Trace-element and Sr–Nd isotopic evidence for the origin of the Sardinian fluorite mineralization (Italy)

Applied Geochemistry 23 (2008) 2906–2921

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Trace-element and Sr–Nd isotopic evidence for the origin of the Sardinian fluorite mineralization (Italy) F. Castorina a,b,*, U. Masi a,b, G. Padalino c, M. Palomba d a

Dipartimento di Scienze della Terra, Università ‘‘La Sapienza”, P.le Aldo Moro, 00185 Rome, Italy Istituto di Geologia Ambientale e Geoingegneria del CNR, Sezione di Roma ‘‘La Sapienza”, Rome, Italy Dipartimento di Geoingegneria e Tecnologia Ambientale, Università di Cagliari, Cagliari, Italy d Istituto di Geologia Ambientale e Geoingegneria del CNR, Sezione di Cagliari, Cagliari, Italy b c

a r t i c l e

i n f o

Article history: Available online 16 April 2008

a b s t r a c t The fluorite-bearing hydrothermal mineralization in Sardinia mainly occurs within Paleozoic volcanic and metasedimentary rocks. Only 3 occurrences are located in volcanic and siliciclastic Cenozoic rocks. Most Sardinian fluorites exhibit relatively high rare earth and Y (REY) contents, strong positive Y anomalies, slightly negative Ce and generally positive Eu anomalies. These features indicate that the REY were mobilized mainly from non-carbonate rocks. Neither Sr nor Nd isotopes can be used to date radiometrically the Sardinian fluorites. However, the measured Sr-isotope ratios of the fluorites hosted by Paleozoic rocks fit mixing lines in the 1000/Sr versus 87Sr/86Sr plot once recalculated at 280 Ma, suggesting that the age inferred for the correction probably represents that of the formation of the fluorite mineralization. Mixing likely occurred between diluted surficial waters and brines circulating mainly through the Lower Paleozoic metasedimentary basement. The Cenozoic fluorites exhibit chemical and isotopic features similar to those of the Paleozoic fluorites, except the Nuraghe Onigu fluorite displaying a possible contribution of Sr from Cenozoic magmatic rocks. The initial eNd values of the Paleozoic fluorites fit the age proposed for the formation of the deposits. Moreover, the values suggest that radiogenic Nd was provided to the fluids from the Ordovician siliciclastic basement, except for 3 deposits where the potential source rocks of Nd were mainly Ordovician acidic magmatic rocks. The initial eNd values of the Cenozoic fluorites suggest a provenance of Nd essentially from the leaching of Variscan granitoids. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The island of Sardinia hosted the largest fluorite deposits of Italy, some of which were mined until 1980. At present, the sole fluorite deposit still being extracted is located at Silius, southern Sardinia (Fig. 1). It can be considered the most important deposit in Europe. The reserves were evaluated at 2 million tons of raw material in 2006, but recent mining exploration has confirmed that the indicated reserves are higher. * Corresponding author. Address: Dipartimento di Scienze della Terra, Università ‘‘La Sapienza”, P.le Aldo Moro, 00185 Rome, Italy. E-mail address: [email protected] (F. Castorina). 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.04.005

The genesis of the Sardinian fluorite mineralization has been demonstrated to be related to Ordovician volcanism (Mazzella et al., 1979), which represents the oldest magmatic activity known on the island. During the Variscan orogeny, F was mobilized from Ordovician metavolcanic and siliciclastic rocks, and precipitated to form: (i) veins, stockworks and lenses in the Lower Paleozoic basement and Variscan granitoids; (ii) infillings of fractures and karst cavities, possibly during the Permian uplift and peneplanation of Sardinia (Padalino et al., 1972). Fluorine was remobilized from the Paleozoic fluorite occurrences and re-precipitated to form small fluorite deposits during Oligocene–Miocene volcanism, which affected more than a third of the island, particularly the western part.

F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

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Fig. 1. Geological sketch map of Sardinia and sampling sites (modified from Palomba et al., 2006). Legend: (1) Arcu Istiddà; (2) Punta Geranule; (3) Castello Medusa; (4) Monte Grighini; (5) Monte Cardiga; (6) Nuraghe Onigu; (7) Silius; (8) Monreale; (9) Bruncu Mannu; (10) Bruncu Ventura; (11) Monte Genis; (12) Bruncu Molentinu; (13) Is Crabus; (14) Santa Lucia; (15) Su Zurfuru; (16) Perda Niedda; (17) Is Murvonis; (18) Nuraghe Perdu Spada; (MB1, MB2, MB3 and MB4) Metasedimentary basement.

The geology of the Sardinian fluorite mineralization has been well studied by several workers, cited in the following paragraphs. In contrast, the geochemistry is very poorly known. In this paper the main chemical and Sr–Nd isotopic

characteristics of fluorite samples from 18 Sardinian occurrences have been determined, in order to constrain the origin of the mineralization. In particular, the REE + Y (REY) may help to define some physical–chemical features of

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the mineralizing fluids, while the Sr and Nd isotopes may provide information on the age of the deposits. The application of radiogenic isotopes to the study of the origin of fluorite is now well established, and the number of papers dealing with Sr and Nd isotopic systematics in fluorite has progressively increased in the last two decades (e.g. Barbieri et al., 1987; Halliday et al., 1990; Chesley et al., 1991, 1994; Galindo et al., 1994; Menuge et al., 1997; Subías et al., 1998; Bau et al., 2003; Schneider et al., 2003). The REY patterns and the Sr–Nd isotope ratios of the Sardinian fluorites have also been compared with those of other fluorite deposits of post-Variscan age in Europe (Menuge et al., 1997; Subías et al., 1998; Bau et al., 2003; Schwinn and Markl, 2005). In particular, the deposits from Schwarzwald (Schwinn and Markl, 2005) and the Pyrenees (Subías et al., 1998) share geological features with most Sardinian fluorite occurrences, because all these areas belong to Variscan Europe. That has allowed drawing a broader picture of the chemical and isotopic characteristics of European Variscan fluorite mineralization. 2. Geological setting and mineralization Sardinian F minerogenesis is strictly associated with the Ordovician metavolcanic complex, the lower part of which (Lower–Middle Ordovician) consists of rhyolitic and rhyodacitic flows (white and grey Porfidi), forming massive bodies affected by low-grade metamorphism during the Variscan orogeny. The Porfiroidi formation of the Middle Ordovician, overlaying the Porfidi, consists of metarhyolites and minor metarhyodacites, exhibiting augen texture and large K-feldspar phenocrysts. The upper part of the metavolcanic complex is composed of the formations of Serra Tonnai and Monte Corte Cerbos (Upper Ordovician), which consist of andesites and dacites with minor rhyolites, respectively. The early episodes of F deposition, mainly occurring as small and thin lenses, were associated with Middle Ordovician volcanism. Besides the Porfiroidi, these lenses are also associated with re-worked materials from both the Porfiroidi and the formations of Serra Tonnai and Monte Corte Cerbos. The raising of the Cambrian–Ordovician terranes before the Variscan orogeny started an intense hydrothermal circulation that mobilized F and redeposited it inside karst occurrences in Cambrian metalimestones, as well as in faults and fractures in both Ordovician terranes and Variscan granitoids. During the Cenozoic, F was again mobilized by the volcanism affecting the Paleozoic terranes, giving origin to a new mineralization inside the Cenozoic volcanites. Fig. 1 shows a geological sketch map of Sardinia along with the locations of the studied fluorite occurrences. These latter are mainly located in central-southern Sardinia, the only area where Ordovician metavolcanics outcrop. The uppermost of the deposits can be clustered into 3 groups based on the age of the host rocks, as follows: group A, composed of karst, lenses and veins in Cambrian rocks; group B, represented by veins and stockworks in Ordovician volcanic and metasedimentary rocks, and group C, composed of veins, lenses and stockworks in Cenozoic

