The Goldfield Mining District is located in west-central Nevada, south of the town of Tonapah. It is described as a large bonanza grade epithermal system which is hosted by Tertiary volcanic rocks.
The host volcanics are within a domed uplift which exposes a series of faulted and hydrothermally altered mid-Tertiary volcanics, unconformably overlying Ordovician black shales and Jurassic quartz-monzonite basement. The volcanics comprise a lower series of rhyodacite to dacite flows and tuffs dated at 31 to 30 Ma, overlain by andesite to rhyodacitic flows and lesser tuffs which are from 22 to 20 Ma in age. These are followed by middle Miocene to Pliocene silicic tuffs, basalts, debris flows and sediments (Reutz, 1987).
The hydrothermal alteration and mineralisation is associated with the second, andesitic to rhyodacitic volcanics and is dated at 21 to 20 Ma. Hydrothermally altered rocks cover an area of approximately 40 sq. km in the Goldfields district. Four alteration assemblages have been recognised, three of which form zoned envelopes of advanced and intermediate argillic alteration adjacent to fractures and faults. These are i). strong silicification composed of fine silica with lesser alunite, kaolinite, pyrite and trace diaspore and pyrophyllite. The bonanza ores are formed in silicified zones, although conversely many silicified zones are barren or low grade. These silica zones are bounded by ii). abundant quartz and alunite with kaolinite, sericite, pyrite and local opal. Farther from the structure there is generally a iii). wide, poorly exposed zone of intermediate argillic alteration that can be divided into an inner kaolinite rich zone with 90% kaolinite, accompanied by quartz-illite-pyrite and local adularia and opal, and an outer montmorillonite rich zone with quartz-kaolinite-illite and pyrite. These zones may extend outwards by up to 100 m from the siliceous sections of the alteration halo. The final alteration type is regional and not zonally related to individual fracture systems. It comprises iv). a propylitic zone which is characterised by calcite and chlorite, and is sulphide poor (Reutz, 1987).
The majority of production has come from the 'Main Vein' that strikes north-south for 410 m and dips to the east at 20 to 25°. The remainder was derived from a number of other adjacent, smaller veins systems. Most of the zones of silicification carry anomalous gold values, although bonanza grades was confined to shoots where previously silicified rocks have been fractured or brecciated. These shoots form irregular, roughly sheet like masses with no sharp boundary. Some 15 bonanza shoots were mined with average strike dimensions of 60 to 130 m, down dip extent of 60 to 100 m and average thickness of 7 m. These averaged 100 000 t of ore each, with grades 30 to 150 g/t Au. The richest contained 12 500 t @ 600 g/t Au (Reutz, 1987).
The ore varied from finely crystalline with disseminated ore minerals, to brecciated or vuggy types with crusts of ore minerals around breccia fragments or vug linings, to massive sooty types that were practically massive ore minerals. Typical un-oxidised ores contain pyrite, bismuthinite, famatinite and fine native gold in a dark grey quartz gangue. Local, subordinate amounts of marcasite, sphalerite, tetrahedrite-tennantite and chalcopyrite are present, as are traces of tellurides. The best ores characteristically exhibit concentric shells of ore minerals and quartz around silicified and pyritised breccia fragments. The grade, degree of silicification, amount of alunite, tellurides, bismuthinite and famatinite all decrease with depth, while pyrite increases. Deeper ore was much richer in Ag, with values of up to 130 g/t Ag and 4% of associated Cu. The Goldfield District has an unusually high Au:Ag ratio of 3:1 (Reutz, 1987).
4.7 Mt @ 28 g/t Au, 9.3 g/t Ag = 130 t Au (Production to 1986, Ruetz, 1987).
0.55 Mt @ 2.1 g/t Au (Reserve at 1986, Ruetz, 1987).
2.4 Mt @ 2.4 g/t Au = 5.7 t Au (Reserve 1984, USBM).
(Porter GeoConsultancy, http://www.portergeo.com.au/, 1996)