THE USE OF FLUORIDE AS A NATURAL TRACER IN WATER, SAN JUAN MOUNTAINS, SILVERTON, COLORADO

A combination of traditional geologic mapping, alteration-type mapping, and geochemical analysis of water samples indicates a relationship between geologic and hydrologic features that may be used to better understand the provenance and evolution of the water. Investigations conducted within the Silverton caldera, in the San Juan Mountains of southwestern Colorado indicates that veins containing fluorite, huebnerite, and elevated molybdenum concentrations are associated with the emplacement of high silica rhyolites and with mineralizing events that produce waters containing elevated concentrations of fluoride along with potassium and beryllium. This unique aqueous geochemical signature can be used to relate water chemistry to key geologic features and mineralized source areas. Two examples that illustrate this relationship are (1) surface-water samples containing elevated fluoride concentrations (greater than 2 milligrams per liter) that closely bracket the extent of several small high silica rhyolite intrusions: (2) water samples containing elevated concentrations of fluoride (greater than 2 milligrams per liter) that spatially relate to mines or areas that contain late-stage fluorite/huebnerite veins. In two additional cases, the existence of high fluoride concentrations in water can be used to (1) infer interaction of the water with mine waste derived from systems known to contain the fluorite/huebnerite association; and (2) relate changes in water quality over time at a high elevation mine tunnel to plugging of a lower elevation mine tunnel and the subsequent rise of the water table into mineralized areas containing fluorite/huebnerite veining. Thus, the unique geochemical signature of the water produced from fluorite veins indicates the location of high-silica rhyolites, mines, and mine waste containing the veins. Existence of high fluoride concentrations along with potassium and beryllium in water in combination with other geologic evidence may be used to better understand the provenance of the water.