rocks. The main geological and mineralogical features of the studied deposits are summarized in Table 1. Group A includes the deposits of Is Murvonis (Uras, 1958), Nuraghe Perdu Spada (Padalino et al., 1972), Su Zurfuru (Pretti and Uras, 1972; Bakos and Uras, 1972), and Monreale (Bakos and Uras, 1972). These deposits mainly occur in Cambrian limestones affected by lowgrade (T ranging between 80° and 150 °C) regional metamorphism. Group B comprises the deposits of Bruncu Mannu (Bakos, 1972a), Bruncu Molentinu (Bakos, 1972a,b,c,d; Cavinato, 1972; Valera, 1978; Belkin et al., 1984), Is Crabus (Bakos, 1972d; Cavinato, 1972), Bruncu Ventura (Bakos, 1972b), Silius (Natale, 1969, 1972), Castello Medusa (Bakos and Valera, 1972a), Santa Lucia (Bakos and Valera, 1972b; Valera, 1978), Punta Geranule (Valera, 1972a) and Arcu Istiddà (Valera, 1972a). These deposits are generally hosted by the Porfiroidi as well as Ordovician siliciclastic rocks. These rocks include several types of metasediments, mainly metarkoses and metagreywakes, which form arenaceous-pelitic layers. The deposits of Santa Lucia, Silius, Punta Geranule and Arcu Istiddà (B group) exhibit some differences with respect to the age and the grade of regional metamorphism affecting the Ordovician sedimentary host rocks. This has resulted in significant variations in the chemical and mineralogical compositions of the materials in each deposit. The Santa Lucia deposit infills faults and fractures in Cambrian metacarbonate and Ordovician siliciclastic rocks. The Silius deposit crosscuts both the Porfiroidi and the metasedimentary rocks, including Silurian black shales and metalimestones. The Punta Geranule and Arcu Istiddà deposits occur only in the Ordovician siliciclastic rocks, which were affected by a higher grade metamorphism than the host rocks of the other deposits. In fact, the grade of metamorphism, induced by the Variscan orogeny, increases from south to north in Sardinia. Group C comprises the deposits of Monte Cardiga (Calvino, 1961; Pani and Valera, 1996), Monte Grighini (Valera, 1972b; Eltrudis et al., 1994; Pani and Valera, 1996) and Nuraghe Onigu (Cavinato, 1954; Pani and Valera, 1996), all occurring in Cenozoic terranes. These deposits are different from the Paleozoic deposits because they have a simpler mineral association, generally consisting of fluorite and chalcedony with subordinate quartz and barite. Their origin was associated with Cenozoic hydrothermalism. Additionally, apart from these 3 groups of deposits exhibiting evidence of a single-staged genesis, there are the multi-staged deposits of Perda Niedda (Padalino et al., 1972; Valera, 1978) and Monte Genis (Boni et al., 1982; Brigo et al., 1982), which occur as skarn and vein deposits, respectively. These deposits can be broadly ascribed to the A and the B groups, respectively. In fact, Perda Niedda was originally hosted by Cambrian limestones, but the contact metamorphism induced by the emplacement of a Variscan pluton, altered the early mineralization to form a skarn deposit. The fluorite of the Monte Genis deposit, hosted by the same leucogranite, occurred originally in the Ordovician metasedimentary rocks. These latter were intruded by the pluton and, thus, early fluorite was mobilized to infill the cooling fractures of the leucogranite.

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F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921 Table 1 Main geological features, mineral association and genesis of the Sardinian fluorite deposits Deposit and Ref. No.

Deposit type

Host rocks

Main

Minor

Genesis

A group Is Murvonis (F17)

Veins

Cambrian metalimestones

Fluorite

Calcite, quartz

Nuraghe Perdu Spada (F18) Su Zurfuru (F15) Monreale (F8)

Veins and stockworks in karst cavities Massive columnar body Sub-vertical veins

‘‘

‘‘

Barite, calcite

F remobilization during the raising of the Variscan peneplain (filling in karst cavities and fractures) ‘‘

‘‘ Fluorite, barite, quartz, pyrite

Skarn

Calcite, quartz Galena, sphalerite, marcasite, calcopyrite Hematite, pyrite, galena, sphalerite

‘‘ F remobilization during Variscan orogeny

Perda Niedda (F16)

‘‘ Upper Cambrian metalimestones/Lower Ordovician siliciclastic metasedimentary rocks Multi-staged deposit, originally hosted by Cambrian metalimestones

B group Bruncu Mannu (F9)

Veins

Fluorite

Quartz, sphalerite, galena, calcite

F remobilization during Variscan orogeny

Fluorite, barite, quartz, calcite, dolomite Fluorite, quartz, calcite barite ‘‘

Galena, sphalerite, pyrite Sphalerite, galena, pyrite ‘‘

‘‘

Fluorite, diopside, wollastonite, garnets, quartz, magnetite

Contact metamorphism due to the Variscan granitoid emplacement, weathering phenomena

Bruncu Molentinu (F12) Is Crabus (F13)

‘‘

Middle Cambrian-Lower Ordovician siliciclastic metasedimentary rocks, Middle Ordovician metavolcanites (Porfiroidi) ‘‘

Veins, brecciated veins

‘‘

Bruncu Ventura (F10) Silius (F7)

Vein

‘‘

System of parallel veins

Fluorite, barite, calcite, quartz

Galena, sphalerite, marcasite

‘‘

Castello Medusa (F3)

Veins

Quartz, fluorite, barite

Veins, lenses

Galena, sphalerite, calcite calcite, dolomite

F remobilization during Variscan orogeny

Santa Lucia (F14)

Punta Geranule (F2)

Stockwork (original steep vein)

Arcu Istiddà (F1) Monte Genis (F11)

‘‘ Vertical veins, brecciated veins, discordant veinlets, stockworks

Middle Ordovician metavolcanites (Porfiroidi) and siliciclastic metasedimentary rocks, Silurian black shales and metalimestones Upper Ordovician siliciclastic metasedimentary rocks Cambrian limestones and dolomitic limestones, Lower Ordovician siliciclastic metasedimentary rocks Middle Ordovician siliciclastic metasedimentary rocks (higher grade metamorphism than the other Ordovician host rocks) ‘‘ Multi-staged deposit in leucogranite, originally hosted by Ordovician metasedimentary rocks

Barite, fluorite, galena, quartz

‘‘ ‘‘

F remobilization during the raising of the Variscan peneplain and orogeny F remobilization during Variscan orogeny

Fluorite, quartz

Barite

Barite, fluorite Barite, fluorite

Quartz Quart, galena, sphalerite

‘‘ F remobilization during Variscan orogeny and the Variscan granitoid emplacement F remobilization of Paleozoic occurrences during OligoceneMiocene volcanism ‘‘ ‘‘

C group Monte Cardiga (F5)

Veins

Upper Eocene arkoses

Fluorite, chalcedony

Barite, calcite

Monte Grighini (F4) Nuraghe Onigu (F6)

Veins, lenses, stockworks Veins, lenses, stockworks

Cenozoic rhyolitic rocks Cenozoic rhyolitic and basaltic tuffs

Fluorite, chalcedony Fluorite, calcite, chalcedony

Barite, quartz Pyrite, galena, quartz, zeolites

For the locations of the deposits (reference numbers) see Fig. 1.

Field evidence shows that the abundance of fluorite in the B-group mineralization is much greater than in the

other occurrences, except the fluorite infilling the karst at Su Zurfuru.

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3. Sampling and analysis Five to 15 samples of fluorite were collected from each mineralization, and were mixed to obtain a representative sample. Samples of the fluorite-hosting rocks were also collected from several occurrences. Mineral identification was performed by X-ray Diffraction analysis. Careful separation of fluorite from the other crystals was carried out by hand picking. The selected fluorite crystals, as well as the host rocks, were ground and then attacked with a mixture of acids (0.2 g sample, 3 mL HF, 3 mL aqua regia, 1 mL HClO4) in a microwave oven (Fadda et al., 1995). The solutions were analyzed for REY, Rb, Sr, Sc, U and Th by ICP–MS, using a Perkin Elmer Sciex (Elan 5000) instrument, with Rh and Re as internal standards, following the procedure of Fadda and Rivoldini (1994). Standard reference materials (GSD from 1 to 8, distributed by the Institute of Geophysical and Geochemical Exploration, Langfang, Hebei, China) were used to test accuracy, that was found to range from ±2% to 5%; precision ranged from ±1% to 2.5% RSD; the sensitivity of the method, expressed by MDLs (method detection limits, Fadda et al., 1995), ranged from 0.0032 to 0.050 mg L 1. The Sr and Nd isotope analyses were performed according to the following procedure: 100 mg of sample were decomposed with a 5:1 mixture of 40% HF and 70% HNO3 in Teflon vessels at 70 °C for 48 h, evaporated to dryness and treated with ultrapure 6 N HCl. The same procedure was repeated at 200 °C to ensure total dissolution of the samples. Strontium was separated in a 3 mL AG 50 W-X8 resin column. The separation of Nd from the other REE was performed in 2-mL columns filled with hydrogen diethylhexylphosphate (HDEHP)-coated Teflon powder, following the procedure of White and Patchett (1984). Isotopic analyses were carried out using a FINNIGAN MAT 262RPQ multicollector mass spectrometer in static mode. Strontium and Nd were run on Re double filaments. The internal precision (within-run precision) of a single analytical result is given as two-standard error of the mean. Repeat analyses of standards gave averages and errors expressed as two-standard deviations (2r) as follows: NBS 987 87Sr/86Sr = 0.710191 ± 13 (n = 20), 86Sr/88Sr normalized to 0.1194; La Jolla 143Nd/144Nd = 0.511861 ± 13 and 146Nd/144Nd normalized to 0.7219. Total procedural blanks were below 2 ng Sr, and below 1 ng Nd, respectively.

