Tectonic and litho-stratigraphic controls on kaolin deposits within volcanic successions: Insights from the kaoliniferous district of north-western Sardinia (Italy)

Ore Geology Reviews xxx (2012) xxx–xxx

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Tectonic and litho-stratigraphic controls on kaolin deposits within volcanicsuccessions: Insights from the kaoliniferous district of north-western Sardinia (Italy)

Giacomo Oggiano, Paola Mameli ⁎Dipartimento di Scienze Botaniche, Ecologiche e Geologiche, Università degli Studi di Sassari, Via Piandanna n°4, 07100 Sassari, Italy

⁎ Corresponding author. Tel.: +39 079 228684; fax:E-mail address: [email protected] (P. Mameli).

0169-1368/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.oregeorev.2012.03.002

Please cite this article as: Oggiano, G., MamInsights from the kaoliniferous district of n

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Article history:Received 4 May 2011Received in revised form 5 December 2011Accepted 1 March 2012Available online xxxx

Keywords:KaolinSardiniaTectonicsVolcanic successions

In northwestern Sardinia, a Tertiary calc-alkaline volcanic succession, linked to the subduction of Insubricoceanic crust and subsequent post-collisional slab break-off, crops out and hosts several kaolin deposits.We have undertaken field and laboratory studies to establish how the Tertiary geodynamic setting of thewestern Mediterranean, and the composition and texture (especially porosity) of the volcanic rocks, controlthe geometry and quality of the deposits. Massive andesites, dacites, and mildly welded rhyodacitic ignim-brites are the precursors of the kaolin. Kaolin deposits from the andesites contain pyrite and jarosite, andthe deposits which formed from the dacites contain up to 5% alunite. The amounts of pyrite and alunite arenegligible in deposits which formed from ignimbrite. Fracture-controlled fluid-dominated systems (Izawa,1986), responsible for the formation of the deposits, are linked to NNW–SSE-trending normal faults whichformed during Burdigalian to Serravallian extension following on from collision between Adria and Europe(Corsica–Sardinia crust). Four types of deposit morphology (bedform, mushroom, fault parallel and funnel)are recognised, based on the attitude, fracturing, and porosity of the precursor rock. The remarkable funnelmorphology was generated by repeated hydrothermal eruptions.

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1. Introduction

Calc-alkaline volcanic activity took place in northwestern Sardiniafrom the Oligocene to the middle Miocene, related to subduction ofthe Insubric oceanic crust beneath the palaeomargin of the southEuropean Plate (Coulon and Dupuy, 1975; Downes et al., 2001). Theonset of subduction is generally placed in the Oligocene, after a sup-posed flip in subduction polarity (from Adria directed to Europe),but this view conflicts with the onset of the calc-alkaline volcanism,which has been dated to the Eocene (Lustrino et al., 2009). Early vol-canic activity involved the development of andesite domes and sub-volcanic quartz-diorite stocks. Subsequently, a thick volcanic andvolcano-sedimentary sequence was emplaced in the Early Miocene,ranging in composition from high-alumina basalt to rhyolite.

Several hydrothermal alteration zones and hydrothermal depositsare hosted within these calc-alkaline volcanics (Fig. 1). The Oligocenequartz-diorite of Calabona, hosting a porphyry Cu system, is the old-est to have been affected by hydrothermal alteration. More recent(Early Miocene) volcanic rocks host either hydrothermal, gold-richvein deposits (Furtei, Osilo), associated with potassic alteration andhigh to low sulphidation deposits (Ruggieri et al., 1997; Simeoneand Simmons, 1999), or barren hydrothermal deposits of kaolin(Mameli, 2000; Palomba et al., 2006), mainly associated with argillicalteration (Romana). The variety of alteration types, the different

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eli, P., Tectonic and litho-storth-western Sardinia (Italy)

morphologies, and the compositions of the deposits in northwesternSardinia make the region suitable for a study of how the parent-rock li-thology and the tectonic and structural framework control the mor-phology and composition of the deposits. To date, the role of regional-scale structures in the formation of these hydrothermal deposits hasnot been properly investigated, despite the fact that, along with litho-logical features, these structures were the prime controlling factor.Depending on the composition of the protolith and its primary andacquired porosity (Neubauer et al., 2005), epithermal deposits formnear the surface in areas where local-scale fault systems allow the for-mation of plumbing networks that provide a link between the surfaceand deeper structural levels (Izawa, 1986; Sibson, 2003).

This paper focuses on the relationships among the hydrothermalkaolin deposits, tectonics, volcanic stratigraphy, and the structuralframework of the district, as well as on the compositional and texturalfeatures of the parent rocks of the kaolin deposits. The objectives areto:

1) Highlight how the interplay between faults and lithology controlsthe morphology and composition of the deposits of industrialminerals derived from hydrothermal alteration.

2) Define the distribution of the deposits in terms of our understand-ing of Tertiary tectonics involving the Sardinia–Corsica Microplate(SCM) and the western Mediterranean.

3) Constrain the timing of hydrothermal activity in the district.

We start by considering some Neogene hydrothermal kaolin de-posits in the district of Romana (NW Sardinia, Fig. 2).

ratigraphic controls on kaolin deposits within volcanic successions:, Ore Geol. Rev. (2012), doi:10.1016/j.oregeorev.2012.03.002

http://dx.doi.org/10.1016/j.oregeorev.2012.03.002

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Fig. 1. Geological sketch map of Sardinia (modified from Carmignani et al., 2001). The samples studied are from the framed area. Note the indication of hydrothermal deposit sites.

