[Sample 1] [Sample 2] [Sample 4] WAIMANU RIVER Alluvial gold was discovered in the Waimanu river, about 20 km NW of Suva, in the early part of this century and it remains the only significant alluvial gold prospect in the islands. Between 1909 and 1935 exploration was carried out by a number of prospectors and syndicates. Further exploration by the Geological Survey and Mines Department in 1960-61 (Rickard and Monro 1960; Rickard and Rodda 1962) included the sinking of 36 pits with an aggregate depth of 79.3 m over an area of 1.3 km2. Greenbaum and Lum (1981) reassessed the findings of Rickard and Rodda (1962) and entirely new exploration has been conducted by Fortress Pacific Mining Co (SPL 1200), Western Mining Corporation (Fiji) Ltd (SPL 1238), Beta Ltd (SPL 1307) and Field Investment Ltd (SPL 1346). Gold occurs in gravels under river flats and in terraces generally overlying volcanic and sedimentary rocks of the Wainimala Group. Bloomstein (1985a & b) described the gold-bearing area of the Waimanu as a 4 km x 2 km graben striking NE-SW, however, further structural investigations are needed to confirm this. Most of the gold is located in the river terraces and investigations by Beta Ltd indicate about 1 x 106 m3 of gravel in terraces 0.5 m to 2 m in thickness. There is generally about a metre of capping soil and locally up to 4 m of sand and clay overburden. The only detailed attempt to assess the grade and tonnage available is that of the Geological Survey and Mines Department. Rickard and Rodda (1962) estimated 1.53 x 106 m3 of auriferous gravel with grades of 40 - 60 mg/m3 of gold, however, Greenbaum and Lum (1981) reassessed this earlier work and gave revised grades of 175 - 262 mg/m3 of gold. Rickard and Rodda (1962) recorded maximum grades of 10.2 g/t Au in a 60 cm gravel horizon in Tom Creek and subsequent work by Fortress Pacific Mining Co has confirmed the high grades in Tom Creek with additional good grades in Jim Creek and the lower reaches of Wainavuru Creek. This company also excavated six test pits in the Waimanu flats using an 8 inch suction floating dredge. The pits reached depths of 4 - 8 ft and sampled 1.5 to 3 ft3 of gravel; up to 4 g/t Au was recovered from the pits (Bloomstein 1985a & b). Beta Ltd estimate about 150 000 m3 of gravel in the river flats, averaging about 1 m in thickness. Gold in the terraces occurs as free flakes and composite grains which may be easily panned from the gravels. The gold is usually distributed in sinuous channels suggesting deposition in a former braided stream system. Composite particles are to 2 mm and contain inclusions of quartz and/or iron oxide grains. Small nuggets are to 0.10 mm, subrounded and cemented by secondary gold. The gold is dark yellow and suggestive of a gold+copper alloy. Panned concentrates are dominantly magnetite and pyrite but also contain ilmenite, leucoxene, rutile, copper sulphides, arsenopyrite and two populations of zircon (McLennan 1992). The source of the gold is enigmatic. Bloomstein (1985a & b) proposed that the Waimanu placer gold represents a reconcentration of gold from a fossil placer in the Wainavuru Valley (Figs 28 and 29).
Figure 29 The fossil placer is represented by the Wainavuru sandy conglomerate, a basal unit of the Namosi Andesite (Medrausucu Group). This basal conglomerate, which is exposed in Tom Creek, is weakly consolidated, clast-supported and contains well-rounded and well-packed pebbles of gabbro, basic tuff, silicified limestone and andesite in a quartz-clay-chlorite-pyrite matrix. These lithologies represent bedrock from the Wainimala Group, the Namosi Andesite and Wainadoi gabbroic stock (Colo Plutonics). The ultimate source of the gold remains unclear but an origin from a hydrothermal vein system is suggested by the gold’s colour combined with the presence of abundant pyrite, lesser chalcopyrite and trace arsenopyrite. Rock-chip sampling and analysis by Beta Ltd of altered float in the vicinity of the Waimanu placer, principally silicified and pyritised Wainimala Group rocks, revealed slightly anomalous Te, Hg, Sb and As contents, but Au was not notably high (Henderson 1989b). Western Mining Corporation (Fiji) Ltd (SPL 1238) looked at gold prospects upstream of the Waimanu alluvials i.e. at Waiturua, Wainiwi-Wainabau including the Waimanu adits, Wainadoi and Wainadoi Northeast which are within a range of units - Wainimala Group, Wainadoi stock of the Colo Plutonic Suite, Namosi Andesites and intrusives possibly synchronous with the Namosi Andesite (Fig. 28) (Mazzoni 1987-1989; Eaton 1990). Altered andesite bedrock close to the Waimanu alluvials showed no apparent gold mineralisation, but upstream the stream sediment anomaly (as defined by the 20 ppb contour for Au) is very extensive and is centred on the porphyry copper system of Wainiwi-Wainabau (Fig. 28) (see section on Wainiwi-Wainabau in the chapter on Disseminated Sulphide Deposits). This raises the possibility of gold in the Waimanu alluvials being sourced from a gold-rich cap to the porphyry copper system rather than just from the peripheral vein occurrences. [Sample 1] [Sample 2] [Sample 4] |
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