The Geology of the Poldark Mine and its surrounding area.

For Advanced Students

N. G. LeBOUTILLIER BSc., PhD., MCSM., EurGeol., CGeol., FGS.

THE GEOLOGY OF POLDARK MINE: PART THREE – MINERALISATION OVERVIEW.

Introduction.

The Cornubian Orefield is the most intensely mineralised belt in the British Isles and it has been exploited continuously for over 3000 years. Early legends of visits by Phoenician traders remain unsubstantiated, but later Greek accounts of trading for tin at the ‘White Hill of Ictis’ (St Michael’s Mount, which 2500 years ago would have stood out in a flat-lying wooded plain close to the coast) in the ‘Cassiterides’ (Tin Isles) are generally accepted. Julius Caesar, writing in the first century B.C, speaks of tin production in Britain and it is likely that the mineral wealth of the island (and its strategic position on the Irish gold trade route) was an added incentive for the Roman invasion in 44 A.D.

Most of this early tin production came from placer deposits. Surface exposures (particularly on the coast) of lodes were also worked, principally by opencast methods or by driving on lode into cliffs. These ‘coffin’ workings are still visible on the coast around St Just [SW370313], close to the workings of Geevor Mine [SW375345] and at Botallack [SW363335]. Underground mining seems to have started around the 12 th century. The granting of royal charters in 1201 and 1305 (setting up the Stannary Parliament, with its independent taxation, legal and control systems) was of major importance to the tin trade, granting miners special privileges with regard to land access, prospecting and mineral extraction. The granting of these rights saw in a major phase of prospecting across the south-west, initially from the alluvial workings on the moorlands, out into the lowland valley floors and the discovery of lode outcrops from steam exposures and exploratory trenches. From perhaps the Roman period until the early 13 th century, Dartmoor was the principal tin producing area in the orefield and during the latter part of the 12 th century it became the main source of the metal in Western Europe. During the early part of the 13 th century Cornwall took over as the major producer (Cornish tin production rose to double that of Dartmoor, at around 500 tonnes per annum), a position it maintained until the closure of South Crofty Mine [SW668412] in 1998. Dartmoor’s production steadily declined (reaching a peak of 285 tonnes in 1515) until the mid 18 th century saw a revival of its fortunes.

Early workings, such as those in the river valleys around Poldark Mine, were chiefly of alluvial and eluvial placer deposits, known as ‘tin streaming’. The tin-bearing sands and gravels were dug out from the riverbed and banks and the heavy cassiterite separated by sluices and crude sediment traps. Underground or opencast workings, prior to the introduction of gunpowder, were worked by a combination of firesetting and manual extraction with picks and chisels. The cassiterite concentrate or rough ore was then taken to a smelter (known as a ‘blowing house’) to be further refined, in the case of the rough ore, by crushing (using water-powered sets of stamps) and hydraulic separation, and was then smelted. The molten metal was often poured into granite ingot moulds, some of which still survive.

The introduction of gunpowder blasting by Bohemian miners (where they worked the tin deposits of the Erzgebirge) during the reign of Elizabeth I saw the rapid development of underground mining in Cornwall. Initially this was for tin, but the manufacture of brass and the use of copper in the national coinage during the 18 th century saw this metal assume prime importance in the orefield. The discovery of large deposits of copper in the Camborne-Redruth and Gwennap districts in the late 1600’s spurred on a further phase of exploration throughout the county, the chief focus of which was now the discovery of lodes, as the alluvial deposits were becoming increasingly exhausted. As production increased, the main barrier to extending the mine workings at depth was the position of the water table and the need to pump out excess water to keep the workings dry. This was overcome by the development of horse-driven, water-powered, and later, steam-powered pumping engines. The use of steam power (building on the work of Newcomen, Boulton and Watt and Trevithick) revolutionised the mining industry and allowed deep mining to expand rapidly during the 19 th century. Steam engines were used to pump water, haul up ore (and, later, men), transport materials and drive sets of stamps. Cornwall became not only a major mining centre, but also a test-bed of new industrial and engineering ideas that were exported across the globe.

