AU593659B2 - Geochemical exploration - Google Patents

Description

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59 1 '6 5 9 ,r-ide untir 9 LODGED AT SUD.O FFICE 1 9 JUN 197 Melbourne I FEE 5 MP TOLUE

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APPLIC ANT: DEPARTMENT OF MINES NUMBER: PH- 6481/86 19 Jun 86 FILING DATE:

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FORM r I COMMONWEALTH OF AUSTRAL 4

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The Patents Act 1952 COMPLETE SPECIFICATION FOR AN INVENTION ENTITLED: GEOCHEMICAL EXPLORATION The following statement is a full description of this invention, including the best method of performing it known to me: -1- This invention relates to geochemnical exploration and, in particular, to a method of geochemnical exploration which provides results of anomalous metal values more quickly and more cheaply than previously known methods.

Geochemical exploration, in many applications, uses sampling of "B" horizon soil but in areas, such as those having wet, temperate, climates and, specifically, those of Tasmania, it has been found that there is strong leaching of soils and horizon soil geochemnistry yields unreliable results.

For this reason, surveys need to be based on the deeper horizon to be effective and gain information by use of existing methods.

In Tasmania, recently, most surveys have been done in this way and such surveys are expensive, as they necessitate drilling into the horizon, which is, itself, time consuming and this is aggravated by and can be difficult to accomplish in the strongly dissected, heavily vegetated terrain of Western Tasmania.

The principal object of the present invention is to provide a method of detailed geochemnical exploration and specifically such a method based on t:tttC sampling the soil horizon.

The invention, in its broadest concept, comprises the method of determining metal values from humic substances extracted firom soil samples comprising the steps of extracting the humic substances and determining the carbon content and the metal quantity in the extract by standard analytical techniques, the metal concentration relative to the carbon content being indicative of the initial metal values in the soil.

-2-A DepOfR&E/22 /Jun87

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In one preferred form of the invention, the determination is done by atomic absorption spectrophotometry.

In a preferred method, the humic substances are extracted by treatment I I I lIZ a a I.

44 4*

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Si a I 4 I*I a *i~*t I. IL I a I 444 4 i I -2a.- DepOfMi nes /22/Jun87 In with ammonium hydroxide and the content of humic substances is obtained from quantitative wet oxidation of an aliquot of the sample with 0.4N

K

2 Cr 2 0 7 In order that the invention may be more readily understood, we shall describe, in some detail, the application of the invention to soil samples.

The conventional exploration geochemistry approach of horizon soil sampling has been found to be unreliable in the wet, temperate, climatic conditions of Western Tasmania. This situation is due to the intensive i leaching of soils under these conditions and it has led explorers to resort to 10 horizon sampling by use of mobile drilling equipment. Whilst this 4, t approach results in the acquisition of reliable data, it is expensive, time consuming, and often difficult to achieve in the heavily vegetated and dissected terrain of Western Tasmania, the area in which the invention has been developed. The invention is applicable to other, similar, areas.

The leaching is believed to be due to the activity of humic substances.

This view is based on field and laboratory studies which indicate that these C, rr 'substances are aggressive solubilizers of sulphide and oxidized zone i: minerals. If this interpretation of leaching is correct, then it follows that metals removed from weathering bedrock will move through the soil profile 1•t 20 as complexes of the humic substances. A study of these substances and ,their metal associates extracted from the soil horizon should thus indicate metal distribution in the substrate.

Two areas in Western Tasmania were readily available for the development and testing of an horizon soil geochemical exploration method. In both areas Pb was the dominant mineralization and the initial study was limited -2{ -3- DepOfMines/22/Jun87 L 1 1 1 1 to Pb, Zn and Cu, but the invention is not limited to these metals.

The design of the analytical scheme was influenced by the need to develop a system that allowed the processing of large numbers of samples at a reasonable rate and that yielded a precision acceptable for geochemical exploration.

Sampling was a simple procedure since the depth rarely exceeded 20 cm.

Excessive root material was removed on site and about 50 cm 3 of the remaining soil was taken for analysis. Where layering of the soil was observed, the darkest layer was selected for sampling. Since the method involved the determination of metals in extracted humic substances, no Spreliminary drying or sieving was necessary.

Approximately 10 cm 3 of the soil samples as collected were transferred to ml capped centrifuge tubes for the extraction step.

tc( Extraction of the mobile fraction of the humic substances was achieved by shaking the soil samples with 50 ml of 0.5 M NH 4 OH for one hour in a laboratory shaker which was modified to accept 100 centrifuge tubes.

After shaking the tubes were centrifuged for 45 minutes at 3000 rpm and the extracted humic substances separated from the insoluble matter by V vacuum filtering through sintered glass filters.

