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Silicification of quartz arenites overlain by volcaniclastic

deposits: an alternative to silcrete formation










Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 165 02 Praha 6, Czech Republic;;;  *


Faculty of Science, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic;


Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic

(Manuscript received December 8, 2005; accepted in revised form March 16, 2006)

Abstract: The origin of the flat-lying body of quartzite at Skalice near Litoměřice, Ohře Rift graben, Bohemian Massif,
is explained by the effect of emplacement of dense tuff of tephritic mineral composition on Eocene quartz sands. In the
upper part of the ca. 10 m thick quartzite body, broad quartz overgrowths on detrital grains are visible in CL images, with
individual zones/bands having variable Al contents. The lower part of the quartzite body features isopachous growth
bands with less variation in Al contents, passing to euhedral quartz precipitation. Porosity of the quartzite decreases
upwards, reaching 3.8 % below the tuff base. Two main silicification stages are inferred: 1. dealkalization of the tuff and
volcanic glass recrystallization, reflected in isopachous precipitation of poorly crystalline silica in deeper parts of the
quartzite profile from fluids with high silica concentration and high amount of impurities; 2. hydrothermal argillization
of the basal tuff portions connected with the origin of euhedral quartz cement deeper in the quartzite profile and opal
coatings in shallower parts of the profile. Simple mass balance calculation shows that the tuff itself could not produce
much silica for the underlying quartzite, and that most silica was produced by the corrosion and dissolution of sand grains
by alkaline fluids in a zone immediately underlying the tuff. The Skalice quartzite should be viewed as a product of
hydrothermal silicification rather than silcretization.

Key words: Ohře Rift, dealkalization, silicification, silcrete, dense tuff, quartz overgrowths, quartzite.


The process of silicification in sedimentary formations of
different age and different lithological composition pro-
duces SiO


-rich rocks – quartzites. This process involves

leaching of SiO


 from silica-rich minerals, its transport in

solution driven by thermal gradient or gravity, and its pre-
cipitation. Even in high-permeability rocks, this process is
multiphase rather than instantaneous, giving rise to char-
acteristic mineral sequences.

Silica solubility increases in the presence of ferric iron,

and with increasing amounts of salts and dissolved organ-
ic compounds (Thiry 1997). In solutions, silica tends to
get oversaturated with decreasing pH and/or decreasing
temperature. The phases precipitated are controlled by the
character of the host rock, prevailing pore-water chemis-
try, SiO


 saturation and the presence of ion impurities.

In general, sedimentary quartzite bodies of three differ-

ent genetic types can be encountered in natural environ-
ments: diagenetic quartzites formed in deeply buried sedi-
mentary formations, hydrothermal quartzites formed at/near
the contact with magmatic bodies, and silcretes formed by
atmospheric effects near the Earth’s surface.

The term “silcrete” has been rather loosely used for quartz-

ites of various origins in the past. Here, we reserve this term
for quartzite formed by silica concentration at or near the
Earth’s surface under specific physicochemical conditions
and low temperatures. Two main types are distinguished. Pe-
dogenic silcretes are typical for regions with distinct wet and

dry periods, and form thin layers with columnar structures
(Summerfield 1983). Vertical zoning ranges from megaquartz
at the top, microquartz in the middle and opal at the base
(Thiry 1997). These features are compatible with dominant
downward movement of fluids. Groundwater silcretes origi-
nate near the groundwater table and show less distinct verti-
cal differentiation: quartz overgrowths in pure sands and
amorphous or fibrous silica formation by replacement of clay
minerals (Thiry & Milnes 1991).

Dissolved silica is a common constituent of hydrother-

mal fluids. It is either leached from lavas and pyroclastic
deposits, especially glass, upon their cooling or from am-
bient silica-rich rocks. Sinters and pore fillings formed by
opal and chalcedony are dominant in modern geothermal
fields, such as those in Wyoming (Yellowstone) and New
Zealand (Kawerau and Taupo—Rotorua). A strong contri-
bution of microbes to opal precipitation is typical for such
settings (Guidry & Chafetz 2003).

The genesis of ancient siliceous deposits can be partly

deciphered by the geological position of the quartzite bod-
ies in question, their geometry and their relation to the pa-
leosurface. While subvertical quartzite bodies can generally
be attributed to the action of hydrothermal systems, the sit-
uation is more complicated with subhorizontal bodies.
Here, silicification may be controlled by hydrothermal fluid
flow following a subhorizontal body of high-permeability
sediment, but may equally be of epigenetic origin.

Subhorizontal quartzite bodies hosted by quartz arenites

are a common feature in the area of the Ohře Rift, a taphro-

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genic structure in the western part of the Bohemian Massif
pertaining to the European Cenozoic rift system (Prodehl et
al. 1995). They have generally been interpreted as products
of tropical weathering (pedogenic and groundwater sil-
cretes), for example by Kužvart (1965) and Malkovský
(1985, 1991) as they are often associated with products of
kaolinization and are of large areal extent. Some quartzite
bodies of smaller extent are, however, overlain by altered
pyroclastic material; this may equally suggest their hy-
drothermal/metasomatic origin (Dittler & Hibsch 1928).
A revision of a typical occurrence of the latter type is the
subject of this study.

Geological setting

The type locality of the Skalice quartzites is situated at

Dlouhý vrch Hill near Skalice, north of Litoměřice, on the
SE slope of the volcanic range of the České středohoří Mts
(Fig. 1). This range represents an erosional remnant of a
volcanic complex inside the Ohře Rift graben and can be
divided into four lithostratigraphic units (Cajz 2000), of
which the most important are the lowermost Ústí Forma-
tion  (36—26 Ma) composed of olivine-basaltic effusions

Fig. 1. The distribution of dense tuff at Dlouhý vrch Hill near Litoměřice with
indicated location of the test-pit excavated in the Skalice quartzites near the tuff
base. Vertical dimension of cross-section A—B is exaggerated by the factor of 2.7.

quartzite blocks can be found over a much larger area.