The 19 th century was the heyday of Cornish mining. After a period of closure in the 1790’s (when cheap copper ore from Parys Mountain on Anglesey almost wiped out the Cornish copper industry) the mines proliferated and during the century over 2500 mines were operated in the orefield as a whole. Copper and tin were the main products of these mines, but considerable tonnages of other metals and minerals were produced (see Table 2 and Figure 12), particularly iron, lead, arsenic, manganese, zinc and tungsten. During the 1860’s copper mining reached its peak with production reaching 15,500 tons of metal; Britain supplied around 40% of world consumption (and was the largest producer). Production of tin reached a peak in 1870 with a little over 10,000 tons of metal. A ruinous fall in metal prices in 1866 (brought about by the discovery of new copper deposits in Michigan (U.S.A) and Chile; and tin deposits in Malaya) saw the mining industry go into a rapid decline with the closure of many mines and the emigration of thousands of miners and their families to the opening mining fields of Australia, South Africa, Mexico, North and South America and the Far East.

World metal prices became increasingly volatile and the industry in Cornwall and Devon became caught in a cycle of ‘boom’ and ‘bust’ with fewer mines surviving each crash in prices. Relatively few mines survived until 1900 (when tin production had fallen to 2000 tons metal per annum; and copper to around 50 tons metal per annum) and after a brief rise in metal prices in the early 1900’s saw prospects improve, the First World War and the loss of labour brought many of the surviving mines to the brink of ruin. Casualties in the immediate post-war years included the famous Dolcoath Mine [SW660401] in 1920, and Carn Brea and Tincroft mines [SW667405] in 1921. By the Second World War only South Crofty Mine and East Pool Mine [SW673415] remained in the Camborne-Redruth

Figure 12. A map of the orefield of South-west England (after Dunham et al., 1978).

Table 2. Estimated total mineral and metal production from South-west England. After Dines (1956), Alderton (1993) and South Crofty PLC (1988-1998).

District with Geevor Mine at St Just, Castle-An-Dinas wolfram mine [SW946623] north of St Austell and Cligga Mine [SW738538] at Perranporth.

East Pool Mine and Cligga Mine closed in 1945 and Castle-An-Dinas closed in 1959. A rise in metal prices during the 1970’s (which saw tin eventually reaching over £10,000 per tonne) saw renewed prospecting in the South-West and the reopening of Wheal Jane Mine [SW771427] near Truro and the opening of Wheal Concord [SW723458] at Blackwater and Wheal Pendarves [SW645383] near Camborne. During this period the tin price was stabilised by the International Tin Council (ITC), formed by the main tin-producing nations, buying and selling metal on the London Metal Exchange to keep the price as high as possible. When Brazil and China (non members) refused to be bound by any quota agreements and flooded the market with tin metal in October 1985, the ITC Buffer Stock Manager was unable to buy all the metal and ran out of money. Its trading was suspended on October 24 th and the price fell overnight from £8,140 per tonne to £3,300 per tonne. Wheal Concord and Wheal Pendarves closed in 1986. Geevor mine managed to survive, in a much reduced form, until 1991 and also in that year Wheal Jane closed. This left South Crofty Mine as the sole surviving mine in the Cornubian Orefield. It was hoped that the tin price would rise, but it fluctuated between £2,900 and £4,300 per tonne (averaging £3,400); with production costs of around £4000 per tonne the mine was continuously losing money, despite every effort to minimise costs. The mine eventually closed on March 6 th 1998, bringing to an end some 3000 years of mining history. The mine was purchased and unabandoned in 2001; at the time of writing (2004) mining has not yet resumed and the venture faces an uncertain future.

Overview of Mineralisation.

Most of the mineralisation present in the orefield can be directly linked to the granite batholith in some way, although some deposits clearly pre-date the granite and a variety of syngenetic sedimentary and SEDEX origins have been ascribed to these. Deposits falling into this category include the manganese deposits of East Cornwall and West Devon and the stratiform Pb-Sb-Cu deposits of the Wadebridge district in North Cornwall. The Sn-W-As-Cu mineralisation for which the region is famous occurs in a variety of forms, but principally in high-angle fissure veins (lodes) in or close to the granites.

