Clay Minerals (1984) 19~ 579-590

H Y D R O T H E R M A L C L A Y M I N E R A L F O R M A T I O N IN A B I O T I T E - G R A N I T E I N N O R T H E R N

S W I T Z E R L A N D

TJ. P E T E R S AND B. H O F M A N N

Mineralogisches-Petrographisches Institut, University of Berne, Balzerstrasse I, Berne, Switzerland

(Received 3 January 1984)

A B S T R A C T: Clay minerals of several hydrothermally altered zones in a 1200-m biotite- granite core from a drillhole in northern Switzerland were studied microscopically, by XRD and by electron microprobe. The minerals principally affected by the hydrothermal alteration were plagioclase (Ans-An20) and, to a lesser extent, biotite. Illite, regularly interstratified illite- smectite and dioctahedral chlorite-smectite, dioctahedral chlorite, trioctahedral chlorite and kaolinite were detected in the alteration products. Commonly, two or more clay minerals occurred together in pseudomorphs after plagioclase. The mineral chemistry of the clay minerals showed a predominance of the substitution KA1 for Si and, to a lesser extent, MgSi for AI. Fluid-inclusion data and the absence of pure smectite and epidote indicated temperatures of -200~ for the fluid that caused this alteration.

Clay minerals are common 'byproducts' in many hydrothermal ore deposits, where they have been used to characterize the fluids (Meyer & Hemley, 1967). Often a species zonation has been observed (Lovering, 1941; Sudo, 1959). Clay mineral formation during superficial weathering has been studied intensively, but due to the scarcity of deep-sited exposures very little attention has been paid to the subsurface alteration of intrusive rocks not associated with major ore deposits. With the growing interest in plutonic rocks for radioactive waste disposal, this attitude has changed rapidly. In several of the driUholes in Canada (Kamineni & Dugal, 1982), the United Kingdom (Storey & Lintern, 1981) and France (Meunier, 1982), argillized granitic rocks have been encountered. In the driUhole Kristal I near Boettstein, Aargau, Switzerland, we had the unique opportunity to study alterations in a fully cored (o 8.5 cm) 1500-m deep drillhole in an almost homogeneous late Variscan granite, which is overlain by about 300 m of Triassic and younger sediments. The rrearest outcrop of crystalline rocks is less than 10 km away in the Black Forest.

P E T R O G R A P H I C D E S C R I P T I O N

The unaltered rock is a porphyritic biotite-granite with large (up to 10 cm) phenocrysts of K-feldspar. The visual mode is quartz 27%, plagioclase 26%, K-feldspar 38%, and biotite 8%. The K-feldspar is perthitic and the plagioclase zoned with a core of oligoclase (An12_20) and an albite rim. Pseudomorphs after cordierite are common. In the least altered rocks the pseudomorphs are isotropic and have the composition of allophane. They are gradually replaced by sericite and chlorite. During a late magmatic stage muscovite formed at the expense of K-feldspar and biotite.

(~ 1984 The Mineralogical Society

580 Tj. Peters and B. Hofmann

Two different types of hydrothermal alteration can be distinguished:

An early hydrothermal alteration which resulted in the formation of flakes of phengitic white mica 1-5 pm in diameter (analysis Boe 491, Table 1) and blobs of calcite in the cores of the plagioclases. Most of the chloritization of biotite must also be attributed to this stage. This sericitization affected the whole granite body and is independent of fissures and veins.

A late hydrothermal alteration which affected the granite much more intensely than the early one but which is restricted to the neighbourhood of fissures (now mineral veins) and tectonically disintegrated parts of the granite (kakirites). The intensity and mode of alteration appears to have been controlled by time as well as the geometry and vicinity of fluid channels. At a depth of about 950 m below surface there is a remarkable change in the way this late hydrothermal alteration has affected the granite. While above this depth the altered zones are intensely coloured by red-stained K-feldspar, green sericite and violet-, red- and green-stained clay minerals, below 950 m the altered rocks are usually pale green.

