Faulting, fluid-rock interaction and hydrothermal mineralisation in ultramafic rocks (Voltri Massif, Ligurian Alps)

This study deals with reverse faults within intensely carbonated metalherzolites, with related gold mineralisations, from the Voltri Massif, and in particular within the Lavagnina Lakes area, in the Gorzente Valley. This area is located in northwest Italy among the municipalities of Casaleggio Boiro, Lerma, Mornese, and Bosio (Alessandria, Italy; GPS coordinates WGS84, 44.600685° N, 8.784286° E). The Voltri Massif is a metaophiolitic complex, which occurs within the Eastern edges of the Ligurian Alps. It is made up of of metaophiolitic rocks associated with metasediments and slices of subcontinental lithospheric mantle. These rocks underwent a complex Alpine tectono-metamorphic evolution, with blueschist- to eclogite- facies peak metamorphism with variable retrogressive overprints. For this study the analysed structures are the upper crustal deformational events (UDC), from late- to post-orogenic linked to the collisional events. These deformational events are linked to different structural regimes, and developed during the late-Alpine to early-Appenine events. The Lavagnina Lakes area is mainly characterised by outcrops of serpentinites, lherzolites, metabasites, metagabbros, lenses of metasediments, and listvenites (in decreasing order of volumetric extension). From a structural point of view, this area is characterised by brittle to brittle-ductile systems of shear zones, with associated carbonates and chalcedony-quartz veins associated with local gold mineralisations. Hydrothermal, carbon-rich fluids permeated the rocks and sustained these deformation stages producing widespread and intense carbonate-rich alteration zones, exclusive meso- and microstructures within carbonates fault zones, called fault pearls, chalcedony shear veins along fault surfaces, widespread quartzchalcedony and carbonates veining, and leading to local gold mineralisation. The geological survey allowed identifying four main structures (Chapter 5) that I studied in detail. In particular I studied the Bisciarelle Creek thrust fault for the occurrence of meso- and microstructures never reported in literature, called fault pearls. Their compositional and textural characteristics, described in chapter 5.3.4, make the fault core of the Bisciarelle fault a peculiar geological object. 2 The data shown in chapter 7 provide constraints on the chemical properties of the fluid that generate the intense carbonation along the Bisciarelle fault. Through the mass transfer profiles was possible to identify and quantify those elements transferred from the fluid to the rock and vice versa. The morphometric image analysis results (Described in chapter 5.3.5) show that the fault pearls have circular shape irrespective to the direction of observation (parallel and orthogonal to the slickenlines), with values that indicate a nearly perfect circular shape. These values are in line with the roundness values, hence the fault pearls in three-dimensions can be considered as nearly perfect spheres or as very spherical textures. Single-spot SEM-EDS (Chapter 8) analyses show that ferroan dolomite makes the pearl bands (CaO: 29.13±2.33 wt%; MgO: 19.03±4.86 wt%; FeO: 1.77±0.87 wt%) and that minor compositional variations across large and thin bands exist, and that silica makes most of the chalcedony veins (SiO2: 97.97±3.23 wt%). Elemental imaging by LA-ICPTOFMS (Chapter 8) provides a detailed account of the distribution of chemical elements within pearls, matrix, and chalcedony shear veins. Mineral paragenesis and elemental imaging by LA-ICP-TOFMS confirm the hypothesis of hydrothermal derivation of the fault rock. This is best evaluated by comparing the concentration ranges of the key trace elements As, Sb, In, Ga, Ag, Zn, and Cu of fault pearls levels and chalcedony with those of the same elements in the average upper crust. Such comparison clearly shows that the peak concentrations of these elements are 5 (Cu, Zn) to 50 (Sb, In) times higher than those of the upper crust, demonstrating that the Bisciarelle fault fluid was capable to transport and deposit a suite of ore elements. These evidences are fundamental to discuss the possible origin of the fluids that developed the carbonation and the fluid-rock interactions along the Bisciarelle thrust fault and along the main fault of the Lavagnina Lakes area. On the basis of all the constraints described, the fault pearl features are compatible with a genesis from a process called “transient” boiling in microfluidics. This process occurs within cavities when a liquid is instantaneously overheated and a vapour phase nucleates and expands up to explosive boiling, and so generates a myriad of vapour bubbles. Such process, which occurred during mixed mode fracturing in the fault, implies that pearls might reflect the liquid-vapour 3 fractionation of chemical elements in a boiling hydrothermal fluid during seismic failure. The close association of fault pearls with seismic-related structures such as submicron carbonates coating of slip surfaces, quartz microtexture related to silica gel deposition, syn-kinematic filamentous phase, injection veins with similar characteristics and overpressure with respect to seismic pseudotachylites along slip zone supports the hypothesis that fault pearls developed during shear events at seismic rates, and hence are considerable as paleo-seismic events indicators. Moreover I discuss the tectonic evolution of the Lavagnina Lakes area linked to the syn-kynematic fluid rock interaction, the origin of the fluids and the gold mineralisations of the area. The occurrence of syn-tectonic fluid flow and consequent fluid-rock interaction (e.g. carbonation) along the two systems of Reverse Shear Zones (RSZ1-RSZ2), is testified by: metasomatic alteration (ALT-1, ALT-2, and ALT-3), at least three main systems of veins (V1, V2, and V3), and hydraulic and cockade breccias. The evidence that the carbonation took place along these structures, in particular along the RSZs systems, and within their damage zones, highlights how these structures acted as important fluid pathways and played a major control in the distribution of the ore deposits. Finally, I stress out further consideration about the comparison between the gold mineralisations of the area with the gold mineralisations of the whole Voltri Massif.