ORE GEOLO(,Y
RFAqEWS
ELSEVIER
Ore GeologyReviews 11 (1996) 33-51
Organic geochemistry of sedimentary uranium ore deposits
P. Landais
CNRS-CREGU BP 23, 54501 Vandoem,re Cedex. France
Received 15 January 1995; accepted 15 September 1995
Abstract
In many sedimentary uranium deposits, close relationships between uranium and organic matter can be observed. They
may be statistical, spatial or chemical. Such relationships entitle the organic matter to be considered as an accurate marker of
the history of uranium deposits. Geochemical analyses of organic matter in uranium deposits often allow the paleoenvironment, the thermal history as well as the different oxidation-reduction processes involved during the diagenesis of the ore to
be reconstructed. Routine techniques such as C - H - O analysis, Rock-Eval pyrolysis, gas chromatography (GC) or infrared
spectroscopy provide basic data that facilitate the characterization of organic matter. However, sophisticated analytical tools
(GC-mass spectrometry, microspectroscopic techniques, L3C nuclear magnetic resonance, kinetic modelling) may be
required in order to obtain more detailed information on the complex behaviour of organic matter associated with uranium
deposits. Data concerning various uranium deposits are presented with special emphasis on the determination of the origin
and the maturation of organic matter as well as the alteration processes involved in its diagenetic history.
1. Introduction
Ore-forming processes in sedimentary environments frequently involve organic material of different origins and chemical composition. Organic matter is a very sensitive marker of the paleoenvironment and diagenetic history as well as alteration
processes. Thus, it may provide useful information
that is not easily studied through other rock components. The development of analytical tools in the
field of organic geochemistry has contributed significantly to the understanding of metallogenic processes and especially of uranium ore formation.
Characterization of organic matter in uranium deposits has been undertaken using various analytical
techniques including infrared, ~3C nuclear magnetic
resonance, and UV fluorescence spectroscopic methods (Landais et al., 1984; Landais and Dereppe,
1985; Turner-Peterson et al., 1993), gas chromatography and pyrolysis G C - M S (Zumberge et al., 1978:
Dahl et al., 1988), and elemental analysis (Gize,
1993). Techniques such as high resolution transmission electron microscopy (Jehlicka and Rouzaud,
1993), carbon isotopic analysis (F~Srster, 1980, pyrolysis G C - M S , and micro-spectroscopic techniques
(Leventhal et al., 1986; Wang et al., 1989; Landais et
al., 1990; Rochdi et al., 1991) as well as the parallel
use of different analytical tools has allowed further
structural investigations to be carried out. The present paper presents the results of different ap-
0169-1368/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved
SSDl 0 169- 1368(95)00l)14-3
34
P. Landais / Ore Geology Reviews l l (1996) 33 51
proaches used for the characterization of the origin,
the maturity, as well as the diagenetic behavior of
organic matter associated with uranium ore deposits.
2. Relationships between uranium and organic
matter
Depending on the observation scale, three types
of relationships between uranium and organic matter
can be noticed: (1) spatial relationships: at the plurimetric (deposit), centimetric (rock sample) or micrometric (thin section) scales when the respective distributions of uranium and organic matter match; (2)
molecular relationships: chemical bonds are established between the functional groups of the organic
matter and uranium compounds; (3) statistical relationships: at the level of a group of samples, a
significant positive correlation coefficient is found
between the total organic carbon content and the
uranium content.
It is very rare to find all these three types of
relationships in the same uranium deposit. For example, in deposits where uranium distribution is controlled by organic matter, barren organic-rich facies
or mineralization devoid of organic material can be
also observed. Such complex relationships are determined by several geochemical and geological factors: (1) type and distribution of the organic matter;
(2) chemistry and migration paths of the plumbing
system of the uranium-carrying solutions; (3) type of
chemical reactions; (4) porosity and nature of fracturating of the host-rock; (5) nature of the uraniferous solutions; (6) reducing properties of the host-rock
and (7) maturity of the organic matter and diagenesis
of the host sediment.
At the sample scale (2-20 cm), the uraniumorganic matter relationships are highly variable from
one deposit to another. They are controlled by the
host-rock lithology and the nature of the organic
material. When the organic matter is dispersed in
black shale, it is possible to observe a uniform
distribution of the uranium inside the organo-mineral
matrix. In this case, the uranium content generally
remains below 1% and has frequently been interpreted as a syngenetic pre-enrichment (Herbosch,
1975). Local remobilization into coarser units or
even inside the black shale are also observed.
Type III organic matter, which is of continental
origin and concentrated in organic debris such as
coal or tree trunks, can display higher uranium enrichment (up to 10%). Uranium minerals compete
with other epigenetic minerals such as sulfides, carbonates or silicates for filling of cell lumens. Migrated organic material is also known to be associated with uranium in various sedimentary environments (Landais, 1993). High uranium concentrations
( > 20%) have been noticed in solid bitumens derived from fluid hydrocarbons. In this case, barren
and mineralized bitumens can coexist in the same
deposit; thus, the occurrence of U and C do not
closely correlate. On the other hand, epigenetic humic compounds can totally control the distribution of
uranium in sandstone deposits (Grants Mineral Belt)
where the uranium concentrations are strictly limited
to the organic-impregnated sandstones (Turner-Peterson et al., 1993). At the thin section scale, similar
variations in uranium distribution are observed: uniform distribution, preferential concentration in fractures, accumulation in coarser facies, localization at
the mineral-organic interface.
From a chemical standpoint, two main processes
can be identified: complexation and reduction. Complexation by ionic exchange is probably the most
frequently observed. The carboxyl functional groups
of humic acids, coals, and kerogens are responsible
for the complexation of uranium by organic matter
(Munier-Lamy et al., 1986). The chemical reactions
describing this phenomenon involve the uranyl cation
and a dehydrogenation of the organic matter:
2R-COOH + UO 2+ --0 RCOO-(UO 2)-OOCR
+ 2H +
It has been demonstrated that the humic acid content
of type III coals and fossil plant debris caused the
fixation of uranium on organic matter from very
dilute solutions of uranium (even at the ppb level).
This fixation is generally a reversible cation-exchange process with a geochemical enrichment factor of about 10,000:1 (Szalay, 1964). However, Disnar and Sureau (1990) concluded that complexation
by organic ligands can play a limited role in the
concentration of uranium and that other phenomena
should be taken into account in order to explain the
higher uranium contents of organic-rich facies.
P. Landais / Ore Geology Reviews 11 (1996) 33-51
Reduction of U 6+ to U 4+ has been experimentally studied by Andreyev and Chumachenko (1964)
and Nakashima et al. (1984). They have demonstrated that the following chemical reaction describes
the oxidation of organic matter during the reduction
of uranium:
2(RH) + UO~ + ~ 2R ° + 2H++ UO 2
Other mechanisms have been suggested (Forbes et
al., 1988) but all of them lead to either a dehydrogenation or an oxidation of the organic material. The
final product and the nature of the chemical reactions
depend on the initial composition of organic matter
involved in the reduction process.
