module 5 - petrology & economic geology ⛏️👷‍♂️🚧

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Concentration factor

The number of times the metal must be concentrated from the average crustal abundance to reach the cut off grade (the lowest economic ore grade for mining)

Concentration factor calculation

cut off grade (%) / average crustal abundance (%)

Average crustal abundance

The amount of metal in the crust of the Earth if it were all evenly spread throughout the crust

Ore

• a rock or minerals which can be mined for metal extraction. 

• Many minerals contain metals but not all will be ores as it may be too expensive or difficult to extract the metal from the mineral

Internal processes

Igneous + metamorphic

Surface processes

Sedimentary + weathering

Resource

• A useful and valuable natural material

• The total quantity of metal in the world which may in the future become economic to mine

Mineral resource

Can be a metallic, non-metallic or industrial mineral

Reserves

The amount of the resource that can be extracted at a profit using existing technology

Consumption

Is the quantity of a metal being used

Ore

The rock containing valuable metal(s) that is economic to mine

Ore deposit

Accumulation of metal that maybe economic to mine

Ore mineral

A mineral containing valuable metal(s)

Gangue mineral

• A low value waste mineral

• e.g. quartz, calcite, pyrites and barytes

Grade

The concentration of valuable mineral within an ore

Cut-off grade

The grade below which it is uneconomical to mine

What controls the cut-off grade?

Value of the metal	The more valuable the metal the lower the cut off grade
Demand	The higher the demand for the metal , the more valuable and the lower its cut off grade
Abundance	If they are useful scarce metals will have a lower cut-off grade
Size of the deposit	Large deposits will be economic to mine at lower cut off grades than smaller deposits
Cost of mining and extraction	If it is costly to mine and extract a metal then the cut off grade will be higher

Cut-off grade can be calculated by rearranging the concentration factor formula:

Average crustal abundance

                             X

Minimum concentration factor to be economic

Spoil

waste material brought up during the course of an excavation or a dredging or mining operation.

Secondary enrichment

• Occurs when metals are leached from surface rocks and precipitated just below the water table

• Not an ore forming process but an ore concentrating process

• Very important for increasing the grade of otherwise uneconomic copper deposits

Leaching

Where elements are dissolved from rocks and carried downwards in solution

Gossan

An insoluble cap of iron oxides at the surface

Oxidising

Describes oxygen rich conditions, allowing elements to combine with oxygen to form oxides

Reducing

Describes oxygen poor anoxic / anaerobic conditions

Enriched Deposit

A zone of high grade ore just below the water table, formed by secondary enrichment

Porphyry

A large igneous intrusion below a volcano with a porphyritic texture

Porphyritic

An igneous rock w/ large crystals, called phenocrysts, surrounded by a matrix of smaller crystals or glass, called the groundmass

Chalcopyrite

Copper sulfide/The main ore mineral for copper

Malachite

Azurite

Chalcocite

Above the water table…

1. Rainwater infiltrates into the exposed copper deposit and percolates downwards through the pore spaces

2. In the zone of oxidation ____ the water table, chemical reactions change insoluble copper sulfides such as chalcopyrite into soluble copper sulfates

3. The copper sulfates are dissolved, taken into solution and carried downwards by the groundwater

4. A barren, leached zone is left near the surface covered by an insoluble iron oxide capping called a gossan

6. Gossans are of no economic value but are useful exploration targets as they may indicate ores deposits underneath

Below the water table…