Clay mineral associations also vary with depth (Fig. 1). The Fe203/FeO ratio varies widely in the upper part but is uniformly low below 950 m, where iron is concentrated in chlorite of the argillized plagioclases. Ore minerals only occur in veins above 950 m.

In plagioclase (Fig. 2) the following transformations can be observed on passing from the weakly altered towards the intensely altered zones. The cores, already containing some sericite from the early alteration stage described above, are transformed first to clay minerals with low (<0.01) birefringence. Consequently, this alteration progrades more and more from the Ca-richer to the Ca-poorer parts. Albite rims are often preserved. The clay

d e p t h Im] 300-

~oo.

500.

600

7oo

aoo.

900-

lOOO-

HOO �9

1200"

i l l i t e i l l i t e / $ m e c t i t e

m

7.hlor i t e / s m e c t . c h l o r i t e

z,~ 7 ?~

m

al

dl

n

k o o l i n i t e

FIG. 1. Distribution of clay minerals formed in hydrothermally altered plagioclase in granite. Ratios in percentages; di = dioctahedral chlorite.

Hydrothermal clay formation in granite 581

FIt. 2. Sample BOE 838. Partly altered plagioclase, showing two generations of day minerals: sericitic white mica in the core (first alteration stage), surrounded by iUite-smeetite interstrati-

fications. Fresh albite rim. Field of view 1.1 ram.

minerals occurring in this stage are, in order of frequency: iUite-smectite interstratification, dioctahedral chlorite-montmorillonite interstratification (more or less regular), trioctahed- ral and dioctahedral chlorite. Even in fully argillized plagioclases the cores are outlined by still-preserved sericitic mica flakes of high (>0.01) birefringence. During this stage, transformation of biotite with ore exsolution as well as pigmentation (iron oxides) and minor alteration (illite and calcite) of K-feldspar are visible. Kaolinite is restricted to the upper part (above 800 m; Fig. 1). Below 800 m, dioctahedral chlorite occurs rather than kaolinite. Kaolinitized plagioclase occurs only in small zones around veins and only in a small proportion of the samples; pure kaolinite was not found. Some admixtures of illite-smectite or dioctahedral chlorite-smectite always occur. The most intense alteration is found directly adjacent to the veins (Fig. 3). Plagioclase is either transformed into a mass of sericitic mica flakes 1-10 #m in size or the plagioclase is albitized, the albite also containing some sericite. Biotite is also sericitized, while K-feldspar shows little evidence of alteration apart from a red pigmentation due to iron oxides. Titanium liberated from biotite has led to the neoformation of anatase.

Simple fractures and tectonically disintegrated parts of the granite (kakirites) acted as channels for the fluids, open spaces being refilled by quartz, calcite, sericite and small amounts of ore minerals of the Co-Ni -Ag-Bi -U association. Often the vein fillings are brecciated. There are also younger veins cutting older ones, producing a second alteration in already altered rock, and transforming, for example, trioctahedral into dioctahedral chlorite.

M E T H O D S

The material for X-ray investigation was scraped out of altered plagioclases and veins. After grinding in an agate mortar, the material was dispersed in distilled water with the aid of ultrasonic treatment. <2 gm fractions were Ca- and Mg-saturated, Mg-saturated samples showing sharper and stronger reflections. From every < 2 gm fraction, oriented

582 Tj. Peters and B. Hofmann

FIG. 3. Representation of mineral zonation around a hydrothermal vein. (A) Vein with microcrystalline quartz, calcite, sericite, rock fragments, often brecciated. Traces of ore minerals (above 950 m), the most common being safflorite, skutterudite, native Bi, As, Ag-Sb and brannerite. Ore minerals are younger than latest deformation in veins. Maximum thickness 20 cm, usually about 1 mm. (B) Albitization of plagioclase, biotite sericitized, K-feldspar stained red (above 950 m). (C) Sericitization of plagioclase and biotite, K-feldspar stained red (above 950 m). Band C is only a few cm wide. (D) Plagioclase altered to kaolinite or dioctahedral chlorite or irregularly interstratified dioctahedral chlorite-smectite. Width of this zone is 2-10 cm. (E) Plagioclase (mainly cores, albite rim often preserved) altered to illite-smectite interstratifications, illite, dioctahedral chlorite-smectite interstratifications (above 950 m) and trioctahedral chlorite + prehnite (below 950 m). Biotite is unaltered to slightly altered, K-feldspar is often stained red (above 950 m). This zone can be several metres thick and zrades into the unaltered granite (F).

