GOLD DEPOSITS Gold was first reported from Fiji in 1868 when Charles Gurney found "colours" in gravels of the Navua River. From this date sporadic exploration usually by lone prospectors or small syndicates, led to a number of finds; however, it was not until 1929 that gold in payable quantities was found in Yanawai, Vanua Levu. In 1932 the first gold was exported from Fiji having been won from the Mt Kasi mine in Yanawai. During the same year gold in economic quantity was discovered near Tavua in Viti Levu. Production from Vatukoula began in 1935 and has continued to the present time with the main activity being centred on the Emperor Mines (including the former Dolphin and Loloma mines). The only other occurrences in Fiji that have produced a significant amount of gold are at Mt Kasi, Natalau in the Vuda valley, and the Mistry mine in Momi. The very considerable amount of gold exploration in the 1980s, which saw most of the land area of Fiji covered by prospecting licences, has dramatically increased the known number of gold prospects. Although a number of these have high grades of gold, none has yet progressed beyond the exploration stage. At the time of writing (1993), Vatukoula apart, the only other prospects at a potentially mineable stage are Mt Kasi where a low-grade eluvial and hard-rock deposit is at the threshold of economic viability, and at Tuvatu in the upper reaches of the Sabeto River. Hardrock gold in Fiji occurs in a variety of deposit types but principally in epithermal veins (e.g. Vatukoula, Mt Kasi) and porphyry copper-gold disseminated deposits (e.g. Waisoi, Kingston). A comparison of the features of these two major types is given in Table 3 and a model incorporating the two deposit types is illustrated in Fig. 5. Gold is also an important constituent in some massive sulphide bodies, polymetallic veins and pyrometasomatic skarn deposits and the reader is referred to the chapter on Polymetallic Deposits for further details. A summary of the gold content of a variety of Fijian mineral deposits is given in Table 4. Non-hardrock gold in Fiji appears to be of limited occurrence with the only significant alluvial and eluvial deposits occurring in the Waimanu river and at Mt Kasi respectively. Gold is also a minor component of the iron sand deposits at Sigatoka and in the Ba Delta and again further details are provided in the chapter on Iron. TABLE 3. Characteristics of epithermal, porphyry-style and plutonic-disseminated deposits in Fiji.
TABLE 4. Average gold values in Fiji deposits. Unless otherwise specified, values are for pre-resource mineralisation.
VUDA The upper Vuda valley is located about 10 km SE of Lautoka and it was the focus of gold exploration prior to the Second World War. Investigations in 1937 proved 2500 to 3000 t of ore containing an average of 8.4 g/t Au mainly in native form. Intermittent mining in the valley, principally at the Natalau workings, between 1938 and 1954 produced 880 oz (22.6 kg) of gold and 195 oz (5.5 kg) of silver. Workings at Natalau reached a depth of 75 m and included a number of adits and shafts (Fig. 14). During the 1960s the Natalau area was covered by PL 991 held by a local prospector, M. H. C. Akhil. In 1969 the lease was transferred to Manganex Ltd who held it until 1975. Subsequently, the upper valley was covered by PLs 1131 and 1132 held by Goldfield Mines Ltd with various joint venture partners and consultants (e.g. Otter Exploration NL; Placer Exploration Ltd; Freeport of Australia Inc; Austpac Resources NL; Beta Ltd; Newmont Pty Ltd). In the latter part of the 1970s these PLs were surrounded by SPL 1116 held by Aquitaine Fidji Ltd with joint venturers Dampier Mining Ltd and Broken Hill Pty Ltd. When this lease was surrendered in 1980 two smaller PLs, 1173 and 1223, were obtained by Nullarbor Holdings Ltd. A comprehensive report on the early history of gold prospecting and mining at Vuda is provided by Greenbaum (1979a). During 1991, all licences at Vuda were surrendered, bringing to an end a major phase of continuous exploration extending back to 1977. The prospect area is located within the Sabeto Volcanics, a member of the Koroimavua Volcanic Group; a Late Miocene - Early Pliocene series of pyroclastic and autoclastic breccias, rare flows, tuffs and volcanic sediments (Fig. 15). The rocks are biotite- and augite-bearing shoshonites and at lower stratigraphic levels coarse-grained, porphyritic dykes of shoshonite are found. Upper stratigraphic horizons consist mainly of shoshonitic sandstones, siltstones and breccias of epiclastic derivation. On the