Geochemical research in the 1960s and 1970s produced exploration methods that could be used to quickly explore large areas from the air using the Barringer-developed AIRTRACE collection and analytical system. Geochemical patterns seen from the analyses of dust from the soil-air interface, known as the soil micro-layer, were thought to be connected to the electrogeochemical model developed by Govett and his students in the early 1970s. This theory was closely linked to H+ release from oxidizing sulphide bodies. Additional studies from the ‘70s to the present by the author have shown that the measurement of pH and detection of pH-related patterns from the soil micro-layer may detect mineralization through appreciable cover and can be done in the field in real time at little expense. Two examples from this research are shown.
The late 1960s and early 1970s saw the introduction of innovative research into geochemical processes that remain at the forefront of applied geochemical technology after nearly forty years. The leading research school at that time was Imperial College in London under the direction of John Webb. His students defined ‘thinking outside the box’ in applied geochemistry. The following retrospective connects a few of these ideas and offers a simple method for mineral exploration through cover that was ultimately distilled from this work.
One of the more interesting aspects that appear to have been disfavored or at least lost in the last forty years was the work of W. Beauford and J. Barber at Imperial College, and G. Curtin at the USGS on the release of heavy metals to the atmosphere from plants. Much of this work was published later in the 1970s (Beauford et al. 1977; Curtin et al. 1974), but had caught the eye of Tony Barringer, one of the leading thinkers in mineral exploration technology in the late 1960s. Barringer had also been introduced to work done on the soil-air interface (the soil micro-layer) that had been shown to be a zone of accumulation of ions. Some of this work was published in the later 1970s (Barringer 1977).
Barringer, ever on the lookout for methods to explore ground quickly (he was the inventor of the proton precession magnetometer, Input Airborne EM system, the Airborne mercury analyzer, the COSPEC airborne gas analyzer amongst many other technologies) thought that if one could collect airborne particles in an aircraft as it flew close to the ground and analyze them for many elements, one could then marry airborne geophysics and geochemistry in one aircraft. Barringer was granted patents on the method itself, which he called AIRTRACE, and on the particle collection method and the method of analyses of the particles (Barringer 1973a, b).
The AIRTRACE system was mounted on either a fixed wing or helicopter equipped with ’aerodynes’ that ingested airborne particles and concentrated this dust on sticky tape that was inside the aircraft. The tape was advanced to a clean position for every five seconds of flight and a record number placed on the tape and the flight path camera film so that the location of the dust spot could be located on the ground.
This tape was analyzed in the Barringer laboratory in Toronto using the first ICP-ES and the first laser ablation system on which Barringer held a patent for many years.
Microscopic examination of ablated particles showed that only the outer layer of dust particles was consistently being vaporized. Barringer knew that AIRTRACE was collecting the particles from the soil micro-layer: the location of ion accumulation in the soil.
This was recognized as being similar to a weak or selective extraction. Very often, the pattern of element responses over mineralization was ’double-peak’ in form and restricted to the immediate vicinity of the deposits. A few published results illustrate these ’donut’ (plan) or ’rabbit-ear’ (profile) anomalies (Barringer 1977).
The Link to Electrogeochemistry
The reason for these patterns was not known. However the 1973 paper by Gerry Govett on electrogeochemical transport of ions was very timely. Govett proposed that ions might move through surface cover in response to a natural electrical force created by oxidizing sulphides. Further research both in the laboratory and the field confirmed that this was not only possible, but measurable, and produced in profile a ’rabbit-ear’ pattern (Govett 1976). Govett & Chork, (1977) showed that this pattern was being controlled by hydrogen ion movement. This work was in part funded by Barringer Research who saw the ’potential’ for explaining the AIRTRACE patterns. Govett showed that the techniques are applicable in an arid environment (Govett et al. 1984).
I studied the electrogeochemical phenomena in the laboratory and field while at the Geological Survey of Canada as part of my Ph.D. work under Govett. The field study area selected was in the varved clay belt of Northern Quebec over a VMS deposit covered by between 5 m and 30 m of till, varved clay and organic material. The varved clay was thought at the time to be a complete mask to geochemical movement.
Fig. 1. Profile from the top of varved clay over a VMS deposit in Northern Quebec showing the relationship between H+ concentration and aqua regia soluble Ca.
(adapted from Smee 1983).
Fortunately the Hudson Bay lowland-derived varved clay is alkaline and Ca-rich and provided an ideal substrate in which to detect hydrogen ion anomalies. This work showed that not only did the surface soil produce a clear hydrogen ion response above and down dip from the VMS, but also moved the pH-sensitive elements (Ca and Fe) in response to changing pH conditions as shown in Figure 1 (Smee 1983). In the case of calcite, for a pH of
This movement of pH-sensitive elements, and Ca particularly has been used by me with success for the past 25 years as a routine exploration tool (Smee 1998, 1999, 2003) and has lead to the discovery of several sulphide bodies. The use of a field pH meter and a bottle of water as a primary exploration method in alkaline terrain appears to be too easy to be true. However one can produce remarkable data with these simple tools and a drop of 10 % HCl.
Fig. 2. Sampling the soil micro-layer in arid conditions. See text for analytical method.
