THE BIPARTITE EARLY PALEOZOIC GONDWANA SHELF: PALEOGEOGRAPHIC CONTROL ON THE SN-W MINERALIZATION ALONG THE VARISCAN-APPALACHIAN OROGENIC BELT

Phanerozoic magmatic Sn-W deposits are concentrated at ancient convergent plate boundaries. The formation of the deposits is controlled by a sequence of sedimentary and tectonic accumulation eventually followed by crustal melting processes. The sedimentary processes include protolith enrichment by intense chemical weathering (loss of Ca and Na) followed by sediment redistribution from the continent interior towards its margin. Hence, the spatial distribution of Sn and/or W mineralization should be determined in the first order by the provenance and the paleogeographic position of the sedimentary source along the continental shelf.

Here, we test the paleogeographic control on the Late Paleozoic belt of the Variscan-Appalachian orogen that hosts several magmatic Sn and/or W deposits and occurrences. The orogenic belt of Europe, North Africa, and North America comprises reworked Precambrian – Early Paleozoic crust of the northern Gondwana shelf. A striking feature is the irregular shape of the belt that is associated with heterogeneously distributed pre-orogenic paleogeographic indicators and spatially separated Sn- and W- dominated mineralization. In analyzing the paleogeographic proxies, the lithostratigraphic profiles and the tectono-magmatic record, we qualitatively assess the initial position of the pre-orogenic units along the Gondwana shelf. In particular, zircon provenance data highlight a bipartite distinction of the Peri-Gondwana shelf into a western and eastern part as had been inferred from sedimentary and faunal affinities.

Although the composition and hence the conditions of chemical weathering for both shelf segments is similar, the ‘western shelf’ is associated with Sn-dominated mineralization, whereas the ‘eastern shelf’ hosts W-dominated occurrences. Orogenic constraints indicate a diachronous tectonic incorporation of the different segments of the Gondwana shelf. The initial paleogeographic position results in contrasting tectonic records and different source enrichment processes, i.e. tectonic accumulation, heating and mobilization, eventually controlling the primary distribution of Sn- and W-dominated deposits in the western and eastern shelf, respectively.