Multiple hydro-fracturing by boron-rich fluids in the Late Miocene contact aureole of eastern Elba...

$: Multiple hydro-fracturing by boron-rich fluids in the Late Miocene contact aureole of eastern Elba Island (Tuscany, Italy)$
Multiple hydro-fracturing by boron-rich fluids in the Late Miocenecontact aureole of eastern Elba Island (Tuscany, Italy)

A. Dini,1 F. Mazzarini,2 G. Musumeci3 and S. Rocchi31Istituto di Geoscienze e Georisorse, CNR - Pisa, Via Moruzzi 1, 56124 Pisa, Italy; 2Istituto Nazionale di Geofisica e Vulcanologia, Sezione di

Pisa, Via della Faggiola 32, 56126 Pisa, Italy; 3Dipartimento di Scienze della Terra, Universita di Pisa, Via S. Maria 53, 56126 Pisa, Italy

Introduction

Contact aureoles are common sites ofintense circulation of fluids releasedby crystallizing magmatic intrusionsand ⁄or derived from metamorphicreactions (e.g. Norton and Knight,1977; Nabelek et al., 1984). Fluidcirculation results in vein systemsand ⁄or metasomatic bodies showingcomplex and mutual cross-cuttingrelationships. In particular, boronmetasomatism and deposition of tour-maline in veins are common processesonce granite intrusions release latemagmatic fluids to the wallrocks (e.g.Shearer et al., 1984; Woodford et al.,2001). Examples of these phenomenaare found in the inner northern Apen-nines, where Neogene intrusive bodieswere emplaced at upper crustal levelsleading to the development of hydro-thermal systems, like in Elba Islandand in the Larderello geothermal field(Dini et al., 2001; Gianelli and Rug-gieri, 2002). We present a case studyfrom a contact aureole of the buriedPorto Azzurro pluton in the south-eastern Elba Island (northern Tyrrhe-nian Sea, Italy), where widespreadcirculation of boron-rich saline fluids,issuing from felsic dykes, led to thedevelopment of tourmaline-bearingmetasomatic bodies and hydrothermal

vein systems. Structural and texturalfeatures of metasomatic-hydrothermaltourmaline, coupled with composi-tional variations, allow determinationof the evolution of fluid–rock inter-

actions. This system is evaluated as apossible exposed proxy of the earlystage deepest part of the Larderello-Travale geothermal field, where tour-maline veins, seismic reflectors

ABSTRACT

In eastern Elba Island (Tuscany, Italy), a shallow crustal levelfelsic, tourmaline-bearing, dyke-sill swarm of Late Miocene ageis associated with abundant tourmaline-quartz hydrothermalveins and metasomatic masses. Development of these veins andmasses in the host rocks demonstrates multiple hydro-fractur-ing by magmatic, boron-rich saline fluid. Tourmalines in felsicdykes are schorl, whereas in veins and metasomatic masses,tourmaline composition ranges from schorl-dravite throughdravite to uvite. This compositional shift is evidence for an

increasing contribution to the magmatic boron-rich fluids by aMg-Ca-Ti-rich external component represented by biotite-richand amphibolite host rocks. This system can be envisaged as anexposed proxy of the high temperature hydrothermal systempresently active in the deepest part of the Larderello-Travalegeothermal field (Tuscany).

Terra Nova, 20, 318–326, 2008

Fig. 1 (a) Geological sketch of the Elba Island (modified after Rocchi et al., 2002).(b) Geological map of south-eastern Elba Island (Calamita Peninsula and PortoAzzurro; modified after Pertusati et al., 1993). The box labelled �Cala Stagnone�indicates the investigated area. The tectonic units derive from both Tethyan Ocean(Ligurian units; Complex IV and V) and Adria continental margin (Tuscan units;Complex I, II and III). Late Miocene intrusive rocks in central-western Elba Islandconsist of a laccolithic porphyry complex, the Monte Capanne pluton and a maficdyke swarm, while in eastern Elba Island, they consist of the Porto Azzurro pluton, afelsic dyke-sill system and a mafic dyke swarm.

