In a polished section examined in reflected light, a soft grain is white, stays dark when the stage is rotated between crossed polars, and shows triangular pits where cleavage fragments were plucked out during polishing. Which mineral is it most likely to be?

Galena, PbS, which is white, isotropic and soft, with perfect cubic cleavage

Pyrite, FeS₂, which is pale yellow, isotropic and very hard, with no cleavage pits

Chalcopyrite, CuFeS₂, which is deep brass-yellow and weakly anisotropic when rotated

Hematite, Fe₂O₃, which is greyish-white and strongly anisotropic, with red internal reflections

In a copper deposit, secondary chalcocite and covellite form a zone below the water table, beneath a leached, iron-oxide-rich cap. Which process formed the chalcocite zone?

Supergene enrichment, in which copper leached from the oxidised zone was redeposited as secondary sulphides

Magmatic crystallisation, in which dense sulphide liquid settled out of a silicate magma under gravity in a layered intrusion

Hydrothermal boiling, in which an ascending brine lost vapour and deposited copper as it cooled

Regional metamorphism, in which copper was remobilised from older sulphides into fold hinges

Supergene enrichment, in which copper leached from the oxidised zone was redeposited as secondary sulphides

A quartz vein hosts gold deposited as an ascending fluid boiled and lost H₂S and CO₂ to a vapour phase. Why does boiling precipitate gold that was carried as the bisulphide complex Au(HS)₂⁻?

Loss of H₂S to the vapour phase removes the sulphur that keeps gold dissolved, so the complex breaks down

Boiling raises the temperature of the residual liquid, and a hotter liquid dissolves less gold

Loss of H₂S to the vapour phase removes the sulphur that keeps gold dissolved, so the complex breaks down

Boiling concentrates chloride in the liquid, which converts the gold into a more soluble chloride complex

Boiling draws in oxygen from the air, which oxidises the fluid and makes the gold insoluble

A primary two-phase aqueous inclusion in quartz homogenises to liquid at 240 °C. Which statement about this temperature is correct?

It is a minimum trapping temperature; the true value is higher by a pressure correction unless the fluid was boiling

It is the highest temperature the crystal reached after trapping, because the bubble cannot vanish below this value

It always equals the trapping temperature, because homogenisation exactly reverses the cooling of the inclusion

It is the temperature at which the last ice crystal melts, which gives the salinity of the fluid

A source rock is rich in woody debris of land plants, with abundant vitrinite and a low atomic H/C ratio. Which kerogen type and main product does it indicate?

Type III kerogen, which is mainly gas-prone because it is hydrogen-poor

Type I kerogen, which is mainly oil-prone, as in algal lake deposits

Type II kerogen, which gives both oil and gas, as in marine shales

Type III kerogen, which is mainly gas-prone because it is hydrogen-poor

Type I kerogen, because woody debris is rich in hydrogen and gives mostly oil

A well on a large, sealed anticline with a good sandstone reservoir finds only water, although a mature source rock lies beside the structure. Burial-history modelling shows that the anticline formed after the main phase of oil expulsion had ended. Which is the best explanation of the dry well?

The trap formed too late: the oil had already migrated elsewhere before the structure existed

There is no source rock in the area, so no hydrocarbons were ever generated

The seal leaked badly, so the hydrocarbons escaped from the trap and reached the surface over time

The reservoir is too porous, which lets oil drain downward out of the trap

The trap formed too late: the oil had already migrated elsewhere before the structure existed

Which pairing of a resource and its Indian occurrence is correct?

Zinc and lead, mined at Rampura Agucha in Rajasthan

Chromite, mined chiefly in the Khetri belt of Rajasthan

Zinc and lead, mined at Rampura Agucha in Rajasthan

Prime coking coal, mined chiefly at Neyveli in Tamil Nadu

Diamond, mined chiefly in the Bailadila range of Chhattisgarh

Which statement about polymetallic nodules is correct?

They are slowly growing Mn–Fe oxide concretions on abyssal plains, rich in Ni, Cu and Co

They are hydrothermal precipitates that form at black smokers on spreading ridges

They are iron–nickel sulphide concretions that form in shallow shelf muds near river mouths

They are slowly growing Mn–Fe oxide concretions on abyssal plains, rich in Ni, Cu and Co

They occur only within the 200-nautical-mile Exclusive Economic Zone and so belong to coastal states

Which of the following statements about ore-forming processes are correct?

Banded iron formations are mostly Archaean to Paleoproterozoic chemical sediments

Beach placers of ilmenite and monazite form by mechanical concentration of heavy minerals

Banded iron formations are mostly Archaean to Paleoproterozoic chemical sediments

Supergene sulphide enrichment takes place above the water table, within the oxidised zone itself

Beach placers of ilmenite and monazite form by mechanical concentration of heavy minerals

Which of the following statements about fluid inclusions are correct?

