| Summary |
The Rancho El Papalote area is 35 km north of Chihuahua City and includes the base metal mines at Terrazas. The oldest rocks are highly fractured, massive, mid-Cretaceous, biomicrites containing rudistid bioherms, that crop out on ridges and peaks surrounded by volcanic rocks of two different eruptive periods. The oldest 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 area. A boulder breccia containing limestone and chert clasts occurs along the limestone-older volcanic contact. The younger volcanics are rhyolite ash flows with minor basalt and andesite flows. There are at least five 8 T 86 cooling units. A 44 m.y. (initial 'Sr/° Sr ratio = 0.7048) old vitric-crystal tuff (22-25% phenos; san/qtz = 4 to 5; san comp = Ab₅₆Or₄₀An₃Fe₁) overlies the older volcanics 1 km west of the ranch buildings. The Red Platy, a vitric ash-flow rhyolite (Or₅₄), overlies the 44 m.y. tuff, and it is a distinctive marker bed. A small dike cutting the 44 m.y. tuff may represent a vent for the Red Platy. The three youngest units, vitric tuffs in the eastern part of the area, dip gently to the northeast and are cut by northwest-trending normal faults. The oldest of these ash flows has a basal lithic zone and contains sanidine phenocrysts (3%; Ab₅₆Or₄₀An₃Fe₁ overlying unit has a vesicular base and contains sanidine phenocrysts (6%; Ab₅₆Or₄₀An₄Fe₂). The youngest ash flow 87 86 in this sequence (28 m.y., initial Sr/ Sr ratio = 0.7082) contains sanidine, quartz, and fayalite phenocrysts (san/qtz = 1; phenos = 4%; san comp = Ab₄₈Or₅₀An₁Fe₁). A 37 m.y. old basalt overlying the Red Platy contains distinctive phaneritic, pyroxene-plagioclase xenoliths. To the south the older volcanics 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. A bedrock geochemical reconnaissance indicates base metal, silver and gold anomalies east of Terrazas in the limestones and older volcanics. The results of a biogeochemical reconnaissance also indicate anomalous metal values in some samples of the plants Prosopis juliflora (mesquite), Rhus virens and Rhus microphylla. |
