ANOXIC SEDIMENTS - AN IMPORTANT ENVIRONMENT FOR PGE - AN OVERVIEW

Anoxic sediments and metasediments have been recently recognized as important host environments for the platinum group elements (PGE). Economically significant PGE concentrations have been reported from marine. metal-rich black shales in China, Yukon (Canada) and Poland. These ore-bearing horizons are characterized by extremely restricted vertical, and sometimes larger horizontal extents. They are associated with the evolution of intracontinental rifting structures without any magmatic activity, suggesting a possible driving mechanism for mineralization processes. Complex PGE associations, including a significant relationship of Pd and Pt with Ni, Bi. Se, Y and also with Sb, Cu, Co, Au, U, and F are suggested using the results of a new statistical evaluation of available geochemical data. These occurrences show more common features with other hydrothermal PGE deposits. The rift-volcanic-related, marine metal-rich black shales of the Czech Republic and Finland, however, display lower PGE values. The mineralized layers are much thicker with a typically limited horizontal extent showing a close association with volcanic centres. Pd reveals a strong correlation with Ni, Zn, Au and V; Pt exhibits a highly significant link with the Cu and Mo phases. Low average Ni/Cu ratios and PGE + Au chondrite-normalized patterns are similar to those of some magmatic PGE deposits hosted in mafic and ultramafic rocks. It is very likely that organic matter played an important role in the genesis of PGE accumulations in metal-rich black shales in both rift and rift-volcanic-related environments.