New Mexico Mineral Symposium — Abstracts

Base metal (Cu-Zn) and precious metal (Au-Ag) mineralization of the Pinos Altos district comprises distinct chalcopyrite and sphalerite-rich assemblages hosted by the Pennsylvanian Syrena Formation and by the overlying Cretaceous Beartooth Quartzite. Replacement of calcareous and siliceous host rocks are common; structurally controlled mineralization is locally important, most notably in the Beartooth Quartzite.

Petrographic studies conducted on samples from the quartzite-hosted KB orebody indicate the presence of locally signifi¬cant quantities of bismuth and silver mineral phases, usually associated with high-grade chalcopyrite+bornite+chalcocite assemblages or occurring in cp+bn+cc associations having hematite of hypogene(?) origin. Preliminary ore petrography has identi¬fied relatively late-stage Ag and Bi minerals from these quartz¬ite-hosted assemblages, comprising native bismuth, bismuthinite (Bi₂S₃), stromeyerite (AgCuS), native silver, wittichenite (Cu₃BiS₃) and emplectite (CuBiS₂).

Paragenetic studies suggest the following sequence of mineral deposition. Early pyrite and pyrite-marcasite were followed by initial copper deposition as chalcopyrite. Zinc was introduced as sphalerite, succeeding chalcopyrite. Apparent reintroduction of copper, as cc+bn+cp, was accompanied by silver (as stromeyerite; ? and native silver?) and, subsequently, by bismuth. Apparently, early base-metal assemblages were succeeded by later introduction of Ag and Bi with attendant Cu and limited S, representing changing oxygen- and sulfur-fugacity conditions during at least two periods of mineralization.