Geology part of Andy Lewis's Mphil
The Lower Carboniferous (Dinantian) rocks that form the Great Orme’s Head have been studied since the later part of last century. Initial studies (Morton 1898) described a sequence dominated by predominately limestones with occasional shales and a single sandstone horizon (figure 6). Further studies this century (Smyth 1925, Neaverson 1937) and more recently (Warren et al. 1984, Davies 1984, Davis 1993) confirmed these earlier studies, although individual stratigraphical studies and lithologies have now been largely reinterpreted and added to (figure 7).
Even though there are now several reports dealing with the geology of the Great Orme, to date there has been little comment on the mineralisation of the ore deposits centred around Vivians shaft in the Pyllau valley and to those at other minor sites on the headland. Early reports (Morton 1898) indicate that mineralised ground was confined to particular layers within the limestones. This arrangement has been recently observed and forms part the present study, indicating a definite relationship between the 18-19th century mine workings and individual rock strata. It has been suggested (Lewis 1994) that there is also a similar link between the prehistoric workings and certain lithological units.
This investigation proposes to study all available documentary evidence on the mineralisation, and to combine this with field investigations to determine the nature of the ore deposit and its relationship with the surrounding limestone country rock. In order to investigate the mineralisation in the detail required it was decided to map the geology and log the varying strata through the surface and underground workings, concentrating primarily on those workings of an assumed early origin. However before any recording could be completed it was necessary to conduct a survey of the general layout of the earlier workings and those of the last two centuries which had been superimposed. Once completed, geological information could then be transferred in plan and elevation representations of the workings, so enabling the relationship between the workings and geology to be determined. These same drawings could also be used to plot locations where evidence for early mining activity was known and also any other points of interest.
The initial studies by Morton (1898) and references in mining reports of the last century describe copper mineralisation being confined around faults or fractures where they cut the more “crystalline beds” within the limestone country rock. Often these horizons occurred as alternating sequences between beds of limestone barren of any mineralisation, so giving the impression of a layered arrangement of mineralised and unmineralised ground. Ore deposits were reported (Vivian 1855-57) to follow this arrangement from surface at 168m A.O.D to the deepest parts of the nineteenth century workings reaching at least 70m below sea level, with greatest concentrations of mineral occurring where veins and fractures intersected one another. These “crystalline beds” were interpreted to be units of dolomite and dolomitised limestone that had provided suitable host areas for mineral bearing solutions.
Reports by the mine manager (Vivian 1855-57) clearly indicate from the listed depths of the working levels that particular rock strata had been mineralised in preference to neighbouring beds above and below. Such information has been useful in determining which set of known workings accessed off the present shafts corresponded to the described accounts, particularly as each mining company last century seemed to rename the access shafts during their term at the mine. For example Vivians shaft has been referred to previously as ‘Copper Ddu’, ‘Engine shaft’ and the ‘Sump shaft’. Some of the documentary descriptions are accurate enough to be relatable to working levels re-entered during the past six years, and this is particularly true at Owens shaft where five levels of workings were surveyed and found to be within 0.3m of the depths recorded in the 1855 report. At other shafts there is less agreement between documentary accounts and workings currently accessible, which is likely to be due to backfilling of the shafts and also to exploratory or productive mining post-dating these written descriptions.
3.3 General Setting and Structure
Both Asbian and Brigantian substages are represented within the limestones of the Great Orme, being termed within North Wales respectively as the ‘Dyserth Limestone’ and ‘Gronant Groups’ (Warren et al 1984). Local formations in these groups have lateral equivalents along the coast with comparable lithologies at the Little Orme, Llandulas and Prestatyn. All formations are well displayed in the cliff exposures on the Great Orme.
The lowest beds are the Llandudno Pier Dolomite which outcrop along the beach between Llandudno pier to a point known locally as “Rock Studio”. Here a unit of mudstone, the Tollhouse Mudstone occurs, which is directly overlain by a series of regular bedded argillaceous limestones. These beds were sought as building stone last century and have consequently been quarried at a number of locations along the outcrop, forming the galleries below Pen-y-dinas, at a small quarry at Tyn-y-coed Road and further west where a small quarry over looking the West shore tollgate displays a reduced thickness of material. The cliffs on this western side of the headland also display the characteristic yellow brown weathering of the Llandudno Pier Dolomite. These dolomites display many of the characteristic features that typify these rock types, such as the presence of vughs and cavities, porous sometimes saccaroidal appearance and distorted bedding due to reductions in volume from the effects of dolomitisation. Smyth (1925) named these quarried limestones which occur above the dolomites as the ‘Lithistrotian affinephillipsi beds’ after the apparent occurrence of this particular coral .
Continuing up the sequence lithologies grade from generally pale grey/grey thin limestones to massively bedded formations with occasional rubbly horizons, these being collectively termed the Great Orme Limestone. Ramsbottom (in Warren et al 1984) identified a series of eleven ‘cycles’ within this formation which give rise to the stepped exposures forming the impressive cliffs seen from the Marine Drive coast road. These ‘cycles’ correspond to the cyclothems of similar rocks in Yorkshire, interpreted as a regressive phases of deposition. Lithologically, a gradual shallowing of marine conditions is observed as sequences of ‘wackestones with packstones passing through into pseuobrecciated packstones and rubbly or nodular beds with infrequent calcrete and rhizoliths forming the upper surfaces of the cycle’ (Warren et al 1984).
The highest beds of the Great Orme Limestone are exposed at the Rofft quarry (SH 7755 8315), represented in part by a brown to purple sandstone, fine to coarse grained with cross bedding. This is overlain by two thickness of limestones one of which has a distinctive shattered appearance, weathering to produce a splintery deposit, the other being a massively bedded pseudobreccia. Together these three distinct units form a useful marker sequence, that can be recognised over large areas of the Orme, especially the pseudobreccia whose outcrop forms a low cliff in many places. The sequence has also been identified at the mine site, directly as a result of detailed study of new rock surfaces, exposed during earth moving operations during the winters of 1990-92. Underground the same sequence has been noted throughout the workings to a depth of 70m below surface. Once identified these rock types together with the overlying thickness of mudstones have proved to be particularly useful in establishing the relationship between the mine workings and the varying lithologies, which will be commented on in the following sections.
The mudstones overlying the pseudobreccia can only be seen on surface at the mine site, elsewhere on the headland they have weathered away and are now grassed over to produce a distinct step or planar area. Within several metres lithologies more typical of the ‘Bishops Quarry Limestone’ are seen to overly these strata. These mudstones are considered to represent an important substage boundary within the Dinantian limestones, dividing the more massively bedded shallower water Asbian lithologies from those of the overlying deeper water tabular bedded Brigantian limestones.
