Ore Geology Reviews 94 (2018) 1–11
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The braunite (3Mn2O3·MnSiO3)-rich mineralization in the metasedimentary succession from southern Apennines (Italy): Genesis constraints
Rosa Sinisia, , Giovanni Mongellia, Francesco Perrib, Giovanna Rizzoa a b
Department of Sciences, University of Basilicata, viale Ateneo Lucano 10, 85100 Potenza, Italy Dipartimento di Biologia, Ecologia e Scienze della Terra, Università della Calabria, via P. Bucci, 87036 Arcavacata di Rende, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Braunite Piemontite Mn ore Mineralizing ﬂuid Southern Apennines
A massive Mn-bearing mineralization, extensively exploited until the 70 s, locally occurs in the metasedimentary succession of the southern Apennines (at Mormanno site, Calabria-Lucania border, Italy). The mineralization mainly consists of three types distinguished by the relative percentages of the principal mineral phases: braunite, quartz, Mn oxi-hydroxides and Mn-rich epidote (piemontite). Petrographic observations revealed that the ore underwent medium- to high-grade metamorphism that was responsible of the growth of braunite at the expense of “primary” amorphous Mn oxi-hydroxides and quartz. During metamorphism, piemontite also formed where altered phyllosilicates are present in addition to braunite and quartz, suggesting that the aluminosilicates were source of Al necessary to the epidote crystallization. The most important geochemical features of the ore are high Mn contents (up to 71.6 wt% of MnO), low concentrations of transitional elements (Co + Ni+Cu + Zn < 0.1 wt %), high Ba (up to 4455 ppm) and variable total REEs (14.7 < ΣREEs < 329.7 ppm) contents. These are the typical characteristics of the ocean-ﬂoor hydrothermal Mn deposits, similarly to what observed in braunite ores occurring in the Alps and northern Apennines. The high LREE/HREE fractionations and the negative Ce and Eu anomalies characterizing the studied ore, suggest the mineralizing ﬂuid was formed by a mixing solution consisting of a Mn-rich hydrothermal ﬂuid and seawater, which attained the ideal condition for the ore precipitation in a distal oceanic area, far from the submarine hydrothermal vent.
1. Introduction Manganese is one of the ten most abundant elements in the Earth’s crust having concentrations commonly higher than 0.1 wt% in the average continental crust. In natural environments the hydrothermal activity, continental weathering, and chemical precipitation in alkaline solutions are the main processes responsible for the manganese mobility. Similarly to Mn oxides/hydroxides, Mn silicates such as braunite (general formula 3Mn2O3·MnSiO3; Ostwald, 1982), are very common minerals in diﬀerent geological environments because their formation take place by a number of geological processes. Braunite ores usually form as a result of hydrothermal activity (Huebner and Flohr, 1990; Nicholson, 1992; Ostwald, 1992a), diagenesis (Roy et al., 1982; Ostwald and Bolton, 1990; Ostwald, 1992b), metamorphism (Bonatti et al., 1976; Velilla and Jiménez-Millán, 2003; Brusnitsyn, 2007) and/ or a combination of these processes (Gutzmer and Beukes, 1997) involving ancient manganese deposits as much as Mn-rich pristine rocks. The most common species of braunite (also known as braunite I, Mn2+Mn3+ 6 SiO12) has a structure deﬁned by alternation of layers
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.oregeorev.2018.01.014 Received 12 July 2017; Received in revised form 8 January 2018; Accepted 15 January 2018 0169-1368/ © 2018 Elsevier B.V. All rights reserved.
parallel to (0 0 1) general plain consisting of [Mn3+O6] octahedra, [SiO4] tetrahedra, and [Mn2+O8] polyhedra (de Villiers and Buseck, 1989; Velilla and Jiménez-Millán, 2003 and references therein). Different stacking sequence of octahedral, tetrahedral and polyhedral layers generate diﬀerent type of minerals that compose the bixbyitebraunite polysomatic mineral group (bixbyite, Mn3+ 2 O3; braunite II, 3+ Ca0.5Mn3+ 7 Si0.5O12; neltnerite, CaMn6 SiO12). In these minerals, it is common to observe minor quantities of Fe3+ and Al in the crystal sites usually occupied by Mn3+ and a very small amount of Mg, Ca and Cu replacing Mn2+ (Bhattacharya et al., 1984; Sen and Dasgupta, 1984; Baudracco-Gritti, 1985). For this reason, diﬀerently from Mn oxides and hydroxides that can adsorb on the crystal surface and include into the lattice several transition metals and rare earth elements (REEs) (Mongelli et al., 2014; Sinisi et al., 2012 and references therein), the Mn silicates are considered minerals with a scarce metal content (Hein et al., 2013). However, signiﬁcant REE enrichments with respect to the upper continental crust have been observed in several Mn silicate ores (Yeh et al., 1999; Zarasvandi et al., 2016 and references therein). The REEs, particularly cerium (Ce) and europium (Eu), are a powerful tool
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Middle Triassic phyllites and metaarenites with intercalations of carbonate rock, that upward pass to metalimestones and dolomites capped by a Norian-Rhaetian slope-to-basin carbonate succession. The upper portion of LVU is mostly made up of a Jurassic cherty metalimestone succession (Calcari con Selce Formation) locally covered by siliceous slates and metaradiolarites. Maﬁc rocks, known in literature as “Limburgites” (Quitzow, 1935; Pierattini et al., 1975), crosscut the Jurassic rocks although their formation was syncronous with the deposition of the Maastrichtian-Paleocene cherty metaconglomerates (Breccia a Selce Formation) (Vitale et al., 2017). These metaconglomerates along with the Eocene-Aquitanian Scaglia-type deposit (Colle Trodo Formation) and the metapelites and metaarenites (Scisti del Fiume Lao Formation) of early Miocene age occur at top of the LVU. The mineralogical, petrographical and sedimentological evidences suggest that the LVU underwent to two main tectono-metamorphic events: (1) a HP–LT metamorphism during the continental subduction after Aquitanian; (2) a decompression and ﬁnal greenschist facies reequilibration during the exhumation occurred from Burdigalian to the end of Tortonian (Iannace et al., 2007). In detail, the studied Mn deposit is in proximity of the Mt. Cerviero (Mormanno village, Fig. 1b), where Triassic dolomites (Norian) and Jurassic limestone and cherty metalimestones (Serra Bonangelo Formation) extensively crop out. Metabasalts and pillow lava are also present in the study area partially bearing a signature of HP-LT metamorphism (Pierattini et al., 1975). The maﬁc rocks lie between the Jurassic carbonate rocks (at the bottom) and the Breccia a Selce Formation (at the top) that consists of coarse metaconglomerates locally associated with cherts or abundant rudist fragments (Fig. 1c; Vitale and Iannace, 2004). Unfortunately, the studied Mn deposit that has been intensively mined for braunite, has now disappeared. Consequently, the exact stratigraphic position of the Mn mineralization in the abovementioned succession is still uncertain and requires further ﬁeld investigations.
