GeolCarp_Vol49_No5_377

GEOLOGICA CARPATHICA, 49, 5, BRATISLAVA, OCTOBER 1998

377387

HYDROTHERMAL ALTERATION OF GRANITOID ROCKS

AND GNEISSES IN THE DÚBRAVA Sb-Au DEPOSIT,

WESTERN CARPATHIANS

MONIKA ORVOOVÁ

, JURAJ MAJZLAN

2,*

and MARTIN CHOVAN

Slovak Museum of Nature Protection and Speleology, kolská 4, 031 01 Liptovský Mikulá, Slovak Republic

Department of Mineralogy and Petrology, Faculty of Sciences, Comenius University, Mlynská dolina,

842 15 Bratislava, Slovak Republic

(Manuscript received October 15, 1997; accepted in revised form September 1, 1998)

Abstract: Three alteration zones are recognized in hydrothermally altered granitoid rocks at the Dúbrava Sb-Au

deposit, Nízke Tatry Mts., Western Carpathians: the outermost chlorite zone, muscovite zone and the innermost illite-

carbonate zone. Chlorite replacing biotite originated at temperature about 320±40

C (according to Cathelineau 1988)

and the frequency distribution of Al

in chlorite suggests its re-equilibration during later hydrothermal events. The

muscovite and illite-carbonate zones are superimposed on the chlorite zone. Muscovite appears to be of both mag-

matic and hydrothermal origin, as deciphered from its TiO

content. Hydrothermal muscovite replaces plagioclase

and chlorite. In the innermost zone, carbonates and sulphides become abundant, muscovite and feldspars are replaced

by illite. Measurements of crystallinity index values of illite gave an estimate of 180200

C for the illite-carbonate

zone. The altered rocks were enriched in K

O, H

O, CO

, Sb and S and depleted in CaO, MgO, Fe

, and Na

Minor changes in Al

and SiO

content are ascribed either to the variations of the composition of the primary

magmatic precursor or local redistribution of these elements. Limited data on alteration of amphibole gneisses and

migmatites suggest that the main processes were chloritization and illitization; SiO

and Na

O wereremoved and K

added to these rocks.

Key words: Nízke Tatry Mts., granitoids, hydrothermal alteration, illite, muscovite, chlorite.

Introduction

Recent effort to understand the processes governing the for-

mation of the Dúbrava Sb-Au deposit, including detailed

mineralogical studies (Chovan 1990), fluid inclusion and sta-

ble isotope investigation (Sachan 1989; Chovan et al. 1995)

as well as structural data (Sasvári 1997) calls also for a clos-

er inspection of the environment that hosts the ore bodies.

This paper presents new data on mineralogical and chemical

changes of the wall rocks in the process of alteration induced

by hydrothermal fluids responsible for the formation of eco-

nomic accumulation of antimony and subeconomic amounts

of other elements (Au, Cu, Ag).

In the past, little attention was paid to the rock alteration

linked to hydrothermal events in the Nízke Tatry Mts. Èillík

& Michálek (1983) dealt with the mutual influence of rock

environment and hydrothermal mineralization and reported

increasing amounts of Pb, Cu, Bi and Ag and decreasing

amounts of K, Na, Ca and Mg with increasing Sb. They found

that sericitization and silicification is typical for granitoids,

while carbonatization, chloritization and saussuritization is

characteristic for migmatites. The authors made no attempt to

compare the results of chemical analyses with petrographic

studies of the rocks. Gubaè (1983) mentioned alteration of the

rocks hosting W-Au mineralization (Jasenie-Kyslá, Nízke Ta-

try Mts.).

The Dúbrava deposit is located in the valley of Kriianka,

on the northern slopes of the Ïumbier part of Nízke Tatry

Mts., Western Carpathians, Slovakia. It has been known for a

long time. It became an important source of Sb in the 1950s

and the exploitation ceased in 1991.

Methods

The studied samples were collected from visible alteration

zones of cm to dm thickness around the ore veins of all min-

eralization stages. Sampling was carried out in four chief

mining works in order to detect any horizontal or vertical

changes of rock alteration. Additional samples were collected

in the periphery of the ore field. The alteration zonality in the

scale of meters was studied in a traverse across the alteration

envelope of Terézia vein, Martin adit with a sampling

distance of 3 m. Changes of mineral composition of the stud-

ied rocks were monitored by planimetric analyses of 20 sam-

ples. The clay material was size-fractionated by gravity set-

tling. XRD analyses were performed on a DRON

diffractometer with Cu K

radiation and a Ni filter. Oriented

samples were prepared by sedimentation of clay suspension

(10 mg/cm

) on glass plates. Samples were analyzed both be-

fore and after saturation by ethylene glycol (8 h at 60

C).

