Namosi - Waisoi, Waivaka
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The Namosi district Waisoi and Waivaka porphyry copper-gold deposits are located ~30 km NW of Suva, the capital and main business centre of Fiji (#Location: 18° 0' 24"S, 178° 9' 20"E).
The major porphyry copper occurrences at Namosi also include Wainabama and Wainiwi, while a number of polymetallic vein and skarn systems are also developed peripheral to the main prospects.
Mineralisation in the area has been known since the early 1930s, although a detailed search for porphyry copper deposits only commenced in 1968. Between 1968 and the early 1980s, Amax Exploration completed over 50 000 m of drilling, and feasibility studies indicated a combined estimate of 590 Mt @ 0.47% Cu at the two main prospects at Waisoi East and West (Fiji MRD, website).
In 1989 WMC conducted a 16 hole diamond drilling programme targeting gold mineralisation. Between 1991 and 1995 Placer Pacific conducted further work, including 12 000 m of drilling. Placer Pacific defined a low grade open pit copper-gold resource in excess of 900 Mt @ 0.43% Cu, 0.14 g/t Au and conducted a feasibility study. Nittetsu was subsequently awarded the exploration licence for the Namosi tenement in early 2001 when the Fiji government tendered the project. Nittetsu has conducted various exploration programs on the Namosi tenement, including a 51 hole diamond drilling programme totalling almost 16 200 m in the Waisoi and Waivaka Corridor. While much of the past exploration activity has centred on the Waisoi area, reconnaissance level exploration within the Namosi tenement has identified more than 15 significant copper and gold prospects which are distributed over a 20 x 12 km NW-SE elongated oval shaped area (Newcrest Market Release, July 2007).
Mineralisation at the main deposits at Namosi, namely Waisoi East and Waisoi West, is associated with a number of small 5.9 to 5.3 Ma diorite porphyry and quartz porphyry stocks and dykes intruded into volcanic rocks of the Oligocene to Mid Miocene Wainimala Group and the overlying Late Miocene to Pliocene Namosi Andesite. Basaltic lapilli tuff of the Wainimala Group surrounds much of the Waisoi East deposit, and appears to be hyaloclastite and pillow breccia. In contrast, the principal wall rock over the remainder of the Waisoi area is the Late Miocene Namosi Andesite. At the Waisoi East deposit, a quartz porphyry body was intruded into the basaltic lapilli tuff of the Wainimala Group at the centre of the deposit, although additional bodies of quartz porphyry are distributed within and around the Waisoi East deposit (Imai, et al., 2007).
In contrast, at Waisoi West diorite porphyry is sporadically exposed along watercourses such as Wainitotoyeuyeu Creek and Wailase Creek, while a small body of quartz porphyry crops out in Wainisovasovatabua Creek (Fig. 10) (Imai, et al., 2007).
Mineralisation is present as broad zones of pyrite-propylite alteration which enclose areas of higher grade chloritesericite, biotite-chlorite-sericite and biotite-quartz-sericite alteration. The total sulphide content decreases inwards from the peripheral propylitic zone, although the copper grade increases as pyrite is superceded by chalcopyrite and bornite (Colley and Greenbaum, 1980).
Minor supergene enrichment at the base of an oxidised zone has been noted (Colley and Greenbaum, 1980). Diorite porphyries at Waisoi have an adakitic character.
Waisoi East is centered on a quartz porphyry intrusion, although the central sections of the deposit are located below the valley, and are covered with alluvial overburden, with only deeply weathered rocks outcropping on the peripheries. In those deeply weathered peripheral sections of the deposit that are exposed, some sulphides are preserved within the quartz veins and veinlets. However, sulphide phases between quartz crystals are not preserved at surface and the frequency of quartz veinlets is not relatively abundant, except for an outcrop in the south. In an example cited by Imai et al., (2007), high grade ore observed in a drill hole in the south-eastern segment of the deposit, is associated with a body of quartz porphyry that intruded the basaltic lapilli tuff of the Wainimala Group. High-grade ore in the basaltic lapilli tuff is intensely altered to hydrothermal biotite, with 2 to 5 cm wide quartz veinlets immediately enveloped by silicification and sericite alteration <1 m wide. The Waisoi East ore that Imai et al., (2007) studied in drill holes, is characterised by a bornite - chalcopyrite - pyrite assemblage accompanied by traces of molybdenite and native gold. An assemblages of bornite, chalcopyrite and pyrite, occasionally with associated molybdenite, occurs in quartz veinlets, and is also disseminated in the host basaltic lapilli tuff accompanied by hydrothermal biotite (Imai, et al., 2007).
