Mining Geology and Mine Development: Minerals, Rocks, Ore Genesis, Structures, Access, Drivages, Drilling and Blasting
1. Minerals and rocks: origin and classification
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered atomic structure; a rock is an aggregate of one or more minerals. Minerals are identified in hand specimen by hardness (the Mohs scale), cleavage and fracture, lustre, streak (the colour of the powder, often truer than the colour of the lump), colour and specific gravity. Ore minerals are usually heavy — galena about 7.5, haematite about 5.3 — against about 2.65 for quartz, which is why gravity methods separate them.
| Hardness | Mineral | Hardness | Mineral |
|---|---|---|---|
| 1 | Talc | 6 | Orthoclase feldspar |
| 2 | Gypsum | 7 | Quartz |
| 3 | Calcite | 8 | Topaz |
| 4 | Fluorite | 9 | Corundum |
| 5 | Apatite | 10 | Diamond |
Rocks are classified by origin. Igneous rocks crystallise from magma: slowly at depth into coarse-grained intrusive (plutonic) rocks such as granite and gabbro, quickly at the surface into fine-grained extrusive (volcanic) rocks such as rhyolite and basalt. By silica content they are acid (more than 66% SiO₂, e.g. granite), intermediate (52–66%, e.g. diorite, andesite), basic (45–52%, e.g. gabbro, basalt) and ultrabasic (below 45%, e.g. peridotite, dunite). Sedimentary rocks form from the weathering, transport, deposition and lithification of older rock or from chemical and organic accumulation — clastic sandstone and shale, chemical limestone, organic coal — and carry bedding. Metamorphic rocks are older rocks recrystallised by heat and pressure without melting: shale to slate, phyllite, schist and gneiss with rising grade, sandstone to quartzite, limestone to marble, and coal to anthracite.
2. Ore genesis and structural geology
An ore is a mineral aggregate from which a metal or mineral can be extracted at a profit, so the word is economic as much as geological. Ore deposits are grouped by the process that concentrated them.
| Process | How the ore is concentrated | Typical deposits |
|---|---|---|
| Magmatic segregation | early-crystallising heavy minerals settle in a cooling magma chamber | chromite, platinum-group metals, titaniferous magnetite |
| Hydrothermal | hot aqueous fluids deposit sulphides in veins and replacements | lead-zinc, copper, gold veins |
| Sedimentary | chemical precipitation or deposition in basins | banded iron formations, manganese, coal, evaporites |
| Mechanical (placer) | running water sorts heavy resistant grains | gold, cassiterite, ilmenite and monazite beach sands |
| Residual weathering | soluble constituents leach away in tropical weathering, leaving the insoluble | bauxite, laterite, residual clays |
| Supergene enrichment | metal leached from the oxidised zone re-precipitates below the water table | enriched copper sulphide blankets |
| Metamorphic | recrystallisation under heat and pressure | graphite, sillimanite, kyanite, garnet |
Structural geology describes the attitude and deformation of rock bodies. A bed’s strike is the direction of a horizontal line on it, and its dip is the angle of steepest slope, measured at right angles to strike. In any other vertical section the bed shows a smaller apparent dip α: tan α = tan δ · sin β, where δ is the true dip and β the angle between the section and the strike. On level ground, an outcrop width w measured perpendicular to strike gives true thickness t = w sin δ and vertical thickness w tan δ = t/cos δ.
- Folds: an anticline is convex upward with the oldest beds in its core, a syncline concave upward with the youngest in its core; a fold whose axis is inclined is plunging. Coal and ore are often preserved in synclines.
- Faults: in a normal fault the hanging wall moves down relative to the footwall (extension); in a reverse fault it moves up (compression), and a low-angle reverse fault (dip below 45°) is a thrust; in a strike-slip fault the movement is horizontal, parallel to strike. A fault displaces a seam, so the miner must know which way to look for it.
- Joints are fractures with no visible displacement; their sets, spacing and orientation govern blasting, support and slope stability. An unconformity is a buried erosion surface — a gap in the record.
3. Access to the deposit and underground drivages
A deposit is reached by one of four openings, and the choice is set by depth, topography and the output to be carried. An adit is a near-horizontal tunnel driven from a hillside into a deposit that lies above the valley floor — the cheapest access, draining by gravity. An incline (or decline) is a sloping drift, suited to shallow seams and to conveyor or trackless-vehicle haulage. A vertical shaft is the shortest route to a deep deposit and carries hoisting, ventilation and services. Shallow flat deposits under level ground may be worked from the surface altogether.
| Method | Principle | Ground |
|---|---|---|
| Conventional (drill and blast) | drill, blast, muck with a grab, line the shaft in stages | dry, competent rock |
| Freezing | brine circulated in boreholes round the shaft freezes the ground into a temporary wall | water-bearing sand, gravel and weak strata |
| Cementation (grouting) | cement grout injected ahead seals fissures | fissured water-bearing rock |
| Caisson and drop shaft | a lining sunk under its own weight as ground is excavated inside it | shallow soft ground |
| Raise boring and shaft boring | a mechanical cutter bores the full section | competent rock, with access below for raise boring |
An underground drivage — a gallery, cross-cut, drift or roadway — advances by a repeated cycle: drill the face, charge and blast, ventilate away the fumes, muck (load out) the broken rock, and support the new roof, then repeat. Progress per cycle is the pull, the depth of the round that actually breaks, usually 85–95% of the hole depth in a good cut. The cut — wedge (V), drag, or burn (parallel-hole with empty relief holes) — opens a free face for the rest of the round to break towards. A road header or continuous miner replaces the drill-and-blast cycle by mechanical cutting in coal and softer rock, with a smoother profile and no blast damage.