Electron microprobe analyses were performed on JXA 840A

(CLEOM, Comenius University) (feldspars, biotite, chlorite,

illite, Ti-oxides) at 15 kV, 10 nA, standards: albite (Na), anor-

thite (Ca), spinel (Al), SiO

(Si), MgO (Mg), hematite (Fe),

adularia (K), MnO (Mn), chromite (Cr) and rutile (Ti); JEOL

*Current address: Department of Geology, University of California at Davis, Davis 95616, USA; E-mail: jmajzlan@ucdavis.edu

378 ORVOOVÁ, MAJZLAN and CHOVAN

733 Superprobe (Geological Survey of Slovakia) (muscovite)

at 15 kV, 20 nA, standards: albite (Na), wollastonite (Ca), co-

rundum (Al), quartz (Si), MgO (Mg), hematite (Fe). Bulk rock

chemistry was determined by wet chemical analysis (major ox-

ides) and combination of AAS and ICP (trace elements). Densi-

ty of the samples was determined by repeated measurements in

pycnometer.

Geological setting and petrography

The Dúbrava deposit is hosted by granitoid rocks; high-

grade metamorphic rocks outcrop to the north of the deposit.

Xenoliths (up to tens of meters) of migmatites and amphibole

gneisses occur as relics of the metamorphic mantle of the

granitoids (Fig. 1).

Emplacement of a large pluton composed of granites, gran-

odiorites to tonalites, took place approximately 300 Ma B.P.

(Petrík et al. 1994 and references therein). The granitoid

rocks represent I-type plagioclase-rich Variscan granitoids

(Petrík et al. l.c.) or IS-type granitoids with affinity to I-type

granitoids (Kohút 1997). The rock-forming mineral assem-

blage comprises Mg-biotite, strongly saussuritized plagio-

clase and quartz (Petrík et al. 1994). Variscan structural

scheme was modified by Alpine tectonometamorphic events

which caused weak retrograde metamorphism in this area.

The large granitoid pluton of the Nízke Tatry Mts. is built

by two principal rock types: the first one (type Ïumbier) be-

ing composed of granodiorite to tonalite, the second one

(type Praivá), dominating around the Dúbrava deposit, of

granitic to granodioritic composition with porphyric K-feld-

spars.

Modal composition of the granitoid rocks (Lukáèik 1983)

is shown in Fig. 2 (column 1, 2). Mafic minerals are not

specified, but according to several authors, biotite greatly

predominates. Chemical composition of biotite in unaltered

rocks of the pluton falls on the boundary between the fields

of Fe- and Mg-biotites (according to Foster 1960, Fig. 3).

Magmatic muscovite and amphibole are scarce. Chemical

composition of plagioclases and K-feldspars is depicted in

Fig. 4. Plagioclases (mostly andesine) are more abundant

than K-feldspars. Their composition ranges between An

25-36

(type Ïumbier) and An

17-34

(type Praivá) (Fig. 4). K-feld-

spar, the principal phase of the phenocrysts, belongs mostly

to microcline (Lukáèik 1983) and contains poikilic inclu-

sions of quartz, biotite and plagioclase. On cooling, plagio-

clase enclosed in K-feldspar phenocrysts developed albite-

rich margins.

Xenoliths of amphibole-biotite gneisses and migmatites

underwent alteration similarly as granitoid rocks. Their neo-

some is compositionally close to biotite granodiorite (quartz,

albiteoligoclase, biotite). Paleosome contains amphibole,

biotite, plagioclase (albiteoligoclase).

Fig. 1. Sketch cross-section of the Dúbrava deposit (after Michálek

1992). 1 granitoids, 2 gneisses and migmatites, 3 ore veins,

4 faults. Inset shows localization of Dúbrava deposit in Slovakia.

Fig. 2. Modal composition of unaltered granitoid rocks (after

Lukáèik 1983, columns 1, 2) and the altered granitoids (columns 3

5) from the Dúbrava deposit. 1 granodiorite to tonalite (type

Ïumbier), 2 granite to granodiorite (type Praivá), 3 chlorite

zone, 4 muscovite zone, 5 illite-carbonate zone. Lukáèik

(1983) did not specify mafic minerals but the dominating one is bi-

otite; amphibole is rare.

HYDROTHERMAL ALTERATION OF GRANITOID ROCKS AND GNEISSES 379

Fig. 3. Composition of biotite in terms of octahedral cations (clas-

sification after Foster 1960): biotite from unaltered granitoids

(Lukáèik 1983, open circles) and biotite from altered granitoids

(chlorite zone, solid squares).

Fig. 4. Composition of feldspars in terms of Na, K, and Ca: feld-

spars from unaltered granitoid rocks (Lukáèik 1983, open circles)

and from altered granitoids (solid squares).

Fig. 5. Macroscopic appearance of altered rocks from chlorite

zone (top), muscovite zone (bottom left) and illite-carbonate zone

(bottom right). Scale in cm.

phases

THtot of fluid

inclusions (ºC)

O (water)

SMOW

stages

scheelite

molybdenite

310-345

+3.5 to +5.1

scheelite

315-355

+3.3 to +5.6

sulfide

pyrite-arsenopyrite

305-350

+5.5 to +8.5

sulfosalt-stibnite

105-170

-9.3 to 1.7

tetrahedrite

118-162

-3.8 to +1.5

barite

105-158

-4.9 to +1.4

Table 1: Mineral stages and phases at the Dúbrava deposit and

values of THtot and

O of the associated fluid inclusions (after

Chovan et al. 1995, completed by new unpubl. data). The stages

and phases are listed in successive order.

