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Enzyme Leach Studies

The enzyme leach is a very selective analytical extraction approach used for detecting extremely subtle geochemical anomalies in B-horizon soils. Many ore bodies are buried beneath thick sequences of overburden, sand, colluviums, and younger volcanic rocks that show no evidence of the resource below. Over time, extremely small amounts of trace elements related to the underlying ore body can migrate to the surface by various methods, and become trapped by oxide precipitates coating mineral grains in the soils. Amorphous MnO2 is one of the most effective oxide traps, even though it is a small portion of the total Mn present in soils. Recognition of patterns is key to the proper interpretation of enzyme leach data, since the anomaly patterns can be quite different from conventional geochemical data and the background leacheable concentrations of many trace elements are in the low parts-per-billion (ppb) range, and the anomalies have a stark contrast above background.

Clark (1997) characterizes Enzyme leach anomalies have been characterized in the following way: Currently Enzyme Leach anomalies can be classified in two ways. Morphologically, there are three commonly recognized anomaly forms: 1. Halo anomalies; 2. Apical anomalies; 3. Combination anomalies. Genetically, there are also three classes: A. oxidation anomalies (sometimes referred to as oxidation halos, where they form a morphological halo); B. diffusion anomalies, which result from the gradual thermodynamic dispersal of a highly concentrated source; C. mechanical/hydromorphic dispersion anomalies. In general the enzyme leach anomalies are based on contrasts with the background levels for certain elements and are interpreted based on the morphology of the anomaly when it is viewed graphically and analyzed within the context of the genetic origin of the anomaly.

Morphologically, oxidation anomalies appear as halos caused by the dispersion of elements around a very subtle electrochemical cell that can develop at the top of a buried reduced ore body. These anomalies are characterized by an “oxidation suite” of elements that includes Cl, Br, I, As, Sb, Mo, W, Re, Se, Te, V, U, and Th. Oxidation anomalies are often in the form of asymmetrical halo or partial halo around the buried reduced ore body that can be up to 2 km beneath the surface. The oxidation halo can be associated with a variety of deposit types including: porphyry copper, massive sulfide, epithermal gold, lode gold, geothermal systems, pyritic shales, or a body that has more oxidizable material than the surrounding rock. At times volatile halide compounds and halogen gases can migrate to the surface along joints and fractures and are “pushed” outwards along electrochemical gradients to form “rabbit ear” pattern to the anomaly for those elements.

Apical anomalies are the most common morphological form of enzyme leach anomalies, and typically are related to faults and trace elements representative of the source are found as an anomaly directly above the source area. Metals and trace elements enriched in the mineralized system may be transported to the surface environment as a consequence of biomethylation by bacterial action. Dimethyl and trimethyl compounds of many elements can be highly mobile in a vapor phase that transports the elements to the surface where it is detected as an enzyme leach anomaly. Many of the apical anomalies manifest themselves as linear anomalies that follow the fault or buried linear structure. Combination anomalies are hybrid varieties that may share characteristics of both oxidation and apical anomalies, and there can be an endless variety, including overlapping anomalies that become very complex to decipher.

The method of utilizing enzyme leach data in the exploration for buried mineral deposits is based on detecting very subtle geochemical signatures using a highly selective extraction process. Activation Laboratories in Ontario, Canada is one of the primary resources for this exploration method and has conducted numerous case studies during exploration of mineral and petroleum deposits throughout the Americas since 1991, including an example of a gold property in Sonora, Mexico.

The preferred method for enzyme leach is to select samples from B-horizon soils, where they are available, samples from the A-horizon or C-horizon. Samples are collected on a grid where sample spacing is determined based on the scale of the deposit that is being looked for. The individual samples are prepared by air drying so as not to allow them to overheat, and then the samples are handled under a process that is described in detail by Clark, using dextrose and glucose oxidase solutions and then leaching the elements that are analyzed by ICP-MS for a package of 60 elements.

In order to detect and interpret the enzyme leach anomalies, the data are presented on the sample grid and the individual sample values for elements are plotted against the background level for those elements. If the sampling is conducted as isolated traverses, then the data can be viewed as a spreadsheet graph that produces a geochemical profile for each element that shows a contrast to the background level. Various factors for environmental setting and the effect of groundwater interactions must be determined on a case by case study, which Activation Laboratories have documented from a number of sites.

Below are shown several examples of enzyme leach data provided by Activation Laboratories from a gold prospect in the Sonoran Desert of Mexico, in an area comparable to the Canasta Dorada property.

The use of oxidation anomalies proved effective during exploration of the Sleeper bonanza-gold deposit near Winnemucca, Nevada during 1989-1991. Initial data collected consisted of sampling from B-horizon soils along traverses across the mineralized structure in areas of where there was 15-37 m of overburden. This data showed both apical and oxidation anomalies. It has been reported that oxidation anomalies often show more subdued contrasts in humid climates but work well in semi-arid and arid environments, such as the Sonoran desert.

Apical anomalies are strongest over the source, rather than forming a halo around the source. These anomalies appears to be one of the most common enzyme leach anomalies, and generally the suite of trace elements reflect the source, forming over a point or a fault which facilitate the movement of trace elements to the surface environment. Interestingly, Au and Hg are not soluble in the enzyme leach, yet these elements can form anomalies within areas of an oxidation anomaly. This was found at the Rabbit Creek deposit in Nevada, where an enzyme leach gold anomaly showed values above 0.1 ppb Au to as high as 0.6 ppb Au in areas above the deposit where there are 50-200 m of overburden. These samples were also anomalous in oxidation elements, Cl, Br, I, and Mo and it is thought that the gold migrated as AuCl3 as a volatile through the overburden.

At Canasta Dorada, vast portions of the property are covered by pediment and colluvium to an undetermined depth. In many cases, this recent cover appears to blanket an undulating bedrock surface that slopes away from the range fronts. The use of enzyme leach has been shown to be effective in the Sonoran Desert environment by Activation Laboratories, and may prove to be an effective method at Canasta Dorada to identify areas where mineralized low-angle structures, faults, and mineralized bodies are present beneath the surface. In particular, extensions of the Big Pit mineralization to the west and possible extension northwest along trend of the thrust faults.

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