A halite daughter crystal in an inclusion at room temperature shows a salinity above about 26 wt% NaCl

Vapour-rich and brine inclusions trapped together that homogenise at the same temperature indicate boiling

Secondary inclusions along healed fractures record the fluid that was present while the host crystal grew

A halite daughter crystal in an inclusion at room temperature shows a salinity above about 26 wt% NaCl

A more negative final ice-melting temperature indicates a lower salinity of the aqueous fluid

Vapour-rich and brine inclusions trapped together that homogenise at the same temperature indicate boiling

Which of the following statements about reserve estimation and geostatistics are correct?

Kriging gives an estimation variance, which simple inverse-distance weighting does not give

In ordinary kriging the weights are chosen freely and need not sum to 1

The nugget effect is the variance found at the range, where samples become uncorrelated

Kriging gives an estimation variance, which simple inverse-distance weighting does not give

Which of the following statements about the distribution of mineral and fuel resources in India are correct?

Sukinda in Odisha is the principal chromite-bearing ultramafic belt of India

Jharia in Jharkhand is noted for prime coking coal of Permian Gondwana age

Sukinda in Odisha is the principal chromite-bearing ultramafic belt of India

Bauxite in India occurs mainly in Jammu and Kashmir and Himachal Pradesh

Jharia in Jharkhand is noted for prime coking coal of Permian Gondwana age

The Khetri belt of Rajasthan is the main tin province of India

Resource Geology: Ores and Their Genesis, Fluid Inclusions, Coal and Petroleum, Marine Resources, Exploration and Reserve Estimation, and Indian Resources

This chapter answers the heading “Resource Geology” of Part B1 of the Geology and Geophysics (GG) paper: “Ore-mineralogy; Ore forming processes - magmatic, hydrothermal, sedimentary, supergene, and metamorphogenic ores; Fluid inclusions in ore genesis; Coal and petroleum geology; Marine mineral resources; Prospecting and exploration of economic mineral deposits - sampling, ore reserve estimation, geostatistics, and mining methods; Distribution of strategic, essential, and critical minerals and fossil fuel deposits in India.” It teaches ore minerals and their metal content, the five ore-forming processes, fluid-inclusion microthermometry, coal and petroleum, marine resources, exploration and reserve estimation, geostatistics and the Indian distribution. The numericals are tonnage and metal content, cut-off grade, kriging, inclusion salinity and the coal fuel ratio.

1. Ore mineralogy

An ore is rock that can be mined at a profit; its valuable ore minerals occur with worthless gangue such as quartz and calcite. The metal content of a mineral follows from its formula: metal % = (atomic mass × atoms) ÷ formula mass × 100. Chalcopyrite CuFeS₂ (formula mass 183.5) holds 63.5 / 183.5 = 34.6% Cu, and galena PbS holds 207.2 / 239.3 = 86.6% Pb. The concentration factor of a deposit is its grade divided by the average crustal abundance of the metal.

Common ore minerals and their metal content
Metal	Principal ore minerals	Metal in the pure mineral (approx.)
Iron	Magnetite Fe₃O₄, hematite Fe₂O₃	72% and 70%
Copper	Chalcopyrite CuFeS₂, bornite Cu₅FeS₄, chalcocite Cu₂S	35%, 63% and 80%
Lead and zinc	Galena PbS, sphalerite ZnS	87% and 67%
Tin	Cassiterite SnO₂	79%
Manganese	Pyrolusite MnO₂	63%
Chromium	Chromite FeCr₂O₄	46% Cr (68% Cr₂O₃)
Aluminium	Gibbsite, boehmite and diaspore (the minerals of bauxite)	Ore graded as % Al₂O₃

Ore minerals are identified in polished sections under reflected light by reflectance and colour (galena white, pyrite pale yellow, chalcopyrite deeper brass-yellow, bornite pinkish brown), by isotropism or anisotropism (cubic galena, pyrite and magnetite stay dark when the stage is rotated between crossed polars, hematite does not) and by hardness and cleavage (galena shows triangular cleavage pits). Textures record history: exsolution (chalcopyrite blebs in sphalerite, pentlandite flames in pyrrhotite), replacement, colloform banding and cross-cutting veinlets. The paragenetic sequence, the order of mineral deposition, is read from these cross-cutting and replacement relations.

2. Ore-forming processes

Ore-forming processes at a glance
Process	How the metal is concentrated	Examples
Magmatic	Early crystallisation and settling (chromite, magnetite layers); unmixing of a sulphide melt from silicate magma (Ni–Cu–PGE sulphides); residual melts (pegmatite, carbonatite); kimberlite carries diamond	Chromite in ultramafic rock; Ni–Cu sulphide; diamond pipes
Hydrothermal	Hot aqueous fluids carry metals as chloride or bisulphide complexes and deposit them on cooling, boiling, mixing or reaction with wall rock	Porphyry Cu–Mo, skarn, epithermal Au–Ag, lode gold, volcanogenic massive sulphide, Mississippi Valley-type Pb–Zn
Sedimentary	Chemical precipitation (banded iron formation, sedimentary manganese, phosphorite, evaporite) and mechanical sorting of heavy minerals (placers)	Precambrian iron formations, beach-sand placers, Witwatersrand paleoplacer
Supergene	Weathering: residual enrichment (bauxite, nickel laterite) and enrichment of sulphides below the water table by copper leached from above	Bauxite; chalcocite blanket beneath a gossan
Metamorphogenic	Metamorphism forms industrial minerals and remobilises older ore into hinges and shears; metamorphic fluids deposit lode gold	Graphite, kyanite–sillimanite, garnet; remobilised sulphides

