Abstract
The Raul mine, situated within the Coastal Mesozoic Belt of Central Peru, consists of Stratabound pyrite-chalcopyrite ores in Upper Cretaceous andesitic volcanics, graywackes, and siltstones. The host rocks and ores are metamorphosed to the upper green-schist-lower amphibolite facies.The 800+ m stratigraphic sequence of the area can be divided into five units, each characterized by a predominant lithology and type of mineralization. Unit I, the oldest exposed rocks in the mine, consists of tuffs and agglomerates of andesitic composition. Unit II is composed principally of graywacke, with minor tuffaceous zones. Andesite lavas and pyroclastics compose Units III and IV. Unit V consists of siltstone with lesser amounts of tuffs and limestones. Layered sulfide-amphibolite ore horizons (mantos) occur intercalated in Units II, III, and V, whereas stringer and disseminated ore occurs in the lavas and pyroclastics of Units III and IV.Analyses of Co, Ni, Zn, Mn, Ag, and Sn content of pyrite and chalcopyrite indicate that significant differences in the minor element content of these minerals exist between units. delta 34 S values of sulfide minerals also show distinctly different distributions between units: Unit I, -10 to +3 ppm; Unit II, 0 to +9 ppm; Unit III, +9 to +23 ppm Unit IV, +8 to +12 ppm; and Unit V, 0 to +14 ppm.Temperature of ore deposition in the manto of Unit II, based on fluid inclusion filling temperatures, was about 320 degrees to 360 degrees C. Thermochemical considerations of coexisting sulfides in Unit V (pyrite + pyrrhotite + chalcopyrite) place a possible temperature range of 70 degrees to 350 degrees C for Unit V mineralization. Silicate mineral assemblages in the host rocks suggest that metamorphic temperatures were between 300 degrees and 500 degrees C. Peak metamorphic temperatures were probably caused by igneous activity related to intrusion of the Coastal Batholith.The data on mineralogy, temperature, and sulfur isotopic composition of sulfide minerals, together with information on the stability of Fe- and Cu-chloride complexes, suggest that manto ores in Units II and III formed at pH conditions of 4 to 7, from a fluid with a delta 34 S (sub Sigma S) value of +23 + or - 3 ppm, a Sigma S content from 10 (super -1) to 10 (super -2) m, and a log (Sigma SO 4 /Sigma H 2 S) ratio from 0 to +2. Manto mineralization in Unit V occurred from fluids with variable delta 34 S (sub Sigma S) values (+3 to +15 ppm). The stringer and disseminated ores in Units III and IV were formed from fluids with delta 34 S (sub Sigma S) values greater than +12 ppm for Unit IV and +23 ppm for Unit III.The delta 34 S (sub Sigma S) values of the fluids involved in pyrite-chalcopyrite deposition suggest a sea-water origin for the sulfur. A model is presented involving circulation of sea water caused by heat associated with submarine volcanic activity. Sea water sulfate was partially or totally reduced to sulfide at elevated temperatures by reaction with Fe (super 2+) -bearing minerals. Precipitation of ore minerals occurred on or near the sea floor, primarily as a result of a decease in temperature, accompanied by increases in oxidation state and pH of the fluid.
