THE AGE AND DURATION OF ANCIENT AND MODERN GEOTHERMAL/HYDROTHERMAL SYSTEMS IN SOUTHERN RIO GRANDE RIFT

Hot springs, geothermal wells, and hydrothermal ore deposits are relatively common along the Rio Grande Rift in southern New Mexico and they encode the complex interactions between basin and surface water waters, structures, lithology, and igneous activity. Direct dating of these systems and determining the duration of their existence has been elusive. We report on recent ⁴⁰Ar/³⁹Ar dating of hydrothermal jarosite in galena-barite-fluorite deposits and cryptomelane-todorokite-ramdellite deposits associated with geothermal systems that places temporal constraints on these systems.

The ore deposits and geothermal systems are commonly located at basement highs along basin bounding faults. Hydrogeochemical and stable isotope data indicate the waters are of meteoric origin in both the ore deposits and geothermal systems that become modified by interactions with host rocks and potential contributions from igneous activity. Mineralization in the ore deposits is initiated by cooling, fluid mixing, or wall rock interaction or any combination. Manganese oxide mineralization associated with modern and ancient hot springs occurs in zones where reduced geothermal fluids discharge: 1. at the surface; 2. into the vadose zone or; 3. mix with an oxygenated groundwater.

⁴⁰Ar/³⁹Ar ages for hydrothermal jarosite from 16 different galena-barite-fluorite deposits indicate they have formed over the last 10 million years. A mineralization period of over 1 million years was noted at the Copiapo mine within the paragenesis of a single vein. At the Hansonburg fluorite deposits we recognize mineralization that has occurred over a time span from 7.97 ± 0.44 to 3.79 ± 0.16 Ma. The ages of manganese deposits associated with modern geothermal systems along the rift were determined at the Riley Travertine (3.2 ± 0.04 Ma), Ellis Manganese deposits at Truth or Consequences (4.86 ± 0.05 Ma), and Tortugas Mountain manganese deposit at Las Cruces (1.8 ± 0.19 Ma). The duration of geothermal activity is similar to those from the hydrothermal ore deposits. These ages are also corroborated by independent paleontological and structural controls as well as uplift/incision history. We present a model to accommodate this data and to illustrate the pathways for geothermal fluid circulation in the rift at the given time scales determined.