The nature of subcontinental mantle and the role of fluid: evidence from unconventional accessory minerals in the Finero complex, Western Alps, Italy

There is ongoing debate about the origin and concentration mechanisms of platinum group elements (PGE) in mafic-ultramafic rocks. This has also attracted particular attention also from the exploration industry in view of rising prices of platinum metals. There is agreement that the Earth`s upper mantle plays a crucial role in this context. There are a few localities world-wide where rocks of the continental upper mantle are exposed. The Finero Complex in the Italian Western Alps is one of them, and it offers a unique opportunity to study transfer of platinum group elements from mantle to the crust and thus the formation of potentially economic platinum group metal deposits. New concentration techniques (hydroseparation) for non-destructive recovery of extremely fine-grained (0.005-0.05 mm) minerals have been employed for tests on two samples, in the preparation of this project. Numerous platinum group minerals (PGE) have been recovered; surprisingly, however uranium and thorium minerals not normally occurring in mantle rocks, have also been encountered. These unusual minerals, which have not been identified before, as well as the occurrence of magnesian mica (phlogopite) point towards the participation of a fluid phase in the evolution of the Finero rocks. It is intended to study the concentrates obtained during the initial tests and to obtain more material in the field. Microscopy in reflected and transmitted light, scanning electron microscopy and quantitative microprobe analyses are to be carried out to identify the minerals in question and to compare them with mineralisation of the East Alpine Kraubath ultramafic massiv. Recently developed technology (LA MC ICP MS) permits the quantitative determination of Os isotope composition a fracer of the source of platinum group elements in small (PGM) grains in the 0.005-0.05 mm grain size range. The result of these multiple methodologies will be of help to understand the role of the fluids in concentrating platinum group elements in the upper mantle as a potential source of platinum group element deposits in the crust.