FACTS & FIGURES — USGS deciphers mineralized locations

Laboratory analyses provide vital geochemical information that is not available in the field. U.S. Geological Survey (USGS) geochemists work with branches within the organization and with other federal and state agencies to decipher mineralized locations, geologic processes, and water-rock interaction. Their research leads to a greater understanding of man’s environmental and mining impact, the origin of mineral deposits, and the location of concealed deposits.

USGS chemists and technicians routinely prepare and analyze 40,000 geologic material samples annually in the complex search for major, minor, trace and ultra-trace elements using both the most modern and traditional techniques. They probe samples for elements existing in a range anywhere from a large percentage to less than one part per billion (ppb).

Our analytical tools for partial- and total-element determinations include atomic and molecular spectroscopy, X-ray spectroscopy, mass spectrometry, neutron activation, and electrochemistry.

One new technique, Induction Coupled Plasma Mass Spectrometry (ICP-MS), has proven valuable in the search for trace elements in enzyme-leached extracts of the north-central U.S. soil and glacial till. This technique is highly sensitive to a variety of elements that may be useful in themselves, or may be pathfinder elements that indicate the presence of a nearby valuable mineral deposit. It is often easier to find these pathfinders than the desired elements. For instance, when geologists search for gold, they almost certainly look for arsenic as well.

In some deposits, arsenic indicates the presence of gold, and because it is more abundant, it is easier to find. Research on the association of ultra-trace pathfinder elements in ground water from the Getchell Trend, Nev., points to water-rock interactions that may be used to look for disseminated gold deposits.

In ICP-MS, a rock sample is digested into liquid solution and nebulized into an intensely hot (18,000 F) plasma. Then, with the aid of a computer, the ICP-MS sorts the ions by their mass and determines their quantity. This method can precisely determine extremely small amounts of rare-earth elements present in all rocks. The relative abundance of these elements creates a pattern that helps us differentiate mineralization sources.

ICP-MS can also determine elemental concentrations spanning six orders of magnitude. You could compare this range of analyses to not only seeing an individual tree in a distant forest, but also counting the whiskers on a squirrel at its base. This tool has permitted greater understanding of world-class platinum-group elements (pge) ore deposits, where concentrations of pges may be a relatively high 350,000 ppb. At the lower extreme, it has aided in the exploration for these elements by detecting as low as 0.5 ppb. Sensitive techniques for small samples have permitted relatively rapid evaluation of non-conventional geo-electrical techniques and have made possible the analysis of minor elements in minute fluid inclusions from ore samples. Because of their small size, these fluid inclusions, trapped like bubbles when the ore was formed, have traditionally been very hard to analyze by previously available techniques.

A new development in the field of ICP-MS includes the introduction of the sample into the spectrometer via laser beam vaporization. While this only allows semi-quantitative analysis for major elements, its superior sensitivity allows the analysis of trace elements in micro-size samples, something that electron microprobes are not capable of doing. Analytical advances such as ICP have led to a need for very precise sample preparation techniques, such as the fluid-digestion process mentioned earlier. The USGS has improved the often labor-intensive stage of chemical preparation through the use of robotics. This technology has improved the production efficiency of high-volume methods such as ICP-atomic emission spectrometry and atomic absorption spectrometry, and has increased the precision of analytical methodologies by maintaining consistency through the sample digestion process.

— J.E. Taggart and David Detra work in the USGS’s geochemistry branch.