Correspondence: A. Dini, Istituto di Geo-

scienze e Georisorse, CNR - Pisa, Via

Moruzzi 1, 56124 Pisa, Italy. Tel.:

0039 050 3152370; fax: 0039 050 3152360;

e-mail: [email protected]

318 � 2008 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2008.00823.x

(bright-spots) and anomalously con-ductive crust have been detected in thecontact aureoles of Plio-Pleistocene

granites (Dini et al., 2005; Bertiniet al., 2006). The study of the fossilhydrothermal systems exposed in east-

ern Elba Island provides useful infor-mation about physico-chemical andstructural conditions attained in the

(a) (b)

(c) (d)

(e) (f)

Fig. 2 (a) Felsic dykes at Cala Stagnone. (b) Metasomatic tourmaline replacing biotite-rich layers in hornfels close to the felsicdyke contact. (c) Tourmaline-rich A-veins cutting across Calamita Schist. Maximum exposed thickness of vein (black arrow) isabout 2 cm. (d) Tourmaline-rich B-veins cutting across Calamita Schist. The distance between the two veins (black arrows) isabout 1.2 m. (e) B-veins (black arrows) cutting across hornfels, felsic dyke and metasomatic tourmaline. (f) Polished slab of a latestage B-vein showing the results of multiple infill. The cataclastic fabric consists of brownish fine-grained uvitic tourmalinecementing clasts of early black schorl-dravite tourmaline and host rock.

Terra Nova, Vol 20, No. 4, 318–326 A. Dini et al. • Hydro-fracturing by boron-rich fluids

.............................................................................................................................................................

� 2008 Blackwell Publishing Ltd 319

contact zone of multiple acidic intru-sions, with implications on the ongoingresearch ⁄exploration of unconven-tional geothermal reservoirs (supercrit-ic fluids) in the Larderello-Travalegeothermal field (Bertini et al., 2006).

Geological outline

In the northern Apennine Cenozoicbelt, the Elba Island tectonic stackwas intruded by magmatic bodiesbetween ca. 8 and 6.9 Ma in westernElba (Pertusati et al., 1993; Dini et al.,2002) and around 5.9 Ma in easternElba (Maineri et al., 2003). In easternElba, a pluton, several felsic tabularintrusions and hydrothermal vein-masses occur within the CalamitaSchist. This unit (Fig. 1) consists ofpsammitic-pelitic hornfels along withminor interlayered amphibolitebodies. The Porto Azzurro pluton isa slightly peraluminous monzogranitebelonging to the Tuscan MagmaticProvince (Innocenti et al., 1992). Sev-eral generations of felsic dykes andsills strike N40E–N160E, with sub-horizontal (sills) to sub-vertical(dykes) attitudes and para-concordantto discordant geometric relationshipswith host rock foliation. They invari-ably have a leucogranitic composition,with ubiquitous tourmaline and mutu-ally exclusive occurrence of primarymuscovite or biotite. Hydrothermaltourmaline-quartz veins cut acrossdykes and sills and are the result ofa hydrothermal circulation affect-ing both the intrusive and hornfelsrocks.

The Stagnone magmatic-hydrothermal system

In the Cala Stagnone area (Fig. 1b),magmatic dykes, hydrothermal veinsand metasomatic masses are hostedwithin biotite–andalusite–feldspar-bearing hornfels. The main hornfelsfabric strikes NW with SW dip, andrepresents the axial plane foliation oftight folds.

Felsic dykes

Sub-vertical coarse-grained pegmatiticto microgranitic felsic dykes trendNNW–SSE with thicknesses of lessthan 1.7 m (Figs 2a and 3). Dykesdiscordantly cut across hornfels fab-ric, and are locally associated with

metasomatic effects. Felsic dykes arecomposed of quartz, K-feldspar, pla-gioclase and rare biotite or muscovite.Tourmaline is a significant early crys-tallizing phase, as testified by its eu-hedral shape (Fig. 4a), reflecting theoriginal high boron content of themagma. Tourmaline pleochroism var-ies from pale brown through darkbrown to dark green, reflectingchanges in mineral chemistry. Theanalysed tourmalines plot in the alkaligroup and can be classified as schorl(low MgO content in the range 0.4–1.9wt%; Table 1; Figs 5 and 6).

