Clay Minerals (1977) 12, 147.
P E T R O G R A P H I C A N D I S O T O P I C S T U D I E S OF
T H E A L T E R E D A C I D V O L C A N I C S OF T H E
TOLFA-CERITE AREA, ITALY:
T H E G E N E S I S OF T H E C L A Y S
G. L O M B A R D I
AND S. M . F . S H E P P A R D *
Istituto di Mineralogia e Petrografia, University of Rome, Italy, and
*Scottish Universities Research and Reactor Centre,
East Kilbride, Glasgow, Scotland
(Received 1 January 1976; revised 29 August 1976)
A B S T R A C T : Alunite-kaolinite deposits occur in argillized zones in acid volcanic rocks
at Tolfa. Over seventy samples were examined by one or more of the following methods:
thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope
analysis. Kaolinite and dickite, of high structural order, are the dominant clays with micamontmorillonite and halloysite subordinate. In many samples kaolinite and dickite coexist;
their relative amounts are variable but the dickite content tends to increase with depth.
SEM studies show that dickite crystals--up to 30/tm--are bigger than kaolinite.
The D/H and x80/160 ratios of six days, a whole rock, two biotites from fresh host rock,
two chalcedony veins and three local meteoric waters indicate that the clays formed in a
meteoric-hydrothermal environment of acid hot spring type at about 80~ The fresh
volcanic host rocks are strongly enriched in 180 relative to 'normal' igneous rocks due to
derivation from or exchange with 1SO-rich sedimentary rocks at depth prior to hydrothermal and solfataric activity. In the absence of present-day geothermal activity the life of
this hot spring system was less than 4 m.y.
GEOLOGICAL
SETTING
Northwest o f R o m e (Fig. 1) extensive outcrops o f acid volcanics o f Late-Pliocene to
Quaternary age (Devoto & Lombardi, 1977) are locally intensely altered to clay minerals
and sulphates o f the alunite group. The igneous rocks which occur in three separate
districts: Tolfa, Cerite and Manziate, intrude and overlie the Cretaceous-Palaeogene
flysch sediments that constitute the local basement and, in places, overlie the psammiticpelitic facies o f the Mio-Pliocene rocks. The volcanic rocks are ignimbrites, in places cut
and uplifted by lava domes o f typical morphology. Petrographically most o f them are
liparites, trachyliparites and quartz latites, with latites and trachyandesites restricted to
a few limited outcrops. Tufts sensu stricto are absent, whilst in the Tolfa district there
are some related hypabyssal rocks in the sediments. Pyroclastic rocks related to the
Quaternary Sabatini volcanic group and Quaternary sediments locally overlie these
volcanics.
Deep alteration o f the volcanic rocks is very c o m m o n in the Tolfa district, m u c h less
in the Cerite and absent in the Manziate district. Volcanic and sedimentary basement
rocks have been silicified and feldspathization occurs at Ripa Maiale ( N W o f Allumiere).
The alunite-kaolinite deposits occur in intensely argillized zones within the more
147
148
G. Lombardi and S. M. F. Sheppard
/
O
8
r
.~
~
4
.-_-
.-=
,z.
..
o "~
E
~9 [""
E
_=
t
~A
O
,..~
%
o
.o
O
:g_
[--
The genesis of Tolfa clays
149
pervasively altered volcanics (see Fig. 1 in Field & Lombardi, 1972). Extensive gossans
are developed locally.
Alunite and other members of this sulphate group are common alteration products in
the areas where sulphides and/or their oxidation products are abundant. Variable quantities of clay minerals, principally kaolinite and dickite, quartz, opal, iron compounds and
relict minerals from the parent volcanic rock are associated with the alunite. Narrow
veins and veinlets of quartz and chalcedony are locally important. From a study of the
distribution pattern of the different alteration minerals, the relationship among alunitesulphide-clay assemblages in a single sample and variation in their relative abundances
in vertical section in some quarries, it is evident that after the volcanic events there were
different alteration processes, which possibly overlapped in space and time.
Sulphur isotopic studies (Field & Lombardi, 1972) have demonstrated that the sulphur
of the alunites was derived from the oxidation of the sulphides (principally pyrite with
minor marcasite, cinnabar, galena, antimonite and tennantite). In this study the petrology,
mineralogy, the hydrogen and oxygen isotope data of the kaolin deposits have been
determined to establish their genesis and relationship with alunite and the post-magmatic
processes.
GENERAL CHARACTERISTICS AND DISTRIBUTION
OF T H E A R G I L L I C R O C K S
The unaltered volcanic rock consists mainly of K-feldspar and plagioclase, glass, quartz,
biotite and pyroxene. The range of alteration is appreciable. Macroscopic and microscopic structural characteristics of the original volcanics often survive the destruction of
much of the primary mineralogy. The groundmass may be argillized, while the phenocrysts retain a transparent appearance with only incipient alteration along their cleavage
planes. In other examples the feldspar phenocrysts or phenoclasts are replaced. The most
intensely altered rocks are white to off-white and friable, containing only small fragments
of the original volcanics and occasional fine-grained compact nuclei with subconchoidal
fracture associated almost exclusively with kaolinite and/or dickite. Silicification frequently accompanies argillization and alunitization. Veins, veinlets and concentrations
of dense, microcrystalline quartz, chalcedony and opal with a characteristic grey-brown
colour occur in areas of intense alteration.
