Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

GEOLOGICAL JOURNAL

Geol. J. (2009)

Published online in Wiley InterScience

(www.interscience.wiley.com) DOI: 10.1002/gj.1178

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoicbasement of SE Sweden

S. MORAD1,2, M. A. K. EL-GHALI 3,4*, M. A. CAJA 5, M. SIRAT 6,K. AL-RAMADAN7 and H. MANSURBEG8

1Department of Petroleum Geosciences, The Petroleum Institute, Abu Dhabi, United Arab Emirates2Department of Earth Sciences, Uppsala University, Uppsala, Sweden

3Department of Petroleum Geosciences, Universiti Brunei Darussalam, Brunei4Department of Earth Science, Faculty of Science, Al-Fateh University, Tripoli, Libya

5Departamento de Petrologıa y Geoquımica, Facultad de Geologıa, Universidad Complutense de Madrid, Madrid, Spain6Schlumberger Oilfield Eastern Ltd, Kuwait

7Department of Earth Sciences, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia8AGR Reservoir Evaluation Services AS, Oslo, Norway

Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel themechanisms and conditions of plagioclase alterations. These alterations include saussuritization, albitization and replacement of plagioclaseby K-feldspar. The hydrothermal alterations, which are inferred to have occurred at ca. 250–4008C, resulted in concomitant formation of Al-rich titanite, epidote, calcite, pumpellyite, prehnite and iron oxides. Replacement of plagioclase by K-feldspar occurs in red-stained zones,which have developed close to thin fractures owing to the precipitation of tiny Fe-oxide pigment particles within the altered plagioclase,whereas saussuritized plagioclase has less systematic spatial relationships to these fractures. Albitization of plagioclase occurred in rocks thatare poor in biotite compared to rocks that suffered extensive saussuritization. The chemical and textural characterization of various types ofplagioclase alterations allows elucidation of the granitic hydrothermal systems. Features of feldspar alteration in the granitic rocks are similarto those encountered in feldspathic sandstones and should hence be considered in studies on diagenetic changes of siliciclastic successionsduring basin evolution. Copyright # 2009 John Wiley & Sons, Ltd.

Received 19 September 2008; accepted 20 July 2009

KEY WORDS saussuritization; hydrothermal alteration; secondary K-feldspar; albitization; granite

1. INTRODUCTION

Feldspars are the most abundant minerals in granitic rocks

and in the Earth’s crust. The elemental compositions and

concentrations of stable and radiogenic isotopes in mag-

matic feldspars are commonly used to determine the age and

cooling conditions of granitic rocks (Giggenbach 1981;

McDowell 1983; Frei 1996; Mathez and Waight 2003; Cole

et al. 2004; Gagnevin et al. 2005; Siebel et al. 2005; Tyrrell

et al. 2006; Takagi et al. 2007). It is, therefore, of

considerable importance to detect chemical and miner-

alogical modifications which may reset the geochemical

* Correspondence to: M. A. K. El-Ghali, Department of Petroleum Geos-ciences, Universiti Brunei Darussalam, Tungku Link, Gadong BE 1410,Brunei Darussalam, Brunei. E-mail: [email protected]

signatures of magmatic feldspars. Detecting such modifi-

cations aids the interpretation of geochemical data and helps

the elimination of erroneous and misleading results (Fiebig

and Hoefs 2002). Moreover, granitic rocks are the major

sources of plagioclase grains in sandstones. Describing the

petrographic and chemical characteristics of feldspars in

granitic rocks (Eliasson 1993; Drake et al. 2008) will help to

distinguish diagenetic alteration from hydrothermal source

rock alteration (Pittman 1970; Trevena and Nash 1981;

Morad et al. 1990; Parsons et al. 2005).

Most work on plagioclase alteration in granitic rocks has

dealt with saussuritization, sericitization and albitization

processes (Jenkin et al. 1992; Garcia et al. 1996; Fiebig and

Hoefs 2002; Boulvais et al. 2007), whereas less attention has

been paid to the replacement of plagioclase by hydrothermal

K-feldspar (Drake et al. 2008). The aim of this paper is to

Copyright # 2009 John Wiley & Sons, Ltd.

Figure 1. Location map of the Aspo hard rock laboratory (after Rhen et al. 1997; Stanfors et al. 1997).

s. morad ET AL.

highlight and discuss the types, timing and conditions of

hydrothermal alteration of magmatic plagioclase in Proter-

ozoic granitic rocks from SE Sweden (Figure 1). These rocks

are currently subjected to extensive studies by the Swedish

Nuclear Fuel and Waste Management Company (SKB) to

evaluate their suitability as a repository for high-level, spent

nuclear fuels.

