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Description
With high grade uranium deposits diminishing and demands for uranium increasing, the abundant low grade vein deposits of Nevada look more promising. Anomalously radioactive areas occur in the Cretaceous quartz monzonite (Toquima pluton) one mile east of Round Mountain, Nevada. A scintillometer grid was correlated with geologic variables noted in an approximate twelve square mile area. Thin sections, colorimetric uranium assays, auto radiographs, and a micro-probe were used to identify and correlate radioactive mineralization. Surface and Subsurface workings were studied for weathering variations. Primary radioactive accessory minerals apatite and zircon were nonsystematic in their occurrence throughout the area studied, but were always in greater abundance in anomalous zones. Radioactive monazite and allanite (?) occurred in trace amounts only in anomalous zones. All four minerals occur either disseminated or associated with opaque/mafic clusters of brookite and dravite. Anomalous radioactivity is due primarily to autunite, a secondary uranium bearing mineral. It occurs intersticially and within sericitized potassium feldspars. Sericitization of potassium feldspars is greatest within fault and shear zones; however, autunite is not necessarily an indicator of a primary uranium deposit. Hydrogeochemical studies indicate autunite precipitates in water with a very low carbonate and sulfate content (approximately 0.2 meq/l.); therefore, autunite mineralization is caused by local hydrogeochemical conditions and not by an exceptionally high uranium content. The Toquima pluton appears to be a typical quartz monzonite averaging 1.9-4 ppm uranium. Background radiation averages 70 c.p.s.. Fault and shear zones range from a low of 35 ppm U308 to a high of 6,100 ppm U308. An average granite averages 4 ppm U308. Fault and shear zones are not traceable on the surface for any great distances due to poor exposure.
