WHERE HAVE ALL THE TAILINGS GONE?

Total production from mines (1871 through 1991) in the Animas River watershed is estimated at 18.1 million tons of ore. Of this total, mills appear to have discharged about 8.6 million tons of tailings (47 %) into streams prior to 1935. Field investigations of the Animas River fluvial deposits downstream from the mill sites and in the Animas River gravels downstream from Silverton have identified only a few isolated pockets of highly contaminated fluvial sediment. Lead isotopic and geochemical data indicate that the tailings can be largely tied to the Eureka graben ores and probably originated from the Sunnyside mills at Eureka (1899-1930). In 1999, Sunnyside Gold, Inc. removed identifiable tailings deposits immediately downstream of their old mill sites and placed them in their repository at the Mayflower mill. Although the Silver Lake mill #2, located near the confluence of Arrastra Creek with the Animas River, also released tailings into the Animas River from 1900-1914, tailings from this mill are more difficult to find in fluvial gravel. Ore from mines within the Silverton caldera processed at the Eureka mills is characterized by 206Pb/204Pb of about 18.33 and high concentrations of Ag, Pb, Mn, and Zn. In contrast, the ores processed at the Silver Lake mill # 2 from mines east of the Silverton caldera have relatively higher concentrations of Cu associated with base metals and 206Pb/204Pb > 18.7. Ore deposits that formed in the older rocks outside the caldera margin have a more radiogenic lead isotopic signature, which reflects the age of the host rocks. Geochemical studies of streambed sediment, both pre- and post-mining, indicate that the discharged tailings are dispersed in the gravel in the stream channel. The paucity of tailings from the Silver Lake mill #2 suggests that much of the tailings passed through the river system. Elevated metal concentrations presently persist in water and streambed sediment, a decade after mining ceased.