The Mineralogical Society of Southern California

Program Notes: Friday, February 13, 2026  at 7:30, join us for Scott Braley presenting Microminerals at the Nacimiento Mine

The Nacimiento Mine (Sandoval Co., NM) is modestly well-known for its copper sulfide-replaced wood and for azurite/malachite nodules.  The deposit was the result of a large log jam, possibly after a significant fire, during the Triassic Period.  Unusually, it was replaced by copper sulfides rather than by silicates, then later partially oxidized. The deposit was first documented in 1860, but was probably known to native peoples, Spanish explorers, and Mexican prospectors.  The mine was a fairly small-scale operation until the 1960’s, when it was converted from an underground mine to an open pit.  No longer in operation, it has been partially remediated.  There is a broad suite of microminerals that can be roughly grouped into two categories.  The first is copper secondaries arising from the weathering of the replaced wood; the second is found in small nodules from a completely separate formation that overlies the copper mineralized layer.  The copper secondaries include fine examples of common minerals like the aforementioned malachite and azurite, but also relative rarities like libethenite and spangolite.  The nodules are sedimentary and include excellent calcite, celestine, barite, and aragonite.  Additionally, pyrite and gypsum crystals can be found loose in some of the soil.  Altogether, roughly 30 minerals (including a few not yet firmly identified) can be found.

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