FORMATION OF ORE DEPOSITS (see Geoscience p. 171 on)
Method of Formation
Processes
Examples
MINERAL DEPOSITS FROMED BY IGNEOUS PROCESSES
Fractional crystallisation of
Magmatic segregation
Pegmatites
Porphyry ore deposits
cooling mafic magma, leading
to deposition of oxides and
silicates
Usually found with granites.
Very large single crystals, e.g.
mica, formed when magma
migrated into cracks, probably
due to low viscosity and cooled
very slowly.
Formed at depth under island
arcs. Large crystals form
slowly, then ion rich melt
escapes and cools quickly in
small cracks leaving veins of
ore
ORE DEPOSITS FORMED BY SEDIMENTARY PROCESSES
Residual and weathered Concentration of resistant
minerals, particularly found in
ore deposits
Placer deposits
Fossil placer deposits
Chemical precipitates
tropical conditions where
silicates have been broken
down leaving insoluble
hydrated aluminium oxide
Concentration of ore, e.g. in
river beds, formed by
differential weathering
Buried placer deposits. These
are harder to extract as they
have been fossilised, but may
be exploited if of sufficient
value
Formed by chemical reaction in
solution, which leaves
insoluble compound on the sea
floor
ORE MINERALS FORMED BY HYDROTHERMAL PROCESSES
Hydrothermal solutions at Minerals are deposited in
or near the earth’s surface hydrothermal veins near basalt
intrusions – e,g, massive
sulphides associated with
black smokers.
Hydrothermal
solutions Solutions carrying ground
water through permeable rocks
from sediments
may deposits minerals from
lower in the sedimentary
sequence
Groundwater penetrates
Hydrothermal veins from
cracks in the rocks and
igneous intrusions
deposits minerals on cooling