Vatukoula, Emperor


Main commodities: Au Ag
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The Emperor gold deposit at Vatukoua is 8 km south of the north coast on the island of Viti Levu in Fiji.   Fiji lies on the circum-Pacific Tertiary volcanic Belt.

The Vatukoula epithermal gold deposit is located on the southwestern margin of the 7 x 5.5 km Tuvua Caldera, which is in turn developed in the central section of the large Tuvua Volcano that dominates the north coast of Viti Levu (Eaton and Setterfield, 1993). It is classed as a quartz-sulphide gold±copper low sulphidation deposit of the type formed at deeper crustal levels close to porphyry intrusions (Corbett, 2002). Major subsidence has occurred along the boundary of the Tuvua Caldera, which is taken at the contact between intermediate caldera fill rocks and the pre-caldera rocks, which are typically mafic absarokites, although more has taken place outward, but between similar absarokites. The subsidence surface dips inwards at an overall >60° angle, made of a series of steep normal faults parallel to the margin of the caldera, separated by flat stratigraphic contacts (at the top of the pre-caldera absarokites). The extrusive rocks at the base of the Tuvua Volcano were laid down at 5.2 Ma, while the collapse occurred at between 4.6 and 4.5 Ma (Eaton and Setterfield, 1993).

The Tuvua Caldera is localised at the intersection of the regional NE-SW Viti Levu Lineament and a generally NW-SE lineament that sweeps across the island, also passing through Namosi, and coincides with the Nasivi Shear. This latter shear zone comprises an en echelon array of discontinuous faults and shears with a significant dip-slip component and some strike-slip displacement. It is a long lived structure that predated the caldera fill and appears to have played an active role in the inner caldera subsidence as well as influencing late intrusive activity. Faults parallel to the Viti Levu Lineament are rare, although the northwest margin of the main caldera is take to represent one such structure. Faults and fractures on the western margin of the caldera, near the intersection of the two lineaments, are generally north to northwest striking normal faults, with steep dips towards the caldera, and mineralised flat faults dipping into the caldera at from 20 to 45°. This pattern is consistent with the Viti Levu Lineament being a major extensional fault (Eaton and Setterfield, 1993).

The sequence deposited within the caldera following collapse is as follows: i). heterolithic mass flow deposits generated by the caldera collapse are known as the Caldera Contact Breccia; ii). Turtle Pool Formation, composed of interlayered lacustrine sediments and intermediate shoshonite (porphyritic plagioclase-pyroxene lavas) with intercalated, hydrovolcanically derived tuffs and lapilli tuffs in the upper sections; iii). A second caldera subsequently formed in the south-west section of the larger caldera and this was filled with the Natolevu Breccia (similar to the Caldera Contact Breccia); iv). The Morrison’s Pool Formation, similar to the Turtle Pool Formation, filled the caldera above this new breccia, although the associated volcanics are of a porphyritic plagioclase-biotite-pyroxene banakite (a more evolved shoshonite) and comprise interlayered lava flows and subaerial ignimbrites; v). The Fused Agglomerate is a 1.2 km diameter vertical cylindrical mass that cuts the Morrisons Pool Formation and has been dated at 4.24 Ma, the youngest event in the Caldera; vi). Several monzonitic intrusions are exposed within and outside of the caldera and have been intersected at depth within the Nasivi shear zone. The multiphase Matkenaka intrusion in the south cuts the Turtle Pool Formation and has been dated at 4.5 to 4.45 Ma, roughly contemporaneously with un-exposed monzonite in the centre of the caldera, which pre-date the Fused Agglomerate. The intrusive mass to the northeast of the caldera is intensely altered to intermediate to neutral argillic assemblages, but is cut by fresh absarokite dykes indicating it is at least in part pre-caldera. Elsewhere in the complex there are a range of varying relationships suggesting continued multipulse intrusive and hydrothermal activity (Eaton and Setterfield, 1993).

