Published on December 23, 2008
Mining Geology 405Drilling and Sampling : Mining Geology 405Drilling and Sampling Joanna Hodge Centre for Exploration Targeting Overview : Overview Drilling Techniques Drilling Equipment Downhole Surveying Sampling Drilling and Sampling : Drilling and Sampling Important mineral exploration procedure Delineate subsurface conditions Expensive Number of techniques Tests ideas and theories developed during prospect and target generation LOCATES AND DEFINES ECONOMIC MINERALISATION Drilling Techniques : Drilling Techniques 3 main techniques used in Goldfields RAB (Rotary Air Blast) RC (Reverse Circulation) Diamond drilling Rotary Air Blast (RAB) : Rotary Air Blast (RAB) Cheapest Least penetrative (~100m) Geochemical sampling to base of regolith (ie only penetrates weathered cover over fresh rocks) Uses compressed air to break ground Air pumped down through drill rod Cuttings blown up hole between rod and hole wall Single barrel technique Advantages : Advantages Cheap Fast Large sample volume Disadvantages : Disadvantages No fresh rock samples Limited depth No structural data Contamination Slide 8: Types of drilling equipment commonly used in mineral exploration (Marjoribanks, 1997) Reverse Circulation (RC) : Reverse Circulation (RC) Moderately priced Good penetration (to ~350m) Samples fresh rock Dual barrel technique High pressure fluid forced down outer pipe and returns chips to surface up inner pipe Slide 10: Schematic of RC drill rig Advantages : Advantages Relatively cheap Quick Large sample Uncontaminated Disadvantages : Disadvantages Limited access No structural data Sample contamination below water table Diamond Drilling : Diamond Drilling Expensive Greatest penetration Whole rock samples Slide 17: Types of drilling equipment commonly used in mineral exploration (Marjoribanks, 1997) Advantages : Advantages Maximum geological information Uncontaminated High quality sampling Disadvantages : Disadvantages Expensive Slow Small sample size Extensive site preparation and water supply required Appropriate Drilling Methods : Appropriate Drilling Methods RAB Early exploration First pass and infill drilling RC Intermediate exploration Delineate ore body Grade control Diamond Late stage Structural controls Drilling : Drilling Marjoribanks, 1997 Directional control : Directional control Important to survey RC and Diamond holes to ascertain exact position of underground features Rod strings can deviate significantly and do not travel straight. Inaccurately surveyed holes can lead to large discrepancies in the actual location of an orebody Surveying is undertaken down the drillhole by a single-shot camera to record dip and azimuth every 30-50m during drilling, or after drilling is completed by a gyro If surveying indicates that a drillhole is deviating significantly off course, the driller can take remedial action. Sampling & Sampling Theory : Sampling & Sampling Theory “Good sampling is like religion – everyone believes in it, but few practice it.” Sampling results are estimates made within bounds of precision (repeatability) and accuracy (lack of bias) A Sample : A Sample Representative part or single item from larger whole Taken for purpose of inspection Evidence of quality Part of statistical population whose properties are studied to gain information about the whole By definition have two elements Typicality Smallness Problems, Bias : Problems, Bias Variable geological environments Erratic metal distribution High unit value of precious metals exacerbates error Low cut-off grades requires greater care when sampling Sampling provides basic data for subsequent resource and reserve calculations Grade control errors can be costly Objectives : Objectives Determine, minimise source of error Determine optimum sample size and density/spacing for In situ mineralisation (core, channel samples) Broken ore (drill chips, ROM material, crushed material) Errors : Errors Core sample split unevenly or divided improperly Density differences in chip samples Hardness differences Slide 28: Where biased or unrepresentative sample data can be recognised, the source of the error can be eliminated Geological bias can be taken into account by separating out sub-populations eg. zoned ore bodies, leaching or secondary enrichment Contamination should be avoided by good sampling methodology Human factors? Different samplers or procedures Fraud Errors : Errors Overall Error Estimation : Overall Error Estimation OE = AE+S(FE+GSE+DE+EE+PE) OE = overall estimation error AE = analytical error (lab precision) FE = fundamental error (in situ variability) GSE = grouping & segregation error (broken ore, drill chips) DE = delimitation error (sample site selection) EE = extraction error (sample selection) PE = preparation error (lab prep) =S(contamination error + sample loss + chemical/physical alteration + human mistakes + fraud) Coefficient of Variability : Coefficient of Variability The higher the CoV, the more difficult ore grade estimation is 1.0 – 1.5 good precision 1.5 – 2.0 fair precision > 2.5 high imprecision – treat with caution Ore Deposit Classification : Ore Deposit Classification Type A: Simple geometry, low CoV Simple geometry, simple grade distribution, eg coal, iron, bauxite, nickel laterite Simple geometry, complex grade distribution, eg, disseminated copper, gold stockworks, Witwatersrand gold Ore Deposit Classification : Ore Deposit Classification Type B: Complex geometry, simple grade distribution, low CoV Base metal deposits Ore Deposit Classification : Ore Deposit Classification Type C: Complex geometry, complex grade distribution, high CoV Lode gold deposits – Yilgarn! Sampling procedures must be tailored to the style of mineralisation Sample size : Sample size Required sample size depends on: In situ concentration of ore mineral In situ grain size of ore mineral Other factors that apply specifically to broken or crushed material Rules of Thumb : Rules of Thumb Sample size limited by physical and economic constraints Bigger is better If samples are poor, then get lots of samples Why Grade Control : Why Grade Control How much ore is going to the waste dump? How much waste is going through the mill? Important to eliminate dilution Even more important not to throw away your ore! Grade Control : Grade Control Carried out routinely at all mine sites to evaluate ore distribution in the pit, or underground Variety of methods used – in open pits commonly RC drilling. Blast holes also logged and assayed. Drilling carried out on fixed pattern to a set depth, unlike exploration drilling Depth determined by size of bench being mined; usually covers two benches at a time. Samples usually only analysed for limited element suite eg. Au only in gold mine, however this is a site-specific decision. Next Week : Next Week Geological logging Geotechnical logging Ore deposit models Summary and review
060807_Joanna_-Drilling_And_Logging |authorSTREAM Drilling and Sampling : Drilling and Sampling Important mineral exploration procedure Delineate ...