New Mexico Geological Society Annual Spring Meeting — Abstracts

Meteoric waters, evolved waters (e.g. basin brines) and magmatic fluids have distinctive nitrogen-argon-helium ratios, hence can be used as tracers of the gaseous components in geothermal fluids. Nitrogen is an indicator specie for magmatic gases, helium for evolved waters, and ratios of N₂-Ar-He similar to air saturated water indicates a meteoric source for these volatiles.

Analysis of N₂-Ar-He in fluid inclusions was performed from ore deposits in New Mexico, adjoining states, and Mexico in order to determine sources of ore fluids. There are strong correlations between fluid inclusion N2 and H2S in many ore deposits indicating a magmatic source for sulfur. Inclusions from Hansonburg, st. Cloud-U. S.Treasury, Hermosa district, Fresnillo (Mexico) and Creede (CO) that are base metal-silver deposits have high amounts of He. The N₂-Ar-He ratios are very similar to inclusion gases from Permian evaporite minerals and those in the oil fields in SE New Mexico. Inclusion gases from Cochiti, Copper Flat-Hillsburro, and sunnyside (CO) gold deposits indicate much higher N₂/Ar ratios than can be explained from a meteoric source. Rather the data indicate a significant contribution of magmatic volatiles.

Overall the data suggest that most ore fluids are mixtures of magmatic derived volatiles and crustal waters. These volatiles must separate from intrusives during crystallization and mix with convectively driven crustal waters. Silver base metal deposits are indicated to be associated with evolved waters that have elevated salinities that have limited additions of H₂S. Gold ore solutions, on the contrary, have high levels of magmatic volatiles and H₂S. The chemical differences between the ore solutions can readily explain the contrast in ore minerals. Gold is principally transported as a H₂S-HS complex, whereas base metals and silver occur in aqueous solutions as complexes with Cl.