HFSE‐REE Transfer Mechanisms During Metasomatism of a Late Miocene Peraluminous Granite Intruding a Carbonate Host (Campiglia Marittima, Tuscany)

Abstract

The different generations of calc‐silicate assemblages formed during sequential metasomatic events make the Campiglia Marittima magmatic–hydrothermal system a prominent case study to investigate the mobility of rare earth element (REE) and other trace elements. These mineralogical assemblages also provide information about the nature and source of metasomatizing fluids. Petrographic and geochemical investigations of granite, endoskarn, and exoskarn bodies provide evidence for the contribution of metasomatizing fluids from an external source. The granitic pluton underwent intense metasomatism during post‐magmatic fluid–rock interaction processes. The system was initially affected by a metasomatic event characterized by circulation of K‐rich and Ca(‐Mg)‐rich fluids. A potassic metasomatic event led to the complete replacement of magmatic biotite, plagioclase, and ilmenite, promoting major element mobilization and crystallization of K‐feldspar, phlogopite, chlorite, titanite, and rutile. The process resulted in significant gain of K, Rb, Ba, and Sr, accompanied by loss of Fe and Na, with metals such as Cu, Zn, Sn, W, and Tl showing significant mobility. Concurrently, the increasing fluid acidity, due to interaction with Ca‐rich fluids, resulted in a diffuse Ca‐metasomatism. During this stage, a wide variety of calc‐silicates formed (diopside, titanite, vesuvianite, garnet, and allanite), throughout the granite body, along granite joints, and at the carbonate–granite contact. In the following stage, Ca‐F‐rich fluids triggered the acidic metasomatism of accessory minerals and the mobilization of high-field-strength elements (HFSE) and REE. This stage is characterized by the exchange of major elements (Ti, Ca, Fe, Al) with HFSE and REE in the forming metasomatic minerals (i.e., titanite, vesuvianite) and the crystallization of HFSE‐REE minerals. Moreover, the observed textural disequilibrium of newly formed minerals (pseudomorphs, patchy zoning, dissolution/reprecipitation textures) suggests the evolution of metasomatizing fluids towards more acidic conditions at lower temperatures. In summary, the selective mobilization of chemical components was related to a shift in fluid composition, pH, and temperature. This study emphasizes the importance of relating field studies and petrographic observations to detailed mineral compositions, leading to the construction of litho‐geochemical models for element mobilization in crustal magmatic‐hydrothermal settings.

From Fossil to Active Hydrothermal Outflow in the Back‐Arc of the Central Apennines (Zannone Island, Italy)

“…The beginning of volcanic activity in the Zannone area has been dated at ∼4.2 Ma (Cadoux et al., 2005), while the onset of extensional tectonics on ZI was constrained to have started at about 7 Ma by U‐Pb dating of the syn‐tectonic carbonates associated with normal faulting and analyzed in this study (Curzi et al., 2020). We therefore propose that the hydrothermal system was active during the late Miocene, roughly coeval with the main ore stage in the Tuscan area (e.g., Brogi et al., 2011; Dini, 2003; Paoli et al., 2019; Rossetti et al., 2011; Figure 1a), and was likely fed by intrusive magmatism at depth before the onset of volcanic activity at ca. 4.2 Ma.…”

“…The Neogene‐Quaternary Tyrrhenian back‐arc region of the Apennine belt represents a key area to investigate fluid‐rock interaction, rock alteration and ore‐forming processes associated with structurally controlled hydrothermal outflow during post‐orogenic magmatism (Brogi et al., 2020; Dini, 2003; Dini et al., 2008; Fulignati, 2018; Minissale et al., 1997, 2000; Paoli et al., 2019; Rossetti et al., 2011; Serri et al., 1993; Vignaroli et al., 2016). Several examples of fossil and active structurally controlled sulfide hydrothermal deposits are documented in the inner sector of the Apennines chain (Tuscan Archipelago and Colline Metallifere of the Tuscan region), where diffuse polymetallic sulphide ores are reported and exploited since historical times (Brogi & Fulignati, 2012; Brogi et al., 2011; Corsini et al., 1980; Cortecci et al., 1985; Dini et al., 2008; Liotta et al., 2010; Martarelli et al., 1995; Rossetti et al., 2011; Figure 1a).…”

Major and rare earth element mineral chemistry of low-grade assemblages inform dynamics of hydrothermal ocean-floor metamorphism in the Dinaridic Neotethys

“…In this contribution, such hydrothermal phases are referred to as ‘metamorphic’ since hydrothermal metamorphism commonly includes partial recrystallization and the formation of clay minerals; these are essentially out of thermodynamic equilibrium and result in broad compositional ranges (Arbiol et al 2021). These hydrothermal processes are best understood as being metasomatic and allochemical, and prone to the selective addition and removal of chemical components (Harlov & Marschall, 2009; Paoli et al 2019). The hydrothermal metamorphic assemblages may therefore provide information on the alteration setting and post-emplacement alteration processes.…”