Late Devonian-Early Carboniferous peak sulphide mineralization in the Western Hercynides

Abstract

The Late Devonian-Early Carboniferous (Dinantian) within the Western Hercynides is marked by the formation of volcanic-hosted massive sulphide deposits: Chessy and Chizeuil in the Brevenne and Somme successions of the French Massif Central; Bodennec and La Porte-aux-Moines in the Châteaulin Basin of the French Armorican Massif; Rio Tinto, Neves-Corvo, Tharsis, etc., in the Volcano-Sedimentary formation of the Iberian Pyrite Belt; and Ketara, Draa Sfar and Hajar in the Jebilet-Guemassa district of the Moroccan Southern Meseta. Although these deposits show a slightly diachronous emplacement in response to a progressive migration of the metalliferous event from Late Devonian in France to Dinantian in Morocco, it is nevertheless possible to define an overall metalliferous ‘‘peak” around 350 Ma. The mineralization of the Armorican, Iberian and Moroccan sectors took place in epicontinental domains of the outer zone of the Hercynian belt, whereas that of the northeastern Massif Central occurred within the inner zone of the belt. This difference is registered by variations both in the geochemical characteristics of the ores (Sn in the outer zone and Mo-Ni in the inner zone) and in their lead isotopic signatures (clear mantle participation exclusively in the inner zone). In many cases the ores appear to be closely related to the felsic member of a bimodal magmatic association, although the massive sulphide deposits in the outer zone are more commonly associated with sedimentary rocks whereas those in the inner zone are hosted by felsic volcanic rocks. Another feature that should be noted is that the host sequences of the massive sulphide deposits commonly seem to be underlain by chaotic formations (notably with olistoliths) reflecting the beginning of Hercynian orogenic activity in the outer zone. It can be concluded that the peak mineralization took place within tensional domains developed during a period of plate convergence, and that it occurred around 350 Ma after a major period of Devonian compression but before the Carboniferous continental closure.