Tricky Mining interview questions

75 questions with answers (MIN-426 to MIN-500). Try each one aloud before reading the answer.

Mining Geology & Exploration

MIN-426 · Is a high-grade drill intersection enough to declare an ore body?

Trap: One spectacular intercept is not a deposit. Continuity, tonnage, metallurgy and economics must be proven by systematic drilling at suitable spacing; isolated high grades often come from a narrow vein or a nugget effect and may not repeat.

MIN-427 · Does a larger number of samples always improve a resource estimate?

Trap: Not if they are biased. Poor sample recovery, contamination, wrong sample preparation or unrepresentative sampling can add many wrong numbers. QA/QC (standards, blanks, duplicates) matters more than raw sample count.

MIN-428 · Is an ore body defined by geology alone?

Trap: Ore is an economic concept. The same mineralised rock can be ore at a high metal price and waste at a low one; cut-off grade, costs and recovery decide the ore boundary, not just the geology.

MIN-429 · Does the arithmetic average of drillhole grades give the deposit grade?

Trap: Simple averaging ignores clustered drilling, sample length and spatial correlation. Composite to equal lengths, decluster and use kriging or other weighted estimation; otherwise dense drilling in high-grade zones biases the average upward.

MIN-430 · Is a geophysical anomaly proof of mineralisation?

Trap: Anomalies show contrasts in physical properties, not ore. Graphite, pyrite, clays, saline water or cultural noise can give the same response. Anomalies must be tested by drilling and interpreted with geology and geochemistry.

MIN-431 · Does a higher grade always mean a more valuable deposit?

Trap: Value depends on tonnage, depth, metallurgy, location, infrastructure and impurities too. A large, low-grade, shallow, easily processed deposit can be worth far more than a small, deep, high-grade one with refractory ore.

MIN-432 · Can inferred resources be included in a reserve if the mine plan needs them?

Trap: No. Under JORC and NI 43-101, only indicated and measured resources can convert to reserves. Inferred material may appear in some scoping studies with caveats, but never in a reserve statement.

Mine Development

MIN-433 · Is the shortest access to an ore body always the best choice?

Trap: The shortest route may cross poor ground, aquifers or future mining zones, or leave the infrastructure inside the subsidence zone. Access design balances length with ground conditions, ventilation, haulage efficiency and the long-term mine layout.

MIN-434 · Is a decline always cheaper than a shaft?

Trap: A decline is cheaper and faster for shallow mines and allows truck haulage, but at depth haulage costs, ventilation and long travel times favour a shaft. Beyond roughly 600–1000 m, a shaft usually wins for high production rates.

MIN-435 · Does driving development faster always shorten the time to production?

Trap: Only if development is on the critical path. Rushing can cause poor ground support, overbreak and rework, and other constraints such as ventilation, power or permits may set the actual schedule.

MIN-436 · Is a bigger tunnel cross-section always better for productivity?

Trap: Larger openings allow bigger equipment and better ventilation, but they cost more to excavate and support and may be less stable in weak ground. The section should fit the equipment and air needs, not exceed them.

MIN-437 · Can development waste rock be dumped anywhere underground?

Trap: Dumping in active drives, near orepasses or without a plan can block ventilation, create stability problems or dilute ore. Waste is usually used as backfill or hoisted, according to a waste handling plan.

MIN-438 · Is the cost of a shaft mainly in sinking?

Trap: Sinking is large, but equipping (headframe, hoist, conveyances, guides, services), commissioning and the time value of a long pre-production period often add as much again. The schedule delay can cost more than the excavation.

MIN-439 · Does a steeper decline gradient always save money?

Trap: A steeper ramp shortens the decline but reduces truck speed, increases fuel use, brake and tyre wear and can be unsafe. Most declines are kept around 1 in 7 to 1 in 9 as a balance.

Surface Mining

MIN-440 · Is the stripping ratio alone enough to compare two open-pit projects?

Trap: No. The value of the ore, haul distances, waste rehandle, slope stability and processing costs all matter. A higher stripping ratio can be fine for high-value ore, and the ratio changes over time with pushbacks.

MIN-441 · Do steeper pit slopes always improve project value?

Trap: Steeper slopes reduce waste stripping, but a slope failure can bury the ramp, stop production and endanger people. The optimum balances stripping savings with an acceptable probability of failure.

MIN-442 · Is the ultimate pit the plan you actually mine?

Trap: The ultimate pit shows the economic limit; the operation mines it through pushbacks and phases that defer waste, reach ore early and keep equipment working. Mining the final pit outline in one go would destroy NPV.

MIN-443 · Does a bigger shovel always reduce cost per tonne?

Trap: Only if the trucks, benches and ore selectivity suit it. An oversized shovel with mismatched trucks idles, and big equipment increases dilution in narrow or complex ore bodies.

