An ash-flow tuff sequence characterizes the rocks of the Moctezuma district, Sonora. Within the sequence, a 50-m-thick unit, the Arenillas Formation, consists of volcaniclastic sedimentary rocks and limestone lenses. Tensional deformation of the region has produced basin-and-range structure. A strong north-trending normal fault forms the western boundary of the principal range of the district, normal faults on the east side of the range define a horst, and the sequence is tilted 10 degrees to 30 degrees east. At the Oposura mine, a Zn-Pb skarn deposit in the limestone of the Arenillas Formation contains galena, sphalerite, magnetite, bustamite, Mn-bearing andradite, hedenbergitic pyroxene, and strong late chloritic alteration. Elsewhere in the district, steep quartz veins contain three characteristic associations: (1) Au with Te, Ag, and barite, (2) Ag with galena, sphalerite, tetrahedrite, and high values of Sb, As, and Mn, and (3) galena, sphalerite, chalcopyrite, and ankerite with low Ag and Au values. Structural reconstructions suggest that these vein deposits developed at different depths, with the first association shallowest, the third association deepest. The skarn may have formed deeper than all the others.

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