Summary:
All the gold deposits are considered to be typical of epithermal vein gold-silver deposits. The Waihi and Wharekirauponga deposits display the following features:
- Gold-silver mineralization is predominantly confined to localized bands within multiphase quartz veins
- Host lithologies for veins are volcanic units of andesitic and/or rhyolitic composition
- Sphalerite, galena, and chalcopyrite commonly occur with gold-silver Mineralization within the Martha Underground (MUG) deposit. This base metal content increases at depth
- Host rock volcanics have undergone pervasive hydrothermal alteration, often with complete replacement of primary mineralogy.
- Characteristic alteration minerals include quartz, adularia, albite, carbonate, pyrite, illite, chlorite, interlayered illite-smectite and chlorite-smectite clays extending over tens of metres laterally from major veins
- Mineralization is structurally controlled.
The gold and silver mineralization occurs within low-sulphidation, epithermal quartz vein systems occupying large, north to northeast trending, normal faults and their subsidiary extensional structures. The vein systems comprise a 3D network of multiple vein sets that collectively strike >1000 m, with a current vertical range of 300 m (Wharekirauponga) and >500 m (Waihi) and include veins typically between 0.5 and 5 m in width (but up to 30 m locally). The main gold bearing minerals are electrum and silver sulphides developed within quartz veins.
The Waihi vein system is hosted within andesitic flows, intrusives and volcaniclastic units of late Miocene age (7.36 to 6.76 Ma) Waipupu Formation. The Waipupu Formation in Waihi can be subdivided into an upper quartz-phenocryst poor unit and a lower quartz-phenocryst rich unit which dip shallowly towards the SE. Some of the veining and gold mineralization in Waihi appears to be better developed within the lower quartz-rich andesite flows, with the exception of the Favona, Moonlight and Gladstone vein systems which are solely hosted within the upper andesite unit. Much of the mineralized andesites in Waihi are overlain by post-mineral rocks including dacite flows of the Uretara Formation (5.23 Ma), Pleistocene ignimbrites and recent ash deposits. Where veining is exposed close to the surface, the quartz-adularia altered andesite appears to have formed resistant paleo-topo ‘highs’ that project through the post-mineral cover sequences.
All known Au and Ag mineralization in Waihi is confined to veining or vein fragment within hydrothermal eruption breccia. The major mineralized 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 mineralized veins within the Waihi area include the Martha Vein System (which incorporates the Martha, Empire, Welcome, Royal, Edward, Rex and Albert veins among many others) in the NW and the Correnso, Daybreak, Union, Trio, Amaranth, Favona, Moonlight and GOP veins progressively southeast.
Martha Vein System
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 hanging wall 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 to 70 degree to approximately 85 degree 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 characterized 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 mineralized structures reaching 18 m width. The hanging wall splays closest to Martha link up with the Martha vein at depth often forming a higher-grade lode at the intersection. The hanging wall 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. Additional, smaller-scale splay veins are present linking the larger vein structures and form a valuable contribution to the mineralization particularly in Martha Open Pit (MOP). These splays typically comprise smaller veins between 5 and 50 cm in width in filling extensional structures with no fault displacement, dipping mode rately towards the northwest. Two steeply dipping, NNE-trending and well mineralized vein structures and form an important part of the overall Martha vein network.
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. Martha ore has silver to gold ratios of > 10:1, The Favona and Trio ores had silver to gold ratios of ~ 4:1, and Correnso ore had a silver to gold ratio of less than 2:1.
Gladstone Vein System
The Gladstone deposit is part of the greater Waihi epithermal vein system located approximately 2 km to the east of MOP. It is situated along the southern strike extent of the Favona and Moonlight deposits. Veining at Gladstone Open Pit (GOP) occurs within the upper 250 m below the surface, hosted within the upper andesite unit (devoid of quartz phenocrysts). The mineralization is characterized by shallow-level, hydrothermal breccias and associated banded quartz veins interpreted to represent the top of the epithermal system. The uppermost mineralized quartz veins flare up into hydrothermal explosion breccias. The Gladstone veins are predominantly steeply dipping veins developed within the hanging wall of the Favona Fault that dips moderately towards the southeast. Gladstone veining trends ENE to NNE between 010 degree and 070 degree and dips steeply towards the southeast.
Wharekirauponga
Low-sulphidation epithermal quartz veins at Wharekirauponga are hosted in Whitianga Group rhyolites, typically rhyolite flow domes to sub-volcanic intrusions within polymict lapilli tuffs. The geology observed in outcrop mapping and diamond drilling indicates the rhyolitic host rocks are partially overlain by strongly magnetic, fresh, andesitic flows, rhyolitic tuffs and recent ash deposits. Deep drilling to the west suggests the rhyolites are underlain by Coromandel Group andesites.
Gold mineralization occurs in association with quartz veining developed along two types of structurally controlled vein arrays. The principal veins, namely the EG, T-Stream and Western Veins occupy laterally continuous, NE trending (025-47 degree), moderately dipping (60-65 degree) 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 NNE strike and appear to lack lateral and vertical continuity compared to the principal veins.
The EG Vein is the largest and most continuous mineralized structure drilled at Wharekirauponga to date. The vein strikes approximately NE (020 degree) for over ~1000 m.