The relation of the Merboltice Formation sandstones to

the overlying quartzites is clearly a parental one, as is
shown by their similar composition and gradual transition.
The presence of well-developed positive-graded bedding
and common cross-bedding in the quartzites, contrasting
with the generally massive Merboltice Formation sand-
stones, suggests fluvial reworking of the Cretaceous detrital
material. Discrete, small basins in the České středohoří Mts
were filled with fluvial to lacustrine sands, sometimes corre-
lated with the Staré Sedlo Formation of the Sokolov Basin
(Knobloch et al. 1996). The age of this process is indicated
by fossil plant remnants in the quartzites. These were first
studied by Engelhardt (1876) and include species, like
Steinhauera subglobosa Presl  in  Sternberg,  Eotrigonobala-
nus furcinervis (Rossmässler) Walther  et  Kvaček,  Sterculia
labrusca  (Unger) Unger, or Daphnogene cinnamomea
(Rossmässler) Knobloch. The presence of the first men-
tioned taxon indicates pre-Oligocene deposition of the pa-
rental sand, most probably in the Middle to Late Eocene
(Knobloch & Konzalová 1998), i.e. at ca. 48—34 Ma.

As the upper contact of the quartzite body is not ex-

posed in outcrops, a test pit was excavated for its detailed
study at Dlouhý vrch Hill (Fig. 3), in an area not affected

confined to the rift valley and the overlying
Děčín Formation  (31—25 Ma) composed most-
ly of explosive tephritic to trachybasaltic vol-
canic products of a composite volcano. The
two younger formations (Dobrná Formation
and Štrbice Formation)  are not present at
Dlouhý vrch Hill.

In the area of Skalice, the Ústí Formation is

absent and the Děčín Formation overlies the
Santonian marine sandstones of the Merboltice
Formation (87—85 Ma), which is the youngest
preserved unit of the Bohemian Cretaceous
Basin. Here, the lowermost unit of the Děčín
Formation is represented by several superim-
posed bodies of dense volcaniclastic rock of
tephritic composition, as revealed by a recent
geological survey (Fig. 2). Conversely, the
quartzites are not developed at places where
other rock types (lahars and lavas) form the
base of the Děčín Formation (Cajz 2004), or
where the base of the volcanic complex is rep-
resented by the Ústí Formation.

Although the paleorelief can hardly be re-

constructed in full detail, the documented ex-
amples of redeposited pre-volcanic sediments,
intravolcanic fluvial sediments (Děčín Forma-
tion)  and quartzite occurrences, indicate the
presence of a shallow, roughly N—S-trending
valley before the deposition of the lowermost
preserved products of the České středohoří
Mts volcanic complex.

The Skalice quartzite body is subhorizontal,

ca. 10 m thick, visible in numerous outcrops at
ca. 460 m a.s.l. along the southern slopes of
Dlouhý vrch Hill. Gravitationally redeposited

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by mass movements. The Merboltice Formation (Unit A in
Fig. 3) sandstones are white or light yellow, fine- to medi-
um-grained, with imperfectly developed horizontal strati-
fication. Some parts of the rock are brown due to the
presence of iron oxyhydroxides. The gradual boundary
against the overlying Skalice quartzites (Unit B) is due to
a different intensity of cementation. The quartzites are
white or light yellow, fine- to medium-grained. In the low-
ermost 7—8 m (Subunit B1), the quartzites have massive
appearance, devoid of cavities. The quartzites in the top-
most 2—3 m (Subunit B2) are rich in small cavities lined
with quartz crystals, and locally contain abundant fossil
flora. Cementation is massive, with no visible quartzite
concretions, columns, pseudo-breccias or other features of
preferred silica precipitation. Sedimentary structures docu-
mented in the ambience include trough cross-bedding and
positive graded bedding. The boundary with the overly-
ing dense volcaniclastics is wavy, represented by a sharp-
based body of white, weakly consolidated, fine-grained
kaolinitic sand with saponite, ca. 0.5 m thick – Unit C.

The sand is overlain by a suite of volcaniclastic rocks.

In their rock type, they are endemic to Dlouhý vrch Hill
and exceptional among other volcanics of the Děčín For-
mation. They are composed of juvenile and cognate vol-
canic clasts “welded” together by basic glass and can,
therefore, be supposed to have originated as products of
very hot pyroclastic flows (“ignimbrite”) passing to lava
flow rich in pyroclastic material of scoria (“clastic lava”),
see Fig. 2. The presence of relatively fresh glass in this
rock type is very rare compared to the Děčín Formation la-
vas, where the original glass is concentrated on the mar-
gins of volcanic bodies, is of much lower proportion, and
is strongly devitrified, analcitized or zeolitized. If of pyro-
clastic origin, this rock type can be regarded as ignimbrite
s.l. (according to earlier petrographic definitions), al-
though this term is now being largely reserved for acidic
rocks. To distinguish this rock from similar-looking prod-
ucts of lahars and common lavas, we use the term dense
tuff or tuff in a non-genetic sense in the text below.

The fundamental discriminative feature between dense

tuffs and lahars at Dlouhý vrch Hill is the much higher de-

Fig. 2. Macrophotograph of the Dlouhý vrch tuff.

