COPPER-ARSENIC ORES AND BRONZE AGE MINING
AND METALLURGY WITH SPECIAL REFERENCE TO THE BRITISH ISLES
University
of Birmingham
Birmingham
B15
2TT
And
R.A.D.Pattrick
Department
of Geology
University
of Manchester
Oxford
Road
Manchester
M13
9PL
There
are two main groups of grey-coloured copper sulphosalts (sulphides with
arsenic, antimony or bismuth), the fahlerz or tetrahedrite group ranging in
composition between tetrahedrite Cu12Sb4S13
and tennantite Cu12As4S13 and the enargite
group with enargite and luzonite, both Cu3AsS4, and
famatinite Cu3SbS4. These are primary minerals and not
the products of secondary processes so are not characteristic of the upper
supergene enrichment or gossan zones of orebodies.
Within
the tetrahedrite group, a wide range of metals including Ag, Fe, Zn, Cd and Hg
can substitute for copper, and these elements, in particular zinc and silver,
together with arsenic and antimony may be carried over into any metal smelted
from these ores.
Tetrahedrite
group minerals are present in many base-metal deposits, hence within the
British Isles they are known from many metalliferous occurrences although they
are only present in sufficient amounts to be considered potential ores in Devon
and Cornwall and in the copper-silver ± mercury deposits of the Munster-Shannon Basin of Ireland.
By contrast the enargite minerals have been recorded only rarely in Britain and
Ireland and world-wide are best known for their association with epithermal
gold deposits.
Amongst
the thirty recognised Bronze Age mine sites in the British Isles, fahlerz
minerals are present in ore quantities only at Ross Island, Killarney, Ireland
where tennantite forms part of a number of mineralogically different,
arsenic-bearing, copper assemblages that were exploited during the Early Bronze
Age. At Western Mine thin veinlets, comprising complex intergrowths of
chalcopyrite-tennantite (with an approximate composition of (Cu10FeZn)12( As3.8Sb0.2)4S13
accompanied by trace amounts of
silver and cobalt phases, cut recrystallised limestones. Coarse-grained
chalcopyrite ± arsenopyrite-rich, tennantite-, or
bornite-rich ores from the same area, but totally removed in the nineteenth
century, may represent a more extensive development of these veinlets.
Approximately 100 metres east of the Bronze Age workings at Western Mine are
further outcrops of mineralisation at Blue Hole. Here, fine-grained
chalcopyrite-pyrite with minor galena and sphalerite pass into finely banded
sphalerite-galena accompanied by lesser amounts of pyrite and chalcopyrite.
Minor to trace amounts of arsenopyrite and tennantite are present in the Blue
Hole ores but arsenides and
sulpharsenides are only of minor importance. Supergene and oxidation zone
minerals are poorly developed and so the availability of secondary minerals
including arsenates was very limited.
Preliminary
evidence from the ongoing archaeological excavations at Ross Island indicates
that both tennantite- and arsenopyrite-bearing ores from Western Mine and perhaps
some fine-grained, banded, chalcopyrite-pyrite ± sphalerite ± galena ores from Blue Hole were exploited in the Early
Bronze Age as copper ores. If this is correct it suggests that fahlerz-bearing
ores played a role very early in Bronze Age metallurgy at least once in the
British Isles.
An understanding of what constituted a
copper orebody and copper ore in the Bronze Age and then finding and
recognising them are fundamental first steps before any interpretation of
Bronze Age metal working practices is possible. It is only after this
interpretation that any meaningful provenance studies of the resultant
metalwork can be undertaken (Ixer, 1995; 1997; 1999; Ixer and Budd, 1998).
The concepts of an orebody and of ore are
sophisticated, indeed both have legal meanings that not only involve geological
and mineralogical considerations but also those from mining, economics and
sociology. The fundamental difference between a mineral occurrence/mineral
association and an ore is not geological but depends on the idea of
benefit/profit to an individual/community or ‘society’ and where this benefit
may be material, commercial or even intangible. Hence an ore must have all of
the following properties –sufficient metal (ore grade) and tonnage, continuity
of supply and be amenable to mining and beneficiation at a profit. If one or
more of these requirements are missing the material cannot be, by definition,
an ore.
.
Metals are extracted from ores, taken
from an orebody. Many orebodies were not formed by a single geological process
but by polyphase mineralisation events, often genetically unrelated to each
other. This is one reason why few orebodies are simple with a single, uniform
ore type but are inhomogeneous that is, the primary ores are zoned. In
addition, most ore deposits show a second type of zoning, one that is
superimposed on any primary zoning. Characteristically this takes the form of a
surficial, friable, limonitic, gossanous zone, rich in metal oxides,
hydroxides, carbonates, sulphates, arsenates etc. but poor in sulphides,
overlying a supergene enrichment zone containing numerous, mineralogically
simple, secondary sulphides. Both these zones overlie the primary ore, which is
itself often zoned in terms of metal content (ore grade), mineralogy (mineral
species), grain size and mining characteristics. An orebody, therefore, can comprise any number of mineral
associations but it is only those assemblages that were exploited as ore for
use in metal production that have significance for archaeometallurgists.
A lack of an appreciation of the
difference between ore and mineralised rock specimens has meant that much
geochemistry and isotopic work has been done on inappropriate material. This
has lead to the so-called ‘Magpie School of Provenancing’ where appearance is
given more scientific value than smelting potential, such that run-of-the-mill
ores are neglected in favour of high-grade or easy to analyse but atypical
mineralogical specimens (Ixer, 1997; 1999). Alongside this, there are
misconceptions in the archaeological literature regarding individual mineral
associations/ores. This is most noticeable in the identity, chemistry, origin
and natural occurrence of the grey copper sulphosalts including the fahlerz and
these errors have compromised suggestions regarding the role of fahlerz in
Bronze Age metallurgy within the British Isles and elsewhere.
In order to counter these errors, the
mineralogy, genesis and occurrence of the fahlerz and enargite groups of
minerals are given in some detail emphasising their wide-ranging chemical
compositions. This is followed by a description of secondary and primary
arsenic-bearing mineral associations within the context of the Bronze Age in
the British Isles and finally a more detailed discussion of the fahlerz and
other arsenic-bearing ores of the Early Bronze Age mine site at Ross Island,
Ireland.
