New Mexico Mineral Symposium — Abstracts

Fluorspar deposits occur on and near Cross Mountain, near Chise, Sierra County, New Mexico. The Chise fluorspar district lies within an inlier of upper Paleozoic rocks nearly surrounded by Tertiary extrusive rocks, composed primarily of flows and tuffs.

The bulk of the sedimentary rocks exposed in the area are Pennsylvanian limestones of the Magdalena Group and Permian sandstones, limestones, and shales of the Abo and Yeso Formations. Pliocene and Pleistocene fluviatile deposits have also been mapped in the area. Igneous rocks include Tertiary intrusive diabase, monzonite, and syenite, as well as late Tertiary andesites, flows, and tuffs.

Cross Mountain is a horst, which has been elevated by movement along two north-trending faults. The easternmost fault (Montoya fault) is more mineralized, and several springs issue along it. There probably were several periods of movement along these faults.

Fluorspar occurrences in the Chise district include jasperoid veins and mantos in Pennsylvanian limestones, open-space filling in brecciated zones along faults and near intrusive contacts, and replacement mantos in Permian rocks. It is postulated that the basement rocks along the eastern margin of the Cordillera have a high fluorine content and that Tertiary vulcanism initiated hydrothermal fluids, which rose along Basin and Range fractures parallel to the Rio Grande rift system. The Chise district is believed to be such an occurrence.

Prospecting guides in the area include the presence of jasperoid, fluorite casts, caliche horizons, and certain shale marker beds.