Economic Geology

The massive sulfide replacement body in carbonate wall rocks east of the Cerro de Pasco volcanic neck contains an early association of pyrrhotite and FeS-rich sphalerite altered by late fluids to pyrite-marcasite and FeS-poor sphalerite. Rapid sulfide deposi-tion from an ore fluid that fluctuated widely in sulfidation-oxidation state is suggested by ' (1) intricate reversals and broad range in X s • ß v s (2) lack of detailed correlation on a local scale between X•es and iron-sulfide association ' (3) evidence from sulfur isotope data for disequilibrium sulfide associations; and (4) superposition and telescoping of mineral stages. The first stage of ore deposition followed the formation of an extensive replacement body of pyrite-quartz in limestone and alteration of adjacent quartz monzonite and volcanic agglomerate to sericite-pyrite-quartz. Upward and outward zoning relative to centrally located pyrrhotite pipes follows the sequence quartz-pyrrhotite-wolframite, pyrrhotite-stannite-chalcopyrite-sphalerite, and pyrrhotite (pyrite)-arsenopyrite-sphal-erite. The latter association represents the main stage of Pb-Zn ore formation and ex