The Emperor mine, Vatukoula, Viti Levu, Fiji.
This article was inspired by a specimen the senior authors acquired at the 2007 Tucson Gem and Mineral Show; it was labeled "Sylvanite, Vatukoula, Viti Levu, Fiji." Dave Bunk observed the crystal's apparent trigonal symmetry and suggested that it might instead be a tellurium crystal. This alternative identification was consistent with tellurium's point group 32, as opposed to sylvanite's 2/m. At his suggestion we took the specimen to Gary L. Zito, Laboratory Manager of the Electron Microscopy Laboratory of the Metallurgical and Materials Engineering Department at the Colorado School of Mines, and he confirmed the identity as native tellurium.
The Republic of Fiji, in the southwestern Pacific Ocean, is a volcanic archipelago consisting of 322 islands, the economy of which is dominated by the sugar cane industry.
The Emperor (or Vatukoula) mine is located at the northern tip of the main island of Viti Levu, in the Nakauvadra Mountains about 100 km northwest of Suva and 8 km inland from the coast. It has been historically the most productive of Fiji's precious metal mines, and Fiji's orebodies have the distinction of being the World's farthest from any continental land mass. Placer gold was discovered there in 1868, and commercial mining commenced in 1932 on Viti Levu's sister island, Vanua Levu. The Mt. Kasi mine there operated until 1946, producing 1800 kg of gold; it was reopened in 1996.
In 1933, a year after the Mt. Kasi discovery, the epithermal veins next to the Tavua collapsed caldera were brought into production. These mines, of which the Emperor is by far the most important, produced ten million ounces of gold from 1935 to 2005 (Rogers, 2005), and they continue in operation today. Since 1993 the operations there have shifted from the lower grade open-pit ore to higher grade ores exploited by underground workings (Mining Journal, 1998). The Emperor mine currently produces about 120,000 ounces of gold per year (Rogers, 2005). The Emperor Mines, Ltd. (and its predecessors) built the town of Vatukoula (meaning "Rock of Gold" in the local language) to accommodate the many mine employees who were brought in; the town has about 5,000 residents.
Compared to the continents, Fiji is very young, dating only from the Eocene (see Denholm, 1967; Ibbotson, 1962). The primary rocks are either extrusive, or intrusive bodies related to the extrusions. Sedimentary cover is limited, and consists largely of early reef and platform limestones. Between about 35 and 12 million years ago, in the middle Miocene, the Pacific plate was subducted under the Tonga Ridge, producing an extensive volcanic suite. In the middle Miocene subduction was reversed; during this time, low-potassium tholeiitic gabbros and tonalites were emplaced. Intense folding and faulting, produced by rifting, began in the late Miocene; this activity led to intense volcanism, and also emplaced a number of plutonic bodies.
The Emperor mine straddles the western border of the Tavua collapsed caldera, which forms a basin about 4.5 X 5 km in area. The parent volcano underwent five intrusive stages, at least two of which were followed by collapses; the volcanic activity spanned the Pliocene, and was followed by shearing and gold mineralization, so the mineralization is quite young.
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The classic epithermal ore deposits occur in steep shear zones in shattered country rock, and in nearly horizontal structures known locally as "flatmakes." The ore occurs in crustified quartz veins and pods.
Mindat.org lists 48 minerals from the Tavua Gold Field. These include 15 minerals containing gold and/or silver and 14 tellurides, among which are the lead telluride altaite, the mercury telluride coloradoite, and the nickel telluride melonite. Other unusual minerals of note in those orebodies are the relatively rare vanadium oxides karelianite, nolanite, and schreyerite, as well as the vanadium mica roscoelite.
The volcanic origin of the Tavua orebodies is analogous to that of the Cripple Creek district but, curiously, Tavua's list of 48 minerals does not include fluorite, which is pervasive at Cripple Creek.
Lawrence et al. (2001) described the various minerals found at the Emperor mine (most of which are not of collector quality, though some have micromount potential); their work is the principal source for the data that follows:
Arsenopyrite was found mainly in the Dolphin East lode, as tiny prismatic crystals on pyrite. The crystals contain sub-microscopic amounts of gold.