2. GEOLOGICAL SETTING

The bedrock on the island of Aspo is composed of reddish to

greyish dioritic to granitic rocks (Figure 2) as well as dykes,

sills and lenses of fine-grained (aplitic) granite, dolerite and

mylonite. This lithological assemblage forms part of the

Proterozoic basement of south-eastern Sweden (Kornfalt

and Wikman 1988; Wikman and Kornfalt 1995; Stanfors

et al. 1999). The igneous suite represents part of the Trans-

Scandinavian Igneous Belt (TIB) and was formed about

1800 Ma (Wahlgren et al. 2008) and episodically

rejuvenated by intrusive bodies at ca. 1450 Ma (Ahall

2001) and 1000–900 Ma (Wahlgren et al. 2007). The

bedrock was affected by deformation events during

the intrusion of the Gotemar and Uthammar granites nearby,

which were associated with the Sveconorwegian (�1000–

900 Ma) and Caledonian (�440-400 Ma) orogenies (Drake

et al. 2007). These processes resulted in a complex history of


ductile and brittle deformation, which affected almost all

rock types (e.g. Stanfors 1996; Munier 1993; Milnes et al.

1998; Stanfors et al. 1999; Drake et al. 2007; Wahlgren et al.

2008). Among the prominent tectonic features are mylonitic,

NE-SW-trending deformation belts up to 100 m wide.

Together with other less extensive fracture zones (up to 50 m

wide), these features form an orthogonal network, with dips

of individual planar zones varying from vertical to sub-

horizontal (Munier 1993).

3. SAMPLES AND ANALYTICAL METHODS

A total of 352 samples were collected from four drill cores

(KAS09, KAS14, KA1131B and KA0575A). Detailed

description of the structural geological features and

lithological variations are given in Sehlstedt et al. (1990).

Thin sections were prepared for all samples and modal

mineralogical compositions were obtained by counting

400 points in each thin section. The chemical compositions

of minerals were determined in polished, carbon-coated thin

sections using a Cameca BX50 electron microprobe (EMP)

equipped with three spectrometers and a back-scattered

electron detector (BSE). Operating conditions were: 20 kV

acceleration voltage, 12 nA measured beam current and a

1mm beam diameter. Standards and count times were

wollastonite (Ca, 10s), orthoclase (K, Ba 5s), albite (Na, Si,

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Figure 2. Geological map of the Aspo Island.

hydrothermal alteration in granitic rocks

5s and 10s, respectively), corundum (Al 20s), MgO (Mg,

10s), MnTiO3 (Mn, 10s) and hematite (Fe, 10s). Detections

limits (3s) were Si 215 ppm, Al 175 ppm, Ti 195 ppm, Mn

240 ppm, Mg 95 ppm, K 140 ppm, Na 110 ppm, Ca 120 ppm,

Sr 95 ppm and Fe 190 ppm. The intimate association

between magmatic and secondary feldspars rendered

obtaining microprobe analyses of pure phases difficult.

The polythermal reaction path model for the stability of

secondary K-feldspar versus albite presented in this work

was constructed over a range of temperatures using the

Geochemist’s Workbench1.

4. RESULTS

4.1. Magmatic minerals: petrography and

chemical compositions

Modal analyses of the studied rocks indicate dominantly

syenogranite, monzogranite to granodiorite compositions.

The primary magmatic minerals include plagioclase, K-

feldspar, quartz, biotite, amphibole, muscovite, titanite,

epidote, magnetite and trace amounts of zircon, apatite and

monazite. The K-feldspar occurs as fresh microcline

perthite, whereas the plagioclase crystals show evidence


of pervasive alteration, which is the focus of this paper. The

quartz crystals display straight to slightly undulose

extinction, except in the vicinity of major deformation

zones where quartz displays strongly undulose extinction.

Biotite, which is fresh or partly to extensively chloritized,

has an average chemical formula: K1.94Na0.01 (Fe2.3M-

g3.1Al0.3Ti0.2)(Al2.4Si5.6)O20(OH)4. Muscovite is rare or

absent in the majority of the samples and has an average

chemical formula of K1.8Na0.1(Fe0.4Mg0.3Al3.3Ti0.04)

(Al1.7Si6.3)O20(OH)4. Amphibole has an average chemical

formula of (Ca1.95Na0.3K0.3)(Fe2.0Mg2.6 Al0.2) (Al1.0Si7.0)

O22(OH)2. Epidote occurs as coarse, subhedral to euhedral,

colourless crystals (100–600mm across) which reveal

chemical zoning in terms of variations in Sr and REE

contents. Titanite occurs as subhedral to euhedral crystals,

up to 800mm across, which contain small inclusions of

magnetite, apatite, epidote, allanite and zircon. Magnetite

occurs in most samples as localized polycrystalline patches.

4.2. Secondary minerals: petrography and

chemical compositions

Secondary minerals include chlorite, prehnite, pumpellyite,

titanite, epidote, albite, K-feldspar, calcite, and fluorite.

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s. morad ET AL.

Prehnite fills fractures and has replaced biotite, K-feldspar

and plagioclase. Fractures filled by prehnite are lined with

albite, K-feldspar, calcite, chlorite and, in some cases,

fluorite, quartz and/or epidote. Pumpellyite occurs within

chloritized amphibole, chloritized biotite and plagioclase.

