978

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Hiinninen, E., T6rnroos, R. and Lahti, S.I. (1986) Lithos,19, 87-93.
Lavrov, M.M. (1979) The Precambrian ultrabasic rocks
and layered peridotite-gabbronorite intrusions qf
North Karelia. Nauka, 136 pp. (in Russian).
McCallum, M.E.. Loucks, R.R., Carlson, R.R., Cooley,
E.F. and Doerge, T.A. (1976) Econ. Geol., 71,
1429-50.
Nilsson, L.P. (1990) Mineral. Petrol., 42, 249-63.
Ohnenstetter, D., Watkinson, D.H. and Dahl, R. (1991)
Amer. Mineral., 76, 1694-1700.
Prichard. H.M., and Tarkian, M. (1988) Canad.
Mineral., 26, 979-90.
Rowell, W.F. and Edgar, A.D. (1986). Econ. Geol., 81,
1272-7.
Rucklidge, J. (1969) Canad. Mineral., 9, 617-28.
Snetsinger, K.G. (1972) Amer. Mineral., 57, 1029-36.
Stumpfl, E.F. (1972) Neues Jahrb. Miner., Mh., 9,
4 0 6 - 15.
Stumpfl, E.F. and Clark, A.M. (1965) Amer. Mineral.,50, 1068-74.
Stumpfl, E.F. and Tarkian, M. (1976) Econ. Geol., 71,
1451-60.

Tarkian, M. and Stumpfl, E.F. (1975) Miner. Deposita,
10, 71-85.
Tarkian, M. and Prichard, H.M. (1987) Miner. Deposita,
22, 178-84.
Thalhammer, O.A.R., Prochaska, W. and Mtihlhans,
H.W. (1990) Contrib. Mineral. Petrol., 105, 66-80.
Volborth, A., Tarkian, M., Stumpfl, E.F. and Housley,
R.M. (1986) Canad. Mineral., 24, 329-46.
Watkinson, D.H. and Ohnenstetter, D. (1992) Canad.
Mineral., 30, 121-36.
Yu, T. and Chou, H. (1979) Science Bull., 24, 310-16(in Chinese).

[Manuscript received 16 January 1995:
revised 10 April 1996]
9 Copyright the Mineralogical Society

KEYWORDS: osmium, rhodium, hollingworthite, cobaltite-gersdorffite, platinum-group minerals, layered
intrusion, Lukkulaisvaara, Russian Karelia.

M I N E R A L O G I C A L M A G A Z I N E , D E C E M B E R 1996, V O L . 60, PP 9 7 8 - 9 8 2Siegenite-bearing assemblages found at the Great
Orme Mine, Llandudno, North Wales
R. A. Ixer

C. J. Stanley

DESPITE the well known presence of siegenite andother thiospinels in the Mississippi Valley-type oresof the Southeast Missouri Lead District, U.S.A.
(llagni, 1986) and the ubiquitous presence of ironnickel-cobalt-bearing sulphides from all of theMississippi Valley-type deposits of Britain (Brown
et al., 1987; lxer and Vaughan, 1993) no thiospinels
have been recorded from British Mississippi Valleytype orefields other than a 'linnaeite' collected fromthe spoil heaps associated with the Great Orme
Mines, Llandudno (Bevins, 1994). Siegenite is,however, reported from South Wales (Bevins andllor~k, 1985) and Central Wales (Mason, 1995) but
in both cases is associated with non-carbonate hosted
mineralization.

