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Mineral Exploration Geology

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Title: Mineral Exploration Geology


1
Mineral Exploration Geology
  • Exploration Geochemistry
  • Reference Marjoribanks 1997. Geological Methods
    in Mineral Exploration and Mining. Chapman and
    Hall. Ch8.

2
Exploration Geochemistry
  • Geochemistry is a fundamental aspect of virtually
    every exploration program
  • It has evolved from simple assaying of the
    commodity in question eg Au, to identifying
    subtle geochemical anomalies using major and
    trace elements contained in surficial sediments
    eg soil, laterite, till, vegetation or
    groundwater
  • The object of geochemistry is to define a
    geochemical anomaly which distinguishes an ore
    deposit from background and insignificant
    mineralisation
  • A geochemical survey can be divided into 5
    phases
  • 1) Planning
  • 2) Sampling
  • 3) Chemical Analysis
  • 4) Interpretation
  • 5) Follow up

Essential Jargon assay, geochemistry, laterite,
till, anomaly
3
Useful Resources
  • Web-resources
  • Geochemistry, Exploration, Environment, Analysis
    http//www.aeg.org/
  • Geochemistry Web Links
  • On-Line Manuals
  • Geol Sci 455 Chapters
  • Exploration GEOCHEMISTRY
  • GEOCHEMISTRY LINKS
  • The Geochemical Society
  • Geochem Online Manual
  • Geochem Resources
  • Texts
  • Evans, A. M. 1995. Introduction to Mineral
    Exploration. Blackwell Science.
  • Govett, G. J. S. 1983. Handbook of Exploration
    Geochemistry series. Elsevier.
  • Evans, A. M. 1997. Introduction to Economic
    Geology and its Environmental Impact. Blackwell
    Science.

4
1. Planning
  • Choice of geochemical surveys and analytical
    methods depends on the commodity sought and the
    nature of the terrain/geomorphology/geology
  • A geologist must start with a knowledge of A) the
    elements associated with a particular deposit
    type (table 8.1), B) an idea of the size of the
    deposit being sought, C) the mineralogical form
    of the elements, and D) the probable size of the
    elemental anomalies around it
  • An ore deposit is usually surrounded by lower
    concentrations of the mined elements, which are
    significantly enriched compared with
    unmineralised country rock. This is known as the
    primary halo. An initial geochem survey must
    atleast be able to pick up an anomaly from the
    primary dispersion.
  • The geochem response at the surface depends on
    the type of terrain and on the type of material
    covering the deposit as well as the element in
    question. Different elements have different
    solubilities eg in a VMS cu/Pb/Zn deposit, Zn is
    more mobile in the surface environm and is
    dispersed further than cu-Pb. This is referred to
    as secondary dispersion
  • Background levels of elements in the medium being
    sampled must be considered. The concentration
    will vary depending on the mode of formation and
    composition of the rock and the processes forming
    the soil eg enriched and depleted zones in a
    regolith profile

Essential Jargon commodity, dispersal,
solubility, terrain, primary halo, primary
dispersion, secondary dispersion, pathfinder
elements, background levels, regolith
5
1.1 Orientation Surveys
  • A key aspect of planning a successful geochemical
    survey is to evaluate which techniques are
    effective for A) the commodity being sought and
    B) the area being explored
  • This involves undertaking an orientation survey
    in which a number of sampling methods are tested
    over a known deposit in a similar environment to
    determine the method which yields the best
    results. This involves 1) Understanding of
    target deposit type 2) Understanding of
    surficial environm of search area 3) Nature of
    primary secondary dispersion 4) Sample types
    available 5) Systematic sample collection
    procedures 6) Sample size requirements 7)
    Sample interval, orientation and aerial density
    8) Field observations 9) Sample preperation
    procedures 10) Sample fraction for analysis 11)
    Analytical method 12) Elemental suite
    (pathfinder elements) 13) Data format for
    presentation and interpretation

