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Lecture 7

Magmatic processes

• Magma: Fluid made up of components leading to the formation of rock-forming minerals

• Crystallization of magma leads to the formation of igneous rocks

– Temperature ranges:

• T > 1300^oC à everything in liquid phase

• 700^oC < T < 1300^oC à crystallization based on melting T of individual minerals

– Differential crystallization: minerals with high melting T crystallize, lowering the content of those elements/minerals in the remaining melt

à Continuous change in melt composition during crystallization

Role of water in magma (Fig. 1)

• Magma is siliceous melt

– At T > 1300^oC, all minerals are liquid

• Siliceous melt with some water in solution

• The maximum of water in melt is 8%

• The presence of water affects the melting temperatures

• As magma cools, minerals with high melting T crystallize first à magmatic stage

– The relative amount of water in the remaining melt increases, lowering the melting T

• When maximum solubility of water is reached, water is forced out, increasing the melting point and causing rapid solidification à pegmatitic stage

• Below 700^oC, all rock forming minerals have solidified, the fluid is now hot, mineral bearing water à hydrothermal stage

Formation of igneous rocks

• Intrusion of magma: magma rises from the lithosphere towards the surface of the crust

• If the magma reaches the surface, cooling is rapid à formation of small crystals

– Volcanic sequence of rocks (basalt à rhyolite)

• If the magma solidifies in the crust, slow cooling due to conduction à formation of large crystals

– Plutonic sequence of rocks (gabbro à granite)

The cooling of magma generates a distinctive sequence of minerals; Bowen’s Reaction Series (Fig. 2)

Ferro-alloy metals

• Metals used as alloys of Fe in order to produce steel with specific characteristics

– V Vanadium

– Cr Chromium

– Co Cobalt

– Ni Nickel

– Mo Molybdenum

– W Tungsten

– Mn Manganese – used for purification of steel

Bushveldt Complex (Fig. 3; Fig. 4)

• Magmatic deposit

• Large complex in South Africa

• Age ~ 2 Ga

• Major mineralization:

– Chromium

– Platinum

– Sulfide ores: (Ni, Fe)S; CuFeS2

– Tin

The formation of the Bushveldt complex represents the differential crystallization of magma

which had intruded a pre-existing sedimentary formation (Transvaal Supergroup) with the formation of distinct, ore rich layers during the cooling and crystallization process (Fig. 5).

From ore to metal

• Mining

– Surface mining: Large Deposits; low grade deposits

– Underground mining: High grade deposits

à Ore (ore minerals + gangue minerals)

• Crushing, sorting

– Done in place, mostly mechanical processesà Ore minerals

à Ore minerals

• Smelting

– Change from ore mineral to metal

• Chemical separation necessary

à Metal

Examples

• Gold: Native Au

• Copper: Sulfide CuFeS2 à Cu

• Iron: Oxide Fe₂O₃ à Fe

• Aluminum: Oxide Al₂O₃ à Al

Gold mining

• Panning: A manual technique of sorting gold:

– separation based on the high density of Au

– same principle as formation of placer deposits

• Hydraulic Mining: Large scale placer mining: Water cannons get the gravel to flow through sluices where Au is sorted out

• Hard rock mining: Gold veins embedded in volcanic rocks or sedimentary deposits

– Most energy needed à high grade deposits

– Often done underground à Witwatersrand

• Cyanide Mining

– Gold (or Ag) can form a compound with cyanide (cyanide ion CN- à extremely toxic)

– Sodium cyanide solution is mixed with finely ground rock à forms gold cyanide solution

– Further steps include addition of Zn, H2SO4 and recovery steps

– Used for low grade gold deposits

– Severe environmental concerns

Copper Production

• Sulfide ores: CuFeS2; Cu2S etc.

• Roasting: Heating the ore removes S (and other volatile impurities)

• Reverberatory furnace: Produces molten Cu-Fe-S

• Converter: oxidizes Fe, S à Blister Cu

From Iron ore to Steel

• Mining: large scale, typically surface mining

• Separation of ore and transport to smelter

• Production of pig iron

– Blast Furnace

• Pig Iron: Fe compound after the initial separation from oxygen

• à contains relatively high amounts of C, not used directly, but starting material for the production of steel (cast iron is form of pig iron)

• Steel: Fe metal containing between 0.5 and 1.5 % of C; often fortified with other Fe-alloy metals (Cr; Mo; W etc.) to give it specific characteristics (toughness; hardness; elasticity etc.)

• Production of Steel

– Steelmaking is the second step in producing steel from iron ore where impurities (e.g. S, P, excess C) are removed and alloying elements (Cr, Ni, V etc.) are added to produce the desired characteristics of steel

– basic oxygen furnace, also known as an LD converter

– An electric arc furnace is a system that heats charged material by means of an electric arc

Production of Aluminum

Essential steps in the conversion of bauxite to Al:

Extraction: Al(OH)₃ and similar compounds in bauxite are converted to Al(OH)₄

Precipitation: Al(OH)₄^- + Na⁺ à Al(OH)₃ + Na⁺ + OH^-

Calcination: 2Al(OH)₃ à Al₂O₃ + 3H₂O

Hall-Heroult process: 2 Al₂O₃ + 3 C → 4 Al + 3 CO₂

Electrolytic process: Al reduced at cathode, C is oxidized at anode.

àNeeds very large amount of electricity (low V, high A).

Recycling 1 kilogram of aluminum can save up to 8 kilograms of bauxite,

4 kilograms of chemical products and 14 kilowatt hours of electricity

Mineral Resources – Summary

• Geologic processes lead to the concentration of ore minerals

– Sedimentary processes

• Banded Iron Formation (Fe)

• Bauxite Formation (Al)

• Placer Deposits (Au)

– Magmatic and hydrothermal processes

• Porphyry Copper Deposits (Cu)

• Massive Sulfide Deposits (Cu, Zn, Pb)

• Magmatic Deposits (Cr, Pt)

• Metals are bound in ore minerals

– Sulfides

– Oxides

• Production of Metals needs large scale operations with severe consequences for the environment

– Mining

• Strip mining

• Underground mining

– Smelter processes

• Future of mineral resources depends on population growth and life style

– Development of per–capita consumption

– Recycling