Finally, both complexation and reduction processes can be involved in the concentration mechanisms of uranium. Their respective efficiency and the
reaction rates depend on the chemistry of the organic
material (amount of available complexing sites) and
on the temperature of reaction (stability of the carboxylate complexes). Meunier et al. (1990) have
clearly shown that during thermal treatment, natural
urano-organic complexes can be destroyed and that
U species can be reduced to U 4+ oxides.
Although spatial and molecular relationships have
been clearly demonstrated in numerous uranium deposits, few of them allow the calculation of a significant U-organic carbon correlation coefficient. In
sandstone-type deposits, typical correlation coefficients range between r = 0.01 (Cottonwood, USA;
Meunier, 1984) and r = 0 . 2 5 (Cerilly, France;
Landais and Connan, 1980). Similar values have
been found by Breger (1974) in several sandstonetype uranium deposits. However, when the calculations are carried out on sets of samples exhibiting U
contents higher than 500 ppm, r is systematically
higher ( r = 0.7, Cerilly, France). There are very few
deposits where correlation coefficients are higher
than 0.7. Meunier et al. (1989) found r values
ranging between 0.75 and 0.94 for the Coutras uranium deposits (France) and Leventhal (1980) determined an r ~ I for the Grants region uranium deposits where the mineralization is strictly controlled
by humic substances. As shown by Pironon (1986),
the statistical relationships between U and organic
matter generally depend on the investigation scale
and on the distribution of both components in the
sedimentary rock. Furthermore, alteration of organic
35
matter related to the formation of the mineralization
may also induce a loss of carbon and, thus, a decrease of the U-organic carbon correlation.
As mentioned earlier, such variable relationships
between uranium and organic material can be partly
controlled by the chemistry and the distribution of
the organic matter in the host-rock. Two major factors determine the chemistry of organic matter: its
origin and its thermal maturity. In the following
paragraphs, results from several investigations on the
chemistry of organic matter associated with uranium
ores will be presented. Emphasis will be put on the
importance of such geochemical data for the reconstruction of the ore-formation history.
3. Analytical procedures
Rock-Eval pyrolysis is a routine technique designed for determining the amounts of free (S1) and
potential (HI) hydrocarbons as well as the quantity
of CO 2 (OI) released from organic matter during
pyrolysis (Espitali6 et al., 1986). S1, HI and OI are
respectively expressed in mg hydrocarbons and mg
CO 2 per g of total organic carbon. Maximum production of potential hydrocarbons occurs at a temperature called Tin,x generally related to the maturation stage of the sample.
13C cross polarization/magic angle spinning nuclear magnetic Resonance (CP/MAS NMR) data
were obtained on a Brucker CXP 100 spectrometer
at the Louvain-la-Neuve University (Belgium). Measurements were made according to the technique
previously described in Dereppe et al. (1983) and
NMR spectra were divided into 10 bands corresponding to oxygen bearing, aromatic, and saturated
carbons (Landais et al., 1988).
Pyrolysis gas chromatography-mass spectrometry
(Py-GC-MS) analyses were performed on approximately 3 mg of sample placed in a solids injector
syringe (SGE Pellitiser P-3) and transferred to the
pyrolysis micro-furnace (SGE Pyrojector II), coupled
directly to the injection port of the gas chromatograph. The furnace was maintained at a constant
temperature of 620°C. Split/splitless mode was used,
with splitting resumed 0.25 rain after injection. For
G C / M S analyses, an HP 5890 Series II Plus gas
chromatograph coupled with an HP 5972 Mass Se-
36
P. Landais / Ore Geology Ret'iews 11 (1996) 33-51
lective Detector was equipped with a JS~W Scientific 60 m DB-5MS column (0.25 mm i.d., film
thickness 0.10 /~m). The GC oven was operated
under the following program: isothermal for 5 min at
40°C; temperature ramped at 5°C/min to 300°C and
then held isothermal for 15 min. The mass spectrometer was operated in full scan mode (50-550 Da, 0.9
scans/s, 70 eV ionization voltage).
Micro-Raman analyses were performed on a multichannel X - Y Dilor spectrometer. The exciting radiation is the 514.5 nm line of an Ar + laser set at a
minimum power (below 5 mW) in order to minimize
alteration of organic matter. An Olympus microscope
fitted with 50X and 100X objectives was used for
observation and spectrum acquisition. Integration
time of 20 s and between 5 and 10 accumulations
were considered adequate for the recording of spectra. Spectral bands have been integrated with emphasis placed on the E2g2 band attributed to C - C
vibrations of aromatic carbons (located at 1575 c m for perfectly crystalline graphite) and the "1350
c m - ~'' band assigned to as a defect band (Tuinstra
and Koenig, 1971).
Micro-infrared spectra were recorded by a Bruker
IFS 88 Fourier transform spectrometer equipped with
a Bruker A-590 microscope. The microscope has its
own 100 /zm diameter narrow-band mercurycadmium-telluride detector (MCT) cooled at 77 K.
The available spectral range lies between 5600 and
600 cm-n with a 4-cm-~ spectral resolution. The
objective magnification (15X) provides an enlarged
image of the sample at the plane of a diaphragm
which restricts the beam between 20 and 80 /~m.
4. Nature and paleoenvironment of sedimentary
organic matter
Most of the uranium deposits classified as sandstone-type (Dahlkamp, 1980) contain organic material. Uranium is disseminated either in continental
fluvial arkosic sandstones or in organic-rich shales.
Depending on the environment of deposition, type I
(lacustrine), type II (marine) or type III (continental)
kerogens are observed. However, sandstone-type deposits from western United States, Europe, and Niger
are mostly associated with type III kerogens. Ore-
800 "
600
"
o
400 -
i
o
•
|n (Tertiary)
In (Jurassic)
In (Paleozoic)
lI (Paleozoic)
l & bitumen (Precambrian)
Jo °'.
200
"
mm
Ao
•
o
i
20
40
60
80
o
100
o
120
OI (mg/g TOC)
Fig. 1. Composition of organic matter associated with uranium
deposits in a Rock-Eval hydrogen index vs. oxygen index diagram. Type Ill: continental origin; type lI: marine origin; type I:
lacustrine origin.
bearing parts of the host-sandstone frequently contain plant debris as well as tree trunks.
Rapid determination of the quality of the organic
matter can be obtained through a routine Rock-Eval
pyrolysis. A plot of the Rock-Eval data (hydrogen
index vs. oxygen index, Fig. 1) obtained from the
pyrolysis of various coals and kerogens from uranium deposits shows that their petroleum potential
(HI in mg hydrocarbons per g of organic carbon) is
generally low and that the oxygen content is highly
variable. However, Landais et al. (1984) have determined that Cottonwood Wash coals (Utah, Jurassic)
associated with U - V tabular ore deposits have high
petroleum potential (HI > 400 m g / g TOC).