MIN-444 · Is reclamation just planting grass at the end of mining?

Trap: Reclamation includes landform design, drainage, erosion control, soil handling, water quality and long-term monitoring against agreed criteria, and is best done progressively. Planting alone often fails on unstable or acid-generating surfaces.

MIN-445 · Is the bench height always set by the shovel?

Trap: Loading equipment influences it, but drilling, blasting, geology (ore selectivity and dilution), slope stability and regulations also set bench height. Ore benches are often lower than waste benches for selectivity.

MIN-446 · Does more blasting energy always reduce total mining cost?

Trap: Finer fragmentation lowers loading, hauling and crushing cost up to a point, but beyond the optimum, explosive cost, flyrock, wall damage and vibration rise. The goal is mine-to-mill optimum, not maximum breakage.

Underground Coal Mining

MIN-447 · Is longwall always the best method for a coal seam?

Trap: Longwall needs regular, fairly flat seams with few faults, enough reserves to repay high capital and acceptable subsidence. Thin, faulted or small deposits may suit room-and-pillar or other methods better.

MIN-448 · Does leaving bigger pillars always make a coal mine safer?

Trap: Oversized pillars waste coal and can transfer high abutment stress to nearby workings, and in deep mines pillar bursts can occur. Pillar size should come from a design method (e.g. ARMPS, Bieniawski) with a suitable safety factor.

MIN-449 · If a methane detector shows 0%, is the area safe?

Trap: Only at that point and moment. Methane layers along the roof, accumulates in cavities and changes with barometric pressure; detectors need calibration and testing at the right height. Other gases (CO, CO₂, H₂S) and oxygen must also be checked.

MIN-450 · Does increasing ventilation always reduce the explosion risk from methane?

Trap: Usually it dilutes methane, but higher air flow over a goaf can increase oxygen supply and the risk of spontaneous combustion, and very high velocities raise dust. Ventilation must be designed for the whole system.

MIN-451 · Is coal dust only a health hazard?

Trap: Fine coal dust is also explosive. A small methane ignition can lift dust and trigger a far bigger explosion, which is why stone dusting (inerting) of roadways is required.

MIN-452 · Does a higher rate of advance always reduce spontaneous combustion risk?

Trap: Faster advance does reduce the time coal is exposed to oxygen in the goaf, which helps, but it can be undermined by poor sealing, leakage paths or high-propensity coal. Monitoring and inertisation are still required.

Underground Metal Mining

MIN-453 · Is the stoping method chosen mainly by ore grade?

Trap: Method selection depends mostly on ore body geometry (dip, thickness, shape), rock strength, depth and value. Grade influences how much selectivity is worth paying for, but geometry and geotechnics drive the method.

MIN-454 · Does block caving give the lowest cost so it suits every large ore body?

Trap: Caving needs rock that will cave and fragment, a large tonnage to repay huge pre-production capital and long lead times, and it causes surface subsidence. Strong, massive rock or limited tonnage can make it unsuitable.

MIN-455 · Is backfill only used to dispose of tailings?

Trap: Backfill's main role is ground support: it stabilises stopes, allows pillar recovery, limits subsidence and provides working floors. Using tailings is a bonus that reduces surface storage.

MIN-456 · Does a larger stope always improve productivity?

Trap: Larger stopes give more tonnes per development metre, but exceed stable spans, increase dilution and can trigger failures. Stope size is limited by the stability graph and dilution targets.

MIN-457 · Is dilution only a problem of bad blasting?

Trap: Dilution also comes from wall slough due to stress or weak hanging walls, mining irregular shapes with regular stopes, poor drilling accuracy and backfill dilution during extraction.

MIN-458 · Does a deeper mine just need more of the same equipment?

Trap: Depth brings high stress, rockbursts, heat, longer hoisting and ventilation demands. Mining methods, support and cooling often have to change fundamentally, not just scale up.

Rock Mechanics & Ground Control

MIN-459 · Is a stronger rock always a safer rock?

Trap: Strong, brittle rock can store large strain energy and fail violently (rockbursts) under high stress, while weaker rock deforms more gradually. Safety depends on the stress-to-strength ratio and failure mode, not strength alone.

MIN-460 · Does adding more rock bolts always solve a ground control problem?

Trap: Bolts only work if they are the right type and length, anchored in competent rock and installed correctly. Wedge failures, squeezing ground or dynamic loading may need mesh, cables, shotcrete or yielding support instead.

MIN-461 · Is a factor of safety of 1.5 the same as a 0% probability of failure?

Trap: A factor of safety is a ratio of mean strength to mean load; with variable rock properties, a slope with FoS 1.5 can still have a meaningful probability of failure. Probabilistic analysis shows the risk the FoS hides.

