Lone Mountain Mining District,
Esmeralda County, Nevada
The Siolfor property consists of 10 unpatented lode mining claims located on the northeast side of Lone Mountain, designated Andrea 1 & 2, the Property encompasses 200 acres. The property is located in an area of highly anomalous to ore grade mineralization of gold, silver, lead, zinc and copper. While several small prospects and underground workings exist in the area, no known detailed exploration activity has been performed.
Location and Access
The claim block lies about 15 miles west of Tonopah, Nevada and 5 miles south of U.S. Highways 95 & 6 in the Lone Mountain mining district (Figures 1 and 2). As shown in Figure 2, the only other claim block in the area is the Fish claims held by Claremont Nevada Mines and consists of 19 unpatented lode mining claims southeast of the SIOLFOR claim block.
History and Production
The Lone Mountain district encompasses 200 mi2 in T1N & T2N, R39 & R40E and T3N, R40E and T1N, R41E. Though numerous small mines occur in this district, the four principal mines are the Alpine (west side of Lone Mountain), Weepah (Sec. 23, T1N, R39E), General Thomas (Sec. 7, 8, 16, 17, 18, T1N, R41E) and Gold Eagle (Sec. 9, T1N, R40E).
Mineralization and ore deposits at all four mines are hosted in carbonate rocks of the Reed Dolomite and Wyman Formation of Precambrian age and Mule Spring Limestone of Lower Cambrian age. Mineralization consists primarily of silver, lead and copper as polymetallic replacement deposits in carbonate rocks. An exception to this is the Weepah Mine which primarily produced gold in a silicified gangue composed of quartz and altered country rock.
Thompson and West (1881) reported the area being prospected in the 1860s by Mexican miners. However, mining did not begin until 1900 when the Alpine Mine was started. Production information from these mines is, at best, sketchy with the most recent information provided by Albers and Stewart (1972) being from the mid 1930s and 1960s with total production from the district of 248,991 tons of precious and base metal ores.
Several small adits and shafts occur in the SIOLFOR claim block. These appear to been used to explore the surface exposure of quartz veins and pods. No information is available on these workings.
Most previous investigations have been limited to specific mines and areas of the Lone Mountain district or reconnaissance level investigations. Among these are Ball (1907) and Spurr (1906) who described the General Thomas and Alpine mines, respectively. Oxnam (1936) reported on the Weepah mine. Albers and Stewart (1972) reported on the geology and ore deposits of Esmeralda County and Phariss (1974) conducted a detailed study of the Alpine Mine.
The most detailed and thorough investigations, to date, in and adjacent to the SIOLFOR claim block were conducted by Bonham and Garside, (1979 and 1982). These investigations involve geologic mapping (1:48,000 scale) and geochemical rock sampling in an area of about 600 mi2 covering portions of Esmeralda and Nye Counties. A total of 632 samples were collected, of these, 39 samples are within or near the SIOLFOR claim block (Figure 3). Analytical results will be presented later. Geologic and geochemical information presented in this report will be drawn primarily from these two investigations.
The Nevada Bureau of Mining & Geology (Bonham and Garside, 1979 and 1982) conducted 63 surface rock sample in the NE part of the district and analyzed for gold, silver, copper among other base metals. Six of these samples were collected as background samples. Results of gold and silver analysis indicated concentrations up to 0.156 and 15.4 oz/ton gold and silver respectively, with background concentrations from 6-samples ranging from non-detectable to 3.2 ppb (0.00009 oz/ton) for gold and 0.002 to 0.25 ppm (0.007 oz/ton for silver. Average concentrations of the 57-samples collected, exclusive of background sampling, show 662 ppb and 78.8 ppm fo gold and silver respectively.
Rock units exposed the district range in age from Precambrian to late Miocene and include sedimentary, metamorphic, granitic and gabbroic intrusive and volcanic rocks. Locally, rhyolitic and lamprophyre dikes intrude most older rock formations.
Precambrian and Cambrian Age Units
The oldest unit is the Precambrian Wyman Formation, which is in unconformable contact with the underlying Cretaceous age Lone Mountain plutonic rocks. The exposed area of the unit is approximately 3 miles long by 1000 ft to 2000 ft wide. The exposed unit underlies approximately half of the SIOLFOR claim block.
