolinite, illite and smectite. Grab samples carry up to 12.2 g/t Au, 20.5 ppm Ag, 1.5% Cu, 21 g/t Te and 500 ppm As. Figure 10 shows gold grades of the area during the time of mining in the 1940s.

Production A brief period of mining in 1941-42 produced 30 oz of gold and 60 oz of silver (Band 1960).

Location The Wainadoi Prospect is centred on a ridge dividing the Wainadoi catchment from the Waimanu in the headwaters of Waisulubonu and Wainiwi, about 1.2 km SSW of Delawaisulubonu trig. at 18° 06 6'S, 178° 14.4'E, VIT 19, N29. Access to the Wainadoi Prospect is by an all-weather gravel road which enters the area from Nabukavesi on the Queens Highway hence by approximately 4-hour bush walk. Alternatively a helicopter from Suva takes you into the area within 15 minutes.

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Figure 10. Working plan of Wainadoi Mine with gold contents of samples.

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Yanawai Goldfield Background Alluvial gold was first found in the Yanawai River in 1885 and for some forty years the area was investigated with prospectors recording 'colours' in the Yanawai River and its tributaries.

It was in November 1929 that two prospectors panning up the Yanawai River, obtaining a few colours, found a large boulder of heavy mineral which was unknown to them. Being curious, they forwarded the sample to Mr Bill Borthwick (the discoverer of the Tavua Goldfield), who mentioned that it might be rich in silver. The sample was sent to Sydney and was identified as baryte and analysed to contain 4 oz of silver per ton. The sample was later analysed for gold and the result was 2 1/3 oz of gold (Anon, 1935). Intensive search found the source to be on a prominent andesite ridge – Mt Kasi. Mt Kasi Mine began gold production in 1932.

Alluvial gold was not considered again until the late 1960s when Ocean Science Capital Corporation, operating as Crawford Marine, which held most of Fiji's offshore areas, conducted investigations for alluvial gold. After a brief exploration programme of sampling, the company concluded that the alluvials were sub-economical at the prevailing gold price of US$42.00.

In 1977, Emperor Gold Mining Co. Ltd investigated the potential of alluvial gold in offshore Yanawai. The company reported low gold assay values, the highest being 480 ppb; the average of 18 samples over 20 ppb was 64.7 ppb. Silver assays ranged from 2 ppm to 10 ppm and tellurium ranged from 0.2 ppm to 32 ppm.

In 1982, Australian Anglo American Limited conducted a hand-augering drilling programme to 6 m depths and a mechanised Banka drilling programme to depths of 18 m. Anglo reported an average assay gold value of 44.2 ppb and that gold values declined markedly with depth. Gold spotting and pan concentrate indicated the presence of uniformly distributed, particulate, gravity-recoverable gold, but the amount was too small to be economically won.

Since 1982, there has been no work on the alluvials at Yanawai. However, intense and vigorous exploration has continued at Mt Kasi for epithermal gold.

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Geology The geology of the Yanawai area is presented by Bartholomew (1959), Rickard (1966) and Taylor (1986) (see Figure 11). The Mt Kasi Mine is located within submarine basic-andesite flows and bedded coarse volcaniclastic sediments of the Natewa Volcanic Group. North of the mine the Natewa rocks are intruded by a number of small hypersthene-hornblende-andesite plugs that are assigned to the Nararo Volcanic Group (Rickard, 1966).

Mineralogy at Mt Kasi appears to be controlled by a northwesterly fault. The Yanawai area shows propylitic-grade alteration with the development of chlorite, calcite, pyrite, sericite and epidote. Silification is widespread and pyrite occurs in disseminated form and in veinlets. At the mine, mineralisation is concentrated in a massive quartz-baryte vein system developed along a steeply dipping northwest shear. Mineralisation is approximately 300 m in length and 12 m in width. An extensive open cut remains from old mining activities.

Microscopic free gold is disseminated throughout the silicified vein system and concentrated at the intersection of cross-shears with the vein system. Individual gold grades up to 92 g/t have been recorded (Taylor, 1987). An eluvial gold occurrence is developed around the mine.