for the geological and geochemical studies of the primary as well as secondary ores. Post-depositional processes, such as hydrothermal activity and/or metamorphism, do not aﬀect the REE behaviour (Bau, 1991). Consequently, the REEs contents and distribution in a chemical sediment reﬂect the deposit type and can be used for the identiﬁcation of ﬂuid source and redox potential of the depositional environment (Bau et al., 2014; Şaşmaz et al., 2014; Sinisi et al., 2012). In this paper, we present the results of a detailed mineralogical and geochemical study performed on a braunite-rich deposit from southern Apennines (at Mormanno site, Italy), extensively exploited until the 70s and here described for the ﬁrst time. All analytical results are discussed with the aim to achieve two principal goals: the assessment of metamorphism that aﬀected the Mormanno Mn ore, and the identiﬁcation of Mn source that gave rise to the mineralization. In particular, the relationships between Mn-bearing minerals, trace elements, and REEs contents were considered and compared with those characterizing Mn ores elsewhere in the world. In fact, although the analysed mineralization no longer has any economic value, it furnishes a geochemical model that may be a good exploration tool for analogous, worldwide occurring deposits. 2. Geological setting The study area is located at the Calabria–Lucania boundary (east of the Pollino Massif, southern Italy, Fig. 1a). It is characterized by a complex tectonic framework resulting from the Late Cretaceous to Quaternary convergence between the Corsica–Sardinia–Calabria block of European margin (to the west) and the Adria plate of African aﬃnity (to the east) (Mazzoli and Helman, 1994, and references therein). This process caused both the disappearance of the Alpine Tethys oceanic lithosphere (originally interposed between the European and African continental palaeo-margins) and the consequent emplacement of a wide accretionary wedge (Ciarcia et al., 2012), considered as the counterpart of the ophiolitic and deep basin successions extensively cropping out in the northern Apennines (Vitale et al., 2013 and references therein). Northern Calabria hosts both the sedimentary and metamorphic carbonate and siliciclastic successions of the external Apennine domains, and the high pressure–low temperature (HP–LT) to greenschist facies Alpine metamorphic ophiolitic units of the Calabria-Peloritani Terrane (CPT, Bonardi et al., 2001; Cello et al., 1996; Iannace et al., 2005; Rossetti et al., 2001; Sansone and Rizzo, 2012; Sansone et al., 2012; Vitale et al., 2013). Recently (Iannace et al., 2005, 2007), the MesoCenozoic succession outcropping in the northern Calabria has been grouped into three juxtaposed tectonic units, the HP–LT Lungro–Verbicaro Unit (LVU, Anisian-Aquitanian) in the hanging wall, the non metamorphic Pollino–Ciagola Unit (PCU, Norian-Langhian) in the footwall, and the Cetraro Unit (Triassic). The Mn deposit object of the present paper occurs within the LVU (Iannace et al., 2007). The LVU consists of a lower part made up of
3. Sampling and methods Diﬀerent type of analyses were performed on ﬁfteen samples of massive Mn mineralization sampled at the entrance of an abandoned mine (Mormanno village, near Mt. Cerviero). The petrographic features of mineralization were studied by optical transmission microscopy (OM) using a Leitz binocular microscope, and by scanning electron microscopy using a XL30 Philips LaB6 ESEM equipped with an energy dispersive X-ray spectrometer (EDS) at Department of Sciences, University of Basilicata (Italy). Bulk-rock mineral composition was studied by X-ray diﬀraction (XRD) analysis of randomly oriented samples, previously powdered by hand in an agate jar, using a Philips X’PERT 3040PW diﬀractometer equipped with a Cu tube and graphite monochromator. The XRD analysis was performed from 5° to 70° 2θ with 0.01° 2 θ/step and 2 s
Fig. 1. Simpliﬁed geological map of the Calabria-Lucania boundary (southern Italy) and stratigraphic sketch of the Mt. Cerviero, in close proximity of the Mormanno site. The red box displays the sampling site. Modiﬁed after Iannace et al. (2005).