The Wyman Formation consists of approximately 45% marble, interbedded in 2- to 5-cm layers with phyllite, quartzite and calc-silicate hornfels. Regional metamorphism is widespread and the unit has locally been affected by contact metamorphism with the plutonic rocks. Contact metamorphism usually only extends a few meters outward from the pluton and has changed limestone and shale to calc-silicate hornfels and garnet-tremolite- diposide skarn.
Conformably overlying the Wyman Formation is the Precambrian Reed Dolomite. In the SIOLFOR claim block area the contact has been defined by Bonham and Garside (1979) as the point where marble and calc-silicate hornfels change to massive, tan dolomite. Exposures of the Reed Dolomite parallel those of the Wyman Formation and cover an area 3 miles long and up to 4,000 ft wide. The exposed unit also underlies approximately 45% of the SIOLFOR claim block. Nearly the entire Reed Dolomite consists of light tan weathering, white sugary, massive recrystallized dolomite.
Overlying the Reed Dolomite, and in conformable contact, is the Precambrian age Deep Spring Formation. Exposed southeast of the SIOLFOR claim block, the unit covers an area 2.5 miles long by 2000 ft wide. Near the base, the unit consists of tan-colored dolomite, similar to the Reed Dolomite, thus distinguishing the contact can be difficult. The formation is composed of alternating gray, thin-bedded marble, mica schist, and lesser amounts of light-gray quartzite.
Throughout its outcrop area, the unit is metamorphosed, marble consisting of calcite, dolomite and varying tremolite. Near intrusive contacts, impure limestone is changed to calc-silicate hornfels, consisting of calcite, tremolite, zoisite, chlorite, sericite and potassium feldspar. Quartzite consist of a mosaic of xenomorphic quartz, potassium feldspar, sericite, calcite and actinolite.
The only Cambrian age unit exposed in the area is what is believed to be the Harkless Formation, which is in thrust fault contact with the underlying Deep Spring Formation and Reed Dolomite. Exposure of the unit is in excess of 8 miles long by 2.5 miles wide. The unit has been extensively intruded by Cretaceous and Tertiary age granitic units north and east of the SIOLFOR claim block. The unit is exposed in the eastern most portion of the claim block. The unit is predominantly a detrital unit that includes dar-brown, black, and olive hornfels with minor amount of thin, gray to black quartzite, impure sandstone and recrystallized, gray-blue limestone. The unit is metamorphosed throughout the area.
Cretaceous Age Units
Cretaceous units exposed in the area are broadly grouped by Bonham and Garside (1979) as Lone Mountain plutonic rocks. These include gabbro, quartz monzodiorite and Lone Mountain Pluton in descending age.
Tertiary age units are the silicic porphyry dikes which, in some locations, have been cut by lamprophyre dikes.
Four Cretaceous age plutonic units occur in the SIOLFOR area. The oldest is a hornblende gabbro (113.0 my, Bonham and Garside, 1979). The gabbro intrudes the Harkless(?) Formation and is intruded by quartz monzodiorite and granite and rhyolite porphyry dikes.
The gabbro appears to be intruded approximately parallel to the regional grain of host rocks. The unit is a medium- to coarse-grained rock. In hand specimen it appears to consist mostly of greenish-black hornblende and very light-gray plagioclase.
Next oldest is quartz monzodiorite which is equigranular and medium-grained. The unit intrudes the Harkless(?) Formation and gabbro and is intruded by Miocene age (22.1 my, Bonham and Garside, 1979) dikes. The quartz monzodiorite and gabbro occur only in the upper thrust plate, which has displaced the Harkless(?) Formation.
Since neither the gabbro or quartz monzodiorite intrude the thrust fault, this relationship suggests the thrust fault developed after emplacement of these two units.
The youngest Cretaceous age unit exposed in the area is the Lone Mountain pluton (63.0 to 71.1 my, Bonham and Garside, 1979). The unit intrudes the Wyman Formation, Reed Dolomite, Harkless Formation and Deep Spring Formation. It is intruded by lamprophyre and rhyolite porphyry dikes. The intrusive contact with the
encompassing rocks is sharp and discordant in detail. Much of the pluton is very light-gray, coarse- to medium- grained, hypautomorphic-granular biotite granite.
