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Fluid inclusion studies of vein, pipe, and replacement deposits, northwestern San Juan Mountains, Colorado

March 4, 1975

Base and precious metal ores valued in excess of $300 million have been mined from vein, pipe, and replacement deposits located on the northwest side of, and structurally related to, the middle Tertiary Silverton caldera. A belt of normal faults radial to the caldera contains veins which have been mined for as much as 15,000 feet (4,600 m) along strike and 3,700 feet (1,100 m) downdip. Structural relations, metal distributions, and fluid inclusion data suggest that many of the veins were open at the same general time and received the same mineralization.

Fluid inclusions in quartz and sphalerite of the main ore stage of the Argentine and adjoining veins have filling temperatures ranging from 315 degrees to 249 degrees C and salinities from 7.9 to 0.1 wt percent NaCl equivalent. However, the majority of inclusions from base metal and gold stages are in a relatively narrow range near 280 degrees to 290 degrees C and salinities are unusually low (less than 2 wt %). Filling temperatures in postsulfide quartz and fluorite are as low as 153 degrees C and salinities are generally less than 0.5 wt percent. A pressure correction of about +25 degrees C should be added to the filling temperatures. Ore fluids were physically homogeneous liquids over the 3,700-foot (1,100 m) vertical and 11,200-foot (3,400 m) lateral range studied and no systematic variations in filling temperatures or salinity in this space are noted, but significant fluctuations are evident locally.

Recently developed base metal replacement deposits occur in the Eocene Telluride Conglomerate adjacent to many metal-bearing veins. Calc-silicate alteration of the conglomerate host was probably by moderately saline fluids that ranged in temperature from 320 degrees to about 400 degrees C. Quartz-bearing clasts in altered zones contain secondary halitebearing inclusions with filling temperatures of about 250 degrees C and salinities about 34 wt percent; such inclusions are not known from the base metal stage and are interpreted to have been preore. Hydrothermal quartz, sphalerite, and calcite from replacement pods contain inclusions with filling temperatures ranging from 204 degrees to 309 degrees C and salinities from 11.2 to 0.2 wt percent. That the replacement zones and throughgoing veins are genetically related is suggested by the similarity of filling temperatures and compositions of base metal-stage fluid inclusions and by structural evidence.

Small high-grade pipe deposits of base and precious metals occur along the ring-fault zone of the Silverton caldera. Filling temperatures for inclusions in quartz, sphalerite, and barite of the main ore stage of these deposits range from 216 degrees to 308 degrees C and salinities range from 1.6 to 0.2 wt percent NaCl. Quartz in altered wall rocks and silicified capping contains moderately saline secondary inclusions with relatively uniform liquid: vapor ratios that suggest filling temperatures of about 225 degrees to 300 degrees C. From the available samples there is no fluid inclusion evidence for boiling. Throttling, which had been hypothesized to explain the telescoped nature of the pipe ores, is neither substantiated nor discredited by the inclusion data.

Publication Year 1975
Title Fluid inclusion studies of vein, pipe, and replacement deposits, northwestern San Juan Mountains, Colorado
DOI 10.2113/gsecongeo.70.8.1448
Authors J. Thomas Nash
Publication Type Article
Publication Subtype Journal Article
Series Title Economic Geology
Index ID 70208898
Record Source USGS Publications Warehouse