Geologic map of the Santa Eulalia District, Chihuahua, Mexico: Supplement 8 from "The Manto Type Limestone Replacement Deposits of Northern Mexico" (Thesis)

A study has been made of the principal manto type limestone replacement deposits of northern Mexico. Three of the districts, Los Lamentos and Santa Eulalia in Chihuahua, and Mapimi in Durango, are described in detail. The sedimentary rocks are correlated with the Texas Comanchean formations. The ore deposits are confined to certain beds of Upper Trinity and Lower Fredericksburg age. Data show that it is the physical and not the chemical properties of the limestone which make some beds more susceptible to replacement than others. Folding and fissuring have been essential in localizing the ore deposits. Mantos have been found to occur, in general, in favorable beds at the crests of folds. The fissure zones are most pronounced in depth. The cross section of a deposit decreases upward from the apparent source although the diminution is not constant. Mantos have yielded most of the ore in the upper deposits; chimneys become more important in depth. The analyses of sections of an ore body are uniform but the deposits show a gradual change in composition in a district. The ores are zoned vertically and laterally from the apparent source and are believed to have been deposited by magmatic waters. The principal deposits of a district were formed at one time. The metallic content of the bedded oxidized deposits is almost identical to that of the primary deposits from which they were derived. Dolomitization has occurred locally during oxidation.