The Rancho el Papalote area is 35 km north of Chihuahua City and includes Minas Terrazes. The oldest rocks are highly fractured (17% dilation), massive Cretaceous biomicrites, containing rudistid bioherms, that crop out on ridges and peaks surrounded by volcanic rocks of two eruptive periods. The older volcanics are altered rhyolite and/or dacite flows, flow breccias, and minor lithic tuffs. The flow breccias are thickest around Cerro Choloma which may represent a vent. A boulder breccia containing limestone and chert clasts is along the limestone-older volcanic contact. The younger volcanics are rhyolite ash flows with minor basalt and andesite flows. There are at least five cooling units. A 44-m.y. (initial ⁸⁷Sr/⁸⁶Sr ratio = 0.7048) old vitric-crys al tuff (22% phenocrysts, sanidine/quartz = 4.5) overlies the older volcanics 1 km west of the ranch buildings. The "Red Platy," an ash-flow rhyolite, overlies the 44-m.y. tuff and is a distinctive marker bed. A small dike cutting the tuff may represent a vent for the Red Platy. The remaining units, vitric tuffs in the eastern part of the area, dip gently east and are cut by northwest-trending normal faults. The first of these ash flows has a basal lithic zone and contains 3% sanidine phenocrysts (Or 45). The next unit is vesicular at its base and has 6% sanidine phenocrysts. The youngest ash flow contains sanidine, quartz, and fayalite phenocrysts (sanidine/quartz = 1, phenocrysts = 4%). A basalt above the Red Platy contains distinctive gabbroic xenoliths. On the south the older volcani s are overlain by an andesite flow. At Minas Terrazas, copper deposits occur in skarns near a felsic intrusive. These skarns are mainly andradite and contain small amounts of pyrite and chalcopyrite.

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