Fig. 3.  Vertical section across the boundary between the Skalice
quartzite and the overlying Dlouhý vrch tuff as seen in the excavat-
ed test pit. For the test-pit location see Fig. 1. Capital letters A—F
correspond to individual lithological units discussed in the text.

gree of solidification of the former. These tuffs are ex-
posed in only several isolated outcrops and differ in terms
of their average size, porosity, shape, colour and origin of
clasts. This, and their alternation with regular terrestric la-
vas and lahar products, suggest that they represent several
partly superimposed bodies. Lower bodies of dense tuff
contain lithics of basanitic composition (Ústí Formation)
while the upper ones contain only tephritic fragments.
Three facies of the dense tuff can be distinguished in the
test pit and its vicinity (Units D—F).

A layer approximately 0.2 m thick is present at the base,

with entrained irregular fragments/lenses of light consoli-
dated kaolinitic sand dispersed in clay matrix showing
peperite-like structure – Unit D.

Tuff in Unit E, ca. 1.5—2 m thick, is altered to brown-

grey kaolinite, smectite and illite clay. Sparse lithic clasts,

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up to 5 cm in size, are also altered in a similar style. Quartz
grains entrained from the underlying sand are present, but
not as common as in Unit D.

Solid dense tuff in Unit F exposed above the test pit

contains lithic clasts and a high proportion of glass in the
groundmass. Large clasts represent cognate lava fragments
and juvenile scoriaceous material. Large volcanic clasts
are mostly subangular to suboval, 10—20 cm in average
size, but considerably smaller and larger clasts can also be
found. The boundary with the altered tuff below has not
been reached by excavation. No prominent grading was
observed in the section, with the exception of the absence
of large volcanic clasts in Unit E in the test-pit. No fi-
amme, glass shards, columnar cooling joints or degassing
pipes were observed in any of the outcrops.


Rock samples (sandstone, quartzite, dense tuff) were

taken from the excavated test pit as well as from other out-
crops and quartzite blocks in the area. A total of 15
polished sections were made for petrographic study.
These were studied in transmitted and polarized light
using an optical microscope. Scanning cathodolumi-
nescence  (SEM-CL) with Cameca SX 100 electron micro-
probe was performed at the Geological Survey of the
Slovak Republic in Bratislava (analyst V. Kollárová), us-
ing accelerating voltage 20 kV, beam current 60 nA for
the discrimination of different silica generations. The sur-
faces of quartz grains from the weakly consolidated sand
were observed using the CamScan CS 3400 scanning elec-
tron microscope at the Czech Geological Survey in Prague
(analyst K.D. Malý, 15 kV, 150  A). The mineral phases of
hand-separated grains from the sandstone and quartzite
were identified using the Philips X’pert X-ray diffracto-
graph at the Institute of Geology, Academy of Sciences of
the Czech Republic in Prague (analyst J. Dobrovolný), us-
ing CuK  radiation, generator voltage of 40 kV, generator
current of 40 mA, 2 = 2—75º, scanning speed of 0.02º · s



The discriminated types of secondary quartz were analy-
sed for the contents of Al, Na, K, Fe and Ti using the Cam-
eca SX 100 microprobe at the Institute of Geology,
Academy of Sciences of the Czech Republic in Prague
(analyst V. Böhmová, 15 keV, 10 nA). The same device
was used for the analyses of the dense tuff. Whole-rock
wet analyses were performed in the Laboratories of the
Czech Geological Survey in Prague (analysts V. Janovská,
J. Šikl) for the main rock types.

Skeletal densities (densities of solid phase) were deter-

mined by means of He pycnometry at the Institute of
Chemical Process Fundamentals, Academy of Sciences of
the Czech Republic in Prague, using the Accupyc 1330
Micrometrics helium pycnometer (analyst H. Šnajdau-
fová), with 5 cycles of purging, equlibration rate of
0.58 Pa/s = 0.005 psig/min.). Statistical evaluation pro-
vides accuracy down to 0.001 g/cm


. Measurements of the

total intrusion volume, median and average pore radii,
bulk density were performed at the same institute using a

high-pressure mercury Micrometrics AutoPore III porosime-
ter (analyst H. Šnajdaufová). This instrument allows high-
pressure mercury intrusion of up to 400 MPa (correspond-
ing to pore radii of 1.5 nm).


Sandstone, Merboltice Formation (Unit A)

The sandstone is well sorted, with subangular to round-

ed detrital grains from 0.1—0.2 (70 %) to 0.3—0.5 mm in
size (30 %). They are composed of quartz (90 %), kaolin-
ized feldspar (K-feldspar >> plagioclase), micas and heavy
minerals (magnetite, ilmenite). Monocrystalline quartz
grains, 60 % with undulatory extinction, prevail over
polycrystalline grains, which constitute 15—20 %. Some
pores are partly filled with kaolinite (Fig. 4A). The overall
degree of cementation is low: diagenetic quartz Q



irregular growth zones, 10—30  m thick, but does not ex-
ceed 5 % of total sample volume. In SEM-CL images, Q


growth zones are dark and with no visible fabric; there-
fore, no signs of their possible corrosion can be identified.

Skalice quartzite (Unit B)

The quartzites contain detrital grains composed of

quartz (95 %), micas and a very small proportion of heavy
minerals. Detrital grains are subangular to rounded, and
their size distribution is similar to that of the Merboltice
Formation sandstone: 0.1—0.2 mm (90 %) and 0.3—0.5 mm
(10 %). The proportion of polycrystalline quartz grains is
3—10 %, 70—80 % of monocrystalline grains show undula-
tory extinction. Some 20 % of grains, especially finer ones,
show concave surfaces indicative of corrosion. Quartz
grains are overgrown with quartz, constituting 20—40 % of
the rock volume.