air-dried (50% r.h.), ethylene glycol sa turated and heated (550~ 1 h) samples were analysed. Non-basa l reflections were obtained with a Guinier de Wolff camera, using

Fe-Kcr radiation. The crystall inity of the illites was measured using the method of Kuebler (1964). The mineral chemistry was obtained from microprobes analyses of polished thin-sections. In a few cases the < 2 /~m fraction of the crushed rock was sedimented on graphite tablets and analysed. The latter method gave identical results but could be applied only if one clay mineral species was present. The microprobe used was an A R L SEMQ model equipped with six spectrometers and EDA. Natura l silicate s tandards were used and analyses were corrected using a full Z A F correction program (program E M M A ; Gubser, 1975). Totals of the water-free analyses varied between 89 and 95%. These low totals are probably due to the abundance of < 0 . 1 - # m micropores between the clay mineral aggregates, as seen during SEM examination of the samples. I t was noted that the coarser the micaceous minerals, the higher the totals.

Hydrothermal clay formation in granite 583

X-RAY I N V E S T I G A T I O N

The phengitie serieite of the first alteration stage is well crystallized and composed of the 2M 1 polymorph. The same sericite (2M 0 with a crystallinity index averaging 4 was detected in the veins where it formed, together with quartz, up to 40% of the tilting. The illite in the kakirites is less well crystallized, with a crystallinity index averaging 11.6. The 1M polymorph predominates. In the late hydrothermal alteration, illite is restricted to the vicinity of fluid channels (veins and kakirites).

Regularly interstratified illite-smectite is abundant. In several samples this mineral is characterized by the following sequence of basal reflections after Mg-saturation and glycollation: 28.5 (001), 13.19 (002), 9.56 (003), 5.215 (005) and 3.339 A (008) (Fig. 4). From the positions of the reflections at 9.56 and 5.215 A and using the tables given by Reynolds (1980), the proportion of the illite component in the mixed-layer structure appears to be between 70 and 80%. The basal reflections indicate a nearly ordered sequence. In many other samples the positions of the basal reflections vary, implying differences in ratios of the illite and smectite components. This is supported by the variations observed in the interlayer charge (Table 2). Within the same sample interstratifications with different layer charges occur, resulting in a broadening of reflec- tions, which makes classification into specific types of interstratifications difficult. The (060) reflection at 1.49 /k points to the dioctahedral character of the mineral, which is confirmed by most microprobe analyses (BOE 318, 320, 747 in Table 2).

Many samples gave diffraction traces without a low-angle reflection (25-30 A) but showed a pronounced reflection between 11 and 12 A (air-dry). As illite was also present it was not possible to determine whether irregular or regular illite-smectite interstrati- tications of the type ISII or ISI (Reynolds, 1980) were present.

The dioctahedral chlorite shows basal reflections at 14.4 (001), 7.19 (002), 4.75 (003), 3.52 (004) and 2.840 A (005) and an (060) reflection at 1.49 A. The positions of the

* 2 0 C u K .

i . . . . , . . . . j . . . . i . . . . i . . . . i . . . . i , , ~

3 0 2 5 2 0 15 10 5

FIG. 4. Sample BOE 747. X R D traces of a regularly interstratified illite-smectite from altered plagioclase. Analysis given in Table 2. Chl = chlorite admixture.

