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New Mexico Mineral Symposium
November 8-9, 2014


Tales from the Crypt(omelane)

Virgil W. Lueth

New Mexico Bureau of Geology & Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM, 87801

Manganese oxide mineralization is commonly associated with hydro- and geothermal systems throughout the western United States and Mexico. A number of manganese oxide phases are potassium-bearing and are amenable to age dating using 40Ar/39Ar techniques. This presentation will highlight the trials and tribulations of a series of studies on manganese oxide age dating along the Rio Grande rift of New Mexico. The results of these studies allow us to understand the complex interactions between basin and surface water waters, structures, lithology, and igneous activity as well as establish the duration of these geothermal systems.

Meet Manganese: Manganese, the 10th most abundant element in the earth’s crust, forms over 250 different species. The large number potential valence states and coordination geometries is responsible for plethora of manganese minerals that comprises up to 6% of all known minerals. The weathering and hydrothermal environments contain the greatest diversity of manganese minerals.

What’s Up with Wad? Wad is old miner’s term for manganese oxide ore and significant amounts were mined from just south of Socorro in the Luis Lopez Manganese District. Mineralization was thought to be caused by a hydrothermal circulation cell centered about a buried intrusion. However, the direct dating of the manganese ores revealed that the mineralization is much younger than originally thought and the source was from elsewhere. The high precision of the dating demonstrates the potential for dating hydrothermal ore deposits.

The Perils of Pyrolusite: Taking the success of the Luis Lopez dating study and applying it to other manganese occurrences up and down the rift resulted in more questions than answers. Manganese oxide mineralization has been considered a pathfinder to sulfide ores for over 100 years. In mining districts where we have significant sulfide mineralization (e.g. Magdalena and Lake Valley); there are also manganese oxide ores nearby. The dating of the manganese minerals revealed them to be much younger than the sulfide ores and thus unrelated to each other.

The Trials of Mice and Man(ganite): A detailed study of the Argentiferous Manganese Oxide Mineralization (AMOM) at Santa Eulalia, Mexico was also attempted with the intent to prove the usefulness of manganese mineralization as a pathfinder to sulfide ore. Detailed geochronology was employed to date the sulfide mineralization at 26.6 Ma. Dating of the AMOM resulted in ages between 10 and 2 Ma. The anecdotal relationship of manganese mineralization to sulfide ore does not appear to hold at this location and other models of formation and timing have to be considered.

Making Sense of Psilomelane: The age and duration of geothermal systems along the Rio Grande rift was part of a recent research study sponsored by the Department of Energy. Manganese deposits in proximity to modern geothermal systems were studied in order to better ascertain the presence blind exploration targets. In all deposits studied, independent age constraints were present at all deposits that allowed for independent verification of manganese oxide geochronology. In all geothermal areas considered, manganese age dating provided accurate ages for determining the age and estimating the duration of the geothermal systems. Accordingly, geothermal exploration in areas of manganese mineralization appears justified if the age of the deposit is less than 10 million years.

pp. 12

35th Annual New Mexico Mineral Symposium
November 8-9, 2014, Socorro, NM