VEIN PETROGRAPHY, PARAGENESIS, AND MINERAL CHEMISTRY OF THE NORTH AMETHYST DEPOSIT, CREEDE MINING DISTRICT, COLORADO: INSIGHTS INTO FLUID EVOLUTION OF AN INTERMEDIATE-SULFIDATION EPITHERMAL DEPOSIT

The Oligocene Creede mining district in southwestern Colorado represents one of the most prolific intermediate-sulfidation epithermal silver and base metal mining districts worldwide. Although the Creede mining district is renowned for its silver and base metal production, zones of significant Au-Ag enrichment are located in the northern end of the district at the North Amethyst deposit. This study examined the precious metal- and base metal-rich epithermal veins intersected by recent exploration drilling at this deposit. Macroscopic study of the drill core, combined with detailed microscopic and microanalytical techniques, resulted in the definition of multiple epithermal vein stages and the identification of a paragenetic sequence of mineral formation. The mineral, mineral chemistry and textural results of this study allowed for the interpretation of boiling and non-boiling conditions and constrain the temperature and sulfidation state of the hydrothermal liquids which formed the various ore-bearing vein stages.

Four sulfide-bearing vein stages were observed at the North Amethyst deposit. The earliest veins belong to a manganese-rich gangue stage which exhibits boiling textures at all depths of the deposit. The hydrothermal liquids that formed these veins are interpreted to have cooled as they ascended from deep to shallow levels of the deposit, acquiring an increasingly higher sulfidation state (1.3 to 0.24 mole % FeS in sphalerite). Formation of the manganese-rich gangue stage was followed by a precious metal (Au-Ag)-rich sulfide stage. Compositional variations in sphalerite from this stage are less pronounced. However, the paragenetic sequence of mineral formation, in particular the observed transition from argentite-acanthite precipitation to native silver, is indicative of a shift towards progressively lower sulfidation states during continued fluid evolution. The two early vein stages at North Amethyst are cut by two base metal-rich vein stages, which appear to correlate with those recorded at the deposits in the central and southern parts of the Creede mining district.