Circular 7—Outlook for Further Ore Discoveries in the Little Hatchet Mountains, New Mexico

By S. G. Lasky, 1940, reprinted from Economic Geology, Vol. 33, No. 4, June-July 1938, 31 pp., 6 figs.

In 1934, the NM Bureau of Mines and Mineral Resources and the U.S. Geological Survey began a joint investigation of the geology and ore deposits of the Little Hatchet Mountains in Grant and Hidalgo Counties, NM. Inasmuch as it is now nearly 6 years since the investigation was started, and it may be two or three years more before the report is published, it appears advisable to release to the prospectors, mining men, and citizens of the state all available information on the district.

The Little Hatchet Mountains contain two mining districts, the Eureka silver-lead-zinc district and the Sylvanite gold district, the deposits of each being associated with a mass of monzonite that intrudes Lower Cretaceous sediments. The same formations crop out in both districts, having been duplicated by a large post-ore fault, and the two monzonite masses and their accompanying mineralized zones lie at essentially the same stratigraphic position in the two fault blocks. The deposits of the Eureka district are mesothermal whereas those of the Sylvanite district are hypothermal, but the two groups are mineralogically similar in many respects.

As the two monzonite exposures are several miles apart, the natural inference is that there are two separate intrusions. Contrary to this inference, the evidence of structure, mineralogy, and petrology indicates that the two monzonite masses are faulted parts of the same body and that the mineralized areas of the Eureka and Sylvanite districts were originally continuous and zonally related, the original igneous mass having been a flat-lying sill-like streamer, 7 mi or more long, that was bordered by a contact-metamorphic halo and that formed the core of a zone of mineralization.

The economic implications of this interpretation are three-fold: (1) the mineralized zone is limited in thickness and is restricted, like a bedded deposit, to a particular sedimentary horizon; (2) the area between the Eureka and Sylvanite districts, hitherto considered barren, should contain mineralized ground at variable depths below the surface; and (3) the deposits change along the trend of the zone from the gold deposits of the one district to the silver-bearing base-metal deposits of the other. In structure and size, the deposits in the hidden parts of the mineralized zone probably are similar to those already known.