The Horse Springs Dacite--An enigmatic pyroclastic deposit at the edge of the Plains of San Agustin
— James C. Ratte, Peter J. Modreski, William C. McIntosh, and Richard M. Chamberlin

The Horse Springs dacite (new, but informal, name) is a local pyroclastic deposit near the southwest end of the Plains of San Agustin and the settlement of Old Horse Springs. The regional Blue Canyon Tuff (Eocene) underlies the Horse Springs dacite, is indistinguishable from it in age (-33.7 Ma), and is petrographically and chemically similar to the dacite; the enigma is the relationship, if any, of these two units. These similarities and the distribution of the Blue Canyon Tuff, which has no known source, suggest the possibility of a genetic relationship and a possible caldera source for both formations beneath the Plains of San Agustin. The Horse Springs dacite consists of pumice flows and block-and-ash flows at least 250 m thick. Outcrops of the dacite near the edge of the plains, closest to the suspected source, contain juvenile, pumiceous dacite blocks, 2- 3 m long, and cognate quartz monzonite blocks of the same size. The dacite also contains accidental inclusions of mineralized jasperoid as large as 0.5 m across, derived from pre-Tertiary sedimentary rocks, including limestone, sandstone, and calcareous argillaceous rocks. The jasperoid inclusions, where found in place, have calcsilicate, skarn-type reaction rims consisting of diopside, rare garnet, and seemingly incompatible clay minerals. Many inclusions also have iron- and zinc-enriched cores that contain as much as 1% zinc, mainly in magnesioferrite. Both the cognate quartz monzonite inclusions and the accidental mineralized jasperoid inclusions are interpreted as having been stoped from the roof of the dacite magma chamber prior to eruption. The skarn reaction rims, however, are believed to have formed after eruption, during the cooling of the enclosing blockand- ash flows, which is indicated by the delicate, granular and friable nature of the rims, the presence of clay minerals, and the absence of unaltered limestone inclusions within the dacite.