THE SWEET HOME RHODOCHROSITE SPECIMEN MINE, ALMA DISTRICT, CENTRAL COLORADO: THE PORPHYRY MOLYBDENUM – FLUORINE CONNECTION

Intermediate sulfidation veins containing quartz-sphalerite-tetrahedrite-rhodochrosite-fluorite in the Sweet Home Mine, Alma district, Colorado were originally mined for silver starting in 1873. For the last thirteen years until October 2004, however, the mine has produced world-class rhodochrosite specimens. Some of these specimens are considered to be among the finest mineral specimens ever produced, and certainly the finest of their species, with values well over $1 million U.S. dollars. The extraction, preparation and marketing techniques pioneered at the Sweet Home operation have revolutionized the minerals specimen industry.

The Sweet Home deposit is interpreted here as a failed (single pulse) variant of a Climax-type porphyry molybdenum hydrothermal system. However unlike Climax-type systems, the hydrothermal system at Sweet Home appears to have consisted of a single, relatively small, pulse of magmatic fluid that slowly cooled and diluted with groundwater. This is inferred to have occurred at moderate depths, on the order of 1.5-2.5 kilometers below the surface.

The fluids that formed the Sweet Home veins were dilute (approximately 2-4% NaCl equivalent), high temperature (up to 370 degrees C) and of magmatic origin. Gem quality ruby red rhodochrosite at Sweet Home is very nearly pure manganese carbonate, with minimal solid solution with Fe+2, Ca or Mg. It formed at higher temperatures and salinities in comparison to lower value, pink rhodochrosite. There is a distinct association of gemmy, ruby-red rhodochrosites with highly evolved silica-rich hydrothermal systems; the high fluorine content typical of such systems suggests that Mn was transported in solution as fluorine complexes, which in turn favors rhodochrosite deposition at above average temperatures and with minimal cation contamination.