New Mexico Geological Society Annual Spring Meeting — Abstracts

The Amethyst vein system is an epithermal caldera-associated Ag-base metal deposit. Historically, the southern end of the system has been the most productive, with ores averaging 1300 ppm Ag.

A petrographic study was made on ore and country rock samples from over 300 m of vertical section at the southernmost end of the Amethyst. The paragenetic sequence observed is complex, dominated by multiple replacement episodes and exhibiting some variations with height within the system.

Veins are primarily quartz, with up to three generations of colorless or white quartz preceding amethystine quartz. The bulk of the sulfide mineralization is within the colorless quartz, the amethyst being generally barren. Rhodochrosite with interstitial sulfides is the dominant Mn mineral in the stratigraphically lowest part of the system sampled, but disappears at higher levels in favor of Mn oxides of occasional Mn sulfides. The sulfide replacement sequence may be summarized as sphalerite to galena to chalcopyrite to pyrite to argentiferous covellite to argentite to hematite, but any or most of these mi.nerals may be minor or absent, and sequential variations appear as well.

Geochemical modelling of the mineralizing solutions indicates that the sequence of replacement observed could most simply be explained by a decrease in system temperature. This would result in thermodynamic instability of one mineral phase relative to another, causing replacement reactions. This explanation correlates well with previous work on Creede which postulated the mixing of hydrothermal solutions with cooler, less saline meteoric waters within the relatively shallow system.