Abstract

The Real de Angeles low-grade, bulk tonnage deposit is located in the southeastern part of Zacatecas, on the western edge of the Mesa Central in transition with the Sierra Madre Occidental volcanic province. Carbonaceous sandstones and siltstones of Cretaceous age host the mineralization, but no associated intrusions are known. Production of Ag, Pb, Zn, and Cd at 10,000 tons per day began early in 1982 by open-pit methods. Calculated reserves are 85 million metric tons at 75 g/metric ton Ag, 1.0 percent Pb, 0.92 percent Zn, and recoverable amounts of Cd.The ore minerals occur in thin (1-3 cm wide), discontinuous veinlets related to sedimentary deformation and diagenesis, and to a lesser extent, in fractures and faults of Laramide age. Cavity-filling textures are common. Sulfide mineralization is overlain by a clay alteration assemblage that is overprinted by oxidation. Alteration within the sulfide zone consists of silicification and thin envelopes of propylitic and potassic alteration around the veinlets. Minor occurrences of hornfelsic alteration are observed in the deeper zones of the orebody.Pyrrhotite, sphalerite, and galena are the most common sulfides, with lesser amounts of arsenopyrite and pyrite. Silver occurs primarily as a solid solution in galena of up to 0.77 wt percent and reflects the coupled substitution 2Pb = Ag + Sb + or - Bi. Other silver phases of microscopic size and trace occurrence include freibergite ((Ag,Cu) 10 (Fe,Zn) 2 Sb 4 S 13 ), stephanite (Ag 5 SbS 4 ), and argentitc (Ag 2 S). The silver, lead, and zinc minerals were emplaced in one mineralizing event.Major gangue minerals include adularia, quartz, fluorite, and calcite. Although adularia is the most abundant, quartz and fluorite accompany ore deposition, and calcite is both pre- and postore.Analysis of fluid inclusions in adularia, quartz, fluorite, and calcite indicates ore and gangue precipitation from hydrothermal solutions of 1.4 to 21.0 equiv wt percent NaCl. Quartz and fluorite fluid inclusion homogenization temperatures indicate a formation temperature of about 290 degrees C.Sulfur isotope variations are from -4 to -9 per mil delta 34 S for sphalerite, pyrrhotite, and galena. A mixing model is proposed of approximately 60 percent sedimentary sulfur with 40 percent magmatic sulfur in the hydrothermal fluid. Comparison of temperatures calculated from sulfur isotope pairs and temperatures of formation from fluid inclusions suggests isotopic disequilibrium during ore formation. This is common in sediment-hosted deposits where there is a mixing of sedimentary and magmatic sulfur.Ore-stage base metal deposition occurred under reducing conditions within the pyrrhotite stability field, although the f (sub O 2 ) -pH conditions of the fluid fluctuated both before and after the ore stage. Sulfur was transported as H 2 S. Temperatures of formation and salinity, textures of ore and gangue minerals, and hydrothermal alteration indicate an epithermal to mesothermal, fissure vein-stockwork deposit.

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