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.

Open Pit Mining
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. Earthworks to stabilise the north wall failure were completed in 2017 over an 8-month period. During this period, the crest of the failure was unloaded and the majority of the excavated material was crushed and conveyed to the waste rock stockpiles adjacent to the tailings storage facilities. The remainder of the material excavated was either tipped over the crest of the pit wall or stockpiled adjacent the crusher. No ore was mined during 2021 and open pit operations currently remain suspended. Because of the wall failure mentioned above, studies are in progress to regain access to the bottom of the pit. There are no Mineral Reserves in the Martha open pit.

Ore and waste are categorised into hard and soft material and mined by conventional drill, blast, load and haul methods from the open pit. Soft material is ripped by D9 dozer whereas hard material is blasted. Strict controls on blast vibration determine the blast hole spacing and the maximum allowable charge weight per delay. Generally, ore is blasted in 5 metre vertical intervals (two flitches), but blast vibration limitations may require blast holes to be drilled at 2.5metre vertical intervals.

All ore and waste is loaded via 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 are stockpiled close to the jaw crusher. Ore is conveyed 1.5 km to the process plant and placed in a stockpile.

The minimum mining width has been set at 3 metres wide, determined by the observed width of many of the small narrow veins that are being mined. Equipment has been sized to suit these design parameters. The selective mining unit developed for the geological block model is a bench height of 2.5 metres, and east west dimension of 3 metres and north south dimension of 10 metres reflecting the drill spacing and the main trend of the mineralised veins in an east westerly direction.

In February 2019, resource consents were granted for Project Martha which includes Mine Operation Plan 4 (MOP4). No mining within MOP4 has occurred to date.

Underground Mining
The Martha underground is accessed via the existing Favona portal through the existing Trio and Correnso workings and shares the ventilation development and shafts as well as the underground workshop, crib room and dewatering systems. Exploration drives were completed on 800 mRL and 920 mRL in 2018. Development of Martha underground commenced in mid-2019 and 2,169 m of lateral development and a 120m ventilation raise were completed by the end of 2019 and a further 16,700 m of lateral development completed up to the end of 2021. Two breakthrough openings into the pit for ventilation and escape were also completed. Development up to end of 2021 has focussed on ramp access for Edward, Empire, Rex and Royal mine areas, ventilation connections, pumping well access drives, drilling platforms and back fill drives.

Based on the proposed mining method and equipment, historical experience and orebody geometries, the development strategy for all underground operations involves mining of declines for access to five main stoping blocks. Access drives will be mined to develop drilling and loading levels, generally intersecting the orebodies centrally. Access drives will be spaced generally at 18m vertically over the height of the mine. Ore drives will be developed in both directions along strike from the access drives. Stockpiles will be mined off the decline and in levels for truck loading.

Four mining methods are proposed for Martha underground mine:
(1) Modified Avoca with rockfill in virgin (previously unmined) areas;
(2) Modified Avoca with rockfill in remnant areas adjacent to collapsed stopes separated by a intermediate pillar;
(3) Modified Avoca with rockfill in remnant areas adjacent historical stopes filled with engineered fill (CRF/ cemented aggregate fill (CAF)); and
(4) Bottom up, side ring method with CRF/CAF/RF where skins adjacent to historical backfill are extracted.

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 ........