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The Bisciarelle fault is a brittle thrust crosscutting lherzolite of the Voltri Massif (Italy) and is similar to the historical gold veins of the area. This is a 16-m-thick fault showing a large damage zone and a 4-m-wide hydrothermal alteration halo. Its fault rocks interacted with a hydrothermal fluid and host peculiar Au-bearing zones made of concentric and circular dolomite aggregates, which we call spherulites, and chalcedony. To constrain spherulite genesis, we quantify fluid-fault interactions at Bisciarelle combining field and microtextural data, mass transfer calculations, and a multi-technique analytical dataset including element imaging by laser ablation-inductively coupled plasma-time-of-flight mass spectrometry. We show that faulting was coupled with significant transfer of H2O, CO2, Ca, Sb, and W from the fault fluid to the protolith, and variable transfers of SiO2 and some trace elements from the protolith to the fluid. This process deposited Au within the spherulites, caused serpentinization and carbonation of the protolith, and weakened the fault localizing subsequent shearing events and providing components for the growth of spherulites and the other fault rocks. We interpret Bisciarelle as a permeable epizonal orogenic vein, which formed fast as a result of dilation and top-to-NE shearing. This fault developed via cycles of fluid pressure build-up, opening, fluid effervescence, and mineral precipitation from a H2O-CO2 hydrothermal fluid. The fluid transported and deposited a suite of metals including Au and interacted with the protolith. A similar mechanism of faulting and reactive fluid flow could have generated the other Au deposits of the area. [ABSTRACT FROM AUTHOR]

Abstract: Geochemical, structural, field, and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths. Models for pulsed intrusion of small magma batches over long timescales (>1Ma) versus transfer of larger magma bodies on shorter timescales predict a different thermal, metamorphic, and rheological state of the crust, highlighting the importance of robust time constraints. This study focuses on a well-characterized upper crustal intrusion, the 15km2 Lago della Vacca complex (LVC), Adamello batholith, N. Italy. Previous studies used structural and petrologic data to argue that the LVC was emplaced through pulsed magma injection and in situ expansion (or, ballooning) of a short lived (∼ 105 yr) magma chamber. We test this model using a dense sampling strategy and high-precision ID-TIMS U–Pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral (U–Pb TIMS-TEA). These data show that the marginal mafic pulses of magma crystallized zircon with primitive Hf isotopes and negligible Eu anomalies during fractional crystallization and ascent through the crust on 10–30ka timescales. Subsequent, more felsic pulses yield individual zircon dates spanning as much as 200ka within single handsamples and restrict the total construction time of the LVC to <300ka. Rim to core solidification of the LVC, as recorded by titanite U–Pb thermochronology, occurred in ≥300ka. U–Pb TIMS-TEA data from these complicated zircon populations limit the sources of antecrystic zircon, constrain AFC processes within host magmas, and illustrate the difficulty in interpreting zircon dates as magma emplacement ages. These data are supportive of growth of the LVC by a pulsed ballooning-type process over ∼300ka, with the restriction that the rims of the LVC had solidified prior to the center-most injection, providing important constraints for thermomechanial models and strain analysis of the LVC and other balloon-like intrusions. [Copyright &y& Elsevier]