| Method | How rock is broken | Machines and use |
|---|---|---|
| Percussive, top hammer | a piston strikes the drill steel at the collar and the bit indents and chips the rock, rotating between blows | jackhammers, jumbos; efficient in short holes, loses energy at every rod joint |
| Percussive, down-the-hole (DTH) | the hammer travels down the hole behind the bit | blastholes in hard rock; energy loss does not grow with depth, holes stay straight |
| Rotary, tricone | heavy thrust crushes rock under rolling toothed cones | large-diameter surface blastholes in medium to hard rock |
| Rotary, drag bit | a cutting edge shears soft rock | coal, shale; roof-bolt holes |
| Rotary-percussive | rotation with heavy thrust plus impact | hard rock at high penetration rates |
4. Explosives, initiation, blast design and ground vibration
An explosive releases gas and heat by a very fast oxidation. A low explosive (gunpowder) deflagrates, burning at subsonic speed and heaving rather than shattering; a high explosive detonates, with a supersonic shock front and far higher pressure. Commercial high explosives are nitroglycerine-based dynamites, ANFO (ammonium nitrate with about 5.5–6% fuel oil, the oxygen-balanced mix — cheap and safe but with no water resistance), and the water-resistant slurries (water gels) and emulsions. Permitted (permissible) explosives are formulated with a cooling salt and tested to be used in gassy coal mines without igniting firedamp. The properties that matter in selection are strength, velocity of detonation, density, water resistance, fume class and sensitivity.
- Initiation systems. Safety fuse with a plain detonator (now largely superseded); electric detonators, instantaneous or with delay elements, fired from an exploder and tested with a circuit tester — vulnerable to stray current, static and radio-frequency energy; non-electric shock-tube detonators, immune to those hazards; electronic detonators with programmable delays accurate to a millisecond; and detonating cord, a PETN core that carries detonation along a line of holes.
- Blasting tools and accessories: exploders, circuit testers (ohmmeters made safe for blasting), tamping rods of wood or non-sparking material, stemming, and the connectors and relays of a non-electric system.
Bench blast design sets the burden B (hole to free face), the spacing S (hole to hole along the row), the stemming (inert collar, about 0.7–1.0 B), the sub-drill below grade (about 0.3 B), and the delay sequence. A hole of diameter d filled with explosive of density ρ holds loading density = (π d²/4) ρ kg per metre. The powder factor is explosive per unit of rock broken, in kg/m³ or kg/t, and one round’s volume is B × S × bench height × number of holes.
Ground vibration is measured as peak particle velocity (PPV, mm/s) and predicted from the scaled distance SD = R/√Q, with R the distance and Q the maximum charge per delay: PPV = K (R/√Q)^−β, K and β being site constants from a trial blast regression. Because only SD enters, keeping SD fixed keeps PPV fixed: the allowed charge per delay grows with the square of distance. The permissible PPV for a structure depends on its type and on the dominant frequency and is set by the regulator; a question will state it. Air overpressure and flyrock are the other two off-site effects, controlled by stemming, burden and confining the charge.
Key takeaways
- Igneous rocks are named by silica (acid above 66%, basic 45–52%) and by texture (coarse intrusive, fine extrusive); metamorphism recrystallises without melting.
- Chromite and PGM are magmatic, bauxite residual, gold and ilmenite placers often mechanical, and base-metal veins hydrothermal.
- tan(apparent dip) = tan(true dip) × sin(angle from strike); true thickness = outcrop width × sin δ on level ground.
- An adit for a hillside deposit, an incline for shallow seams, a vertical shaft for depth; freezing for water-bearing loose ground.
- Loading density = (π d²/4)ρ; powder factor = charge/rock broken; vibration scales with R/√Q, so allowed charge per delay grows as distance squared.
Practice questions (14)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
On the Mohs scale of hardness, the mineral with hardness 7 is:
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Answer: B — Quartz
Quartz is 7, between orthoclase (6) and topaz (8); apatite is 5. Because quartz is so common and so hard, a steel blade (about 5.5) scratches feldspar with difficulty and quartz not at all — a quick field test.A fine-grained igneous rock with about 48% SiO₂ is most likely:
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Answer: C — Basalt
48% silica is basic (45–52%), and fine grain means it cooled quickly at the surface — basalt. Gabbro has the same chemistry but is coarse-grained and intrusive; rhyolite and granite are acid, above 66% silica.Bauxite deposits are formed chiefly by:
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Answer: B — Residual weathering in a tropical climate
Intense tropical weathering leaches silica and bases out of aluminous rock and leaves insoluble aluminium hydroxides behind — a residual concentration, like laterite. Magmatic segregation gives chromite, and hydrothermal fluids give sulphide veins, not aluminium ores.Which of the following statements about faults are correct? (Select all that apply.)