Fahlerz (or fahlore) is an 'informal' group name for
tetrahedrite-tennantite a series of copper sulphosalts (sulphides with As, Sb
or Bi); the name is derived from the German word meaning 'pale ores'. The
tetrahedrite group of minerals have a cubic structure based on that of
sphalerite (ZnS) (Wuensch, 1964) and are so called because crystals take the
form of distinctive tetrahedra as shown in figure 1. Tetrahedrite group
minerals are economically important, as copper ores and are the world's most
important primary source of silver.
Tetrahedrite group mineral chemistry
Tetrahedrite group minerals have the most
varied chemistry of any of the sulphides and have been extensively analysed by
electron microprobe (Springer, 1969; Charlat and Levy; 1974; Pattrick and Hall,
1983; Johnson et al, 1986). The tetrahedrite structure has sites available for
uni- di- and trivalent cations such that the general unit formula is:
M+10M2+2M43+S132-
and thus a typical formula of a natural
tetrahedrite might be Cu10Zn2Sb4S13.
The combination of these varied sites in the structure (figure 2) and the formation
in a wide range of deposits leads to an enormous range of compositions and a
number of 'mini' solid solution series. If all elements found in the group are
taken into account, the general formula can be described as:
(Cu,Ag)10(Zn, Fe, Cd, Cu, Mn, Hg)2(As,Sb,Bi)4(S,Se,Te)13
Antimony and arsenic are the common M3+
elements found in the mineral with Sb
end-members termed tetrahedrite [1]* (also used as the name for the whole
group and often written in the plural) and the As end-members termed tennantite
[2], with complete solid solution between them [3]. Bismuth is a rare
constituent but has been found in significant concentrations in Bi-rich
assemblages such as found in pegmatites [4]. Many tetrahedrites contain only
copper [1-4] as the M+ element, although silver can replace up to 4
of the 10 Cu atoms per unit formula [5], rarely up to 7 atoms [6], and very
exceptionally more than 7 atoms [7]. Silver-bearing varieties are termed
'argentian tetrahedrite' [4] but those with more than 5 silver atoms of the 10
M+ atoms are termed freibergite [6,7]. There is a very close positive correlation between a high Ag
content and a high Sb content due to structural limitations on Ag substitution
in As-rich varieties (Johnson and Burnham, 1985; Charnock et al, 1989a).
The commonest M2+ elements in
tetrahedrite are Zn and Fe, one or both of these are omnipresent [1-11]; there
is a correlation between high Ag and high Fe in tetrahedrites (Pattrick and
Hall, 1983). Zinc and iron are replaced to lesser or greater extent by several
other elements in relatively restricted occurrences. Mercurian (Hg-bearing)
tetrahedrites (8) have been recorded where Hg is found in both Sb- and As-rich
varieties. A general positive Hg/Sb correlation was recognised by Charlat and
Levy (1974) but locally positive Hg - As correlations occur, such as in the
Gortdrum Cu-Hg-As deposit, Ireland. Cadmium, although common in hydrothermal
fluids, occurs rarely in tetrahedrites, preferring to substitute into
sphalerite, ZnS. The first discovery of Cd-tetrahedrite was as exsolved grains
from galena in a low temperature base-metal deposit where up to 11.7 wt% Cd was
recorded in silver-rich tetrahedrites (Pattrick, 1978; 1985)[9]. Very rarely,
Pb (Bishop et al., 1977; Ixer and
Stanley, 1983) [10] and Mn [11] (Burkhart-Baumann, 1984) have been recorded in
tetrahedrites. In some tetrahedrites, copper (as Cu+) in excess of
10 atoms per unit formula of M2+ is present, the valence deficiency
is compensated for by Fe3+ instead of Fe2+ (Charnock et al., 1989b). Recently, an
indium-bearing tennantite has been reported from the Neves Corvo mine in the
Iberian Pyrite Belt, Portugal (Pinto et al., 1994).
Selenium and tellurium only replace
sulphur in small amounts but a rare variety of tetrahedrite called goldfieldite
contains significant Te4+ which replaces the M3+ elements
- the charge imbalance is restored by the omission of the M2+ elements giving an ideal formula of Cu10Te4S13
[13] (Shimizu and Stanley, 1991).
It
should be noted that, despite the wide range of metals that can be present in
tetrahedrites, lead, nickel and cobalt are very rare or unknown components and
hence cannot ‘carry through to the smelted metal’ (Craddock, 1995) if only pure
fahlerz is used.
Tetrahedrite group mineral associations
The main occurrence of tetrahedrite group
minerals is as primary (hypogene) precipitates from hydrothermal
solutions, either in base- or precious-metal sulphide deposits. They vary in
abundance from a minor component of ores, such as inclusions exsolved from galena
(the cause of much 'argentiferous' galena), to being a major ore mineral.
Tetrahedrites can be associated and co-exist with a very, very wide range of
minerals including the base-metal sulphides (chalcopyrite, galena and
sphalerite), other Cu-Ag sulphosalts, pyrargyrite-proustite (Ag3(As,Sb)S3)
and precious metal tellurides (Seal et
al.,1990). Tetrahedrite is often found as a replacement of earlier formed
sulphides or sulphosalts, due to the reaction of minerals such as sphalerite
and chalcopyrite with later As and Sb (Cu)-bearing hydrothermal fluids. The
composition of the tetrahedrite often reflects the chemistry of the associated
sulphide minerals. Where it forms as a replacement of existing phases, the
chemistry of those phases is inherited (e.g. tennantite after arsenopyrite
(FeAsS) or freibergite after miargyrite (AgSbS2); it is under such
conditions that 'exotic' compositions can form. However, it should be stressed
that the replacement of earlier minerals by tetrahedrite is part of the primary
mineralisation and not part of the later processes of supergene enrichment or
weathering, as widely reported in the archaeological literature (Craddock,
1995). Indeed, during weathering, tetrahedrite group minerals breakdown to
secondary sulphides such as chalcocite, chalcopyrite, (as found in
sulphide-rich supergene 'blankets'), covelline (CuS), acanthite (Ag2S)
and Zn-, As- and Fe-hydroxyoxides and carbonates and arsenates like olivenite
(Cu2AsO4.OH).
