Mining geology and exploration

In short

Mining geology is the application of geology to finding, defining and extracting mineral deposits - it covers how ore bodies form, how they are shaped and disturbed by structures, and how a prospect is traced from a surface clue to a drilled, sampled and modelled ore body. Exploration is the staged, cost-escalating sequence - reconnaissance, prospecting, general exploration, detailed exploration - that converts geological possibility into a measured deposit on which mine design and investment rest. In India the work is governed by the MMDR Act, 1957 (as amended), the Minerals (Other than Atomic and Hydro Carbons Energy Minerals) Concession Rules, 2016 and the Minerals (Evidence of Mineral Contents) Rules, 2015, with resources reported on the UNFC framework used by the Indian Bureau of Mines. It is examined in GATE Mining Engineering (Geology section), university papers on mining geology and ore deposits, and it is the core technical round for graduate geologist/mining engineer roles at GSI, MECL, Coal India, NMDC, Hindustan Zinc, Tata Steel and Vedanta. Practical competence - reading a geological map, logging a core, computing true thickness - is what interviewers actually test.

Explained

Rocks, minerals and the geological basis

• Mineral: a naturally occurring, inorganic, homogeneous solid with a definite chemical composition and ordered atomic structure. Rock: an aggregate of one or more minerals. Ore: a mineral aggregate from which a metal can be extracted at a profit - so "ore" is an economic, not purely geological, term.
• Gangue: the valueless minerals associated with ore; tenor/grade: the concentration of the valuable constituent (%, g/t, ppm, kcal/kg for coal).
• Mineral identification properties: crystal form, cleavage, fracture, hardness (Mohs 1-10), lustre, colour, streak, specific gravity, magnetism, effervescence with HCl.
• Rock classes: (i) igneous - from magma; plutonic/intrusive (granite, gabbro, dolerite) and volcanic/extrusive (basalt, rhyolite); (ii) sedimentary - clastic (sandstone, shale), chemical/organic (limestone, coal, evaporites); (iii) metamorphic - from heat/pressure (slate, schist, gneiss, quartzite, marble).
• Why it matters in mining: rock type controls strength and support, drillability, blastability, water inflow, dust and silicosis risk, and the washability of coal.
• Indian geology in brief: Archaean cratons and greenstone belts (Dharwar - gold, iron, manganese; Singhbhum - copper, uranium), Proterozoic belts (Aravalli-Delhi - lead-zinc at Zawar/Rampura Agucha; Cuddapah - barytes, limestone), Gondwana basins (almost all Indian coal - Jharia, Raniganj, Korba, Talcher, Singrauli), Deccan Traps (basalt, bauxite on laterite caps), Tertiary of the northeast, Tamil Nadu and Rajasthan (lignite at Neyveli, Barmer).

Ore genesis and types of deposits

• Magmatic: crystallisation from magma - orthomagmatic/liquid immiscibility (Ni-Cu sulphides, Sukinda-type is laterite but chromite at Sukinda/Byrapur is magmatic), gravity settling/layered intrusions (chromite, PGE), pegmatites (mica, beryl, Li, rare earths - Bihar/Jharkhand and Rajasthan mica belts).
• Hydrothermal: deposition from hot aqueous fluids in fissures and replacements - cavity filling (quartz-gold veins, Kolar and Hutti), replacement/metasomatic, porphyry (low-grade, large-tonnage Cu-Mo; Malanjkhand in Madhya Pradesh), skarn (contact metamorphic, W-Cu-Fe), VMS (volcanogenic massive sulphide, Cu-Zn), SEDEX (sediment-hosted Pb-Zn, Rajasthan belts).
• Sedimentary: placer deposits (gold, diamond, monazite-ilmenite-rutile beach sands of Kerala/Tamil Nadu/Odisha), evaporites (gypsum, rock salt, potash), bedded iron formations (BIF/BHQ of Odisha-Jharkhand), coal (peat → lignite → sub-bituminous → bituminous → anthracite by increasing rank).
• Residual and supergene: laterite/bauxite (Panchpatmali, Kolhapur-Belgaum, Jharkhand), nickeliferous laterite (Sukinda), supergene enrichment of Cu below a leached cap with a gossan above.
• Metamorphic: graphite, garnet, kyanite-sillimanite, asbestos, marble.
• Form of deposits: vein/lode (tabular, steep), bed/seam (stratiform, gently dipping - coal), lens/pocket, stock/pipe (kimberlite pipes - Panna diamonds), disseminated/stockwork, saddle reef, placer sheet. Form dictates the mining method - seams favour longwall or bord-and-pillar, steep narrow veins favour shrinkage or sub-level stoping, massive bodies favour block caving or open pit.

