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Article

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

by
Simone Vezzoni
1,
Diego Pieruccioni
2,*,
Yuri Galanti
3,
Cristian Biagioni
2 and
Andrea Dini
1
1
Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, I-56124 Pisa, Italy
2
Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy
3
Dipartimento di Pistoia, ARPA Toscana, Via dei Baroni 18, I-51100 Pistoia, Italy
*
Author to whom correspondence should be addressed.
Geosciences 2020, 10(10), 399; https://doi.org/10.3390/geosciences10100399
Submission received: 13 September 2020 / Revised: 28 September 2020 / Accepted: 29 September 2020 / Published: 5 October 2020
(This article belongs to the Special Issue Subduction and Exhumation of the Lithosphere: The Contribution of Structural Geology, Petrology and Geochronology)
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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.

Graphical Abstract

1. Introduction

Polymetamorphic basement complexes may show a large variety of lithotypes that are usually interpreted as primary variations in sedimentary inputs and/or in the composition of magmas that fed igneous bodies, as well as metasomatic changes experienced during metamorphism/subduction (e.g., [1,2]). Little attention has been given to the possibility that some peculiar lithotypes actually derive from the metamorphism of hydrothermally-altered protoliths formed during magmatic-hydrothermal events preceding the last metamorphic event (e.g., [3,4]). The identification of hydrothermal alterations predating metamorphism is relevant for the reconstruction of old magmatic-hydrothermal systems and the correct interpretation of past geodynamic settings (e.g., [5]). In addition, they can be used as potential guides to the localization of hidden ore deposits.
Hydrothermal alteration (triggered or not by magmatism) can produce very peculiar mineralogical assemblages and geochemical fingerprints, significantly deviating from the usual sedimentary and/or magmatic realms. These features can still be recognized after metamorphic recrystallization. It is well-known that low-grade metamorphism can be a nearly isochemical process, and thus, the mobility of chemical components is largely restricted to volatiles (e.g., [6,7,8,9]). Consequently, the metamorphic processes can modify the texture and mineralogy of the protoliths, but the bulk composition and some refractory minerals (e.g., tourmaline, zircon) are commonly preserved.
This contribution provides geochemical and mineralogical evidence that a pre-Alpine hydrothermal event related to ore genesis is recorded within the Paleozoic succession of the Alpi Apuane, affecting both the Cambrian-Early Ordovician phyllitic rocks and the felsic magmatites of Permian age.

2. Geological Background

2.1. Regional Geology

The Northern Apennine (Italy) is a collisional belt related to the convergence between the Europe and Adria plates active since Late Cretaceous ([10] and references therein). The Alpi Apuane is the largest tectonic window of the Northern Apennine in which the deepest structural levels are exposed (Figure 1). The tectonic units are representative of the distal part of the Adria continental margin (Tuscan Domain) and they lie below the westerly-derived oceanic Ligurian and sub-Ligurian accretionary wedge units ([11,12,13] and references therein).
Three main stratigraphic (Tuscan Nappe) and tectono-metamorphic units (Massa and Apuane units) are traditionally distinguished in the Alpi Apuane region, all of which are derived from the Adria paleo-margin. The Tuscan Nappe is a sequence formed by Mesozoic carbonates, siliceous rocks and Tertiary deep-water and turbidites, mainly detached from their original basement along the decollement level of the former Carnian and Norian evaporites (e.g., [15,16] and reference therein). The Massa Unit consists of a Paleozoic metasedimentary and metavolcanic basement and an upper Permian-Upper Triassic metasilicoclastic succession (Figure 1; e.g., [16,17]). The Apuane Unit includes a litho-stratigraphic sequence made up of a Paleozoic basement (e.g., [18]) intruded by post-Variscan magmatic rocks (Fornovolasco Metarhyolite Fm; [19,20,21]), unconformably overlain by an Upper Triassic-Oligocene metasedimentary succession (Figure 1 and Figure 2; e.g., [16]).
The Tertiary tectonic evolution of the Alpi Apuane includes an early stage of underplating and antiformal stacking, associated with peak metamorphism and isoclinal folding (D1 event according to [11]). This stage was followed by deformation associated with syn-contractional exhumation, during which folding and subhorizontal crenulation cleavage was developed (D2 event according to [11,23]). The latest stages of geological evolution, associated with the final exhumation and uplift of the Alpi Apuane, were characterized by brittle faulting (low- and high-angle faults) during “post-orogenic” regional extension of the inner part of the Northern Apennine wedge (e.g., [24,25,26]).
In the metamorphic units of the Alpi Apuane, peak conditions are roughly estimated between 350–450 °C and 0.4–0.8 GPa ([27] and references therein). The early deformation D1 occurred during early Miocene at 27–20 Ma [28], whereas syn-metamorphic D2 structures developed at temperatures higher than 250 °C and predated cooling at 11 Ma, according to zircon fission-track ages [29].
The Sant’Anna tectonic window is located in the southern sector of the Alpi Apuane (Figure 1). Notwithstanding its small exposed area, it is a key zone for the study of ore deposits. Indeed, several small pyrite ± baryte ore bodies, associated with Pb-Zn-(Ag) and iron oxides ores, occur in this area and were formerly exploited in some mines (Pollone, Verzalla, Monte Arsiccio, and Zulfello; Figure 2). Its complex tectonic setting is characterized by the occurrence of the polymetamorphic Paleozoic basement, intruded by post-Variscan magmatic rocks, and the Triassic-to-Oligocene metasedimentary cover belonging to the Apuane Unit. Ore bodies are located within the Paleozoic succession or close to the contact with Triassic metadolostone (e.g., [30,31]). The metamorphic rocks are overlain by the Tuscan Nappe (Figure 2).