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recrystallized quartz showing undulate extinction, irregular boundaries and diﬀerent grain size. In some samples, the quartz new grains form large bands where triple junctions are evident (Fig. 2b). In addition to quartz, variable amounts of opaque minerals and Mn-rich epidote are observed. Regardless of their relative abundances, a close relationship between opaque minerals and quartz was detected. All samples, in fact, show mosaic textures and symplectites of opaque minerals after quartz (Fig. 2c and d). Where present, the Mn-rich epidote (piemontite) coexists with opaque minerals and quartz (Fig. 2e and f) forming prismatic hypidioblasts of millimetre size and/or aggregates of very ﬁne crystals. Both piemontite blasts and aggregates are well distinguished from the other minerals by their typical high relief, characteristic birefringence, and pleochroism (Deer et al., 2013). On the basis of SEM-EDS analysis, two diﬀerent Mn-bearing phases (corresponding to the opaque minerals observed by optical microscopy) were identiﬁed: (a) Mn silicate (namely braunite), which forms both idioblasts in the mosaic texture (Fig. 3a) and very ﬁne crystal aggregates in the symplectites (Fig. 3b); (b) Mn oxides (namely cryptomelane) that form massive microscopic aggregates of primary minerals (Fig. 3c) as well as secondary acicular crystals into voids or veins crosscutting the rock (Fig. 3d). The textural relationships between braunite, cryptomelane, and quartz are shown also in Fig. 3e where granular crystals of braunite seem to form by reaction between poorly crystalline cryptomelane and quartz.
counting time. To compare the study samples, an estimation of wholesample mineral abundances has been carried out using corundum as internal standard and the reference intensity ratios (RIR) values listed in the PDF-4 database included into the X’Pert HighScore Plus software (PANalytical). Bulk-rock chemical analysis of major and trace elements (including rare earth elements) was determined by inductively coupled plasma-mass spectroscopy (ICP-MS) at Activation Laboratories Ltd., Canada. Analytical uncertainties were less than ± 5%, except for elements at a concentration of 10 ppm or lower, for which uncertainties were ± 5–10%. Total loss on ignition (LOI) was determined gravimetrically after heating overnight at 950 °C. For the discussion, the rare earth element (REEs) concentrations were normalized to chondrite standard (Evensen et al., 1978). The Ce and Eu anomalies were calculated as Ce/Ce∗ = 2Cech/(Lach+Ndch) and Eu/ Eu∗ = 2Euch/(Smch+Gdch) respectively (Yeh et al., 1999), where the subscript “ch” refers to normalized values for chondrite. 4. Results 4.1. Petrography Optical microscopy observations revealed that the studied mineralization is characterized by a granoblastic texture typical of metamorphosed rocks (Fig. 2a). Such a texture mainly consists of
Fig. 2. Microscopic features of the mineralization. OM images displaying (a) granoblastic texture (+N), (b) triple junctions of quartz (+N), (c) mosaic textures and (d) symplectites of opaque minerals after quartz (+N), (e) millimetre prismatic hypidioblasts and (f) aggregates of very ﬁne crystals of piemontite (//N). O-M = opaque mineral; Qz = quartz; Pm = piemontite.
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Fig. 3. BSE images showing (a) braunite that forms idioblasts in the mosaic texture and (b) very ﬁne crystal aggregates in symplectites, (c) primary crystals of cryptomelane that forms massive microscopic aggregates and (d) secondary acicular crystals within a vein crosscutting the rock, (e) granular crystals of braunite formed by reaction between poorly crystalline cryptomelane and quartz, (f) relict chlorite partially replaced by braunite. Br = braunite; Cry = cryptomelane, Qz = quartz; Pm = piemontite, Chl = chlorite. The insets of a and b represent the EDS results identifying the braunite and cryptomelane respectively.
Based on their mineralogical aﬃnities, studied samples were grouped in three subsets: SB1 including samples with Mn oxi-hydroxides > 50% (MR1, MR6, MR14, MR16), SB2 containing samples with Mn oxi-hydroxides < 20% (MR2, MR3, MR8, MR11, MR15), SB3 comprising samples with piemontite and scarce Mn oxi-hydroxides (MR5, MR7, MR9, MR10, MR12). Because of its mineralogical composition, the sample MR4 (consisting of braunite and quartz, without Mn oxi-hydroxides) was not included into the previous subsets and it will be discussed separately.
Phyllosilicates were detected by SEM observations as well. They mostly correspond to chlorite group minerals and are present as very small relict crystals locally showing evidences of braunite replacing (Fig. 3f). 4.2. Bulk mineralogy To deﬁne the mineralogical composition carefully, the X-ray diffraction analysis was performed on randomly-oriented powders of each sample. As shown in Table 1, the XRD results conﬁrm the petrographic observations showing a mineralogical assemblage mainly composed by braunite and quartz with highly variable amounts. Apart from braunite, other Mn-bearing phases such as Mn oxi-hydroxides (cryptomelane, general formula Kx[Mn4+, Mn3+]8O16; hollandite, general formula Bax[Mn4+, Mn3+]8O16; todorokite, general formula [Mg, Ca, Na, K]x[Mn4+, Mn3+]6O12·3.5H2O; birnessite, general formula [Na,Ca] Mn7O14·2.8H2O) and the Sr-rich manganese epidote (Sr-piemontite, general formula CaSrAl2Mn3+Si3O12[OH]) were identiﬁed. In addition, only in a few samples 2:1 type phyllosilicate (chlorite, in MR7, MR9 and MR12 samples), clinopyroxene (diopside, in MR12 sample), and Ba sulfate (barite, in MR2, MR14 and MR16 samples) were also detected.