S* 20 The carbon content of the extracts was determined by quantitive wet t i oxidation with K 2 Cr 2 07. Aliquots of 2 ml of the extracts were transferred to 125 ml Erlenmeyer flasks, taken to dryness on a hot plate and 10 ml of 0.4N K 2 Cr 2 0 7 in 10M H 2

SO

4 were added. The flask contents were then boiled for 5 minutes to achieve oxidation of the humic substances. After cooling, the residual K 2 Cr 2 0 7 was titrated with 0.1N -4- DepOfMi nes/22/Jun87 l I

(NH

4 2 Fe(S0 4 2 using 0.2% phenanthranilic acid in 0.2% Na 2

CO

3 as indicator. During the early stage of the titration, the solution appeared a murky greenish brown but, as the end point was approached, a distinctive violet colour developed which changed sharply to a bright green at the end point. For this titration 1 ml of the (NH 4 2 Fe(S0 4 2 solution was equivalent to 0.0003 g carbon. If the amount of carbon in the humic substances is known a factor may be used to convert the values to the quantity of humic substances present, although this step is not necessary for geochemical exploration applications.

Metal determination was carried out by flame atomic absorption spectrophotometry. A study of the effect of an alkaline extract of humic substances on absorbance values for Cu, Pb and Zn indicated that this matrix was benign and, as a result, standards made up with IM HNO 3 were used for calibration. It was found, however, that the humic substances extracts had to be shaken before analysis since layering developed if they were left standing for a few hours. This was probably due to the wide range of high molecular weight components in the extracts.

The procedure was applied to ten replicates of two soil samples in order to establish the precision. As can be seen from the following Table, the results are adequate for geochemical exploration.

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j (O DepOfMines/22/Jun87 .pont Fr hi ttrtin m o th (N-).F(C)7slto wa Tablo 1. Results of replicate determination of Pb, Zn and Cu in soil organic extracts 44 It~t 4 Sample Ref.

waratah 100 1 2 3 4 6 71 8 9 Mean S. Deviation

R.S.D.%

Waratah 1OOS/140E 1 2 3 4 5 6 7 8 9 10 Mean S. Deviation

R.S.D.%

192 21 11.0 Pg/gC C C t 4 C CCC C IL 4 4 4 64 I 44 U 4444 CCC C C C C CC

CCC'

5680 5390 5350 5470 5120 5260 5100 5780 5550 5060 5376 247 4.6

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h C 4 II 4 42 1 The invention was tested in relation to two areas in Western Tasmania, one near Zeehan and one near Waratah (Figure The Zeehan area was the site of the former Oceana Mine where high grade Pb ore with subordinate Zn was mined from fissure filling and replacement deposits in Ordovician limestone. Limited horizon data were available from previous surveys but, as the area was heavily disturbed by costeaning (trenching), sampling was limited to two traverses at a 10 m interval to yield 55 samples. The vegitation over the area was largely of button grass (Gymnoschoenus sphaerocephalus) which is known to produce strongly aggressive humic substances. The Waratah area was the scene of an extensive horizon survey undertaken to follow up a previously recorded Dighem anomaly.

The horizon sampling was carried out at a 20 m interval on traverse spacings of 100 m to yield 320 samples. No mineralisation had been recorded from the area but it is underlain by Cambrian volcanics which host a small Pb-Zn orebody at the former Magnet Mine, 4 km to the north. The vegetation over this area includes button grass, tea-tree (Melaleuca ericifolia) thickets and rain forest consisting largely of myrtle (Nothofagus cunninghami) sassafras (Atherosperma moschatum) and horizontal (Andopetalum biglanduiosum) which provide abundant litter for the development of substantial quantities of relatively mobile humic substances.

The results of analysis of the Zeehan samples are plotted in Figure 2. It can be seen that, in general, the horizon organics reflect both the "C" horizon results and the surface projection of the orebody. Close comparison between the two sets of results is not possible since tile "C" horizon sampling was carried out at a 50 m spacing and lacks the detail of the current sampling program. The horizon data show a slight spread -7- DepOfMines/22/Jun87

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r Z-ZTZ. T T J ii; 1 i m of values compared with that for the horizon which is to be expected since the samples concerned have been subjected to extensive biochemical activity during soil formation. The anomaly developed in the horizon organics is surprisingly strong for Pb and Zn with respective values of 11.0% and 2.1% on the 3700 N traverse. The expression of Cu is far weaker and this is probably due to the fact that this metal is only a minor component of underlying mineralisation. The strong horizon result for Cu could well be due to the presence of sporadically distributed Cu minerals which would be dispersed during soil forming processes.

For the Waratah area, the results are given in the form of a typical traverse (Figure 3) and anomaly maps (Figure The anomalies are of low order, being based on the median plus two standard deviations for the raw I data which has been processed by the method of Yufa and Gurvich (1964).

