New Mexico Geological Society Annual Spring Meeting — Abstracts

Cryptomelane, KMn₈O₁₆, samples from the Nancy and MCA mines in the Luis Lopez manganese district were collected from a similar paragenetic stage of mineralization at both mines. The hydrothermal characteristics of these deposits have been documented by previous workers using fluid inclusion and stable isotope analysis. Cryptomelane precipitation occurred at the onset of calcite mineralization following banded manganese ore deposition. Both samples consisted of arborescent growths of acicular crystal bundles projecting into calcite. Composition and sample purity were confirmed by x-ray diffraction analysis. ⁴⁰Ar/³⁹Ar dating of cryptomelane at the MCA mine yielded well-behaved age spectra with a weighted mean age of 6.31 ± 0.08 Ma. Age spectra from Nancy mine cryptomelane reveal a pattern suggestive of argon loss in the first 25% of ³⁹Ar released during heating of the sample. A minimum age of 6.03 ± 0.08 Ma for cr~tomelane precipitation is calculated from the remaining 75% of the age spectrum. The ⁴⁰Ar/³⁹Ar apparent ages fall within the bracket of ages (3 to 7 Ma) proposed from
geologic age relationships in the district by previous workers. Cryptomelane has previously been used to successfully date residual manganese deposits and manganiferous soil horizons in laterites by the ⁴⁰Ar/³⁹Ar method. This study demonstrates that the dating of hydrothermal manganese mineralization by this technique is accurate with respect to the geologic time constraints observed in the district.