Secondary epidote occurs as euhedral crystals (<100mm),

which line and/or extensively fill fractures and voids

together with calcite, and within chloritized biotite and

plagioclase. Titanite occurs as small (2–15mm) lenticular to

elongated crystals arranged along traces of the cleavage

planes in chloritized biotite. Calcite occurs mainly as

fracture filling and as partial replacement of magmatic

quartz and feldspar. Fluorite occurs as cubic crystals that line

the fracture walls and are intimately associated with and/or

enclosing, and hence post-dating, quartz, K-feldspar, epidote

and prehnite. Quartz occurs as the sole or main mineral in

some fractures. Further detailed work on the hydrothermal

alterations of the studied granitic rocks has been presented by

Morad and Aldahan (2002), Eliasson (1993), and Drake et al.

(2007, 2008, 2009).

Figure 3. A and B. Optical photomicrographs (crossed-polarized light)showing plagioclases that have suffered increasing extents of saussuritiza-tion (i.e. formation of albite, sericite and epidote) accompanied by oblit-

4.3. Texture and chemistry of secondary feldspars

Secondary feldspars, which include albite and K-feldspar,

have replaced magmatic plagioclase and are most abundant

in red-stained (originally grey coloured) granitic and

granodioritic rocks that occur as zones (10–25 cm wide)

adjacent to fractures (cf. Drake et al. 2008). The amounts of

secondary feldspar and degree of red staining decrease with

increasing distance from the fracture. Secondary feldspars

include:

eration of twin laminae. Note the deformed twins of the plagioclase in 3A,which occur close to a small fracture filled with fine-grained epidote (Ep).

(i) A

Saussuritization is most extensive in samples rich in chloritized biotite.

Copy

lbite, which together with variable proportions of

epidote and sericite, has replaced plagioclase; this suite

of secondary minerals is typically found in saussur-

itized plagioclase (Eliasson 1993). In rare cases, the

plagioclase is mainly sericitized. The degree of saussur-

itization varies widely from slight to pervasive, in the

latter case resulting in nearly complete obliteration of

the twin planes of plagioclase (Figures 3A and 3B). The

twin planes of saussuritized plagioclase are, in some

cases, bent, presumably owing to ductile or semi-

ductile deformation (Figure 3A). The amounts of saus-

suritized plagioclase, which occurs throughout the

granitic body, are greatest in rocks containing abundant

partly to completely chloritized biotite (the average

chemical formula of which is (Fe3.9Mg5.8Al2.1)

(Al1.8Si6.2)O20(OH)16.

EMP analyses of sericite reveal the presence of

higher amounts of Fe (av. 0.53 atoms per formula unit

(apfu)), and are hence somewhat more phengitic in

composition than typical magmatic muscovite. Tetra-

hedral and octahedral Al range in abundance from 1.69

right # 2009 John Wiley & Sons, Ltd.

to 1.77 apfu and from 3.28 to 3.35 apfu, respectively.

The interlayer sites are strongly dominated by K,

whereas Na (i.e. paragonite solid solution) is low

(av. 0.1 apfu). The average chemical formula of the

sericite is K1.6Na0.1(Fe0.5Mg0.2Al3.3Ti0.04 Al1.7Si6.3)

O20(OH)4. Epidote is colourless to yellowish–green

with an average chemical formula of Ca1.9Fe0.8Al2.2

Si3.0O12.5 assuming that all iron occurs as Fe3þ. Albite

is virtually pure NaAlSi3O8 (Ab> 98 mole%).

(ii) A
lbite that has solely replaced magmatic plagioclase
(i.e. albitized) partly to pervasively. Pervasively albi-

tized plagioclase crystals are vacuolated (i.e. riddled

with micro-pores) and/or variable amounts of Fe-oxide

pigment (Figure 4A), which induce a turbid appearance

and red staining. Vacuolization is often restricted to

patches of albitized plagioclase, whereas the non-

albitized patches display little or no vacuolization

Geol. J. (2009)

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(Figures 4B and 4C). Albitized plagioclase may contain

a few scattered crystals of epidote, sericite and, in

some cases, Fe-rich chlorite. The twin planes of albi-

tized grains, which are, in many cases, dislocated

owing to brittle fracturing, are partly to completely

obliterated. Some of the saussuritized and albitized

plagioclase crystals are partly replaced by prehnite.

Magmatic K-feldspar crystals embedded in albitized

plagioclase (i.e. in perthite) have also been subjected to

partial albitization (Figure 4C). Albitized plagioclase

of nearly pure albite end-member (NaAlSi3O8) com-

position (Table 1) is most abundant in rocks that

contain little or no biotite.

(iii) K
-feldspar that has partly replaced plagioclase occurs
as red-stained, polysynthetically twinned crystals

(Figure 5A) that are restricted to red-stained zones

adjacent to thin fractures. Replacement by K-feldspar

evidently initiated along the twin and fracture planes in

plagioclase and proceeded inwards into the host plagi-

oclase resulting in patches that are completely replaced

by K-feldspar (Figures 5B and 5C). In some cases, the

replacement of plagioclase by albite and K-feldspar has

resulted in a chessboard-like texture (Figure 5D). Sec-

ondary K-feldspar patches vary in composition from

nearly pure KAlSi3O8 composition (Or> 99 mole%) to

variably barium-rich (Figures 5D and 5E; Table 1).

Plagioclase crystals that are replaced by secondary K-

feldspar display petrographic and chemical evidence of

extensive albitization.