4.3. Elemental distribution in the ore The studied mineralization is a medium to high-grade Mn ore because of its average MnO contents higher than 40 wt% (Table 2). Major element composition of the bulk-rock clearly reﬂects the mineralogy of the samples. Furthermore, the relative concentrations of various major elements are generally in proportion to the relative concentration of respective minerals (see Table 1 for comparison). The ore is mainly composed by MnO and SiO2. These major element oxides have largely variable contents in the individual subset as much as between the three subsets. The other major elements have very low 4
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Table 1 Semi-quantitative mineralogical composition (in percentage) of the Mn-rich mineralization. Samples were grouped by their mineralogical aﬃnities. See the text for further details. Subset
19 10 19 23
MR4 MR1 MR6 MR14 MR16
26 35 20 30 22
35 24 37 25
MR2 MR3 MR8 MR11 MR15
32 12 70 69 25
20 19 6 19 5
MR5 MR7 MR9 MR10 MR12
61 58 53 69 48
Total of Mn oxi-hydroxides
Birnessite 74 6 29 8 12
14 6 24 21 5
8 5 4 5
0 59 51 56 61
45 69 34 18 70
9 15 8 6 19
20 19 6 19 5
13 6 11
5 8 9 4 5
Table 2 Concentrations of major, trace and rare earth elements of the Mormanno Mn ore. MR4
wt.% SiO2 TiO2 Al2O3 Fe2O3(T) MnO MgO CaO Na2O K2O P2O5 LOI tot Mn/Fe
47.80 0.007 0.20 0.22 43.32 0.14 1.06 0.02 0.08 0.03 1.49 94.37 218.03
4.57 0.044 1.73 0.89 71.64 0.17 0.91 0.31 1.93 0.15 11.72 94.06 89.13
7.81 0.023 0.93 0.96 66.90 0.77 0.83 0.36 2.28 0.03 13.78 94.67 77.16
4.11 0.03 0.98 0.86 64.41 0.22 0.93 0.35 1.68 0.12 11.65 85.34 82.93
5.63 0.041 1.67 1.03 65.54 0.24 0.95 0.29 2.11 0.14 12.55 90.19 70.46
29.11 0.011 0.40 0.24 55.94 0.06 0.69 0.12 1.08 0.05 6.14 93.84 258.09
50.39 0.011 0.38 0.20 33.85 0.06 0.40 0.11 1.04 0.06 5.51 92.01 187.41
16.70 0.005 0.24 0.09 67.47 0.08 0.97 0.03 0.18 0.01 2.11 87.89 830.09
15.96 0.019 0.50 0.47 72.53 0.04 0.49 0.10 0.91 0.04 4.22 95.28 170.87
50.05 0.024 0.43 0.25 37.81 0.13 0.69 0.05 0.30 0.00 2.68 92.42 167.46
16.94 0.215 2.87 5.84 53.43 0.14 1.96 0.02 0.12 0.04 7.38 88.96 10.13
16.92 0.147 3.77 3.71 60.83 2.66 3.10 0.13 0.47 0.00 2.80 94.54 18.16
19.19 0.066 1.01 1.89 65.82 0.34 1.83 0.03 0.06 0.00 1.93 92.17 38.56
15.41 0.029 0.81 0.73 68.04 0.06 0.34 0.01 0.03 0.00 1.02 86.48 103.20
26.63 0.237 5.17 6.42 44.10 1.58 4.21 0.91 0.03 0.03 3.32 92.64 7.61
ppm V Cr Co Ni Zn Cu Ba Sr Rb Pb Th U Zr Y Co/Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ΣREEs (La/Yb)ch Ce/Ce* Eu/Eu*
77 20 7 75 82 78 270 65 2 5 0.1 1.7 2 2 0.09 5.1 1.1 0.9 3.8 0.9 0.2 0.7 0.1 0.9 0.2 0.4 0.1 0.3 0.1 14.7 11.5 0.11 0.81
144 20 99 120 140 310 4455 1001 23 8 0.8 3.2 10 50 0.71 105.0 35.2 20.2 70.6 13.3 2.9 12.0 1.8 10.2 1.9 4.9 0.6 3.7 0.5 282.9 19.2 0.17 0.71
80 30 90 180 120 320 4047 564 40 6 0.6 3.4 5 28 0.75 50.8 23.1 8.9 33.5 6.1 1.4 6.7 1.0 5.5 1.0 2.5 0.3 1.9 0.3 143.0 18.1 0.24 0.67
147 20 79 170 140 320 1418 972 19 5 0.5 3.3 6 23 0.56 39.6 13.0 6.9 24.9 4.6 1.1 5.3 0.8 4.4 0.8 2.2 0.3 1.7 0.2 105.8 15.7 0.17 0.68
137 30 109 180 180 350 5197 924 30 11 1.0 2.8 7 61 0.61 124.0 36.0 23.5 83.7 15.6 3.6 14.4 2.3 12.6 2.3 5.9 0.8 4.4 0.6 329.8 19.0 0.15 0.74
224 20 39 140 130 220 1084 370 9 5 0.1 2.0 2 6 0.30 7.4 1.3 1.3 5.8 1.1 0.3 1.2 0.2 1.1 0.2 0.5 0.1 0.5 0.1 21.1 10.0 0.09 0.77
72 20 10 40 70 90 705 241 9 6 0.4 2.3 4 3 0.14 10.6 7.4 1.8 6.2 1.2 0.3 1.1 0.2 1.1 0.2 0.6 0.1 0.5 0.1 31.4 14.3 0.37 0.72
59 30 50 60 80 100 207 177 2 5 0.1 7.5 2 6 0.63 3.8 2.1 0.8 3.9 1.1 0.3 1.0 0.2 0.9 0.2 0.5 0.1 0.3 0.0 15.2 8.6 0.27 0.76
59 30 86 80 80 350 432 236 11 5 0.2 6.0 2 4 1.08 8.4 6.5 2.1 8.9 1.9 0.4 1.8 0.3 1.6 0.3 0.8 0.1 0.7 0.1 33.9 8.1 0.35 0.71
50 30 25 60 70 170 235 248 4 5 0.4 1.9 3 2 0.36 4.8 4.0 0.9 3.6 0.7 0.2 0.8 0.1 0.6 0.1 0.4 0.1 0.3 0.0 16.6 10.8 0.42 0.78
42 40 251 110 120 1260 1092 736 4 22 4.7 4.0 35 28 2.09 85.2 55.5 13.0 45.1 7.6 1.6 6.6 1.0 5.6 1.1 2.8 0.4 2.4 0.4 228.2 24.0 0.35 0.69
36 40 93 200 160 810 737 3655 17 28 2.5 1.6 23 4 0.58 11.1 23.5 2.8 11.0 2.2 0.5 2.1 0.3 1.8 0.3 0.9 0.1 0.9 0.1 63.2 8.3 0.97 0.75
24 40 139 120 150 400 1258 1699 2 8 0.8 1.6 11 8 0.93 15.3 30.2 3.7 14.0 3.1 0.7 2.4 0.4 2.4 0.5 1.2 0.2 1.1 0.2 75.3 9.4 0.92 0.75
122 40 187 100 100 710 237 32 2 5 0.3 4.7 5 3 1.87 8.0 10.6 1.9 8.1 1.7 0.4 1.7 0.3 1.5 0.3 0.9 0.1 0.8 0.1 36.5 6.8 0.62 0.72
48 40 327 260 190 1320 532 9337 2 64 4.5 1.8 46 19 1.72 33.2 61.2 9.1 34.1 7.9 1.7 6.5 1.0 5.7 1.1 2.8 0.4 2.4 0.4 167.5 9.3 0.81 0.74
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Table 3 Comparison of REEs concentrations and signiﬁcant element ratios between the studied mineralization and some important worldwide Mn deposits. Deposits
Mormanno Mn ore China braunite-rich ore Turkysh hydrothermal braunite ore Mn deep-water hydrothermal layers Fe-Mn shallow sea nodules Indian Ocean Fe-Mn abyssal nodules Deep-water central Paciﬁc Fe-Mn hydrogenetic crusts
14.70–329.74 68–113 34.30 54 93–168 770–2158 1750
6.76–23.99 6.0–10 5.63 4.6 6.4–7.2 5.8–7.4 6.6
0.09–0.97 2.2–2.9 1.12 0.69 0.73–0.91 1.2–3.5 2.6
