Summary:
Tuvatu is a high grade alkaline gold deposit forming a small part of the 7 km diameter Navilawa Caldera on a corridor of high grade alkaline gold systems in Fiji.
The Project lies within the Fiji Gold Trend, which is a northeast trending extensional fault zone across northern Viti Levu and Vanua Levu (the two main Fijian islands). Virtually all significant gold occurrences in Fiji occur along the Fiji Gold Trend in clusters surrounding igneous craters such as the Tavua Caldera hosting the Vatukoula Gold Mine and the Navilawa Caldera hosting Tuvatu.
The main characteristics of these gold deposits/occurrences are:
1. The gold is igneous hydrothermal (250°C) in origin, introduced by the potassic, shoshonitic tertiary intrusives, which represent the feeders and latest phases of the volcanic rocks that dominate northern Fiji. Hydrous mineral phases (biotite and hornblende, particularly) in the intrusives develop whenever the hydrothermal mineralization systems have developed.
2. Gold is generally vein-controlled and can be restricted to narrow bonanza-grade lodes within weakly altered host rocks. Primary gold is fine-grained and can be in the following forms:
- Gold-silver tellurides
- Electrum
- Native gold
- Gold-bearing pyrite
3. Common minerals associated with mineralization are:
- Quartz
- Carbonates
- Adularia - K-feldspars
- Pyrite
- Roscoelite (green vanadium-titanium micas)
- Smectite clays
4. Magnetite in the adjacent host rock has been converted to pyrite by the bisulphide complex, which carries gold in the alkaline-rich fluids, but alteration selvages are commonly very narrow (less than 0.5 m).
The main mineralized zone (Upper Ridges) comprises eleven principal lodes with a strike length in excess of 500 m and a vertical extent of more than 300 m.
Another major zone of mineralization (Murau) strikes east–west and consists of two major lodes with a mapped strike length in excess of 400 m. A total of 47 different lode structures were identified in the resource area including 11 lodes in the Upper Ridges area, 7 lodes in the Murau area, 7 lodes in the West area, 7 lodes associated with Snake and Nasivi lodes, 4 lodes in the Tuvatu area, and 9 stockwork veins in the SKL area.
Tuvatu is an alkaline hosted gold deposit. Mineralization is structurally controlled and occurs as sets and networks of narrow veins and cracks, with individual veins generally ranging from 1 to 200 mm wide. Zones of veining, which comprise the lodes, may be up to 5 m wide.
Although gold mineralization is primarily hosted in monzonite, it can also occur in the volcanic units. Veins are narrow, generally less than 1 m up to a maximum of 7 m, and can contain significant gold grades. Lode mineralogy is varied, with most veins containing quartz, pyrite, and base metal sulphides.
A very high proportion of the gold occurs as either free gold or is contained in quartz or pyrite composite particles that can be floated. Free gold present is both fine and coarse grained. Mineralization is clean with respect to deleterious elements such as arsenic, selenium, and uranium.
In addition, a number of other lodes have been identified in the local area but remain untested The grades ofindividual lodes vary considerably due to the “spotty” nature of the gold and the variability in width of the host structures. Average grades for the lodes range from 2.0 to 10.0 g/t. Gold mineralization tends to be quite coarse, and visible gold can be observed in mineralized sections of core.
Tuvatu is an alkaline gold system related to the intrusion and subsequent cooling of a local monzonite. Stress regimes within intrusion systems can be quite complex. The resulting veins and stockwork zones will pinch and swell along various strike orientations. This style of emplacement will always result in a risk to the tonnes and grades of any model developed. The mineralization is typical of this style of deposit in being confined to narrow structures with little wall rock alteration, which are hence “blind” outside of the mineralization.
Mineralization is generally hosted in a series of sub-vertical, north and north-northeast striking trending veins as well as shallow, south-dipping veins (locally referred to as “flatmakes”). In spite of the narrow widths of individual veins, the gross lode structures appear to be continuous for over 100 m. The majority of lodes vary in width from 0.5 to 5.0 m with an average width of 1.1 m (individual vein intercepts have been recorded as low as 4 cm).
The Tuvatu and H Lodes are up to 5 m wide and are characterized by intrusive related gold mineralization with potassic alteration.