The Bishops Quarry Limestones fall within the ‘Gronant Group’ (Warren et al 1984) with the lowest beds exposed at the eastern Bishops quarry. Cliff faces at both the quarries display typical lithologies, consisting of alternating regularly bedded argillaceous limestones and thin shale partings. Notable brachiopod fauna exist throughout both limestones and shales, including Productina margaritacea and Gigantoproductus edelburgensis.
The sequence between the Bishops Quarry and Great Orme Limestones are considered to represent the boundary between the Asbian (D1) and Brigantian (D2) sub stages, whose exact position has been the subject of debate for some years. At first the division was decided on faunal evidence (Smyth 1925, Power & Somerville 1975) and later to include lithological changes (Somerville & Strank 1984). Davies (1984) opted for the latter, placing the boundary at the top of his thirteen cycles ‘within the Middle White Limestone’ (Great Orme Limestone), correlating it with the upper surface of the Craig Rofft Sandstone. More recent studies debate this boundary position further (Davis 1993).
Logged strata produced as part of this study, are likely to become an important component in assisting with the boundary debate, as they represent the only complete record through this rock sequence. For the purposes of this study, the Asbian-Brigantian boundary will be taken as the introduction of the first major mudstone above the massive pseudobreccia.
The Bishops quarry beds give way to the Summit Limestone, a series of brown to grey limestones and dolomites, with nodules of chert. These beds occur around the summit with other exposures near the road side on the route between the halfway tram station and St. Tudno’s church.
The relationship between the mineralised strata and extent of identified workings is known to be complicated mainly due to the interconnecting system of veins (Morton 1898), and there is a total lack of former mine plans. Previous investigation and simple survey of many of the explored passages of both Bronze Age and recent mining confirm this complexity. Often mined tunnels are observed to be connected to steeply inclined chambers or stopes that corresponded to the worked out mineral veins or lodes. In many respects these mined areas resemble a labyrinth of interconnecting workings. There are two purposes to this part of the study. One is to study and report on the geological conditions of the ore deposit and its relationship to the parent rock. The other is to determine the form and extent of the prehistoric workings and their relationship to those workings from the 18/19th centuries. The ideal method of achieving this would be to present the form and extent of the workings and their geological conditions in some three dimensional form. To do this however, would have been beyond the capabilities of this study, and it was therefore decided the best and most practical means of representing the geometry of the workings would be to produce sets of two-dimensional plans and sections. When complete, geological detail in the from of logged strata could then be plotted on to these, so that the relationship between the ore deposit and parent rock could be understood. The same drawings would also be useful for plotting on other information, such as important archaeological features and artefacts.
Fortunately a small number of limited underground surveys already existed,compiled over the last seven years of exploration. Many of these had been incorporated within a later more extensive underground survey partly funded by CADW, which had been undertaken by the author for Great Orme Mines Ltd (Lewis 1993b). The purpose of this was to highlight and describe all known locations where evidence for early mining activity was recognised, to determine their relationship with the more recent workings and so provide a preliminary set of documents to which new information could be later added. The present study utilised this previous data and added to it by surveying further areas of mine workings. These drawings then provided a base survey which enabled the form and extent of the workings to be determined, how they related to geological conditions and whether there were distinctions between the earliest and most recent workings.
The intricate and variable size of the workings, combined with the presence of collapsed and back-filled spoil, make surveying underground difficult. Abrupt differences in elevation, restrictions in size, the presence of running/standing water and muddy conditions also add to this. Therefore when surveying began it quickly became apparent that using an EDM (electronic distance measurement) would be very time-consuming and could damage the equipment. A simpler method became necessary. This entailed the more practical use of a sighting compass and measuring tape, allowing horizontal-lateral distances and angles to be measured. Differences in elevation were either measured vertically using a tape or determined by using a clinometer to measure the slope angle and thus calculate the vertical differences. A series of reference stations were established through the principal routes between the four shafts: Vivians, Owens, Treweeks and Roman. Normally these consisted of a steel pin secured into the roof of the passage from which a siting string was hung; when this was not possible a pin was positioned on the floor. When surveys of the main routes were complete additional information was then added by referencing the more remote side workings to the principal survey route. Using this method it was then possible to continually add further survey information from future visits to these areas. The tape and compass method also allowed direct sight lines through the workings to be more easily maintained, so minimising the removal of spoil and consequent disturbance to areas of potential archaeological interest, which may have otherwise been affected if an EDM survey system had been used. The accuracy of the surveys generally conformed with Grade 3/4 of the British Cave Research Association, although some areas were recorded in more detail and graded as level 2.
When the survey was complete, magnetic compass data were plotted onto draft plans, then corrected to grid data (5 degrees in 1993) and transferred to the main plans at a scale of 1:500 (appendix B). Where possible underground survey traverses were linked between internal shaftways and other recognised shafts open to surface. This provided a useful means to check the accuracy of the survey. It was found that only slight adjustments were necessary to obtain the best line fit between known positions coinciding, with the shafts. For example the western traverse from Vivians to Treweeks shaft (figure 11) required an adjustment of about 0.5m, which is quite reasonable considering the difficulties of surveying over these distances, difference in elevation and restrictive conditions encountered through the workings.
All reference points on the completed plans were later computer digitised and corrected to actual grid co-ordinates. This enabled the eastings and northings of each reference point to be calculated, so providing a method of plotting survey data onto Ordnance Survey mapping. Levels in metres above ordnance datum (A.O.D.) were determined at most reference points and included with the remaining survey data (appendix A). The main set of drawings at 1:500 scale (appendix B) were later reduced to produce further sets of drawings (figures 8,9,10,11) at scales of 1:1250 and 1:1000 on to which information could be plotted. This included details of 35 locations which were of particular importance and mainly displayed evidence for early mining, as well as areas with notable geological features and evidence for 18-19th century mining (appendix C).
It is hoped that the recorded survey data may eventually be utilised as part of another study to produce a three dimensional plot of the mine passages, onto which relevant data can be included as overlays. This could include geological/mineralogical detail, position of artefacts/features and perhaps even be used as a predictive tool for locating undiscovered areas of workings.
The general lithostratigraphy of the Great Orme is documented in section 3.3, which is based mainly on studies by Smyth (1925), Warren et al (1984) and Davies (1984). This information was, however, not sufficiently detailed to meet the requirements demanded by the present study, so a series of more detailed recordings were necessary. As regards the general geology of the mine site very little had been recorded, partly because some of the present surface rock exposures had only recently been uncovered and access to certain underground workings had only been known for a few years. Information that did exist dealt more with the simple listing of rock and mineral types, lacking the necessary detail required here. Therefore geological-mineralogical conditions needed to be recorded in detail at both surface exposures and underground at the mine. The logged strata covered formations (after Warren et al 1984) ranging from the Summit Limestone, Bishops Quarry Limestones and the Craig Rofft and Creigiau Cochion upper members of the Great Orme Limestone (figure 7).