In the upper part of the Skalice quartzite body (Subunit

B2), the main silica phase is megaquartz and microquartz
(in small cavities). Grey varieties of quartzite B2 also con-
tain aggregates of hematite crystals on diagenetic quartz.
The lower part of the Skalice quartzite body (Subunit B1)
contains only megaquartz. The proportion of quartz ce-
ment increases upwards within the whole quartzite body,
and the increase in relative porosity in the same direction
is only due to the elevated cavernosity in quartzite B2.
Three types of quartz cement were distinguished:

1 – Minor growth zones of diagenetic quartz Q


, 10—50  m

thick, were observed directly on the grains, much like in
the sandstone. These overgrowths are mostly invisible in
transmitted light, but are discernible in SEM-CL images.
They are of uneven thickness, generally darker than the
younger Q


 overgrowths, and usually do not completely

overgrow the grains. This indicates some corrosion before
the formation of Q


. Q


 zones are homogeneous, unbanded,

but display ghosts of fibrous textures in some places, which
might indicate opal-CT/chalcedony recrystallization.

2 – Younger Q


 overgrowths, found in quartzites only,

are up to 100  m thick and completely enclose the detrital

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Fig. 4. A – Sandstone of the Merboltice Formation, Unit A. Crossed nicols, Sample 312. Quartz grains are partly cemented by kaolin-
ite clay. B – Dense tuff overlying the quartzite, Unit F. Crossed nicols, Sample 308. Larger clinopyroxene xenocrysts and smaller plagio-
clase xenocrysts in the tephritic glassy groundmass. C, D – Quartzite, Subunit B1, Sample 306. C – Crossed nicols, dark core of the grain
in the centre of the photo represents detrital quartz grain rich in fluid inclusions. In contrast to the core, the overgrowth  zone of this grain
is  free  of  fluid/solid  inclusions.  D  –  SEM-CL  image.  The  grain  in  the  centre  is  overgrown  by  a  dark  Q


  zone,  isopachous  growth  bands



 and euhedral growth zones Q


E, F – Quartzite, Subunit B2. SEM-CL images, Sample 303. E – Euhedral to isopachous Q



and topmost bright zone of microquartz Q


. F – Spherulitic Q


 growth bands on a tourmaline grain (black). G – Quartzite, Subunit B2.

SEM  image,  Sample  303.  Microquartz  Q


  crystals  filling  a  triple-junction  cavity.  H  –  Kaolinic  sand,  Unit  C.  SEM  image,  Sample  315.

Corroded surface of a detrital quartz grain.

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grains. These overgrowths are formed almost exclusively
by megaquartz. The proportion of grains with crystal faces
increases upwards in the section, reaching 80 % in Sub-
unit B2.

In the lower part of the Skalice quartzite body (Subunit

B1), this type is represented by  ~ 50  m thick quartz over-
growths, apparently homogeneous in transmitted light. In
SEM-CL images, Q


 and Q


 types of fabric can be identi-

fied. The Q


 fabric consists of multiple (max. 15) isopac-

hous growth bands, 1—2  m thick, alternating bright and
dark, roughly copying grain surfaces and their Q



(Fig. 4E). They may represent recrystallized amorphous sili-
ca coatings. Growth bands of the Q


 fabric are euhedral to

subhedral, mostly grow over those of Q


, sometimes being

separated by a distinct truncation surface. At least 9 bands
can be distinguished, each 3—5  m thick, and fill all the
voids in the rock. Rarely, Q


bands are overgrown by




bands of the same character as the Q



In the upper part of the Skalice quartzite body (Sub-

unit B2), this type consists of six quartz growth bands,
each 5—20  m thick, visible in transmitted light. The low-
ermost three bands are sometimes fused. In SEM-CL imag-
es, these bands are manifested by alternating dark and
bright colours. The lowermost four bands often show saw-
like patterns caused by the growth of minute quartz crys-
tals, while the topmost – and the broadest – two bands
display less acute angles which may indicate recrystal-
lized amorphous silica coatings. Very rarely, the topmost
two bands were observed to form spherules up to 35  m in
diameter on surfaces of tourmaline detrital grains (Fig. 4F).

3 – The youngest Q


 generation consists of micro-

quartz (crystals 30—150  m) and is present in Subunit B2
only. The crystals are euhedral (Fig. 4G). They commonly
grow on the topmost zone of Q


 quartz, with the serrated

contact indicating some degree of corrosion of the latter.
The thickest microquartz layers were observed on the sur-
faces of small cavities at triple junctions of detrital grains.
This type was commonly found in small cracks in the

Weakly consolidated sand (Unit C)

The sand of Unit C at the contact with the tuff is well

sorted, dominated by subangular to rounded quartz grains,
0.1—0.2 mm in size. Surface morphology indicates grain
corrosion (Fig. 4H). Interstices are filled with coarse ka-
olinite (crystals 10—20  m in diameter) and saponite.

Solid dense tuff (Unit F)

The rock contains pyroxene and plagioclase xenocrysts

and volcanic fragments in the tephritic glassy ground-
mass. Xenocrysts of clinopyroxenes (0.8—3 mm in size) are
automorphic, zoned, those of possible cumulate origin
form crystals with pressure lamellae and reaction rims up
to 0.1 mm thick. Clinopyroxene xenocrysts enclose auto-
morphic grains of titanian magnetite (0.05—0.2 mm) and
ilmenite. Magnetite grains have concave, corrosional mar-
gins with glass embayments. Plagioclase xenocrysts

(0.15—0.2 mm) are vitrified and corroded along cleavage
planes. Enclaves of iddingsitized olivine(?) are rare.

