Summary:
All the gold deposits are considered to be typical of epithermal vein gold-silver deposits.
All known Au and Ag mineralisation in Waihi is confined to veining or vein fragment within hydrothermal eruption breccia. The major mineralised veins are typically coincident with dip-slip, normal faults believed to have formed in an extensional setting related to early, back-arc rifting of the TVZ dated at ca.6.1 Ma (Mauk et al 2011).
Some of the main mineralised veins within the Waihi area include the Martha vein system (which includes the Martha, Empire, Welcome, Royal, Edward, Rex and Albert veins among many others) in the Northwest (NW) and the Correnso, Daybreak, Union, Trio, Amaranth, Favona, Moonlight and Gladstone veins progressively SE.
Martha
The Martha vein system is the largest and most documented of the vein networks in Waihi. It has historically been mined from underground and more recently mined as an open pit and is currently being mined from underground once again. 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 to 15 m wide. Increased vein widths are closely associated with the steepening of vein dips from an average of 65 to 70 degrees to approximately 85 degrees to the southeast. 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, smallerscale 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 and 50 cm in width infilling extensional structures with no fault displacement, dipping moderately towards the northwest. Two steeply dipping, northnorth-east 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 base metal sulphide content is low but is observed to increase in concentration with depth within all the Waihi veins. Sphalerite and galena are the most abundant base metal sulphides while chalcopyrite is less common and pyrrhotite is rare. Correnso ore has higher base metal content than other Waihi veins. Oxidation extends down the vein margins to over 250 m below surface however the andesite host rocks can appear only weakly weathered at or near the surface.
Correnso
The Correnso epithermal vein system is part of the greater Waihi epithermal vein system. It trends northerly and lies between the Martha Hill deposit to the west and the Union/Amaranth/Trio and Favona deposits to the south and southeast. The dominant host lithology is quartz phyric andesite lava, also the main host lithology for the Martha Vein System. The quartz andesite unit attains thicknesses in excess of 400m in the Union Hill – Waihi East area with only minor variation in texture or modal composition. The Correnso system comprises a main lode with smaller splays veins on both the hanging wall and footwall sides. The main mineralized lode is interrupted to the north by a NE trending calcite-quartz (barren) structure. The grade distribution to the north is increasingly complicated by bands / lobes of low grade calcite-quartz which appear to have flooded the system post Au mineralization. The main gold mineralisation lies between 900mRL and 775mRL. Gold-silver mineralisation is dominantly hosted in localized bands within multiphase quartz veins.
Wharekirauponga
The Wharekirauponga project is located approximately 10 km to the north of Waihi. The low-sulphidation epithermal quartz veins at Wharekirauponga are hosted in Whitianga Group rhyolite flow domes to sub-volcanic intrusions within polymict lapilli tuffs.
Gold mineralisation occurs in association with quartz veining developed along two types of structurally controlled vein arrays. The principal veins, namely the East Graben (EG), T-Stream and Western Veins occupy laterally continuous, northeast trending (025-47°), moderately dipping (60-65°) fault structures reaching up to 10 m in width. More subsidiary, extensional veins (1-100 cm wide) are developed between or adjacent to the principal fault hosted veins. These veins often form significant arrays that are moderate to steeply dipping with a more northerly to north-northeast strike and appear to lack lateral and vertical continuity compared to the principal veins.
The rhyolites have undergone pervasive hydrothermal alteration, often with complete replacement of primary mineralogy by quartz and adularia with minor illite and/or smectite clay alteration.
The EG Vein is the largest and most continuous mineralised structure drilled at Wharekirauponga to date. The vein strikes approximately northeast (020°) for over ~1000 m although the extent of veining to the north and south remains open due to limited drill data. Veining dips steeply to the west and is still considered to be open up-dip. Veining and grade are seen to decrease at depth (at approximately -180 mRL. Veins observed in drill core is characterised by multiphase white quartz/chalcedony with textures including colloform banding, brecciation, vein sediments and quartz replacing platey calcite.
The T-Stream Vein is a breccia zone within rhyolite flows containing mineralised quartz veins located approximately 500 m to the west of the main EG Vein. This structure strikes approximately northeast (020°) and dips moderately (65°) towards the west. The brecciated vein zone is exposed at the surface and appears oxidised and often broken at depth.