584 Tj. Peters and B. Hofmann

reflections did not change on glycollation. After heating at 550~ the (001) spacing was preserved at 14.14 A but the other basal spacings disappeared. This dioctahedral chlorite is comparable to the dioctahedral chlorite found by Mueller (1963) and redescribed by Eggleston & Bailey (1967). Hydrothermal alteration is thought to have given rise to the original mineral in the rhyolitic tuff near Kesselberg in the Black Forest (Mueller, 1963).

A regular interstratification of smectite--dioctahedral chlorite (tosudite) shows the following reflections (Mg-saturated, air dried): 29.0 (001), 14.29 (002), 9.58 (003), 7.21 (004), 5.71 (005), 4.79 (006), 3.56 (008), 3.21 (009), 2.86 (00.10) and 2.036 ,A (00.14). On glycoUation, the (001) peak shifts to 31.6 .A. After heating the (002) peak contracts to 11.71/~. A (060) reflection at 1.498/~, confirms its dioctahedral character. Diffractometer traces of this mineral are illustrated in Fig. 5. This interstratification is very similar to one described by Eggleston & Bailey (1967) in a clay from the Geneva-Davis mine, Michigan.

Irregular interstratifications ofsmectite-dioctahedral chlorite have been identified by a main basal reflection in Mg-saturated samples at 14.1 /~ that shifts to 15.5 .~ on glycollation and collapses to 12.45 A after heating at 550~ The (060) reflection at 1.49 /~ indicates its dioctahedral character. No ordered sequence of basal reflections is detectable; these also vary in position and intensity ratios from sample to sample, indicating the existence of random interstratifications and different ratios of smectite to dioctahedral chlorite. An example of a diffraction trace is shown in Fig. 6 and on analysis is given in Table 3.

The trioctahedral chlorites usually show much sharper peaks than the dioctahedral chlorites. The (060)reflection lies around 1.53/~.

The kaolinites encountered are well ordered, showing a good resolution of (021), (111) and 131), (201) reflections. No other polymorphs were found.

*20 CuK=

~0 2$ 20 15 Io s

FIG. 5. Sample BOE 649. XRD traces of a regularly interstratified dioctahedral chlorite- smectite (tosudite) in altered plagioelase. Analysis given in Table 2.

Hydrothermal clay formation in granite

d - -

* 2 8 C u K ~

30 25 20 15 10 5

FIG. 6. Sample BOE 1011. XRD traces of irregularly interstratified dioctahedral chlorite- smectite from altered plagioclase. Analysis given in Table 3.

585

M I N E R A L C H E M I S T R Y

Some representative analyses of the first micaceous mineral that appears in the cores of the plagioclase are shown in Table 1 (Boe 325 and 672). Their interlayer occupancy varies from 1-86 down to 1.80, with potassium as the main cation. Their layer charge originates mainly from the substitution KAI for Si. They are slightly phengitic, comparable to the micas formed at the expense of K-feldspars during the late magmatic stage. The late magmatic micas (Boe 491, Table 1) do have a significantly higher paragonite content than the micaceous material formed during the hydrothermal alteration. In the MR 3-2R 3-3 R 2 diagram (Fig. 7) (Velde, 1977) these analyses plot very near the 'ideal muscovite composition' because of their almost pure dioctahedral character and high interlayer charge. This is because the MR3-2R3-3R 2 plot does not take into account the Tschermak's exchange for which the presentation of Thompson (1982) would be more suitable.

TABLE 1. Chemical compositions of magmatic muscovite, sericites of the first alteration stage in plagioclase, and illites in the veins and their immediate vicinity. Microprobe analyses based on 22 oxygens.