2.2. The Paleozoic Succession of Sant’Anna Tectonic Window

The Paleozoic succession of Sant’Anna tectonic window (southern Alpi Apuane) is formed by two geological formations: the “Filladi Inferiori” and the Fornovolasco Metarhyolite Fms (Figure 2).
The “Filladi Inferiori” Fm is made up of light- to dark-grey quartzite, dark-grey and/or grey-greenish phyllitic quartzite and phyllite, interpreted as a metamorphic product of an original alternance of quarzitic sandstone and pelite [16,32,33]. The top of this formation is locally marked by the presence of a discontinuous layer of matrix-supported metamorphic paraconglomerate that contains quarzitic pebbles and grains of magmatic quartz and feldspar in a quarzitic-feldspathic matrix [18,33]. In the southern Alpi Apuane, a peculiar tourmalinite-bearing facies, spatially associated with orebodies, has been referred as the “Fornovolasco Schists” Fm; it was attributed to a different lithostratigraphic unit (e.g., [34]). However, this interpretation has been strongly debated (e.g., [19]). A Cambrian-early Ordovician depositional age of the “Filladi inferiori” Fm was proposed through the correlation with the Paleozoic successions of Sardinia (e.g., [18,35]); this dating was recently confirmed by Paoli et al. [36] and Pieruccioni et al. [37], who suggested a depositional age between early Cambrian and middle-late Ordovician times, based on LA-ICP-MS U-Pb zircon data.
The Fornovolasco Metarhyolite Fm is formed by fine-grained massive subvolcanic rocks with a granular to porphyritic texture, locally showing the widespread occurrence of cm-sized tourmaline + quartz orbicules [19,20]. On the basis of LA-ICP-MS U-Pb zircon dating, Vezzoni et al. [20] dated this formation to Permian (c.a. 270 Ma). The Fornovolasco Metarhyolite Fm occurs as decameter-sized lensoid bodies embedded within the “Filladi Inferiori” Fm, commonly associated with the facies previously referred to as “Fornovolasco Schists” [21,34].

2.3. The Alpi Apuane Ore District

The Alpi Apuane hosts several small polymetallic orebodies [14,38] (Figure 1 and Figure 2) discontinuously exploited almost from the Renaissance to the end of the 1980s. The main orebodies widely crop out in the southern Alpi Apuane and are preferentially hosted in the “Filladi Inferiori” Fm. Among them, the pyrite ± baryte ± Fe-oxides deposits are the most important ones, representing the volumetrically largest orebodies. The pyrite, baryte, and Fe-oxides ores are an example of metamorphosed orebodies (following the definition reported in e.g., [8,39]) in which the primary mineralogy and texture were modified by the Alpine metamorphism and associated polyphasic deformation (e.g., [40] and references therein). Previous authors debated the origin and age of the ores (e.g., [41] and references therein), and recently, a magmatic-hydrothermal origin, likely Permian age, was suggested [20] on the basis of the following observations:
  • the discovery of tourmaline-bearing Permian felsic shallow-intrusive rocks (Fornovolasco Metarhyolite Fm) [19,20] spatially associated with the orebodies;
  • the elemental association of Tl, Hg, As, and Sb in pyrite ores, a typical geochemical feature of low-temperature hydrothermal systems (e.g., [40,41,42]);
  • Pb isotope data [41], showing similar values with late Paleozoic-Triassic ores of Sardinia.
However, the effects of hydrothermal activity on the rocks hosting the orebodies are largely unknown, and the occurrence of tourmaline veins have been considered as the only evidence of these hydrothermal processes (e.g., [20,34,41]).

3. Materials and Methods

In total, 61 samples of phyllites and quarzitic phyllites belonging to the “Filladi Inferiori” Fm [18,22] were collected in the Sant’Anna tectonic window (Figure 2). The samples were collected following the method of the channel sampling (e.g., [43]). The channels were 50 cm long and ca. 5 cm wide, and were cut perpendicular to the main foliation, avoiding quartz veins, tourmalinite levels, and sulfide enriched-volumes. The samples were collected using a grid of around 150 m × 150 m covering the whole outcrop of the Paleozoic succession of Sant’Anna tectonic window. Finally, an oriented block sample was collected near all the channels and sixteen thin sections covering the whole petrographic and geochemical variability of the “Filladi Inferiori” Fm were made. For the sake of comparison, all samples, thin sections, and geochemical data of the Fornovolasco Metarhyolite Fm, reported in Vezzoni et al. [20], were used. The database of metarhyolite samples was increased by other samples and thin sections and bulk geochemical data. Sample locations and geochemical data are reported in Table S1.
The petrographic features of the rock samples were investigated by optical microscopy. Chemical analyses of “Filladi Inferiori” Fm samples were performed at ACTLABS (Ancaster, Ontario, Canada). Major and trace elements were determined by ICP-OES and ICP-MS, respectively, following lithium metaborate/tetraborate fusion and dissolution with diluted HNO3. Additional samples of the Fornovolasco Metarhyolite Fm were analyzed for major and trace elements at the Dipartimento di Scienze della Terra, University of Pisa. Major elements were determined by XRF Philips PW1480 using fused glass discs following the method reported in [44]. Trace elements were determined by ICP-MS (VG PQII Plus STE). Samples were dissolved in perfluoralkoxy (PFA) vials at about 120 °C with a HF + HNO3 mixture following the standard lab protocols.
The distribution of Ishikawa Alteration Index (AI, see chapter 4 for further details) has been gridded and contoured throughout the study area using the Natural Neighbor interpolation method [45] available in Surfer software code [46]. We opted for this method because it provides reliable results with data sets that have an inhomogeneous distribution of data. The Natural Neighbor interpolation estimates the grid node value by finding the closest subset of input data points to a grid node and then applying weight to each. The Natural Neighbor method does not extrapolate the Z-grid values beyond the range of data and does not generate nodes in areas without data. In order to take into account the anisotropic distribution of the different lithotypes (controlled by the Alpine deformation as constrained by geological mapping) and the variable density of data, an anisotropy ratio of 1.5 was used coupled with an anisotropy angle of 45° E and 40° W, respectively, in the south-eastern and in the northern sector of the study area. This approach provided a contour map of the distribution of the AI that is remarkably consistent with the real distribution of the geological formations (whitish and/or tourmalinized schists, ores, metarhyolites) as also reported in the geological map of Orberger [47].

