Summary:
The principal rock units at Bell Mountain are stratified rhyolitic ashflow tuffs. The ashflow tuff sequence is relatively monotonous, varying only in the intensity of welding. Geologic mapping by BMEC geologists show that individual units can be broken out based on lithology, welding features, and alteration. BMEC mapped three surficial deposits, two intrusive units, three extrusive tuff units and features controlling mineralization at the property.
Mineralization
At the Bell Mountain deposit gold-silver mineralization is strongly structurally controlled. The primary control is an east-northeast trending (~070o) zone of faulting, named the Varga-Spurr fault, which can be traced for more than 6000 feet (1.8 km). The Varga-Spurr fault dips steeply to the south and has experienced normal and dextral displacement. It is offset slightly in a right lateral sense by a set of northwest trending, steeply dipping faults of similar strike length. Both fault sets have quartz-calcite veins and stockworks, gold-silver mineralization and pervasive silicification. Minor disseminated mineralization is present in silicified wallrocks. The intersection of the NE and NW vein sets, particularly in the Varga area, localized a significant volume of mineralization.
The quartz-calcite veining is rarely displayed as large planar veins, rather it is seen as variably intense stockwork zones of braided veins and veinlets which may be up to 40 meters wide. Within the stockwork the dips of individual veins are highly variable, but the overall dip of the body of mineralization as a whole is nearly vertical.
Mineralization at the property is separated into four deposit bodies – the Spurr deposit on the western end of the Varga-Spurr fault, the Varga deposit in the central part, the Sphinx deposit approximately 2000 feet (600 meters) southeast of the Varga on a northwest trending structure and the East Ridge deposit on an east-northeast trending structure approximately one mile (0.6 km) northeast of Varga. All four are composed of complex structurally controlled veins, stockworks and hydrothermal breccias. Between the Varga and the Spurr deposits, the east-northeast structure persists, but appears narrow, and it has had very little drilling. There were several other target areas which had returned attractive precious metal values, but had not been drilled.
Spurr Deposit
Calcite is the most abundant vein mineral in the Spurr deposit, with lesser amounts of quartz occurring as 1 to 20 centimeter veins concentrated near the vein walls. The calcite vein is generally strongly banded. The vein material is completely oxidized to depths of current drilling.
The values from the sampling of sixteen crosscuts in the Spurr adit range from nil to 11.2 g./t Au and nil to 385 g/t Ag, averaging 1.6 g/t Au and 50.5 g/t Ag. Sampling results from eight crosscuts in the Lovestedt adit range from nil to 5.5 g/t Au and 10 to 138 g/t Ag, averaging 0.6 g/t Au and 31.8 g/t Ag (Payne, 1982). Surface and underground sampling suggests that the mineralization is largely confined to the vein, although adjacent altered wall rocks carry lower precious metals values which may be minable in an open pit mining scenario.
Varga Deposit
The Varga vein can be separated into two parts. The western 120 meters (eastward from the adit portal) is a relatively simple and planar vein structure ranging in width from 5 meters near the portal, to 14 meters (eastward) where it is cut by a N60W trending fault. This vein segment strikes N60E and dips 50 degrees to the south.
The values from the sampling of nine crosscuts in the Varga adit range from nil to 4.1 g/t Au and nil to 143 g/t Ag, with an average grade of 0.4 g/t Au and 27.7 g/t Ag. Trench sampling by Payne in 1980 near the east end of this vein segment produced 6.1 meters (20 ft) grading 2 g/t Au with 10 g/t Ag and 8.2 meters (27 ft) grading 2.1 g/t Au with 24 g/t Ag. An ECU sample of the vein at surface nearby produced a grade of 1.48 g/t Au across 7 meters (23 ft). Another 24-meter (79 ft) surface sample interval by ECU, including both hanging-wall and footwall rocks, averaged 0.82 g/t Au and 5.3 g/t Ag. This suggests that, unlike the Spurr zone, mineralization in the western portion of the Varga zone does extend some distance into the wall rocks. The Varga is about 500 meters (1640 ft) long, with its ends poorly defined.
Sphinx Deposit
The Sphinx vein system can be traced for more than 900 meters along strike by prospect pits, vein quartz float and a few trenches. The Sphinx deposit contains at least two sub-parallel veins with other smaller splits which trend approximately North 70° West. Vein and stockwork widths in the crosscuts ranged from 10 to 30 feet (3 to 9 meters) and from nil to 5.1 g/t Au (Payne, 1982). Veins here are quartz with little calcite, are often banded and have a bluish tinge (Pinet, 1996). Minor silicification is present, surrounded by argillic alteration, which is stronger than elsewhere on the property. The veins dip steeply toward the southwest. The Sphinx deposit may be exposed at a somewhat deeper erosion level in the epithermal system due to the relative lack of calcite and better gold grades.
East Ridge Deposit
The East Ridge Deposit consists of a single east-northeast trending quartz-calcite vein which dips steeply to the south. Quartz is the predominant vein material with lessor calcite. The width of the vein is 1 to 4 meters. The vein is exposed in outcrops and surface cuts for approximately 250 meters.
The vein is cut by sparse northwest northeast trending fractures that locally host quartz-calcite veinlets and may continue out into the hanging wall for several meters. These crosscutting veins and fracture sets have not yet been tested by drilling. The west and east ends of the deposit are not well defined and are interpreted as weakening sheeted veinlets and stockwork zones.
Deposit Types
The following section on the deposit type is modified from Durgin (2010). The Bell Mountain deposit is characterized as a low-sulfidation epithermal vein system. Hydrothermal alteration in the upper levels of veins such as at Bell Mountain is expressed as broad irregular zones of argillic (kaolinite, illite) alteration with localized to extensive silicification and bleaching of the host rocks. Vein deposits can exhibit highly variable gold and silver contents and metals are vertically zoned. The geometry of both vein and disseminated mineralization can be complex and is a function of pre- and post-mineral faulting, host rock permeability, and intensity of hydrothermal fracturing.
Multiple phases of vein infilling, brecciation, and hydrothermal fracturing are common in many such deposits. Mineralization occurs as electrum in banded colloform/crustiform quartz or quartzcalcite veins, veinlet stockworks, and hydrothermal breccias. In the upper levels of many veins including those at Bell Mountain, coarsely bladed calcite, deposited during fluid boiling, is replaced by chalcedonic to sucrose quartz and usually represents higher grade parts of the deposit. Adularia and sericite are common gangue minerals. Generally, there is no close spatial or genetic relationship to larger intrusive bodies, although felsic dikes are often associated with mineralization. In western Nevada, many epithermal vein districts are associated with subaerial volcanic centers such as the Fairview Peak caldera.
Sulfide minerals are present in sparse amounts, but are largely pyrite, marcasite, and acanthite. Gold and silver occur along sulfide crystal surfaces, as electrum, and locally as grains of native silver and gold. Other associated trace elements include arsenic, antimony, barium, manganese, mercury or selenium. At higher levels of most epithermal veins, base metals (copper, lead, zinc) are typically absent or present in sub-economic amounts.