Sericite Illite in veins lllite bordering veins Muscovite BOE 491 BOE 325 BOE 672 BOE 400 BOE 654 BOE 395 BOE 395 BOE 667

Si 6.20 6.34 6.35 6.83 6.75 6.52 6.51 6.30 A1 Iv 1.80 1.66 1.65 1.17 1.25 1.48 1.49 1.70

A1 vl 3.72 3.77 3.61 3.44 3.47 3.68 3.59 3.62 Fe 0.14 0.12 0.22 0.20 0.22 0.08 0.15 0.21 Mg 0.18 0.16 0.18 0.39 0.29 0.24 0.31 0.26

K 1-74 1.72 1.81 1-58 1-70 1-72 1.73 1-63 Na 0.19 0.08 0.05 0.03 0.03 0.03 0.2 0.10

586 Tj. Peters and B. Hofmann

* illir tn ot~ oro~t~d veins o ~/sm interstrotitico|ions

. . . . . . . . . . ~a S r~ ~Cd, ~ hlo I ;Ires? It rtoii I,lc~r2 ino n s

2R 3 3R 2

FIG. 7. MR3-2R3-3R2 diagram (Velde, 1977) showing plots of analyses of hydrothermal alteration products of plagioelase in biotite-granite and of illite in veins. Abbreviations: b =

beidellite, s m = smectite.

TABLE 2. Chemical compositions of illite-smectite interstratifications in hydrothermaUy altered plagioclase. Microprobe analyses based on 22 oxygens.

BOE318 BOE318 BOE318 BOE318 BOE319 BOE320 BOE320 BOE320

Si 7.33 7-09 7.33 7.38 7.38 7.33 7.07 7.01 AI w 0.67 0-91 0-67 0.62 0.62 0.67 0.93 0.99

A1 vl 3.33 3.40 3.34 3.39 3-41 3.48 3.66 3.62 Fe 0.19 0.18 0.23 0.21 0.20 0.18 0.13 0.16 Mg 0.57 0.55 0.49 0.54 0.55 0.45 0.42 0.43

K 1.08 1.14 1.22 0.85 0.85 0.81 0.86 0.70 Na 0.01 0.09 0.05 0.00 0.00 0.00 0.00 0.00

BOE320 BOE320 BOE325 BOE325 BOE325 BOE491 BOE633 BOE633 BOE747

Si 6.95 6.77 7.02 7.16 7.17 6,91 6.87 7.17 6.91 A1 w 1.05 1.12 0.98 0.84 0.83 1-09 1-13 0-83 1-09

AI vI 3.69 3.86 3.58 3.47 3.49 3.76 3.59 3.36 3.58 Fe 0.15 0.16 0.20 0.19 0.20 0.00 0.00 0.29 0.11 Mg 0.44 0.39 0.38 0.41 0.44 0.26 0.40 0.52 0.24

K 0.67 0.56 1.07 1.18 0.92 1.02 0.80 1.08 1.30 Na 0.00 0.02 0.02 0.02 0.01 0.05 0.05 0.03 0.10

The sericites that form after plagioclase, bordering the veins and in the veins themselves, have an interlayer occupancy of 1.70-1.75 (Table 1). Only part of this is due to the exchange KA1 for Si. The substitution" (MgFe)Si for AlVIAI Iv must be responsible for the low charge in the tetrahedral layer and the high Mg content of the octahedral layer.

The interstratified illite-smectites have quite variable chemistries as demonstrated by the analyses in Table 2. Their interlayer occupancy, predominantly potassium, ranges from 0.56 to 1.25. Chemically these minerals fall between the vermiculites and smectites according to the nomenclature of the AIPEA Nomenclature Committee. In the MR 3-2R 3-3 R 2 presentation of Velde they plot in a band from theoretical montmorillonite

Hydrothermal clay formation in granite

TABLE 3. Chemical compositions of irregular interstratifications of dioctahedral chlorite--smectite and a regular interstratification of smectite-dioctahedral chlorite in hydrothermally altered plagioclase.

Microprobe analyses based on 22 oxygens.

