Mining Geology 405 Geological Logging

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Mining Geology 405Geological Logging

• Joanna Hodge
• Centre for Exploration Targeting

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Geological Logging

• Critical part of the drilling process
• Provides basis for compiling ore deposit models
  during exploration (exploration drilling
• Allows reassessment and improvement of model in
  mining environment (grade control drilling)
• Provides important information required for
  day-to-day mine reconciliation

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Drill hole logging

• An example of a graphic diamond drill log. Most
  logging is typically done on palm-top computers,
  however these lack detail which is contained in
  hand-written logs

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Drill hole Logging

• Important information
• Structural features (fracture spacing and
  orientation, faults, foliation, joints)
• Lithological description (colour, texture,
  mineralogy, alteration, rock type)
• Geotechnical information (RQD, hardness)
• Weathering data
• Water table

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Regolith

• Regolith profile also a factor in mine stability
  and in operating cost assessments
• Regolith is the weathered rock overlying the
  bedrock which is often over 100m thick
• Different parts of the profile have different
  characteristics and must be mined and milled
  according to these characteristics

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Regolith

• Transported overburden may be consolidated or
  unconsolidated
• Completely oxidised bedrock
• Partially oxidised bedrock
• Essentially fresh bedrock

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Regolith

• Density variations
• Transported material s.g. 1.6-2.0
• Clay s.g. 2.2-2.6
• Bedrock s.g. gt 3.0
• Significant when deciding on earth moving contract

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Regolith

• Hardness
• Extremely variable
• Depends on whether consolidated or
  unconsolidated, cemented or uncemented
• Somewhat subjective during initial drill hole
  logging, but should be evaluated during
  metallurgical testing
• Has a significant impact on mining and metallurgy
  costs if you get it wrong
• Oxide material significant in most mills

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Geotechnics

• In civil engineering, geotechnical investigations
  are part of site evaluation and a basis for site
  selection
• In mining, site selection is impossible the ore
  deposit is where it is
• Epigenetic ore deposits are usually located in
  sites of secondary porosity poor ground,
  unstable.
• Syngenetic ore deposits are not often the
  dominant lithology in a package and so are also
  associated with discontinuities and anisotropy

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Geotechnics

• Why is geotechnical information important?
• Mine Design
• Pit wall failure is relatively common on a small
  scale in most mines, but large scale failure is
  disastrous
• Can lead to significant down-time (costly), early
  mine closure, as well as injury or death

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Factors Affecting Site Stability

• Rock fails in response to stress
• Structural discontinuities strongly influence
  whether an underground mine working or open pit
  wall fails or stands
• Intersecting joints, faults and bedding planes
  dominate open pit stability as overburden removal
  changes confining pressure increased sliding
• Faults, joints and cleavage planes are less
  resistant to shear stress and tend to open after
  blasting in underground mines underground mine
  planning is more complex as these structures need
  to be used, not fought

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Joints

• Number of different joint types
• Cooling joints (post-emplacement cooling, esp. of
  igneous rocks
• Tectonic joints (related to crustal movement)
• Sheet joints (unloading erosion)
• Stress joints (unloading mining)
• Shears (joints which have moved, faults)
• Joint interaction causes structural weaknesses

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Critical Data

• Fractures
• Orientation
• Spacing
• Length
• Filling
• Planarity (planar, wavy, irregular)
• Roughness (rough, smooth)
• Water-bearing Characteristics

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Fractures
Marjoribanks, 1997
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Where do the data come from?

• Predominantly diamond logging
• Drill core logged on site before moving or
  cutting
• Fracture parameters carefully logged in detail,
  as well as RQD

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RQD

• Rock Quality Designation
• of core recovered, counting only pieces of
  intact core 10cm or longer

RQD Quality
0-25 Very Poor
25-50 Poor
50-75 Fair
75-90 Good
90-100 Excellent
Peters, 1987
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RQD

• For example
• Total length drilled 130cm
• Total core recovered 104cm
• Core recovery 104/130 80
• Summed core lengths gt10cm 71.5cm
• RQD 71.5/130 55

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Rock Strength

• A number of other methods are also employed to
  quantify rock strength
• Unconfined compressive strength testing
• Shear strength testing (shear box)
• Triaxial compressive shear testing
• During feasibility stage, prior to mining,
  diamond holes are drilled specifically for
  geotechnical evaluation

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Summary

• Geological assessment complex
• Data from numerous sources, gained during
  different stages of development and mining
• Although it appears to be a less than exact
  science, GOOD GEOLOGY IS CRITICAL TO SUCCESSFUL
  MINE OPERATION

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Next Week

• Grade control
• Sampling methodology
• Geochemistry