M I N E R A L O G I C A L MAGAZINE, JUNE 1985, VOL. 49, PP. 451-456

Pumpellyite-dominated metadomain alteration
at Builth Wells, Wales evidence for a
fossil submarine hydrothermal system?
R. E. BEVINS
Department of Geology, National Museum of Wales, Cardiff, CF1 3NP, Wales

ABSTRACT. Two metamorphic alteration styles havebeen observed in basic lavas and volcaniclastic rocks of
the Buitth Volcanic Series of Ordovician age exposed at
Llanelwedd Quarry, in the Builth lnlier, Wales. The firstalteration style is characterized by the developmentof a mineral assemblage comprising albite+chlorite+
sphene+ pumpellyite _ prehnite + calcite + white mica,
although original textures are largely preserved. This
alteration pattern corresponds with that developed in
volcanic and volcaniclasticrocks elsewherein Wales, andwhich is attributed to the effects of burial. The second
alteration style is characterized by the development of
metadomains dominated by pumpellyite or, more rarely,
prehnite. In the metadomains no original textures areseen in hand specimen.This alteration pattern has, so far,not been observed elsewhere in Wales, and it is heresuggested that it might result from a submarine hydrothermal circulation system.
KEVWOROS: pumpellyite, metadomains, metamorphism, Builth Wells, Wales.
THE Builth Inlier, Powys, Wales, exposes a
sequence of sedimentary, volcanic and volcaniclastic rocks which accumulated at the margin of the
Welsh Basin during Ordovician times. The extensive quarry at Llanelwedd (National Grid Ref. SO
051 521), 1 km northeast of Builth Wells, provides
an excellent section through westward-dipping
basic lavas and associated volcaniclastic rocks of
Llanvirn age, which form a part of the Builth
Volcanic Series (Jones and Pugh, 1949). Elsewhere
within the series, but not exposed at Llanelwedd,
are andesitic to dacitic lavas and intrusions, andrhyolitic tufts. Minor intrusions of basic composition occur in the inlier, but these invade strata of
Caradoc age, and are not related to volcanism
associated with the development of the Builth
Volcanic Series.
Basic lavas of the Builth Volcanic Series are
typically feldspar-phyric, strongly vesicular andgenerally massive, although pillowed flows andhyaloclastites are locally developed. This, coupled~) Copyright the Mineralogical Society

with the nature of the associated volcaniclastic
sediments both above and below the lavas, suggests
that, for much of the time, lava eruption andemplacement occurred in a submarine environment. Jones and Pugh (op. cit.), however, described
the presence of a shore-line environment during
accumulation of the sequence, and it is likely that'volcanic islands' existed periodically during
development of the pile. The volcaniclastic sandstones which overlie the lava pile (the Newmead
Sands) appear to have been deposited in a shallow
marine environment and provide clear evidence ofcontemporaneous erosion.
Chemically the basalt-andesite-dacite-rhyolite
volcanic rocks of the Builth Volcanic Series show
calc-alkaline affinities (Furnes, 1978), which contrasts with the predominantly tholeiitic basaltrhyolite character of lavas from other Ordovician
sequences in the Welsh Basin (Kokelaar et al.,
1984; Bevins et al., 1984).
The Lower Palaeozoic rocks of the Welsh Basinand adjacent areas have suffered low-grade regional
metamorphism, generally within the prehnitepumpellyite facies (Bevins and Rowbotham, 1983),
although there is probably a complete range fromzeolite facies through to greenschist facies (Bevins
et al., in press). Metamorphic alteration has been
attributed to the effects of burial (Bevins and Rowbotham, op. cit.), although it has been suggested
that in Snowdonia and Ll~n the grade may havebeen influenced, at least in part, by the intensityof deformation (Merriman and Roberts, 1985).
Metamorphic alteration in the Builth lnlier. Twometamorphic alteration styles can be observed
within the volcanic and volcaniclastic rocks of the
Builth Volcanic Series at Llanelwedd. First, andmost widely developed, is a mineral assemblage inbasic lavas and volcaniclastics comprising albite +chlorite + sphene + pumpellyite • prehnite + calcite + white mica, sometimes with relict clinopyroxene. These secondary minerals occur in varying