4. Results Tables 2 and 3 report the REY, Sr, Sc, U and Th contents in the samples of fluorite and selected host rocks, respectively. The RREE in fluorite range widely from 5.36 ppm (Is Murvonis) to 222 ppm (Is Crabus). The samples exhibiting the highest RREE come from the occurrences of Arcu Istiddà, Punta Geranule, Castel Medusa and Monte Grighini, all located in the north-western area along a line parallel to the main NE–SW tectonic lineation of northern Sardinia (Fig. 1). Yttrium contents also range widely from 4.2 ppm (Is Murvonis) to 354 ppm (Bruncu Molentinu). The distri-

bution of the REY is illustrated in Fig. 2 by REYn patterns, i.e. normalized to PAAS (Post Archean Average Shale; McLennan, 1989) because the mineralization is regarded as a process of intracrustal element redistribution. Yttrium is inserted between Dy and Ho according to its ionic radius. The Sardinian fluorites display variable patterns. In particular, the (La/Sm)n ratio ranges from 0.09 to 1.28, while the (Sm/Yb)n ratio ranges from 0.69 to 7.72. The Eu/Eu* anomalies range from 0.40 to 4.26. All fluorites exhibit positive Y anomalies except that from Monreale; they also display slightly negative Ce anomalies (Ce/Ce* = 0.73 0.98), except the Arcu Istiddà fluorite which has Ce/Ce* = 1.17. Taking into account the REY patterns of the Sardinian fluorites with respect to the 3 groups of deposits (Fig. 2), it appears that, apart from the Perda Niedda fluorite, the A-group fluorites exhibit poorly humped patterns with only slightly depleted HREE and positive Eu anomalies. The B-group fluorites display humped patterns, except the Arcu Istiddà fluorite, which exhibits a nearly flat pattern. Moreover, most B-group fluorites exhibit positive Eu anomalies, while those from Bruncu Ventura, Bruncu Molentinu, Santa Lucia and Monte Genis display negative Eu anomalies. Lastly, the C-group fluorites show patterns characterized by variable increase from the LREE to the HREE. In particular, the Nuraghe Onigu fluorite displays a nearly flat pattern, and the fluorites from Monte Cardiga and Monte Grighini show positive Eu anomalies. Among other cations present in the Sardinian fluorites, Sr is more abundant than Sc, U and Th. However, Sr contents are relatively low, ranging from 5 ppm (Bruncu Molentinu and Is Crabus) to 64 ppm (Monte Genis). Scandium ranges from 0.23 ppm (Punta Geranule) to 0.97 ppm (Nuraghe Perdu Spada), but the Cenozoic fluorites from Monte Grighini and Nuraghe Onigu display the highest values (2.22 and 3.04 ppm, respectively). Uranium contents are low, ranging from 0.05 ppm (Monte Grighini) to 0.45 ppm (Nuraghe Onigu). Thorium contents are also low, ranging from 0.11 ppm (Bruncu Molentinu, Is Crabus and Monreale) to 0.68 ppm (Monte Cardiga). The U/Th ratios vary from 0.19 (Monte Cardiga) to 1.97 (Nuraghe Onigu). Table 4 shows the Rb, Sr, Sm and Nd contents as well as the measured Sr and Nd isotope ratios of the fluorite samples. Sr-isotope ratios range from 0.711764 (Santa Lucia) to 0.716922 (Su Zurfuru), except Monte Grighini fluorite exhibiting a higher value (0.724208). Neodymium isotope ratios range narrowly from 0.512026 (Punta Geranule) to 0.512451 (Monte Genis).

5. Discussion 5.1. REY patterns It is known that the REE patterns of fluorite are the result of many factors and processes, the significance of which is not yet very well understood (Möller et al., 1998). Fleischer (1969) showed that the fluorites from a single location may exhibit different REE abundances, controlled by the variability of the parameters governing the

Table 2 REE and trace elements (ppm) of representative samples of the Sardinian fluorites ICP–MS analyses Sample*

La 32.79 18.29 16.82 17.51 5.12 12.23 3.28 1.13 3.79 2.56 6.73 6.96 12.79 3.85 3.93 4.23 0.99 10.35

Ce

Pr

68.64 37.09 33.98 30.93 10.31 16.60 8.64 2.64 10.30 6.87 15.27 14.64 44.16 9.03 6.28 10.06 1.78 21.01

5.18 5.02 4.60 4.40 1.40 2.10 1.35 0.35 1.86 1.32 2.47 2.34 9.36 1.54 0.91 1.70 0.23 2.91

Nd 18.56 21.15 19.92 18.37 5.91 7.65 6.64 1.48 10.31 7.99 13.23 12.60 54.03 8.39 3.36 8.99 0.91 12.23

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

Sc

U

Th

4.21 5.89 6.48 5.55 5.02 1.39 2.63 0.58 4.93 4.03 6.22 6.76 21.44 6.13 0.81 6.22 0.26 3.72

1.60 2.69 3.76 2.24 2.88 0.37 1.73 0.23 2.17 0.64 1.31 1.77 6.50 1.51 0.33 0.67 0.11 1.43

4.42 8.52 10.87 9.44 2.02 2.01 5.71 0.73 8.10 7.79 12.69 15.05 26.83 11.96 1.00 10.16 0.36 5.49

0.63 1.49 2.22 2.06 0.33 0.32 1.06 0.15 1.31 1.11 2.12 2.72 3.84 2.16 0.18 1.95 0.06 0.95

4,13 9,69 15,90 14,45 1,98 2,09 6,82 0,97 7,49 5,88 12,81 17,15 21,55 12,20 1,06 11,74 0,29 5,77

126.51 91.49 137.97 122.13 14.08 20.58 184.46 5.82 120.75 94.74 244.81 354.39 241.24 200.46 8.03 104.15 4.20 41.23

0.93 2.09 3.33 3.07 0.39 0.49 1.37 0.20 1.36 0.99 2.38 3.45 4.10 2.08 0.20 2.05 0.06 1.12

2.53 6.00 9.67 8.59 0.97 1.58 3.32 0.62 3.12 1.99 5.11 8.64 10.28 4.55 0.55 5.48 0.15 2.89

0.29 0.76 1.33 1.30 0.12 0.26 0.34 0.09 0.32 0.18 0.43 1.00 1.09 0.47 0.09 0.84 0.02 0.37

1.33 4.53 8.08 8.01 0.65 1.68 1.55 0.70 1.56 0.69 1.44 5.30 5.18 2.06 0.51 5.77 0.12 2.19

0.15 0.68 1.17 1.20 0.09 0.31 0.19 0.12 0.20 0.09 0.16 0.68 0.70 0.24 0.07 0.84 0.02 0.32

0.48 0.23 0.44 2.22 0.48 3.04 0.30 0.62 0.68 0.29 0.28 0.63 0.28 0.72 0.26 0.25 0.27 0.97

0.11 0.16 0.10 0.05 0.13 0.45 0.10 0.14 0.14 0.08 0.09 0.11 0.09 0.26 0.16 0.09 0.22 0.18

0.26 0.40 0.25 0.13 0.68 0.23 0.15 0.11 0.15 0.28 0.13 0.11 0.11 1.31 0.14 0.21 0.59 0.35