About 70~ of the outcrops of volcanic rocks in the vicinity of the villages of Allumiere
and Tolfa are argillized, producing a characteristic scenery of whitish hill sides and quarry
faces with locally red patches of iron oxides. Relatively fresh volcanic rocks occur along
the north and north-east sides of the complex. The intensity of argillization increases
progressively to the south, together with an increase in alunite and sulphide-bearing
veins. The complete range alunite ~ clayey alunitic rocks ~ clays is present in the
deposits. At depth clays appear to prevail.
There are minor outcrops isolated from the main massif (Fig. 1). At La Montagnola
argillization is weak, and at Tolfaccia alteration is of minor significance. An outcrop of
a few hectares at Fosso Eri (near S. Severa) is intensely argillized and was exploited until
recently.
Argillized volcanics are found only at the western end of the Cerite massif. The intensity of the alteration processes is such that complete destruction of the original structural
characteristics of the volcanics has frequently occurred. The kaolins are worked at the
D
150
G. Lombardi and S. M. F. Sheppard
large quarry of Mount Sughereto. Alunite and disseminated sulphides, which are present
in the altered rocks, are absent from the fresh volcanics, emphasizing again the connection between argillization, alunitization and the presence of sulphides.
The intense alteration observed in the other districts is completely lacking in the
Manziate district. Minor alteration of the volcanics is related to the later Sabatini
volcanism and to superficial effects.
200
400
600
800
1000
Temperature (~
FIG. 2. D T A curves o f s o m e typical kaolinitic-dickitic rocks f r o m t h e Tolfa-Cerite district.
The dickite/kaolinite ratio increases from H18 to H41. Apparatus based on Mackenzie &
Mitchell (1959); Chromel-alumel thermocouples; nickel sample-block; h.r. 10~
Sensit. 70 pV/in; N2 atmosphere; sample weight 150 rag; ref. material: calcined kaolinite.
Few data are available on argillization in the Tolfa-Cerite district. Fuchs & De Launay
(1893) suggested the alunite deposits were formed by 'solforic' solutions attacking the
volcanics producing sulphate and then kaolin. De Launay (1913) proposed that alunite
and kaolinite formation was related to 'secondary' alteration of the volcanics. When
sulphide-rich dykes were present alunite formed. Studies by Moretti & Peretti (1938)
considered the industrial uses of the kaolin, and Negretti & Morbidelli (1963), Negretti
et al. (1966) and Caneva & Mattias (1977) give data on some samples from various
deposits.
The genesis of Tolfa clays
151
M I N E R A L O G Y OF T H E A R G I L L I C R O C K S
In this investigation seventy samples, representative of the different rock types, were
selected from four quarries (up to about 50 m deep) and examined by XRD, DTA, TG,
SEM, IR and chemical methods. The principal clay minerals found were kaolinite and
0
H42
VVo " L
LI,
49 ~
,=,
~,,
l[
[
40 ~
I
32 ~
I
i
I
I
I
l
I
I
I
22 ~
FIG. 3. X R D traces for s o m e typical kaolinitic-dickitic rocks f r o m t h e Tolfa-Cerite district.
Philips diffractometer with proportional counter; CoKc~ radiation; s c a n n i n g speed 1 ~
iron filter on b e a m exit.
dickite, with occasionally a regular mixed-layer mineral of the illite-montmorillonite
type, an open structure illite (11 •) and halloysite.
Kaolinite and dickite are normally present in very variable proportions. DTA curves
of whoIe rocks are shown in Fig. 2. The dehydroxyIation peak temperature of kaolinite
152
G. Lornbardi and S. M. F. Sheppard
ranges from 530 ~ (H46) to 575 ~ (H10), with higher values corresponding to higher degrees
of crystallinity, as found by X-ray diffraction. The dehydroxylation peak temperature of
dickite is between 650 and 660~ For both minerals the exothermic high temperature
peak occurs between 990 and 1010~ Results of D T A on different size fractions demonstrate that in the natural mixtures dickite is dominant in the > 5 #m fraction, whilst
kaolinite crystals are of smaller dimensions. The X-ray diffraction patterns (Fig. 3) show
4000 3750 5500 1350
1200
I000
800
600
400
Wavenumber ( c m - I )
FIG. 4. Infrared absorption spectra of kaolinitic-dickitic rocks of the Tolfa-Cerite district.