The Vatukoula Gold deposit is located at the intersection of the Viti Levu Lineament and the Shatter shear zone, (which is believed to be part of the broader Nasivi shear zone), and is almost entirely to the south of the Shatter shear zone. All of the significant gold mineralisation associated with the Viti Levu Lineament is found along the north-western margin of the Caldera and ranges from the deeper level Vatukoula in the south to high level epithermal mineralisation of the Northern Prospects towards the north. The Nasivi shear zone hosts extensive alteration zones and a series of mineral occurrences, ranging from the small, sub-economic Nasivi 3 porphyry copper occurrence to epithermal veining further to the west, closer to the Viti Levu Lineament. The porphyry style Korovou Hill and Matekenaka prospects are in the larger alteration zone in the south-central part of the caldera (Eaton and Setterfield, 1993).

At Vatukoula, the mineralisation occurs as narrow, very high grade, lodes carrying 4 to 60 g/t and locally up to 2.7% Au as gold-silver tellurides. The lodes are generally less than 1 m in width and are either in steep faults or ‘flatmakes’ (<45° dip faults/shears), or as bulk mineable ‘shatter zones’ which occur as structurally complex zones of brecciation at intersections between flatmakes and steep shears. Lode structures are usually continuous, although grades are not. Some flatmakes can be traced for up to 2 km along strike and 1 km down dip. The lodes are single to multiple fractures, infilled with quartz, lesser calcite, dolomite, ankerite and adularia, typically as multistage veining. At higher levels in the mine, minor amounts of chalcedonic and opaline silica appear. The gold bearing minerals are concentrated on fracture margins, as coarse tellurides within the veins and associated with pyrite and roscoelite in the wall rock alteration (Eaton and Setterfield, 1993).

Lodes are typically surrounded by broad +8 ppb anomalous zones for up to 100 outwards (compared to -4 ppb backgrounds) and Hg anomalies. Two types of alteration are evident, namely: i). a narrow halo of epithermal alteration, symmetrical and parallel to the vein margins, varying from a few mm to 2 m in width, characterised by ubiquitous quartz, with carbonate, adularia, pyrite, sericite-illite, chlorite, telluride, smectite, illite-smectite and roscoelite; ii). Low temperature propylitic alteration, which is widely, but irregularly distributed, unrelated to individual lodes and composed of chlorite, carbonate and quartz, with associated sericite-illite, pyrite, smectite and illite-smectite (Eaton and Setterfield, 1993).

Dating of the flatmakes alteration has yielded reliable dates of 3.71±0.13 Ma, suggesting it is the youngest event in the system (Eaton and Setterfield, 1993). Approximately 4 km to the north, on the northwestern margin of the caldera, higher level epithermal mineralisation has been tested at the Northern Prospects. Mineralisation occurs on steep caldera bounding faults, some radial steep fractures, hangingwall structures in the Turtle Pool Formation, siliceous flatmakes and interpreted intrusive breccias. Vein filling is generally chalcedonic to opaline quartz. Hydrological studies indicate this veining is connected to the Vatukoula system to the south and that it is a higher level of the same hydrothermal/ epithermal system. Various observations confirm that this mineralisation is generally coeval with that at Vatukoula (Eaton and Setterfield, 1993).

The Nasivi 3 porphyry copper-gold occurrence is hosted by a fault zone of the Nasivi shear zone and is capped by a prominent silica-alunite cliff near the centre of the Caldera. This silica-alunite is taken to be an advanced argillic lithocap. The mineralisation is associated with a steeply plunging, upward narrowing, faulted monzonite stock cutting a shallowly dipping succession of intercalated caldera fill deposits of the Morrisson’s Pool Formation, Natolevu Breccia and Turtle Pool Formation. This stock and the associated mineralisation has been dated at 4.57±0.13 Ma and as such is older than both the Fused Agglomerate and the subsequent Vatukoula gold phase (Eaton and Setterfield, 1993).

Mineralisation at Nasivi 3 is found above and within the stock, occurring in all lithologies, although strongest in the monzonite and in faults directly above the intrusive within the Natolevu Breccia. The mineralised faults are strongly anomalous in gold at the surface, with grades of as much as 2.2 g/t Au, which decrease with depth (Eaton and Setterfield, 1993).