4. Results

4.1. “Filladi Inferiori” Fm

4.1.1. Field and Petrographic Investigation

The prominent feature of the “Filladi Inferiori” Fm of the Sant’Anna tectonic window is the wide variability in color, texture, and modal mineralogy (Figure 3). Two end-members facies have been recognized: (i) a tourmaline-poor to tourmaline-free facies (FAF; Figure 3a,b) and (ii) a whitish facies with variable abundance of tourmaline (w-FAF; Figure 3c,d). The whitish facies was recognized in other area of the southern sector of the Alpi Apuane (e.g., [19,34]).
We observed a continuous transition between the FAF and w-FAF end-members, at all scale of observation, from field to hand (Figure 3) and microscale (Figure 4). The FAF facies consists of light- to dark-grey fine-grained quartzite, dark-grey and/or grey-greenish phyllitic quartzite, and phyllite. At the macroscale, this facies is characterized by a pervasive foliation and a very fine-grained texture and, sometimes, it shows alternance of centimeter-thick layers with different colors (e.g., Figure 3a,b). Deformed smoky and milky quartz veins are common (Figure 3a), as are late and vuggy quartz veins (Figure 3b). At the microscale, it consists of an alternation of millimeter-sized granoblastic and lepidoblastic levels. The granoblastic levels are dominated by quartz with minor albite and rare carbonates, while the lepidoblastic levels are mainly formed by fine-grained oriented white mica and a variable amount of “chlorite” and carbonaceous materials (Figure 4a,b). Common accessory minerals are opaque minerals associated with rare sulfides (typically pyrite) and tourmaline supergroup minerals (hereafter tourmaline). Tourmaline shows clastic texture, and their crystal size does not usually exceed 60 μm in length (Figure 4b). Rarely, tourmaline porphyroclasts (up to 3 mm) occur.
The w-FAF facies is represented by quarzitic-phyllites and quartzite with a characteristic whitish color due to the occurrence of abundant quartz and large crystals of white mica, up to 3 mm across. The mica crystals are iso-oriented, developing a penetrative foliation (Figure 3c,d). The w-FAF facies is ubiquitously associated with pyrite and baryte and contains variable amounts of tourmaline-rich veinlets and bodies, from millimeters to decimeters in size. Furthermore, the w-FAF facies is spatially associated with the main orebodies of the Sant’Anna tectonic window as, for instance, those previously exploited at the Pollone and Monte Arsiccio mines. At the microscale, the w-FAF consists of granoblastic levels, mainly formed by quartz, and lepidoblastic layers of white mica associated with variable amounts of “chlorite”, and rare carbonaceous materials. Petrographic investigation reveals the absence of feldspars, while pyrite and baryte are widespread, even if their modal abundance is highly variable. Tourmaline can be found as very small relicts (<50 μm), or as zoned euhedral to anhedral porphyroclasts, up to 5 mm, sometimes forming tourmalinite levels and pods (Figure 4e,f).

4.1.2. Geochemistry

The “Filladi Inferiori” Fm can be classified using chemical methods for (meta-)sedimentary rocks (Figure 5a–c). For the sake of comparison, the geochemical data of “Filladi Inferiori” Fm outside the study area available in literature (from Andreotti [48] and Dini [49]; green cross symbols) were reported.
The classification diagrams stress the intra-unit variability in SiO2, Al2O3, and Fe2O3(tot) associated with a very low (CaO + MgO) concentration (Figure 5a–c). The samples have a SiO2 concentration varying from ≈55 to 83.5 wt%, whereas the Al2O3 concentration ranges between 10.5 and 23.5 wt%. The Fe2O3(tot) concentration is low and comprises between 1.0 to 8.5 wt%. The Na2O/K2O ratio is generally low to very low, with a negative correlation between Na2O and K2O. Their concentrations range from around 0.2 to 3.3 wt% and 1.9 to 6.5 wt%, respectively. In particular, the w-FAF samples have a Na2O/K2O ratio lower than 0.1, a value considered the common lowest limit for sedimentary rocks (see [51,55]). These samples also show the higher Alteration Index (AI) values (Figure 5b). This index was defined by Ishikawa et al. [56] to quantify the intensity of sericite and chlorite alteration in magmatic rocks that occurs in the footwall rocks proximal to Kuroko hydrothermal deposits (see [57] for further details); it is expressed by the following formula:
A I = 100 ∗ ( K 2 O + M g O ) ( K 2 O + M g O + N a 2 O + C a O )
The AI can be used for the geochemical discrimination between FAF and w-FAF facies. Indeed, the former has lower AI values than the latter. For this reason, the use of AI in the plots (e.g., Figure 6 and Figure 7) shows us immediately which facies the sample belongs to.
Figure 6 shows the major elements and L.O.I. variations integrated by the chemical compositions of the main rock-forming minerals. The wt% of these phases are recalculated from the quartz, albite, “andesine” (An40), and calcite stoichiometric formula, whereas selected chemical electron microprobe (EPM) data measured on white mica and “chlorite” [47,58] from “Filladi Inferiori” samples were used. The plots in Figure 6 indicate that the variability of the major elements is restricted within the albite-muscovite-chlorite-quartz field in agreement with the petrographic investigation. Furthermore, the alteration box plot, a combination of the AI and the Chlorite-Carbonate-Pyrite Index (CCPI) plotted in the x-axis and y-axis, respectively, is reported (Figure 6a) [57]. The CCPI is based on major elements and was defined as:
CCPI = 100 × ( M g O + F e O ) ( M g O + F e O + N a 2 O + K 2 O )
where FeO is the total FeO + Fe2O3 concentration of the rock. The CCPI was developed for geochemical exploration perspective (e.g., [59,60,61]), with a focus on Volcanic-Hosted Massive Sulfide (VHMS) deposits. This index is related to the hydrothermal fluid temperature and H2O/rock ratios. The CCPI varies between around 25 and 60, not showing obvious correlations with the AI.
Finally, the K and Rb concentrations show a positive correlation with the AI (Figure 7a,b); a similar but more scattered correlation occurs also for Ba, Tl (Figure 7c,d) and other metals and metalloids (Cu, Zn, As, Ag, Sb, Pb, Bi; not reported in figure).