(iv) K
-feldspar crystals that are zoned in terms of variations
in barium contents (BaO¼ trace to 4.1%; Figure 5F;

Table 1) and have developed around albitized and

saussuritized plagioclase crystals in a few of the studied

samples. These plagioclase crystals, which are com-

monly embedded within magmatic K-feldspar, are

partly replaced by quartz and contain dissolution voids

that are partly filled by microcrystalline, Fe-rich chlor-

ite (Figure 5F).

(v) F

Figure 4. (A) Optical photomicrograph (crossed-polarized light) showingplagioclase in which the twin planes have been obliterated by albitization.Note the local presence of Fe-oxide pigment. The opaque crystals aremagnetite inclusions that have been subjected to partial oxidation. (B) BSEimage showing albitized (lighter grey area) plagioclase (grey areas); notethat the albitized patches are riddled with micropores and tiny particles of

racture-filling albite and K-feldspar. Albite lines the

fracture walls, followed by K-feldspar and then other

minerals, such as fluorite, prehnite, chlorite and

calcite that successively fill the fracture. Small

fractures (< 20mm across) that are filled solely by

K-feldspar cut across, and hence post-date, albitized

plagioclase and small fractures filled solely by albite.

K-feldspar crystals also occur within chloritized bio-

tite, and are closely associated with pumpellyite

[Ca4Si6Al3Fe2O24(OH)3.2H2O] and Al-rich titanite

(grothite, Al2O3¼ ca. 7%).
Fe-oxide pigment (black arrows). The bright crystal to the right is epidote.(C) BSE image of albized plagioclase showing that the albitized patch (darkgrey) is richer in micropores than the plagioclase (light grey). The light
patches are K-feldspar.

EMP analyses of plagioclase crystals that have been

subjected to various types of replacement processes reveal

that the amounts of CaO in plagioclase vary between 4.2 and

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Table 1. Electron microprobe analyses of primary and secondary feldspars based on eight oxygen atoms

Type Wt. % Atom/Formula

SiO2 Al2O3 CaO Na2O K2O BaO SrO Total Si Al Ca Na K Ba Sr

A 68.5 19.8 0.08 11.9 0.04 nd nd 100.3 2.98 1.02 — 1.01 — — —A 68.5 19 0.03 11.6 0.01 nd nd 99.14 3.01 0.99 — 0.99 — — —A 68.9 19.3 0.06 11.9 0.05 0.03 nd 100.2 3.00 0.99 — 1.01 — — —A 68.1 20 0.41 11.9 0.08 nd 0.07 100.6 2.97 1.03 0.02 1.01 — — —A 68.8 20.7 0.32 11.2 0.22 nd nd 101.2 2.97 1.05 0.02 0.94 0.01 — —A 68.6 19.6 0.02 11.4 0.02 nd 0.02 99.66 3.00 1.01 — 0.97 — — —A 69.1 19.9 0.02 11.9 nd nd 0.01 100.9 2.99 1.02 — 1.00 — — —A 67.7 19.3 0.11 11.6 0.16 nd 0.03 98.9 2.99 1.01 0.01 0.99 0.01 — —A 69.0 19.3 0.02 11.4 0.02 nd 0.01 99.75 3.01 0.99 — 0.97 — — —A 68.4 19.2 0.06 11.2 nd nd nd 98.86 3.01 1.00 — 0.96 — — —B 60.1 24.4 5.9 8.2 0.02 nd nd 98.62 2.71 1.30 0.29 0.72 — — —B 59.8 22.7 5.8 8.4 0.14 nd 0.27 97.11 2.74 1.23 0.29 0.75 0.01 — 0.01B 59.7 23.1 5.8 8.5 0.11 nd 0.3 97.51 2.73 1.25 0.28 0.75 0.01 — 0.01B 60.2 23.2 5.9 8.2 0.18 nd nd 97.68 2.84 1.29 0.01 0.75 0.01 — —B 65 21.9 4.2 8.7 0.05 nd nd 99.85 2.86 1.14 0.20 0.74 — — —B 59.6 24.8 6.5 7.8 0.03 0.04 nd 98.77 2.69 1.32 0.31 0.68 — — —B 61.8 23.8 5.4 8.7 0.06 nd 0.14 99.9 2.75 1.25 0.26 0.75 — — —B 60.7 24.5 5.9 8.3 nd nd nd 99.4 2.71 1.29 0.28 0.72 — — —B 59.7 25.0 6.7 8.0 0.01 0.02 0.02 99.45 2.68 1.32 0.32 0.70 — — —B 61.2 23.5 5.4 9.2 nd nd nd 99.3 2.74 1.24 0.26 0.80 — — —B 62.3 23.1 4.8 9.1 0.02 0.02 0.01 99.35 2.78 1.21 0.23 0.79 — — —B 61.4 23.9 5.7 8.6 0.02 nd 0.01 99.63 2.74 1.26 0.27 0.74 — — —B 60.5 24.1 5.5 8.9 0.01 nd 0.01 99.02 2.72 1.28 0.27 0.78 — — —B 59.8 24.8 6.2 8.1 0.02 0.01 0.01 98.94 2.69 1.31 0.30 0.71 — — —B 59.8 24.7 6.0 0.03 nd nd nd 90.53 2.82 1.37 0.30 — — — —B 60.5 24.2 5.8 8.7 0.01 nd nd 99.21 2.71 1.28 0.28 0.76 — — —B 60.2 24.4 5.6 9.1 0.02 0.02 0.01 99.35 2.70 1.29 0.27 0.79 — — —H 64 18.4 nd 0.1 16.2 0.65 0.1 99.48 2.99 1.01 — 0.01 0.97 0.01 —H 63.1 18.1 0.19 0.11 15.6 1.2 0.43 98.73 2.98 1.01 0.01 0.01 0.94 0.02 0.01A 68.1 19.1 0.12 11.9 0.09 nd 0.06 99.37 3.00 0.99 0.01 1.02 0.01 — —H 61.3 19 nd 0.26 14.7 4.1 nd 99.36 2.93 1.07 — 0.02 0.90 0.08 —C 64.3 18.2 0.03 0.19 16.3 0.16 0.06 99.24 3.00 1.00 — 0.02 0.97 — —H 64.3 18.4 0.06 0.24 16.4 0.64 nd 100 2.99 1.01 — 0.02 0.97 0.01 —H 65.0 19.9 0.02 0.03 16.8 0.84 0.02 102.6 2.95 1.06 — — 0.97 0.02 —C 65.0 18.7 nd 0.02 16.9 0.3 0.02 100.9 2.99 1.01 — — 0.99 0.01 —C 63.9 17.5 0.01 0.07 16.4 0.07 nd 97.95 3.02 0.98 — 0.01 0.99 — —C 64.8 20.0 0.03 0.02 16.7 0.02 0.02 101.6 2.95 1.07 — — 0.97 — —C 64.9 18.3 nd 0.09 16.6 0.02 0.05 99.96 3.00 1.00 — 0.01 0.98 — —D 64.5 17.6 0.08 0.17 16.3 0.08 0.03 98.76 3.02 0.97 — 0.02 0.97 — —E 68.6 20.8 1.5 9 0.15 nd nd 100.1 2.98 1.06 0.07 0.76 0.01 — —F 65 18.3 nd 0.23 15 0.27 0.12 98.92 3.02 1.00 — 0.02 0.98 0.01 —F 64.2 17.7 0.01 0.06 16.7 0.06 0.04 98.77 3.01 0.98 — 0.01 1.00 — —G 65.1 18.8 nd nd 16.8 nd nd 100.7 2.99 1.02 — — 0.99 — —

