New Mexico Geological Society Annual Spring Meeting — Abstracts

Hazen et al. (2008) identified 10 stages of planetary “mineral evolution” based on observations and geochemical principals. The Earth formed from a relatively homogeneous material and has differentiated over 4.5 billion years of time. Mineral evolution is a consequence of changing temperature, pressure, and composition during this differentiation process that is unique to tectonically active planets. Hazen et al. (2009) noted that the geochemistry of uranium is highly sensitive to some of these “evolutionary stages” and the resulting uranium mineralogy can be used to define four phases of uranium mineralization that have operated within the 10 stages of mineral evolution. Six of the first 10 stages of U mineral evolution occurred prior to the formation of crust that eventually formed New Mexico ca 1.8 Ga. Accordingly, two of the four uranium mineral evolution phases occurred prior to the formation of the oldest rocks in the state. Vestiges of the first two stages may be recognizable in some New Mexico rocks as detrital uraninite or thorite grains or as mineral inclusions in zircons which demonstrate partial U-Th solid solution compositions. The assembly of New Mexico produced the state’s first home-grown uranium minerals that were formed in ore deposits associated with late stage magmatic activity. A relatively simple assemblage of uranium-bearing oxides and silicates were deposited, mainly in pegmatites and some copper veins, with some secondary uranium minerals formed via less conventional processes of auto-oxidation. The formation of New Mexico is also coincident with the “Great Oxidation Event” which gave rise to abundant formation of the uranyl ion (U⁶⁺) and an explosion of secondary U minerals, estimated at over 200 (Hazen et al., 2009). Since 2 Ga, U⁶⁺ geochemistry dominates the mobility and mineralogy of the element in both the formation and natural destruction of U ore deposits. Complexation and crystal chemistry results in intricate paragenetic mineralization sequences that are often reversible. Mineralogical change in single ore specimens have been documented over very short time periods (e.g. dehydration of tyuyamunite to metatyamunite) in addition to crystallization of new phases upon extraction from their natural environment (e.g. crystallization of liebigite on mine rock surfaces). Current mineralogy at any deposit (or waste/tailings pile) does not necessarily reflect the dominant geochemical processes that operated over the longest period of time. The additional consideration of biological influences on U mobility further complicates forensic types of research as well as imparting geochemical ambiguities in environmental considerations. Microbes have been documented to either oxidize or reduce uranyl ions in near-surface environments thus influencing uranium mobility irrespective of inorganic chemical controls.

References:

1. Hazen, R.M., Ewing, R.C., and Sverjensky, D.A., 2009, Evolution of Uranium and Thorium minerals: American Mineralogist, v. 94, p. 1293-1311.
2. Hazen, R.M., Papineau, D., Bleeker, W., Downs, R.T., Ferry, J.M., McCoy, T.J., Sverjensky, D.A., and Yang, H., 2008, Mineral Evolution: American Mineralogist, v. 93, p. 1693-1720.