Bulletin-95—Geology and mineral deposits of the Gallinas Mountains, Lincoln, and Torrance Counties, New Mexico

By R. M. Perhac, 1970, 51 pp., 7 tables, 11 figs., 2 plates, 1 index.

The Gallinas Mountains comprise a transgressive sedimentary sequence of Lower Permian rocks intruded by middle(?) Tertiary rhyolite and trachyte laccoliths and associated bodies. Iron and fluorite-copper-bastnaesite deposits are associated with the trachyte laccolith; iron also occurs as replacement bodies in Yeso carbonates.

The Gallinas Mountains comprise Lower Permian sedimentary rocks that were intruded by middle Tertiary hypabyssal intrusives. The sedimentary sequence is typically transgressive, grading upward from Abo arkoses through continental feldspathic sandstones of the Yeso Formation into the beach or shallow-water, marine Glorieta orthoquartzites. A southward facies change into lagoonal and marine beds indicates a northward-advancing sea throughout Early Permian time. By Middle Permian, the shoreline had migrated from southern NM northward to the vicinity of the Gallinas Mountains.

The Permian beds were uplifted, domed, and faulted by middle Tertiary intrusives, primarily alkalic trachyte and rhyolite laccoliths. Despite extensive igneous activity, contact metamorphism is minor. The presence of these and a number of other similar hypabyssal intrusives in central NM mines a distinct Tertiary subsilicic alkalic intrusive province in this part of the state.

Many iron and fluorite-copper-bastnaesite deposits are associated with the trachyte laccolith. The iron occurs as magnetite-hematite replacement lodes in Yeso carbonate beds. The fluorite-copper-bastnaesite deposits are typical low-temperature epithermal veins and breccia fillings in Yeso sandstones. Despite the large number of deposits, the economic potential of the Gallinas area is limited. The better iron deposits have been almost completely exhausted, and, although fluorite, copper minerals, and bastnaesite are common, total reserves of these are small.The Gallinas Mountains form one of the larger domal uplifts in an intrusive belt in central NM.

The range is, thus, ideally suited for a study of the igneous geology and of the uplifted Permian sedimentary rocks. Associated with the intrusive activity are many epithermal mineral deposits that lend themselves to a study of the epigenetic mineralization of iron, copper, fluorite, and bastnaesite.