Mineralisation across the Cornubian Orefield can be divided into the following, chronologically arranged, groups: (1) pre-granite orebodies of sedimentary/sedimentary-exhalative type; (2) syn-granite intrusion orebodies – skarns and pegmatites; (3) early post-granite intrusion orebodies – greisens and sheeted vein complexes; (4) main stage polymetallic orebodies – Sn-Cu-As-Zn-Pb lodes and carbonas, etc; (5) late post-granite mineralised (Zn-Pb-Ag-Co) and unmineralised fissure veins – crosscourses. The last two occur at Poldark Mine and are described in more detail below.

Main-Stage Lode Mineralisation.

The typical lodes of the province are steeply dipping (most >70 o) fracture-infill veins, which are concentrated along the axis of the batholith and are closely associated with elvan dykes. The lode system as a whole has produced almost all the metallic output of the orefield and has produced not only tin and copper, but a range of metals including tungsten, iron, lead, zinc, silver, etc (see Table 2). The origin of these metals is still in debate; the tin (and tungsten) is likely to have been derived by fractionation, from tin-rich sediments or protolith at the point of anatexis, though some authors point to the possibility of derivation from the mantle. Though this seems less likely than a crustal source, researchers have found traces of mantle helium in fluid inclusions from the orefield, attesting to some mantle involvement in mineralisation. The origin of the Cu-Zn-Pb mineralisation is thought to be due to a combination of xenolith assimilation and hydrothermal leaching of basic rocks (and pelites); it has been calculated that the volume of basic rocks and their copper content could easily supply the amount of copper extracted in the province.

Though on the scale of the orefield, the lode system is extremely complex; within localised areas a number of fairly simple relationships can be established. Mineralogically (as a rule) the lodes show decreasing complexity with depth. Close to surface they are truly polymetallic and may show a mixed oxide/sulphide assemblage that, in many cases, has been modified by supergene activity (see Figure 13).to give a large potential list of secondary minerals. These include secondary sulphides, hydroxides, oxides, sulphates, arsenates, carbonates and native metals; many of these were first described in Cornwall and the area has been the focus of professional and amateur mineral collectors for centuries.

Figure 13. A section through a typical Sn-Cu lode, showing the relative position of the gossan, supergene and primary sections and the zoning seen in some of the major structures in the Camborne-Redruth District (after Hosking, 1988).

Within this near-surface zone the ores of tin, arsenic, copper and zinc were worked (though often in separate stages, depending on the economics of the time); many mines also produced minor amounts of lead and silver (Dolcoath) and occasional U, Fe, Bi, Mn, Ni and Co (Wherry Mine [SW470294], Penzance) ores. In the Camborne-Redruth and Gwennap areas (as elsewhere) the lodes close to surface were dominated by copper mineralisation. In the supergene zone original simple sulphides (chalcopyrite, pyrite) were replaced by malachite, azurite, tenorite, cuprite and native copper. Rare secondaries, such as olivenite and liroconite, etc, are known from the Gwennap area and Porthleven area (where Cu and Pb ores occur together). Below the oxidised zone secondary chalcocite, bornite, enargite and covellite were deposited before passing back into primary sulphides below the water table. These very shallow (often < 50 metres) rich deposits were mined at an early date (1700 onwards) and the high financial returns gained were responsible for the proliferation of mining activity across the orefield during the 18 th century.

With increasing depth the zinc and lead mineralisation died away leaving a zone of simple sulphides dominated by copper and arsenic. As the granite/killas contact was approached tungsten became locally important, reaching its greatest development immediately below the contact (e.g. Rogers Lode, East Pool Mine). Below the contact copper declined and tin became increasingly important, and at depth (~500 metres from surface) cassiterite is often the sole ore mineral present.

