Contact metasomatic and hydrothermal minerals in the SH2 deep well, Sabatini volcanic district, Latium, Italy

0375-6505/87 $3.00 + 0.00 Pergamon Journals Ltd. © 1987 CNR.

Geothermics, Vol. 16, No. 2, pp. 127-145, 1987. Printed in Great Britain.

CONTACT METASOMATIC AND HYDROTHERMAL MINERALS IN THE SH2 DEEP WELL, SABATINI VOLCANIC DISTRICT, LATIUM, ITALY G. C A V A R R E T T A

and F. T E C C E

Centro di Studio per la Geologia dell'Italia Centrale del C.N.R. c/o Dipartimento di Scienze della Terra, Universitgt degli Studi di Roma "'La Sapienza'" 1-00185 Roma (Received April 1986, accepted for publication October 1986)

Abstract--Metasomaticand hydrothermal minerals were logged throughout the SH2 geothermal well, which reached a depth of 2498 m in the Sabatini volcanic district. Below 460 m of volcanics, where the newly formed minerals were mainly chlorite, calcite and zeolites (mostly phillipsite), drilling entered the Allochthonous Flysch Complex. Evidence of the "Cicerchina facies" was found down to 1600 m depth. Starting from 1070 m, down to hole bottom, a contact metasomatic complex was defined by the appearance of garnet. Garnet together with K-feldspar, vesuvianite, wilkeite, cuspidine, harkerite, wollastonite and apatite prevail in the top part of the contact metasomatic complex. Vesuvianite and phlogopite characterize the middle part. Phlogopite, pyroxene, spinel and cancrinite predominate in the bottom part. The 1500 m thick metasomatic complex indicates the presence at depth of the intrusion of a trachytic magma which released hot fluids involved in metasomatic mineral-forming reactions. Minerals such as harkerite, wilkeite, cuspidine, cancrinite, vesuvianite and phlogopite indicate the intrusive melt had a high volatile content which is in agreement with the very high explosivity index of this volcanic district. The system is at present sealed by abundant calcite and anhydrite. It is proposed that most, if not all, of the sulphates formed after reaction of SO2 with aqueous calcium species rather than from sulphates being remobilized from evaporitic (Triassic) rocks as previously inferred. The hypothesis of a CO2-rich deep-derived fluid ascending through major fracture systems and contrasting cooling in the hottest areas of Latium is presented. INTRODUCTION T h e S a b a t i n i volcanic district, l o c a t e d in n o r t h e r n L a t i u m , is p a r t o f the p e r p o t a s s i c R o m a n C o m a g m a t i c R e g i o n ( W a s h i n g t o n , 1906) b o r d e r i n g the e a s t e r n T y r r h e n i a n sea basin. B r o a d extensional tectonic activity, starting in U p p e r M i o c e n e a n d reaching its m a x i m u m in L o w e r Pliocene, f o r m e d h o r s t - g r a b e n structures o r i e n t e d N N W - S S E in the coastal belt f r o m T u s c a n y to C a m p a n i a . W h i l e the d e b a t e a b o u t the g e o d y n a m i c i n t e r p r e t a t i o n of the R e g i o n is still active, most current literature defines this tectonic style as post-collisional a n d the associated m a g m a t i s m as s u b d u c t i o n - r e l a t e d r a t h e r t h a n i n t r a p l a t e rift-related (Di G i r o l a m o , 1978; Peccerillo et al., 1984; Rogers et al., 1985; a n d therein cited literature). T h e s a m e c o a s t a l belt is c h a r a c t e r i z e d by a strong positive t h e r m a l a n o m a l y which is of e c o n o m i c interest for g e o t h e r m a l energy. Besides the widely k n o w n g e o t h e r m a l field o f L a r d e r e l l o - T r a v a l e (Tuscany), o t h e r fields are currently being exploited a n d / o r explored by the Joint Venture A G I P - E N E L , such as Mt. A m i a t a , L a t e r a , C e s a n o , A l b a n Hills and P h l e g r a e a n Fields on m a i n l a n d Italy, plus the A e o l i a n Islands (from n o r t h to south). W i t h i n the S a b a t i n i volcanic district (Fig. l a ) , the a b o v e m e n t i o n e d J o i n t Venture ( E N E L as o p e r a t o r ) has c a r r i e d out a drilling p r o j e c t starting in the C e s a n o - B a c c a n o caldera, which is c h a r a c t e r i z e d b y the highest g e o t h e r m a l g r a d i e n t o f the area. Since 1975 14 deep wells have been drilled, f o u r o f which are p r o d u c t i v e . T h e recovered fluid is a s u p e r s a t u r a t e d chlorides u l p h a t e b r i n e at t e m p e r a t u r e near 200°C a n d T . D . S . ( T o t a l Dissolved Salts) up to 400 g/1 ( C a l a m a i et al., 1975; F u n i c i e l l o et al., 1979). T o search for h o t fluids with lower T . D . S . values, deep drilling has also been e x t e n d e d well o u t s i d e the C e s a n o - B a c c a n o caldera, b u t with little success (see Fig. l a for l o c a t i o n o f S H 2 a n d C e s a n o wells). 127