Type II kerogens hosted in tidal or marine sedimentary formations can also be spatially associated
with uranium mineralization. They have been recognized in European (Lod~ve, France; Randstadt, Sweden), African (Arlit-Akouta, Niger), Russian
(Siberia, USSR) and US (Chattanooga) deposits.
Most of the U concentrations associated with black
shales are believed to be syngenetic and range between tens and thousands of ppm. More interesting is
the oil potential (300 m g / g < 600 m g / g ) of several
black shales (Fig. 1) that can generate significant
amounts of liquid hydrocarbons during catagenesis.
After expulsion and secondary migration, these hydrocarbons can participate in the formation of miner-
P. Landais / Ore Geology Rer,iews I I (1996) 33-51
SATURATES
POLARS
50
AROMATICS
Fig. 2. Composition of the chloroform extracts in a triangular
diagram of saturated, aromatic and polar compounds from different uranium deposits.
alization in reservoir facies (Landais and Connan,
1986). Type II and type III kerogens can be easily
differentiated using a triangular diagram displaying
the composition of the chloroform extract in terms of
saturates, aromatics and polars (Fig. 2). Obviously,
type II kerogen extracts are enriched in hydrocarbons
whereas type III kerogen and coal extracts are mainly
composed of polar compounds.
The chemistry of the organic matter is very sensitive to the conditions of deposition and of early
diagenesis. Furthermore, as a function of its origin
(algal, planktonic or terrestrial), the organic matter
will exhibit very different characteristics. These variations in chemical composition can be used to identify the conditions of deposition of the host sediments and provide evidence for reworking or early
oxidation processes. As a function of the type of
organic matter, information concerning not only oil
potential and complexation ability (amount of carboxyl-type functional groups) of organic matter but
also the sedimentological conditions can be determined. More detailed data can be obtained on the
paleoenvironment and early diagenesis by analyzing
the chloroform-soluble and residual organic material
from uranium deposits. Two examples are presented
and show that such data can contribute to the reconstruction of the ore formation process.
interbedded with sandy units < 100 m thick can be
observed. The upper layer contains the main uranium
mineralization that is suspected to be syngenetic or
early-diagenetic. The deposition of this upper layer
was partly controlled by synsedimentary tectonics
that allowed the sedimentation to remain active during the Autunian. In this case, uplifted zones played
the role of detrital source for the subsiding areas.
Both shales contain detrital organic matter mostly
composed of lignitic debris. The chloroform extractyield (0.3% < O E / T O C < 3%) and the oil potential
(HI < 100 rag/g) are low. The vitrinite reflectance
averages 0.52. However, several differences can be
observed between the two layers. The upper layer
organic-matter is characterized by: (1) higher C 2 / C 1
and C3+/C1 ratios in the volatiles; (2) higher percentages of saturates in the chloroform extract (1030% vs. 2-10% in the lower layer, Fig. 3); (3)
higher Rock-Eval Tm~X; (4) different maceral composition characterized by dominant inertinite and the
presence of colloidal organic matter; and (5) specific
bimodal distribution of n-alkanes in the GC traces of
saturates associated with a pristane/phytane ratio
close to 1 (Pr/Ph > 5 in the lower level).
These geochemical and petrographic characteristics of the upper shale are associated with a clay
mineral association marked by the predominance of
authigenic kaolinite (kaolinite > 90%) while the
lower shale is rich in detrital illite (kaolinite < 40%)
(Fig. 4). The specific organic characteristics of the
upper layer have been related to the reworking of
already buried type III organic matter during the
synsedimentary tectonic activity. This reworking
100 "
In the Permian Cerilly basin (French Massif Central), two layers of type III organic matter-rich shale
• Wood
o
Lower level
•
Upper level
e~
10
%o
65
o~
,l
0
4.1. Reworking process. The del;elopment of inertinite
37
10
20
Saturates
30
40
(%)
Fig. 3. Differentiation of the two levels of organic matter in the
Permian Cerilly basin (France) in a saturate (%) vs.
aromatic/saturates diagram. Reference is made to woody debris
collected in the same basin (Wood).
P. Landais/ Ore Geology Reviews 11 (1996) 33-51
38
caused the development of inertinite and the formation of authigenic kaolinite. The peculiar chromatographic characteristics of the saturates fraction of the
kerogen in the upper shale can be interpreted as the
result of a bacterial input that overprints the distribution of the alkanes extracted from the detrital organic
fraction (lower pristane/phytane ratio). The C 20-C 30
alkane distribution which shows a slight even predominance can be attributed to the microbial synthesis of higher molecular weight alkanes during the
reworking and the degradation of the vitrinite-rich
organic matter into inertinite (Dembicki et al., 1975;
Tissot and Welte, 1984).
These organic geochemistry data reveal two different paleoenvironments for the two organic-rich
levels. The depositional environment of the upper
shale layer is composed of low drainage swampy
depressions in which detrital material originating
from granitic and older Autunian rocks accumulated.
The synsedimentary tectonics associated with a multiple accretion-migration process described by
Gruner (1956) and Webb (1969) may have played a
major role in the formation of the U mineralization
of the upper shale (Landais and Connan, 1980).
4.2. Biomarkers
The geochemical characterization of the extractable hydrocarbons can also be used to differentiate between several types of paleoenvironments or
organic matter. Provided that the maturity of the
organic matter is determined, simple biological
markers such as pristane and phytane are able to
discriminate between two types of organic matter. In
the Lod~ve basin (Permian, France) where the major
100
•~
75
"''~Z!Z!~iZi
[]
•
[]
[]
50
Chlorite
[
Mixed-Layer
Illite
[
Kaolinite
0,6 ©
•
O
0,5"
•Q
0
0,4"
di~ 0,3"
0
0
0
0
|•
O•
•
• MIXEDTYPE ]
SAPROPELIC
0,2"
@•
0,1
0,2
0,3
(3
0,4
0,5
0,6
0,7
0,8
0,9
Pr/nC17
Fig. 5. Plot of the pristane/nCl7 vs. phytane/nCl8 ratios for the
organic matter from the Permian shales of the Lod~ve basin
(France). The two different trends are related to two different
types of kerogen(mixedtype and sapropelic).
U-mineralization is associated with migrated bitumens, types I, II and III kerogens can be recognized.
Mutual occurrence of these types is associated with
tidal sedimentation which allows marine, continental,
and tidal environments to alternate. Pr/nC17 and
P h / n C 18 ratios facilitate the discrimination between
mixed-type kerogen and sapropelic-type kerogen
(Fig. 5). Palynological analyses as well as other
geochemical data (petroleum potential, amount and
type of extractable components) confirm the results
based on the pristane and phytane ratios. Chromatograms of total alkanes show a predominance of
low molecular weight n-alkanes (C16-C24 range),
their distribution maximum being located at C19C20. On the basis of these gas chromatographic
characteristics, it seems likely that the kerogen of the
corresponding sediments is mostly of type II organic
matter. The distribution of both steranes (m/z = 217)
and terpanes (m/z = 191), confirms this hypothesis.