The dickite/kaolinite ratio increases from H18 to H41. Grubb Parsons 'Spectromaster'
infrared spectrometer; 12 mm diameter pellet containing 1 mg sample in 170 mg KBr
pressed at 8 ton/in2; one run at ambient temperature, one after heating at 100~
a progressive shift in the position of the reflections from predominantly kaolinitic rocks
(H18) to those with dickite as the main mineral (H41, 42). The shape and height of the
reflections and the doubling of the peaks at dhk~4"06--4"13 and dhk~2"31--2"34 for sample
H10 are characteristic of a well-crystallized kaolinite. Attempts to quantify the 'crystallinity' of the kaolinite by measuring the integrated line width of the (001) peak and also
the crystallinity index of Hinckley (1964) were largely unsuccessful due to the presence
of variable amounts of dickite.
Infrared analyses show a progressive, regular variation in the position, shape and peak
The genesis of Tolfa clays
153
dimension ratios of the absorption bands, when passing from kaolinite-rich to dickiterich samples (Fig. 4). Height and shape of the absorption bands in the 1104-1121 cm -1
region can vary, according to Farmer & Russell (1964), because of variations in crystal
dimensions. In this study DTA, SEM and X-ray analysis confirm that the dickite
crystals are larger than those of kaolinite and consequently have sharper absorption
bands.
Fie. 5. Scanning electron micrographs of fracture surfaces of kaolinitic-dickitic rocks from
the Tolfa-Cerite district. Explanation in the text.
154
G. Lombardi and S. M. F. Sheppard
Fracture surfaces of the rocks (Fig. 5) were examined by scanning electron microscopy.
In the dickite-rich samples (Figs. 5a and 5b) large crystals, up to 30 pro, are prevalent
whereas kaolinite crystals, although well formed, are generally smaller and in less ordered
stacks and packets (Fig. 5c). Dickite often occurs in regular intercrossing veins (Fig. 5d)
consisting of packets of crystals with a well-developed hexagonal shape. A few elongated
prismatic crystals, with a strong tendency for euhedral development, are also present
(Fig. 5e). They are very similar to dickite crystals observed in limestone of the Pennsylvanian of Kansas, U.S.A. (Bohor & Hughes, 1971) and a microprobe analysis showed
that their composition is compatible with dickite.
Halloysite is common in the alunitic assemblage of the Tolfa district. It tends to be
more abundant than kaolinite when the sulphate content is high (Lombardi, 1967). In
the argillic rocks investigated in this study there was no evidence for halloysite using
DTA and X-ray methods. However, aggregates of elongated, tubular crystals, strongly
resembling halloysite as identified in the alunitic rocks, were observed (Fig. 5f) on some
of the scanning electron micrographs.
Two other phyllosilicates are normally present in these argillic rocks and are particularly
abundant in the Sughereto quarry. Caneva & Mattias (1976) identified them as: (i) a
regular mixed-layer mineral of the illite-montmorillonite type (untreated sample peaks
at 23.2 and 11'6 A, glycerolated at 27.1-13.6 and 9"3 A, after 2 h at 550~ one peak at
9.8 A; (ii) a random mixed-layer mineral of the illite-montmorillonite type; untreated
samples show a peak at 10.7 A, when treated with glycerol they resolve in a double peak
at 9.8 and 12.6 A, at 550~ for 2 h only the 10 A peak remains.
The following conclusions are drawn for the argillic rocks of the Tolfa-Cerite district:
(a) The mineral assemblages, the relative proportions of the minerals and their structural characteristics are variable and can be found in all the quarries.
(b) In vertical section the minerals tend to be distributed as follows: 1--enrichment
of alunite in the upper levels; 2--kaolinite is dominant and uniformly distributed;
3--dickite occurs mainly in the deeper parts of the quarries; 4--halloysite is more
abundant in the alunitic rocks.
(c) On the scale of a few millimetres kaolinite, dickite and alunite coexist.
(d) Kaolinite is the most abundant clay mineral, followed by dickite. Both have either
low or nil degree of structural disorder; dickite consists of larger crystals and has a higher
degree of crystallinity.
(e) Halloysite is common in the alunite-rich rocks, but rare in the argillic assemblages.
As suggested by Lombardi (1967) the formation of halloysite is probably favoured by
the strongly acid environment that is implied by the formation of alunite.
ISOTOPIC ANALYSES
Samples
Six clay-rich samples from four quarries in the Tolfa-Cerite districts were selected for
hydrogen and oxygen isotope analyses (Table 1). Their mineralogy was established by
XRD, and the amount of quartz relative to kaolinite and/or dickite determined semiquantitatively from random preparations using the boehmite peak as a standard (Thompson et al., 1972). TG and DTA were also used with quartz as an internal standard. The
amorphous silica and alumina contents are less than 1~ (Na2 CO3 solution treatment
The genesis of Tolfa clays
155
method). Some of these samples were enriched in clays by centrifuging off the < 2/tin
fraction in deionized water.
Three spring waters, including two flowing out of the Tolfa volcanics, a whole rock
TABLE 1. Mineralogy of samples analysed
Kaolinite minerals
Other phyllosilicates
Quartz
Feldspars
Alunite
Amorphous SiO2
Amorphous A1203
H18
Tolfa
Casalavio
H28
Tolfa
Cim. Allum
H8
Cerite
Sughereto
H10
Cerite
Sughereto
H12
Tolfa
C. Vecchie
H43
Tolfa
Cim. Allure.