2 G. Oggiano, P. Mameli / Ore Geology Reviews xxx (2012) xxx–xxx

2. Materials and methods

Geological mapping of the Romana area has been undertaken at ascale of 1:10,000. To distinguish between the products of differentvolcanic episodes, the various volcanic units have been identified as“Unconformity-Bounded Stratigraphic Units” (UBSU; Pasquarè et al.,1992). Accordingly, any evidence for breaks in volcanic activity, suchas palaeosols, epiclastites, and basal vitrophyric layers, has beentaken into account.

The textural and mineralogical features of the volcanic rocks thatwere precursors to the kaolin deposits have been determined inthin section under a polarising microscopy (PM), and the rocks havebeen classified on the basis of chemical analyses of rock-powder pel-lets, using XRF with a Philips PW 1400 spectrometer equipped withRh andW tubes. X-ray counts were converted into elemental concen-trations with a computer programme based on the matrix correctionmethod according to Franzini et al. (1972, 1975) and Leoni and Saitta(1976). Maximum errors are as follows: ±1% for SiO2, TiO2, Al2O3,Fe2O3, CaO, K2O, and MnO; and ±4% for MgO, Na2O, and P2O5. Maxi-mum errors for trace elements vary according to their concentrations:2%–3% at 1000 ppm; ±5%–10% at 100 ppm; ±10%–20% at 10 ppm.The detection limit is 3 ppm for the majority of measured trace ele-ments. Loss on ignition (LOI) was determined at 1100 °C.

To constrain the role of precursor textures on the genesis of kaolin,textural parameters were obtained from porosity determinations.Values for the open porosity were obtained according to the UNI9724–7/1992 rule (Pa (%)=[(Ci×Mv)/1000]where Pa=open porosity,

Please cite this article as: Oggiano, G., Mameli, P., Tectonic and litho-stInsights from the kaoliniferous district of north-western Sardinia (Italy)

Ci=water absorption, Mv=dry density) and using amercury intrusionThermo Finnigan Pascal 240 porosimeter.

Finally, X-ray powder diffraction (XRPD) was used to qualitativelyand quantitatively examine the crystalline phases present in the kao-lin samples. For this purpose, we used a Siemens D5000 with a CuKαradiation diffractometer. For the qualitative analyses, the patternswere recorded in the 2°–70° 2θ range (step size 0.02° and 1 s count-ing time for each step). For the quantitative determinations, rawmaterials, previously admixed with α-Al2O3 50 wt.% as internal stan-dard and micronized in a McCrone micronizing mill, were analysedusing the Reference Intensity Ratio (RIR) method (Bish and Chipera,1988) taking from literature I/Ic values (JCPDS — XRD database).

3. Geology

The kaolin district of Romana encompasses a calc-alkaline volcanicsuccession that was broadly described by Deriu (1962), and then inmore detail by Coulon (1977), who also assessed the petrogenesisof the calc-alkaline suite on the basis of rock chemistry and isotopecontents, and recognised an anatectic component in the late felsicproducts. Dostal et al. (1982) suggested that only the andesiticrocks had a sub-crustal origin, whereas other authors have suggesteda sub-crustal origin for the entire suite, using magmatic differentia-tion as the process (e.g., Morra et al., 1997).

Our field work around the kaolin district allows the reconstructionof a succession that partially matches that described by Coulon(1977), and which confirms that the less-evolved volcanic products



Fig. 2. Geological map of the Romana mining district (modified from Mameli, 2000).

3G. Oggiano, P. Mameli / Ore Geology Reviews xxx (2012) xxx–xxx

occupy a lower stratigraphic position, with the exception of somelavas that belong to the so-called “terminal magmatic activity”.

The older effusive rocks (Coulon, 1977) consist of basalts (MonteSeda Oro basalts, labelled A1), and these are capped by the MonteTiloromo andesitic flows (A2). These volcanics are massive in struc-ture, but they sometimes, probably close to vents, exhibit autobrec-ciated structures, scoriaceous beds, and dykes.

Wide, thick dacitic flows rest on the basaltic and andesitic rocks,from Monte Pizzinnu in the west to the Thiesi Road in the east. Thedacite lavas ofMonte Pizzinnu (D1) are red-purple in colour and exhibita roughly defined flow-banding that dips south-eastwards (25°/150°).More acidic volcanic rocks, consisting of pyroclastic flows, lava flows,and lava domes, overlie the lower basaltic–dacitic complex. These felsicrocks showmarked differences that easily allow them to be categorisedin the following units.

1) The pyroclastic complex. This composite complex is linked to theexplosive activity that was dominant in western Sardinia duringBurdigalian time. In the northwestern sector of the area, the com-plex lies between the Monte Tiloromo andesite and the MonteFrusciu lava dome. It consists of weakly welded pumice and ashflows with variable amounts of pumice and lithics, representedby millimetre- to centimetre-sized xenoliths of andesite and raremetamorphic rocks. Besides the variable amounts of pumice andlithics, a constant feature is an abundance of biotite phenocrysts.In addition to the ash flow deposits, a fall deposit also occurs,represented by laminated ash–lapilli tuffs and blocky breccias.

2) The Monte Frusciu lava dome. This unit consists of a flat-toppedextrusion of rhyodacite (D2), made up of reddish coloured lavaswith a well-developed flow banding and flow lineation.

3) The Monte Traessu Dome. This unit consists of a rhyolite dome witha flat-topped surface, probably due to reworking and erosion asso-ciatedwith the upper Burdigalian transgression. The rocks (R2) arered in colour and massive. At the base, the Monte Traessu Dome is


rimmed by volcanic breccias made up of blocks up to 100 cm in di-ameter that are black and glassy, rich in biotite, and sometimesscoriaceous. The blocks are embedded in a welded ash matrix,and they may represent a collapse that resulted from the explosivedestruction of the Monte Traessu Dome, or the collapse of an adja-cent older dome such as the Cossoine Ridge.

