Ore Geology Reviews 47 (2012) 110–125

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Different Mn-ores in a continental arc setting: Geochemical and mineralogicalevidences from Tertiary deposits of Sardinia (Italy)

R. Sinisi a, P. Mameli a,⁎, G. Mongelli b, G. Oggiano a

a Dipartimento di Scienze Botaniche, Ecologiche e Geologiche, Università degli Studi di Sassari, 07100 Sassari, Italyb Dipartimento di Chimica, Università degli Studi della Basilicata, Campus di Macchia Romana, 85100 Potenza, Italy

⁎ Corresponding author. Tel.: +39 079 228684; fax:E-mail address: mamelip@uniss.it (P. Mameli).

0169-1368/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.oregeorev.2012.03.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 November 2010Received in revised form 14 October 2011Accepted 21 March 2012Available online 29 March 2012

Keywords:ManganeseGeochemistryOresHydrothermalMarine sedimentsSardinia

This contribution examines two types of Mn deposits, which were exploited in the past in Sardinia. Both typesare Tertiary in age, but not strictly coeval and they are both linked to a continental arc setting characterized bythe presence of calcalkaline volcanics and shallow marine sediments. Mn-deposits formed in terrestrial orshallow marine environments are unusual compared to the most common deep oceanic ones, and thereforepoorly studied. TheMn-ores of Sardinia occur either as stratabounddepositswith stratiformgeometry associatedto rudites with littoral facies or as vein-type deposits hostedwithin volcanic rocks. In the stratiform deposits theMn ores form the matrix or cement of a transgressive conglomeratic bed overlain by shallow water limestonesand underlain by calc-alkaline volcanics. The vein-type deposits consist ofMnoxides hosted by latite lava domes,comenditic lava flows and rhyolitic ignimbrites. Although these geological settings are markedly different, themain Mn-bearing phases are the same, namely pyrolusite and hollandite. The deposits differ in the presence ofaccessory minerals, such as todorokite, manganite, coronadite, cryptomelane and gangue minerals.Rare earth element geochemistry is themost effective tool in discriminating the two types ofMn deposits. Theseanalyses indicate that the vein-type deposits were formed from geofluid mixtures linked to the Oligocene–Miocene calcalkaline volcanic cycle. Mineralogical and geochemical data indicate that the crystallization of theMn oxides in the stratabound deposits was diagenetic, related to sudden changes in Eh and pH in coastal,meteoric waters that came in touch with the marine coastal water after having flushed the calcalkalinevolcanics.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The redox potential and pH of an aqueous system are the maincontrols on the geochemical behaviour of Mn and, ultimately, thecompetition between its affinity for solution or for the growth of solid-state Mn phases (Brookins, 1988). Manganese occurs in three oxidationstates: Mn2+, which is soluble under reducing and acidic conditions;and Mn3+ and Mn4+, which are less soluble and are stable underrelatively oxidizing and alkaline conditions. The accumulation of Mn insolution and its deposition occur in a variety of modern and ancientgeological settings (Roy, 1992). Supergene and hydrothermal pro-cesses, active in both marine and continental environments, areresponsible for the origin of most Mn ores (Nicholson, 1992). Theoccurrence of Mn-ores and their classification have been the subject ofmany studies (Roy, 1981, 1997; Nicholson, 1992; Fan and Yang, 1999;Liakopoulos et al., 2001; Shah and Moon, 2004, 2007; Brugger andMeisser, 2006), which have shown that each deposit has uniquecharacteristics and must be studied using a multidisciplinary approach.

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In Sardinia (western Mediterranean) different types of Mn-depositsoccur: a) sediment-hosted Mn deposits that are hosted by a clast-supported conglomerate deposited on calc-alkaline volcanic rocks duringa Miocene marine transgression, and b) volcanic-hosted Mn accumula-tions that cross-cut volcanic rocks ranging in composition from andesiteto rhyolite and comendite. Only a few studies have focused on thesedeposits (Redini, 1940; Guerin, 1983), which differ from the widespreadand better-knownMn ores that occur in sedimentary and hydrothermalocean-floor settings.

The unusual occurrence of these SardinianMn deposits makes themworthy of detailed analysis; in the present study, we investigated thegeology, geochemistry and mineralogy of different types of the Mndeposits from a few localities in Sardinia, with the aims of (1) verifyingwhether the different geological occurrence of these deposits producedMn-mineral accumulations with different geochemical and mineralog-ical characteristics, and (2) contribute to constrain the genesis of thesedeposits.

2. Geological setting

The geological framework of Sardinia comprises of a segment ofthe western European Variscan metamorphic basement, capped by

111R. Sinisi et al. / Ore Geology Reviews 47 (2012) 110–125

continental Upper Carboniferous–Permian sedimentary and volcanicdeposits, and covered by Mesozoic–Eocene carbonatic shelf deposits(Fig. 1). From the Oligocene to middle Miocene, calc-alkaline volcanicarc activity produced thick volcanic and volcano-sedimentary se-quences, ranging in composition from basaltic to rhyolitic. Thisvolcanic activity was related to subduction of the Ligurian–Piedmontoceanic domain beneath the south European paleomargin (Downes etal., 2001). During Burdigalian time, after collision between the Europeand Adria plates, which led to the formation of the northernApennines (Carmignani et al., 1995; Oggiano et al., 2009), back-arcspreading resulted in the counter-clockwise rotation of the Sardinia–Corsica block. Radiometric data indicate that most of the volcanicactivity occurred between 21 and 17 Ma, during the drift of theSardinia–Corsica microplate (Vigliotti and Langenheim, 1995) and therelated opening of the Liguro–Provençal back-arc basin (Malinvernoand Ryan, 1986). The eastward slab rollback (Beccaluva et al., 1994)and slab breakoff generated an asthenospheric window (Serri et al.,1993), resulting in the climax of the volcanic activity and inanomalous heat flow below the western part of the Sardinia–Corsicamicroplate, which is still apparent today (Carmignani et al., 1995;Della Vedova et al., 1995). Such geodynamic setting favoured theoccurrence of hydrothermal activity and related ores (de Boorderet al., 1998; Chen et al., 2009b).

After its rotation, the Sardinia–Corsica block experienced awidespread transgression, within a system of shallow half-grabensinfilled by carbonatic and siliciclastic sequences of upper Burdigalian–Tortonian age. At this stage the volcanism was almost quiescent andthere are no lavas or tuffites interbedded the mainly carbonatic-sequences of upper Burdigalian–Tortonian age (Funedda et al., 2000).

Fig. 1. Simplified geological map of Sardinia (modified after Carmign

2.1. Geology of the Mn deposits

2.1.1. Sediment-hosted Mn deposits (SH group)In northwestern Sardinia, several sediment-hosted Mn deposits

occur. The host-rocks were deposited during a transgressive phase inseveral N–S-trending intra-arc basins related to the upper Burdigalianextension. The Mn-mineralization at the base of a marine transgressivecarbonate sequence deposited within the Logudoro Basin duringthe upper Burdigalian–Langhian (Funedda et al., 2000). Manganeseminerals occur as matrix and cement of a transgressive conglomeratedeposited in littoral–facies, which include rounded, decimetre-sizedcobbles and rare boulders, derived from the underlying volcanic calc-alkaline succession (Fig. 2). The Mn-bearing conglomerate is continu-ous over a distance of 5 km and its thickness rarely exceeds 1.5 m. Theconglomerate layer is capped by a coarse bioclastic layer of oyster shells(Ostrea lamellosa), which are locally cemented by sparitic calcite andMn oxides. Fine, reworked Mn oxides occur in silty layers that locallycap the conglomerate (Fig. 2).

