25th International Meeting on Organic Geochemistry IMOG 2011

P-050
Information provided by the Organic Matter contained in cherts
of the Ras-Draâ phosphate deposit (Tunisia) on their
depositional environment
Aïda Ben Hassen 1 , Jean Trichet 2 , Jean-Robert Disnar 2 , Habib Belayouni 3
1 Centre de Recherche et des Technologies de l'Energie, Technopôle Borj Cédria, Hammam-Lif, Tunisia,
2 Institut des Sciences de la Terre d'Orléans, Orléans, France, 3 Faculté des Sciences de Tunis, El Manar,
Tunisia
In the Paleocene-Early Eocene Ras-Draâ (Tunisia)
phosphate outcrop formation a laminated (siliceous)
chert level is interbedded between two phosphatic
strata (CVI and CVII phosphorites; P2O5 > 18%). The
study of the organic matter (OM) associated to the
chert aims at bringing some light on the nature and
the geochemical properties of this OM and then to
draw some conclusions on the genesis of these
sediments.
The main results of this study can be summarized
as follows:
1. The Total Organic Carbon (TOC) content, given
by Rock-Eval pyrolysis, is about 1.6%.
2. The geochemical characterization of the OM
contained in cherts, by Rock-Eval (RE) pyrolysis
concludes to: (i) a high aliphatic character (Hydrogen
Index IH value ~ 499 mg d‘HC/g TOC) and thus to a
marine planktonic origin of the OM (Type II), (ii) a low
degree of OM maturity (Rock Eval Tmax = 429 °C)
and (iii) to the presence of little oxidized organic
components due to the lower Oxygen Index IO (about
50 mg de CO2/g TOC).
3. The chemical extraction of the humic substances
(HS) associated to the cherty sediments (following the
<strong>International</strong> Humic Substances Society protocol)
evidences significant amounts of extractable humic
compounds (~20% TOC) and higher proportions of
non extractible residue or "humin" (~ 80% TOC). Such
significant amounts of HS were reported by Kahouach
(1989) (up to 27% TOC) in chert interlayers from
tunisian phosphatic series. However, higher HS
proportions were found by Bein et Amit (1982), in
cherts associated to Negev phosphorites (more than
40% of OM are in humic form). According to Kolodony
(1969), such results suppose that the cherts formation
is due to the silicification of phosphatic materials that
were probably rich in HS. In our study, RE parameters
– markedly high HI and distinctly low OI values –
respectively suggest the autochthonous origin of this
OM and, the absence of oxidative destruction of the
initial organic material during chert formation.
4. The main geochemical and spectroscopic
features of the humic acids (HA) extracted from the
cherts are the following ones: (i) atomic O/C and H/C
ratio values of 0.27 and 1.53% signify that these
humic compounds originated from marine aliphatic
OM; (ii) FTIR analysis of cherts-extracted HA show
that these organic compounds are rich in aliphatic
structures that is typical for an algae derived material.
5. The occurrence of fossil molecules in chert lipidic
extract (C22 predominance in n-alkane distributions,
presence of steroids and benzohopanoids) confirms
the autochthonous marine origin and the low degree
of maturity of the OM. With regards to chert age, the
abundance of HS implies their good preservation in a
reducing depositional environment (Belayouni et
Trichet, 1983).
Finally, the study of the OM associated to the
cherts interbedded within P-rich levels brings clear
evidence that these sediments were deposited in a
reducing marine environment marine and that they
only experienced limited diagenesis (that led them to
the humic stage).
References
[1] Bein, A., Amit, O., 1982. Depositionnal
environments of the Senonian chert phosphorites and
oil shales sequence in Israel as deduced from their
organic matter composition. Sedimentology 29, 81-90.
[2] Belayouni, H. and Trichet, J. [1983] Preliminary
data on the origin and diagenesis of the organic
matter in the phosphate basin of Gafsa (Tunisia).
Advances in Organic Geochemistry 1981, 328-335.
[3] Ben Hassen, A. [2007] Données nouvelles sur la
matière organique associée aux séries du bassin
phosphaté du sud-tunisien (Gisement de Ras-Draâ)
et sur la phosphatogenèse. Thèse de Doctorat,
Université d‘Orléans. France.
[4] Kahouach, H., 1986. Abondance, nature et
composition de la matière organique contenue dans
les sédiments de la série phosphatée du gisement de
Ras-Draâ (Nefta-Tozeur). Application à la valorisation
des minerais phosphatés. Rapport de DEA. Université
de Tunis. Tunisie.
[5] Kolodony, Y. 1969. Petrology of siliceous rocks in
the Mishtah Formation (Negev, Israel). J. Sedim.
Petrol. 39, 166-175.
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