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Mining Geology 405 Geological Logging

Mining Geology 405 Geological Logging. Joanna Hodge Centre for Exploration Targeting. Geological Logging. Critical part of the drilling process Provides basis for compiling ore deposit models during exploration (exploration drilling

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Mining Geology 405 Geological Logging

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  1. Mining Geology 405Geological Logging Joanna Hodge Centre for Exploration Targeting

  2. 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

  3. 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

  4. 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

  5. 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

  6. Regolith • Transported overburden – may be consolidated or unconsolidated • Completely oxidised bedrock • Partially oxidised bedrock • Essentially fresh bedrock

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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

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

  14. Fractures Marjoribanks, 1997

  15. 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

  16. RQD • Rock Quality Designation • % of core recovered, counting only pieces of intact core 10cm or longer Peters, 1987

  17. RQD • For example Total length drilled = 130cm Total core recovered = 104cm Core recovery = 104/130 = 80% Summed core lengths >10cm = 71.5cm RQD = 71.5/130 = 55%

  18. 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

  19. 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

  20. Next Week • Grade control • Sampling methodology • Geochemistry

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