Re-examination of the Aïn Allegua-Aguiba celestite-Pb-Zn salt diapir-related Mississippi Valley-type deposits (Nappes zone, Northwestern Tunisia): New conceptual model

The celestite-rich and Pb-Zn-poor Ain Allegua deposits are located at the northeastern extremity of the Dougas diapir in the Nappes zone of northern Tunisia. The Sr-Ba-Zn-Pb mineral system studied in this paper is enclosed in a Triassic breccia, labeled cap rock, located between the Triassic diapir and the surrounding Cretaceous to Lutetian marine sediments. It consists of evaporite dissolution float breccia where Triassic dolostone clasts are floating in a Triassic matrix of anhydrite and argillites. Five mineralizing stages are recognized, which are from early to late: celestite (stage 1); hydrothermal dolomite (stage 2); crosscutting galena-barite or sphalerite-galena-barite-calcite (stage 3); (4) fracture and karst filled by barite, celestite, and galena (stage 4); and late supergene mineralization (strontianite, smithsonite, and cerussite) (stage 5). The present study shows that the celestite and Pb-Zn mineralization formed from the circulation in the cap rock of at least three fluids: (1) fluid A (100-150 degrees C; 10 to 25 wt% NaCl eq.) deposited the celestite; (2) fluid B (125 to 185 degrees C; 29 to 34 wt% NaCl eq.) deposited the hydrothermal dolomite, and (3) fluid C (temperatures around 90 degrees C; 14 to 22 wt% NaCl eq.) deposited the calcite that accompanied sphalerite and galena precipitation. The 834S values for sphalerite and galena fall within a narrow range from + 10.4%o to + 18.9%o (avr. 13%o) indicating a homogeneous source of sulfur. The 834S values of celestite and barite are also relatively homogeneous from + 14.7 to + 23%o (ave. + 16.5%o) and match those of Tunisian Triassic gypsum and Triassic marine sulfates (834S = +15%o). The temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of Triassic sulfate. The lead isotopes in galena show that Pb, and probably Zn and Ba were extracted from the upper crustal rocks. Part of the strontium is derived from the Triassic anhydrite and the other part is probably transported to the cap rock in formation waters. The tectonic setting, the carbonated nature of the host rocks, the epigenetic style of the mineralization, and the mineral associations, together with fluid inclusions data and sulfur and lead isotopes show that the Ain Allegua deposit belongs to the diapir-related Mississippi Valley-type (MVT) deposits of the Eastern Maghreb salt diapirs province, that formed, as the most of the MVT deposits and districts in the world, from basinal brines with no mixing with metamorphic and/or magmatic fluid or involvement of a heat source from a concealed sill or magmatic intrusion.

Automated summary

This study reveals the rock composition and formation mechanism of the Ain Allegua deposit in the Dougas diapir in northeastern Tunisia. The deposit consists primarily of celestite and Pb-Zn minerals and is located in the cap rock between the Triassic diapir and the surrounding Cretaceous to Lutetian marine sediments. The mineralization process underwent five stages involving the precipitation of various hydrothermal fluids. Sulfur isotope data suggest that the reduced sulfur in sulfides mainly originated from thermochemical sulfate reduction of Triassic sulfates. Lead isotope analysis reveals the source of lead, zinc, and barium in upper crustal rocks. This study provides important clues for understanding the genesis of the deposit and the formation of the minerals in the region.