MIN-462 · Is RQD a complete measure of rock mass quality?

Trap: RQD only measures intact core pieces over 10 cm. It ignores joint condition, orientation, water and stress, and depends on drilling quality. Classifications such as RMR or Q combine RQD with these other factors.

MIN-463 · If convergence stops increasing, is the ground permanently stable?

Trap: Stabilised convergence is good, but later mining nearby, stress redistribution, water or weathering can restart deformation. Monitoring must continue as conditions change.

MIN-464 · Is in-situ stress always vertical and equal to rock weight?

Trap: Vertical stress roughly equals overburden weight, but horizontal stresses are often higher than vertical at shallow to moderate depths due to tectonics. Underground designs that assume low horizontal stress can fail in the roof.

Drilling & Blasting

MIN-465 · Does a higher powder factor always give better fragmentation?

Trap: Beyond a point, more explosive mainly creates fines, flyrock, noise and wall damage. Fragmentation also depends on burden-spacing geometry, timing and rock structure, so better design can beat more explosive.

MIN-466 · Does a longer delay between holes always reduce vibration?

Trap: Delays reduce charge per delay, but timing that coincides with dominant structure frequencies or produces constructive interference can increase vibration. Delay choice must consider wave superposition, not only separation.

MIN-467 · Is ANFO the best explosive for all conditions because it is cheapest?

Trap: ANFO has poor water resistance and lower density and energy per hole volume. In wet holes or hard rock, emulsion or heavy ANFO is often cheaper overall because it performs better.

MIN-468 · Is air blast only a nuisance issue?

Trap: Air overpressure can rattle windows, cause complaints and in extreme cases damage glass. It is regulated, and is controlled through stemming, avoiding exposed detonating cord and considering weather conditions.

MIN-469 · Does more stemming always improve a blast?

Trap: Proper stemming confines gas, but too much leaves the collar zone under-charged, producing oversize boulders at the top of the bench. Stemming length is a trade-off between flyrock control and top fragmentation.

MIN-470 · Can you assume all holes in a blast detonate at their nominal delay?

Trap: Pyrotechnic delays have scatter of several percent, so nominal sequences can overlap and fire out of order, increasing vibration and poor breakage. Electronic detonators reduce this problem.

Mine Ventilation & Gases

MIN-471 · Does doubling fan speed double airflow and fan power?

Trap: By the fan laws, airflow doubles but pressure rises by 4× and power by 8×. Large airflow increases are very energy-expensive, which is why ventilation on demand saves so much.

MIN-472 · Is a reading of 19.5% oxygen always safe?

Trap: It meets many minimum limits, but the shortage of oxygen may indicate that another gas (CO₂, nitrogen or methane) is displacing air. The cause should be identified and other gases checked.

MIN-473 · Is carbon monoxide only produced by fires?

Trap: CO also comes from diesel exhaust, blasting fumes and early low-temperature oxidation of coal. A rising CO trend in a coal mine can be the first sign of spontaneous heating, well before visible fire.

MIN-474 · Is more airflow always better for workers' comfort in a hot mine?

Trap: Higher air velocity helps evaporative cooling, but if the air is already hot and humid, extra air carries more heat in and adds dust. Cooling the air is often needed in deep mines.

MIN-475 · Do auxiliary fans solve all face ventilation problems?

Trap: A poorly placed auxiliary fan can recirculate contaminated air if the primary airflow is too low. The fan must sit in fresh air with sufficient through-flow, and duct leakage must be controlled.

MIN-476 · Is a self-rescuer the same as a breathing apparatus for rescue work?

Trap: A self-contained self-rescuer is for escape only and lasts about 30–60 minutes. Rescue teams use long-duration closed-circuit breathing apparatus and are trained for working in irrespirable atmospheres.

Mine Machinery & Transport

MIN-477 · Does the truck with the largest payload always give the lowest haulage cost?

Trap: Big trucks reduce labour per tonne, but need wider roads, stronger pavements, a matching shovel and enough production to use them. In small or narrow pits, mid-size trucks can be cheaper.

MIN-478 · If a truck fleet shows 90% availability, does that mean it is productive 90% of the time?

Trap: Availability only says the trucks are mechanically ready. Utilisation, queueing at the shovel, travel delays and breaks reduce productive time, so effective use can be 60% or less. Productivity needs availability × utilisation × efficiency.

MIN-479 · Does a conveyor always beat trucks on cost?

Trap: Conveyors have low operating cost per tonne-kilometre but high capital, need a crusher and fixed routes, and lose flexibility. Short, changing hauls usually favour trucks.

MIN-480 · Is a machine breakdown mainly a maintenance department problem?

Trap: Many failures come from operating practices: overloading, poor roads, aggressive driving and missed pre-start checks. Reliability depends on operations and maintenance working together.