0.67–0.77 0.06–0.63 0.67 0.59 0.66–0.82 0.62–0.86 0.70
This study Yeh et al. (1999) Öksüz (2011) Hein et al. (1990) Szefer et al. (1998) Nath et al. (1992) Hein et al. (1999)
Jiménez-Millán, 2003; the Andros Mn ore in Greece, Reinecke et al., 1985). In Italy, whose geodynamic history was characterized by continental collision that gave rise to the Alps and Apennines orogenic belts, several Mn ore deposits have been documented in the northern part of the Italian peninsula (Marescotti and Frezzotti, 2000; Tumiati, 2006; Tumiati et al., 2010, 2015 and references therein). Such ores are part of the wider Gambatesa (northern Apennine, eastern Liguria) and Praborna (western Alps, Aosta Valley) Mn deposits of hydrothermal origin, for which a metamorphic imprint was recognized (Marescotti and Frezzotti, 2000; Tumiati et al., 2010). In these deposits, similarly to the Mormanno Mn mineralization, braunite and quartz are the main mineral phases of the Mn ores. Such an assemblage remained stable during the entire tectono-metamorphic evolution of the orogen, up to prehnite-pumpellyte facies conditions (T = 275 ± 25 °C; P = 2.5 ± 0.5 kbar; Lucchetti et al., 1990). According to Marescotti and Frezzotti (2000), the co-occurrence in the studied samples of poorly crystalline cryptomelane and quartz along with coarse-grained braunite crystals (Fig. 2e) suggests braunite has grown at the expense of “primary” amorphous phases via the following reaction:
concentrations (mostly lower than 1 wt%), except for Al2O3, Fe2O3 and CaO in the SB3, and Al2O3 and K2O in the SB1. Highly variable contents of trace elements have been observed as well. In general the SB1 and SB3 subsets are more enriched of transition elements, mainly Co, Ni, Zn and Cu, than the SB2 one. Such elements show concentrations up to 15X (i.e. Co and Cu in the SB3) those of the upper continental crust (GLOSS, Global Subducing Sediments, Plank and Langmuir, 1998). Uranium is also enriched in all subsets, whereas Y and Pb have higher concentrations relatively to the GLOSS in some samples of the SB1 (MR1 and MR16) and SB3 (MR5, MR7 and MR12) subsets, respectively. On the contrary, Cr, Rb, Th and Zr have contents constantly lower than those of the standard. A wide range of total rare earth element concentrations (ΣREEs) outlines the analysed mineralization. The ΣREEs values vary between 14.70 and 329.70 ppm that are in the typical REEs content range of worldwide hydrothermal and braunite-rich Mn ores (Table 3). However, the total REEs contents tend to be diﬀerent in the three subsets with a general ΣREEs increase from the SB2 to SB3 and SB1. In detail, the ΣREEs values of the SB1 samples are up to ∼20–30 times higher to those of the SB2 samples although a certain overlap between concentration ranges is clearly recognized. Regardless of variations of the total REEs contents, the Mormanno ore shows signiﬁcant REE enrichments (up to 300X) when compared to the chondrite values (from Evensen et al., 1978; Fig. 4a). The light REEs (LREEs: La to Sm) and heavy REEs (HREEs: Eu to Lu) are strongly fractionated in all samples causing high values of the (La/Yb)ch ratio (6.80–23.90). Besides the MR5 sample of SB3 subset ((La/Yb)ch = 23.90), the highest (La/Yb)ch ratio values are of the SB1 samples. Mn ore is also characterized by negative Eu anomalies, with Eu/Eu∗ values varying in the 0.67–0.77 narrow range, and negative Ce anomalies deﬁned by Ce/Ce∗ ratios between 0.09 and 0.97. 5. Discussion 5.1. Metamorphic imprint of the mineralization Based on petrographic observations metamorphism is thought to be the main process that aﬀected the studied ore during and after its formation. Granoblastic texture, quartz triple junctions, symplectites of braunite and quartz, and the braunite + quartz + piemontite mineralogical assemblage are the major evidences of intense recrystallization during a medium-/high-grade metamorphism (Deer et al., 2013; Marescotti and Frezzotti, 2000; Tumiati, 2006; Tumiati et al., 2010). Considering the geological framework in which the studied mineralization occurs, we may assume that the petrographic features of the ore are due to metamorphic events which occurred during subduction and subsequent tectonic exhumation of the southern Apennine wedge. At the Calabria-Lucania boundary as well as in the study area, in fact, diﬀerent type of metasediments crop out conﬁrming that metamorphic events strongly aﬀected signiﬁcant portions of the southern Italy (Sansone and Rizzo, 2012; Vitale et al., 2013; Laurita et al., 2015). The braunite + quartz ( ± piemontite and pyroxene) mineral assemblage characterizes numerous worldwide Mn ores developed along passive continental margins (e.g. the Ossa-Morena ore in Spain, Velilla and
Fig. 4. Chondrite-normalized REE distribution patterns of (a) Mormanno Mn ore and (b) Mn ores from literature shown for comparison.
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Fig. 5. Al vs Si and (Co + Ni + Cu) vs (Co/Zn) binary plots used to distinguish hydrothermal and hydrogenous ores.
