The deposits discovered by Waihi Gold to date are considered to be typical of epithermal vein gold – silver deposits. In the opinion of the Qualified Persons (QPs), features that the Waihi deposits display that are typical of epithermal gold deposits include:
• Host lithologies for veins are andesite flows and volcaniclastics.
• Gold-silver mineralisation is hosted in localized bands within multiphase quartz veins.
• Host andesitic volcanics have undergone pervasive hydrothermal alteration, often with complete replacement of primary mineralogy.

The major gold - silver deposits of the Waihi District are classical low sulphidation adularia-sericite epithermal quartz vein systems associated with north to northeast trending faults. Larger veins have characteristically developed in dilational sites in the steepened upper profile of extensional faults with narrower splay veins developed in the hanging wall of major vein structures. Moderate to steeply dipping veins or vein systems are characterised by 200 to 2000m of strike, 170 to 700m vertical range and upwards of 30m individual vein widths; – but more typically 1-5m. The main ore minerals are electrum and silver sulphides with ubiquitous pyrite and variable though usually minor sphalerite, galena and chalcopyrite in a gangue consisting of quartz, locally with calcite, chlorite, rhodochrosite and adularia. Base metal sulphide content generally increases with depth.

The Correnso epithermal vein system is part of the greater Waihi epithermal vein system. It trends northerly and lies between the Martha Hill deposit to the west and the Union/Amaranth/Trio and Favona deposits to the south and southeast. The dominant host lithology is quartz phyric andesite lava, also the main host lithology for the Martha Vein System. The quartz andesite unit attains thicknesses in excess of 400m in the Union Hill – Waihi East area with only minor variation in texture or modal composition. The Correnso system comprises a main lode with smaller splays veins on both the hanging wall and footwall sides. The main mineralized lode is interrupted to the north by a NE trending calcite-quartz (barren) structure. The grade distribution to the north is increasingly complicated by bands / lobes of low grade calcite-quartz which appear to have flooded the system post Au mineralization. The main gold mineralisation lies between 900mRL and 775mRL. Gold-silver mineralisation is dominantly hosted in localized bands within multiphase quartz veins. There is an association of sphalerite, galena and chalcopyrite with gold-silver mineralisation throughout the deposit. The lower part of the deposit is base metal rich with galena (up to +3% Pb) and sphalerite (up to +1% Zn).

Correnso is accessed via the existing Favona Mine and Trio Mine, the portal is close to the Process Plant.

Mining options available for Correnso were limited because of the permit conditions, blasting and backfill constraints and AVOCA mining was selected as the preferred mining method. Correnso has been designed with a 15m level spacing, floor to floor primarily to limit blast vibration but this also assists hanging wall and footwall stability.

Access to the Correnso underground is via a decline from previously mined areas, and also serves as a fresh air intake. Two primary exhaust raises and a single fresh air raise has been raise bored to surface and equipped.

Conventional cross cut accesses are designed for Avoca stoping levels. Ore and waste passes are planned to assist with efficient materials handling. Exhaust ventilation is provided from the 972 Trio development to the existing Union Hill ventilation rise. Mining is located approximately 200metres below residential housing and the mining reserve is approximately 120metres high by 600metres long.

The Permit and mining method requires all stopes and selected development to be backfilled. Mine waste and supplement from Waste Rock Embankment would be used. The open contractor has supplied to Waihi Gold costing to move the material from the Waste Rock Embankment to the portal waste stockpile, estimated at $7.32/tonne.

Ore processing consists of five stages: comminution, leaching/adsorption, elution, electro-winning and smelting.

Underground stockpile ore is reclaimed at 80 tonnes per hour by front end loader and fed onto a static grizzly with an aperture of 150 mm. The final conveyor from the ore handling circuit transports the ore into the grinding circuit, which is enclosed in a steel clad building for sound protection.

Prior to entry into the feed chute of the SAG mill, the ROM ore is further reduced in feed size via a jaw crusher to a P80 of 110 – 130 mm. The SAG mill make up ball size is 125 mm and the mill operates typically with a 10% ball load. The SAG mill draws between 2100-2150 kW of power when operating at 15 rpm or 77% of the critical speed.

The SAG mill discharge is sized using a trommel attached to the SAG. The +12mm oversize material is conveyed to a HP100 cone crusher with a closed side setting of 9mm and is recycled back to the SAG mill. The cone crusher has a power draw of approximately 30 kW. The undersize slurry from the SAG trommel is pumped to two 500 mm diameter inclined Weir Warman Cavex cyclones. The cyclone underflow reports to the SAG mill feed chute. The cyclone overflow gravitates to the ball mill discharge hopper, whereby the slurry is pumped to a cyclone distributor, which consists of fourteen 250 mm diameter Weir Warman Cavex cyclones. The cyclone underflow reports back to the ball mill for further grinding and the cyclone overflow reports to the pre-leach thickener.

The pre-leach thickener increases slurry density from approximately 15% solids to approximately 37 to 40% solids prior to the leach/adsorption circuit, which comprises six leach and six CIP adsorption tanks. The leaching tanks capacity are 700 m3 and the adsorption tanks have 300 m3, providing a total residence leach/adsorption time of 24 hours for Martha ore and 48 hours for Correnso ore. Wedge wire cylindrical inter-stage screens with mechanical wipers are installed in each adsorption tank. The interstage screens retain carbon in the tank but let the slurry pass through to the next stage. A bleed stream is pumped from an adsorption tank to the previous tank in the circuit, the carbon contained in the bleed stream is retained in the previous adsorption tank in the circuit, this provides counter current flow whereby the slurry flows from adsorption tank 1 to 6 while the carbon flows from adsorption tank 6 to 1. This allows for maximum carbon loading in adsorption tank 1 and maximum scavenging of gold solution in adsorption tank 6. From adsorption tank 6 the slurry passes over a carbon safety screen to collect any carbon that may have leaked from the adsorption circuit, the barren tailings slurry is then pumped to the tailings storage facility.

Cyanide is delivered and mixed on site, via a sparging system to a concentration of 21 % wt./vol. The cyanide is dosed into the first leach tank and the concentration is maintained at 280 ppm for Martha and 240 ppm for Correnso. Oxygen is added to the first leach tank by a shear reactor to enhance the leach kinetics and reduce cyanide consumption.

Loaded carbon from the adsorption circuit is fed into an elution column where the carbon is washed at high temperature and pressure to remove the gold and silver from the carbon and into a pregnant eluate. The pregnant eluant is then passed through electrowinning cells where gold and silver is electroplated onto stainless steel cathodes. Following elution the barren carbon is reactivated and recycled to the adsorption tanks.

The cathodes are periodically harvested and rinsed to yield a gold and silver bearing sludge which is dried, mixed with fluxes and put into a furnace at 1200OC. Once the sludge is molten it is poured as bars of doré (alloy of gold and silver) ready for shipment to the Mint.