128

G. Cavarretta and F. Tecce

Study of the newly-formed minerals found in the well samples may provide useful information for an attempt to reconstruct the evolution of the local geothermal system and obtain a better knowledge of the structure and evolution of the Sabatini volcanic district. G E O L O G I C A L BACKGROUND Volcanological setting The Sabatini volcanic district is characterized by the presence of several (more than 20) eruptive centers; the majors are apparently oriented approximately E-W and a few minor ones show an approximate Apenninic (NW-SE) orientation (Fig. la). The products of this volcanic district are widespread, from the Monti della Totfa (to the west) to the Valle del Tevere (to the east), covering a total 50 km across. Activity began 0.6 M yr ago in the Morlupo center with pyroclastic flows; the youngest dated products are 0.083 M yr old (Baccano pyroclastic flow); stratigraphic evidence indicates that the latest products were erupted from Martignano and nearby centers (Di Filippo et al., 1984). The main eruptive center was Sacrofano, a true strato-volcano whose structure and evolution have been described by De Rita et al. (1983); important centers were also Morlupo, Baccano and Martignano. Widespread cinder and scoria cones, late-stage craters .f .~.f"

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Fig. lb. Tentative cross-section along the Bracciano lake and the SH2 drilling site, as a best fitting of residual gravity anomaly and stratigraphic data (courtesy of M. Di Filippo). (1) Volcanic cover; (2) Allochthonous Flysch Complex; (3) Carbonate Basal Complex; (4) Contact metasomatic complex; (5) Trachytic intrusion; (6) Top of crystalline basement; (7) Residual gravity anomaly.

130

G. Cavarretta and F. Tecce

of phreatomagmatic origin, the majority of which formed after a single event, give the district a characteristic "crater field" morphology. Such a morphology induced Locardi and Mittempergher (1967) to hypothesize the existence of a rather flat, elongated magma chamber. The petrography of the products erupted within the Sabatini volcanic district varied from trachytes to leucitites (Cundari, 1979). Lava flows (mainly tephrites) are less abundant than volcanoclastic products of various type and particularly localized in the surroundings of the Bracciano lake depression, especially on its northern side. The volcanic units outcropping in the area of the SH2 drilling site (as mapped in Di Filippo et al., 1984) are described as follows: (a) "pyroclastic products and lavas of the central sector from local centers"; (b) "pyroclastic units from non distinguished centers"; (c) "leucitic tephrite laval flows. In the northern sector phonolitic lavas are also present". Stratigraphy o f sedimentary units A comprehensive reconstruction of the stratigraphy of the sedimentary units underlying the volcanic cover of the northern Roman Comagmatic Region is reported in Baldi et al. (1974). Funiciello and Parotto (1978) used more recent deep drilling data and examined numerous sedimentary ejecta contained in the volcanoclastic units to improve the detail and extend the reconstruction to southern Latium. The lithology of the Triassic carbonate units deserves some special comments: Mariotti (1980) reported marked facies variations in Upper Triassic terrains underlying the Cesano-Baccano area; in a range of a few kilometers, shelf-lagoon evaporitic deposits (to the east) and open-sea deposits (to the west) were interpreted in the light of compressive tectonics. The presence of thin layers of anhydrite, which were detected only in the eastern Cesano geothermal wells, played a major role in such an interpretation. No evidence for a thick evaporitic sequence is reported. The lithology is represented by black marly limestones which are in places highly dolomitized. In summary, the sedimentary basement of the area can be described as follows: - - a n Allochthonous Flysch Complex (Upper-Cretaceous to Oligocene) recognizable as "Flysch della Tolfa". Thickness is up to 3000 m; lithology varies from alternating calcarenites, marly limestones and clays to alternating calcarenites and quartz-feldspathic sandstones. - - a Carbonate Basal Complex (with the oldest units dated Upper-Trias) showing sedimentological characters intermediate between those of the "Tuscan series" and the "Umbrian series". The units referred to this complex are: (a) "Scaglia", marls or marly limestones, UpperCretaceous to Oligocene in age; (b) " M a r n e a Fucoidi"; (c) "Maiolica"; (d) "Calcari with Filaments": the last three units are limestones, marls or marly limestones and their age goes from Upper Lias to Lower Cretaceous; (e) " C o r n i o l a " , a cherty limestone dated Middle Lias; (f) "Calcare Massiccio", a detrital limestone of Lower Lias; (g) "Calcari a Raethavicula", a marly limestone with alternating dolomitic levels dated Upper Trias. Deep structure Geophysical investigations carried out in the region primarily for geothermal exploration indicate horst-graben deep structures dominate, superimposed upon by sub-circular structures of volcanic origin (Toro, 1978). The area covered by the Bracciano lake is itself a problem: even though contoured by abundant lava and pyroclastic flows, it is described in the literature (Locardi and Sommavilla, 1974; Baldi et al., 1976) as being only a collapsed area; no evidence for it being a (or the) main eruptive center is at present available. A recent gravimetric and magnetometric survey carried out in the area (Di Filippo et al., 1982, 1983), supports this interpretation. The marked positive gravimetric anomaly present in the SE sector of the lake has been interpreted as a structural " h i g h " of the carbonate basement as present in the Cesano area and thus aligned with Apenninic orientation;