Tertiary Age Units
Two units are exposed in the SIOLFOR area, the older is silicic porphyry dikes of early Micoene age, dated 22.1 my (Bonham and Garside, 1979) and younger lamprophyre. Silicic porphyry dikes consist of a swarm of quartz monzonite, granodiorite, granite, and rhyolite units. These dikes intrude all older rock units in the area.
Quartz monzonite and granodiorite dikes are light gray to yellowish gray and contain prominent phenocrysts of plagioclase plus smaller phenocrysts of quartz, orthoclase and biotite in a felsitic matrix of quartz, orthoclase and plagioclase. The granite porphyry is pale red, pinkish gray, or grayish orange pink and has phenocrysts of orthoclase, quartz, plagioclase and biotite in a micrographic or aplitic matrix of quartz and alkali feldspar. The rhyolite porphyry is light gray, pinkish gray, grayish orange pink, pale red or very pale orange and contains sparse phenocrysts of quartz, orthoclase, plagioclase and biotite in a spherulitic matrix of quartz and alkali feldspar.
All dikes are hydrothermally altered. The alteration ranges from propylitic to argillic and phyllic. Most of the dikes contain sparse disseminated pyrite. Hydrothermal alkali feldspar occurs in quartz veinlets in rocks displaying phyllic alteration.
Bonham and Garside (1979) suggest that a subjacent pluton was the magmatic source of the dike swarm and that the emplacement of this pluton caused doming and extensional fractures of its roof. The dike swarm was emplaced in fractures developed as a result of the extension of the roof.
Lamprophyre dikes are the youngest exposed rock unit and intrudes all units older. The dikes are dark greenish gray and contain prominent hornblende phenocrysts in a fine-grained pilotaxitic matrix of plagioclase, augite, and alkali feldspar. Magnetite and apatite occur as accessory minerals. The dikes are deuterically altered and contain abundant calcite, chlorite, and epidote replacing the primary minerals.
Two types of faults have been mapped in the area 1) a thrust fault and 2) high angle normal faults. Thrust faulting at Lone Mountain emplaced the Harkless(?) Formation over the Reed Dolomite and Deep Spring Formation, cutting out 3,000 ft to 6,000 ft of section. The thrust fault plane seems nearly parallel to beddin in the upper and lower plates. In some locations, this would result in a near vertical fault plane. Major offset is indicated by younger-over-older relations and progressive crosscutting of units. The originally low-angle thrust has been tilted by high-angle faults of Tertiary ages. Previously mentioned observations suggest the thrust is younger that Mesozoic intrusive units.
As noted by Bonham and Garside (1982), most of the ore bodies in the Lone Mountain district are predominantly replacement bodies in Precambrian and Cambrian rocks. The ore in the replacement bodies is largely oxidized and is composed of hemimorphite, smithsonite, cerussite, chrysocolla and tenorite. The large Cretaceous stock og granite intruding the Precambrian and Cambrian rocks is essentially unaltered and does not appear to have any direct relationship to the metallization in the district.
In the SIOLFOR claim block area, the ore bodies are spatially and perhaps genetically related to the northwest- trending granite porphyry dikes. The dikes are hydrothermally altered and now consist essentially of K-mica, K- feldspar and quartz. Unoxidized ore at the Alpine Mine has been described by Phariss (1974) as galena, sphalerite, molybdentie, enargite and pyrargyrite in a maganosiderite gangue.
Ore grade lead-zinc-silver mineralization is present as replacement bodies along fracture zones in carbonate rocks of the Reed Dolomite and Deep Spring Formation in the area. Geochemical anomalies of several metals are shown in Table 1. These anomalies are associated with the porphyry dike complex. These dikes exhibit pervasive hydrothermal alteration and the dike swarm almost certainly is the upper portion of an intrusive complex which might contain porphyry molybdenum mineralization.
Also mentioned by Bonham and Garside (1982), the area north of the SIOLFOR claim block contains anomalous amounts of arsenic, bismuth, silver, barium, mercury, and manganese indicating possible silver-gold mineralization.
Possible Mineralization Models
Given the dearth of information available, developing mineralization models can be problematic. Surface rock sampling shows high values for base and precious metals associated with quartz veining and replacement deposits in carbonate rocks. As mentioned above, three possible ore deposit models have been put forth by Bonham and Garside (1982). These are:
Porphyry molybdenum, also possible copper or copper-molybdenum
Tertiary epithermal silver-gold (north of area)
Since polymetallic replacement deposits do occur in the area and these types of deposits are more likely associated with copper porphyry or copper-molybdenum deposits. To these models possible other models could be added, skarn deposits related to porphyry.
