NEW HORIZONS — Soil gases as exploration guides

Most of the ore deposits mined in the past, as well as those being mined today, were discovered because they cropped out at the surface. In the U.S., most mineral deposits that crop out at the surface have probably been discovered. Now we must search for deposits that are concealed beneath the surface. One way to discover these is to measure the naturally occurring gases associated with them.

Gases are associated with most mineral deposits, both as primary constituents of ore-bearing hydrothermal fluids and as products of chemical reactions of ore minerals with ground water. Gases migrate from zones of high pressure (depth) to zones of lower pressure (the surface) and, when measured at or near the surface, can indicate the presence of buried deposits. Anomalous concentrations of sulphur gases, for example, are found above oxidizing sulphide orebodies. Carbon dioxide gas is produced when acid ground water reacts with calcareous minerals, such as calcite; oxygen is consumed in this reaction, resulting in lower levels of oxygen which coincide with high levels of carbon dioxide. Thus, antithetical anomalies of carbon dioxide and oxygen can also indicate oxidizing sulphide deposits. Hydrocarbon gases, such as methane, ethane and propane, are often associated with base and precious metal ore deposits and have been used successfully as guides for discovering concealed deposits.

In a method developed by the U.S. Geological Survey, gases are sampled by driving a metal probe into the ground at a depth of 0.75 metre and extracting the interstitial soil air. The gases contained in the soil air are analyzed at the field site using a truck-mounted mass spectrometer. Gas atoms or molecules are bombarded with electrons, producing positively charged ions which are separated according to their atomic mass unit (AMU) under the influence of electrostatic and radio-frequency fields. The ions at each AMU produce a signal that is proportional to the abundance of the gas species. Sequential scanning of AMU 2-100 takes about 45 seconds.

Soil gas studies have been conducted at several gold deposits in Nevada, and, in most cases, gas anomalies detected at the surface reflect buried deposits.

Soil gases have also been used to locate and map concealed faults, which, in turn, can be useful in the exploration for mineral deposits.

— The author is a member of the U.S. Geological Survey, based in Reno, Nev.