Abstract

The high-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits of northern Mexico occur in thick carbonate-dominant Jurassic-Cretaceous basinal sedimentary sequences floored by Paleozoic or older crust. The districts lie within, or on the margins of, a major fold-thrust zone; structures parallel to the trends of this belt control mineralization in many districts. Altered and/or mineralized volcanic and intrusive rocks of Tertiary age are found in all but three of the districts; mineralization apparently occurred during a restricted period 47 to 26 m.y. ago.The deposits are characterized by irregular ore lenses that commonly reveal strong structural controls and are stratigraphically discordant. Orebodies are of three principal morphological types: mantos, chimneys, and pods and are composed of massive sulfides and/or calc-silicate skarn. Sulfide mantos and chimneys are enclosed within carbonate rocks and are generally remote from intrusive bodies. Sulfide mantos commonly grade downward into sulfide chimneys, which may in turn grade downward into skarn chimneys cored by dikes. Podiform skarn and sulfide orebodies principally occur along stock, dike, or sill contacts, but they may also be developed at some distance from an intrusive contact. The associated intrusions are not widely mineralized.Despite the variations between ore types, major similarities are seen in structural controls, host-rock characteristics, and sulfide mineralogy. Structural controls include intrusive contacts, faults, fold axes, fractures, fissures, and cavern zones. Intrusive contacts and intrusion-related faults are most important in the skarns, whereas regional fault, fold, and fracture systems are dominant controls on mantos and chimneys. Mineralization commonly occurs below relatively impermeable shaly units, sills or volcanic rocks, but some extensive orebodies show remarkable restriction to specific horizons within apparently homogeneous carbonate successions. Studies of mineralized and unmineralized strata have found no consistent links between ore deposition and carbonate composition, facies, organic content, or insoluble components. However, zones of secondarily enhanced porosity and permeability induced by recrystallization, heating, deformation, or hydrothermal dolomitization are important mineralization controls.Geochemical data for the deposits are sparse but indicate strongly contrasting conditions among some of the major districts. Fluid inclusion data indicate temperatures in the range 200 degrees to 500 degrees C and salinities ranging from i to 60 equiv wt percent NaCl. The hotter, more saline, solutions are typically from skarn zones. Evidence for boiling has not been reported. The skarn ores of Naica, Concepcion del Oro, and Velardefia show tight clustering of sulfide sulfur isotope values near 0, whereas the skarn and massive sulfide ores of Santa Eulalia show a wide range of values. In most cases there is isotopic disequilibrium between galena and sphalerite, pyrrhotite, or pyrite. Calcite veinlet stockworks above mineralization and limestone wall rocks adjacent to mineralization show shifts of carbon and oxygen isotopes to lighter values. The tightly clustered sulfur isotopes and high-temperature, high-salinity fluid inclusions have been interpreted as indicating a strong magmatic component in the mineralizing fluids at Naica, Concepcion del Oro, and Velardena, but Santa Eulalia and other districts show evidence suggesting extensive mixing between magmatic and meteoric fluids.It appears that the spectrum of mineralization styles shown by these deposits represent differing responses to variations in intrusive associations, depth of emplacement, host-rock characteristics, and geochemical evolution of the individual systems.

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