A challenge for the geochemist is to develop and test transport theories and near surface sampling techniques for use in areas overlain by cover of exotic origin, i.e., where the surficial cover is not in any way related to the underlying bedrock. Selective extraction methods that dissolve certain minerals or attack specific binding sites in soil suppose that some form of upward migration of ions must be taking place. Chemical components related to mineralization must travel from the bed-rock-overburden interface to the surface, where they are immobilized, or change the surface chemistry in some detectable fashion. Upward migration is likely caused by a combination of diffusion down a concentration gradient, capillary action or “wicking” from the water table to the dry surface, osmotic action carrying ionic species, vegetation roots carrying elements to the surface portions of the plant, then back to the soil, or electrical fields transporting ions through the soil by cation exchange.
A geochemical orientation survey over two buried epithermal gold 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. The 5 North deposit is covered by 25-40m of alluvial fill, and the 8 North deposit is covered by about 100m of fill. Soil gas, CHIM, vegetation, lag, caliche, and soil samples were included in the survey. The soil samples were subjected to a variety of chemical attacks and analyses. This report will present some of the results from the soil samples and propose a theory for the formation of surface geochemical responses.
The pH of the soil in the Marigold area was extremely alkaline, but still produced nearly a 100-fold difference in hydrogen ion concentration over the deposits. The alkaline condition suggests that iron and manganese should be stable, but that calcium may move in response to a shift in soil pH conditions. Aqua regia soluble calcium, hydroxylamine hydrochloride soluble calcium and strontium, sodium acetate soluble calcium and strontium and enzyme leach soluble strontium produced clear double-peak responses to mineralization regardless of overburden thickness. In fact, the type of analytical attack used did not really affect results. Low detection limit gold in soil was significantly correlated with calcium and strontium.
The proposed geochemical model suggests that the release of H+ during the oxidation of buried sulphide mineralization may migrate directly to the surface, or may react with surrounding carbonate, thus producing CO2 which migrates to the surface. In either case, a disequilibrium in pH sensitive compounds or elements such as CaCO3, MgCO
3, Fe and Mn will occur at the soil surface in places of accumulation of H+ or CO 2. This disequilibrium will correct itself over time by the migration of these compounds or elements away from the stimuli toward the ambient pH condition. A precipitation of these pH sensitive components will occur at the positions where chemical stability is re-established-at the margin of the stimuli-thus producing double-peak patterns over the margins of the underlying mineralization. Co-precipitation of ambient ions in the soil will produce element patterns that may appear to have moved through the overburden column, but have not. Calcium and magnesium may be the controlling ions in alkaline soils such as are found in desert environments, whereas iron and manganese may control anomaly patterns in acid to neutral soil conditions found in temperate climates.
The use of calcium in closely spaced soil samples in alkaline environments and selected element ratios, together with low detection limit gold, arsenic and antimony, is recommended as an exploration method for buried or blind mineral deposits.
Results of a test of partial extraction techniques
In: Exploration 97 Symposium Volume, Toronto: 301-314.