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Tuvatu |
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Fiji |
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Au Ag
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Super Porphyry Cu and Au
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The Tuvatu low sulphidation gold-silver telluride deposit is the second largest in Fji with reserves of 14.9 t Au. It is located in the upper reaches of the Sabeto Valley, ~24 km NE of Nadi on the west coast of Viti Levu island (#Location: 17° 42' 49"S, 177° 34' 39"E) and is 50 km SW of the largest gold deposit in the country, Emperor (production + reserves of 280 t Au) .
Both deposits occur along the >250 km ENE-trending Viti Levu lineament. They are spatially associated with alkaline rocks of almost identical age (~5.4 to 4.6 Ma) and having a shoshonitic affinity. The gold mineralisation in both deposits is spatially and genetically related to monzonite intrusions and to a low-grade porphyry copper-style system. The Tuvatu deposit is interpretted to occur adjacent to an eroded shoshonite volcano.
The upper Eocene to lower Oligocene (35 to 40 Ma) Yavanu Group are the oldest rocks in Fiji, composed of extrusive and intrusive tholeiitic rocks, a trondjhemitic and a tonalite stock, and minor volcaniclastic rocks and limestone. From the middle to late Miocene (7 to 12.5 Ma), large-scale faulting and folding of the upper Oligocene to middle Miocene (13 to 32 Ma) Wainimala Group accompanied the the Colo orogeny, accompanied by the emplacement of the plutonic suite of gabbro and minor tonalite intrusions. At the end of the Colo orogeny, Viti Levu a period of widespread and voluminous volcanism from 6.5 to 2.5 Ma ensued on Viti Levu, while magmatic compositions changing from island-arc tholeiite and calc-alkaline andesite to shoshonite at ~5.5 Ma. During the early Pliocene, the prominent shoshonite and high K calc-alkaline Tavua and Rakiraki volcanoes were erupted. The Koroimavua Group, which occurs to the northeast of Nadi, consists of basal shoshonitic volcaniclastic sandstone and mudstone overlain by the Sabeto Volcanics, which include shoshonitic lava, breccia, rudite, and sandstone.
The oldest geologic unit in the Tuvatu area is the 12 to 26 Ma Nadele Breccia, which belongs to the Wainimala Group and constitutes approximately 60% of the 1 sq. km deposit area. It consists of andesitic to basaltic reworked, polymict volcanic breccias, pillow lavas and sedimentary rocks. The polymict volcanic breccias appear to be the dominant unit of the Nadele Breccia. Thin layers of volcanic sandstone and siltstone are commonly interbedded with the breccias. The Wainimala Group is unconformably overlain by members of the 5.4 ± 0.1 Ma Sabeto Volcanics, the basal unit of the Koroimavua Volcanic Group, which comprises augite-biotite flows and breccia with a basal sequence of andesitic and dacitic lithic and crystal tuffs, grits and agglomerates with minor flows. The Nadele Breccia was intruded by the 4.9±0.1 Ma multi-phase Navilawa Monzonite, which has been interpreted to be the intrusive equivalent of the Sabeto Volcanics, and includes an early fine-grained micromonzonite, a medium-grained monzonite, basaltic-andesite dykes and late-stage pegmatitic dykes. The medium-grained monzonite is surrounded by an envelope of micromonzonite and contains inclusions of micromonzonite, which together cover an elliptical, NE-SW elongated, 3 x 2 km area. Porphyry copper-style mineralisation occurs in the core of the Navilawa Monzonite near the transition zone between micromonzonite and monzonite, 1.8 km north of the Tuvatu gold deposit. Faults in the Tuvatu area strike north to northeast and northeast to southeast with near-vertical dips, although in places they dip shallowly to the west.
Porphyry-style copper mineralisation occurs in the northern part of the deposit in the NW-SE trending H and Tuvatu lodes, which are 5 to 40 m wide and dip moderately to the northeast. The Tuvatu lode is characterised by potassic alteration of the wall rock consisting of coarse grained apatite, orthoclase, magnetite, pyrite, chalcopyrite and optically and compositionally zoned biotite. In the H lode, epithermal gold veins locally cross-cut porphyry copper-style mineralisation. In contrast to the Tuvatu lode, where magnetite is very common, magnetite only occurs in trace quantities in the H lode. Other differences between the Tuvatu and H lodes include a marked decrease in grain size of the ore minerals in the H lode and the presence of trace amounts of native gold, calaverite, petzite and tennantite. However, while these two lodes are overprinted by gold-bearing epithermal style veins and associated propylitic and phyllic alteration, most gold occurs in epithermal style veins unrelated to porphyry-style mineralisation in the Navilawa Monzonite, basaltic-andesite dykes and, to a lesser extent, the Nadele Breccia.
At Tuvatu low-sulphidation, epithermal gold telluride mineralisation occurs in flat-lying veins, steep faults, shatter zones, stock-works, and hydrothermal breccias which have been grouped into three lode types: i). 'steep-dipping veins' striking NE (e.g., the Nasivi and Upper Ridges lodes), ii). shallowly dipping veins (<45°) or flatmakes representing reactivated oblique thrust faults (e.g., the Murau lode), and iii). irregular brecciated bodies or shatter zones (e.g., the SKL lode) that occur at the intersection of the other two lode types. The mineralisation is characterised by the presence of quartz-roscoelite telluride veins in which gold-rich tellurides were deposited prior to silver-rich tellurides. Gold tellurides and vanadium minerals were deposited at approximately 250°C from moderately saline fluids. Oxygen and hydrogen isotope compositions of ore fluids are similar to the composition of waters exsolved from arc magmas.
Each lode may consist of up to nine individual flatmakes or vertical veins that are generally no more than 1 m wide. In most places, alteration zones are narrow and do not extend for more than 1 m into the wall rocks. Individual veins contain between 2 and 100 g/t Au, with the Upper Ridges veins containing the highest grades (5 to 100 g/t Au). Most veins are silicified and include base metal sulphides (pyrite, chalcopyrite, sphalerite, and galena), native gold and/or electrum, tellurides, and gangue minerals (quartz, chalcedony, sericite, adularia, and roscoelite). Quartz (medium-grained cockade and fine-grained banded) in the veins is intergrown with or replaced by chalcedony, while adularia and white mica occur within the medium- to fine-grained quartz.
Published Mineral Resources are (Ewart et al., of P&E Mining Consultants Inc, 2010):
Oxides
Indicated resource - 0.76 Mt @ 7.05 g/t Au;
Inferred resource - 2.502 Mt @ 5.78 g/t Au.
Sulphides
Inferred resource - 0.116 Mt @ 4.15 g/t Au.
TOTAL
Indicated resource - 0.76 Mt @ 7.05 g/t Au;
Inferred resource - 2.618 Mt @ 5.71 g/t Au
The information in this summary has been drawn from the reference(s) listed below. See also Ewart et al., of P&E Mining Consultants Inc, 2010, for X-Tal Minerals Corp. - Technical Report and Resource Estimate on the Tuvatu Gold Property Viti Levu, Fiji, an NI-43-101 Technical Report, available from http://www.sedar.com.
The most recent source geological information used to prepare this decription was dated: 2006.
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.
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Selected References:
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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
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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