Geology and mineral deposits of the El Boleo copper mine, Santa Rosalía District, Baja California Sur
The El Boleo mine, second largest copper producer in Mexico during the 1920s, lies near the town of Santa Rosalia, about halfway down the east coast of Baja California Peninsula. The copper deposits occur within a recently uplifted coastal belt of Pliocene sediments that form stream-cut mesas. Mineworks are spread over an area 11 km long by 0.5 to 3 km wide.
The climate is hot and dry; rainfall is generally torrential at intervals in the course of several years. The xerophyte vegetation consists mostly of cacti and other thorny desert plants. The region’s main natural resources are copper, manganese, and nonmetallic deposits, together with prosperous fisheries in the Gulf of California and the Pacific Ocean.
The oldest rock, exposed only at small erosional windows, is a Cretaceous or pre-Cretaceous quartz-monzonite. This is followed unconformably by the mid(?)- and upper Miocene Comondú Volcanic Formation, probably at least 500 m thick, of nonmarine andesite and basalt flows, tuffs, breccias, agglomerates, conglomerates, and tuffaceous sandstones(?). The facies changes indicate that the formation was derived from the present emplacement of the Gulf of California.