Hydrothermal deposition has four triggers: a fall in temperature; boiling, which strips H₂S and CO₂ into a vapour phase; mixing with cooler or oxidised water; and reaction with wall rock (neutralisation, sulphidation of iron-bearing minerals). In a porphyry system alteration zones run outward from the intrusion as potassic, phyllic (quartz–sericite–pyrite), argillic and propylitic, and copper is concentrated in the inner potassic–phyllic zones.

⚠️ Where the supergene blanket sits

Sulphides oxidise above the water table, leaving a leached, iron-oxide-rich gossan, a guide to ore below. The dissolved copper moves down and is redeposited as secondary chalcocite and covellite below the water table, so the enriched blanket lies under the leached cap, not in it.

3. Fluid inclusions in ore genesis

A fluid inclusion is a droplet of ore fluid trapped in a crystal. Primary inclusions form along growth zones, secondary inclusions along fractures healed after growth, and pseudosecondary inclusions along fractures healed during growth; only primary and pseudosecondary ones date the ore fluid. At room temperature an inclusion shows liquid, a vapour bubble and sometimes a halite daughter crystal (salinity above about 26 wt% NaCl) or liquid CO₂.

Heating gives the homogenisation temperature Th (the bubble vanishes), a minimum trapping temperature that needs a pressure correction unless the fluid was boiling. Freezing gives the first-melting temperature (about −21 °C for H₂O–NaCl, which identifies the salt system) and the final ice-melting temperature Tm. With θ = −Tm in °C, the salinity in wt% NaCl equivalent is S = 1.78θ − 0.0442θ² + 0.000557θ³ (Bodnar, valid up to the eutectic θ = 21.2 °C, S = 23.2). Example: Tm = −5.0 °C gives S = 8.90 − 1.11 + 0.07 = 7.86 wt%.

Porphyry Cu–Mo: about 300 to 600 °C, brines of tens of wt% NaCl equivalent beside vapour-rich inclusions (boiling).
Orogenic (lode) gold: about 250 to 400 °C, low salinity (a few wt% NaCl equivalent), H₂O–CO₂ fluid.
Epithermal Au–Ag: about 150 to 300 °C, low salinity, often boiling.
Mississippi Valley-type Pb–Zn: about 80 to 200 °C, saline basinal brines.

⚠️ Th is a floor, not the trapping temperature

A fluid trapped under pressure is a single phase; on cooling at constant volume it unmixes into liquid and vapour, so Th is the minimum temperature of trapping. Coexisting vapour-rich and brine inclusions that homogenise at the same temperature, at different phases, show that the fluid was boiling, and then Th is the trapping temperature itself.

4. Coal geology

Coal forms from plant debris preserved in waterlogged peat and then coalified by burial: rank rises with temperature, not time alone, through peat, lignite, sub-bituminous, bituminous and anthracite, as carbon increases and moisture and volatile matter fall. Macerals are the microscopic constituents: vitrinite (woody tissue), liptinite (spores, resin, cuticle, algae; hydrogen-rich) and inertinite (charred or oxidised matter). Lithotypes are the visible bands: vitrain, clarain, durain and fusain. Vitrinite reflectance R₀ rises with rank and measures thermal maturity.

Proximate analysis gives moisture M, volatile matter VM, ash A and fixed carbon FC in percent, with FC = 100 − (M + VM + A) on the as-received basis; ultimate analysis gives C, H, N, S and O. The fuel ratio is FC / VM. Example: M = 8%, VM = 28% and A = 24% give FC = 100 − 60 = 40% and a fuel ratio of 40 / 28 = 1.43. Coking coal needs low ash and sulphur and good caking behaviour.

⚠️ Rank is not grade, and the basis must match

Rank is the degree of coalification (carbon, calorific value); grade is the amount of mineral matter, measured by ash. Proximate results may be given as received, air-dried or dry ash-free, and FC = 100 − (M + VM + A) works only when all four figures share one basis.

5. Petroleum geology

A petroleum system needs a source rock, a reservoir, a seal, a trap and correct timing, meaning that the trap existed before the oil migrated. Source rocks are fine-grained and organic-rich (TOC of about 0.5 wt% or more). The kerogen type sets the product: Type I (algal, lacustrine; hydrogen-rich, oil), Type II (marine; oil and gas) and Type III (terrestrial plants; low H/C, mainly gas). Burial heating runs through diagenesis (biogenic methane), catagenesis (the oil window, roughly 60 to 120 °C or vitrinite reflectance of about 0.6 to 1.3%, with wet gas at its base) and metagenesis (dry gas).

