Permian Hydrothermal Alteration Preserved in Polymetamorphic Basement and Constraints for Ore-genesis (Alpi Apuane, Italy)

Abstract

The reconstruction of the polymetamorphic history of basement rocks in orogens is crucial for deciphering past geodynamic evolution. However, the current petrographic features are usually interpreted as the results of the metamorphic recrystallization of primary sedimentary and/or magmatic features. In contrast, metamorphic rocks derived by protoliths affected by pre-metamorphic hydrothermal alterations are rarely recognized. This work reports textural, mineralogical and geochemical data of metasedimentary and metaigneous rocks from the Paleozoic succession of the Sant’Anna tectonic window (Alpi Apuane, Tuscany, Italy). These rocks were recrystallized and reworked during the Alpine tectono-metamorphic event, but the bulk composition and some refractory minerals (e.g., tourmaline) are largely preserved. Our data show that the Paleozoic rocks from the Alpi Apuane were locally altered by hydrothermal fluids prior to Alpine metamorphism, and that the Permian magmatic cycle was likely responsible for this hydrothermal alteration. Finally, the Ishikawa Alteration Index, initially developed for magmatic rocks, was applied to metasedimentary rocks, providing a useful geochemical tool for unravelling the hydrothermal history of Paleozoic rocks, as well as a potential guide to the localization of hidden ore deposits in metamorphic terranes.

Sulfates from the Pyrite Ore Deposits of the Apuan Alps (Tuscany, Italy): A Review

“…Here, a microcrystalline pyrite ore body is embedded within whitish quartzitic phyllites, mineralogically formed by quartz and mica with variable amounts of chlorite, with tourmalinite layers representing a common feature. As reported by Vezzoni et al [77], these rocks are enriched in K 2 O, likely as a consequence of a Permian hydrothermal alteration event. This K 2 O enrichment is likely the main reason for the abundance of K-sulfates in the assemblage from Monte Arsiccio, where the action of H 2 SO 4 is able to cause the leaching of aluminosilicates, that may be the main source of K [23].…”

Lithium Occurrence in Italy—An Overview

“…Evidence of a Middle Permian magmatism (tourmaline-bearing felsic metaigneous rocks) was recently found in the basement of the Apuan Alps (Northern Tuscany; [75,76]).…”

“…The Permian magmatism mostly affected the Variscan basement, and possibly Permian sediments, producing widespread tourmalinization and phyllic alteration, with local formation of small Pb-Zn-Ag and barite-pyrite orebodies [76,77]. Geochemical analyses and mineralogical studies [78,79] highlighted the Sn-W-Be anomalous character of the ores, but lithium potential was not addressed.…”

Secondary Sulfates from the Monte Arsiccio Mine (Apuan Alps, Tuscany, Italy): Trace-Element Budget and Role in the Formation of Acid Mine Drainage

“…In addition, the composition of these latter phases is also influenced by the geochemistry of host rocks. No data are available for the Monte Arsiccio mine; however, Vezzoni et al [41] reported major-and trace-element contents in the Paleozoic formations cropping out in the Sant'Anna tectonic window, where the Monte Arsiccio mine is located. In particular, the host rocks are strongly enriched in K, resulting in the relatively high abundance of K-sulfates in the studied assemblage.…”