Faulting, fluid-rock interaction and hydrothermal mineralisation in ultramafic rocks (Voltri Massif, Ligurian Alps)
Creators: name: SCARSI, MARCO
Publisher: name: Universit� degli studi di Genova
Year: 2018

description: 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.
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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
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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.