Vast diversity of sedimentary rocks and environments
has produced suitable environments as a host for
mineral reserves. In sedimentary environments,
metallic and non-metallic reserves are available
indicating sedimentary structure and texture. A wide
variety of factors are observed in mineral
sedimentation. Chemical and mechanical factors,
sedimentary environment, living things, temperature,
pH and Eh of sedimentary environments are all factors
that have significant role in formation of different
ore deposits. Considerable amount of sedimentary
Copper, lead, zinc, uranium, gold, iron and aluminum.
Sandstone-host uranium ore deposits
The most important uranium reserves are formed in
sandstones holding riverine origin. 45% of uranium
reserves discovered in western countries and 95% of
the USA uranium have sandstone host rock. These ore
deposits have formed in the South Africa from
Carboniferous to Triassic, in the western US and
eastern Europe within Mesozoic and in Australia during
The host rock for mineralisation has been sandstone,
Arkose and tuff, which have formed in alluvial and
shallow basins. Primary uranium of pegmatites, acidic
volcanic tuffs and regional granites are oxidized by
oxygen-bearing surface waters and transported in form
of solution. After penetration in ground and in form
of subsurface waters rich of uranium, it moves through
sandstone porous layers in terms of topographic dip.
In the meantime, it changes the oxidation environment
and moves near available organic matters in sandstones
in form of uraninite and deposits in sandstone pores.
Important and effective factors of uranium
concentration are; high permeability of host rock,
availability of uranium absorbers, such as coal, iron
and manganese oxides and clay minerals, redox
conditions of availability of organic matters and
Uranium important minerals in these reserves are;
carnotite, uraninite, pitchblende and uranium-bearing
organic complexes. The average carat of U3O8 of these
ore deposits varies between 0.1 to .035%. The reserve
of each ore deposit varies between 25000 to 30000 ton.
There are some minerals in nature that have low carat
in their early stages of formation, therefore they do
not have any economic value. For instance, zircon as
an accessory mineral is crystallized in granites, but
its carat is too low to be exploited. The number of
these minerals is a lot in nature, but only small
number of them have sufficient mechanical and chemical
resistance. After weathering of host rock by water,
wind and sometimes glacier transportation, these
minerals are concentrated in appropriate secondary
environments of river beds, deserts or glacial
sediments. These reserves form placer sediments.
Minerals holding high chemical and mechanical
resistance, such as clastic gold, platinum, ilmenite,
diamond, zircon, cassiterite and garnet are
transported by water and deposited in suitable
locations based on specific gravity, shape and
particles’ sizes. The most important conditions to
form placer reserves are; suitable source rock, warm
and humid climate producing weathering and releasing
resistant minerals from rock’s groundmass, and at last
low topographic dip. For instance, diamond placer
from kimberlites, tin placers from granites, and
garnet placer originate from garnetschists.
About 95% of produced tin are obtained by Malaysia,
Thailand and Indonesia. Its age is Cenozoic. After
rocks’weathering, available cassiterite (SnO2) in
pegmatites and granites are released, then they are
transported by water and in river beds and suitable
environments form tin placers.
Bauxite is a rock rich of aluminum which are mainly
formed by aluminum hydroxides and small amount of
quartz and clay minerals. Bauxite mineralogical
composition is variable and is in relation to primary
source rock. About 96% of the world Al is supplied by
bauxite. The average Al of crust rocks is 13.8%,
whereas an economic reserve for Al 30% of Al2O3 is the
minimum exploitable carat. Among igneous rocks,
nepheline-syenite holding 3.21% Al2O3 and among
sedimentary rocks, shales holding 7.14% Al2O3 have
maximum amount of Al in crusts’ rocks. If rocks have
high content of Al and low amount of SiO2 and go under
chemical weathering process resulting from alternating
rain (so-called hydrolysis), K, Mg, Ca and Si contents
of rock will convert to solution and go out of the
region by surface and subsurface waters. Remaining
parts are Al2O3 and slightly Fe2O3 assist the thick
layer resulting from weathering overlies primary
source rock. The mentioned layer is red. Important
factors, which have a significant role on bauxite
reserves formation, are as follows;
1.Chemical and mineralogical composition of source
3.High amount of rain, snow and temperature.
4.Suitable topography and high drainage.
Nepheine-syenite, shale, clayey limestone and basalt
are the rocks, which are suitable for bauxite
formation. To convert to bauxite, appropriate rocks
must be subject to hot and humid weather of tropical
regions. The annual average rain fall is 1200 to
1400mm, and the average temperature is 26C. This
situation together with the low-topographic conditions
produces maximum chemical and minimum mechanical
erosion. Waters of this region bearing a pH between 7
and 8 can disintegrate silicates and all leached
minerals. Then they change to a solution. Only
aluminum hydroxides (gibsite, bohmite and diaspore)
are not soluable and deposit.
The most important ore deposits of bauxite are found
in Brazil, Australia, Guinea Bisau and Jamaica. The
Al2O3 carat varies between 35 to 55% for bauxite and
their reserve is between 1 to 700 million ton.
