Circular 121—Geochemical background values in iron-bearing rocks of Rio Arriba County, New Mexico

By D. F. McLeroy, 1972, 11 pp., 1 table, 2 figs.

A study of geochemistry of iron-bearing rocks indicate potential for additional mineral deposits in the area. Precambrian banded iron-formation occurs within the Moppin Formation and the Ortega Quartzite at Cleveland Gulch, Iron Mountain and Cañon Plaza in the San Juan Mountains. At Cleveland Gulch and Iron Mountain, magnetite selectively replaced bands in a metavolcanic rock. At Cañon Gulch Plaza the iron formation occupies a shear zone in the Ortega Quartzite, and the banding is a shearing phenomenon causing segregation of specularite and aluminum silicates. Whole rock spectrochemical analyses of the host rocks and of the iron deposits provide information on the addition and subtraction of ions during hydrothermal replacement and excellent geochemical background values. Only 26 elements were detected in one or more samples. Because the potential for hidden mineral deposits in Rio Arriba County is very good, establishing general trace element background values will greatly assist in future geochemical prospecting in this area. Mineral deposits at Cleveland Gulch, Iron Mountain, and Cañon Plaza are 17 to 30 mi south of the Colorado border within the bounds of the Carson National Forest in Rio Arriba County, north-central NM. Cleveland Gulch is 38 mi northwest of Taos and 70 mi north-northwest of Santa Fe, NM. These deposits are situated wholly within the Las Tablas quadrangle bounded by long 106º and 106º15'W and lat 36º30' and 36º45'N. No all-weather roads extend into the area of the deposits. Cleveland Gulch is on unpaved Route 111, 10 mi west-southwest of Tres Piedras, a small community on paved Route 285. Iron Mountain is 12 mi northwest of Cleveland Gulch and 2 mi north of Hopewell Lake over fair-weather roads. Cañon Plaza deposit is 8 mi south-southwest of Cleveland Gulch and 1 mi south of the community of Cañon Plaza on unpaved Route 111.

The study of the geochemistry of the iron-bearing rocks in Rio Arriba County was undertaken as part of a larger study of the geology of the Precambrian rocks and the genesis of the banded iron-formation within these rocks. During this larger study the potential for additional mineral deposits in Rio Arriba County became apparent. If ore deposits are to be found within the Precambrian of this area, it will be necessary to utilize geochemical and geophysical exploration methods because much of the area has considerable overburden. Geochemical values and the overall geological picture are presented here to assist geochemical or geophysical prospecting for hidden ore deposits in Rio Arriba County. Determining what constitutes a true geochemical anomaly in a given test area is often difficult, expensive, and time consuming. With the geochemical background values presented in this report, any geochemical prospecting data can be better interpreted to locate areas with true geochemical anomalies for more detailed exploration.