Modeling near-surface processes in mineral systems

by Richard Wanty, Dennis Helsel, and Karen Kelley

As part of its basic mission, the U.S. Geological Survey Mineral Resources Program studies the geochemistry of mined and unmined mineral deposits as they weather. The goal of this project is to develop models that describe the effects of several mineral deposit types on surrounding soils and water in various climatic and geologic settings. By understanding and quantifying these processes, the levels of metals and other weathering products to be expected before and after mining can be forecast as a function of deposit type, climate, and geology of the surrounding land surface. These models will allow us to evaluate the relative importance of key characteristics to the eventual effects of each deposit type

While the spatial extent of a mineral deposit can be determined with some certainty by exploration drilling, mining, and other methods, the extent of the natural and anthropogenic effects of a deposit depends on a number of processes and on the properties of the physical system. Thus, the 'footprint' of a mineral deposit will vary with climate, geology, and composition of the deposit, among other factors. The models that are developed will forecast characteristics of this footprint, an expressed need of Federal land-use management agencies, among others. One need most often expressed is the delineation of natural background versus geochemical effects due to mining. Areas with undeveloped mineral deposits may be expected to have high background levels of the metals present in the deposit, and remediation goals following mining should take these levels into consideration. The models undertaken here will provide a more accurate estimate of background levels.

The Redwell Basin in southwest Colorado shows influences from mining activities (the white portion of the stream in the left side of the photo) as well as natural acidic drainage from an iron-depositing spring (the red mound in the right-center portion of the photo). Resolving the influences of these different effects can be difficult (photo by Richard Wanty, USGS).

The project is divided into four main tasks:

• Develop a modeling database that will provide a home for data collected which facilitates analysis and model building.
• Source characterization to better characterize the primary mineral deposit geochemistry.
• Near-surface processes characterization to better understand the processes involved as deposits undergo weathering.
• Develop prototype quantitative models to demonstrate that effects can be forecast based on sound science.

There are a variety of volcanogenic massive-sulfide type deposits that show a variation in geochemistry as they weather. In this figure, it is apparent that the dissolved Zn:Cu ratio of waters draining the various deposits reflects, in general, the Zn:Cu ratios of the deposits from which they drain. Using data relationships such as these allows us to understand the chemistry of deposit drainages but also may allow us to formulate predictions of the geochemical behavior of newly discovered or newly developed deposits (figure from Robert Seal, USGS).

Once developed, models will be useful to government agencies interested in managing lands that include mineral deposits, to the mining industry to help develop mitigation, remediation and monitoring strategies, and to the public to understand the key influences on mineral deposit weathering. The geographic scope of the project will be at several sites across the United States, with intense data developed at the deposit scale, and less intensive data measured at the regional watershed scale. Measures will include critical variables such as climate, hydrologic setting, mining practices, and tailings disposal methods.

For more information

Visit our web sites: http://crustal.cr.usgs.gov/projects/gem/index.html and http://pubs.usgs.gov/of/2002/of02-195/

Contacts

Richard Wanty
U.S. Geological Survey
MS 973, Denver Federal Center
Denver, Colorado 80225
303-236-1819
rwanty@usgs.gov

or

Dennis Helsel
U.S. Geological Survey
MS 964, Denver Federal Center
Denver, Colorado 80225
dhelsel@usgs.gov

or

Karen Kelley
U.S. Geological Survey
MS 973, Denver Federal Center Denver, Colorado 80225
kdkelley@usgs.gov