Quiruvilca, Peru; mineral zoning and timing of wall-rock alteration relative to Cu-Pb-Zn-Ag vein-fill deposition

Abstract

Quiruvilca district Cu-Pb-Zn-Ag veins have produced over 8 million tons of ore since 1789. The veins occupy tension gashes associated with left-lateral strike-slip faults that cut the central facies of a Miocene andesitic stratovolcano complex. The district has four broadly concentric mineralogic zones from district center to edges which are based on dominant vein fill: enargite, transition, lead-zinc, and stibnite. All veins, regardless of zonal position in the district, share the same four gross paragenetic stages: pyrite, base metal, sulfosalt, and carbonate. Veins closest to the center of the district contain relatively greater proportions of the earlier assemblages. Mineralogic zoning commonly is focused on vein intersections and on a local scale reflects changes in vein width.Wall-rock alteration types from the vein margin outward and from most intense to least intense are intense sericitic, strong sericitic, moderate sericitic, strong argillic, weak argillic, and propylitic. Petrographic observations indicate that at any given location, propylitic alteration is oldest, followed by argillic, and then sericitic alteration. The zoned alteration halos formed as each inner assemblage advanced, overprinted, and replaced its adjacent outer precedent. The width and intensity of alteration halos enclosing veins increase with depth and toward the district center.Correlation between paragenetic stages of vein fill and wall-rock alteration is based upon the mineralogy and wall-rock alteration of single-stage veinlets that are zoned about major veins. The veinlet sequence in time correlates with the veinlet distribution in space; this also matches the time-space sequence documented for major veins. Most alteration occurred contemporaneously with the pyrite stage of mineralization. Ore deposition followed in the base metal stage. Thus, ore deposition in the veins in large part postdated formation of the alteration halos associated with those veins. This time sequence is corroborated by breccia dikes which cut alteration halos but precede the base metal stage.Temperatures of ore deposition, deduced from the stability of various mineral assemblages, decreased through time from >320 degrees to <230 degrees C. Fugacity of sulfur (f (sub S 2 ) ) decreased (from approximately 10 (super -6) to <10 (super -12.5) ) both with time and laterally away from the district center. The pH is inferred to have increased laterally away from the center of the district and to have decreased in time at any one spot in the wall rock.