These Mn deposits occur in the Padria (SH1 subgroup), Ittiri–Uri(SH2 subgroup) and Monte Jorzi (SH3 subgroup) areas, where theywere mined during the 20th century. The transgressive conglomer-ates that host the Mn deposits comprise of ignimbrite clasts at Ittiri–Uri and Monte Jorzi, whereas at Padria andesite clasts prevail. AtIttiri–Uri and Monte Jorzi, the Mn-bearing conglomerate and itscalcareous cap are cut by E–W-striking normal faults that are possiblyrelated to Serravallian (~11–15 Ma) extensional tectonics. Thesefaults post-date the calc-alkaline volcanic activity in this part of theisland. There is no evidence to suggest the faults acted as conduits forhydrothermal fluids.

ani et al. (2001)) and S. Pietro Island (after Morra et al., 1994).

Fig. 2. (a) Stratigraphic scheme of Upper Burdigalian–Langhian littoral–facies conglomerate (not in scale) and (b) field photo of sediment-hosted Mn ores from Padria.

112 R. Sinisi et al. / Ore Geology Reviews 47 (2012) 110–125

2.1.2. Volcanic-hosted Mn deposits (VH group)In the Anglona district of northwest Sardinia, volcanic-hosted Mn

mineralization (the VH1 subgroup) occurs within a latite lava dome and,to a lesser degree, within a welded pyroclastic flow that is 18±0.6 Maold (Oudet et al., 2010; Fig. 3a). The area is characterized by a volcanic–sedimentary succession consisting of andesite domes, lacustrine tomarine Oligocene–Aquitanian tuffites, welded ignimbrites, latite to high-K basaltic andesites and lacustrine cherty tuffites capped by a weaklywelded pyroclastic flow. This succession is uncomformably overlain bytransgressive marine deposits of late Burdigalian age. Manganesemineralization occurs both as vein fill (Fig. 3b) and as stockwork inzones oriented approximately parallel to NNE–SSW-trending faults. TheMnminerals in these deposits are commonly associatedwith opal CT andquartz gangue, which also fills the stockwork.

At San Pietro Island, in southwest Sardinia (Fig. 1), vein-type Mnoccurs within both comenditic lavas and a rhyolitic ignimbritecomplex, which Arana et al. (1974) attributed to the final phases ofMiocene magmatism (~15 Ma).

San Pietro Island contains a further two types ofMn ore : 1) in the LaPiramide area, Mn mineralization occurs within a welded ignimbrite(VH2 subgroup) as individual centimetre-wide veins (Fig. 4a) and asaggregates, which mimic eutaxitic structure (Fig. 4b); opal-CT andquartz are the main gangue minerals; 2) in the Cala Fico area, the Mnminerals (VH3 subgroup) are hosted within comenditic lavas (Cioni

Fig. 3. (a) Stratigraphy of the Anglona volcanic sequences (not in scale). Note the occurrence(b) hydrothermal mineralization (VH1) within the Anglona andesites. Mn oxihydroxides a

and Funedda, 2005), consisting of vein swarms of massive ore orientedparallel to the main faults and of Mn-rich bands oriented parallel torheomorphism-related anisotropies (Fig. 4c).

Both types of mineralization are developed along an E–W-strikingfault that separates the lower part of the comenditic lava flows from theunderlying Matzaccara Ignimbrite unit, both of which are Langhian(~15–16 Ma) in age (Cioni and Funedda, 2005). Along the vein selvagesclinoptilolite often occurs as a gangue mineral (Fig. 4d).

3. Sampling and analytical methods

Eighteen samples of sediment-hostedMn ores and nineteen samplesof volcanic-hosted Mn ores were analysed to identify their textural,mineralogical and chemical characteristics. The samples were collectedfrom natural exposures or disused mines. Care was taken to preventcontamination of the samples during their collection, transport andsubsequent storage. At DISBEG Laboratories of the University of Sassari(Italy) whole-rock samples were first dried and then reduced to finepowder in a planetary mill (Retsch) equipped with two agate jars andagate milling balls; the mineralogy was analysed from randomlyoriented whole-rock powders recorded at 2°–70° 2θ with a step size of0.02° 2θ, using a Siemens D5000 diffractometer (Cu-Κα radiation, 40 kVand 30mA). At CIGAS Laboratories of University of Basilicata (Italy), thepetrography was studied by optical microscopy, using a binocular

of Mn-bearing stockworks and veins within the Burdigalian ignimbrites and lava flows;nd opal fill the fracture.

Fig. 4. (a) Centimetre wide vein-type Mn mineralizations and (b) Mn oxihydroxide aggregates within a welded rhyolitic ignimbrite at La Piramide district (VH2 ores); (c) Mn-richbands and (d) fracture infill within a comendite lavas at the Cala Fico site (VH3 ores).

113R. Sinisi et al. / Ore Geology Reviews 47 (2012) 110–125

microscope (OM Leitz), and by scanning electron microscopy (ESEMXL30 Philips LaB6) equipped with an energy-dispersive X-ray spec-trometer (SEM–EDS).

The abundances of major and trace elements were determined byinductively coupled plasma-mass spectrometry (ICP-MS) and instru-mental neutron activation (INAA) analysis, at the Activation Laborato-ries, Ancaster (Canada). Analytical uncertainties were less than ±5%,except for those elements occurring at a concentration of 10 ppm orlower, for which the uncertainties were ±5% to 10%. Total loss onignition (LOI) was gravimetrically estimated after overnight heating at950 °C.

Quantitative electron microprobe analyses of mineral phases wereacquired by a scanning electron microscope TESCAN Vega 3 LMUequipped with an X-Ray dispersive analyser TEAM EDS System withApollo X SDD installed at the Department for the Study of theTerritory and its Resources, University of Genoa. Operative conditionswere 15 kV accelerating voltage and 2.20 nA beam current. Theestimated uncertainties are: 0.1 wt.% for SiO2, TiO2, Al2O3, Cr2O3, FeO,MgO, MnO, P2O5; 0.05 wt.% for CaO and K2O; 0.25 wt.% for Na2O.

4. Results

4.1. Mineralogy and petrography

It is very difficult to recognize the Mn oxides on the base of X-raypowder diffraction scans alone, because they are composed often byamorphous matter or complex solid solutions. Most of the SardinianMn-accumulations, in particular, consist of hollandite, cryptomelaneand coronadite, which have very similar crystal structures. Therefore,the XRD analyses were combined with electron microprobe analyses

of few Mn minerals occurring in the studied ores. Several point EDSanalyses were performed on each mineral phases whose averagecomposition is given in Table 1.

4.1.1. SH Mn oresSamples RS4, RS5 and RS8 show a paragenesis dominated by

pyrolusite [MnO2] with minor hollandite [general formula Bax(Mn4+,Mn3+)8O16]. Todorokite [(Mg, Ca, Na, K)x(Mn4+,Mn3+)6O12·3.5H2O]andmanganite [general formula MnO(OH)] occur as accessory phasesin samples RS9 and RS1 (tokorokite), and RS3 (manganite). Gangueminerals are quartz, calcite, plagioclase and K-feldspar. The XRD dataalso indicate trace amounts of 2:1 clay minerals in samples RS4, RS5,RS8, RS9, Mn4 and Mn5.

Microscopic (OM and/or SEM) examinations reveal that the Mnoxides occur as microcrystalline cement that contains fragments ofvolcanic material. The texture of the cement varies with the differentMnminerals. Samples dominated by pyrolusite are characterized by aheterogeneous texture consisting of large prismatic and acicularpyrolusite crystals (20–60 μm) within smaller pyrolusite crystals(Fig. 5a). Where hollandite occurs as the main mineral phase, it formsbotryoidal nodules and concretions (Fig. 5b).