Sabatini Volcanic District, Latium, Italy

131

the NE sector, in contrast shows an elongate negative anomaly with anti-Apenninic orientation. The light positive magnetometric anomalies detected around the lake perimeter have been interpreted by the same authors as related to near-surface lavas or to dykes uprising along fractures. A tentative N-S cross section within the Sabatini volcanic district is shown in Fig. lb (courtesy of M. Di Filippo). It was calculated fitting the residual gravity anomaly profile with deep stratigraphy and density data for the SH2 well, as reported by Calamai et al. (1983) and this paper. ANALYTICAL METHODS Polished thin sections of four drill cores and cuttings from every 10 m have been examined under the polarizing microscope to compile a litho-stratigraphic and newly-formed minerals log and to check textural relationships. Relative abundances of minerals were only estimated under the microscope. Ambiguities in mineral recognition have been resolved by using both the X-ray diffractometer and the electron microprobe (Cambridge Geoscan and Stereoscan equipped with ORTEC ED S i - L i detector). Quantitative analyses of minerals have been obtained with the WDS probe and corrected by using Micro-ELEXA, a standard ZAF procedure coded by the first author for an on-line microcomputer. Precision is about 2o7o for major elements and 10O7o for minor ones. The structural state of alkali-feldspar has been determined from refined cell dimensions by using X-ray powder diffraction data. The least-squares program by Appleman and Evans (1973) and standard cell parameters for orthoclase and high-sanidine as reported by Wright and Stewart (1968) have been used. Thermodynamic data for minerals, gases and aqueous species have been computed at the temperatures and pressure of interest by means of the SUPCRT program and data file as published by Helgeson et al. (1978) and subsequent updates. Data for SO2(g) are from Robie et al. (1978). STRATIGRAPHY AND NEWLY-FORMED MINERAL DISTRIBUTION The SH2 deep well was drilled approximately 2 km north of Bracciano Lake (localith Vicarello, ground surface above sea level 325 m) reaching a bottom hole depth of 2498.7 m (below surface level) and a temperature of 290°C with no evidence of the presence of exploitable fluids (Calamai et al., 1983). As these authors report, well SH2 showed the existence of a thick (from - 1140 to bottom hole) contact metasomatic and hydrothermal altered complex intruded by (mainly) trachytic and (occasionally) leucitic dykes. The intrusion of a trachytic magma, and related hot fluids, into the sedimentary units underlying the volcanics in "localith Vicarello", resulted in extensive metamorphic-metasomatic phenomena, the records of which are preserved almost throughout well SH2. Different metamorphic stages or events correspond to different mineral assemblages; relics of previous ones may survive reequilibration so that the entire range of newly-formed minerals detected in cuttings and core samples may represent a significant part of the recent history of these units. The observed mineral assemblages add little that is new to the knowledge of the mineralogy of the Roman Comagmatic Region beyond the study of the ejecta contained in volcanoclastic products (Barbieri et al., 1977; and therein cited literature). The new evidence from well SH2 includes the depths of such assemblages and the stratigraphy of the well; for many, this is their first reported "on site sampling". The SH2 litho-stratigraphic log is illustrated in Fig. 2. The well first penetrated about 460 m of volcanic products, then entered rocks recognizable as the Allochthonous Flysch Complex

132

G. Cavarretta and F. Tecce NEWLY FORMED MINERALS

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Fig. 2. Lithostratigraphic log of well SH2: (2) volcanic cover Is m a d e up of lava flows (1), pyroclastic flows (2), volcanoclastic breccias (3); (b) allochthonous flysch complex consisting of alternating calcarenites, marly limestones and clays to alternating calcarenites and quartz-feldspathic s a n d s t o n e s (4); (c) contact m e t a s o m a t i c complex intruded by (mainly) trachytic d y k e s (5). Newly-formed mineral distributions and their e s t i m a t e d relative a b u n d a n c e s ((6), (7), (8) in the legend c o r r e s p o n d i n g to scarce, m e d i u m and high mineral a b u n d a n c e respectively), in-hole m e a s u r e d t e m p e r a t u r e s and core sampling levels (open circles) are also shown. CC = calcite, A N = anhydrite, B C = barite-celestite s.s., P Y = pyrite, H M = h a e m a t i t e , Q Z = quartz, Z E = zeolites, KF = K-feldspar, M U = K-mica/sericite, R E = reyerite, C L = chlorite, G R = garnet, V E = vesuvianite, P H = phlogopite, BI = biotite, CP = clinopyroxene, W O = wollastonite, SP = spinel, C A = cancrinite, FL = fluorite, A P = apatite, W I = wilkeite, C U = cuspidine, H A = harkeritc.

