Circulation of fluids in faults controls precipitation of minerals, which can also be of economic interest. The interaction between circulating fluids and host rock may also affect the mechanical properties of faults by processes of dissolution and precipitation. Fluid-rock interaction processes thus produce local variation of fault rock mineralogy that may control reactivation of faults. Previous studies demonstrate that the permeability of fault zones varies depending on the distribution of impermeable fault cores and fractures. However, a full comprehension on how mineralizations (and hence fluids) are distributed at multiple scales of observation and the relationship between their distribution and the internal structure of the fault zone is still lacking. The proposed research project aims at contributing on this topic through a multi-technique and multi-scale approach on various fault zones in the central and northern Apennines. The km-scale geometry of fault zones and characterization of mineralizations will be accomplished by fieldwork. The outcrop scale (10m-1km) distribution of fluids within fault zones will be investigated at centimetric to metric resolution through structure-from-motion multi-view stereophotogrammetry coupled with the collection of structural data on the field and mineral composition of fault rocks and veins. The study of mineralogy of veins associated with faults and fault rocks will be used to reconstruct the geochemistry of paleofluids that circulated and reacted with the host rock. This combined structural-geochemical approach will provide crucial information on the structural control on the distribution of mineralizations, and hence on the physico-chemical properties of the circulating fluids. Preliminary data and our expertise in the topics and techniques that we will adopt make us confident for the completion of the research project.
Due to the high social and economic importance, fluid circulation and mineralization within fault zones is extensively studied (Sibson et al., 1988; Gudmundsson et al., 2001; Cox, 2016). However, due the complexity of fault zones, several aspects of the relationship between fault zone structure and fluid circulation are still unclear. The following are just some of the scientific queries that have not yet been fully understood and that necessitate the multiscale and multidisciplinary approach we propose for this research project.
1) How do fluids and mineralization compartmentalize at km-scale along faults?
2) What is the relation between mineral assemblage zoning and internal architecture of fault zones and, more specifically, with the fracture orientation and distribution within the damage zone? How do these structures control fluid migration and precipitation of minerals in the fault architecture?
3) How do the active deformation processes and fluid-rock interaction processes guide the mobility of elements along faults?
4) Why do faults in the same tectonic context (e.g., different carbonate-hosted faults in the extensional area of the Apennines) show or lack of mineralizations (i.e., veins)? How does fault architecture and circulation path affect mineralization? Does along-fault pulverization constitute an impermeable or permeable element of the fault zone?
5) Which are the transported ions that can be used as evidence of seismic activity?
In order to answer these questions, we individuated several case studies in the central and northern Apennines. The distribution of mineralizations at km scale (open question n.1) will be studied in the case studies of Monti della Tolfa and Gavorrano-Boccheggiano area, where low-sulphidation epithermal deposits occur at a regional scale. Both case studies also offer spectacular outcrops where the distribution of mineralization within fault zones (open question n. 2) can be studied also at the outcrop scale with a millimetric to metric resolution. Two key outcrops are represented by an abandoned Caolino quarry in the Allumiere village (Monti della Tolfa mineralization) and by an outcrop with sulphide mineralization close to the Capanne area (Gavorrano-Boccheggiano area). The first is characterized by rhyolitic igneous rocks affected by kaolinitization, which is evidently guided by the migration of hydrothermal fluids within fault zones. In the second case the sulphide mineralization is hosted in the Calcari a Palombini Fm. and the mineralization extent is controlled by regional scale faults. The Gavorrano-Boccheggiano case study, coupled with another case study which we identified in the Gubbio fault will also be useful to investigate the controlling factors for the mobilization of chemical elements within fault zones (open question n. 3) and transported ions during seismic events (open question n. 5). The Gubbio fault also offers an incredible example of mineralization controlled by fault architecture (open question n. 4), therefore its structure will be used to study the relationship between fault architecture and fluid circulation. Although fluids circulate within fault zones, an intriguing observation is the lack of mineralized fractures (i.e., veins) in some faults in the central Apennines whilst others, such as the Monte Gorzano fault are pervaded by cm-thick calcite veins. We will contribute to answering the open question n. 4 through the comparison of the results coming from the combination of chemical-mineralogical and structural analysis conducted on M. Gorzano fault with other vein-free previously studied faults (Tre Monti fault, Mercuri et al., 2020a,b; Campo Felice fault, Schirripa et al., 2021). Finally, the distribution and the permeability of pulverized rocks, very common in carbonate-hosted faults (e.g., Fondriest et al., 2015), will be studied for the Roccapreturo fault (central Italy), where a pulverized damage zone crops out with an impressive extension on map (>300 from the fault core).
Our expertise in structural mapping, especially at the outcrop scale (Marchesini et al., 2019; Mercuri et al., 2020a,b), in the fluid-rock interactions (Marchesini et al., 2019a,b), and in carbonate-hosted faults (Mercuri et al., 2018, 2020a,b; Schirripa et al., 2021; Marchesini et al. 2021) make us confident in the completion of the above mentioned tasks.
References:
Cox, S. F. Economic Geology, 111(3), 559-587 (2016)
Fondriest, M. et al. Tectonophysics 654, 56¿74 (2015)
Gudmundsson, A.et al. Journal of Structural Geology 23, 343¿353 (2001)
Marchesini, B. et al. Solid Earth 10, 809¿838 (2019a)
Marchesini, B. et al.. SIMP-SGI-SOGEI 2019 (2019b)
Marchesini B. et al. SGI 2021 (2021)
Mercuri, M. et al. Journal of Structural Geology 109, 1¿9 (2018)
Mercuri, M., et al. Journal of Structural Geology 130, (2020a)
Mercuri, M., et al. Journal of Structural Geology 104085 (2020b)
Schirripa et al. Tectonics (2021)
Sibson, R. H. et al. Geology, 16(6), 551-555 (1988)