4.1.2. VH Mn oresThe parageneses of the ores from San Pietro Island (VH2, VH3) are

different from those from Anglona (VH1). In the VH2 and VH3 orespyrolusite, cryptomelane [general formula Kx(Mn4+,Mn3+)8O16] andhollandite are the main Mn minerals. Coronadite [general formulaPbx(Mn4+,Mn3+)8O16] was detected only in few samples from the VH3subgroup. Pyrolusite is the only Mn mineral in the VH1 subgroup. Theores in the Anglona area are also characterized by the occurrence of

Table 1Electron microprobe analysis of selected elements in representative samples of SH and VH Mn ores.

RS5 RS8 RS12 Mn4 RS17 LP5 CF4 CF7

cry pyr hol hol pyr cry hol cry cor hol cor

Na2O nd 0.13 0.09 0.28 0.11 0.70 0.39 0.38 nd 0.36 ndMgO nd 0.51 0.22 0.33 0.08 0.28 0.20 nd nd nd ndAs2O3 1.43 nd nd nd nd nd nd nd 0.67 nd 0.33Al2O3 0.32 nd 0.62 0.98 0.08 0.32 nd 2.00 0.21 0.19 0.08SiO2 0.19 0.9 0.26 0.39 0.8 0.39 0.34 0.68 0.71 0.30 0.51SrO 0.34 nd 0.60 0.93 nd 0.48 0.34 0.35 nd 0.32 ndSO3 0.52 nd nd nd nd nd nd nd nd nd ndP2O5 nd nd nd nd nd 0.55 0.89 nd nd 0.85 ndCl 0.11 nd nd nd nd nd 0.10 0.10 nd nd ndPbO Nd nd nd nd nd nd nd nd 23.62 nd 22.39K2O 6.28 nd 0.25 0.31 nd 4.47 1.18 4.64 0.27 0.90 0.34CaO 0.36 0.38 0.27 0.32 0.44 0.48 0.41 0.27 0.74 0.31 0.50BaO 0.87 nd 12.05 10.32 nd nd 7.00 0.39 1.36 6.53 1.15MnO2 87.13 95.6 77.16 77.19 95.75 88.11 81.23 85.60 65.36 87.27 68.57FeO 0.60 1.07 0.77 0.59 1.18 0.51 0.90 1.29 2.33 0.69 1.78CuO 0.40 nd nd nd nd nd nd nd nd nd ndZnO 0.19 nd nd nd nd nd nd nd 0.91 nd 0.61Tot 98.75 98.59 92.29 91.65 98.44 94.99 92.99 95.70 96.17 97.72 96.26

Note: Samples from RS5 to Mn4 are of SH group, the other samples are of VH group; all analysis are in percent; nd = not detected.

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feldspars, baryte and opal-CT. Baryte was detected only by SEM–EDSobservation,which also reveals that the Ba-bearing phase crystallized incavities within the massive pyrolusite. Opal-CT was observed both inoutcrop and in thin sections but its association with Mn mineralsprecludes the possibility to establish a precipitation sequence based ontextural features.

Fig. 5. BSE images showing the heterogeneous texture of pyrolusite (a) and a botryoi

Fig. 6. (a)OM image of a VH sample composed of K-feldspars partially replaced byMnoxihydroxicrystals are also detected. Ap = apatite; Bt = biotite; Cry = cryptomelane; Zrn = zircon.

Micromorphological observations reveal that, in all VH samples, Mnminerals form a muddy matrix containing accessory phases, such asquartz, plagioclase andK-feldspar. Inmost of the VH samples, K-feldsparis inter-grown with or replaced by Mn oxides or Fe-bearing minerals(Fig. 6a). Some samples also contain calcite, biotite, chlorite andclinoptilolite. The accessoryminerals detected by the EDSmicroanalyses

dal hollandite concretion (b) of SH ores. Cry = cryptomelane; Hol = hollandite.

des; (b) BSE image showing the biotite lamellae replaced by cryptomelane. Zircon and apatite

Table 2Chemical composition of sediment-hosted Mn ores.

SH1 (Padria) SH2 (Ittiri–Uri) SH3 (Mt. Jorzi)

RS1 RS3 RS4 RS5 RS6 RS8 RS9 RS10 RS11 RS12 RS13 Mn1 Mn2 Mn3 Mn4 Mn5 RS20 RS21

wt.%SiO2 2.7 4.3 16.6 32.2 17.3 32.4 15.4 3.9 4.6 2.9 16.2 4.2 4.0 10.1 25.9 19.3 10.7 8.6Al2O3 0.8 1.6 5.7 12.9 5.6 13.1 4.3 2.0 2.1 1.1 5.0 1.4 1.3 3.1 8.0 5.6 3.1 2.3Fe2O3 0.3 3.8 3.3 5.8 1.8 6.1 6.8 0.9 1.4 0.7 0.3 3.9 3.5 2.0 1.2 0.5 0.4MnO 73.3 76.5 50.3 16.6 51.7 27.4 53.0 65.0 76.7 80.8 30.7 7.7 41.0 4.3 41.0 44.4 40.8 58.1MgO 0.5 0.4 0.9 1.8 1.1 1.6 1.3 0.4 0.4 0.2 0.7 0.4 0.4 0.4 1.8 0.7 0.6 0.5CaO 5.3 0.5 4.5 9.5 5.6 2.7 2.1 8.4 0.8 1.2 21.0 48.5 24.6 42.7 4.5 10.2 18.8 9.6Na2O 0.2 0.1 0.8 1.1 0.6 1.1 0.4 0.1 0.1 0.9 0.1 0.1 0.4 0.5 0.9 0.8 0.7K2O 0.2 0.4 4.0 3.1 1.3 2.0 1.0 0.7 0.4 0.2 2.3 0.2 0.1 0.9 2.5 1.9 0.8 0.5TiO2 0.1 0.4 0.5 0.2 0.5 0.2 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1P2O5 0.2 0.1 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.3 0.2 0.1 0.5 0.2 1.0 0.3 0.5 0.3LOI 16.4 12.4 12.8 16.3 14.5 12.7 15.2 17.1 13.4 13.3 21.0 36.9 24.1 34.0 12.5 15.2 23.2 18.3Total 99.8 100.1 99.5 100.1 99.9 99.8 99.9 98.8 100.1 100.1 98.7 99.7 100.1 99.6 99.9 99.8 99.9 99.4Mn/Fe 319.8 22.3 16.9 3.2 31.3 4.9 8.6 80.0 58.6 17897.5 48.6 32.3 11.5 1.4 23.2 40.4 93.5 163.7SiO2/Al2O3 2.9 2.4 2.6 2.2 2.7 2.2 3.2 1.7 1.9 2.3 2.9 2.7 2.7 2.9 2.8 3.0 3.1 3.3

ppmAs 706 1560 2350 777 653 643 786 1170 1880 226 304 21.3 178 99 125 82.2 46 82Cr 23 27 32 43 32 35 42 35 33 36 31 4.5 7 12 12 4.5 59 40Sb 63.3 276 91.4 19.4 168 25 43.3 918 1190 10.5 17.7 2.9 11.9 3.4 9.4 15.7 1.8 15.1Sc 0.9 7.1 6.6 11.8 5.8 12.6 7.1 2.7 5.4 0.6 2.9 0.9 1.4 2.3 5 5.8 1.9 1.3Be 1 4 2 2 3 2 2 21 10 2 3 0.5 3 2 3 4 0.5 3Co 20 121 493 51 84 22 27 57 138 11 20 0.5 0.5 4 33 12 7 26Ni 308 86 42 21 61 29 67 76 67 113 32 25 71 15 107 90 53 77Zn 306 202 94 67 93 81 119 264 331 72 34 20 67 23 85 62 61 80Cd 1.8 0.8 0.6 0.4 1.1 0.8 1.2 1.1 0.4 1.5 1.6 6 1.9 0.6 2.4 1.9 2.7 2.1S 410 160 360 100 700 200 240 110 40 40 130 170 90 90 210 180 250 70Cu 256 3190 2750 744 3790 127 104 293 70 27 111 32 71 23 42 57 24 23Ag 18.5 21 13.2 2.2 12.6 5.6 14.2 18.3 19.7 14.3 7.2 1.8 9.5 0.3 14.4 15.1 9.2 15.5Ga 89 115 90 59 81 68 86 96 118 121 76 10 40 4 29 32 90 97

