Reactions of fluorine- and beryllium-bearing hydrothermal fluids with Cretaceous limestone formations in brecciated zones near dikes of rhyolite porphyry, under conditions of low temperature and low pressure, produced sizeable fluorspar deposits, some of which are potential economic sources of beryllium in the Aguachile district, Coahuila, Mexico.
Bertrandite (Be4OH2Si12O7) is intermingled with fluorite in the “lower” contact zone along a ringdike which encircles a subsidence basin (cauldron collapse) in the central part of Aguachile Mountain. Fluorspar deposits outside Aguachile Mountain contain 5 to 20 ppm beryllium in ionic (?) combination with fluorite.
A pluglike mass of quartz microsyenite, younger than the rhyolite dikes, intrudes the floor of the subsidence basin; a few still younger small dikes and irregular-shaped bodies of analcite diabase are present elsewhere in the district.
The hydrothermal fluids probably were late emanations from a parent magma which gave rise to one or more of the types of intrusive igneous rocks. Both the quartz microsyenite and the bertrandite probably came from the same parent magma. Bertrandite mineralization followed two generations of fluorite and appears to have been essentially contemporaneous with a third, weak generation of fluorite mineralization. The ionically combined beryllium probably accompanied the earlier, stronger fluoritization. Greater concentration of beryllium in deposits around Aguachile subsidence basin may be the result of more open connections with a deep-seated, beryllium-enriched alkalic magma.