REGIONAL MN HORIZON AND MN-ANDALUSITE MINERALIZATION IN PROTEROZOIC QUARTZITES OF THE SOUTHWESTERN U.S.A

Thick (>1 km), ultra-mature quartzites of Proterozoic age (ca. 1.7.Ga) outcrop across the southwestern USA. Many overlie 1.7 Ga high-Silica rhyolite. Similar quartzites of similar age occur in the mid-continent (U.S.A.), Australia, and several other continents. An anomalous Mn-rich horizon occurs near the rhyolite-quartzite contact in most exposures in the Southwest, including northern New Mexico, Needles Mtns., CO, Squaw Peak, AZ and others. The layer typically contains distinctive green, Mn-bearing andalusite along with oxides of anomalous trace-element-rich composition. In the Tusas Mts., NM, Mn-bearing andalusite occurs along crossbeds in the lower 10s of meters of the Ortega Formation quartzite section. The origin and evolution of the Mn horizon is still debated, but the presence of Mn-andalusite on crossbeds suggests a depositional origin, possibly related to fluids in the underlying (and locally contemporaneous) volcanics. Where secondary cleavages have developed, particularly in large-scale fold hinges, Mn-andalusite appears to bleed into cleavage planes, and textural evidence suggests that at least two generations of Mn-andalusite are present. Aluminum silicate textures and phase relations and Fe-Mn partitioning among aluminum silicates can help to answer on-going questions about the early deformation and metamorphism of the quartzite sequence. The nearly ubiquitous monazite included in, and associated with, Mn-andalusite may place absolute timing constraints on the Mn-mineralization and on deformation and metamorphic events. In addition, the geochemistry (and oxidation state) of the Mn-horizon may shed light on conditions at the time of quartzite deposition and allow the horizon to be used as a marker horizon around the Southwest and beyond.