A-Albitised plagioclase, B-magmatic plagioclase, C-K-feldpsar replacing plagioclase, D-magmatic K-feldspar, E-perthitic albite, F-Fracture-filling K-feldspar,G-Secondary K-feldspar in chloritized biotite, H- Ba-rich, secondary K-feldsparnd¼ not detected.

s. morad ET AL.

7.4% (An¼ 20 to 36 mole %) with an average chemical

formula of Na0.7Ca0.3Al1.3Si2.7O8 (Table 1). The average

composition of plagioclase is An25Ab74Or1 and that of

microcline is Or92Ab8An0. Modal compositional analyses of

the granitic rocks reveal that the replacement of magmatic

plagioclase by secondary albite and K-feldspar has resulted

in significant changes to the type of granite, from

granodiorite and monzogranite towards syenogranite and

alkali feldspar granite (Figure 6A and 6B).


5. DISCUSSION

5.1. Overall conditions of mineral paragenesis

The close association of secondary feldspar and Ca-Al

silicates (prehnite, pumpellyite, epidote and titanite)

suggests that the mineral assemblage was formed at

temperatures of ca. 300–4008C (e.g. Pascal 1979; Tulloch

1979; Liou et al. 1983; Frey et al. 1991; Inoue and Utada

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Figure 5. Plagioclase crystals replaced by albite (dark grey) and K-feldspar (light grey). (A) Optical photomicrograph (crossed-polarized light) showing aplagioclase crystal which is replaced by K-feldspar and albite along the twin planes and riddled with tiny particles of Fe-oxide pigment. (B) BSE image showingalbitized plagioclase replaced by K-feldspar along one set of twin planes whereas the other set is replaced by albite; also note that replacement by K-feldspar hasalso occurred along a thin fracture. (C) BSE image showing albitized plagioclase in which replacement by K-feldspar proceeded inward. (D) BSE imageshowing that the replacement of plagioclase by albite and K-feldspar results in a chessboard-like texture. (E) Higher magnification of Figure 5D showing that thegrey tone in K-feldspar varies slightly owing to small variations in Ba contents. (F) BSE image showing albitized plagioclase (dark and slightly lighter grey)rimmed by K-feldspar containing Ba-rich zones; the albitized plagioclase has also been replaced by quartz (Q & solid arrows) and partly dissolved with pores

filled by Fe-rich chlorite (dashed arrow).


1991; Eliasson 1993; Recio et al. 1997; Freiberger et al.

