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Hansen, Taylor Ranch, Boyer, Noah, High Park, Picnic Tree |
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Colorado, USA |
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U
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Hansen/Taylor Ranch group of sandstone-hosted uranium deposits, which include the Hansen, Taylor, Boyer, Noah, High Park and Picnic Tree deposits are located ~140 km SE to SSE of Denver and 35 km NW of Canon City in the Tallahassee Creek District of Colorado, USA. This district was the focus of several small scale uranium mining operations that produced ~225 t of U3O8 at grades of 0.20 and 0.30% U3O8 between 1954 and 1972.
These deposits are hosted within Eocene sedimentary rocks that unconformably overlie an unevenly eroded basement of 1.7 Ga Palaeoproterozoic schists, gneisses and granodiorite, and 1.4 Ga Mesoproterozoic quartz monzonite. Exposures of intervening Palaeozoic and Mesozoic sediments are virtually absent within the district, although they are present nearby. Uplift and associated faulting formed grabens and palaeovalleys where the Tertiary sediments were deposited and the uranium mineralisation concentrated.
The oldest preserved Tertiary rocks known in the Tallahassee Creek District are widely scattered remnants of the Echo Park Alluvium, which is primarily composed of Precambrian detritus, initially deposited in depressions and subsiding basins above a widespread Eocene erosion surface of low relief that covered most of south central Colorado (Epis et al., 1976). Thick sequences of Echo Park Alluvium were developed, consisting predominantly of poorly sorted colluvium as sheet-wash and fanglomerate facies material intertongued with fluvial facies (~5 to 30% of the sequence). The fluvial deposits were deposited in channels, and contain carbonaceous material, grading upward from a lower conglomerate, through a middle fairly well sorted pebbly sandstone, to an upper clayey sandstone and mudstone unit (Cyprus Mines, 1980). These sediments were largely deposited within the Echo Park graben, a complex subsidence structure controlled by major strike-slip faults that progressively developed during the deposition of the Echo Park Alluvium (Chapin and Gather, 1981; Epis and Chapin, 1974). The graben extends from Echo Park near the Arkansas River, northwestward for about 70 km to the vicinity of Hartsel in Park County (Scott et al., 1978). The graben is about 8 km in width at its widest point. The Tallahassee Creek uranium district lies in the southern part of this graben, which appears to have been partly or completely detached and moved relatively westward along a strike-slip fault. The Echo Park Alluvium ranges from 0 to 350 m in thickness, and have been assigned an age of late early to early middle Eocene (50 to 45 Ma) on the basis of palynomorph assemblages.
The Echo Park Alluvium is overlain by rocks of the Thirtynine Mile volcanic field which commenced with the emplacement of ~30 m of early Oligocene (34.7±0.7 to 36.2±0.8 Ma) rhyolitic ash-flows of the Wall Mountain Tuff. The tuff contains phenocrysts of sanidine and is is mostly devitrified, but is partly black vitrophyre (Epis and Chapin, 1974). The distribution of palaeovalleys suggests that the Wall Mountain Tuff was sourced to the west, possibly near Mount Aetna in the Sawatch Range (Epis and Chapin, 1974). A period of erosion ensued, during which time major palaeochannels were incised into the underlying rocks and filled by sediment of the 50 to 100 m thick Tallahassee Creek Conglomerate. The main development of this unit contains boulders up to 5 m in diameter in an arkosic groundmass, and occurs within a roughly meandering channel that crosses the Echo Park graben in a southeasterly direction. A substantial quantity (5 to 30 m) of "bentonitic" montmorillonite and kaolinite altered volcanic ash and andesitic debris (subangular to angular boulders of andesite, locally dated at 34.3±1.5 Ma) are interbedded in the upper part of the conglomerate from early local vent eruptions of the overlying Thirtynine Mile Andesite lower member.
Continued eruptions of andesite, from scattered vents and breccia cones, formed the 100 to 250 m thick Thirtynine Mile Andesite lower member composed of a coalescing sheet of laharic and flow brecciated material. In the north, this volcanic activity followed the emplacement of intermediate composition domes, flows and pyroclastic rocks, where the Thirtynine Mile Andesite lower member is partially coeval with, and older than, hypabyssal rhyolitic to andesitic domes and andesitic to basaltic dykes.
