The Favona epithermal gold-silver deposit, Waihi, New Zealand

Abstract

The Favona deposit is a new epithermal vein discovery (>595,000 oz Au, >2,367,500 oz Ag resource), located less than 2 km east of the world-class Martha deposit (>6.7 million oz Au, >42.1 million oz (Moz) Ag), in the Hauraki goldfield of New Zealand. The concealed Favona vein is hosted in late Miocene andesitic lava flows and occurs to the east of the nearby historically mined Silverton, Amaranth, Mascotte, and Union veins.Hydrothermal alteration is characterized by a variety of alteration minerals that display distinct zonation along four subsurface cross sections (800 m long x 450 m deep) that transect the Favona, Cowshed, and Amaranth veins. Hydrothermal quartz and pyrite are ubiquitous. Adularia is widespread and envelopes veins; it locally coexists with hydrothermal albite adjacent to the Amaranth vein at depth. Both adularia and albite are variably replaced by illite, which, together with broadly coextensive chlorite, are generally restricted to the footwall of the Favona and Cowshed veins, although both minerals appear locally at depth in the hanging wall. Interstratified illite-smectite containing 10 to 70 percent smectite and rare discrete smectite are mostly confined to the hanging wall of the Favona and Cowshed veins.Late-stage kaolinite, cristobalite, and rare alunite locally overprint the above minerals at shallow levels, whereas late-stage overprinting calcite occurs at depth near the Amaranth, Cowshed, and southern portion of the Favona veins.Aeromagnetic data show that the Favona and Martha deposits both fit,within a broad zone of hydrothermally induced demagnetization that is approximately 3.7 x 3.0 km. Alteration is broadly similar at each deposit, although albite and calcite are more abundant at Martha. Fluid inclusion data indicate that both the Favona and Martha deposits formed at similar levels with veins developed over extensive vertical intervals exceeding 400 and 600 m, respectively. Based on these data, we propose that the veins comprising the Favona and Martha deposits formed at similar shallow levels (< 1 km) in a single paleogeothermal system. It is nuclear if the Martha and Favona veins systems formed within a single upflow zone, a migrating upflow, or within two separate upflow zones. Regardless widespread alteration surrounding the veins includes quartz, adularia, albite, chlorite, illite plus pyrite, and reflects formation from alkai chloride waters that contained moderate concentrations of aqueous CO2. Focused and episodic boiling of these chloride waters along extensive open conduits resulted in the formation of gold- and silver-bearing colloform- and crustiform-banded quartz veins. Steam-heated waters formed above and on the margins of the paleogeothermal system, and during the eventual thermal collapse resulted in overprinting by kaolinite, cristobalite, and alunite from descending steam-heated acidsulfate waters and late calcite from localized steam-heated CO2-rich waters.The discovery of the Favona deposit next to the large Martha deposit highlights the importance of determining the physical limits of hydrothermal alteration in a system in order to identify the limits of paleogeothermal activity and define the area that may be explored for veins. The veins at Favona, Martha, and Golden Cross (Empire vein), combined with a resource at Karangahake, collectively total more than 3.5 Moz Au and should encourage the exploration of known epithermal deposits throughout the Hauraki goldfield for additional resources.