(La/Sm)n

(Sm/Yb)n

(La/Nd)n

(La/Yb)n

(Y/Ho)n

(La /Lu)n

Eu/Eu*

1.61 0.66 0.41 0.35 3.92 0.42 0.86 0.42 1.61 2.98 2.20 0.65 2.10 1.51 0.82 0.55 1.08 0.87

1.57 0.77 0.75 0.85 0.77 1.42 0.44 0.67 0.33 0.28 0.45 0.49 0.21 0.41 1.04 0.42 0.96 0.75

Sample*

RLREE

RHREE

RREE

RREE+Y

LREE/HREE+Y

Ce/Ce*

F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 F16 F17 F18

129.38 87.43 81.81 76.76 27.76 39.97 22.54 6.18 31.18 22.77 43.93 43.30 141.77 28.94 15.29 31.20 4.17 50.22

14.41 33.77 52.56 48.12 6.55 8.74 20.37 3.58 23.46 18.71 37.14 53.99 73.55 35.71 3.66 38.82 1.09 19.09

145.39 12.89 138.13 127.12 37.19 49.08 44.63 9.99 56.81 42.13 82.38 99.06 221.83 66.16 19.29 70.69 5.36 70.74

27.90 215.37 276.10 249.25 51.27 69.66 229.09 15.81 177.27 136.87 327.20 453.45 463.06 266.62 27.31 174.85 9.57 11.97

0.99 0.85 0.54 0.55 1.75 1.55 0.15 0.82 0.30 0.30 0.22 0.15 0.61 0.19 1.56 0.32 0.94 1.04

1.14 0.88 0.87 0.80 0.88 0.72 0.89 0.97 0.77 0.70 0.78 0.75 0.74 0.76 0.78 0.78 0.85 0.88

1.13 0.45 0.38 0.46 0.15 1.28 0.18 0.28 0.11 0.09 0.16 0.15 0.09 0.09 0.70 0.10 0.56 0.40

1.82 0.30 0.15 0.16 0.58 0.54 0.16 0.12 0.18 0.27 0.35 0.10 0.18 0.14 0.57 0.05 0.60 0.35

5.01 1.61 1.52 1.46 1.33 1.54 4.93 1.07 3.27 3.52 3.78 3.78 2.16 3.53 1.46 1.87 2.43 1.35

2.44 0.30 0.16 0.17 0.64 0.45 0.19 0.11 0.21 0.33 0.47 0.12 0.21 0.18 0.61 0.06 0.46 0.37

1.75 1.79 2.11 1.46 4.26 1.04 2.10 1.66 1.62 0.54 0.70 0.83 1.28 0.83 1.73 0.40 1.66 1.49

F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 F16 F17 F18

Legend: *Reference No in Fig. 1; Ce anomaly (denoted as Ce/Ce*) calculated using the equation Ce/Ce* = 3xCePAAS/(2xLaPAAS+NdPAAS) as given by Elderfield and Greaves (1982); Eu anomaly (denoted as Eu/Eu*) calculated using the equation Eu/Eu* = EuPAAS/(SmPAASxGdPAAS)1/2 as given by Taylor and McLennan (1985). 2911

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Table 3 REE and other trace elements (ppm) of some representative samples of the Sardinian fluorite host rocks ICP-MS analyses R2

R6

R8

R9

R10

R11

R12

R13

R16

R17

R18

Lithotype

Metasiltstone

Rhyolitic tuff

Metamudstone

Metasiltstone

Metasiltstone

Granitoid rock

Metasiltstone

Metasiltstone

Skarn in granitoid

Metalimestone

Dolomitic metalimestone

Age

Lower Ordovician

Tertiary

Lower Ordovician

Lower Ordovician

Lower Ordovician

Variscan

Lower Ordovician

Lower Ordovician

Variscan

Cambrian

Cambrian

La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Sc Th U RLREE RHREE RREE RREE + Y LREE/HREE+Y Ce/Ce*

12.31 20.26 2.82 11.04 2.64 0.67 2.77 0.38 2.08 13.34 0.40 1.24 0.18 1.22 0.21 10.05 7.60 1.44 49.07 8.48 58.22 71.55 2.76 0.79 0.66 1.10 0.99 0.59 1.22 0.67 1.15

26.41 38.28 4.77 17.65 3.54 0.92 4.54 0.65 3.82 31.50 0.84 2.50 0.37 2.47 0.43 6.31 4.45 4.32 90.65 15.62 107.19 261.62 6.98 0.76 0.64 0.73 1.33 0.55 1.38 0.71 1.06

52.55 102.42 11.64 42.43 8.94 2.42 8.45 0.94 4.44 21.94 0.76 2.23 0.28 1.88 0.30 21.03 13.79 2.48 217.98 19.28 239.68 261.62 6.98 0.96 1.09 2.42 1.10 1.93 1.05 2.04 1.31

32.47 63.67 7.72 28.60 5.82 1.21 5.43 0.61 2.90 12.70 0.52 1.42 0.20 1.31 0.22 10.14 11.02 2.11 138.26 12.62 152.09 164.79 7.29 0.94 1.11 2.26 1.01 1.72 0.89 1.70 1.01

27.42 42.69 5.83 22.33 4.84 1.36 4.84 0.55 2.86 23.05 0.52 1.51 0.22 1.44 0.25 21.74 15.38 2.97 103.11 12.19 116.66 139.70 3.60 0.77 0.93 1.71 1.09 1.05 1.62 1.26 1.32

38.54 67.73 9.11 33.65 7.07 0.70 7.21 0.99 5.49 33.77 1.06 3.35 0.51 3.68 0.59 4.28 22.16 3.54 156.11 22.89 179.70 213.48 3.32 0.85 0.80 0.97 1.02 0.65 1.17 0.75 0.46

62.36 85.33 13.00 48.40 9.67 1.93 8.77 1.01 4.68 23.64 0.80 2.19 0.28 1.75 0.29 11.33 12.23 2.30 218.76 19.77 240.46 264.09 6.63 0.69 1.13 2.81 1.14 1.73 1.09 2.52 0.99

22.54 45.80 5.55 20.64 4.46 1.11 4.31 0.58 3.25 22.18 0.64 1.93 0.27 1.73 0.28 14.55 10.81 2.71 98.99 12.98 113.08 135.26 3.38 0.96 0.84 1.31 0.97 0.94 1.28 0.94 1.19

13.56 21.50 4.56 17.51 4.97 0.18 4.66 0.72 4.18 25.94 0.79 2.31 0.35 2.42 0.40 3.10 17.35 2.25 62.08 15.83 78.08 104.03 1.80 0.66 0.56 1.04 0.69 0.32 1.20 0.40 0.17

39.26 80.90 9.62 36.86 7.42 1.62 6.70 0.71 3.02 13.41 0.53 1.62 0.23 1.57 0.29 14.92 12.85 3.07 174.06 14.66 190.34 203.74 8.54 0.97 1.19 2.41 0.95 1.83 0.93 1.59 1.08

6.61 12.83 1.43 5.15 1.51 0.62 1.47 0.21 1.24 9.60 0.23 0.68 0.09 0.56 0.08 2.19 1.69 0.85 27.53 4.57 32.73 42.32 2.33 0.97 0.59 1.37 1.14 0.81 1.50 0..91 1.97

(La/Sm)n (Sm/Yb)n (La/Nd)n (La/Yb)n (Y/Ho)n (La/Lu)n Eu/Eu*

d.l.

Legend: *Reference No in Fig. 1; Ce anomaly (denoted as Ce/Ce*) calculated using the equation Ce/Ce* = 3xCePAAS/(2xLaPAAS+NdPAAS) as given by Elderfield and Greaves (1982); Eu anomaly (denoted as Eu/Eu*) calculated using the equation Eu/Eu* = EuPAAS/(SmPAASxGdPAAS)1/2 as given by Taylor and McLennan (1985); d.l. = detection limit.