Metasomatic masses andhydrothermal veins

Tourmaline at Cala Stagnone iscommon in the magmatic rocks anddominant in the related hydrothermaland metasomatic products, mainlyoccurring as tourmaline-quartz hydro-thermal veins along with tourmaline-quartz metasomatic bodies.Metasomatic bodies have developed

at the contact between leucogranitesand the host hornfels over a distanceof few centimetres up to 1 m, withpreferential replacement of biotite-rich layers by black microgranulartourmaline and quartz (Figs 2b and4b). The metasomatic bodies occur asblack massive rocks with relics of

folded quartz veins maintaining thefabric of the hornfels. Metasomaticbodies are associated with small tour-maline veins (A-veins, see below) thatpropagate from the felsic dykes intothe hornfels.Two sets of tourmaline-quartz veins

(A-veins and B-veins) characterize thehydrothermal system (Fig. 3); bothvein sets cut across felsic dykes, horn-fels rock and metasomatic bodies.They show sharp contacts with hostrocks and off-shoots. The observedmutual cross-cutting relationshipsindicate a coeval development of thetwo vein sets.The A-veins are centimetre to deci-

metre-spaced N- to NNW-strikingveins moderately to steeply dipping tothe W (Figs 2c and 3). Vein widthsrange from 0.2 to 8 cm and exposedlengths are between 0.9 and 3 m. Theseveins, with nearly constant width andsingle tapering terminations, are filledwith fine-grained black tourmaline andminor quartz with isotropic fabric.The B-veins consist of decimetre to

metre spaced E–W to WNW–ESEveins dipping gently to S–SW(Figs 2d and 3). Thicknesses rangefrom 0.1 to 25 cm and exposedlengths are between 3 and 15 m.These veins show marked variationsof width along-strike, with singletapering, forked terminations,

N

Poles to felsic dykes; n = 20Poles to A-veins; n = 32Poles to B-veins; n = 38

Fig. 3 Lower hemisphere equal area stereonet showing poles to felsic dykes, A-veinsand B-veins.

Hydro-fracturing by boron-rich fluids • A. Dini et al. Terra Nova, Vol 20, No. 4, 318–326

.............................................................................................................................................................


branching structures, off-shoots andintense cataclasis of host rock in theintervening space (Fig. 2d–f). The B-veins are characterized by variablefabrics and infilling materials. Someveins are filled only by fine-grainedblack tourmaline and quartz andtheir width is commonly about2 cm, while most of the B-veins showa composite fabric. The margins ofmost of the B-veins consist of fine-grained isotropic black tourmalineand quartz (as for A-veins), and theinner parts exhibit a coarse-grainedcataclastic fabric with angular frag-

ments of host rock and early tour-maline vein infill set in a very fine-grained matrix of brownish tourma-line and quartz (Figs 2f and 4c,d).Hydrothermal and metasomatic

tourmalines are petrographically andchemically different from magmaticones. Metasomatic bodies are made offine-grained (100–500 lm) clusters ofsubhedral to euhedral tourmalinecrystals with short prismatic habit(Fig. 4b). Tourmaline crystals showconcentric zoning with progressiveMg enrichment towards the rim withschorl-dravite brown cores and drav-

itic dark-brown to green rims. Thecalcium content is higher (0.4–0.9wt%) than in magmatic tourmalinefrom felsic dykes (0.1–0.4 wt%)(Table 1; Figs 5 and 6).The black tourmalines from A-veins

and the first generation in B-veins areprismatic and their aggregates displayvariable textures fromfine-grained equi-granular (100–500 lm) to very fine-grained (50–200 lm), with intergrownelongated crystals (Fig. 4c).Tourmalinefrom the coarser type shows bothreverse concentric zoning (from schorlto dravite) and patchy-zoned grains;