Tetrahedrite group minerals- occurrence
and formation
Tetrahedrite is stable over very wide
range of ore-forming conditions and is therefore found in almost all types of
hydrothermal sulphide deposits throughout the world. Tetrahedrite group
minerals are usually precipitated from acidic, saline mineralising fluids in
the temperature range 200 to 400oC and over a wide range of redox
conditions. They are common accessory minerals in volcanogenic massive sulphide
(VMS) deposits, especially in the black ore (sphalerite-galena-pyrite) of
Kuroko deposits (such as at Avoca, Ireland and Parys Mountain, Wales), but also
in their chalcopyrite-rich yellow ores and in Cyprus-type deposits.
Tetrahedrite minerals are recorded in present day active hydrothermal vent
systems found at mid-oceanic ridges, forming within the black smokers chimney
structures (Meecham, 1990). Sedimentary exhalative (SedEx) and associated
deposits contain significant tetrahedrite, as for example, at Mt Isa,
Queensland (Riley, 1974) whilst the similar Irish base-metal deposits; Tynagh
and Abbeytown, have significant tennantite (figure 3), with silver-rich pods in
Silvermines containing minerals of the freibergite-argentotennantite series
(Zakrzewski,1989). Argentian tetrahedrite and freibergite also occur in the
enigmatic Co-As assemblages in Cobalt-Gondwana type ores, which are possibly
associated with basic igneous activity. The Tsumeb breccia pipe in Namibia
contains abundant tennantite.
Tetrahedrite group minerals are
ubiquitous in polymetallic veins and skarns formed by hydrothermal activity associated
with high level intermediate or acid igneous rocks - especially in the meso-
and epithermal deposits of the western Americas related to Andean (the
Cordilleran Veins) and Laramide orogenesis. Mineralisation related to
hydrothermal convection associated with igneous intrusive (I-type) activity
often contains tetrahedrite group minerals. Copper porphyry deposits, the main
source of the world's copper, have chalcopyrite as the main Cu-ore but many
contain tetrahedrite minerals in significant concentrations as at Butte,
Montana; Bingham, Utah and Chuquicamata, Chile. Mineralisation is often
associated with evolved S-type granites and tetrahedrite group minerals are
common components of spatially related Cu-rich veins. Examples include those
formed close to the Variscan granites of Europe; in the Bohemian Massif (Czech
Republic-Slovakia) at PéÍbram
and Kutn« Hora (where mercurian tetrahedrite is
common), and the sulphide-rich veins of the Cornubian tin-copper province of
Southwest England.
In addition to the localities already
mentioned, tetrahedrite is found widely in sulphide mineralisation of the
British Isles although, apart from the Irish deposits and in Cornubia, it is
never abundant. There are many occurrences in the granite-related mineralisation
of the Caledonides of Scotland (the Lagalochan and Tomnadashan Porphyry Copper
Deposits, figure 4), the Lake District and Ireland. One group of deposits
world-wide, in which tetrahedrites are rare, are the Mississippi-Valley type
deposits because fluid temperatures are often too low to carry copper. However,
in deposits hosted by the Carboniferous limestones of the Pennines and Mendips
(the F-Ba-Pb deposits), tetrahedrite is present in microscopic amounts, most
notably in early high temperature assemblages (Ixer, 1986). Tetrahedrite group
minerals also occur in copper veins hosted by Triassic carbonates at Clevedon,
Avon (Ixer et al., 1993).
Large scale zoning of As/Sb in
tetrahedrites has been noted from several deposits and usually As-rich
varieties precede Sb-rich varieties (Johnson et al., 1986; Wu and Petersen,
1977). At Bingham, Utah, the zoning is from As to Sb plus Ag rich varieties;
whereas in Kuroko deposits, tennantite is more common in the lower, yellow ores
with (silver-bearing) tetrahedrite in the upper, black ores. Similar zoning
sequences can occur on the micro-scale, with major changes in chemistry taking
place in individual grains over a few microns. Hence fahlerz taken from
different places within the same orebody may show quite disparate compositions.
Enargite group
Enargite, Cu3AsS4
is also a grey-coloured copper sulphosalt. It has an orthorhombic
sphalerite-based structure and limited As-Sb solid solution (up to a maximum of
10 wt% Sb). Luzonite, is the low
temperature tetragonal dimorph of enargite, which in contrast, has a complete solid solution with an antimony
end-member, famatinite, Cu3SbS4. Famatinite (synonymous
with stibioluzonite) and intermediate members of the series [Cu3(As,Sb)S4]
are stable over a wider temperature range than luzonite. In the literature, enargite is very often reported where the term
enargite group would be more appropriate (Mihaly and Buseck, 1993; Clark, 1993;
Feiss, 1974).
As with tetrahedrite, enargite is a
primary precipitate from hydrothermal solutions. Although more restricted in
occurrence than tetrahedrite it is found in similar assemblages in copper-rich
ores and the two minerals can be intergrown. The classic occurrence of enargite
group minerals is in 'high sulphidation' (or 'acid sulphate') gold vein
deposits, precipitated from high temperature (300-400oC), very
acidic, hydrothermal fluids, typically associated with high level intrusions,
subvolcanic igneous activity, and collapsed calderas (Perello, 1994). These
deposits have sometimes been called 'enargite-type' as the mineral is one of
their characteristic features and are found the world over. The best examples
are in the Circum-Pacific volcanic ring; such as in the Philippines (including
Luzon), Indonesia, Japan, the Andean and Rocky Mountain vein deposits,
including those in the San Juan Mountains of Colorado.