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Table 2 (continued)

SH1 (Padria) SH2 (Ittiri–Uri) SH3 (Mt. Jorzi)

RS1 RS3 RS4 RS5 RS6 RS8 RS9 RS10 RS11 RS12 RS13 Mn1 Mn2 Mn3 Mn4 Mn5 RS20 RS21

Ge 13 14 16 9 10 5 8 12 22 6 2 0.5 3 0.5 2 3 4 3Rb 5 8 49 46 24 32 51 6 7 6 52 10 8 31 81 52 26 15Sr 800 443 2600 640 567 478 1803 1898 1011 136 776 241 147 121 1907 1021 334 815Y 1.5 23 30 27 17 25 26 71 42 2 7 12 15 8 33 22 23 2Zr 3.5 3.5 3.5 63 3.5 75 3.5 3.5 3.5 3.5 3.5 3.5 3.5 71 3.5 3.5 3.5 3.5Nb 0.5 1 1 5 4 6 2 0.5 0.5 0.5 1 0.5 0.5 2 4 0.5 0.5 0.5Mo 6 20 32 63 46 32 6 18 8 28 25 7 21 2 6 1.5 3 4Sn 0.5 0.5 0.5 0.5 0.5 2 0.5 0.5 3 0.5 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5Cs 0.6 1.2 2.8 2.7 1.8 2.7 7.1 0.4 1.5 0.6 1.3 0.9 0.8 1 6.4 1.4 1.3 0.5Ba 1162 2644 13630 4411 1658 837 520 16830 5020 8978 5045 3004 739 911 9700 4119 635 4257Hf 0.1 0.5 1.3 3.8 1.7 3.8 1.2 0.4 0.4 0.1 1.3 0.6 0.4 1.7 2.3 1.1 1 0.8Ta 0.05 0.05 0.05 0.4 0.2 0.5 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.4 0.05 0.05 0.05W 2 13 0.5 3 110 10 7 47 54 1 45 4 14 12 31 5 10 25Tl 49.6 20.8 868 538 68.9 64.4 8.4 7.1 2.2 0.9 16.3 1.9 0.8 0.8 48.4 2.8 0.4 1.6Pb 39 55 35 15 36 24 38 38 46 35 20 7 24 10 32 32 25 44Th 0.4 0.7 2.3 6.1 2.7 5.7 2.3 0.7 0.9 0.6 3.5 0.8 0.8 2.4 7.8 4.2 1.9 1.4U 3.6 4.6 2.8 1 3.7 1.5 3.5 7.9 9.3 10.3 5 1.9 7.1 1 8 10.3 9.1 8.9La 4.2 12.3 17 32.7 20.1 28.5 16.6 50.7 16 2.7 13.6 2.9 7.5 4.5 26.4 23.8 23.8 6.7Ce 8.8 39.1 30 78.2 49.6 94.3 31.6 29.3 20.3 3.5 20.2 3.8 8.9 9.6 41.1 46 27.9 13.4Pr 0.8 3.49 4.3 8.31 4.85 9.49 4.36 4.15 2.84 0.35 2.28 0.46 1.1 0.85 5.41 5.64 3.3 1.05Nd 2.7 12.5 15.4 29 15.6 33.9 14.9 15.2 10.4 1.1 7 1.6 3.5 3.1 17.2 18 10.4 3.3Sm 0.6 2.8 3.4 6.1 3.5 7 3.2 2.6 2.5 0.2 1.4 0.4 0.7 0.6 3.4 3.8 2.1 0.6Eu 0.08 0.57 0.86 1.43 0.6 1.52 0.77 1.13 0.83 0.13 0.42 0.07 0.18 0.19 0.91 0.78 0.37 0.18Gd 0.5 2.7 3.8 5.9 3 6.2 3.2 5.1 3.6 0.3 1.3 0.5 0.9 0.7 3.5 3.7 2.2 0.6Tb 0.05 0.6 0.7 0.9 0.6 0.9 0.6 0.7 0.7 0.05 0.2 0.1 0.2 0.1 0.7 0.7 0.4 0.1Dy 0.6 3.8 4.1 5.4 3.6 5.2 3.8 4.1 4.4 0.3 1.3 0.8 1.2 0.7 4.3 4.1 2.5 0.6Ho 0.1 0.9 0.9 1.1 0.8 1.1 0.8 0.9 1 0.05 0.3 0.2 0.3 0.2 0.9 0.8 0.6 0.1Er 0.4 3.2 3.3 3.5 2.4 3.1 3 2.8 3.4 0.4 1 0.7 1.1 0.5 3.1 2.6 1.9 0.5Tm 0.07 0.55 0.54 0.53 0.39 0.48 0.49 0.34 0.48 0.08 0.16 0.13 0.19 0.08 0.5 0.41 0.31 0.08Yb 0.4 3.9 3.5 3.5 2.6 3.2 3.3 1.8 2.9 0.6 1 0.9 1.3 0.6 3.3 2.7 1.9 0.5Lu 0.06 0.63 0.56 0.53 0.39 0.47 0.51 0.24 0.47 0.1 0.15 0.16 0.22 0.09 0.53 0.42 0.28 0.07ΣREE 19.36 87.04 88.36 177.10 108.03 195.36 87.13 119.06 69.82 9.86 50.31 12.72 27.29 21.81 111.25 113.45 77.96 27.78Eu/Eu* 0.61 0.87 1.01 1.00 0.78 0.97 1.01 1.31 1.16 2.23 1.31 0.66 0.95 1.23 1.11 0.88 0.72 1.26Ce/Ce* 1.18 1.55 0.90 1.22 1.32 1.53 0.97 0.38 0.71 0.80 0.88 0.76 0.72 1.18 0.87 1.04 0.72 1.19

Note: Ce/Ce*=(3Ce/CeGloss) / (2La/LaGloss+Nd/NdGloss). Eu/Eu*=EuGloss /√(SmGloss ∙GdGloss).

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Table 3Chemical composition of volcanic-hosted Mn ores.