Barite is found in clusters of tiny white grains and in colorless, rhomb-shaped euhedral crystals commonly found attached to the termination faces of quartz crystals.
Calcite occurs at the Emperor mine in much smaller amounts than quartz, and is only rarely observed as colorless, poorly formed crystals of the "nail-head" habit. Ore minerals are never present as inclusions, indicating that calcite crystallized after the ore-forming phase.
Late-stage chalcopyrite in sphenoidal microcrystals and reniform blebs lining cavities in telluride ore has been found on the 12th level of the Emperor mine.
Coloradoite has been identified in polished sections from the Cardigan West shaft, in grains to 1 mm along quartz boundaries and as inclusions in krennerite (?) and calaverite (?).
The rare silver telluride empressite has been reported as a minor component of Emperor mine ore.
Although gold is present in smaller amounts than the tellurides, some surprisingly attractive specimens have been recovered. Small, elongated crystals of gold to more than 1 cm occur on drusy quartz at the Emperor mine, and are found intergrown with sylvanite and pyrite in the Loloma mine (adjacent to the Emperor mine). The crystals are square in cross-section, and finely striated by alternating octahedron faces perpendicular to the axis of elongation. Equant crystals to about 1 mm are also known, in some cases covering several square centimeters of vein surface. Thin filaments (in sub-parallel nests like a bristle brush) and thin wires twisted and intertwined with quartz have been found; these wires can be up to 3 cm long. Earthy, brownish yellow "mustard gold" occurs lining near-surface joints as a result of the oxidation of other gold-containing minerals.
Hessite crystallizes in cubic symmetry above 149[degrees] C. but is monoclinic below that temperature. Emperor mine hessite formed above the inversion temperature. It was found more commonly in the 1940's, associated with sylvanite and gold. The rare macroscopic crystals are small and of a dark lead-gray color, usually surrounding sylvanite crystals.
Krennerite is known from the Emperor mine as a 3 X 4-cm specimen that used to be on display in the mine office of the former Mine Manager, P. Schmidt. The specimen consists of cleaved, silvery masses showing patches of violet iridescence.
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Rock Currier visited the Emperor mine some years ago, and reports that, "They had a watch glass full of krennerite (?) crystals up to about an inch long. Thin, striated, silvery crystals, slightly rough but apparently terminated."
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Melonite has been found in polished-section studies of massive ore, in rare particles up to 20 microns across associated with chalcopyrite, sylvanite and hessite, and as inclusions in sylvanite and tellurium.
Nagyagite from the Emperor mine has been known since the 1944 edition of Dana's System of Mineralogy, but has not been reported since then.
Diamond-shaped low-temperature crystals of orthoclase have been reported in mineralized vugs, but require confirmation.
Pyrite and Marcasite Fe[S.sub.2]
Pyrite, the most abundant of the Emperor mine metallic minerals, occurs as minute grains and disseminations, as microcrystals in vein like aggregates, and as small crystals and clusters in the interstices between bladed sylvanite crystals. The cube and pyritohedron are the dominant forms. Late-stage marcasite blades have also been found associated with pyrite.
Some of the mineralized vein quartz (carrying gold, stibnite or hessite) in the Emperor mine is colored dark gray to black by inclusions of pyrolusite.
A few small grains of pyrrhotite have been seen in polished sections of telluride ore.
Quartz typically encrusts the walls of fractures, fissures and vugs at the Emperor mine, generally as drusy layers of transparent and colorless to white crystals up to around 4 mm.In some areas it is tinted greenish, presumably by chloritic inclusions.
Roscoelite [KV.sub.2]Al[Si.sub.3][O.sub.10] [(OH).sub.2)
Roscoelite, a rare vanadium-mica, is found as tiny flakes up to 0.5 mm in size, in wallrock close to quartz veins. Interestingly, roscoelite also occurs in association with telluride minerals at Cripple Creek, Colorado.