2001; Boulvais et al. 2007; Drake et al. 2008). Variations

in the type and distribution pattern of plagioclase alteration

in the Proterozoic granitic rocks can be attributed to

variations in the chemistry of hydrothermal fluids. There

are several lines of evidence suggesting that the types and

extents of alteration have probably occurred through mass

redistribution by fluid-rock interaction (cf. Eggleton and

Banfield 1985; Janeczek 1994; Fiebig and Hoefs 2002;

Boulvais et al. 2007). Drake et al. (2008) concluded that

fluid circulation was associated with the intrusion of the

nearby Mesoproterozoic Gotemar and Uthammar granites.

The presence of tiny particles of hematite pigment in the

red-stained zones (Drake et al. 2008) has been taken to


indicate large-scale fluid-rock interaction (Putnis 2002;

Putnis et al. 2007).

Thus, the alteration processes and products are strongly

controlled by the mineralogical composition of the granite.

For instance, saussuritization of plagioclase is most

extensive in samples that are rich in chloritized biotite,

which, it has been suggested, provided the necessary Kþ and

Fe2þ (Veblen and Ferry 1983; Eggleton and Banfield 1985;

Meunier et al. 1988; Janeczek 1994). Paragenetically,

albitization of plagioclase and formation of fracture-filling

albite pre-dates the replacement of plagioclase by K-

feldspar and formation of fracture-filling K-feldspar. The

formation of K-feldspar within chloritized biotite was

presumably controlled by a local geochemical environment

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Figure 6. (A) Original modal magmatic mineralogical composition of the studied rocks obtained by counting 400 points in each thin section and plotted onquartz (Q), plagioclase (P) and alkali feldspar (A). (B) Modal compositions of the same samples in Figure 6A plotted considering the alteration (mainlysaussuritization and albitization) of magmatic plagioclase. Note that these alterations have resulted in considerable changes in the types of granitic rocks from

granodiorite and monzogranite towards syenogranite and alkali feldspar granite (IUGS 1973 classification).

s. morad ET AL.

provided by the biotite, in particular a high aþK/aþH ratio

(Boles and Johnson 1982; Freiberger et al. 2001).

Hydrothermal minerals formed later than K-feldspar include

fluorite, prehniteþ pumpellyite, chlorite and calcite. This

overall paragenetic sequence indicates that the hydrothermal

fluids underwent successive decrease in aþK/aþH ratios and

increase in a2þCa /aþH ratios (Parry 1998).

5.2. Saussuritization of plagioclase

Initial types of hydrothermal alteration that occurred soon

after crystallization of the Proterozoic granitic rocks and the

associated release of magmatic fluids include saussuritiza-

tion of plagioclase at a temperature of ca. 4008C (Ferry

1979; Eggleton and Banfield 1985; Turpault et al. 1992; Lee

and Parsons 1997; Boulvais et al. 2007). The link between

the saussuritization of plagioclase and chloritization of

biotite (Eliasson 1993), which is in agreement with

petrographic observations of the abundances of these two

alteration features, can be tentatively written as follows,

assuming conservation of Al among the solid phases (Ferry

1979):

1:0 plagioclase þ 0:3 biotite þ 2:2 Hþ

¼ 0:6 albite þ 0:2 epidote þ 0:2 sericite

þ 0:7 H4SiO4 þ 0:3Fe2þ þ 0:7 Mg2þ þ 0:2 Kþ: (1)

Thus, the reaction acts as a source of Si, Fe, Mg and K

ions, the amounts of which are controlled by the proportions

of albite, epidote and sericite formed at the expense of

plagioclase. Reaction 1 was probably enhanced by HF

formed upon leaching of F- from the biotite and titanite

(Cerney and Povandra 1972; Coombs et al. 1977). Excess


silica may account for the formation of small amounts of

quartz in saussuritized plagioclase (Figure 5F), whereas

excess Fe could have precipitated as fracture-filling epidote

(Figure 3A) and red-staining induced by the formation of

tiny particles of hematite pigment (Drake et al. 2008). It

should be pointed out that the latter authors have shown that

Al is depleted in the red-stained zone relative to the greyish

granitic rocks further away. Thus, the assumption of Al

conservation among the solid phases is fraught with

uncertainties. Excess Al could have precipitated as

fracture-filling silicates.

The mineralogical modifications encountered agree well

with the precipitation temperature range of ca. 250–4008Cdeduced by Eliasson (1993) and Drake et al. (2008). The Fe-

rich composition of the secondary epidote suggests

formation temperatures of �3508C, under elevated fO2

and low PCO2 (Liou et al. 1983). The presence of pseudo-

hexagonal hematite within chloritized biotite supports

elevated oxygen fugacity in the hydrothermal fluids.

Although the phengitic composition of sericite indicates

elevated formation temperatures, it is difficult to determine

the precise formation temperature.