Sabatini Volcanic District, Latium, Italy

133

down to a depth of about 1600 m. Below this depth the products of extensive contact metasomatic phenomena inhibit any attempt at defining original lithology. As shown, what can be called the "contact metasomatic complex" extends from 1070 m to bottom hole and is itself affected by several dyke intrusions. As far as lithological a n d / o r newly-formed mineral association variations are concerned, the most significant intervals can be summarized as follows (depth values are below ground surface level): - - d o w n to 290 m the volcanic cover consists of lava flows (phonolites, leucitites and leucitic tephrites) and subordinate pyroclastic flows; starting below 170 m, chlorite appears as replacement and calcite as vug filling in pyroclastics; - - f r o m 290 to 460 m the well encountered volcanoclastic breccias. These show abundant calcite, disseminated and as substitution of primary minerals; subordinate chlorite and zeolite (mostly phillipsite) are present, mainly as glass alteration; - - f r o m 470 m downward, the well entered the limestones, marls and sandstones of the Allochthonous Flysch Complex. Down to 860 m the newly-formed minerals are represented only by abundant calcite and minor quartz, K-feldspar and zeolite. A leucitic dyke was detected at 760 m depth; - - a t 870 m anhydrite appears and, together with calcite, remains one of the most abundant newlyformed mineral throughout the well. Pyrite and K-feldspar are common; as an expected effect of the host rock control, quartz also appears as newly-formed crystals; - - a t 1070 m the sudden appearance of abundant garnet marks the top limit of the "contact metasomatic complex"; here calcite, anhydrite and pyrite occur in paragenetic relationship, commonly constituting completely mineralized cuttings; - - a t 1140 m the country rock is almost completely replaced by the last mentioned mineral assemblage. Abundant relics of acicular crystals occur, identified as wilkeite from better preserved samples from greater depth. At 1190 m a rare lime-silicate, cuspidine, appears and occurs continuously downward for about 360 m; barite is also associated. Garnet is present also as a vein mineral; - - a t 1240 m vesuvianite first appears and is found almost continuously down to 2210 m; several trachytic dykes are present between 1300- 1400 m, in which K-feldspar is the only primary mineral left after the extensive transformation to sericite, chlorite, calcite and subordinate anhydrite and pyrite; sericite is present as alteration in almost all the dykes throughout the well; - - f r o m 1450 to 1560 m the lithology of the country rock is recognizable as a sandstone or microconglomerate containing coarse grains of quartzite ("Cicerchina facies"; Losacco, 1963). Harkerite appears as relatively big (up to 5 mm across), diamond-shaped crystals altered by carbonates; it is abundant for approximately 350 m and around 1700 m many chips are almost monomineralic. From 1550 m phlogopite and biotite (rare) add to the recurrent calcite-anhydritevesuvianite-garnet association; phlogopite becomes increasingly abundant with depth down to well bottom. Authigenic K-feldspar becomes rare; --below about 1700 m depth, grossularitic garnet decreases significantly in abundance; trachytic dykes occur in this interval approximately every 100 m but the country rock is not now recognizable being replaced by newly-formed calcite, anhydrite and silicates; - - f r o m 2200 m down to well bottom vesuvianite is absent; clinopyroxene (fassaitic augite) and melanitic garnet become increasingly more abundant, especially in the last 100 m where spinel and cancrinite are added. Calcite and anhydrite decrease in the last 200 m. Sericitized trachytic dykes are also very abundant in this interval. The size of igneous K-feldspar crystals increases significantly with depth (see Fig. 3). In summary the newly-formed mineral distribution along well SH2 can be described as follows: (i) calcite, anhydrite, pyrite, sericite are almost ubiquitous; (ii) a near-surface chlorite _+ phillipsite domain occurs in the volcanoclastic units;

134

G. Cavarretta and F. Tecce 500-

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(iii) a K-feldspar _ garnet (grandite) + vesuvianite ± wilkeite ± cuspidine ± harkerite + wollastonite _ apatite domain is present in the top part of the "contact metasomatic complex"; (iv) a vesuvianite ± phlogopite domain occurs in the middle part of the same complex; (v) a phlogopite + pyroxene ± spinel + cancrinite mineral assemblage is present in the bottom part. MINERAL CHEMISTRY AND TEXTURAL EVIDENCE Spinel Spinel has been found in the deepest horizons of the well. In thin section it appears as green, anhedral crystals. Electron probe analyses show it is characterized by an X-hercynite of about 0.3 (pleonaste, Table 1). It is associated with calcite, phlogopite, andraditic garnet, cancrinite, fassaite, but with no evidence for equilibrium. Spinel occurs in ejecta of volcanoclastic units of the Sabatini district (Stoppani and Curti, 1982); here its occurrence in geodes in association with fassaitic pyroxene confirms its metasomatic origin. Cancrinite The minerals of the cancrinite group ((Na,Ca,K)6_8[AI6Si6024] (CO3, SO4, C1)I_z"1-5H20), are widely represented in the Latium volcanic complexes. In the anhydrite-rich SH2 environment there is a sulphatic cancrinite (microsommite). It is observed in equilibrium with carbonate-apatite and associated with phlogopite, fassaite, spinel, andradite in close similarity with previously reported findings in ejecta (Leoni et al., 1979; Burragato et al., 1980). Cuspidine Cuspidine (Ca4Si207. ((OH,F)2) is a calcium silicate commonly found in Latium, in ejecta in volcanoclastic deposits. In well SH2 it constitutes monomineralic cuttings or else is in paragenetic relationship with wilkeite, equally abundant, and subordinate pyrite. Crystals are spear-shaped

Sabatini Volcanic District, Latium, Italy

135

Table 1. Representative electron microprobe analyses of spinel, cuspidine, wilkeite, sericite, reyerite and cancrinite from different levels of well SH2 (averages of spots on single crystals). Total iron as FeO or Fe203. Atomic proportions calculated on the basis of 4, 9, 26, 22, 62.5 oxygen atoms respectively; cancrinite atomic proportions on the basis of Si + A1 = 12. Spinel 2450

Cuspidine 1300

Wilkeite 1300

Sericite 2250

Reyerite

Cancrinite

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2495

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9.912 0.332 1.911

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0.101 0.0 0.030

0.023 0.0 0.006

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1.988 4.568 1.356 1.797 0.012 0.086

(Fig. 4b); sector extinction is frequent. Calcite pseudomorphs on cuspidine are quite common. Alteration by fan-shaped reyerite or partial substitution by microcrystalline grossular have also been observed. Electron probe analyses indicate that the formula is perfectly stoichiometric with the anion sites, in the sub-sorosilicate structure, totally filled by fluorine (see Table 1). Cuspidine occurs typically in limestones metamorphosed under pneumatolytic conditions; it has been hydrothermally synthesized from crystallizing melts at temperature above 500°C and by solid state reactions at 1000°C (Van Valkenburg and Rynders, 1958).

