Martha deposit is considered to be typical of epithermal vein gold-silver deposits.

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 fo ........

In February 2019, consents were granted for Project Martha comprising the Martha Underground mine and a small cutback of the north wall of the Martha open pit called Phase 4 (MOP4). Mining commenced in the Martha Underground mine in July 2019 with the development of access tunnels. There has been no mining undertaken in MOP4 at this stage. Production in the Martha mine commenced in June 2021.

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 Correnso workshop, Trio cribroom and dewatering systems. Exploration drives were completed on 800 m RL and 920 m RL 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 2022 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 areas involves mining of declines for access to five main stoping blocks. Access drives will be mined to develop drill ........

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.

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