Fig. 6. Ternary discrimination diagrams for the identiﬁcation of the origin of the Mn-rich mineralizing ﬂuid.
K(Mn8O16) + SiO2 + 2H2 O = Mn6Si O12 + 2MnO−4 + 4H+ + K+ cryptomelane
dissolved components from seawater; 3) diagenetic, namely Mn deposits formed by diagenesis of a Mn-rich protolith. In order to distinguish Mn ores of diﬀerent origin, several major and trace element discrimination diagrams as much as elemental ratios have been proposed (Bonatti et al., 1972; Choi and Hariya, 1992; Hein et al., 1992; Öksüz et al., 2011; Toth, 1980). Some of the most common binary and ternary diagrams are shown in Figs. 5 and 6. They show the aﬃnity of the studied mineralization with modern hydrothermal Mn-rich crusts deposed on the present-day oceanic ﬂoor. Most of the ore samples, plot within the hydrothermal ﬁeld with a few exceptions in the triangular Fe–(Si∗2)–Mn and Ni–Zn–Co diagrams. Five samples (MR11 of SB2 and MR5, MR9, MR10, MR12 of the SB3, Fig. 6a and b) fall out of the hydrothermal and hydrogenous ﬁelds because of their higher Co and lower Zn contents. Co and Zn, whose chemical aﬃnity relates to seawater and hydrothermal solutions respectively (Zarasvandi et al., 2016 and references therein), are trace metals commonly used to identify the nature of chemical sediments. According to Toth (1980), Co/Zn ratio of 0.15 indicates hydrothermal type deposits, while Co/Zn ratio of 2.5 deﬁnes hydrogenous type deposits. The Co/Zn values of the Mormanno Mn ore are between 0.07 and 2.09 suggesting that the mineralization has a possible hydrothermal origin, which however, in some samples could be masked by their mineralogical features. In particular, it is noteworthy that Co-rich units are characterized by high percentages of Mn oxi-hydroxides and phyllosilicates that in marine as well as in continental environment, are the most common mineral phases to exert uptake on dissolved trace elements favouring their accumulation into sediments (Mameli et al., 2008; Sinisi et al., 2012). This ﬁnding is supported also by the low contents of Co and other trace metals in the MR4 sample, where braunite and quartz are the sole minerals present. The mineralogical composition of these rocks also may explain the other trace element enrichments (in respect to the average continental crust) of the studied ore. In fact, the highest Sr enrichments are observed in those rocks containing Sr-piemontite (SB3 subset); Ni, Cu and U are associated with high percentages of Mn oxi-hydroxides (SB1
Likewise, metamorphic processes can be assumed for the Sr-piemontite crystallization. Piemontite represents the Mn-rich component of the epidote mineral group and it is considered a typical metamorphic mineral (Bonazzi and Menchetti, 2004). It occurs as an accessory or minor phase in a wide variety of rocks of diﬀerent bulk composition that form under diﬀerent physicochemical conditions. However, its occurrence chieﬂy is associated with highly oxidized manganiferous rocks that were aﬀected by low- to high-pressure metamorphism in a wide temperature range (Bonazzi and Menchetti, 2004 and references therein). It is worth noting that not all the samples have piemontite as the main mineral phase. The epidote is observed in samples where phyllosilicates and other aluminium-bearing minerals are also present. This is because piemontite formation takes place in a system including, in addition to Ca, Mn and Si, Al as major element. In the studied ore, altered phyllosilicate minerals detected by SEM analysis (Fig. 3f), i.e. chlorite, could be the most feasible source of Al to the system, thus promoting the piemontite crystallization. 5.2. Origin of the Mn-rich mineralizing ﬂuid It is always diﬃcult to deﬁne the origin of Mn ores because Mnbearing minerals can be ubiquitous and therefore present in diﬀerent geological settings and environments. Several type of Mn deposits can be distinguished according to their depositional environment (marine or continental), formation process (supergene or hydrothermal), chemical and mineralogical compositions (Aplin, 2000; Nicholson, 1992). However, three types of Mn ores are commonly considered (Hein et al., 1997): 1) hydrothermal, namely manganese oxides deposited directly from geothermal waters around hot springs and pools in continental environments, or sedimentary-exhalative manganese mineralization deposited in marine environments; 2) hydrogenous (or hydrogenetic), that are Mn deposits formed by slow precipitation or adsorption of 7