This change from a simple oxide-dominated assemblage at depth, passing into mixed oxide/sulphide assemblages close to the contact and complex polymetallic assemblages at surface was the foundation for the theories of hydrothermal zonation formulated during the early 20 th century. However, the relationship between the various phases was not always as clear cut and within a single lode there is often evidence of a protracted history of mineralisation, brecciation, shearing and further mineralisation, that negates the idea of mono-ascendant fluids. Most lodes do not show a single continuum of pressure/temperature-controlled mineralisation, they show a series of punctuated events, with the later sections of the lodes showing lower temperature assemblages. In this way some lodes initially worked for copper may later have had the walls of the existing stopes reworked for their tin or tungsten content (e.g. the North Tincroft Lode of South Crofty Mine).

The gangue minerals associated with the ores also vary with depth and are also temperature dependent. At depth (associated with tin ore) the main gangue minerals are tourmaline (fine-grained, powdery to flinty, Prussian blue to dark blue) and quartz. At higher elevations, lower temperatures and in lower energy environments (in areas reactivated by further fracturing) this gives way to a chlorite-dominated assemblage (though initially in places still retaining a proportion of tourmaline) with quartz and fluorite. Lower temperature phases are dominated by quartz, siderite, fluorite, marcasite and rare calcite. While this trend to lower temperature mineral assemblages and lower energy environments over time and proximity to the surface is broadly correct, recent studies have shown that some shallow deposits can also be tourmaline-dominated and that some of the chlorite assemblages record violent brecciation events with clasts transported considerable distances. This again suggests a series of punctuated mineralisation events, utilising fluids from a variety of sources and under a variety of physiochemical conditions.

Many lodes show a complex interplay between tectonically-driven episodes of mineralisation and remobilisation of constituents by convecting hydrothermal fluids. This last particularly applies to copper and uranium mineralisation (pitchblende and coffinite are themselves a late-stage infill in some lodes, e.g., No4 Lode at South Crofty Mine), which, in many secondary phases, are highly mobile and readily dissolved. An environment in which a cyclical system of pressure/temperature changes occurs may see the deposition of the rare fibrous form of cassiterite known as ‘wood tin’, if the fluids are supersaturated with respect to tin.

Often in contrast to the mineralogy (particularly at depth) the structural history of many lodes is complex and shows a series of brecciation and shearing events responsible for depositing a variety of individual assemblages in the lode over time. Brecciation textures are common in lodes in the deeper workings of many mines and occasionally at surface. In the deeper workings of South Crofty Mine many lodes showed an early tourmaline (‘blue peach’)/quartz ± cassiterite breccia with a cassiterite/quartz cement (Plate 3). This was sometimes followed by other brecciation events, but was more often followed by further lower energy dilational episodes, giving the lode a banded appearance (some of these bands were, occasionally, microbreccias, emplaced within the lode), particularly along the hangingwall. Some of the reactivation episodes lead to the deposition of later chlorite-dominated assemblages, while other events lead to fine fracturing across the lode and the deposition of low temperature chalcedony-marcasite-siderite assemblages.

Plate 3. The Dolcoath North Lode, 380 fathom level, South Crofty Mine. The lode is predominantly composed of brecciated blue peach and quartz with minor (<1%) cassiterite. Some later lode-parallel shears carry minor fluorite and haematite. The wallrock adjacent to the lode contacts is irregularly tourmalinised. The width of the lode is ~1 metre.

This brecciated texture is due to hydraulic fracturing and explosive decompression. The majority of lodes appear to be extensional faults and would have communicated with areas of lower pressure, which were accessible during movement, along their dip-length. With the loss in pressure boron-rich (or silico-stanniferous) fluids, that had previously been building up pressure in the fluid reservoir, were suddenly released and travelled upwards as wallrock pore fluid pressures caused spalling off of fragments along the sides of the lode fracture. It is difficult to ascertain the amount of transport that took place; lode textures are fine-grained and indicative of very rapid nucleation and a number of clasts appear to have moved very little distance before being arrested in the crystallising fluid. Occasionally some clasts show evidence of entrainment (rounded, rolled clasts with ‘debris trails’ in their wake), but again the distance travelled cannot be quantified.

