N.W. DUNBAR1, D.J. ENNIS2 AND C.E. CHAPIN1
1New Mexico Bureau of Geology, New Mexico Tech, Socorro, NM, 87801, USA (nelia@nmt.edu)
2Souder, Miller and Associates, 401 17th St, Ste 4, Las Cruces, NM, USA (dje@millereng.com)
Potassium metasomatism, thought to be caused by deep circulation of alkaline-saline
brines, affects an area of roughly 1000 km2 near Socorro, New Mexico, increasing
the K2O content of local volcanic rocks by up to 10 wt.%. Petrographic and microbeam
analyses indicate that during the chemical changes associated with metasomatism,
Na-rich phases, primarily plagioclase, are dissolved and replaced by secondary
mineral phases, mainly adularia and clay minerals. Bulk geochemical analyses
of altered rocks compared to fresh rocks demonstrate consistent chemical patterns,
including enrichments of K2O, Fe2O3, As, Rb, Pb, Sb, Ba, and Cs and depletions
of MgO, CaO, Na2O, Eu and Sr. Analyses of alteration phases hand-picked from
relict plagioclase grains provide the clearest picture of the chemical processes
that occur during alteration, and allow quantitative correlations between the
presence of alteration phases and the abundance of elements such as K2O, Rb,
CaO, Na2O, Eu and Sr. This analysis also suggests that variations of elements
such as As, Pb, Sb, Ba, and Cs are related to hydrothermal alteration, either
overprinting, or overprinted by, potassic metasomatism. A final chemical signature
observed in hand-picked alteration mineral suites is that the REE content of
samples from a low-REE rock unit increase during metasomatism, whereas there
is a significant decreases in samples from a unit with higher initial REE contents,
suggesting that the variation in REE contents may be related to equilibration
between the rocks and the metasomatizing fluid. Results suggest that the enrichment
of REE may be roughly related to the abundance of metasomatism-derived clay
minerals in the sample.
The chronology of metasomatism determined by 40Ar/39Ar analysis of hand-picked
secondary potassium feldspar, indicates that metasomatic alteration began at
around 15 Ma, and continued to around 7 Ma. The areas that show the most intense
effects of alteration yield the youngest ages, suggesting that this may represent
the deepest, most central and longest-lived part of the playa system. The age
range determined here is consistent with inferred timing of playa deposition
from independent geological evidence.