The presence of metamict multiple oxide and silicate minerals (“radioactive blacks”) in many stream placers in west-central Idaho has long been established (e.g. Shannon, 1922), and in 1952—1954 the geology of these deposits was studied in detail (Mackin and Schmidt, 1956; summarized in Heinrich, 1958). The radioactive placer minerals have been derived chiefly from the quartz monzonitic facies of the Idaho batholith, in which they occur locally as accessory species. “Segregations” of quartz monzonite containing marked concentrations of these minerais range in size from a few inches to a few tens of feet and are megascopically identical with enveloping quartz monzonite that has a much lower content of these species.

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