As with the Zeehan results, it is clear that the horizon organics reflect the metal distribution found in the horizon. A slight westward dispersion is apparent for Pb and Zn whereas the weaker Cu anomaly lies close to that given by the horizon data. The results suggest that the mineralization is dominated by Pb with subordinate Zn and Cu which is S, compatible in type with the Magnet orebody. Diamond drilling of the 1 20 southern most anomaly has intersected a 25 m zone of altered and mineralized volcanics. The zone contains veinlet and massive sulphides up to I m in thickness which contain up to 41.3% Pb Zn, 460 g/t Ag and 5.6 y, g/t Au. This is taken as fundamental evidence that the Huminex system is effective.

This study suggests that the analysis of horizon organics may prove a useful addition to soil based methods of geochemical exploration. Under .i.

DepOfMines/22/Jun87 conditions of strong leaching, the last vestiges of evidence of the presence of metals is likely to bc associated with that component responsible for much of the lear ;-ng the humic substances. The usual soil geochemistry approach of drying, sieving and analysis based on the total sample may yield erratic results under the former conditions due to the fact that a substantial inorganic component included in the analysis has little bearing on the metal content. Positive features of the horizon organics approach are that it is very cost effective with respect to sampling and it can be carried out with little disturbance to an area. Analytical costs are comparable with standard methods since, although carbon analysis is an extra factor, there is no preliminary preparation of samples and the extraction is with cold NH4OH.

*h Finally, comment must be made on the role of humic substances in the generation of false and true anomalies. These substances exhibit adsorption and ion exchange phenomena and these features of their activity have been subject to numerous studies. Under some physiographic S conditions, such as internal and impeded drainage, the interaction between ground water draining a low level dispersed source of metals and humic substances may result in an accumulation of these metals to yield a false anomaly. In such cases there is generally good correlation between the amount of humic substances present and the metal values obtained through adsorption and ion exchange. It is not believed that this behavious is commonly the cause of anomalies since, in nature, it is rare to have a system where cations are free to migrate in an aqueous system until they interact with humic substances at some point during migration. These substances, as part of the soil forming p. ,cess, are always present in the soil water and, under wet, temperate, conditions, may attain -9- DepOfMines/22/Jun87 concentrations of the order of 500 ug/ml. Reaction occurs as the soil-rock interface and the complexes formed by the extraction of metals from their host mineral lattices migrate through the hydrosphere until such time as the physico-chemical conditions cause their destruction.

The amount of metal complexed is not constant and is dependant on the mineralization in the substrate. Thus, in this case, high concentrations of metals associated with humic substances in the horizon are indicative of higher than average metal content in the substrate a true anomaly) and are not an artifact of ionic chemistry. At the scale of an exploration target the soil forming process tends to be uniform and, as a result, the humic substances are evenly distributed. Since metal values in the same target area are varying widely, the correlation between the contents of j metals and of humic substances in the sample set exhibit poor correlation. For the two areas discussed herein, the correlation Sr t. coefficients for humic substances and Pb, Zn and Cu are -0.19, -0.39 and -0.34 at Zeehan whilst, at Waratah, they are 0.08, -0.23 and -0.29.

Whilst in this specification we have referred specifically to exploration for Pb, Zn and Cu values, this is purely exemplary and any other metals which form complexes with the humic substances can equally as well be considered by the use of the process of the invention.

v ^i .Further, whilst we have also described one specific manner of extraction r f I and analysis, these could readily be changed should this be so desired without departing from the scope of the invention.

epf ns2-10-8 2 DepOfMines/22/Jun87

Claims (8)

1. A method of determining metal values from humic substances extracted from soil samples comprising the steps of extracting the humic substances and determining the carbon content and the metal quantity in the extract by standard analytical techniques, the metal concentration relative to the carbon content being indicative of the initial metal values in the soil.

2. A method as claimed in claim 1 wherein the soil sample is from the S. soil horizon.

3. A method as claimed in claim 1 or claim 2 wherein the extraction is effected by ammonium hydroxide. t r

4. A method as claimed in claim 3 wherein the extract is separated by a* centrifuging and vacuum filtering. a

5. A method as claimed in claim 4 wherein the metal values are ascertained by flame atomic absorption spectrophotemetry. t a C t k a

6. A method as claimed in anyone of claims 3 to 5 wherein the carbon content of the extract is ascertained by drying and oxidised by the addition of K 2 Cr 2 O 7 in H 2 SO 4 and heating and titrating the residual K 2 Cr 2 07 with(NH 4 2 FE(SO 4 2 -11- e DepOfR&E/ 2 Jun 87 1 Y !r St

7. A method as claimed in claim 6 when appended to claim 4 whereby the metal values can be considered in relation to the quantity of humic substances present.

8. A method of determining metal values from humic substances extracted from soil samples substantially as hereinbefore described with reference to the accompanying figures. 4~ t 4AY~ IC C I DATED this 19th day of Jun~e, 1987 DEPARTMENT OF RESOURCES AND ENERGY By its Patent Attorneys A. TATLOCK ASSOCIATES 1144 I 14 II I 41 4 4 4* 4 444444 4 448444 4 .4 .4 4 444 444444 4 -12- DepOfR&E/22 /Jun87