In porphyry copper deposits, enargite
forms late in the main hydrothermal mineralising episode within a high
sulphidation zone effected by acidic fluids which produced a retrograde
assemblage showing for example the replacement of pyrite and bornite by
enargite and tennantite (Guilbert and Park, 1985). Enargite is found in many other deposit types, but is not as
widespread as tetrahedrite, hence it has rarely been reported from the British
Isles. However, enargite group minerals have been described from a low
temperature deposit at Clevedon, Bristol where typical intermediate
luzonite-famatinite occur [15] (Ixer et al., 1993); the North Pennines (Vaughan
and Ixer, 1980; Ixer and Stanley, 1998); the South Pennines (Cooper, 1995) and
the copper deposit of Alderley Edge (Ixer and Budd, 1998). In all of these
occurrences the minerals are present in microscopic amounts and cannot be
considered as ore.
Copper-arsenic secondary mineral assemblages
Unlike the restricted number of copper
arsenic sulphides, there are quite a number of copper arsenates and even more
copper-bearing lead, uranium, calcium, rare earth element, iron or zinc
arsenates. However, ‘most are so rare that they are seldom seen’ (Blackburn and
Dennen, 1994). Typically these copper-arsenic-bearing secondary minerals are
brightly coloured and found in minor to trace amounts in the upper oxidised
zones (gossans) of arsenical copper deposits, or even on mine spoil, having
formed as part of their post-mining oxidation by local groundwaters.
The rarity, bright colours and unusual
crystal habits of these arsenates are attractive to mineral collectors
(especially those who specialise in micromounts) so that the minerals have been
reported from a large number of mineral localities including many in the
British Isles. In particular, a wide range of minerals are known from mine
sites in parts of the Caldbeck Fells in the English Lake District (Cooper and
Stanley, 1990), locally within the Central Wales Orefield (Rust and Mason,
1994) or even from individual mines like Alderley Edge in Cheshire
(Braithwaite, 1994). However, the greatest concentration of copper and
copper-bearing arsenates is reported from Devon and Cornwall (Embrey and Symes,
1987), where locally, as for example at Wheal Gorland in the St Day district of
central Cornwall, the mine dumps are well known for their arsenate assemblages
(C. Stanley, pers.comm.).
Supergene oxidation of tennantite-tetrahedrite or mixed
copper-iron-sulphur-arsenic ores (for example chalcopyrite-arsenopyrite) can
lead to the formation of green copper arsenates including olivenite (Cu2AsO4(OH)),
clinoclase (Cu3(AsO4)(OH)3), cornwallite (Cu5(AsO4)2(OH)4.H2O) and cornubite(Cu5(AsO4)2(OH)4) and liroconite (Cu2Al(AsO4)(OH)4.4H2O)
alongside the more common minerals arsenolite (As2O3) and
scorodite (FeAsO4.2H2O). These copper arsenates have no
present day economic value other than as mineralogical specimens. However, some
workers, notably Charles (1967, 1994), have suggested that within the British
Isles green arsenates (originally mistaken for malachite) became important
Early Bronze Age copper ores capable of producing arsenical copper (Group 1)
metal without any by-product slag.
Characteristically, metal slags are absent from British and Irish Bronze
Age mine sites, which is difficult to explain as smelting of sulphides ± arsenides or fahlerz-bearing ores should
leave a residual slag (Craddock, 1995). It was to explain this absence that
Charles, (1967) first proposed the use of copper arsenates as ore.
More recently Pollard et al (1991) and Budd et al (1992) employing theoretical and
experimental chemistry have suggested that arsenical copper was smelted from
secondary copper minerals (malachite, azurite) mixed with copper arsenates in
an essentially non-slagging process. Although it may be possible to produce
arsenical copper in the laboratory from copper arsenates this does not make
them an ore. Minerals must be present, not just in traces but in sufficient
amounts to be exploitable and this does not appear to be true for copper
arsenates in the British Isles (Ixer and Budd, 1998).
.
The distribution of fahlerz and other arsenic-bearing mineral assemblages within the Bronze Age mine sites of the British Isles
Earlier speculations that Bronze Age mine
sites in the British Isles were fahlerz-bearing (Mount Gabriel, Jackson 1968,
1980) or contained arsenical ores (Alderley Edge, Pollard et al., 1991; Budd et al.,
1992) have been shown to be incorrect. Ixer and Budd (1998), in a review of the
mineralogy of the mineral associations and copper ores from the major
prehistoric mines of Britain and Ireland, have shown that some are free of
fahlerz (Mt. Gabriel and Cwmystwyth) and in others that fahlerz is only present
in microscopic amounts (Alderley Edge and the Great Orme). Elsewhere, where it
is present it is restricted to ores that could not be beneficiated (the process
of physically separating wanted ore minerals from each other and from unwanted
gangue) using Bronze Age technology (Parys Mountain).
Similarly, despite the statement of Craddock, (1995) that
‘olivenite is not uncommon in the upper oxidised zones of copper deposits’, for
most ore deposits in the British Isles and for all known Bronze Age mine sites,
arsenates are mineralogical curiosities (Ixer, 1995; Ixer and Budd, 1998)
without sufficient tonnage (i.e. kilograms to tens of kilograms) to be treated
as copper ore. In particular, contrary
to the suggestion of Pollard et al
(1991), the copper ores of Alderley Edge have insufficient amounts of arsenates
(or arsenides or sulpharsenides) to produce arsenical copper (Ixer and Budd,
1998). It is only in the unglaciated, tin-copper metallogenic province of
Cornubia (Devon and Cornwall) with its abundance of arsenopyrite- and
fahlerz-bearing, primary copper ores and their secondary gossans (Alderton,
1993) that arsenates may have been of importance, but even here their
distribution is restricted and uneven.
Ross Island in Munster, western Ireland
stands alone as a known Bronze Age mine site with sufficient quantities of
arsenic-bearing minerals amongst its ores/potential ores to make the production
of Group I metal possible.
O'Brien (1995 and this volume) has
described the archaeology and mining activity at Ross Island, Killarney, Co.