Range Resources (1987) estimated Mt Kasi Mine to have 500 000 t of hardrock at 6 g/t Au and 700 000 t of eluvial material at 2 g/t.

Production No gold has been produced from the alluvials at Yanawai but at Mt Kasi, some 4 km away, from 1932 to 1946, a total of 63 592 oz (1.8 t) of gold and 4850 oz of silver was produced from 266 200 t of ore (Colley, 1976). The mine closed in 1946.

Location The Yanawai alluvial deposit is located approximately at 16° 46' S and 179° 02'E in the lower reaches of the Yanawai River. Access is by a gravel road leaving the Savusavu Highway. The alluvials are some 60 km away from the town of Savusavu.

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Figure 11. Mount Kasi.

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SOLOMON ISLANDS All Solomon Island’s gold production comes from alluvial mining. Solomon Islands produces approximately 40 000 grams of gold per year by landowners panning along the Chovohio River and at Gold Ridge (see Figure 12). The Chovohio River and the Matepono River (a tributary of the Chovohio River), both which contain detrital gold, are described below.

Chovohio River Background Gold discovery in the Solomon Islands dates back to 1568 when the Spanish explorer Mendaña discovered alluvial gold in the mouth of Matepono River into which the Chovohio River drains, on Guadalcanal Island. To date the only gold produced in the Solomon Islands comes from the Chovohio River environs. Total raw gold production in the river to 1992 amounted to 835.453 kg valued at $A7.5 million (Ministry of Natural Resources, 1993). In 1992, some 32.957 kg of raw gold valued at A$341 248 was produced. Gold has been reported to have been produced in the early 1930s but records are not available. During 1939 to 1941 sparse information shows that alluvial gold was being produced by numerous leaseholders. Mining activity stopped during the World War II period. During the war, soldiers found nuggets when digging trenches (Mining Magazine, 1985). Up to 1985, gold has been produced exclusively by local islanders using basic panning methods. In 1984, Zanex Ltd formed a joint venture with a local mining company, Mavu Gold Development Ltd, to explore alluvial gold deposits at Chohovio River. The joint-venture partners conducted extensive hand pitting and geophysical surveys. Exploration by the company identified a resource potential of approximately 20 million m 3 with grades of between 0.3 g/m 3 and 2 g/m 3 (Mining Magazine June 1985). In 1985, Solomon Island's first mining lease, SML 001, was issued to the joint venture for a period of 12 months. A 100-m 3/h treatment plant was installed to process both river gravels and material from perched terraces. The plant used a skid-mounted scrubber trommel and a skidmounted Inverell jig concentrator. Earth-moving equipment consisted of a 40-t excavator, a 25-t bulldozer and a 19-t front-end loader. Capital costs for the plant were approximately A$1 million. In 1986-1987, the joint venture produced 39.65 kg of gold, worth to A$614 966. In mid-1988, the joint venture relinquished the licence. Mining operations were hampered by a series of frustrations. The joint venture blamed poor management, threats of take-over within the company, land ownership problems, government

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Figure 12. Placer potential of the Solomon Islands.

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Figure 13. Chovohio River.

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impositions and logistic problems such as slow pitting progress, inadequate pumping capacity, large, heavy ground boulders, numerous breakdowns with earth-moving equipment and continuous expenditure with no real return. Figure 13 shows areas of mining activity along the Chovohio River.

Geology The alluvial gold at Chovohio River is derived from Gold Ridge, which is a large gold prospect with gold values spread over an area of some 4 km 2. Some 60 t to 90 t of gold is thought to have been shed from the primary volcanic exhalative gold deposits into the gravel deposits (Mining Magazine, 1985). The gravels consist of closely packed 'boulder beds' deposited by the river. These gravels are derived from intrusive igneous rocks, metasediments, lavas, and pyroclastic material. The boulders are well rounded and average 0.3 m to 1 m in diameter. A poorly consolidated basal sandstone devoid of gold lies below the boulder beds. Sediment varies in thickness from 2.4 m to 14 m. The alluvium consists of silt and clay at the surface (2 to 8 m thick) with enriched gold in the underlying river gravels.

