Radioactive ages of minerals from the Brown Derby Mine and the Quartz Creek Granite Near Gunnison, Colorado

Abstract

Radioactive age determinations have been made on 16 minerals from the Brown Derby pegmatite and the Quartz Creek granite. A high degree of consistency has been found among the Rb-Sr ages measured on micas and feldspars. A similar degree of consistency exists in the K-A ages of micas. These consistencies are indicative of the usefulness of measurements of the decay of K40 and Rb87 in determining relative ages of igneous rocks. U-Pb and Th-Pb ages determined from minerals obtained from the pegmatite and granite were not concordant. Different minerals showed different types of partial separation of parent and daughter elements. Comparisons of Rb-Sr and K-A ages with concordant U-Pb ages have been made on other pegmatites. The measurements are most consistent with an age of 1350±100 m.y. for the pegmatite and the granite.

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