4.2. Fornovolasco Metarhyolite Fm

4.2.1. Field and Petrographic Investigation

Field evidence and petrographic investigation of the Fornovolasco Metarhyolite Fm are described by Vezzoni et al. [20]. In addition to the main rock bodies cropping out close to the Fornovolasco hamlet [19,21], several smaller bodies occur in the Sant’Anna tectonic window (Figure 2). They are decimeter- to decameter-sized and are usually flattened on the main field foliation. The primary magmatic features are variably preserved. The Fornovolasco Metarhyolite bodies are preferentially hosted in the w-FAF, spatially associated with pyrite-baryte ore, tourmaline-rich veins, and tourmalinite. However, they were also found embedded in the FAF facies. Similar field relationships were observed in all the southern Alpi Apuane [20].
Usually, the metarhyolite bodies hosted in the FAF facies show well-preserved primary features characterized by granular to porphyritic texture with tourmaline-quartz orbicules, up to 4 cm, surrounded by a leucocratic halo. The primary mineralogical features are partially preserved, as shown by phenocrysts of quartz (sometimes showing magmatic embayments), feldspars (usually completely sericitized), “biotite” (locally replaced by “chlorite”), white mica, and tourmaline minerals (see [20] for further details). In contrast, the rock bodies embedded within the w-FAF facies are usually more altered and strongly deformed. In addition, they can be more commonly associated with baryte + pyrite orebodies, as well as tourmalinite layers. At the mesoscale, these samples can be identified due to the occurrence of dark-blue to black tourmaline-rich spots in a whitish foliated matrix. The tourmaline-rich spots are a textural feature distinguishing them from all the other rocks occurring in the Sant’Anna tectonic window. In thin section, the primary magmatic texture (e.g., porphyritic texture; Figure 8a,b) is largely lost, and only the spots are partially preserved with euhedral quartz in dendritic- to radiated-aggregates of tourmaline crystals. The tourmaline crystals are strongly fractured, and the tourmaline-rich spots are wrapped by foliation (Figure 8c,d). The matrix consists essentially of granoblastic quartz and lepidoblastic white mica with large grain-size, i.e., commonly up to 3 mm, associated with a minor “chlorite”. The altered rock hosts dispersed pyrite and baryte as accessory phases and trace amounts of other sulfides and sulfosalts.

4.2.2. Geochemistry

The classification of the Fornovolasco Metarhyolite Fm has been recently discussed [20], whereas in this paper, the geochemical variability of this rock is investigated. The classification diagrams (Figure 5d–f) show the variability of the concentration of major elements while the considered immobile elements in the Zr/TiO2 × 0.0001 vs SiO2 are substantially constant. This variability is mainly related to the alkali content, that is correlated with the alteration of the sample. The less altered samples (those having lower AI values) have an alkali content of around 6 wt%, a value typically found in calc-alkaline rhyolites. In contrast, the altered samples (those having higher AI values) show a lower alkali content (around 4 wt%). It is worth noting that the alteration is related to lower Na2O and higher K2O concentrations. Thus, the AI can be used for the geochemical discrimination of least and most altered samples, like in the “Filladi Inferiori” Fm.
The major elements and CCPI show similar trends to those observed for the “Filladi Inferiori” Fm (Figure 6), although some components have a lower variability (e.g., SiO2, MgO, L.O.I.). As previously described for the “Filladi Inferiori” Fm, the observed major elements variability falls within the albite-muscovite-chlorite-quartz field, in agreement with the petrographic investigation (for white mica and chlorite EPM data see [20]).
Potassium and Rb concentrations show a clear positive correlation with the AI (Figure 7a,b) while Tl and Ba show a more complex trend, although the higher values are related to the most altered samples (Figure 7c,d).