Fig. 4 Thin section microphotographs of Stagnone tourmalines. (a) Doubly terminated euhedral crystals of schorl in felsic dyke.All crystals are distinctly zoned, both in sections cut perpendicular and parallel to the c-axis of the tourmaline grains. In (0 0 1)sections, the crystals have irregular cores, pale-brown in colour, with darker euhedral rims. Sections parallel to the c-axis show adifferent texture, with a wedge-shaped zone on either side of the crystal, recalling the hourglass zoning. In complete tourmalinecrystals (see inset), this hourglass consists of a short green wedge on the analogous c) pole enriched in Ti (TiO2 = 0.9–1.0 wt%), amore developed pale-brown wedge on the antilogous c+ pole depleted in Ti (TiO2 = 0.3–0.4 wt%), embedded in a darker browntourmaline (TiO2 = 0.6–0.7 wt%). (b) Polygonal aggregate of metasomatic tourmaline with interstitial quartz. The crystals show aconcentric zoning with a progressive Mg enrichment towards the rims. Quartz, and rarely tourmaline, contains numerous fluidinclusions (�10–30 lm) with halite crystals (see inset). (c) Cataclastic texture in a B-vein: fragments of the first tourmalinegeneration (coarse grained; schorl-dravite) cemented by the very fine-grained tourmaline of second generation (dravite-uvite);(d) Detail of the second tourmaline generation in B-veins: note the difference in grain size and crystal shape of this tourmalines withrespect to the first hydrothermal, tourmaline generation and the metasomatic tourmaline.


.............................................................................................................................................................


pleochroic colours are similar to thoseof metasomatic tourmalines. Finally,very fine-grained elongated tourmalinecrystals display an intermediate schorl-dravite composition with low calcium

content (0.3–0.7 wt%) and negligiblezoning (Figs 5 and 6).The very fine-grained brownish

tourmalines (second generation inB-veins) display an isotropic inter-

growth of elongated wedge-shapedcrystals (50–150 lm) with triangularsection and skeletal terminations, setin a cryptocrystalline groundmass (5–30 lm; Fig. 4c,d). Pleochroism ranges

Table 1 Representative analyses of tourmalines from Stagnone magmatic-hydrothermal systems and Porto Azzurro pluton.

Sample no. PA04 ⁄ 1 PA04 ⁄ 2 PA101 ⁄ 4 PA101 ⁄ 7 PA102 ⁄ 1 PA102 ⁄ 2 PA104 ⁄ 6 PA104 ⁄ 7 PA104 ⁄ 2 P104 ⁄ 10 PA104 ⁄ 1

Rock

type

Porto

Azzurro

pluton

Porto

Azzurro

pluton

felsic

dyke

c+ pole

felsic

dyke

c) pole

Metasomatic

masses

Metasomatic

masses A-vein A-vein

B-vein,

early

black infill

B-vein, late

brown infill

B-vein, late

brown infill

SiO2 35.58 35.43 35.60 34.77 36.13 36.97 35.88 36.11 36.48 35.77 35.97

TiO2 0.07 1.24 0.36 1.01 0.59 0.43 0.77 0.70 0.36 1.69 1.84

Al2O3 31.42 31.70 35.17 34.26 32.72 32.91 33.04 33.88 34.34 29.91 27.95

Cr2O3 0.01 0.00 0.00 0.00 0.00 0.03 0.11 0.01 0.00 0.00 0.15

FeO 12.04 8.38 11.35 11.37 9.99 3.32 7.52 7.55 7.46 6.13 2.45

MgO 3.46 5.66 1.56 1.79 3.63 8.69 5.40 4.81 4.25 8.17 12.08

CaO 0.81 0.85 0.18 0.31 0.42 0.93 0.73 0.47 0.31 1.98 4.16

MnO 0.17 0.11 0.12 0.09 0.03 0.00 0.00 0.01 0.03 0.00 0.00

Na2O 2.05 2.01 1.34 1.83 1.60 1.81 1.80 1.79 1.71 1.56 0.78

K2O 0.05 0.06 0.03 0.03 0.08 0.02 0.06 0.02 0.05 0.09 0.03

F 0.36 0.30 0.54 0.36 0.29 0.17 0.08 0.21 0.33 0.32 0.77

Cl 0.00 0.02 0.03 0.00 0.16 0,00 0.11 0.00 0.25 0.00 0.00

H2O* 3.39 3.47 3.36 3.42 3.44 3.64 3.58 3.57 3.45 3.48 3.32

B2O3* 10.33 10.49 10.50 10.42 10.47 10.77 10.58 10.62 10.63 10.55 10.67

Li2O* 0.28 0.31 0.35 0.44 0.35 0.28 0.36 0.40 0.46 0.35 0.40

Total 100.03 100.08 100.49 100.11 99.89 99.97 100.01 100.13 100.12 100,00 100.55

O=F 0.15 0.13 0.23 0.15 0.12 0.07 0.03 0.09 0.14 0.14 0.32

Total* 99.88 99.95 100.26 99.95 99.77 99.90 99.98 100.05 99.97 99.87 100.23

Sites Structural formula based on 31 anions (O, OH, F)

