EES 119/219

Lecture 3

 

Changes in production and use of minerals reflect growth of population and changes in standard of living.  The relative amount of metals coming from production in developed countries has generally decreased, as shown by the examples of USA and Japan. 

Metal resources are not uniformly distributed, in spite of a relatively small land area, South Africa has unusually high levels of important metals (Fig. 1).

Resources and Reserves;

•         Reserves: Known occurrences of metals, which can be extracted for a profit under current economic conditions

•         Resources: Estimate of metal concentrations present in the crust 

McKelvie Diagram on Reserves and Resources (Fig. 2)

 

Crustal concentration and cutoff grades for selected metals

 

Metal

Crustal conc.

%

Cutoff grade

%

Concentration factor

Al

8

30

4

Fe

5.6

30

6

Cr

0.0096

1

100

Zn

0.0082

1.5

150

Ni

0.0072

1.5

150

Cu

0.0058

0.5

100

Pb

0.001

2

2000

U

0.00016

0.2

1200

Ag

8x10-6

0.08

10,000

Hg

2x10-7

0.02

100,000

Au

2x10-7

0.0008

4,000

 

•         Average concentrations of metals are too low for mining

•         Minable concentrations depend on geologic and economic parameters

•         Concentration factors vary between 3 and 100,000

ΰNatural processes are necessary to increase concentrations in order to form an ore deposit

 

Principal natural processes leading to concentration of metals in the crust (Fig.3)

  • Differentiation in place
  • Removal of matrix
  • Transport and deposition of ore minerals

 

Make-up of an Ore Deposit:

 

  • Ore minerals: Minerals containing the metal or metals to be extracted for profit
    • Galena (PbS); Hematite (Fe2O3)
  • Gangue minerals: Minerals of no economic value which have been deposited together with ore minerals
    • Quartz SiO2; Pyrite FeS2
  • Host formation

 

Ore minerals: Ore minerals contain one or more metals which can be extracted for a profit

 

Major types of ore minerals;

 

  • Native Occurrences: Au, Pt (Fig.4)
  • Sulfides MexSy: Galena PbS (Fig.5)
  • Multi-metal Sulfides Me(1)xMe(2)ySz : Chalcopyrite CuFeS2 (Fig. 6)
  • Oxides MexOy : Hematite Fe2O3 (Fig. 7)

 

Gangue Minerals: Deposited together with ore minerals, but of now economic value

 

•         Often similar types of minerals (oxides, sulfides, sulfates) as ore minerals

•         Commonly much more abundant than the ore minerals

ΰUseful for the determination of formation processes

 

Host Formation: large-scale formation hosting the ore deposit; can be pre-existing or contemporaneous with ore-formation

Sedimentary layers; igneous intrusion

ΰ    Metal constitutes only small component of ore deposit

ΰ    Extraction of metals produces large amount of ‘waste rock’

 

 

Distinction between major and minor elements in the crust:

•         Major elements: Found as main constituent of rock forming minerals: O, Si, Fe, Al, Na, K

•         Minor elements: Present in minerals not part of the main rock sequences or in traces in other minerals

 

Consequence for mining considerations: Mining for major elements will incrementally move to formations with lower concentrations while mining for metals with low crustal abundance might encounter a mineralogical barrier (Fig. 8)