Kaolinite
> 90~
tr
--.
0-22
0.43
K a o l ~ Dick Kaol>>Dick Kaol>>Dick Kaol > Dick Dick~>~ Kaol
> 90~
> 85~
> 85~
> 90~
> 85~
tr
tr
tr
tr
tr
4.5~
--4-0~
--5~
5~
--.
.
.
.
7~
0"57
0-04
0-04
0.20
0'15
0.50
0"42
0-31
0.49
0'39
TABLE 2. Description, location and hydrogen and oxygen isotope analyses of samples
Sample
No.
T1
K2
T3
T4
H8
H10
H12
H18
H28
H43
IW3
IW4
IW5
Description and location
Quartz latite lava; Poggio Casalavio (Fig. 1)
Ignimbrite; Sasso della Strega, 2-5 km N E of Tolfa
1-2 cm wide vein cutting altered volcanics; La Cavaccia, E. of
Allumiere
1 cm wide vein cutting altered volcanics; Cimitero di Allumiere
(Fig. 1)
Altered volcanics; Sughereto quarry; 8 m from surface
Altered volcanics; Sughereto quarry; 20 m from surface
Altered volcanics; Cave Vecchie quarry; 3 m from surface
Altered volcanics; Poggio Casalavio quarry; 5 m from surface
Altered volcanics: Cimitero di Allumiere quarry; N 16 m from
surface
Altered volcanics; Cimitero di Allumiere quarry; ~ 4 8 m from
surface
Spring mineral water; La Rota, E. of Tolfa
Spring water from volcanics; La Bianca, near Allumiere
Spring water from volcanics; Tolfa
Mineral*
W.R.
Biotite
Biotite
0Dr
018Or
- 70
- 75
16.2
14'3
14'2
Chal.
15-3
Chal.
K>~ D
K>~D
K>D
K
-36
-37
-37
-52
13.2
13-4
13-5
13.1
12-1
K~>~D
- 45
10'5
D >),K
Water
Water
Water
- 44
- 37
--38
-37
10.4
- 6'7:~
--6'8
-6'7
* Abbreviations: W.R. = whole rock; c h a l . = chalcedony; K = kaolinite; D = dickite.
t Analytical precision: +0.1-0.2 per mil for oxygen; _+1 per mil for hydrogen.
~: Calculated value using meteoric water relationship.
and two biotites from the fresh volcanics, and two chalcedony samples from veins within
t h e a l t e r e d v o l c a n i c s w e r e a l s o a n a l y s e d ( T a b l e 2).
Experimental
Hydrogen and oxygen were extracted quantitatively from the mineral, whole rock and
G. Lombardi and S. M. F. Sheppard
156
water samples by well-established analytical methods (Bigeleisen et al., 1952; Clayton &
Mayeda, 1963).
All adsorbed and/or interlayer water was removed from the clay minerals and biotites
at temperatures up to 200~ while under vacuum. Isotopic ratios were determined on
H 2 and CO2 gases in McKinney-Nier type mass spectrometers with a general analytical
precision of 0"1-0"2 per mil for oxygen and 1-2 per mil for hydrogen. Our 6180 value
for NBS 28 is 9.60 per rail.
*
Chalcedony veins
I
0
[]
x
- 20
-
9 SMOW
8iotite- Tolfa
Whole Reck-Tolfa
9 Waters
A Kanlinites from Hot Springs
Tolfa - Cerite
9- ~k-,
Tolfa
40
if
-
/ H43
J?~H~
s oo-"
sJ
',~wQ , '
/
f
/
"Normal"
u)
Igneous
1-:] X
[]
f
/
80
S
Sulphur
,--', Rank
Rocks
C~
Lassun
-
~
H t~
iJ
6O
0
-
*
Q Clay samples-Tolfa Cerite
100
A
,~x)
f ;.~
J ~
,/
/
/-~
-120
-
//
%o~- . "
,~'..
/
140
/
Yellowstone
-160
-20
-16
-12
-8
-4
s
0
4
8
12
16
20
24
(%o)SMOW
Fit. 6. Plot of 6D versus 01sO for all the kaolinite, dickite, biotite, whole rock and water
samples and 0180 of the chalcedony veins from the Tolfa-Cerite districts. Meteoric water
(Craig, 1961) and kaolinite-weathering (Savin & Epstein, 1970) lines are given for reference.
Also plotted are clays and waters from acid hot spring environments (Craig, 1963 ; Sheppard
et aL, 1969) and the range for 'normal' igneous rocks.
The data (Table 2) are presented as 6 values defined as the difference in D/H or
180/160 ratios between the sample and standard mean ocean water (SMOW), in parts
per thousand. The fractionation of isotopes between any two phases A and B is given as
A A _ B = 1000 In ~ A-B ~--- 6A--OBwhere ~ is the fractionation factor.