School of Earth Sciences,
University of Birmingham,
Edgbaston, Birmingham, B I 5 2TT
Department of Mineralogy,
Natural History Museum,London, SW7 5BD
The North Wales 'linnaeite" specimen (National
Museum of Wales specimen NMW 83.41G. MI489)
has been investigated as part of a programme tocharacterize the ores of the Great Orme Mine
especially those that may have been exploited in
the Bronze Age.
The Great Orme Mine (SH772831) lies above andto the west of Llandudno, Gwynedd. Here epigenetic
copper mineralization cuts Asbian carbonates andbelongs to the copper-dolomite class of deposits
despite falling within the western extremity of theNortheast Wales Orefield, one of B r i t a i n ' s
Mississippi Valley-type orefields. Indeed, thiscopper mineralization is the first example of thecopper-dolomite association to be recorded from

SHORT COMMUNICATIONSGreat Britain. Within the mine area, copper ores
accompanied by minor lead and trace amounts ofcobalt and nickel mineralization, have been exploited
for at least four millenia, approximately 1880
8 c - 1 8 8 0 At (Dutton et al., 1994). Notwithstanding
the longevity of the mining activity descriptions of
the ores are sparse and scant. The presence ofchalcopyrite as the main copper-bearing sulphidewith malachite, azurite, cuprite, tenorite and native
copper as secondary phases, and minor amounts ofgalena have been noted (Dewey and Eastwood, 1925;
Lewis, 1990; Jenkins and Lewis, 1991). A brief,
preliminary description of the ores and theirmineralogies is given in Ixer, 1994, and Joel et al.,
1996.
Parnell (1988) described chalcopyrite, malachiteand saddle dolomite from the nearby Ty Gwyn Mine
(SH782829) together with hydrocarbons containing
cobaltian niccolite (Nio.82Coo.ls)0.95As and Eakin (inBevins, 1994) noted gersdorffite (also associatedwith hydrocarbons) from the Great Ormes Head.

979

three generations of supergene calcite can be
distinguished by cathodoluminescence. These
calcite generations are interbanded with botryoidal
limonite, accompanied by native copper, cuprite,
tenorite, spionkopite, malachite and manganeseminerals plus hydrocarbons. Lead, zinc, cobalt andnickel-bearing minerals are absent from this ore.
In situ galena ores

Galena is rare at the Great t r i n e Mines but
centimetre-diameter crystals occur along a fault/
joint plane cutting fine-grained dolostones. Small,5 - 6 0 I.tm diameter, pyrite, nickeloan pyrite, marcasite, millerite and chalcopyrite crystals are present
within galena but sulphosalts are absent. Galena isaltered initially to thin rims of digenite and djurleite
followed by spionkopite and covelline and then tobanded cerussite. Late-stage voids are infilled bymalachite and calcite. The mineralization is similar tothat found in the rest of the Northeast Wales Orefield(Ixer and Vaughan, 1993).

Methods

Representative material from the full range ofwallrocks and rock types were collected fromsurface spoil and in situ underground workings
supplemented by material from the National
Museum of Wales and Great Orme Mines Ltd.
Routine transmitted and reflected light petrography
supplemented by X-ray diffraction and cathodoluminescence were performed. Electron microprobe
analyses on siegenite were performed using a
Cameca X50 employing an accelerating voltage of
20 kV with pure elements and FeS standards.
Copper-dolomite association ores

Close to the mine coarse-grained limestones and finer
grained argillaceous bioclastic limestone, interbedded with shale/mudstone horizons and raresandstone units, arc cut by a number of sub-parallel,high angle, north-south trending faults. Adjacent tothese faults the limestones have suffered extensive
dolomitization and the resultant dolostones carry thecopper mineralization.
The ores arc dolomite-altered chalcopyritccalcite-malachite-rich and are present within very
vughy coarse-grained dolostones. In these ores saddle
dolomite crystals are overgrown by coarse-grained
untwinned chalcopyrite intergrown with minor,
cuhedral, unzoncd pyrite and marcasite to form
veins and vugh-infills up to ccntimctrcs in diameter.
The primary sulphidcs show alteration to bandedlimonite accompanied by thin rims of digenitc,
djurleite, spionkopitc, covclline and trace amountsof photosensitive silver-beating minerals. At least