T

Si 5.985 5.870 5.891 5.799 6.000 5.964 5.896 5.909 5.966 5.895 5.860

Al 0.015 0.130 0.109 0.201 0.000 0.036 0.104 0.091 0.034 0.105 0.140

B 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

Z

Al 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 5.704 5.227

Mg 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.296 0.773

Y

Al 0.214 0.060 0.751 0.534 0.403 0.221 0.296 0.444 0.587 0.000 0.000

Ti 0.009 0.154 0.045 0.127 0.074 0.052 0.095 0.086 0.045 0.210 0.226

Mg 0.867 1.398 0.385 0.445 0.899 2.091 1.323 1.173 1.037 1.712 2.160

Mn 0.024 0.015 0.017 0.013 0.004 0.000 0.000 0.001 0.004 0.000 0.000

Fe2+ 1.693 1.161 1.571 1.586 1.387 0.447 1.034 1.034 1.021 0.844 0.333

Li* 0.192 0.205 0.232 0.296 0.233 0.185 0.238 0.261 0.306 0.234 0.262P

Y 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

X

Ca 0.147 0.151 0.032 0.055 0.075 0.161 0.129 0.083 0.055 0.349 0.726

Na 0.669 0.646 0.430 0.592 0.516 0.565 0.572 0.566 0.542 0.500 0.246

K 0.011 0.013 0.006 0.006 0.017 0.004 0.012 0.004 0.010 0.019 0.006

h 0.173 0.191 0.532 0.346 0.392 0.270 0.287 0.347 0.393 0.133 0.022

OH 3.807 3.837 3.709 3.810 3.806 3.915 3.929 3.893 3.759 3.831 3.604

F 0.193 0.157 0.283 0.190 0.150 0.085 0.041 0.107 0.173 0.169 0.396

Cl 0.000 0.006 0.008 0.000 0.044 0.000 0.03 0.000 0.068 0.000 0.000

Mineral name Schorl Dravite Schorl-foitite Schorl Schorl Dravite Dravite Dravite Dravite Dravite-uvite Uvite

*B2O3, H2O and Li2O calculated by stoichiometry; B = 3 apfu, OH + F = 4 apfu and Li = 15-total(T + Z + Y). Chemical analyses of tourmalines were performed

using a JEOL-FXA-8600 electron microprobe (CNR–IGG, Firenze) in wavelength-dispersive mode with acceleration voltage = 15 kV, sample current = 10 nA and

4 lm beam size. WDS element peaks were measured with counting time of 10 s (Na), 15 s (Si, K), 30 s (Fe) and 40 s (Al, Cr, Mg, Mn, Ca, F); background was

counted for 10 s. Albite was used as reference standard for Si and Na (TAP); plagioclase for Al (TAP); olivine for Mg (TAP); sanidine for K (PET); diopside for Ca (PET);

ilmenite for Ti (PET) and Fe (LIF); chromite for Cr (LIF); bustamite for Mn (LIF) and apatite for F (TAP). The detection limit under the specified conditions was

approximately 0.05 wt%.


.............................................................................................................................................................


fluid exsolved from felsic dykes andconcentrated along the dyke–hostrock boundaries, leading to the for-mation of centimetre- to metre-thickmetasomatic masses through perva-sive, local fluid circulation at the grainscale. Accumulation of boron-richfluids locally built-up fluidoverpressure conditions favouringhydro-fracturing and development ofthe various tourmaline vein sets.According to Oliver (1996), both veinsets are channelized fractured opensystems at the mesoscopic scale withina closed wall rock system, as sup-ported by the lack of any significantrecrystallization-metasomatism in thewall rock. The close spatial relation-ships of the two vein sets (Figs 2 and3) document a switch in the orienta-tion of the local minimum compres-sive stress (r3 nearly sub-horizontalfor A-veins and nearly vertical for

