Bulletin-25—Mica Deposits of the Petaca District, Rio Arriba County, New Mexico

With brief description of the the Ojo Caliente district, Rio Arriba County, and the Elk Mountain district, San Maguel County, by R. H. Jahns, 1946, 294 pp., 28 figs., 26 plates, 1 index.

Wartime increases in demand for mica of strategic quality, beryllium, tantalum, columbium, tin, lithium, and uranium have focused attention on pegmatite deposits more sharply during recent years than ever before. Not only have numerous technical developments of the past few decades involved new uses for these commodities, but during the recent period of emergency these and other uses were greatly expanded. The attendant rapid increases in demand and a heavy dependence on foreign sources of supply soon confirmed the need for a careful appraisal of domestic resources of the minor pegmatite minerals. This need had been foreseen prior to 1939, when the Federal Geological Survey (GS) began a program of pegmatite investigations that attained nation-wide scope by the spring of 1942.

Pegmatite deposits in the Petaca district of northern NM have been sources of commercial mica since the 17th century, and some of the mines may well represent the oldest systematic operations for sheet mica in this country. Although an irregular but appreciable production of sheet, punch, and scrap mica has been maintained to the present time, little has been recorded concerning the geology of these deposits, the distribution and richness of mica concentrations within them, or their potentialities for future production of mica and other pegmatite minerals. A helpful background of general geology has been provided for the district by Just and several of the mines were briefly examined and described by Holmes in 1899, by Sterrett in 1923, and by Just in 1937. At no time, however, have all the more promising deposits been examined, and no geologic map of any mine in the district has been available heretofore.

The attention of the GS was turned toward the Petaca district in the fall of 1943, when a comprehensive program of pegmatite mapping and examination was begun. Detailed mapping, structural correlation, and mineralogic study demonstrate that the occurrence of mica and other minor constituents in the pegmatites follows certain rules and that a working knowledge of these rules can be applied to prospecting, evaluating, developing, and mining the deposits.

The Petaca pegmatites are remarkably consistent in their physical and mineralogical features, and appear to have a common origin. This is in accord with recent findings elsewhere in this country and in South America by members of the GS and others; their work has shown that most pegmatites, though erratic and unpredictable in detail, obey certain rules and within a given district or area can be analyzed and classified in much the same manner as many other mineral deposits. Although such classifications are basically empirical, they appear to be sound from both theoretical and practical standpoints, and tend to confirm the objections of some geologists to strong emphasis on the irregularity of pegmatite deposits with little or no recognition of broad consistencies that might be present.