MIN-481 · Is a hoist's capacity limited only by its motor power?

Trap: Rope strength and safety factors, skip size, depth, acceleration limits, shaft compartment space and loading/unloading times also limit capacity. Cycle time often matters more than motor rating.

MIN-482 · Do battery-electric vehicles remove all ventilation requirements?

Trap: They remove diesel exhaust and much heat, which can greatly reduce airflow needs, but ventilation is still needed for blasting fumes, strata gases, dust, heat from rock and people's breathing air.

Mine Planning, Valuation & Economics

MIN-483 · Is the pit with maximum undiscounted profit the best pit?

Trap: Maximum undiscounted profit ignores the time value of money. The best pit often has a slightly smaller final shell because deep, late waste and ore add little NPV.

MIN-484 · Should cut-off grade stay constant over the life of the mine?

Trap: A constant cut-off is simple but usually not optimal. Lane's approach shows a higher cut-off early to bring forward cash flow and a gradually lower one later, with low-grade stockpiles processed at the end.

MIN-485 · Does a positive IRR mean the project should go ahead?

Trap: IRR must exceed the cost of capital, and multiple sign changes in cash flow (e.g. closure costs) can give multiple IRRs. NPV at the right discount rate and risk analysis are more reliable.

MIN-486 · Is a mine's life simply reserves divided by production rate?

Trap: That gives a nominal figure, but grade changes, mining constraints, ramp-up, pushback timing and reserve growth or loss change it. Taylor's rule also suggests optimal production rate grows with reserve size.

MIN-487 · Is a lower operating cost per tonne always better?

Trap: Cost per tonne can fall by mining more waste or lower-grade material while cost per unit of metal rises. Compare cost per unit of product and margins, not cost per tonne alone.

MIN-488 · Does a high metal price justify any expansion?

Trap: Prices are cyclical; expansions take years and may come online in a downturn. Projects should be tested at long-term consensus prices, and capital discipline protects value through the cycle.

Mine Safety & Legislation

MIN-489 · If a mine has no reported accidents this year, is it safe?

Trap: Low lagging indicators can hide under-reporting or luck. Leading indicators (critical control verification, hazard reports, inspections) and near-miss data give a better picture of real risk.

MIN-490 · Does PPE make a hazardous task safe?

Trap: PPE is the last line of defence in the hierarchy of controls. Eliminating, substituting or engineering out the hazard is far more effective; PPE only reduces consequences when other controls fail.

MIN-491 · Is compliance with legislation enough to manage mine safety?

Trap: Laws set minimum standards. Major hazards need site-specific risk assessments, critical controls and a safety culture that goes beyond compliance.

MIN-492 · Is human error the root cause of most mining accidents?

Trap: Human error is usually a symptom. Root causes include poor design, unclear procedures, fatigue, production pressure and missing controls. Blaming people stops learning.

MIN-493 · If gas monitors are installed, is gas detection fully covered?

Trap: Fixed monitors only measure where they are placed and must be calibrated, maintained and alarmed to someone who acts. Portable detectors and inspections cover locations and changes fixed monitors miss.

MIN-494 · Do refuge chambers mean workers should always stay underground in an emergency?

Trap: Escape to the surface or a fresh-air base is usually preferred when a safe route exists. Refuge chambers are for when escape is impossible, and their use must be part of a trained emergency plan.

Mineral Processing & Mine Environment

MIN-495 · Does grinding finer always increase recovery?

Trap: Finer grinding improves liberation, but very fine particles float poorly, create slimes that consume reagents and raise energy cost sharply. The optimum grind size balances recovery with cost and throughput.

MIN-496 · Does higher recovery always mean better plant performance?

Trap: Pushing recovery can lower concentrate grade, increase smelter penalties and raise reagent costs. Plants are optimised on net smelter return or margin, not recovery alone.

MIN-497 · Is acid mine drainage only a problem during mining?

Trap: AMD can start or worsen decades after closure as sulphides oxidise in dumps and workings, and may need treatment for centuries. Prevention by design and covers is essential.

MIN-498 · Is a tailings dam safe if it has never failed in 20 years?

Trap: Past performance does not prove future safety. Raising the dam, changes in water level, liquefiable tailings and extreme events can cause sudden failure. Monitoring and independent review are essential.

MIN-499 · Is water from a mine always contaminated?

Trap: Many mines discharge water that meets standards after settling or treatment, and some is used for irrigation or supply. Water quality depends on geology, ore type and management; it must be measured, not assumed.

MIN-500 · Does reclamation return the land to exactly its original state?

Trap: Usually it creates a safe, stable, non-polluting landform suited to an agreed post-mining land use, which may differ from the original. Topography and soils are rarely identical to pre-mining conditions.

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