Download Uranium in magmatic processes

Uranium, vanadium, niobium,
tantalum
Uranium
Universe: 0.0002 ppm (by weight)
Sun: 0.001 ppm (by weight)
Carbonaceous meteorite: 0.010 ppm
Earth's Crust: 1.8 ppm
Seawater: 3.13 x 10-3 ppm
Uranium in lithosphere
Uranium chiefly appears in minerals with valences 4+ and
6+. The coordination number of oxygen around U4+
is six or eight. The coordination number of U6+ around
oxygen is six, seven, or eight. Crystallochemical properties
of U4+ are very close to those of Th4+ (ionic radii 0.94 A
and 1.05 A) and LREE3+ (ionic radii 1.03-0.98 A and
1.16-1.10 A, from La to Nd).
The geochemistry of this element in igneous rocks is
strongly coherent with that of Th and LREE. In
hydrothermal and supergene processes, however, uranium
is partially or totally oxidized to U6+, and does not bear any
coherence with the above elements.
Uranium in magmatic processes
About 110 uranium minerals are known. The most
abundant is the oxide uraninite (its simplified formula is
UO2). It can forms solid solution with thorianite. When
uraninite is strongly oxidized it is called pitchblende.
Uraninite is widespread in acidic magmatics either as
minute inclusions in major rock-forming minerals or as
large grains (up to several mm) in granites and pegmatites.
Some common accessory minerals may contain
appreciable uranium contents. The most important are
thorite (~1-35 wt% UO2 ), thorianite (varies from ThO2 to
UO2), xenotime and zircon (up to 5 wt% of UO2 ),
monazite (100-20 000 ppm U), allanite (10-2000 ppm U).
Uranium in magmatic processes
Ultramafic and mafic rocks have very low concentrations
(average 10 ppb). The concentration of U from the
ultramafic to acidic series increases steeply with silica
contents, the granites normally having about 4-5 ppm U. It
occurs by Ca/REE substitution in silicates (allanite, thorite),
and phosphates (monazite, xenotime, apatite).
Characteristic enrichment is the pneumatholitic processes,
mainly in the Co-Ni-Sn-Bi-As ore deposits. It forms various
oxides with Nb-Ta-REE in pegmatites (e.g. pyrochlore,
aeschynite group minerals), alkali magmatic rocks, and
especially in carbonatites.
Uranium in weathering and
sedimentary rocks
In supergene conditions uranium is invariably oxidized to
uranyl ion, which is easily mobilized. For this reason, most
uranium deposits, regardless of their origin, contain an
important assemblage of supergene U6+ minerals
consisting of silicates (coffinite, uranophane), carbonates
(andersonite, cejkaite), phosphates (torbernite, autunite),
arsenates (novacekite, zeunerite), sulfates (uranopilite,
zippeite, johannite), vanadates, molybdates, niobates,
tantalates. The largest amounts of secondary mineral
associations is found in the oxidation zone of uranium ore
deposits. Characteristic phases in abandoned mines, and
waste dumps, too. Most of the secondary phases rather
instable, after weathering the U moves to hydrosphere.
Uranium in weathering and
sedimentary rocks
Distribution of U in sedimentary rocks: evaporites have very
low U contents, usually < 100 ppb. Limestones contain
between 0.5 and 3 ppm U. In terrigenous rocks, U
generally increases as the grain size decreases.
Sandstones contain about 0.5-2 ppm U, and shales
between 2 and 8 ppm.
Black shales may have very high U contents, with values
higher than several hundred ppm being not uncommon.
Phosphate rocks, coals, bitumens can also have very high
U contents, in some cases greater than 1000 ppm. The
high U content correlate with the amounts of organic
matter, which adsorbed not only U, but Th, V, Ge, Nb etc.
Vanadium
Universe: 1 ppm (by weight)
Sun: 0.4 ppm (by weight)
Carbonaceous meteorite: 62 ppm
Earth's Crust: 190 ppm
Seawater: 1.1 x 10-3 ppm
Vanadium in magmatic processes
Vanadium can take on a 2+, 3+, 4+, and 5+ charge,
it is typically in the 3+ valence state in primary terrestrial
minerals. In the 3+ valence state it is most often
octahedrally coordinated.
Vanadium is a lithophile element at low-pressure, but may
be siderophile at the elevated pressures suggested for core
formation in the Earth. It is incompatible in most silicate
minerals, although it may be moderately compatible in
some pyroxenes. It has an estimated abundance of 103
ppm in the bulk Earth, 98 ppm in present-day bulk
continental crust, 53 ppm in the upper crust, finally 149
ppm in the lower crust.
