Novodneprite (AuPb3), anyuiite [Au(Pb, Sb)2] and gold micro- and nano-inclusions within plastically deformed mantle-derived olivine from the Lherz peridotite (Pyrenees, France): a HRTEM–AEM–EELS study

Abstract

To contribute the problem of the missing (“invisible”) gold fraction in mantle rocks, olivine grains separated from orogenic lherzolite of the peridotite body of Lherz (Eastern Pyrenees, France) have been investigated by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The results indicate the presence of micrometric inclusions of novodneprite, AuPb3, and anyuiite, Au(Pb,Sb)2, together with nanometric clusters of metallic gold. Both minerals have been recognised on TEM images as darker contrast inclusions and identified through selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) analyses. Gold clusters have been indirectly identified in randomly distributed nano-sized rectangular areas that occur in TEM images obtained from the edges of olivine crystals. Within these volumes the EDS analyses reveal a constant presence of Au (0.1–0.2 wt %). High-resolution TEM (HRTEM) investigations evidence series of regularly alternating sigmoidal and ellipsoidal domains developed along [110]. The EELS investigations revealed that the Au signal (M-series lines) arises from the ellipsoidal domains. It is proposed that novodneprite and anyuiite are the result of subsolidus recrystallization of the Pyrenean lherzolites accompanied by a secondary olivine grains growth that trapped inter-granular components. Likely, a process of plastic deformation favoured the formation of edge dislocations within olivine grains and thus, the circulation through them of Au-enriched fluids. A mass balance calculation of the missing gold percentage within this lherzolite points to olivine as one of the potential hosts for about the 80 % of the “invisible” gold in form of nano-inclusions, whereas only 20 % of the whole-rock Au-budget, would be hosted within assemblages of Cu–Fe–Ni sulphides.