A very weak partial leach that is based on an enzyme reaction strongly enhances the contrast of subtle trace-element anomalies in soils that have formed on glacial overburden. In many cases the anomalies cannot be detected with stronger leaching techniques. The products of the enzyme reaction preferentially leach part of the manganese oxide coatings on mineral grains without attacking their matrix. Glucose oxidase reacts with dextrose, oxygen and water to produce trace amounts of hydrogen peroxide and gluconic acid. Dilute hydrogen peroxide reduces and dissolves the more reactive forms of manganese oxide (primarily, amorphous manganese dioxide), releasing trace elements trapped in the coatings. Gluconic acid complexes the metals and holds them in solution. Once the products of the enzyme reaction are no longer being consumed the reaction slows. The concentration of hydrogen peroxide probably never exceeds 40 mug/cm3, and sufficient gluconic acid is produced to complex the metals that are solubilized by the process. This self-limiting characteristic of the enzyme reaction minimizes the leaching of mineral substrates and is responsible for the enhanced contrast of trace-element anomalies. The concentrations of most trace elements in the resulting solutions are in the parts per trillion to low ppb range. Consequently, instrumental analytical techniques that have very low detection limits, such as inductively coupled plasma-mass spectrometry (ICP-MS), are required for many trace-element determinations.
Pilot studies were conducted in northern Minnesota, U.S.A., in which A- and B-horizon soils, aspen and willow stems and soil-gas samples were collected and analysed. The analyses of B-horizon soils produced more consistent data than those of A-horizon soils and B-horizon soils were found to be a more practical sample medium than vegetation or soil gases. Enzyme-leach data from B-horizon soils show greater anomaly contrasts for many trace elements than data from stronger leaching techniques that were tested. In many instances the source for an anomaly seems to be either basal till or bedrock. Groundwater flow is probably the most important mechanism for transporting metals towards the surface, although ionic diffusion, electrochemical gradients, vapour transport of volatile phases and capillary action may play a role in the dispersal of elements to produce anomalies. It appears that a complex interaction between groundwater, vegetation and soil produces high-contrast B-horizon soil anomalies.
The enzyme leach has been used in a U.S. Geological Survey regional mineral assessment programme in northern Minnesota. In that area crystalline Archaean basement is covered by two basal tills, which were deposited by ice advances from two different directions, and by lake sediments that were deposited in Glacial Lake Agassiz. Analyses of B-horizon soil samples collected throughout the region revealed Co, Tl and Ag anomalies that are apparently related to basement structures. The anomalies suggest the presence of a previously unsuspected Proterozoic vein-Ag district that is similar to those at Thunder Bay and Cobalt, Ontario, Canada. Anomalies of Ag, Bi, Te and W, which are spatially associated with known quartz-chlorite-carbonate-altered shear zones buried beneath the transported overburden, are interpreted as indicating a potential for lode-Au deposits.