Siegenite-bearing ores

Rarely, hand specimens collected from the spoil
heaps consist of a second and distinctly different ore
assemblage. This assemblage is polymetallic andassociated with fine-grained argillaceous dolostones
that show widespread dedolomitization. Here, the ore
minerals are disseminated throughout the dolostonerather than infil!ing visible void spaces. The clay
component comprises kaolinite, minor detrital quartz
and TiO2 minerals plus pyrite, chalcopyrite andstrongly zoned nickeloardcobaltian pyrite. Within the
carbonate component fine-grained saddle dolomite is
replaced by calcite and the junction is marked bydiscontinuous veinlets of chalcopyrite and pyrite but
mainly by a subhedral to euhedral, up to 5 ~tm indiameter, white, cubic, high reflectance phase withthe optical properties of gersdorffite/cobaltite.
Elsewhere this phase forms 2 - 5 ~tm diameter
grains that infill small void spaces or forms
inclusions in galena and chalcopyrite, or forms
2 - 1 0 btm pentagonal dodecahedral cores surrounded
by a 2 ~tm wide lilac-coloured zone within theearliest generation of pyrite, itself forming euhedral
crystals up to 40 Ixm in diameter. The samegeneration of pyrite encloses 2 - 5 ~tm cores oflilac-coloured pyrite.
Pale pink twinned, euhedral to subhedral siegenite
crystals, 20 to 250 Ilm in size, grow out fromdiscontinuous veinlets of pyrite, chalcopyrite andgersdorffite into small void spaces or into clear,coarse-grained calcite. Siegenite is shattered and is
enclosed and cemented by late coarse-grained pyrite
and twinned marcasite so forming aggregates up to980

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millimetres across or it forms euhedral cuboctahedral
crystals rarely up to 4 mm in diameter (specimen
NMW 83.41G.M1489), that enclose minor amountsof chalcopyrite. Microprobe analyses of these large
siegenite crystals (Table 1) show them to be
restricted in c o m p o s i t i o n from (Col.84Ni1.18
Cu0.osFeo.o2)S4 to (COl.69Nil.3oFeo.o6Cuo.o4)S4 andwith up to 1.08 wt.% Cu; 1.01 wt.% Fe; 0.17 wt.%
As and 0.09 wt.% Sb. The mean composition (n = 19)
is (Co i .77Ni 1.24Cuo.05Feo.04)S4.
The main generation of chalcopyrite forms up to
200 Ixm diameter euhedral crystals and occupies asimilar paragenetic position to that of siegenite. It is
the most abundant phase and encloses iron-nickelcobalt sulphides, galena and trace amounts of 2 - 3 0~tm tennantite and up to 60 ~tm diameter aggregatesof pale honey-coloured inclusion-free sphalerite that
infills voids between chalcopyrite crystals.
Galena forms euhedral to anhedral crystals up to
centimetres in length that enclose gersdorffite, ironcobalt-nickel sulphides plus very rare tennantite andsphalerite up to 10 I~m across. It is free of any otherinclusions. The paragenetic position of galena isdifficult to determine especially its relationship withthe main generation of chalcopyrite. Much galena isintergrown with chalcopyrite and appears to be
replaced along its cleavage by the copper-ironsulphide and by late stage pyrite and marcasite.
Unzoned pyrite and marcasite enclosing all otherphases are the final primary sulphides.

Supergene alteration of the primary phases isminor but the iron-nickel-cobalt sulphides are altered
to limonite, chalcopyrite is pseudomorphed bymalachite and limonite, and galena by cerussite.

Discussion
Siegenite is present in Wales in three quite different
mineral associations and as Table 1 shows they areeach compositionally distinctive.
Siegenite forms part of a widespread association ofmillerite, galena sphalerite, chalcopyrite and organic
matter within voids in clay-ironstone nodules fromthe South Wales Coalfield (Bevins and Hor~ik, 1985).
It has a mean composition of Coj.30Nij.6oFeo.o9
Cuo.oz)S4 and hence is nickel-rich but cobalt-poor
when compared with the siegenite analyses from theCarboniferous rocks of North Wales which have amean of (COl.77Nil.21Cuo.05Feo,o4)S4 and wheremillerite is absent from the assemblage.
In central Wales siegenite is intergrown withcobaltian pentlandite as part of a polymetallic,minor gold-bearing sulphide assemblage cutting
Lower Palaeozoic rocks (Mason, 1995). Here it hasa mean composition of (Coo.95Nil.53Fe0.56)S4 andhence is iron-rich and cobalt-poor compared witheither stoichiometric siegenite or siegenites from theCarboniferous rocks of Wales.
The copper ores within vughy dolostones at theGreat Orme and Ty Gwyn Mines (Parnell, 1988) are

TABLE 1. Analyses of Welsh siegenite (all data in wt.%)
Specimen No.

