New Mexico Mineral Symposium — Abstracts

The Petaca mining district in Rio Arriba County of north-central New Mexico includes probably the oldest systematically mined group of pegmatites in North America. Muscovite has been produced from the district since the 17th Century. More recently, muscovite as well as minor amounts of beryl, columbite-tantalite, and quartz have been mined sporadically from 1870 to the present. The pegmatites of the district are complex and zoned. The essential minerals are perthitic microcline, quartz, albite, and muscovite. Relatively common accessory minerals include garnet, fluorite, columbite-tantalite, monazite, bismutite, beryl, and samarskite. In addition, 36 other accessory minerals were identified from the district by R.H. Jahns in 1946. Two theories have been proposed for the genesis of the Petaca pegmatites. R.H. Jahns has proposed that they are due to the late-stage crystallization of a volatile-rich granitic magma associated with a larger granite body. The postulated large granite source does not outcrop in the area. The other possible genesis, as proposed by R.L. Gressens in 1967, is from an accompanying intergranular alkali-chloride fluid phase characterized by the reaction: qtz + musc + (K+, Na )--perthite + H+. The necessary beryllium and rare earth elements are presumed to have been introduced by contamination from hydrothermal solutions. This theory is less tenable than genesis of the Petaca pegmatites from a deep-seated granitic intrusive.