Reservoirs are porous, permeable sandstones and carbonates; seals are shale or evaporite; traps are structural (anticline, fault), stratigraphic (pinch-out, unconformity, reef) or combined. Oil leaves the source rock by primary migration and moves up carrier beds by secondary migration, floating on water, until a seal stops it; in a filled trap gas lies over oil over water. Unconventional resources include coalbed methane, shale gas and gas hydrates. With a surface temperature of 25 °C and a gradient of 30 °C/km, the oil window (60 to 120 °C) lies between (60 − 25) / 30 = 1.2 km and (120 − 25) / 30 = 3.2 km.

⚠️ Timing beats geometry

A large, sealed anticline with a good reservoir beside a mature source kitchen can still be dry if it formed after the oil had been expelled and had migrated elsewhere. Always compare the age of trap formation with the age of expulsion.

6. Marine mineral resources

Marine resources lie inside a country’s Exclusive Economic Zone (to 200 nautical miles) or in the international Area beyond national jurisdiction, which the International Seabed Authority regulates. The main kinds are these.

Polymetallic nodules: Mn–Fe oxide concretions on abyssal plains carrying Ni, Cu and Co; they grow very slowly, by millimetres per million years. India holds an exploration contract for them in the Central Indian Ocean Basin.
Cobalt-rich ferromanganese crusts coat seamounts, and seafloor massive sulphides form at hydrothermal vents on spreading ridges and in back-arc basins.
Nearshore deposits: placers of ilmenite, rutile, zircon, monazite and garnet, shelf phosphorites, and sand, gravel and lime mud.
Gas hydrates in continental-margin sediment are a potential fuel.

7. Prospecting, sampling, reserve estimation, geostatistics and mining

Exploration narrows the search in stages: regional reconnaissance (geological mapping, stream-sediment geochemistry, remote sensing, airborne geophysics), target testing (soil geochemistry, ground geophysics, trenching) and then drilling (diamond core, reverse circulation) to outline the body. Sampling must be unbiased: channel or chip samples across a face, split drill core, bulk samples for erratic ore. Safe sample weight rises with the square of the largest particle size (Richards–Czeczott, Q = kd²), and a high-nugget ore such as gold needs larger or more samples.

Resources are occurrences with reasonable prospects of economic extraction, graded inferred, indicated and measured; reserves are the economically mineable part, graded probable and proved. Tonnage = volume × bulk density, with the tonnage factor 1 / density in m³ per tonne, and metal content = tonnage × grade. Grade over several intersections is weighted, not a plain mean: 2 m at 4%, 6 m at 1% and 2 m at 3% give (8 + 6 + 6) / 10 = 2.0%. Example: 200 m × 50 m × 20 m = 200,000 m³ at 3.2 t/m³ is 640,000 t, which holds 16,000 t Cu at 2.5% Cu. Methods are cross-section, polygon, triangle, inverse-distance weighting and kriging.

Cut-off grade is the grade below which material does not pay for its mining and processing: cut-off = cost per tonne ÷ (price per unit of metal × recovery). Raising it lowers the tonnage and raises the average grade (the grade–tonnage curve). Example: $48 per tonne, $60 per gram and 80% recovery give 48 / (60 × 0.8) = 1.0 g/t. In an open pit the stripping ratio is waste ÷ ore, and the pit deepens until it reaches the break-even ratio at which an ore tonne just pays to remove its waste. Underground methods suit deep, steep or narrow bodies: room and pillar, cut-and-fill, shrinkage and sublevel stoping, block caving, and longwall for coal.

Geostatistics treats grade as a regionalised variable that becomes less alike as the separation h grows. The experimental semivariogram is γ(h) = [1 / 2N(h)] Σ [z(xᵢ) − z(xᵢ + h)]². It rises from the nugget (variance at near-zero lag: measurement error and micro-scale variability) to the sill (total variance) at the range, beyond which samples are uncorrelated. Ordinary kriging estimates a point or block as a weighted sum of nearby samples whose weights sum to 1 and minimise the estimation variance, which it also reports. Example: grades 2, 4, 3, 5, 6 at 1 m spacing give squared differences 4, 1, 4, 1 at lag 1 m, so γ(1) = 10 / (2 × 4) = 1.25. For samples at 0 m and 10 m, a linear variogram with no nugget and an estimate at 2.5 m, the kriging weights are 0.75 and 0.25.

8. Strategic, essential and critical minerals and fuels of India

Strategic minerals matter for defence and nuclear security (uranium, thorium, beryllium, rare earths); essential minerals are those industry needs in bulk (iron ore, coal, limestone); critical minerals combine high economic importance with supply risk (lithium, cobalt, nickel, graphite, rare earth elements are on the official list). The labels describe use and supply, not geology, and one mineral may carry several. The table gives widely known occurrences only.

