Gold in pyrite revisited: insights into remobilization during deformation using electron backscatter diffraction and LA-ICP-MS

Abstract

In a gold deposit near Nassara, southern Burkina Faso, gold occurs closely associated with pyrite within a network of veins hosted by metavolcanic and metasedimentary rocks. Using SEM and LA-ICP-MS analyses, we identified three generations of pyrite with distinct roles in gold mineralization. Pyrite 1 (Py1) formed early during mineralization, replacing alteration minerals like ankerite in metabasalt. Pyrite 2 (Py2) developed around Py1 in pressure shadows caused by localized micro-shear zone reactivation during successive micro-seismic events. Pyrite 2 is enriched in As and Au, unlike Py1. Pyrite 3 (Py3), unrelated to mineralization, formed at a later stage. Gold occurs in pyrite as micro-inclusions (in Py1 and Py2), fracture-fillings (mainly in Py2), and within the pyrite structure as invisible gold, including nanoparticles (predominantly in Py2). Combining electron backscatter diffraction (EBSD) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis reveals that deformation-induced misorientation of pyrite facilitated the remobilization of invisible gold, which subsequently re-precipitated as colloidal particles along sub-grain boundaries and within fractures, mimicking visible inclusions. These findings demonstrate that gold perceived as inclusions (visible or invisible) often precipitates within micro/nano-fissures and sub-grain boundaries during remobilization. This highlights the critical importance of thorough ore characterization for accurately determining gold deportment. Such insights advance our understanding of mineralization processes and support the development of more efficient recovery strategies.