Dark fresh glass dominates the groundmass and also lines

pyroxene xenocrysts and volcanic fragments. Its proportion
locally reaches 40 % of rock volume. Besides glass, the
groundmass also includes clinopyroxene microphenocrysts
(0.1—0.4 mm) and fragments of larger xenocrysts. Small ac-
icular and skeletal anorthoclase crystallites, 0.02—0.05 mm
in size, are ubiquitous in most samples (ca. 25 % of ground-
mass) and completely surrounded by glass.

Volcanic fragments ( ~ 30 vol. %) form clasts, 2—3 mm

in diameter. They contain clinopyroxene phenocrysts
(0.5—0.7 mm),  plagioclase (max. 0.5 mm), opaque miner-
als and holocrystalline groundmass. Some of the frag-
ments have voids filled with zeolites and silica (opal), and
some of them have partly zeolitized groundmass.

Chemical composition

Bulk analyses of main rock types are shown in Table 1

and microprobe analyses of the solid tuff are summarized
in Table 2.

Skalice quartzite

Variations in the Al, Na, K, Ti and Fe contents in sec-

ondary quartz overgrowths were studied on electron mi-
croprobe in both quartzite subunits, B1 and B2, along
grain-to-overgrowth profiles. The contents of K, Ti and Fe
were mostly below the detection limits of 0.025 wt. %,
0.020 wt. % and 0.100 wt. %, respectively. Profiles with Al
and Na contents representative for each subunit are shown
in Fig. 5.

Clear differences were observed in Al contents, for which

the detection limit was 0.016 wt. %. Al contents in detrital
grains depend on the presence of fluid inclusions, inclu-
sions of feldspars, micas and clay minerals. Overgrowths in
quartzite B1 show only a small variation in Al, with its con-
tents not exceeding 0.020 wt. %. Here, Al contents corre-
spond to the luminescence colours, with lighter bands
being richer in Al compared to darker bands. In quartzite
B2, however, a steady increase in Al contents can be seen
from the grains outwards, with values in excess of
0.020 wt. % in the youngest Q


 bands. A clear increase in

Na contents to exceed the detection limit (0.019 wt. %) was
observed from the detrital grains outwards.

These trends of Al and Na distribution are, however, not

reflected in the bulk-rock composition of the B1 and B2
quartzite. The grey, hematite-containing variety of quartz-
ite B2 shows elevated Fe content (0.23 wt. %) in the bulk-
rock analysis.

Solid dense tuff and argillized tuff

The bulk composition of the dense tuff need not reflect

the parental magma composition as a result of the air-sort-
ing of pyroclastic material during transport and alteration.
Solid tuff from Skalice plots in the field of alkali basalt in

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Table 1: Representative bulk analyses of the main rock types.

Table 2: Average microprobe analyses of the main phases in the solid ignimbrite, Sample 308.

the TAS (total alkali vs. silica) diagram (Table 1). As re-
vealed by microprobe analyses (Table 2), glass in the
groundmass of the solid tuff is of prevailing basaltic com-
position, with SiO


 values ranging between 47.9 and

52.2 wt. % and total alkali content between 2.5 and
5.3 wt. %. The alkalinity index (Na


O + K






) varies

within the range of 0.14—0.27. Both sodic and potassic
glasses are present (K




O = 0.17—2.23). Average Mg-

number [MgO/(MgO + 0.15 FeO) 100]  for glass corre-
sponds to the value of 64.3 (55.5—75.0).

Small anorthoclase crystallites are scattered in the glass

in most samples of the solid tuff. In microprobe analyses,
their composition is contaminated by Ca, Fe and Ti from
the surrounding groundmass glass. Not only alkali con-
tents (average 10.8 wt. % K


O + Na


O) but also Al contents

(average 19.4 wt. % Al




) exceed those in the glass. So-

dium slightly predominates over potassium in most crys-
tallites, although extreme K




O ratios were also en-

countered (0.03—6.65). Plagioclase phenocrysts are mostly
of andesine to labradorite composition. Magnetite in the
solid tuff is enriched in Ti, with 14.3—22.4 wt. % TiO



To characterize the style of alteration, the chemistry of

tuff from Unit F was compared with that of argillized tuff

from Unit E. Argillized tuff is markedly depleted in Ca,
Na, Mg and P and enriched in Si (Table 1), thus indicating
the decomposition of plagioclase and pyroxene, with Al
being locked in the newly formed kaolinite. Although
some release of silica from glass and plagioclase can be
expected, the true extent of the silica depletion is ob-
scured by the admixture of detrital quartz grains. This ex-
plains the extremely high Si contents in the argillized tuff.

Density and porosity measurements

Samples of sandstone (Unit A, Sample 312) and quartz-

ites from the lower part (Subunit B1, Sample 306) and up-
per part of the Skalice quartzite body (Subunit B2, Sample
303) were analysed for skeletal densities (densities of sol-
id phase) and bulk densities, plotting their pore-size distri-
butions. Skeletal densities were also determined for
altered dense tuff (Unit E, Sample 314) and fresh dense
tuff (Unit F, Sample 313). The data are summarized in Ta-
ble 3 and Fig. 6.

Both quartzite samples show much lower total porosi-

ties than the sandstone, with the highest porosity reduc-

background image



tion found in quartzite of Subunit B2 (porosity 3.84 %). As
shown by the pore-size distribution diagrams (Fig. 6), the
infrequent large corrosive pores (3—50  m) in all quartzites
have no secondary mineral fills. Nevertheless, quartzites
of Subunit B2 – unlike those of Subunit B1 – are
characterized by almost complete filling of pores in the
size category of 0.1 to 2  m with quartz of the youngest




Dense tuff alteration: devitrification or hydrothermal

The time gap of over 4 Myr between the Eocene fluvial

deposition of fossiliferous sand and the Late Oligocene
onset of tephritic volcanic activity (Děčín Formation)
must have brought relative uplift and partial erosion of the
tectonic block at Dlouhý vrch Hill, as shown by the local
absence of the Ústí Formation products. The former river
channels may have been abandoned and the Dlouhý vrch
tuff was deposited on dry land. This is suggested by the
absence of signs of interaction with water, such as chilling

and quenching phenomena or fumarolic
pipes, in the outcrops and by the absence of
palagonitization or fumarolic alteration in mi-
croscopic scale. Breccia with peperite-like
structure in Unit D is viewed as a product of
mechanical incorporation of loose sediment
into the basal portions of the pyroclastic flow.

The average values of SiO


 and alkalis

(48.4 wt. % and 3.7 wt. %, respectively) for
the studied volatile-free tuff, differ from
43.7 wt. % SiO


 and 5.2 wt. % alkalis in aver-

age volatile-free tephrite of the Bohemian
Massif (Shrbený 1995), and project into the
field of alkali basalt in the TAS diagram. The
discrepancy between the tephritic habitus and
mineral content of the solid tuff and its basal-
tic chemistry is explained by the loss of alka-
lis and by the high proportion of glass
(50.2 wt. % SiO


 on average), which adds ex-

tra silica to the contents typical of tephrites. A
part of the free alkalis was locked in Na- and
K-rich feldspar (anorthoclase) crystallizing
from glass, but a significant part must have
been removed from the rock. The presence of
anorthoclase crystallites in relatively fresh
glass suggests that these represent products of
early post-emplacement devitrification in-
duced by the thermodynamic instability of
the glass (Cas & Wright 1987). Although fi-
brous silica is a common product of acidic
glass devitrification, devitrification spheru-
lites (cf. Lofgren 1971) are absent from the
tuff. Chalcedony coatings in voids in the rock
are, much like the zeolites, therefore rather at-
tributed to downward seepage of rainwater

Fig. 5. Al




 and Na


O contents in detrital quartz grains and their overgrowths,

and the corresponding SEM-CL photos with analytical profiles. A – Lower part
of the Skalice quartzite body (Subunit B1). B – Upper part of the Skalice quartz-
ite body (Subunit B2).

through the tuff body. No major SiO


 losses can be as-

sumed during the devitrification stage as the newly
formed anorthoclase is not depleted in silica relative to
the glass.

The extensive argillization of the lowermost 1.7 to

2.2 m of the dense tuff body (Units D—E) and clayey sand
(Unit C) was, however, associated with a much more ex-
tensive solute transport than the devitrification process
observed in Unit F. Therefore, it must have entailed some
degree of fluid involvement. In the absence of surface wa-
ter, the most probable solute carrier was rainwater perco-
lating through the tuff. The argillization either post-dated,
or was synchronous with, the devitrification stage. Volca-
nic glass was transformed into a mixture of smectite, ka-
olinite and illite, which implies considerable losses of


. Large-scale Ca and Na removal during the argilliza-

tion process is compatible with the decomposition of pla-
gioclase and clinopyroxene. This transformation could
have proceeded in two steps, with smectite crystallization
at temperatures of 20—150 ºC (Aoki et al. 1996) and its
subsequent hydrothermal alteration into a mixture of ka-
olinite and illite at approximately  80—100 ºC. The latter
reaction took place in an alkaline environment with a con-
comitant release of additional SiO


. The increase in vol-

background image



tal quartz in Units D—E and the unknown degree of grain
corrosion in Unit C, the amounts of transported silica can
merely be estimated.

Assuming that the proportions of glass, plagioclase and

pyroxene (the main phases subject to decomposition) in
the solid tuff are 3.5 : 2.5 : 4, together constituting 85 % of
the rock (based on thin sections and mineral chemistry),
and that these phases in the argillized dense tuff are com-
pletely transformed into kaolinite with 46.5 wt. % SiO



the mass of released SiO


 on a unit area of the tuff/sand

contact can be calculated:



= (0.85A/100) ·


· T



where A is the weighted average of the difference in



 contents in these three mineral phases and kaolinite

in wt. % (available excess silica), 


 is the skeletal densi-

ty of the argillized dense tuff, and T


 is the thickness of the

argillized dense tuff.

The potential amount of available silica is approximate-

ly 4.2 wt. %, skeletal density of the laboratory sample of
argillized dense tuff is 2630 kg · m


, and the approximate

thickness of the argillized dense tuff is 2 m (Units D—E).
Under such conditions, the mass of the released SiO



be estimated at 188 kg · m



The gains of silica in the quartzite (Unit B) underlying

the tuff can be quantified as follows:



= (P




) ·


· T


where P


 and P


 are relative porosities of unaltered sand-

stone and quartzite, respectively, 


 is the density of

the newly formed silica phases in the quartzite, and T



the thickness of the quartzite. Relative porosities mea-
sured in laboratory samples are 0.286 for sandstone, 0.046
for B1 quartzite, and 0.038 for B2 quartzite. The 



ue of 2600 kg · m


 was used for B1 quartzite, dominated

by quartz overgrowths, while the density of 2100 kg · m


was used for B2 quartzite, dominated by recrystallized
opal overgrowths. Thicknesses T


 of B1 and B2 quartzite

are 7.5 m and 2.5 m, respectively. The mass of the newly
precipitated SiO


 can then be estimated at 4680 kg · m


for Subunit B1 and 1302 kg · m


 for Subunit B2, namely

5982 kg · m


 in total.

A comparison of the approximate supply (188 kg · m



vs. gain (5982 kg · m


) of silica, however imprecise these

estimations may be, suggests that the tuff could not serve
as a sole source for the secondary silica cement in the

Table 3: Results of helium pycnometry and mercury porosimetry of the main rock types.

Fig. 6. Diagrams of differential pore-size distribution in sandstone
(Unit A) and quartzites from the lower part of the Skalice quartzite
body (Subunit B1) and upper part of the Skalice quartzite body
(Subunit B2).

ume associated with smectite alteration resulted in the
crystallization of coarse kaolinite in Unit C below the tuff
base. The late-stage timing of kaolinite and saponite crys-
tallization in Unit C sand is supported by the fact that the
unit must have been fully permeable for the hydrothermal
fluids until the late stages of downward silica transport.

Mass balance and the source of silica

Knowledge of mineral transformations taking place dur-

ing the tuff alteration permits us to calculate whether the
tuff itself could serve as the source of secondary silica ce-
ment below its base. As the devitrification stage released
possibly no silica from the tuff itself, only the argilliza-
tion/silicification stage is considered. The observation
that the quartzites are developed only beneath the dense
tuff allows us to eliminate the horizontal component of
solute transport in the mass balance calculations, and to
consider only mass fluxes in a vertical section. As precise
calculations are impossible due to the admixture of detri-

background image



quartzite, even if all the main mineral phases
were completely kaolinized. The main process
of silica production for the quartzite must
therefore be seen in the alkaline dissolution of
quartz grains beneath the pyroclastic flow.

The solubility of quartz in the sand beneath

the tuff could have been controlled by higher al-
kali contents in fluids generated during the al-
teration of feldspar, clay minerals and alkali
glass (Levandowski et al. 1973). Nevertheless,
quartz is equally dissolved by fluids of normal
pH at elevated temperatures (Rimstidt 1997).
Also, the presence of organic acids in fluids of
normal pH increases the dissolution rate of
quartz considerably even at 25 ºC (Bennett et al.
1988). All these factors can be held responsible
for quartz dissolution in the sand beneath the
Dlouhý vrch tuff.

Silicification process in sand

Silica cement precipitated in pure quartzose

sand after disintegration and redeposition of
the underlying Merboltice Formation sand-
stone. The high purity of the target lithology,
devoid of kaolinized feldspar and clay miner-
als, is a necessary pre-requisite for the quartz
overgrowths to form (McBride 1989). The pres-
ence of albite, talc and amphibole reported
from feldspathic and clayey sandstones at their
contact with volcanic rock (Brauckmann &
Füchtbauer 1983; McKinley et al. 2001) was
not encountered in the Skalice quartzites.

The succession of the silicification process

as documented by microscopic observations is
illustrated in Fig. 7. The earliest, sporadic
quartz overgrowths Q


 are developed also in

the non-silicified Merboltice Formation  sand-
stones, which suggests that they are products
of pre-Eocene burial diagenesis. The main
phase of silicification with the formation of subhedral
overgrowths in quartzite B2 and isopachous and euhedral
overgrowth in quartzite B1 relates to the circulation of flu-
ids activated by the emplacement of the hot pyroclastic
flow. Petrographic differences between B1 and B2 quartz-
ites can be most easily explained by their different posi-
tions relative to the groundwater table. Descending fluids
in B2 were providing silica for broad overgrowths discern-
ible in CL images, where each zone/band represents a sep-
arate batch of fluid seeping from the tuff to its basement.
In contrast, the zone of B1 quartzites, lying below the
groundwater table, precipitated silica of much more homo-
geneous chemical composition, producing compound sets
of mostly bright isopachous growth bands with less varia-
tion in Al contents. Conversion of amorphous silica precip-
itation into quartz precipitation in the B1 quartzite can be
explained either in terms of decreasing silica concentration
in fluids or by the presence of impurities (clay minerals,
e.g.) on quartz grains that prevented early crystallization of

quartz. Later recrystallization of amorphous silica into
quartz in the whole profile may be due to the sinking of
the groundwater table during progressive incision of relief
or due to the dehydration upon sediment burial.

The isopachous character of the growth bands of Q


from the lower part of the Skalice quartzite body allows us
to speculate on early fluids with high SiO



and high amounts of impurities, giving rise to poorly crys-
talline silica cement. A release of fluids with such parame-
ters seems to be compatible with the process of volcanic
glass alteration in the dense tuff. Subsequently formed eu-
hedral growth zones of Q


 in the lower part of the Skalice

quartzite body and Q


 in its upper part precipitated, on the

other hand, from relatively pure fluids with lower SiO


concentration, compatible with the process of smectite
transformation into kaolinite and illite in the basal portion
of the tuff. Precipitation of euhedral quartz was probably
concomitant with the recrystallization of earlier, poorly
crystalline phases into megaquartz.

Fig. 7. A schematic diagram of the succession of silicification stages in the Skalice
quartzite. Note the different silicification histories of its lower part (Subunit B1)
and upper part (Subunit B2). The distinction is explained by their respective posi-
tions below and above the groundwater table.

background image



below the tuff base, the silica transport must have been
completed before cooling of the pyroclastic flow.

In summary, the process of the Skalice quartzite forma-

tion can be described as rapid silica mobilization induced
by hydrothermal alteration of a tephritic pyroclastic flow
and sudden heating and alkalization of the sand in the
proximity of a groundwater table. Such hydrothermal pro-
cess clearly outpaces the classical process of groundwater
silcrete formation driven by mixing of meteoric water with
silica-rich groundwater and silica precipitation in water
discharge areas. Therefore, the Skalice quartzite body
should be regarded as a hydrothermal quartz deposit, de-
spite its geometry defined by the course of the groundwa-
ter table at the time of the pyroclastic flow emplacement.