Surveying underground presented its own set of difficulties, similarly the recording of geological detail. It was decided that information from surface rock exposures in and around the opencast as well as from the underground workings would need to be recorded in order to fully interpret and understand the geology and mineralogy of the mine site. The method, was to individually log lithostratigraphic units starting at an arbitrary point and then working up or down the sequence. Any indications of mineralisation were also recorded, including the extent and degree of dolomitisation and presence of copper and associated ores. At surface this was quite straightforward and entailed recording details of the strata above the tourist entrances, then through the opencast, to finish at the uppermost part of the sequence near to the Engine House situated to the north east of the site (figure 12).
A similar exercise was completed underground, this presented a number of difficulties. The main one was the complicated nature and constricted size of the workings which meant that any logging would be very time consuming. In order to overcome this it was decided that the best means of recording information would be to log the strata through three of the shafts - Vivians, Owens and the Roman. The only safe means to obtain this information was by controlled abseiling on a fixed rope suspended within the shaft mouth, beginning at the top and recording the various lithologies down the sequence, at the same time taking note of the position of the different working levels and also any other point of interest. Owens shaft was particularly suited to this type of recording and a complete sequence was logged over the length of the shaft to the 140ft level. Vivians shaft proved to be more awkward due to the instability of the shaft walls as a result of loose spoil and areas of unstable rock, therefore only a limited amount of detail could be recorded. Remaining detail was obtained by logging strata through the workings adjacent to the shaft, this included information from the near-by tourist route. At the Roman shaft information was recorded in the main shaft and then through the workings to link with a cross cut to Treweeks shaft known as ‘Wagon Gate’.
Further geological/mineralogical information was obtained from observations in many of the passages previously surveyed, with information referenced to either the recorded locations or the surveyed reference points. Within certain rock units and sequences, the degree of dolomitisation and resulting mineralisation was far more pronounced than in adjacent rock units. Where much of the original rock fabric had been altered and replaced it was difficult to describe lithologies prior to dolomitisation. Therefore where possible, the affected horizon was followed to where the original rock was less altered, so enabling proper identification. In some cases this was achieved by tracing surface exposures to where they existed outside the zone of dolomitisation.
Initially all logs were plotted at a scale of 1:20 so that as much detail as possible could be plotted, however for the purposes of this study these had to be reduced to a scales of 1:200 and 1:150 so that they could be conveniently included in the report (figures 11,12,13,14 & 16). In addition to the logged sections and field data from the mine site, similar but less detailed geological information was recorded in the surrounding area. From this, a geological plan and section (figure 15 &17) indicting the rock types and structure of the site was produced. When all the geological logs and maps were complete and the extent of mineralised ground determined, attempts were made to interpret the relationship between the extent of the mine workings and the recorded geological conditions.
3.5 Results and Interpretations
The purpose of the two surveys was firstly to map the extent of the known workings thought to contain evidence for early mining, and secondly to record and interpret the lithological-mineralogical and structural conditions of the mined area. This information could then be used to perhaps explain the scale and nature of the early workings, what factors dictated the types of mining methods used and how might the copper ores have been identified. The following sections will detail these recordings and propose their most likely interpretations.
Throughout the recorded sequence (figures 12,13,14), particular rock units were found to be more easily recognisable in the field than others. They provided useful ‘marker horizons’ within the workings, so enabling the position in the overall sequence at the mine to be determined. The two most prominent horizons were the ‘sandstone’ (Craig Rofft Sandstone) and ‘thick or Pyllau mudstone’. Each of these units could clearly be followed on the surface (plate 6) and also underground where their basal surface typically formed the ceiling of stopes and passages, as described in sections 3.5.3 and 4.2. Other, thinner mudstones also became useful as minor marker horizons particularly when their position with respect to these main markers was known. These thinner horizons also frequently corresponded with the ceilings of worked out areas. By superimposing these marker horizons onto survey sections it became possible to extrapolate between recorded areas, slowly building up a picture of the relationship between mined areas and geological detail (figures 15 & 18).
When viewed from the south the limestone strata that form the headland are seen to be gently folded into a shallow syncline, with a north-south axis centred just east of the Great Orme summit. Dips on average are about 10-12 degrees but occasionally up to 25 degrees. A similar structure is seen from the east with a plunging fold axis trending east-west producing dips of 25 degrees in the south of the Pyllau valley and around 10 degrees in the north. Overall this arrangement describes a structural basin, centred roughly in the region of former mining activity.
A large number of fractures are developed in the mining region, with which the dolomitisation and copper mineralisation are intimately associated. The main trend of these fractures is north-south with hades of 80-90 degrees to the west. At least forty are known but, they display few signs of displacement and could more correctly be described as master joints, although some displacement of about 1m is recorded from the more pronounced fractures, so constituting faults. Often these fractures occur in pairs or sets of up to 2m width, but more generally between 0.5-1.0m. Occasionally fragmentation has occurred between the two planes resulting in a fault breccia. Normally one of these surfaces forms the foot or hanging wall to a mined passage or chamber. Additional less well developed fractures follow an east -west trend, and also are associated with the dolomitisation. Both the north-south and east-west fractures are encountered at other locations on the headland, typically with associated dolomitisation but not always copper mineralisation. Prospecting last century at these locations has resulted in a number of trials and minor workings.
A number of north-west to south-east oblique trending faults also exist within the mining region, some of which can be traced over the surrounding area. Typically they display greater displacement, lower angles of hade and few indications of dolomitisation compared with the other fracturing. One in particular has assisted with exposing mineralised beds at surface, taking a line across the Pyllau valley forming the scarp feature where the tourist entrances are situated. This fault appears to have a hinge component with a displacement measurable in tens of metres near to Tyn-y-coed (SH 774 828), while near the northern margin of Bishops Quarry (SH 767 832) it is measurable in a few metres. Displacement is down to the north at an angle assumed to be between 80-90 degrees.
In order to clarify the fracture pattern at the mine site a system of referencing was devised, taking Vivians shaft as a zero point. North-south fractures to the west of the shaft were given a prefix of W (west) then numbered consecutively from one onwards. Fractures on the eastern side followed a similar system, and likewise for the east-west fractures. Once established, this system could be used to relate surface features with those underground, further assisting with the interpretation of mining with respect to mineralisation.
No definite age is ascribed to folding or fracturing, though generally it is considered to be associated with activity during the Variscan orogeny towards the late Carboniferous and early Permian.
Previous sections have briefly commented upon how the dolomitisation and copper mineralisation are closely related with respect to structural fractures and country rock lithology. Both field observation and the study of the lithostratigraphic logs have confirmed this association, also agreeing with those references from the last century which stated that ore was confined to the “crystalline beds” (~ dolomitised).
Because of the exclusive association between dolomitisation and ore mineralisation any controls on their distribution through the country rock can be considered together. They will be discussed in the following sections that cover their relationship with structural and lithological control.
Dolomitisation and Structural Control
The chapter dealing with structure states that dolomitisation is normally associated with the north-south and east-west fractures, implying these lines of weakness have provided the appropriate routes for migrating magnesium-rich fluids responsible for this alteration. Dolomitisation is a metasomatic or replacement process , occurring when calcite, the constituent mineral of limestone, is partially or totally replaced by the mineral dolomite. The reaction advances by magnesium replacing calcium, represented by the following equations:
Mg2+ + 2CaCO3 = CaMg(CO3)2 + Ca2+
CaCO3 + Mg2+ + CO32- = CaMg(CO3)2
Experimentation (Sperber et al 1984) indicates that two main dolomite groups are generally recognisable, one at 10-30% dolomite giving dolomitised limestone, and another at 90-100% dolomite termed dolostone (dolomite). Dolomitised limestones typically have some unreplaced matrix with indications of sedimentary structure, lithological changes, and fossil remains that can be preserved as aragonite a component mineral which is less susceptible to dolomitisation than calcite. Dolomites are well indurated in their unweathered form, consisting of tight interlocking crystals, lacking fossils and any original structure. On weathering they may become friable with a saccaroidal nature.
Dolomitising solutions are normally of marine or meteoric origin, and are able to be transmitted laterally over distances measurable in kilometres through appropriate rock types. With regard to the Great Orme though, little information is available as to the likely source, though a fluid direction from below rather than above is the more likely, the reasoning being that the lowest beds in the Great Orme succession, the Llandudno Pier Dolomite are almost entirely dolomitised while the overlying Great Orme Limestone shows few signs of any alteration. The dividing horizon between these two formations, the Tollhouse Mudstone therefore appears to have acted as a barrier to the rising dolomitising solutions. However, in the region of the mine and at a few scattered sites on the headland, penetration of near vertical fractures have allowed the upward migration of these magnesium-rich solutions, so giving rise to dolomitisation. Diagenesis is considered to be late rather than early, however this is often difficult to diagnose because it can occur progressively from connate and ground water. Typically dolomitisation is associated with uplift, which resulted in folding, faulting and shearing to which it is closely related, as is the case at the Great Orme.
The actual process of dolomitisation produces a decrease in volume in the affected rock, so causing shrinkage of up to 12% (Hatch et al 1971), taking the form of increased porosity as well as creating cavities and vughs. Dissolution by further fluids can increase this effect. All of these voids provide suitable sites for subsequent deposition by mineralising solutions, notably those responsible for the copper-bearing ores.
Many of the observed dolomites are stained buff to yellow-brown, due to the oxidation of ferrous iron replacing magnesium. A complete range of colours can result, from white, yellow, red to brown, including varieties described as ‘pearlspar’, ‘brownspar’ and ‘rhombspar’. Eventually iron content can increase to a level corresponding to the composition of the mineral ankerite, which is also known from the site (Lewis 1990a).
Dolomitisation is generally considered to be related to the north-south fracturing, with progressively less development in the east-west and oblique fractures.
Dolomitisation and Lithological Control
Dolomitisation occurs via a fabric-selective process, with replacement of calcite in preference to any other carbonate. Therefore there will be a tendency to replace those lithologies with a higher proportion of calcite in preference to those with increasing amounts of argillaceous (clay) or arenaceous (sand) material (Hatch et al 1971). This characteristic is further emphasised if there is a confining horizon of a none or semi calcareous lithology overlying one of distinct calcareous composition. An arrangement of this type causes the flow of the rising dolomitising fluid to be restricted, so allowing replacement in a lateral direction beneath the confining horizon. Typically mudstones, and to a lesser extent sandstones, produce the most prominent confining strata at the Great Orme. Examples are observed above the tourist entrances and in the main stope off Vivians shaft at 13m level where a sandstone bed has confined dolomitisation to the beds beneath it.
Closer examination of the limestone lithologies indicates a further feature of the fabric selectivity of dolomite. There is a tendency for the coarser sparry limestones (calcarenites) to be preferentially dolomitised with respect to the finer grained (micritic) types. This is particularly noticeable at a number of locations underground and at surface, notably the eastern side of the main opencast feature. Here limestone lithologies show obvious dolomitisation in the vicinity of the mineralised fractures, while mudstones display few effects of this process. Typically dolomitisation is more pronounced in the thicker or massively bedded limestones that also tend to be composed of coarser more calcareous material, while thinner beds are marked by higher contents of finer material. Sometimes the dolomitising solutions do not totally replace the selected limestone lithology, so producing a feature where cores of unaffected limestone exist within a largely dolomitised unit. This is particularly well displayed in one horizon locally termed the ‘shattered bed’ , with exposures in the main opencast, above the tourist entrances (plate 5) and at numerous locations underground, notably in Owens shaft.
Many of the characteristics mentioned above can be seen in the logged sections where extraction of the more strongly mineralised thicker strata bordering a fracture constitute the mined galleries (figure 15).
Ore Mineralogy
The previous section highlights the controls on dolomitisation and thus the creation of suitable depositional sites for subsequent mineralisation by metallic ores. Two forms of ore emplacement are recognised, one occurring as an infill to suitable cavities or fissures producing a vein or lode, the other replacing areas within the dolomite in proximity to the cavity infill that have been confined by overlying lithologies, producing laterally extending ore bodies as flats or pipes.
All primary mineralisation is considered to be of hydrothermal origin, with deposition of predominantly chalcopyrite (CuFeS2, plate 8) and to a far lesser extent galena (PbS), which was apparently exploited in small quantities last century in certain parts of the mine (Williams 1979), possibly near to surface as indicated by small quantities of ore recently exposed in the northern opencast. Chalcopyrite mineralisation occurs throughout the mine complex and appears to have been the chief copper ore mined in previous centuries, recorded to depths of 210m below surface, including at least 70m below sea level. The chalcopyrite occurs in both massive forms assumed to be at least several centimetres across, as reported last century with “some lumps of very pure ore......as big as three men could lift” (Williams 1979), and also as well developed crystals encrusting vughy dolomite. Ore is largely confined to the north-south fractures, and to a lesser and decreasing extent in the east-west and north west-south east fractures. It is likely that these north-south veins represent the two major features described last century as the ‘Cyllell’ (knife), the ‘Great Slide’ and ‘Hanging Mawr’ (great hanging) where “good amounts” of ore were obtained. Lesser veins were termed ‘strings’, such as ‘Y Cefn’, or presumably named after the miner who made the first discovery such as ‘Ellis’s’ and ‘Jones’s String’. Particular concentrations of mineral occurred at the intersection of these lines of weakness, and often constituted some of the larger bodies of ore recorded last century (Williams 1979). This is also true for the Bronze Age workings where, at some locations, networks of interconnecting passages display a definite enlargement where mineralised zones cross one another (eg. large stope - location 18)
When observed in hand specimen, many samples of chalcopyrite are seen to be surrounded and cut through by many irregular, often radiating lines of a dark earthy mineral. On analysis (D.A. Jenkins pers. comm.) this proved to be the hydrated iron oxide goethite (xFeO(OH)), having resulted as an alteration product due to the breakdown of the chalcopyrite (plate 8). One possible explanation is oxidation with the iron component undergoing alteration to goethite, while the sulphide and copper components are lost to solution, as the following equation indicates:
CuFeS2 + 5O2 + H2O = FeO(OH) + Cu2+ + 2SO42- + OH-
A reaction of this type is typical of the oxidised zone of mineralisation, and results in the development of the ‘oxide ores’ (secondary mineralisation). A number of ores of this form are known from the mining region, and are likely to have been some of the first exploited in early times, due to the vivid colourations they produce in the surface zones of dolomitisation. The most common ore is the green copper carbonate malachite (CuCO3.Cu(OH)2, & plate 9) with lesser amounts of the blue carbonate azurite (2CuCO3.Cu(OH)2). They have formed by copper in solution combining with carbonate material derived by solution of calcite from surrounding limestones/dolomitised limestone and/or by breakdown of dolomite itself.
2Cu2+ + CO32- + 2OH- = CuCO3.Cu(OH)2
The sulphide component is also oxidised and goes into solution as sulphate (SO42-) where it may combine with calcium to produce gypsum (CaSO4.2H2O), which is often observed as acicular crystals (a variety termed selenite) typically seen growing from beds of mudstone.
Ca2+ + SO42- + 2H2O = CaSO4.2H2O
The formation of the pale yellow mineral jarosite (KFe3(SO4)2(OH)6) often found accompanying the selenite, is also produced as a result of similar oxidation.
Typically the copper carbonates are closely associated with the altered chalcopyrite, occurring as coatings to the sulphide or more often as infilling to vughs, cavities, fractures,joints and small scale solution features within the dolomite and as nodules at and within the basal layer of some mudstones. Analyses of some of the mudstones has given 0.52% copper, from which malachite was found to be concentrated readily by light crushing and panning (Jenkins & Lewis 1990).
Both dolomite and mudstone-bound carbonate ores can be observed in particular underground workings and at surface from the tourist entrances to the opencast. What ore that does remain is very limited, generally appearing as colourations along fracture planes, pore fillings or as small nodules and nuggets up to 20mm across. On rare occasions hand-size lumps of partly altered chalcopyrite were found during rock excavation for the tourist mine. The same work also uncovered a mudstone horizon (500mm thick) on the eastern side of the opencast, parts of which contained numerous concretionary nodules of azurite. Nearby, to the north east corner of the opencast the only known in situ galena was found, this consisted of irregular pockets (up to 150mm across) along a joint surface through a thick mudstone (‘Pyllau Mudstone’). On closer inspection it was noted that at least 80% of the galena was oxidised to the lead carbonate cerussite (PbCO3). Dolomite surfaces in the same vicinity exhibited further traces of azurite and malachite as coatings up to 5mm thick; the area had clearly been worked, implying there must at one time been a reasonable amount of ore at this location. From the nature of the mined surfaces here, together with spoil stratigraphy, this location is considered to be of Bronze Age origin.
Carbonate ores are found through the entire zone of mineralisation including the prehistoric and recent workings to depths of at least 200m below surface. This mode of occurrence does not agree with the ideal model of oxidised zone mineralisation as typically such ores are confined to a near surface zone and rarely reach those depths where malachite was recorded last century, to 130m depth. The limit of the zone of oxidation is governed by the position of the water table, with oxidised ores above and primary ore below. In the case of the Great Orme ore body there would have existed a series of perched water tables trapped by mudstone units in the limestone sequence. There therefore could have existed a number of discrete oxidised zones, that in turn were connected through the mineralised fractures down to a depth where a definite water table occurred, probably corresponding to sea level. Such an arrangement may then help to explain the occurrence of oxidised ores at depth.
Additional partially oxidised ores found last century include the copper oxide tenorite (CuO) or melaconite (Morton 1898) and native or arborescent copper recorded by the then mine captain William Vivian (1859 & 1868), who reported finding microscopic quantities in certain mudstone horizons.
More recently a number of secondary ores from the zone of enrichment have been found in workings (location 16) 20m below Bryniau Poethion accessed via Roman shaft. They include the copper arsenates olivinite (Cu3As2O8.Cu(OH)2) and clinoclase (Cu3AsO4(OH)3), the pink cobalt arsenate erythrite (Co3As2O8.8H2O) and the green copper sulphate brochantite (Cu4(SO4)(OH)6) which occurs in mudstone bands exposed off Treweek’s shaft (Jenkins & Johnson 1993). The occurrence of such ores indicates that appropriate conditions for their formation therefore exist at the site, so implying that larger quantities of the same ores could have once existed, although this does seem unlikely as these ores typically occur in only very minor quantities. It is not unreasonable to assume that a number of related ore types may also have resulted, including enargite (Cu3AsS4) and tennanite ((Cu,Fe)12As4S13). There are now active arguments to suggest that smelting of secondary copper (II) arsenate ores in the Copper and Early Bronze Ages could begin to explain the noticeable arsenic content of implements from this period (Budd et al 1992, Budd 1992) as discussed in section 4.
More recent and continuing study of mineral samples by reflected light microscopy at Birmingham University (Ixer & Davies 1996) has confirmed the oxidation of the chalcopyrite to limonite (goethite) and identified a number of intermediate reaction products. Here the copper sulphides digenite, covelline (CuS) and blaubleibender covelline form reaction rims to the grain boundaries and fractures to the chalcopyrite. In places the limonite containing small areas of the pale blue copper sulphide djurleite. Minor amounts of pyrite and marcasite were also identified, occurring within the chalcopyrite and similarly oxidising to limonite.
A secondary study (Ixer & Davies 1996) of other altered ores additionally identified cuprite,native copper, tenorite, chalcocite and trace amounts of possible tetrahedrite and enargite group minerals. Investigation of galena ores indicated an extensive alteration to the lead carbonate cerussite, with very minor amounts of pyrite, marcasite and chalcopyrite being replaced by limonite.
Both studies also confirmed minor amounts of malachite and azurite and noted the presence of undetermined minor manganese oxides or hydroxides termed ‘wad’. On particular joint surfaces of the dolomite as well as the limestone the manganese minerals produce dendritic forms, likely to be pyrolusite.
A final mineral curiosity is a black bituminous material that occurs as irregular segregations (10-15mm across) and as thin infills to joint surfaces in the dolomite. Occasionally larger lenses of material occur, the largest found in workings off Roman shaft is about 300mm in length. On analyses the ash from a burnt sample was found to have concentrated vanadium and uranium such that it was detectably radioactive. Traces of other minerals from the surface tips have also recently been recorded (Bevins 1994).
No age to the emplacement of the mineralisation has been determined, but comparable lead and zinc deposits in the Llanrwst orefield in the Conwy valley related to the ‘Variscan foreland’ have been dated (K-Ar isotope) to 280 million years old (Ineson & Mitchell 1970). It is possible that the Great Orme mineralisation could relate to reactivated or renewed mineral injection with characteristics that are akin to ‘Mississippi Valley’ type deposits (Sangster 1976), and may be similar to those of the Pennine ore fields, having resulted from saline hydrothermal solutions at temperatures less than 6000 C (Evans 1980). The secondary ores are concluded to be post Variscan (or Hercynian) and are likely due to the oxidation of primary sulphide ores by percolating connate and meteoric waters during the Tertiary and Quaternary periods. Hagerty (1994) suggests that the copper-dolomite mineralisation of the Great Orme is peripheral to the more typical ‘Mississippi Valley’ type deposits of north Wales and the Pennines.
Dolomite-Dolomitised Limestone
Particular areas within the zone of dolomitisation are seen to have undergone a degree of disaggregation producing a loosely bound granular rock that readily disintegrates to produce a sandy deposit. This apparent rotting of material tends to concentrate along lines of weakness such as joints, fractures and mineral veins, and is particularly prominent on surface at a number of locations, for example to the immediate south west of Vivians shaft where archaeological excavation has uncovered a number of early trench-like features. Here the dolomite was very friable, often crumbling away to a yellow-brown granular sand and easily removed by gentle scraping with a piece of wood, bone or even the finger nail.
This rotting is also noted at numerous sites below surface, once more tending to follow the lines of mineral veins and associated lateral ore bodies. Generally its extent is limited to the surface and near surface, progressively decreasing with depth to around 40m below the collar of Vivians shaft. This distance also coincides with the depth of the evidence for Bronze Age workings recorded here. However the deterioration is not totally confined to this zone, as similar but smaller areas are recorded in the nineteenth century workings off the Penmorfa drainage level, which lie at a depth 130m below surface, to the south west of Vivians shaft.
A microscopic examination of the rotted dolomite revealed a matrix of individual crystals with very little cementing material. Component minerals consisted largely of iron stained dolomite, with clear calcite and in some cases dark brown ankerite. The impression was given that either the dolomite-dolomitised limestone had undergone some form of recrystallisation with the part or total exclusion of carbonate mineral cement between individual grains. Alternatively, and more likely, is that no recrystallisation has occurred, instead original grain cement (calcite) is being lost to solution and or that solution is acting along the grain boundaries and slowly attacking the component crystals, so reducing them in size. However, the calcite crystals are euhedral in shape and therefore if they have reduced by solution effects then this must have been closely followed by secondary growth which accounts for their present form.
The mechanism for the rotting is considered to relate in part to the process of weathering, as its occurrence is far greater at or near surface than at depth. This was particularly well seen during the main excavation and spoil clearance of the site for the tourist development in the years 1990-92. During this time rock exposures that had previously been buried for some considerable time were exposed. This then required cleaning to achieve a surface that would stand without further deterioration. In places thicknesses of material up to 250mm were removed with ease, quickly revealing the unaffected dolomite beneath and continued rotting along particular lines of weakness. This observation helped to substantiate the idea that a deposit with these characteristics could have been worked with the simplest of tools, so providing an explanation for why early man had been able to exploit copper ores at this location.
If a process of weathering is considered, one possible mechanism for the attack on the crystalline structure of the dolomite-dolomitised limestone could be by acidified groundwaters. Acidification occurring during the solution of carbon dioxide from the air into rainwater, with humic acids from overlying soils contributing to the decrease in pH. Such a process may account for rotting to depths of 40m, with ground water flowing through a system of fractures and previously rotted zones bordering the mineral veins. This follows a very similar process to that of the formation of caves and their associated calcite formations.
The arrangement of the rock strata in the Pyllau valley also has an influence on ground water flow. A basin like structure is described here, with the affect of directing water at surface from a wide area (260,000 m2) underground along dipping bedding planes, numerous mineral veins and other fractures , towards an area coinciding with the open cast north of Vivians shaft. Many routes for the passage of surface derived water therefore exist , so aiding rotting in appropriate regions of the dolomite with corresponding oxidation of chalcopyrite and production of secondary and enrichment zone minerals.
However the weathering mechanism does not fully explain the extent of the rotting, especially at depth, suggesting some additional process is aiding its production. One possibility is related to the oxidation of the chalcopyrite. During this reaction (see section 3.5.3) sulphur is taken into solution to eventually form the sulphate component of gypsum and sulphide component of some of the enrichment zone minerals. In the early stages of these reactions it can be assumed the oxidised sulphide in solution could produce low concentrations of sulphuric acid (H2SO4). These acids could then assist with the solution of the carbonate cements, and along grain boundaries in the dolomites-dolomitised limestones, contributing to the rotting.
The extent, and degree of deterioration of the dolomitic ore-bearing rock, are the two main factors governing the respective depth and size or form of the Bronze Age workings at the Great Orme. These subjects will be dealt with in section 4.2.
Mudstone
A degree of rehydration is noted in certain of the mudstone horizons, particularly where they are intersected by mineral veins. Often lithologies at these locations have become softened, and absorb water easily, sometimes producing glutinous sticky masses of clay that flow from the bedding planes. A change in colour accompanies the softening, typically to paler shades of the original. Generally, unaltered mudstones are grey to dark grey occasionally approaching black due to increasing organic content, while altered equivalents are light grey to white and reddish brown to yellow. The changes in colour are due to oxidation of ferrous iron with an overall reduction in colour intensity where iron has been leached out. Mudstones exhibiting these effects can be seen throughout the workings, with particularly good examples in workings off the ‘Wagon gate’ (location 12 to 15 and 16, in figures 8,9,10) where complex oxidation has produced deposits of varying consistency and colour. Similar, though less developed rehydration of mudstone was also noted in workings (locations 9-11, in figures 8,9,10) off Owens shaft.
Like the rotting in the dolomite, the exact mechanism for the alteration of the mudstone is uncertain. However it is likely to be related to those reactions already described above. The extent of alteration of the mudstone is somewhat similar to the rotting in the dolomite, although it does not have the same prominence near to the surface as the dolomite, instead having a slightly more even distribution at depth. This is observed in the typically nineteenth century workings accessed off the Penmorfa drainage level, where a number of generally thin mudstone horizons have deteriorated and concentrated to produce flows of material up to 1m thick. However, this softening is not always associated with mineralogical features and may be the result of ground water flow along lines of weakness such as unmineralised joints and fractures.
The deterioration of the mudstone is likely to involve at least two processes. Firstly, during emplacement of the copper mineralisation ground waters were likely to be slightly acidic and may have partly oxidised any iron oxides in the mudstone. Then at some later period percolating ground waters were able to pass along lines of weakness, and at the same time become slightly acidified due to possible oxidation of chalcopyrite so causing further rotting of the mudstones.
3.5.5 Superficial Deposits and Surface Development
The surface development of the Great Orme has not been studied in any detail, though it is generally thought to have evolved during the Tertiary period with modifications during the Pleistocene and Postglacial. Brown (1960) suggested that the Orme, being a similar height to surrounding coastal features (600ft or 183m), was part of an ancient ‘coastal platform’ and likely to date to the early Pleistocene. This was subsequently fragmented and isolated by water courses from the Conwy valley, with following phases of glaciation accentuating these features so that ice from the north was divided to the south-east and south-west (Whittow & Ball 1970).
Generally, the drift deposits of the Great Orme are considered to be the result of glaciation during the Quaternary period, comprising a number of phases of advancing and retreating ice sheets. Through the north Wales area two source areas for the glacial deposits are identified, one from the south, centred around Snowdonia, producing blue-grey material, the other from from the Irish Sea direction to the north (Whittow & Ball 1970). This northern ice brought with it Triassic sediments, producing the characteristic reddish-brown colour of the drift that skirts the Great Orme, particularly to the south west from the West Shore to Llys Helig Road, forming cliffs at least 50m high. Deposited material comprises predominantly sandy clays with mixtures of pebble, cobble and occasional boulder size clasts. They conform to the description of tills with some horizons of glacifluvial origin.
Typically the Irish Sea drift overlies earlier north Wales deposits, although this is not a simple relationship and further study is necessary to establish their inter-relationship (Addison et al 1990). For example a section exposed in the Ty-Gwyn mine adit near to the Llandudno pier gates displays an interleaving of the two deposits, suggesting some contemporaneity to the events (Barker 1993), however this interpretation is not conclusive as no detailed petrographical examination has been made. Overall, the till deposits are thought to relate to the Devensian stage of glaciation, with much of those presently exposed probably deposited during the later part, around 18,000 years ago. Subsequent weathering denuded the tills and caused frost shattering of exposed rock to produce periglacial deposits now present on the headland. Exposures of frost shattered limestone were observed in the Ty-Gwyn mine adit and were sufficiently well cemented as to be self supporting.through the tunnel. This deposit continued over a distance of 50m being bounded by limestone to the north-west and glacial tills to the south-east. Similar cemented scree deposits can be seen above the beach to the north of the entrance toll gate of the Marine Drive. Further deposits and also areas of glacial striated rock were exposed around the Great Orme during the flooding and subsequent landslips that occurred in the summer of 1993. It is also likely that a proportion of the Postglacial deposits originated as loess.
It is difficult to establish how the Pyllau valley or hollow might have appeared prior to the first stages of early mining. We can assume that the surrounding contours of the limestone country rock are largely the same as they are now, with strata predominantly dipping into the area of dolomitised and mineralised ground forming a hollow of between 15-20m deep. This feature would therefore have provided a focus for surface waters to collect from the surrounding catchment, estimated to cover at least 260,000 sq m. This focus lies just to the south of Vivians shaft, and overlies the major north-west south-east fault that is assumed to pass through this area. It is therefore likely, that the accumulated surface and ground water may have contributed to the rotting of the dolomite and mudstones in proximity to the veins. Such an arrangement is also likely to have produced an underground karst drainage system in the unaltered limestones bordering the zone of dolomitisation. There is however, only one known area of natural cave passage within this zone, and comprises a partly clay-filled bell-shaped chamber (location 28) accessed from a prospecting cross-cut off the 132ft (40m) level from Vivians shaft. The large size (150mm) and shape of scalloping on the roof of the cavern would indicate that formational water flow was slow moving or stagnant and would suggest this feature is a solution chamber rather than the result of an active cave system. It is also interesting to note that this chamber has formed at the junction of the zone of dolomitisation with dolomite forming one wall while the remainder has formed in the limestone (Lewis 1993b). This clearly displays the differing solution characteristics of the two rock types with dolomite being virtually insoluble. It is possible that other chambers such as this may exist in this area, although present evidence from similar 19th century cross-cuts through barren limestone would seem to dispute this. The position of the chamber at location 28 also coincides with the limit of the dolomitisation vertically at around 90m AOD and also correspondingly the extent of both prehistoric and recent workings. No other workings have been identified below this area, even though Vivians shaft continues for a further 54m (39m AOD) where it is blocked by infill. The main areas of 19th century workings lie beneath the ‘Great Shale Bed’ (figure 17) or the 60m AOD level, but are situated 130m to the south-west beneath the fields of Pyllau Farm on the opposite side of the main fault through the valley.
An alternative or contributory factor to account for the solution chamber, is that this karst feature is the result of Interglacial/Preglacial activity, and formed when the water table was at an intermediate depth. This means of formation does not, however explain the existence of other larger solution chambers which were encountered by miners last century in workings below sea level. One of these was of an immense size and its contained water when the chamber was breached resulted in the flooding of a large area of workings (Williams 1995). The existence of solution chambers at these depths would suggest that the water table has fluctuated, at times falling to levels below the existing sea level. Alternatively, as suggested above, the formation of solution cavities or caves may be the result of a series of perched water tables linked through the system of mineralised fractures that form the mineral veins.
From the above observations it is speculated that the Bronze Age, and indeed the workings up to 1805 (Williams 1995), did not extend below the 40m from surface or 93m AOD level in Vivians shaft. It is also unlikely that an underground karst drainage system developed to enable ground waters to quickly drain from the zone of dolomitisation and copper mineralisation. Instead, it would seem that the permeable dolomitic ground could have acted as a reservoir for ground water behaving as a perched water table, which correspondingly aided rotting in the dolomites and mudstones. Water was likely to have been slowly released from the system along the north-south and east-west fractures and joints and probably more so from the main north-west south-east fault crossing the valley. This suggestion would also agree with the descriptions (Parry 1863) of dams found in the “old Welsh miners works” that were used to drain water from the workings (see section 2.5) and also from the descriptions by Lewis Morris in 1748 (Williams 1979) who describes the workings which are known to have not reached 40m below surface, as being “drowned out”. During the early part of the 19th century the shafts and workings were deepened and eventually linked with the Penmorfa adit so allowing free drainage to all areas above sea level. It can be assumed from the above that the Bronze Age workings were limited by the extent of mineralised ground and correspondingly the resulting drainage.
It is likely that intensive weathering during the warmer Tertiary period resulted in zones of oxidation and enrichment, with chalcopyrite altering to form prominent exposures of green malachite and to a lesser extent blue azurite. Because of the permeability of the host dolomite and period of time involved it is likely that these carbonate ores developed to great depths along the veins and other ore bodies. To what extent these ores were removed in glacial times is uncertain, however, from the hollowed form of the Pyllau valley it is quite possible that ice sheets may not have been able to remove these deposits completely and so remnant material may have survived as the first conspicuous indicator of the presence of copper ores. It is likely that these ores would also have affected vegetation in the immediate area, with stunted or decreased growth due to the increased copper content of derived soils. Other plants known as metallophytes, with a tolerance to these derived soils may also have been present. It is possible that early man may even have been aware of these plants, as indicators of metal ores buried beneath the surface. Such plants can be found scattered over the present day spoil tips and include thrift (Armeria maritima), spring sandwort (Minuartia verna) and sea campion (Silene vulgaris ssp. maritima). During Postglacial times it is not unreasonable to assume that continuing surface weathering and oxidation of chalcopyrite bearing dolomite produced further carbonate ores which would have added to the already exposed coloured ores.
By studying the surveyed workings and recorded geological sections it has been possible to conclude that all the known prehistoric workings at the Great Orme are exclusively confined to areas of dolomitised limestone. Two main controlling factors governing the extent of dolomitisation and ore mineralisation have been identified. The first factor is structural control, where the predominant north-south trending fractures and to a lesser extent the minor east-west fractures have provided routes along which mineralising solutions were able to pass. This process is likely to have occurred in two stages, beginning with the dolomitisation of the limestones and other calcareous lithologies adjacent to the fractures. This was then followed by or possibly accompanied by the hydrothermal emplacement of copper bearing minerals, principally chalcopyrite within exclusively the dolomitised lithologies. The reason for this being the increased porosity of dolomite over limestone and also increases in small scale fracturing associated with the primary fractures, both of which provided ideal areas for the crystallisation of the chiefly copper ores. The fractures mainly occur as single features inclined to the west at 80-900, however, in certain areas, particularly the wider worked out features (trench veins-stopes) twin sets of fractures are noted (figure 23). When viewed closely, the rock mass confined by the fractures is seen to be highly fractured and may be described as a fault breccia. Such fracture zones provide some of the best areas for copper ores to have been deposited, which is demonstrated by the wider worked veins centred around and to the north of Vivians shaft (plate 3).
The second and combining factor to structural control is the selective nature of the dolomitisation on differing rock lithologies. As described earlier, dolomitisation predominantly affects only calcareous lithologies, with the process becoming more pronounced as the calcite mineral component increases. This was clearly observed during the field studies with limestones displaying a greater degree of dolomitisation and corresponding ore minerals than the more argillaceous limestones and calcareous mudstones. The size of the calcite crystals constituting the limestones is also important in affecting the extent of dolomitisation, with coarser grained limestones-calcarenites displaying more pronounced alteration than the finer grained limestones-micrites. This is due to lesser amounts of energy being required to replace calcium by magnesium in larger calcite crystals than in smaller ones.
Differences in lithology can further accentuate the process of dolomitisation where more impervious or non calcareous rock types occur. This is most commonly seen where mudstones or shales overlie limestone strata, particularly those of a more spary type with larger component calcite crystals. It appears that these more impervious strata have restricted the upward migration of the mineralising solutions along the fractures, and instead mineralisation has extended laterally beneath these confining horizons. This arrangement of dolomitised limestone and overlying mudstone is the most common feature seen through both the prehistoric and more recent workings. Typically the base of mudstone units forms the ceiling of passages and stopes, a notable example being the Pyllau Mudstone which has controlled areas of mineralisation and corresponding workings off Roman (locations 13-15) and Owens shafts. The same unit exposed in the opencast has also controlled and resulted in a large area of ore bearing ground, which is thought to have been worked in early times and to have formed the existing feature. The Craig Rofft Sandstone has behaved in a similar way to the mudstones in affecting mineralisation, and is responsible for the production of the large stope (location 18) and other early workings accessed off Vivians-Treweeks shafts (locations 33 &35) and Owens shaft (location 22 &23).
Field observations and limited documentary evidence indicate at least two phases of ore mineralisation. The primary phase is considered to be related to the Variscan or Hercynian orogeny during the Permo-Triassic period, which is responsible for the dolomitisation and principal chalcopyrite ores and very minor related sulphides of galena and pyrite. Ixer (pers comm) considers there may also be a secondary but minor phase of primary mineralisation, which has produced the galena-chalcopyrite assemblages confined to the finer mudstone lithologies observed in the northern opencast.
The second phase of mineralisation is a consequence of the oxidation of the primary sulphide ores during the Tertiary and Quaternary periods. Resulting ores include chiefly malachite and minor amounts of azurite, with rarer tenorite and secondary sulphides. The oxidation of the sulphide ores is also considered to be major contributory factor for the rotting or dedolomitisation of the ore bearing dolomites. The most likely explanation is that decreases in pH (more acidic) due to the oxidation of sulphide material have attacked calcite cements and component grains within the dolomites and dolomitised limestones. The resulting rotting and softening of the ore bearing material would therefore have made such a deposit very easy to mine with simple tools. This is probably the single most important factor why early miners were able to remove copper ores to the extent which is seen today at the Great Orme mines.
The logged strata through the mine complex also represent the only exposed sequence spanning the boundary between the Great Orme Limestones of Asbian age and the Bishops Quarry and Summit Limestones of Brigantian age. This boundary represents a transgressive phase of deposition with a gradual deepening of marine conditions resulting in the more argillaceous and alternating limestone-mudstones lithologies as displayed by the Bishops Quarry Limestones. Until recent it was not known whether the boundary was transitional or well defined, due to the lack of surface exposure. The field work presented here will therefore be important in deciding the position and type of boundary change between the Asbian and Brigantian, which will be the subject of a forthcoming paper.