Much more simply, the reducing properties of the
environment of deposition can be related to the total
organic carbon content. Pironon (1986) has shown
that in the St. Hyppolyte basin (Vosges, France), the
total organic carbon content variations are correlated
with the extract yield, P r / P h ratio and distribution of
n-alkanes.
25
o
U~/"
UPPER LEVEL
LOWER LEVEL
Fig. 4. Composition of the clay fraction of the two stratigraphic
levels of organic matter in the Permian Cerilly basin (France)
determined by X-ray diffraction.
5. Source and origin of bitumens associated with
uranium ores
Examples of migrated organic matter-mineralization relationships have also been widely observed in
P. Landais / Ore Geology Reviews 11 (1996) 33-51
uranium ore deposits (Hawley et al., 1965; Haji-Vassiliou and Kerr, 1972; Upenskii et al., 1973; Landais
and Connan, 1986; Rouzaud et al., 1981; Bonnamy
et al., 1982; Curiale et al., 1983; Landais and
Dereppe, 1985; Cortial et al., 1990; Landais et al.,
1987 Landais et al., 1988, 1990; Parnell, 1988;
Eakin,
1989). H o w e v e r , the s o u r c e - r o c k
maturation/hydrocarbon genesis and migration concept is less frequently applied to uranium deposits
than to lead-zinc deposits. This is probably due to
the fact that organics related to many economic
uranium ores are generally of humic origin (Breger,
1974).
Bitumens are generally viscous or solid, brown to
black and brittle. When mineralized, they are scarcely
soluble in normal solvents, dehydrogenated, sometimes oxidized, and display high aromaticities and
low pyrolysis yields. The uranium content of the
bitumens can range between a few ppm to 30%. The
origin of the bitumens associated with uranium ores
is not always easy to determine, because their original characteristics have been drastically modified
either by radiolytic alteration a n d / o r by the combined effects of time and temperature. This is probably why the origin of several well known occurrences of bitumens in Precambrian U deposits such
as Oklo (Gabon), Northern Saskatchewan (Canada)
or Witwatersrand (South Africa) is still controversial.
Finally the most debated case history concerning
migrated organic matter associated with U deposits
is probably that of the Grants Mineral Belt mineralization (New Mexico, USA) hosted by the Westwater Canyon Member of the Upper Jurassic Morrison
Formation. The amorphous organic matter, which
generally fills interstitial pore spaces in the sandstones and includes most of the uranium, is slightly
hydrogenated (0.4 < 0.7), more or less oxygenated
(0.1 < 0.25), and totally insoluble in the usual solvents. The most significant peaks obtained in infrared spectroscopy are representative of type III
organic matter, i.e., -OH, -COOH and aromatic
C = C groups (Squyres, 1980). Pyrolysis GC traces
provide a hump of poorly resolved peaks (Leventhal,
1980). Comparing the kerogen ~3C NMR spectra of
the " K shale" (located immediately over the Westwater Canyon Member) and amorphous organic matter, Turner-Peterson et al. (1986) noticed a similarity
of spectra showing a well developed aromatic band
39
with aromatic C/aliphatic C ratios higher than 3.
They suggested that this organic matter could have
been formed by oxidation and radiolysis of epigenetic humic acids generated from organic carbon
derived from the shale (Turner-Peterson et al., 1993).
The "petroleum hypothesis" proposed by Birdseye
(1977) regarding a hydrocarbon genesis for the organics in the Lower Jurassic Todilto Limestone and
their subsequent migration into the Westwater
Canyon sandstones is less convincing and is not
generally considered.
5.1. Bitumen-source rock correlation in the Lodkce
basin (France)
In several deposits, the occurrence of both oil
a n d / o r bitumen together with a suspected sourcerock facilitates the investigation of the origin of the
migrated organic material. The Autunian shales at
Lod~ve are carbon rich with total organic carbon
contents ranging from 1% to 5%. The kerogen is
mostly type II; some samples contain more than 80%
sapropelic organic matter. Rock-Eval pyrolysis data
confirm the good quality of the kerogen, showing
high hydrogen indices (HI > 500 mg/g). Sorbed
gaseous hydrocarbons are relatively wet and chloroform extract compositions are characteristic of good
oil source-rocks (35 to 50% of saturates). The mean
vitrinite reflectance values range between 0.65 and
0.85% and correspond in terms of maturity to the oil
formation zone.
Oil occurs in the Autunian reservoir facies as well
as in the Autunian conglomerate and the Cambrian
karstified basement. Geochemical results have established a correlation between the Autunian shale extracts and the oils. When the soluble organic matter
yield is high enough, correlations can be based on
the chromatographic fingerprints of the hydrocarbons. Total alkane chromatograms from a source
rock, an impregnated sandstone, and an unmineralized bitumen collected in the same layer show similar chromatographic fingerprints (Landais, 1993).
Furthermore, on the basis of the sterane and terpane
fingerprints, the oil to source rock correlation can be
confirmed; the C29ceaS to C 2 9 a a R sterane ratio
remains close to one and terpane and sterane distributions in both oil and source rock extract are similar despite mild biodegradation of oil (Landais and
Connan, 1986).
40
PI Landais / Ore Geology Ret,iews I I (/996) 33-51
500
5.2. Precambrian bitumens in Oklo, Northern
Saskatchewan and Witwatersrand uranium deposits
L9
In the unconformity-type deposits ~from Northern
Saskatchewan, the U mineralization occurs either in
the Athabasca sandstones or in the basement graphitic
gneisses. It is controlled by mylonite zones and by
the alteration processes taking place at the unconformity. The major deposits are dated between 1150
and 1050 Ma (Bell, 1985). The Oklo (Gabon) mineralization occurs in the clastic Lower Proterozoic
Francevillian series and is marked by the presence of
natural fission reactors. Ages for the major U deposits range between 1.8 and 2.1 Ga (Nagy, 1993).
The Witwatersrand uranium-gold deposits (South
Africa) are associated with carbon-rich levels interbedded with conglomerates and quartzite strata.
Ages for the main mineralization events as well as
the origin of the different organic matter occurrences
are still controversial.
Bitumens associated with these Precambrian uranium deposits are generally highly aromatic and
show ~3C nuclear magnetic resonance aromaticity
factors ranging between 60 and 90%. Because these
organics are totally insoluble in the usual solvents, it
is impossible to derive oil to source-rock correlations
based on the analysis of the hydrocarbon fraction.
Thus, the characterization of the different bitumen
occurrences can only be carried out by analytical
techniques that allow the solid phase to be investigated. In the present study, ~3C nuclear magnetic
resonance, infrared spectroscopy as well as pyrolysis
techniques (Rock-Eval and pyrolysis gas chromatography-mass spectrometry) were combined in
0,9"
0,8-.
•
~ •
0,7-
..
