The Waisoi deposits are located 30km north west of Suva (the capital city of Fiji). The Waisoi deposits composed of the Waisoi East and Waisoi West deposits are porphyry-type copper deposits. In this prospecting area, core drilling of 10 holes, total 3, 192.55m, was carried out in 2003 for exploration of middle to high grade copper ore body. Based on the results of this survey, the characteristic of mineralization in the Waisoi deposits was confirmed as follows;
(1) All of 10 drill holes have intersected copper mineralization zone of more than 1.0% copper grade. Among them, remarkable copper mineralization were observed in 5 holes. The results of chemical analysis of prospective holes are as follows. Waisoi East deposit:
Drill hole NWS-3: Cu 0.85%, Au 0.41g/t, ECu: 1.06% (from 12.00 to 156.00m, 144m interval)
Drill hole NWS-10: Cu 0.89%, Au 0.64g/t, ECu: 1.21% (from 93.00 to 267.00m, 174m interval)
Waisoi West deposit:
Drill hole NWS-6: Cu 0.78%, Au 0.42g/t, ECu: 0.99% (from 243.00 to 440.00m, 197m interval)
Drill hole NWS-7: Cu 0.94%, Au 0.60g/t, ECu: 1.24% (from 273.00 to 339.00m, 66m interval)
Drill hole NWS-8: Cu 0.91%, Au 0.46g/t, ECu: 1.14% (from 33.00-174.00m, 141m interval)
The result of the drilling program, low angle holes with vertical direction of dominant fractures in Waisoi East deposit indicated more than 20% higher grade than other old vertical holes. This suggests possibility of major improvement of grade, which was reported by previous licence holders.
(2) Conceptual model of copper mineralization in the Waisoi deposits are established.
In the Waisoi East Deposit, the centre of mineralization can be characterised by the bornite-chalcopyrite mineral assemblage. The ore body seem to be developed in Wainimala Group, especially around the margin of the quartz porphyry, which is located in the centre of Waisoi East Deposit. From the results of microscopic observation, rock chemistry and fluid inclusion microthermometry, the ore mineralization with bornite-chalcopyrite mineral assemblage and strong potassic alteration probably occurred in the early stage by magmatic fluid with high temperature(>500°C) and high salinity. After this early stage mineralization, chemical and physical condition of ore fluid was changed by circulation along fractures in the quartz porphyry and in its marginal part, and phyllic alteration was formed associated with pyrite-chalcopyrite precipitation in the later stage at lower temperature.
In the Waisoi West Deposit, the centre of mineralization can be characterised by the bornite-chalcopyrite mineral assemblage accompanied by strong silicification and/or quartz vein stockwork and the mineralization is mainly related to the diorite porphyry which is located in the central part of Waisoi West Deposit. The shape of the ore body is seemed to be controlled by ENE-WSW and NW-SE trending fracture system developed around diorite porphyry. Early mineralization with bornite-chalcopyrite assemblage and potassic and phyllic alteration probably occurred in the early stage by magmatic fluid with high temperature(>500°C) and high salinity. It formed strong silicification and quartz vein stockwork in and around diorite porphyry. In the later stage, phyllic and argillic alteration which was formed associated with pyritechalcopyrite precipitation took place.