5. Discussion

5.1. Pre-Alpine Hydrothermal Alteration Recorded by the Alpi Apuane Paleozoic Rocks

As reported above, several ore deposits occur in the Sant’Anna tectonic window. Usually, the genesis of hydrothermal ore deposits may be associated with a pervasive alteration of the country rocks (e.g., [62,63,64,65]). However, only the occurrence of a widespread tourmalinization is currently recognized as evidence of a hydrothermal process related to ore genesis (e.g., [41,66]). No other hydrothermally-altered rocks, typically associated with orebodies, have been reported so far.
However, the data given in this work clearly indicate the occurrence of different facies in the Paleozoic rocks characterized by variable degrees of hydrothermal alteration. This is particularly evident for the Permian metarhyolite belonging to the Fornovolasco Metarhyolite Fm, which may play a central role in the identification of the hydrothermal alteration affecting these rocks. The main factors which make it possible to use metarhyolite to assess the occurrence of hydrothermal processes recorded in the Alpi Apuane are:
  • Permian metarhyolite experienced only the Alpine metamorphic event;
  • the geochemistry of a calc-alkaline rhyolitic rock is less variable than that of a sedimentary protolith like the “Filladi Inferiori” Fm (Figure 5);
  • several diagrams for the investigation of hydrothermal alteration were developed for igneous rocks (e.g., [63]).
The two different facies recognized within the Fornovolasco Metarhyolite Fm may be related to different alteration degrees. Indeed, one facies is characterized by a relatively well-preserved magmatic texture and mineralogy and display low AI values, associated with high Na2O/K2O ratios, typical of calc-alkaline rhyolite. The other facies is typically strongly deformed and foliated, and few remnants of the pristine magmatic textures and mineralogical features are preserved. These rocks display the highest AI values, with a K2O enrichment and a strong depletion in Na2O.
Both rock facies can be classified as (meta-)rhyolite/rhyodacite-dacite on the basis of the immobile elements (Figure 5), whereas, taking into account the mobile elements (i.e., the alkali content), they show a strong data dispersion (TAS and AFM diagrams; Figure 5). However, the very low Na2O concentration is not compatible with an “unaltered” rhyolite (e.g., [67] and reference therein), supporting the occurrence of post-crystallization hydrothermal alteration.
Since this process does not affect the facies having low AI values, and both facies suffered the Alpine metamorphism, we hypothesize that the Alpine metamorphism has not significantly affected the geochemistry of the protoliths. This could be in agreement with previous works suggesting a limited fluid-rock interaction during syn-metamorphic processes in the Alpi Apuane metamorphic complex (e.g., [68,69,70,71]). This assumption was valid for the metasedimentary Triassic-Miocene cover, whereas Cortecci et al. [69] suggested extensive fluid flow in the basement units. However, these latter authors recognized the possibility that this extensive fluid flow could pre-date the Apennine orogeny. Indeed, other data supported the existence of only small vein systems of Alpine age that represented localized drainage systems in the basement rocks (e.g., [72]). The metamorphism of the Alpi Apuane was therefore substantially isochemical, and thus, the chemistry of the pre-Alpine history may be preserved.
To support this interpretation, we report a litho-geochemical approach based on major element variability—the alteration box plot (Figure 9). This diagram was developed to discriminate among geochemical trends due to diagenetic and/or hydrothermal alteration [57].
The first end-member plot in the less altered rhyolitic field of the alteration box plot, while the other one falls toward the right side of the plot, in agreement with the common hydrothermal processes that shift the bulk composition toward this side [57]. A petrographic investigation suggests that the geochemical shift is related to the disappearance of albite and the modal increase of white mica, explaining the decrease and increase of Na2O and K2O concentrations, respectively. The role of “chlorite” and pyrite is less evident, as shown by the slight increase in the CCPI (the most sensible index for the increase of these phases), while quartz is weakly enriched, as observed in the TAS diagram (Figure 5).
The metarhyolite bodies are hosted within the “Filladi Inferiori” Fm. It is therefore reasonable to look for hydrothermal alteration also in this formation. Indeed, the tourmalinization and the presence of pyrite disseminations and pods support this hypothesis. In addition, two different facies, interpreted as the results of the different alteration degrees, have been observed. The FAF facies is the least hydrothermally altered, and occurs in a distal position with respect to the main orebodies and metarhyolite bodies outcropping in the Sant’Anna tectonic window. The features are similar to those described in other outcrops of the Alpi Apuane [16,32,33], where these rocks are characterized by the occurrence of albite and the absence of significant amounts of pyrite and tourmaline pods and veins (Figure 3a,b). The samples from the FAF facies have low AI (50–65), associated with a variable CCPI (25–60). A transition between this facies and the w-FAF was observed. The latter is characterized by the presence of tourmaline-rich veins and pods associated with a dispersed baryte + pyrite mineralization. The w-FAF is spatially associated with the orebodies and is characterized by the highest AI values (>85) and variable CCPI (30–60). The CCPI variability could be a relict of the primary inhomogeneity of the sedimentary protolith as supported by:
  • field and petrographic data (e.g., alternance of centimeter-thick layers with different colors, variable modal “chlorite” content in the less altered samples; Figure 3a–c and Figure 4a–d);
  • similar variability of the CCPI value for both FAF and w-FAF facies (Figure 9).
In summary, the hydrothermal processes yielded similar mineralogical and geochemical features both on the Fornovolasco Metarhyolite and on the “Filladi Inferiori” Fms. This similarity is probably derived from the simple and similar main mineralogy of the two rocks. Both formations were affected by two main hydrothermal processes, corresponding to (i) tourmalinization, and (ii) sericitization. The former is easily recognizable in the field due to the occurrence of millimeter- to decimeter-sized tourmalinite bodies. These particular hydrothermal rocks are usually associated with the whitish color of the host rocks, making it possible to distinguish among the most altered samples (i.e., w-FAF facies). The whitish color is indeed one of the main field features of the w-FAF, resulting from the modal increase of white mica and quartz. The mineral proportion explains the significant increase of the AI with no or moderate increase in the CCPI (Figure 9) and the largely constant Al2O3 concentration during the alteration (Figure 6f). Finally, the slight increase of SiO2 is more evident for the Fornovolasco Metarhyolite samples (Figure 5 and Figure 6; e.g., TAS diagram). These features are typical of sericite alteration (e.g., [57]). The relative timing of tourmalinization and sericitization is not clear. In similar ore deposits, tourmalinization postdated sericitization in a “short” time interval (e.g., [73]). However, further studies are needed to clarify this issue in the Alpi Apuane.

