Abstract

The Sierra de Santa Maria is located in the Velardena mining district in northeastern Durango in the Republic of Mexico. The Pb-Zn-Ag ore deposits have been exploited since the sixteenth century. Production from the Santa Maria dome is, at the time of writing, 800 metric tons per day from two principal mines, the Los Azules mine and the Santa Maria mine, both located within the northeastern flank of the sierra.The deposits are located along a Mesozoic basin-uplift margin at the junction between the Sierra Madre Oriental, the Sierra Madre Occidental, and the Mesa Central. Host sediments are the Lower Cretaceous Aurora and Cuesta del Cura limestones and the Upper Cretaceous Indidura and/or Caracol Formations. Mineralization is spatially and temporally associated with a 33-m.y.-old quartz latite porphyry stock and associated quartz latite porphyry and rhyolite dikes.Major sulfide minerals existent within the dome include pyrite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, and galena. Minor sulfides are boulangerite and freibergite. Traces of geocronite, argentite, polybasite, proustite, and Bi sulfosalts are found locally. Sulfide mineralization occurs as skarn-replacement, carbonate-replacement, disseminated, breccia, and epithermal fissure vein ore. The ore zones are divided and named on the basis of the dominant sulfide peculiar to their respective assemblages: the pyrrhotite ore zone (skarn-replacement ore), the arsenopyrite ore zone (disseminated ore in quartz latite porphyry), and the pyrite ore zone (carbonate-replacement ore). The other two ore zones in the sierra are the epithermal ore zone (fissure vein ore) and the breccia ore zone (quartz latite porphyry breccia ore).A variety of alteration assemblages are recognized within the dome. These are, in order of occurrence, presulfide-stage prograde skarn, potassic and propylitic alteration; sulfide-stage retrograde skarn; late sulfide-stage phyllic alteration; and postsulfide-stage argillic alteration. Prograde and retrograde skarn, potassic, propylitic, and phyllic alteration are all attributed to hypogene hydrothermal activity while argillic alteration is probably a supergene effect.Fluid inclusion analyses reveal that two distinct fluids caused prograde skarn alteration (550 degrees C, 0.75 kb, 10 equiv wt % NaCl) and sulfide mineralization (375 degrees C, 0.12 kb, 40-60 equiv wt % NaCl). A fracturing and related boiling event after skarn formation but prior to sulfide mineralization is proposed to explain the pressure and compositional variations between fluids.Sulfur isotope analyses of the sulfides (0ppm vs. CDT) and host Mesozoic sedimentary rocks (-7.3-3ppm vs. CDT) suggest that the sulfur was derived from a juvenile (i.e., magmatic) source. No delta 31 S compositional variations were observed paragenetically or spatially.Lead/zinc ratios in the Santa Maria mine indicate that fracture intersections with the Santa Maria dike were the major hydrothermal fluid pathways. This is further supported by higher Bi/Sb ratios in galenas within the central portions of the inferred hydrothermal fluid channels than along the distal fringes. Finally, the variation in sulfide mineralogy between ore zones is attributed to zonation, apparently controlled by increasing sulfur and oxygen fugacities as the hydrothermal fluid migrated away from the stock. This effect was probably due to increasing small-scale meteoric ground-water interaction with magmatically derived hydrothermal fluids distally from the stock.

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