EXOGENIC CONTROL ON THE DISTRIBUTION OF ENDOGENIC SN-W-TA, AU, AND U MINERALIZATION IN THE APPALACHIANS AND THE VARISCIDES

Magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization along the plate boundary zone between Laurussia and Gondwana results from the superposition of a series of exogenic and endogenic processes. Exogenic processes control (i) the enrichment of the ore elements in sedimentary protoliths, (ii) the spatial distribution of these rocks, and, thus, eventually (iii) the distribution of zones with possible mineralization. Source rocks within the Laurussia-Gondwana plate boundary zone include U-rich Early Silurian black shale, whose occurrence is restricted to Gondwana, and intensely weathered Gondwana-derived Cambro-Ordovician siliciclastic rocks for Sn-W-Ta and Au. Pre-Variscan volcanic arc rocks that formed on older continental crust are restricted to the Laurentian side of the future plate boundary zone. They are important Au sources.

Whether endogenic mineralization develops within these zones depends on the mobilization of Sn-W-Ta, Au, and U from fertile protoliths, i.e., on the metamorphism and partial melting of these rocks. On Gondwana, the fertile protolith have essentially the same spatial distribution, but mineralization of the various metals is spatially separated and followed separate paths. Gold mineralization dominantly includes quartz-Au veins in low-grade metamorphic rocks, whereas Sn, W, and Ta mineralization it related to areas where the same protoliths as for Au experienced high-grade metamorphism and partial melting during continental collision or post-collisional crustal extension. The spatial separation of Sn, W, and Ta mineralization is related to different melting conditions and contrasting tectonic history before crustal melting. Although Sn and U were mobilized by the same granitic melts, mineralization differs in style and age: Sn mineralization is directly related to granite emplacement, whereas U mineralization is related to later leaching of U-bearing minerals in the granite. At the Laurussia margin, Au, W, Sn, and Ta mineralization related to the accretion of terranes (Au) and subduction (Sn, W, and Ta) is bound to different protoliths and is regionally diachronous. After the collision between Laurussia and Gondwana, Au mineralization on Laurussia is bound to shear zones that became reactivated synchronously with changes in collisional style on Gondwana.