Production Table 1 Alluvial Gold Production

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Raw Gold grams 120000 100000 80000 60000 40000 0

Figure 14. Gold Production from the Chovohio River (Ministry of Lands, Energy and Natural Resources).

1992

20000 1990

Aus$ 94 779.5 255 811.9 593 105.8 539 846.7 377 331.7 440 610.1 517 245.0 640 326.7 1 177 026.0 871 866.0 891 298.0 553 682.1 644 275.5 549 520.0 341 248.5 335 597.6

1988

Exported g 23 350.7 33 455.8 45 474.9 60 373.9 41 000.8 40 469.4 65 226.8 65 995.4 97 807.8 69 415.5 74 949.9 53 529.6 54 658.9 49 658.2 31 681.3 36 057.3

1986

g 22 642.5 30 123.3 45 553.0 62 155.7 58 014.6 58 224.5 76 982.3 76 531.2 99 761.7 56 217.0 62 508.9 47 473.3 47 739.8 58 576.2 32 957.4 43 960.0

Value

1984

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993

Refined Gold g 17 807.1 25 590.0 35 281.0 45 763.5 30 361.7 26 996.6 40 646.6 44 699.7 65 807.1 43 473.2 50 588.0 36 241.3 40 954.2 36 885.0 23 230.2 26 646.2

1982

Raw gold

1980

Raw Gold

1978

Year

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Location Chovohio River prospect is located in north central Guadalcanal. Access is by sealed road and allweather gravel road from Honiara to Turaana Clinic/School and thence by walking for about half a kilometre to Kavahmbe.

Matepono River Background Gold at the mouth of the Matepono River was reported as early as 1568 when Mendana reported traces of gold. In the early 1930s, gold was traced to Gold Ridge where intermittent mining began. With a known source of gold at Gold Ridge and with mining activities in the Chovohio River, the sediments at the mouth of the Matepono River became a target for placer gold.

In 1975, SOPAC (then CCOP/SOPAC) and the Mineral and Water Resources Division of the Solomon Islands carried out an offshore sampling programme consisting of a nearshore bottomsediment sampling programme and a sub-bottom reflection-profiling survey. The survey collected 172 samples for analysis, of which three contained gold above detection limit. The report suggested further work in specific areas (Turner and others 1977).

In 1990, SOPAC conducted a geomorphological study as a basis for resource assessment and planning for economic development. In 1992, SOPAC further undertook a beach sampling programme. The report concluded that gold is present in the beach in uneconomic amounts (Collins 1993). In 1994, SOPAC carried out a magnetometer survey to define targets for detrital gold. The study identified some 10 elongated, continuous, narrow heavy-mineral deposits parallel to the present shoreline. These deposits are up to 300 m long and 50 m wide, and extend as far as 800 m inland where the traverse lines ended. In March of 1995, SOPAC and the Solomon Islands Mineral Resources Divison conducted the follow-up study to test the anomalies. A magnetometer and auger survey were undertaken in the lower reaches of the Matepono River. The augering program consisting of 16 holes (6 m depth per hole) confirmed the presence of magnetite layers and gravels beds in the survey area. Magnetite layers are up to 25 cm in thickness and gravel beds consist mainly of rounded basalt and quartz gravel. Samples were collected at one-metre intervals and some 100 samples weighing approximately 3 tonnes were collected for gold analysis. The samples will be analysed by both fire assay and chemical analysis

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Figure 15. Matepono River.

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for their gold content and reported in a SOPAC Technical Report. The technical report is expected to be released in the year following the receipt of the gold results.

Geology The deltas of the Matepono River and seven other river systems have amalgamated to form the Guadalcanal coastal plain, which is some 50 km long and 10 km wide (see Figure 15). Hackman (1980) described the Guadalcanal coastal plain as being built up from the progressive coalescing of the main river deltas which are still expanding.

