EXPLORING HYDROLOGIC CONNECTIONS IN THE UPPER CRUST – THE ORE DEPOSITS OF THE RIO GRANDE RIFT

Extension and uplift in the Rio Grande Rift over the last 30 million years has exposed hydrothermal ore deposits that formed concurrently with rifting. Recent ⁴⁰Ar/³⁹Ar age date studies of Rio Grande Rift (RGR)-type galena-barite-fluorite and manganese oxide (MnOx) deposits document hydrothermal mineralization events spanning 10 million years that continues to be active to present time. Geochemical and isotopic evaluation of these mineralizing events allows for the understanding of the pathways involved in the transfer of components from the mantle, to the upper crust, and ultimately to the surface where they occasionally affect water quality in the region.

Most significant mineralization events are concentrated on block boundary faults within and on the margins of the rift. These fault zones provide the pathway for both ascending and descending fluids often mineralized multiple times. Oxygen and hydrogen isotope determinations on various minerals in these deposits indicate the largest component of the mineralizing fluid is meteoric with some evidence of contribution from igneous sources. Sulfur isotopes demonstrate mixing between potentially magmatic, bio- and/or thermochemical reduced, and Permian age sulfate sources. Particular alteration assemblages associated with the hydrothermal events constrain the compositions and temperatures of the fluids and indicate both sulfuric acid and hydrofluoric acid were involved in some of interactions. Mineralogy of the deposits strongly indicates mixing of fluids of diverse origin.

The overlap in space and time of these deposits allows for construction of a model that accommodates the features of ancient ore deposits and modern geothermal occurrences. RGR deposits retain some mantle signatures and MnOx deposits represent distal fluids that discharge at geothermal springs or are encountered in wells. The near surface manifestations of mineralization are significantly influenced by climate, tectonics, and sedimentation.