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MIME 329 – Notes Lecture 1: Techniques of Mineral Exploration Part a: Prospecting – Exploration – Compilation – Mapping/Field Work – Geophysical Surveying

Phases of Development of a Mining Project Most Mines typically • Start out as a Showing Discovered by Prospectors • Defined by Junior Exploration Companies (e.g. Drilling) • Developed by Intermediate or Senior Companies into Viable Mineral Deposits

(Mining)

First Two Stages of the Life of a Mine - Definitions • Prospecting: § § §

Initial Search for Mineral Occurrences that have the Potential of Being Mined Target Generation ± Based on Scientific Principles

• Exploration: Advanced Evaluation of a Prospect to Determine: §

Size, Grade & Geometry of the Deposit o Define Resources - Profit Potential

When does Prospecting turn into Exploration? • No easy Answer • Partial Overlap • Generally when Project Requires: Systematic Examination of a Deposit (Drilling);

Large Amount of Money to Advance the Project (e.g. Airborne Geophysics, Logistics,..) – (turning point often at drilling stage) Prospecting – Strategy 1. Decide which Commodity to Search for: • Fe, Au, Rare Earths, Lithium, Uranium, Diamonds,... 2. a) Select the Country/Area based on: • • •

(Perceived) Mineral Potential Mining Tradition, Laws (Canada, Ghana, Chile,....) Political, Economic, Logistical,... Considerations

2. b) Most Prospectors Focus on Areas with: • • • • •

Recent Mineral Discoveries or Staking Activity (“Follow the Crowd”; Gold Rush) Minerals with Expected Rising Demand (e.g. Lithium, Co-Ni) New Roads (Improved Access, New Rock Exposures) Newly Released Geoscientific Data by Government, Mining Companies Presence of Outcrops with Clues to Mineralization § “Favourable” Minerals (Quartz Veins and Pyrite - Au) § Oxidized Rocks (Au, Ni, Cu,..), Alteration (Chlorite, Epidote,..) § Topographic Expression, Structure,.. (Folds, Shear Zones Commonly visible on Aerial Photographs) o Rock Type § Vegetation

3. Compilation • Review of Historical Data: § Maps, Reports (Government, NI 43-101,..) o Previous Exploration Data § Data from Closed down Mines § Old Drill Core (Occasionally left in the field) • Provides

§ §

Major Competitive Advantage Added Value to the Property

• Ensures that Mineral Exploration Investment: § Does not Duplicate Past Effort § Builds Upon Accumulated Knowledge

4. Acquire Title (Claim, Exploration Permit,..) 5. Field Work • Start with Geological – Structural Mapping • Interpret Topographic Map, Air Photos • Plots Rocks, Minerals, Shear Zones, Folds,.. (Strike, Dip, Plunge) • Determine Position (GPS) of all Data Points 6. Select Exploration Technique • Reconnaissance Mapping • Sampling (Grab, Auger,..) • Magnetic Survey,..... Special Case : Quaternary Sediments Cover/Weathered Rocks : • • • • •

Trenching Pitting (“Orpailleurs”) Outcrop Stripping to Expose Bedrock Blasting Outcrops Drilling (Hand or Mechanized Auger, Portable Drills)

Prospectors – Requirements • Knowledgeable about the Mining Industry o Price & Demand for Metals o Trends (Iron Ore, Rare Earth, Uranium,....) o Economic Size (10-cm Quartz Vein !!) o Grades of Economic Potential (vs. Visible Gold) o Negotiating Mining Agreements (e.g. Fred Schumacher) • Working knowledge of: o Geology: Identification of Rock Type, Mineral, Structure o Mining Act, Claim Staking, Relevant Legislation (Permits) o Mineral Exploration Techniques o Bush Craft o Geography & History of the Area they are Working in

Prospectors – Role • •

Part of Preliminary Stage of Mineral Exploration (Prospectors, Artisanal Miners) Market, Promote, Sell Mineral Occurrences To Mining Companies, for a Profit and Interest in the Mineral Properties