4) The Cossoine Ridge (Coulon, 1977). The ridge consists of a rhyoda-citic (R1) lava dome that coalesces with the Monte Traessu Dome.A distinctive characteristic of the Cossoine Ridge is the attitude ofthe flow banding, which changes from near vertical or very steepin the core, to subhorizontal at the borders.

5) Late extrusions (“Magmatisme terminale” of Coulon, 1977). The re-gion is characterised by “extrusions” of rhyodacite and andesitelavas (D3) that form several conical or cylindrical upstandingmasses of rock that emerge from middle Miocene sediments. Thegeometric relations between these late extrusions and the marinedeposits of upper Burdigalian–Langhian point to sub-volcanic bod-ies that were intruded within the Langhian marls rather than “ex-truded”. The contact between the wall of the lava bodies and themarine sediment is sharp and vertical, indicating the “intrusive”nature of the rocks, because it is clear that the Miocene sedimentscould not have lapped onto these volcanics as a pre-existing volca-nic morphology. Moreover, these rocks are never found asreworked fragments at the base of the transgressive deposits.This field evidence is in good agreement with recent 40Ar/39Ar dat-ing (14.14±0.07 Ma; Ferrandini, pers. comm.) on the Bonvei ex-trusion. This rhyodacitic lava has thin flow banding deformedinto rheomorphic folds with near-vertical axes that are cross-cutby columnar joints at intervals of 30 cm (Fig. 3). Of note, silicifiedLanghianmarls are located close to the contact with the Bonvei ex-trusion; thismetasomatism is the only contact-effect in the hostingsediments. The marine Miocene deposits record the submergenceof the region during the uppermost Burdigalian, when two silici-clastic and carbonate sequences were deposited (Martini et al.,



Fig. 3. Monte Pedru dacite with intersecting columnar joints and magmatic foliation (Bonvei extrusion).


1992). The first sequence starts with 1–20 m thick conglomeratesand sandstones, followed by a few metres of bioclastic limestonesof upper Burdigalian age. Alternating silty and sandy marlstonesoverlie the limestones.

6) Intra-plate Pliocene–Quaternary basalts. In Sardinia, during thePliocene and Pleistocene, an intra-plate volcanic cycle took place,

Table 1Chemical analyses of the volcanic rock precursors of the kaolin deposits. The analyses are r

Sample P 24 P 25 P 12 P 42 P 33 P 41B P 41A

(wt.%) A1 A2

SiO2 50.64 55.78 56.20 59.31 59.45 60.27 61.51TiO2 1.00 0.82 0.82 0.75 0.77 0.68 0.68Al2O3 20.24 18.09 18.27 16.32 17.63 16.29 16.82Fe2O3 1.12 0.95 0.90 0.90 0.87 0.83 0.79FeO 7.51 6.32 5.98 5.99 5.79 5.55 5.30MnO 0.20 0.17 0.15 0.13 0.13 0.13 0.12MgO 4.16 4.21 3.97 4.03 2.70 3.85 2.33CaO 10.54 8.24 7.96 7.08 6.48 6.56 6.00Na2O 2.94 2.87 2.93 2.25 3.00 2.29 2.56K2O 1.27 2.32 2.56 3.01 2.94 3.32 3.67P2O5 0.39 0.24 0.26 0.22 0.24 0.24 0.21Total 100.00 100.00 100.00 100.00 100.00 100.00 100.0(ppm)Pb 55 52 22 10 17 16 12Zn 75 62 63 67 94 66 61Ni 11 20 15 8 11 8 8Co 30 24 23 20 19 18 16Cr 14 64 45 14 20 15 13V 271 201 201 181 152 162 156Rb 30 80 97 97 102 111 108Sr 725 444 424 394 396 374 339Ba 218 270 297 341 375 339 321Th 1 7 8 11 7 12 11Nb 4 7 8 11 13 11 11Zr 81 148 132 78 188 180 175Y 21 21 21 26 26 27 26La 20 19 28 31 30 30 33Ce 53 59 55 62 51 61 56

Key: A1: Monte Seda Oro basalts; A2: Monte Tiloromo andesites; D1: Monte Pizzinnu dacitrhyolites; D3: Bonvei andesites and rhyodacites.


linked to the opening of the south Tirrenian basin and characterisedby alkaline to transitional basalts (Beccaluva et al., 1985 and refer-ences therein). These basalts crop out at Monte Traessu and nearPozzomaggiore above the Miocene marine sediments (Macciottaand Savelli, 1984). Hydrothermal activity has not affected thesevolcanics, so they are not considered any further in this paper.

eported on anhydrous basis.

P 43 P 32 P 15 P 01 P 14 P 56 P 17

D1

62.47 65.33 65.76 65.77 65.89 66.03 67.140.63 0.58 0.69 0.66 0.68 0.71 0.6616.59 16.50 16.94 16.98 16.97 16.30 16.920.68 0.49 0.57 0.58 0.55 0.70 0.454.52 3.30 3.84 3.86 3.70 4.69 2.990.13 0.09 0.04 0.04 0.05 0.04 0.032.97 2.07 0.50 0.64 0.63 0.83 0.464.94 3.69 3.10 2.89 3.25 3.06 2.773.42 3.80 3.85 3.80 3.99 3.41 4.143.35 3.87 4.42 4.54 4.02 3.99 4.200.28 0.28 0.30 0.24 0.27 0.24 0.24

0 100.00 100.00 100.00 100.00 100.00 100.00 100.00

n.d. 21 19 19 18 n.d. 2048 65 58 67 59 57 345 3 4 5 4 4 4n.d. 7 8 7 5 n.d. 33 7 4 4 3 3 674 44 58 63 59 55 55109 138 153 136 145 129 148367 329 307 257 325 324 307399 461 468 452 459 476 470n.d. 12 10 10 11 n.d. 129 13 17 9 14 15 17176 216 250 175 241 217 25329 29 28 29 32 29 3335 34 35 37 37 41 5070 61 73 73 68 68 66

es; D2: Monte Frusciu rhyodacites; R1: Cossoine Ridge rhyodacites; R2: Monte Traessu