Structural geology for the mining engineer

• Attitude of a bed: strike is the horizontal line on the bed's surface; dip is the maximum inclination measured perpendicular to strike (dip amount and dip direction); apparent dip in any other vertical section is always less than true dip.
• Outcrop, overburden, subcrop: where a bed meets the surface; concealed deposits are found by drilling or geophysics, not mapping.
• Fold: bending of beds - anticline (arch, oldest in core), syncline (trough), monocline, isoclinal, overturned, recumbent; parts are limb, axial plane, hinge/crest, plunge. Folds repeat seams in section - a borehole can cut the same seam twice.
• Fault: a fracture with displacement. Normal (hanging wall down, extension), reverse/thrust (hanging wall up, compression, low angle), strike-slip/wrench, step and trough/graben faults. Terms: fault plane, hade (deviation from vertical), throw (vertical displacement), heave (horizontal), slickensides, drag, gouge.
• Effect of faults on workings: seam lost at the face; rule of thumb - with a normal fault the seam is found by going "down on the downthrow side"; faults bring water, gas and bad roof; fault zones are left as barriers or crossed by stone drifts.
• Joints: fractures without displacement; cleat in coal is a natural vertical joint set (face cleat and butt cleat) that controls cutting direction, methane flow and blockiness of the cut coal.
• Unconformity: a buried erosion surface - a break in the record; washout in coal is a channel that has eroded the seam; dykes and sills are intrusions that burn, devolatilise or displace coal and create hard-to-cut ground.
• Dip terms underground: full dip (true dip), apparent dip (along an inclined roadway), strike drive (level), rise and dip workings.

Stages of mineral exploration (and the Indian framework)

1. Reconnaissance (G4): regional appraisal - toposheets, geological maps, satellite imagery, airborne geophysics; identifies belts and targets over thousands of km². Outcome: *reconnaissance mineral resource* (334).
2. Prospecting (G3): target-scale geological mapping (1:25,000 to 1:5,000), pitting, trenching, stream-sediment and soil geochemistry, ground geophysics, wide-spaced drilling. Outcome: *inferred* resource (333).
3. General exploration (G2): systematic drilling on a defined grid, bulk sampling, preliminary metallurgical tests and a preliminary feasibility input. Outcome: *indicated* resource (332).
4. Detailed exploration (G1): close-spaced drilling, exploratory mining (adits, shafts, drives), full density/moisture/quality data, geotechnical and hydrogeological work. Outcome: *measured* resource (331) and, after a feasibility study, proved mineral reserve.
5. Reporting: India uses the UNFC three-digit code (economic axis E, feasibility axis F, geological axis G) adopted by the Indian Bureau of Mines (IBM); reserves and resources are consolidated in IBM's National Mineral Inventory. Note that UNFC itself was updated internationally (UNFC 2019/2024 generation), while Indian statutory reporting follows the IBM-notified version - cite the current IBM guideline rather than a remembered code string.
6. Statutory basis: MMDR Act, 1957 as amended in 2015, 2016, 2021, 2023 and 2025 - auction-only grant of mineral concessions, Mineral Exploration Licence (EL) introduced by the 2023 amendment for deep-seated and critical minerals, Composite Licence (PL-cum-ML) and Mining Lease (ML). Minerals (Evidence of Mineral Contents) Rules, 2015 fix the minimum extent of exploration (G4 for a composite licence, G2 for a mining lease, with a 2021 relaxation allowing G3 blocks to be auctioned for a mining lease) - check the current notification, as these thresholds have been amended.
7. Institutions: Geological Survey of India (GSI) - national geoscience mapping and baseline data (NGCM, NGPM); Mineral Exploration and Consultancy Ltd (MECL) - drilling and detailed exploration; Atomic Minerals Directorate (AMD) - uranium; CMPDI - coal; NMET (National Mineral Exploration Trust) funds exploration from a levy on royalty; IBM regulates conservation and reporting; DGMS regulates safety. Mining safety law now sits under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025, which repealed the Mines Act, 1952; the coal and metalliferous mine regulations continue under it until replaced.

