By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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Hidalgo County, in the southwest corner of the State, was part of Grant County until 1920. Its chief mineral products are copper, gold, silver, and lead; through 1952 it is credited with a gold production of about 227,000 ounces. More than nine-tenths of this production has come from the Lords-burg district, which has yielded gold as a byproduct of base-metal ores.
LORDSBURG DISTRICT
The Lordsburg district is 3 to 8 miles south-southwest of the town of Lordsburg at the north end of the Pyramid Mountains. It includes the Pyramid camp on the south end of the district and the Virginia (Shakespeare) camp on the north end. Copper has been the most important metal mined, and gold and silver have been valuable byproducts.
Recorded gold production of the Lordsburg district from 1904 through 1959 was about 223,750 ounces, of which about 189,000 ounces was produced before 1933. Prior to 1904 the total production of silver, copper, lead, and gold from the district had a value of about $500,000 (Lasky and Wootton, 1933, p. 72), but the amount of gold has not been ascertained.
Prospecting for silver began about 1870; however, the major early interest in the area was generated after it had been salted with diamonds. The ensuing stampede resulted in discovery of a few silver deposits, but development lagged until the copper potential of the area was considered. After several years of sporadic exploration, the Emerald vein was developed in the early 1900's, and by 1932 workings on this vein were the deepest in the State. The Emerald vein yielded nine-tenths of the ore mined in the district (Lasky, 1938, p. 25-27). Activity continued at a moderate rate through 1959.
Rocks of the Lordsburg district consist mainly of basalt, intrusive rhyolite breccia, and rhyolite volcanic necks of early Cretaceous age and a stock of granodiorite of Late Cretaceous or early Tertiary age. Numerous dikes of quartz latite and felsite cut the granodiorite and older rocks (Lasky, 1938, p. 9-11).
The ore deposits in the district are characterized by abundant tourmaline and chalcopyrite. The ore occurs in veins along faults that were repeatedly reopened during mineral deposition. Six stages of mineral deposition are recognized, but only the second stage yielded exploitable ore deposits (Lasky, 1938, p. 31-33). The important ore minerals are chalcopyrite, pyrite, specularite, sphalerite, and galena; the most common gangue minerals are quartz, calcite, tourmaline, barite, and mangano-siderite.
The Emerald vein (Lasky, 1938, p. 28) has been traced on the surface for more than 6,000 feet, mined to a vertical depth in the Eighty-five mine of 1,900 feet, and explored to a depth of 2,250 feet. Lasky (1938, p. 34) pointed out that the grade of ore mined and the ratio between the different metals are remarkably uniform and that the roots of the main shoot in the Emerald vein are almost identical in grade with the average grade of the last 850,000 tons of ore mined.