The same cannot be said of the ‘breccia lodes’ that outcrop (and subcrop), most commonly, in the Gwinear District southwest of Camborne, along the line of a buried granite ridge that appears to be an extension of the Carn Brea ridge between Camborne and Redruth. These bodies consist of rounded pebbles and cobbles of a variety of rock types (including granite, up to 1 metre in size, though the contact is some 750 metres below surface) set in a matrix of comminuted rock fragments of various sizes. The bodies are chaotic and disordered and in places the clasts (primarily slate) are so finely ground that the rock appears similar to a sandstone in texture, indicative of violent, high-energy emplacement. The walls of these fractures are often scoured and polished by the passage of material and small breccia fragments are found forced into cracks in the walls. Some of these breccia bodies carry (later, infilling) chlorite, cassiterite and chalcopyrite in the matrix and were mined from surface as early as Tudor times (e.g. Relistien Mine [SW601368] at Wall, near Gwinear). The lode at Trevaskis Mine [SW607378], nearby, is in excess of 10 metres wide and lies between intensely brecciated and silicified wallrocks of metadolerite. The ore consists of angular slate clasts cemented by chlorite and fine rock fragments. Within this are veins of quartz carrying a chlorite-arsenopyrite-chalcopyrite-chalcocite-cassiterite assemblage and also a chalcopyrite-sphalerite-galena assemblage that may be later.

Both Relistien and Trevaskis breccias carry clasts of internally brecciated elvan and other examples are also closely associated with elvan dykes. It appears that the elvans and breccia bodies are roughly contemporaneous; clasts of elvan moulded around other clasts found at Trevaskis suggest that still-plastic elvan was utilising the same fracture pathways as the breccia lode at close to the same time. In other examples host slates were brecciated before the intrusion of an elvan dyke. These fluidised explosion breccias appear to have formed under conditions of very high pressure where the system had instantaneous connection with areas much closer to surface than other lodes in the district. Rapid boiling and the development of a gas-fluid medium, similar to that seen in volcanic breccia pipes (e.g., Ardsheal Hill, near Kentallen, Scotland) lead to a violent explosive reaction with material entrained and blown up along the fracture. This appears to have been a largely barren episode, as in almost every case the mineralisation that accompanies these structures post-dates their emplacement. Some lodes preserve evidence that they originated as breccia lodes and were later reactivated during later phases of mineralisation, with large-scale replacement of original textures and materials.

Lodes (or later assemblages in a pre-existing structure) emplaced in a lower-energy environment typically show a banded appearance, due to repeated opening of the lode fracture. Some of these lode sections may be mylonised by fault movements, while others appear to be open-space dilational infillings with vugs and druses of crystals (and occasionally, as at Dolcoath Mine, pockets of carbon dioxide in sealed vugs). They may show a range of assemblages of various temperature/pressure characteristics, or may have been crack-sealed in a single mineralising event.

A second, later, sequence of lodes occur in many districts, which cut across and displace the earlier lodes. These later caunter lodes generally carry a lower temperature, mesothermal, assemblage (dominated by copper mineralisation) and strike E-W (in the Camborne-Redruth-Wendron District). Rotation of the stress field saw these lodes emplaced in a fracture set offset from the dominant lode trend by ~30°. Field evidence from South Crofty Mine and other locations around the northern margins of Carnmenellis show that while the main-stage lodes are typically associated with dip-slip or oblique dip-slip movements, caunter-orientation structures are associated with horizontal to sub-horizontal slickenlines formed by shear movements..

Some of the 050°-060° (dominant lode trend) lodes were also reactivated during the deposition of the caunter lodes. At South Crofty Mine segments of caunter lode orientation were opened up within existing lode systems (e.g. Roskear D Lode) and infilled with an assemblage dominated by low temperature quartz, earthy chlorite, haematite, kaolinite, fluorite and chalcedony. These ‘caunter jogs’ were economically barren and were left as pillar areas along the strike of the lode.