Co

Ni

Fe

Cu

As

Sb

Great Orme
83.41G.M1489
83.41G.MI489
83.41G.M1489
83.41G.M1489
83.41G.M1489

34.04
34.86
34.02
32.15
33.34

23.18
22.27
23.34
24.67
23.78

0.66
0.38
0.60
1.01
0.71

0.87
1.08
0.90
0.79
0.86

0.05
0.01
0.17
0.09
0.02

0.05
0.04
0.06
0.00
0.00

41.03 99.88
41.34 99.98
41.54 100.63
41.36 100.07
41.24 99.95(Col.s1Nil.23Fe(i.(}4Cno.o4)54(COl.84Nil. 18Cno.05Feo.02)S4(Co1.78Ni) .23Cuo.04Feo.03)S4(Co 1.69Ni) .30Feo.06Cno.o4)S4(Co i .76Ni1.26Feo.o4Cuo.o4)54

Central Wales Orefield
Erglodd Mine
16.60 31.90
Erglodd Mine
1 8 . 5 1 27.19
Coveden Mine
19.30 28.60

9.63
12.11
8.99

n.a.
n.a.
n.a.

n.a.
n.a.
n.a.

n.a.
n.a.
n.a.

41.80
41.75
41.50

(Coo.ssNi 1.66Fe0.52)84(Coo.95Ni i.42Fe0.67)84(Co t.02Ni i .52Fe(i.49)54

South Wales Coalfield
83.42G.M3
83.42G.M3
37.239G.RI

S

Total

99.93
99.27
98.39

Formula(Co 1.31Ni1,59Feo.o8Cuo.o3)S4(Co 1.32Ni1.57Feo.09Cuo.02)84(Co 1.26Ni1.65Fe0.09)54

Great Orme analyses are all from one specimen but different analyses.
Analysis S. Wales Coalfield from Bevins et al. (1985); Central Wales Orefield from Mason (1995)
n.a. not analysed

SHORT COMMUNICATIONS
the result of a simple, primary dolomite-sulphidemineralization followed by multiple generations ofsupergene enrichment. The presence of saddle
dolomite, chalcopyrite, minor pyrite and marcasite
and the paucity of galena and absence of sphalerite
separate these ores from the galena-sphalerite
dominated Mississippi Valley-type ores of theNortheast Wales Orefield (Ixer and Vaughan,1993). Rather these ores belong to the world-wide
copper-dolomite association. These deposits, foundin association with evaporites, hydrocarbons andMississippi Valley deposits, are interpreted as being
formed by fluids expelled during late-stage basin
dewatering and hence later than their associatedMississippi Valley type ores (Andrew, 1993).
However, in situ Mississippi Valley-type style ores
belonging to the main mineralization of the Northeast
Wales Orefield do occur at the Great Orme Mines,
albeit as a single galena-bearing vein cutting finedolostones. The presence of minor amounts ofchalcopyrite and Co-Ni-bearing pyrite is characteristic, although that of millerite is unusual for theseores.
The polymetallic siegenite-bearing assemblages
collected from the spoil, although chalcopyrite-richand associated with baroque dolomites show greateraffinities in their mineralogy to British Mississippi
Valley-type ores than to the copper-dolomite
association. This is exemplified by the presence ofgalena, minor sphalerite, trace amounts of tennantite
plus Co-Ni-bearing pyrite, siegenite and gersdorffite.
In addition the textural relationships of galenachalcopyrite and tennantite are very similar to thosedescribed from assemblages seen at Clevedon, Avon,
mineralization associated with the Mendip Orefield(lxer et al., 1993).
Although it has not been possible, so far, to showthat the siegenite-bearing ores were mined at theGreat Orrne Mines, they are hosted by dolomitized
Carboniferous Limestone and so belong to theNortheast Wales Orefiel& Indeed during the nineteenth century small, irregular pockets of cobalt- andnickel-bearing (up to 4.5% Co and Ni) oxidized ironores were mined from the orefield (Foster, 1882;
Warren et al., 1984). These ores may have been the
result of the oxidation of siegenite-bearing assemblages similar to those described here.

