Weak chemical extractions on soils are being promoted throughout the exploration world as a method for detecting mineralization that is blind to surface (Clark, J.R., 1993, Mann, A.W. et al, 1995). These techniques are being used in a wide variety of geological and climatic terrains, with apparently little regard for geological substrate, or changing soil conditions.
Early studies with weak extractions showed that the geological substrate influences the type and form of chemical response, and resulted in warnings from the authors (Bradshaw, P.M.D., et al, 1974, Fletcher, W.K., 1981) that these important factors should not be ignored. The recent resurgence of weak extractions has prompted similar warnings from experienced analysts and geochemists. The recently published Special Issue of the Journal of Geochemical Exploration on Selective Extractions contained a warning from Gwendy Hall (Hall, G.E.M., 1998a) that unbuffered extractions such as Enzyme Leach will be sensitive to changing soil pH. Neutralization of the extractant by the soil will cause a change in the amount of element extracted. Similar problems and warnings were made in the same volume by Gray et al, and Fonseca and da Silva. These warnings also form part of the Distinguished Lecture talks given by Hall, as reported in the recent issue of Explore (Hall, G.E.M., 1998b).
The Selective Extraction volume also contains results from a study I did in 1994 (Smee, B.W., 1998) over two buried epithermal gold occurrences in the semi-arid desert of Nevada, U.S.A. The two gold occurrences are known as the 5 North deposit, overlain by about 30 m of alluvial valley fill, and the 8 North deposit, covered by about 100 m of alluvium. That work compared a number of chemical extractions, including aqua regia (AR), hydroxlyamine hydrochloride (HYDHCl), sodium acetate/acetic acid (NaOAc), and enzyme leach on surface soils. The soils were classified as typical aridisols, containing little to no discernable horizon development, with the exception of a poorly developed pedogenic carbonate layer. The pH of the soils was strongly alkaline. The reader is referred to that paper for the deposit location, geological setting, and details of sampling and analysis.
Both deposits showed an increase in Ca in soils on the margins of the underlying mineralization, thus creating a double-peak or “rabbit-ear” pattern of Ca distribution. This pattern was seen in all chemical extractions used. The results of that study lead to the presentation of a hypothesis to explain these patterns. This hypothesis called for the mobilization of near-surface Ca over oxidizing mineralization in response to the release and upward movement of H+. Supporting evidence for this mobilization of Ca from over the mineralization and the re-precipitation on the margins of mineralization came from the ratio of weakly bound Ca to total Ca (as derived from the acetic acid/sodium acetate soluble Ca divided by the aqua regia soluble Ca). It was assumed that re-mobilized and re-precipitated Ca would have a slightly different composition, i.e. contain less impurities, than that of ambient pedogenic Ca and therefore would be more easily dissolved by the weaker extraction. Positive residuals from this ratio should coincide with the “rabbit-ear” pattern. This positive residual pattern did occur over both deposits.
These same samples have recently been reanalyzed using much improved ICP-MS techniques that offered lower detection limits and a larger number of elements than in the initial study. Chemex Labs of Vancouver donated the analysis as part of the up-coming I.G.E.S. field trip to Nevada, where comparative selective extraction data from a number of buried and blind deposits will be shown. The reanalysis included the same analytical extractions used in the initial study, with the exception of the enzyme leach method. As part of this reanalysis, Brenda Caughlin, Director, Laboratory Services, had the foresight to measure the pH of the leach solution for each sample after leaching had been completed but before analysis. These pH measurements were reported with the normal analytical data.
The hydroxylamine hydrochloride (HYDHCl) extraction is designed to dissolve iron and manganese oxy-hydroxide species, similar to enzyme leach, but a bit stronger. HYDHCl also has a very low neutralization potential, and should be sensitive to changes in pedogenic CaCO3. The HYDHCl leach solution pH was plotted against AR Ca (Figure 1a) and HYDHCl Ca (Figure 1b) to determine what effect, if any, a change in Ca concentration had on the leach solution strength.
The effect of a change in soil Ca concentration was dramatic. A change of only 0.5% AR Ca, from 0.5% to 1%, raised the pH of the HYDHCl leach solution from 2.7 to nearly 5, at which point the rate of increasing pH begins to flatten with increasing Ca. An almost identical pattern occurs for HYDHCl soluble Ca. This change in leach solution pH possibly explains the inverse relationship found between enzyme leach Sr (a substitute for Ca), and Fe reported in the initial study (Smee, 1998). It is apparent that the soil substrate plays an important, and possibly controlling, role in the concentration of elements leached from a soil if a low neutralization potential leach is used. Spurious element distribution patterns, completely unrelated to underlying mineralization, are probably formed where ever the concentration of alkaline soil compounds change.
