Cyclic fault-controlled mass transfer during gold mineralisation in carbonated ultramafites (Bisciarelle stream, Voltri Massif, Ligurian Alps)

In this work, we examine a thrust fault developed within carbonated ultramafites (lherzolite and serpentinite) of the Voltri Massif (Ligurian Alps), exposed along the Bisciarelle Stream. Different rock types characterise the damage zone of this fault, namely carbonate and chalcedony-quartz veins at the footwall and intensely fractured and hydrothermally altered lherzolite at the hangingwall. The fault core hosts unusual carbonate-coated grains (CCGs), in a ca. 50 cm-thick level, cut by chalcedony shear veins at the top. We have combined petrographic data with SEM-EDS and LA-ICP-TOFMS (Laser Ablation-ICP Time of Flight-Mass Spectrometry) analyses, and with mass transfer calculations in order to: 1) determine the chemical properties of fault core rocks, and 2) monitor chemical exchanges that took place between hydrothermal fluids flowing within the fault at the time of its slip and the host lherzolite. The used samples were collected at the fault damage zone and along a sampling profile orthogonal with respect to the fault strike. Fault core CCGs are mm to cm-sized, have a surprisingly regular round shape, and are rarely in contact with each other. The largest CCGs show complex internal textures, in which relatively thick and fibrous dolomite bands alternate with thinner, massive, and partly laminated dark carbonate bands, suggesting a cyclic growth process; the smallest CCGs show simpler internal textures. Two dimensional, multi-elemental compositional maps generated by LA-ICP-TOFMS analyses highlight systematic compositional difference between the thin massive bands and the fibrous dolomite bands; the first ones have relatively high concentrations of Mn, Al, K, Pb, Cr, Co, and Cu, whereas the second are enriched in only a small number of metals (i.e. Fe, Sr, and W). Chalcedony shear veins contain Al2O3, Na2O, and K2O in addition to SiO2, and show a peculiar enrichment in Sb, Ag, In, Pb, and Au. Mass transfer calculations show that the fluid transferred consistently volatiles (CO2 and H2O) and Sb to the rock, whereas the rock transferred Si, Fe, Co, and Cr to the fluid. Our dataset shows that the thrust hosted a gold-bearing hydrothermal fluid, which was responsible for the rhythmic growth of the CCGs. Such growth was syn-kinematic and at equilibrium with the fault fluid, but was not coupled with any substantial grain-size reduction of the fault core. Mass transfer data show that hydrothermal formation of CCGs and chalcedony was entirely fault-controlled, and that the host rock provides a poor record of this circulation. The evidence for transfer of Si, Fe, Co, and Cr from the protolith to the fluid provides a strong indication that the enrichment of these elements within CCGs and chalcedony was made possible by fluid-rock interaction. In conclusion, the occurrence of CCGs with a peculiar composition, their texture, and the lack of grain-size reduction suggest that the hydrothermal precipitation controlled the composition of the Bisciarelle fault rocks.