452

R. E. BEVINS

amounts in rocks which, although altered,generally preserve original textures which arevisible in both hand specimen and thin section. The
second alteration style, occurring in basic lavas andassociated volcaniclastics, is characterized by thepresence of metadomains dominated by pumpellyite or, more rarely, prehnite. In these rocks, virtually no original textures are seen in hand specimen,
but are visible when observed microscopically.
The first alteration style corresponds with that
present in volcanic rocks (particularly those ofbasic composition) from other parts of Wales,
including the basic intrusions of Caradoc age in the
Builth Inlier; this alteration is attributed to theeffects of burial (Bevins and Rowbotham, 1983). In
the Builth lavas, as elsewhere, prehnite and pumpetlyite occur in a variety of forms, most commonly assmall inclusions or aggregates of crystals in albitized feldspar and, more rarely, in the groundmass.
When calcite is abundant, prehnite and pumpellyite
are usually absent and the secondary mineral
assemblage is then calcite-chlorite-albite-sphene_+
white mica, which is non-diagnostic of metamor-

phic grade. Chlorite is found in cloudy, albitized
feldspar grains, as well as lining vesicles, and also in
the groundmass, whilst sphene forms small, dusty,
granular aggregates of high relief, usually in, or
closely associated with, altered ore grains, or inchloritic areas within the groundmass. Original
mafic minerals are invariably pseudomorphed bychlorite, although relict clinopyroxene has been
identified. Both actinolite and epidote are rare toabsent and the mineral assemblage is diagnostic of
the prehnite-pumpellyite facies. In contrast, however, the Ordovician shales exposed in the inlier,and lying both above and below the lavas of the
Builth Volcanic Series, contain white micas with
illite crystallinity values characteristic of the diagenetic zone (D. Robinson and R. E. Bevins,
unpublished data), i.e. below the prehnitepumpellyite facies. Some support for a lower
metamorphic grade in the Builth Inlier is thepresence of rare laumontite-calcite veins cutting thebasic lavas in Llanelwedd Quarry; however, these
are interpreted as being related to a late-stage,
retrogressive event, although no such veins have

FIG. 1. (a) Plagioclase feldspar phenocryst and groundmass replaced by fine grained pumpellyite. Sample BW82/3.
Magnification x 55. PPL. (b) Plagioclase feldspar microphenocryst and groundmass microlites entirely pseudomorphed by pumpellyite. Sample BW82/3. Magnification • 140. PPL.

PUMPELLYITE-DOMINATED ALTERATION
been observed so far elsewhere in Wales. In viewof the widespread occurrence of prehnite andpumpeltyite in the inlier, not only in the basic lavas
but also in the associated volcaniclastic sediments
and in the basic intrusions of Caradoc age, it isconsidered that the metamorphic grade in the
Builth Inlier lies in the prehnite-pumpellyite facies,
and is related to burial of the sequence under acover of younger Ordovician and Silurian strata.
The second alteration style contrasts markedly
with that described above and has, so far, not been
observed elsewhere in Wales. It is characterized by
the development of metadomains dominated bypumpellyite or prehnite and is found in basic lavas
and associated volcaniclastic rocks exposed in theeastern part of the main level, Llanelwedd Quarry.
Nicholls (1958) described aspects of this alteration,
attributing the development of the Ca-rich areas tothe segregation of an immiscible liquid from themagma. This suggestion is not thought to betenable and an alternative model is suggested here.
In hand specimen the rock is fine grained andstructureless with no original textures and a distinctive pale blue-green or a milky, pale green hue
when pumpellyite or prehnite, respectively, aredominant. Contacts between the alteration metadomains and normal basic lava (showing the firstdescribed alteration style) are generally sharp andare readily seen in the loose blocks on the quarry
floor. The metadomains commonly define large,
crudely ovoid, alteration patches, which show
poorly defined concentric zones. Irregular calcite
veins, some containing carbon, cut through these
metadomains and in most cases appear to postdate the pumpellyitization and prehnitization episodes; in a number of cases, however, the evidence
for the timing of events is ambiguous. Whenextensively developed these veins give the rock a
brecciated appearance, particularly on weatheredsurfaces, following leaching of the calcite. No field
evidence has so far been found to demonstrate therelationship of these veins with the laumontitecalcite veins.
In thin section the pumpellyite metadomains arecharacterized by the extensive development of darkgreen to colourless, strongly pleochroic pumpellyite which replaces virtually all of the primaryminerals. Under crossed polars no primary igneous
textures can be seen, but in plane light original
crystal outlines can be observed (fig. 1). In the
pumpellyite metadomains sparse prehnite ispresent along with calcite and chlorite in vesicles
and also large, irregular patches that resemble
vesicles, but which, in detail, are more vein-like.
Such 'veins' are seen only in lavas which havesuffered pumpellyitization and these features arethought not to be primary vesicles, but to have453