Ore mineralization

The mineralization occurs in veins, stockworks and impreg-

nations in granitoid rocks and xenoliths of metamorphic rocks

(Fig. 1). Two major phases of mineral deposition were identi-

fied by the mineralogical studies (Chovan 1990): an earlier

scheelite phase and later sulphide phase, both of them further

subdivided into several stages (Table 1). The distinction be-

tween phases and stages is based on mineral assemblages and

structural arrangements of the veins bearing different assem-

blages. Quartz from scheelite, molybdenite and pyrite-arse-

nopyrite stage contains CO

-rich and aqueous fluid inclusions

with relatively high total homogenization temperature (TH

tot

)

(Table 1). Oxygen isotope data suggest endogeneous source of

these fluids. Fluids in quartz of the later stages are richer in

dissolved salts and they lack CO

; contemporaneous decrease

in TH

tot

and salinity and oxygen isotope data indicates the in-

troduction of meteoric waters into the hydrothermal system.

Fluids of the last, barite stage contained appreciable amounts

of CaCl

. Similar evolution of the fluids is expected in the case

of Au-W deposit Jasenie-Kyslá (Nízke Tatry Mts.) where the

D and

O values of water extracted from fluid inclusions in

arsenopyrite fall in the field of metamorphic water close to the

boundary with the field of magmatic water (Bláha & Bartoò

1991). There is no direct geological or other evidence about

the age (Variscan or Alpine) of the Dúbrava deposit. Chovan et

al. (1995) argue that the major mineralization event was

Variscan, with Alpine remobilization. The Sb-Au deposits of

380 ORVOOVÁ, MAJZLAN and CHOVAN

Fig. 6. Alteration halo of a quartz-stibnite vein (in the left part of the photograph). ill-carb illite-carbonate zone, mus muscovite

zone, chl chlorite zone. Scale bar = 2 cm.

Table 2: Electron microprobe analyses of biotite, muscovite, chlorite and illite from the altered rocks in Dúbrava. Analyses of chlorite

recalculated on the basis of 14 oxygen atoms, analyses of other minerals on the basis of 22 oxygen atoms.

Nízke Tatry Mts. appear to be genetically close to the Variscan

Sb-Au mineralization of Massif Central, Iberian massif (e.g.,

Boiron et al. 1989, 1996; Ortega et al. 1996) and elsewhere.

Results

There are no unaltered wall rocks left in the Dúbrava de-

posit. Field work, petrographic observation, mineralogical

studies and chemical analyses supplied evidence of hydro-

thermal alteration of the studied rocks. Samples collected in

the periphery of the ore field show also weak alteration.

Thickness of the inner alteration zones reaches centimetres

to decimeters. The alteration envelope is best developed

around the major veins with economic accumulation of met-

als, but it is found also around minor (few cm thick) veinlets

or even fractures without ore mineralization. The study of

alteration at the scale of meters was limited due to strong tec-

Sample

A-9

A-45

A-23

A-7

A-21

A-9

A-45

A-22

A-16

A-14

A-7

A-27

A-28

A-37

A-38

avg. of

Chl

Ill

SiO

36.85

35.34

36.85

26.86

26.84

25.29

27.03

26.35

46.27

45.24

46.19

45.90

45.92

48.04

51.43

48.54

TiO

3.10

2.89

3.10

0.09

0.10

0.08

0.13

0.25

0.57

0.66

1.55

1.24

2.12

0.11

0.05

0.16

16.30

16.53

16.30

20.82

21.05

21.19

19.03

21.27

29.29

33.67

31.06

31.24

30.09

34.24

30.32

33.41

0.06

0.10

0.07

n.a.

0.00

0.08

0.06

0.02

FeO

20.64

19.97

20.64

28.15

20.88

25.40

24.83

22.82

5.63

3.47

4.27

4.01

2.50

1.75

2.87

MnO

0.41

0.90

0.41

0.11

0.48

0.51

0.61

0.60

0.00

0.02

0.04

0.05

MgO

9.47

9.69

9.47

11.35

17.88

13.70

13.92

15.43

2.29

0.61

0.98

1.11

0.29

1.84

1.30

CaO

0.00

0.02

0.01

0.08

0.03

0.08

0.06

0.05

0.00

0.05

0.10

0.03

0.05

0.15

0.06

0.01

0.00

0.02

0.01

0.15

0.48

0.28

0.30

0.25

0.26

0.36

9.61

9.66

9.62

0.30

0.06

0.11

0.45

0.52

11.47

11.16

11.14

11.11

10.93

9.60

8.52

Total

96.49

95.24

96.53

87.76

87.31

86.39

86.07

87.28

95.67

95.29

95.49

95.07

94.61

96.51

95.43

95.27

5.59

5.45

5.58

2.86

2.77

2.71

2.90

2.76

6.33

6.13

6.27

6.26

6.29

6.34

6.76

6.41

2.41

2.55

2.42

1.14

1.23

1.29

1.10

1.24

1.67

1.87

1.73

1.74

1.71

1.66

1.24

1.59

T site

8.00

4.00

8.00

0.50

0.46

0.49

1.47

1.33

1.40

1.31

1.39

3.06

3.50

3.24

3.28

3.15

3.67

3.46

3.60

0.35

0.34

0.35

0.01

0.02

0.06

0.07

0.16

0.13

0.22

0.01

0.00

0.02

0.01

n.a.

