THE FORMATION OF GOLD:
Formation Gold is formed in a wide variety of hydrothermal deposits, which may then be eroded and the gold re-deposited as sedimentary placer deposits. In hydrothermal deposits, gold is most commonly associated with silver and the base metals, copper, lead and zinc. Most of these deposits are quartz veins deposited from hydrothermal fluids in fault zones at medium (mesothermal) or shallow (epithermal) depths in the crust.
Gold deposit within quartz rock
GEOLOGY OF THE THAMES GOLD FIELD:
Epithermal quartz lodes in Cenozoic volcanic rocks Epithermal gold-silver deposits in Northland and in the Hauraki Goldfield were formed in past geothermal systems associated with volcanism that was active in the MiocenePliocene, whereas gold is being deposited today in active geothermal systems in the Rotorua-Taupo area, in association with Quaternary volcanism.
The Hauraki Goldfield contains about 50 known epithermal gold-silver deposits that were deposited by geothermal systems associated with volcanism that was active during the Miocene and Pliocene.
About 1360 t of gold-silver bullion were produced between the 1860s and 1952 (about 34% of New Zealand's total gold production), mostly from deposits hosted by andesite and dacite.
The Martha Mine is the largest producer in the Hauraki Goldfield with 1100 t of bullion mined from underground workings between 1878 and 1952. The mine was reopened in 1988, and is the second largest in New Zealand, currently producing about 3 t of gold and 25 t of silver from open pit mining. Four major lodes (Martha, Welcome, Empire and Royal) and numerous smaller veins strike in a northeasterly direction and form a braided vein system over 2.5 km long by 600 m wide extending to a depth of over 600 m.
At the Golden Cross deposit, northwest of Waihi, mining of a resource of about 19 t Au began in 1991. By 1993 Golden Cross was New Zealand's biggest mine, producing 3.2 t gold and over 10 t silver, but the mine was closed in 1997.
Small quantities of ore-grade gold-silver mineralisation have been deposited by geothermal fluids in several active geothermal fields of the Taupo Volcanic Zone, including Ohaaki (Broadlands), Rotokawa, Waiotapu and Kawerau.
Highly mineralised precipitates have also been found in discharges associated with geothermal exploration and development. Silver-gold mineralisation is also present in fossil geothermal systems in the Taupo Volcanic Zone. Several hot springs type epithermal deposits are known in Northland.
Small quantities of silver and mercury have been produced at Puhipuhi and mercury at Ngawha. Reconnaissance drilling to intersect feeder quartz veins at Puhipuhi intersected 10 m averaging 5.3 g/t Au and 18.5 g/t Ag. At Te Pene, Te Mata and Puketotara, quartz veins and silicified breccias have geochemically anomalous values gold and silver.
* Maps and text have been excerpted from > NZ Minerals website