B-veins) related to fluctuation in fluidoverpressure (Collettini et al., 2006).Tourmaline textures and grain sizereflect the different conditions of crys-tallization, ranging from fine-grainedpolygonal equigranular (metasomaticbodies) to very fine-grained inter-growth with elongated wedge-shapedskeletal crystal set in cryptocrystallinegroundmass (veins). The former sug-gests a progressive replacement ofbiotite-rich layer in the hornfels, thelatter points to fast crystallizationbecause of cyclic pressure release ina fault valve behaviour (e.g. Sibson,2000) with cataclasis of early infilling.The multiple opening-infill process inB-veins is emphasized by at least twomain stages of pressure build-up andrelease as testified by the cataclasis ofthe early black tourmaline infill sealedby the very fine-grained brownishtourmaline.

Evolution of fluid chemistry and thetourmaline record

The multiple hydro-fracturing pro-cesses at Stagnone is further charac-terized by variable tourmalinecompositions that provide informa-tion on the fluids involved. The tour-maline hosted in felsic dykes hastypical schorl composition reflectingthe very low Mg content of the acidicmagma (Figs 5 and 6). In contrast, theschorl-dravite composition of themetasomatic tourmaline indicateschemical interaction with biotite-richlayers in hornfels rocks. Indeed, thereverse zoning of tourmaline recordsthe progressive biotite replacement.The same tourmaline composition isfound in the A-veins and in the earlystage of B-veins. This suggests a directlink between the metasomatic fluids,equilibrated with hornfels and fluidsinvolved in the early stage of hydro-fracturing.The drastic change in texture from

first- to second-generation of tourma-line in the B-veins is coupled with asignificant shift towards uvite compo-sition that indicates Ca-rich fluid cir-culation. In particular, Ca enrichmentis positively correlated with Ti content(Fig. 6), suggesting a different fluidsource distinct from the magmaticone. Interaction with psammitic horn-fels of the Calamita schist (CaO �0.3wt%; TiO2 �0.8 wt%; Puxeddu et al.,1984) could not account for Ca and Tienrichment of fluids. Alternatively,the interaction of boron-rich fluidswith mafic rocks, as the amphibolitebodies interbedded within the hornfels(CaO �9.2 wt%; TiO2 �2.3 wt%;Puxeddu et al., 1984), can be regardedas an effective mechanism.

Evolutionary model for the Stagnonesystem

The examined system is inferred toresult from multiple magma pulsesinteracting with different host rocks,thus producing distinct hydrothermalfluids recorded by multiple hydro-frac-turing events and different tourmalinecomposition. The evolution of Stag-none magmatic-hydrothermal systemcan be summarized as follows (Fig. 7):

1 felsic dyke emplacement in thehornfels rocks with boron-rich fluidrelease;

Fig. 7 Conceptual model of the Stagnone magmatic-hydrothermal system (sectionview). Letters indicate A- and B-veins, while different arrows show the sequentialstages of fluid circulation as discussed in the text. Metasomatism occurred early in thishistory, as boron-rich fluids were made available by felsic dyke crystallization.Pervasive infiltration of fluids (modified through the reaction with biotite-rich rocks)at the dyke ⁄hornfels contacts led to local fluid overpressure giving rise to the firsthydro-fracturing event and deposition of black schorl-dravititic tourmalines intoA-veins and early B-veins. Later, new fluid pressure build-up occurred, presumably bythe emplacement of new felsic dykes into an amphibolite-bearing portion of theCalamita unit, and Ca-Ti-rich fluids re-exploited B-vein structures, producingcataclastic textures cemented by brownish tourmaline (dravite-uvite).


.............................................................................................................................................................


2 development of a confined metaso-matic-hydrothermal system withfluid evolution towards Mg-richcomposition;

3 build-up of fluid pressure andhydro-fracturing with A-vein andearly B-vein development;

4 build-up of fluid pressure andhydro-fracturing triggered by Ca-Ti-rich fluids with growth of uvitictourmaline in the late B-veins.