5.3. Albitization of plagioclase

Albitization of plagioclase and, to very limited extent, K-

feldspar is most substantial in granitic rocks that contain

little or no biotite. This observation further supports the

importance of local geochemical environment in the style of

plagioclase alteration. The small amounts or lack of

chloritized biotite did not favour the formation of sericite

and epidote, and hence resulted merely in albitization

rather than saussuritization of plagioclase. Albitization of

plagioclase can be written tentatively as follows, assuming

Geol. J. (2009)

DOI: 10.1002/gj


the conservation of Al among the solid phases (Ferry

1979):

1:0 plagioclase þ 1:2 SiO2 þ 0:6 Naþ

¼ 1:3 albite þ 0:3 Ca2þ: (2)

The reaction is thus favoured by increase in aþNa/a2þCa ,

which could have been achieved through: (i) the precipi-

tation of calcite, which has replaced quartz, K-feldspar,

plagioclase and/or (ii) flux of NaCl-rich brines, which were

derived either from external sources (Barton and Johnson

1996) or exsolved from the granitic magma (Hay et al. 1988;

Aslund et al. 1995). The increase in the volume of feldspar

caused by the albitization reaction, which counteracts the

presence of vacuoles in albitized plagioclase crystals, may

account for the precipitation of fracture-filling albite.

Albitization of plagioclase in sandstones is suggested to

occur via a dissolution-reprecipitation process and not via

solid-state ionic diffusion; evidence for dissolution-repre-

cipitation is provided by the formation of numerous, tiny,

euhedral albite crystals at the expense of single plagioclase

grains (Boles 1982; Morad 1986; Saigal et al. 1988; Morad

et al. 1990). On the basis of SEM studies, Morad (1986)

concluded that diagenetic albitization, which occurs at

relatively low temperatures (commonly> 908C) does not

involve the complete breakdown of feldspar into dissolved

ions in pore fluids, a conclusion later confirmed by Hirt et al.

(1993). Hirt et al. (1993) and Lee and Parsons (1997)

concluded that plagioclase albitization occurs along sub-

microscopic cleavages, which result from changes in the

stress regime on the grain scale owing to dissolution of an

adjacent phase. Albitized plagioclase crystals in the

Proterozoic granitic rocks are petrographically and chemi-

cally similar to plagioclases produced by diagenetic

albitization (i.e. vacuolated, untwinned and with a pure

NaAlSi3O8 composition), and are thus likely to have formed

by a similar process (Lee and Parsons 1997).

Figure 7. Polythermal reaction path obtained by using the Geochemist’sWorkbench1 showing that decrease in temperature in granitic hydrothermalsystem results in increase in K-feldspar and concomitant decrease inalbite, which is in agreement with the paragenetic sequence in the Proterozoicgranitic rocks. The reaction is based on 3 cm3 muscovite, 20 cm3 quartz,

20 cm3 albite, 10 cm3 microcline, and assuming fluid with 1 molal NaCl.

5.4. Replacement of plagioclase by K-feldspar

Subsequent shift in the style of alteration of plagioclase from

albitization/ saussuritization to replacement by K-feldspar

was presumably caused by an increase in aþK/aþH ratio as

temperature decreased (cf. Morad et al. 1989). A possible

internal source of Kþ is chloritization of biotite. As with

albitization, it has also been suggested that the replacement

of plagioclase by K-feldspar during sandstone diagenesis

occurs by a dissolution-reprecipitation process (Morad et al.

1989). The formation of K-feldspar subsequent to albite

agrees well with the polythermal path modelling of granite-

water (I molal NaCl) interaction performed in this study

using the Geochemist’s Workbench1, which has revealed


that initially albite rather K-feldspar is the dominant phase at

temperatures greater than 3008C, whereas at a temperature

of ca. 2508C K-feldspar is stabilized at the expense of albite

(Figure 7). The restriction of this alteration phenomenon to

red-stained zones close to thin fractures may underline the

importance of fluid flux for the reaction. The closure

temperatures for argon diffusion in fracture-filling K-

feldspar in granitic rocks in the study area were ca. 125–

3508C (Drake et al. 2009). The latter authors concluded, on

the basis of petrography and 40Ar/39Ar dating, that red

staining and formation of K-feldspar in granitic rocks from

the study area resulted from intrusion of the nearby granites

at Gotemar and Uthammar (1452 Ma and 1441 Ma,

respectively; Ahall 2001), the associated post-magmatic

fluid circulation, and the Danopolonian orogeny to the south

(cf. Cecys and Benn 2007; Bogdanova et al. 2008).

The relatively high porosity encountered in the red stained

zones in comparison to the unstained zones (Drake et al.

2008) may suggest mineral leaching by circulating fluids.

Nevertheless, Drake et al. (2008) concluded that the red-

stained zones display moderate K depletion and Ca

enrichment. Partial albitization of the plagioclase probably

enhanced the later replacement of plagioclase remnants by

K-feldspar, because albitization results in the formation of

micropores that enhance ionic diffusion and hence reaction

rates (Caussiux et al. 2006). Moreover, relaxation of elastic

strain energy caused by albitization may also have been a

driving force for later replacement by K-feldspar (Hirt et al.

1993; Lee and Parsons 1997). Other explanations for the

formation of K-feldspar at the expense of plagioclase

include fluid mixing and release of CO2 from the

hydrothermal system (Parry 1998).