~1
o
•
0,60,5
0,4
0,3
-
0,2
0,0
• Cigar-Lake
•
•
A g~S~,
~e,
O• •
O
Cluff-Lake
,9
•
Oklo
t~
Witwatersrand
•
me el
•
•
•
•
•
,
,
,
0,1
0,2
0,3
O/C
at.
Fig. 6. Elemental composition of organic matter from different
Precambrian uranium deposits plotted on an atomic H / C vs.
atomic O / C diagram.
[~ Saskatchewan ]
490
Witwatersrand
480
~A
tX
470
A
460
A
E
[,,,
450
4 o
A
•
440
e•
430
4
I
I
20
40
60
HI
80
(mg/g
100
120
140
TOC)
Fig. 7. Rock-Eval characteristics of the organic matter from the
Saskatchawan (Cluff Lake and Cigar Lake) and Witwatersrand
deposits. HI: hydrogen index.
order to obtain information on the structure of these
organics. Furthermore, bitumens may occur as micron- to millimetre-size blebs that do not allow
conventional techniques to be used. Thus, in situ
microspectroscopic techniques, such as Raman and
infrared, are needed to provide additional information.
C, H, O analysis of Precambrian bitumens plotted
on an atomic H / C vs. atomic O / C diagram (Fig. 6)
reveals that, despite their high aromaticity, significant hydrogen and oxygen contents are still present.
Higher O / C ratios are generally related to an oxidation process associated with the formation of the
uranium mineralization (Cortial et al., 1990). RockEval data indicate that the petroleum potential of the
bitumens (HI) decreases with the increasing Tm~,×
(Fig. 7). Such an evolution may be interpreted as a
"thermal maturation" effect. However, the scattering of the data within a single deposit suggests that
other alteration processes (radiolysis or oxidation)
are responsible for the observed variations.
When analyzed by Py-GC-MS, the Precambrian
bitumens display spectra that are dominated by low
molecular weight aromatics (alkyl-benzenes) (Fig. 8,
Table 1). Indenes, naphtalenes and phenanthrenes are
subordinate. As already pointed out by Nagy (1993),
such chromatographic fingerprints do not allow either the origin of the oil or an oil to source-rock
correlation to be established. Most of the variations
recorded can be related to the uranium content of the
bitumen and to the radiolytic alteration that resulted
in a decrease of the pyrolyzate yield and a modification of the substitution pattern of the aromatic rings.
P. Landais / Ore Geology Reviews 11 (1996) 33-51
41
SO2
+4,A4
B1
OKLO
B2
+5, A5 03
B3
i AA, 04
NI
iji
/
4#1
I1
NO
IB~!,+12,
m
N2
A0
N3
4
9
14
19
24
29
34
qq
FI0
H1
39
44
49
39
44
49
/i +4.A4
WITWATERSRAND.
BI
i
B2
! AA, 04
B3
il,
!
'1
! B0
'~
B4
NO
i01
+1, l
,b,I
4
9
14
19
N1
+r.
!i,, ~
24
N1
L
N2
i
29
34
Fig. 8. Typical pyrolysis-GC-MS total ion current chromatogram of two organic-rich samples from Oklo and Witwatersrand uranium
deposits. Identification of the main peaks are given in Table 1.
Py-GC-MS results can also be cross-checked with
spectroscopic data in order to discriminate different
bitumen generations. In Fig. 9, the alkylation index
of the benzene rings vs. the absorbance of the
aliphatic CH infrared band is plotted for several
Oklo bitumens. A good correlation is obtained, suggesting that most of the aliphatic groups are substituted on aromatic rings. Furthermore, because significant variations of chemical composition cannot be
explained either by the uranium content, by the
Table t
Pyrolysis-GC/MS peak identification for Fig. 8
Symbol
Compound class
Ions used in quantitation ( m / z )
Bn
Kn
On
qb n
1n
Nn
n
An
Om
Am
+m
AA
S°
Benzenes
Cyclopentadienes
Thiophenes
Phenols
Indenes
Naphthalenes
Phenanthrenes
Anthracenes
Alkanones
n-Alkenes
n-Alkanes
Acetic acid
Elemental sulphur
78(BO),92(B 1),106(B2),120(B3)
66(K0),80 (K1)
84(O 0),98(O 1), 112(O 2)
94( q~0), 108(q~ 1)
116(I0), 130(I 1)
128(N0),I42(N 1),156(N2)
178(~ 0), 192(~ 1)
178(A0), 192(A l )
58(03),72(04)
55
57
60
(not quantitated)
" n " indicates the extent of alkyl substitution (0 = none, 1 = methyl, 2 = dimethyl or ethyl etc.). " m " indicates carbon number.
42
P. Landais / Ore Geology Reviews l 1 (1996) 33-51
0.8"
by radiation effects for example - - or to analyze
small spots of organic matter that cannot be extracted from the host-rock. Such techniques were
applied to the characterization of the organic material in U deposits from the Witwatersrand province
(South Africa). The origin of Witwatersrand kerogen
is controversial and several hypotheses have been
presented (Button and Adams, 1981): the polymerization of light hydrocarbons around uraninite grains
(Liebenberg, 1955; Schidlowski, 1981), accumulations of algae similar to B o t r y o c o c c u s (Snyman,
1965), prokaryote precursors (Mossman and Dyer,
1985), or biochemical products derived from microorganisms (Zumberge et al., 1978). Furthermore, late
carbon seams and carbon granules known as "flyspeck" (Hallbauer, 1975) that occur in the conglomerates and in the surrounding granites, can also be
observed.
Typical Raman spectra of the Witwatersrand organic material exhibit the C - C vibration band (E2g2)
0.6"
¢0 0.4"
I
'~
!
0,2"
I
J
1.2
0.0
1,0
,
1.4
i
1.6
,
1.8
Alkylation index
Fig. 9. Correlation between the aliphatic CH band intensity deduced from infrared spectra and the alkylation index (sum of Co
to C3-alkylbenzenes/benzene+toluene) calculated from Py-GCMS traces for different Oklo bitumens.
vicinity of the fission reactors or by thermal maturity
variations, the occurrence of different generations of
bitumens is suggested.
Microspectroscopic investigations may be required to select homogeneous areas - - not perturbed
0.10
~SEAM
0.05 -
~
_
=
IF\, '
0.00-
-0,05 -
0,15
b
s
o
--
FLY-SPECK
0.10
#
b
a
n
c
'°°' l
000
"
:
l
-0,05
A
RANITENODULE
0.10
o05~ ~
~
~
0.00 i
~' 111
005 1
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
Wavenumbers (cm-1)
Fig. 10. Micro-infrared spectra obtained on three different occurrences of organic matter in the Witwatersrand region.
800
P. Landais/ Ore Geology Reviews 11 (1996) 33-51
as well as the band attributed to defects in the carbon
structure (1350 cm-1). Schift of the E2g2 band to
higher wave numbers (1605-1610 cm -1 ) and broadening of the two bands are characteristic features of
Raman spectra of poorly organized organic matter
(Friedel and Carlson, 1972; Oh, 1987). Landais et al.