4. Tectonics

The SCM in the main geodynamic models dealing with the Tertiarygeodynamics of the Alpine–Apennine system is still a matter of de-bate. In some classical reconstructions, the SCMcrust acted as the fore-land of the Alpine Chain. Then, by the Oligocene, after a supposed flipin subduction polarity (fromAdria to Europe directed), the subductionof Insubric oceanic crust beneath the southern Europeanmarginmadethe Sardinia–Corsica crust evolve as an Andean-type arc, which mi-grated oceanwards, leading to the opening of the Liguro–Provençalback-arc basin. The rotation and drifting of the SCM ended in its colli-sion with Adria, which lasted until the Tortonian (Giglia, 1973); thenthe SCM became part of the hinterland of the Northern Apennines(Boccaletti and Guazzane, 1972; Boccaletti et al., 1990). The subduc-tion flip is also used in other models that deal with the geodynamicsof the western Mediterranean, where the retreat of the Insubric oce-anic slab is thought to have acted as the engine for the opening ofboth the Liguro–Provençal and south Tirrenian basins (Malinvernoand Ryan, 1986). According to Gueguen et al. (1997), there was nocollision between Adria and Europe because of rapid slab retreatenhanced by westwards-directed mantle flow. In this model, theSCM originated by lithospheric necking as a continental lithospheric“mega-boudin” between the South-Tyrrhenian and Liguro–Provençalback arc basins. However, among other incongruities, this modelfails to provide an explanation for the occurrence of calc-alkalinesubduction-related volcanism dating back to the Eocene and lowerOligocene.

According to Carmignani et al. (1995), Oggiano et al. (2009), andPrincipi and Treves (1984), the Europe-directed subduction startedin the Upper Cretaceous, and the collision with Adria occurred duringthe Oligocene, leading to the formation of the north Apennine Chain;no flip in subduction is invoked. The Burdigalian drift followedthe collision, concomitant with the collapse of the north Apennineorogenic wedge. Post-collision slab break-off (Vos et al., 2007) and

P 49 P 31 P 30 P 20B P 35 P 34 P 18A P 18B

D1 D2 R1

67.90 68.31 71.60 68.06 67.81 69.12 67.51 68.570.76 0.45 0.44 0.35 0.49 0.44 0.50 0.4816.73 16.38 14.39 16.40 16.98 16.14 16.38 15.660.37 0.52 0.48 0.47 0.46 0.46 0.48 0.442.48 3.46 3.22 3.12 3.04 3.10 3.21 2.950.03 0.02 0.05 0.09 0.05 0.06 0.03 0.040.44 0.56 0.26 0.77 0.82 0.88 0.61 0.742.70 2.76 3.11 3.64 3.14 2.37 3.55 3.664.10 3.60 3.31 3.30 2.90 2.55 3.24 3.264.28 3.75 2.95 3.64 4.16 4.76 4.32 4.050.21 0.19 0.20 0.17 0.15 0.11 0.16 0.15100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.0

n.d. 28 17 22 n.d. n.d. 24 2146 57 41 52 38 43 51 466 3 3 5 4 6 4 4n.d. 3 6 4 n.d. n.d. 6 5

5 10 6 0 4 7 743 42 48 45 0 54 57 53138 142 101 88 138 181 167 143293 271 296 624 233 212 269 240497 458 403 493 0 534 538 562n.d. 8 7 17 n.d. n.d. 13 914 10 6 8 11 14 11 11232 188 159 122 223 215 239 19033 27 21 22 31 33 26 2642 51 18 50 0 33 31 3571 70 46 96 0 60 60 61


lower lithosphere delamination generated “asthenospheric windows”(Flower et al., 1998, 2001) accompanied by mantle upwellings,which, migrating eastwards (Serri et al., 1993), caused anomalousheat flows on the western side of the SCM, in the Thyrrenian Basinand in Tuscany (Della Vedova et al., 1995). This anomalous heatflow is still evident today. Such a geodynamic context favours theproduction of hydrothermal systems and deposits (Chen et al.,2009; de Boorder et al., 1998; Kesler, 1998; Sillitoe, 1997), particular-ly when the reactivation of deep-lithospheric faults provides an easypath for the transport of fluids and heat. According to this model, thestrike-slip faults linked to the north Apennine collision, particularlythe transtensional ones, and the subsequent faulting that was con-temporaneous with the Apennine collapse and the opening of thenorth Thyrrenian basin, could have exerted a strong control on thedistribution of hydrothermal alteration.

5. Structural framework of the study area

The Tertiary tectonic assemblage of north Sardinia is the conse-quence of the following events.

1) Oligocene Aquitanian collision between the Sardinia–Corsica crustalblock and the Adria Plate in north Sardinia and south Corsica, whichresulted in both transtensional and transpressional structures asso-ciated with regional strike-slip faults oriented between E–W andENE–WSW (Carmignani et al., 1994, 1995; Oggiano et al., 1995;Oudet et al., 2010).

2) The rifting-to-drifting of the SCM during the middle–upperBurdigalian, resulting in extensional tectonics with NNW–SSE-trending normal faults that bound several basinsfilled bymarine de-posits in the western part of the island (Funedda et al., 2000).