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hydrothermal Mn deposits (Zarasvandi et al., 2016). Fractionation of Fe from Mn during the ore formation is due to precipitation kinetics of Febearing minerals more promptly than that of Mn-bearing minerals (Toth, 1980; Ruhlin and Owen, 1986; Jach and Dudek, 2005). Accordingly, proximal deposits generally exhibit high Fe values, whereas distal deposits are characterized by high Mn content. This characteristic is also reﬂected in the lack of signiﬁcant Fe minerals in the analysed Mn ore (Table 1). However, the strongly variable ΣREEs contents of the studied samples could suggest two alternative hypotheses about the genesis of Mormanno Mn ore: (a) the mineralization precipitated from ﬂuids with diﬀerent Mn sources, or (b) according to Sinisi et al. (2012), the chemistry of the ore was inﬂuenced by interaction between mineralizing ﬂuid and diﬀerent rock types through which it migrated. In the case of our study, however, it is evident that total REEs concentrations are associated with the relative amounts of Mn-bearing phases and quartz, suggesting a direct control of these minerals on REEs distribution. Previous authors have highlighted that braunite, piemontite and mostly Mn oxi-hydroxides, can host signiﬁcant REEs amounts thereby playing an important role in the chemistry of diﬀerent Mn ores (Franz and Liebscher, 2004; Bonazzi and Menchetti, 2004; Maynard, 2010). On the other hand, quartz is a trace element-poor mineral phase because of its crystallographic properties that hinder signiﬁcant element inclusions and substitutions (Wenk and Bulakh, 2004). Consequently, the high abundances of quartz dilute the chemical composition of the ore resulting in a relative decreasing of REEs and trace element contents. In Fig. 7 the quartz percentages have been compared to concentrations of total REEs plus the trace metals (Cu, Ni, Co, Zn) signiﬁcant for the studied ore. Samples are broadly distributed along a mixing line between the MR12 sample, having the lowest quartz content, and the MR4 sample, which has the highest quartz amount, conﬁrming the strong correlation between the considered variables. Abundances of quartz and/or of its polimorphs (mainly chalcedony and opal) are common in deep-ocean ﬂoor sediments, mostly as cherts and radiolarites that frequently are found in ophiolite sequences from passive continental margins, including those of the northern and southern Apennines (Iannace et al., 2007; Vitale et al., 2017). Cherts are the product of silica remobilization that can be caused by several processes (Murray, 1994). In modern submarine and sub-lacustrine hydrothermal vents, for example, chert is formed by precipitation of silica gel aided by changes in temperature and/or pH (Jonasson and Perﬁt, 1999; Hein et al., 1994; Posth et al., 2008). Geochemical features of marine siliceous sedimentary rocks have been studied largely in order to obtain information about their sources and depositional environments (Murray et al., 1991; Kato et al., 2002 and references therein). In detail, the Al/(Al + Fe + Mn) and MnO/TiO2 ratios are used to evaluate the relative contributions of terrigenous and
subset); whereas the highest Ba concentrations are associated with the presence of barite and hollandite (SB1 subset). Regarding Ba, it is worth noting that high concentrations are observed in all samples. Volcanic activity and sedimentation can aﬀect the Ba concentrations in mineralizing ﬂuids (Monnin et al., 2001; Öksüz, 2011; Şaşmaz et al., 2014). In particular, hydrothermal solutions commonly have higher Ba contents than seawater (Bau et al., 2014). In the Mormanno Mn ore Ba concentrations range from 207 to 5197 ppm reﬂecting the chemical feature of submarine hydrothermal Mn deposits. It has to be considered that post-depositional processes, and especially metamorphism, can be responsible of changes of chemical and mineralogical composition of rocks and sediments. Such compositional changes hide the distinctiveness of the pristine rock and consequently prevent the understanding of its origin. However, it has long been recognized that the geochemical behaviour of REEs, diﬀerently from that of the other trace elements, is not necessarily aﬀected by secondary processes (Bau, 1991; Yeh et al., 1999) suggesting that REEs are an useful geochemical tool to evaluate the origin of a rock/sediment. Mnrich deposits and mineralizations are not an exception as comprehensively described in scientiﬁc papers dealing with the genesis of modern and ancient Mn-rich deposits from diﬀerent geological settings (Mishra et al., 2007; Chetty and Gutzmer, 2012; Zarasvandi et al., 2016 and references therein). Despite the high variability of total REEs concentrations, the chondrite-normalized distribution patterns of the analysed samples, including MR4, are homogeneous (Fig. 4a). Conversely, they diﬀer from patterns reported in literature, especially when compared to the average concentrations of REEs in hydrogenous Mn-rich deposits, crusts, and nodules (Toth, 1980; Hein et al., 1997; Nath et al., 1997; Maynard, 2003; Shah and Moon, 2004). A wider overlap between REEs concentrations of studied mineralization and literature data of hydrothermal Mn ores is observed (Fig. 4b) suggesting a major aﬃnity with deposits of hydrothermal origin. This ﬁnding is consistent with: (1) the large LREE/HREE fractionations, typical of hydrothermal solutions (Shah and Moon, 2007; Xie et al., 2013) because of the greater stability of HREE complexes in hydrothermal ﬂuids (German and Von Damm, 2010), (2) the negative Ce anomalies