1. Exploration & Environmental Geochemistry Emmanuel Arhin

2. Introduction-this module will be in two parts Part 1-consist of Course work In this course we will learn about: –Dispersion and behaviour of elements (e.g. Au) in stream sediments, soils and in the regolith –Geological controls on geochemical data –Geochemical survey of streams, soils and termite: planning and execution. –Guidelines for sampling (e.g. soil sampling) –Geochemical data interpretation or analysing geochemical data –Analytical methods in geochemistry –Quality assurance and control of geochemical data. Part 2- Fieldwork

3. Assessment Assessment will be in two ways: –Every 2 nd week (starting in week 4) there will be a 20 minute multiple choice test on ALL materials you will have covered so far (not just the material covered in the previous 2 weeks) –Your marks will be posted by candidate/student number on the Department notice board the following week so you can see how you progressing. –The average of the multiple choice tests will contribute 40% of the mid trimester course mark. –The remaining 60% marks will be for the Final examination marks.

4. What is Dispersion? Refers to: Distribution of a particular element in a particular rock, regolith soil or streams etc. During the dispersion processes the concentration of the element gradually reaches the average value from high concentrations wrt space. Average value=Background of that element It can be considered as primary or secondary This is based on formational environment.

5. Dispersion explained in pictures

7. Dispersion explained In residual soils: –High geochemical assays tend be closer to source –This implies the farther away from source mineralisation the lower the assay In transported sediments –Element distributions or dispersions appear erratic –This implies higher assays may not indicate closeness of the source Complex regolith environments also have similar characteristics like transported sediment

8. Anomalies formed by Chemical weathering often are residual and relates to the underlying mineralisation. However, in nature it is rare to have only chemical weathering dispersing elements.

9. Physical weathering & Dispersion Physical or Mechanical Processes contribute to every stage and every scale to the evolution of landscapes : –From the initiation of weathering –To their destruction by erosion. The same processes are involved in –Geochemical dispersion and –Hence it is important in determining the nature of the surface expression of mineralisation Main agents here are: –Water, and to a lesser extent, wind and organisms operating mainly under the influence of gravity and temperature gradients. Note: Gravitational and thermal effects alone have only a minor role in mechanical dispersion. Many of the processes are common to all climatic zones, but, nevertheless, climate, relief, lithology and weathering history determines their relative significance in any region.

10. Some illustrations

11. Dispersion halo from primary mineralisation- Source: Arhin’s PhD thesis

13. Au as an example-mobilisation in the regolith Au mobilisation e.g. in regolith is through: –Chemical and/or physical processes The processes can be hampered during these processes. The hindrance of element mobilisation occur : –When laterite caps serve as barriers against Au upward migration –When precipitated clay minerals filled void spaces during dissolution processes –These chemical processes reduces the permeability. Transported sediments from depositional environments dilute or enhance the surficial Au geochemical signatures. Also transported sediments from depositional environments –dilute or enhance the surficial Au geochemical signatures.

14. Mechanism of metal transport from source mineralisation Most minerals formed under igneous and metamorphic conditions are: –unstable at low temperature and –under near-surface hydrous conditions and Why are they unstable in the near surface or in the oxidised environments? (Assignment, it will collected in the next meeting) To achieve stability they: – react to form dissolved components and –Consequently form new mineral precipitates (Scott and Pain, 2008).

15. More on metals transport The mobility of elements from primary minerals depends: –on the nature of regolith (i.e. on pre and post–weathering conditions): –its depth, and –aquifer characteristics (i.e. aquifer grain size and evaporation rates). In savannah regions (Nahon and Tardy, 1992) noted: –Rise in temperature activates chemical reactions and –Changes in climate accelerates or retards lateritization processes. In rainforest regions increase in rains enhances chemical weathering Question: in your own words explain the behaviour and characteristics of e.g. Au migrating from the bedrock mineralisation to the surface regolith, discuss all the processes that may act on it till its signature is expressed in the surface regolith (about a page or 500 words)

16. Metals mobility in oxidised environments Metal migration Surficial Processes Hydromorphic dispersion Acts downwards Chemical dispersion Geochemical interactions Physical dispersion Effective in surface layers via groundwater and infiltrated water Move elements in solution via pore spaces

17. Processes and elements migrations differ Each of these element dispersion mechanisms results in different element signatures in the regolith Modified after Butt et al., 2000

18. Critical to Know Nature of dispersion haloes displayed in weathering profiles relies on a)flow directions of groundwater, b)infiltrated water, c)solution properties, d)aquifer heterogeneity, e)adsorption of elements onto specific regolith minerals, f)elements forming complexes with appropriate ligands as well as g)inherent geochemical interactions of elements and regolith material types (Anand 2001; Anand et al., 2005). Qu. List seven properties in a complete weathering profile that have controls on the metal dispersions. Explain how any of these two properties can influence elements mobility.

19. Element mobility in savannah regions Modified after McQueen and McRae (2004); Taylor and Eggleton (2001).

20. Grade control sampling in a mine pit in Ghana-

21. QUESTIONS END OF LECTURE-MODULE 1

22. Lecture 2-Geological Controls on Geochemical Data The first question is “Does geology control geochemistry? The answer is yes, it does –Meaning the fruitfulness of geochemical investigation hinges upon the clear understanding of the geological relationships. Thus any successful geochemical investigation must be based upon the proper understanding of the geology and not on high elements concentrations? This implies that the crustal elemental levels or backgrounds need to be known. It is then that anomalous concentrations or depletions can be detected.