(1990) showed that spectral characteristics of "fly
speck" carbon were roughly similar to those of the
carbon seams and that a broadening of the E2g2
band was observed with increasing uranium content.
Besides this, carbon nodules in granite exhibit quite
different Raman characteristics including higher
1600/1350 band ratios and a broader 1350 cm - 1
band. Micro-infrared spectra of the three types of
organic matter are displayed in Fig. 10. While "flyspeck" and carbon seam spectra are very similar, the
granite nodule spectrum is characterized by higher
aliphatic CH absorbance and aliphatic CH/aromatic
C = C ratio as well as a shift of the aromatic C=C
band towards higher wave numbers (1603 vs. 1597
cm- ~). This suggests that the structure of the organic
matter in the granite nodules is less aromatic and less
condensed. However, comparison of aromatic CH
bands in the 700-900 cm-~ range is not possible
because mineral bands perturb this spectral region.
Such spectroscopic data together with C, H, O analyses may indicate that "fly-speck" carbon and carbon
seams originate from the same material (Robb et al.,
1994). Granite nodules probably originate from the
secondary migration of lower molecular weight hydrocarbons as suggested by their lighter carbon isotopic composition (-36.3%o) compared to that reported for carbon seams ( - 25.3%o to -34.0%o) by
F6rster (1986).
Carbon isotopic compositions were also used to
propose a genetic pathway for the bitumens associated with the Precambrian unconformity-type uranium deposits from Northern Saskatchewan
(Canada). A possible genetic link between the
graphite from the metamorphic basement and the
bitumens has been suggested (Hoeve and Sibbald,
1978; Landais and Dereppe, 1985). This hypothesis
was first rejected on the basis of isotopic evidence.
The carbon isotopic composition of the basement
graphite ( - 23.5%c > 6 r3C > - 29.6%0) is very different from the isotopic composition of the uranium
rich bitumens ( - 4 3 % o > 6J3C > - 4 4 . 7 % o )
(Leventhal et al., 1987). However, Landais et al.
43
(1993) showed that isotopic composition of barren
bitumens (U < 500 ppm) is close to that of the
basement graphite ( - 27.3%o > 6 t3C > - 30.2%0).
Such variations in carbon isotopic composition between barren and mineralized bitumens cannot be
explained by radiolytic phenomena which cause a
preferential concentration of the ~3C isotope in the
solid residues (Leventhal and Threlkeld, 1978). ~3C
NMR and Py-GC-MS data provide evidence for
structural differences between barren and mineralized bitumens; i.e., aromatic tings in barren bitumens
are more often substituted by shorter alkyl chains
than in mineralized bitumens (Landais et al., 1993).
Evidence for progressive lattice disorder of
graphite along a basement profile and the presence
of amorphous carbon in pit structures inside graphite
flakes close to the unconformity (Wang et al., 1989)
suggest a genetic relationship between graphite and
bitumens. Barren ~3C enriched bitumens could have
formed directly from hydrogenation of amorphous
carbon, whereas, mineralized, isotopically light bitumens (613C = -48%~) can result from the polymerization of light hydrocarbons around uraninite grains.
Such examples illustrate that the very condensed
structure of Precambrian bitumens associated with
uranium ores can only be investigated through the
combined application of analytical tools. However,
more information is still needed in order to better
constrain the conditions of formation of these bitumens, their diagenetic behavior, and their possible
role in the mineralizing processes.
6. Thermal maturation of organic matter associated with uranium deposits
When temperature increases during the burial of
sediments, the organic matter experiences major
modifications that include: loss of functional groups,
oil and gas genesis, and aromatization and condensation of the solid residue. These different chemical
transformations modify the reactivity of the organic
matter and its subsequent response to interactions
with uranium or uranium-bearing solutions. Besides
this, organic matter is a very sensitive marker of the
temperature regimes in sedimentary basins and can
efficiently be used to help reconstruct the geological
history of uranium deposits.
P. Landais / Ore Geology Retdews 11 (1996) 33 -51
44
800"
I
o in (Tertiary)
• lll~Iurassic)
• I I l (Paleozoic)
]
• U(Paleozoic)
[] (&bitumen (Precambrian
I
600
o
b. 400
",o'.
•
200
i I
0
400
,
420
,
,
440
460
T m a x (°C)
480
500
Fig. 11. C o m p o s i t i o n o f o r g a n i c m a t t e r a s s o c i a t e d with u r a n i u m
deposits plotted on a R o c k - E v a l h y d r o g e n i n d e x vs. Tmax diag r a m . T y p e III: c o n t i n e n t a l origin; t y p e II: m a r i n e origin; t y p e I:
lacustrine origin.
Routine techniques such as vitrinite reflectance
determination, Rock-Eval pyrolysis, and infrared
spectroscopy provide basic data that allow thermal
maturity to be roughly estimated. Plotting the
Rock-Eval data on an HI vs. Tmax diagram (Fig. 11)
for organic material originating from different uranium deposits shows that Tmax generally ranges between 420 and 450°C for type III kerogen and coals,
thus indicating a maturation stage corresponding to
the end of the diagenetic stage and the beginning of
the catagenetic stage. For this type of material, HI
generally remains below 100 m g / g but, as stated
earlier, type II kerogens as well as some coals
10000
IMPREGNATIO
ARIZONA
~¢~ 1000
URCE
ARLIT
°f
TAH
z
lOO
10
.........
,1
i
........
i
........
I
10
100
O R G A N I C C A R B O N (%)
Fig. 12. E v a l u a t i o n o f the p e t r o l e u m potential o f s e d i m e n t a r y
o r g a n i c m a t t e r a s s o c i a t e d with different u r a n i u m deposits in a
h y d r o c a r b o n s (pprn) vs. total o r g a n i c c a r b o n d i a g r a m . O n l y the
A r i z o n a , L o d ~ v e a n d Arlit deposits contain g o o d s o u r c e - r o c k s for
oil.
(Landais et al., 1984) exhibit much higher petroleum
potentials (HI > 400 m g / g ) (Fig. 11).
The maturity or rank of organic matter may be
important when considering oil genesis. In Fig. 12
the source-rock evaluation of various uranium deposits is reported. It appears that most of the organic
material is classified as type III kerogen (Cerilly,
Vosges, and US coals) and displays low to moderate
petroleum potential either because of the low quality
of the kerogen or because of the thermal history of
the host sediments. Only the Arlit (Niger), Arizona
(USA), and Lod~ve (France) deposits contain good
source-rocks for oil. Similarly, the pristane/nCl7
and phytane/nC 18 ratios, that decrease with increasing maturation, are highly variable depending on the
maturity of the organic matter. Pristane/nC17 ratios
range between 4-5 for low maturity kerogens
(Cerilly, Utah) and 0.4-0.5 for kerogens that have
reached the oil genesis stage (Lod~ve, Arlit, Arizona
pipes). Lower values (0.1-0.3) can be found in
higher maturity kerogens from Carboniferous deposits in the Vosges Mountains in France. Other
molecular parameters based on biomarkers or on the
distribution of the different isomers of aromatic hydrocarbons (methyl-phenanthrene indexes, Garrigues
et al., 1990) may also be calculated in order to
provide more accurate information. In the Lodbve
basin, the sterane distribution suggests that the Autunian shales are mature (oil-generating zone) (Landais
and Connan, 1986).