•

Despite all his Hard Work, 99% of the Time, the Prospectors don’t Find Anything Significant! Statistically, hardly 1 in 10,000 mineral showings ever becomes a mine (1) It takes 25,000 claims staked to find 500 worth diamond drilling to find one mine (2) However

• •

Prospectors Make Tremendous Contributions to the Development of Mineral Resources 1945-1965 : 57% of the Mines put into Production in Canada were Discovered by Conventional Prospecting (Lang 1970)

Time Evolution of the Discoveries of a Mineral Deposit

General Trends • •

Decline in Discovery Performance : Inability to Effectively Explore and Find Deposits Under Deep Cover Most Grassroot (Greenfield) Discoveries : § Continue to be Made at- or Near- Surface (Average Depth of Cover over 20052014 = 64 m) § Nearly Half of These Deposits Outcrop § Over Time This Search-space Becomes Progressively Depleted

• Brownfield Discoveries: Different; shows a Steady Trend Towards Deeper Discoveries

Geophysical Surveying Definition: Application of the Principles of Physics to the Study of the Earth Employs Quantitative Physical Methods Purposes: Characterization of the Subsurface Geology, Structure, Minerals

Description (All geophysical Techniques ) • • •

Based on the Detection of Contrasts in Physical Properties of Materials Remotely sensed, Non-invasive Technologies Determine what is Directly Below the Surface or to Depths of 100s m

Classification •

•

Passive Methods: Measure Natural Properties or Fields of the Earth (Gravity, Magnetic, Radioactive, Self Potential,...) Active Methods: Introduce Artificial Signals into the Earth and Measures the Response after being Modified by the Earth Materials (Electrical, Electromagnetic, Induced Polarization, Seismic, Radar,..)

Measurement of the Physical Properties of Rocks • • • •

Magnetic Field Electrical and Electromagnetic Methods Gravitational Field (Density) Natural Radiation (Remote Sensing) § Passive : Reflected Sunlight (from Visible to Gamma Ray Electromagnetic Spectrum), Radioactivity,.. § Active : Radar Survey, .. • Wave Field (Density & Elastic Modulus) [Seismic, Radar]

Data Collection • Ground Surveys • Borehole • Airborne Surveys - Helicopter or Fixed-Wing Airplane, (Drone) § Fast § Several Surveys Methods at Once o Relatively Inexpensive per Unit Area o Only Used to Cover Large Areas § Limitations o Terrain & Weather o Accurate & Visible Ground Control Features Required o Wider Line Spacing o Signal Attenuation (Better Details or Sharper Signal in Ground Survey)

Airborne Surveys: Helicopter vs. Fixed-Wing vs, Drone • Heli-borne § § § § § §

More Constant Ground Clearance Maintained Above Rugged Terrain Detailed Work or Reconnaissance Only a Third as Many Line-Kilometers per Day Covered at best Relatively Short Range of Operation Much More Expensive to Operate than Fixed-Wing Aircraft More Maintenance Work Required per Flying Hour

• Drone capabilities § Ultra High Resolution Imagery, Digital Elevation Models (Lidar) § Magnetic survey: replaces labour-intensive ground surveys, no lines to cut, §

cheaper and quicker, smaller environmental footprint 50-minute flight time

Geophysical Surveying - Magnetic Method Measures the Total Magnetic Field (Vertical Component) • •

Terrestrial Field (Earth = Dipole) Buried Magnetic Body (Anomaly) § Remanent (Permanent) Field (Direction may Differ Radically from Induced Field - Paleomagnetism) § Induced Field : Ferro-Magnetic Material Aligns Along the Ambient Terrestrial Magnetic Field (Susceptibility to Magnetization)

Geophysical Surveying - Gravity Method Definition • •

Measure of the Vertical Component of the Gravity Field Force of Gravity due to: § Total Mass of the Earth § Buried Mass of Different Density