VH1 (Anglona) VH2 (La Piramide) VH3 (Cala Fico)

RS17 RS18 RS19 RS22 LP 1 LP 2 LP 3 LP 4 LP 5 LP mc LP v CF 1 CF 2 CF 3 CF 4 CF 5 CF 6 CF 7 CF 8

wt.%SiO2 7.7 5.6 5.5 25.5 32.7 27.6 23.1 21.0 25.1 25.4 31.4 42.8 42.0 45.0 3.2 1.9 6.5 22.9 29.9Al2O3 1.4 0.4 1.2 0.9 7.9 5.6 5.9 4.3 5.2 5.7 6.1 6.6 6.7 7.4 1.9 1.4 1.4 3.6 4.6Fe2O3 0.2 0.0 0.4 0.2 2.4 1.5 1.7 1.3 3.1 1.8 1.7 1.8 1.7 2.0 0.1 0.0 0.2 1.6 2.6MnO 78.1 81.7 79.3 62.3 40.8 48.1 53.2 58.4 52.2 50.0 45.6 31.3 35.4 31.2 75.9 80.0 72.9 56.8 49.1MgO 0.1 0.1 0.2 0.1 0.4 0.5 0.4 0.1 0.2 0.6 0.3 0.2 0.1 0.1 0.2 0.1 0.6 0.1 0.1CaO 0.4 0.3 0.5 0.4 0.8 0.7 0.8 0.2 0.4 1.0 0.7 0.2 0.1 0.1 0.2 0.1 0.4 0.1 0.2Na2O 0.2 0.2 0.1 2.5 1.0 1.6 1.1 0.4 1.1 1.7 1.8 2.6 2.5 0.5 0.4 1.2 1.9K2O 0.2 0.2 0.2 4.5 4.2 3.2 4.1 4.6 2.9 4.7 4.9 4.7 4.9 5.2 3.9 2.0 3.8 4.6TiO2 0.3 0.3 0.4 0.1 0.2 0.3 0.3 0.1 0.1 0.1 0.1 0.1P2O5 0.2 0.2 0.1 0.1 0.5 0.2 0.2 0.3 0.3 0.2 0.4 0.1LOI 11.1 11.9 12.4 9.8 7.2 10.1 9.6 9.1 8.0 10.9 7.0 10.3 6.2 6.3 11.7 11.7 16.0 9.1 6.8Tot 99.7 100.2 100.0 99.6 99.9 99.8 100.1 100.1 99.8 99.7 99.8 99.9 99.7 99.7 99.1 99.5 100.0 99.4 99.7Mn/Fe 466.7 18229.7 230.7 442.6 19.0 34.5 33.8 48.4 18.8 31.2 30.1 18.9 22.9 16.9 976.3 16989.6 385.0 39.2 21.1SiO2/Al2O3 4.8 11.8 4.0 24.0 3.7 4.4 3.4 4.4 4.2 4.0 4.5 5.7 5.6 5.4 1.5 1.2 4.1 5.6 5.7

ppmAs 74 79 70 114 193 172 159 165 159 116 213 93.1 101 87.5 207 399 756 297 294Cr 96 91 77 4.5 11 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 8 7 12 4.5 4.5Sb 31.4 31.9 54.2 87.9 6.2 3.9 5.4 4.5 4.5 8.2 3.5 4.1 3.6 8.3 19.7 50.2 61.3 13.6 4.5Sc 5.3 4.9 3.6 3 6.2 3.5 2.6 4.6 5.9 3.2 6.2 1.6 1.7 1.5 1.5 1.2 0.9 0.9 1Be 15 13 17 22 38 35 67 47 38 43 42 23 41 27 46 61 50 75 37V n.d. n.d. n.d. n.d. 33 4.5 29 4.5 29 35 7 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5Co 12 21 16 17 23 37 125 26 26 69 32 10 28 4 19 21 13 88 31Ni 37 53 37 32 36 45 58 55 49 41 35 40 54 36 77 69 62 49 46Zn 125 121 93 45 256 419 343 490 438 231 187 1120 1540 1260 1180 869 597 1150 778Cd 3.9 3.7 3.7 1.8 39.8 50.5 23.5 44.8 54.4 29.5 49.8 9.5 13 17.1 21.7 19.8 33.1 113 18.3S 3 2 2 1 90 30 70 50 40 80 20 100 50 120 170 80 150 50 10Cu 520 1330 258 956 3 5 31 4 4 6 5 13 7 28 31 11 21 21 3Ag 20.2 20 21 15 9.6 12 13 14.3 13.3 11.1 8.9 6 8.4 6.6 20.7 19.1 19.7 14.5 12.7

(continued on next page)

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Table 3 (continued)

VH1 (Anglona) VH2 (La Piramide) VH3 (Cala Fico)

RS17 RS18 RS19 RS22 LP 1 LP 2 LP 3 LP 4 LP 5 LP mc LP v CF 1 CF 2 CF 3 CF 4 CF 5 CF 6 CF 7 CF 8

Ga 107 129 130 103 16 13 16 18 16 16 15 25 24 28 24 17 13 25 18Ge 11 9 13 6 3 2 4 6 4 2 10 5 5 7 11 12 8 20 9Rb 9 2 7 3 89 107 79 94 97 69 115 170 178 187 64 45 22 109 134Sr 377 401 529 462 1987 2174 1159 1741 2118 1780 1640 712 833 497 1790 1009 805 1568 978Y 72 60 60 27 53 72 150 120 150 61 41 239 353 254 171 187 184 123 216Zr 3.5 4 4 4 136 75 79 142 333 17 3.5 311 364 545 3.5 3.5 7 86 283Nb 2 1 2 1 31 18 17 39 6 2 2 10 52 120 0.5 18 4 1 16Mo 16 101 10 12 101 101 81 101 53 13 16 11 23 101 23 101 52 24 101Sn 0.5 2 1 1 2 4 4 3 2 2 3 4 7 7 0.5 0.5 0.5 0.5 6Cs 1.4 0 1 1 3.3 17.5 11.4 3.7 2.7 15.2 2.5 4 4.6 3.3 1.6 0.9 0.4 4.3 3.1Ba 2209 1882 2828 4786 6824 6939 34370 10870 11160 18020 9871 2558 4223 2771 1224 1112 262 14030 7351Hf 0.7 0.1 0.9 0.1 4.9 4.3 4.7 6.4 4.5 3.9 3.3 12.4 11.7 14.6 3.1 2.3 2.6 6.9 12.4Ta 0.05 0.05 0.05 0.05 1.6 1.1 1.1 2.3 0.8 0.4 0.7 3.4 3.8 4.8 0.05 0.05 0.05 0.2 2.4W 179 237 49 505 11 3 5 53 1 0.5 0.5 0.5 0.5 69 0.5 18 11 0.5 0.5Tl 7.7 3.6 7.6 2.9 4.6 5 47.6 67 75.6 16 68.1 2.8 1 4.7 14.5 5.1 7.1 1001 263Pb 48 48 49 35 76 88 325 52 48 321 48 38 79 99 180 69 46 170 45Th 1.0 0.6 1.2 0.1 8 7.2 7.2 8.4 8.2 7 6.9 21.9 20.8 32 5 1.8 1.1 11.8 18.1U 26.0 28.1 24.4 32.7 9.2 5.3 13.5 14.1 10.5 10.6 8.6 23.9 22.5 18.5 26.8 63.2 107 32 31.5La 78.4 32.7 64.3 42.7 77.6 75.5 124 127 182 84.5 48.6 479 516 464 797 744 695 502 281Ce 65.4 30.4 60.5 24.1 305 207 629 328 233 539 100 620 503 977 1420 563 634 1370 289Pr 14.1 6.0 11.4 4.6 15.8 15.9 20.8 25 31.8 17.5 9.95 97.6 96.7 98 187 187 197 116 52Nd 48.6 21.6 37.1 14.5 54.4 58.3 74 87.6 116 62.6 36.6 345 349 336 594 612 687 397 193Sm 9.8 4.7 7.1 2.5 11.4 13.7 17.6 20.7 24.3 14.3 7.9 79.7 81.9 76.6 122 128 145 87.3 45.6Eu 1.37 0.70 1.02 0.31 1.53 2.78 2.97 1 1.22 2.21 1.63 0.5 0.6 0.51 0.67 0.7 1.15 0.35 0.42Gd 11.6 6.3 7.4 3.1 9.1 12.7 20.9 19.7 22.8 13.6 7 83.6 105 74.6 89.2 102 83.7 63.3 68.4Tb 1.8 1.1 1.2 0.5 1.9 2.6 4.9 4.5 4.7 2.7 1.4 11.8 14.6 10.8 11 11.4 11.6 7.6 9.7Dy 11.5 7.1 7.8 3.5 10.9 14.5 31.2 26.7 26.9 15.4 8.6 53.1 67.7 51 47.4 51.6 54.7 30.4 47Ho 2.6 1.6 1.7 0.9 2.2 3 7.3 5.6 5.3 3.1 1.8 8.6 11.1 8.6 7.3 8 8.9 4.7 8.3Er 7.9 5.5 5.7 3.2 6.4 8.4 22 15.9 14.7 8.7 5.3 20.2 25.8 20.8 17.7 19.4 21.5 11 19.2Tm 1.12 0.82 0.84 0.55 1.04 1.1 3.29 2.22 2.07 1.26 0.74 2.7 3.25 2.8 2.39 2.57 2.71 1.4 2.46Yb 6.70 4.90 5.10 3.90 6.3 6.5 20.3 13.4 12.5 8.2 4.7 14.1 15.7 14.6 12.1 12.9 13.9 7.2 12.1Lu 0.99 0.76 0.76 0.65 0.9 1 3.32 2.09 1.89 1.28 0.72 1.77 1.95 1.85 1.39 1.52 1.74 0.9 1.58ΣREE 261.88 124.17 211.92 105.02 504.47 422.98 981.58 679.41 679.18 774.35 234.94 1817.67 1792.30 2137.16 3309.15 2444.09 2557.90 2599.15 1029.76Eu/Eu* 0.54 0.54 0.59 0.47 0.63 0.89 0.65 0.21 0.22 0.67 0.92 0.03 0.03 0.03 0.03 0.03 0.04 0.02 0.03Ce/Ce* 0.47 0.52 0.54 0.36 2.16 1.46 2.90 1.42 0.72 3.45 1.11 0.71 0.54 1.15 0.96 0.40 0.45 1.45 0.57