Using pH to highlight targets through cover in Arid Areas
Alkaline soil conditions exist in most semi-arid to arid conditions. Oxidizing sulphides should produce a change in pH in the surface soil as confirmed by Hamilton et al. (2004), and especially in the soil micro-layer where an upward moving front of H+ accumulates, as shown nearly 40 years ago. If this soil micro-layer is sampled (Fig. 2), slurried with distilled water and the pH measured, the response should be obvious.
However experience has shown that other factors such as a break-in-slope or a water-bearing structure can produce single-peak anomalies. These are somewhat confusing when attempting to interpret a soil survey.
The actual pattern in alkaline soils related directly to oxidizing sulphides should include two variables: positive H+ surrounded by an increase in Ca concentration where the mobilized calcite re-precipitates. Both of these variables can be detected by initially measuring the soil slurry pH, then by adding a drop of 10 % HCl, stirring the solution for about 10 seconds and taking the pH again.
Soils of lower pH (high H+ molar concentration) have mobilized the Ca and therefore the soil slurry will be relatively unbuffered. The addition of HCl to the solution will immediately drop the pH in these samples where calcite has been removed, but will have little effect where calcite been precipitated. These buffered soils should be on the edge of the low pH (high H+). The pattern from an oxidizing sulphide should therefore be an H+ high and surrounded by a small or no change in H+ concentration when HCl has been added.
Plotting a variable that is significant when small is rather difficult. The method developed to clearly highlight the areas of calcite precipitation on a chart or plan map involves some manipulation. To do this, the acidified H+ in moles (converted from pH) is subtracted from the non-acidified H+. The least differences are the areas of calcite precipitation. The inverse difference (1/difference) is calculated and plotted. This variable produces positive peaks which are far more pleasing to the eye. For ease these are called Inverse Difference Hydrogen (IDH) anomalies.
Two examples are shown: one from the porphyry copper-gold Hugo South Oyu Tolgoi area, Mongolia and one from a quartz reef hosted gold deposit south of the Bulyanhulu mine, Tanzania, Africa. Figures 3a-d over Line 475800 N at the south end of Hugo South shows the step-wise progression of data handling to produce both the H+ and IDH patterns in profile. The acid peak surrounded by the calcite halo is easily seen.
Fig. 3. Profiles of the step-wise handling of the pH data illustrated by Line 475800 N of the Hugo South Cu-Au deposit. The thick black line at the bottom of each figure is the surface projection of the mineralization. Figure 3a is the H+ moles *10^8 from the soil micro-layer. Figure 3b is the acidified soil slurry converted to H+ moles *10^8. Figure 3c is acidified H+ minus original H+. Figure 3d is the inverted difference from 3c. (IDH). Calcite is absent over the projection of mineralization and H+ anomaly, but occurs outboard from the mineralization. Alluvium thickness c. 40 m.
The Tanzanian example in Figure 4 is a plan contour map of the IDH anomalies together with drill holes shown as dots. There is sparse to no outcrop with much of the area covered by fluvial and alluvial deposits. The drill holes were positioned using geological mapping and geophysics rather than the pH survey. The predominance of holes shows the location of gold mineralization. The soil IDH shows the areas of mineralization in many instances, and other areas which have yet to be tested by drilling.
Fig. 4. Plan view of IDH in soils over extension of mineralized horizon from Bulyanhulu Tanzania.
Dots are drill locations. The interpretation of soil pH was done after the drilling was completed.
This inexpensive technique was developed because of imaginative and multi-disciplinary thinking nearly 40 years ago.
I would like to thank Ivanhoe Mines and Mr. Dale Sketchley for assisting in the collection of the soil samples and allowing the data over Oyu Tolgoi to be released. Barrick Gold collected the information and allowed the data from Bulyanhulu South to be shown. Tony Barringer showed me that doing what other people have done will result in finding what other people have found. Gerry Govett inspired the initial research and has remained a mentor over the years.
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Barringer, A.R. 1973b. Method and apparatus for sensing substances by analyses of adsorbed matter associated with atmospheric particles. US Patent # 3768302.
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Govett, G.J.S., Dunlop, A.C., & Atherden, P.R. 1984. Electrogeochemical techniques in deeply weathered terrain in Australia. Journal of Geochemical Exploration, 21, 311331.
Hamilton S.M., Cameron E.M., McClenaghan M.B., & Hall, G.E.M. 2004. Redox, pH and SP variation over mineralization in thick glacial overburden. Part II: field investigation at Cross Lake VMS property. Geochemistry: Exploration, Environment, Analysis, 4, 45-58.
Smee, B.W. 1982. Laboratory and field evidence in support of the electrogeochemically enhanced migration of ions through glaciolacustrine sediment. Journal of Geochemical Exploration, 19, 277-304.
Smee, B.W. 1998. A new theory to explain the formation of soil geochemical responses over deeply covered gold mineralization in arid environments. Journal of Geochemical Exploration, 61, 149-172.
Smee, B.W. 1999: The effect of soil composition on weak leach solution pH: a potential exploration tool in arid environments. Explore, 102, 4-7.
Smee, B.W. 2003. Theory behind the use of soil pH measurements as an inexpensive guide to buried mineralization, with examples. Explore, 118, 1-19.