• Lateral Variations in Rock Density Give Rise to Gravity Anomalies (Regional versus Residual) Uses Exploration for • •

Massive Sulphides (d=4.1) Banded Iron Formation (d=3.4) vs. Granite (d=2.6)

Geophysical Surveying - Electrical Methods - Resistivity

Principles • •

Most rocks devoid of metallic minerals are electrically resistive Variations in the Resistivity and Distribution of Subsurface Material Produce Variations Between Applied Current and Potential Measured on Surface

•

Relationship : Potential – Current – Resistance (Ohm’s Law) V=IR

•

Resistance (R) Calculated from § Known input current (I) § Measured voltage (V) § Geometry of the electrode array

• •

Resistivity (Ohm-meter) is the Resistance times Area divided by Distance “Apparent Resistivity”: average value of the different materials within the volume (halfspace) of materials being measured Methodology

§ § § §

Use four electrodes driven into the ground Apply an electrical current to two electrodes (Current Electrodes) using a generator Measure the voltage (Difference of Potential) between the other electrodes (Potential Electrodes) using a voltmeter The Magnetic Field may be Measured Instead of Potential

Measurements: Different Electrode Arrays (Schlumberger, Wenner, Dipole-Dipole) § §

2 D Profiling (Constant Separation - Lateral Changes in Resistivity) Vertical Electrical Sounding (Depth Sounding)

Applications § § §

Depth to Bedrock, Stratigraphy (Contacts between Two Formations) Massive Sulphides Faults, Shear Zones

Geophysical Surveying - Electrical Methods – Induced Polarization (IP) Principle

Certain Bodies in the Ground Become Polarized by Electric Current and Act as a Battery Discharging After a Charge Period (“IP Effect”) Potential Difference Measured Between the Potential Electrodes Gradually Decays when the Current is Turned Off Measurements • Decrease in Potential (Voltage) when the Current is Turned Off (Time Domain; Chargeability) • Drop in Resistance to an AC Current with Increasing Frequencies (Frequency Domain – Frequency Effect or Metal Factor) • Amplitude and Phase Shifts at Different Frequencies (Spectral IP) Uses of IP Survey • Disseminated Sulphide Mineralization (Non-Connected Grains): § §

Porphyry Copper Halo Around Massive Sulphides Deposit

Geophysical Surveying - Electrical Methods – Self Potential (SP) Principle Measurement of the Difference in Potential Between Two Points on the Ground Produced by Currents (Galvanic) Naturally Generated by Conductive Mineralised Bodies Acting as Batteries Partially Immersed Below the Water Table (Oxido-Reduction?) Geophysical Surveying – Electro-Magnetic Methods - EM Active vs. Passive EM Systems • •

Active: Use Transmitter to Induce Ground Current Passive: Use Natural Ground Signals § Magneto-Telluric Method: Correlated with Variations in the Geomagnetic Field Caused by Solar Emissions § Audio Magneto-Telluric Uses Thunderstorms Worldwide as a Source

Principles •

AC Injected Through a Transmitting Loop (Coil) Generates a Primary Magnetic Field

• •

This Primary Magnetic Field Induces Secondary Electrical Eddy Currents (Also Called Foucault Currents) in the Subsurface The Eddy Currents, in Turn, Generate a Secondary Magnetic Field in a Conductive Body

Measurements • •

Receiver Measures Sum of Primary & Secondary Fields Geometry, Size, Electrical Properties of the Conductor Causes: § Difference between Resultant and Primary Filed § Phase Lag of Secondary Magnetic Field Relative to Primary Field

Survey Systems Frequency-Domain § §

Amplitude and Phase of the Secondary Field Measured [ In-Phase and Out-of-Phase (Quadrature) Components ] Variations in Amplitude and Phase Depends on the Electrical Conductivity and Depth of the Medium Time-Domain Current Pulse Generated by Transmitter