basis of facies variations from primary volcanic rocks to volcanic-derived sediments and the occurrence of widespread alteration in the Natalau area, Dickinson (1968) suggested that the upper Vuda valley was an eruptive centre for Koroimavua shoshonites. He alluded to the possibility of a monzonitic intrusion at shallow depth, but apart from occasional dykes, no intrusive rocks are seen at surface. However, Lawrence (1978) interprets holocrystalline monzonite containing xenoliths of fine-grained shoshonite cropping out in Nadaka Creek as a probable intrusive. In drilling, possible intrusives and pebble breccia dykes were found by Nullarbor (Table 9). Deeper level intrusives are indicated by the gravity data of JICA-MMAJ (1991-1992). Medium-wavelength gravity data indicates a large elliptical anomaly of 46 mgals centred on Navilawa but with a weak ridge extending west-northwest beneath the Nawainiu intrusive complex (Tarawai Creek area) to beneath the Vuda alteration zone (Fig. 12). The data were interpreted by JICA-MMAJ as reflecting magma chambers, about 5 km below the surface, consisting of the differentiated residua after extrusion of the magmas now exposed as the Koroimavua and Ba Volcanic Groups and intrusive hypabyssal monzonite. Estimates of the area of alteration vary from 3 - 4 km2 (Colley 1976b; Brook 1982a) to 6 - 8 km2 (Rush 1979; Greenbaum 1979b; Lawrence 1978) (Fig. 16). Alteration mineral assemblages have been described as potassic, phyllic, argillic and propylitic with sericite, pyrite, kaolinite, calcite, zeolite, gypsum/anhydrite and epidote being widespread (Dickinson 1968; Colley 1976b; Rush 1979; Anonymous 1986a). In addition, a cap-rock assemblage contains silica (quartz), alunite, diaspore, limonite, sericite, pyrophyllite and kaolinite (Rush 1979; Storey 1981; Fiani and Keshwan 1982; 1983). Lawrence (1984) has questioned whether there is a silica cap-rock preferring to regard the assemblage as part of an old, gossanous erosion surface in which there is an Fe-Mn assemblage (hematite, limonite and pyrolusite) with a low silica content (c. 43% Si02). Jones (1979) also refers to a lateritic (duricrust) cap-rock. Alteration and mineralisation also occur along late-stage, linear structures. Brook (1984) records quartz-alunite-diaspore-kaolinite and Merchant and Fiani (1987) report a similar sequence recorded in drilling a linear structure. The arrangement of alteration zones into mushroom-shaped areas has been proposed by Henderson (1985) and Merchant and Fiani (1987). Much of the gold at Vuda occurs in puggy, clay-rich zones in northerly-striking shears (e.g. Natalau and Delainasomo workings). Gold can be panned from the clays and average grades are around 8 g/t Au. These gold-bearing shears are regarded as late-stage features and in addition to gold they carry quartz-alunite "reefs", adularia and may be intruded by dykes. The size of the structures is disputed with some authors (e.g. Henderson 1984) quoting structures 2.5 km long and several hundred metres wide (including the Natalau shear) and others (e.g. Rush 1979) reporting the Natalau linear structure as 75 m long, 10 m wide with a vertical extent of 45 m. Rush (1979) estimates that the Natalau shear carried 90 000 t of ore containing 6 - 8 g/t Au. Henderson (1989a) refers to the Natalau structure as a mineralised "dyke" with a strike length of 80 m and average width of 6 m. Gold occurs in disseminated form in base metal veinlets occurring in argillised, porphyritic shoshonite. Minerals recorded in the veinlets include pyrite, rutile, galena, sphalerite, chalcopyrite, tetrahedrite, covellite, digenite, chalcocite and native gold. Brook (1982a) gives values of 2 to 10 g/t Au for this disseminated mineralisation with highs between 120 and 330 g/t Au. Primary gold in basemetal veinlets from Natalau workings has a fineness of 850 (Lawrence 1984). Gold also occurs in supergene form at Vuda with isometric crystals of gold up to 100 microns in diameter but averaging 0.02 mm occurring in oxidised, saprolitic soils (Lawrence 1984). Geochemical analysis of stream sediments, soils and bedrock in the last decade or so has generally proved to be of limited use in prospecting. Areas of known mineralisation have registered no, or weak anomalies, and different geochemical prospecting programmes in the same location have given conflicting results. The variable assay results from the same sample sites appear to be the result of several factors, including the erratic distribution of particulate supergene gold and of poor sampling and analytical procedures with