The Matepono River is one of the smaller rivers but has a steeper overall channel gradient. The river is described by Roy (1990) as 'narrow, steep-sided, meandering channels and a sediment load that is fine grained'. Collins (1992) characterises the beach sediments adjacent to the mouth of the Matepono River as 98 percent sand. The beach at the mouth of the river is relatively flat, being the result of a low-wave-energy system. In spite of the low energy, a berm ½ m high has developed along the beach. Patches of heavy minerals are common on the beach, notably at the mouth of the Matepono River. These heavy minerals, mainly magnetite, occur as layers approximately 25 cm from the surface. The beach is sandy, lacking pebbles and boulders, and extends unbroken along the coast. It is generally 20–30 m wide with indications of further extension inland. Roy (1990) states that the coastal plain is 10 km wide. Vegetation on the coastal plain consists of grass, pandanus trees and coconut and oil palms. The coastal plain adjacent to the Matepono River is relatively flat, showing no relief such as indications of paleo-strand lines. Gold panning results by Eade et al (1975), Roy (1990), Collins (1992) and Lum (1993) all confirmed the presence of gold in the strand lines. Gold grains are very fine (but visible to the naked eye) and sub-angular in shape.

Production There has been no reported gold production in the lower reaches of the Matepono River.

Location Matepono River is located on the north coast of Guadalcanal. Access to the mouth of the river from Henderson Airport is by a 21-kilometre dirt road to Tetere Bay and then by a kilometre walk along the beach.

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VANUATU

There are at least two areas where placer gold has been identified in Vanuatu (see Figure 16 and description below). Currently exploration is being undertaken by SOPAC in these areas.

Figure 16. Placer potential of Vanuatu.

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Big Bay Background In July 1985, visible gold was panned in the SE Cumberland Peninsula area of Big Bay by United Resources Ltd, while the company was exploring for epithermal gold deposits in the area. The presence of gold and the substantial volume of beach deposits made Big Bay a target for placer gold. During 1985-1988 the exploration company conducted pan-concentrate and bulk sampling at the mouths of major drainages in Big Bay. Altogether 78 samples were collected from the Jordon, Lower Ora, Lape, Riovi and Apuna Rivers at approximately one kilometre sample spacing. The maximum calculated grade of the in-situ samples was 0.01662 g/m 3. A grade of 0.3 g/m 3 is required for a normal viable alluvial gold operation. The company concluded that although it is evident that a very large volume of alluvials is present and the alluvials contain gold, the concentrations in the areas sampled are far below economic grades.

In 1994, SOPAC undertook a sampling programme along the western part of Big Bay and confirmed that although gold is present, it occurrs below economic mining grades (Lum et al 1994).

Big Bay is considered a prime target by virtue of its size and location.

Geology Quaternary sediments extent for more than 25 km along the coastline of Big Bay and 0.5 km to 6 km inland from the coast (see Figure 17). The bay is fed by the Jordan, Ora, Sale, Raovi, Apuna, Tavol and Puluma Rivers. It is believed to have been a shallow platform area during the Early Pliocene following the deposition of the Tawali calcarenite formation (Macdonald, 1986). Uplift led to erosion and deposition of the Sale Formation of marine silts, gravels and conglomerates. The conglomerate member is composed of well-rounded pebbles, cobbles and occasional boulders (up to 35 cm in diameter) of mainly volcanic rock, with subordinate limestone in a matrix of silt, sand and gravel. The conglomerates thin and wedge out to the north and north-east, indicating that they formed in a shallow-water environment contemporaneous with the silts which settled further offshore than the coarse sediments and at a greater distance to the north and north-east of the emergent land areas (Mallick and Greenbaum, 1977). These conglomerates are stratified and clasts are well rounded, suggesting that the deposits

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Figure 17. Big Bay.

resulted from erosion by torrential rivers. The embayment was formed following a major marine transgression at the end of the Pliocene and the emergence of the reef limestones during the Pleistocene (Macdonald, 1986).