Acknowledgements
Mr Tony Hammond and the staff of the Great Orme
Mines are thanked for their hospitality, access to themine workings and donation of material. Dr Richard
Bevins of the National Museum of Wales provided
material from the Great Orme and elsewhere in theorefield. Mr John Davies is thanked for material anduseful discussions.

981

References
Andrew, C.J. (1993) Mineralization in the Irish
Midlands. In Mineralization in the British Isles.
R.A.D. Pattrick and D. Polya (eds), Chapman andHall, 208-69.
Bevins, R.E. (1994) A Mineralogy of Wales, 146 pp.
National Museum of Wales Geological Series 16,
Cardiff.
Bevins, R.E. and Hor~ik, J.M. (1985) Siegenite in clayironstone nodules from the South Wales Coalfield. J.
Russell Soc., 1, 83-5.
Brown, G.C., Ixer, R.A., Plant, J. and Webb, P.C. (1987)
Geochemistry of granites beneath the north Pennines
and their role in orefield mineralization. Trans. Inst.
Mining Metall., Sect. B. Appl. Earth Sci., 96.
B65-B77.
Dewey, H. and Eastwood, T. (1925) Copper ores of theMidlands, Wales, the Lake District and Isle of Man.
Spec. Rep. Mines Resour. Mem. Geol. Surv. G.B. vol.
23.
Dutton, A., Fasham, P.J. and Jenkins, D.A. (1994)
Prehistoric copper mining on the Great Orme,
Llandudno, Gwynedd. Proc. Prehistoric Soc., 60,
245-86.
Foster, C. Le Neve (1882) On the occurrence of cobalt
ores in Flintshire. Trans. Roy. Geol. Soc. Cornwall,
10, 107-12.
Hagni, R.D. (1986) Mineral paragenetic sequence of thelead-zinc-copper-cobalt-nickel ores of the southeast
Missouri Lead District, U.S.A. In Mineral
Parageneses (S.S. Augustithis, ed.) Theophrastus
Publications, Athens, 93-132.
Ixer, R.A. (1994) The ores of the Great Orme Mines(abstract). Research Colloquium on the Great Orme
Mines, Llandudno.
Ixer, R.A. and Vaughan, D.J. (1993) Lead-zinc-fluoritebaryte deposits of the Pennines, North Wales and theMendips. In Mineralization in the British Isles.
(R.A.D. Pattrick and D.A. Polya, eds). Chapman andHall, London, 355-418.
Ixer, R.A., Pattrick, R.A.D. and Starkey, R. (1993)
Lead-zinc-copper-arsenic-baryte mineralization at
Clevedon, near Bristol. J. Russell Soc., 5, 23-30.
Jenkins, D.A. and Lewis, C.A. (1991) Prehistoric
Mining for copper in the Great Orme, Llandudno.
In P. Budd et al. (eds) Archaeological Sciences 1989.
Oxbow Monograph 9, Oxford, 151-61.
Joel, E.C., Taylor, J.J., Ixer, R.A. and G0odway, M.
(1996) Lead isotope analysis and the Great Orme
Mine. In A. Sinclair (ed.) Archaeological Sciences
1995 Oxbow Monograph Oxford (in press).
Lewis, C.A. (1990) Underground exploration of theGreat Orme Copper Mines. In P. and S. Crew (eds)
Early Mining in the British Isles Plas Tan-y-Bwlch
Occasional Paper 1, 5-10.
Mason, J.S. (1995) Regional paragenesis for the Central