F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

Sample*

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F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

A-Group 100

Rock/PAAS

10 Nuraghe Perdu Spada Is Murvonis

1

Su Zurfuru Monreale Perda Niedda

0.1

0.01

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

B-Group 100 Silius Bruncu Ventura Bruncu Molentinu Monte Genis Bruncu Mannu Santa Lucia Is Crabus Castello Medusa Punta Geranule Arcu Istiddà

Rock/PAAS

10

1

0.1

0.01

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

C-Group

100

Rock/PAAS

10

Monte Grighini Monte Cardiga Nuraghe Onigu

1

0.1

0.01

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

Fig. 2. PAAS-normalized REY plots for the fluorite samples. For the 3 groups of deposits see the text.

mineralizing process. However, comparing the REY patterns of the Sardinian fluorites with those of the most common types of fluorite of various origins (e.g. Möller et al., 1998), it is evident that the Sardinian fluorites were deposited by hydrothermal fluids, in agreement with field evidence. Among physical–chemical conditions controlling REE concentrations, it is considered that Tb and Gd are the REE which establish the most stable complexes with F. This leads to some enrichment of Gd and Tb in hydrothermal fluids (Wood, 1990a,b). Therefore, a Gd and Tb-rich fluorite

can crystallize during the late stage of the mineralizing process. From this, a normalized convex REY pattern may indicate crystallization of fluorite from late hydrothermal fluids (Schwinn and Markl, 2005). This may apply to the B-group fluorites. In contrast, the patterns of the other Sardinian fluorites may be explained by a weaker F complexation of Gd and Tb, probably because the fluorite deposition took place at early stage of the hydrothermal process. Another parameter to be considered is the pH of solutions. It is known that the total REE concentration in

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F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

Table 4 Rb–Sr and Sm-Nd isotope data for the Sardinian fluorites and representative rocks from the basement. Sr and Rb from DCP data Sample A-group F8 F15 F16 F17 F18

fluorites Monreale Su Zurfuru Perda Niedda Is Murvonis Nuraghe Perdu Spada

B-group F1 F2 F3 F7 F9 F11 F12

F14

fluorites Arcu Istiddà Punta Geranule Castel Medusa Silius Bruncu Mannu Monte Genis Bruncu Molentinu Is Crabus Bruncu Ventura Santa Lucia

C-group F4 F5 F6

fluorites Monte Grighini Monte Cardiga Nuraghe Onigu

F13 F10

87

53 24 38 17 18

0.109 0.362 0.305 0.511 0.482

0.722322 ± 30 0.716922 ± 9 0.711347 ± 23 0.713308 ± 23 0.714627 ± 15

3 3 3 3 4 3 2

42 48 17 23 32 16 5

0.207 0.181 0.511 0.377 0.362 0.543 1.158

0.712744 ± 10 0.716025 ± 7 0.717729 ± 10 0.712176 ± 17 0.713024 ± 13 0.714257 ± 8 0.712263 ± 13

3 3

15 24

3

19

Sr (ppm)

2 3 4 3 3

Rb/86Sr

Sr/86Sr ± 2se (87Sr/86Sr)i*

87

Rb (ppm)

Nd (ppm)

147

0.721888 0.715484 0.710136 0.711277 0.712709

0.58 0.81 6.22 0.26 3.72

1.48 3.36 8.99 0.91 12.23

0.236 0.145 0.417 0.172 0.183

0.512195 ± 33 0.512125 ± 30 0.512726 ± 7 0.512092 ± 26 0.512288 ± 8

8.68 10.05 +1.68 10.69 6.87

10.10 8.22 6.20 9.83 6.40

0.711922 0.715306 0.715698 0.710675 0.711586 0.712099 0.707660

4.21 5.89 6.64 2.63 4.93 6.22 6.76

18.56 21.15 19.92 6.64 10.31 13.23 12.60

0.137 0.168 0.201 0.239 0.288 0.283 0.323

0.512157 ± 14 0.512026 ± 7 0.512235 ± 22 0.512866 ± 14 0.512283 ± 7 0.512394 ± 6 0.512446 ± 6

9.42 11.98 7.90 4.96 6.96 4.80 3.78

7.29 10.96 8.06 6.47 10.24 7.90 8.32

0.579 0.362

0.712865 ± 13 0.710564 0.713124 ± 15 0.711686

21.44 4.03

54.03 7.99

0.239 0.304

0.512125 ± 4 0.512389 ± 13

10.05 4.90

11.57 8.74

0.457

0.711764 ± 13 0.709947

6.13

8.39

0.440

0.512512 ± 6

2.50

5.55 5.02 1.39

18.37 5.91 7.65

0.182 0.512 0.110

0.512227 ± 12 0.512141 ± 5 0.512277 ± 5

7.6 8.0 8.2 7.9

34.9 39.3 46.6 41.0

0.131 0.122 0.106 0.108

0.512120 ± 7 0.512080 ± 9 0.511963 ± 7 0.511829 ± 6

2 4 3

49 28 32

0.118 0.413 0.271

(87Sr/86Sr)i** 0.724202 ± 5 0.724152 0.714257 ± 14 0.714081 0.709892 ± 9 0.709776

Metasedimentary basement MB1 Budonia 142 87 MB2 Riu Mannua b 155 MB3 Sinsicola c 98 MB4 Seulo

171 202 430 128

2.403 1.246 1.040 2.216

0.725494 ± 10 0.723224 ± 12 0.715185 ± 15 0.726211 ± 10

0.715940 0.718270 0.711050 0.717400

Sm/144Nd

Nd/144Nd ± 2se eNd(T)

11.21

eNd i**

*Calculated at 280 Ma; **Calculated at 30 Ma; recalculation of 87Sr/86Sr to 280 Ma using k87Rb = 1.42E 11 a i k147Sm = 6.54E 12 a 1, (147Sm/144Nd)0, CHUR = 0.1967 and (143Nd/144Nd)0, CHUR = 0.512638. a Siliciclastic rocks. b HT–LP rocks. c Low-grade rocks.

hydrothermal fluids is controlled by the pH and bulk chemical composition of solutions. Generally, REE concentrations increase with decreasing pH (Michard, 1989). In acidic solutions with low concentrations of complexing ligands (i.e. hydroxy- or carbonate species and halogens) the LREE are enriched in solution with respect to the HREE which are preferentially adsorbed onto surfaces. In alkaline fluids with carbonate species and/or halogens as complexing ligands, the HREE are enriched in solution and the REE patterns show a (La/Lu)n ratio <1 (Schwinn and Markl, 2005). From this point of view, it is assumed that the fluorite-bearing fluids were alkaline and HREE-enriched with respect to the LREE as calcite is a gangue mineral in the fluorite deposits at several locations. Therefore, the fluorites deposited by those fluids should also be HREE-enriched. According to Möller (1991) an enrichment of the HREE in the REE patterns would indicate low Ca2+/F ratios of the parent fluids. This means that the origin of the fluids must be from a F-rich source or from mobilization of previously existing fluorite mineralization in deeper parts of the crust (Lüders, 1991). This may apply to most fluorites of the A and C groups. In contrast strong F complexation may have affected the chemical composition of the B-

143

eNd i*

Sm (ppm)

1

; eNd

i

8.06 9.73 7.08

7.98 10.91 6.72

10.14 10.92 13.21 15.82

calculated for 280 Ma using

group fluorites, with no major HREE enrichment. Therefore, an origin by remobilization cannot be proven for the B-group fluorites, despite the fact that most of these occurrences are richest in fluorite. Fluorites not associated with calcite might have been deposited by less alkaline solutions. The flat REY patterns of the C-group fluorites from Monte Cardiga and Nuraghe Onigu suggest primary deposition from a fluid with Ca/F ratio > 1. Therefore, that would rule out any remobilization of previously deposited fluorite. Mineral paragenesis may significantly affect the REE patterns of fluorite, as the REE can also enter other minerals and, thus, their abundance in hydrothermal fluids may depend on the order of deposition of the REE-bearing minerals, independently of the ultimate REE source. In view of this, garnet, in particular, is a competitor for the HREE. Therefore, early crystallization of garnet would leave the fluorite-bearing fluids depleted in the HREE with respect to the LREE. However, garnet occurs only in the Perda Niedda skarn, the fluorite of which exhibits no HREE depletion. Calcite and barite can also compete with fluorite for uptake of the REE. However, both minerals are generally precipitated later than fluorite in the Sardinian