3) The opening of the southern Tyrrhenian basin, triggered byrollback of the subducting Ionian oceanic lithosphere during thePliocene. This event resulted in faults that trend N–S, caused the

P 18C P 13 P 27 P 28 P 52 P 23 P 38

R2 D3

67.39 73.33 72.81 73.56 61.76 67.83 72.150.49 0.18 0.20 0.18 0.84 0.36 0.2916.67 14.44 14.86 14.16 17.38 16.49 15.180.44 0.27 0.29 0.23 1.01 0.42 0.292.95 1.82 1.93 1.57 6.77 2.78 1.970.08 0.03 0.07 0.03 0.17 0.11 0.111.21 0.57 0.67 0.28 0.84 1.19 0.074.00 1.35 1.41 1.39 4.07 4.27 2.333.48 3.12 3.01 3.56 2.68 3.35 3.113.13 4.87 4.73 5.02 4.25 3.00 4.380.15 0.02 0.00 0.03 0.22 0.20 0.12

0 100.00 100.00 100.00 100.00 100.00 100.00 100.00

23 26 22 27 n.d. 17 n.d.48 40 46 33 80 36 427 2 3 4 8 4 58 3 2 2 n.d. 5 n.d.13 2 6 4 21 4 150 10 9 10 170 48 31162 176 169 171 142 89 150258 129 140 134 361 622 431592 481 485 485 567 548 53712 14 15 13 n.d. 20 n.d.10 13 14 11 12 11 12187 157 166 157 211 81 13927 25 28 24 32 21 2231 39 32 36 28 55 4262 62 61 72 77 100 65




reactivation of older fractures, and determined the present coastaloutline of the island.

Funedda et al. (2000) providedmore evidence for a tectonic phase ofSerravallian age that caused local emergence of the Langhian sequences,and that was accompanied by the outpouring of the most recent calc-alkaline lavas.

In the study area, the most important fault array is oriented NNW–

SSE (Fig. 2) and is related to the extensional Burdigalian phase. Thisfault array is therefore coeval with the drifting of the SCM and the cli-max of the volcanic activity (Beccaluva et al., 1994). The main fault ofthis array strikes N140, and runs from the Bonvei lava dome to thesedimentary remnants that overlie the Monte Frusciu dacite. The di-rection of strike is typical of the syn-rift Burdigalian faults. Beforethe Burdigalian transgression, the early movement of this fault causeduplift on the SW side, leading to erosion of the Monte Pizzinnu daciteand the Monte Tiloromo andesite. Displacement upon the fault wassubsequently reversed, uplifting the base of the Burdigalian trans-gressive deposits by about 200 m.

A second family of faults strikes between E–W and NE–SW. Thesefaults have the same strike direction as the Oligocene Aquitaniantranscurrent faults that, east of the study area, deeply affect thebasement and the pre-Burdigalian cover. In the mapped area, aENE–WSW-trending fault runs across the Monte Traessu Dome,down-throwing its northern part by several metres; this is an older,reactivated, strike-slip fault that affects the basement beneath thevolcanic cover.

The Serravallian to Pliocene reactivation of all the faults generatedvertical movements of at least 400 m, and this is the actual height dif-ference between the base of the Langhian strata at Romana and atMonte Traessu.

Fig. 4. a) Total alkali-silica diagram (TAS) showing the composition of the volcanic rocks in thK2O diagram (from Beccaluva et al. 1991) showing the high potassium content of the analystrachyandesites; TD— trachydacites; L— latites; R— rhyolites; prefix HK— high potassium. SMonte Frusciu; R1 — Cossoine Ridge; R2 — Monte Traessu; D3 — Monte Larenta and Bonvei


6. Compositional and textural features of the precursor rocks

In thin section, the Monte Seda Oro basalt displays a glomeropor-phyritic seriate texture with phenocrysts of resorbed plagioclase (An>80), pyroxene, and olivine in a fine-grained intergranular ground-mass. The Monte Tiloromo andesitic rocks display a variety of tex-tures. The dominant texture is glomeroporphyritic with plagioclaseand augite phenocrysts set in a groundmass composed of fine-grained plagioclase, pyroxene and magnetite, together with abundantinterstitial glass. The Monte Pizzinnu dacite exhibits a porphyriticstructure defined by plagioclase phenocrysts set in a groundmass oforiented microcrystalline plagioclase and glass. Mafic minerals aredeeply altered to chlorite. The pyroclastic flow displays a vitroclastictexture with very few crystals, abundant pumice and shards, andlithics. The welding of this deposit is weak.

TheMonte Frusciu dacite has mm-scale flow bands and a porphyrit-ic texture with a microcrystalline groundmass. The number of pheno-crysts is small, and they consist of biotite, amphibole, clinopyroxene,orthopyroxene and relatively abundant, often altered, plagioclase. Theporphyritic texture of the Monte Traessu rhyolite is evident to thenaked eye. In thin section, by PM observation, the rock shows abundantphenocrysts of sanidine and plagioclase (oligoclase), with resorbedquartz xenocrysts and minor biotite in a vitrophyric groundmass. Therhyodacitic lavas of the Cossoine Ridge differ from those of MonteTraessu by having plagioclase with a higher An content (at the bound-ary between oligoclase and andesine) and abundant mafic minerals.The dacite of the Bonvei extrusion exhibits a porphyritic texture withphenocrysts of plagioclase, green-brown hornblende, biotite, andresorbed quartz, all of which are set in a mainly glassy groundmass.Late crystallisation of layers of polycrystalline quartz mimics, and par-tially seals, the discontinuities presented by the flow laminae.

e Romana District. TAS diagram classification according to Le Bas et al. (1986). b) SiO2 vsed rocks. Legend: B — basalts; BA — andesitic basalts; A — andesites; D — dacites; TA —

amples from: A1—Monte Seda Oro; A2—Monte Tiloromo; D1—Monte Pizzinnu; D2—

Castle.



Table 3Average chemical compositions of the kaolin deposits. XRF analyses normalised to100%.