Vanadium in magmatic processes
It concentrates in spinel minerals in early magmatic
differentiation (e.g in Ti-magnetite, in V-containing spinel
coulsonite). It substitutes Fe, Al, Ti in some rock-forming
silicates (e.g. pyroxenes, micas). However, it forms
independent phases, mainly sulphides in the postmagmatic processes, as patronite, VS4, and sulvanite –
Cu3VS4.
Vanadium in magmatic rocks
Vanadium in weathering and sedimentary
processes
It has many secondary minerals in sedimentary
environments. Various vanadates found in the oxidation
zone of V-bearing ore deposits. The most important are:
vanadinite [Pb5(VO4)3Cl], carnotite [K2(UO2)(VO4)2•
3H2O), descloizite PbZn(VO4)(OH). The vanadates show
some similarities to arsenates and phosphates. There are
some substituting possibilities between P-As-V in these
compounds. Characteristic Al – V substitution is known in
some clay minerals, e.g. in illite (so-called vanadium-illite).
.
Vanadium in sedimentary rocks
Vanadium in sedimentary processes
It enriched by adsorption in organic-matter-rich sediments
in reductive condition: coals, lignites, black shales,
bitumens, sandstones. There are some independent Vbearing minerals in these rocks: oxides, sulphates, etc.
In bauxites and clays the V4+, V5+ cations concentrated by
adsorption on the surface of Fe-Mn-Al-oxides/hydroxides or
clay minerals, too. Glauconite- and chlorite-bearing
sediments can also show some V enrichments.
Niobium
Tantalum
Universe: 0.002 ppm
Sun: 0.004 ppm
Carbonaceous meteorite:
0.19 ppm
Earth's Crust: 17 ppm
Seawater: 9 x 10-7 ppm
Universe: 8 x 10-5 ppm
Carbonaceous meteorite:
0.02 ppm
Earth's Crust: 2 ppm
Seawater: 2 x 10-6 ppm
Niobium and tantalum
in magmatic processes
They are chemically very similar and often occur together
in some pegmatites, alkaline rocks and carbonatites.
The most important mineral of niobium is pyrochlore,
NaCaNb2O6 (OH,F), and columbite (Fe,Mn)(Nb,Ta)2O6,
while Ta is tantalite (Fe,Mn)(Ta,Nb)2O6. Nb and Ta
commonly substitute for Ti in rutile (Nb-rich rutile so-called
ilmenorutile, Ta-rich rutile so-called strüverite), titanite,
perovskite and ilmenite, and for Zr, W and Sn in other
minerals (e.g. eudialyte, astrophyllite). Alkaline rock
complexes (e.g. syenites, nepheline syenites and alkaline
ultrabasites) have the highest Nb content of all magmatic
rocks; niobium is, therefore, mainly recovered from
carbonatites and associated alkaline rocks.
Niobium and tantalum
in magmatic processes
High enrichment in Nb and Ta in carbonatite-alkalic rock
complexes results in differentiation of partial melts of the
asthenosphere (carbonatite) and the metasomatized
mantle (alkalic rocks). The high enrichment in Nb and Ta in
pegmatites is the result of extreme fractionation of granitic
magma. Specialized granites (alkali granites, biotite and/or
muscovite granite, lepidolite-albite granites) often present
associated niobium and tantalum mineralization, but the
strong niobium enrichment is characteristic of alkali
granites. The most important Nb/Ta enrichment is known in
pegmatites (with many complex Nb-Ta oxides in the
pyrochlore, aeschynite, columbite, tantalite groups).
Tantalum in magmatic processes
Ta-dominant minerals (e.g. tantalite-columbite series,
wodginite, microlite, tapiolite) are mainly found in the highly
fractionated rare element granitic pegrnatites. The most
important economic sources of tantalum are alkali granites,
greisenized granites, rare element granitic pegmatites and
tantalum-bearing cassiterite (SnO2 ) deposits.
From the most primitive to the highly fractionated rare
element granitic pegmatites there is an increase in the
Ta/Nb ratio of the (Nb,Ta) mineral species.
TheTa enrichment trend is continued in most of the
alteration products that replace these pre-existing mineral
species in highly fractionated pegmatites.
Niobium and tantalum in weathering and
sedimentary processes
Weathering of carbonatites enriches the alluvial sediments
in Nb/Ta minerals, because of the most abundant Nb/Ta
minerals are rather stable compounds.
A few portion of Nb/Ta phases, after chemical weathering
connect by adsorption to clay minerals, or Fe-Al-Mn oxides.
Relative enrichment of Nb/Ta is known in deep-marine Mnnodules.