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 vei ........

The underground operations referred to as Waihi underground, comprises the two main mining areas of Correnso and Martha Underground. There is no more ore planned to be extracted from Correnso and backfilling of current drives in line with consent conditions is the only work being carried out. Development and Production activities continue in Martha Underground.

There are multiple orebodies within the underground mine including Correnso, Daybreak, Empire, Trio, and Louis. All mining areas share the same stoping method and have very similar modifying factors and assumptions and design criteria applied.

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

Mining options available for Correnso are limited because of the consent conditions which include blasting and backfill constraints. Modified Avoca long hole bench mining with waste rock backfill was selected as the preferred mining method for extraction of Correnso.

Small areas of the Louis orebody use an overhand cut and fill method due to the narrow, shallow dipping nature of the orebody.

Access to underground workings is via a decline from previously mined areas, which also serves as a fresh air intake. The primary ventilation is exhausted through a raise bored shaft to surface and a return air drive breakthrough into the Martha pit. An escapeway rise that has been raise bored to surface and equipped also serves as a ........

Comminution
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.

Prior to entry into the feed chute of the semi autogenous (SAG) mill, the ore is further reduced in feed size via a jaw crusher to a P80 of 110 – 130 mm. The SAG mill ball size is 125 mm and the mill will operate typically with a 10% ball load. The SAG mill draws between 2.1 and 2.5 MW of power.

The SAG mill discharge is sized using a trommel attached to the SAG. The +12mm oversize material is conveyed to a 30 kW cone crusher and is recycled back to the SAG mill. The undersize slurry from the SAG trommel is pumped to two 0.5 m 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.

Normally, the comminution process is set-up as a closed-circuit, but the plant has an ability to operate to an open circuit system if required.

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