F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

mineralization, and, thus, their crystallization has no effect on the REE contents of the fluorites. As a whole, the mineral paragenesis did not control the REE patterns of the Sardinian fluorites. Considering interaction between fluids and the fluorite host rocks, according to Schwinn and Markl (2005), REY patterns of fluorite similar to those of the host rock may mean that the host rock is the source of REY. This may be the case in the A- and C-group mineralization, where fluorite and host rock exhibit roughly similar REY patterns in each occurrence (Fig. 3). In view of this, the low REY concentrations in Is Murvonis fluorite support the hypothesis of a REY provenance from marine limestone, i.e. the fluorite host rock. As demonstrated by Parekh et al. (1977), the REY contents of aluminosilicate rocks may be several orders of magnitude higher than those of marine limestone and would thus produce higher REY concentrations in hydrothermal fluids. In contrast, the B-group fluorites and their host rocks generally exhibit quite different patterns (Fig. 3). This would suggest that the host rocks are not the source rocks of the REY of these fluorites. Alternatively, the different patterns could be reflecting the strong influence of chemical complexation and physical–chemical conditions of leaching on the REE pattern of the hydrothermal fluid. According to Möller et al. (1998), Eu, Y and Ce anomalies in fluorite represent the mode of deposition of the mineral. In particular, the Eu anomaly is controlled thermally and chemically, mainly by redox conditions. Generally, the Eu3+/Eu2+ redox potential of hydrothermal fluids depends strongly on temperature, slightly on pH and is almost unaffected by pressure. Oxygen fugacity and fS2 may also affect the presence of the Eu anomaly. At T > 200 °C (Schwinn and Markl, 2005, but according to other authors at a somewhat higher temperature of about 250 °C) Eu3+ is reduced to Eu2+, which is very mobile and, thus, accumulates in the fluid. The fluid exhibits a positive Eu anomaly, while the leached rock displays a negative Eu anomaly. Therefore, the fluorite deposited by such a fluid will show no or a slightly positive Eu anomaly at T < 200 °C, and a negative Eu anomaly at higher temperature. Moreover, at T > 200 °C the size of Eu2+ prevents its incorporation into the fluorite lattice and, thus, the mineral will show a negative Eu anomaly due to a crystallographically controlled fractionation during precipitation (Bau, 1991; Möller et al., 1998; Möller and Holzbecher, 1998). Lastly, the Eu anomaly in the fluorite can also reflect the REE pattern of the parent fluid (e.g. Schwinn and Markl, 2005). In view of this, all Sardinian fluorites exhibiting positive Eu anomalies have crystallized either at temperatures below 200 °C or from fluids with positive Eu anomalies. As the crystallization for most B-group fluorites occurred between 250° and 350 °C (Valera, 1974; Belkin et al., 1984), it can be assumed that their positive Eu anomalies were not related to a crystal–chemical control during precipitation, but they were produced by a positive Eu anomaly in the fluid. Such an anomaly may suggest the premobilization of Eu in a gneiss-derived hydrothermal fluid, as experimental work carried out on fluids leaching gneiss proves that these fluids display positive Eu anomalies (Schwinn and Markl, 2005). Generally, the lack of

2915

strong positive Eu anomalies in the uppermost Sardinian fluorites suggests that the hydrothermal fluids had never attained the higher temperature, even prior to fluorite deposition (Bau and Möller, 1992). Therefore, the parent fluids of these fluorites derived from relatively shallow crustal levels. The negative or null Eu anomalies displayed by a few Sardinian fluorites can be explained by crystallization either above 200 °C from a fluid with or without a Eu anomaly, or below 200 °C from a fluid with a negative Eu anomaly. This latter may have been inherited by interaction of the fluids with felsic rocks, which generally exhibit negative Eu anomalies because of feldspar fractionation during magmatic differentiation. The negative Eu anomaly of the Perda Niedda fluorite was likely controlled by a fluid exhibiting any or no Eu anomaly, as the crystallization took place at T > 350 °C (Padalino and Valera, 1978). In contrast, the negative Eu anomalies of the other fluorites are likely explained by interaction of the fluids with Eu-depleted felsic rocks, as there is no evidence of formation at T > 200 °C. This is reasonable, as the fluorites from Bruncu Molentinu, Bruncu Ventura and Santa Lucia occur within siliciclastic rocks derived from mainly acidic volcanics, and the Monte Genis fluorite is hosted by the leucogranite. Overall, however, the explanations given above for estimating the temperature based on Eu anomalies may be meaningless if the fluorite-depositing fluids were of mixed origin. Finally, the presence of sulfides in most Sardinian fluorite deposits indicates that the Eu2+-enriched fluids depositing fluorite were reduced. Yttrium anomalies reflect fluid complexation with F (Möller et al., 1998). In particular, positive Y anomalies indicate strong F complexation and a provenance of fluids from a source relatively far from the fluorite-hosting rocks. Both these characteristics may apply to the B-group fluorites. The decoupling of Y from the HREE is a common feature in hydrothermal fluids dominated by F-complexes (Möller et al., 1998). The slightly negative Ce anomalies of the uppermost of the Sardinian fluorites and associated host rocks suggest that the parent fluids interacted with rocks under reducing conditions (Möller et al., 1998). This is consistent with the hydrothermal origin of the fluorite-bearing fluids and the presence of sulfides in most deposits. Alternatively, the negative Ce anomalies may be inherited from seawater, assumed to be present in the hydrothermal fluids. The positive Ce anomaly of the Arcu Istiddà fluorite may indicate interaction between rocks and fluids in an oxidizing environment, where Ce3+ oxidizes to Ce4+ and becomes immobile, or very mobile if it is strongly complexed (Möller et al., 1998). The fluid becomes Ce4+ enriched, developing a positive Ce anomaly, while the rock has a negative Ce anomaly. In view of this, it is noted that at Arcu Istiddà the mineralization lacks sulfides and contains barite. Concentrations of Sr, Sc, U and Th fall within the ranges of fluorites from a variety of occurrences (e.g. Eppinger and Closs, 1990; Subías and Fernández-Nieto, 1995; Menuge et al., 1997; Bau et al., 2003) and, thus, they are not worthy of further comment. However, the high U content of the volcanic-hosted Nuraghe Onigu fluorite may suggest that the parent fluids were strongly oxidized, as U is very mobile under oxidizing conditions. But this hypothesis is not

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F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

A-Group Monreale Rock/PAAS

Rock/PAAS

1

0.1 0.01 La Ce

Is Murvonis

10

10

1 0.1

0.01 Pr Nd Sm Eu Gd Tb Dy

Y

La Ce

Ho Er Tm Yb Lu

Pr Nd Sm Eu Gd Tb Dy

Nuraghe Perdu Spada Rock/PAAS

Rock/PAAS

1

0.1 La Ce

Nd Sm Eu Gd Tb Dy

Y

La Ce

Ho Er Tm Yb Lu

1

Y

Ho Er Tm Yb Lu

Y

Ho Er Tm Yb Lu

Y

Ho

Is Crabus

100

Rock/PAAS

Rock/PAAS

Pr Nd Sm Eu Gd Tb Dy

B-Group

Monte Genis

10

1

0.1

0.1 La Ce

Pr

Nd Sm Eu Gd Tb Dy

Y

La Ce

Ho Er Tm Yb Lu

Bruncu Ventura

Pr Nd Sm Eu Gd Tb Dy

Bruncu Mannu

10.00

Rock/PAAS

10

Rock/PAAS

1

0.1 Pr

10

1

0.1

1.00

0.10

0.01 La Ce

Pr Nd Sm Eu Gd Tb Dy

Y

La

Ho Er Tm Yb Lu

Punta Geranule

1

0.1

La Ce

Pr

Nd Sm Eu Gd Tb Dy

Y

Ce

Pr

Nd Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu

Er Tm Yb Lu

Bruncu Molentinu

100.0

Rock/PAAS

10

Rock/PAAS

Ho Er Tm Yb Lu

Perda Niedda

10

10

Y

10.0

1.0

0.1

La Ce

Pr

Nd Sm Eu Gd Tb Dy

Y

Ho Er Tm Yb Lu

C-Group Nuraghe Onigu

Rock/PAAS

10

1

0.1 La

Ce

Pr

Nd Sm Eu Gd Tb Dy

Y

Ho

Er Tm Yb Lu

Fig. 3. PAAS-normalized REY plots for the samples of fluorite and associated host rocks from selected deposits. Legend: full square = host rock; full triangle = fluorite.

supported by the negative Ce anomaly and the presence of sulfides in the Nuraghe Onigu mineralization. Alternatively, it is possible that the fluids interacted with the vol-

canics, which may be the source of U. This may find support in the parallel REY patterns of both the fluorite and the host rock.