0.00

0.01

0.00

2.62

2.58

2.62

2.51

1.80

2.28

2.23

2.00

0.64

0.39

0.48

0.49

0.46

0.28

0.19

0.32

0.05

0.12

0.05

0.01

0.04

0.05

0.06

0.05

0.00

0.01

2.14

2.23

2.14

1.80

2.75

2.19

2.23

2.41

0.47

0.12

0.20

0.23

0.06

0.36

0.26

O site

5.67

5.73

5.66

5.80

5.93

5.84

5.87

4.23

4.09

4.08

4.09

4.06

4.02

4.03

4.21

0.00

0.01

0.00

0.01

0.00

0.01

0.00

0.01

0.04

0.02

0.00

0.04

0.13

0.07

0.08

0.07

0.06

0.07

0.09

1.86

1.90

1.86

0.04

0.01

0.02

0.06

0.07

2.00

1.93

1.94

1.84

1.61

1.44

A site

1.87

1.95

1.88

2.04

2.05

2.01

2.02

2.01

1.91

1.69

1.54

HYDROTHERMAL ALTERATION OF GRANITOID ROCKS AND GNEISSES 381

tonic fracturing (and consequent alteration) in the surround-

ing of the veins. In the thin sections, secondary trails of CO

rich as well as aqueous fluid inclusions, corresponding to the

two major phases of deposit formation, can be found.

Changes of mineral composition of granitoids

The least altered rocks are greenish in color, equigranular,

medium- to coarse-grained with occasional transitions into

porphyric types (Figs. 5, 6). Illitization of plagioclase in this

zone is weak; illite represents only about 12 vol. % of the

feldspar (Fig. 2). Acidic reaction rims between plagioclase

and K-feldspar contain no illite (Fig. 7a). Chessboard albite

and various forms of pertites are common in the K-feldspar

phenocrysts but they show no signs of alteration due to hy-

drothermal metasomatic processes. The Mg/(Mg+Fe) ratio of

biotite in the altered rocks (0.450.48) is identical to the ratio

for biotite from unaltered granitoid rocks (0.450.49)

(Table 2, Fig. 3). Biotite is partially or completely converted

to chlorite (brunsvigite, ripidolite, according to Foster 1962)

(Figs. 7b, 8a, Table 2) and needle-like crystals or fine-grained

aggregates of rutile, rarely also epidote or sphene. Chlorites

(Mg/(Mg+Fe) = 0.390.64) are formed exclusively at the ex-

pense of biotite. Chemical composition of feldspars from al-

tered samples and unaltered rocks (Lukáèik 1983) does not

differ (Fig. 4).

Decomposition of biotite and chlorite leads to bleached ap-

pearance of the rocks (Figs. 5, 6) closer to the contact with

ore veins. The rocks appear as equigranular, medium- to

coarse-grained. Illitization is still not intensive and the

proportion of illite does not increase (Fig. 2). Chlorite is

preserved only rarely and is mostly replaced by muscovite

with inherited cleavage planes clearly marked by fine-grained

aggregates of Ti-oxides, identified by electron microprobe

analyses. Sporadic epidote and sphene are products of

decomposition of biotite and plagioclase. Minute anhedral

grains of Fe-dolomite and calcite become more abundant,

replacing plagioclase and chlorite. K-feldspars are still

relatively resistant to alteration.

Strongly altered rocks, confined to immediate vicinity of ore

veins, are grayish or white in color, medium to fine-grained

(Figs. 5, 6). Relics of muscovite flakes, replaced by illite (Fig.

7c), Ti-oxides, epidote and carbonates represent the original

biotite crystals. In thin sections, muscovite and illite were dis-

tinguished on the basis of crystal size; fine-grained aggregates

were considered to be illite and separate large flakes musco-

vite. Other rock-forming minerals, except of quartz, are almost

totally transformed to illite which represents up to 33 vol. % of

the rock (Fig. 2). Chessboard albite and pertites in K-feldspars

are preferentially replaced by illite. Increasingly abundant car-

bonates form aggregates of veinlets in plagioclase, less in K-

feldspars. Strongly altered rocks bear larger quantities of euhe-

Fig. 7. (a) partially sericitized plagioclase crystals in unaltered K-

feldspar, note the unaltered rims on the plagioclase crystals; (b) bi-

otite bt flakes partially altered to chlorite chl; (c) muscovite

mus replaced by fine-grained aggregate of illite ill.

382 ORVOOVÁ, MAJZLAN and CHOVAN

Fig. 8. a chemical composition of chlorites in the classification di-

agram of Foster (1962); b histogram of Al

content in chlorites.

Fig. 9. Ternary diagram of TiO

, Fe

, and MgO in muscovite (cir-

cles) and illite (squares) from the altered rocks from the Dúbrava

deposit. Fields after Monier et al. (1984): 1 magmatic muscovite,

2 late to post-magmatic muscovite, 3 hydrothermal illite.

dral crystals, veinlets and impregnations of pyrite, arsenopy-

rite, and stibnite.

