Uranium, Thorium, Rare Earths and Other Metals in Cretaceous Age Basement Rocks: A Source for New Uranium District in Tertiary Age Sediments of the McCarthy Basin (A New Middle Cretaceous Age Impact Crater?), Seward Peninsula, Alaska - 2018

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

M. D. Campbell, et al., reported in 2018 that mineral (uranium and REE) exploration conducted during the summers of the latter 1970s and early 1980s by personnel of United Resources International (URI) on behalf of the Omega Energy Corporation confirmed the existence of mineralized zones containing anomalous uranium, thorium, and rare earths, and other metals in the Kachauik area and Death Valley area of the Eastern Seward Peninsula of Alaska.

According to the policy recently announced by the senior author (Campbell, 2017), this paper is based on data from the field work produced by the senior author and associated URI personnel some 40 years ago and the work continue today. The geological mapping, sampling, and resulting analytical data, considered in the light of the detection limits, precision and accuracy of the analytical methods available at that time, remain relevant to mineral exploration today. Substantial work has been conducted on the metamict mineral referred to as allanite, and on the regional geology and geophysics over the past 40 years. This new information has also been incorporated in this paper.

Field reconnaissance and sampling, and petrographic, chemical, XRD, microprobe, cathodoluminescence and metallurgical analyses conducted in the late 1970s indicate that the mineralized zones occur within a composite alkaline intrusive complex related to, but separate from, the Darby pluton of the Darby Mountains area. The areas sampled contain uranium that has been leaching into the groundwater below wherever uranium is available. The rocks sampled also contain thorium and rare-earth elements associated with allanite and common accessory minerals. The major zone of mineralization examined appears to be related to prominent phonolite dikes that occur along its margin in monzonitic country rock. Metasomatic introduction of uranium, thorium, and rare earths related to dike intrusion is postulated as the mechanism of metallogenesis. Areas with associated faulting and favorable host rocks, e.g., contact metamorphosed rock within fractured carbonate and graphitic rocks, as well as other favorable rock types, occur in the immediate area.

Of the total uranium contained in the whole-rock allanite-rich samples, approximately 88% is in one or more leachable mineral phases. Uranium present in one or more of the mineral phases is concentrated in the < 3.3% specific gravity fraction of the sampled examined. Approximately 85 to 90% by weight of the total uranium present in the rock occurs in this fraction, which averages approximately 0.15% cU₃O₈.

Uranium occupies lattice or inter-lattice positions within hornblende and feldspar and/or can also be present in separate uranium-bearing phases as minute inclusions within essential, varietal, and accessory minerals. The remainder of the uranium is present in the greater-than 3.3% specific gravity fraction, which also includes abundant allanite, the accessory minerals zircon, apatite, monazite, sphene, and other heavy minerals, such as siderite, barite, and magnetite. As a group, the heavy fraction contains approximately 10 to 15% by weight of the total uranium present in the whole rock.

Based on the analytical work completed, chemical uranium appears to be in equilibrium with its radiogenic daughter products. And based on the mineralized samples examined to date, the rare-earth content consists predominately of the cerium (heavy) group. The distribution of the rare-earth elements is similar to uranium (i.e., the rare-earth elements occur in both specific gravity fractions. However, some segregation is apparent between the rare earths and uranium in the heavy fraction. The rare-earth elements preferentially occur within the allanite subfraction, while uranium is associated with other minerals of very low magnetic susceptibility. Strong geochemical anomalies exist within within and around the areas in the Kachauik area, including bismuth, lead, niobium, molybdenum, copper, nickel, chromium, vanadium, lithium, fluorine, scandium, silver, cesium, tin, and arsenic, among other elements defined as threshold anomalies. In general, the data indicate that the region might be a new metallogenic province.

Interpretations of the geochemical and geophysical data presently available also reveal the possibility that the alkalic intrusive bodies might also include carbonatites, which would, if confirmed, expand the potential for the occurrence for other classes of mineralization as well. Based on the anomalous uranium content of the intrusive rocks, the plutons are also likely source rocks of leachable uranium to the groundwater system in a small, closed Tertiary basin, i.e., McCarthy Marsh, (created initially by an impact crater?), which is some 30 miles in diameter and is covered by tundra underlain by as much as 16,000 feet of fluvial and paludal sediments and lignite dating from early Tertiary to recent. The eastern part is underlain by a prominent dipole magnetic anomaly with a likely source in the shallow basement rocks below.

The potential has been recognized for the likely presence of classical sedimentary uranium roll-front occurrences west of the northern pluton evaluated by URI personnel, designated as the Dry Canyon Pluton. Nearby and to the northeast, a smaller basin (i.e. a southern extension of Death Valley, a subsidiary impact crater?) contains uraninite in block-faulted Tertiary sediments along its southern boundary, which was originally discovered by geologists of the Houston Oil & Minerals Corporation (HO&M) in 1977, and anomalous base metals in much older faulted rocks to the north around the northern periphery of Death Valley east of the Windy Creek pluton. Death Valley fluvial sediments may also offer a suitable environment for the occurrence of uranium in the form of roll fronts, the classic type of uranium occurrence in Tertiary sediments. 

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