The horizontally lying body of the Skalice quartzites

near Litoměřice, Ohře Rift graben, shows no signs of su-
pergene origin. On the contrary, its spatial association
with the overlying body of the Dlouhý vrch tuff, the pres-
ence of hydrothermal kaolinite in sand at the quartzite/tuff
boundary, and the lack of features characteristic for sil-
cretes suggest silica mobilization beneath the tuff layer
during its deposition. The documented vertical section
across the tuff/quartzite boundary, petrographic observa-
tions and chemical analyses of the main rock types indi-
cate three stages of interaction between the tuff and its
basement during a progressive temperature decrease:

1 – dealkalization of the tuff synchronous with glass

recrystallization (origin of anorthoclase crystallites), re-
sulting in quartz grain corrosion beneath the tuff base; the
released silica was deposited in the form of opal coatings
on quartz grains deeper in the profile (Subunit B1);

2 – hydrothermal desilicification (smectitization of

volcanic glass) and continued dealkalization in the basal
part of the tuff body connected with massive silicification
of the sand  beneath the tuff base; origin of euhedral
quartz cement deeper in the profile (Subunit B1) and opal
coatings in shallower parts of the profile (Subunit B2);

3 – transformation of smectite in the basal part of the

tuff body into kaolinite and illite; kaolinite crystallized
on top of the sand profile prevents further fluid movement
and solute exchange between the tuff and the sand.

The boundary between the shallower, loosely cemented

and partly corroded quartzite B2 and the deeper, densely
cemented quartzite B1, can be identified with the ground-
water table at the time of quartzite formation. As the mass
balance calculations imply the silica supply from the tuff
itself would not be sufficient for the Skalice quartzite for-
mation in its total thickness of ca. 10 m. Additional topog-
raphy-controlled quartz-grain dissolution in the underlying
sands is also shown by the frequent signs of corrosion, espe-
cially above the groundwater table.

The style of cementation in the Skalice quartzite was – to

a certain degree – controlled by the paleorelief (tuff base)
and the depth of the groundwater table. In this sense, it can
be considered a special type of a groundwater silcrete. The

The last stage, characterized by Q


 precipitation, was pre-

ceded by the corrosion of Q


 quartz in quartzite B2. The

clarity and small size of Q


 quartz crystals indicate a steady

fluid chemistry (lower SiO


 concentration, low content of

impurities) but a certain decline in the flushing rate.

Diagnostic features for volcanic alteration vs. silcretiza-
tion of quartz arenites

Quartzite bodies developed in the sands of the Staré

Sedlo Formation in other parts of the Ohře Rift graben
bear signs of pedogenic silcretization: distinct vertical
profiles with pseudo-brecciation horizons, vertical succes-
sion of silica phases, and TiO


 accumulation. The approxi-

mately coeval Skalice quartzites, much smaller in areal
extent, were believed to be a result of the same silicifica-
tion process by earlier authors (e.g. Malkovský 1985).

In fact, the Skalice quartzites lack the features character-

istic of pedogenic silcretes summarized by Thiry (1997).
Instead, they can be briefly described by a simple three-
storey vertical profile with decreasing intensity of corro-
sion and increasing intensity and variety of silica
precipitation in a downward direction (Fig. 7). No inherit-
ance of micromorphological features is observed in the
uppermost horizons as an indication of the ‘progressively
sinking profile’ of Thiry (1997). In contrast, the Skalice
quartzites share many petrographic features indicative of
groundwater silcretes, the best described example of
which are the Fontainebleau quartzites (Thiry & Maréchal
2001). Especially the isopachous quartz overgrowths de-
veloped on detrital grains and rarely on euhedral over-
growths, most probably representing recrystallized opal
coatings, indicate precipitation in stagnant or slowly cir-
culating silica-supersaturated groundwater. However, no
elongated or air-foil morphologies of quartzite bodies sug-
gesting preferred fluid flow direction are visible, and the
Skalice quartzite rather forms one thick, tabular body with
no internal structuring.

The Fontainebleau model presumes a long-distance sili-

ca supply by groundwater, the silica oversaturation of
which results from alteration of clay minerals in the over-
lying formation, and only occasional dissolution within
the quartzite body itself is admitted. In contrast, the Skal-
ice quartzites show very strong dissolution in the topmost
portion (loose sand with corroded grain surfaces) and me-
dium dissolution in the upper part of the quartzite body
(dissolution cavities in quartzite B2). This, and the fact
that the tuff itself could not produce enough silica for the
cement in quartzite, documents a progressive downward Si
transport from Unit E to Unit B as far as to the B1/B2
quartzite boundary. Below this boundary, quartzite B1
displays only rare, mild corrosion features. On the basis of
these observations, the B1/B2 boundary should be viewed
as the local groundwater table at the time of the silicifica-
tion process.

The process of Si leaching from the tuff and the underly-

ing sand and its deposition around the groundwater table
must have been rather rapid. As suggested by the presence
of post-dissolution hydrothermal kaolinite in Unit C sand

background image



distinction from the groundwater silcrete model of Thiry &
Maréchal (2001) lies in the short-term, heat-driven silica
mobilization, and in the short-distance, essentially vertical
silica transport. The thoroughly documented case of the hy-
drothermal Skalice quartzite is to warn against hasty genet-
ic interpretations of horizontal quartzite bodies where the
overlying strata have not been preserved.


The authors wish to thank J.K. Novák

and J. Ulrych (Inst. Geology AS CR Prague) for their help in
the processing of electron microprobe analyses. Porosity
measurements were kindly provided by O. Šolcová (Inst.
Chem. Process Fundamentals AS CR Prague). The study of
sandstone silicification is supported by the Grant Agency of
the Academy of Sciences CR, Project No. A3013302 and
falls within the Academic Research Plan AV0Z 30130516.


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