SIMS measurements of δ 34S in sulfide minerals from adjacent vein and stratabound ores

Abstract

The effects of sample matrix and secondary ion energy on the instrumental fractionation of sulfur isotopes have been studied for troilite, pyrite, and galena using the Cameca IMS-3f ion microprobe. An analytical procedure is described for the measurement of δ 34S on negatively charged sulfur ions from pyrite and galena with a reproducibility better than ±1.5%. Sulfur isotope ratios were measured in pyrite and galena from adjacent stratabound manto orebodies and crosscutting veins in the Hualgayoc district of northern Peru. Isotopic compositions (δ 34S CDT) of sulfur in pyrite and galena from vein and manto deposits have a total range from −20 to 10%.. Sulfur in manto galenas has a wide range of isotopic compositions and is usually isotopically heavier than pyrite when both minerals have been analyzed in the same sample. Isotopic disequilibrium between pyrite and galena suggests precipitation from separate fluids, consistent with textural evidence that galena is paragenetically later than pyrite and often replaces it. Formation of ore minerals in the veins appears to have overlapped temporally with the later stages of manto mineralization and continued after manto formation ceased. Sulfur isotope values in pyrite and galena from veins are lower by about 5%. on average than the same sulfides in the mantos, and δ 34S values of vein galenas decrease over the course of vein formation. This shift of δ 34S toward lower values with time may be the result of increased mixing with sulfur from diagenetic sulfides or a decrease in the δ 34S value of aqueous sulfide due to oxidation of the ore fluid in the later stages of deposition. The latter hypothesis is favored because the lead isotopic compositions of the same vein minerals do not indicate an influx of sediment-derived lead and because of the appearance of barite in the final stages of several vein deposits. The majority of pyrite and galena analyses from vein and manto ores are compatible with precipitation from ore fluids having δ 34S of aqueous sulfur of 0 ± 5%.. This result indicates that the associated igneous rocks may have been the source of sulfur in the ore minerals (as well as of lead). Low values of δ 34S in manto galena may be accounted for by incorporation of sulfur derived from diagenetic iron sulfides.