Systematic variation in Galena solid-solution compositions at Santa Eulalia, Chihuahua, Mexico

Argentiferous galena is the main silver-bearing phase at both the East and West camps of the Santa Eulalia district, Chihuahua, Mexico. The silver occ urs as a coupled substitution of Ag and Sb for Pb in PbS with compositions ranging from 0.04 to 5.9 atomic percent (at. %) Ag and 0.10 to 7.2 at. perc ent Sb. Correlation analysis between Ag and Sb resulted in r values of 0.97 and 0.99 (significant at the 98% confidence level) for direct-current plas ma-atomic emission spectroscopy (DCP-AES) and microprobe analysis of galena , respectively. Discrete and crystallographically oriented inclusions of di aphorite (usually 1 mum or smaller) were common in high silver-antimony gal enas and rare in low silver types (when present, tend to be larger). These inclusions were most abundant in the core zones of the crystals and rare on the edges. Spatial and temporal variations in Ag-Sb concentrations and ratios in galen a were found mainly in the West camp of the district. High silver galenas ( maximum 5.9 at. %) are confined to skarn zones or deep manto and chimney ar eas. The Ag/Sb ratio increases in galena from depth (0.88) to the surface ( 1.04) and from south (0.76) to north (0.94), following zonation and flow pa tterns established in previous investigations. Differences in Ag-Sb substit ution in galena are also seen in different mineralization types: breccia zo nes, deep mantos, and chimneys contain more Ag-Sb (3.5 at. % avg) and have lower Ag/Sb ratios (avg ratio = 0.88) than the upper mantos, silicate and c alc-silicate orebodies, which have lower Ag-Sb concentrations (2.2 avg at. %) but higher ratios (avg ratio = 1.00). Silver and antimony substitution also correlates with sulfur isotope variat ion in the district. Within individual orebodies, the delta S-34 value incr eases with decreasing silver and antimony concentrations in galena. The sol id solution compositional variations in galena coupled with sulfur isotope values are a useful tool for inferring fluid paths and appear to reflect fl uid evolution. At Santa Eulalia, silver is disseminated to various degrees throughout the orebodies and galena solid-solution concentrations appear to be controlled by subtle physical and/or chemical gradients. This contrasts with volcanic- epithermal systems in which significant physiochemical perturbations (larg e thermal and pressure gradients, boiling, etc.) lead to dumping of preciou s metals and semimetals as sulfosalts in bonanza zones. Relatively low conc entrations of other potential ore-forming elements, namely copper and bismu th, also precluded the formation of a complex suite of silver sulfosalts an d antimonian- argentiferous galena precipitated instead. Sulfosalts at Sant a Eulalia are localized to upper mineralized zones as breccia void fillings , indicative of limited zones of boiling.