Irregularly shaped sub-horizontal or pipe-like replacement bodies sometimes occur at the junction of two, or more, lode structures. These are called carbonas and commonly consist of a fine network of veins, in altered granite, and bunches of ore. They reach their greatest development in the Lands End Granite and the Great Carbona of St Ives Consols is arguably the most famous. This body (10-20 metres thick by 230 metres long, dipping at 20°) carried cassiterite, copper sulphides and fluorite (a mineral not found in the lodes of this mine) in tourmalinised/chloritised/sericitised granite. The average grade of the ore was 1.5% Sn, which compares very well with the average grade in the lodes; most Cornish mines operated at grades of between 0.70% and 2%; at South Crofty Mine the average R.O.M grade was 1.5%, but varied up to 2.5%, while individual lode grades over short strike lengths could reach as high as 40% Sn.

The main-stage lodes throughout the province are accompanied by wallrock alteration of varying type and intensity. The most common types of alteration are tourmalinisation (the progressive replacement of chlorite, micas and feldspars by tourmaline, which may lead to the development of a quartz-tourmaline rock; in pelites replacement of phyllosilicates may be extensive and pervasive), chloritisation (replacement of micas and feldspar by chlorite, due to the influx of Fe and Mg in solution), haematisation (due to the breakdown of existing chlorite, although some textures appear due to primary replacement of feldspars and micas) and sericitisation (the replacement of feldspars by white mica). At South Crofty Mine wallrock alteration haloes could extend in excess of 3 metres from the lode in either direction and sometimes showed overprinting of one type of alteration (particularly haematisation after chloritisation) on another. Such alteration was often barren, but it was not uncommon for Sn grades in the wallrocks (these metasomatic haloes mirror the mobility of Sn, seen in the country rocks around the granite) to exceed that in the lodes and the alteration zone was often included in stoping patterns and formed a significant part of the material extracted. The impregnation of granite with cassiterite is not uncommon in the Camborne-Redruth District; the Great Flat Lode, south of Carn Brea, consisted primarily of tourmalinised/chloritised granite (known to the miners as ‘capel’) carrying cassiterite over widths of up to 5 metres around a narrow quartz leader vein.

The dating of the main-stage lodes has seen a revolution over the past decade. Halliday (1980) obtained dates for main stage Sn-bearing lodes of between 279±4 and 269±4 Ma. These dates were obtained from muscovite and orthoclase, using Rb-Sr methods, associated with various lodes across the Lands End and Tregonning-Godolphin granites. Using a previously published age of 295-300 Ma for the emplacement of the granite batholith, he envisaged a 20 million year hiatus between the emplacement of the granite and the onset of mineralisation. Between these two events, and intimately associated with the mineralisation, he placed the intrusion of the elvan dykes. This model became refined by later workers with a ‘second magmatic event’ (the emplacement of the elvan dykes) some 20 million years after granite emplacement, followed by main-stage mineralisation over a protracted period. This model was reinforced by later workers, who obtained Nd-Sm dates of 259 ± 7 and 266 ± 3 Ma for fluorites from South Crofty Mine and Wheal Jane respectively (though their samples were not paragenetically constrained, and came from a late stage during mineralisation).

This model was radically altered by Chen et al. (1993), who, in dating the granites and mineralisation, were able to show that each pluton of the Cornubian Batholith had its own discrete history of magmatism and mineralisation. They were able to show that the batholith was made up of discrete bodies, intruded between 293-274 Ma, and that mineralisation was diachronous across the orefield, instead of being related to a series of pan-province events; mineralisation and magmatism also overlapped, with mineralisation related to one magmatic pulse occurring (e.g. Carnmenellis) prior to later renewed granite magmatism.

Chen et al. (1993) give dates of 286 Ma for main stage lodes at South Crofty Mine, 272 ± 4 Ma for the lodes of the St Just area and 278 ± 6 Ma for the Sn lodes of central Dartmoor. This data indicates that mineralisation started in the Carnmenellis area some 3 million years after the emplacement of the early granites (10 million years before the emplacement of the fine-grained Boswyn granite in northern Carnmenellis) and was almost complete before the intrusion of the oldest (Zennor Lobe) of the Lands End granites at 274 Ma. The emplacement of the elvan dykes now appears to be related to later pulses of granite being tapped by extensional fractures (rather than the radiogenically-driven remelting of a single large intrusion), some of which were later utilised by ascending hydrothermal fluids.