At Waisoi West, Cu mineralisation is associated with a diorite porphyry intrusion that is exposed along the creeks. Namosi Andesite is the dominant outcrop, although basaltic lapilli tuff of the Wainimala Group appears in deeper drilling and underlies the area. A swarm of sheeted quartz veinlets 1 to 3 cm wide is exposed for approximately 30 m along one of the creeks, while an approximately 20 m wide quartz vein zone is exposed at another site, grading into a swarm of horizontal sheeted quartz veinlets 1 to 5 cm wide. Copper minerals are absent in these quartz veinlets in general, and the grades were reported to be 0.02% Cu and 0.2 g/t Au. Stockwork and sheeted quartz veinlets are also observed in hydrothermally altered (chlorite and/or sericite alteration) diorite porphyry along another Creek, situated at the southwestern periphery of the deposit. The quartz veinlets commonly carry chalcopyrite and bornite as primary sulphide minerals in un-weathered sections, whereas malachite coatings are also commonly observed where weathered. Magnetite is disseminated along with chalcopyrite and bornite. Locally, grades of up to 0.72% Cu and 0.31 g/t Au have been reported in creek outcrop. In drilling, the high-grade ore zones are found in the basaltic lapilli tuff of the Wainimala Group along the intrusive contact of the diorite porphyry, associated with a stockwork of quartz veinlets, as stringer veinlets and disseminations of bornite and chalcopyrite accompanied with pyrite. The basaltic lapilli tuff of the Wainimala Group is intensely altered to hydrothermal biotite, whereas the diorite porphyry underwent chlorite - sericite alteration. At Waisoi West, stockwork quartz veinlets are associated with a bornite - chalcopyrite – pyrite assemblage although bornite is partly replaced by chalcocite. Molybdenite and native gold have not been observed (Imai, et al., 2007). In both the Waisoi East and Waisoi West deposits, the primary bornite - chalcopyrite - pyrite assemblage in the high grade zone was deposited at the upper stability limit of chalcopyrite with respect to sulphur fugacity. Thus, the principal Cu mineralisation at the Waisoi deposits, particularly at Waisoi West which is associated with the diorite porphyry, occurred at a high sulphur fugacity, representing a high-sulphidation environment (Imai, et al., 2007).
The Waivaka resource is concentrated in the Waivaka Corridor. Several small high grade deposits, associated with bodies of diorite porphyry, are found in zones of intense quartz veining and potassic alteration, surrounded by widespread, weak propylitisation. Copper is chiefly present in the form of chalcopyrite with subordinate bornite in both the porphyry and in the volcanic country rocks and is closely associated with quartz veining. Magnetite is the principal Fe phase in the main mineralised zone of the corridor (Colley and Greenbaum, 1980).
At Wainambama, low grade copper mineralisation is associated with a large, composite porphyry intrusion (believed to be the larger intrusive immediately to the southwest of Waisoi on Fig. 9) which shows argillic and weak potassic alteration. Chalcopyrite, bornite and pyrite with minor molybdenum occurs as disseminations, veins, fracture fillings and coatings. Higher grades are apparently confined to zones of strong quartz veining (Colley and Greenbaum, 1980).
Reserves and resources have been quoted as follows:
Waisoi East - 150 Mt @0.5% Cu; Waisoi West 300 Mt @ 0.48% Cu (Colley and Greenbaum, 1980).
963 Mt @ 0.435% Cu, 0.014% Mo, 0.14 g/t Au at a 0.3% Cu cut-off at Waisoi East and Waisoi West (USGS);
23 Mt @ 0.75% Cu, 0.25 g/t Au at Waivaka (USGS);
1.1 Gt @ 0.4% Cu, 0.13 g/t Au total resource (Fiji MRD, website);
344.1 Mt @ 0.64% Cu equiv = 0.54% Cu, 0.19 g/t Au at Waisoi East and Waisoi West (Nittetsu Mining, 2004).
Ore Reserves and Mineral Resources at 31 December, 2016 (Newcrest Mining Reserves and Resources statement, 2017) were:
Indicated resources - 1837 Mt @ 0.34% Cu, 0.11 g/t Au;
Inferred resources - 311 Mt @ 0.41% Cu, 0.10 g/t Au;
TOTAL resources - 2148 Mt @ 0.35% 0.11 g/t Au.
Ore Reserves (included in Mineral Resources)
Probable reserves - 1328 Mt @ 0.37% Cu, 0.12 g/t Au.
The most recent source geological information used to prepare this summary was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
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