originated in the alteration episode. More rarely
prehnite along with calcite and minor pumpellyite(fig. 2) infills former plagioclase phenocrysts. It is
typically coarse and forms large, radiating, sometimes bow-tie shaped, masses. In thin section, whencoarse, it is colourless but where fine grained it
assumes a pale brown colour. Areas which havesuffered prehnitization are relatively rare in comparison with the pumpellyite-dominated metadomains. However, loose blocks of coarse
volcaniclastic sandstone, most probably from theunderlying Pebbly Feldspar Ash (Jones and Pugh,
1949), collected from the main level at Llanelwedd
Quarry, are pale green in colour with milky white topale-green, irregular to ovoid prehnitized patches,
up to 2 cm in diameter. Thin section examination
reveals variable prehnite development, both in theclasts and the matrix. Many clasts are totallyreplaced by large, radiating prebnite crystals.
Original textures and clast outlines are, for the mostpart, only observed in plane polarized light and

FIG. 2. Plagioclase feldspar phenocrysts pseudomorphed
by coarse, radiating, colourless prehnite. Groundmass
between feldspar phenocrysts composed predominantlyof fine-grained pumpellyite. Specimen BW82/3. Magnification • 23. PPL.

454

R. E. BEVINSTABLE I .

RepresenCatlve

analysesof pumpellylte

6 i02

36.42

33.32

3 5 . O0

35.72

35.88

35.50

35.74

35.35

35.24

35.16

Ti02

n .d.

n.d,

n.d.

n.d.

n,d.

0.18

n.d.

n.d.

n.d.

n.d.

n.d,

A1203

15.36

16.57

16.73

18,67

18,44

16.22

16, Q6

16.21

16.51

15.79

18.30
14.58

35.86

Fe203*

16.21

16.93

15.23

14.04

13.74

15.84

15.48

13.82

15.81

16.10

MgO

2.73

2.14

2.26

1.97

2.08

1.98

2.39

2.40

2.18

2.04

2.06

CaO

21.58

21.29

20.98

21.82

22.07

21.72

21.39

21.36

21,40

21.48

22.04

Na20

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.43

n,d.

n.d.

K20

n.d.

n,d.

n.d.

n,d.

n.d.

n.d.

n,d,

n.d.

n.d.

n.d,

n.d.

Total

92.30

91.25

90.20

92.22

92.31

91.44

91.06

91.13

91.57

90.57

92.84

6.04

6.06

6.11

6.16

6.09

6.02

6.12

6.03

8ecalculatlon

Si

based

on 16 c a t i o n s

6.21

6.09

6.08

A1

-

-

~Z

6.21

6.09

6.08

8.04

6.06

6.11

6.16

6.09

6.02

6.12

A1 Y

3.09

3.37

3.43

3,72

3,67

3.29

3.27

3.29

3.33

3,24

3.63

Fe u

O. 9 1

O. 6 30.57

0,28

O. 3 3

0.69

0.73

0,71

O. 6 7

0.76

0.37

-

-

4,00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

Fe*

1.17

1.44

1.42

1.51

1.42

1.36

1.28

1.34

1.36

1.35

1.48

Mg

0.69

0.55

0.59

0.60

0.52

0.51

0.61

0.62

0.56

0.53

0.52

~X

1.86

1.99

2.01

2.01

1.94

1.87

1.89

1.96

1.92

1.88

2.00

Ca

3.94

3.93

3.91

3.95

3.99

4.01

3.95

3.94

3.92

4.01

3.97

4.01

3.97

Tl u

ZY

6.03

0.02

AI*

Mn

-

Na

-

K
~W

0,14
0,02

3,94

* Total
n.~,

-

-

iron

as

3.93

3.91

3.95

3.99

4.01

3.95

3.96

4.06

Fe203

= not detected

pumpellyite is only sparsely developed in theseprehnite metadomains.
Mineral Chemistry. Electron probe microanalysesof the important mineral phases in a sample (BW77/3) of a pumpellyite metadomain from Llanelwedd Quarry have been determined at the Department of Geology, University of Manchester, using a
Geoscan Mk. II, operating at 15 kV and 3.5 • 10 -8
amps.
Pumpellyite analyses (Table I) have been recalculated assuming a total of sixteen cations (seeCoombs et al., 1976). Recalculated in this manner Si
is generally close to the ideal six cations. Fe
contents (assumed to be Fe3+; see Liou, 1979) are
noticeably high compared with other pumpellyites
in Wales (reported in Bevins and Rowbotham,1983), with up to 16,2 wt. ~ Fe20 3 and, indeed, arethe most Fe-rich samples reported from Wales(fig. 3). Slight variations are present in Fe contents
in BW 77/3 pumpellyites and are reciprocal tovariations in AI. MgO is generally low, reaching amaximum of 2.7 wt. ~o. The pumpellyites compare
closely with those from other low-pressure zeolite
and prehnite-pumpellyite facies terrains, including
Vancouver Island (Surdam, 1969; Kuniyoshi andLiou, 1976) and the East Taiwan Ophiolite (Liou,

