Mineral Exploration Geology

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Mineral Exploration Geology

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

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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
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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.

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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
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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

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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
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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.

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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)

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Analytical Methods
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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

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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

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Statistics Daisy Ck
90
210
Straight line
Curve
100ppm
90
210
Straight line
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Element correlation -Daisy Ck
Weak correlation
Strong correlation
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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

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Mineralzn along lithological strike
Known mineralzn
Mineralzn along fault strike
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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

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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

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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

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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

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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

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Soil Profiles and Geochemistry
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Element Mobility
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Topography and Geochemistry
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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.

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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

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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

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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
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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