Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Simpson, M. P. Articles by Mauk, J. L. Search for Related Content GeoRef GeoRef Citation

The Favona Epithermal Gold-Silver Deposit, Waihi, New Zealand

Mark P. Simpson and Jeffrey L. Mauk

School of Geography, Geology and Environmental Science, University of Auckland, Private Bag 92019, Auckland Mail Centre, Auckland 1142, New Zealand

Corresponding author: e-mail, ma.simpson{at}auckland.ac.nz

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 foot-wall 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 unclear 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 alkali 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 acid-sulfate 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.

This article has been cited by other articles:

				A. B. Christie, M. P. Simpson, R. L. Brathwaite, J. L. Mauk, and S. F. Simmons Epithermal Au-Ag and Related Deposits of the Hauraki Goldfield, Coromandel Volcanic Zone, New Zealand Economic Geology, August 1, 2007; 102(5): 785 - 816. [Abstract] [Full Text] [PDF]

				J. L. Mauk and M. P. Simpson Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand Economic Geology, August 1, 2007; 102(5): 841 - 871. [Abstract] [Full Text] [PDF]