136

G. Cavarretta and F. Tecce

Fig. 4. (a) Cuspidine relics (spear-shaped, substituted by calcite), grossularitic garnet and anhydrite in a chip from 1380 m depth; (b) relics of zoned harkerite crystals in a calcitized chip from 1700 m depth; (c) late-stage vein of anydrite and phlogopite from 2168 m depth; (d) vesuvianite crystals bordering a vein filled by spathic calcite (1493 m depth): (e) anhydrite and phlogopite (most abundant) in a vein cross-cutting a sample almost completely transformed into fassaitic augite, from 2498 m depth. All pictures taken with parallel polars.

Sabatini Volcanic District, Latium, Italy

137

Apatite group: wilkeite, carbonate-apatite Wilkeite is a calcium silicate sulphate apatite occurring in well SH2 for a relatively long interval near dykes. Besides the last mentioned paragenesis, it occurs as euhedral or subhedral elongated crystals in association with calcitized relics of harkerite and very subordinate apatite. Wilkeite is frequently replaced by calcite (often completely). Electron probe analyses (Table 1) show that the monovalent anion site is completely filled by fluorine, a condition already seen for cuspidine. The low total oxide wt.% and the values of atomic proportions suggest the possible existence of CO3 substituting for PO,. Mineral species isotypic to apatite (formed under hydrothermal conditions) have been already reported from the Sabatini volcanic district (Cavarretta et al., 1981). Lath-shaped crystals recognizable under the microscope as apatite are present in almost all the "contact metasomatic complex". Crystal size increases with depth from 0.05 to 0.75 mm; microprobe analyses on the bottom hole core sample indicate a carbonate-apatite end-member composition with some SiO2 replacing PO,. Harkerite Harkerite is a rare calcium-magnesium borosilicate-carbonate first described by Tilley (1951). Its occurrence in Latium has been reported by Barbieri et al. (1977) in ejecta of the Alban Hills area in which it is associated with cuspidine (mostly) and grossular, phlogopite, vesuvianite, biotite plus other minor phases. In well SH2 it occurs close to the dykes as very abundant octahedral crystals showing a marked oscillatory zoning (Fig. 4b). Extensive modifications of the early assemblages do not allow proper determination of its original paragenesis: the little available evidence suggests harkerite was in equilibrium with wilkeite, cuspidine and calcite. Its abundance must have been more conspicuous during the early (hottest) stages of pneumatolytic metamorphism; advanced substitution by calcite (mostly) and anhydrite (subordinately) left numerous relics which have harkerite morphology. Crystal cores are usually isotropic or almost completely calcitized while rims are better preserved and exhibit a weak birefringence. An incomplete (missing boron) electron probe analysis on a typically zoned crystal' showed that the outer parts correspond to a higher Si content in the mineral formula with a value typical for harkerite. Crystal core contains only about 4.5% SiO~ which is a figure approaching the sakhaite/harkerite limit (2%). Harkerite is not the only borosilicate found in the Sabatini district. Incompatible element bearing minerals such as a thorium-rich hellandite ((Ca,RE,Th,U),~(A1,Fe,Ti)~(OH)4SisBsO40 (OH)4) have been recently reported by Della Ventura et al. (1985) in a "sanidinitic" ejecta. Phlogopite Phlogopite, together with vesuvianite, is among the most characteristic newly-formed minerals of well SH2. It takes the place of K-feldspar as K-bearing phase from about 1600 m downward. Being very abundant in the last 900 m of the well, it occurs in a variety of assemblages; the vein association of phlogopite together with calcite and anhydrite at 2168 and 2498 m depth indicates also a noteworthy late-stage (hydrothermal) occurrence (Fig. 4c and e). The electron probe analyses show a low to moderate intra and inter-sample variability of the Fe/Fe + Mg ratio. Typical analyses are reported in Table 2. Clinopyroxene Scattered occurrences of clinopyroxene are from the central part of the well downwards; close to well bottom it becomes very abundant. Table 2 shows a chemical variability ranging from near augitic to fassaitic composition. Typical assemblages include phlogopite, spinel, garnet, cancrinite. At 1530 m a fassaitic form was found in association with garnet (Gro75) and cuspidine. Occurrence of such mineral assemblages within the Sabatini volcanic district has been reported by Barrese et al. (1983) in ejecta related to the Sacrofano center: the authors proposed

138

G. Cavarretta and F. Tecce

Table 2. Representative electron microprobe analyses of phlogopite, clinopyroxene and vesuvianite from well SH2. Total iron as FeO. Atomic proportions on the basis of 22 oxygen f~toms, 4 and 25 cations respectively, Fe2+/Fe ~+ ratio for pyroxenes according to Vieten and H a mm (1978) Phlogopite

Pyroxene

Depth (m)

2250

2498

2450

2498

SiO,~ AI20~ TIP., Fe~O~ FeO MnO MgO CaP Na_,O K_,O F Sum O =- F Total

40.0 15.0 0.4

36.9 17.7 0.4

1.6 0.07 26.1 0.02 0.05 10.5 2.3 96.04 0.97 95.07

3.3 0.1 24.5 0.01 0.1 10.4 1.6 95.01 0.67 94.34

43.7 11.8 1/.7 5.5 1.0 0.0 11.0 25.2 0.0 0.0

46.1 5.1 1.3 8.6 2.7 0.5 11.4 23.6 0.4 0.0

98.9

99.7

Vesuvianite 1550 35.1 13.3 0.6 4.6 0.1 5.5 33.8 0.0 0.0 0.2 93.2 0.1 93.1

Atomic proportions Stoichiom. Si A1w A1vt Ti Fe ~+ Fe z+ Mn Mg Ca Na K F

22 O

22 O

5.576 2.424 0.042 0.037

5.261 2.739 0.235 0.043

0.187 0.008 5.422 0.003 0.014 1.867 1.014

0.393 0.012 5.205 0.001 0.028 1.891 0.721

4 cat. 1.641 0.359 0.163 0.020 0.156 0.031 0.0 0.616 1.014 0.0 0.0

4 cat. 1.751 0.228 0.0 0.037 0.246 0.086 0.016 0.645 0.961 0.029 0.0

25 cat. 8.785 0.215 3.709 0. I 13 0.963 0.021 2.051 9.064 0.0 0.0 0.158

a "skarn-like" genetic environment with a moderate thermal range ( 5 0 0 - 650°C) and low pressure (1 kbar). Garnet