23. Then Exploration SMARTNESS is not: Submitting large numbers of samples to the laboratory if the relationship between the samples is unknown and their relationship to the regional geology is unclear. Example 1: Consider a Greenstone belt in which there are several generations of melt and some have associations with gold mineralisation. A regional study in which samples are collected on a grid pattern may have a statistically accurate feel and yet will provide limited information on the source of the gold. Example 2: Consider a migmatic terrain in which there are several generations of melt produced from a number of possible sources. A regional study in which samples are collected on a grid pattern may have a statistically accurate feel and yet will provide limited information on the origin of the migmatite Complex.

24. So WHAT SHOULD BE DONE !! It is normal to use GEOLOGY to interpret GEOCHEMISTRY. Rarely is the CONVERSE true. ALWAYS HAVE THIS IN MIND Let’s get to the question about “what should be done…..”

25. WAY FORWARD >>>>>>> Map the geology to establish the age relations between the units present at an appropriate scale Sample each unit separately What is the benefit then>>>> –This will allow chemical variations within the units to be identified –Selection of an appropriate models for the different units can be applied. This approach leads naturally to the way in which geochemical data are presented.

26. Geochemical Data Presentation Can take the form of: Bivariate plot or Trivariate plot In these plots the variables are the geochemical data

27. Examples of Bivariate plots

28. If geology is also considered then: Additional variables of time and space may be plotted Note: it can be informative on some occasions to examine chemical variations with time in a sedimentary pile or mine tailings or on refuse dumps. Similarly it is often valuable to consider geological controls for anomaly delineation for Mineral Exploration or Potential toxicological elements detection for environmental health purpose.

29. Anomaly delineation: examining spatial geochemistry of stream sediments Source: Nude & Arhin (2009)

30. Anomaly delineation: examining spatial hydrogeochemical data from wells in Bongo area From this interpretation: a. areas with prevalent high and low fluorine can be defined. b. The source rocks releasing fluorine in the area can also be detected We can do same for all elements of interest either for mineral exploration or environmental health purpose. Source: Arhin & Affam M.,(2012)

31. Now how do we get the data to analyse and interpret Conduct geochemical field survey. Typical for mineral exploration are:  Stream and stream sediment survey  Soil survey  Trench survey  Rock or lithological survey  Termitaria survey  For environmental health we may conduct surveys for different types of water e.g. bottled mineral water, shallow and deep wells survey etc. The heart of Exploration and Environmental Geochemistry

32. Stream sediment survey Read: Nude & Arhin 2009, Overbank sediments as appropriate geochemical sample media in regional sediment surveys. Journal of Geochemical Exploration, vol. 103, p. 50-56. Fletcher & Wenqin 1988, Size distribution of gold in drainage sediments: Mount Washington, Vancouver Island. Geological Fieldwork, 1988, Paper 1989-1 To begin think about the following: –Size distribution of gold in stream>>this is useful in deciding on where to sample –The size of sample (e.g. 50g, 1500g etc., and –What size fraction to analyse in stream sediment and heavy mineral surveys for gold (250 µ, 150 µ )

33. Stream sediment & Heavy mineral Surveys Used in the early stages of gold exploration Results from these survey are often extremely ERRATIC Difficult to reproduce or confirm if area is re-sampled The non reproducibility of results arises from the problems inherent in obtaining samples that are statistically representative of the true abundance of rare particles. –I cannot see how careful one can be to collect more representative sample in stream sediments BUT we try

34. Perhaps the non-reproducibility- the notion is a result of Hydraulic processes that preferentially: –Sort and –Deposit Heavy minerals during sediment transport These processes can lead to development of heavy mineral placers.

35. What should be the SIZE of the sample? Large or Small !!! The sporadic presence of one or more particles in small samples will give very STRONG, non-reproducible anomalies To reverse this high density survey may be needed but this will be unnecessary for any new survey.

36. Fundamental notion of stream sediment survey Is based on the assumption that samples collected from the stream channel represent the products of weathering upstream from the sample site. –True for homogeneous regolith areas. Caution: This may not be so………… –In a complex regolith terrains (Nude & Arhin, 2009) a)If this is the case then let’s plan a stream sediment survey for an area in northern Ghana (make a copy of the Wa field sheet). b)Plan your own stream sediment survey on the map provided. c)Give reasons why your selected sites are most appropriate for anomaly detection. Fieldwork is necessary for this !!!!!

37. This is part of Goaso field sheet. Use this to plan stream sediment survey for gold exploration. Give reasons for your site selections (5 marks)

38. Thematic Au plot of stream sediments survey at Mehame area of Goaso field sheet is presented in the map below. Demarcate the anomalous areas and prioritise them in order of importance for soil survey (10 marks for correct answer). Colour plot of this field sheet is preferred

39. Thematic As plot of stream sediments survey at Mehame area of Goaso field sheet is presented in the map below. Demarcate the potential toxicological areas in As and prioritise them in order of mitigating toxicity of As in soils (10 marks for correct answer)

40. Thank you for your audience

41. Lecture 3: Soil Survey Soil surveys are normally conducted in grid patterns. –The grid or the systematic survey often depends on the trend of mineralisation if known or the strike of the supposed mineralisation is assumed to be parallel to the regional trend of the underlying rocks. The soil survey traditionally are conducted after stream sediment surveys. Often after an orientation survey has been conducted. Soil samples are a widely used geochemical sampling medium.

42. Plan soil geochemical survey at these target areas assuming no information on mineralisation trend

43. Plan soil geochemical survey at these target areas

44. Soil Sampling Soil sampling is generally recommended in the following situations: Areas of residual soil over sap rock; Areas with soil developed on in situ regolith; Areas with soil developed over transported regolith that is less than 5 m thick.