1979). Pumpellyites of similar composition also
occur in the zeolite and pumpellyite zones of the
Del Puerto Ophiolite submarine hydrothermal
system (Evarts and Schiffman, 1983).
Analyses of prehnite in BW 77/3 (Table II)
similarly show high Fe contents, with between 6.2
and 8.2 wt. ~o Fe203, again in excess of thatreported from elsewhere in Wales (Bevins andRowbotham, 1983). The high Fe 3+ contents of thepumpellyites and prehnites described here probably reflect high oxygen fugacity at the time of
crystallization.
Two analyses of chlorite have been made and,following the classification of Hey (1954), arediabantites. They are relatively Si-rich in comparison with other chlorites from Wales, which
generally plot in the brunsvigite, pycnochlorite, or
ripidolite fields (see Bevins and Rowbotham, 1983).
Chlorites similar in composition to those reported
here have been recorded from other low-pressurezeolite or prehnite-pumpellyite facies terrains,
including the zeolite and pumpellyite zones of the
Del Puerto Ophiolite (Evarts and Schiffman, 1983).
Origin of the Builth metadomains. As the regional
metamorphic grade in the Builth Inlier is in theprehnite-pumpellyite facies, the simplest model isPUM P E L L Y I T E - D O M I N A T E D A L T E R A T I O N
to relate generation of the metadomains to theburial episode. However, the field and petrographic
evidence described above strongly suggests that thetwo alteration types belong to separate events. The
metadomain alteration is not regional in extent,
being only observed in Wales to date at Builth; it ismore typical of a hydrothermal type of alteration
event. This implies an origin at an early stage in thealteration history of the Builth sequence.

AI

95
90

85

80

TABLE I f .

sl~
Tl~
,~2o3

70
65

455
of prehnlte

analyses

1

2

3

4

5

6

41.97

42.09

42.21

41.98

42.18

42.05

n.d.

n.d.

u.d.

n.d.

n.d.

n.d.

1 8 . O0

18.37

18.66

16.60

19.42

19.23
6.57

8.23

7.79

7.02

7.11

6.35

n.d.

n.d.

n.d.

n.d.

0.25

n.d.

CaD

25.38

35.60

35.46

23.49

26.91

25.96

NasO

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

K20

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Tot
T
ot~l

93.58

94.05

93.35

93.17

94.01

93.81

based

on 22 oxygens

6.06

6.04

6.09

6.07

6.03

6.03

~Z

6.06

6.04

6.09

6.07

6.03

6.03

A1v l

3.07

3.14

3.16

3.17

3.27

3.25

0.90

0.84

0.76

0,77

0.67

0.71

3.97

3.98

3.92

3.94

3.94

3.96

Fe30 *

S

Pumpellylte

10

15

9
S ti rr u c t t ~ r a l ~ o r ~ l a e
20
Sr

75

Representative

25

A1I v
0
35
Tt

60

40

Fe3+
Zu

50

50

F~

%Mg

-

Mg

0.05

-

Ca

3.93

3.94

3.94

3.95

3.97

3.99

FIG. 3. A portion of the Al:total Fe:Mg diagram, R~
showing compositions of pumpellyites from Builth Wells(triangles), along with the field of other pumpellyites from K
Wales (shaded).
zx

3.93

3.94

3.94

3,95

4.02

3.99

Pervasive alteration, involving the productionof monomineralic metadomains or metadomains
dominated by a limited number of phases, as
described here, provides evidence of extensive
element mobility (Smith, 1968), involving highwater/rock ratios. Elsewhere, these alteration patterns have been related to areas of increased
permeability, such as in the vicinity of faults or
fractures, or at the vesicular margins of lava flows,
as described by Smith (1968) and Jolly and Smith(1972). At Llanelwedd Quarry, no relationshipbetween the development of metadomains and flow
tops or bases is observed. In addition, althoughfaults are an attractive method for control of fluidpassage, and there are a number of important faults
in the quarry area, field evidence to support this is,again, lacking.
If the metadomain alteration pattern at Llanelwedd Quarry is the result of hydrothermal alteration, a heat source must have been available todrive a small, localized, hydrothermal circulation
system. Evidence for this heat source can be found* total

iron

as

n.d.