The general characteristics and possible grade and tonnage expectations will be discussed for each model. Much of this discussion is based on USGS mineral deposit models as defined in USGS Bulletin 1693 (Cox and Singer, 1986), USGS Bulletin 1930 (Theodore, et. al., 1991), USGS Bulletin 1857-E (Shawe and Ashley, 1990), USGS Survey Bulletin 1857-C (Shawe and Ashley, 1989) and 1857-F (Shawe and Ashley, 1990). Table 2 summarizes the grade and tonnages of these deposits.
Polymetallic Replacement Deposits. Sometimes referred to as manto deposits, these deposits occur as massive lenses, pipes and veins of silver, lead, zinc and/or copper in carbonate rocks near igneous intrusions. Most deposits occur in mobile belts that have undergone moderate deformation and have been intruded by small plutons. Examples include Eureka, Cortez and Spruce Mountain in Nevada. Associated deposits include base metal skarns and porphyry copper.
Porphyry Copper Deposits. These deposits, along with copper-molybdenum, are among the largest mineral deposits and have by-product gold, silver and molybdenum. Associated rock types include tonalite to monzogranite intruding granitic, volcanic, calcareous sedimentary and other rocks. Depositional environment is high-level intrusive rocks contemporaneous with abundant dikes, breccia pipes and faults. Also cupolas of batholiths. Associated deposits are base-metal skarn, epithermal veins, polymetallic replacement, volcanic- hosted massive replacement. Examples are numerous and include Yerrington and Ely in Nevada.
Porphyry Copper-Molybdenum Deposits. Stockwork veinlets of quartz, chalcopyrite, and molybdenite in or near porphyritic intrusion. Rock types and depositional environment are similar to porphyry copper deposits. Associated deposits include Cu, Zn or Fe skarns, which may be gold rich. Volcanic-hosted massive replacement and polymetallic replacement deposits. Examples of this type of deposit are Morenci, Ray and Sierrita-Esperanza in Arizona.
Gold Skarn Deposits. These are sometimes divided into two classes 1) gold skarns where gold and silver are major commodities and 2) gold and silver are by-product commodities. Gold-bearing skarns are generally calcic exoskarns associated with intense retrograde hydrosilicate alteration. Additional economic metals include copper, lead, zinc and others. Most deposits are in orogenic-belt and island-arc settings and are associated with felsic to intermediate intrusive rocks of Paleozoic to Tertiary age. Associate deposits include porphyry copper skarn- related, porphyry copper-molybdenum or copper-gold, polymetallic replacement andpolymetallic veins, distal disseminated silver-gold, W skarns, among other deposit types. Examples of this type of deposit are Fortitude, Buffalo Valley and McCoy in Nevada.
Epithermal Gold-Silver Deposits. This is a rather broad definition as it includes both volcanic and sediment hosted deposits. Though the potential for this type of deposit lies north of the SIOLFOR area and is the least likely of the deposit models discussed, the wide-spread occurrence of high mercury and arsenic values, as well as, the proximity to the epithermal deposits of Tonopah demands some attention. Rhyolite and intrusive breccia occur 2-3 miles north of the SIOLFOR claims. In addition to mercury and arsenic, this area also hosts slightly anomalous silver, bismuth, zinc and barium. This setting suggests two possible models, carbonate-hosted gold- silver and Comstock epithermal veins.
Carbonate-hosted deposits, as the name implies, are hosted in sedimentary carbonaceous rocks intruded by felsic dikes. Associated deposits are porphyry molybdenum and tungsten-molybdenum skarn. Examples are numerous and include Carlin, Cortez, Getchell, Gold Quarry, Jerritt Canyon and others, in Nevada.
Comstock epithermal vein deposits are lated to calc-alkaline or bimodal volcanism. Host rocks are rhyolite, rhyodacite, quartz latite, dacite, andesite and sedimentary. Examples are Tonopah, Olinghouse, Aurora, Comstock and Divide, all in Nevada.
Ophir Canyon Property, Nye Country, Nevada