The source of gold at Big Bay is most likely to be from the Peak Santo Region, the Middle Ora Valley, and the Middle Apuna Valley where mineralisation is known. In the Peak Santo Region, in the headwaters of Ora River, the Ora-Navaka Intrusion Complex carries disseminated iron and

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copper sulphides. Skarn mineralisation has also been identified in the area (Mallick and Greenbaum, 1977). Blocks of red jasper are also common in rivers and tributaries associated with these areas of mineralisation. During 1969-71, the Bureau de Recherche Géologique et Minière identified a sample with microscopic gold in the Ora River during its search for porphyry copper. In 1973-75 Kennecott Exploration, also in search for porphyry copper, reported a rock sample containing 2.4 ppm Au. In the Middle Ora Valley, galena and pyrite occur with a small skarn occurrence. In the Middle Apuna Valley, numerous pyritic joints and minor stockworks are present in the Apuna Intrusive Complex.

Production There has been no gold production at Big Bay.

Location Big Bay is located in the northern part of Espiritu Santo. Access from Lugainville is along a 65 km spur road to Matantas, thence by boat or foot to areas along the bay.

Pealorai Prospect Background Copper-magnetite mineralisation was first discovered in the early 1950s by a local company, D.J. Gubbay and Co. In 1968, Conzinc Rio Tinto Australia (CRA) traversed the area looking for porphyry-copper mineralisation but failed to locate intrusions on the Cumberland Peninsula. Detailed mapping in the area by Bureau de Recherche Géologique et Minière in 1969-71 delineated four copper-anomalous areas. The company's final report concluded that there are broad petrographic similarities between the diorites and microdiorites of the area and those of typical porphyry-copper deposits such as Bougainville. In 1973-75 Kennecott Exploration of Australia also explored for porphyry-copper deposits in the vicinity. Their investigations showed that mineralisation is associated with magnetite skarn bodies containing chalcopyrite, bornite, and malachite. These skarns were considered to be too small and discontinuous to have economic potential. Also during the same period, the then New Hebrides Geological Survey conducted complementary investigations (Radford 1974, 1975; 1976, Mallick and Greenbaum, 1977). The Geological Survey reports classified the mineralisation in the area as skarn and magnetite lode mineralisation. Radford (1976) estimated some 625 000 tonnes of magnetite in the beach sands

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northwards for 9.5 km from Maloeta on the Big Bay coast. In 1977-79 Melanesia Exploration Ltd (MEL) conducted reconnaissance exploration and established a low-order gold anomaly of 1-2 ppm in the upper Pealorai River. In 1981, MEL formed a joint venture with AMAX and Getty Oil to search for gold porphyry-style mineralisation and associated skarns, and/or epithermalhydrothermal gold mineralisation. The joint-venture partners concluded that the area was discouraging in that it lacked signs of a significant hydrothermal system or associated mineralisation. During 1985 to 1990, United Resources (Vanuatu) Ltd conducted exploration in the area. Their exploration outlined an area in the headwaters of the Pealorai, between the Peamaeto and Peameghu Rivers, shedding anomalous gold. Visible gold was common in pan-concentrate samples. Thirty-eight 1-m 3 bulk samples were collected to test for pannable gold in stranded beach terraces bordering active drainages and along the coast between the Peavovoko River and Vasi Village. The average grade of the samples was 0.05 grams per cubic metre. The company concluded that although it was evident that a very large volume of alluvials is present, and the alluvials contain gold, the concentrations in the areas sampled were far below economic grades. Currently the area is being explored for porphyry-copper deposits by Placer Pacific Exploration Ltd.