Isotopic results
The data are plotted in Fig. 6. The meteoric water line representing the isotopic
composition of most present-day meteoric waters (Craig, 1961) and the kaolinite line
The genesis of Tolfa clays
157
defining the isotopic variations of kaolinite from surface weathering environments (Savin
& Epstein, 1970) are given for reference. The field representing the relatively restricted
range of D/H and 180/160 ratios of 'normal' igneous rocks is also given (Taylor, 1974).
The biotites and whole rock samples from the unaltered volcanic rocks are extremely
enriched in 180 compared with 'normal' igneous 8180 values. The D/H ratios are
'normal'. The six kaolinitic samples, with quite similar ~ D and ~180 values, plot within
a restricted field that lies between the meteoric water line and the kaolinite line. They are
enriched in deuterium relative to the fresh volcanics. Two chalcedony samples from
veinlets within the altered vocanics are slightly enriched in 180 relative to the clays. The
spring waters plot by the meteoric water line; the small displacement is similar to that
observed for eastern Mediterranean surface waters (Dansgaard, 1964).
ARGILLIZATION
The large gap between the field for the kaolinitic samples and the kaolinite line (Fig. 6)
establishes that these clays formed in a hydrothermal environment (Sheppard et al.,
1969; Sheppard, 1977). Kaolinite in equilibrium with the Tolfa spring waters would have
compositions on the order of6D _~ -70%0 and 6180 -~ +20~o (Savin & Epstein, 1970).
Thus the clays are dramatically out of equilibrium with present-day local meteoric waters
and this also argues strongly against any significant retrograde oxygen or hydrogen
isotope exchange.
Two clays were analysed from one of the Tolfa quarries (H28, H43) and one of the
Cerite quarries (H8 and H10). In both cases these sample pairs are essentially identical
isotopically. From Tolfa, H43 is dickite rich, while H28, from an upper level, is kaolinite
rich. It would be expected that dickite and kaolinite that formed under identical conditions
of T, 6Dwatet, (~lSOwate r t o be isotopically very similar. These data therefore suggest that
factors other than temperature probably controlled the formation of dickite with crystals
of larger dimension and higher crystallinity relative to kaolinite. For example differences
in the chemistry or pH of the fluids may be critical.
ISOTOPIC COMPOSITION OF THE HYDROTHERMAL
SOLUTIONS
The hydrogen and oxygen isotope kaolinite-water fractionation factors in Table 4 of
Sheppard et al. (1969) are applied to calculate the isotopic composition of water in
equilibrium with the clays over a range of temperatures. For dickite H43 this is shown as a
band on Fig. 7. These kaolinite-water fractionation factors are not known precisely so
the band represents an estimate of the uncertainties. The upper temperature of 300~
is given by the upper stability limit for kaolinite (Hemley, 1959). The lower temperature
limit is given approximately by the point where the calculated hydrothermal water band
intersects the meteoric water line. More 180 depleted water at this ~ D value are geologically unrealizable. Hence a minimum temperature of formation for dickite H43 is about
70~
Waters in equilibrium with biotites from the unaltered volcanics at magmatic temperatures would have 6 D values of about - 5 0 per mil--well within the range of 'normal'
158
G. Lombardi and S. M. F. Sheppard
magmatic w a t e r - - a n d 6~aO values of about +17 per mil (Taylor, 1974). Thus, the
D / H and 180/~60 ratios of magmatic-hydrothermal solutions are distinctly different
from our calculated hydrothermal waters responsible for argillization. Therefore,
meteoric-hydrothermal waters were dominant in the Tolfa-Cerite geothermal systems.
From the study of active hot spring systems Craig (1963) has shown that such waters
are of meteoric origin. In acid hot springs the isotopic composition of the waters become
enriched in both deuterium and oxygen relative to the initial meteoric water composition
due to non-equilibrium evaporation at temperatures of about 70-100~ The composition
of the waters typically plot about a line of slope about 3. Therefore, meteoric waters
during geothermal activity could have been more deuterium depleted than our point of
-I0
/'~-\'-~
/Band forH20 in equilibrium withclayH43
T-'x ~ ~ ~ ~
-30
s
To'zIfCerite
t/H~3
~x~''
-50
-70 I
-8
Cetit-~(~]/t
eT~
lI
~*\
I
I
I
I
I
-4
0
4
8
12
t
8180 (~) SMOW
FIG. 7. ~D versus 61sO diagram for the kaolinites, dickites and water samples from the
Tolfa-Cerite districts, showing the field for the calculated isotopic composition of H20
that would be in equilibrium at the stated temperatures with clay H43. The acid hot spring
water trend is drawn through the Tolfa water samples.
intersection (6 D ~ 20). A maximum limit of deuterium depletion is given approximately
by present-day meteoric waters, and on Fig. 7 a line of slope 3 is drawn through this
point. Similar trends for some other acid hot spring areas are shown in Fig. 6. Note that
a coexisting kaolinite and water from Yellowstone Park, Wyoming, is included. These
samples came from a mud pot at 80~ in the acid hot spring part of the system.