Exploration methods: geological, geochemical, geophysical, remote sensing

• Geological/direct methods: prospecting by outcrop and float tracing, geological mapping on toposheets with strike-dip readings, pitting (shallow vertical holes in soil cover), trenching (across strike, to expose and channel-sample a vein), adits and exploratory shafts/drives (expensive, give bulk samples and in-situ geotechnical data).
• Geochemical methods: detect anomalies - soil, stream-sediment (best for regional reconnaissance), rock-chip/lithogeochemical, water (hydrogeochemical), vapour (Rn, Hg) and biogeochemical (plant) surveys. Background, threshold, anomaly - the threshold is commonly taken as mean + 2 standard deviations of background; pathfinder elements (As for Au, Hg for base metals, He/Rn for uranium) are used when the target element is hard to detect.
• Geophysical methods - measure a physical property contrast:
  • Magnetic: total field/gradient; targets magnetite, BIF, kimberlite; cheap and fast, airborne over large areas; affected by diurnal variation and cultural noise.
  • Gravity: density contrast; good for massive sulphides, chromite, salt domes and basin shape; needs elevation and terrain (Bouguer, latitude, free-air) corrections.
  • Electrical - resistivity: for groundwater, overburden thickness, lignite; induced polarisation (IP): the premier method for disseminated and porphyry sulphides; self-potential (SP): natural voltage over sulphide bodies.
  • Electromagnetic (EM, TEM, VLF, airborne EM): for conductive massive sulphides and graphitic/conductive structures; works through cover; graphite and salt water give false anomalies.
  • Seismic: reflection for coal and oil basin structure, faults and seam continuity; refraction for overburden/rockhead depth; costly but highest resolution of structure.
  • Radiometric: gamma-ray spectrometry (K, U, Th) for uranium, REE-bearing sands and placers.
• Remote sensing and GIS: Landsat/Sentinel and Indian IRS/Resourcesat, Cartosat imagery; lineament and alteration mapping with band ratios and hyperspectral (AVIRIS-NG, Hyperion) data; Bhukosh (GSI geoscience data portal) and NGDR (National Geoscience Data Repository) host Indian baseline data for GIS-based targeting.
• Choice of method: geophysics finds a *physical anomaly*, geochemistry a *chemical anomaly*; only drilling confirms *ore*. Direct methods are definitive but costly - hence the funnel from cheap, wide, indirect methods to expensive, narrow, direct ones.

