Geology and Geophysics
The Excelsior Mountains
The Excelsior Mountains are defined sharply by range front faults along the south and east sides. The trend of the faulting is north east; recent work has identified many more north east trending faults with conjugate northwest trending structures; The precious metal bearing quartz and quartz–tungsten veins in the area of the north east portion of the Black Mountain Group are thought to be associated with the north west trending structures.
Great Western Mining's District is located in the Excelsior – Coaldale Block of the major structural zone – the Walker Lane Belt (WLB), which is a complex rift zone, 100-300 km wide and 700 km long; hosts a number of bulk tonnage porphyry copper and molybdenum deposits; may extend into copper producing districts of Arizona – the WLB could have hosted more than one billion tonnes of copper resources.
The Golconda thrust fault is a major structural feature spanning the length of Nevada. The large, disseminated silver deposits that were mined at Candelaria are intimately associated with the Golconda thrust fault. The Candelaria fault extends westward into California and passes through the southern Great Western claims.
The Excelsior Mountains consist of granitic rocks of late Cretaceous age intruded into by sediments of Permian to Jurassic age. The oldest sediments are of the Permian Mina formation, present in south western end of the range, in the area of the Huntoon mine and consists of interbedded volcanogenic sedimentary rocks; chert; igneous breccia and local intrusions of mafic porphyry. Clastic rocks consist of medium to thick bedded massive and laminated sandstone consisting of: pyroxene; plagioclase; clasts of feldspar-pyroxene porphyry; thin to medium bedded feldspathic turbidite; red mudstone; pebbly volcanogenic sandstone with mud clasts; fragments of porphyry and chert.
The Jurassic Dunlap formation overlies the Mina formation, and consists of quartz sandstone; quartz chert feldspar sandstone; volcanic and chert clast breccia; red mudstone.
Clastic units are overlain by volcanic rocks, including greenstone, greenstone breccia, felsite and tuffs with interrelated tuffaceous sandstone. Erosional remnants occupy approx. 160 hectares north of Marietta and approx. 65 hectares west of Teels Marsh, in the eastern portion of the Black Mountain Group.
The youngest rocks exposed in the Excelsior Mountains are intermediate volcanics of Tertiary age, mainly consisting of Andesite flows; tuffs; breccia characterised by the presence of phenocrysts of hornblende, augite or plagioclase and an absence of megascopic quartz.
Basalt flows of Tertiary age are found in close proximity to the range front fault that defines the eastern flank of the range. Quaternary deposits consist of alluvium and pediment gravels and the evaporates of Teels Marsh – these consist of: halite and various borax minerals.
Black Mountain Claim Group
Granitic rocks in north east; 30% consist of medium to coarse grained leucocratic granite, made up of: 30% quartz; 31% plagioclase; 36% potash feldspar; 3% mafic minerals.
Granitic rocks in north west; 15% consist of pinkish grey medium to coarse grained porphyritic biotite granite – mineral composition: 26% quartz; 31% potash feldspar; 37% plagioclase; 6% mafic minerals.
Granitic rocks in western end of the range, near the Huntoon Mine, consist of both the Whiskey Flat granite and the Huntoon Valley granodiorite the latter unit consists of light to medium grey medium to coarse grained equigranular to porphyrytic hornblende granodiorite, mineral composition 19% quartz; 17% potash feldspar; 49% plagioclase feldspar; 15% mafic minerals. Granodiorite is distinguished from adjacent igneous rocks by biotite crystals as large as 8mm in diameter and the formation of grey topsoil. K-Ar ages of 86 million years (m.y.), 100 m.y. and 101 m.y. have been obtained from the samples collected in the area.
The results of airborne magnetic and land based bouguer gravity surveys of the Walker Lake region were published in the mid 1980’s by the Nevada Bureau of Mines & Geology – Great Western obtained copies of the maps from that agency.
The aeromagnetic mapping shows a north east – south west aligned series of high magnetic anomalies parallel to the north west side of Little Huntoon Valley.
The anomaly located immediately south west of the Huntoon Mine is believed to be particularly significant and indicative of a shallow intrusion that may be mineralised with copper. A similar pattern of high magnetic anomalies exists proximate to the south east boundaries of Huntoon and Little Huntoon Valleys.
Prominent magnetic high anomalies are mapped coextensive with the Marietta, Moho Mountain and Camp Douglas Mining Districts. High gravity anomalies are coextensive with the magnetic highs located at Camp Douglas and 13km north west of Marietta is strongly indicative of buried igneous intrusive. Gravity low in Little Huntoon Valley suggests presence of deep alluvium.
Great Western Mining contracted with Christopher Ludwig and Image 2 Map Services, Inc. to complete a more detailed aeromagnetic survey coupled with rock alteration studies in October 2010. Image 2 Map Services identified areas of hydrothermal alteration by spectral processing of ASTER high altitude infrared imagery.
Aeromagnetic study and ASTER study succeeded in identifying numerous linear and circular features – the circular features could be volcanic vents or breccia pipes. 16 anomalies were identified of which 8 are of interest to Great Western: 2 of the targets have potential for hosting Candelaria-style disseminated silver deposits; 4 of the targets are permissive for epithermal precious metals; the 7th target suggests a south west extension of the skarn copper deposit at the Huntoon Mine; the 8th expands a skarn hosted copper target in the north west portion of the Black Mountain Claim Group.