Geology, vein petrography and mineral chemistry of the North Amethyst deposit, Creede mining district, Creede, Colorado

Abstract

The Oligocene Creede mining district represents one of the most prolific intermediate sulfidation-state epithermal silver and base metal mining districts worldwide. The district is located in the Central San Juan Mountains of southwestern Colorado. Since 1889, the mining at Creede yielded approximately 4.1 million tonnes of ore containing 2,400 t Ag and approximately 139,000 t Pb, 41,000 t Zn and 2,500 t Cu. The base metal and silver production has come from a number of major base metal and silver rich deposits in the central and southern parts of the Creede district located close to the town of Creede. However, the North Amethyst deposit, located at the northern end of the Creede district is known to contain significant precious metal (Au-Ag) mineralization. The ores of the North Amethyst deposit are crustiform banded, intermediate sulfidation-state epithermal veins. The veins filled dilatant zones of the Amethyst and Equity faults at or near the intersection of the two major structures. The vein zones occur at elevations from 11,288 to 8,820 ft which are hosted in a flow-laminated to massive dacite to rhyolite lava dome and a rhyolitic ash flow tuff. Mineral exploration drilling conducted from 2011-2013 at the North Amethyst deposit allowed for the unique opportunity to reevaluate how the Au-rich veins of the North Amethyst deposit formed. Macroscopic study of the drill core, combined with detailed microscopic and microanalytical techniques resulted in the definition of multiple epithermal vein stages that occurred at the North Amethyst deposit and the identification of a paragenetic sequence of mineral formation. The mineralogical and textural characteristics of the vein stages were determined by optical microscopy and back-scatter electron imaging on a scanning electron microscope. The petrographic analyses included the documentation of quartz, adularia and calcite textures to determine boiling and non-boiling conditions and interpret correlations between texture and metal grades. Electron microprobe analysis was performed on sulfide minerals to determine the geochemical characteristics of the vein stages. Particular emphasis was placed on the compositional analysis of sphalerite to 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 and are each punctuated by a breccia or a gangue stage. The earliest of the four sulfide bearing veins is the Alpha stage which was observed from the deep to shallow elevations of the deposit. Alpha stage veins exhibit boiling textures at all depths. The hydrothermal liquids forming the Alpha stage are interpreted to have cooled as they ascended from deep to shallow levels of the deposit, acquiring a higher sulfidation state (1.3 to 0.24 mole % FeS). Following the Alpha stage, the precious metal (Au-Ag) bearing vein stage known as Beta stage was formed. The Beta stage is weakly mineralized at depth but is well developed in the shallow portions of the deposit. Beta stage veins are spatially associated with Alpha stage veins. Compositional variations in the sphalerite are less pronounced. However, the paragenesis of Beta stage indicates a shift from high to low sulfidation states through the transition from argentite-acanthite to native silver at the end of the mineral deposition sequence. The late base metal sulfide-rich Stage-1 was observed in the deep part of the deposit and the Fe-poor Base Metal Sulfide stage was observed at mid-elevation of the North Amethyst deposit. These two base metal and silver stages correlate with those recognized in the central and southern parts of the Creede mining district.