Summary:
The Kilgore deposit is a zoned, low sulfidation (LS) epithermal hot spring precious metals (Au, silver [Ag]) deposit the result of caldera-related volcanic activity. These deposits are commonly bulk-tonnage, lowgrade, and amenable to open-pit mining. Numerous scientific articles have been written and published on this deposit type concerning its origins, physical, chemical, and geological settings, recognition criteria, major- and trace-element geochemistry, zoning, alteration types, ore mineralogy, ore grades and distribution of ore, and mining characteristics. Models are described in papers by Buchanan (1981), Silberman (1982), and Berger (1985), among others, and the reader is referred to these for more information on the subject.
Gold mineralization at Kilgore occurs within two suites of receptive host rocks: 1) in rocks of volcanic or subvolcanic origin, including the Tlt, Tap, and the sub-vertical granitoid dikes, dike swarms, and granite to granodioritic bodies that intrude it, and 2) the sedimentary turbidites composed of arkosic sandstones and siltstones, and carbonaceous shales of the Aspen Formation.
Gold mineralization in the volcanic and related intrusive rocks contains high grade zones as a result of weak to moderate vein development and open space fracture-fill, together within a broad, low grade halo of disseminated gold within variably silicified and argillically altered rocks. Gold content appears to decrease rapidly to lower grades (less than 50 to 100 parts per billion [ppb] gold [Au]), with corresponding decrease in quartz or quartz adularia as silicification and increase in argillic alteration. Exceptions occur in strongly oxidized rock near the topographic surface where strong to pervasive iron-oxide, yellow-orange to brown staining is accompanied by high gold grades.
Mineralization in the volcanic and associated intrusive rocks accounts for an estimated 85% of the known mineral resource, with the remaining 15% occurring in the underlying Aspen Formation sediments.
The Kilgore deposit is a zone of mineralization approximately 2,600 feet (800 meters) long, 2,000 feet (600 meters) wide, and 1,070 feet (325 meters) deep from ground surface to the maximum inferred mineral resource depth. Mineralized intercepts generally average 130 feet (40 meters) and range up to 300 feet (90 meters) in thickness in the Mine Ridge core and North Target areas.
Significant mineralized zones within the Project area are typically associated with structures and the mineralized halos around them in the surrounding rocks. The geology and apparent detailed mineralization controls vary from one area to the next. Mine Ridge comprises the core of the Kilgore deposit and the bulk of the gold mineralization contained within it. Major geologic controls include the northwest-trending fault swarms and the northeast-trending radial fault structures. Gold mineralization is spatially associated with the intrusive rocks and their contacts with porous and permeable lithic tuffs. Some of the higher-grade mineralization is localized in sub-vertical to vertical fissure, shear, and fault/fracture zones. The Tlt hosts significant disseminated mineralization that forms more extensive zones away from the dike contacts.
The system of northwest-trending faults in the areas of the north and B roads (locally identified at the north road/rhyolite fault, northwest B-Road fault, northwest central fault, and northwest western fault) may represent a fault or shear zone several hundreds of meters wide and comprised of several sub-parallel structures. These faults and the cross-cutting Cabin, Mine Ridge, and other east-northeast trending faults contribute significantly to the overall distribution of mineralization.
The upper 100 to 200 feet (30 to 60 meters) of the Aspen Formation serves as a major host environment to gold mineralization in the Mine Ridge area, especially at the upper contact of the Aspen Formation with the overlying Tap. Here the unit is variably silicified, displaying quartz microveining, development of iron oxides along micro fractures, oxidation of sulfides, the presence of pyrite stringers, and chlorite, ankerite alteration. Quartz veins and sheared quartz vein zones cutting Aspen rocks, as well as the edges and margins of mafic dikes intruded into the Aspen Formation, all serve as environments for the deposition of higher-grade gold values.
Sulfide and precious metal mineral species identified in core from the Mine Ridge area include pyrite, electrum, native gold (some visible), galena, arsenopyrite, sphalerite, stibnite, cinnabar, naumannite (Ag2Se), aguilarite (Ag4SeS), argentite-acanthite, chalcopyrite, wolframite (ferberite), and rare pyrargyrite (Ag3SbS3) (Benson C. , 1986; Otis Gold Corp., 2012). Benson’s (1986) scanning electron microscope and energy dispersive X-ray spectrometry studies on heavy mineral concentrate grains from historic Bear Creek drill samples in the area found additional mineral species, including an unknown mineral composed of Ag-Pb-Bi- Se-S, galena with minor selenium and silver, gersdorffite (NiAsS), and cobalt-nickel-pyrite ((Ni, Co, Fe)S2) with minor chalcopyrite. All of the silver minerals, electrum, and gold occur as discrete grains and within pyrite. Panned concentrate studies of gold grains conducted by Hazen Research, Inc. (1995) for Echo Bay found spongy, lacy, rectangular, splinter, and amoeboid morphologies with rich yellow color and sizes in the 25- to 150-micron range.