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Understanding Our Planet Through Chemistry

Exploration for Covered Ore Deposits

Ore deposits covered by transported overburden, such as gravels, are more difficult to locate than ore deposits that are buried in the host rock in which they formed. New research using super-sensitive analytical techniques provide scientists with a way to see through that covering.

This research is based on the idea that buried ore deposits may release trace amounts of ore-related elements that are transported through the overburden. These trace elements that are found at the surface, however, may have been originally introduced with the transportation of the overburden and don't necessarily indicate the presence of a covered ore deposit. The ability to distinguish between the trace elements already in the overburden and those migrating from an ore deposit would provide a powerful tool for subsurface exploration. Two of the methods that are currently being researched by the USGS are ground-water analysis and selective chemical extractions of overburden samples for the loosely bonded migrating elements on the surface of the gravel fragments.

Ground water collected from wells, springs, and drill holes may provide clues to the presence of covered deposits. This water moves very slowly through the overburden until it discharges at the surface as a spring or seeps into a body of water. Subsurface flow rates vary from almost zero to over 100 feet per year. The slower rates cause water to have a longer contact time with the subsurface gravels, rocks, and, if present, ore deposits, permitting minute amounts of metals to be leached from the rocks.

Drawing showing how sampling water from drill holes can be used to detect halos of ore deposits. Geochemists can sample water from previously drilled holes to detect the "halo" of an ore deposit. [24k] [77k]

Detecting gold in a ground-water dispersion pattern requires an extremely sensitive analytical technique. The USGS has developed a method for detecting gold in water at the one-part-per-trillion (ppt) determination level. One ppt could be represented by one marble on 20,000 football fields (almost 39 square miles) covered with marbles.

In this technique, gold ions are removed from relatively large-volume water samples by the use of anion-exchange resin, in a manner similar to the exchange of ions that takes place inside a commercial water softener. Later, the gold ions are stripped from the resin and analyzed using graphite-furnace, atomic- absorption spectroscopy. (AAS is discussed in the Maps of natural contamination section)

Photo of drill in the field. The USGS is working on a new method to gather information from nonproductive drill holes.[51k] [173k]

Photo of scientists in field recovering ground water sample.Using a simple device, a ground-water sample is recovered from the drill hole in hopes that it will show proximity to an ore deposit.[50k] [145k]

Photo of scientists filtering water sample with a syringe filter.The relatively large dilute water sample is filtered and stabilized prior to being transported to a mobile laboratory for analysis.[44k] [153k]

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