Note: Ce/Ce*=(3Ce/CeGloss) / (2La/LaGloss+Nd/NdGloss). Eu/Eu*=EuGloss /√(SmGloss ∙GdGloss).

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Fig. 7. GLOSS normalized major, trace elements and REE patterns of SH and VH samples.

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include sub-euhedral zircon crystals, commonly associated with biotiteand fluorapatite, which are probably remnants of the wall rock, andsecondary ilmenite and kaolinite. The latter mineral appears at themargins of cryptomelane cores, and cryptomelane occurs as pseudo-morphs after biotite (Fig. 6b).

4.2. Geochemistry

The elemental composition of the SH and VH Mn-rich ores isprovided in Tables 2 and 3, respectively. The elemental distributions,normalized to the GLOSS upper continental crust model (Plank andLangmuir, 1998), are shown in Fig. 7.

4.2.1. Major elementsAll studied Mn ore samples have Mn contents that are commonly

equal to or higher than 100XGLOSS. However, the SH andVHores showa certain degree of variability, likely due to varying amounts of ganguematerial. In the VH samples, the MnO content ranges from 31.2 to81.7 wt.%; in the SH ores, it varies between 4.3 and 80.8 wt.%. For theother major elements, the Anglona ores (VH1) are characterizedby concentrations lower than GLOSS values, except for P2O5, which

occurs at concentrations similar to those of the standard. All the VH2and VH3 samples, except for CF6, show moderate enrichment of K2O(1–2×GLOSS) and moderate depletion of SiO2, Al2O3, Fe2O3, Na2O andTiO2. MgO and CaO are always depleted relative to the standard values.The P2O5 concentrations are highly variable; within VH2 ores, theconcentrations are similar to or higher than those of GLOSS, whereasthey are depleted within VH3 samples.

The GLOSS-normalized distribution of major elements in thesediment-hosted Mn accumulations is similar to that in the volcanic-hosted ores, although most SH samples contain CaO concentrationshigher than that in GLOSS, and K2O concentrations are similar to orlower than that of the standard. The SH samples also show greatervariability in SiO2, Al2O3 and Fe2O3 than in the vein type Mn ores.

In summary, despite some variability, the major elements geo-chemistry is not suited to distinguishing the different Mn-bearingdeposit types.

4.2.2. Trace elementsThe trace element geochemistry has been a diagnostic tool to

distinguish volcanic-hosted from sediment-hosted Mn deposits(Nicholson, 1992). The VH2 and VH3 samples are characterized by a

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wide range of trace element contents. Co, Zn, Sr, Y, Ba, Hf, Pb and Uare enriched relative to GLOSS, whereas the transition elements(mainly Cr, Sc, Cu and V) are depleted. Rb, Zr, Nb, Cs, Ta and Th arevariably depleted or enriched, whereas the Ni content is alwayscomparable to that of the standard. In the VH1 samples, large-ionlithophile elements (LILE) and Zn are enriched, whereas most of thehigh-field strength elements (HFSE) are depleted. Ni concentrationsare usually similar to that of GLOSS, whereas Cu is strongly enriched.

Contrary to expectation, the trace element geochemistry of SHores is similar to that of the Anglona deposits (VH1). The VH2 andVH3 ores, however, strongly differ from SH ores, mainly in terms oftheir Ni, Zn, Rb, Cu and Ta contents. The pattern of these elements inthe SH samples is the opposite of that in the samples from San PietroIsland (VH1 and VH2 groups, Fig. 7).

4.2.3. Rare earth elementsFig. 7 also shows the GLOSS-normalized REE patterns of VH and SH

Mn ores. REE concentrations in both types of Mn ores are very highwith respect to the reference standard, and the volcanic-hosted oreshave ΣREE values (105.02–3309.15 ppm) that are several orders ofmagnitude higher than those of most of the sediment-hosted deposits(9.86–195.36 ppm).

Large negative Eu anomalies are observed in the samples of VHgroup and Eu/Eu* values range from 0.02 to 0.92. Positive Ceanomalies are observed in VH2 ores, whereas most of the VH3 andall of the VH1 samples have a negative Ce anomaly. The SH samplescontain both negative and positive Ce anomalies (0.38bCe/Ce*b1.55)and Eu anomalies (0.61bEu/Eu*b2.23).

5. Discussion

The aim of the discussion is to compare deposits with obviousdifferences of geological occurrence in order to identify their distinctivefeatures compositional characteristics.

5.1. Sediment-hosted Mn ores

Redox processes govern Mn behaviour in the sedimentary environ-ment. Eh–pH diagrams indicate that Mn2+ is soluble over a wide rangeof Eh and pH values and that it precipitates only when the environmentbecomes alkaline and/or highly oxidizing (e.g., Brookins, 1988; Takeno,2005). Reconstructing the depositional environment of Mn depositsis, however, very difficult because other Mn ore minerals, such asMn4+ oxihydroxides form under an array of conditions. Consequent-ly, the mineral assemblages of hydrogenous deposits cannot be usedto constrain the environment of mineralization (Nicholson, 1992).In particular, mineral assemblages similar to those of the Sardiniandeposits have been documented in mineralization related to bothmarine (El-Hasan et al., 2008) and hydrothermal environments (Post,1999; Glasby et al., 2005; Fitzgerald and Gillis, 2006). These studiesindicate that the mineral assemblages per se cannot be used todiscriminate the environment in which they form.

