The Martha Vein System is the largest and most documented of the vein networks in Waihi. The veins are numerous and form a large network that extends for more than 1600 m along strike and 600 m below the surface. The vein network although complex in detail, simply comprises the dominant southeast-dipping Martha vein and several northwest-dipping hangingwall splays including the Empire, Welcome, Royal and Rex veins. The Martha vein is the largest vein structure reaching up to 30 m in thickness in places but averages 6-15 m wide. Increased vein widths are closely associated with the steepening of vein dips from an average of 65-70 degrees to approximately 85 degrees to the SE.

Steeper portions of the vein tend to contain higher concentrations of Au and Ag. The vein itself comprises mainly intact brecciated quartz vein material evidence for vein emplacement during the late stages of dip-slip faulting. The quartz is characterised by multiphase brecciation and banding (colloform and crustiform) and quartz textures are highly variable from a fine, microcrystalline and chalcedonic character to more coarsely crystalline particularly at depth. Apart from the main Martha vein, the hanging wall splay veins are also significant mineralised structures reaching 18 m in width (e.g. the Empire Vein). The hangingwall splays closest to Martha link up with the Martha vein at depth often forming a higher-grade lode at the intersection. Hangingwall splays further away from Martha either thin out at depth or are not drilled deep enough to make out their relationship with Martha at depth (e.g. the Rex and Ulster Veins). Additional, smaller-scale splay veins are present linking the larger vein structures and form a valuable contribution to the mineralisation particularly in the Martha Open Pit. These splays typically comprise smaller veins between 5-50 cm in width infilling extensional structures with no fault displacement, dipping moderately towards the NW. Two steeply dipping, NNE-trending and well mineralised vein structures known as the Edward and Albert veins also form an important part of the overall Martha vein network.

The andesitic host rocks within proximity to veining have often undergone pervasive hydrothermal alteration, sometimes with complete replacement of the primary mineralogy. Characteristic alteration assemblages of the host rocks are dominated by argillic alteration (quartz+adularia+pyrite+illite) closest to veining and propylitic alteration (weak quartz+weak pyrite+ carbonate+ chlorite+ interlayered illite-smectite and chlorite-smectite clays) extending over tens of metres laterally from major veins. The degree of alteration within the Waihi District is variable and often dependent on the host rock lithology and the nearby veining. On rare occasions, some host rocks at or near the contact of large veins appears only weakly altered, for example the “hard bars” identified during the early historical mining of the Martha vein. Volcaniclastic units tend to have increased clay alteration compared to the flow units.

Gold occurs mostly as small inclusions of electrum (averaging 38% silver) occurring as both free grains in the quartz and as inclusions in sulphides such as pyrite, galena, sphalerite and less commonly chalcopyrite. Free gold is rarely observed. Acanthite associated with pyrite and galena is the main silver mineral.

Permits and consents have been granted for Martha underground and all selected mining methods are to be accordance with the license, permit and consent conditions, principally related to placement of backfill, blast vibration limits, methods of working and hydrogeological controls.

The Martha open pit operations were suspended following a localised ramp failure in April 2015 and a larger failure of the north wall in April 2016.

The north wall of the open pit will be laid back to create a stable and safe pit wall. The haul road inside the open pit will be re-established to provide access to the pit floor. We expect this work to last approximately 10 years. However, it will not be continuous over that time. There will be periods of time when no work will be undertaken on this part of the project.

Stope optimisation results were used as a basis for the underground mine design. The top of the mineralisation is approximately 120 m below surface and extends to a depth of approximately 500 m below surface.

Mineral Resources extend below and outside of the existing open pit mine. There are multiple orebodies within the MUG Mine including Edward, Empire, Martha, Rex, and Royal. All mining areas share similar stoping methods and have very similar modifying factors and assumptions and design criteria applied. For simplicity all these areas are hereafter collectively referred to as Martha underground.

Avoca long hole mining has been selected as the mining method for much of the orebody. This method requires upper and lower access drifts to be developed with a proposed 15 m to 20 m vertical spacing between levels. The overall open stope dimensions are 15 m wide x 20 m high x 15 m long (maximum length).

Backfilling of all stoping voids forms a major component of the mining consent condition for underground mining in Waihi. Mining of stopes in virgin ground will utilise loose rockfill for backfilling of stope voids. This is in line with current practices on-site.

A cemented fill type will be required for mining of remnant and near-remnant mineralisation. CRF has been proposed for use and costed in this study. Some loose rockfill may be used in selected historical voids or remnant mining where further exposure of the fill mass is not expected.

CRF will be required to enable exposure of the fill mass during the life cycle of the mining front. Due to the predominantly mining sequence of the remnant mining (particularly within the Edward and Empire lodes), the CRF may need to be exposed due to adjacent stoping, as well as exposed due to underhand mining or for tunnelling through. To this end, it is expected that, on the basis of stopes being mined typically in 15 m panels, with an average width of eight (8)m, cement addition rates of ~6-7% are anticipated. Dilution of 0.5 - 1 m or more from undercutting can be expected, and less for sidewall exposure.

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 +12 mm oversize material is conveyed to a 30kW 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.

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

Leaching and Adsorption
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 carbon in pulp (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 inter-stage 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 th ........