Geology The rocks in the Pealorai area have been geologically divided into the following: 1. Recent alluvium and coastal platform limestone 2. Sale Formation 3. Tawoli Calcilutite Formation 4. Pialapa Greywacke Formation 5. Intrusive Complex 6. Ilava Volcanic Complex

This summary of the geology of the area is extracted mainly from Carney and Macfarlane (1986) (see Figure 18). The oldest rocks belong to the Ilava Volcanic Complex which crops out in the western part of the area. The Ilava Volcanic Complex is dominated by basaltic breccias and paraconglomerates, and arenites with intercalated palagonitic tuffs and pillow lavas (Carney and Macfarlane, 1986). The intrusive complex is represented by a number of small, fault-controlled

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Figure 18. Peolorai Prospect.

stocks of basalt, andesite and microdiorite which are variably mineralised. The Peavovoko-Pialapa Intrusive Complex occurs as a fault-controlled sliver some 10 km in length and 1.5 km in width. Mineralisation consists dominantly of pyrite in shear breccia and crackle zones. The emplacement is believed to be related to a major episode of block-faulting which brought development of basins to the east, in which medium- to fine-grained volcaniclastic sediments of the Middle Miocene Pialapa Greywacke Formation accumulated. These sediments were flexed and eroded during a phase of uplift that followed in Late Miocene times by subsidence and the deposition of hemipelagic carbonate mudstones and marls of the latest Miocene-early Pleistocene Tawoli Calcilutite Formation. Succeeding these hemipelagites is the Sale Formation, a Plio-Pleistocene

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sequence of terrestrial sands, gravels and conglomerates. Recent deposits are present-day beaches which occur along much of the peninsula. They consist of unsorted gravels and conglomerates derived from lava, paraconglomerates and occasional reefal limestone. Fluviatile deposits occupy alluvial fans along the shore and have coalesced as an extensive plain to the south of the Pialapa River.

Production There has been no gold production at Pealorai Gold Prospect.

Location The Pealorai area is located in the eastern part of the Cumberland Peninsula and lies approximately 65 km northwest of Luganville at 15° 00' S and 166° 45' E. Access to Pealorai from Luganville is along a 65-km spur road to Matantas, thence by boat along the bay.

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CONCLUSION

The placer potential for the Solomon Islands, Vanuatu and Fiji is promising. The possible major reason why an economic placer deposit has not been discovered is because there is little exploration in this area. There are no reasons to expect that economic placer deposits will not form in these island environments. In fact, there is every reason to believe that many large deposits should occur in nearshore environments of the region for several reasons. (i) World-size gold deposits occur on or near coastlines. These gold deposits are either residual gold caps or epithermal deposits, i.e. both occurring near the surface exposed by erosion. (ii) Being in a tropical region, the islands are subject to heavy erosion. (iii) Seasonal storms and cyclones provide the energy for placer gold reworking and concentration, and moreover, fringing reefs prevent gold from leaving the system.

Due to the difficulties involved in the evaluation and assessment of placer gold deposits mining companies have generally kept away from exploring for them. Unless a significant placer gold deposit is discovered, the mining industry will continue to shy away. Governments should continue to explore and provide incentives so as to encourage further exploration in this area.