5.2. Constraint on the Permian Age of Hydrothermal Alteration

The hydrothermal alteration recognized in the Paleozoic rocks occurs only in the pre-Triassic “Filladi inferiori” and Fornovolasco Metarhyolite Fm.
There are further constraints on the actual dating of this hydrothermal process. Indeed, tourmalinization and sericitization affecting the Fornovolasco Metarhyolite, dated at ca. 270 Ma [20], give a lower limit for the hydrothermal event. The higher limit is given by the occurrence of tourmalinite clasts in the upper Carnian-lower Norian metasedimentary rocks of the Alpi Apuane (e.g., [19,74]), as well as the occurrence of Ba-rich mineralized clasts in the Late Triassic–Lower Jurassic formation [75]. This suggests that hydrothermally-altered rocks were partially exposed and eroded at least during the Middle–Late Triassic. Taking also into account that no evidence of hydrothermal alteration is recorded in the Upper Triassic–lower Miocene rocks, the hydrothermal event should be likely constrained between the emplacement of the metarhyolite bodies and the pre-Carnian metasilicoclastic rocks. However, we hypothesize that the hydrothermal alteration developed in a short-time interval with respect to the emplacement age of Fornovolasco Metarhyolite bodies, based on:
  • the type of hydrothermal alteration (i.e., tourmalinization) typically associated to the shallow intrusion of tourmaline-bearing felsic magmatic rocks (e.g., [76,77]);
  • the clear spatial distribution of hydrothermally altered “Filladi Inferiori” and Fornovolasco Metarhyolite bodies (Figure 10);
  • comparison with other post-Variscan Permian ore deposits and magmatic rocks in Europe with similar features (e.g., [78,79,80,81]);
  • the description of several magmatic-hydrothermal systems in which the hydrothermal alteration follows, in a short-time interval, the emplacement of magmatic rocks (e.g., [82,83,84]).
Consequently, a Permian hydrothermal event may be recorded in the Alpi Apuane Paleozoic rocks. This seems to have close relationships both with the post-Variscan magmatic cycle and the ore genesis.

5.3. Hydrothermal Alteration and Ore Genesis

Among the different genetic models proposed for the ore deposits from the Alpi Apuane, recent data seem to support a hydrothermal origin, as suggested, for instance, by the association of Tl-Hg-As-Sb, typical of low-temperature hydrothermal systems [41]. The identification of hydrothermally-altered rocks in the mineralized areas of the Sant’Anna tectonic window allows us to refine this scenario. In fact, although a detailed geological mapping of the entire study area is still lacking, the AI contour map (Figure 10) shows with a good approximation the real distribution of the w-FAF (higher AI values and widespread tourmalinization) within the Sant’Anna tectonic window, showing their close spatial relationship with both metarhyolite (in red) and the main ore bodies (in black, generally formed by baryte + pyrite with minor amount of Pb-Zn-Ag sulfides).
The w-FAF is the host rocks of the ore bodies, as previously reported by other authors (e.g., [34,47,72]). Our study has pointed out an increase in the alteration degree toward the ores. Figure 10 visualizes such a variation of the AI index around the ore bodies. The close spatial relationships between rock alteration and ore bodies suggest a genetic link between these two geological features. Indeed, the occurrence of alteration halos around the orebodies is a typical feature reported in hydrothermal ore-forming systems worldwide (e.g., [62,63,64,65]); it is characterized by a decrease in the alteration moving away from mineralization, as observed in the Sant’Anna tectonic window. These geological data allow us to hypothesize that the ore deposits could be genetically linked to a Permian hydrothermal event, very likely associated to the emplacement, in the shallow crust, of felsic magmatic bodies.
This interpretation is also supported by the correlation between the AI values and the Ba and Tl concentrations in the samples (these two elements being the most characteristic elements of the Alpi Apuane ores; [40,41]; Figure 7), as well as with other metals and metalloids (Cu, Zn, As, Ag, Sb, Pb, Bi) typically occurring in these mineralizations.

6. Conclusions

The pre-Alpine formations cropping out in the Sant’Anna tectonic window (Alpi Apuane, Italy) recorded the occurrence of a hydrothermal alteration event predating the Alpine metamorphism. This event locally produced tourmalinization and sericitization, affecting both the Cambrian-Early Ordovician phyllitic rocks (FAF), giving rise to whitish schists (indicated as w-FAF), and the felsic magmatites of Permian age. In contrast, the Triassic to Miocene metasedimentary cover was not involved in such an alteration process.
The age of the hydrothermal event is bounded between the age of the younger magmatic rocks (around 270 Ma) and the occurrence of tourmalinite clasts in the upper Carnian-lower Norian metasedimentary rocks of the Alpi Apuane. On the basis of field evidence, a Permian age is suggested, being related to the Permian magmatic cycle, in analogy with similar geological contexts.
The spatial distribution and geochemical features of hydrothermally altered rocks point to a genetic link with the ore bodies occurring in the Sant’Anna tectonic window, whose Permian age is thus hypothesized. This conclusion is possible owing to the near isochemical nature of the Alpine metamorphism in the Alpi Apuane, in agreement with previous authors (e.g., [68,69,70,71]), that preserved the original geochemical variability of the Paleozoic rocks.
At a larger scale, the present study reveals that the Alpi Apuane have recorded an hydrothermal post-Variscan event with similar features to those recognized in many ore districts in Europe (e.g., [77,78,79,80,81,85]). Further studies are needed to specify this scenario.
Finally, it is worth noting that the Ishikawa Alteration Index, originally developed for magmatic rocks, proved to be a useful geochemical tool for unravelling hydrothermal alteration, also in polymetamorphic basement rocks with sedimentary protoliths, resulting in a potential guide for the localization of hidden ore deposits.

