Controls on Formation of Low-Sulfidation Epithermal Deposits in Mexico

Fluid inclusion and metal-ratio data have been compiled for 52 low-sulfidation precious metal and base metal-rich low-sulfidation epithermal deposits in Mexico. Precious metal deposits typically have inclusion salinities that range from 0 to 7.5 wt percent NaCl equiv, whereas base metal-rich deposits have salinities that are as high as 23 wt percent NaCl equiv. Salinities are typically high in fluids included in sphalerite, suggesting a genetic relationship between base metal mineralization and saline fluids. Silver/gold and Ag + Au/Pb + Zn + Cu ratios correlate with fluid inclusion salinity, a relationship that underscores the importance of chloride complexing in base metal transport and polymetallic mineralization.Fluid inclusion gas chemistry of 21 low-sulfidation epithermal deposits plotted on N2-Ar-He and N2-Ar-CH4 diagrams indicate that meteoric, mantle or evolved crustal, and magmatic fluids were present in the ore-forming hydrothermal systems, although in different proportions in individual deposits. The N2/Ar ratios of sulfide mineral fluid inclusions are all higher than that of air-saturated water, indicating a mag-matic source, whereas a significant proportion of inclusions in barren gangue minerals have N2/Ar ratios near that of air-saturated water. Plots of N2/Ar vs. H2S/Ar show a correlation between N2 and H2S concentrations. The data suggest that low-sulfidation epithermal deposits in Mexico comprise both meteoric waters and magmatic waters, with a significant contribution of H2S of magmatic origin.New oxygen and hydrogen isotope data are presented for seven deposits. Fluids responsible for precious metal and base metal deposition contain consistently heavy oxygen isotope signatures and shifts as high as +10 to +20 per mil from the meteoric water line, regardless of host rock type. Boiling and/or water-rock interaction processes alone cannot explain adequately the consistently heavy oxygen isotope signatures of Mexican low-sulfidation deposits. Rather, these results are best accounted for by a significant contribution of magmatic waters to the deep fluid, subsequently modified by water-rock interaction, boiling, and mixing with meteoric water.A classification of low-sulfidation deposits of Mexico is presented based on depth of formation and whether or not boiling is thought to have occurred in the system. Three end-member types are recognized: shallow with boiling, deep with boiling, and deep without boiling. In shallow-formed deposits boiling fluids rise to depths of <500 m below the paleowater table, and ore occupies a vertical range of a few hundred meters. In deep-formed deposits, boiling occurs at temperatures that may exceed 300°C, and ore is generally deposited between 400 and 1,000 m from the paleowater table as fluids rise within the area of liquid-vapor immiscibility.