Bulletin-19—Manganiferous Iron-Ore Deposits Near Silver City, New Mexico

By L. P. Entwistle, 1944, 70 pp., 11 figs., 10 plates, 1 index.

For many years the potential importance of the manganiferous iron ores at Boston Hill, near Silver City, NM, has been recognized. As recent economic developments have greatly increased the demands for manganese and manganiferous iron ores, these deposits have become of commercial value. Detailed geologic information on the area, with its direct relation to ore localization, is scant; therefore the writer made a survey of the manganiferous iron-ore deposits, and also of the related silver deposits in the Chloride Flat mining district, about one mile north of Boston Hill. Results of the survey make up the chief part of this report.

A detailed study of a much larger area than that covered in this report is necessary for full comprehension of the broader geologic features. Previous work by competent geologists appears to have correlated adequately the sedimentary rocks and the structural features of the Silver City quadrangle. Many interesting possibilities in the chemistry of the hypogene and supergene ore solutions can't be presented fully at this time because of limited library facilities and insufficient experimental data.

Field work was started in the summer of 1936, when the writer mapped and studied in considerable detail the surface and underground geology of the most productive part of the Chloride Flat mining district. Field work was then continued intermittently through 1942 in order to complete mapping the Boston Hill mining district and to correlate its geologic features with those at Chloride Flat. Much of the mining at Boston Hill has been near the surface, and many of the deeper workings are now flooded; consequently most of the time was spent mapping and studying the areal geology. Topography and geology were mapped simultaneously, with transit and stadia. The original field maps were made on a scale of one inch to 200 ft.

The report outlines in a general way the most favorable areas for prospecting, and describes the geologic structures that have controlled or localized the deposition of the ore minerals. No attempt has been made to show developed ore reserves with indicated tenor of ore, or to discuss metallurgical problems which might arise if selective concentration of the ores should be attempted.

The Boston Hill mining district lies at the south extremity of the Silver City Range. The district, which is a triangular area covering about 2 mi², lies in Grant County, NM. It is situated immediately southwest of the town of Silver City, the county seat. Silver City was the first town to be incorporated by the Territory of NM, and has been the county seat sine December 29, 1871.