Pge-bearing Minerals in Silurian Sedex Deposits in the Poblet Area, Southwestern Catalonia, Spain

The Lower Llandoverian metasedimentary series of the Prades Mountains, in southwestern Catalonia, Spain, consists of interbedded massive feldspar beds, composed of almost pure anorthite, with chert, phosphate beds, sulfide-rich black shale, massive sulfides and minor calc-silicate beds. The sulfides mainly consist of pyrrhotite with minor chalcopyrite. The metasedimentary rocks are anomalously Vand Cr-rich and contain disseminated minerals bearing precious metals, e.g., sperrylite and palladian löllingite. On the basis of the geological setting of ore formation and the textural relationships among the minerals, a sedex model is proposed for the primary origin of these deposits and occurrences. However, textural patterns and mineral compositions were modified during several episodes. Firstly, Hercynian deformation and associated regional low-grade metamorphism produced cleavage and small-scale veining in the primary associations. Later, contact metamorphism related to Late Hercynian intrusions annealed the mineral associations. A sulfidation stage of mineralization caused replacement of Pd-bearing löllingite by arsenopyrite. During this process, the arsenopyrite structure was unable to accommodate such high Pd contents and, consequently, Pd was precipitated as PdAs2 and scarce small grains of native Pd, disseminated along the löllingite – arsenopyrite contact, in association with argentian gold, and tellurides and selenides of Au, Ag, Bi and Pb.