F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

5.2. Sr–Nd isotopic compositions The Sardinian fluorites have Rb contents in the range of 2–4 ppm, and Sr contents of 5–64 ppm (Table 4). The measured Sr and Nd isotope ratios of the Sardinian fluorites are heterogeneous. If the Paleozoic fluorites can be considered as one coeval mineralization, then the Sr isotopic data cannot be used to determine any isochron age because of the lack of any significant covariation in the 87Rb/86Sr versus 87 Sr/86Sr diagram (not shown here). Therefore, information on the age may be obtained indirectly by correcting the measured isotopic values with respect to the ages of the main geological events, potentially responsible for the formation of the studied deposits. In view of this, a reference age for the Paleozoic fluorites of 280 Ma, the late stage of the Variscan orogeny when leucocratic granitoids were emplaced along with a large amount of hydrothermal fluids (Boni et al., 1992), is assumed. In contrast, the measured Sr isotopic ratios of the Cenozoic fluorites were recalculated at 30 Ma, which represents the mean age of Cenozoic magmatic activity in Sardinia (Savelli et al., 1979). Age-corrected Sr-isotope ratios are shown in Table 4. Of the Paleozoic fluorites, only that from the Perda Niedda skarn has a value close to the initial Sr-isotope ratio of the Variscan granitoids (0.70906 0.71020 Tommasini et al., 1995; Castorina and Petrini, 1989). This suggests that the radiogenic Sr carried by the fluorite-bearing fluids was magmatic in origin or leached from Variscan granitoids. In contrast, marine limestone (0.7075 0.7085; McArthur et al., 2001) was the only source for the radiogenic Sr carried by the fluorite-bearing fluids at Bruncu Molentinu, where geologic evidence shows that the siliciclastic host rocks are thrust below a marine metalimestone. All the other Paleozoic fluorites exhibit initial Sr-isotope ratios >0.7106, suggestive of a Sr origin from leaching of the siliciclastic basement (>0.711; Tommasini et al., 1995 and Table 4). Among these fluorites, that from Monreale is

2917

distinguished by its higher initial Sr-isotope ratio; that suggests the presence of 87Sr-rich potential source rocks in the local basement. More information can be obtained from the 1000/Sr versus 87Sr/86Sr plot of Fig. 4, which illustrates the situation of the Paleozoic fluorites at 280 Ma. The points of the A- and B-group fluorites fall along 3 distinct straight lines that may be considered as binary mixing lines. In particular, most B-group fluorites fit line 1, suggestive of mixing between a Sr-rich (Sr = about 180 ppm) component with a relatively high isotopic ratio (87Sr/86Sr = about 0.712) and a Sr-poor (Sr = about 8 ppm) component with a low isotopic ratio (87Sr/86Sr = about 0.707). The high Sr isotopic component derived radiogenic Sr by leaching the Lower Paleozoic siliciclastic basement, while the low Sr isotopic component was represented by seawater or waters circulating through carbonates. Line 1 follows a regional trend, as it clusters all fluorites from the Sarrabus–Gerrei district, suggesting a common mineralization process, although the proportions of the two mixing components are variable at the deposit scale. The Arcu Istiddà fluorite is distinguished from the fluorites from the other two nearby deposits located in central Sardinia, i.e. Punta Geranule and Castello Medusa, as it shows an initial Sr-isotope ratio close to the fluorites from the Sarrabus–Gerrei area, suggesting an analogous source of radiogenic Sr. In contrast, the fluorites from Punta Geranule and Castello Medusa cluster along line 2, which may represent a mixing line between two components of analogous characteristics to those of line 1, although with higher Sr isotopic ratios. The Sr-rich component likely derived radiogenic Sr from the siliciclastic basement, while the Sr-poor component may represent surficial fluids which acquired their Sr isotopic ratio circulating through the siliciclastic basement. Lastly, all A-group fluorites except that from the Perda Niedda skarn fit line 3, that also represents a mixing line between two components, one derived from the siliciclastic basement (high Sr-isotope ratio) and the other from

Fig. 4. 1000/Sr versus 87Sr/86Sr ratio plot for the Paleozoic fluorites. The figure also shows the fields for marine limestones, Variscan granitoids and the Lower Paleozoic siliciclastic basement (for references see text). All isotopic data are corrected at 280 Ma (see text).

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F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

marine limestones (low Sr-isotope ratio). In agreement with the skarn genesis, the Perda Niedda fluorite plots near the field of Variscan granitoids and thus off line 3, characteristic of the carbonate-hosted fluorites. To summarize, it can be said that the Paleozoic fluorites were deposited by Sr-rich fluids circulating in the various levels or through different lithotypes of the Lower Paleozoic metasedimentary basement. These fluids were diluted, to various extents, by Sr-poor fluids at the scale of each mineralization. This suggests the existence of two different mineralizing fluid systems in Late Variscan Sardinia. The one is likely to have been basement brines, comparable with the fluids described for other hydrothermal systems of the European Variscan basement (e.g. Subías and Fernández-Nieto, 1995), while the other system was probably composed of convecting surficial waters, modified chemically and isotopically. The Cenozoic fluorites have initial Sr isotopic ratios overlapping the range of the Paleozoic fluorites. However, the isotopic ratios are quite different from those of Ceno-

zoic volcanics (average 0.706, Lustrino et al., 2004), ruling out that the radiogenic Sr present in the fluorite-bearing fluids was derived only from the magmas. A significant magmatic contribution is possible for the Nuraghe Onigu fluorite, the remainder of the Sr being supplied by a source with a higher Sr isotopic ratio, i.e. the Paleozoic metasedimentary basement and/or Variscan granitoids. Paleozoic rocks were the only sources of the radiogenic Sr in the Monte Grighini fluorite, the measured Sr isotopic ratio of which is meaningless, even if corrected at 280 Ma. This makes the isotopic value of the Monte Grighini fluorite close to that of the Monreale fluorite, suggesting the existence of similar potential source rocks of Sr in the two nearby localities. Lastly, the radiogenic Sr of Monte Cardiga fluorite likely resulted from dilution of Sr provided by the metasedimentary basement and/or Variscan granitoids, with the Sr from marine carbonates and/or seawater (87Sr/86Sr = 0.709). Neodymium contents vary between 0.91 ppm (Is Murvonis) and 21.2 ppm (Punta Geranule), except for the Is Cra-

Fig. 5. Age (Ma) versus eNd plot for the Paleozoic fluorites. Dashed lines show the temporal evolution of the eNd of the fluorites from the present-day values as far back as 400 Ma. Solid lines show the temporal evolution of the eNd of the main facies of the Lower Paleozoic siliciclastic basement from the Present back to 400 Ma. Also shown is the field of the eNd of Variscan granitoids today and at 280 Ma. (a) A-group fluorites; (b) B-group fluorites.

F. Castorina et al. / Applied Geochemistry 23 (2008) 2906–2921

bus fluorite (54 ppm) (Table 4). The 147Sm/144Nd ratios range from 0.11 (Nuraghe Onigu) to 0.512 (Monte Cardiga). Information about potential source rocks of Nd involved in the genesis of the Paleozoic fluorites can be gained from the age versus eNd plots (Fig. 5). It appears that most fluorites straddle the field of the basement at about 280 Ma, indirectly confirming the proposed age of formation. A few fluorites fall within the field of Variscan granitoids. Therefore, a Nd provenance from the basement is probable for most fluorites, while it is suggested that a few others derived Nd from Variscan magmatic rocks. Field evidence may support this latter hypothesis for the fluorites from Perda Niedda and Silius. Moreover, a contribution of Sr from magmatic fluids cannot be excluded a priori for the Nuraghe Perdu Spada fluorite, as Variscan granitoids are widespread at depth throughout Sardinia (e.g. Di Vincenzo et al., 2004). The lack of Nd-isotope data for marine limestones prevents any evaluation of the role played by these rocks as a potential source of Nd. The range of the initial eNd of the Cenozoic fluorites ( 7.08 to 9.73) overlaps those of the Paleozoic fluorites and Variscan granitoids ( 8 to 10 Tommasini et al., 1995; Castorina et al., 2005). In contrast, the range is distinguished from those of Cenozoic volcanics (average – 3.7, Lustrino et al., 2004) and the Lower Paleozoic metasedimentary basement ( 10, 15, Tommasini et al., 1995 and Table 4). Therefore, the radiogenic Nd was essentially provided by the leaching of Variscan granitoids. Uncoupling of potential source rocks of Nd and Sr is thus apparent in the same mineralization process.