The possibility that the leach solution pH is influenced to a greater degree by easily dissolved pedogenic Ca was tested by plotting the ratio of HYDHCl Ca/AR Ca against the HYDHCl leach solution pH as shown in Figure 2. A direct relationship between the Ca ratio and leach solution pH occurs over the two deposits. This clearly linear relationship raises an interesting possibility: if pedogenic carbonate is being mobilized and re-precipitated in response to buried or blind sulphide mineralization, can a simple leach solution pH measurement be used to detect this mineralization? If so, an inexpensive and rapid field technique could be used to detect mineralization in semi-arid and arid environments.
This theory was tested by plotting the Ca ratio and the HYDHCl solution pH together along the four test lines over the 5 North and 8 North deposits as shown in Figures 3 a-d. The correlation between the two variables is remarkable. The solution pH is as good or better an indicator of underlying mineralization as is the Ca ratio. Each line has a clear “rabbit-ear” response on the margins of mineralization. Lines 1 and 2, over the 5 North deposit each show a single positive response on the western and eastern end of the lines. These responses are thought to occur in the vicinity of faults that bound the mineralized block (Leinz, R.W. et al, 1998). The solution pH reveals the location of the 8 North deposit through nearly 100m of alluvial fill much better than does the Ca ratio on Line 3. Line 4 is located over a thicker portion of the 8 North deposit, but is also covered by nearly 100m of overburden. Both the Ca ratio and solution pH produces unambiguous responses over the margins of the mineralization.
Soil chemistry, particularly the Ca content, is clearly influencing the pH of poorly buffered weak leach solutions. Changes in soil Ca will produce corresponding changes in leached element concentrations that may be unrelated to mineralization. A large amount of time and money is probably being wasted in following up “anomalies” that are, in fact, simple changes in soil conditions. Anyone using weak leach methods should insist on being provided with the pH of the leach solution after the leach has been performed but before the analysis. All weak leach analysis should include the elements Ca or Sr as a check on soil variability.
These data suggest that leach solution pH may itself be a useful tool to detect covered sulphide mineralization in semi-arid to arid environments. Much more research should be done in this regard, however the cost for conducting field tests is small in comparison with analytical costs now being charged by companies providing proprietary leach analysis. A simple soil scoop, a bottle of distilled water (no buffering capacity), a test tube and a pH meter may be all that is necessary to detect blind mineralization in desert environments.
by: Barry W. Smee
Smee and Associates Consulting Ltd.
4658 Capilano Road, North Vancouver B.C. V7R 4K3
Phone 1 (604) 929-0667 Fax 1(604) 929-0662
Bradshaw, P.M.D., Thomson, I., Smee, B.W., and Larsson, J.O., 1974: The application of different analytical extractions and soil profile sampling in exploration geochemistry. J. Geochem. Explor., 3: 209-225
Clark, J. R., 1993: Enzyme-induced leaching of B-horizon soils for mineral exploration in areas of glacial overburden. Trans. Inst. Min. Metall., 102: B19-29.
Fletcher, W. K., 1981: Analytical methods in geochemical prospecting. Handbook of exploration geochemistry, Elsevier, Amsterdam, 225pp.
Fonseca, E.C., and da Silva, E.F., 1998: Application of selective extraction techniques in metal-bearing phases identification: a South European case study. J. Geochem. Explor. 61: 203-212
Gray, D. J., Lintern, M.J., and Longman, G.D., 1998: Readsorption of gold during selective extraction- observations and potential solutions. J. Geochem. Explor., 61: 21-37
Hall, G.E.M., 1998a: Analytical perspective on trace element species of interest in exploration. J. Geochem. Explor., 61: 1-19
Hall, G.E.M., 1998b: Selective leaching- a tool in identifying an element’s provenance. Explore, 101: 12
Leinz, R.W., Hoover, D.B., Fey, D.L., Smith, D.B., and Patterson, T., 1998: Electrogeochemical sampling with NEOCHIM â€” results of tests over buried gold deposits. J. Geochem. Explor., 61: 57-86.
Mann, A.W., Gay, L..M., Birrell, R.G., Webster, J.G., Brown, K.L., Mann, A.T., Humphreys, D.B., Perdrix, J.L., 1995: Mechanism of formation of mobile metal ion anomalies. Miner. Energy Res. Inst. Western Aust. Rep., 153: 407pp.
Smee, B. W., 1998: A new theory to explain the formation of soil geochemical responses over deeply covered gold mineralization in arid environments. J. Geochem. Explor., 61: 149-172
Figure 1: The relationship between aqua regia soluble Ca (a), hydroxylamine hydrochloride soluble Ca (b), and hydroxylamine hydrochloride leach solution pH after the leach has been completed but before solution analysis.
Figure 2: A linear relationship between hydroxylamine hydrochloride leach solution pH and easily leached soil Ca, as indicated by the ratio between weakly bound hydroxylamine hydrochloride soluble Ca and “total” (aqua regia soluble) Ca.
Figure 3: A comparison of the spatial responses of the Ca ratio and hydroxylamine hydrochloride leach solution pH over: (a) and (b) the 5 North deposit (approximately 30m of alluvial cover); and (c) and (d) the 8 North deposit (approximately 100m of alluvial cover).