Sulfates from the Pyrite Ore Deposits of the Apuan Alps (Tuscany, Italy): a Review
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minerals Review Sulfates from the Pyrite Ore Deposits of the Apuan Alps (Tuscany, Italy): A Review Cristian Biagioni, Daniela Mauro * and Marco Pasero Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy; [email protected] (C.B.); [email protected] (M.P.) * Correspondence: [email protected]; Tel.: +39-050-221-5789 Received: 9 November 2020; Accepted: 2 December 2020; Published: 5 December 2020 Abstract: The occurrence of sulfate minerals associated with the pyrite ores of the southern Apuan Alps has been known since the 19th century but modern mineralogical studies started only in the last decade. Sulfate assemblages were identified in all the pyrite ore deposits from the studied area but the more impressive associations were discovered in the Fornovolasco and Monte Arsiccio mines. Their study allowed to improve the knowledge of the sulfate crystal-chemistry and to achieve a better understanding of the acid mine drainage (AMD) systems associated with pyrite oxidation. More than 20 different mineral species were identified and, among them, four sulfates (volaschioite, giacovazzoite, magnanelliite, and scordariite) have their type localities in the pyrite ore deposits of the Apuan Alps. A review of the mineralogical results of a ten-year-long study is given here. Keywords: sulfate minerals; pyrite oxidation; new mineral species; crystal-chemistry; Apuan Alps; Italy 1. Introduction The occurrence of secondary sulfate assemblages associated with the pyrite ore bodies from the Apuan Alps (Tuscany, Italy) has been reported since the second half of the 19th century (e.g., [1]). However, few species were described and some of them were doubtful. For instance, D’Achiardi [1] reported the presence of gypsum, melanterite, halotrichite, alum-(K), goslarite, and, possibly, coquimbite. Since the second half of the 2000s, the mineralogical study of the pyrite baryte iron ± ± oxide ore deposits from the southern Apuan Alps led to the identification of some interesting sulfate assemblages in which rare or even new mineral species were identified. The sulfate assemblages from the Fornovolasco mine were the first to be systematically studied using modern analytical techniques, spurred by the identification of the new mineral species volaschioite, Fe3+ (SO )O (OH) 2H O[2]. 4 4 2 6· 2 Later, Mauro et al. [3] identified a new hydrated iron phosphate-sulfate from the Buca della Vena mine and named it bohuslavite, Fe3+ (PO ) (SO )(OH)(H O) nH O. The most recent discovery is 4 4 3 4 2 10· 2 a sulfate assemblage found at the Monte Arsiccio mine, where well-crystallized sulfate specimens were discovered, including three new mineral species [4–6]. In this paper, the results of a ten-year-long study are reviewed and summarized, along with some still unpublished results. 2. Geological Background The pyrite baryte iron oxide ore deposits of the southern portion of the Apuan Alps ± ± (northern Tuscany, Italy) are located along a NE–SW belt, 10 km in length, from the hamlet of Fornovolasco, in Garfagnana, to the Valdicastello Carducci village, in Versilia (Figure1). Five main mining sites are known. From NE to SW they are the Fornovolasco, Buca della Vena, Canale della Radice, Monte Arsiccio, and Pollone mines. At these localities, the relative abundance of pyrite, baryte, and iron oxides (magnetite, hematite, “limonite”) is widely variable. For instance, at Fornovolasco and Minerals 2020, 10, 1092; doi:10.3390/min10121092 www.mdpi.com/journal/minerals Minerals 2020, 10, x FOR PEER REVIEW 2 of 31 Radice,Minerals 2020 Monte, 10, 1092 Arsiccio, and Pollone mines. At these localities, the relative abundance of pyrite,2 of 31 baryte, and iron oxides (magnetite, hematite, “limonite”) is widely variable. For instance, at Fornovolasco and Canale della Radice, baryte occurs only as an accessory mineral, whereas it forms largeCanale ore della bodies Radice, at the baryte Monte occurs Arsiccio only and as an Pollone accessory mine mineral,s. Pyrite whereas is widespread it forms largein all orethese bodies localities, at the althoughMonte Arsiccio its abundance, and Pollone texture, mines. and Pyrite geochemistry is widespread are inhighly all these variable. localities, George although et al. its[7] abundance, described thetexture, textural and and geochemistry trace element are highly evolution variable. of pyrite George ores et from al. [7] these described deposits, the textural showing and the trace occurrence element ofevolution several ofpotentially pyrite ores toxic from heavy these elements, deposits, showingconfirming the previous occurrence data of severalreported potentially by Biagioni toxic et heavyal. [8] andelements, D’Orazio confirming et al. [9]. previous data reported by Biagioni et al. [8] and D’Orazio et al. [9]. Figure 1. 