Supplementary Materials

The following are available online at https://www.mdpi.com/2076-3263/10/10/399/s1, Table S1: major and trace element analyses of “Filladi Inferiori” and Fornovolasco Metarhyolite Fms.

Author Contributions

A.D. and S.V. conceived and designed the experiment; Y.G., D.P. and S.V. collected samples and field geological data on the Paleozoic rocks and prepared samples for geochemical analysis; S.V. performed ICP-MS analysis at the Dipartimento di Scienze della Terra—Università di Pisa; A.D. and D.P. realized the geostatistical interpolation of chemical data; S.V. wrote the original manuscript, with contributions by C.B., A.D. and D.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received support by MIUR (Ministero dell’Istruzione, dell’Università e della Ricerca) through the project SIR 2014 “THALMIGEN—Thallium: Mineralogy, Geochemistry, and Environmental Hazards”, granted to Cristian Biagioni (Grant No. RBSI14A1CV), and PRIN 2017 “TEOREM—deciphering geological processes using Terrestrial and Extraterrestrial ORE Minerals”, granted to Cristian Biagioni and Andrea Dini (prot. 2017AK8C32).

Acknowledgments

We wish to thank Giancarlo Molli for the fruitful discussions about the geology of the southern Alpi Apuane, Massimo D’Orazio and Rolando Matteoni for the ICP-MS analysis at the Dipartimento di Scienze della Terra—Università di Pisa. The constructive criticism of two anonymous reviewers helped us improving the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geological sketch map (modified after [11]) of the Alpi Apuane massif with location of the main orebodies (modified after [14]).
Figure 1. Geological sketch map (modified after [11]) of the Alpi Apuane massif with location of the main orebodies (modified after [14]).
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Figure 2. Geological map of the Sant’Anna tectonic window (modified after [22]) with locations of the main abandoned mines. The “Filladi Inferiori” and Fornovolasco Metarhyolites samples are also shown.
Figure 2. Geological map of the Sant’Anna tectonic window (modified after [22]) with locations of the main abandoned mines. The “Filladi Inferiori” and Fornovolasco Metarhyolites samples are also shown.
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Figure 3. Polished slabs of “Filladi Inferiori” specimens from Sant’Anna tectonic window. (a) Phyllitic quartzite with alternance of quartz- and white mica+chlorite-rich levels with deformed milky quartz vein. (b) Phyllitic quartzite with intermediate features between least (e.g., a) and most altered (e.g., c,d) samples. See text for further details. (c,d) Whitish quarzitic phyllites with levels and pods of tourmaline and pyrite. AI: Alteration Index of Ishikawa. Mineral abbreviation: Py, pyrite; Qz, quartz; Tur, tourmaline; Wmca, white mica.
Figure 3. Polished slabs of “Filladi Inferiori” specimens from Sant’Anna tectonic window. (a) Phyllitic quartzite with alternance of quartz- and white mica+chlorite-rich levels with deformed milky quartz vein. (b) Phyllitic quartzite with intermediate features between least (e.g., a) and most altered (e.g., c,d) samples. See text for further details. (c,d) Whitish quarzitic phyllites with levels and pods of tourmaline and pyrite. AI: Alteration Index of Ishikawa. Mineral abbreviation: Py, pyrite; Qz, quartz; Tur, tourmaline; Wmca, white mica.
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Figure 4. Petrographic features of the “Filladi Inferiori” from Sant’Anna tectonic window. (a) Alternance of granoblastic and lepidoblastic levels in a phyllitic quartzite. (b) Albite crystals in a granoblastic level. Note the occurrence of small clastic tourmaline crystals. (c) Tourmaline pod aligned along the main foliation with pyrite crystals. (d) Folded levels with granoblastic and lepidoblastic texture. (e,f) Tourmaline porphyroclasts associated with pyrite in a white mica + quartz matrix. Mineral abbreviation: Ab, albite; Carb, carbonates; Chl, chlorite; Py, pyrite; Qz, quartz; Tur, tourmaline; Wmca, white mica.
Figure 4. Petrographic features of the “Filladi Inferiori” from Sant’Anna tectonic window. (a) Alternance of granoblastic and lepidoblastic levels in a phyllitic quartzite. (b) Albite crystals in a granoblastic level. Note the occurrence of small clastic tourmaline crystals. (c) Tourmaline pod aligned along the main foliation with pyrite crystals. (d) Folded levels with granoblastic and lepidoblastic texture. (e,f) Tourmaline porphyroclasts associated with pyrite in a white mica + quartz matrix. Mineral abbreviation: Ab, albite; Carb, carbonates; Chl, chlorite; Py, pyrite; Qz, quartz; Tur, tourmaline; Wmca, white mica.
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Figure 5. Classification diagrams for metasedimentary (a–c) and magmatic rocks (d–f) with data from “Filladi Inferiori” and Fornovolasco Metarhyolites Fms, respectively. (a) Ternary diagram modified from Turekian [50]. (b,c) Classification diagrams from Herron [51]. (d) Ternary AFM diagram from Irvine and Baragar [52]. (e) TAS diagram from Le Bas et al. [53]. (f) Zr/TiO2 × 0.0001 vs SiO2 diagram modified from Winchester and Floyd [54].
Figure 5. Classification diagrams for metasedimentary (a–c) and magmatic rocks (d–f) with data from “Filladi Inferiori” and Fornovolasco Metarhyolites Fms, respectively. (a) Ternary diagram modified from Turekian [50]. (b,c) Classification diagrams from Herron [51]. (d) Ternary AFM diagram from Irvine and Baragar [52]. (e) TAS diagram from Le Bas et al. [53]. (f) Zr/TiO2 × 0.0001 vs SiO2 diagram modified from Winchester and Floyd [54].
