Systematic Variation in Galena Solid-Solution Compositions atSanta Eulalia, Chihuahua, Mexico

Abstract

Argentiferous galena is the main silver-bearing phase at both the East and West camps of the Santa Eulalia district, Chihuahua, Mexico. The silver occurs 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. percent 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 plasma-atomic emission spectroscopy (DCP-AES) and microprobe analysis of galena, respectively. Discrete and crystallographically oriented inclusions of diaphorite (usually 1 μ m or smaller) were common in high silver-antimony galenas 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 galena 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 areas. 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 patterns established in previous investigations. Differences in Ag-Sb substitution in galena are also seen in different mineralization types: breccia zones, 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 calc-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 variation in the district. Within individual orebodies, the δ 34S value increases with decreasing silver and antimony concentrations in galena. The solid-solution compositional variations in galena coupled with sulfur isotope values are a useful tool for inferring fluid paths and appear to reflect fluid 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 (large thermal and pressure gradients, boiling, etc.) lead to dumping of precious metals and semimetals as sulfosalts in bonanza zones. Relatively low concentrations of other potential ore-forming elements, namely copper and bismuth, also precluded the formation of a complex suite of silver sulfosalts and antimonian-argentiferous galena precipitated instead. Sulfosalts at Santa Eulalia are localized to upper mineralized zones as breccia void fillings, indicative of limited zones of boiling.