Granditic garnet (grossular-andradite solid solution) is a common newly-formed mineral especially in the top part of the "contact metasomatic complex". Two generations of grandite are clearly distinguishable: the earlier is characterized by small disseminated grains with a higher grossular mole percent and is clearly related to the proper contact metasomatic stage. The later produced coarser crystals formed along fractures in the host rock and are related to a "high" hydrothermal stage. Although grossular mole contents of both types are similar (on average), the hydrothermal garnet shows a wider intra-crystal compositional variation with the grossular mole fraction significantly increasing towards the crystal rim. A very similar pattern was observed in garnets from the Latera geothermal field in the Vulsini volcanic district (Cavarretta et al., 1985). The SH2 vein association includes anhydrite, calcite and vesuvianite. An individual drill chip might contain the two garnets, evidencing the superposition of different events. A slight trend of the andradite end member increasing with depth is shown in Table 3; the garnet detected close to well bottom has the highest Ti and Fe content, resulting in a dark-brown colour in thin section (melanite variety). Melanitic garnet is also very abundant in ejecta from all the Latium volcanic districts. The above referenced pattern of compositional variation may be interpreted in terms of a fluid uprising through fractures, characterized by a high ferric iron content. After fluid pressure and Fe supply decrease, the pattern follows a Rayleigh fractionation model.

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139

Table 3. Garnet compositional range in three levels of well SH2:1380 refers to disseminated crystals in metamorphosed limestones, 1530 to vein crystals, 2450 to the so-called "sanidinitic" mineral association. Total iron as Fe203. Atomic proportions on the basis of 12 oxygen atoms

Depth (m) SiO2 A1203 TiO2 Fe203 MnO MgO CaO Sum

1380

1380

38.0 17.6

39.0 17.4

7.2 0.5 0.4 35.5 99.2

7.8 0.7 0.3 35.6 100.8

Garnet 1530 38.3 16.0 1.0 8.8 0.1 0.6 35.5 100.3

1530 38.6 15.7 0.7 9.7 0.2 0.4 35.6 100.9

2450

2450

37.9 14.5 1.1 10.0

36.8 12.3 2.3 12.4

0.3 36.0 99.9

0.4 35.4 99.6

Atomic proportions Stoichiometry based on 12 O Si A1w AI vl Ti Fe Mn Mg Ca

2.955 0.045 1.568

2.986 0.014 1.556

0.421 0.033 0.046 2.958

0.449 0.045 0.034 2.920

2.959 0.041 1.416 0.058 0.512 0.007 0.069 2.938

2.972 0.028 1.397 0.041 0.562 0.013 0.046 2.937

2.965 0.035 1.302 0.065 0.589

2.918 0.082 1.068 0.137 0.740

0.035 3.017

0.047 3.008

1.1

1.7

64.2 29.4 3.1

51.4 37.0 7.0

End members % Pyr Spe Gro And Sch

1.4 1.0 76.2 21.0

1.3 1.4 74.7 22.6

2.4 0.3 67.9 25.6 3.0

1.5 0.4 67.0 28.2 2.1

Vesuvianite

Vesuvianite is widespread and quite abundant in the mid part of the "contact metasomatic complex"; it occurs as deposits on fracture walls, with relatively large (up to 5.9 mm) euhedral or subhedral crystals containing apatite inclusions. As reported for other early newly-formed minerals, vesuvianite also shows substitutions by microcrystalline calcite. The pulsating character of the growth environment is evidenced by layered sorted crystals, oscillatory zoning and growth rims. The mineral assemblage includes grandite, which is often interlayered with vesuvianite, plus apatite, forming also individual crystals, and sulphides. Late-stage anhydrite and spathic calcite seal intercrystal voids and the central part of the veins (Fig. 4d). The probe analysis reported in Table 2 shows that its composition fits the formula proposed by Rucklidge and Kochman (1973) and that it is similar in composition to those reported by Cavarretta et al. (1985) for the Latera geothermal field. Probe analyses evidenced that the oscillatory zoning, easily detectable under the microscope, is actually very weak as far as chemical data are concerned. According to Valley et al. (1985) its presence indicates H20-rich fluids (Xco2 < 0.2) and should not be stable over 600°C.

K-feldspar Non-igneous K-feldspar is relatively less abundant than expected in the considered sequence. It occurs mainly as minute vein crystals or as patchy impregnations in the top half of the "contact metasomatic complex".

G. Cavarretta and F. Tecce

140

Igneous K-feldspar, as already mentioned, is almost the only primary mineral left after the intense metasomatism and hydrothermal alteration. X-ray diffraction data of samples from four levels ranging from 1300 to 2480 m depth have been refined by a least square routine. According to Wright and Stewart (1968) plotting b vs c unit cell parameters, the shallowest samples fall closer to the high-sanidinc area, while the deepest samples plot closer to the orthoclase area (Fig. 5). It is therefore evident that st continuous ordering increase occurred with depth.

725

2480 -4- i

720

Orth

2310

+ 1600 +

1300 •

o,~

Hi-San

OI

7.15

i

12 95

13.00

i

b /~,

13.05

Fig. 5. Igneous K-feldspar " b " vs " c " unit cell parameters for four different depth levels (m) along well SH2. Bars represent standard errors. High-sanidine and orthoclase values according to Wright and Stewart (1968). The ordering increase with depth is shown.

K-mica (sericite) All the trachytic dyke intrusions are deeply altered into K-mica (sericite), which coexists with subordinate chlorite, pyrite and calcite. A typical analysis is given in Table 1. Reyerite Reyerite, (Na,K)aCa14(Si,A1)2406o(OH)~'5H20, is a sheet silicate that occurs as radiating fibrous aggregates in vein or vug fillings. It was found in contact with cuspidine from which it could have formed at relatively low temperatures during late stage alteration. A typical analysis is reported in Table 1.

DISCUSSION Well SH2 showed the existence of a considerable contact metasomatic complex within the Sabatini volcanic district: previous evidence of this feature in Latium comes from the Latera geothermal wells (Vulsini volcanic district) as described by Cavarretta et al. (1985). The metasomatism was there related to the intrusion of a syenitic body which was discussed in detail by Durazzo et al. (1982) and by Barberi et al. (1984). It is certainly difficult to give an exact evaluation of the volume or areal extension of such sin intrusion. If we take the appearance of garnet as the marker for the beginning of the contact

Sabatini Volcanic District, Latium, Italy

141

metasomatic complex, the thickness of this lithotype cross by well SH2 is greater than any other in any Latera well. Comparison with the thickness of the contact-metasomatic rocks crossed by the Latera wells would then indicate the presence of a "comparatively large" (Barberi et al., 1984) igneous body. The extent of metasomatism or the amount of hot volatiles supplied to the carbonate rocks considered indicates that either the cooling intrusion had a relatively high volatile content or its volume was at least comparable with that evidenced at Latera. As a matter of fact, volatile-rich newly-formed minerals are very abundant in well SH2 but are less so in the Latera wells. Although high temperatures were locally reached at the dyke-host rock interface, the overall stratigraphy of newly formed minerals did not significantly change. The in-out sequence (in order of increasing depth) of garnet, vesuvianite, phlogopite, fassaitic augite, spinel as pictured in Fig. 2, indicates a thermal gradient which cannot be ascribed only to dyke intrusions but requires a major heat source to be present at depth. This hypothesis is also supported by the size of igneous K-feldspar crystals which significantly increases with depth as already shown: no phenocrystsgroundmass distinction could be made on the deepest samples; moreover the structural state of the same shows a significant ordering with depth. The remarkable abundance of volatile-bearing minerals, such as those found within the 1200-1800 m interval, indicates that the mineralizing fluid was enriched in F, B, P, S species (as well as H20 and CO2), i.e. the saturation levels of such species with respect to apatite and uncommon minerals such as wilkeite, harkerite and cuspidine were, at least, reached. A fluid of this type was probably released by a melt undergoing fractional crystallization. The Roman Comagmatic Region undersaturated perpotassic products are known for their high content of incompatible elements (i.e. hygromagmatophile: whose concentration in the liquids increases continuously with fractionation). These have been interpreted as contaminatedmantle derived. The primitive contaminants were subducted terrigenous sediments, possibly carbonate-rich (Beccaluva et al., 1985). The perpotassic melts then inherited a high volatile content: their explosivity index is in fact very high. They also show a relatively high oxygen fugacity as evidenced also by Rogers et al. (1985), who suggest that the "unusually high Fe3+/Fe2+ ratios" of fresh Italian lavas represent a "feature of the primary magma and once the source". Among volatiles, COs must have played a major role in the partial melting process which generated K-rich, undersaturated melts (Wendlandt and Eggler, 1980) and always characterized such melts. An open-system behaviour for perpotassic melts generation processes has been proposed by Capaldi et al. (1982) who studied radioactive disequilibria in Vesuvian products. According to these authors, a supercritical mantle-derived fluid recently "contaminated" Vesuvian materials. Such a fluid was presumed to be "mainly composed of H20 and/or CO2". It can be deduced that, at low pressure, a significant igneous CO2 component was added to the metamorphic-produced CO2 in the metasomatizing fluid. As no evidence supports an alternative hypothesis, it is likely that this fluid first interacted with the deepest units of the "Carbonate Basal Complex" which, according to Mariotti (1980), could have been Triassic marly-limestones and dolomites. This interaction resulted in the intense M g - C a - K metasomatism which characterizes the deepest 1500 meters of the well. The several mineral forming reactions can be related, as a function of space and time (the distance from the contact of the intrusion and the cooling period respectively), to three stages: (i) pneumatolytic or metasomatic; (ii) high hydrothermal; (iii) low hydrothermal. Relying on textural and stratigraphic evidence, the group of minerals which are believed to pertain (not exclusively in some cases) to the first stage includes spinel, cancrinite, clinopyroxene, biotite, harkerite, cuspidine, wilkeite, apatite, phlogopite and garnet. The second stage includes vesuvianite, garnet and phlogopite. The last stage produced K-feldspar, sericite, chlorite, and reyerite; phlogopite also formed in the deepest, hottest horizons during this stage. Calcite, anhydrite (plus barite-celestite s.s.) and pyrite, although almost

