Abstract

The paragenesis of the asymmetrically zoned, Li-rich, topaz-bearing portion of the Brown Derby No. 1 pegmatite, Gunnison County, Colorado, has been reconstructed on the basis of observed textural relationships in the light of the recent model for pegmatite genesis by Jahns and Burnham (1969).

The coarse-grained, K-rich “hood” formed in the presence of an aqueous fluid phase during the crystallization of the wall zones; topaz was originally abundant in the hood assemblage. The finer-grained texture of the footwall unit suggests crystallization directly from the silicate melt. Large topaz crystals concentrated at the base of the core unit imply gravitational settling of topaz in the melt. Lepido-lite and cleavelandite crystallized later from the aqueous fluid phase, replacing pre-existing assemblages and forming the observed core unit. Topaz was armored and preserved by the residual melt trapped largely at the base of the core unit which crystallized to form multiple quartz pods.

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