A new theory to explain the formation of soil geochemical responses over deeply covered gold mineralization in arid environments

Abstract

Mineral exploration is extending into geologically prospective areas which are overlain by surficial cover. Geochemical techniques using chemical reagents or analytical procedures that selectively dissolve certain minerals or attack specific ion-binding sites in soil have been suggested to enhance the detection of buried mineral deposits. Few of these techniques have been tested one against the other in a controlled environment, and no mechanism of transporting elements through overburden cover has been proposed that explains observed patterns. A geochemical orientation survey over two buried epithermal Au deposits at Marigold, Nevada was completed during 1994–95 in an attempt to determine the effectiveness of various analytical extractions, and develop a theory to explain observed geochemical responses. Soil samples were subjected to a variety of chemical attacks and analyses, including aqua regia, hydroxylamine hydrochloride, sodium acetate, enzyme leach, water-soluble anions, pH, soil conductivity, and `low detection limit' Au, As and Sb. Results from the enzyme leach method showed that Fe is being dissolved in addition to Mn. Enzyme leach- and water-soluble anions were correlated with soil conductivity. Calcium and its substitute, Sr, were correlated with soil-Au (total), as has been found in other desert environments, and these are the only elements which produced positive responses to mineralization even where mineralization was covered by 100 m of alluvial valley fill. Double-peak responses to mineralization occurred regardless of the chemical extraction used. The proposed element migration model suggests that H + released during the oxidation of sulphide mineralization travels directly to the surface, or may react with carbonate in wall rocks, producing CO 2. The CO 2, in turn, migrates to the surface over time. In either situation, a disequilibrium in pH-sensitive compounds or elements such as CaCO 3, Fe and Mn oxides occurs at the soil surface in places of accumulation of H + or CO 2. This disequilibrium corrects itself over time by the migration of these compounds or elements away from the stimuli and towards the ambient pH condition. Precipitation of these pH-sensitive components occurs at the positions where chemical stability is re-established—at the margins of the stimuli—thus producing double-peak patterns over the buried mineralization. The use of Ca and Sr in closely spaced soil samples in alkaline environments, together with the measurement of Au, As and Sb to low detection limits, is recommended as an exploration method for buried or blind epithermal Au deposits.