Widely known Indian occurrences
Resource	Main occurrences
Iron ore (hematite, magnetite)	Banded iron formations of Odisha–Jharkhand, Chhattisgarh (Bailadila), Karnataka, Goa
Manganese	Odisha; Madhya Pradesh–Maharashtra (Balaghat–Nagpur belt); Karnataka
Chromite	Sukinda ultramafic belt, Odisha, the bulk of Indian resources
Bauxite	Odisha (largest), Gujarat, Jharkhand, Maharashtra, Chhattisgarh
Copper	Khetri (Rajasthan), Malanjkhand (Madhya Pradesh), Singhbhum (Jharkhand)
Lead–zinc	Rajasthan: Zawar, Rampura Agucha, Rajpura–Dariba
Gold	Karnataka (Hutti; Kolar field now closed), Andhra Pradesh (Ramagiri)
Uranium	Jaduguda (Singhbhum, Jharkhand), Tummalapalle (Andhra Pradesh), Domiasiat (Meghalaya)
Beach-sand minerals (ilmenite, rutile, zircon, monazite, garnet)	Coasts of Kerala, Tamil Nadu, Odisha and Andhra Pradesh; monazite is the thorium source
Diamond	Panna, Madhya Pradesh
Coal	Mostly Permian Gondwana coal (Damodar, Son, Mahanadi, Godavari valleys) with high ash; Jharia has prime coking coal; Tertiary coal in Assam and Meghalaya; lignite at Neyveli (Tamil Nadu), Rajasthan and Gujarat
Petroleum and gas	Mumbai Offshore (Bombay High), Assam (Digboi), Cambay (Gujarat), Krishna–Godavari, Cauvery, Rajasthan (Barmer)

Key takeaways

Metal in a mineral = (atomic mass × atoms) ÷ formula mass; chalcopyrite 35% Cu, galena 87% Pb. Ore microscopy uses reflectance, colour, isotropism, hardness and cleavage; textures such as exsolution and replacement give the paragenetic sequence.
Magmatic, hydrothermal, sedimentary, supergene and metamorphogenic processes concentrate metals; hydrothermal ore drops out on cooling, boiling, mixing or wall-rock reaction. The supergene blanket lies below the water table, under the leached gossan.
Th is a minimum trapping temperature unless the fluid boiled. Salinity from final ice melting: S = 1.78θ − 0.0442θ² + 0.000557θ³ wt% NaCl equivalent. Porphyry fluids are hot brines, orogenic gold fluids low-salinity H₂O–CO₂, MVT fluids cool basinal brines.
Coal rank rises from peat to anthracite; FC = 100 − (M + VM + A) and fuel ratio = FC / VM. Petroleum needs source, reservoir, seal, trap and timing; Type III kerogen is gas-prone; the oil window is about 60 to 120 °C. Nodules, crusts, sulphides, placers and hydrates are the marine resources.
Tonnage = volume × density, grade is weighted by thickness, cut-off = cost ÷ (price × recovery), and the stripping ratio is waste ÷ ore. A semivariogram has nugget, sill and range; kriging weights sum to 1 and give an estimation variance.
India: Sukinda chromite, Odisha bauxite and iron, Khetri and Malanjkhand copper, Rajasthan lead–zinc, Hutti gold, Jaduguda uranium, Panna diamond, beach-sand monazite, Gondwana coal (Jharia coking coal), Neyveli lignite and Mumbai Offshore, Assam, Cambay and Krishna–Godavari hydrocarbons.

Practice questions (18)

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.