Exploratory drilling, sampling and logging

• Drilling types: diamond core drilling (DDH) - continuous core, essential for structure, grade and rock mechanics; reverse circulation (RC) - fast, clean chip samples, common for grade control; rotary/tricone and auger - soft ground, coal, lignite; percussion/DTH - cheap, for blastholes and shallow definition.
• Core sizes (metric, Indian and international practice): BQ (~36.5 mm core), NQ (~47.6 mm), HQ (~63.5 mm), PQ (~85 mm) - larger core gives better recovery and representativeness.
• Core recovery (%) = length of core recovered / length drilled × 100; RQD = sum of intact core pieces ≥ 100 mm / total run length × 100 - the basis of rock-mass classification (RMR, Q).
• Core logging: depth-by-depth record of lithology, colour, grain size, structures (dip of bedding/foliation relative to core axis), alteration, mineralisation, joints, recovery, RQD, sample intervals; core is oriented or structural dips are measured with alpha/beta angles. Core is kept in trays, photographed, half-cored for assay, half retained as reference.
• Borehole geophysics (well logging): gamma, density, neutron, caliper, resistivity and sonic logs - standard for Indian coal exploration (CMPDI) to pick seam roof/floor and estimate ash from density.
• Sampling: grab (unreliable), chip, channel (a continuous groove across the full width, perpendicular to strike - the standard vein sample), bulk (for metallurgical and washability tests), core. Sampling must be representative; sample reduction follows coning-and-quartering or riffle splitting, and QA/QC uses duplicates, blanks and certified reference standards.
• Drill-hole layout: holes are planned perpendicular to the ore body wherever possible; grid spacing tightens with each stage (for example hundreds of metres at G2 to tens of metres at G1 for a complex body). Spacing is set by the deposit's geological continuity, not by a universal number.
• True thickness: a borehole rarely cuts a dipping bed at right angles, so intersected (apparent) thickness must be corrected - an error that inflates tonnage if ignored.
• Deliverables: borehole database, cross-sections and level plans, isopach (thickness), structure contour and grade-contour plans, a 3D geological/block model, and the exploration report in the IBM/UNFC format.

Key points

• Mining geology - geology applied to locating, defining and working mineral deposits, including rock behaviour in excavation.
• Ore vs mineral - ore is economic; a change in price or technology can turn waste into ore and back.
• Strike and dip - strike is horizontal on the bed; dip is maximum inclination at right angles to strike; apparent dip < true dip.
• Fault terms - throw is vertical displacement, heave horizontal, hade the deviation of the fault plane from vertical.
• Normal fault rule - the seam lost at the face is sought downward on the downthrow side.
• Cleat - natural vertical jointing in coal (face and butt cleat) controlling cutting direction and gas drainage.
• Washout and dyke - a washout erodes the seam away; a dyke burns and displaces it - both cause unexpected seam loss.
• Exploration stages - reconnaissance (G4) → prospecting (G3) → general (G2) → detailed (G1), with cost per km² rising and area falling at each step.
• UNFC - the three-axis (E, F, G) code used by IBM for Indian resource and reserve reporting.
• MMDR Act, 1957 (amended 2015-2025) - auction-based concessions; the 2023 amendment added the Exploration Licence for deep-seated and critical minerals.
• NMET - trust funded by a levy on royalty that finances regional and detailed exploration in India.
• Geochemical anomaly - a value above threshold (commonly mean + 2σ of background); pathfinder elements stand in for hard-to-detect targets.
• IP survey - the standard geophysical tool for disseminated sulphide and porphyry copper targets.
• Magnetic and gravity - fastest regional coverage; magnetic for magnetite/BIF/kimberlite, gravity for dense massive bodies and basin form.
• Seismic reflection - best resolution of coal seam continuity and faults, used before longwall layout.
• Core recovery and RQD - recovery measures sample completeness; RQD (pieces ≥ 100 mm) feeds RMR and Q rock-mass ratings.
• Channel sampling - continuous groove across full vein width, perpendicular to strike; grab samples are not representative.
• True thickness correction - essential before tonnage estimation; uncorrected borehole thickness overstates the deposit.

Formulas and examples

Example 1 - true thickness and tonnage of a coal seam. A vertical borehole intersects a coal seam from 148.00 m to 152.50 m. The seam dips at 20°. Coal density 1.45 t/m³. The seam is proved over 800 m along strike and 500 m down dip (both measured in the plane of the seam).

• Intersected (vertical) thickness t_v = 152.50 − 148.00 = 4.50 m.
• True thickness t = 4.50 × cos 20° = 4.50 × 0.9397 = 4.23 m.
• Area in the plane of the seam = 800 × 500 = 400,000 m².
• Volume = 400,000 × 4.23 = 1,692,000 m³.
• Tonnage = 1,692,000 × 1.45 ≈ 2.45 million tonnes.
• If true thickness had not been corrected, tonnage would come to 400,000 × 4.50 × 1.45 ≈ 2.61 Mt - an overstatement of about 6.4%.

Example 2 - apparent dip in an inclined roadway. A seam has a true dip of 12°. A development roadway is driven in a vertical plane making 55° with the true dip direction.

