Summary:
The oldest unit in the Tuvatu Project area is the Nadele Breccia (26-12 Ma), part of the basaltic sequence of the Wainamala Group.
Sabeto Volcanics (5.5 to 4.8 Ma) unconformably overlie the Nadele Breccia and represent the basal unit of the Korroimavua Volcanic Group. The unconformity can be observed in the field and is often accompanied by a distinct change in soil types with the red brown Nadele Breccia contrasting with the grey sandy soils of the Sabeto Volcanics. High ridges and cliffs emphasize this gradation due to the greater resistance of the Sabeto Volcanics to weathering (Vigar, 2000, 2009).
The monzonite intrudes the Nadele Breccia and Sabeto volcanics. The monzonite exhibits considerable local variation in composition, with changes in grain size and inclusion of country rock. The overall intrusive complex is elongated in a northeast orientation. Late small intrusive stocks and dykes, dominantly composed of micromonzonite, commonly referred to as andesite dykes, intrude monzonite and the Sabeto volcanics, and strike dominantly N to NE.
Tuvatu is the largest known of several alkaline hosted gold prospects from the Sabeto area of northwestern Viti Levu.
Locally, the geology is structurally complex with the area cut by at least two 20 to 60m-wide WNW striking cataclasite fault zones referred to as the Core Shed Fault (CSF) and Cabex Fault. These are inferred to be late caldera collapse structures with normal fault displacement. Reverse displacement can also be observed along discrete structures, interpreted as local small-scale adjustment features.
Deposit Types
The Tuvatu Gold deposit is an alkalic gold system (Hennigh 2019; Holden 2019, Kelley et al, 2016). An alkalic epithermal gold system, according to early definitions by Jensen and Barton (2000), is a particular class of epithermal gold deposit wherein the potential scale and grade is considerably larger than a typical ‘hotspring’ epithermal gold system. Hot-spring deposits are bonanza grade systems, due to the narrow vertical extent of boiling horizons, and do not show large vertical or lateral continuity. By contrast, alkalic gold systems mainly form vertically extensive vein and pipe-type deposits. Alkalic gold systems, as a class in the broadest definition of epithermal systems, can show large tonnages and vertical extents beyond 1,000 m (Hedenquist 2000, Kelley et al, 2022). Related definitions of alkalic systems are presented by Jensen &Barton (2000), Kelley & Luddington (2002), and Kelley et al, 2020.
Mineralization
Tuvatu is an alkaline (alkalic) hosted gold deposit. Mineralization is structurally controlled and occurs as sets and networks of narrow veins, cracks, and vein breccias, with individual structures generally ranging from 1 to 200 mm wide. Sets of veins are commonly up to 5 m wide with blow-out zones up to 20m wide forming at the intersection of multiple structures. Schmidt (2023) described an early phase of mineralization as high-temperature porphyry style as indicated by coarse secondary biotite and potassic feldspar. The later, main phase of Au mineralization occurred as episodic ‘flashing’ of mineralizing fluids during uplift and unroofing of the monzonite. This later epithermal mineralization may have been fed by fluids from a younger and deeper intrusion which exploited the same plumbing system provided by faults and pre-conditioned structures (Schmidt, 2023).
Structural controls
The dominant “Upper Ridges” lodes are oriented generally north with a steep east dip and are intersected by east and south striking structures such as the Murau lodes and flat-lying to shallow-dipping narrow sets. Individual veins and zones pinch and swell both along strike and down dip and can go from a zone several meters wide to a narrow veinlet over a short distance.
High-grade ‘blow-out’/stockwork zones are located at the intersection of multiple structures and generally plunge steeply south.
With recent underground development, new mapping and analysis has been carried out. A high-grade stockwork zone on the intersection of URW1 with the Murau Lodes has been exposed on the 1141 Level. This mineralized stockwork zone has a steep southerly plunge.
Detailed structural work by company geologists indicates that although there is a dominant north to NNE, steep east-dipping series of structures (UR main and splays), high grade gold is also found in east-striking structures dipping south (Murau lodes), and shallow south- and north-dipping micro-fractures associated with these zones (SKL lodes). The HT Zone strikes NW and contains late gold emplaced within earlier, potassic altered (base-metal) mineralization.
Mineralogical Associations
Gold mineralization occurs in narrow quartz-carbonate-sulphide veins with local association with roscoelite. Individual veins and zones can exhibit distinct variations in mineralogy and particularly in sulphide content.
Vein selvages are generally bleached and although the silica-argillic-sulphide wallrock alteration generally extends only a few centimeters from single veins, it can be pervasive throughout breccia and stockwork zones (Schmidt, 2023; Jefferson, 2023).
Schmidt (2023) noted three epithermal alkaline vein events:
• Epithermal I: veins consisting of pyrite, chalcopyrite, galena, and sphalerite (base-metal veins) and potassium metasomatism (gold content likely an overprint of Epithermal II).
• Epithermal II: veins crossing and reactivating earlier base-metal veins with a dominant quartzsericite-pyrite and associations with roscoelite (vanadium-bearing mica). This phase also contains arsenian marcasite and arsenian pyrite.
• Epithermal III: albite and chlorite association with late calcite and other carbonate phases (not associated, generally, with gold mineralization).
Whilst gold is generally associated with sulphides, it occurs dominantly as native gold within the veins and coating the outside of sulphides. Some gold associated with arsenian pyrite can be bound as nano-particles within the sulphide whereas Clarke (2022) noted also the gold telluride calaverite, as well as other Te-bearing phases in minor quantities.
Dimensions and Continuity
Overall, the main mineral system, made up of multiple individual zones, is approximately 800m N-S by 400m E-W, and with the West Zone, extends a further 500m to the west. Mineralized zones extend from surface to a depth more than 1,000 meters.
Individual veins/zones have continuity over several hundred meters. However, the ‘pinch and swell’ of veins suggests that they may not be economic over their entire length. The stockwork zones such as URW1 observed on the 1141 level show an east-west width 20 to 25m and are elongate north-south for up to 40m. The URW1 stockwork zone is currently understood to extend steeply down plunge to the south for over 100m.