Crosscourse Mineralisation.

A distinctive set of fractures which are orientated at roughly 90° to the main-stage lodes are known by a variety of local names (fluccans, trawns, guides), but are commonly referred to as crosscourses. In mining circles only mineralised structures (usually chalcedony with quartz and minor haematite, etc) are referred to as crosscourses (fluccans are gouge-filled structures), while those carrying economic mineralisation are called lodes. They played an important part in mining as they represented ‘easy ground’ to work in and were selectively mined (being considerably easier to mine than granite or hornfelsed killas) as crosscuts into the workings on main-stage lode structures. The lodes (which have produced a wide range of metals, including Pb, Zn, Ag, Ba, Sb, Co, Ni, Fe, Mn, Bi, and U) belonging to this group are patchy in their distribution (only occasional structures of this type occur in the Camborne-Redruth District, e.g. Cobalt Lode of Pedn an Drea Mine at Redruth), the most important areas being the Menheniot and Tamar Valley Pb-Zn districts.

The development of the crosscourses was controlled by pre-granite tension joints and wrench faults (many of these fractures appear to have a movement history that is pre-, syn- and post-granite emplacement, only becoming mineralised during the final stages of the development of the orefield), oriented NNW-SSE to N-S. In the St Just area the lodes trend NW-SE and the crosscourses run N-S to NE-SW. Crosscourses typically reach a few metres in width (but may range from ~1 cm to >100 metres) and often have dextral throws from a few metres to tens of metres.

Plate 4. A sub-vertical crosscourse; 290 fathom level, Main Lode drive, South Crofty Mine. The crosscourse is infilled with banded chalcedony, with minor haematite and earth chlorite, and shows evidence of repeated movements and dilation. Though predominantly wrench-style faults, the offset quartz floor is evidence of a vertical displacement component in the movement (metre rule for scale).

At South Crofty Mine a large number of chalcedony-filled crosscourses faulted the main-stage lodes (Plate 4). These were typically sub-vertical structures with a banded appearance, (due to repeated infilling over a protracted period) typically 0.5 metres in width (but commonly ranging from <1 cm to ~1 metre) and infilled with chalcedony, quartz and occasional fluorite, siderite, earthy chlorite and soft haematite; kaolinite and bitumen were rare accessories. They ranged from compact to open vuggy structures (the largest on 315 fm Pryce’s Lode drive measured some 4 metres high by 2 metres (maximum) wide) with quartz/ fluorite druses infilling vugs. Wallrock alteration was typically absent or confined to minor kaolinisation, with the exception of the Great Crosscourse which was heavily kaolinised. Movements were sometimes on the order of a few metres, but were generally <3 metres and often a few centimetres; both dextral and sinistral displacements were recorded with horizontal or sub-horizontal slickenlines.

In economic term the Pb-Zn ± F, Ba deposits of the Tamar Valley and Menheniot areas are the most important. These lodes carry galena, sphalerite, fluorite and quartz with minor barite, calcite and siderite; occasional Cu and Fe sulphides also occur. The deposits are often small and shallow (with the exception of the Pb-Zn-F lode at Wheal Mary Ann [SX294635] at Menheniot, which was mined to 600 metres from surface), though occasionally very rich.

The reactivation of the crosscourse fracture system was due to the onset of E-W crustal extension after the deposition of the main-stage lodes. The fracture system intersected the sedimentary rift basins to the north and south of Cornubia and fluids from these basins, charged with Pb, Zn, U, etc penetrated into the massif. Rb-Sr isotopic dating of fluorites from the Tamar Valley has demonstrated a mid-late Triassic age of 236 Ma; this date effectively signals the end of mineralisation in the province, with only minor hydrothermal remobilisation of U, etc taking place afterwards in the late Mesozoic and Tertiary.