Abstract

At La Encantada, three mines produce Pb-Ag ores from deposits that occur in fracture zones in the Cretaceous Aurora limestone, within the north-northwest–trending Sierra Madre Oriental of northern Mexico. The host limestone is remarkably monotonous, almost totally lacking in diagnostic marker beds. The limestone unit is gently folded into a major anticline, the Sierra de La Encantada, which is cut by major frontal strike faults and dissected into blocks by cross-fractures and faults.

Because of the uniformity of the limestone (apart from micro-fossil assemblages), the area was studied by repeated sampling, thin sectioning, and detailed lithofacies and biofacies identification. A very high proportion of all thin sections contained diagnostic or partially indicative fossils. Regional mapping, begun in the field by recognizing only two rock units, was accordingly refined by laboratory analyses, and ten subdivisions of the local sequence were ultimately traceable. These units were based on local refinements of F. Bonet's planktonic biozone scheme for the Cretaceous System of eastern Mexico. In consequence, the areas on the western flank of the anticline, where ore deposits are known, can be more precisely related to the prevalent step faulting and flank grabens of the region.

There is a preferential association of regional-scale ore-bearing fractures with sparry calcite–cemented, skeletal calcarenites. All the ore worked to date has been from heavily iron-stained, secondary, oxidized ore of complex composition. The primary ore is likely to have been argentiferous galena with sphalerite, magnetite, pyrite, and proustite. It occurs as oxidized remnants in two high-grade chimneys, originally with about one million tons in La Prieta and 350,000 tons in El Escondida; in addition, 500,000 tons occurred in a series of veins and small mantos. Recent work has uncovered a zone of lower grade but extensive sulfide tactile ore, possibly associated with the contact zone of an intrusive mass that occurs at depth. A halo of contact metamorphism, marmorization, and bleaching surrounds the ore. The alteration halo is so variable in its development as to be of little value in the search for more ore.

The most important ore-controlling structures appear to be the northeast-trending cross-fractures with which the chimneys and veins are associated. Major faults do not appear to be mineralized, presumably because they have been resealed with fault gouge. Fractures without apparent movement of the opposing surfaces and, therefore, of minor tectonic significance, at surface at least, seem to retain their dilatancy long enough to allow mineralizing fluids to surface.

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