Thus it is proposed that argillization occurred in an acid hot spring environment at
temperatures on the order of ~ 80~ Clays H43, H28 and H8, H10, H12 fall on a trend
approximately parallel to the acid hot spring trend and hence could all have formed at
a similar temperature, with the isotopic variation representing different degrees of
evaporation. H18 may have formed at a slightly lower temperature ( ~ 2 0 ~ lower) from
waters isotopically like those associated with H43 or from slightly different waters that
had been modified by processes such as mixing and exchange with the rock reservoir
system.
The genesis of Tolfa clays
159
SILICIFICATION
As a partial check on the proposed conditions during argillization, two chalcedony
samples were analysed from narrow subvertical veins which cut the heavily argillized
and alunitized volcanics. Chalcedony T-4 comes from the same quarry as samples H43
and H28. Although little is known specifically about the paragenetic relationships
between the chalcedony and the kaolinite, these veins probably represent the channelways
tbr the hydrothermal solutions responsible for the alteration of the volcanics. Assuming
6180,,ater ~-- --1 for the hydrothermal fluids, as derived from the data for H43, and
combining this with the estimated quartz-water oxygen isotope fractionation expression
(Knauth & Epstein, 1975), a temperature on the order of 140_+25~ and 120+25~ is
given for the two chalcedony samples if equilibrium was attained (estimated uncertainties). These calculated temperatures assume that chalcedony and quartz have essentially
identical isotopic properties. Also no adjustment has been made for the possible effect
of salinity on the isotopic behaviour of 'water' (Truesdell, 1974). Minimum temperatures
of I00 and 80~ are given respectively by taking our most ~80-depleted estimate of the
ratio in the meteoric water.
Considering the uncertainties associated with both the kaolinite-water and quartzwater fractionation factors the chalcedony data are interpreted to support the proposed
meteoric-hydrothermal environment. The small differences in the 'temperatures ~may be
real. They could imply that chalcedony crystallized at a higher temperature than the
kaolinites or that they formed at different times from hydrothermal solutions with
different 180/160 ratios (different degrees of evaporation effect). Very minor retrograde
exchange of the clays could also be possible.
CONCLUSIONS
In the Late Pliocene-Quaternary acid volcanic rocks, strongly enriched in ~80 relative
to 'normal' igneous rocks (Fig. 6), were extruded in the Tolfa-Cerite region. The unusual
oxygen isotope values suggest that the magmas were derived from or exchanged with
180_rich meta-sedimentary rocks at depth. The 'normal' D/H ratios are not incompatible
with either of these origins. Taylor & Turi (1976) have presented oxygen isotope evidence
for large-scale crustal assimilation during magma genesis in the nearby Tuscan and
Roman Province.
Hydrothermal activity followed with the deposition of sulphides, quartz, calcite, barite
and fluorite at temperatures up to 300~ (Ferrini, 1975). During the waning stages of the
geothermal system and possibly before the major argillization processes took place silica
was deposited in the veins from the meteoric-hydrothermal solutions. Argillization and
alutinization developed at temperatures of about 80~ near the end of the life of the
geothermal system. The net outflow of water from depth must have been low because
meteoric water near the boiling point was undergoing evaporation at the surface before
moving downwards through the permeable volcanics that were undergoing acid attack.
The areas of most intense argillization were probably localized over local heat sources
like those observed in active geothermal systems (Elder, 1965). There is no isotopic
evidence for a significant magmatic water contribution during argillization. Acids were
generated either from hydrogen sulphide rising with the thermal meteoric waters or from
sulphide minerals during oxidation processes in this near surface environment. The
160
G. L o m b a r d i a n d S. M . F. S h e p p a r d
p r o b l e m o f recognizing rising vs descending t h e r m a l fluids in such surficial locations has
been discussed b y Schoen e t al. (1974). The d e p t h o f the w a t e r table p r o b a b l y controls
the d e p t h to which a c i d - a l t e r a t i o n can take place. (Mining has n o t exceeded depths o f
a b o u t 50 m f r o m the surface.)
I n the Tolfa district the sulphur i s o t o p e c o m p o s i t i o n o f the sulphate minerals (alunite
a n d barite) a n d sulphides, principally pyrite, are similar. F i e l d & L o m b a r d i (1972)
i n t e r p r e t e d the f o r m a t i o n o f the alunite a n d barite as a consequence o f the superficial
o x i d a t i o n o f pre-existing sulphides d u r i n g supergene processes. T h e y f a v o u r e d a low
t e m p e r a t u r e weathering e n v i r o n m e n t for this. Nevertheless they n o t e d t h a t the d a t a were
also c o m p a t i b l e with a ' m o d i f i e d supergene hypothesis t h a t involves relatively high
t e m p e r a t u r e near surface o x i d a t i o n o f sulphide to sulphate'. M a n y o f their samples c o m e
f r o m the same quarries as the clay samples used in this study a n d clearly alunitization
a n d argillization are processes which are p a r t o f a m a j o r g e o t h e r m a l system a n d are
interrelated.
