Geology Reference
In-Depth Information
sandstones and more reduced assemblages of shales, carbonates and evaporates.
The metals were transported in basin-derived fluids that were set into motion by
rapid rifting and subsidence. The metals were leached from detrital minerals such as
magnetite, biotite and hornblende and they were transported as chloride complexes.
Ore deposition occurred at the redox interface between oxidised and reduced
sedimentary rocks. As for SEDEX deposits, there is considerable debate about
the precise process, and particularly whether the ore metals were primary
precipitates or epigenetically replaced sedimentary iron sulfides.
4.4.2 Uranium Deposits
Uranium is very different from the other elements discussed in this chapter: it is an
energy source, and not a metal used in industry or finance like the copper, zinc, or
gold; and because it is radioactive, used in bombs, it is the target of the ire of
ecologists (a moustachioed politician who became famous for tearing down
a MacDonald's and brandishing his roquefort at anti-capitalism demonstrations,
learnt his trade in anti-nuclear protests). Although a trace element, uranium is found
in a large range of crustal rocks and forms a wide variety of deposits. A brief
description of the more important types in given in Table
4.4
. Those in magmatic
rocks and in purely sedimentary settings are mentioned in other chapters; here we
discuss just two types, unconformity-related deposits and sandstone deposits, both
of which formed from hydrothermal fluids, to continue the theme of ore deposition
related to redox reactions.
The primary uranium ore mineral in these and other deposits is uraninite
(UO
2
) or pitchblende (UO
3
,U
2
O
5
). Other uranium minerals include carnotite
(K
2
(UO
2
)
2
(VO
4
)
2
ยท3H
2
O) and complex oxides or titanates rich in rare trace elements
such as davidite-brannerite-absite, and the euxenite-fergusonite-samarskite group.
Secondary uranium minerals such as torbernite and autunite have brilliant yellow or
green colours and are fluorescent under ultraviolet light.
The key to the formation of uranium deposits is the vastly different solubility of
this element in oxidized and reduced fluids. Uranium occurs in two valence states,
the reduced form U
4+
and the oxidised form U
6+
. The latter is highly soluble in
oxidised fluids where it forms stable complexes with fluoride, phosphate or carbon-
ate ligands; in this condition it is readily transported in the fluids that circulate in
sedimentary basins. The reduced form, in contrast, is highly insoluble, such that
when an oxidised fluid comes into contact with a reductant, the U precipitates.
The richest uranium ore bodies are the
unconformity-related deposits
in the
Athabasca Basin, in Saskatchewan, Canada. These deposits are not large, almost
always less than one million tons of ore, but their relatively small size is compen-
sated by high grade; Cigar Lake contains about 875 000 t of ore at an average grade
of 19% uranium oxide and McArthur River a slightly smaller amount at an average
grade of 24%. Similar deposits in the Northern Territories of Australia are larger but
have far lower grade, averaging 0.4%.
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