Deposittype

Bedform(Donigazza)

Bedform(Badu e'Ludu)

Mushroom Faultparallel

Funnel

(wt.%)SiO2 71.63 77.03 72.75 58.45 72.21TiO2 0.23 0.19 0.42 0.32 0.30Al2O3 19.22 15.37 18.23 29.23 18.23Fe2O3 0.63 0.89 0.29 1.56 0.29MnO 0.00 0.00 0.00 0.00 0.01MgO 0.07 0.01 0.01 0.01 0.23CaO 0.25 0.27 0.31 0.02 0.31Na2O 0.03 0.61 0.01 0.01 0.33K2O 0.05 0.18 0.06 0.01 0.67P2O5 0.07 0.03 0.17 0.07 0.17LOI 7.82 5.42 8.04 10.33 7.54Total 100.00 100.00 100.00 100.00 100.00


Twenty-nine XRF analyses were made to classify the rocks of theTertiary volcanic sequences of the Romana district, and to character-ise their chemistries (Table 1). The sequences are classified firstaccording to the total alkali vs. silica (TAS) and the K2O vs. SiO2 dia-grams (Fig. 4). In the TAS diagram, the Monte Seda Oro and MonteTiloromo lavas plot in the sub-alkaline basalt and basaltic andesitefields. Lavas from the upper sequence have more silica, more alkalis,and less titanium than those from the lower sequence (Table 1).They are therefore quartz normative, whereas the older basalts arehypersthene normative. Apart from the silica contents, the main dif-ference between the Seda Oro basalts and the Tiloromo basaltic an-desites is the variability in composition of the latter compared withthe more uniform composition of the former. Potassium contentsare higher than sodium, placing most of the rocks within the high-Kcalc-alkaline series, as shown on the K2O vs. SiO2 diagram (Fig. 4b).

Besides the textural parameters derived from petrographical obser-vations, further parameters were obtained by determining porosities,coupled with bulk and apparent densities, in order to constrain therole of precursor textures on the genesis of kaolin. The rocks involvedin kaolinisation, namely the Monte Pizzinnu dacite lava flow, theMonte Tiloromo andesite, the pyroclastic flow, and the Bonvei daciticlava, were sampled from unaltered (not kaolinised) outcrops using aportable corer in order to avoid shocks (which may significantlymodify the porosity of the undisturbed rock; Tullborg and Larson,2006). Results are shown in Table 2.

7. The kaolin deposits

The deposits of the Romana district are the most important accu-mulations of kaolin in Sardinia and the Italian mainland. The esti-mated reserves exceed 2 Mt (Ligas et al., 1997). Previous studiesfocused on the conditions relating to fluid–rock interactions (Mameli,2001; Palomba et al., 2006; Simeone et al., 2005) and on the technolog-ical properties of the material (Cara et al., 2006; Ligas et al., 1997;Mameli, 2000; Mameli et al., 2003), which are beyond the scope ofthe present work.

Over an area of at least 500 km2, based on morphological features,we have distinguished the following types of deposits with slightlydifferent compositions whose average chemical compositions arereported in Table 3.

7.1. Bedform-shaped deposits

Both the Donigazza and Badu e'Ludu deposits are developedwithinthe pyroclastic flow, parallel to the NNW–SSE-trending faults. Themorphologies of the deposits are typically bedform (Fig. 5), and thethicknesses of the deposits roughly coincide with the thicknesses ofthe weakly welded tuffs that belong to the Pyroclastic Complex.

Table 2a) Rock and pore characteristics determined with mercury porosimeter. b) Rock characterist2/1990, respectively.

a

Total porosity(%)

Average poreradius (μm)

Cumulative vo(cm3/g)

M. Tiloromo 3.8239 0.003132 0.0139M. Pizzinnu 13.8433 0.027479 0.0654Bonvei 22.7074 0.058742 0.1058Pyroclastic flow 24.1623 0.058731 0.1474

b

Open porosity (%)

M. Tiloromo 1.09M. Pizzinnu 5.36Bonvei 9.01Pyroclastic flow 29.83


Toward the top, these kaolin deposits are sealed by a horizon of green-ish bentonite, which formed during the Burdigalian transgression byinteraction of marine water and volcanic glass. Limestone of upperBurdigalian age in turn lies above the bentonite level. At Donigazza,the grade of kaolin is the highest among the different deposits ofthe district, containing 48% kaolinite (including dickite), 51% opal-CT or cristobalite, and 1% quartz on average, with rare or no alunite.The grade is lower at Badu e'Ludu, which contains 34% kaolinite, 61%cristobalite or opal-CT, 3% quartz, and 2% of other phases such asalunite, feldspar, and chlorite.

7.2. Mushroom-shaped deposits

This type of deposit is present on the Bonvei dacitic extrusion, closeto its contact with the host marls. The contact is locally faulted, and thefaults deform the kaolin, indicating that the late fault movements post-date the hydrothermal alteration. The deposits have a vertical, half-mushroom-shaped geometry with limited lateral extension. The aver-age composition is 40% kaolinite, 54% opal-CT, 3% quartz, and 3% alunite.

7.3. Fault-parallel alteration zones

TheMonte Tiloromo basaltic andesite is a dark porphyritic lavawith avitrophyric groundmass, which makes it compact and impermeable.Rock–fluid interactions are restricted to cross-cutting fault gouges.Generally, the gouges of the main faults are 5 m thick, although thehydrothermally altered area can stretch over 10 mdue to hydraulic brec-ciation. The nature of the alteration is strongly controlled by the maficcomposition of the precursor. Kaolinite is abundant, even if chalcedonyand vuggy silica deposits bearing pyrite and other sulphides are

ics determined according to the rules UNI 9724–7/1992, UNI EN 13755/2002 and 9724-

lume Specific surface area (m2/g)

Bulk density(g/cm3)

Apparent density(g/cm3)

5.761 2.74594 2.855124.584 2.11750 2.457736.237 2.14575 2.776148.944 1.63948 2.16182

Imbibition coefficient (%) Apparent density (g/cm3)

0.38 2.832.34 2.613.86 2.32

19.33 1.59



Fig. 5. Badu e'Ludu bedform deposit.


common; jarosite, if present, replacing alunite. The deposits are tele-scoped, with propylitic assemblages in the outer zones and argillic alter-ation restricted to the fault core. The composition of these deposits variesmarkedly, and some spot samples contain up to 74% kaolinite, 24%quartz, and 2% pyrite.