Regardless of the depositional environment and processes offormation, Mn4+ oxihydroxides are characterized by high surfacecharge, which influences the mineralogy and geochemistry of Mndeposits. Murray (1975) and Nicholson and Eley (1997) argued thatthe adsorption of elements onto the surface of Mn4+ oxihydroxides isthe main process leading to the accumulation of cations (mainlyheavy metals) in Mn deposits. The pH of the solution also influencesthe adsorption mechanism: the adsorption capacity of Mn4+

oxihydroxides increases with increasing pH (Nicholson and Eley,1997). In alkaline seawater, dissolved cations of transition metals areeffectively absorbed onto oxihydroxide surfaces, resulting in themarked enrichment of Cu, Ni, Co, Zn, Ba and Pb (Koschinsky et al.,2003; Maynard, 2003).

Trace elements can be absorbed within the Mn-bearing mineralsby a series of mechanisms including adsorption, absorption, replace-ment through ion exchange and co-precipitation with Mn oxides(Koschinsky and Hein, 2003). These mechanisms typically occur inthe presence of Mn-bearing minerals with a tunnel crystal structure(Chen et al., 2009a; Randall et al., 1998).

Pyrolusite, hollandite and todorokite, which are the main Mnminerals in the studied SH ores, are characterized by a tunnel structureconsisting of MnO6 octahedra units shared by apexes or edges (Post,1999). Therefore, the occurrence of these minerals in the sediment-hosted Mn ores of Sardinia may explain, at least in part, the highcontents of certain alkali and alkaline earth metals that also occur inother minerals, such as feldspars. The availability of dissolved ions insolution may also affect the mineralogy of Mn oxides (Nahon et al.,1983), promoting the precipitation of todorokite at some of the studiedsites. Furthermore, the absence of calcite in samples containingtodorokite suggests that this mineral may have played a significantrole in the removal of Ca from solution.

The occurrence ofmanganite requires a separate discussion. Althoughit is metastable, manganite is themost stable of the Mn3+ oxihydroxidesin a surface environment. As with the other Mn oxides, manganite isubiquitous, occurring as a weathering product from parent rocks ofvarying origin andmineralogy (Nahon et al., 1983), or, as is most likely inthe present case, as an authigenicmineral formedduring the initial stagesof oxidation of dissolved Mn2+ (Roy, 2006). Accordingly, the precipita-tion of manganite requires a solution with lower Eh values than thosefrom which pyrolusite forms (Takeno, 2005). Correlation plots mayelucidate relationships among manganese and trace elements andsignificant trace elements versus MnO plots are shown in Fig. 8. Despitesome negligible differences among sets of samples, it is observed thatsome heavy metals (Ni, Zn, Pb) and U are positively correlated to theMnO concentration. In contrast, other distinctive elements of magmaticrocks, such as Rb and Th, show a negative correlation to Mn oxide,confirming that these elements are sourced from the host rock.

Sediment-hosted Mn deposits contain high concentrations of As, Sband U. The behaviour of As and Sb depends largely on redox conditions.Because of their similar physicochemical properties, these elementsform similar aqueous species, complexes andminerals. Previous studieshave argued that isomorphic replacement, adsorption, and oxidizationprocesses may be responsible for the uptake of these trace elements inMn ores (Ouvrard et al., 2005; Mohapatra et al., 2006; Landrum et al.,2009). This suggestion is supported by the weak correlation betweenS and As or Sb in the studied ores (As versus S: r=0.14; Sb versusS: r=−0.19). SEM–EDS observations did not reveal the presence ofa U-bearing mineral, hence the adsorption mechanism exerted byMn-oxides is probably responsible for uranium enrichment observedin SH deposits (Koppi et al., 1996; Webb et al., 2006).

REEs may also be adsorbed onto the surface of Mn oxides (Toth,1980; Fodor et al., 1994; Koppi et al., 1996). REE geochemistry hasbeen widely used to estimate the redox conditions of marinedepositional environments and sedimentary processes (Hein et al.,1997; Dia et al., 2000). The concentrations and distribution patternsof REEs may indicate a change in the redox potential of the solutionand could be linked to changes in the sorption properties of solidphases (El-Hasan et al., 2008). However, REEs are relatively immobile,resulting in low REE contents in seawater (e.g., Temple and Walsh,1994). Thus, the apparent lack of correlation observed between MnOand REEs in the present study (MnO versus La: r=−0.02; MnOversus Eu: r=−0.10; MnO versus Yb: r=0.07) suggest that the veryfine detrital component trapped within the ore yield most of the REEsin the Sardinian Mn ores of marine origin. Furthermore, it isconceivable that the REE geochemistry of these ores is also influencedby accessory minerals. The occurrence of REE-containing minerals,such as feldspars and carbonates, inevitably leads to a dilution effectof the LREE, MREE and HREE concentrations. In fact, in SH ores theflattened shape of REE distribution patterns and the positive correlation

Fig. 8. Correlation plots of some trace elements vs MnO contents in sediment-hosted samples.

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of La, Yb and ΣREE vs SiO2 contents, as shown in Fig. 9, may be theevidence of a clear dependence of the rare earth element contents bysilicate phases rather than by manganese oxides.

5.2. Volcanic-hosted Mn ores

In Sardinia, there are numerous examples of metalliferous depositsoriginated by hydrothermal activity. As mentioned above, during theMiocene the geodynamic context of the island was favourable to theformation of hydrothermal deposits (Sillitoe, 1997; de Boorder et al.,1998; Glasby et al., 2006). In fact, hydrothermal ore deposits, includingCu-porphyry, Carlin-type gold deposits and minor hydrothermal veins,are widespread within the Early Miocene volcanic rocks of Sardinia.Probably, in western Sardinia, the Mnmineralization was also promotedby hydrothermal activity, which generally occurred within lava flows ofvarious compositions and in acidic pyroclastic flows of Miocene age. Thebrecciated texture of the vein infill, characterized by wall rock fragmentsenclosed in a Mn-rich cement and the occurrence of clinoptilolite alongthe wall rock selvages are the main proof of their hydrothermal origin.The occurrence of zeolite, in particular, may indicate the presence ofslightly alkaline solutions produced by interaction between fluids andalkaline/peralkaline rocks, such as rhyolites and comendites (Zhang et al.,2001).

The geochemistry of the volcanic-hosted Mn ore examined in thepresent study is markedly different from that of the widespread andbetter-known marine hydrothermal Mn ores. In fact, the values ofMn/Fe and SiO2/Al2O3 ratios, the Ce anomalies values, the concentra-tions of transition metals and ΣREE, which characterize the hydro-thermal Mn-rich crust, such as described by several authors (Bonattiet al., 1972; Toth, 1980; Nath et al., 1997), are different to thoseobtained for the volcanic-hosted Mn ores of Sardinia, indicating thatthe two ore types have different origins.

Few studies have examined vein-style hydrothermal Mn miner-alization within a continental environment (e.g., O'Reilly, 1992), butseveral models have been proposed to explain the geological settingin which the required hydrothermal fluids may form (Robb, 2005,and references therein). Of all the models, the mixing of magmaticfluids with meteoric water in the shallow crust appears most relevantto the geological setting of the Sardinian vein-type Mn ores. The highconcentrations of some important trace elements, such as W, Mo, Cu,Be, U and Pb in the studied ores, suggest that the mineralizing fluidwas magmatic in origin (O'Reilly, 1992). The dilution and cooling ofmagmatic fluids and mixing with meteoric water are commonphenomena in geological contexts where hydrothermal convectivecells form during and after magma emplacement (Mangas andArribas, 1988). During convection, the heated fluid may be enriched

Fig. 9. Correlation plots of La, Yb and REE vs SiO2 contents in SH samples and versus MnO in VH samples.

