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Location: 112 km SE from Auckland, New Zealand
43 Moresby AvenuePO Box 190 Waihi 3641WaihiNew Zealand3610
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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 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.