that strongly diﬀer from the positive ones common in oceanic nodules and crusts (Indian Ocean Fe-Mn abyssal nodules, 1.2 < Ce/Ce∗ < 3.5, Nath et al., 1992; deep-water central Paciﬁc Fe-Mn hydrogenetic crusts, Ce/Ce∗ ∼ 2.6, Hein et al., 1999; Table 3). Negative Ce anomalies are also distinctive of seawater (Tostevin et al., 2016 and references therein) reﬂecting oxidizing conditions that promote Ce depletion by Ce3+ to Ce4+ oxidation and the consequent CeO2 precipitation (Mongelli et al., 2014 and references therein). Accordingly, ores precipitated from such a solution, inheriting its chemical signature, result in turn Ce-depleted (Mongelli et al., 2015). In Fig. 4b, the REEs distribution pattern of seawater was added to those of the studied samples to evaluate a possible relationship between them. The negative Ce anomaly and the LREE distribution of seawater are comparable to Ce anomalies and LREE patterns of the studied samples supporting the hypothesis of a seawater solution as mineralizing ﬂuid. The negative Eu anomalies also point to this conclusion because they are common in oceanic water, becoming more considerable when the water deepens (Henderson, 1984; James et al., 1995; Douville et al., 1999). Negative Eu anomalies also can occur in hydrothermal ﬂuids when they are cold (< 200 °C) and oxidizing, in contrast to hot (> 300 °C) and reducing ﬂuids that are characterized by substantial positive Eu anomalies (Bau et al., 2014; Tostevin et al., 2016 and references therein). This is because the Eu mobility in hydrothermal ﬂuids strongly depends on the redox and temperature conditions (Michard et al., 1993; Bau and Dulski, 1999). Therefore, a distal hydrothermal source, such as a hydrothermal ﬂuid diluted with seawater, could be taken into account as solution from which the Mormanno Mn mineralization formed. This ﬁnds support in the high Mn/Fe values (Table 2) that are one of the most common geochemical features of submarine
Fig. 7. Diagram showing the dilution eﬀect of the quartz amounts on contents of total REEs and selected trace elements.
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Fig. 8. The multidisciplinary approach used to characterize the ore and assess the origin of Mn-rich mineralizing ﬂuid.
variable total REEs concentrations, the dilution eﬀect of quartz has been suggested and conﬁrmed by Quartz% vs. ΣREE+(Cu, Ni, Co, Zn) diagram showing the good correlation between the considered variables.
hydrothermal input of marine sediments, with Al and Ti being representative of a terrigenous source and Mn and Fe of a hydrothermal one (Aitchison and Flood, 1990 and references therein). Very low Al/ (Al + Fe + Mn) values (lower than 0.06) and very high MnO/TiO2 values (between 186 and 13,494) exclude the terrigenous input for the Mormanno Mn ore, suggesting that silica in the studied mineralization, especially in the form of quartz, can be regarded as further evidence of a hydrothermal origin.
Acknowledgments This work was ﬁnancially supported by a Prof. G. Mongelli grant (RIL 2016, Italy). The authors are indebted to Dr. S. Laurita for the valuable work during sampling and to A. Laurita, University of Basilicata, and M. Davoli, University of Calabria, for their support during the SEM-EDS analysis. Many thanks to Editor and three anonymous reviewers for their fruitful suggestions that improved the ﬁnal version of the manuscript.
6. Conclusions In this paper, it is documented for the ﬁrst time the occurrence of a braunite-bearing mineralization in a metasedimentary succession of the southern Apennine belt (the Lungro-Verbicaro Unit). A multidisciplinary approach based on petrographic, mineralogical and chemical analyses was used with the aim to characterize the ore and assess the origin of Mn-rich mineralizing ﬂuid (Fig. 8). The textural features of the ore are typical of metamorphic rocks formed during medium- to high-grade metamorphic events. Signiﬁcant mineralogical aﬃnities between studied mineralization and Mn ores developed in passive continental margins, such as the northern Apennines and Alps Mn deposits, were observed. In particular, the co-occurrence of granular braunite and piemontite with poorly-crystalline quartz and Mn oxihydroxides ( ± chlorite) suggests crystalline phases are the result of metamorphic recrystallization processes. The principal discrimination diagrams used for determining the genesis of Mn-rich mineralizing ﬂuids show that the Mormanno Mn ore is chemically similar to submarine hydrothermal Mn deposits. However, some signiﬁcant diﬀerences concerning the REE concentrations and distributions are evident. High LREE/HREE fractionations and negative Ce and Eu anomalies characterize the studied ore suggesting a possible distal hydrothermal source. In more detail, based on the chondrite-normalized REE distribution patterns, a mixing solution consisting of a hydrothermal ﬂuid and seawater can be considered as the mineralizing ﬂuid from which Mn ore formed. As for the strongly
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