OF-01-11 History, Geology, and Environmental Setting of the Tweed Mine, Pike/San Isabel National Forest, Chaffee County, Colorado


This publication describes the history, geology, and environmental setting of the Tweed mine near Monarch Pass west of Salida in Chaffee County. Includes: abbreviations and symbols, introduction, location, historical overview, mining history, geology, site description, waste and hazard characteristics, migration pathways, conclusions, references, and appendix. Digital PDF download. OF-01-11D

From the Author’s Notes:

During an abandoned mine inventory in 1994, the CGS assigned Environmental Degradation Ratings (EDRs) of 2 (significant environmental degradation) to the Tweed Mine and its associated waste-rock pile. This work was done as part of a statewide inventory of abandoned mines on or having potential environmental impacts to U.S. Forest Service-administered lands. In 1999 the U.S. Forest Service requested more information regarding the Tweed Mine.

Mine features discussed in this report are west of Salida near Monarch Pass (Figure 1) and fall within the “Southwest of Hoffman Park” inventory area (USFS-AMLIP form 12-02-384/4269-3). Adit #100 and associated waste-rock pile #200 are described in detail; other mine features within this inventory area were not considered significant environmental problems and were not included in this study (Appendix).

Information revealed in this study suggests that adit #100 is the lower working of the Tweed Mine. Adit #101 is also considered part of the Tweed Mine and is higher on the hill above adit #100. Mineral Survey No. 18695 indicates that both workings are on patented mining claims (Figure 2). Apparently, this block of patented mining claims was not plotted correctly on the Garfield PBS map (Figure 3). An accurate survey is needed to confirm the location of the patented claim block. Although the Tweed Mine is probably on private land, effluent draining from adit #100 flows onto USFS-administered land and into the Middle Fork of the South Arkansas River.

While production records are not available, the district is believed to have produced lead-silver-gold-zinc-copper ore. This publication also includes tables summarizing analytical data from water discharging from the mine adit and waste rock. The Tweed Mine discharges a moderate volume of degraded water. Effluent sampled at adit #100 exceeded water-quality standards in aluminum, manganese, zinc, iron, cadmium, fluoride, lead, and sometimes copper. However, samples from Middle Fork of the South Arkansas River both above and below the confluence with the effluent from the Tweed Mine were similar. Zinc slightly exceeded standards at both locations, but increased about 10 percent below the confluence with mine waters. This publication provides useful data for future water quality investigations or restoration.