This model emphasizes the transi-tion from confined, pervasive to chan-nelized fluid circulation, reflected bythe tourmaline fabric and composi-tion. Moreover, the model highlightsthe role of fluid circulation and over-pressure associated with, and gener-ated by, the emplacement of multiplesheet-intrusions in the formation ofvein systems.

Conclusions

The hydrothermal system associatedwith the eastern Elba igneous com-plex, affecting an area exceeding20 km2, represents a major hydrother-mal event characterized by boron-richsaline fluids that circulated close totheir magmatic source, as indicated bythe spatial distribution of tourmalineveins in the Stagnone area. Such aconfined circulation of high tempera-ture saline fluids could have tempo-rarily increased the conductivity of thecrustal layer hosting this hydrother-mal system.Similar highly saline hydrothermal

fluids and conductive upper crustcharacterize the contact aureole inthe currently active Larderello-Tra-vale geothermal field (Cathelineauet al., 1994). There, the occurrence ofa fluid phase is testified by a seismicreflector with a �bright-spot�-typestrong amplitude signal at a depth ofaround 4000 m (K-horizon; Batiniet al., 1983) and by the abundantfragments of quartz–tourmaline veinserupted by a geothermal well thatreached this seismic reflector (Gianelliet al., 1997). Such a deep fluid reser-voir could represent a hydrothermalcircuit confined at the top of a recentgranite intrusion (Bertini et al., 2006),not directly connected with the over-lying meteoric-dominated fluid reser-voirs that are currently exploited forgeothermal fluids (Cathelineau et al.,1994; Ruggieri et al., 1999).

Thus the Stagnone metasomatic-hydrothermal system could representthe high temperature stage of hydro-thermal circuits that characterize LateNeogene contact aureoles in the uppercrust of the northernApennines. In thisview, the Stagnone system is a proxy ofthe high temperature system currentlyactive in the deep portion of the Larde-rello-Travale geothermal field. Theunderstanding of these deep-seatedunconventional geothermal reservoirsis a major challenge in the futureexploitation of geothermal energy.

Acknowledgements

This research has been funded by MIUR,PRIN Project no. 2005-040-459 (scientificcoordinator F. Sani). The authors thank S.Philipp and an anonymous referee for thehelpful reviews and A. Gudmundsson andA. Nicolas for editorial handling.

References

Batini, F., Bertini, G., Gianelli, G., Pan-deli, E. and Puxeddu, M., 1983. Deepstructure of the Larderello field: contri-bution from recent geophysical andgeological data. Mem. Soc. Geol. Ital.,25, 219–235.

Bertini, G., Casini, M., Gianelli, G. andPandeli, E., 2006. Geological structure ofa long-living geothermal system, Larde-rello, Italy. Terra Nova, 18, 163–169.

Cathelineau, M., Marignac, C., Boiron,M.C., Gianelli, G. and Puxeddu, M.,1994. Evidence for Li-rich brines andearly magmatic fluid rock interaction inthe Larderello geothermal system. Geo-chim. Cosmochim. Acta, 58, 1083–1099.

Collettini, C., De Paola, N. and Goulty,N.R., 2006. Switches in the minimumcompressive stress direction induced byoverpressure beneath a low-permeabilityfault zone. Terra Nova, 18, 224–231.

Dini, A., Tonarini, S., Leeman, W.P.,Ertan, I.E. and Pezzotta, F., 2001.Geochemical behavior of boron at themagmatic-hydrothermal transition:insights from d11B of tourmaline inperaluminous granites (Elba Island,Italy). In: Water–Rock Interaction.Proceedings of the 10th InternationalSymposium on Water–Rock Interaction(R. Cidu, ed.), pp. 1513–1516. Balk-ema, Lisse, The Netherlands.

Dini, A., Innocenti, F., Rocchi, S., Tona-rini, S. and Westerman, D.S., 2002. Themagmatic evolution of the Late Miocenelaccolith–pluton–dyke granitic complexof Elba Island, Italy. Geol. Mag., 139,257–279.

Dini, A., Gianelli, G., Puxeddu, M. andRuggieri, G., 2005. Origin and evolution

of Pliocene–Pleistocene granites fromthe Larderello geothermal field (TuscanMagmatic Province, Italy). Lithos, 81,1–31.

