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 quartz vein system at Martha is hosted by hydrothermally altered quartz bearing andesite flows and flow breccias inter-bedded with thin tuffaceous sediments, dipping South-East at about 40 degrees. These are unconformably overlain by a post-mineral sequence of late Pliocene to Quaternary ignimbrite and alluvial units. These units thicken to the south and east and are inferred to infill a caldera-like structure. Oxidation extends down the vein margins to over 250m below surface.

The system comprises of four main northeast trending veins (Martha, Welcome, Empire and Royal) and a two north trending cross-cutting vein structures, the Edward and Albert. The main veins are enveloped by a stockwork of
subsidiary veins. Mineralisation extends for 1600 metres along strike with a width of 500 metres and was historically mined to over 600 metres below surface. Multiple stages of vein filling are ecognised with sulphide bearing crustiform-banded quartz being the main ore bearing type. Electrum (averaging 38% silver) is the main gold mineral and occurs as both free grains in the quartz and as inclusions in sulphides (pyrite, chalcopyrite, and sphalerite). Acanthite associated with pyrite and galena is the main silver mineral.

A localised failure of the north wall occurred in April 2015 which undercut the main access ramp. Operations were suspended in April 2015 and the open pit mining contract terminated in June 2015. A 1 million tonne failure of this wall occurred in April 2016 and studies to Feasibility Study level have now been completed to regain access to the Mineral Reserve. Permits and consents to commence the remedial works to access the Mineral Reserve were received in December 2018. The following refers to operations between 1988 and 2015 and will be relevant to the extraction of the open pit Mineral Reserve.

The open pit mining process at Martha was determined largely by the consents granted to the Company. Ore and waste were mined by conventional drill, blast, load and haul methods from the open pit. Waste and ore were categorised into hard and soft material and waste further categorised into potentially acid forming or non-acid forming rock. Ore sampling was conducted in-pit by RC drilling. Ore blocks were defined on the basis of this sampling and took into account the capacities of the equipment to selectively mine these blocks. Soft material was ripped by D9 dozer whereas hard material was blasted. Strict controls on blast vibration determined the blast hole spacing and the maximum allowable charge weight per delay. Ore was generally blasted in 5 metre vertical intervals (two flitches).

The presence of historic workings in the open pit required probe drilling to identify voids or weak pillars which created both a safety hazard and an operating constraint. Underground voids were either bunded off or marked with hazard tape.

All ore and waste were loaded by 190 tonne backhoe excavators into 85 tonne rear dump trucks and trucked via a 1 in 10 ramp and generally direct tipped to a jaw crusher or Stamler breaker station. Small quantities of ore and waste were stockpiled close to the jaw crusher.

Ore was conveyed 1.5 km to the process plant and placed in a 40,000 t stockpile. A surge (Polishing Pond) stockpile (up to 1.2Mt) is available close to the water treatment plant for excess ore. Crushed waste was conveyed a further 1.5 km and used to construct the tailings embankment.

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.