By Virginia T. McLemore and Cynthia Connolly
May 31, 2024
Many people have heard of the Nacimiento sedimentary-hosted copper mine, in the Nacimiento Mountains east of Cuba, especially mineral collectors that collect azurite rosettes or botryoids (https://www.mindat.org/photo-1146260.html) and chalcocite and malachite replacing wood fragments (figure; https://www.mindat.org/photo-374580.html). On Sunday, May 5, K-12 teachers from around the state met in Cuba to tour the Nacimiento mine as part of the New Mexico Science Teachers Association program's field trip offerings and the Cuba Independent School District outreach program.
After a short lecture by Virginia McLemore on the geology and history of the mine at Cuba High School, the group toured the mine.
The copper deposits were mined by Native Americans and Spanish miners perhaps as early as the 1500s. Larger-scale mining in the Nacimiento district did not occur until the 1880s. Earth Resources Company built a 2722 metric ton/day mill to handle the estimated reserves in the 1970s. There was also a floatation circuit to concentrate the copper sulfides during Earth Resources operations. The deposit was mined by open-pit methods.
In 1973, there was release from the tailings dam into Senorito Creek, followed by issues with high-wall instability in the open pit and falling copper prices; in 1975, Earth Resources Company ceased production. The property was operated by various companies through the 1970s. In the early 1980s, Leaching Technology Inc. attempted to mine the deposit by in situ recovery methods using sulfuric acid, but poor results, low recovery from wells, and environmental concerns have hampered the project. Contaminated groundwater in the open pit area was pumped and effectively treated onsite by the USDA Forest Service beginning about 2007 and the treated water was discharged to Senorito Creek. A plume of sulfuric acid remains in the subsurface.
Copper deposits found at the Nacimiento mine are classified as sediment-hosted strata bound copper deposits, which are bodies of copper minerals found as disseminations, cement, and veinlets in bleached sandstones, siltstones, shales, and limestones within or marginal to typical thick sedimentary red-bed sequences that were deposited in stream and river environments. Similar deposits are found throughout New Mexico and are of Triassic or Permian age; the Nacimiento mine is one of the largest of these deposits in New Mexico.
The host rock is white, poorly cemented arkosic conglomeratic sandstone, 23 to 30 m thick. Copper is associated with carbonaceous material. Much of the deposit occurs at the surface; the deepest mineralized zones are at least 274 m and deeper. Pyrite and native silver are present locally throughout the deposit, and the oxidized portion contains malachite, chrysocolla, azurite, cuprite, antlerite, spangolite, native silver, silver sulfides, and native copper. Large, mineralized fossil logs up to several meters long have been replaced by chalcocite, locally preserving the woody cell structure. Historic reserves are reported for the sediment-hosted stratabound copper deposits at the Nacimiento mine amounting to 5.4 million metric tons of ore at a grade of 0.56% Cu and an additional 11.8 million metric tons of ore at a grade of 0.48% Cu as of May 2, 1980.
Electron microprobe analyses of chalcocite in replaced wood from the Nacimiento mine show an average of ~400 ppm Ag (up to ~600 ppm Ag) substituting within the mineral. Chalcocite and pyrite are the dominant sulfide minerals present in the replaced wood from the Nacimiento mine.
Selected samples from the Nacimiento mine have elevated concentrations of copper and other critical minerals such as heavy rare earth elements (REE), vanadium, cobalt, and arsenic, but additional studies are required to fully characterize these areas to determine if there is any economic potential for critical minerals.