Gianelli, G. and Ruggieri, G., 2002.Evidence of a contact metamorphicaureole with high-temperature metaso-matism in the deepest part of the activegeothermal field of Larderello, Italy.Geothermics, 31, 443–474.

Gianelli, G., Manzella, A. and Puxeddu,M., 1997. Crustal models of the geo-thermal areas of southern Tuscany(Italy). Tectonophysics, 281, 221–239.

Hawthorne, F.C. and Henry, D.J., 1999.Classification of the minerals of thetourmaline group. Eur. J. Mineral., 11,201–215.

Innocenti, F., Serri, G., Ferrara, G., Ma-netti, P. and Tonarini, S., 1992. Genesisand classification of the rocks of theTuscan Magmatic Province: thirty yearsafter Marinelli�s model. Acta Vulcanol.,2, 247–265.

London, D., 1999. Stability of tourmalinein peraluminous granite systems: theboron cycle from anatexis to hydro-thermal aureoles. Eur. J. Mineral., 11,253–262.

Maineri, C., Benvenuti, M., Costagliola,P., Dini, A., Lattanzi, P., Ruggieri, C.and Villa, I.M., 2003. Sericitic alterationat the La Crocetta mine (Elba Island,Italy): interplay between magmatism,tectonics, and hydrothermal activity.Mineral. Deposita, 38, 67–86.

Nabelek, P.I., Labotka, T.C., O�Neil, J.R.and Papike, J.J., 1984. Contrastingfluid ⁄ rock interaction between the NotchPeak granitic intrusion and argillites andlimestones in western Utah: evidencefrom stable isotopes and phase assem-blages. Contrib. Mineral. Petrol., 86,25–34.

Norton, D. and Knight, J., 1977. Trans-port phenomena in hydrothermal sys-tems: cooling plutons. Am. J. Sci., 277,937–981.

Oliver, N.H.S., 1996. Review and classifi-cation of structural controls on fluidflow during regional metamorphism.J. Metam. Geology, 14, 477–492.

Pertusati, P.C., Raggi, G., Ricci, C.A.,Duranti, S. and Palmeri, R., 1993.Evoluzione post-collisionale dell�Elbacentro-orientale. Mem. Soc. Geol. Ital.,49, 297–312.

Puxeddu, M., Saupe, F., Dechomets, R.,Gianelli, G. and Moine, B., 1984.Geochemistry and stratigraphiccorrelations – application to theinvestigation of geothermal and mineralresources of Tuscany, Italy. Chem. Geol.,43, 77–113.

Rocchi, S., Westerman, D.S., Dini, A.,Innocenti, F. and Tonarini, S., 2002.Two-stage growth of laccoliths at ElbaIsland, Italy. Geology, 30, 983–986.


.............................................................................................................................................................


Ruggieri, G., Cathelineau, M., Boiron,M.C. and Marignac, C., 1999.Boiling and fluid mixing in thechlorite zone of the Larderellogeothermal field. Chem. Geol., 154,237–256.

Shearer, C.K., Papike, J.J., Simon, S.B.,Laul, J.C. and Christian, R.P., 1984.Pegmatite ⁄wallrock interactions, BlackHills, South Dakota: progressive boronmetasomatism adjacent to the Tip Top

pegmatite. Geochim. Cosmochim. Acta,48, 2563–2579.

Sibson, R.H., 2000. Fluid involvement innormal faulting. J Geodynam., 29,469–499.

Tonarini, S., Dini, A., Pezzotta, F. andLeeman, W.P., 1998. Boron isotopiccomposition of zoned (schorl-elbaite)tourmalines, Mt. Capanne Li-Cspegmatites, Elba (Italy). Eur. J. Mineral.,10, 941–951.

Woodford, D.T., Sisson, V.B. andLeeman, W.P., 2001. Boronmetasomatism of the Alta stockcontact aureole, Utah: evidence fromborates, mineral chemistry andgeochemistry. Am. Mineral., 86,513–533.

Received 31 July 2007; revised versionaccepted 22 May 2008


.............................................................................................................................................................


Multiple hydro-fracturing by boron-rich fluids in the Late Miocene contact aureole of eastern Elba...

Documents

Transcript of Multiple hydro-fracturing by boron-rich fluids in the Late Miocene contact aureole of eastern Elba...