New Mexico Mineral Symposium — Abstracts

The Nacimiento Mine east of Cuba, in Sandoval Co., NM, is well-known among New Mexico collectors for its copper sulfide-replaced fossil wood and for azurite/malachite nodules. However, Nacimiento hosts a broad suite of microminerals that have not been extensively documented, though familiar to interested collectors.

During the Triassic Period (~230 MYA), a large logjam of conifers, which had burnt during a forest fire, formed along one of the channels of the Chinle River system and was quickly covered with mud. This wood was eventually substantially replaced by copper sulfides, primarily chalcocite. The overall mineralized formation is the Agua Zarca Member of the Chinle Formation, consisting of sandstone and conglomerate (Antony 1972). Later oxidation, perhaps during the uplift of the local Nacimiento mountains, led to the formation of secondary copper minerals, such as malachite and azurite (Fig. 1).

The replacement itself is significantly more involved than a simple replacement by chalcocite, with plant cell structures preferentially mineralized by marcasite, hematite, and aragonite (Mustoe 2018) in addition to chalcocite, bornite, and djurleite.

The deposits were first documented in the 1860's, but were probably rediscovered repeatedly by native peoples, Spanish explorers, and Mexican prospectors prior to the area becoming part of the United States. The mine was initially operated sporadically on a small scale as an underground mine but was converted to an open-pit mine until it became unprofitable (with an interesting side-venture into in-situ leaching via sulfuric-acid injection, which is still being cleaned up today) (Mustoe 2018). Currently, the mine pit is partially water-filled, and the old dumps have been substantially remediated, limiting collecting opportunities.

While most attention is given to the fossil wood and azurite/malachite nodules, local collectors have been well-aware of the other minerals found at the mine. Pyrite and gypsum crystals have been commonly found. Septarian nodules and concretions originating in a different, overlying member of the Chinle Formation and unrelated to the copper mineralization have been widely collected at the mine site. Micro collectors have noted several less-common copper secondaries, such as spangolite and libethenite. Small clusters of hollow concretions ("knobby concretions," Fig. 2) with calcite crystals lining the cavities are also sought out.

This work surveys the microminerals found at the Nacimiento mine, divided between 1) the copper secondaries and other minerals associated with the fossil wood, and 2) the minerals found in the "knobby concretions."

Secondary copper minerals positively identified include malachite, azurite, spangolite, libethenite, and cyanotrichite. Of special note are the malachite curls (Fig. 3). The "knobby concretions" produce an interesting suite of photogenic minerals including calcite, celestine (Fig. 4), quartz, and barite.

References:

1. Antony, J.J. (1972) Geology and Copper Deposits of the Nacimiento Mine – Eureka Mesa Area, Sandoval and Rio Arriba Counties, New Mexico [Master's Thesis, Colorado School of Mines], Mines Repository. https://repository.mines.edu/entities/publication/659eaad9-bf9f-490d-a92f-a5f4d43bb870
2. DeMark, R.S. (1992) New Mexico: Mineral Locality Index. Rocks & Minerals, 67(5):314-331
3. Mustoe, G.E. (2018) Mineralogy of Non-Silicified Fossil Wood. Geosciences, 8(3):85
4. Ottea, J.M., Uhl, B.H. (2020) Copper-Mineralized Fossil Wood from the Nacimiento Mining District, Sandoval County, New Mexico. Rocks & Minerals, 95(6):548-554
5. Talbott, L.W. (1974) Nacimiento pit, a Triassic strata-bound copper deposit. In New Mexico Geological Society guidebook, 25th Field Conference, Ghost Ranch, New Mexico, 301–03.
6. Woodward, L.A. (1987) Geology and mineral resources of Sierra Nacimiento and vicinity, New Mexico. Memoir 42, New Mexico Bureau of Mines & Mineral Resources.
7. Woodward, L.A., Kaufman, W.H., and Schumacher, O.L. (1974). Sandstone copper deposits of the Nacimiento region, New Mexico. In New Mexico Geological Society Guidebook, 25th Field Conference, Ghost Ranch, New Mexico, 295–99.

pp. 28-30