The origin of the zoned, Ba-rich K-feldspar crystals that

rim albitized plagioclase crystals is not well understood. It is

Geol. J. (2009)

DOI: 10.1002/gj

s. morad ET AL.

thought that Ba-rich K-feldspar can form under various

geological conditions, including diagenesis and low-grade

metamorphism of sedimentary successions (Milliken 1992;

Essene et al. 2005). Moro et al. (2001) inferred that the

formation of Ba-rich K-feldspar in barite deposits occurs at

3508C and 1-2 kbar, which is in general agreement with the

paragenetic sequence of hydrothermal alterations in the

Proterozoic granites. Barium is an important component in

hydrothermal systems, and Ba-feldspar (celsian) can thus

form solid solution with orthoclase in secondary K-feldspar

during alteration of the Proterozoic granitic rocks.

Parameters controlling small-scale variations in Ba content

of the secondary K-feldspar are poorly constrained.

5.5. Implications for diagenesis and provenance studies

on sandstones

Similarities in petrographic and chemical characteristics

between hydrothermally altered plagioclase in the Proter-

ozoic granitic rocks and diagenetically altered feldspars

(Boles 1982; Walker 1984; Morad 1986; Saigal et al. 1988;

Morad et al. 1989, 1990) requires that attention should be

paid when determining the origin of altered feldspars in

sandstones. The distinction between diagenetic and detrital

origins of feldspar alteration has important implications for

mass flux in sedimentary basins (Boles 1998; Aagaard et al.

1989; Wilson et al. 2000) and for provenance studies

(Pittman 1970; Trevena and Nash 1981; Lee et al. 2003).

Diagenetically albitized plagioclase shows some features

that can be used to distinguish it from hydrothermally altered

plagioclase in granite, including: (i) the presence of tiny,

euhedral albite crystals that are arranged parallel to twin and

cleavage planes of plagioclase, (ii) systematic increase in the

extent of albitization with increase in burial depth and

temperature (Morad et al. 1990), as well as in sandstones

that have not been cemented by early-diagenetic carbonates

(Saigal et al. 1988), (iii) more frequent and greater volume

of intragranular pores (Milliken 1989; Morad et al. 1990)

and (iv) absence of sericite, epidote and tiny particles of

hematite pigment.

The distinction between detrital feldspars replaced by

diagenetic K-feldspar and magmatic plagioclase replaced by

hydrothermal K-feldspar is less straightforward. However,

the replacement of feldspar by diagenetic K-feldspar

commonly influences both detrital plagioclase and K-

feldspar, whereas replacement in granitic rocks influences

only the magmatic plagioclase. Moreover, as in the case of

albitization, diagenetic K-feldspar occurs as numerous,

small, euhedral crystals that are aligned parallel to the

cleavage and twin planes (Morad et al. 1989). These textural

features have not been detected by scanning electron

microscopy in any of the studied granitic rocks containing

plagioclase replaced by secondary K-feldspar.


6. CONCLUSIONS

Petrographic and mineral chemical investigations of granitic

basement rocks from SE Sweden reveal a number of

important conclusions regarding the types, extent and

distribution patterns of plagioclase alterations, including:

(i) S
aussuritization and albitization of plagioclase and
replacement of plagioclase by K-feldspar were

strongly controlled by the mineralogical composition

of the rock.

(ii) T
he extent of saussuritization of plagioclase crystals
(i.e. replacement by albite, epidote and sericite) is

proportional to the amounts of chloritized biotite,

which provided the Kþ and Fe2þ needed for the

formation of sericite and epidote, respectively.

(iii) G
ranitic rocks containing little or no chloritized biotite
show only albitization of plagioclase and simultaneous

precipitation of fracture-filling albite.

(iv) D
ecrease in temperature to ca. 2508C and increase
in aþK/aþH ratio are inferred to have led to a shift in the

style of plagioclase alteration from albitization and

saussuritization to replacement by K-feldspar with

variable barium content. Simultaneously, K-feldspar

was precipitated in fractures and within chloritized

biotite.

(v) T
he replacement of plagioclase by K-feldspar is most
common in red-stained granitic rocks which were

originally grey coloured. Staining is caused by pre-

cipitation of tiny particles of Fe-oxide pigment within

plagioclase replaced by albite and K-feldspar, and is

hence indicative of circulation of oxic, hydrothermal

fluids.

(vi) T
he textural and chemical characteristics of altered
plagioclase in Proterozoic granitic rocks should be

taken into account in sandstone studies in order to

avoid misinterpretation of these features as being of

diagenetic origin. Such misinterpretations can have

serious implications on mass flux in sedimentary

basins if linked to the burial-diagenetic evolution of

sandstone successions. Recognition of hydrothermally

altered plagioclase is also useful in studies on sand-

stone provenance.

(vii) T
his study has revealed that chemical and textural
characterization of various types of plagioclase altera-

tion allows constraining the granitic hydrothermal

alteration systems.

ACKNOWLEDGEMENTS

We are grateful for the detailed comments and suggestions

provided by two anonymous referees, which helped us

improving the paper significantly. This study was funded

Geol. J. (2009)

DOI: 10.1002/gj


by the Swedish Nuclear Fuel and Waste Management Co.

(SKB).

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Scientific editing by Giles Droop

Geol. J. (2009)

DOI: 10.1002/gj

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Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden