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Origin of siderite deposits from the Lombardy Valleys, northern Italy: a carbon, oxygen and strontium isotope study
Chemical Geology (Isotope Geoscience Section), 105 (1993) 293-303
293
Elsevier Science Publishers B.V., Amsterdam [PD]
Origin of siderite deposits from the Lombardy Valleys, northern Italy: a carbon, oxygen and strontium isotope study G. Corteccia and P. Frizzo b aDipartimento di Scienze della Terra dell'UniversitY, Via S. Maria 53, 1-56126 Pis~ Italy bDipartimento di Mineralogia e Petrologia dell'Universith, Corso Garibaldi 37, 1-35122 Padova, Italy (Received December 3, 1991; revised and accepted November 19, 1992 )
ABSTRACT Corteeci, G. and Frizzo, P., 1993. Origin of siderite deposits from the Lombardy Valleys, northern Italy: a carbon, oxygen and strontium isotope study. Chem. Geol. (hot. Geosci. Sect.), 105: 293-303. In the Lombardy Valleys, siderite mineralization is widespread as stratiform orebodies in Lower Triassic terrigenouscarbonate rocks as well as vein orebodies in Permian volcanics and continental sequences and in the metamorphic basement. A number of these orebodies and host limestone/dolomite rocks have been analysed for C, O and Sr isotope compositions, in order to constrain geochemically the origin of the siderite. The intermediate ~'3C-, ~sO- and STSr/S6Sr-valuesof stratiform siderite relative to vein siderite and host limestone and the very good linear correlations between &-~ and ~-STSr/S6Srdata points concur to support a metasomatic origin for stratiform siderite after calcite. In addition, stratiform and vein siderite should have formed from the same kind of fluids, and possibly are the products of the same mineralizing event. The &values of carbonate rocks are discussed in terms of recrystallization/dolomitizationeffects during diagenesis. These processes occurred earlier than siderite mineralization.
1. Introduction In the area between Lake Como and Trompia Valley in Lombardy (northern Italy), the principal manganiferous siderite deposits consist of stratiform/strata-bound orebodies, and are typically associated with calcareous rocks of the Servino Formation (Scythian). The origin proposed for these deposits ranges from sedimentary-diagenetic to sedimentary-exhalative to diagenetic-metasomatic. Vein orebodies are also present both in the Servino Fm., the underlying Permian terrains and in the crystalline basement. The present work isotopically investigates a Correspondence to: G. Cortecci, Dipartimento di Scienze della Terra dell'Universit~t,Via S. Maria 53, 1-56126 Pisa, Italy.
number of stratiform and vein siderite deposits from the Servino Fm. and the crystalline basement, as well as a number of country carbonate rocks from both the mineralized and unmineralized series. It was aimed mainly to provide constraints on the source(s) of carbon, oxygen and strontium in the siderite, and then on the origin of the deposits.
2. Geological setting The geological framework of the area under study is extensively described in several papers (Frizzo, 1984, and references therein), and it will only be briefly summarized here. The area is part of the South Alpine Complex that includes the Hercynian metamorphic basement and the Permo-Triassic sedimentary cover. The basement mainly consists of garnetiferous
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
294
mica-schists and phyllites; carbonate rocks are absent or very rare. It is unconformably overlain by a Permian sequence of volcanics, conglomerates and sandy-silty-argillaceous rocks. The sequence is followed by the Scythian sediments of the "Servino Fm."; their depositional environment can be depicted as an epicontinental, shallow seawater terrigenous platform fed by a continent undergoing progressive erosion. The main lithology includes sandstones, siltstones and calcareous-dolomitic banks and layers being often oolitic and bioclastic. Arenaceous-pelitic material displays ripple-marks, flaser structures and mud cracks. During early diagenesis, all sediments from the formation underwent beachrock-type cementation, carbonate recrystallization, replacement dolomitization and pressure solution processes. Two orogenetic events (Saalica and Montenegrina phases) developed prior and after the deposition of the Servino sediments (Casati, 1969 ) between 260 and 240 Ma ago according to the time scale of Odin (1982). The last event caused numerous distensive faults trending EW, N-S and NE-SW. The Alpine metamorphic cycle did not significantly affect the area. The magmatic activity is represented by Upper Carboniferous diorites and by large masses of porphyries of Early-Middle Permian and Middle Triassic ages. Alpine plutonic activity is well documented in the northeastern sector of the area (Adamello quartz-dioritic massif).
3. Siderite mineralization On a regional scale, the discontinuous distribution of the siderite deposits can be subdivided into two sublatitudinal belts (Fig. 1 ). The belts correspond to regional tectonic lineaments and possibly represent the margins of the Scythian sedimentary basin. The stratiform/strata-bound siderite orebodies ("banks") within the Servino Fm. show lateral and vertical transitions, both gentle and
G. CORTECC1 AND P. FRIZZO
sudden, with calcareous host rocks. Paragenesis is essentially composed of massive and coarse-grained manganiferous siderite and variable quantities of clastic quartz. Rutile and white mica are common detrital accessories; locally, some pyrite and minor chalcopyrite, cinnabar and sulfosalts occur. The "banks" can generally be identified with originally stratoid or lens-shaped oolitic-bioclastic bodies replaced to different extents by the siderite. Host rocks show features typical of shallow-water marine sediments, with carbonatic and terrigenous deposition under high-energy (bar, open lagoon) to tidal-flat and sabkha conditions (Frizzo and Scudeler Baccelle, 1983 ). A number of vein-type manganiferous siderite deposits were exploited in the Servino Fro. and in the underlying terrains and basement. The orebodies in the Servino Fm. are thinner when crossing the more plastic terrigenous lithotypes and thicker when intersecting carbonatic layers. Paragenesis is predominantly spathic manganiferous siderite, with little quartz and some pyrite, chalcopyrite and tetrahedrite. All the vein orebodies are emplaced along subvertical faults, mainly trending N-S and subordinately E-W; thus the mineralization may be placed in correspondence with the Montenegrina orogenetic phase, at ~ 240 Ma ago. Finally, the "bank" mineralization appears to be subsequent to early diagenesis of host sediments, and its interdependence with vein mineralization is often obvious. These features led Frizzo (1984) to conclude an epigenetic origin of stratiform siderite.
4. Samples and analytical techniques Stratiform and vein siderite samples show some supergenic limonitization. They contain comparable Mn (1.7-4.5%), Mg (0.7-6.4%) and Ca (0.3-1.8%). For isotopic analyses, any visible impurity and vein material was removed by crushing and hand-picking; X-ray
ORIGIN OF SIDERITEDEPOSITSFROMTHE LOMBARDYVALLEYS,NORTHERN ITALY
295
EDOLO
Servino Formotion (Scythion) with stratiform and stratcl-bound siderite mineralization
/
Unmineral/zed Servino Formation
Vein type siderite crossin9 Perrnlon ond pre-Permlon formctfions
~'LAV ~ALONNO
M.Tor$oleto ~ " M.Lorio
LINEA
DEL
sA~o.~
PAISCO - LOVENO
PasSo d. ScQlett a
~ge
( /V,L.
"
I
__ J vE
i
R~ Ba£1ile
MAN
\ \
~
~
O
E
Rz° deUa Presolana
2~21
3RENO
i
LLA
Fig. 1. Geological sketch map of eastern Lombardy Valleys in northern Italy, showing locations of the studied siderite deposits and country carbonate rocks.
296
diffractometry ( X R D ) on separates did not reveal carbonate minerals other than siderite. Country rock samples consist of a carbonate fraction and a silicoclastic fraction; as from XRD, the carbonate fraction from all but one rock essentially consists of dolomite or calcite. Further details on the studied material are given in the Appendix. Carbon dioxide was extracted from siderite, dolomite and calcite by reaction with strictly anhydrous H3PO4 (McCrea, 1950), and massspectrometrically analysed for ~3C/~2C and 80/160 ratios. Siderite was reacted at 100 ° C in Pyrex ® break-offduring 3 days basically following the procedure of Rosenbaum and Sheppard (1986), and dolomite and calcite at 25 °C during 3 days and 3 hr, respectively. Before reaction with phosphoric acid, dolomites and siderites were treated with 5% acetic acid for 30 min at room temperature to remove any calcite, and stratiform siderite samples with traces of bituminous matter were roasted at 400 ° C. Only rock sample PG24, showing comparable proportions of dolomite and calcite, was submitted to the fractional extraction method (see Cortecci et al., 1985). The carbon and oxygen isotope ratios are reported in 6 (%0) notation relative to the PDB and V-SMOW standards, respectively. Duplicate analyses for both carbon and oxygen agreed to within _+0.1-0.2%o for siderite, and better than _+0.1%0 for dolomite and calcite. In calculating the 6~80-values, the phosphoric acid fractionation factors of 1.00881 at 100°C for siderite (Rosenbaum and Sheppard, 1986 ), 1.01109 at 25°C for dolomite (Friedman and O'Neil, 1977) and 1.01025 for calcite (modified from Sharma and Clayton, 1965) were used. A few siderites and carbonate rocks were analysed for the total strontium 87Sr/86Srratios, by complete dissolution of samples with HC1HC104-HF. The measured ratios were normalized to 86Sr/88Sr=0.1194. The Sr and Rb contents were determined by X-ray fluorescence.
~; CORTECCI AND P FRIZZO
5. Isotope results Locations of the siderite orebodies and country carbonate rocks studied are shown in TABLE I Carbon and oxygen isotope compositions of siderite from stratiform and vein deposits Sample No.
Locality
,5~ ~(' (%o vs. PDB)
~ ~80 (%o vs. S M O W )
Strat([brm orebodies (Servmo Fro., northern belO: LAVI LAV36 ES(1) ES(e )
Alta Val Camonica (Lava mine ) Val Paisco (ErbignoCuel mine )
5.8
+ 16.7
- 5.2
+ 18.9
-5.3 - 5.5
+ 19.1 + 19.1
Stratt/brm orebodies (Servmo Fro.: southern bdO: FS5
FS20
CT4 CTO
Bassa Vat 3.,~ Camonica ( Fusio . . . . 4.0 Pisogne mine ) Val T r o m p i a .... 4.6 (Carlo -. 5.0 Iassara mine, Collio )
+ 18.0 + 17.8
+ 16.9 + 17.3
Vein orebodies (Servino Fro.. northern belO: MAN9 MANI3
Passo della Manina (Manina mine )
- ¢~.8
+ 15.8
~. ~
+ 15.6
¢~,in orebodies (basement, northern belO: SC9 TOR4 LOR4
Passo della Scaletta Torsoleto Lake Monte Lorio
-9 L
+17.8
-- 7.2 --7.4
+ 15.8 + 14.9
l'ein orebodie~ (basement, southern belO: T5 PIN3 REG 10
Torgola/Collio Pineto/ Bovegno Regina/ Pezzaze
-6.4 ~- 4.4
+16.5 +16.2
-- 7.4
+16.2
ORIGIN OF SIDERITE DEPOSITS FROM THE LOMBARDY VALLEYS,NORTHERN ITALY
297
TABLE 2 C a r b o n a n d oxygen isotope c o m p o s i t i o n s o f u n m i n e r a l i z e d c a r b o n a t e rocks f r o m t h e Servino Fro., associated or n o t with the siderite-bearing belts Rock s a m p l e *~
Carbonate m i n e r a l o g y .2
Isotopic c o m p o s i t i o n .3 t~13C (%0 vs. P D B )
3*80 (%0 vs. S M O W )
WR
C
D
WR
C
D
D>C
+0.2
-0.4
+0.6
+21.8
+20.0
+22.8
D D D C C C C C
- 2.5 -2,6 +0,2 - 1,3 -0,4 +0.4 -2,8 -0.6
- 1.2 -0.4 +0.4 -2.8 -0.6
- 2.4 n.d. +0.3 -
+ 25.2 +24.7 +23.2 + 18.7 + 19.5 + 19.0 +20.9 + 17.6
+ 18.4 + 19.2 + 18.8 +20.8 + 17.3
+ 26.4 n.d. +23.5 -
-3.4 -0.7 -0.7 0.0 - 1.8
n.d. - 1.7
-3.4 -0.8 -0.8 + 0.2 -
+20.1 +24.1 +24.3 + 24.2 + 18.2
n.d. + 17.9
+20.1 +24.4 +24.8 + 25.2 -
S I D E R I T E - B E A R I N G BELTS:
Northern belt: PG24 Southern belt: FOI4 FO18 F039 F050 CTX IS7
IS18 FS6
BARREN OUTCROP:
ANIO AN16 AN22 AN35 AN53
D D D D >> C C
- = no data, as the c a r b o n a t e fraction in the s a m p l e s is nearly 100% d o l o m i t e or calcite; n.d. = n o t d e t e r m i n e d .
*1Locality: P G = P a s s o Garzetto; FO, C T X = V a l T r o m p i a - V a l Fontanelle; IS, FS = Iseo-Pisogne; A N = A n f u r r o . A p p r o x i m a t i v e spatial relationships with s t r a t i f o r m siderite deposits (see Table 1 ): P G = ES; FO, C T X = CT; IS, FS = FS. *2Data f r o m X R D . C h e m i c a l analysis yielded m i n o r Fe a n d M n in all rock samples. C = calcite; D = dolomite. *3WR = whole rock ( C + D ) . Whole-rock d e c o m p o s i t i o n was p e r f o r m e d by 3-day reaction with H3PO4, a n d the acid fractionation factor for t h e d o m i n a n t or prevalent c a r b o n a t e p h a s e was u s e d in calculating the c~SO-values.
TABLE 3 Isotopic c o m p o s i t i o n o f total s t r o n t i u m in siderite a n d c a r b o n a t e rock s a m p l e s f r o m the s o u t h e r n belt Sample
T5 CT4 CT6 CTX
F050 F039
vein siderite stratiform siderite s t r a t i f o r m siderite limestone limestone dolomite
Rb (ppm)
Sr (ppm)
(STSr/SeSr)m
S7Rb/86Sr
(S7Sr/S6Sr)o*
4 16 12 16 12 55
8 517 92 1,066 641 40
0.72017 0.71404 0.71433 0.70846 0.70915 0.72195
1.412 0.087 0.368 0.042 0.053 3.884
0.71536 0.71374 0.71308 0.70831 0.70897 0.70871
*Corrected for decay o f STRb a s s u m i n g a n age o f 240 M a for the siderite ores (see text) a n d the value o f 1.42-10 - I t a -1 for the decay c o n s t a n t o f STRb.
298
(;.
Fig. 1. The results of the isotopic analyses are reported in Tables 1-3. 5. I. Carbon and oxygen isotopes Stratiform siderite from the northern and i. . . . . . .
o!
-6
0_" -
I
.
/O i; t
t
/4
~_
,
16
! /8
±
L.L
l_
20
l
I
L
24
22
.[ 26
i.
,~/80 (SMOW)
Fig. 2. Plot of 6'3C vs. dlso for vein siderites (filled circles), stratiform siderites (open circles) and carbonate rocks of the Servino Fm. (filled triangle = calcite; open triangle= dolomite). Groupings of data contoured by dashed and solid lines refer to samples from the mineralized northern belt and southern belt, respectively. Uncontoured data points refer to carbonates from the barren outcrop at Anfurro locality. [. . . . . . . . . . . . . . . . . . . . . . . . . . . .
*/ [
F039 A
L
~'C 72~"
-, -
FO50,t
"
7
i
/ '
! F O /4 1
/
a
L PfN,J
zx
FO/8
Oo/ -sPot4 ;30r6 /
- 7"~
@ REG/0 16
18
20
Z2
24
26
leO(*/..)
Fig. 3. Plot of •t3C vs. ~tsO of vein siderites (T, REG, PIN ), stratiform siderites (CT) and carbonate rocks (CT, FO) from nearby occurrences in the southern belt (see Fig. 1 and Tables 1 and 2). Key to symbols as in Fig. 2. The straight line interpolates stratiform siderite CT6, hostrock calcite CTX close to the front of mineralization, and the nearest vein siderite T5. Vein siderite REGI0, stratiform siderite CT4 and calcite F 0 5 0 fit such line well, whereas vein siderite PIN3 and dolomite FO samples do not.
CORTECCIANDP. FRIZZO
southern belts is fairly uniform isotopically, with mean 613C=-4.9+0.7°/oo and 6 ' 8 0 = + 18.0 + 1.0%o. Analogously, vein siderite from the basement shows mean 6 ' 3 C = -7.0+1.5°/o0 and 6180=+16.2+0.9°/oo. A vein siderite from a silty-micritic level of Servino (Manina mine) has 6~3C=-6.7°/00 and 6 ' 8 0 = + 15.7°/0o. Country carbonate rocks have 6 ~ 3 C = - 3 . 4 to +0.4%0 and 6~80= + 17.6 to +25.2%0. In the PG24 specimen, dolomite is enriched in heavy isotopes relative to calcite coexisting in comparable proportion in the rock. In the other predominantly dolomitic or calcitic rock specimens, a similar isotopic enrichment is suggested, especially for ~80. From comparison of the results in Fig. 2, the main features are: ( 1 ) stratiform siderite is on average enriched in 13C and 180 with respect to vein siderite from the same belt; (2) on the whole, vein and stratiform siderite is depleted in 13C relative to calcite and dolomite, as well as in '80 relative to dolomite. When samples from the two belts are compared separately, it is also depleted in 180 relative to calcite, but in some cases comparable 180 contents are observed; (3) dolomite and calcite from rocks both associated and not associated with siderite mineralization have quite similar 613C-values but two distinct groupings of internally consistent 61SO-values, with dolomite being on average enriched in 180 by 5-6%0 relative to calcite (they also show comparable 6~3C and 6 ' 8 0 ranges of variation); and (4) a positive 6' 3C-6180 relationship is suggested between vein siderite, stratiform siderite and carbonate rocks. When data from close occurrences are compared, a very good linear correlation involving calcite instead of dolomite is found (Fig. 3). 5.2. Strontium isotopes The results of the isotope analyses on total strontium from a number of siderite and carbonate rock specimens are reported in Table 3,
299
ORIGIN OF SIDERITE DEPOSITS FROM THE LOMBARDY VALLEYS,NORTHERN ITALY
"T4
0.716 0.715 0. 714 O.7/3
TS)C! -~ CT4
O'~ 0 712 ~_ 0.711
(2fflO 0.7O9
F'O39 A C TX
CTX\
0.7"O8 I
-7
i
-5
i
i
-3 -I d /3C(PDB)
i
÷1
i
16
i
i
t
18 20 2~ d/aO(SMOW)
i
24
Fig. 4. Plots of (STSr/a6Sr)o vs. c$13C and ~ 1 8 0 for stratiform siderite (open circles), vein siderite (filled circles) and associated carbonate rocks (filled triangles = limestone; open triangles = dolomite) from the southern belt. Interpolations do not include sample F039. Key to samples and their spatial relationships as in Fig. 3.
along with the corrected (87Sr/S6Sr)0 ratios assuming an age of 240 Ma for the vein (and stratiform) siderite mineralization. Carbonate rocks show comparable (87Sr/ 86Sr)0 ratios averaging 0.70866_+0.00033, which are much lower than those of associated stratiform siderite (ave. 0.71341 _+0.00033 ); these latter, in turn, are notably lower than that of vein siderite (0.71536). When plotted on a (87sr/a6Sr)o vs. t~3C or 5180 diagram, limestones and siderites correlate very well; dolomite does not (Fig. 4). 6. Discussion
In the following, the genetic implications from the isotopic data on siderite deposits and country carbonate rocks will be discussed separately.
6.1. Siderite deposits The comparable
(~13C-
and ~180-values of
vein siderite occurrences from both the basement and the Servino terrains suggest a common mineralizing event. Along with the absence of carbonate rocks in the basement, the range of g13C-values points towards a mainly deep-seated origin for carbon, that may be related with the (Early?) Triassic "geothermal" activity in the area. However, it appears to be too light for most deep-seated CO2 sources (t~3C= - 8 to -50/00; e.g., Taylor, 1986; Hoefs, 1987), suggesting organic matter of low g'3C in the basement as a possible additional source of carbon for the ore-forming fluids. The c~lSO signature agrees with that reported by Timofeyeva et al. (1976) for a number of European vein hydrothermal siderite deposits. The high 87Sr content of sample T5 from the crystalline basement is as expected for Sr leached out from old sialic rocks (e.g., Faure, 1986 ). The tj13C-cJlsO-S7Sr/a6Sr straight-line correlations in Figs. 3 and 4 suggest that stratiform siderite effectively crystallized from vein
300
G. CORTECCI AND P. FRIZZO
siderite-depositing fluids after replacement of host limestone. According to the absence of chamosite and the rare iron oxides and pyrite in the deposits, siderite precipitation should have occurred under low Eh and negligible sulfide activity from slightly acidic to moderately basic solutions (see Curtis and Spears, 1968 ). Acidity in the metasomatizing fluids was not such as to dissolve dolomite appreciably. Finally, most Fe in vein and stratiform siderite should have derived from the same source, i.e. leaching of iron minerals in the basement and overlying rocks (Frizzo, 1984), followed by transport possibly as ferrous bicarbonate. Therefore, relevant chemical equations for the replacement process might have been: Fe (HCO3) 2 ~ Fe2 + + 2HCO~ CaCO3
+ H + ~- Ca 2+ + HCO~
A metasomatic* origin of stratiform siderite matches with its paragenesis with carbonate rocks (unknown for typical sedimentary siderite; e.g., Timofeyeva et al., 1976), the presence through the siderite bodies of vein infillings of carbonates including siderite, and the very close and quite uniform ~-values in the ore deposits from the two belts. Further support is provided by the 6~80-values, which are in keeping with those obtained by Timofeyeva et al. (1976) for siderite metasomatic after carbonate rocks, and typically ranging between + 14 and + 20%0. These values, in turn, are definitively lower than those between + 22 and + 28°/00 reported by the same authors for either diagenetic siderite from terrigenous-marine and terrigenous-brackish lagoon sediments or for siderite from marine sediments associated with underwater volcanism.
HCO~- ~ H + +CO~-
6.2. Country carbonate rocks'
Fe 2+ +CO~- =FeCO3
Rock calcite and dolomite associated with or away from the mineralized belts are isotopically indistinguishable for both oxygen and carbon, thus excluding any influence of the oreforming fluids on the isotopic composition of the former carbonates. In addition, the ~3C and Jso enrichments observed in calcite with respect to associated siderite are the opposite of what one obtains for cogenetic minerals by combining the appropriate fractionation factors available from Bottinga (1969), O'Neil et al. (1969) and Carothers et al. (1988). Along with petrographic observations by Frizzo and Scudeler Baccelle (1983), these features suggest that recrystallization, replacement dolomitization and lithification of sediments were basically completed before mineralization took place. On the whole, the 613C of calcites and do-
We can imagine dissolution of calcite to provide H C O 3 - and S r 2+ which mix with H C O 3 and Sr 2+ carried by the dissolving fluid, followed by precipitation of siderite with intermediate 87Sr/a6Sr and ~13C-values. Whereas the STSr/86Sr ratio simply corresponds to the weighted mean of the ratios in calcite and vein siderite (i.e. in the vein siderite-depositing fluid), the t~~3C should depend on that of total H C O 3 - in the resulting solution plus fractionation during precipitation. In turn, the ~ ~3C of total H C O 3 - c a n be related to the contributions and the ~ 3 C of H C O 3 - from the two sources, these latter values corresponding to those of calcite and vein siderite when corrected for the isotopic fractionations involved in the dissolution/precipitation processes. Finally, the intermediate t~sO of stratiform siderite would reflect deposition from vein-siderite fluid somewhat enriched in ~80 by interaction with limestone, and/or at lower temperature.
*The term "metasomatic" or "metasomatism" is possibly not applied correctly in the interpretation here. Perhaps, the genetic process for stratiform siderite is more accurately described as incongruent dissolution of calcite for siderite.
ORIGIN OF SIDERITE DEPOSITS FROM THE LOMBARDY VALLEYS, NORTHERN ITALY
lomites varies between - 3 . 4 and +0.6%0, all but four values being well within the range from - 2 to + 4%0 typically shown by recent marine (e.g., Land, 1989 and references therein) and ancient (also Triassic) marine analogs (Veizer and Hoefs, 1976; Land, 1980). Four t~13C-values are between - 3 . 4 and -2.4%0; they refer to both calcite and dolomite from rocks containing notable quantities of terrigenous material, and might represent a significant contribution of biogenic carbon to the sedimentary carbonate precursor and/or during diagenesis. Unlike g l3C, calcite and dolomite display two well-separated groups of tJlSO-values' close to + 19 and + 24%0, respectively. The exception is dolomicrite ANIO, with a ~ 180 lower by 4-5%0 relative to replacement dolomite from the same outcrop. Both carbonate phases are considerably depleted in 1sO relative to modem (Gross, 1964; McKenzie, 1981; Land, 1980, 1989) and most Triassic analogs (Veizer and Hoefs, 1976). All these low glsO signatures may be attributed to recrystallization (and oxygen isotopic exchange) of calcium carbonate and its dolomitization during diagenesis under the influence of water (meteoric?) depleted in 1sO relative to seawater or/ and at elevated temperature. An increase in temperature may be inferred as a precursor to the hydrothermal activity responsible for the vein (and stratiform) siderite mineralization in the area. Indication on the temperature (and 5180 of water) cannot be obtained from the l8O and '3C enrichments shown by the only one available dolomite-calcite pair PG24, as the relative geothermometers are far from being firmly established (e.g., Friedman and O'Neil, 1977; Matthews and Katz, 1977; Land, 1980). We can only point out that temperatures between 80 ° and 100°C are calculated from the calcite ~lSO-values, assuming seawater of gls 0 = 0%0 and using the well-establishedfractionation curve of O'Neil et al. ( 1969 ). When compared with the thickness of 150-250 m of the Lower-Middle Triassic rocks in the area (Frizzo, 1984 ), these thermometric estimates
301
may account for an anomalously high heat flow regime during diagenesis of the carbonate sediments under study. Finally, the (S7Sr/86Sr)o ratios of carbonate rocks are notably greater than that of seawater strontium of Early Triassic age [not higher than ~0.7075 (Burke et al., 1982) and time scale of Odin ( 1982 ) ]. This can be attributed to the contribution of strontium rich in 875r from the non-carbonate fraction of samples during the analytical procedure and, at least partly, to the incorporation into the carbonate fraction of radiogenic 878r from continental sources during sedimentation and diagenesis. 7. Conclusions
The genetic model we propose for the vein and stratiform siderite mineralizations of Lombardy Valleys from the isotopic results obtained in the present study is the following: ( 1) Vein siderite would have deposited from hydrothermal fluids, where oxidized carbon should have been largely provided by a deepseated source, and Sr (and Fe) by leaching of rocks passed through. (2) Stratiform siderite would be metasomatic, replacing calcite rather than dolomite in the host carbonate rocks. Carbon and oxygen in the parental solution should have been contributed by the metasomatizing fluid and host limestone, and the isotopic composition of siderite controlled by the glSO(H20) and 13C ( H C O 3 - ) as well as by the temperature of precipitation. Strontium can be regarded simply as a mixture. (3) Vein siderite and stratiform siderite should have deposited from the same kind of fluids, and possibly are the products of the same mineralizing event. Finally, the isotopic features of country carbonate rocks match with their deposition in a terrigenous-marine environment, followed by early diagenetic recrystallization and dolomitization possibly under the influence of meteoric water and/or at elevated temperature.
302
Specimens from siderite-bearing outcrops or from barren outcrops are isotopically indistinguishable; in addition, calcite and associated siderite appear to be in isotopic disequilibrium for both carbon and oxygen. These features concur to support that the mineralization is later than early diagenesis of the host sediments.
(i, CORTECCI AND P. FRIZZ()
Vein siderite.trom the crystalltne basement SC9, LOR4 and TOR4: spathic+cataclased siderite, with local recrystallization and recementing by siderite, quartz _+ankerite; traces of sulfide minerals. REG 10, PIN3 and T5: spathic + cataclased siderite, locally recrystallized and recemented by veinlets of quartz+_carbonates; traces of sulfide minerals, namely pyrite and chalcopyrite. Country carbonate rocks (Servino Fm.)
Acknowledgements This paper benefited from discussion with G. Leone. Many thanks are due to G.F. Del Chicca (Istituto Internazionale per le Ricerche Geotermiche, CNR, Pisa) for carbon and oxygen mass-spectrometric analyses, M. Menichini for X-ray fluorescence analyses, S. Tonarini (Istituto di Geocronologia e Geochimica Isotopica, CNR, Pisa) for Sr isotope analyses, R. Tosi for drafting all but one figures and Kristina Unterauer for having improved the English. Appendix - - Description of the siderite and carbonate rock specimens studied in the present work Strat(form siderite.from the Servino Fm. LAVI and LAV36: massive medium-grained siderite with silicoclastic relics, rare pyrite granules, traces of chalcopyrite and of interstitial clayey-bituminous matter; later veinlets of quartz +-ankerite + hematite recement the fractured siderite in LAV36. ES(1) and ES(e): massive siderite with widespread micritic relics and some silicoclasts. FS5 and FS20: massive spathic siderite with traces of interstitial organic and clayey matter; quartz in detritic granules with syntaxial growth and cherty aggregates; later barite veinlets. CT4 and CT6: massive, coarse-grained, spathic siderite with local micrite relics and ghosts of oolitic-bioclastic structures; silicoclast relicts; detritic quartz with syntaxial intergrowths and chert; traces of interstitial organic matter. Vein siderite from the Servino Fm. MAN9 and MAN 13: spathic siderite cementing a breccia of siltite with micrite; quartz + ankerite recement later fractures.
PG24 :dolosparite. FO14 :recrystallized and dolomitized oosparite with sparse silicoclasts. FOI8 :silty-sandy dolosparite. F039 :crystalline dolomia with scattered silicoclasts and glauconite. F050 :Myophoriae limestone with oolites and micritic cement. CTX :oolitic-intraclastic biosparite with sparitic cement (adjacent to the siderite "bank" CT6); silicoclats (quartz with syntaxial growth) and chert; traces of organic matter. IS7 :packstone with microsparitic cement with ooids, bioclasts and intraclasts. IS18 :sandy-silty microsparite. FS6 :oolitic-bioclastic packstone with micritic cement. ANIO :silty dolomicrite. A N I 6 :dolomitic-arenaceous limestone. AN22 :dolomitic-arenaceous rudstone, oolitic with gasteropods and local silicization. AN35 :dolomitized oolite with gasteropods. AN53 :silty-sandy limestone.
References Bottinga, Y., 1969. Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxide-graphite-methane-hydrogenwater vapor. Geochim. Cosmochim Acta, 33: 49-64. Burke, W.H., Denison, R.E., Hetherington, E.A., Koepnick, R.B., Nelson, H.F. and Otto, J.B., 1982. Variations of seawater 87Sr/86Sr throughout Phanerozoic time. Geology, 10:516-519. Carothers, W.W., Adami, L.H. and Rosenbauer, R.J., 1988. Experimental oxygen isotope fractionation between siderite-water and phosphoric acid liberated CO2-siderite. Geochim. Cosmochim. Acta, 52: 24452450. Casati, P., 1969. Le fasi orogenetiche nelle Alpi Meridionali, Artigianelli Paviniani, Monza, 49 pp. Cortecci, G., Lattanzi, P. and Tanelli, G., 1985. C- and Oisotope and fluid inclusion studies of carbonates from pyrite and polymetallie ore deposits and associated
ORIGINOF SIDERITEDEPOSITSFROMTHELOMBARDYVALLEYS,NORTHERNITALY country rocks (southern Tuscany, Italy). Chem. Geol. (Isot. Geosci. Sect.), 58: 121-128. Curtis, C.D. and Spears, D.A., 1968. The formation of sedimentary iron minerals. Econ. Geol., 63: 257-270. Faure, G., 1986. Isotope geology of strontium in sedimentary rocks. In: G. Faure (Editor), Principles of Isotope Geology. Wiley, New York, N.Y., pp. 183-199. Friedman, I. and O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. U.S. Geol. Surv. Prof. Pap., 440K: 1-12. Frizzo, P., 1984. Le mineralizzazioni a barite e a siderite nel Permiano Superiore e nello Scitico della Val Camonica (Lombardia). Ind. Miner., 4:21-43. Frizzo, P. and Scudeler Baccelle, L., 1983. Rapporti strutturali e tessiturali fra mineralizzazione a siderite e litotipi carbonatici nel Servino (Scitico) delle Valli Lombarde. Mere. Sci. Geol. (Padova), 36: 195-210. Gross, M.G., 1964. Variations in the ~sO/160 and lac/ ~2C ratios of diagenetically altered limestones in the Bermuda Islands. J. Geol., 72:170-194. Hoefs, J., 1987. Stable Isotope Geochemistry. Springer, Berlin, 241 pp. Land, L.S., 1980. The isotopic and trace element geochemistry of dolomite: the state of the art. In: D.H. Zenger (Editor), Concepts and Models of Dolomitization. Soc. Econ. Paleontol. Mineral., Spec. Publ., 28: 87-110. Land, LS., 1989. The carbon and oxygen isotopic chemistry of surficial Holocene shallow marine carbonate sediments and Quaternary limestone and dolomite. In: P. Fritz and J.Ch. Fontes (Editors), Handbook of Environmental Geochemistry, Vol. 3, The Marine Environment, A. Elsevier, Amsterdam, pp. 191-217.
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Matthews, A. and Katz, A., 1977. Oxygen. isotope fractionation during dolomitization of calcium carbonate. Geochim. Cosmochim. Acta, 41: 143 l - 1438. McCrea, J.M., 1950. On the isotope chemistry of carbonates and a paleotemperature scale. J. Chem. Phys., 18: 849-857. McKenzie, J.A., 1981. Holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of Abu Dhabi, U.A.E.: a stable isotope study. J. Geol., 89: 185-198. Odin, G.S., 1982. The Phanerozoic time scale revisited. Episodes, 3: 3-9. O'Neil, J.R., Clayton, R.N. and Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys., 51: 5547-5558. Rosenbaum, J. and Sheppard, S.M.F., 1986. An isotopic study of siderites, dolomites and ankerites at high temperatures. Geochim. Cosmochim. Acta, 50: 11471150. Sharma, T. and Clayton, R.N., 1965. Measurements of tsO/~60 ratios of total oxygen of carbonates. Geochim. Cosmochim. Acta, 29:1347-1353. Taylor, B.E., 1986. Magmatic volatiles: isotopic variation of C, H, and S. In: J.W. Valley, H.P. Taylor, Jr. and J.R. O'Neil (Editors), Stable Isotopes in High Temperature Geological Processes. Mineral. Soc. Am., Rev. Mineral., 16: 185-225. Timofeyeva, Z.V., Kuznetsova, L.D. and Dontsova, Y.I., 1976. Oxygen isotopes and siderite formation. Geochem. Int., 13(5): 101-112. Veizer, J.D. and Hoefs, J., 1976. The nature of ~sO/~60 and~3C/~2C secular trends in sedimentary carbonate rocks. Geochim. Cosmochim. Acta, 40:1387-1395.
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Elsevier Science Publishers B.V., Amsterdam [PD]
Origin of siderite deposits from the Lombardy Valleys, northern Italy: a carbon, oxygen and strontium isotope study G. Corteccia and P. Frizzo b aDipartimento di Scienze della Terra dell'UniversitY, Via S. Maria 53, 1-56126 Pis~ Italy bDipartimento di Mineralogia e Petrologia dell'Universith, Corso Garibaldi 37, 1-35122 Padova, Italy (Received December 3, 1991; revised and accepted November 19, 1992 )
ABSTRACT Corteeci, G. and Frizzo, P., 1993. Origin of siderite deposits from the Lombardy Valleys, northern Italy: a carbon, oxygen and strontium isotope study. Chem. Geol. (hot. Geosci. Sect.), 105: 293-303. In the Lombardy Valleys, siderite mineralization is widespread as stratiform orebodies in Lower Triassic terrigenouscarbonate rocks as well as vein orebodies in Permian volcanics and continental sequences and in the metamorphic basement. A number of these orebodies and host limestone/dolomite rocks have been analysed for C, O and Sr isotope compositions, in order to constrain geochemically the origin of the siderite. The intermediate ~'3C-, ~sO- and STSr/S6Sr-valuesof stratiform siderite relative to vein siderite and host limestone and the very good linear correlations between &-~ and ~-STSr/S6Srdata points concur to support a metasomatic origin for stratiform siderite after calcite. In addition, stratiform and vein siderite should have formed from the same kind of fluids, and possibly are the products of the same mineralizing event. The &values of carbonate rocks are discussed in terms of recrystallization/dolomitizationeffects during diagenesis. These processes occurred earlier than siderite mineralization.
1. Introduction In the area between Lake Como and Trompia Valley in Lombardy (northern Italy), the principal manganiferous siderite deposits consist of stratiform/strata-bound orebodies, and are typically associated with calcareous rocks of the Servino Formation (Scythian). The origin proposed for these deposits ranges from sedimentary-diagenetic to sedimentary-exhalative to diagenetic-metasomatic. Vein orebodies are also present both in the Servino Fm., the underlying Permian terrains and in the crystalline basement. The present work isotopically investigates a Correspondence to: G. Cortecci, Dipartimento di Scienze della Terra dell'Universit~t,Via S. Maria 53, 1-56126 Pisa, Italy.
number of stratiform and vein siderite deposits from the Servino Fm. and the crystalline basement, as well as a number of country carbonate rocks from both the mineralized and unmineralized series. It was aimed mainly to provide constraints on the source(s) of carbon, oxygen and strontium in the siderite, and then on the origin of the deposits.
2. Geological setting The geological framework of the area under study is extensively described in several papers (Frizzo, 1984, and references therein), and it will only be briefly summarized here. The area is part of the South Alpine Complex that includes the Hercynian metamorphic basement and the Permo-Triassic sedimentary cover. The basement mainly consists of garnetiferous
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
294
mica-schists and phyllites; carbonate rocks are absent or very rare. It is unconformably overlain by a Permian sequence of volcanics, conglomerates and sandy-silty-argillaceous rocks. The sequence is followed by the Scythian sediments of the "Servino Fm."; their depositional environment can be depicted as an epicontinental, shallow seawater terrigenous platform fed by a continent undergoing progressive erosion. The main lithology includes sandstones, siltstones and calcareous-dolomitic banks and layers being often oolitic and bioclastic. Arenaceous-pelitic material displays ripple-marks, flaser structures and mud cracks. During early diagenesis, all sediments from the formation underwent beachrock-type cementation, carbonate recrystallization, replacement dolomitization and pressure solution processes. Two orogenetic events (Saalica and Montenegrina phases) developed prior and after the deposition of the Servino sediments (Casati, 1969 ) between 260 and 240 Ma ago according to the time scale of Odin (1982). The last event caused numerous distensive faults trending EW, N-S and NE-SW. The Alpine metamorphic cycle did not significantly affect the area. The magmatic activity is represented by Upper Carboniferous diorites and by large masses of porphyries of Early-Middle Permian and Middle Triassic ages. Alpine plutonic activity is well documented in the northeastern sector of the area (Adamello quartz-dioritic massif).
3. Siderite mineralization On a regional scale, the discontinuous distribution of the siderite deposits can be subdivided into two sublatitudinal belts (Fig. 1 ). The belts correspond to regional tectonic lineaments and possibly represent the margins of the Scythian sedimentary basin. The stratiform/strata-bound siderite orebodies ("banks") within the Servino Fm. show lateral and vertical transitions, both gentle and
G. CORTECC1 AND P. FRIZZO
sudden, with calcareous host rocks. Paragenesis is essentially composed of massive and coarse-grained manganiferous siderite and variable quantities of clastic quartz. Rutile and white mica are common detrital accessories; locally, some pyrite and minor chalcopyrite, cinnabar and sulfosalts occur. The "banks" can generally be identified with originally stratoid or lens-shaped oolitic-bioclastic bodies replaced to different extents by the siderite. Host rocks show features typical of shallow-water marine sediments, with carbonatic and terrigenous deposition under high-energy (bar, open lagoon) to tidal-flat and sabkha conditions (Frizzo and Scudeler Baccelle, 1983 ). A number of vein-type manganiferous siderite deposits were exploited in the Servino Fro. and in the underlying terrains and basement. The orebodies in the Servino Fm. are thinner when crossing the more plastic terrigenous lithotypes and thicker when intersecting carbonatic layers. Paragenesis is predominantly spathic manganiferous siderite, with little quartz and some pyrite, chalcopyrite and tetrahedrite. All the vein orebodies are emplaced along subvertical faults, mainly trending N-S and subordinately E-W; thus the mineralization may be placed in correspondence with the Montenegrina orogenetic phase, at ~ 240 Ma ago. Finally, the "bank" mineralization appears to be subsequent to early diagenesis of host sediments, and its interdependence with vein mineralization is often obvious. These features led Frizzo (1984) to conclude an epigenetic origin of stratiform siderite.
4. Samples and analytical techniques Stratiform and vein siderite samples show some supergenic limonitization. They contain comparable Mn (1.7-4.5%), Mg (0.7-6.4%) and Ca (0.3-1.8%). For isotopic analyses, any visible impurity and vein material was removed by crushing and hand-picking; X-ray
ORIGIN OF SIDERITEDEPOSITSFROMTHE LOMBARDYVALLEYS,NORTHERN ITALY
295
EDOLO
Servino Formotion (Scythion) with stratiform and stratcl-bound siderite mineralization
/
Unmineral/zed Servino Formation
Vein type siderite crossin9 Perrnlon ond pre-Permlon formctfions
~'LAV ~ALONNO
M.Tor$oleto ~ " M.Lorio
LINEA
DEL
sA~o.~
PAISCO - LOVENO
PasSo d. ScQlett a
~ge
( /V,L.
"
I
__ J vE
i
R~ Ba£1ile
MAN
\ \
~
~
O
E
Rz° deUa Presolana
2~21
3RENO
i
LLA
Fig. 1. Geological sketch map of eastern Lombardy Valleys in northern Italy, showing locations of the studied siderite deposits and country carbonate rocks.
296
diffractometry ( X R D ) on separates did not reveal carbonate minerals other than siderite. Country rock samples consist of a carbonate fraction and a silicoclastic fraction; as from XRD, the carbonate fraction from all but one rock essentially consists of dolomite or calcite. Further details on the studied material are given in the Appendix. Carbon dioxide was extracted from siderite, dolomite and calcite by reaction with strictly anhydrous H3PO4 (McCrea, 1950), and massspectrometrically analysed for ~3C/~2C and 80/160 ratios. Siderite was reacted at 100 ° C in Pyrex ® break-offduring 3 days basically following the procedure of Rosenbaum and Sheppard (1986), and dolomite and calcite at 25 °C during 3 days and 3 hr, respectively. Before reaction with phosphoric acid, dolomites and siderites were treated with 5% acetic acid for 30 min at room temperature to remove any calcite, and stratiform siderite samples with traces of bituminous matter were roasted at 400 ° C. Only rock sample PG24, showing comparable proportions of dolomite and calcite, was submitted to the fractional extraction method (see Cortecci et al., 1985). The carbon and oxygen isotope ratios are reported in 6 (%0) notation relative to the PDB and V-SMOW standards, respectively. Duplicate analyses for both carbon and oxygen agreed to within _+0.1-0.2%o for siderite, and better than _+0.1%0 for dolomite and calcite. In calculating the 6~80-values, the phosphoric acid fractionation factors of 1.00881 at 100°C for siderite (Rosenbaum and Sheppard, 1986 ), 1.01109 at 25°C for dolomite (Friedman and O'Neil, 1977) and 1.01025 for calcite (modified from Sharma and Clayton, 1965) were used. A few siderites and carbonate rocks were analysed for the total strontium 87Sr/86Srratios, by complete dissolution of samples with HC1HC104-HF. The measured ratios were normalized to 86Sr/88Sr=0.1194. The Sr and Rb contents were determined by X-ray fluorescence.
~; CORTECCI AND P FRIZZO
5. Isotope results Locations of the siderite orebodies and country carbonate rocks studied are shown in TABLE I Carbon and oxygen isotope compositions of siderite from stratiform and vein deposits Sample No.
Locality
,5~ ~(' (%o vs. PDB)
~ ~80 (%o vs. S M O W )
Strat([brm orebodies (Servmo Fro., northern belO: LAVI LAV36 ES(1) ES(e )
Alta Val Camonica (Lava mine ) Val Paisco (ErbignoCuel mine )
5.8
+ 16.7
- 5.2
+ 18.9
-5.3 - 5.5
+ 19.1 + 19.1
Stratt/brm orebodies (Servmo Fro.: southern bdO: FS5
FS20
CT4 CTO
Bassa Vat 3.,~ Camonica ( Fusio . . . . 4.0 Pisogne mine ) Val T r o m p i a .... 4.6 (Carlo -. 5.0 Iassara mine, Collio )
+ 18.0 + 17.8
+ 16.9 + 17.3
Vein orebodies (Servino Fro.. northern belO: MAN9 MANI3
Passo della Manina (Manina mine )
- ¢~.8
+ 15.8
~. ~
+ 15.6
¢~,in orebodies (basement, northern belO: SC9 TOR4 LOR4
Passo della Scaletta Torsoleto Lake Monte Lorio
-9 L
+17.8
-- 7.2 --7.4
+ 15.8 + 14.9
l'ein orebodie~ (basement, southern belO: T5 PIN3 REG 10
Torgola/Collio Pineto/ Bovegno Regina/ Pezzaze
-6.4 ~- 4.4
+16.5 +16.2
-- 7.4
+16.2
ORIGIN OF SIDERITE DEPOSITS FROM THE LOMBARDY VALLEYS,NORTHERN ITALY
297
TABLE 2 C a r b o n a n d oxygen isotope c o m p o s i t i o n s o f u n m i n e r a l i z e d c a r b o n a t e rocks f r o m t h e Servino Fro., associated or n o t with the siderite-bearing belts Rock s a m p l e *~
Carbonate m i n e r a l o g y .2
Isotopic c o m p o s i t i o n .3 t~13C (%0 vs. P D B )
3*80 (%0 vs. S M O W )
WR
C
D
WR
C
D
D>C
+0.2
-0.4
+0.6
+21.8
+20.0
+22.8
D D D C C C C C
- 2.5 -2,6 +0,2 - 1,3 -0,4 +0.4 -2,8 -0.6
- 1.2 -0.4 +0.4 -2.8 -0.6
- 2.4 n.d. +0.3 -
+ 25.2 +24.7 +23.2 + 18.7 + 19.5 + 19.0 +20.9 + 17.6
+ 18.4 + 19.2 + 18.8 +20.8 + 17.3
+ 26.4 n.d. +23.5 -
-3.4 -0.7 -0.7 0.0 - 1.8
n.d. - 1.7
-3.4 -0.8 -0.8 + 0.2 -
+20.1 +24.1 +24.3 + 24.2 + 18.2
n.d. + 17.9
+20.1 +24.4 +24.8 + 25.2 -
S I D E R I T E - B E A R I N G BELTS:
Northern belt: PG24 Southern belt: FOI4 FO18 F039 F050 CTX IS7
IS18 FS6
BARREN OUTCROP:
ANIO AN16 AN22 AN35 AN53
D D D D >> C C
- = no data, as the c a r b o n a t e fraction in the s a m p l e s is nearly 100% d o l o m i t e or calcite; n.d. = n o t d e t e r m i n e d .
*1Locality: P G = P a s s o Garzetto; FO, C T X = V a l T r o m p i a - V a l Fontanelle; IS, FS = Iseo-Pisogne; A N = A n f u r r o . A p p r o x i m a t i v e spatial relationships with s t r a t i f o r m siderite deposits (see Table 1 ): P G = ES; FO, C T X = CT; IS, FS = FS. *2Data f r o m X R D . C h e m i c a l analysis yielded m i n o r Fe a n d M n in all rock samples. C = calcite; D = dolomite. *3WR = whole rock ( C + D ) . Whole-rock d e c o m p o s i t i o n was p e r f o r m e d by 3-day reaction with H3PO4, a n d the acid fractionation factor for t h e d o m i n a n t or prevalent c a r b o n a t e p h a s e was u s e d in calculating the c~SO-values.
TABLE 3 Isotopic c o m p o s i t i o n o f total s t r o n t i u m in siderite a n d c a r b o n a t e rock s a m p l e s f r o m the s o u t h e r n belt Sample
T5 CT4 CT6 CTX
F050 F039
vein siderite stratiform siderite s t r a t i f o r m siderite limestone limestone dolomite
Rb (ppm)
Sr (ppm)
(STSr/SeSr)m
S7Rb/86Sr
(S7Sr/S6Sr)o*
4 16 12 16 12 55
8 517 92 1,066 641 40
0.72017 0.71404 0.71433 0.70846 0.70915 0.72195
1.412 0.087 0.368 0.042 0.053 3.884
0.71536 0.71374 0.71308 0.70831 0.70897 0.70871
*Corrected for decay o f STRb a s s u m i n g a n age o f 240 M a for the siderite ores (see text) a n d the value o f 1.42-10 - I t a -1 for the decay c o n s t a n t o f STRb.
298
(;.
Fig. 1. The results of the isotopic analyses are reported in Tables 1-3. 5. I. Carbon and oxygen isotopes Stratiform siderite from the northern and i. . . . . . .
o!
-6
0_" -
I
.
/O i; t
t
/4
~_
,
16
! /8
±
L.L
l_
20
l
I
L
24
22
.[ 26
i.
,~/80 (SMOW)
Fig. 2. Plot of 6'3C vs. dlso for vein siderites (filled circles), stratiform siderites (open circles) and carbonate rocks of the Servino Fm. (filled triangle = calcite; open triangle= dolomite). Groupings of data contoured by dashed and solid lines refer to samples from the mineralized northern belt and southern belt, respectively. Uncontoured data points refer to carbonates from the barren outcrop at Anfurro locality. [. . . . . . . . . . . . . . . . . . . . . . . . . . . .
*/ [
F039 A
L
~'C 72~"
-, -
FO50,t
"
7
i
/ '
! F O /4 1
/
a
L PfN,J
zx
FO/8
Oo/ -sPot4 ;30r6 /
- 7"~
@ REG/0 16
18
20
Z2
24
26
leO(*/..)
Fig. 3. Plot of •t3C vs. ~tsO of vein siderites (T, REG, PIN ), stratiform siderites (CT) and carbonate rocks (CT, FO) from nearby occurrences in the southern belt (see Fig. 1 and Tables 1 and 2). Key to symbols as in Fig. 2. The straight line interpolates stratiform siderite CT6, hostrock calcite CTX close to the front of mineralization, and the nearest vein siderite T5. Vein siderite REGI0, stratiform siderite CT4 and calcite F 0 5 0 fit such line well, whereas vein siderite PIN3 and dolomite FO samples do not.
CORTECCIANDP. FRIZZO
southern belts is fairly uniform isotopically, with mean 613C=-4.9+0.7°/oo and 6 ' 8 0 = + 18.0 + 1.0%o. Analogously, vein siderite from the basement shows mean 6 ' 3 C = -7.0+1.5°/o0 and 6180=+16.2+0.9°/oo. A vein siderite from a silty-micritic level of Servino (Manina mine) has 6~3C=-6.7°/00 and 6 ' 8 0 = + 15.7°/0o. Country carbonate rocks have 6 ~ 3 C = - 3 . 4 to +0.4%0 and 6~80= + 17.6 to +25.2%0. In the PG24 specimen, dolomite is enriched in heavy isotopes relative to calcite coexisting in comparable proportion in the rock. In the other predominantly dolomitic or calcitic rock specimens, a similar isotopic enrichment is suggested, especially for ~80. From comparison of the results in Fig. 2, the main features are: ( 1 ) stratiform siderite is on average enriched in 13C and 180 with respect to vein siderite from the same belt; (2) on the whole, vein and stratiform siderite is depleted in 13C relative to calcite and dolomite, as well as in '80 relative to dolomite. When samples from the two belts are compared separately, it is also depleted in 180 relative to calcite, but in some cases comparable 180 contents are observed; (3) dolomite and calcite from rocks both associated and not associated with siderite mineralization have quite similar 613C-values but two distinct groupings of internally consistent 61SO-values, with dolomite being on average enriched in 180 by 5-6%0 relative to calcite (they also show comparable 6~3C and 6 ' 8 0 ranges of variation); and (4) a positive 6' 3C-6180 relationship is suggested between vein siderite, stratiform siderite and carbonate rocks. When data from close occurrences are compared, a very good linear correlation involving calcite instead of dolomite is found (Fig. 3). 5.2. Strontium isotopes The results of the isotope analyses on total strontium from a number of siderite and carbonate rock specimens are reported in Table 3,
299
ORIGIN OF SIDERITE DEPOSITS FROM THE LOMBARDY VALLEYS,NORTHERN ITALY
"T4
0.716 0.715 0. 714 O.7/3
TS)C! -~ CT4
O'~ 0 712 ~_ 0.711
(2fflO 0.7O9
F'O39 A C TX
CTX\
0.7"O8 I
-7
i
-5
i
i
-3 -I d /3C(PDB)
i
÷1
i
16
i
i
t
18 20 2~ d/aO(SMOW)
i
24
Fig. 4. Plots of (STSr/a6Sr)o vs. c$13C and ~ 1 8 0 for stratiform siderite (open circles), vein siderite (filled circles) and associated carbonate rocks (filled triangles = limestone; open triangles = dolomite) from the southern belt. Interpolations do not include sample F039. Key to samples and their spatial relationships as in Fig. 3.
along with the corrected (87Sr/S6Sr)0 ratios assuming an age of 240 Ma for the vein (and stratiform) siderite mineralization. Carbonate rocks show comparable (87Sr/ 86Sr)0 ratios averaging 0.70866_+0.00033, which are much lower than those of associated stratiform siderite (ave. 0.71341 _+0.00033 ); these latter, in turn, are notably lower than that of vein siderite (0.71536). When plotted on a (87sr/a6Sr)o vs. t~3C or 5180 diagram, limestones and siderites correlate very well; dolomite does not (Fig. 4). 6. Discussion
In the following, the genetic implications from the isotopic data on siderite deposits and country carbonate rocks will be discussed separately.
6.1. Siderite deposits The comparable
(~13C-
and ~180-values of
vein siderite occurrences from both the basement and the Servino terrains suggest a common mineralizing event. Along with the absence of carbonate rocks in the basement, the range of g13C-values points towards a mainly deep-seated origin for carbon, that may be related with the (Early?) Triassic "geothermal" activity in the area. However, it appears to be too light for most deep-seated CO2 sources (t~3C= - 8 to -50/00; e.g., Taylor, 1986; Hoefs, 1987), suggesting organic matter of low g'3C in the basement as a possible additional source of carbon for the ore-forming fluids. The c~lSO signature agrees with that reported by Timofeyeva et al. (1976) for a number of European vein hydrothermal siderite deposits. The high 87Sr content of sample T5 from the crystalline basement is as expected for Sr leached out from old sialic rocks (e.g., Faure, 1986 ). The tj13C-cJlsO-S7Sr/a6Sr straight-line correlations in Figs. 3 and 4 suggest that stratiform siderite effectively crystallized from vein
300
G. CORTECCI AND P. FRIZZO
siderite-depositing fluids after replacement of host limestone. According to the absence of chamosite and the rare iron oxides and pyrite in the deposits, siderite precipitation should have occurred under low Eh and negligible sulfide activity from slightly acidic to moderately basic solutions (see Curtis and Spears, 1968 ). Acidity in the metasomatizing fluids was not such as to dissolve dolomite appreciably. Finally, most Fe in vein and stratiform siderite should have derived from the same source, i.e. leaching of iron minerals in the basement and overlying rocks (Frizzo, 1984), followed by transport possibly as ferrous bicarbonate. Therefore, relevant chemical equations for the replacement process might have been: Fe (HCO3) 2 ~ Fe2 + + 2HCO~ CaCO3
+ H + ~- Ca 2+ + HCO~
A metasomatic* origin of stratiform siderite matches with its paragenesis with carbonate rocks (unknown for typical sedimentary siderite; e.g., Timofeyeva et al., 1976), the presence through the siderite bodies of vein infillings of carbonates including siderite, and the very close and quite uniform ~-values in the ore deposits from the two belts. Further support is provided by the 6~80-values, which are in keeping with those obtained by Timofeyeva et al. (1976) for siderite metasomatic after carbonate rocks, and typically ranging between + 14 and + 20%0. These values, in turn, are definitively lower than those between + 22 and + 28°/00 reported by the same authors for either diagenetic siderite from terrigenous-marine and terrigenous-brackish lagoon sediments or for siderite from marine sediments associated with underwater volcanism.
HCO~- ~ H + +CO~-
6.2. Country carbonate rocks'
Fe 2+ +CO~- =FeCO3
Rock calcite and dolomite associated with or away from the mineralized belts are isotopically indistinguishable for both oxygen and carbon, thus excluding any influence of the oreforming fluids on the isotopic composition of the former carbonates. In addition, the ~3C and Jso enrichments observed in calcite with respect to associated siderite are the opposite of what one obtains for cogenetic minerals by combining the appropriate fractionation factors available from Bottinga (1969), O'Neil et al. (1969) and Carothers et al. (1988). Along with petrographic observations by Frizzo and Scudeler Baccelle (1983), these features suggest that recrystallization, replacement dolomitization and lithification of sediments were basically completed before mineralization took place. On the whole, the 613C of calcites and do-
We can imagine dissolution of calcite to provide H C O 3 - and S r 2+ which mix with H C O 3 and Sr 2+ carried by the dissolving fluid, followed by precipitation of siderite with intermediate 87Sr/a6Sr and ~13C-values. Whereas the STSr/86Sr ratio simply corresponds to the weighted mean of the ratios in calcite and vein siderite (i.e. in the vein siderite-depositing fluid), the t~~3C should depend on that of total H C O 3 - in the resulting solution plus fractionation during precipitation. In turn, the ~ ~3C of total H C O 3 - c a n be related to the contributions and the ~ 3 C of H C O 3 - from the two sources, these latter values corresponding to those of calcite and vein siderite when corrected for the isotopic fractionations involved in the dissolution/precipitation processes. Finally, the intermediate t~sO of stratiform siderite would reflect deposition from vein-siderite fluid somewhat enriched in ~80 by interaction with limestone, and/or at lower temperature.
*The term "metasomatic" or "metasomatism" is possibly not applied correctly in the interpretation here. Perhaps, the genetic process for stratiform siderite is more accurately described as incongruent dissolution of calcite for siderite.
ORIGIN OF SIDERITE DEPOSITS FROM THE LOMBARDY VALLEYS, NORTHERN ITALY
lomites varies between - 3 . 4 and +0.6%0, all but four values being well within the range from - 2 to + 4%0 typically shown by recent marine (e.g., Land, 1989 and references therein) and ancient (also Triassic) marine analogs (Veizer and Hoefs, 1976; Land, 1980). Four t~13C-values are between - 3 . 4 and -2.4%0; they refer to both calcite and dolomite from rocks containing notable quantities of terrigenous material, and might represent a significant contribution of biogenic carbon to the sedimentary carbonate precursor and/or during diagenesis. Unlike g l3C, calcite and dolomite display two well-separated groups of tJlSO-values' close to + 19 and + 24%0, respectively. The exception is dolomicrite ANIO, with a ~ 180 lower by 4-5%0 relative to replacement dolomite from the same outcrop. Both carbonate phases are considerably depleted in 1sO relative to modem (Gross, 1964; McKenzie, 1981; Land, 1980, 1989) and most Triassic analogs (Veizer and Hoefs, 1976). All these low glsO signatures may be attributed to recrystallization (and oxygen isotopic exchange) of calcium carbonate and its dolomitization during diagenesis under the influence of water (meteoric?) depleted in 1sO relative to seawater or/ and at elevated temperature. An increase in temperature may be inferred as a precursor to the hydrothermal activity responsible for the vein (and stratiform) siderite mineralization in the area. Indication on the temperature (and 5180 of water) cannot be obtained from the l8O and '3C enrichments shown by the only one available dolomite-calcite pair PG24, as the relative geothermometers are far from being firmly established (e.g., Friedman and O'Neil, 1977; Matthews and Katz, 1977; Land, 1980). We can only point out that temperatures between 80 ° and 100°C are calculated from the calcite ~lSO-values, assuming seawater of gls 0 = 0%0 and using the well-establishedfractionation curve of O'Neil et al. ( 1969 ). When compared with the thickness of 150-250 m of the Lower-Middle Triassic rocks in the area (Frizzo, 1984 ), these thermometric estimates
301
may account for an anomalously high heat flow regime during diagenesis of the carbonate sediments under study. Finally, the (S7Sr/86Sr)o ratios of carbonate rocks are notably greater than that of seawater strontium of Early Triassic age [not higher than ~0.7075 (Burke et al., 1982) and time scale of Odin ( 1982 ) ]. This can be attributed to the contribution of strontium rich in 875r from the non-carbonate fraction of samples during the analytical procedure and, at least partly, to the incorporation into the carbonate fraction of radiogenic 878r from continental sources during sedimentation and diagenesis. 7. Conclusions
The genetic model we propose for the vein and stratiform siderite mineralizations of Lombardy Valleys from the isotopic results obtained in the present study is the following: ( 1) Vein siderite would have deposited from hydrothermal fluids, where oxidized carbon should have been largely provided by a deepseated source, and Sr (and Fe) by leaching of rocks passed through. (2) Stratiform siderite would be metasomatic, replacing calcite rather than dolomite in the host carbonate rocks. Carbon and oxygen in the parental solution should have been contributed by the metasomatizing fluid and host limestone, and the isotopic composition of siderite controlled by the glSO(H20) and 13C ( H C O 3 - ) as well as by the temperature of precipitation. Strontium can be regarded simply as a mixture. (3) Vein siderite and stratiform siderite should have deposited from the same kind of fluids, and possibly are the products of the same mineralizing event. Finally, the isotopic features of country carbonate rocks match with their deposition in a terrigenous-marine environment, followed by early diagenetic recrystallization and dolomitization possibly under the influence of meteoric water and/or at elevated temperature.