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in metals and other trace elements leached from the host rocks. Thehigh concentrations of trace elements in the Sardinian Mn oressuggest they formed from a solution previously enriched in such traceelements. Abrupt changes in the physical and chemical conditions ofthe mineralizing solution may result in the precipitation of the Mn4+

oxihydroxides and the closely associated fractionation of elementsbetween fluid and mineral. Fractionation may occur not only byprecipitation but also by particle scavenging, which is important inthe presence of Mn4+ oxihydroxides (Nicholson and Eley, 1997;Mohapatra et al., 2009). Consequently, the high adsorption capacityonto the Mn oxides surface and high cations uptake into the crystalstructure likely play a key role in the hydrothermal environments asmuch as in supergene ones. These geochemical properties of Mn ores,combined with the lack of minerals that contain high concentrationsof Co, Cu, Zn, Sb and As, can explain the high contents of theseelements in the VH samples. Conversely, the high contents of K, Baand Pb may reflect the partitioning of these elements into the latticesof cryptomelane, hollandite and coronadite, respectively.

Binary diagrams of some trace elements vs MnO contents ofVH group mineralization were constructed (Fig. 10) to identify theinfluence of Mn oxides on the distribution of trace elements. Despitethe VH1 subgroup showing markedly different elemental concentra-tions and patterns compared to VH2 and VH3 sample sets, the plotsshow a general positive correlation for Ni, Zn and U, confirming theirclose association with Mn4+ oxihydroxides, and a clear negative

correlation for Rb, Th and Zr likely related to volcanic host rock relictswithin VH accumulations.

The three subgroups of Sardinian volcanic-hosted Mn ores havedifferent geochemical composition. The Anglona ores (VH1) aremarked by relatively high Cu contents, possibly reflecting the presenceof a hydrothermal fluid that was previously enriched in Cu duringinteraction with one of the Cu-porphyries that are widespread in NWSardinia. It is also interesting to note the elevated Zn concentrations forVH2 and VH3 samples. A possible source of Zn can be envisaged in thePaleozoic basement, below the volcanic rocks, of the San Pietro Islandwhich contains the well-known Sulcis–Iglesiente Zn deposits. Theseenrichments could be due to hydrothermal fluids that have been incontact with the Zn-rich basement rocks.

Concentration and distribution of REE are characteristic for eachsubgroup of the SardinianMnores and they differ frompatterns reportedin literature, especially when compared to the average concentrations ofREEs in economically important Mn deposits worldwide (Toth, 1980;Hein et al., 1997; Nath et al., 1997; Maynard, 2003; Shah and Moon,2004). The deposits of Anglona and San Pietro Island have different REEpatterns. The concentrations and distribution of REEs in the Anglonasamples, as well as Eu anomalies, are comparable to those of an andesiticlava, suggesting that the mineralizing solution retained the chemicalsignature of the rocks through which it migrated. Alternatively, the REEgeochemistry of the Anglona Mn ore samples is determined by smallamounts of host rock fragments that dilute the ore. The negative Ce

Fig. 10. Correlation plots of some trace elements vs MnO contents in volcanic-hosted samples.

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anomalies in these ores, as well as those observed in the VH3 ores, mayindicate oxidizing conditions for the fluids, which promoted Ce depletionprior to Mn mineral precipitation. Furthermore, the VH3 ores havehigh REE contents, high LREE/HREE values and strongly negative Euanomalies,whichmay reflect the chemical signature of afluid related to ahighly evolved comenditic lava. The moderate Eu anomalies and flat REEpatterns observed in the VH2 subgroup may indicate a fluid related tothe less-evolved rhyolitic ignimbrite. However, a portion of REE contentsof volcanic-hosted accumulations is likely due to adsorption process onMn4+ oxihydroxides. As shown in Fig. 9, the La, Yb and ΣREE vs MnOplots show a positive correlation of elements confirming the key role ofMn oxides in the REE uptake.

6. Summary and conclusions

This paper reports, for the first time, mineralogical and geochem-ical data regarding sediment-hosted and volcanic-hosted Mn depositsof Sardinia. These data provide a series of important constraints onthe genesis of these deposits, which are summarized below.

1. With several exceptions, the two types of Mn deposits in Sardiniacontain two main Mn-bearing mineral: pyrolusite and hollandite.The Mn minerals in the sediment-hosted group include pyrolusite,

hollandite, manganite and todorokite; calcite and clay mineralsoccur as accessory minerals. The volcanic-hosted ores consist ofpyrolusite, hollandite, cryptomelane and coronadite as Mn min-erals, and quartz, opal-CT and baryte as gangue minerals.

2. Both the SH and VH Mn deposits are characterized by a highlyvariable chemical composition. Relative to GLOSS, most of thesamples are enriched in As, Co, Zn, Sr, Ba, Pb and U, regardless ofmineralogy. This finding suggests that the Mn oxides had aninfluence on trace element distribution in both deposit types.Previous studies have proposed that metal enrichment occurs viaadsorption, cation exchange and co-precipitation reactions. Thisview is supported by SEM–EDS observations in the present study,because the trace elements are not associated with a specificmineral phase; instead, they are uniformly distributed throughoutthe ores samples. The two types of deposit also show clear chemicaldifferences, especially in terms of REE patterns. Compared with theSH deposits, the VH deposits have higher values of total REE contentand larger Eu anomalies. These differences are related to thecompositional diversity of the solutions fromwhich the oremineralsformed, rather than to the contrasting uptake capacities of thevarious Mn minerals.

3. Based on geochemical and mineralogical data, backed by fieldobservations, we propose that the SH deposits formed during

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the lower Miocene transgression when continental, acidic, poorlyoxygenated, phreatic waters of a coastal aquifer mixed with themarine water that saturated the highly permeable conglomerate.The rapid increase of pH favoured the precipitation of pyrolusiteand other Mn-bearing minerals that now constitute the cement ofthe littoral rudite, as evidenced by the abundance of concretionarytextures and nodules (Fig. 5).

Similar diagenetic origin of Mn deposits is described in Morocco atImini (Force et al., 1986; Rhalmi et al., 1997; Gutzmer et al., 2006)where Mn ores formed within coastal dolomitic sandstones thatfavoured the interaction between groundwater, which flushed igneousrocks, and marine water. The direct precipitation and accumulation ofMn minerals from marine water can be excluded at least in the open,wave dominated, shore where conglomerates are formed. Anexhalative-sedimentary genesis of SH ore can also be ruled out for thefollowing reasons: i) in north Sardinia no volcanic activity coeval to theupper Burdigalian transgression is recorded; ii) the SH deposits arestrictly associated with the transgression-related conglomerate thatrepresent the interface between continental andmarine conditions andno Mn concentrations occur in the overlaying, frank marine deposits;iii) no hydrothermal-related gangue mineral, such as opal or quartz,accompany this mineralization.

The VH ores most likely originated from the mixing of magmaticfluids and meteoric waters, as indicated by field observations andgeochemical data. Additional isotope-based studies are required toclarify the nature of the Mn-bearing fluids. Our data, however, revealedthat race elements and REE patterns are the most appropriate tool indistinguishing among these Mn deposits.

Acknowledgements

We thank Cristina Persano for her constructive comments and forcorrecting and improving the English, and also Jens Gutzmer and histwo anonymous referees for important comments and constructivecriticism.

We are also very grateful to Laura Negretti of the University ofGenova for assistance in mineral data acquisition and AlessandroLaurita of the University of Basilicata for the SEM image acquirement.

Financial support was provided by Banco di Sardegna Foundationgrant.

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