Changes of mineral composition of migmatites

The collected migmatite samples do not display as pro-

found mineral changes as in the case of granitoid rocks. The

chief processes induced by hydrothermal fluids are chloriti-

zation and illitization. Muscovitization of mafic minerals

was not observed. Alteration intensity may be assessed ac-

cording to the degree of amphibole preservation and the vari-

able volume proportion of the secondary minerals.

Changes of chemical composition of granitoids

Investigation of chemical composition of the rocks is an-

other way of alteration assessment. In this study, alteration

index (A.I. = (MgO + K

O) / (Na

O + K

O + CaO + MgO))

defined by Hashiguchi et al. (in Vivallo 1987) was found to

be a useful tool. The degree of alteration estimated from

petrographic study correlates well with the A.I. The least

altered rocks are typical of A.I. < 50, while the most altered

ones reveal A.I. > 60, with a maximum at more than 80.

Chemical analyses of the altered rocks are listed in Table 4.

The first 16 analyses (A-9 to A-11) represent granitoid rocks

and are arranged, with an exception of A-9, in the order of in-

creasing A.I. The sample A-9 was selected as the least altered

one according to petrographic observation, (the degree of bi-

otite preservation), even if it does not hold the lowest A.I. val-

ue. The last three analyses are migmatite samples, with in-

creasing degree of alteration from A-23 to A-34. Approaching

to the ore veins, density of the samples slightly decreases and

then increases sharply due to the abundant sulfide grains in the

immediate vicinity of the veins.

Discussion

The principal wall-rock alteration products in the Dúbrava

Sb-Au deposit are chlorite, muscovite and illite, together with

lesser amounts of rutile, epidote, Fe-dolomite, calcite, and other

minerals formed by decomposition of biotite, plagioclase and

K-feldspar. Progressive alteration caused transformation of orig-

inal magmatic biotite to chlorite, muscovite and illite. The three

latter sheet silicate minerals are abundant and typical for a

specific alteration zone and govern the textures and color of the

rocks. Three zones of alteration are recognized in the Dúbrava

deposit:

chlorite zone,

II. muscovite zone,

III. illite-carbonate zone.

Chlorite is preserved only in the most external (chlorite)

zone. Recently, De Caritat et al. (1993) gave critical evalua-

tion of available empirical and thermodynamic chlorite ther-

mometers and concluded that none of them performs satisfac-

HYDROTHERMAL ALTERATION OF GRANITOID ROCKS AND GNEISSES 383

Fig. 10. Distribution of K/O

(OH)

in muscovite and illite from

the altered rocks.

torily in the full range of chlorite composition, bulk rock min-

eralogy and P-T conditions examined. Empirical thermome-

ters, such as the one developed by Cathelineau (1988) are val-

id only for specific geological environment and produce large

errors if used in geochemically different environments. Later

thermal events may also override the original chemistry of

chlorite, thus affecting any thermometric data. Although there

was undoubtedly a later event related to the formation of sul-

fidic stages the studied chlorite seem to record, at least partial-

ly, action of the higher thermal fluids linked to the older miner-

alization stages. In the case of alteration, chlorite may inherit

compositional features of its precursor. Application of chlorite

thermometers to the samples from Dúbrava gives wide range

of formation temperatures, summarized in Table 3. Asymme-

try of Al

frequency distribution (Fig. 8b) in chlorite suggests

re-equilibration of studied chlorite during later mineralization

stages. We believe that the calculated average temperature

320±40

C (Cathelineau 1988; Jowett 1991; Table 3) is rea-

sonable when compared to the previous results of fluid inclu-

sion investigation (Chovan et al. 1995).

Several authors investigated reactions of rock-forming min-

erals under hydrothermal conditions. Ferry (1979) reports

breakdown of biotite to chlorite, sphene and release of silica,

and Na

into the fluid. In their experimental work with

iron chloride complexes, Fein et al. (1992) found that no

annite was formed in the presence of sulfur in agreement with

the data of Hammarback & Lidqvist (in Fein et al. 1992) who

suggested that annite component is destabilized by sulfur.

Destabilization of biotite and amphibole by S-rich fluid is

reported also by Tso et al. (1979) and Popp et al. (1977).

Furthermore, if the fluid contains As-bearing complexes, the

reaction will proceed according to Heinrich & Eadington

(1986) as follows:

biotite + chlorite + H

AsO

+ H

→

muscovite + quartz + arsenopyrite + water + O

K-feldspar + chlorite + H

AsO

+ H

→

muscovite + quartz + arsenopyrite + water + O

Bryndzia & Scott (1987) mention also destabilization of Fe-

chlorite by sulfidation, although at temperatures and pressures

higher than those assumed for the Dúbrava deposit. All these

reactions may account for release of silica in the hydrothermal

fluid and development of impregnations of pyrite and arse-

nopyrite in the altered rocks.