Kerry and, in particular, the Bronze Age mine and work camp at the Western Mine
area. This mine site is potentially the most important for our understanding of
Early Bronze Age mining and metallurgy within the British Isles, and even for
Northwest Europe. It is unique amongst the thirty or so known Bronze Age mines
in the British Isles for two reasons:
firstly, mining activity has been securely dated to 2500-2000 BC,
beginning in the Final Neolithic/Copper Age and continuing through all the time
that arsenical copper (Group 1/Group A metal) was being produced and in
circulation (O'Brien 1995). These are
the earliest dates recorded for any British or Irish mine site.
Secondly, only Ross Island had sufficient quantities of
chalcopyrite-tennantite or chalcopyrite ± arsenopyrite ± pyrite that these can
be considered to be copper ores, and so it is alone in having the potential to
produce arsenical copper or copper metal with significant amounts of
arsenic.
Preliminary descriptions of the geology
and genesis of the mineralisation at Ross Island - Muckross are given by Weaver
(1838) and Ixer and Patrick (1995) whilst the mineralisation as Bronze Age ore
has been discussed by Ixer et al. (1995)
and Ixer and Budd (1998). Tennantite and trace amounts of enargite-bearing
copper ores at the Ross Island site are part of a discontinuous zone of
mineralisation stretching south-eastwards from Ross Island (Western Mine and
Blue Hole) to Crow Island to Muckross Mine and beyond. Historical mining has
taken place along a strike length of two kilometres. At Ross Island,
stratabound Cu-Pb-Zn-As plus minor silver and cobalt mineralisation is present
as sulphides, arsenides and sulpharsenides. These are intermittently developed
in 'beds' up to 3 metre thick within a 20 metre thick, highly brecciated,
dolomitised, partially silicified, and intensely recrystallised Lower
Carboniferous limestones, close to the present day land surface (Weaver, 1838;
Ixer and Pattrick, 1995). The
mineralisation is epigenetic and displays a number of different styles and
mineral associations (ores). Amongst
the most important are: -
1. Thin, cross-cutting veinlets or
disseminations in limestones- these are found in situ in the Western Mine. The veinlets comprise complex
intergrowths of chalcopyrite (CuFeS2) - silver-poor tennantite ((Cu10FeZn)12(As3.8Sb0.2)4S13)
with up to 0.3 wt % silver, together with trace amounts of nickeliferous
cobaltite ((CoNi)AsS), molybdenite (MoS2) and pyrite (FeS2),
plus a little secondary stromeyerite (AgCuS) and native silver, all within a
calcite gangue (Ixer and Pattrick 1995).
2. Richer, coarse-grained copper ores
were mined during the nineteenth century from the Western Mine area, although
there is almost no trace of them today and nothing is found in situ. Weaver (1838) gives the only account of these ores at Ross Island
(and at Muckross) before their exhaustion and perhaps total removal in the
1820s. In particular he lists three
copper ores with metal contents of 10 to >15% Cu; these are "rich, grey
copper sulphuret" (fahlerz-rich ore); "purple sulphuret"
(bornite-rich ore) and "yellow sulphuret" (chalcopyrite-rich
ore). As these ore types are
distinguished from “copper pyrites,” a lower grade, chalcopyrite-pyrite ore,
these three rich ores are here interpreted to mean coarse-grained, non-banded,
massive sulphides. Weaver noted that
these rich copper ores were common in the pre-1820 spoil and suggested their
collection and removal - the extreme paucity of this material in the post-1820
spoil (only one or two examples of each have been found) suggests that his
advice was acted upon. Despite their rarity, examples of all three of Weaver's
ore types have been collected and examined.
They form a continuum from tennantite plus minor chalcopyrite and trace
amounts of molybdenite, pyrite, cobaltite and stromeyerite ('grey sulphuret
ore') to chalcopyrite or chalcopyrite-arsenopyrite plus minor to trace amounts
of tennantite, galena and molybdenite ('yellow sulphuret ore'). They also share
the same mineralogy, mineral textures and paragenesis as the in situ chalcopyrite-tennantite veinlets
from the Western Mine area. The third ore type 'purple sulphuret' is a
coarse-grained mixture of bornite (Cu5FeS4) - 'idaite' (a
fine-grained crystallographic intergrowth of bornite-chalcopyrite-covelline)
-chalcopyrite plus minor amounts of molybdenite, pyrite and tennantite; the
extensive replacement of the iron-copper phases in this ore type by spionkopite
(Cu39S28) may be a post-mining, weathering
phenomenon.
3. Blue Hole, which is approximately 100
metres from the Bronze Age workings at Western Mine, may also have been the
site of Bronze Age mining activity (O'Brien 1995). Here the mineralisation
comprises fine-grained, banded, massive sulphides infilling void spaces within
karstic surfaces (A. Bowden pers.comm.).
However, the detailed field relations between the mineralisation and its host rocks
have yet to be interpreted and so the exact style of mineralisation at Blue
Hole remains unclear. The sulphides vary from yellow-coloured
chalcopyrite-pyrite with minor galena and sphalerite and trace amounts of
arsenopyrite (FeAsS) and tennantite to finely banded, dark grey-black- coloured
ores comprising sphalerite (ZnS) and galena (PbS) with lesser amounts of
pyrite, chalcopyrite and trace amounts of arsenopyrite and tennantite.
4. The historical account of Weaver
(1838), plus inspection and sampling of six drill cores, as well as outcrop
show most of the mineralisation to be unaltered sulphides/sulphosalts with only
a minor development of a secondary enrichment zone or a gossan. The absence of any gossan is especially
marked in the Western Mine area and is explained by the lack of pyrite, for it
is the initial oxidation of pyrite that begins the process leading to gossan
formation.
However, a wide range of secondary
minerals have been recorded from Ross Island including malachite, azurite,
cuprite and native copper (Weaver, 1838); serpierite Ca(CuZn)4(SO4)2(OH)6.3H2O
from Blue Hole (Russell, 1927), smithsonite ZnCO3, cerussite PbCO3,
brochantite Cu4(SO4)(OH)6 and olivenite Cu2(AsO4)(OH)
(confirmed by X ray diffraction) and hydrozincite Zn5(CO3)2(OH)6
and erythrite Co3(AsO4)2.8H2O
(Ryback and Moreton, 1991). Most occur
in microscopic amounts on oxidised spoil samples and very commonly on
mineralised material recovered from the excavations. Ryback and Moreton, (1993)
and Moreton et al. (in press) (who
have recorded an even greater list of secondary minerals from Muckross Mine
including nickel, iron, calcium and mixed (Co,Ni,Mg,Ca,Fe) arsenates) state
that these arsenates formed during post-mining oxidation of mine waste and that
most are exceedingly rare; these statements apply equally well to Ross Island.
Petrographic
investigations of material taken from nineteenth century spoil, drill core and
the archaeological excavation clearly demonstrate that there are a number of
distinctive mineral associations present at Ross Island. They further show that the mineralisation at
the Western Mine area (massive chalcopyrite-tennantite ± bornite) is different from the bedded
sulphides (chalcopyrite-pyrite-sphalerite-galena) found in the Blue Hole area.
However, for these mineral associations to become ore, they have to fulfil the
criteria described earlier. That is, they need to be readily mineable, to have
sufficient tonnage and metal grade, and to be amenable to beneficiation in
order to produce a clean (free of harmful or unwanted impurities), metal-rich
concentrate that can be smelted easily.
All the mineralisation lies close to the
surface lying at a maximum depth of less than twenty metres and is hosted
within the 'blue limestone' (Colleen Baun Limestone Member) a hard, heavily
fractured but recrystallised, bioclastic limestone mineable by firesetting
(O'Brien 1995). Archaeological and
historical evidence shows that the primary mineral assemblages at Ross Island
had a sufficient tonnage to sustain both Bronze Age and Early Christian Age
metal mining and still provide a final 3000 tons of dressed copper ore in the
1820s.
Thin chalcopyrite-tennantite veinlets
found in situ in the Western Mine
area constitute the most abundant mineralised material recovered during the
excavations of the work camp where they are present as comminuted fragments
(O'Brien 1995). The ore-grade of this
mineralisation is variable but hand cobbing of thicker veinlets (> 1
centimetre in width) and of bunches of ore could produce a mixed
chalcopyrite-tennantite copper concentrate with more than 10% copper metal, but
also carrying major iron and arsenic and trace amounts of antimony, silver and
cobalt.
The copper ores described by Weaver,
which may have underlain the chalcopyrite-tennantite veinlets (Weaver 1838), or
were richer, coarse-grained versions of them (Ixer and Pattrick, 1995), have
metal grades of >10% Cu. These, therefore, should be regarded as potential
Bronze Age ores - indeed examples of all three have been recovered during the
excavation of the work camp area. They would not require beneficiation but only
crushing before being smelted. Any copper metal produced from them could
contain the following impurities iron (present in chalcopyrite, bornite, pyrite
and arsenopyrite), plus variable but significant amounts of arsenic (present in
tennantite, arsenopyrite and cobaltite) and trace amounts of molybdenum
(molybdenite), silver (tennantite and stromeyerite), cobalt and nickel
(cobaltite).
It is difficult to determine if the
fine-grained, chalcopyrite-pyrite-rich, massive sulphides from the Blue Hole
area were Bronze Age ores but the presence of two caches, one up to 2.7
kilograms in weight, of this material within the work camp suggest that, at
least, they were regarded with interest.
Ten samples taken from the larger cache are between 2.5 to 5 centimetres
in diameter (mainly 3 to 4 centimetres) with an orange-brown limonitic crust up
to five millimetres thick. When broken they range in colour from yellow
(chalcopyrite) to dull yellows (mixed chalcopyrite-pyrite) and most show
visible galena. They share a very
uniform petrography with the relative abundance of the minerals being
chalcopyrite > pyrite > sphalerite > galena >> tennantite >
arsenopyrite. All show complex
intergrowths between the minerals and some comprise alternating chalcopyrite-
and pyrite-rich bands and galena-rich areas.
Four samples from the other cache are very similar to those of the first
cache but a little coarser grained. The grain size and complex nature of the
intergrowths between the minerals means that beneficiation would be impossible
so that any copper concentrate would carry iron, zinc, lead and arsenic as
major impurities. In neither cache were there examples of the fine-grained,
galena-sphalerite-rich sulphides present at Blue Hole nor the coarse-grained,
tennantite-chalcopyrite ores from the Western Mine area.
The fine-grained, sphalerite-galena-rich sulphides from Blue Hole
cannot be considered to be Bronze Age copper ore, the copper content is low
(<2%), the grain size too fine and the mineral intergrowths too complex for
a copper concentrate to be produced by any physical means. Indeed these sulphides were unsaleable in
the 1820s, as were the very similar fine-grained, fahlerz-bearing, lead-zinc
ores from Parys Mountain, Wales ('bluestone ore') at the end of the same
century. Only a single small piece of
sphalerite-galena ore has been recovered from the excavations of the Bronze Age
work camp at Ross Island.
Weaver noted the presence of the
secondary minerals malachite, azurite and cuprite plus minor amounts of native
copper but stated they occur 'much more rarely' and were restricted to a small
part of the mine. Copper arsenates are
only present in ‘academic’ amounts and as at Muckross Mine, they may be
post-mining in origin (Moreton et al.
in press). Hence the amount of copper arsenates compared to fahlerz- or
arsenopyrite-bearing ore at Ross Island was trivial and so they are not
considered to be ore.
Although excavations up to September 1995
have revealed much Early Christian Age slag, neither Bronze Age copper metal
nor slag have been recovered from a securely dated prehistoric context. A single sample, however, which was
initially identified as a vuggy, limonitic gossan but now interpreted as
partially roasted ore may give a clue to at least one of the ores used by the
Beaker People. The sample (find number
1242) comprises a mixture of yellow copper-iron sulphide (some of which
corresponds in composition to cubanite (CuFe2S3) plus
minor amounts of a white iron arsenide close to lØllingite
(FeAs2) plus cellular organic matter (wood/charcoal). Very minor amounts of neomorphic albite and
magnetite, itself enclosing traces of copper metal, line the circular walls of
the vughs, which are infilled by coarse-grained, post-Bronze Age, secondary
cerussite and minor olivenite. The
copper-iron sulphides and iron arsenides are heavily altered to limonite.
Cubanite, lØllingite,
magnetite and albite have not been recognised from any of the ore mineral
assemblages from Ross Island nor would they be natural alteration products from
these ores. Although cubanite and lØllingite
are unlikely results of sulphide-arsenide roasting (T.Rehren pers.comm.) the chaotic mixture of
organic matter and ore minerals, the presence of circular vughs (gas bubbles?)
and magnetite suggest that specimen 1242 represents a poorly roasted
sulphide-arsenide ore. The exact nature of this ore is speculative but a very
few fragments of a coarse-grained chalcopyrite-arsenopyrite-rich ore akin to
Weaver’s ‘yellow sulphuret ore’ have been recovered during the excavations and
may represent the pre-roasting charge.
Despite the rarity and geographical restriction within the British
Isles of fahlerz in sufficient quantities that it could constitute an ore, the
data from Ross Island provide evidence that fahlerz did play a significant and
early role in Early Bronze Age metallurgy at least once, in Munster. Although
the evidence is preliminary it also seems likely that a number of different ore
types were being exploited in Ireland during the Early Bronze Age including
chalcopyrite-tennantite, chalcopyrite-arsenopyrite and fine-grained, mixed, chalcopyrite-pyrite
± base-metal sulphides. An assessment of
the importance of each ore type will only be possible after the relative
amounts of ores types (as found in the work camp) have been quantified together
with the integration of the post-1995 excavation material. This is currently
being studied and includes rare metallurgical residues found in association
with Beaker period pit furnaces in the work camp location. Notable amongst
these finds are a small number of roasted and altered ore and a metal droplet
(O’Brien pers. comm.)
The present work broadly supports O’Brien
(1995). He, in a thoughtful and thought provoking contribution, states that at
Ross Island, the oldest known copper mine in northwest Europe, people were
mining and processing primary fahlerz-bearing ores rather than secondary copper
ores to supply Group I (As-Sb-Ag-bearing) metal from 2500-2000 BC, namely until
early tin-bronze times. He further
suggests that data from Ross Island strengthen the ideas of Case (1966) and
Northover (1980, 1982) that Munster (especially counties Cork and Kerry) was
the main source of early copper in Ireland and Britain. Ixer and Budd (1998) concur but, partially
based on the presence of the two caches of fine-grained, chalcopyrite-pyrite ± galena nodules, propose that sulphide
ores from Blue Hole may have been processed together with or alongside
fahlerz-bearing ores from the Western Mine area. They also note that all the mineral associations from Ross Island
are antimony- and silver-poor (reflecting tennantite rather than tetrahedrite
as the main copper sulpharsenide) so that Group I metal with its characteristic
As, Sb, Ag trace element impurity pattern, cannot have been made from Ross
Island ore alone. This contrasts with O'Brien who comments on the uniformity of
Group I metal and wonders if Ross Island were the sole provider of Group I
metal to Ireland before urging caution because of the presence in Munster of
other fahlerz-bearing mineral assemblages notably Ballycummisk near Mt Gabriel
(Ixer, 1990) and Ardtully in the Kenmare Valley as well as arsenopyrite-bearing
ores from Dhurode (Ni Wen et al.
1991).
Elsewhere
in the British Isles, Bronze Age people could have exploited arsenic-bearing
ores from within the copper-tin-arsenic mineral province of Cornubia but, as is
well known, there are no recognised Bronze Age mine sites from southwest
England. In addition the widespread
presence of primary fahlerz-bearing and chalcopyrite-arsenopyrite-rich ores
(plus, perhaps, local concentrations of secondary copper arsenates) makes it
impossible to determine what sort of ore, if any, was being exploited. It is
worth noting, however, that the evidence from Ross Island would indicate that
both fahlerz-rich and chalcopyrite-arsenopyrite primary ore types were of
interest to Bronze Age metallurgists, so reinforcing Cornubia as an important
potential orefield.
Indeed, mineral deposits from Cornubia
and especially Cornwall west of the small city of Truro, include native copper
from the Lizard Peninsula, hardrock sources of
chalcopyrite-arsenopyrite/fahlerz ± copper arsenates and
chalcopyrite-cassiterite ores from numerous coastal and inland exposures plus
placer cassiterite from streams. These
localities could have supplied metal for the British Isles from the inception
of the Copper Age well into tin-bronze times. Although the combined absence of
evidence for early mining, and the negligible amounts of Copper Age metalwork
from Cornubia, will continue to be important stumbling blocks to our
understanding of Bronze Age mining and metallurgy in mainland Britain, current
investigations at Ross Island are ensuring that this is no longer true for the
British Isles.
Acknowledgements
The Ross Island mineralisation study is
part of an inter-disciplinary investigation undertaken by University College,
Galway in collaboration with other bodies. The Irish Government through the
Royal Irish Academy funded the ore analyses. Paul Budd is thanked for stimulating
discussions and Paul Craddock for his patience.
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Table 1. Representative electronprobe
microanalyses of fahlerz, in wt% and calculated to unit formula using S=13.
|
Cu |
Ag |
Zn |
Fe |
Hg |
Cd |
Pb |
As |
Sb |
Bi |
Te |
S |
Total |
[1] |
38.1 |
0.0 |
7.3 |
0.0 |
- |
- |
- |
0.0 |
24.6 |
- |
- |
23.9 |
99.5 |
[2] |
3.70 |
0.11 |
6.85 |
1.44 |
- |
- |
- |
19.80 |
0.20 |
- |
- |
27.92 |
100.03 |
[3] |
38.90 |
0.50 |
4.82 |
2.80 |
0.00 |
0.07 |
- |
7.67 |
19.11 |
0.27 |
|
25.08 |
99.22 |
[4] |
37.4 |
0.4 |
4.8 |
1.9 |
- |
- |
|
11.1 |
0.0 |
19.2 |
|
24.3 |
99.1 |
[5] |
32.7 |
8.9 |
2.3 |
4.5 |
- |
- |
- |
2.2 |
26.7 |
- |
- |
23.7 |
101.00 |
[6] |
13.0 |
33.0 |
1.10 |
3.8 |
- |
- |
- |
0.3 |
26.5 |
- |
- |
21.9 |
99.6 |
[7] |
2.4 |
47.8 |
0.8 |
5.1 |
- |
- |
- |
0.0 |
26.9 |
- |
- |
20.0 |
99.7 |
[8] |
21.3 |
16.9 |
0.61 |
0.0 |
18.3 |
- |
- |
2.75 |
21.0 |
- |
- |
21.6 |
102.46 |
[9] |
30.41 |
6.60 |
0.07 |
0.34 |
Mn |
11.70 |
0.25 |
1.12 |
27.14 |
0.13 |
|
22.42 |
100.18 |
[10] |
39.58 |
2.60 |
2.27 |
1.29 |
- |
- |
4.64 |
12.53 |
11.03 |
- |
|
26.27 |
100.21 |
[11] |
44.51 |
0.00 |
0.02 |
1.63 |
5.15 |
- |
- |
18.42 |
2.32 |
- |
- |
26.32 |
98.37 |
[12] |
|
47.6 |
|
- |
- |
- |
- |
8.1 |
1.3 |
|
17.0 |
26.0 |
100.00 |
[13] |
41.1 |
7.1 |
- |
- |
- |
- |
- |
- |
28.0 |
- |
- |
25.3 |
101.5 |
[14] |
46.56 |
0.09 |
0.00 |
0.41 |
- |
0.16 |
- |
11.54 |
10.69 |
- |
- |
30.71 |
100.16 |
[1] Cu9.95Zn1.86Sb3.92S13
(Tetrahedrite, Siegenland; Springer, 1969)
[2] Cu10.27Ag0.01Zn0.39Fe1.57As3.94Sb0.03S13
(Tennantite, Casapalca, Peru; Wu and Petersen, 1977)
[3] Cu9.90Ag0.08Zn1.19Fe0.81Cd0.01As1.66Sb2.54Bi0.02S13.00
(Tetrahedrite-tennantite, Tomnadashan, Scotland; Pattrick, 1984)
[4]Cu10.1Ag0.1Zn1.6Fe0.6As2.5Bi1.6S13
(Bismuthian tennantite, Manualde, Portugal; Oen and Kiefe, 1976)
[5] Cu9.4Ag1.4Zn0.6Fe1.5As0.5Sb3.9S13
(Argentian tetrahedrite, Coeur d'Alene, Idaho; Knowles, 1987)
[6] Cu3.39Ag5.82Zn0.32Fe1.29As0.08Sb4.14S13
(Freibergite, Mt Isa, Queensland; Riley, 1974,)
[7] Cu0.78Ag9.23Zn0.25Fe1.92Sb4.04S13
(Freibergite, Janggun, Korea; Imai and Lee, 1980)
[8] Cu6.47Ag3.02Zn0.18Hg1.76As0.72Sb3.33S13
(Mercurian tetrahedrite, Chiprovtsi, Bulgaria; Atanasov, 1975,)
[9] Cu8.90Ag1.14Zn0.02Fe0.11Cd1.94Pb0.02As0.28Sb4.2Bi0.01S13
(Cadmium tetrahedrite, Tyndrum, Scotland; Pattrick, 1985)
[10] Cu9.6Ag0.4Zn0.5Fe0..4Pb0.6Sb2.6As1.4S13 (Plumbian tennantite, Sark, Channel
Is; Bishop et al., 1977).
[11] Cu11.1Fe0.5Mn1.5As3.9Sb0.3S13
(Manganiferous tetrahedrite, Quiruvilca, Peru; Burkhart-Baumann, 1984)
[12] Cu12.02As1.73Sb0.17Te2.14S13
(Goldfieldite, Butte, Montana; Springer, 1969)
[13] Cu10.7Ag1.1Sb3.8S13
(M2+ deficient tetrahedrite, Ilímaussaq, Greenland; Karup-Møller,
1974)
[14] Cu3.06Fe0.03Cd0.01As0.64Sb0.37S4
(Enargite (Luzonite-famatinite),
Clevedon; Bristol; Ixer et al., 1993)
Figure and Plate Captions
Figure
1. The structure of tetrahedrite group
minerals. This figure shows the structure (molecular model) of a tetrahedrite
and demonstrates the coordination environment of the atoms. This example is of
a tetrahedrite with a chemical formula of Cu10Cd2Sb4S13.
The copper occurs in two sites, a four-fold coordinated site (CuIV)
and a three-fold coordinated site (CuIII). The Me2+ (in
this case Cd) replaces two of the CuIV atoms while Ag preferentially
replaces the CuIII atoms. The Me3+ element (Sb) is in
three-fold coordination adjacent to a ‘hole’ in the structure. The S atoms are
also in two sites, one site (the SIV site) has four-fold S atoms
bonded to the CuIV atoms, and another site in which the six-fold S
atoms (the SVI) are bonded to the CuIII atoms. (From
Pattrick et al., (1993) and after
Pauling and Neumann (1934).
Colour
Plate 1. Tennantite from veins at Casapalca, Peru. The classic tetrahedra that
give the mineral group its name can be seen, growing from massive tennantite.
Specimen size 8cms across.
Colour
Plate 2. Complex intergrowths of
tennantite (green-grey, bottom left) – galena (white) – chalcopyrite (deep
yellow) – sphalerite (light grey, top right).
Rhombic crystals of dolomite (dark grey, centre) are enclosed in the
mixed sulphides. Fine- grained ores
like this are very difficult to beneficiate using physical methods. Tynagh, Ireland. Field of view 0.7mm. Reflected Light, plane polarised.
Colour
Plate 3. Tetrahedrite-pyrite-chalcopyrite from Tomnadashan, Scotland. Pyrite, (very pale yellow, bottom right) is
partially replaced along its cleavage planes by chalcopyrite (deep
yellow). Tetrahedrite (green-grey,
left) locally has totally replaced pyrite.
The ore minerals are in a calcite gangue (dark greys). Field of view 1.3mm. Reflected Light, plane polarised.