Isotopic constraints on the classification of the Jales Au district (Northern Portugal)

The Jales Au district encompasses three areas with epigenetic mineralization of Variscan age. Mineralization is hosted in syn- to late-tectonic Variscan granites and in greenschist facies metasedimentary rocks, ranging in age from Cambrian to Lower Devonian. Sulfide paragenesis proceeds from early Fe-As sulfides to late base-metal sulfides. The deposits proximal to the intrusion (Campo and Gralheira) are relatively sulfide-rich and contain paragenetically late electrum associated with significant amounts of base-metal sulfides, whereas the deposit distal to the intrusion (Três Minas) is sulfide-poor and contains high-fineness Au that is paragenetically early. In spite of displaying characteristics related to Orogenic Au deposits, the regional association with W-Sn deposits, close temporal relationship of mineralization with granitic intrusions, metal associations characterized by significant Bi contents or Bi-Au correlation, a paragenesis defining a low fO2 environment and S (and Pb) isotope studies suggest that the mineralization is related to syn- to late-tectonic reduced granites and could also be classified as intrusion-related.