Organic geochemical parameters may also be
cross-checked with geological data or other geothermometers to evaluate the timing of oil genesis. This
may be achieved by using simple time-temperature
normographs established by Connan (1974) and by
Lopatin (1976). The different scenarios, corresponding to the geological histories of the source-rocks
from two different uranium deposits where the mineralization is spatially associated with migrated oils
and bitumens (Lod~ve and the Arizona breccia pipes),
are reported on a time-temperature diagram in Fig.
13 (Connan, 1974). This diagram allows the timing
as well as the average temperature of oil genesis to
be estimated. Further cross-checking of these estimates with the average homogenization temperatures
derived from the analysis of aqueous fluid inclusions
associated with oil-beating fluid inclusions (80-90°C
for the Arizona breccia pipes and 100-110°C for
45
P. Landais / Ore Geology Reviews 11 (1996) 33-51
275"
250" ~ ~
.-:. 225"
200"
~ '~,~
175"
150" Arizo
125"
75
20
/
~
/
date for the threshold of oil genesis to be fixed at
120 __+10 Ma which corresponds to a maximum age
for the migration of the hydrocarbons in the pipe
structures (Landais, 1993). This date can be related
to the isotopic ages for the Orphan Mine breccia pipe
mineralization, based upon U - P b systematics, that
average 100 to 120 Ma (Miller and Kulp, 1963).
OIL Z O N ~
/
40 60
80 100 120
TEMPERATURE (°C)
140
Fig. 13. Burial histories of two uranium deposits plotted on a
time-temperature diagram. Reference is made to the oil generation zone and the statistical time-temperature relationship for oil
generation determined by Connan (1974).
Lodbve) confirms the temperature range derived from
Fig. 13. For the Lodbve basin, maximum oil generation occurred between 200 and 160 Ma ago depending on the selected thermal history (Fig. 13). These
results suggest that the migration of oil occurs synchronously or earlier than the two major mineralization stages dated by P b / P b and U / P b techniques at
175 Ma and 100-110 Ma, respectively (Lancelot et
al., 1984; Vella, 1989).
Additional information can be obtained when
modelling the thermal maturation of organic matter
by using adapted computer models. Meyer et al.
(1989) developed a multidisciplinary approach for
assessing the thermal history of the Permian
Toroweap Formation associated with uranium deposits in the Arizona breccia pipes (Wenrich, 1986).
The time-temperature profile for the Permian
Toroweap Formation was reconstructed from average minimum trapping temperatures of fluid inclusions (Gomitz and Kerr, 1970; Landais, 1986) and
with fission track analyses. This profile was introduced in the computer model in order to calculate
the organic matter maturation state in terms of
Rock-Eval Tmax and transformation ratio (amount of
generated hydrocarbons versus potential hydrocarbons). Computed results yield averages of 430-445°C
and 0.75-0.85, respectively and correspond to a
slightly more mature kerogen than the Toroweap
kerogens (424°C < Tmax < 432°C). Thus, a slightly
different computed time-temperature profile was selected as a best fit for observed organic matter
parameters. Additional computer calculations allow a
7. Alteration phenomena
Alteration phenomena have frequently been recognized in uranium deposits. They can be related to
different interactions between organic matter and
meteoric waters or bacteria as well as radiolytic
processes. Most of the organic matter closely associated with uranium mineralization displays geochemical characteristics which are very different from
those expected from their origin and thermal history.
Oxidation, biodegradation, radiolysis, and thermal
degradation have been recognized as the main alteration processes of organic matter occurring in uranium deposits (Landais et al., 1987). When an adequate sample set is available, it may be possible to
ascertain, in the environment of a uranium deposit,
the effects of various alteration processes. For example, in the Akouta deposit (Niger), the combination
of different geochemical investigations allowed 3
types of degradation phenomena to be recognized
(Fig. 14): (1) a burial-independent thermal effect
associated with a major fault in the vicinity and
70.
~
~
~
,~
~
E~ Unaltered
O Unaltered (ore)
• Oxidized
• Mineralized
50"
40"
302010430 440 450 460 470 480 490 500
T m a x (°C)
Fig. 14. Rock-Eval TmaX vs. oxygen index diagram showing
different types of alteration processes that occurred in the Akouta
uranium deposit (Niger). (1) Thermal maturation; (2) oxidation;
(3) mineralization + radiolysis.
46
P. Landais/ Ore Geology ReL,iews 11 (1996) 33 51
probably the circulation of hot fluids; (2) a slow-rate
diagenetic oxidation; and (3) a combined effect of
both mineralization and radiolytic processes (Forbes
et al., 1988).
Oxidation is probably the most common alteration
process occurring in uranium deposits (Granger and
Warren, 1979). It can be related to weathering,
leaching by meteoric oxygen-rich waters, bacterial
alteration, reduction of oxidized species, metal fixation or even radiolysis. Oxidation of organic matter
may also indicate the involvement of the organic
material in maintaining a reducing environment as
well as participating in the formation and the preservation of the mineralization.
Supergene oxidation of type III organic matter
can be responsible for the formation of very compact, humic-like macromolecules that are able to fix,
complex, and transport uranium and, thus, promote
the concentration of uranium (Bach, 1980). Late
oxidation may also be responsible for the preferential
alteration of organic matter associated with sandstones and conglomerates relative to silts and shales.
In the Cerilly basin (France), a significant increase in
vitrinite reflectance (from 0.5% to 1.5%) was observed during the late oxidation of the organic material hosted in sandstones (Landais et al., 1987).
Diagenetic oxidation has also been observed in
uranium deposits. In this case, the effects of oxidation are combined with those associated with thermal
alteration due to burial. A typical case was observed
in the Cottonwood Wash coals of the Morrison
Formation (Utah, USA) by Landais et al. (1984).
Data derived from C, H, O analysis, infrared spectroscopy, Rock-Eval pyrolysis and ~3C NMR spectroscopy (Landais et al., 1988) showed that the geochemical evolution of the samples could not be
explained either by a thermal maturation process or a
late oxidation. Further comparison of the evolution
of the Cottonwood coals with an oxidation trend
obtained by the simulation of a late oxidation in a
ventilated oven confirmed this hypothesis (Landais
et al., 1984). The action of an oxidation-maturation
process was proposed to explain the chemical composition of the coals; the oxygen released during
thermal maturation was replaced by oxygen derived
from diagenetic, oxidizing solutions.