302
Specimens from siderite-bearing outcrops or from barren outcrops are isotopically indistinguishable; in addition, calcite and associated siderite appear to be in isotopic disequilibrium for both carbon and oxygen. These features concur to support that the mineralization is later than early diagenesis of the host sediments.
(i, CORTECCI AND P. FRIZZ()
Vein siderite.trom the crystalltne basement SC9, LOR4 and TOR4: spathic+cataclased siderite, with local recrystallization and recementing by siderite, quartz _+ankerite; traces of sulfide minerals. REG 10, PIN3 and T5: spathic + cataclased siderite, locally recrystallized and recemented by veinlets of quartz+_carbonates; traces of sulfide minerals, namely pyrite and chalcopyrite. Country carbonate rocks (Servino Fm.)
Acknowledgements This paper benefited from discussion with G. Leone. Many thanks are due to G.F. Del Chicca (Istituto Internazionale per le Ricerche Geotermiche, CNR, Pisa) for carbon and oxygen mass-spectrometric analyses, M. Menichini for X-ray fluorescence analyses, S. Tonarini (Istituto di Geocronologia e Geochimica Isotopica, CNR, Pisa) for Sr isotope analyses, R. Tosi for drafting all but one figures and Kristina Unterauer for having improved the English. Appendix - - Description of the siderite and carbonate rock specimens studied in the present work Strat(form siderite.from the Servino Fm. LAVI and LAV36: massive medium-grained siderite with silicoclastic relics, rare pyrite granules, traces of chalcopyrite and of interstitial clayey-bituminous matter; later veinlets of quartz +-ankerite + hematite recement the fractured siderite in LAV36. ES(1) and ES(e): massive siderite with widespread micritic relics and some silicoclasts. FS5 and FS20: massive spathic siderite with traces of interstitial organic and clayey matter; quartz in detritic granules with syntaxial growth and cherty aggregates; later barite veinlets. CT4 and CT6: massive, coarse-grained, spathic siderite with local micrite relics and ghosts of oolitic-bioclastic structures; silicoclast relicts; detritic quartz with syntaxial intergrowths and chert; traces of interstitial organic matter. Vein siderite from the Servino Fm. MAN9 and MAN 13: spathic siderite cementing a breccia of siltite with micrite; quartz + ankerite recement later fractures.
PG24 :dolosparite. FO14 :recrystallized and dolomitized oosparite with sparse silicoclasts. FOI8 :silty-sandy dolosparite. F039 :crystalline dolomia with scattered silicoclasts and glauconite. F050 :Myophoriae limestone with oolites and micritic cement. CTX :oolitic-intraclastic biosparite with sparitic cement (adjacent to the siderite "bank" CT6); silicoclats (quartz with syntaxial growth) and chert; traces of organic matter. IS7 :packstone with microsparitic cement with ooids, bioclasts and intraclasts. IS18 :sandy-silty microsparite. FS6 :oolitic-bioclastic packstone with micritic cement. ANIO :silty dolomicrite. A N I 6 :dolomitic-arenaceous limestone. AN22 :dolomitic-arenaceous rudstone, oolitic with gasteropods and local silicization. AN35 :dolomitized oolite with gasteropods. AN53 :silty-sandy limestone.
References Bottinga, Y., 1969. Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxide-graphite-methane-hydrogenwater vapor. Geochim. Cosmochim Acta, 33: 49-64. Burke, W.H., Denison, R.E., Hetherington, E.A., Koepnick, R.B., Nelson, H.F. and Otto, J.B., 1982. Variations of seawater 87Sr/86Sr throughout Phanerozoic time. Geology, 10:516-519. Carothers, W.W., Adami, L.H. and Rosenbauer, R.J., 1988. Experimental oxygen isotope fractionation between siderite-water and phosphoric acid liberated CO2-siderite. Geochim. Cosmochim. Acta, 52: 24452450. Casati, P., 1969. Le fasi orogenetiche nelle Alpi Meridionali, Artigianelli Paviniani, Monza, 49 pp. Cortecci, G., Lattanzi, P. and Tanelli, G., 1985. C- and Oisotope and fluid inclusion studies of carbonates from pyrite and polymetallie ore deposits and associated
ORIGINOF SIDERITEDEPOSITSFROMTHELOMBARDYVALLEYS,NORTHERNITALY country rocks (southern Tuscany, Italy). Chem. Geol. (Isot. Geosci. Sect.), 58: 121-128. Curtis, C.D. and Spears, D.A., 1968. The formation of sedimentary iron minerals. Econ. Geol., 63: 257-270. Faure, G., 1986. Isotope geology of strontium in sedimentary rocks. In: G. Faure (Editor), Principles of Isotope Geology. Wiley, New York, N.Y., pp. 183-199. Friedman, I. and O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. U.S. Geol. Surv. Prof. Pap., 440K: 1-12. Frizzo, P., 1984. Le mineralizzazioni a barite e a siderite nel Permiano Superiore e nello Scitico della Val Camonica (Lombardia). Ind. Miner., 4:21-43. Frizzo, P. and Scudeler Baccelle, L., 1983. Rapporti strutturali e tessiturali fra mineralizzazione a siderite e litotipi carbonatici nel Servino (Scitico) delle Valli Lombarde. Mere. Sci. Geol. (Padova), 36: 195-210. Gross, M.G., 1964. Variations in the ~sO/160 and lac/ ~2C ratios of diagenetically altered limestones in the Bermuda Islands. J. Geol., 72:170-194. Hoefs, J., 1987. Stable Isotope Geochemistry. Springer, Berlin, 241 pp. Land, L.S., 1980. The isotopic and trace element geochemistry of dolomite: the state of the art. In: D.H. Zenger (Editor), Concepts and Models of Dolomitization. Soc. Econ. Paleontol. Mineral., Spec. Publ., 28: 87-110. Land, LS., 1989. The carbon and oxygen isotopic chemistry of surficial Holocene shallow marine carbonate sediments and Quaternary limestone and dolomite. In: P. Fritz and J.Ch. Fontes (Editors), Handbook of Environmental Geochemistry, Vol. 3, The Marine Environment, A. Elsevier, Amsterdam, pp. 191-217.
303
Matthews, A. and Katz, A., 1977. Oxygen. isotope fractionation during dolomitization of calcium carbonate. Geochim. Cosmochim. Acta, 41: 143 l - 1438. McCrea, J.M., 1950. On the isotope chemistry of carbonates and a paleotemperature scale. J. Chem. Phys., 18: 849-857. McKenzie, J.A., 1981. Holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of Abu Dhabi, U.A.E.: a stable isotope study. J. Geol., 89: 185-198. Odin, G.S., 1982. The Phanerozoic time scale revisited. Episodes, 3: 3-9. O'Neil, J.R., Clayton, R.N. and Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys., 51: 5547-5558. Rosenbaum, J. and Sheppard, S.M.F., 1986. An isotopic study of siderites, dolomites and ankerites at high temperatures. Geochim. Cosmochim. Acta, 50: 11471150. Sharma, T. and Clayton, R.N., 1965. Measurements of tsO/~60 ratios of total oxygen of carbonates. Geochim. Cosmochim. Acta, 29:1347-1353. Taylor, B.E., 1986. Magmatic volatiles: isotopic variation of C, H, and S. In: J.W. Valley, H.P. Taylor, Jr. and J.R. O'Neil (Editors), Stable Isotopes in High Temperature Geological Processes. Mineral. Soc. Am., Rev. Mineral., 16: 185-225. Timofeyeva, Z.V., Kuznetsova, L.D. and Dontsova, Y.I., 1976. Oxygen isotopes and siderite formation. Geochem. Int., 13(5): 101-112. Veizer, J.D. and Hoefs, J., 1976. The nature of ~sO/~60 and~3C/~2C secular trends in sedimentary carbonate rocks. Geochim. Cosmochim. Acta, 40:1387-1395.