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Location and Land
The Eureka property is located along the west flank of the Wassuk Range and is approximately 4 miles southwest of Mt. Grant (11,239 feet), the topographic high point of the range. The property occurs at an elevation of about 7,800 feet within the Mount Grant mining district, Mineral County, Nevada. Access to the mouth of Lapon Canyon is via paved and graded roads 40 miles west from Hawthorne.
Previous Exploration and Development
The Eureka claims were originally located along the surface trace of the vein for approximately a mile, although the only significant development occurred at the Eureka shaft. High-grade ore was mined and milled at a stamp mill located on Lapon Creek.
In 1939, a cross-cut adit was driven to the vein at 150 feet below the collar of the original Eureka shaft. Ore was mined and shipped to the Dayton Consolidated mill in Silver City, Nevada for processing. The mine was closed in 1942 (Executive Order L-208) during World War II and apparently never resumed production after the war.
Total production from the mine is unknown and production records prior to 1939 do not exist. Recorded production beginning in 1939, is approximately 300 tons at 0.8 oz/t (-240 oz) Au.
Development on the property consists of a vertical shaft (caved), a crosscut to the main shaft at the 150 foot level (caved near vein), and drifts along the vein (north and south) for at least 300 feet according to the previous owner, George Wilson a mining engineer. The north workings were open until the mid-1970's when the old lagging collapsed, sealing the drifts just west of the crosscut adit and vein intersection, thereby rendering the entire subsurface vein zone inaccessible.
Geology
The Wassuk Range is underlain by a large composite batholith of silicic igneous rocks petrologically similar to and coeval with, the Sierra Nevada Batholith. The intrusive rocks are considered by Stewart et al. (1981) to be of Jurassic to Cretaceous age. The range in composition for the intrusions is from granite to granodiorite. Numerous roof pendants and xenoliths of metavolcanic and metasedimentary rocks occur in the intrusive mass and are believed to be composed, principally of the Permian Excelsior Formation (Ross, 1961).
The main vein zone on the Eureka property is hosted by a medium-grained equigranular to porphyritic quartz monzonite that is part of the Granitic Rocks of Butler Mountain intrusion. Sedimentary rocks are comprised of quartz, feldspar, diopside, and magnetite. Metavolcanics in the area are intermediate in composition and exhibit relict igneous textures.
South of the Eureka shaft, the northeast-trending mineralized structural zone appears to strike into, and follow, the contact developed between the quartz monzonite intrusion (west) and a large pendant of metavolcanic rocks (east). This contact zone could provide an ideal localization feature for hydrothermal mineralization.
Hydrothermal Alteration and Mineralization
The quartz vein portion of the main zone is typically 18-24" wide at surface exposures, although sheared and crushed wallrock granite containing varying amounts of quartz, chlorite, iron oxides, jarosite, copper oxides, MnOx, and secondary silicate minerals yields an aggregated vein zone width of >6 feet. Underground vein widths in excess of 4 feet and up to 6 feet wide have been reported. The quartz-rich portion of the vein lies on the footwall side of the structural zone. The hangingwall, above the more massive quartz, is intensely crushed and sheared and is comprised of chloritic and argillic clay-altered intrusive rocks. These rocks contain varying amounts of introduced silica and disseminated limonitic FeOx, jarosite, and magnetite in addition to the clay minerals. The chloritic intrusion commonly contains vugs lined with crystallized quartz, coarse chlorite, and FeOx. The wallrock alteration, specifically the copper and manganese oxides, appear to be a valuable exploration guide to precious metals vein mineralization where the vein is poorly exposed.
The quartz ranges in character from vuggy, crystalline and crystallized (crystal-lined open spaces) to massive, dense white crystalline quartz. Crystal-lined open spaces are common and contain FeOx. No sign of obvious banding was observed, however, much of the vein material exhibits a distinct epithermal character. Limonitic iron oxides are ubiquitous as surface stains, disseminations, open space lining and infillings. Virtually all FeOx is yellow-brown to dark-brown and occurs in association with minor jarosite. Hematite was not observed. Visible gold is relatively common in the oxidized portions of the high-grade vein material.
All exposed vein material is oxidized at the surface and most of the dump material is oxidized. Rarely, material containing sporadic pyrite was noted on the dump. Sulfide minerals were reported from the lower workings, described as pyrite, chalcopyrite, bornite, and possibly argentite in the mineralized vein zone.
Geochemistry
Surface rock sampling in 2000, confirmed the reported high gold grades at the property. Two chip samples and two select samples from suspected ore piles at the mine contained up to 2.3 oz/t Au. Rock chip samples were collected from the Eureka mine workings and along strike of the exposed vein zone (prospect pits) for over 500'. A sample collected across the exposed vein zone approximately 375 feet south of the main shaft contained 3.859 oz/t Au over a 2.0' chip. The northern-most prospect pit (approximately 150 feet north of the main shaft) contained 0.355 oz/t An. These samples, in addition to others collected along the vein zone and at the main shaft, indicates that the Eureka vein zone contains high gold grades (>0.50 oz/t Au) along much of its length.
In addition to gold, the high-grade samples contain 1-3+ oz/t Ag. The Ag/Au ratios are quite low and are commonly around 1.0 and seem to range between 0.5 and 1.8. Arsenic is moderately anomalous (-20 to >100ppm), copper is moderately to strongly anomalous (<100 to 4,000 ppm; one sample 1.74%), and lead is weakly anomalous (<100 to 1,000 ppm). In addition, bismuth (up to 525 ppm) and molybdenum are anomalous (up to 165 ppm).
Exploration Potential
The Eureka property exhibits good exploration potential for a high-grade gold vein system. Historic reports indicate that the now inaccessible portion of the vein zone contains high gold grades (up to 2.3 oz/t Au) and chip samples of 3-6 feet wide yield gold grades between 0.170 to 1.70 oz/t; suggesting that mining widths of 0.5 oz/t material is not unreasonable. Surface exposures and underground documentation indicates that the vein zone averages 3-6 feet wide to a depth of approximately 150' down-dip on the vein; the deepest level of exploration.
The vein can be followed by surface exposures for more that 500' along strike. The vein disappears under cover to the north and to the south we have not attempted to follow it beyond about 400' south of the mine, although copper oxide-stained and quartz veined zones occur along strike approximately 1500 feet south of the shaft.. In addition, historic reports suggest the vein zone can be traced for at least a mile (and possibly up to 3 miles) to the south.
After some additional geologic mapping, reconnaissance, and geochemical sampling (rock and soil), a modest drilling program could be formulated to test this zone. However, because much of the gold in the oxidized zone is free milling careful sampling will be needed to obtain a valid test of the vein.
REFERENCES CITED
Ross, D.C., 1961, Geology and mineral deposits of Mineral County, Nevada: Nevada Bureau of
Mines Bulletin 58, 98 p.
Steward, J.H., Reynolds, M.W., and Johannesen, D.C., 1981, Geologic map of the Mount Grant
Quadrangle, Lyon and Mineral Counties, Nevada: U.S. Geological Survey Map MF
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