1. MapMap showing showing the localities the localities cited citedin the intext. the Labels: text. BdV Labels: = Buca BdV della= VenaBuca mine; della CdR Vena = Canalemine; CdRdella= RadiceCanale mine; della For Radice = Fornovolasco mine; For mine= Fornovolasco; MA = Monte mine; Arsiccio MA mi=ne;Monte Pol = ArsiccioPollone mine. mine; Pol = Pollone mine. The main orebodies are hosted within the Apuane Unit and, in particular, in a Paleozoic metavolcanic-metasedimentaryThe main orebodies are hosted sequence, within locally the Apuanetourmalinized, Unit and, and in close particular, to the in contact a Paleozoic with overlyingmetavolcanic-metasedimentary Triassic metadolostone sequence, belonging locally to tourmalinized,the “Grezzoni” and Formation. close to the A contact review with of overlyingthe main Triassicgeological metadolostone features of these belonging deposits to is the given “Grezzoni” in [9,10]. Formation. A review of the main geological featuresThe ofprevious these deposits mining is activity, given in along [9,10]. with the setting of the ore deposits, located close to the contactThe between previous a miningvirtually activity, impermeable along with phylladic the setting basement of the oreand deposits, a strongly located fractured close and to the karstified contact carbonaticbetween a virtuallycover (metadolostone, impermeable phylladic marble),basement favoured and the adeep strongly oxidation fractured of some and karstified ore bodies, carbonatic giving origincover (metadolostone,to some limonitic marble), lenses that favoured were, thein some deep cases, oxidation exploited of some for ore iron bodies, production. giving originThe pervasive to some groundwaterlimonitic lenses circulation, that were, enhanced in some cases,by high exploited rainfall forcharacterizing iron production. the Apuan The pervasive Alps (more groundwater than 3000 mm/ycirculation, [11]), enhancedis also responsible by high for rainfall the formation characterizing of some the secondary Apuan Alps sulfate (more assemblages, than 3000 mmdiscovered/y [11]), onlyis also in responsible the last decade. for the These formation assemblages of some may secondary play a sulfate key role assemblages, in determining discovered the budget only inof the heavy last metalsdecade. and These acidity assemblages of acid mine may playdrainage a key (AMD) role in determiningsystems occurring the budget at some of heavy former metals mining and sites acidity in theof acid Apuan mine Alps drainage [12]. (AMD) systems occurring at some former mining sites in the Apuan Alps [12]. 3. Secondary Secondary Sulfate Sulfate Mineralogy Mineralogy from the Apuan Alps Sulfate minerals occur in all the studied pyrite de depositsposits (Table 11),), althoughalthough onlyonly somesome localitieslocalities have been been investigated investigated in in detail detail so so far. far. Conseque Consequently,ntly, the the large large number number of species of species reported reported from from the Fornovolascothe Fornovolasco and andMonte Monte Arsiccio Arsiccio mines mines may may be bedu duee to toa asampling sampling bias, bias, owing owing to to the the finding finding of sulfate assemblages with well-crystal well-crystallizedlized phases; on the contrary, such phases may be absent or still undiscovered at the Buca della Vena, CanaleCanale delladella Radice,Radice, andand PollonePollone mines.mines. Minerals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/minerals Minerals 2020, 10, 1092 3 of 31 Table 1. Secondary sulfate minerals from the pyrite ore deposits of the southern Apuan Alps. Mineral BdV CdR For MA Pol Alum-(K) × × Alunogen × × Anhydrite × Copiapite group × × × × × Coquimbite × × Epsomite × × Fibroferrite × Giacovazzoite × Goldichite × Gypsum × × × × Halotrichite × × × Jarosite subgroup × × × × Khademite × Krausite × × Magnanelliite × Melanterite × × × × × Pickeringite × Rhomboclase × Römerite × × Scordariite × Volaschioite × Voltaite × × Wilcoxite × Note: BdV = Buca della Vena mine; CdR = Canale della Radice mine; For = Fornovolasco mine; MA = Monte Arsiccio mine; Pol = Pollone mine. Crosses indicate that the mineral has been identified in the corresponding locality. Copiapite group and jarosite subgroup minerals have not been fully characterized from all the occurrences and consequently they are reported as unspecified members of their own group or subgroup. 3.1. Alum-(K) Alum-(K), ideally KAl(SO4)2(H2O)12, is a common sulfate first described by Pliny the Elder in 77 CE [13] and typically represents the result of the action of H2SO4 formed during the weathering of sulfides (mainly pyrite) on (K,Al)-rich silicate rocks or it forms through the action of S-bearing vapors in fumaroles [14].