The core of the alteration is a zone of what is described as potassic, characterised by alkali feldspars that vary from sodic orthoclase to pure albite, accompanied by biotite and magnetite which comes to within 20 m of the current surface. This zone is accompanied by disseminated chalcopyrite and either pyrite, magnetite or hematite, and has a peak grade of 1.3% Cu (Eaton and Setterfield, 1993). The albite/potassic zone has been overprinted in the centre of the deposit and to depth by a lower temperature, anhydrite rich retrograde zone that is unrelated to the advanced argillic cap. It is characterised by anhydrite veins up 1 m thick with minor quartz and alkali feldspar within a zone flooded by anhydrite (up to 60%), biotite and alkali feldspar. Copper in this zone occurs as bornite, tetrahedrite and chalcocite as fracture fillings within the anhydrite veins or on the margins of solution cavities within the altered wall rocks, as well as coatings on fine pyrite and chalcopyrite. A maximum 2.8% Cu has been encountered in these veins. Gold tellurides and stannite-group minerals are associated with the bornite and tetrahedrite, and is proportional to copper grades. This assemblage is taken to infer an association with the epithermal telluride rich Vatukoula deposit (Eaton and Setterfield, 1993).

The albitic/potassic primary core is overprinted by a peripheral phyllic alteration zone (quartz-sericite-illite with kaolinite, chlorite, smectite), particularly along fault zones. Within strongly siliceous zones, an assemblage of bornite, tetrahedrite and chalcocite is observed, generally on vug margins. There is an outer zone of neutral argillic alteration (quartz, smectite, illite-smectite with carbonate, pyrite, chlorite, sericite-illite and kaolinite). At the surface, the individual faults are intensely altered to an erosion resistant advanced argillic alteration assemblage (quartz-alunite with sericite-illite, kaolinite, iron oxides, pyrite, illite-smectite, pyrophyllite and diaspore). These fault controlled cappings are flanked by kaolinite rich intermediate argillic alteration (kaolinite-quartz with smectite, smectite-illite, sericite-illite, pyrite, chlorite, carbonate and alunite). All of the above are flanked by a broader phyllic zone. The presence of typically high temperature pyrophyllite and diaspore and local persistence of alunite to depths of 400 m imply the alunite is hypogene. These same faults are those described with high gold grades at surface and appear to represent an acid-sulphate eithermal setting (Eaton and Setterfield, 1993).

In conjunction with the overprinting retrograde anhydrite core, this occurrence is taken to represent a collapsing porphyry system (Eaton and Setterfield, 1993).

JORC compliant reserves and resources at Vatukoula are reported by Vatukoula Gold Mines plc (2008 ?) as follows:
Proved + probable reserves - 2.26 Mt @ 11.41 g/t Au;
Measured + indicated + inferred resources - 11.66 Mt @ 16.99 g/t Au.

Historic production is quoted at near 220 tonnes of gold and over 60 tonnes of silver from 22.5 Mt of ore representing a recovered grade of around 9.65 g/t Au, 2.75 g/t Ag.

The most recent source geological information used to prepare this summary was dated: 2006.     Record last updated: 18/8/2017
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Ahmad M, Solomon M, Walshe J L  1987 - Mineralogical and geochemical studies of the Emperor Gold Telluride deposit, Fiji: in    Econ. Geol.   v82 pp 345-370
Eaton P C, Setterfield T N  1993 - The relationship between epithermal and porphyry hydrothermal systems within the Tavua Caldera, Fiji: in    Econ. Geol.   v88 pp 1053-1083
Muller D and Groves D I  1993 - Direct and indirect associations between potassic igneous rocks, shoshonites and gold-copper deposits : in    Ore Geology Reviews   v8 pp 383-406
Pals D W, Spry P G, Chryssoulis  2003 - Invisible Gold and Tellurium in Arsenic-rich Pyrite from the Emperor Gold deposit, Fiji: implications for Gold distribution and deposition: in    Econ. Geol.   v98 pp 479-493
Richards, J.P.,  2009 - Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere: in    Geology   v.37, pp. 247-250.
Scherbarth N L and Spry P G,  2006 - Mineralogical, Petrological, Stable Isotope, and Fluid Inclusion Characteristics of the Tuvatu Gold-Silver Telluride Deposit, Fiji: Comparisons with the Emperor Deposit : in    Econ. Geol.   v101 pp 135-158
Setterfield T N, Mussett A E, Oglethorpe R D J  1992 - Magmatism and associated hydrothermal activity during the evolution of the Tavua Caldera: 40Ar-39Ar dating of the volcanic, intrusive, and hydrothermal events: in    Econ. Geol.   v87 pp 1130-1140

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