Use atomic masses Cu 63.5, Fe 55.8, S 32.1, C 12.0, O 16.0 and H 1.0. Which of the following ore minerals contains the highest percentage of copper by weight?
1. Chalcopyrite, CuFeS₂
2. Bornite, Cu₅FeS₄
3. Chalcocite, Cu₂S
4. Malachite, Cu₂CO₃(OH)₂
Show answer
Answer: C — Chalcocite, Cu₂S
Cu percentages: chalcopyrite 63.5 / 183.5 = 34.6%; bornite 317.5 / 501.7 = 63.3%; chalcocite 127.0 / 159.1 = 79.8%; malachite 127.0 / 221.0 = 57.5%. Chalcocite is the richest copper mineral. Bornite is the tempting second, because it is copper-rich, and chalcopyrite is the most common copper mineral but has the lowest copper content here because iron and sulphur make up most of its mass.
In a polished section examined in reflected light, a soft grain is white, stays dark when the stage is rotated between crossed polars, and shows triangular pits where cleavage fragments were plucked out during polishing. Which mineral is it most likely to be?
1. Galena, PbS, which is white, isotropic and soft, with perfect cubic cleavage
2. Pyrite, FeS₂, which is pale yellow, isotropic and very hard, with no cleavage pits
3. Chalcopyrite, CuFeS₂, which is deep brass-yellow and weakly anisotropic when rotated
4. Hematite, Fe₂O₃, which is greyish-white and strongly anisotropic, with red internal reflections
Show answer
Answer: A — Galena, PbS, which is white, isotropic and soft, with perfect cubic cleavage
A white, soft, isotropic mineral with triangular cleavage pits is galena, whose perfect cubic cleavage leaves those pits when fragments pluck out. Pyrite is also isotropic and is the tempting alternative, but it is pale yellow, very hard and has no cleavage pits. Chalcopyrite is a deeper yellow and weakly anisotropic, and hematite is strongly anisotropic with red internal reflections.
In a copper deposit, secondary chalcocite and covellite form a zone below the water table, beneath a leached, iron-oxide-rich cap. Which process formed the chalcocite zone?
1. Magmatic crystallisation, in which dense sulphide liquid settled out of a silicate magma under gravity in a layered intrusion
2. Hydrothermal boiling, in which an ascending brine lost vapour and deposited copper as it cooled
3. Regional metamorphism, in which copper was remobilised from older sulphides into fold hinges
4. Supergene enrichment, in which copper leached from the oxidised zone was redeposited as secondary sulphides
Show answer
Answer: D — Supergene enrichment, in which copper leached from the oxidised zone was redeposited as secondary sulphides
Chalcocite and covellite below the water table and under a leached gossan are the signature of supergene sulphide enrichment: copper dissolved by oxidation above the water table moves down and replaces primary sulphides below it. Magmatic settling, hydrothermal boiling and metamorphic remobilisation would not produce a blanket whose position is tied to the water table and to an overlying leached cap.
A quartz vein hosts gold deposited as an ascending fluid boiled and lost H₂S and CO₂ to a vapour phase. Why does boiling precipitate gold that was carried as the bisulphide complex Au(HS)₂⁻?
1. Boiling raises the temperature of the residual liquid, and a hotter liquid dissolves less gold
2. Loss of H₂S to the vapour phase removes the sulphur that keeps gold dissolved, so the complex breaks down
3. Boiling concentrates chloride in the liquid, which converts the gold into a more soluble chloride complex
4. Boiling draws in oxygen from the air, which oxidises the fluid and makes the gold insoluble
Show answer
Answer: B — Loss of H₂S to the vapour phase removes the sulphur that keeps gold dissolved, so the complex breaks down
In gold-bearing fluids much gold travels as Au(HS)₂⁻, so it depends on dissolved reduced sulphur. Boiling partitions H₂S and CO₂ into the vapour, which destabilises the bisulphide complex and deposits gold. Boiling is also accompanied by heat loss, so the liquid cools rather than heats, the fluid does not take in atmospheric oxygen at depth, and the loss of vapour does not create a more soluble chloride complex.
A primary two-phase aqueous inclusion in quartz homogenises to liquid at 240 °C. Which statement about this temperature is correct?
1. It is a minimum trapping temperature; the true value is higher by a pressure correction unless the fluid was boiling
2. It is the highest temperature the crystal reached after trapping, because the bubble cannot vanish below this value
3. It always equals the trapping temperature, because homogenisation exactly reverses the cooling of the inclusion
4. It is the temperature at which the last ice crystal melts, which gives the salinity of the fluid
Show answer
Answer: A — It is a minimum trapping temperature; the true value is higher by a pressure correction unless the fluid was boiling
A fluid trapped under pressure is a single phase, and on cooling at nearly constant volume it unmixes into liquid and vapour. On heating, the bubble disappears at Th, which is therefore the minimum trapping temperature; a pressure correction raises it unless the fluid was boiling, when Th is the trapping temperature. The final ice-melting temperature, measured on freezing, gives salinity, and Th is not a maximum.
A source rock is rich in woody debris of land plants, with abundant vitrinite and a low atomic H/C ratio. Which kerogen type and main product does it indicate?
1. Type I kerogen, which is mainly oil-prone, as in algal lake deposits
2. Type II kerogen, which gives both oil and gas, as in marine shales
3. Type III kerogen, which is mainly gas-prone because it is hydrogen-poor
4. Type I kerogen, because woody debris is rich in hydrogen and gives mostly oil