Muscovite is found in all alteration zones, but it dominates

only in the muscovite zone. The least altered rocks contain

muscovite flakes only along grain boundaries of plagioclases;

in the course of alteration, muscovite replaces chlorite and

plagioclase. Electron microprobe analyses (Table 2) revealed

that the textural variety of muscovite found in the least

altered samples with still preserved biotite contains the

largest amounts of Ti. This muscovite is considered to be of

magmatic or late magmatic origin (Fig. 9). Muscovite that ob-

viously replaces chlorite contains less Ti, representing the

product of hydrothermal activity. Fig. 9 depicts comparison of

chemical composition of muscovite from Dúbrava with com-

positional fields of magmatic and hydrothermal muscovite in

the Millevaches Massif outlined by Monier et al. (1984). Posi-

tion of our analyses outside of the magmatic muscovite field is

most probably due to differences in bulk rock chemistry; the

general trend is clearly preserved, however. In more altered

samples, distinction between magmatic and hydrothermal

muscovite, based exclusively on the petrographic observa-

tions, becomes ambiguous. Beside TiO

, Monier et al. (1984)

showed that magmatic muscovites exhibit a higher Na

(Na

O + K

O) ratio (0.070.12) while hydrothermal white mi-

cas are characterized by substantially lower ratio (0.020.06).

On the other hand, in their discrimination of magmatic and hy-

drothermal muscovites in granites, Villa et al. (1997) found no

differences between the two groups in Na

O and K

O content,

although the difference in TiO

concentration is obvious. A lit-

tle variation in Na

O/(Na

O + K

O) ratio with changing TiO

concentration in muscovite was observed also in the studied

samples from the Dúbrava deposit (Table 2).

Illite from Dúbrava is characterized by lower content of K/

(OH)

and Al

than muscovite, FeO

tot

content up to 3 wt.

% and low TiO

content (Table 2, Fig. 9), comparable to the

published data on illite composition (rodoñ & Eberl 1984).

The K/O

(OH)

content shows distribution with two maxima

between 0.60.7 and 0.80.9 K/O

(OH)

while muscovite

contains 1.02±0.02 (1

) K/O

(OH)

(Fig. 10). It is not clear

whether the analyses correspond to mixed-layer illite-smectite

or merely to a mixture of illite with later smectite. The end-

member illite was found to contain 0.88 (Yates & Rosenberg

1997) or 0.85 (Aja 1991) K/O

(OH)

. Aja (1991) implies also

range

mean (1

Cathelineau (1988)

215–376

319(39)

K ranidiotis & MacLean (1987)

135–195

179(14)

Jowett (1991 in De Caritat et al. 1993)

217–380

324(39)

Table 3: Estimated temperature of chlorite formation according to

various chlorite thermometers.

384 ORVOOVÁ, MAJZLAN and CHOVAN

the existence of stable solubility-controlling phase with 0.69

K/O

(OH)

Table 5 lists the results of XRD analyses of the most altered

granodiorites, performed to determine their mineral composi-

tion and the crystallinity index (CI) value. The measured val-

ues, compared to the data of ucha & Eberl (1992) gave an es-

timate of illite crystallization interval of 180200

C. The CI

values are similar to those measured for hydrothermal illite in

Silverton caldera by Eberl et al. (1987) who determined crys-

tallization temperature from

H and

O data to 170320

Some XRD patterns reveal the presence of minor amounts of

smectite which probably originated during final, lower, ther-

mal stages of the alteration process.

Mylonite zones, apparently reactivated during the Alpine

orogeny, including the ¼ubelská fault, a major structure run-

ning throughout the deposit, contain mostly illite and traces

of smectite and kaolinite.

The isocon method of Grant (1986) was adopted in the cal-

culation of gains and losses during wall-rock alteration. Field

observations suggest little or no volume change during alter-

ation, but its quantification is not feasible. The isocons de-

fined for no mass change are almost identical with those for

constant volume. The solution of Gresens (1967) equation

gave no single datum for the volume change for the pairs of

the least altered sample and other analyzed samples, a con-

siderable scatter of values was obtained instead. The isocons

for constant alumina imply volume changes in range between

8.2 to +9.5 %, but aluminum needs not to be immobile (cf.

Robert & Brown 1986; Boiron et al. 1989), although in other

cases it seems to be (Ferry 1985;MacLean & Kranidiotis

1987; Ortega et al. 1996). Enrichment or depletion was quan-

tified on the basis of constant volume assumption.

MacLean & Kranidiotis (1987) advise investigation of the

relationship of pairs of elements, where one of them serves as

a reference and is shown or considered to be immobile. If the

data points for the two elements form a linear array with high

positive correlation passing through the origin then the

second element is also immobile, provided that the two

elements are not decoupled geochemically. The most suitable

reference element in most cases appears to be zirconium.

High positive correlation coefficients were found only in the

case ZrTiO

(r = 0.92, Fig. 11a) and ZrSc (r = 0.83,

Fig. 11b), suggesting that these three elements behaved as

immobile throughout the alteration process. Contrary to this,

rutile is a common constituent of the pyrite-arsenopyrite as-

semblage of hydrothermal veins and the trends for both ele-

ments (TiO

, Sc) are but a copy of the magmatic trends

(Figs. 11a, b). Investigation of the relationships ZrAl

(Fig. 11c) and ZrK

O (Fig. 11d) shows that they follow the

magmatic trends as well, with an evident gain of K

(Fig. 11d). The application of this method seems to be ques-

tionable. We suppose that TiO

was released from primary

magmatic minerals (biotite) but its migration was limited. We

are unable to judge the degree of Sc mobility.