Irregular dioctahedral chlorite-smectite

Regular smectite- dioctahedral chlorite

BOE 1011 BOE 1011 BOE 649

Si 6.10 6.14 5.94 AI Iv 1.90 1-86 2.06

A1 vl 4.38 4.08 3-99 Fe 0.00 0.00 0.39 Mg 0.11 0.27 0.07

K 0.70 0.80 0.25 Na 0.06 0.11 0.27

587

to illite, near the MR3-3R 2 sideline. The interlayer charge mainly results from the substitution KAI for Si. The additional MgSi for A1Al substitution lowers the layer charge of the tetrahedral layer and increases the charge of the octahedral layer. Some analyses indicate an almost pure dioctahedral occupancy. Others, especially those with low interlayer occupancies, show a tendency towards trioctahedral character. Here the substitution 3Mg for 2AI must have taken place.

The kaolinites formed at the expense of plagioclase contain no detectable (<0.01%) constituents other than AI:O 3 and SiO 2. Those formed after biotite contain 0.04-0.05 wt% TiO 2, even where no other phase was detected microscopically. Pseudomorphs after cordierite showed kaolinite compositions but were X-ray amorphous, suggesting the presence of aUophane.

Biotite transformation

The original biotite of the granite is quite Fe-rich (Table 4). During the later stages of crystallization some biotite was transformed into white mica of the same composition as the phengitic white mica formed contemporaneously after K-feldspar. In the first alteration phase (sericite/illite) of the plagioclase, biotite was partly chloritized. During the main hydrothermal alteration of plagioclase (iUite-smectite interstratifications) the biotite was also partly transformed into iUite. The first stages of this transformation were studied in a 50-#m profile within a large biotite grain. The analyses at different positions along this profile shown in Table 4 indicate an increase in Si and A1 and a decrease in Fe, Mg, Ti, Mn and Na; potassium decreases slightly.

D I S C U S S I O N

The progressive transformation of the plagioclases in the granite results in a sequence of clay minerals with decreasing potassium content: illite--interstratifications ofiUite-smectite

588 Tj. Peters and B. Hofmann

TABLE 4. Chemical compositions of successive zones of biotite transformed into illite. Microprobe analyses based on 22 oxygens.

Biotite Illite

Si 5.47 5.52 5.83 6.28 6.42 6.60 6.78 A1 Iv 2-53 2.48 2.17 1.72 1.58 1.40 1.22

AI vI 0.64 0.74 1.26 2.37 2-48 2.78 3-05 Fe 2.53 2.85 1.94 1.63 1.27 0.90 0.58 Mg 1.86 1.61 1-52 0.68 0.79 0.54 0.52 Ti 0.45 0.40 0.42 0.12 0.13 0.13 0.10 Mn 0.05 0.03 0.04 0.01 0.02 0.00 0.00

K 1.89 1.56 1.60 1.47 1.42 1.64 1.43 Na 0.05 0.11 0.10 0.03 0.02 0.01 0.02

corresponding chemical analyses in wt%

SiO 2 35.68 34.71 37.35 42.32 44.25 46.13 48.67 A1203 17.46 17.15 18.63 23.39 23.74 23.76 25.98 Fe203 20.01 21.44 14.87 13.13 10.44 7.50 4.99 MgO 8.27 6.78 6.53 3.10 3.66 2.53 2.52 TiO 2 3.96 3.97 3.57 1.06 1.12 1.25 0.96 MnO 0.33 0.24 0.27 0.06 0.15 0.00 0.00 K20 9-56 7.68 8.05 7.75 7.66 8.99 8.07 Na20 0.25 0.37 0.32 0.10 0.09 0.05 0.08

- - in te rs t ra t i f ica t ions of smect i te -d ioc tahedra l ch lo r i t e - -d i - and t r ioctahedral chlorite or kaolinite. The al terat ion does not proceed to a complete kaolinit ization of plagioclase; kaolinite is always associated with other clay minerals. Within the same rock sample, and even within the same plagioclase pseudomorph, two or more clay minerals occur together. The absence of pure smectite and epidote minerals indicates formation temperatures above 100~ (Velde, 1977) and below 350~ This is confirmed by fluid inclusion da ta (H. A. Stalder, pers. comm.) which suggest temperatures around 200~ and ore-microscopical studies which indicate 105 < T < 250~

