Studies of textural relationships, color, internal features, and morphology of zircons in four samples of gneiss from the southern Bighorn Mountains indicate that two distinct generations of zircon are present. Older dark-colored, strongly zoned, inclusion-bearing, commonly subrounded zircons are mantled, partly or completely, by younger colorless, unzoned or weakly zoned, largely inclusionfree, subhedral zircon of high clarity. The younger colorless zircon is also represented by subordinate amounts of discrete core-free crystals which occur along with the dark zircons in several samples.

The morphologic characteristics of the dark, pre-metamorphic zircons, from two of the samples in which these zircons exhibit little or no later alteration effects, tentatively suggest a sedimentary parentage for the gneiss. The modes of occurrence and character of the colorless zircon suggest formation by solution of the older zircon, under alkaline conditions, and redeposition during the episode of metamorphism represented by the gneiss.

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