Gold in Those Hills

Analytical laboratory offers opportunity to mining industry

By James R. Weldy

James R. Weldy is a research engineer in the CNWRA. His responsibilities include conducting research into dose assessments for the proposed high-level radioactive waste disposal site at Yucca Mountain, Nevada. Additionally, he helped develop the Nuclear Regulatory Commission (NRC) regulations and guide against which the Yucca Mountain license application will be evaluated. Weldy also provides technical support to the NRC for projects that include uranium recovery activities.

Nineteenth-century prospectors in California and Alaska spotted gold as a shiny metal trapped in quartz or as glimmering flakes in fast-running streams. Now geologists use infrared satellite images, aerial photographs, historical records or geological surveys to identify likely gold-producing sites without ever seeing any physical sign of gold.

To determine if this potential site has sufficient gold to warrant expensive mining operations, these modern-day prospectors go out to the field, dig or drill for samples of rock and examine samples to see if they contain gold, at what depth and of what quality or grade. They then ship promising samples to an analytical laboratory to determine the concentration of gold at the site.

Using a variety of laboratory equipment and techniques, analysts can find vanishingly small traces of gold in these samples, down to the parts-per-trillion level. Although potentially accurate to the extreme, commercial laboratory analyses may require weeks to complete. In today's highly competitive mineral exploration industry, such delay and off-site analysis creates opportunities for modern-day "claim jumpers" to obtain the mineral rights before test results are returned.

Dr. English C. Pearcy, James R. Weldy and James D. Prikryl, all of the Center for Nuclear Waste Regulatory Analyses (CNWRA) at Southwest Research Institute, believed they could develop a faster and more secure analysis technique that could quantify trace amounts of gold in the field. SwRI's Advisory Committee for Research, which administers the Institute's internal research and development program, funded their effort to build a field-portable neutron irradiation chamber and develop an analytical technique to perform neutron activation analyses on samples in the field. For the technique to be commercially viable, the CNWRA team had to achieve 100 parts-per-billion (ppb) level detection capability or better. In effect, the researchers needed to be able to find 1 ounce of gold in more than 300 tons of earth.

Vials of irradiated gold ore are placed in a high-purity germanium detector. The gamma rays emitted by the decaying isotope are converted to electrical signals, which are processed as counts in the energy spectrum.

The team designed and built a portable apparatus to irradiate geological samples, converting a fraction of the normal gold (¹⁹⁷Au) in a sample into its radioactive isotope (¹⁹⁸Au). The apparatus consists of a neutron source surrounded by a high-density polyethylene case. In this instance, californium-252 (²⁵²Cf), a commonly used, short-lived radioactive isotope, served as the neutron source. The polyethylene quickly reduces the energy of the neutrons emitted by the californium, making the neutrons more likely to interact with the gold atoms. An off-the-shelf, high-purity germanium (Ge) detector counts the gamma rays released by the unstable, short-lived gold isotope. An electronic multichannel analyzer counts the number of gamma ray hits at specific energy levels, allowing the concentration of gold in the sample to be determined.

This procedure, however, results in the creation of other radioactive materials within the sample, such as sodium-24 and manganese-56, which emit high-energy gamma rays. The random scattering of these higher energy gamma rays, known as Compton scattering, causes a continuous background noise in the energy spectrum. This noise raises the lower detection limit for gamma rays in the medium-energy range, obscuring the gold peak.

Early in this project, the researchers used a 26-millicurie (mCi) capsule of ²⁵²Cf as the neutron source. To verify the concept, staff members irradiated pure silicon dioxide samples doped with gold and successfully detected gold at the 10-ppb level. They had more of a challenge, however, with U.S. Geo-logical Survey (USGS) gold ore samples. The Compton scattering from the activated soil trace elements interfered with the analysis of the gold peak. Although gold was detected in the USGS samples at concentrations of 50 ppb, the process required more than a week and still offered a large degree of uncertainty. While this effort demonstrated the validity of the field-portable, nuclear-activation analysis concept, more work was needed to develop a technique the mining industry would find useful.

In the first year's effort, the research team constructed a field-portable neutron source chamber, using high-density polyethylene to slow down the neutrons created by the 252Cf energy source. The inset shows a sample of gold ore that undergoes irradiation in the neutron source chamber. Because of the low activity and short half-life of activated radionuclides, an irradiated sample can be discarded safely in the field without special handling.

The team bought a stronger, 170-mCi ²⁵²Cf neutron source to increase the amount of radioactivity in the gold with less exposure time. The new neutron source enabled the ¹⁹⁸Au peak to be distinguished from the Compton background spectrum more easily, allowing the device to detect 36 ppb of gold in only three days with a high degree of certainty.

The CNWRA team proved in the laboratory that the system works. Exploration crews can safely and economically transport the new system in a small flatbed truck to possible gold-bearing sites, where company geologists can analyze potential ore samples quickly and accurately. This onsite analysis enables the company that first discovered the gold-bearing site to obtain mineral rights quickly, without fear of the information being leaked to competing organizations.

The Institute is negotiating with an organization that provides field services to mining companies to develop the technique further. With additional support from a client, the team plans to reduce the size and weight of the system, further shorten the time required to determine gold content and decrease the level of expertise required to operate the equipment. The Institute has applied for two patents and has published several papers based on this work.

In addition to finding and quantifying precious metals, SwRI scientists believe this technology may be useful in detecting minute amounts of harmful elements such as mercury, copper, zinc or lead in contaminated soil to aid in clean up of toxic sites.

Comments about this article? Contact Weldy at (210) 522-6800.

For information about further development of this technology, contact Dr. English Pearcy at (210) 522-5540 or epearcy@swri.org.

Published in the Summer 2001 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

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