6. Comparison between the Sardinian fluorites and other hydrothermal fluorites from Europe Schwinn and Markl (2005) studied REE systematics of fluorite from a large number of hydrothermal veins, generally hosted by the Variscan basement of the Schwarzwald area, Germany. They explained the origin of the veins as due to meteoric fluid influx into the basement rocks after the Variscan orogeny. Then remobilization of fluorite may have occurred in the Mesozoic and Tertiary. Both the Sardinian fluorites and those from Schwarzwald display similar PAAS-normalized REY patterns. Therefore, it appears that the fluorite-bearing fluids shared similar physical–chemical conditions of formation. In particular, the strong F complexation of the REY appears to be the dominant process characterizing most deposits from the two areas. Similar conclusions can be drawn when comparing the Sardinian fluorites and those from the Pennine mineralization of northern England (Bau et al., 2003). These latter were probably formed in multiple episodes from the Late Carboniferous to the Triassic. Both the Sardinian and Pennine fluorites share similar PAAS-normalized REY patterns; in particular, the B-group fluorites resemble those from the northern Pennine district because of the relatively high REY contents and slightly negative to null Ce anomalies. However, they are also generally similar to fluorites from the southern Pennine district because of the lack of strong positive Eu anomalies. According to Bau et al. (2003), these features rule out an origin of the REY from pure marine

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carbonates and suggest that the hydrothermal fluids originated from shallow crustal levels. The few available Sr and Nd isotope data for the Pennine fluorites fall within the range of the Sardinian fluorites suggesting similar potential source rocks for Sr and Nd in the both areas. The comparison between the Sardinian fluorites and those from hydrothermal veins formed about 231 Ma in a leucogranite at Galway, Ireland is also interesting (Menuge et al., 1997). The Irish fluorites are thought to be precipitated in a process where hot, dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous limestone overlaying the granite. In fact, the initial Sr-isotope ratios of the fluorite are much lower (0.7101–0.7139) than those of the host granite. The variable Nd isotope ratios of fluorite reflect the Nd derivation from the various wallrocks and subordinately from the limestone. Therefore, the comparison confirms the existence of possible uncoupling of Sr and Nd source rocks, especially if limestone is involved in the mineralizing process. Moreover, the combined application of Sr–Nd isotopic ratios to study the origin of hydrothermal fluorite deposits is strongly supported. Finally, the REY patterns and the Sr-isotope ratios of the Sardinian fluorites are compared with those of post-Variscan hydrothermal fluorites from Pyrenean vein deposits (Subías and Fernández-Nieto, 1995; Subías et al., 1998). This comparison is particularly interesting because until the early Tertiary, Sardinia was closely tied to southern France and north-western Spain. Therefore, it is possible that the geological event responsible for the genesis of the Paleozoic fluorite mineralization in Sardinia was similar to that for the Pyrenean fluorites. It is interesting to note that of the Pyrenean fluorites, those from Portalet exhibit PAAS-normalized REE patterns similar to those of fluorites from Nuraghe Perdu Spada, Is Murvonis and Su Zurfuru. The Portalet fluorites are thought to have formed by remobilization of previously formed fluorite veins by hydrothermal-meteoric fluids. In contrast, the nearly flat REY patterns of the Lanuza fluorites are similar to the those from Nuraghe Onigu and Arcu Istiddà. The Lanuza fluorites are regarded as being formed from primary precipitation from basement brines. Moreover, the Sr-isotope ratios of Pyrenean fluorites (0.7086–0.7104) indicate that radiogenic Sr leached from the Lower Paleozoic siliciclastic basement and Variscan granitoids was added to non-radiogenic Sr derived from the fluorite-hosting limestones.

7. Conclusions Three types of REY patterns that are similar to those of hydrothermal fluorites from a variety of European deposits, particularly Late Variscan, have been recognized. That suggests that the physical–chemical conditions of the fluorite formation were broadly similar in Variscan Europe. The most common pattern exhibits evidence of strong F complexation of Gd and Tb, and significant depletion in both the LREE and the HREE. This pattern, that is typical of fluorites deposited by late-stage hydrothermal fluids, is particularly characteristic of all the occurrences located in the Sarrabus–Gerrei district. A second pattern is charac-

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terized by increasing contents from the LREE to the HREE, suggesting that the fluorites were probably formed by remobilization of former fluorite occurrences. A third pattern generally shows no REE fractionation, suggesting fluorite deposition from primary fluids. The Sr–Nd isotope ratios of the Sardinian fluorites hosted by Paleozoic rocks indicate a provenance of the two radiogenic isotopes mainly from brines circulating through the siliciclastic basement. Such brines mixed to various extents with dilute surficial waters circulating through carbonates and/or the siliciclastic basement to produce the various occurrences. Uplift of Sardinia in Late Variscan–Permian times may have allowed surficial waters to penetrate to depth, where they mixed with brines. Such fluids may have mobilized fluorite from the Ordovician mineralization, reprecipitating it in later occurrences. The potential source rocks of Sr and Nd were likely magmatic in a few deposits, as suggested by field evidence. Lastly, a Sr provenance from marine limestones is dominantly recognized for the Bruncu Molentinu fluorite. The Sr–Nd isotopic range of the Sardinian fluorites overlaps that of other post-Variscan fluorites of Europe, indicating broadly similar potential source rocks for the two radiogenic isotopes. The Sr–Nd isotopic data of the Sardinian fluorites hosted by Paleozoic rocks cannot be used to define any isochron ages, assuming that the fluorites belong to one coeval mineralization. Nevertheless, the isotopic data strongly suggest that the formation of most occurrences probably occurred during or just after the late stages of the Variscan orogeny. This inference is broadly consistent with that resulting from studies conducted on other post-Variscan fluorite occurrences in Europe. Finally, the main chemical and isotopic characteristics of the Cenozoic fluorites are generally similar to those of the Paleozoic fluorites. A significant contribution of Sr from Cenozoic magmas is recognized only for the Nuraghe Onigu fluorite; the remainder was supplied mainly by the Paleozoic metasedimentary basement and/or Variscan granitoids. These latter were the main potential source rocks of Nd. References Bakos, F., 1972a. Le mineralizzazioni fluoritiche di Bruncu Mannu (Sarrabus-Gerrei, Sardegna sud orientale). Atti della giornata di studio su: Le fluoriti italiane, Torino 16/12, 1972. Ass. Min. Subalp., Special issue, vol. II, 197–224. Bakos, F., 1972b. Le mineralizzazioni fluoritiche di Bruncu Ventura (Sarrabus, Sardegna sud orientale). Atti della giornata di studio su: Le fluoriti italiane, Torino 16/12, 1972. Ass. Min. Subalp., Special issue, vol. II, 173–196. Bakos, F., 1972c. Le mineralizzazioni fluoritiche di Bruncu Molentinu (Sarrabus, Sardegna sud orientale) Atti della giornata di studio su: Le fluoriti italiane, Torino 16/12, 1972. Ass. Min. Subalp., Special issue, vol. II, 99–138. Bakos, F., 1972d. Le mineralizzazioni fluoritiche di Is Crabus (Sarrabus, Sardegna sud orientale). Atti della giornata di studio su: Le fluoriti italiane, Torino 16/12, 1972. Ass. Min. Subalp., Special issue, vol. II, 141–172. Bakos, F., Uras, I., 1972. Le mineralizzazioni fluoritiche di Monreale e Perda Lai (Sardara, Campidano di Cagliari) Atti della giornata di studio su: Le fluoriti italiane, Torino 16/12, 1972. Ass. Min. Subalp., Special issue, vol. II, 261–281. Bakos, F., Valera, R., 1972a. Il campo filoniano di Castel Medusa (Asuni, Sardegna centrale). Atti della giornata di studio su: Le fluoriti italiane, Torino 16/12, 1972. Ass. Min. Subalp., Special issue, vol. II, 283–298.

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