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BIBLIOGRAPHY

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Taylor, J.F.A. 1951. Department of Mines. [Annual report for 1950.] Legislative Council of Fiji, Council Paper 23 of 1951. 13 pp, 2 figs. Taylor, J.F.A. 1952. Department of Mines. [Annual report for 1951.] Legislative Council of Fiji, Council Paper 6 of 1952. 11 pp. Turner, C.C.; Eade, J.V.; Danitofea, S.; Oldnal, R. 1977. Gold-bearing sediments on the continental shelf northern Guadalcanal, Solomon Islands. South Pacific Marine Geological Notes 1(6): 55-70. Various. [Various documents, reports, maps, etc., on the Yanawai Goldfield.] Fiji Mineral Resources Department Exploration Reports 30/.../377/ML 2-1 to -4. (Unpublished). Various. [Miscellaneous correspondence and reports on gold prospecting and mining in the Tavua (Vatukoula) area, 1932 to 1949.] Fiji Mineral Resources Department Exploration Report 207/.../302/ML 1/3 to 5-1. 112 pp + 2 figs, 2 plans. (Unpublished). Various. [Miscellaneous correspondence, reports and plans concerning the Homeward Bound Mine, Tavua, 1937 to 1948.] Fiji Mineral Resources Department Exploration Report 222/.../455/ML 12-1. 41 pp + 5 figs, 4 plans. (Unpublished). Various. Miscellaneous correspondence and reports on prospecting at the Nabila gold prospect, Momi, 1935 to 1939. Fiji Mineral Resources Department Exploration Report 241-1. 14 pp + 3 figs. (Unpublished). Various. [Miscellaneous correspondence, reports and data concerning prospecting for gold at the Wainadoi (Delaiwaisulubonu) prospect, 1935 to 1939.] Fiji Mineral Resources Department Exploration Report 283/386/411-1. 99 pp + 4 figs, 1 plan. (Unpublished). Various. [Miscellaneous correspondence, reports and data concerning prospecting for gold at the Wainadoi (Delaiwaisulubonu) prospect, 1940 to 1954.] Fiji Mineral Resources Department Exploration Report 283/386/411-2. 85 pp + 3 figs. (Unpublished). Various. Prospecting at Momi (later Mistry Mine) 1935-1948. Fiji Mineral Resources Department Exploration Report 342/.../476-1. 37 pp + 11 figs, 2 plans. (Unpublished). Various. [Miscellaneous reports, data and correspondence concerning prospecting at Kingston Mine, 1940 to 1965.] Fiji Mineral Resources Department Exploration Report 392/.../522-1. 60 pp + 11 figs, 1 plate, 6 plans. (Unpublished). Various. Miscellaneous reports, data and correspondence on operations at Mistry (Senilabuli) Mine, Nabila, Momi, 1952 to 1960. Fiji Mineral Resources Department Exploration Report 518/578/IPM 10/ML 23/26-1. 80 pp + 3 figs, 2 plans. (Unpublished). Various. [Miscellaneous reports on prospecting in PL 764 and other parts of Viti Levu 1954 to 1958.] Fiji Mineral Resources Department Exploration Report 764-1. 15 pp + 4 figs, 1 plan. (Unpublished). Various. [Exploration for gold in the Tavua Basin, 1964 to 1970: reports on collaborative studies by the Geological Survey Department. Volume 2. Various Geological Survey reports.] Fiji Mineral Resources Department Exploration Report 987/992-2. 73 pp + 26 figs. (Unpublished). Various. [Exploration for gold in the Tavua Basin 1962 to 1968. Miscellaneous correspondence and reports.] Fiji Mineral Resources Department Exploration Report 987/992-3. 73 pp + 1 fig. (Unpublished). Various. [Various documents giving prospecting results from 30 September 1965 to 30 September 1974.] Fiji Mineral Resources Department Exploration Report 991-1. 164 pp + 11 plans. (Unpublished).

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Various. Vuda gold study - Kennecott. Fiji Mineral Resources Department Exploration Report 991-4. 74 pp + 6 plans. (Unpublished). Various. [Results of offshore prospecting in Area 6 (Mt Kasi, Yanawai). A collection of various documents.] Fiji Mineral Resources Department Exploration Report 1023-1. 50 pp + 4 plans. (Unpublished). White, F.T.M. 1937a. Report on P.L. 348 - Aranachalam Pillay. [Vuda.]. Mines Department, Suva. 4 pp. Available in Fiji Mineral Resources Department Exploration Report 296/.../857/ML 10/18-1. (Unpublished). White, F.T.M. 1937b. Report on the Natalau Gold Prospecting Syndicate. Mines Department, Suva. 7 pp. Also available in Fiji Mineral Resources Department Exploration Report 296/.../857/ML 10/18-1. (Unpublished). White, F.T.M. 1940. G.H. Middenway - P.L. 400, Vuda Mining Area. Mines Department, Suva. 1 page. Also available in Fiji Mineral Resources Department Exploration Report 296/.../857/ML 10/18-1. (Unpublished).

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