New Mexico Mineral Symposium — Abstracts

The mine known as the Phil Sheridan lode was discovered in the early days of prospecting, about 1860. The surface of the vein was worked for the very fine flour gold that resulted from decomposition of gold minerals: sylvanite, calaverite, and krennerite. Initially it was thought that this was all the gold that could be won from the veins. Trenches were dug along the veins following the strike of the vein. Several feet of overburden contained the fine flour gold they sought. The waste gravel was discarded downhill covering the exposed vein, and the telluride crystals were not recognized as containing gold.

Then in 1872 it was learned that the silvery crystals in the horn quartz were sylvanite. A new gold rush and boom resulted, and most of the claims that exist today were initiated at that time—the Phil Sheridan lode was one of these. However, the work done in previous years had covered many of the rich sources of flour gold along the strike of the vein.

In 1965 the vein was uncovered when the owner of the Phil Sheridan claim gave the Boulder County road crew permission to use the gravel to widen the road into Sunshine. It was not immediately recognized, however, until the property was sold to the present owner who investigated the area with a keen interest in geology. A 1-inch-wide vein of nearly solid "rusty gold" was discovered at the present site of the mine. This vein was at the junction of the Washburn and Phil Sheridan veins, which run parallel south of this junction. Ore occurs sporadically in these veins, so it is not practical to mine the entire vein, only the cymoid structure that carries the richest ore.

The host rock of the Phil Sheridan and Washburn veins is the Boulder Creek granodiorite, which lends simplicity to the implantation of hydrothermal ore. The fissure stands out in good contrast to the surrounding wall rock. In the older workings, the altered wall rock is covered with secondary minerals, which delineate the vein. Mineralization caused hydrothermal alteration, which produced a sericitic envelope on both sides of the vein.

The major silver telluride in these veins is petzite along with minor hessite. The most abundant gold telluride is sylvanite along with some calaverite and minor krennerite. Associated minerals include coloradoite, nagyagite, altaite, melonite, and native tellurium. The associated minerals appear to have been deposited in their own suite and at different periods of vein opening. The base-metal sulfide minerals are very scarce; however pyrite or marcasite, each of which appears in its own suite, are quite commonly associated with the tellurides. Marcasite is much less common in the veins that contain abundant gold tellurides and often is auriferous.
The slides to be presented will clarify much of the information contained herein.