The fluids responsible for the t ransformat ions must have been aqueous chloride solutions ( 2 - 5 % NaC1 equivalent) as indicated by fluid inclusions. The K / N a rat io of the solution was determined by the albi te /K-feldspar equilibrium, as both feldspars were stable during the alteration. The very scarce illitization of K-feldspar indicates K + / H § ratios above or near the K-feldspar --, muscovite equilibrium. F r o m the mineral associat ions and fluid-inclusion da ta a p H of about 5.5 can be inferred. Most of the elements part icipating in the clay-mineral-producing reactions remained in the granite, even the Ca and Si released in many mineral reactions precipitated in neighbouring calcite and quar tz veins. The altered

granite was only significantly impoverished in Na. The hydrothermal alteration presented here is much less complete than the alteration

that p roduced the kaolinite in the Cornish granites (Bristow, 1977) and the Vend6e Massi f granites (Meunier & Velde, 1982) or the more porous rocks in many geothermal fields (Browne & Ellis, 1970; Steiner, 1968).

Hydrothermal elay forrnation in granite 589

The following scheme summarizes the different alterations that affected the granite and

produced the argillaceous layer-silicates:

I. Intrusion of granite; early hydro thermal

alteration of granite

II. Late hydrothermal alteration near paleosurface

Mineralogical changes

(a) K-feldspar --, muscovite

biotite ~ muscovite

(b) plagioclase cores --, sericite + calcite biotite - chlorite

plagioclase - clay minerals

Stage, t ime

magmat ic stage

335 -330 M a

hydro thermal event related with faulting 280-220 Ma

This s tudy also shows the coexistence and probable co-formation of illites with high

Kuebler indexes and well-crystall ized illites with low Kuebler indexes at the same pressure and temperature . The latter are found primari ly in veins and adjacent to them, where the K concentrat ion in the solutions was higher, the former in kakirites. The same mechanism could be envisaged for porous sediments with diagenetic illite cement. In such cases extreme care must be taken in the applicat ion of illite crystall inity as an indicator of increasing low-grade metamorphism. Use should be limited to impermeable clays and shales.

A C K N O W L E D G M E N T S

The authors wish to express their thanks to the NAGRA (Nationale Genossenschaft fuer die Lagerung radioaktiver Abfaelle) for the opportunity to study the cores and permission to publish the results. Prof H. A. Stalder kindly supplied us with the firsl~ results of his fluid-inclusion studies, Drs J. C. Hunziker and R. Steiner with K/Ar dates of illites, Dr R. Oberhaensli helped with the microprobe analyses and C. Blaeuer carefully drew the figues. The anonymous reviewers are thanked for their positive criticism and helpful suggestions.

REFERENCE S

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Contr. Mineral. Petrol. 75, 235-250. KAr~nNENI D.C. & DUGAL J.J.B. (1982) A study of rock alteration in the Eye-Dashwa Lakes Pluton, Atikokan,

Northwestern Ontario, Canada. Chem. Geol. 36, 35-57. KUEBLER B. (1964) Les argiles, indicateurs de m&amorphisme. Rev. Inst. franc. Pdtrole 19, 1093-1112. LOVERING T.S. (1941) The origin of the tungsten ores of Boulder County, Colorado. Econ. Geol. 36, 229-279. MEUNIER A. (1982) Superposition de deux alt+rations hydrothermales dans la syenite monzonitique du Bac de

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altered granite. Clay Miner. 17, 285-299, M~VER C. & HEMLEY J.J. (1967) Wall Rock Alteration. Pp. 166-235 in: Geochemistry of Hydrothermal Ore

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Chlorit-Gruppe). Proc. Int. Clay Conf., Stockholm, 1, 121-130.

590 Tj. Peters and B. Hofmann

REYNOLOS R.C. (1980) Interstratified clay mineral. Pp. in: Crystal Structues of Clay Minerals and their X-ray Identification (G. W. Brindley & G. Brown, editors). Mineralogical Society, London.

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