• tan(δ_app) = tan 12° × cos 55° = 0.2126 × 0.5736 = 0.1219.
• δ_app = arctan(0.1219) = 6.95°, say 7° - the gradient the roadway will actually climb, about 1 in 8.2.
• A drive exactly along strike (θ = 90°) gives cos 90° = 0, so apparent dip = 0 - a level drive, which is why strike drives are used for haulage.

Example 3 - fault throw and the lost seam. A seam is cut off at a face by a normal fault whose plane dips 65° (hade = 25°). Drilling shows the net dip-slip displacement along the fault plane is 22 m.

• Throw V = 22 × cos 25° = 22 × 0.9063 = 19.9 m (vertical displacement).
• Heave H = 22 × sin 25° = 22 × 0.4226 = 9.3 m (horizontal separation across the plane).
• Check: throw/heave = 19.9/9.3 = 2.14 = tan 65° ✓ (tan 65° = 2.145).
• Action: being a normal fault, the seam continues downwards on the downthrow side - the stone drift is driven down through about 20 m of vertical section (plus the 9.3 m horizontal offset) to pick the seam up again.

Example 4 - core recovery, RQD and length-weighted grade. In a 1.50 m core run, the recovered core totals 1.38 m. Intact pieces measure 0.28, 0.12, 0.45, 0.09, 0.22, 0.14 and 0.08 m.

• Core recovery = 1.38 / 1.50 × 100 = 92%.
• Pieces ≥ 100 mm: 0.28 + 0.12 + 0.45 + 0.22 + 0.14 = 1.21 m (0.09 and 0.08 m are excluded).
• RQD = 1.21 / 1.50 × 100 = 80.7% - "good" rock on the Deere classification.
• Three assay intervals from the same hole: 1.5 m at 3.2% Zn, 2.5 m at 6.0% Zn, 1.0 m at 1.5% Zn.
• ḡ = (1.5×3.2 + 2.5×6.0 + 1.0×1.5) / 5.0 = (4.8 + 15.0 + 1.5)/5.0 = 21.3/5.0 = 4.26% Zn over 5.0 m. (The arithmetic mean of the three grades, 3.57%, is wrong because it ignores sample length.)

Example 5 - geochemical threshold and drill targeting. A soil survey over a gold prospect gives background Au values with mean 8 ppb and standard deviation 3 ppb; three stations read 14, 22 and 61 ppb.

• Threshold T = 8 + 2×3 = 14 ppb.
• 14 ppb is at the threshold (marginal), 22 ppb is anomalous, 61 ppb is a strong anomaly (about 17.7σ above the mean).
• Follow-up: trench the 61 ppb station across strike, channel-sample the exposure, run an IP/magnetic line, then drill - the anomaly is a chemical signal, not proof of ore.

Where it is used

• Coal: CMPDI and Coal India subsidiaries (ECL, BCCL, SECL, MCL, WCL) run borehole grids with geophysical logging to prove seams, pick washouts and faults, and lay out longwall panels; Singareni Collieries in Telangana does the same.
• Metals: NMDC (iron ore, Bailadila and Donimalai), Hindustan Zinc (Rampura Agucha, Sindesar Khurd - deep IP/EM and definition drilling), Hindustan Copper (Malanjkhand, Khetri), Tata Steel and JSW (captive iron and coal blocks), Vedanta (bauxite, Lanjigarh).
• Public exploration agencies: GSI (national mapping, Bhukosh/NGDR data), MECL (contract drilling and detailed exploration), AMD (uranium at Jaduguda, Tummalapalle), ONGC/OIL (seismic and well logging for hydrocarbons).
• Job roles: graduate geologist, exploration/resource geologist, mine geologist (grade control, face mapping, seam correlation), geotechnical engineer (RQD, RMR), geophysicist, GIS/resource modeller, competent person for statutory exploration reports.
• Deposit evaluation for auctions: since auction-only grant under MMDR, the geological report and UNFC category of a block decide its reserve price and bid premium - exploration quality is directly a commercial variable.
• Software and standards practice: Surpac, Datamine, Micromine, Leapfrog, Vulcan and QGIS for logging, modelling and plans; reports in IBM/UNFC format, with JORC/NI 43-101 style reporting used for overseas listings.

