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Uranium mineralization in Miocene sediments at the Anderson Mine, 70 km northwest of Wickenburg, Arizona, is interpreted to have been volcanogenic on the basis of geologic setting, absence of uranium-depleted source rocks in the vicinity, and geologic similarities to the Aurora uranium prospect in the McDermitt caldera, Nevada.
The Anderson deposit formed in moat sediments within the McLendon caldera. The caldera is identified by a sediment-filled basin coincident with a circular, -25 mgal gravity low centered 5 km (3 mi) south of the mine. A thick apron of andesite, near-source lahar, and rhyodacite forms a crescentic outcrop pattern that partially encircles the gravity low. Ashflow tuff, interpreted to have erupted during caldera collapse, crops out approximately 30 km (18 mi) south of the mine.
Contrary to previous interpretations, the volcanic rocks of McLendon caldera are unlikely source rocks for uranium in the Anderson deposit. The lavas and ash-flow tuff from the volcano have average Th:U ratios of 4.5 and 2.4, respectively. Both ratios are close to or within the magmatic Th:U range of 2.5-5, indicating minimal uranium depletion. If the uranium did not come from volcanic rocks, it could have been provided to the sediments through hot-spring systems from a late-stage, uranium-enriched differentiated source.
The occurrence of the Anderson and Aurora deposits within caldera moat sediments strongly suggests a genetically similar, volcanogenic model. Other geologic similarities include silicified zones, fossil hot springs, thin-laminar bedding, stacked ore bodies, association of anomalous manganese and molybdenum, and the presence of carnotite and coffinite.
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