By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Grant County, in southwestern New Mexico, contains several highly productive mining districts and ranks first in the State in the production of mineral wealth. The ore deposits are diverse and have yielded copper, lead, zinc, silver, gold, iron, manganese, and molybdenum.
Total gold production of the county through 1959 was about 501,000 ounces. Before 1900 placers and oxidized ores were the chief sources of the gold, but from 1912 through 1959 much gold has been a byproduct of base-metal mining. The major districts are the Central, Pinos Altos, and Steeple Rock.
The Central district, which includes Santa Rita, Hanover, Fierro, and Bayard, is in eastern Grant County and has produced copper, zinc, iron, lead, and small amounts of gold and silver. From 1904 through 1959 the entire district produced about 140,000 ounces of gold, mostly as a byproduct from base-metal ores. Gold production before 1904 is not known but presumedly was negligible.
Copper ores at Santa Rita were probably known to the Indians at an early date and to the Mexicans before the American occupation. Small amounts of copper were produced in this district intermittently for a century beginning in 1804, and large-scale copper mining began in 1911 (Spencer and Paige, 1935, p. 5-9; Lasky, 1936, p. 102-105; Graton, in Lindgren and others, 1910, p. 306).
In 1906 a detailed study was undertaken to determine the feasibility of open-pit mining of the copper- deposits of the Santa Rita basin in the Central district. In September 1910 stripping began and by 1912 copper was being produced in large quantities (Spencer and Paige, 1935, p. 8-9). Silver-bearing lead carbonate ores were mined in the Bayard area as early as 1870, and lead-zinc deposits in the Hanover area were developed on a large scale by 1912. The district continued to be a significant producer of copper, lead, and zinc through 1959. Gold and silver were byproducts.
The following summary of the geology and ore deposits is from Spencer and Paige (1935) and Lasky (1936).
The Central district is underlain by a relatively complete section of Paleozoic sedimentary rocks, in which every system of the Paleozoic Era is represented. These rocks are unconformably overlain by Cretaceous sedimentary rocks. Folding and faulting related to igneous activity took place in Cretaceous and Tertiary time. The igneous rocks include sills, stocks, and dikes of quartz diorite and granodiorite and also include younger quartz latite tuffs and andesitic basalt flows. Locally, the sedimentary and intrusive rocks are intensely altered. Additional faulting occurred after the latest igneous activity.
Ore deposits are of three principal types: contact metamorphic deposits, veins, and disseminated copper deposits in granodiorite porphyry. The contact metamorphic deposits are in certain limestone beds adjacent to the Hanover-Fierro stock. Some deposits are commercially valuable magnetite bodies with subordinate amounts of pyrite, chalcopyrite, and chalmersite. Others are masses of sphalerite, galena, hedenbergite, ilvaite, and manganese-bearing calcite, which are valuable for their zinc content. The veins are mineralized fractures related to the quartz diorite and granodiorite intrusions. Sphalerite, pyrite, chalcopyrite, and galena are the chief sulfide constituents of these deposits. Quartz and sericite are common gangue minerals. The disseminated copper deposit at Santa Rita is in the upper highly fractured part of a granodiorite stock and in the invaded sedimentary rocks near the contact. Primary pyrite and chalcopyrite were converted to rich deposits of chalcocite by supergene enrichment processes. The chalcocite is overlain by a blanket of malachite, azurite, chrysocolla, cuprite, and native copper (Spencer and Paige, 1935, p. 64-75). The gold content of all the primary ore is very low; however, the enriched and oxidized parts of the veins and disseminated copper deposits yield considerable amounts of gold as a byproduct.
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