Figs. 12ag demonstrate gains and losses in the analyzed

samples arranged in the order of increasing alteration index.

Variations in alumina and silica content (Figs. 12a,b) are as-

cribed either to the variable chemical composition of the

magmatic precursor, or, more probably, to local redistribu-

tion of Al

and SiO

. Chlorite is occasionally found in the

fractures of the vein quartz, indicating extraction of Al from

the ambient rocks into the veins. Therefore, calculations of

chemical changes on the basis of constant Al

were avoid-

ed and the constant volume approach was used instead.

From the change in mineralogy across alteration zones it is

obvious that K

O, H

O, Sb, S were added to the rocks during

alteration, whereas CaO, Na

O, MgO, Fe

were removed

(Figs. 12cg). Conversion of biotite to chlorite in the chlorite

Table 4: Bulk chemical composition of altered rocks from the Dúbrava deposit. Oxides in wt.%, trace elements in ppm. LOI = loss on ig-

nition, D = density, A.I. = alteration index (see text). Analyses of altered granitoids are arranged in the order of increasing A.I.

A-9 A-38 A-44

807

806

A-7 A-27 A-37 A-16 A-14 A-32 A-6a A-25 A-10

A-11

A-23 A-22

A-34

granitoids

migmatites

SiO

66.06 67.88 68.45 66.51 68.72 65.69 64.05 54.67 69.05 71.20 64.50 64.85 66.80 70.08 65.87

68.62 56.53 54.10

51.06

15.28 15.04 14.57 14.79 15.01 15.70 16.46 14.69 14.49 14.27 16.97 16.57 16.23 15.90 17.26

17.09 15.06 15.54

14.69

3.70

3.29

3.36

4.11

2.93

2.44

3.33

6.45

2.22

3.28

2.14

3.94

2.99

1.58

3.45

2.00

8.05

9.20

8.13

TiO

0.582 0.470 0.521 0.676 0.465 0.614 0.549 0.580 0.380 0.188 0.514 0.304 0.513 0.570 0.375

0.480 0.916 1.089

0.971

CaO

2.93

3.06

2.51

2.57

2.23

2.55

1.68

4.51

1.35

0.32

2.49

0.67

1.48

0.65

1.06

0.83

5.36

5.32

6.47

MgO

1.55

1.24

1.38

1.89

1.29

1.26

1.53

3.36

0.79

0.52

1.53

1.29

1.22

0.73

1.20

0.89

5.33

5.26

5.35

MnO

0.069 0.049 0.045 0.060 0.043 0.061 0.107 0.230 0.057 0.098 0.037 0.150 0.176 0.021 0.140

0.067 0.146 0.182

0.150

0.37

0.35

0.24

0.35

0.20

0.41

0.25

0.63

0.16

0.23

0.21

0.26

0.22

0.27

0.10

0.23

0.43

0.79

0.47

3.53

3.67

3.37

3.48

3.38

2.78

2.38

0.62

2.52

2.77

1.09

2.43

1.13

2.00

1.21

0.45

3.20

2.67

1.78

3.36

2.72

2.62

2.73

3.27

4.13

4.24

4.08

4.85

4.43

4.42

4.31

4.46

5.04

4.55

5.33

2.39

2.44

2.63

0.18

0.13

0.09

0.06

0.08

0.14

0.69

2.77

0.94

0.59

0.27

0.13

1.21

0.15

0.45

0.27

0.06

2.07

1.41

LOI

1.75

1.73

2.66

2.13

2.18

3.43

4.99

9.53

2.75

2.54

5.73

4.49

4.62

2.48

4.85

3.69

1.59

2.37

7.43

0.27

0.28

0.26

0.18

0.25

0.30

0.35

0.25

0.28

0.45

0.43

0.31

0.44

0.42

0.30

0.32

0.23

0.34

Total

99.18 99.50 99.73 99.30 99.72 99.07 99.57 99.35 98.65 99.85 99.90 99.26 99.84 99.33 100.10

99.68 99.00 98.96

99.13

63.95 69.95 54.88 87.37 41.95 37.28 32.00 24.95 20.40 15.83 16.74 21.88 26.04 33.84 34.86

22.13 119.20 158.30 107.30

118

103

143

163

192

135

191

161

174

224

142

193

214

127

130

9.456 8.178 9.445 10.13 9.037 9.324 8.178 11.060 5.776

5 9.148

5 7.903 5.975 7.407

6.129 23.830 24.610 27.670

4.786 4.711 3.396 5.537 3.044 3.452 4.885 12.090 3.479 5.461 2.709

2.78 2.861

1.763 15.990 12.430 19.630

7.761 5.836 4.645 7.021 3.921 3.235 3.778 20.450 2.562 9.886 1.716 4.455 1.582 3.552 1.468

4.478 12.850 6.271 25.810

730

610

560

680

500

490

280

270

610

160

250

280

300

240

320

230

1610

1350

890

237

217

195

250

179

230

196

218

175

110

193

177

192

172

173

228

288

251

1990

1040

1000

1420

1380

4010

420

820

4750

460

390

1930

1290

2020

550

290

2040

1530

900

2.707 2.703 2.714 2.709 2.696 2.791 2.713 2.738 2.659 2.682 2.732 2.751 2.731 2.706 2.744

2.723 2.759 2.798

2.789

A.I.