6
2. Analysis
  • A geologist will generally see little of the
    processes of analysis, which is usually done at a
    commercial laboratory. As a result geochemical
    data tend to be used uncritically, which can be
    be risky and expensive. Errors do happen! It is
    important for the geologist to monitor and check
    the quality of the data produced by the labs
  • 2.1 Accuracy and Precision
  • The critical question for a geologist is, how
    reproducible the analysis is (precision eg or -
    5ppb Au) and how representative of the correct
    concentration the result is (accuracy)
  • Precision is tested by analysing samples in
    duplicate
  • Accuracy requires the analysis of a sample of
    known composition -reference material

Essential Jargon accuracy, precision,
duplicates, reference material, contamination
7
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8
Sample Collection and Preparation
  • Samples should be collected in non-metallic
    containers to avoid contamination
  • Most sample prep carried out in field eg sieving
    soils
  • Drying, disaggregation and sieving to obtain the
    desired fraction
  • Imperative to eliminate contamination in this
    process eg clean crushing equipment between
    samples, dont wear gold jewellery when
    collecting samples.

9
Analytical Methods
  • It is impossible at present to analyse for all
    elements simultaneously at required levels and
    each new analysis brings added expense so in
    exploration compromises have to be made
  • Expensive High-tech
  • ICP-ES - Inductively Coupled Plasma Emission
    Spectrometry
  • ICP-MS - Inductively Coupled Plasma Mass
    Spectrometry
  • XRF - X-ray Fluorescence (non-destructive)
  • NAA - Neutron activation analysis
  • Inexpensive Low-tech
  • AAS - Atomic Absorbtion spectrophotometry
    (solvent extraction aqua-regia)
  • Fire Assay - precious metals are extracted into a
    small button (melting) which is seperated from
    the slag and determined by AAS, ICP-ES/MS
  • The analysis of precious metals is different from
    base metals in that large sub-samples (30-50g)
    are analysed as opposed to base metals (0.25-1g)
    to overcome the grainy or nuggety effect
  • Element association eg Ni occurs in ultramafic
    rocks in either olivine (uneconomic) or sulfides
    (economic). Therefore acids that digest sulfides
    (nitric or HCl) are prefered over those that
    attack silicates (HF)

10
Analytical Methods
11
3. Interpretation
  • Geochemical results are usually multi-element and
    a large number of samples. This will invariable
    require the use of statistical analysis on
    computers to check for precision and accuracy

12
Statistics
  • The object of geochem exploration is to define
    significant anomalies
  • The best way to evaluate geochemical data is by
    graphical representation using histograms and box
    plots (Fig 8.5)
  • Homogenous data will form a log-normal
    distribution whereas if the data falls into
    distinct groups they will be multi-nodal
  • Contour plots using the intervals from the
    histograms (Fig 8.8)
  • Anomalous areas then need to be investigated
    using detailed geology and regolith mapping,
    topography and general site investigation to look
    for possible contaminant sources eg mine dumps,
    tailings dams

13
Statistics Daisy Ck
90
210
Straight line
Curve
100ppm
90
210
Straight line
14
Element correlation -Daisy Ck
Weak correlation
Strong correlation
15
4. Reconnaissance Techniques
  • 4.1 Stream sediment sampling
  • Relies on active physical weathering to obtain a
    representative sample of an entire catchment
  • Sample fraction depends on the environment.
    Temperate terrains - max contrast for trace
    metals is found in the fine fraction (80 mesh or
    lt190um) due to the high organic content, clays,
    Fe-Mn oxides
  • Base metals (0.5 kg), Au (10 kg of sieved -2mm
    material)
  • Sample usually taken as a number of sub-samples
    20-30m along the stream at a depth of 10-15cm to
    avoid excess Fe-Mn oxides
  • Avoid upstream contaminant sources such as roads,
    factories, farms, mines
  • Panning for heavy mineral concentrates can be a
    useful way of enhancing weak signals but tends to
    be less quantitative and more subjective but can
    be very useful in remote areas where access to
    labs is restricted or when the exploration budget
    dries up

16
Mineralzn along lithological strike
Known mineralzn
Mineralzn along fault strike
17
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18
4.2 Lake sediments
  • Glaciated areas are often accompanied by small
    lakes (tarns) which provide an ideal sampling
    medium for reconnaissance exploration as they are
    accessible by air and can be sampled by dropping
    a heavy sampler into the lake sediment and
    retrieving it
  • Data is interpreted in a similar manner to stream
    sediments
  • Lake sediment data is only useful if glacial
    material is locally derived and is ineffective in
    glacio-lacustrine environments

