FOSSIL BIVALVES FROM THE UPPER CRETACEOUS CARBONATES ASSOCIATED TO FLUORITE DEPOSITS FROM THE VICINITY OF MUZQUIZ COUNTY, NORTHEASTERN MEXICO

In this abstract, fossil bivalves associated to fluorite deposits are preliminarly described. Fossils were collected from a fluorite mine located at the “La Encantada” District, at approximately 200 km north to the Muzquiz County, in the state of Coahuila, NE Mexico. Fluorite is present as dikes or parallel-to-bedding lenses, and is embedded in decimetric (20-30cm) calcareous strata of the Boquillas Formation (Upper Cretaceous). Bivalves consist of unidentified inoceramids as well as giant forms preliminarly assigned to Cladoceramus undulatoplicatus ROEMER, which suggest a Coniacian-Santonian age. The specimens here reported are preserved in two ways: 1) as external moulds, 2) as partially dissolved specimens exposing fragments of a recristalized shell. Energy Dispersed Spectroscopy (EDS) analysis reveals that both external moulds, as well as partially dissolved shells were affected by fluorite mineralization. Specimens that were collected directly from dikes or lenses expose approximately 50% wt to 40% wt of fluorine, whereas those specimens collected in the calcareous beds affected by fluorite mineralization, the fluorine values range from 20.96% wt to 26.98% wt.

So far, it is known that during the migration of mineralizing fluids an equilibrium between these fluids and the carbonates could be stablished; such condition could avoid the total dissolution of carbonates and fossils, and allows the incorporation of fluorite to them. However, more detailed studies are required to stablish the thermodynamic conditions that allow this exceptional style of preservation.