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The Sweet Home rhodochrosite specimen mine, Alma District, Central Colorado: the porphyry molybdenum–fluorine connection

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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 13 years up until 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 the finest of their species with values well over $1 million US 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 as a single pulse variant of a Climax-type hydrothermal system. Evidence for this includes (1) an age of mineralization (25.8 ± 0.3 Ma) that coincides with the age of the end stages of mineralization of the Climax molybdenum deposit approximately 7.5 km to the northeast; (2) a geochemical (Mn, W, F) and mineralogical (topaz, fluorite, hubnerite, greisen muscovite) signature typically associated with Climax-type systems; (3) the presence of porphyry rhyolite dikes, a breccia dike, and local quartz–molybdenite veins in the nearby area; (4) a small pegmatite within the mine with an age (25.9 ± 0.3 Ma) coincident with mineralization, which also contains minor amounts of disseminated molybdenite; and (5) the presence of similar-appearing gemmy, red rhodochrosites at Climax and other high-silica rhyolite systems. A significant difference is that unlike Climax-type systems, the Sweet Home hydrothermal system 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 in the order of 1.5–2.5 km below the surface. The fluids that formed the Sweet Home veins were dilute (salinity in the order of 2–4 wt% NaCl equivalent), high-temperature (temperatures of homogenization up to 370°C), and initially of magmatic origin. Gem quality ruby-red rhodochrosite at the Sweet Home Mine is 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. Gemmy, ruby-red rhodochrosite is distinctly associated with highly evolved silica-rich igneous/hydrothermal systems. The high fluorine content typical of such systems suggests that Mn was transported in solution as fluoride complexes, which, in turn, favored rhodochrosite deposition at above-average temperatures and with minimal cation contamination.

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Kate Barbá provided some vein and wall rock samples from her work at the Sweet Home Mine; the use of these samples is greatly appreciated. The other mappers of the Alma quadrangle, Beth Widdman and Richard Madole, along with field assistants Kate Barbá and Marilyn Moll, provided much useful feedback in the field and office. Joel Brugger constructed and provided the log fO2/pH diagrams. Reviews by Graham Closs, Kalin Kouzmanov, and Joel Brugger greatly improved the paper.

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Correspondence to Paul J. Bartos.

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An erratum to this article can be found at http://dx.doi.org/10.1007/s00126-006-0116-0

Editorial handling: M. Chiaradia

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Bartos, P.J., Nelson, E.P. & Misantoni, D. The Sweet Home rhodochrosite specimen mine, Alma District, Central Colorado: the porphyry molybdenum–fluorine connection. Miner Deposita 42, 235–250 (2007). https://doi.org/10.1007/s00126-006-0103-5

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  • Rhodochrosite
  • Climax-type system
  • Sweet Home mine
  • Fluorite