The h y d r o g e n , oxygen a n d sulphur isotope d a t a are all c o m p a t i b l e with the a l t e r a t i o n
o f the volcanic rocks a n d sulphides to argillic a n d alunitic assemblages, in an acid h o t
spring or solfatara environment. A l t h o u g h some o f the alunite m a y have f o r m e d at
similar t e m p e r a t u r e s to the clays, p a r t has f o r m e d in a supergene-weathering environment. Sulphides at the surface t o d a y are being oxidized to alunite. In the absence o f
p r e s e n t - d a y hot spring activity in the Tolfa complex, the life o f this g e o t h e r m a l system
was less t h a n 4 m.y.
ACKNOWLEDGMENTS
The mineralogical analyses for this work were carried out at the Macaulay Institute for Soil Research,
Aberdeen, Scotland, in 1973 by G. Lombardi under the sponsorship of the Royal Society. The first author
is deeply indebted to both Institutions for their help and support.
The Isotope Geology Unit is supported by the Scottish Universities and a N.E.R.C. research grant.
We are particularly grateful to J. Borthwick for his assistance in the laboratory.
We are grateful to B. D. Mitchell (Macaulay Institute), C. Lauro and G. C. Negretti (University of
Rome) and a reviewer for critical reviews of the manuscript.
REFERENCES
BIGELEISEN J.J., PERLMANM.L. & ROSSERH.C. (1952) Analyt. Chem. 24, 1356.
BOHOR B.F. & HUGHESR.E. (1971) Clays Clay Miner. 19, 49.
CANEVAC. & MATTIASP.P. (1977) Boll. Soc. GeoL It. (in press).
CLAYTONR.N. & MAYEDAT.K. (1963) Geochim. cosmochim. Acta 22, 43.
CRAIG H. (1961) Science 133, 1702.
CRAIG H. (1963) In: Nuclear Geology on Geothermal Areas (Tongiorgi E., editor), p. 17. Spoleto, Italy.
DANSGAARDW. (1964) Tellus 16, 436.
DE LAUNAYL. (1913) Traite de Metallogenie 2, 245.
DEVOTO G. & LOMBARD1G. (1977) Acad. Lincei It. Quaderno n. 227.
ELDER J.W. (1965) Geophys. Mon. 8 NAS-NRC No. 1288. Amer. Geophys. Union, 211.
FARMERV.C. & RUSSELLJ.D. (1964) Spectrochim. Acta 20, 1149.
FERRINt V. (1975) Ist. Petrograf. Univ. Rome, p. 55.
FIELD C.W. & LOMBARDIG. (1972) Mineral. Deposita 7, 113.
FucHs E. & DE LAUNAYL. (1893) Traitd des Gftes Mindraux et Mdtalliferes, Vol. 1, p. 607. Paris.
HEMLEu J.J. (1959) Am. J. Sci. 257, 241.
HINCKLEV D.N. (1964) Clays Clay Miner. 11, 229.
KNAU:rHL.P. & EPSTEINS. (1975) Earth Planet. Sci. Letters 25, 1.
The genesis o f Tolfa clays
161
LOMBARDI G. (1967) Period. Mineral. (Rome) 36, 399.
MACKENZIE R.C. & MITCHELL B.D. (1959) Clay Miner. Bull. 4, 31.
MORETTI A. & PERETTI L. (1938) Ind. Mineraria (Rome) 12, 405.
NEGRETTI G.C., LOMBARDIG. & MORBIDELLI L. (1966) Ist. Petrograf. Univ. Rome, p. 172.
NEGRETTI G.C. & MORBIDELLI L. (1963) Ist. Petrograf Univ. Rome, p. 91.
SAVlN S.M. & EPSTEIN S. (1970) Geochim. cosmochim. Acta 34, 25.
SCHOEN R., WHITE D.E. & HEMLEY J.J. (1974) Clays Clay Miner. 22, 1.
SHEPPARD S.M.F., NIELSEN R.L. & TAYLOR H.P. JR (1969) Econ. Geol. 64, 755.
SH~PPARO S.M.F. (1977) In: Volcanic Processes in Ore Genesis, p. 25. Inst. Min. and Metall. and
Geological Society of London.
TAYLOR H.P. JR (1974) Econ. Geol. 69, 843.
TAYLOR H.P. JR & TURI B. (1976) Contrib. Mineral. Petrol. 55, 33.
THOMPSON A.P., DUTHIE D.M.L. & WILSON M.J. (1972) Clay Miner. 9, 345.
TRUESOELL A.H. (1974) Earth Planet. Sci. Letters 23, 387.
R I ~ S U M I ~ : Des gisements d'alunite-kaolinite se pr6sentent en zones argilis6es dans des
roches volcaniques acides h Tolfa. Plus de soixante-dix 6chantillons ont 6t6 analys6s ~t raide
d'une ou de plusieurs des m6thodes suivantes: coupe mince, DTA, Rayons-X, IR, TG,
SEM, chimique, et analyse d'isotopes d'hydrog6ne et d'oxyg6ne. La kaolinite et la dickite
d'ordre hautement structurel sont les argiles dominantes avec le mica-montmorillonite et
l'halloysite adventifs. Darts de nombreux 6chantillons, on constate la coexistence de
kaolinite et de dickite; leurs quantit6s relatives sont variables, mais la teneur en dickite tend
~t augmenter avec la profondeur. Les 6tudes de SEM d6montrent que la dimension des
cristaux de dickite d6passe celle des cristaux de kaolinite jusqu'b. 30 am.
