Summary:
Deposit Types
There are currently four separate mineralized zones known on the Gabbs Property: the Sullivan, Lucky Strike, Gold Ledge and Car Body Zones. The Sullivan, Lucky Strike and Gold Ledge Zones are considered to be gold-copper porphyry deposits, whereas the Car Body Zone is considered to be a low-sulphidation epithermal gold deposit.
Gold-copper porphyry deposits form in subduction-related settings (oceanic or cratonic crust, extensional or compressional regimes), associated with steep cylindrical stocks intruding volcanic piles. Cu and Au are largely introduced during potassium-silicate alteration. Low-sulphidation epithermal Au-Ag deposits develop more distally from intrusions, in near-neutral fluids often in dilational, rift-style settings, with smaller magmatic contribution. These deposits feature banded quartz veins carrying pyrite, sphalerite, galena, and chalcopyrite, accompanied by carbonate and illite-type wall rock alteration.
The Gabbs Property exhibits quartz-sulphide Au ± Cu style mineralization, which is characterized by quartz and by pyrite as the main sulphide phase. Quenched, very-fine grained pyrite locally exhibits difficult metallurgy, whereas coarser sulphides are typically associated with the near-surface supergene Au enrichment.
Mineralization
Mineralization and hydrothermal alteration at the Gabbs Property occurs in two principal styles:
1) Porphyry gold-copper-molybdenum mineralization with associated potassic, phyllic and propylitic alteration; and
2) Volcanic-hosted gold-mineralized hydrothermal breccias with associated phyllic and argillic alteration.
There are four separate mineral deposits, three of which (Gold Ledge, Lucky Strike and Sullivan) are considered to be porphyry gold-copper deposits. The Car Body Deposit is considered to be a nuggety epithermal gold deposit.
Porphyry Gold-Copper Mineralization
Porphyry copper deposits are among the largest and most valuable mineral-deposit types on earth and are the most important source of global copper supply. The deposits typically contain hundreds of millions of tons of mineralized rock and millions of tons of copper, with smaller amounts of molybdenum, gold, and (or) silver.
Porphyry copper deposits form in subduction-related magmatic arcs and northern Nye County contains parts of at least three such arcs: 1) Late Triassic to Jurassic age; 2) Cretaceous to Palaeocene age; and 3) Oligocene and Miocene age. Although large porphyry copper deposits are not known in the northern Nye County region, at least two sites provide specific analogues to deposits that may exist. The Royston Deposit is 40 km northwest of Tonopah, on the NyeEsmeralda County line, and the Sullivan Deposit occurs on the Gabbs Property. The Lucky Strike and Gold Ledge Deposits are also considered to host porphyry-style mineralization.
Sullivan, Lucky Strike and Gold Ledge Zones
The Sullivan Deposit, also known as Cuervo, is located approximately four km northeast of the Paradise Peak epithermal gold deposit (Ludington et al., 2009), and is exposed at the surface where the monzonite “sill” outcrops. The Deposit is a vein stockwork hosted in Late Cretaceous monzonite porphyry. The veins contain copper and gold. Glamis Gold Ltd. excavated 30,000 tons of mineralized material from a surface pit for test leaching purposes in the late 1980s.
Porphyry gold-copper-molybdenum mineralization occurs in two shallow dipping sill-like monzonite porphyry bodies at the Sullivan and Lucky Strike Deposits, and a vertically continuous body, possibly a plug, at Gold Ledge. The “sills” range from 1 to >100m thick and are laterally extensive. Average orientation at Lucky Strike is N46°E dip 25°SE and Sullivan varies from N140°E dip 31°SW (upper contact) to N94°E dip 24°SW (lower contact). The “sills” may be rotated dykes or tectonically emplaced slabs of a porphyry stock.
Porphyry-style mineralization at Gabbs is characterized by stockworks, grain boundary filling and disseminations of early sulphide ± biotite veinlets. These bodies are mostly cut by quartzchalcopyrite “A” veins and less common “B” veins accompanied by potassic alteration (biotite and K-feldspar). Quartz-sericite-pyrite (phyllic) alteration is common and generally accompanied by thick, quartz-pyrite-chalcopyrite-molybdenite “D” veins. Thick, massive to coarsely crystalline, sometimes ribbon-textured, pinching and swelling, mesothermal quartz-chalcopyrite-chalcocite “D” veins occur in monzonite porphyries and in surrounding Triassic metavolcanic and metasedimentary country rocks. Visible gold was observed in one such vein. Late veins of pink manganoan calcite cut mineralized monzonite porphyry in places and selenite (after anhydrite) was observed at Lucky Strike.
Epithermal Gold-Silver Mineralization
Epithermal gold-silver deposits are important sources of gold and silver worldwide (Simmons and others, 2005). They form at depths of < 1.5 km depth and temperatures of < 300°C, mainly in subaerial hydrothermal systems (Henley and Ellis, 1983; Hedenquist and Lowenstern, 1994). These hydrothermal systems developed in association with calc-alkaline, alkaline and, less commonly, tholeiitic magmatism, generally in volcanic arcs at convergent plate margins, and also in intra-arc, back-arc, and post-collisional rift settings. In addition, some non-magmatically heated epithermal deposits formed by deep circulation of meteoric water along steep extensional faults are present in northern Nevada.
Epithermal gold-silver deposits have highly variable characteristics, including mineralized material and alteration mineralogy and gold, silver, and base metal (Cu, Pb, Zn) contents, and formed in diverse geologic environments (Hedenquist and others, 2000; Sillitoe and Hedenquist, 2003; Simmons et. al., 2005). Two principal types of deposits are low-sulphidation deposits (also called quartz-adularia or adularia-sericite type deposits) and high-sulphidation deposits (also called quartz-alunite or acid-sulphate deposits).
Epithermal deposits have been the largest producers of gold-silver in northern Nye County since discovery of silver-rich veins in the Tonopah District in 1900. Round Mountain has the largest total production and is the largest current producer in the region. It has produced >373,000 kg of gold and 311,000 kg of silver since 1907.
In northern Nye County, isotopically dated epithermal gold-silver mineralizing systems range in age from approximately 26 Ma to 17 Ma. High-sulphidation deposits generally form in or proximal to eruptive/intrusive centres and have a larger magmatic component than lowsulphidation deposits. Their formation is related to degassing of shallow, oxidized magma bodies and circulation of acidic hydrothermal fluids released from these magmas.
Low-sulphidation deposits are common in the western half of northern Nye County and are widespread throughout much of the northern Great Basin. On the Gabbs Property, the Car Body Deposit is an epithermal gold deposit hosted in similar Tertiary volcanic rocks to the Paradise Peak Deposit. Whereas Paradise Peak was a high-sulphidation epithermal gold deposit, Car Body is of the low-sulphidation type. The Gold Ledge area also has potential to contain an epithermal gold deposit.
Car Body Zone
The Car Body Zone at the Gabbs Property is hosted in intrusive, magmatic-hydrothermal breccias. The breccias occur in Miocene upper andesite-dacite and middle rhyolite volcanic and intrusive lithologies best exposed in the adjacent Paradise Peak Mine. Breccia textures were recognised previously in petrographic studies of RC drill hole chips from the Car Body Deposit. Coarse gold is reported in RC drill chips from Car Body. However, the gold values are variable and difficult to reproduce between RC and drill core, which indicates a strong gold “nugget effect”.