142

G. Cavarretta and k\ Tecce

ubiquitous, are typical of the second and third stage. The deposition of calcite and anhydritc is dominant over other minerals in the last stage. This observation is supported by the widespread occurrence of hot, CO2-rich springs in the area; by the high CO2 pressure (over 20 bar) measured in the Cesano wells and by the lack, or very rare occurrence, of void-filling calcite and anhydrite in ejecta containing the same calc-silicate mineral assemblages as discussed abovc. This last stage is then virtually active at present and kills any attempt of tectonics to improve permeability by promptly sealing fractures. A pan-sedimentary (Triassic) origin for sulphates in the Sabatini brines (Calamai et al., 1975; Cortecci et al., 1980) seems to be contrasted by stratigraphic evidence as previously mentioned. The magmatic system then counts as the major (if not only) source of sulphur species among which SO., should predominate during the hydrothermal stages. The reaction: 4SO2(g) + 4H20(1) = H2S(g) + 6H + + 3SO~ 2 under hydrothermal conditions, i.e. below 4 0 0 - 350°C, shifts to the right side and aqueous species such as HSOs, KSO4 and NaSO~ tend to become the dominating oxidized sulphur species with decreasing temperatures. If the interaction of SO-, with carbonate rocks is considered by the reaction: 4SO2(g) + 3CACO3 + H20 = 3CaSO4 + 3CO2(g) + HzS(g) even without an evaluation of CO2, H2S, SO2 fugacities, it can be inferred from thermodynamic data that at any temperature (within the considered range) the reaction is strongly shifted towards the right-side (see Fig. 6). Following from the above, the massive self-sealing process which seems to be the rule in the Sabatini district may result from a still uprising (hydrous) CO2 and SO2-rich fluid. It may be hypothesized that a continuous heat transfer through the supply of a hot fluid of deep origin contrasted the cooling of the area after melt emplacement and thus maintained the high values of geothermal gradient as measured of present. The deep source of this fluid could be represented by a primary magma chamber of a mantle region. The striking similarities between explored Latium geothermal fields suggest they had a common large-scale model. CONCLUSIONS The lithostratigraphy and sequence of metasomatic and hydrothermal minerals evidenced by means of deep well SH2 in the Sabatini volcanic district allow the following considerations to be proposed: (1) A magma of possible trachytic (or less differentiated) composition intruded into a subvolcanic environment at a depth greater than 2500 m in the northern Bracciano lake area. (2) The high volatile content of this magma, enhanced by fractional crystallisation, resulted in extensive metasomatic products in a 1500 m thick sedimentary (mainly carbonate) rock pile. (3) The mineral assemblages recorded throughout the SH2 well closely reflect those already known from the ejecta contained in volcanoclastic products of the Sabatini and/or other Latian volcanic districts. Abundant late-state calcite and anhydrite constitute remarkable exceptions to the above, however. (4) Relying on lithostratigraphic evidence, the hypothesis of a massive remobilization of sedimentary (Triassic) sulphates in a low-pressure environment might be rejected. Most (if not all) anhydrite deposited in fractures and vugs seen in samples from well SH2 probably formed after interaction of a CO_,- SO-,- rich fluid with the host rocks or a shallow Ca-rich fluid of phreatic origin. (5) It is proposed that in addition to the proper melt-released fluid from the shallow magma chamber, a fluid of deep origin (a deeper seated magma chamber or a mantle region) continued mass and heat transfer in the considered region. The present high thermal anomalies in Latium may depend on such fluid, which would rise through major fracture systems.

Sabatini Volcanic District, Latium, Italy

143

Log K 30.

20~ 0~

\ -10

-20

~

~

\ \

-30-

~

\ -40-

~

-50 150 260 250 360 3,50 T °C 460 Fig. 6. Log K (equilibrium constant) vs temperature (degree °C) for the reactions: 4SO2 + 4H20 = H2S + 6H + + 3SOl(dashed) 4SO2 + 3CACO3 + H20 = 3CASO4 + 3CO2 + H2S (solid) calculated at 500 bar by means of SUPCRT program and data file. Acknowledgements--The Joint Venture AGIP-ENEL is acknowledged for supplying the data and samples discussed

here. The authors are indebted to M. Di Filippo for permission to publish the cross-section based on gravimetric data and to G. Buonasorte, E. Pandeli and P. Valenti of ENEL for their valuable collaboration. The manuscript was improved after critical reading by G. Lombardi and G. Gianelli. M. Serracino performed most of the electron probe analyses. M. Albano and M. Salvati helped during preparation of art work.

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a n d F. T e c c e

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86,230-24(1. Robie, R. A., Hemingway, B. S. and Fisher, J. R. (1978) Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (11)5 Pascals) pressure and higher temperatures. U.S. Geol. Surv. Bull. 1452, 1-456. Rogers, N. W., Hawkesworth, C. J., Parker, R. J. and Marsh, J. S. (1985) The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region. Contrib. Mineral. Petrol. 90, 244-257. Rucklidge, J. C. and Kochman, V. (1973) On the crystal structure of vesuvianite. Geol. Soc. A m . Abstr. Prog. 5, 787. Stoppani, F. S. e Curti, E. (1982) I minerali del Lazio. Editoriale Olimpia. Firenze. Tilley, C. E. (1951) The zoned contact skarns of the Broadford Area, Skye: a study of boron--fluorine metasomatism in dolomites. Mineral. Mag. 29, 6 2 1 - 666. Toro, B. (1978) Anomalie residue di gravit~ e strutture profonde nelle aree vulcaniche de Lazio settentrionale. Geologica Rom. 17, 35-44.

Sabatini Volcanic Distinct, Latium, Italy

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