fine gold in microfractures being easily missed in channel sampling and if sampled possibly being lost in fine dust during pulverising. In view of the unreliability of geochemical prospecting, non-recognition of supergene versus primary gold, possible separation of supergene gold from its source and the problem of distinguishing shoshonitic dykes from very similar shoshonitic country rocks, siting of drill-holes has been difficult and assay results have generally not been encouraging. Only very limited geophysical prospecting has been carried out in the Vuda Valley with magnetometer surveys aimed mainly at delineating magnetic lows resulting from hydrothermal destruction of magnetite (Nettle 1983). Drilling at Vuda has been aimed principally at assessing the potential for surface/near-surface mineralisation, with at least 18 diamond drill-holes and about 150 RC drill-holes, but with most attention focussed at the Natalau mine (Table 9). At the Natalau mine, pitting followed by shallow (to about 25 m) RC drilling have shown that a near-surface, bulk tonnage, low-grade gold deposit is unlikely to occur. The most extensive drilling programme, 134 holes sunk by Freeport (Nettle 1983) had only 18 holes registering values greater than 1 g/t Au with best result of 48.7 g/t being from RC 48 on the west side of Nakerekere Creek. However, in 18 follow-up holes at this location, only two holes had "significant" gold at 0.91 g/t. Henderson (1989a) reported that eight RC holes drilled to test the Natalau structure all returned low gold values with the best intersection being 2 m at 0.35 g/t Au in RCN8. Pitting, channel sampling and shallow drilling by other companies gave a similar picture of isolated high gold values in the supergene zone, but with overall subeconomic values. Newmont had an average grade of 0.315 g/t Au with a maximum of 0.726 g/t Au (Jones 1979), Nullarbor recorded a maximum of 0.32 g/t Au in silica cap-rock with a range of 0.05 - 0.25 g/t Au being more usual (Fiani and Keshwan 1983; Merchant and Fiani 1987). Goldfield Mines drilled six drill-holes at Natalau, reaching depths of 44 m to 62 m and with the best values of 10 m at 7.5 g/t Au from 30.0 m in DDH 1 and 6 m at 7.5 g/t Au in from 36.0 m in DDH 4, with mineralisation in the dyke and selvages (Lawrence and Govett 1982) (Table 9). In the most recent diamond drilling, i.e. since 1981, there have been 20 drill-holes that have progressed beyond a depth of 100 m (Table 9). Most of these deeper holes have been angled beneath supergene gold anomalies or to test extensions of planar structures at depth. The deep drilling by Nullarbor yielded disappointing results, with a best intersection of 3.3 m at 5.12 g/t Au from 92.0 m in VD 3, and tended to reduce the prospectivity of the district. The cores from the holes showed argillic and quartz-sericite alteration and the geochemistry has a distinct Au-Cu-Zn association, which was not evident in the oxidised and supergene zones. Cu ranges up to 0.85% over intervals of several metres. A number of models of mineralisation have been suggested for Vuda, however, the basic problem with these is that interpretations are based mainly on important, but isolated observations and there is no adequate geological mapping to support the models. Few of the reports produced during the 1980s presented detailed geological maps. The most popular model can be described as porphyry-epithermal with porphyry-style alteration zones, a collapse caldera setting and the probability of intrusions at depth. Fluid inclusion studies by Austpac indicate homogenisation temperatures in the range 235 - 335oC with boiling of fluids occurring (Anonymous 1986b). Variations of the model are illustrated in Figures 17 and 18. In the late 1970s a number of writers (e.g. Rush 1979; Jones 1979) supported a stratabound zone of alteration and mineralisation between 30 m and 150 m thick. Lawrence and Govett (1982) suggested that this flat-lying alteration zone might represent an old, laterised erosion surface with a mineral assemblage of kaolinite, hematite, limonite and pyrolusite. There has also been a suggestion of maar-breccia pipe mineralisation, however, recent investigations have not located unequivocal breccia pipes. It is possible to incorporate aspects of these models into a porphyry-epithermal system having localised "stratabound" alteration zones related to the local land surface and with the possibility of localised incipient brecciation in some hydrothermal centres. [Sample 2] [Sample 3] [Sample 4] |
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