Les rapports D / H et ~80/t60 de six argiles, une roche enti6re et deux biotites de roche
h6te nouvelle, deux veines de chalc6doine et trois eaux m6t6oriques locales indiquent que
les argiles se sont form6es dans un environnement m6t6orique-hydrothermal de type source
d'acide chaud h environ 80~ Les roches hftes volcaniques nouvelles sont fortement
enrichies dans des roches ign6es ~80 relatives ~. 'normales' en raison de la d6rivation de, ou
d'un 6change avec des roches s6dimentaires riches 180 en profondeur pr6alablement
l'activit6 hydrothermale et solfatarienne. En l'absence d'activit6 g6othermale actuelle, la
vie de ce syst6me de sources chaudes 6tait de moins de 4 millions d'ann6es.
KURZREFERAT:
Ablagerungen von Alunit-Kaolinit kommen in kaolinisierten
Zonen tier sauren Vulkangesteine in Tolfa vor. t3ber 70 Proben wurden nach mindestens
einem der folgenden Verfahren untersucht: Dtinnschliff, DTA, Radiographic, IR, T G und
SEM; auch chemische und Wasserstoff- sowie Sauerstoffisotopenanalyse wurde angewandt.
In Glimmer-Montmorillonit sind Kaolinit und Dickit, deren struktureller Ordnungsgrad
hoch ist, die vorherrschenden Tone, wiihrend Halloysit eine untergeordnete Rolle spielt.
In vielen Proben trifft man sowohl Kaolinit als auch Dickit gleichzeitig an. Ihre relativen
Mengen sind verlinderlich, doch nimmt der Diekitgehalt in der Regel mit der Tiefe zu.
SEM-Untersuchungen haben erwiesen, dass Dickitkristalle bis zu 30 a m gr6sser sind als
Kaolinitkristalle.
Die D / H und 180/160 Verhiiltnisse von sechs Torten, einem ganzen Felsen und zwei
Biotiten aus frischem Wirtgestein, zwei Chalcedong~ingen und drei 6rtliehen Meteorgewiissern weisen darauf hin, dass die Tone in einer meteorisch-hydrothermischen Umgebung in der Art saurer Thermalquellen bei einer Temperatur yon fund 80~ entstehen.
Die frischen vulkanischen Wirtgesteine sind im Vergleich zu 'normalen' Eruptivgesteinen
stark mit 1s O angereichert. Dies ist darauf zuriickzuftihren, dass sic vor hydrothermischer
und solfatarer Aktivit~it in der Tiefe aus lSO-reichen Sedimentgesteinen entstammen oder
durch diese ersetzt wurden. In Abwesenheit geothermischer Aktivitiit betrug die Lebensdauer dieses Heissquellensystems weniger als 4 Millionen Jahre.
162
G. Lombardi and S. M. F. Sheppard
R E S U M E N : E n Tolfa se e n c u e n t r a n dep6sitos de alunita-caolinita en zonas argilizadas
de rocas volc/micas gtcidas. Mgts de setenta m u e s t r a s se h a n e x a m i n a d o por u n o a mils de
los siguientes m 6 t o d o s : Anillisis de secci6n delgada, D T A , rayos X, IR, T G , SEM, quimicos
y con is6topos de h i d r 6 g e n o y oxigeno. L a caolinita y la dickita, de orden estructural alto,
s o n las arcillas d o m i n a n t e s , est~indoles s u b o r d i n a d a s la m i c a - m o n t m o r i l l o n i t a y la halosita.
E n m u c h a s m u e s t r a s coexisten la caolinita y la dickita; sus cantidades relativas son
variables, pero el contenido de dickita tiende a a u m e n t a r en funci6n de la profundidad. Los
estudios S E M m u e s t r a n que los cristales de dickita de h a s t a 3 0 / t m s o n mils grandes que los
de caolinita.
Las relaciones D / H y ~80/~60 de seis arcillas, u n a roca entera, dos biotitas de la roca
h o s p e n d a n t e no expuesta, dos venas de calcedonia y tres a g u a s mete6ricas locales indican
que las arcillas se f o r m a r o n en u n a m b i e n t e mete6rico-hidrot6rmico del tipo de fuente
caliente ~lcida a alrededor de 80~ Las rocas volcilnicas h o s p e d a n t e s no expuestas a la
intemperie est~in fuertemente enriquecidas en 2*0 c o n relaci6n a l a s rocas volc~inicas
' n o r m a l e s ' debido a la derivaci6n de o el intercambrio con roeas sedimentarias ricas en 180
a profundidad, con anterioridad a la actividad hidrot6rmica y solfatfirica. E n ausencia de
la actual actividad geot6rmica la vida de este sisterna de fuente caliente era de m e n o s de
4 millones de afios.