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Answer: A — In a normal fault the hanging wall moves down relative to the footwall; C — A thrust is a reverse fault with a low dip
A normal fault drops the hanging wall and records extension, not compression; a reverse fault lifts it, and a reverse fault dipping below 45° is called a thrust. Strike-slip movement is horizontal, parallel to the strike, so the last option describes a dip-slip fault instead.A coal seam dips at 30°. In a vertical section making 60° with the strike, the apparent dip in degrees, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 26.57
tan α = tan δ sin β = tan 30° × sin 60° = 0.57735 × 0.86603 = 0.5, so α = tan⁻¹ 0.5 = 26.57°. Using cos 60° instead of sin 60° (the angle from the dip direction, not from strike) gives tan α = 0.289 and 16.1°.On level ground, the outcrop of an ore bed dipping at 30° measures 40 m perpendicular to its strike. The vertical thickness of the bed, in m, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 23.09
True thickness t = w sin δ = 40 × 0.5 = 20 m, and vertical thickness = t/cos δ = 20/0.86603 = 23.09 m, which is also w tan δ = 40 × 0.57735. Answering 20 gives the true thickness, measured perpendicular to the bedding, not the vertical one a borehole would cut.A mineral deposit outcrops on a hill slope well above the adjoining valley floor. The cheapest means of access is usually:
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Answer: B — An adit
An adit driven in from the slope reaches the deposit with no hoisting at all, and water and ore leave by gravity. A shaft or a decline from the hilltop would have to lift or carry everything back out through the height the adit avoids.A shaft is to be sunk through 60 m of water-bearing running sand. The most suitable special method is:
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Answer: C — Freezing
Freezing turns the loose saturated sand into a strong, watertight frozen wall until a permanent lining is in. Drill and blast needs competent dry rock, raise boring needs access below and stable ground, and grouting (cementation) suits fissured rock rather than running sand.The main advantage of a down-the-hole (DTH) hammer over a top-hammer drill for deep blastholes is that:
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Answer: B — Its impact energy is not lost through the rod joints, so penetration does not fall with depth
The DTH hammer sits directly behind the bit, so every blow reaches the rock whatever the hole depth, and the holes stay straighter. A top hammer loses energy at each coupling, which is why it wins only in short holes; the DTH still needs compressed air for flushing and uses percussion, not rotation alone.A 115 mm diameter blasthole is filled with ANFO of density 850 kg/m³. The loading density, in kg per metre of hole, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 8.83
Loading density = (π d²/4)ρ = 0.7854 × 0.115² × 850 = 0.7854 × 0.013225 × 850 = 8.83 kg/m. Forgetting to convert 115 mm to 0.115 m gives an absurd 8.8 million, and using d instead of d²/4 gives about 77 kg/m.A bench round has 20 holes on a 3 m burden and 3.5 m spacing in a 10 m bench, each charged with 84 kg of explosive. The rock density in situ is 2.5 t/m³. The powder factor, in kg/t, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 0.32
Rock broken = 3 × 3.5 × 10 × 20 = 2100 m³ = 2100 × 2.5 = 5250 t, and explosive = 20 × 84 = 1680 kg. Powder factor = 1680/5250 = 0.32 kg/t (0.8 kg/m³). Quoting 0.8 answers per cubic metre, and dividing one hole’s charge by the whole round’s tonnage gives 0.016.A vibration study at a quarry shows that a scaled distance of at least 50 m/kg^½ (square-root scaling) keeps the peak particle velocity within the limit for a village. The village is 250 m from the blast. The maximum charge per delay, in kg, is ____.
Numerical answer — type the value.
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Answer: 25
SD = R/√Q ≥ 50 gives √Q ≤ 250/50 = 5, so Q ≤ 25 kg per delay. Taking Q = R/SD = 5 forgets the square root, and cube-root scaling (R/Q^⅓) is a different law that the question did not specify.With square-root scaled distance, if the charge per delay at a site is to be increased four times without raising the peak particle velocity at a structure, the blast must be moved:
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Answer: B — to twice the distance
PPV is fixed by R/√Q, and √(4Q) = 2√Q, so R must double to hold the ratio. Four times the distance would be needed only if PPV scaled with R/Q. Adding delays keeps the charge per delay down, which is the opposite of the change the question asks for.Which of the following statements about explosives and initiation are correct? (Select all that apply.)
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Answer: A — ANFO has poor water resistance; B — Permitted explosives are formulated for use in gassy coal mines; C — Shock-tube detonators are immune to stray current and radio-frequency energy
Ammonium nitrate dissolves in water, so ANFO fails in wet holes; permitted explosives carry a cooling salt and are tested against firedamp ignition; a shock tube carries a low-energy detonation inside a plastic tube with no electrical path. The oxygen-balanced mix is about 94 : 6 AN to fuel oil, not 80 : 20 — excess fuel gives CO-rich fumes.