Oxidation also appears as a process resulting from
other forms of alteration. During biodegradation of
the oils associated with the Temple Mountain (Utah,
USA), Lod~ve (France) and Arizona breccia pipes
(Arizona, USA) ore deposits, a noticeable oxygen
increase is observed in the insoluble residues resulting from the oil degradation (Landais and Connan,
1986; Landais, 1993). This oxidation is closely associated with the progressive removal of the saturated
hydrocarbons and an increase in the aromatic carbon
concentration (Landais et al., 1988). Likewise, the
abnormally high organic oxygen content of the Precambrian bitumens of the Cluff (Saskatchewan,
Canada) and Oklo (Gabon) deposits is considered to
be a consequence of the diagenetic removal of
urano-organic complexes (Rouzaud et al., 1981).
Meunier et al. (1990) demonstrated that during the
artificial maturation of lignites (Coutras, France)
containing 6 to 15% complexed uranium, a rapid
dehydrogenation was observed along with the growth
of uraninite crystals. This hydrogen loss was interpreted as the result of uranium reduction to form
uraninite.
In uranium deposits, radiolytic degradation remains one of the major alteration processes affecting
the organic matter closely associated with the uranium ore. Various geochemical parameters exemplify the general consequences of the radiolytic alteration: (1) decreases of the solvent extract yield,
petroleum potential, H / C atomic ratio; (2) increases
of the Rock-Eval T,.ax, oxygen content, aromaticity,
reflectance; and (3) variations of the carbon isotopic
composition. Variations in the distribution of the
hydrocarbons generated during pyrolysis have also
been noticed when comparing mineralized and bar-
tTl
0,8"
z
F.l.l
>.
mm
0,7"
E.-,
~J
•
mmMkm
0,6-
mlm
0,5
I
I
I
I
I
I
2
4
6
8
10
12
URANIUM (%)
Fig. 15. Effects of radiolytic degradation on organic matter from
the Witwatersrand deposits as shown by a plot of the uranium
content vs. methyl/methylene ratio deduced from 13C nuclear
magnetic resonance spectra.
P. Landais / Ore Geology Reviews 11 (1996) 33-51
ren organic material from the same deposit (Leventhal
and Threlkeld, 1978; Zumberge et al., 1978; Sassen,
1984; Landais et al., 1987, 1988). Different analytical techniques can be combined in order to provide
more precise information on the effects of radiolysis.
A complete set of organic-rich samples from the
Witwatersrand deposits was studied by both solid
state ~3C NMR and Py-GC-MS. Parameters ob0.16 -
tained from both techniques ( 1 3 C NMR
methyl/methylene ratio and Py-GC-MS unsubstituted aromatics/substituted aromatics ratio) indicate
that the average length of the aliphatic substituents
decreases with increasing uranium content (Fig. 15).
However, whole rock analyses only give an averaged picture of the effects of radiolysis that may be
combined with the aspects of associated alteration
I
®
0.12
47
I
A
lar°mati~
-
C=C
m
0,08 -
0.04
-
aliphatic
CH
C=O
0.00 -
-0.04
~a
L)
Z
<
¢=
-
-0.08-0.12 m
0
0.16 -
®
<
0.12 -
0.08 -
O.04 -
0.00
-
-0.04
-0.08
-0.12
I
I
I
I
3000
2500
2000
1500
I
10(30
WAVENUMBERS
Fig. 16. Micro-infrared spectra recorded on the same bitumen sample from the Oklo deposit: (a) far from uranium minerals: (b) in the high
reflectance halo surrounding a uraninite inclusion.
P. Landais/ Ore Geology Reviews 11 (1996)33-51
48
processes (oxidation for example). In fact, radiolytic
degradation is generally limited to a 50-100 /zm
high reflectance halo surrounding uranium minerals.
Therefore, only microspectroscopic techniques that
allow pinpoint in situ analysis of the organic matter
can give an accurate description of radiolytic effects.
In an Oklo bitumen, two microinfrared spectra have
been recorded, one far (500 /zm) from any uranium
mineral inclusion and another in a high reflectance
halo around a uraninite grain. The two spectra are
reported in Fig. 16 and clearly show the effects of
the radiolytic degradation; major decrease of the
aliphatic CH band in the 3000-2800 c m - l spectral
region, relative increase of the aromatic C=C and
C = O bands, and decrease of the aromatic CH bands
in the 700-900 cm-I spectral region. These results
are comparable to those obtained by Rochdi et al.
(1991) when studying the radiolytic alteration in the
Witwatersrand carbon seams and indicate that radiolysis is not only responsible for a preferential consumption of aliphatic moieties (Colombo et al., 1964)
but also for possible oxidation of the organic matter.
Various types of organic-matter alteration
recorded in uranium-deposit environments can
strongly modify the structure and the functionality of
the organic matter. The chemical modifications resulting from these alteration processes can be conveniently summarized on an H / C vs. O / C diagram
2.t
(~ BIODEGRADATIONOF CRUDE OILS
DIAGENETIC OXIDATION
(~ LATE OXIDATION
2.
(~ EARLY ALTERATION
(~ BITUMEN POLYMERIZATION
ABNORMAL MATURATION
where dehydrogenation, oxidation and the combination of both mechanisms can be observed in a deposit (Fig. 17). Thus, when investigating the structure and the chemical composition of organic matter
in uranium deposits, it should be kept in mind that
the original fingerprint of the organic matter may
have been drastically modified by the combined
effects of different alteration processes. Consequently, the actual composition of the organic matter
may not accurately reflect either its origin, its rank,
or maturity.
8. Conclusion
Organic matter is a major constituent of many
sedimentary uranium deposits. The spatial and chemical relationships between mineralization and organic
matter are often considered proof of organic matter's
involvement in the mineralization process. The roles
of organic matter in uranium ore-forming processes
are various: transport, remobilization, reduction, concentration, and preservation. Organic geochemistry
techniques are able to provide general as well as
detailed information on the history and formation of
uranium ore deposits. Depositional environment, reworking, and thermal regimes may be accurately
assessed through the analysis of organic matter. Analytical techniques including molecular analysis by
GC-MS or Py-GC-MS as well as microspectroscopic investigations are frequently required to solve
the complex evolutionary paths of organic matter in
uranium deposits. Furthermore, it is frequently necessary to combine different approaches in order to
overcome problems arising from the superposition of
various alteration processes that occur during diagenesis and the mineralization processes.
.2
E
o
~ 1.o
Acknowledgements
e;
0.5
I
0.0
I
I
0.2
I
i
0.4
Atomic O/C
Fig. 17. A summary plot, in atomic O / C vs. atomic H / C space,
of the effects of the different alteration processes recorded by
organic matter from uranium deposits.
The author gratefully acknowledges the CREGU
for supporting the present study and M. Kruge and
L. Mansuy for the Py-GC-MS analyses. Reviews by
Drs. P. Hansley and B. Nagy and editing by Dr. T.H.
Giordano are appreciated. Part of the financial support has been provided by the EEC contract N ° FI
2W-CT90-0026.
P. Landais / Ore Geology Reviews 11 (1996) 33-51
References
Andreyev, P.F. and Chumachenko, A.P., 1964. Reduction of
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