7.4. Funnel-shaped deposits

The Locchera deposit, which is the most extensive in the area, ishosted within the Monte Pizzinnu dacite and is centred around theintersection of two NNW–SSE and one ENE–SSW-trending faults.The deposit consists of stony, intensely bleached material contain-ing 39% phyllosilicates of the kaolinite group, including dickite(Mameli, 2000), 48% cristobalite or opal-CT, 8% quartz, and asmuch as 5% alunite. The kaolin was developed from breccia, andthe kaolinised breccia is spread over an area of 30,000 m2. The

Fig. 6. Panoramic photo showing the


thickness of the deposit increases toward the intersection of thefaults, where it reaches approximately 50 m, as visible in the Loc-chera quarry where the funnel shape of the deposit is also evident(Fig. 6). The base of the deposit consists of Monte Pizzinnu dacite,which is affected by a stockwork of veins of pure kaolin (1–2 cm thick)and fractures (10–30 cmwide) filled with kaolinised, immature, brecciaswith a jigsaw texture (Fig. 7). Such features, according to Lorilleux et al.(2002), suggest an origin by hydraulic expansion and brecciation of thehost rock.

The breccias forming the core of the deposit are very poorly sorted(clasts range from a few mm up to a few dm) and are supported by amuddy matrix. The kaolinised clasts are mostly angular, and anylateral or vertical grading is usually poorly developed or absent(Fig. 8). On the other hand, some clear bedding (Fig. 9) is found inthe peripheral zone, where the thickness of the brecciated materialis limited to a couple of metres.

funnel shape of Locchera deposit.



Fig. 7. Hydrobrecciated dacite with fractures filled with kaolinite breccias at the base of the Locchera deposit.


Alteration has erased most of the original features of the precursormaterial, but some ghost textures of sub-rounded fine clasts (Fig. 10)indicate that most of the kaolinised clasts were derived from theMonte Pizzinnu dacite. A silica-rich (opal-CT and quartz) layer, 1 mthick, caps the deposit (Fig. 11), which in turn is overlain by a con-glomerate composed of pebbles of kaolin-rich rock and dacite set ina smectite-rich matrix, which passes upwards into the transgressivemarine limestones.

8. Discussion

The kaolin deposits in the Romana district are associated withfaults that are linked to Early–Middle Miocene tectonics. These fault

Fig. 8. The dominant texture of the Locchera kaolin mad


zones conveyed fluids that, under intermediate water/rock ratios andlow pH conditions, reacted with the country rock to generate argillicalteration (Mameli, 2001) and produce kaolin deposits.

The effectiveness of the fluid–rock interaction responsible for thekaolin formation was controlled by the open porosity of the protolith.The higher the porosity, and hence the permeability, the wider thearea over which fluids migrate from the fault zones, and react withthe country rock. When a fluid rises up along a vertical fault, it may en-counter a formation that is more porous and permeable than thoseabove and below. In such a case, the alteration selectively affects thepermeable formation, giving rise to bedform-type deposits. This conceptfits well with the morphologies of the Donigazza and Badu e'Ludu de-posits. In fact, the protolith of these deposits is the weakly welded and

e up of massive unbedded hydrothermal breccias.



Fig. 9. Bedded and graded kaolinised breccias at the border of Locchera deposit. The different colour does not correspond to different mineralogical compositions.


highly porous pyroclastic flow that permitted fluids to expand later-ally, resulting in alteration over a wide area. The thickness of thesedeposits simply reflects that of the pyroclastic flow, which has anaverage thickness of 15 m. Hydrothermal explosions did not occurbecause the high porosity and relatively large average pore radius(Table 2) of this precursor allowed the easy escape of the vapoursideways during subsurface boiling.

Within the basalts and andesitic basalts, the areas involved in hy-drothermal alteration consist of narrow, vertical belts that practicallycoincide with the fault gouges. In these rocks, the development oflarger kaolin deposits was prevented by their low porosity. Moreover,the small amount of reactive, glassy material did not assist the fluid–rock interactions. In addition to kaolinite, phases such as pyrite andjarosite occur, and this reflects the mafic composition of the protolith.We cannot exclude the possibility that these deposits represent the

Fig. 10. Microphotograph showing kaolinised sub-rounded, fluidis


feeders for overlying bedform deposits that were eroded before theBurdigalian transgression.

In the case that rising fluids are vertically constrained by anoverburden or cap rock, and if a moderate amount of lateral expan-sion is possible due to primary fracturing or a moderate porosity, amushroom-shaped alteration zone can result. The mushroom-shapedmorphology is enhanced by subsurface boiling or by mixing withmeteoric water at depth. These were the conditions under whichthe Cuguruntis deposit formed at the expense of the Bonvei dacite,which is embedded within low-permeability Langhian marlstones.The mushroom shape of the Cuguruntis deposit was further modifiedby the reactivation of faults.

In low–moderate porosity rocks, the discontinuities, close to afault zone, allow infiltration of pressurised fluids, which in turn in-duces hydraulic brittle fractures with a wide variety of orientations.

ed clasts with ghost textures typical of Monte Pizzinnu dacite.



Fig. 11. Silica-rich layer capping Locchera deposit.


