The Martha low sulphidation, adularia-sericite epithermal quartz vein gold-silver deposit is located within the township of Waihi on the Coromandel Peninsula, North Island, New Zealand, some 110 km south-east of the city of Auckland. Historically the mine, which is the largest within the extensive Hauraki Goldfield, produced 167.5 tonnes (5.4 Moz) of gold and 1050 tonnes (33.7 Moz) of silver from 12.2 Mt of ore (at an average grade of 13.73 g/t Au, 86 g/t Ag) between 1878 and 1952 before re-opening in 1988 as an open pit operation which continues mining to the present. The historic underground mining followed major veins over a length of 1600 m and vertical interval of 575 m, while the open pit resource comprises zones of quartz veining between the originally worked major veins. Production between 1988 and 2000 yielded 31 tonnes (1 Moz) of gold and 193 tonnes (6.2 Moz) of silver.
The Favona low sulphidation epithermal vein deposit is located less than 2 km to the east of the centre of the Martha deposit. It is hosted by late Miocene andesitic lava flows, immediately to the east of the historically exploited Silverton, Amaranth, Mascotte and Union veins.
Both the Martha and Favona deposit occur towards the margins of a 3.7x3.0 km, roughly circular zone of hydrothermally induced demagnetisation, as apparent on aeromagnetic data.
Martha and Favona are situated near the southern end of the 200 km long, north-south trending Hauraki Goldfield which is defined by a series of epithermal gold deposits that are associated with the subaerial Miocene to early Quaternary Coromandel volcanic zone. This zone is part of a late Cenozoic continental margin volcanic arc related to the boundary between the convergent Australian and Pacific plates.
A regional NNW trending structural high of Jurassic lithic-volcanic greywacke is overlain by Miocene to Pliocene andesites with lesser dacites and rhyolites of the Coromandel volcanic zone, which young to the south where they merge with the currently active Taupo volcanic zone. Some porphyry style mineralisation is associated with dykes and sills of dioritic porphyry cutting the basement and lower volcanics, while epithermal Au-Ag mineralisation is mainly associated with andesites and dacites.
The Martha Hill quartz vein system is hosted by hydrothermally altered andesites of the late Miocene Waipupu Formation, principally composed of plagioclase porphyritic two pyroxene andesite (with prominent but minor quartz phenocrysts), minor interlayered dacitic tuffs and a few thin carbonaceous lacustrine beds. These hosts are unconformably overlain by un-altered late Miocene to early Pliocene dacite and rhyolite (5.6 to 4.8 Ma). All of the above were buried by extensive late Pliocene to early Quaternary ignimbrites.
The quartz veins at Martha form as an extensive braided pattern of wall rock andesite lenses enveloped by quartz veins, including four main, continuous, sub-parallel, north-east striking quartz veins, namely the Martha, Welcome, Empire and Royal lodes. Most dip steeply north, although the Martha Lode dips 70 to 80 degrees SE to intersect the other lodes at depth. Other less prominent sets strike east-west while there are a few veins trending north-south. The major lodes comprise multiple vein phases filled by microcrystalline to medium grained quartz, quartz-vein breccias, and sulphide bearing bands of pyrite, sphalerite, chalcopyrite and acanthite in fine to medium grained quartz. The vein quartz consists of a main sulphide bearing phase and late stage amethyst. Remnant platy calcite is evident in some veins indicating an early quartz-calcite veining subsequently replaced by quartz and sulphides. Vein textures include colloform and crustiform banding and botryoidal characteristics on a millimetric scale separating quartz/sulphides of different grain sizes. Adularia is mainly restricted to the wall rock margins of veins, while fine grained quartz bands commonly contain comb-quartz lined vugs.
Three zones are recognised within the pervasive alteration halo that envelopes the vein system. These are i). an illite zone and adularia-quartz-illite subzone adjacent to the quartz lodes and associated with veinlets of quartz +calcite +pyrite ±adularia ±illite, encompassed by ii). a broader, pervasive interlayered illite-smectite zone and iii). an outer pervasive smectite zone of smectite-chlorite-calcite-pyrite which passes out into poorly altered andesite.
The Favona mineralisation is also hosted by the 7.36 to 6.76 Ma Waipupu Formation porphyritic andesite flows which comprise phenocrysts of plagioclase, augite, hypersthene and minor amphibole set in a pilotaxitic to trachytic matrix of plagioclase laths, Fe-Ti oxides and altered interstitial glass. To the south-east the hosts sequence is unconformably overlain by hornblende dacite of the post-ore, 5.23 Ma Uretara Formation. Both of these units are concealed under a thick sequence of rhyolitic pyroclastics of the 2.9 Ma Owharoa Ignimbrite and the widespread 1.5 Ma Waikino Ignimbrite.
The Favona veins dip steeply to the east and parallel the Silverton, Amaranth, Mascotte and Union veins which are immediately to the west and trend NE to NNE, although the Silverton and section of the Favona vein trend NW. The Favona veins comprise the main east-dipping footwall vein, which with the hangingwall splays, extends over a strike length of more than 1 km, with a vertical extent of over 400 m. The zone averages 1 to 3 m in width, although locally it is up to 25 m thick. The hangingwall splays are best developed where the footwall vein dips more shallowly.
At shallow depths (>1050 m RL) the Favona veins comprise cryptocrystalline, concentrically banded quartz that grade at intermediate depths into dominant colloform banded quartz (92 to 96% quartz, with minor illite and illite-smecite bands and lesser adularia) with vein breccia and locally platy calcite. With increasing depth (<700 m RL), crystalline quartz with local bands of chlorite dominate, carrying <3% disseminated base metal sulphides (pyrite, lesser galena, sphalerite, chalcopyrite and marcasite). The gold is only found as very fine grained (1 to 2 µm), although silver is also found in tetrahedrite, naumannite and augilarite. Adularia alteration has been dated at around 6.0 to 6.2 Ma.
Most of the host volcanic rocks have been intensely altered in the vicinity of the deposit with all igneous minerals, with the exception of rare quartz phenocrysts and accessory zircon, being completely replaced, although the primary texture remains. Hydrothermal quartz and pyrite alteration are ubiquitous, while adularia is widespread and envelopes the veins, locally coexisting with hydrothermal albite. Both adularia and albite are replaced by illite, which with broadly co-extensive chlorite, are usually restricted to the footwall of the major vein zone. Interstratified illite-smectite are mostly found in the hangingwall of the Favona vein, while late stage kaolinite, cristabolite and rare alunite overprint the alteration assemblage at shallow levels. Late stage calcite predominates at depth.
Proven + probable reserves at Martha (at the beginning of 2003) were to 6.22 Mt @ 3.9 g/t Au. In 2002 the mine produced 1.04 Mt of ore @ 3.1 g/t Au from the open pit mine, with the removal of 5.14 Mt of waste (Newmont website, 2004). The Martha mine is operated by the wholly Newmont owned Waihi Gold Mining Co. Ltd.
Open pit production between 1988 and the end of 2006 totalled 17.38 Mt of ore for 55.584 t Au and 381.513 t Ag, representing recovered grades of 3.2 g/t Au, 22 g/t Ag (Simpson and Mauk, 2007). The average Au:Ag ratio is 6.9, although within the mine it varies from 6.2 to 11.2.
The Favona deposit contains a resource of 1.793 Mt @ 10.3 g/t Au, 41.0 g/t Ag for 18.4 t Au and 73.5 t Ag (Simpson and Mauk, 2007).
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2011)