Circular 54—Magnetite taconite rock in Precambrian formations in Rio Arriba County, New Mexico

By W. E. Bertholf II, 1960, 24 pp., 8 tables, 9 figs., glossary.

In the summer of 1959 it became evident that a Precambrian magnetite taconite formation of unknown extent occurs in the Moppin metavolcanic series in Rio Arriba County, New Mexico. A geologic map of the Las Tablas quadrangle, the area within which this formation was observed, is presented by Barker (1958). Exploratory dip-needle and magnetometer traverses were made, and sparsely distributed outcrops and float rock of the magnetite taconite rock were examined in secs. 19 and 29 T29N R7E, and sec 29 T28N R8E. The aforementioned localities are on the northeast limb of a synclinal structure and are separated by approximately 10 mi. Lower Precambrian formations continue to the southeast on the same structural trend.

The magnetite taconite formation examined is within the Carson National Forest. Samples were collected in the SW ¼ sec 29 T28N R8E, and the results of the study to date are presented hereinafter. Field work was discontinued after November 29, 1959 because of frozen ground and deep snow on the back roads.

With the use of a dip needle of the Lake Superior type, exploratory traverses were made in the sections in which Precambrian magnetite taconite rock formation was thought to occur. The results of the dip-needle survey indicated that an exploratory magnetometer survey probably would yield worthwhile data, because the area is covered with trees and bushes, and the pattern of rock outcrops leaves large areas to conjecture. It is common experience that Precambrian iron-rich formations weather down more rapidly than other Precambrian rocks. In preparation for the magnetometer survey, a suitable area was sought for a magnetic base station, and for a traverse to establish the regional level for nonmagnetite rocks.