Petrographical differentiation between Palaeozoic oolitic ironstones from France, Belgium and Germany and application to the provenance study of archaeological artefacts – preliminary results

Samples of Palaeozoic oolitic ironstone beds susceptible of having being used as
raw materials for Neolithic red ochres, have been petrographically investigated. The
preliminary results of this first comparative analysis are quite encouraging:
microfacies differences have been observed between Ordovician oolitic ironstones
from Normandy (France), late Upper Devonian oolitic ironstones from Belgium and
uppermost Lower Devonian to lowermost Middle-Devonian (Emsian-Eifelian) oolitic
ironstones from the Eifel area (Germany).
Petrographical differentiation is based upon contrasting grain size, mineralogy
(hematite/chlorite ratio) and typology of the ferruginous ooids, besides differences in
mineralogy, diagenetic history and lithologic nature of the host sediments. Most
conspicuous are differences in ferruginous ooid typology, including “true” concentric
ooids, superficial ooids, algal oncoids and pseudo-ooids (ferruginized cortoids and
rounded bioclasts). “Flax seed” or Clinton-type iron ores (rich in flattened ooids) and
“fossil iron ores” (essentially composed of ferruginized bioclasts) can be identified as
well as transitional or mixed types.
Homogenous and well-sorted, often flattened and fine-grained ferruginous “true”
ooids (flax seed ore) with alternating hematite and chlorite cortices in a sideritic-
chloritic or fine siliciclastic matrix, are characteristic for the Ordovician (Llanvirn)
oolitic ironstones of Normandy (basal part of the Urville Shales). Locally, weathered
levels exist, enclosing limonitic (goethitic) crusts. Medium-sorted, fine-to coarse-
grained ferruginous hematitic pseudo-ooids (ferruginized bioclasts) in a bioclastic
limestone matrix (fossil ore) characterize the Lower-Middle Devonian boundary oolitic
ironstone beds (Heisdorf and Lauch Formations, Eifel Synclines). Finally, well- to
medium-sorted heterogenous, fine- to medium-grained, pure or mixed flax seed- and
fossil ore-type hematitic oolitic ironstones in siliclastic and/ore carbonate matrices,
characterize the Belgian Latest Upper Devonian (Famennian) ironstone deposits
(Hodimont Formation, Famenne Shales Group). Several stratigraphic levels do exist
within the Lower Famennian and basal part of the Upper Famennian in the Namur,
Dinant and Vesdre Synclinoria, but the lowermost Famennian one is the only level
that has been mined. Within some of the younger Famennian oolitic ironstone levels,
proximal and distal facies can be distinguished on the basis of microfacies
differences and mineralogy of the ferruginous pseudo-ooids. Only the proximal
hematitic facies of the lowest stratigraphical oolitic ironstone level (level I) is
supposed to have been used in prehistoric times for the manufacturing of ochre.
Diagenetic sideritization and dolomitization, particle deformation as well as sulphide
mineralizations, affect most of the studied oolitic ironstones. However, the intensity of
these mineralizations varies strongly (even within the same deposit) and depends on
local tectonics.
A distinction can be made between the Emsian-Eifelian and Famennian fossil iron
ores, based on the nature of the bioclasts and other ferruginzed components).
Eifelian oolitic ironstones contain ferruginized crinoids, bryozoans, trilobites,
brachiopods, goniatites besides ferruginized siliciclastic intraclasts, whereas the
Famennian ones are dominated by ferruginous ooids and algal oncoids, mixed with
ferruginized bioclasts including crinoids, bryozoans, brachiopods, ostracods, algae
and incertae sedis, and locally intraclasts (ferruginized stromatolitic crusts). Distal
facies contain slightly Fe-impregnated bioclasts only such as crinoid ossicles and
display a higher chlorite/hematite ratio.
Thin sections have been made in archeological objects (red ochre), allowing a first
comparative petrographical analysis indicating their probable geological and
geographical provenance.
References
Ph. Joseph, 1982. Le minerai de fer oolithique Ordovicien du Massif Armoricain: sédimentologie et
paléogéographie. Thèse présentée à l’Ecole Nationale Supérieure des Mines de Paris. 325 p.
R. Dreesen, 1989. Oolitic ironstones as event-stratigraphical marker beds within the Upper Devonian
of the Ardenno-Rhenish Massif, in: Young, T.P. & Taylor, W.E.G. (eds), Phanerozoic Ironstones.
Geological Society Special Publications, n°46, pp. 65-78
Rath, S., 2003. Die Erforschungsgeschichtede Eifel-Geologie. Ph.D. Dissertation, Rheinisch-
Westfälische Technische Hochschule Aachen, 239 p.