Interview questions

• Q: What is the difference between a mineral, an ore and gangue? A: A mineral is a naturally occurring crystalline inorganic solid of definite composition; an ore is a mineral aggregate that can be mined at a profit; gangue is the worthless mineral matter accompanying the ore.
• Q: Distinguish true dip from apparent dip. A: True dip is the maximum inclination of a bed, measured perpendicular to strike; apparent dip is the inclination seen in any other vertical section and is always smaller, given by tan(δ_app) = tan(δ_true)·cos θ.
• Q: A coal seam is suddenly lost at the face - what could have happened and how do you locate it? A: A fault, a washout, a dyke, or the seam simply pinching out; mapping and short exploratory drilling identify which, and for a normal fault the seam is picked up downward on the downthrow side.
• Q: What are throw, heave and hade? A: Throw is the vertical component of fault displacement, heave the horizontal component across the fault plane, and hade the deviation of the fault plane from vertical (90° minus fault dip).
• Q: Why is true thickness correction necessary for a borehole intersection? A: A hole rarely meets the bed at right angles, so the intersected length overstates the thickness; the correction t = L·sin α (or t_v·cos δ for a vertical hole in a dipping bed) prevents an inflated tonnage.
• Q: Which geophysical method would you choose for a disseminated copper target and why? A: Induced polarisation, because disseminated sulphides are chargeable but not continuously conductive enough for EM, and their density and magnetic contrasts are weak.
• Q: What is RQD and why does a mining engineer care? A: The percentage of a core run made up of intact pieces 100 mm or longer; it quantifies rock-mass quality and is an input to RMR and Q classifications, which govern support design and excavation method.
• Q: What is cleat in coal and how does it affect mining? A: The natural vertical joint system (face cleat and butt cleat); cutting along the face cleat is easier, and cleat controls permeability for methane drainage and the size of the coal produced.
• Q: Explain the exploration stages and what each produces. A: Reconnaissance (G4) narrows regions to targets, prospecting (G3) gives an inferred resource, general exploration (G2) an indicated resource on a drill grid, and detailed exploration (G1) a measured resource that supports a feasibility study.
• Q: Under which law are Indian mineral concessions granted today, and what did the 2023 amendment add? A: The MMDR Act, 1957 as amended - concessions are auctioned; the 2023 amendment introduced the Exploration Licence to let private agencies explore deep-seated and critical minerals, with specified critical minerals auctioned by the Central Government.
• Q: Why is a grab sample unacceptable for grade estimation? A: It is selectively collected and unweighted, so it represents neither the full width nor the average of the body; channel or core samples across the full width are required.
• Q: Geochemistry shows a strong soil anomaly but the drill hole is barren - what are the possible reasons? A: The anomaly may be displaced by soil creep or dispersion, the source may be at depth or offset laterally, the hole may have missed a narrow steep body, or the anomaly may come from a non-economic source such as barren pyrite.

Common mistakes

• Treating apparent dip as true dip, or measuring dip in a direction not perpendicular to strike - every downstream thickness and depth calculation then fails.
• Using the borehole-intersected thickness directly in a tonnage calculation instead of correcting for the angle between hole and bed.
• Averaging grades arithmetically when sample lengths differ - length (and sometimes density) weighting is mandatory.
• Confusing throw with the net slip along the fault plane, or hade with dip (hade = 90° − dip).
• Counting all recovered core towards RQD - only intact pieces 100 mm or longer qualify, so RQD is always ≤ core recovery.
• Claiming a geophysical or geochemical anomaly proves an ore body; it only defines a target that drilling and assaying must confirm.
• Quoting resource figures without the UNFC category or the stage of exploration behind them, and mixing "resource" with "reserve" in exam answers and reports.

Related topics

Mineral economics and reserve estimation · Mineral processing · Rock mechanics and ground control · Surface and underground mining methods · Mine surveying · Mine legislation and safety (Mines Act and DGMS regulations)