§

When the Pulse is Off : Secondary Voltage Only Measured at the Receiver

§

Decay of the Signal Measured Tilt-Angle Receiver Measures the Angle of the Secondary Magnetic Field (Difference in Orientation between the Primary and Secondary Magnetic Fields)

Characteristics • No Electrode in the Ground: Transmitting Coil - Receiving Coil § §

Speed of Operation Airborne Survey Possible

• Large Depth of Penetration • Area of Investigation • Small or Large Loop: § Depth of Penetration § Area of Investigation

§ Loops : Horizontal Co-Planar, Vertical co-Planer, Vertical Coaxial,.. Electro-Magnetic Survey Targets • •

Massive Sulphides (Interconnected Sulphides) Shear-Hosted Gold (Conductive clay, graphite)

Geophysical Surveying – Radar/Seismic Methods Elastic Modulus – Density • •

Ground Penetrating Radar (Radio Waves) Seismic (Refraction, reflexion)

Part b: Remote-sensing – Geochemical Surveying – Data Interpretation – Data Mining Remote-Sensing Definition Observation and measurement of objects from a distance Use of satellite imagery to look at variations on the earth's surface Types Passive instruments detect radiation naturally emitted or reflected (sun’s energy) by the object • • •

Optical sensors – measuring the portion of the visible spectrum (Photography) Multispectral scanner collects data over a variety of wavelength ranges beyond the visible wavelengths (..., X Rays, UV --- Infrared, Radar,..) Hyperspectral sensors collect data within very narrow spectral bands (very high spectral resolution)

Active instruments provide their own energy (electromagnetic radiation) to illuminate the scene and receive the reflected or backscattered radiation • Satellite RADAR Photos

• •

Illuminate an Area on the Ground and Take a Picture at Radio Wavelengths (Microwaves) Removes Masking Effect of Vegetation

• LIDAR (Light Detection And Ranging) • •

Measures the Distance to a Target by Illuminating it with Laser Light Main Uses • Altimetry (Airborne Surveys) • Geological Interpretation (from Outcrop Patterns) (*) • Digital Terrain (Elevation) Model (DTM)

Radiometric (Gamma-Ray Spectrometry) Survey Definition •

•

Record the Intensity of Gamma Radiation (End of Electromagnetic Spectrum) from rocks and soils that comes from the natural decay products of radio-isotopes • Uranium • Thorium • Potassium Sensors on Satellite/Aircraft Platforms or Ground Survey

Applications • Maps the Radioelements in the upper rock/soil layer (Surface Geology, Weathering, Alteration, Granite Intrusions) Geochemical Survey – Introduction Definition - Purposes • • •

Detect Abnormally High Concentrations of Specific Chemical Elements In Rocks, Soil, Water (Lakes, Rivers), Vegetation, Radon,.. A Deposit is a Geochemical Anomaly Reduce the Area of Investigation by Looking for: • Metal Itself (“The best indicator of gold is gold”) • Pathfinders, Indicator Minerals-Elements (Cu, Ag, As,.. for Gold Mineralization) • Enlarged Anomaly (More Mobile Elements, e.g. Antimony & Arsenic associated with Gold)

Method • •

Collect Samples of Rocks, Soil, Sediments, Vegetation,.. Analyse - Plot Results on a Map - Interpret

Geochemical Anomaly : • • •

A concentration of one or more elements in rock, soil or vegetation that is markedly higher (or lower) than background Contrast between two Populations : Background and Anomaly Absolute Values not Critical (25 ppb Au can be Anomalous Against 5 ppb Background) Background

• •

The “Natural” Abundance of an Element in any Material where the Equilibrium has not been Upset by the Presence of Mineralized Material Corresponds to the Largest Number of Values Within a Restricted Range

Geochemical Survey – Stream Sediments Definition • •

Sampling of Stream Sediments from Active or Ancient Drainage Courses Samples Reflect • Bedrock Geology / Mineralization of the Catchment Area • Overburden Cover

