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PARTIAL LEACHES AND TOTAL LEACHES A major drawback with total leach analytical methods is that they often limit the use of some of the new modern analytical technologies, such as ICP-MS, due to high solution ionic strengths, thereby restricting lower assay limits to relatively high levels (~1ppm). By selective or partial leaching unmilled material and dissolving a more weakly bound component, solution ionic strengths are much lower and detection limits can be significantly lowered. Partial and selective leach techniques normally only take into solution a small proportion of a particular analyte occurring in a sample (the total value). They dissolve weakly bound ions (cations or anions) from the sample matrix (soil, weathered rock fragments) and those added to the sample from ground water, or other sources and weakly attached to the matrix. Selective leaches are designed to only attack specific components within a sample. Some methods dissolve precipitated carbonates or other salts, while others attack Mn-oxides or amorphous Fe-oxides, or strip ions held in organic complexes. Partial leaches, which often have a low pH (acid), might attack all of these to some extent. The degree of dissolution depends to a large extent on the intensity of weathering a sample has undergone and resultant matrix mineral species in the sample.
Depending on the local superficial environment the anomalous component can remain associated with amorphous Fe or Mn oxide or its aggregated equivalent (eg. laterite, gossan etc), be partitioned into a carbonate or sulphate phase or be adsorbed or react with some organic phase. Partial leaches that remove the anomalous component from all of these probably best determines the "total" partial leachable component transported from depth to the surface and are also less susceptible to local superficial environment variations especially in areas where extensive recent sheet wash erosion occurs.
The more robust partial extraction techniques will always see or produce an anomaly related to mineralisation where reported by total assay methods. (In some circumstances, Au may provide an exception to this). The reverse is however not true. There are many instances where total assays report below limit of detection (BLD) but partial leach assays produce good coherent multi-element responses at much lower levels. Some leach methods are better than others for different elements and in different environments hence the detection limits for different leach procedures can vary.
From Grey (1999) - Soil data above mineralisation under transported cover.
From Grey (1999) Soil data (red is bedrock total assay and site of mineralisation under cover) Partial leach methods, because of their much lower detection limits, also reliably measure background variation and this is seen in line profile data as changes in the apparent noise level along a line. We are so used to seeing flat essentially featureless backgrounds from total assay methods that when presented with the real character of background populations from partial leach data we tend to blame the method rather than acknowledging what the real world variation in natural geochemistry is like.
From Seneshen (1999) - Note anomaly zone relationship to background even with what is regarded as a strong leach (aqua regia) compared with a weak selective leach (enzyme leach). While partial leach analysis offers advantages over conventional soil assay methods in weathered terrains or in areas of cover there is a trade off. The greater analytical sensitivity means greater susceptibility to superficial factors, (or at least the effects of this are more obvious), and these need to be assessed by assaying for a number of elements that most strongly reflect surface geochemical processes to assist in data interpretation. The most important in this regard are iron, manganese, calcium (calcretes), sodium, magnesium, and sulphur (sulphate e.g. gypsum). The presence of Fe or Mn oxides, calcretes or gypsum gives an indication of likely soil pH values for example, or sites with seepages etc. This in turn influences the dispersion, or lack of dispersion, of ''mobile'' elements transported in ground water.
From Seneshen (1999) - Depicts elevated Co associated with Mn at sites with thin Ao soil cover. The source of any partial or selective leach anomaly may be from beneath or to one side of it. There is no way to discriminate the source direction other than by eliminating one or the other by logging sample sites carefully. In deeply weathered terrains, for example, the re-distribution of products such as lateritic pisolites, ferricrete and lag by sheet wash erosion into drainage systems creates a supply of potentially anomalous material that can release analytes of interest (Cu, Zn, Pb, As, Sb etc). Buried accumulations of these weathering products may break down under hydration, redox change or pH change to release ore elements into the ground water system and ultimately into the soil profile where they may report as anomalism. Such anomalism may be many kilometres removed from the original source of mineralisation from which the original pisolite
From Seneshen (1999) - Depicts elevated Co associated with Mn at sites with thin Ao soil cover. Areas with thin cover probably have deeper bedrock oxidation and this permits greater Mn mobility. Mn is reduced/precipitated adjacent to these sites. Vegetation sampling offers another way in which loosely bound anions and cations transported into the superficial environment may be measured. The very low matrix effects in analysing vegetation by neutron activation (INAA) permits levels of detection comparable to those achieved for partial leaches for many trace elements. For some key elements, such as Au, As and Sb, INAA detection limits are well below those routinely quoted for total leach assays (Cohen et al., 1998). |
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