Siderite occurs in the Emperor mine as the bright brownish red, tellurium-rich variety coating fracture surfaces in basalt near sylvanite-hessite veins, and as tiny salmon-pink globules with tellurides in quartz cavities.
Stibnite has been found occasionally; the best crystals (stellate groups of radiating prisms to 6 mm) have come from the 6th level of the Emperor mine.
Sylvanite is the principal telluride mineral at the Emperor mine, and occurs in a range of associations, intergrown on a fine scale with pyrite, tetrahedrite, arsenopyrite, hessite, native tellurium and occasionally gold. Habits include (1) elongated, striated prisms on drusy quartz, (2) lustrous, well-formed prisms stacked in parallel orientation in and on blackish quartz, and (3) as prismatic crystals to 2 cm in vugs lined with drusy quartz and ultra-fine-grained masses of tellurides and sulfides that sometimes encase sylvanite crystals. Numerous specimens of this type were collected in the 1970s.
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Tellurium in bright, silvery crystals is common in small vugs lined with tiny quartz crystals. As a component of massive ore it occurs intimately mixed with sulfides, tellurides and gold. Single crystals and crystal clusters of tellurium rank with native gold as some of the finest specimens from the Emperor mine.
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Slender, terminated, trigonal prisms to 4 cm have been reported in quartz cavities. The crystals are usually striated, occasional bent, and may show a colorful iridescent luster. It is only rarely associated with other metallic minerals.
Our specimen was analysed using an electron microscope (a Quanta 600 FEI), and the EDX analyzer (a PGT-Prism 2000). Imaging was performed in the backscatter mode at 20Kv. The EDX spectra was collected at 20Kv and 40mA. This remarkable system can handle macro targets (our specimen is 6 X 6 cm) with a resolution down to 100 nanometers, and demands no special preparation (e.g. carbon coating).
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Tetrahedrite in sharp, lustrous, modified tetrahedral crystals to 8 mm has been found rarely on drusy quartz vein linings. The crystals often have the points of smaller crystals protruding from their faces, suggestive of penetration twinning.
Polished section studies have also noted the presence of galena, sphalerite, bornite, covellite and chalcocite. The Crown Crescent lode in the Emperor mine has yielded very minor amounts of bournonite, pyrargyrite, proustite and polybasite. Calaverite was tentatively reported but has not been confirmed. Dolomite and ankerite have also been reported.
Oxidation products of the primary minerals (usually as thin veneers) have also been reported from the upper levels of the mine. These include apple-green emmonsite, and a vanadium-rich variety of the rare scandium phosphate kolbeckite in tiny lime-green crystals associated with sylvanite.
Our thanks to Dave Bunk and Gary Zito for positive identification of the tellurium specimen. We thank Ed Raines for providing most of the geologic literature. And we thank Jeff Scovil for his photography.
DENHOLM, L. S. (1967) Lode structure and ore shoots at Vatukoulo, Fiji. Proceedings of the Australian Institute of Mining and Metallurgy.
IBBOTSON, P. (1962) Petrology of the Tertiary caldera, Tavua Gold Field. Geological Survey of Fiji Mem no.3.
LAWRENCE, L. J., SHARPE, J. L., and WILLIAMS, R. A. (2001) Minerals of the Vatukoula gold mines, Fiji. Australian Journal of Mineralogy, 7, 63-72.
LYDAY, T. (1997) The Mineral Industry of Fiji. USGS Open Files.
MINING JOURNAL (1998) Fiji--A leading light in Pacific mining. Mining Journal Country Supplement, 330 (8466), February 6,12 p.
ROGERS, G. (2005) Alcaston Mining NL, Activities Report for the quarter ended 31 March 2005. 22 April 2005.
Bill Smith and Carol Smith
1721 Daphne Street
Broomfield, Colorado 80020
Wendell E Wilson
4631 Paseo Tubutama
Tucson, Arizona 85750
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|Author:||Smith, Bill; Smith, Carol; Wilson, Wendell E.|
|Publication:||The Mineralogical Record|
|Date:||Jul 1, 2008|
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