Applications Early Stage Exploration (e.g. Reconnaissance Survey, About 1 Sample per 1.5 sqkm) Method • • •

Select Appropriate Size Fraction (Orientation Survey) to Enhance the Anomalies Collect Sediments, Sieve, Pan Start (*) from as Low an Order as Possible (Smallest Streams: 1) and Upstream from Confluence with Higher Order (Larger: 2, 3, 4) Streams

Geochemical Survey – Soil Sampling Definition Use of Soil Samples for Geochemical Analysis Method – Planning • Determine: •

Residual Soil / Regolith over Bedrock (“In Situ”)

•

Transported Material

• Spacing (e.g. Reconnaissance : Typically 400 by 400m) • Orientation Survey (Material With the Strongest Response) • • •

Soil Horizon (“B” Horizon; Laterite, Saprolite,... in Tropical Environment) Fraction to be Sampled (Grain size) Sample Size

• Method of Sampling • Sample Preparation & Analysis Geochemical Survey – Geo-Botanical Survey • Rationale : Plants Growth Modified by Certain Metals • • •

Thrive Tolerate (Stressed) Poisoned

• Methodology: Visual Investigation of Plant Cover Types that May Indicate Mineralization • •

Nature and Distribution of Plants Modification in the Appearance of Foliage or Vegetation

Rationale • Certain plants can take up and concentrate elements selectively from soil solutions (More precisely : Phyto-geochemical survey) Method • •

Analysis of plant material that has accumulated trace elements without producing visible morphological or physiological effects Media: leaves, twigs, bark, humus,..

Geochemical Survey – Lithogeochemical Survey Definition Analysis of Major Element in Rocks Uses Verify that an Area is in : • •

Favorable Geochemical (Metallogenic) Provinces Similar Stratigraphy known to host Mineralization

•

Rocks Likely to be Associated with Mineralization (Cr, Pt,.. in Ultramafic Rocks)

Rocks Likely to be Associated with Mineralization : •

•

Different Granite Types : - Tin-Areas Enriched in CaO in Granite; - Zoning in Mo-Cu-Au Porphyry-type Deposits (Zn-Au-Cu-K-Mo) Hydrothermal Alteration Associated with Massive Sulphides Deposits (Na-Ca Depletion and Enrichments in Fe, Mg, Si, K,... in Area Proximal to Discharge Zone )

• •

VMS in Calc-Alcaline rather than in Tholeitic Basalts Strongly Metamorphosed Rocks (Sedimentary or Volcanic Precursor)

Geochemical Survey – Mobile Metal Ions Rationale • •

Ions Released from Mineralized Material Ions Travel Upward from Mineralization to Unconsolidated Surface Materials and are Adsorbed by • Soil • Till (Glacial Sediments) • Sand • Laterite (Nodules, Gravel)

Method • • •

Careful Soil Sampling (Constant Depth) Sophisticated Chemistry (Light Leach) Ultra Sensitive Instrumentation

Benefits Detection of Mineralization : • •

Under Deep Cover Transported Anomaly (“False Anomaly)

Problems • •

Expensive Sensitive to Field Conditions (Rain,..)

Geochemical Survey - Interpretation

Background/Threshold/Anomalous Values • Determine Threshold • • • •

Per Element Per Area (Domains) Per Sample Type (Populations: Soil-Humus; Laterite-Saprolite-Fresh) Per Analytical Method (FA, AA)

• Methods • • • •

Upper 5% (Over 95th Percentile) Mean+2 Standard Deviations Cumulative Frequency Curve – Histogram Contours (Start of Erratic Behaviour)

Geochemical Survey - Validation Validating a Geochemical Anomaly: 1. Define • •

Residual Anomaly Transported Anomaly

2. Validate •

Start with RAB Drilling (Bedrock Samples will Delineate In Situ Anomaly)

•

Follow with RC/Core Drilling

Special Case, Several Populations Several Thresholds for the Same Element (e.g. Use of Different Analytical Methods) or Several Sa...