Paleozoic gold in the Appalachians and Variscides

The distribution of Au mineralization in the Appalachians and Variscides is irregular. Major segments of the belt do not show significant Au mineralization. Segments with major Au deposits, however, show a complex history of repeated endogenic and exogenic metal redistribution. Major sources for Au occurrences in the Appalachians and Variscides are Cambrian to Ordovician sedimentary rocks and Cambrian to Ordovician magmatic arcs. (i) Gondwana-derived Au forms paleo-placer deposits in Cambrian to Ordovician sedimentary rocks on stable continental crust and may be preserved in low-strain or upper plate domains of the Appalachians and Variscides. The erosion of these sediments during rifting of peri-Gondwana led to the redeposition of these Au-anomalous protoliths as fan and turbiditic sediments at the continent margin. (ii) The magmatic arcs are built on Laurentian or Gondwana crust that earlier had separated from the main continents. Gold mineralization is mainly bound to volcanic massive sulfide deposits that formed in suprasubduction zone ophiolites. Sedimentary rocks earlier deposited at the continent margin may significantly contribute to the Au budget of suprasubduction zone ophiolites. The early Paleozoic accretion and obduction of the magmatic arcs to Laurentia and Laurussia resulted in a first redistribution of Au, in part in close spatial relation with the older deposits, in part in relation to structural elements that were active during accretion. With the onset of the collision of the Armorican Spur (Gondwana) with Laurussia at c. 400-380 Ma, style of Au mineralization and redistribution becomes highly diverse within the plate boundary zone between Gondwana and Laurussia with (i) magmatic Au mineralization (New Brunswick), (ii) shear zone related mineralization (e.g., Southern Alleghanian, Newfoundland, Ireland, Scotland), and (iii) quartz-Au veins in greenschist to amphibolite facies metasedimentary rocks (Nova Scotia). Only in Maritime Canada Au mineralization is related to subduction, in all other areas Au mineralization is related to reactivation of strike-slip shear zones and to redistribution within older deposits. In areas of continental collision with subduction of continental crust, Au is restricted to shear-zone bound vein-type mineralization and to magmatic rocks in structurally high allochthonous units (Spain, Armorica, Central Bohemian Plutonic Complex). The age of Au mineralization in supra-subduction ophiolites is very variable along the Appalachians and Variscides and reflects the formation of oceanic crust followed by the accretion or obduction of these rocks. In contrast, shear-zone bound quartz-Au veins in low-grade metamorphic domains and in areas with older Au mineralization, as well as granite-bound Au mineralization, formed along the entire belt in distinct pulses that are controlled by reorganizations within the plate boundary system and by reactivations of old structures in different stress systems.