B-54 Natural Acid Rock Drainage Associated with Hydrothermally Altered Terrane in Colorado


This study examined specific areas in Colorado that have natural surface-water of poor quality because of the local geological regime. Streams in eleven headwater areas of Colorado where surface water is acidic and has high concentrations of metals well upstream of any significant human impact areas were mapped and sampled. Through the detailed geologic field work, the type and intensity of hydrothermal alteration was mapped and the geology was correlated with surface-water chemistry. Many of the areas exhibiting intense hydrothermal alteration also contain historic mine sites. Because acid rock drainage from natural sources and mine sites combine to cause severe downstream water quality problems, it is important to differentiate the natural, or background water quality so that realistic cleanup goals for total water quality may be set. Digital PDF download. B-54D

Water can be naturally degraded by flowing through saline geologic deposits, such as geologic shallow basin evaporite deposits. This can show up at the land surface as saline springs or deeper in drilled wells.

Waters can also be naturally degraded by acid drainage generated in hydrothermally altered geologic areas. Acid drainage is acidic water (pH <5.0), laden with sulfate and metals that forms under natural conditions when geologic strata containing pyrite are exposed to the atmosphere or oxidizing environments. The typical metals include iron, manganese, aluminum, cadmium, and zinc. The water may be visibly discolored (usually red due to the high iron content). Areas of naturally degraded water commonly have names such as Red Mountain, Red Cone, Iron Creek, and Cement Creek (named for the ferricrete deposits that line its banks).

Hydrothermal activity often creates natural zones of mineral alteration that extend well beyond the immediate location of concentrated ore mineralization. In fact, large areas of hydrothermally altered terrane extend from mined areas to areas without any associated mineral production. Mineral alteration can be a source of hydrologic contamination on a scale that is not immediately evident. Thus, the extent of environmental impact derived from hydrothermally altered areas unaffected by economic mineral production or other significant anthropogenic influence is a subject that historically has not received much attention.

UPDATE: The Association of American State Geologists announced that their annual John C. Frye Memorial Award for 2012 is granted to the CGS and the staff members who authored B-54.