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Figure 6. (a–f) Major elements variability plots of the “Filladi Inferiori” and Fornovolasco Metarhyolites Fms. (a) Alteration box plot (see text for further details), (b) SiO2 vs AI, (c) L.O.I. (Loss On Ignition) vs SiO2, (d) MgO vs SiO2, (e) Na2O vs Al2O3, and (f) K2O vs Al2O3. The composition of the main rock-forming minerals is also reported (see text for further details). The red and blue areas represent the composition of rock-forming minerals exclusively by variable proportion of Ab, Wmca, Chl, and quartz, for “Filladi Inferiori” and Fornovolasco Metarhyolites Fms, respectively. Mineral abbreviation: Ab, albite; An40, plagioclase “Andesine”; Cc, calcite; Chl, chlorite; Qz, quartz; Wmca, white mica. The major elements are expressed in wt%.
Figure 6. (a–f) Major elements variability plots of the “Filladi Inferiori” and Fornovolasco Metarhyolites Fms. (a) Alteration box plot (see text for further details), (b) SiO2 vs AI, (c) L.O.I. (Loss On Ignition) vs SiO2, (d) MgO vs SiO2, (e) Na2O vs Al2O3, and (f) K2O vs Al2O3. The composition of the main rock-forming minerals is also reported (see text for further details). The red and blue areas represent the composition of rock-forming minerals exclusively by variable proportion of Ab, Wmca, Chl, and quartz, for “Filladi Inferiori” and Fornovolasco Metarhyolites Fms, respectively. Mineral abbreviation: Ab, albite; An40, plagioclase “Andesine”; Cc, calcite; Chl, chlorite; Qz, quartz; Wmca, white mica. The major elements are expressed in wt%.
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Figure 7. Plots showing correlation trends between (a) K2O, (b) Rb, (c) Ba, and (d) Tl vs. AI. The major and trace elements are expressed in wt% and ppm, respectively.
Figure 7. Plots showing correlation trends between (a) K2O, (b) Rb, (c) Ba, and (d) Tl vs. AI. The major and trace elements are expressed in wt% and ppm, respectively.
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Figure 8. Petrographic features of the Fornovolasco Metarhyolite. (a) Porphyritic texture, showing phenocrysts of quartz, altered feldspars (sericite), biotite (partially altered in chlorite), and tourmaline. (b) Quartz and feldspar phenocrysts in a recrystallized white mica + quartz matrix. (c,d) Tourmaline + quartz porphyroclasts wrapped by the Alpine schistosity highlighted by white mica crystal orientation. Mineral abbreviation: Ab, albite; alt-Fld, altered feldspar phenocryst; Bt, biotite; Chl, chlorite; Qz, quartz; Tur, tourmaline; Wmca, white mica.
Figure 8. Petrographic features of the Fornovolasco Metarhyolite. (a) Porphyritic texture, showing phenocrysts of quartz, altered feldspars (sericite), biotite (partially altered in chlorite), and tourmaline. (b) Quartz and feldspar phenocrysts in a recrystallized white mica + quartz matrix. (c,d) Tourmaline + quartz porphyroclasts wrapped by the Alpine schistosity highlighted by white mica crystal orientation. Mineral abbreviation: Ab, albite; alt-Fld, altered feldspar phenocryst; Bt, biotite; Chl, chlorite; Qz, quartz; Tur, tourmaline; Wmca, white mica.
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Figure 9. Alteration box plot for metasedimentary (“Filladi Inferiori” Fm) and metaigneous (Fornovolasco Metarhyolites Fm) samples. The red arrow represents the typical effect of sericite alteration. Fields for diagenetic (lower left) and hydrothermal (upper right) alteration are also reported (see [57] for further details).
Figure 9. Alteration box plot for metasedimentary (“Filladi Inferiori” Fm) and metaigneous (Fornovolasco Metarhyolites Fm) samples. The red arrow represents the typical effect of sericite alteration. Fields for diagenetic (lower left) and hydrothermal (upper right) alteration are also reported (see [57] for further details).
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Figure 10. Sant’Anna tectonic window AI distribution map (see text for further details) based on geochemical data from “Filladi Inferiori” Fm. The main ores and Fornovolasco Metarhyolites bodies are also reported.
Figure 10. Sant’Anna tectonic window AI distribution map (see text for further details) based on geochemical data from “Filladi Inferiori” Fm. The main ores and Fornovolasco Metarhyolites bodies are also reported.
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Vezzoni, S.; Pieruccioni, D.; Galanti, Y.; Biagioni, C.; Dini, A. Permian Hydrothermal Alteration Preserved in Polymetamorphic Basement and Constraints for Ore-genesis (Alpi Apuane, Italy). Geosciences 2020, 10, 399. https://doi.org/10.3390/geosciences10100399

AMA Style

Vezzoni S, Pieruccioni D, Galanti Y, Biagioni C, Dini A. Permian Hydrothermal Alteration Preserved in Polymetamorphic Basement and Constraints for Ore-genesis (Alpi Apuane, Italy). Geosciences. 2020; 10(10):399. https://doi.org/10.3390/geosciences10100399

Chicago/Turabian Style

Vezzoni, Simone, Diego Pieruccioni, Yuri Galanti, Cristian Biagioni, and Andrea Dini. 2020. "Permian Hydrothermal Alteration Preserved in Polymetamorphic Basement and Constraints for Ore-genesis (Alpi Apuane, Italy)" Geosciences 10, no. 10: 399. https://doi.org/10.3390/geosciences10100399

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