19
Panning
  • Density contrasts
  • Heavy mineral concentrates
  • Enhance weak signals
  • Collects precious metals, gossanous fragments
    rich in metals, cassiterite, zircon, cinnabar,
    barite and most gemstones diamond, sapphire
  • Useful in remote areas where lab turn-around
    times are high
  • Not particularly quantitative

20
Overburden Geochemistry
  • Often used as a follow-up to stream sediment
    surveys in areas of residual overburden
  • Method of sampling depends upon the nature of the
    overburden. The cheap option of sampling soil is
    only used when the chemistry of the surface soil
    is known to reflect the geology below. Soil
    sampling does not always work in glaciated,
    windblown and lateritic environments where trace
    metals have from or to the subsurface layers

21
Surface Soil Sampling
  • The type of soil present represents the surface
    processes operating and these are quite varied.
  • The most effective soil samples are generally
    from the B-horizon located some 30cm depth (Fig
    8.12). Formed by the downward movement of clays,
    organic material and Fe-oxides
  • It is essential that the characteristics of the
    soil sample are recorded and that the same
    horizon is consistently sampled
  • 100-200g sample for base metals 0.5-2kg for Au
  • Spacing depends upon the size of the primary halo
    and the strike length and width of the potential
    target

22
Lag Sampling in Lateritic Profiles
  • Pisolites formed in the ferruginous zone of a
    regolith profile tend to concentrate trace
    elements and can provide an effective sampling
    medium in highly weathered lateritic terrains
  • The large (0.5-3cm) pisoliths are swept up off
    the surface and the results are treated similar
    to soil geochemistry except the background levels
    tend to be elevated
  • The potential anomaly is dispersed over a greater
    area and will be of a higher magnitude than that
    of soil, however it tends to be less consistent
    than soil surveys

23
Soil Profiles and Geochemistry
24
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25
Element Mobility
26
Topography and Geochemistry
27
4.4 Hydrogeochemistry
  • Uses water as a sampling medium
  • Useful in arid regions with poor outcrop or deep
    surficial cover
  • Deep water wells can tap into deep aquifers
    (ground water) that can be used to explore in the
    subsurface
  • Extremely low detection levels are required -
    precious metals 2 ppt, base-metals ppm.

28
4.5 Gases
  • Gases can be a useful sampling medium in areas of
    thick overburden but have not proved to be
    particularly useful in mineral exploration
  • Mercury is the only metal to form a vapour at
    room temperature
  • Radon is generated during the decay of U and has
    been used with some success
  • The enrichment of carbon-dioxide and the
    depletion of oxygen caused by the weathering of
    sulfide deposits is being tested in Western USA

29
4.6 Vegetation
  • Two methods 1) the presence of a particular plant
    species can indicate the presense of a particular
    rock/ore type and is referred to as Geobotany
    2) the elemental content of a particular plant is
    measured and is referred to as Geochemistry

30
5. Follow-up Sampling
  • Required to define the source of the anomaly in
    more detail
  • 5.1 Rock-chip geochemistry
  • Used where there is good outcrop
  • Most effective when used in conjunction with
    drillcore
  • Very useful in delineating disseminated ore
    bodies but can be hit and miss with high-grade
    veined or stockwork orebodies
  • I kg sample is sufficient usually with chips
    taken as a composite over a specific distance eg
    1 m composites in order to get a representative
    sample

Essential Jargon
31
6. Summary
  • Exploration geos are more likely to be directly
    involved in geochem surveys than geophysical ones
    so they must have a good knowledge of its
    application
  • Geochemical programs require
  • Planning choice of appropriate field survey and
    analytical methods for a particular commodity
  • Orientation survey to test a variety of sampling
    methods over a deposit of similar geology to that
    of the target and in similar topographical
    conditions
  • Analysis of samples should be monitored by
    submitting standards and duplicates to check for
    accuracy and precision
  • It must be determined whether the overburden
    material being sampled is residual or transported
  • Overburden geochem is often a follow-up to stream
    sediment surveys
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