Circular 202—Geochemistry of micas from Precambrian rocks of northern New Mexico

By J. L. Post and G. S. Austin, 1993, 20 pp., 7 tables, 6 figs.

Average concentrations of 10 minor elements were determined for 53 mica samples from eight mining districts, areas, and isolated mines in northern New Mexico. Minor-element composition of muscovites from the entire Petaca mining district is uniform, suggesting that all pegmatites in that district formed metasomatically during regional metamorphism. In five of the remaining seven areas muscovite composition is similar, hinting of similar emplacement. The exception is the Harding pegmatite, which appears to have formed by a syntectonic emplacement. Differing minor-element concentrations among the eight areas suggest that the sources were not interconnected. Minor-element geochemistry of micas may be useful in the study of the origin and sources of pegmatites.

Regionally, California muscovites exhibit more uniform minor-element composition than do NM muscovites, but a regional shift in TiO₂ and BaO concentrations in California could not be identified in New Mexico muscovites. However, when compared with the isolated and compositionally complex California Isinglass pegmatite, NM pegmatitic muscovites show little minor-element variation within each deposit. In general, the MnO and ZnO concentrations of NM muscovites are much higher, with weight percent of MnO up to 0.45 and ZnO up to 0.12, in contrast with California values of 0.06 and 0.02. Substitution of smaller ions in New Mexico2M1 muscovites has resulted in a smaller average unit cell with basal spacings of about 19.920 Å, as compared with the standard 20.058Å. Little biotite is present in pegmatites of northern NM, but analyses of the two biotites included in this geochemical study show a lower equivalent FeO content than in California biotites. Minor-element geochemistry of micas may be useful in the study of the origin and sources of pegmatites. Variations in elemental composition may indicate the same or different sources and/or times of emplacement and may help in the identification of extensions to known pegmatites and in the discovery of new pegmatites.