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Location: 16 km NW from McDermitt, Oregon, United States
5470 Kietzke Lane, Suite 3000RenoNevada, United States89511
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Local GeologyThe Aurora Project area is covered by a thin layer of alluvium over lakebed sediments. These sediments are mostly tuffaceous and interbedded with Aurora dacitic flows. In some areas, the contact between the lake sediments and Aurora flows is abrupt, while in others it gradually increases in volume and thickness of dacitic flows and tuffs. The flows generally become more massive or compact near the contact with the underlying rhyolitic welded tuffs and flow domes. Cross-sections in the Aurora area illustrate the generalized geologic relationships between the different units and the variability in thickness of the units. The Aurora lavas were deposited upon an irregular surface of rhyolitic rocks, which appear in part to be intrusive based on porphyritic textures, and may represent local volcanic domes (Roper, 1979).The Quaternary alluvium is composed of a variety of alluvial, colluvial and in-situ debris consisting of volcanic boulders, cobbles and gravel derived from adjacent highlands and finer material derived from the lake sediments. The thickness of the gravels varies from 0 to more than 50 feet (15 meters), and averages about 20 feet (6 meters).The lake sediments are Miocene in age and are composed of poorly-consolidated, subaerial tuffaceous material, interstratified with fine-grained non-descript bedded layers and discontinuous lenses and nodules of chalcedony. Tuffaceous material within the lakebeds includes devitrified glass fragments and fine to coarse-grained crystal and lithic fragments. Lake sediments vary from finely laminated clay-shales, siltstones and tuffaceous sandstones, to more massively bedded rhyolitic air-fall ash tuffs (Roper, 1979). The lake sediments are up to 600 feet (183 meters) thick in the drillholes, being thickest on the north edge of the mineralized zone in a graben-like growth basin. The sediments probably originated from local volcanic vents and were deposited in moat-like basins within the caldera margins.The Aurora lava flows and tuffaceous units consist of a complex interbedded sequence of dark colored dacitic flows with vesicular to scoriaceous flow tops with some interbeds of ash. The cores of the flows are dense and black with rare plagioclase phenocrysts. The dacitic lavas contain high total iron, high calcium, sodium, and potassium and 60- 62% silica (Roper, 1979). Individual flows range in thickness from 5 to 50 feet (1.5 to 15 meters). The lava sequence contains a variety of breccia layers, which include flow breccia, laharic (mudflow) breccia, pyroclastic breccia and local fault breccia (Roper, 1979). Cumulative thickness of the Aurora lava sequence is variable, but generally is 100 to 300 feet (30.5 to 91.5 meters).Rhyolitic rocks are, at least in part, intrusive and may represent several generations of extrusive and intrusive flow dome and vent breccia events. Whole-rock chemical analyses are very similar to the dacitic rocks of the Aurora lava flows (Roper, 1979). The flow banded rhyolite may be a portion of a flow dome complex in the area. Extrusive rhyolitic welded tuffs are exposed on the margin of the project area north and east of the Bretz pits, along the mountain front marking the caldera rim. These rocks were deposited as thick ash flow layers, erupted during successive collapse periods as part of the evolution of the caldera complex (Roper, 1979).MineralizationThe mineralization at Aurora uranium forms stratabound and cross-cutting bodies in the lake sediments and dacitic flow units, forming an irregular mineralized zone approximately 5,000 feet (1,524 meters) long by 1,000 feet (305 meters) wide. The mineralized horizons range from a true thickness of a few feet to more than 100 feet (30.5 meters) thick. The mineralized beds are nearly horizontal to moderately dipping, up to 40°. The beds are spatially related to, and partially controlled by, possible growth faults or graben bounding structures, primarily on the northeast margin of the mineralization. The diamond drill core logs show that the uranium mineralization includes some primary deposition associated with volcanic and hydrothermal activities. The spatial distribution of uranium within sediments and broken, permeable zones of volcanic rocks suggests mechanically and chemically transported zones of mineralization are common. Several of the secondary or tertiary basins, within the lake sediments and graben block, show thin repeating beds of mineralization, within zones of the more permeable rocks, which are isolated by clay-rich zones. Thicker and higher-grade mineralization may indicate high angle structures that served as hydrothermal feeders or enrichment zones. Drillhole AUR_DDH-495 is the only angle core hole and confirms the approximately horizontal nature of the mineralization.Geologic analysis shows moderate and low-grade mineralization (<0.05% or 500 ppm eU3O8) has lateral continuity, while high-grade mineralization (>0.08% or 800 ppm eU3O8) is sporadic. Local feeder zones may explain this uneven high-grade distribution. High-grade areas have not been tested with angled drilling. Exploring these zones could boost the overall average grade of mineralization.The mineralized zone trends northwest, aligning with a dome of rhyolitic tuff and porphyry. The drill logs provide limited descriptions of the volcanic rocks and alteration assemblages.Mineralization is associated with the porous and permeable volcanic rocks and includes pyrite-bearing clays with uranium minerals, leucoxene, marcasite, and arsenopyrite. Uranium minerals which have been identified in various studies include uraninite (uranium oxide), coffinite (hydrous uranium silicate), phosphranylite (hydrous calcium uranium phosphate), umohoite (hydrous molybdenum uranium oxide) and autenite (hydrous calcium uranium phosphate) (Dudas, 1979b), (Dudas, 1979a) and (Roper & Wallace, 1981)).Pyrite is abundant and occurs in two forms. A coarser, crystalline variety is disseminated throughout the Bretz area and appears to be the earliest formed. Euhedral marcasite and arsenopyrite are also associated with the coarser pyrite. Fine grained, framboidal pyrite occurs in the Aurora area and is associated with uranium mineralization (Dudas, 1979b), (Dudas, 1979a). Framboidal pyrite is formed in areas rich in bacteria and organic material, these reducing conditions are favorable for the precipitation of uranium from oxidized solution. The precise identification of a source rock for mineralization remains unclear. The distribution of uranium within the more porous units indicates the remobilization of primary mineralization by oxidizing fluids. followed by lateral transport and re-deposition in flow and tuff units under reducing conditions. The assemblage of uranium and alteration minerals observed, along with textural evidence, implies the potential for colloidal mineral deposition through a relatively low-temperature aqueous mechanism (Dudas, 1979b), (Dudas, 1979a).
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