The result is a “structural mash”, which, according to Sibson (2003),“self-generates” high permeability.

Although the large Locchera deposit formed at the expense ofdacites with relatively low porosities, it stretches over a wide area,has a thickness of about 50 m, and is larger than the other deposits.The characteristics of the Locchera breccias, and the funnel-shapedgeometry of the deposit, indicate brecciation was followed by hydro-thermal, probably multiple, eruptions (Browne and Lawless, 2001;Jebrak, 1997). Alteration of the brecciated rock started before erup-tion took place, and the rock had probably been simmering before

Table 4Mineralogical composition of the various parent rocks and the correlated kaolin units.

Site Deposit type Paren

Donigazza Bedform Qtz; PBadu e'Ludu Bedform Qtz; PBonvei Mushroom Pl; HM. Tiloromo Fault parallel Pl; AuLocchera Funnel Pl; Ch

Abbreviations follow Whitney and Evans (2010).


the brecciated material was ejected. Table 4 summarises the miner-alogical composition of the various kaolin deposits and parent rocks.

In an extensional tectonic regime, hydraulic fracturing startswhen the fluid pressure Pf exceeds the least average stress compo-nent of a quantity equal to the tensile strength. In practice, failureoccurs when the conditions expressed by the following relation aresatisfied (Hulen and Nielson, 1988; Phillips, 1972):

Pf ¼ Pl þ 2Phð Þ=3

t rock Kaolin unit

l; Bt; Glass Kln±Dck; Opl-CT±Crs; Qztl; Bt; Chl; Glass Kln; Opl-CT±Crs; Qzt; Alu±Fsp±Chl

bl; Bt; Qtz; Glass Kln; Opl-CT±Crs; Qzt; Alug; Mag; Glass Kln; Qzt; Pyl; Glass Kln; Opl-CT±Crs; Qzt; Alu



Fig. 12. Temperature vs. depth diagram with the boiling point curve for a fluid withsalinity like that of Romana district (after Haas 1971, modified). Faint lines point thedepth interval at which boiling happens at Locchera, considering a fluid temperaturebetween 150 and 200 °C.


where Pf is fluid pressure, Pl is lithostatic charge, and Ph is hydrostat-ic pressure. The fluid pressure needed for brecciation at a depth of100 m is 1.537 MPa, assuming rock with a mean density of 2.7 g/cm3.

Considering the boiling–depth curve (after Haas, 1971) of a fluidwith the same salinity and temperature as that responsible for theLocchera deposit (between 150 and 200 °C; Mameli, 2001; Padalinoet al., 2003; Simeone et al., 2005), boiling could have flashed at adepth between 40 and 100 m (Fig. 12), causing the explosive expul-sion of hydraulically fractured and altered host rock. The growth ofalteration minerals increases the chances of producing the overpres-sure needed for a blowout of the altered rock (Berger and Henley,2011); in particular, quartz and a second generation of very compact,porcelain-like, kaolinite (Fig. 13), would have sealed the fractures,and opal-CT and quartz would have formed a cap rock, hinderingsteady degassing. Seismic failure of the cap rock could have triggeredan explosion, as observed in present-day hydrothermal eruptions(Ohba et al., 2007). The presence of pressurised fluids would haveled to grinding and a significant increase in the reactive surface-areas, and this would have enhanced the further alteration of theMonte Pizzinnu dacite fragments over a wide area.

Fig. 13. Porcelain-like kaolinit


9. Conclusions

Miocene extensional/transtensional tectonics controlled the cyclichydrothermal systems in the Sardinia–Corsica Microplate during andafter its drifting. The syn-to-post-drift E–Wextension, which generat-ed NNW–SSE-trending faults, controlled the formation of most of thekaolin deposits.

Porositywas themost important factor in controlling the intensity ofwater–rock interactions, and the composition of the precursor rock hada major influence on the quality of the kaolin deposits, so that a maficprecursor, for example, favoured the precipitation of relatively largeamounts of FeS2, making the kaolin unsuitable for ceramics production.

The following four types of deposit, which match the geometry ofthe alteration zones described by Inoue (1995), can be recognised,depending on the structural and stratigraphic frameworks that char-acterise the original volcanic succession:

1) Bedform-shaped deposits, which developed parallel to faults thatcross-cutweaklywelded pyroclasticflowsof rhyodacitic composition.These deposits contain higher-grade kaolin than the other deposits.

2) Small mushroom-shaped kaolin deposits, which developed alongthe intrusive contacts between the impermeable Langhian marl-stones and the Bonvei Serravallian sub-volcanic rhyodacite domes.

3) Vertical bands of mixed deposits of kaolin and vuggy silica, devel-oped along fault gouges within the andesites and andesitic basalts.

4) Funnel-shaped deposits which developed as a result of repeatedhydrothermal eruptions. The very large deposits of Locchera areof this type, and they resulted from repeated hydrothermal erup-tions within the dacite of Monte Pizzinnu.

Volcanic clasts with different types of hydrothermal alteration, in-cluding kaolin, occur in the conglomerate at the base of the marinetransgression, suggesting that some kaolin deposits formed priorthe upper Burdigalian. However, the kaolin deposits that formed atthe expense of the Serravallian Bonvei dacite provide evidence thatthe hydrothermal alteration responsible for kaolin genesis lasted atleast until the Serravallian.

Acknowledgments

We thank Ore Geology Reviews Editor-in-Chief Nigel Cook and theanonymous referees for important comments and constructive criticism.

e vein (Locchera quarry).




We are also grateful to Prof. M. de'Gennaro and collaborators for theMercury Porosimetry determinations. Financial support was providedby CNR and Banco di Sardegna Foundation grants.

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Tectonic and litho-stratigraphic controls on kaolin deposits within volcanic successions: Insights from the kaoliniferous district of north-western Sardinia (Italy)

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