190 Peter Stille and Graham Shields
The shale layers are also enriched in organic carbon, which can make up to 15%
of the whole rock weight. This series is also well known for the richness of
exploitable uranium deposits there, in particular Oklo where natural nuclear
reactions took place 2 Ga ago. On the basis of isotopic investigations (K-Ar, RbSr) we know that, as with the "Gunflint Banded Iron Formation", the last
hydrothermal events took place around 2.0 to 1.8 Ga ago (Bonhomme et al. 1982).
The sediments themselves must be older than 2.0 Ga. The most important detrital
mineral components that occur in the fossil-oil (i.e., solid bitumen )-rich black
shales are quartz and large, up to 500 lam biotite s, which are aligned parallel to the
primary sedimentary bedding and 'float' in a much finer rock matrix (Fig. 7.3A).
Silicate minerals such as chlorite and illite formed during diagenesis. They are
found in the matrix of the rock and reach vain-sizes of just 0.5 lam, i.e. 1000
times smaller than the large detrital biotites (Fig. 7.3B). Mineralogic
investigations show that these chlorites and illites derived from detrital Kfeldspars (microclines) (Fig. 7.3E).
Light- and electron-microscope investigations show that organic carbon plays
an important role in feldspar replacement and is also involved in the new
formation of chlorite and illite (Fig. 7.3D). The organic matter lies on top of the
crystal faces and in the surface cracks of the often strongly corroded and silicified
feldspars, thus mimicking the original microcline twinning crystal structure. The
influence of organic hydrocarbons on long-term diagenetic processes and on the
new formation of illites and chlorites will be discussed more fully later on. The
following discussion of the results of various Nd isotopic analyses will allow us to
recognize that the Nd isotopic system in these rocks provides a very. important
tool to the understanding of their genesis. Coupling of the Nd isotope data with
mineralogic data also permits us to understand chemical exchange processes
which have taken place between the silicate phases and oil.
Four black shales from various depths in one bore hole, and pure bitumen,
which migrated out of the clay shales and ended up in the joints and cracks of the
overlying sandstone horizon, were used in these isotopic investigations. Various
grain-size fractions: 2-0.8, 0.8-0.4, 0.4-0.2 and <0.2 ~tm were extracted and
concentrated from two of these shales. Concentrating these various clay fractions
made it possible to separate the diagenetically produced clay minerals with grain
size <0.5 p.m from the detrital clay minerals with grain sizes up to 500 ktm.
Leaching experiments were carried out using IN HCI acid on the smallest
fractions (<0.4 lam). Clauer et al. (1993) assumed that the leachates were
isotopically and geochemically identical to the fluid phase, with which the clay
minerals last experienced exchange. If the clay minerals crystallized within this
fluid medium then they would have existed in isotopic equilibrium with this fluid
phase. Leachates would thus produce leachate-residue internal isochrons that
define the age of crystallization. The leachate represents the diagenetic fluid
phase. Similar considerations can be brought to bear on the Sm-Nd isotopic
system. Interpretations may be problematic due to the possible existence of small
The shale layers are also enriched in organic carbon, which can make up to 15%
of the whole rock weight. This series is also well known for the richness of
exploitable uranium deposits there, in particular Oklo where natural nuclear
reactions took place 2 Ga ago. On the basis of isotopic investigations (K-Ar, RbSr) we know that, as with the "Gunflint Banded Iron Formation", the last
hydrothermal events took place around 2.0 to 1.8 Ga ago (Bonhomme et al. 1982).
The sediments themselves must be older than 2.0 Ga. The most important detrital
mineral components that occur in the fossil-oil (i.e., solid bitumen )-rich black
shales are quartz and large, up to 500 lam biotite s, which are aligned parallel to the
primary sedimentary bedding and 'float' in a much finer rock matrix (Fig. 7.3A).
Silicate minerals such as chlorite and illite formed during diagenesis. They are
found in the matrix of the rock and reach vain-sizes of just 0.5 lam, i.e. 1000
times smaller than the large detrital biotites (Fig. 7.3B). Mineralogic
investigations show that these chlorites and illites derived from detrital Kfeldspars (microclines) (Fig. 7.3E).
Light- and electron-microscope investigations show that organic carbon plays
an important role in feldspar replacement and is also involved in the new
formation of chlorite and illite (Fig. 7.3D). The organic matter lies on top of the
crystal faces and in the surface cracks of the often strongly corroded and silicified
feldspars, thus mimicking the original microcline twinning crystal structure. The
influence of organic hydrocarbons on long-term diagenetic processes and on the
new formation of illites and chlorites will be discussed more fully later on. The
following discussion of the results of various Nd isotopic analyses will allow us to
recognize that the Nd isotopic system in these rocks provides a very. important
tool to the understanding of their genesis. Coupling of the Nd isotope data with
mineralogic data also permits us to understand chemical exchange processes
which have taken place between the silicate phases and oil.
Four black shales from various depths in one bore hole, and pure bitumen,
which migrated out of the clay shales and ended up in the joints and cracks of the
overlying sandstone horizon, were used in these isotopic investigations. Various
grain-size fractions: 2-0.8, 0.8-0.4, 0.4-0.2 and <0.2 ~tm were extracted and
concentrated from two of these shales. Concentrating these various clay fractions
made it possible to separate the diagenetically produced clay minerals with grain
size <0.5 p.m from the detrital clay minerals with grain sizes up to 500 ktm.
Leaching experiments were carried out using IN HCI acid on the smallest
fractions (<0.4 lam). Clauer et al. (1993) assumed that the leachates were
isotopically and geochemically identical to the fluid phase, with which the clay
minerals last experienced exchange. If the clay minerals crystallized within this
fluid medium then they would have existed in isotopic equilibrium with this fluid
phase. Leachates would thus produce leachate-residue internal isochrons that
define the age of crystallization. The leachate represents the diagenetic fluid
phase. Similar considerations can be brought to bear on the Sm-Nd isotopic
system. Interpretations may be problematic due to the possible existence of small