Platinum-group elements in the ultrabasic rocks of the Braganca and Morais massifs, northern Portugal

• Main Author: Bridges, John Charles
• Published: Open University 1991
• Subjects: 551; Geology
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.306152

The ultrabasic rocks studied in this thesis are located in the Upper Allochthonous Thrust Complex nappes of the Braganca and Morais massifs, Träs-os-Montes, N. Portugal. They are relicts of residual mantle and cumulates formed at a destructive continental margin. In the Braganca massif, the ultrabasic rocks consist mainly of a harzburgite formation, which is residual mantle. This formation contains lenses of a separate chromitite-bearing dunite formation. The Morais massif assemblage, at an equivalent structural level, is made up of peridotite and gabbroic/troctolitic cumulates. The chromitite-bearing dunite formation crystallised within small magmatic masses. This chromite crystallisation acted to fractionate the associated Platinum-Group Element (PGE) assemblage. Os, Ir and Ru were incorporated as laurite and irarsite minerals during chromite grain crystallisation, and are located in textural positions from the centres to the margins of the chromite grains. In contrast Pt-bearing arsenides and sulpharsenides started crystallising after the Os, Ir, Ru group, and are only located at the margins of or in between chromite grains. More fractionated assemblages show positive slopes on chondrite normalised whole rock PGE plots. Pd is not directly associated with this fractionation. Instead its mineralisation is associated with that of the base-metal sulphide. Serpentinisation has caused recrystallisation of much original pentlandite to heazlewoodite and magnetite. At the same time Pd-bearing alloys were created, mostly adjacent to sulphide grains with the source of the Pd being in solid solution within the base-metal sulphide. Several factors suggest that the chromite mineralisation was derived from melts of boninitic affinity. The relatively high Pd/Cu ratios calculated for a silicate melt from which the chromite crystallised are consistent with this. The composition of the chromite grains, having 1000r/(Cr+Al) ratios clustering around 75, is typical of boninitic magmas. In addition the refractory composition of the harzburgite formation, and the high tenors of Pd within sulphide, show that it was a possible source for such boninitic melts. The chromite mineralisation took place down to depths of 30km within the mantle wedge of a destructive continental margin.