The ~400 m thick, 34.1±1.1 Ma Thirtynine Mile Andesite upper member is mainly composed of andesite flows that appear to represent the remnants of a composite stratovolcano responsible for the latter volcanic rocks described above (Epis and Chapin, 1968). This volcanic activity apparently disrupted the northern end of the Echo Park graben complex. Within and to the west of the Tallahassee Creek district, a sequence of major ash-flow tuffs, sourced from the west, were deposited over the Thirtynine Mile Andesite lower member from 34 to 29 Ma (Epis and Chapin, 1974). The most recent activity is represented by remnants of the Fear Creek conglomerate (27.8±0.6 Ma) and Miocene (18.9±1.2 Ma) bimodal volcanic rocks (Epis and Chapin, 1974).
Uranium deposits within the district are of two types, hosted by the Echo Park Alluvium or the Tallahassee Creek Conglomerate respectively.
Mineralisation within the Echo Park Alluvium has a tabular geometry, and is characterised by the presence of locally carbonaceous fluvial sediments resting unconformably on the much older igneous and metamorphic basement, capped in turn by generally thick sequences of volcanic and/or volcaniclastic rocks. Uranium occurs as uraninite, coffinite, and meta-autunite, associated with carbonaceous material and pyrite. The Echo Park Alluvium contains 80 to 90% of the known uranium resources in the district, although the majority of uranium occurrences occur within the Tallahassee Creek Conglomerate.
Mineralisation within the Tallahassee Creek Conglomerate occurs in bentonitic tuffaceous sediments, as crudely tabular or lenticular bodies, where uranium appears to have been localised by accumulations of fragmental carbonaceous material associated with diagenetic pyrite. Pitchblende with minor coffinite are the primary ore minerals reported, while coatings and fracture fillings of autunite associated with hematite, limonite, and gypsum occur as oxidation products.
The Hansen deposit, the largest of the resources within the Echo Park Alluvium, is ~1400 m long x ~500 m wide, and is hosted by a flat-lying sequence of clayey sandstones, sandy mdustones, and pebble to boulder conglomerates in the lower portions of the Echo Park Formation, with the high grade portion of the deposit being from 9 to 45 m in thickness (Black Range Minerals, 2012). The depth to the mineralised zones varies from 150 to ~190 m below the surface. The orebody is reduced, but shows alteration of feldspars to clay throughout and is surrounded by oxidised sediments, with at least some deposition having taken place in roll-like bodies with uranium enrichment along redox contacts (Cyprus Mines, 1980; Chenoweth, 1980). Carbonaceous material within the host rocks at the time of ore deposition appears to have been the prime factor in concentration of the uranium. This material mostly comprises wood fragments deposited in fluvial channels, but may also have included carbonaceous palaeosols. Pyrite is commonly found in association with the carbonaceous material. The presence of impervious beds or facies that restricted and channelled circulation of oxidising waters carrying uranium is another important factor in the deposition of uranium, but also in preventing the destruction of the deposit and reductant which may have occurred if circulation had been unrestricted (Dickinson, 1984).
The preservation of the deposits is affected by the sluggish ground-water movement through the host rock and by the large amount of carbonaceous material in those rocks, maintaining reducing conditions in the orebodies and, consequently, the immobility of the uranium. The present uranium content of groundwater is as much as 900 ppb (Chenoweth, 1980). The limited groundwater movement was apparently the result of the fault offset and isolation of the southeast end of the graben, hindering the efficient discharge of ground-water into the Arkansas River drainage system.
It is thought that the overlying Wall Mountain tuff was the source of the uranium minerals that were dissolved into the groundwater and transported through the Echo Park formation. The carbonaceous material then acted as a reducing agent allowing the uranium mineralisation to be concentrated and deposited in the sedimentary sandstone layer.
JORC compliant resources for the Noah, Hansen, North West Taylor, Boyer and Picnic Tree deposits are (Black Range Minerals, 2012):
At a 250 ppm U3O8 cut-off,
Indicated resource - 28.93 Mt @ 0.062% U3O8
Inferred resource - 40.06 Mt @ 0.058% U3O8
Total Indicated + inferred resources - 68.99 Mt @ 0.060% U3O8 for 41 240 t of U3O8.
At a 750 ppm U3O8 cut-off,
Indicated resource - 7.71 Mt @ 0.121% U3O8
Inferred resource - 8.86 Mt @ 0.119% U3O8
Total Indicated + inferred resources - 16.58 Mt @ 0.120% U3O8 for 19 890 t of U3O8.
This summary was based on Hon, 1984 (USGS), Dickinson, 1984 and Black Range Minerals website in 2012
The most recent source geological information used to prepare this decription was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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