not

detected=

Fe203

in the Felsite Agglomerate of Jones and Pugh
(1949), which is one of the volcaniclastic horizonsinterbedded with the lava pile, and which forms thewestern face of the main level at Llanelwedd
Quarry. The origin of this unit is problematical (seeFurnes, 1978; Baker and Hughes, 1979), but ofrelevance here is the fact that it contains abundant
clasts, generally in the range 10-15 cm in diameterbut reaching up to 1 m, of intrusive and extrusive
igneous rocks which vary in composition frombasic through intermediate to acid. The intrusive
clasts suggest the presence of an igneous complex or
magma chamber at depth. Heat from this source
may have been responsible for driving a small,
localized, hydrothermal circulation system, theeffects of which were possibly concentrated along a
fault or fault system(s) with fluid being derived
either from seawater which had penetrated into theroof rocks overlying the heat source or from the still456

R. E. BEVINS

wet, unlithified sediments and volcaniclastics. Support for this model is provided by the fact that someof the clasts incorporated in the Felsite Agglomerate
are extensively altered, containing abundant
pumpellyite, in contrast to adjacent clasts which,although of similar composition, do not show suchintense alteration. This is here interpreted as indicating an early stage of pumpellyite development,
occurring before the clasts were incorporated into
the Felsite Agglomerate deposit.
Conclusions. It is suggested here that generationof the pumpellyite- and prehnite-dominated metadomains in basic lavas of Ordovician age in the
Builth Inlier was related to a submarine hydrothermal circulation system, the heat to drive thesystem being provided by a high level magma
chamber or intrusion complex. A similar mode1 hasrecently been forwarded to account for extensivealteration of the Del Puerto Ophiolite (Evarts and
Schiffman, 1983) and although the Builth volcanic
rocks do not form part of an ophiolite system, asimilar heat source to that at Del Puerto can beenvisaged. Evarts and Schiffman (op. cit.) suggested
that the Del Puerto Ophiolite was generated in an
island arc environment, with an intrusion of basic
magma within 3 km of the floor resulting inclosed-system fractionation. This caused heating ofseawater which had descended through the roof
rocks, leading to the development of a hydrothermal circulation system. Although the scale ofalteration is much larger in the case of the Del
Puerto Ophiolite than at Builth there are obvious
similarities.
More detailed work is in progress, aimed particularly at gaining an understanding of the nature of
the fluids responsible for the metadomain alteration at Builth.

Acknowledgements. The author is grateful to Dr D.

Robinson for his comments on an early draft of this paper.
Professor J. Zussman kindly made available microprobe
facilities at the University of Manchester, whilst Dr G.
Rowbotham determined the analyses.
REFERENCES
Baker, J. W, and Hughes, C. P. (1979) Proc. Geol. Assoc.
90, 65 79.
Bevins, R. E., and Rowbotham, G. (1983) Geol. J. 18,
141-67.
Kokelaar, B. P., and Dunkley, P. N. (1984) Proc.
Geol. Assoc. 95, 337-47.
- - O l i v e r , G. J. H., and Thomas, L. J. (in press) Earth
Evolution Sciences.

Coombs, D. S., Nakamura, Y., and Vuagnat, M. (1976) J.
Petrol. 17, 440-71.
Evarts, R. C., and Schiffman, P. (1983) Am. J. Sci. 283,
289 340.
Furnes, H. (1978)Unpubl. D.Phil. thesis, Univ. of Oxford.
Hey, M. H. (1954) Mineral. Ma 9. 30, 277-92.
Jolly, W. T., and Smith, R. E. (1972)J. Petrol. 13, 273 309.
Jones, O. T., and Pugh, W. J. (1949) Q. J. geol. Soc.
London, 105, 65-99.
Kokelaar, B. P., Howells, M. F., Bevins, R. E., Roach,
R. A., and Dunkley, P. N. (1984) In Marginal Basin
Geology: Volcanic and associated sedimentary and tectonic processes in modern and ancient marginal basins.

Spec. Publ. Geol. Soc. London, 16, 245 69.
Kuniyoshi, S., and Liou, J. G. (1976) Am. J. Sci. 276,
1096 119.
Liou, J. G. (1979) Am. Mineral. 64, 1-14.
Merriman, R. J., and Roberts, B. (1985) Mineral. Mag.
49, 305-19.
Nicholls,G. D. (1958) Q. J. geol. Soc. London, 114, 137 62.
Smith, R. E. (1968) J. Petrol. 9, 191-219.
Surdam, R. C. (1969) Am. Mineral. 54, 256-66.
[Manuscript received 8 November 1984]