Show answer
Answer: C — Type III kerogen, which is mainly gas-prone because it is hydrogen-poor
Terrestrial higher-plant material is vitrinite-rich and hydrogen-poor, which is Type III kerogen, and it generates mainly gas and little oil. Types I and II are hydrogen-rich, from algal and marine organic matter, and give oil. The option that calls woody debris a Type I source repeats the Type I error by assuming that woody tissue is hydrogen-rich, which it is not.
A well on a large, sealed anticline with a good sandstone reservoir finds only water, although a mature source rock lies beside the structure. Burial-history modelling shows that the anticline formed after the main phase of oil expulsion had ended. Which is the best explanation of the dry well?
1. There is no source rock in the area, so no hydrocarbons were ever generated
2. The seal leaked badly, so the hydrocarbons escaped from the trap and reached the surface over time
3. The reservoir is too porous, which lets oil drain downward out of the trap
4. The trap formed too late: the oil had already migrated elsewhere before the structure existed
Show answer
Answer: D — The trap formed too late: the oil had already migrated elsewhere before the structure existed
A petroleum system works only if the trap exists before or during migration. The source rock is mature and the structure is large and sealed, so the best explanation is a timing failure: the oil migrated before the anticline formed and bypassed it. The stem excludes the other causes, since a mature source rock is present and the structure is described as sealed, and a high porosity does not drain oil from a sealed trap.
Which pairing of a resource and its Indian occurrence is correct?
1. Chromite, mined chiefly in the Khetri belt of Rajasthan
2. Zinc and lead, mined at Rampura Agucha in Rajasthan
3. Prime coking coal, mined chiefly at Neyveli in Tamil Nadu
4. Diamond, mined chiefly in the Bailadila range of Chhattisgarh
Show answer
Answer: B — Zinc and lead, mined at Rampura Agucha in Rajasthan
Rampura Agucha in Rajasthan is a major zinc–lead mine. Chromite is concentrated in the Sukinda belt of Odisha, whereas Khetri is a copper belt. Prime coking coal is associated with Jharia, while Neyveli is a lignite field. Diamond is associated with Panna in Madhya Pradesh, and the Bailadila range is known for iron ore.
Which statement about polymetallic nodules is correct?
1. They are hydrothermal precipitates that form at black smokers on spreading ridges
2. They are iron–nickel sulphide concretions that form in shallow shelf muds near river mouths
3. They are slowly growing Mn–Fe oxide concretions on abyssal plains, rich in Ni, Cu and Co
4. They occur only within the 200-nautical-mile Exclusive Economic Zone and so belong to coastal states
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Answer: C — They are slowly growing Mn–Fe oxide concretions on abyssal plains, rich in Ni, Cu and Co
Polymetallic nodules are Mn–Fe oxide concretions that grow by millimetres per million years on abyssal plains and carry Ni, Cu and Co. Hydrothermal precipitates at ridge vents are seafloor massive sulphides, not nodules. Nodules are oxide, not sulphide, and they occur on the deep sea floor largely beyond the EEZ, in the international Area regulated by the International Seabed Authority.
Which of the following statements about ore-forming processes are correct?
1. In a porphyry system the alteration zones run outward from the intrusion as potassic, phyllic, argillic and propylitic
2. Banded iron formations are mostly Archaean to Paleoproterozoic chemical sediments
3. Supergene sulphide enrichment takes place above the water table, within the oxidised zone itself
4. Beach placers of ilmenite and monazite form by mechanical concentration of heavy minerals
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Answer: A — In a porphyry system the alteration zones run outward from the intrusion as potassic, phyllic, argillic and propylitic; B — Banded iron formations are mostly Archaean to Paleoproterozoic chemical sediments; D — Beach placers of ilmenite and monazite form by mechanical concentration of heavy minerals
Porphyry alteration is zoned from a potassic core outward through phyllic and argillic to propylitic. Banded iron formations are mostly Archaean to Paleoproterozoic chemical sediments, and beach placers are heavy-mineral concentrations made by waves and currents. Supergene enrichment occurs below the water table, because copper dissolved in the oxidised zone above is redeposited beneath it, so the statement that places it above is wrong.
Which of the following statements about fluid inclusions are correct?
1. Secondary inclusions along healed fractures record the fluid that was present while the host crystal grew
2. A halite daughter crystal in an inclusion at room temperature shows a salinity above about 26 wt% NaCl
3. A more negative final ice-melting temperature indicates a lower salinity of the aqueous fluid
4. Vapour-rich and brine inclusions trapped together that homogenise at the same temperature indicate boiling
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Answer: B — A halite daughter crystal in an inclusion at room temperature shows a salinity above about 26 wt% NaCl; D — Vapour-rich and brine inclusions trapped together that homogenise at the same temperature indicate boiling
Halite saturation in H₂O–NaCl is near 26 wt% NaCl at room temperature, so a halite daughter crystal means a more saline fluid. Coexisting vapour-rich and brine inclusions with the same Th are the standard evidence for boiling. Secondary inclusions record later fluids, not the one present during growth, and a more negative Tm means a larger freezing-point depression and therefore a higher salinity, so those two statements are wrong.
Which of the following statements about reserve estimation and geostatistics are correct?
1. Raising the cut-off grade lowers the tonnage and raises the average grade of the ore above the cut-off
2. In ordinary kriging the weights are chosen freely and need not sum to 1
3. The nugget effect is the variance found at the range, where samples become uncorrelated
4. Kriging gives an estimation variance, which simple inverse-distance weighting does not give
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Answer: A — Raising the cut-off grade lowers the tonnage and raises the average grade of the ore above the cut-off; D — Kriging gives an estimation variance, which simple inverse-distance weighting does not give
Raising the cut-off removes low-grade material, so tonnage falls and the mean grade of what remains rises. Kriging yields an estimation variance together with the estimate, which inverse-distance weighting cannot. Ordinary kriging weights are constrained to sum to 1 for unbiasedness, and the nugget is the variance at near-zero lag, not at the range, so those two statements are wrong.
Which of the following statements about the distribution of mineral and fuel resources in India are correct?
1. Sukinda in Odisha is the principal chromite-bearing ultramafic belt of India
2. Bauxite in India occurs mainly in Jammu and Kashmir and Himachal Pradesh
3. Jharia in Jharkhand is noted for prime coking coal of Permian Gondwana age
4. The Khetri belt of Rajasthan is the main tin province of India
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Answer: A — Sukinda in Odisha is the principal chromite-bearing ultramafic belt of India; C — Jharia in Jharkhand is noted for prime coking coal of Permian Gondwana age
Sukinda holds most Indian chromite, and Jharia in the Damodar valley is the source of prime coking coal from the Permian Gondwana measures. Indian bauxite is concentrated in Odisha and neighbouring peninsular states, not in the Himalaya, and Khetri is a copper belt, not a tin province.
A tabular copper ore body has a mean thickness of 12 m over a plan area of 25,000 m². The ore has a bulk density of 3.0 t/m³ and a grade of 2.4% Cu by weight, and 90% of the ore is recovered in mining. The mass of copper in the recovered ore is ____ tonnes.
Numerical answer — type the value.
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Answer: 19440
Volume = 12 × 25,000 = 300,000 m³. Ore tonnage = 300,000 × 3.0 = 900,000 t. Copper in place = 900,000 × 0.024 = 21,600 t. With 90% recovered, the copper in the recovered ore is 21,600 × 0.90 = 19,440 t. Forgetting the recovery gives 21,600, and forgetting the density gives 7,200.
The cost of mining and processing is $30 per tonne of ore, the price of copper is $8,000 per tonne of metal and the metallurgical recovery is 85%. Using cut-off grade = cost per tonne of ore ÷ (price per tonne of metal × recovery), the cut-off grade is ____ % Cu (give the answer to two decimal places).
Numerical answer — type the value.
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Answer: 0.44
The metal that must be recovered from each tonne of ore to cover the cost is 30 / 8,000 = 0.00375 t per tonne of ore if recovery were complete. With 85% recovery the grade must be 0.00375 / 0.85 = 0.004412, that is a fraction by weight, and as a percentage this is 0.44% Cu. Omitting the recovery gives 0.375%, and forgetting to convert the fraction to a percentage is another slip.
Two samples lie on a line, one at 0 m with a grade of 8 g/t and one at 10 m with a grade of 3 g/t. The semivariogram is linear, γ(h) = 0.5h with no nugget. Using ordinary kriging, the estimated grade at the point 4 m from the sample at 0 m is ____ g/t.
Numerical answer — type the value.
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Answer: 6
The ordinary kriging equations are λ₁γ(0) + λ₂γ(10) + μ = γ(4) and λ₁γ(10) + λ₂γ(0) + μ = γ(6), with λ₁ + λ₂ = 1. With γ(h) = 0.5h these become 5λ₂ + μ = 2 and 5λ₁ + μ = 3. Subtracting gives 5(λ₁ − λ₂) = 1, so λ₁ − λ₂ = 0.2, and with λ₁ + λ₂ = 1 the weights are λ₁ = 0.6 and λ₂ = 0.4. The estimate is 0.6 × 8 + 0.4 × 3 = 4.8 + 1.2 = 6.0 g/t. Using inverse-squared-distance or equal weights gives other values.
In an aqueous NaCl-type fluid inclusion the final ice-melting temperature is −8.0 °C. Use S = 1.78θ − 0.0442θ² + 0.000557θ³, where θ is the freezing-point depression in °C (θ = 8.0 here) and S is the salinity in wt% NaCl equivalent. The salinity is ____ wt% NaCl equivalent (give the answer to two decimal places).
Numerical answer — type the value.
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Answer: 11.70
With θ = 8.0: 1.78 × 8.0 = 14.240; 0.0442 × 64 = 2.8288; 0.000557 × 512 = 0.2852. So S = 14.240 − 2.8288 + 0.2852 = 11.6964, which is 11.70 wt% NaCl equivalent. Taking θ = −8.0 without using its magnitude gives a negative nonsense value, and dropping the cubic term gives 11.41.
A coal sample analysed on the as-received basis has moisture 7%, volatile matter 30% and ash 15%. The fixed carbon is the remainder after these three, and the fuel ratio is fixed carbon ÷ volatile matter. The fuel ratio is ____ (give the answer to one decimal place).
Numerical answer — type the value.
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Answer: 1.6
Fixed carbon = 100 − (7 + 30 + 15) = 48%. The fuel ratio is 48 / 30 = 1.6. Using 100 − (30 + 15) = 55 as fixed carbon, which leaves moisture in, gives 1.8, the usual slip.