0.432 0.370 0.405 0.433 0.448 0.503 0.587 0.592 0.593 0.616 0.624 0.644 0.685 0.685 0.717

0.829

HYDROTHERMAL ALTERATION OF GRANITOID ROCKS AND GNEISSES 385

zone is responsible for K

O loss in the less altered samples

(Fig. 12d). Illitization accounts for most of the gain of K

and H

O. Loss of Na

O (Fig. 12c) and CaO (Fig. 12g) is due

to decomposition of plagioclases. MgO (Fig. 12f) and Fe

(Fig. 12e) were removed during destruction of biotite and

chlorite. Fixation of Ca, Mg and Fe in carbonates was not suf-

ficient to counterpart their release; only a single anomalous

sample (A-37) shows a positive change (gain) for these ele-

ments (Figs. 12eg). The samples were not analyzed for CO

but according to abundant occurrence of carbonates, the rocks

were also enriched in CO

Evaluation of the element migration in migmatites is limit-

ed due to few analyzed samples. It seems that SiO

and Na

were removed and K

O added to these rocks (Table 4). Mi-

nor losses were determined also in the case of Fe

, MgO

and CaO. The same fluids drove hydrothermal alteration of

both granitoids and migmatites and therefore the differences

in alteration patterns are caused only by the variability of the

primary material.

Conclusions

Changes of chemical and mineralogical composition of

rocks altered by hydrothermal fluids at the Dúbrava Sb-Au

deposit, Nízke Tatry Mts. enabled to distinguish three zones

of alteration: I. chlorite zone (outermost), II. muscovite zone,

III. illite-carbonate zone (innermost).

Every wall rock sample from the deposit represents an al-

tered rock. Increasing degree of alteration results in progres-

sive decomposition of plagioclase, K-feldspar and complete

disappearance of biotite. Primary magmatic minerals and tex-

tures are completely erased in the most internal alteration zone.

The presented data indicate a temperature decrease from

the external chlorite zone (320±40

C, chlorite thermometry)

toward the inner zone rich in illite and carbonates (180

200

C, crystallinity index). The initial stages of alteration

Table 5. Crystallinity index values of illite from the altered rocks

from the Dúbrava deposit (in º2Q).

Sample

CI (air dried)

CI (EG)

A-37 (1)

0.55

A-37 (2)

0.40

A-10

0.44

0.38

A-27

0.22

0.225

A-6A

0.46

0.38

A-5

0.36

0.35

A-101

0.46

0.415

A-100

0.62

0.41

A-8

0.36

0.385

A-25

0.52

0.42

829

0.70

0.65

Fig. 11. Plots of Zr vs. TiO

, Sc, Al

, and K

O for unaltered granitoids (Cambel and Janák, unpubl.data, solid circles) and altered grani-

toids from the Dúbrava deposit (open squares). Zr and Sc in ppm, oxides in wt. %.

Fig. 12. Percentage gains and losses of oxides in altered granitoid rocks from the Dúbrava deposit based on data listed in Table 4 arranged

in order of increasing alteration index. The calculation was performed from 15 samples, the sample A-9 was used for normalization.

386 ORVOOVÁ, MAJZLAN and CHOVAN

bore pervasive character, altering mostly biotite in large vol-

umes of rocks within the ore field. Muscovite and illite-car-

bonate zones are superimposed on the most external chlorite

zone. They are a product of action of cooler thermal fluids

whose circulation was restricted to open tectonic structures

and channelways. Gradual sealing of the walls of the veins

and temperature decrease led to isolation of external chlorite

zone and preservation of its mineral assemblage even during

later events.

In the course of alteration, the rocks were enriched in K

O, CO

, Sb, S and depleted in CaO, Na

O, MgO and

(Figs. 12cg). Local redistribution and/or variation in

the chemical composition of the primary magmatic precursor

caused small changes in Al

and SiO

content (Figs. 12a,

b). Zr, TiO

and Sc seem to be the least mobile elements, al-

though rutile appears in pyrite-arsenopyrite assemblage of

the quartz veins.

Similar features of alteration accompanying the ore min-

eralization are expected, according to our observations, also

in other Sb-Au deposits (Magurka, Dve Vody) in the Nízke

Tatry Mts.

Acknowledgements: This paper includes results of Master

thesis of M. Orvoová. We are grateful to doc. V. ucha for

the help with interpretation of XRD patterns of clay minerals.

We thank Dr. J. Kritín (CLEOM, Comenius University) and

Dr. M. Köhlerová (GS SR) for electron microprobe analyses

of minerals. The preliminary versions of the paper benefited

greatly from critical remarks of Dr. J. rodoñ, Prof. Z. Pertold,

Prof. I. Kraus and doc. I. Rojkoviè. We are indebted to Dr. I.

Petrík for advice and valuable remarks with the problems on

granitoid rocks. The field work and bulk rock analyses were fi-

nanced by Dr. M. Arvensis and Dr. J. Michálek (ENVIGEO).

The work was financially supported also from VEGA Grant

Project 1/2172/95.

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