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Abstract

In Tunisia, as in most of the world's metalliferous provinces, the problem of the origin of lead-zinc-barite-fluorite deposits is far from being resolved, and only hypotheses can be formulated.

The one certainty is the existence, in the Miocene which overlies the stratiform mineralizations of the Cretaceous, of pebbles of mineralized Cretaceous (Djebel Chambi). However, in the north this same Miocene, as at Sidi Bou Aouane, Djebel Semene, etc., contains stratiform lead deposits. Examples of the redistribution of mineralization, even within historic time, also are known. It is impossible, therefore, to determine age, and consequently an origin, for lead-zinc mineralization.

Observed facts, nevertheless, do permit the rigorous determination of the relations between lead-zinc mineralizations and their geological setting as follows:

a. All deposits (100%) are at well-defined stratigraphic horizons (stages or formations). All are along axes of diapirism. All are in fault breaks or folds. Almost all (90%) are in zones of subsidence during Upper Cretaceous time, even if the mineralized formation does not belong to the Upper Cretaceous. Nearly all (80%) are near the borders of Pliocene-Pleistocene plains. The majority (62%) are near outcrops of Triassic.

b. On the scale of a stage or a formation: All are in biostromal facies either of the Lutetian, the Jurassic or the Aptian. They correlate with formation isopachs. They are near ancient shore lines, etc. Mineralizations of fluorite or barite follow rules of distribution either similar to those for lead and zinc, or in some cases different.

Finally, one must realize that the natural empirical facts in the knowledge of ore deposits involve many more elements than the stated theories.

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