982

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Unpublished M.Phil. thesis,
University of Wales, Aberystwyth.
Parnell, J. 0988) Mineralogy of uraniferous hydrocarbons in Carboniferous-hosted mineral deposits, GreatBritain. Uranium 4, 197-218.
Warren, P., Price, D., Nutt, M.J.C. and Smith, E.G.
(1984) Geology of the country around Rhyl and

Denbigh. Mere. Br. Geol. Surv. Sheets 95 and 107,217 pp.
[Manuscript received 6 March 1996."
revised 10 April 1996](~ Copyright the Mineralogical Society

KEYWORDS:siegenite, thiospinel, galena, Great Orme mine, Wales.

M I N E R A L O G I C A L MAGAZINE, DECEMBER 1996, VOL. 60, PP 982-986

Solid solution among members of theaenigmatite group
B. B. Jensen

SEVERALminerals exist which are very closely related
to aenigmatite in terms of chemistry, structure andmode of occurrence. This contribution offers a new
explanation for the lack of solid solution between
aenigmatite and other members of the group.
Aenigmatite has the end member formula
Na2TiFe2+Si6020. It is considered to be isostmctural
with the following minerals which have the same
stoichiometry (M1402o) and together constitute theaenigmatite group:-

Mineralogical-Geological Museum,
Oslo University,
Sars Gate 1,
N-0562 Oslo 5,
Norwayonly seven co-ordinated at A. A closely related
structure is sapphirine where the stoichiometry is thesame but both A and B cations are six co-ordinated(Merlino, 1980). A number of synthetic phases havebeen prepared which have the aenigmatite structuretype: SFCA (SiFeCaA1) described by Hamilton et al.
(1989); CSVA (CaSiVA1) described by Arakcheeva
and Ivanov (1993); CaAl-ferrite (Arakcheeva, 1994);
and NaMgFe-germanate (Barbier, 1995). No detailed
structural data are available for the minerals
wilkinsonite, hOgtuvaite, welshite or dorrite.
Krinovite
Na2Cr2MgaSi6020
The main chemical differences between members
Wilkinsonite
Na2Fe3+Fe42+Si6020
ofthe group are: (a) The predominance of either Na
HCgtuvaite(Ca,Na)z(Fe2+,Fe3+,Ti,Mg,Mn,Sn)6or Ca in the A position; (b) Very different cations in(Si,Be,A1)602o
the six co-ordinated position. (All contain both small
Welshite
CaeSbFea+Mg4BezSi4020
RhOniteCaz(Ti,Fe3+,FeZ+,Mg)6AlaSi3020high valency cations and larger divalent cations in
the B sites, but in different proportions.)
Serendibite
CazA13Mg3BI 5All 5Si3020
Three of these minerals occur in magmatic
DorriteCaeFe~+MgzAl6020
environments and might be expected to form solid(The synthetic
material
baikovite
solution series; they are aenigmatite, rh6nite andCa2Ti4+Ti3+Mg3A14Si2020 can be added to this list.) wilkinsonite. However, the large number of analyses
The structure of the chain silicate aenigmatite is available for aenigmatite show only small departures
well studied and complex. The co-ordination is from the ideal formula. (Larsen, 1977; Mahood and
AVIII DVIvlVr-~z ,,6 A6 '-'2o with two structurally different A Stimac, 1990; Bonaccorsi et al., 1990).
positions, seven B positions and six X positions
There is general agreement that a miscibility gap(Cannillo et al., 1971). Structural studies indicate the occurs between aenigmatite and rhOnite (Yagi, 1953;
same co-ordinations for krinovite (Bonaccorsi et al., Grtinhagen and Seck, 1972; Johnston and Stout,1989) and rh6nite (Bonaccorsi et al., 1990) although1985) and an intergrowth of rh6nite/aenigmatite has
the latter has a more distorted A site co-ordination been reported (Yagi, 1953). Mumme (1988) was thepolyhedron, serendibite (Van Derveer et al., 1993) is first to suggest that it was the substitution of Ca for