Mineralisation in Cornwall - Lode Development, Structure and Geometry

Contents

Lode Geometry

Lode Development

Structural Controls on Mineralisation

Lode Formation and Development.

Lode Geometry.

Lodes in the Cornubian Orefield typically occupy near-parallel, steeply-dipping normal, reverse or strike-slip faults, with offsets up to a few decametres (Lucas andWilkinson, 1998). They range in size from 1 - 2 metres in width to >10 metres in width across Carnmenellis, occasionally reaching (e.g. Dolcoath Main Lode) widthsof over 15 metres (Taylor, 1965; Farmer, 1991; LeBoutillier, 1996). Lodes in the St Just area are generally much narrower, with widths between 0.45 - 0.60 metresbeing the average (Garnett, 1962).

Individual lodes may have strike lengths of several kilometres (the Great Flat Lode has a strike length of over 5 km) and dip lengths of many hundreds of metres(Dolcoath Main Lode was worked to a vertical depth of around 1000 metres from surface), though such structures tend to be exceptional.

Examination of mine sections and plans (Taylor, 1965) reveals the relationship that the structures with the longest strike length also have the greatest dip height (andalso width and, often, payability); thus lodes such as Dolcoath Main Lode, Highburrow Lode, Pryce's Lode, and Reeve's Lode (all in the Camborne-Redruth District)have strike lengths in excess of a kilometre and can be traced from surface for vertical depths of over 800 metres; they also lie both within killas and granite. Thesemajor structures contrast with other lodes that fall into two major groups: (1) lodes hosted within the granite that peter out just beyond the granite/killas contact(though this was never put to the test at South Crofty Mine, as it was policy to halt development once the contact was reached. Lodes at the contact were reduced inwidth, were uneconomic and were structurally poor, but were never developed to see if this was a temporary condition or if the lode returned to pay in the killas afterthe example of Geevor Mine); (2) those lodes within the killas that fail before the granite contact is reached. The former produced mainly tin ores (with tungsten andarsenic around the granite/killas contact) and the latter mostly copper ores, with some tin at depth and, occasionally, close to surface (Dines, 1956).

The majority of lodes are not simple planar features; in section (see Figure 6) they can be seen to be curved bodies occupying listric faults, which tend to flatten in dip(often from near vertical at surface or highest point up dip) with increasing depth (e.g. Dolcoath Main Lode, the No8 and No4 lodes of South Crofty Mine). Manylodes also branch up and down dip, forming horsetail geometries, particularly in the granite.

Figure 6. Long section through New Cook's Kitchen Shaft, South Crofty Mine (from original by Nick LeBoutillier).

On a smaller scale, irregularities such as rapid changes in dip and strike (or both) were responsible for the selective opening of lode segments during normal or reversemovements on the fault plane. This resulted in the formation of low pressure zones (Garnett, 1961, 1966a; Taylor, 1966) in the open spaces created, where rapiddeposition of minerals could take place (see Figure 7). Collins (1912) noted that the richest sections of a lode were usually its steepest. This was also seen on lodes,such as Dolcoath South Lode, at South Crofty Mine, where steep sections of the lode were very rich and values died away rapidly following a change to more gentledips (which often took place at a sharp, angular bend in the lode). This scenario is consistent with open space formation during normal (extensional) faulting; if themovement is strike-slip or oblique dip-slip, then changes in strike can also be important as sites of open space creation. The position of these open space sites on

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irregular fault planes was critical in the economic deposition of cassiterite and other metallic minerals (Taylor, 1966) during the various phases of mineralisation.

Figure 7. The effect of fault movements on fracture width (after Garnett, 1966).

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Lode Development.

A number of structural studies of lode formation were produced in the 1960's (Garnett, 1961, 1962, 1966a, 1966b; Taylor, 1965, 1966), but it was Moore (1975) whocreated the first integrated account of the province as a whole. He argued that the lode and elvan dyke fissures were propagated in a regional stress field, withconsiderable local horizontal anisotropy, by internal fluid overpressures, generated in the still-fluid cores of the major plutons, during the later stages of magmaticemplacement and crystallisation. In his model (see Figure 8) he used pre-granite faults, joints and fractures in the granite carapace and cover rocks (generated by fluid'pressure cells') as channels for both magma and mineralising fluids. The principal stress axes remained in approximately the same orientation during successiveepisodes of brittle deformation, but the principal stresses were interchanged many times during the evolution of the lode-dyke system. In his model the regional stressfield controlled batholith emplacement and the generation of the E-W and NW-SW fracture systems; he explained the 'radial pattern' of lodes in the St Just area of theLands End Granite in terms of the internal hydraulic pressures overcoming the regional stress field.

Figure 8. Moore's fracture geometry models for (A) the porphyry dykes, and (B) main-stage lodes, based on his 'fluid pressure cell' model (after Moore, 1975).

Each major pluton was mechanically independent, with some overlap of individual stress fields, leading to the development of superimposed and curvilinear


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structures. Moore envisaged failure taking place on the flanks of the plutons, rather than across the roof (thus explaining the 'emanative centres' of the northern flankof Carnmenellis and Land's End) and also the contemporaneous development of normal faults at higher levels and oblique and strike-slip faults on the flanks of theplutons.

Halls (1987, 1994) studied the mechanics of lode formation and was able to distinguish two distinct lode systems on the basis of structural, textural and mineralogicalevidence. In his findings he took the opposite view to Collins (1912), who thought that lode formation was a passive process, with gravitational collapse andrelaxation of pressure the main forces responsible. Halls looked at the early W-As veins and the main-stage Sn-Cu lodes, which had previously been describedtogether as a single group in the paragenetic frameworks of previous authors. He was able to show, on the basis of cross-cutting relationships, that the W-As 'greisen'veins were of an earlier generation, but was also able to show that they formed under very different conditions.

During the later stages of crystallisation of the granite magma, incompatible elements were selectively partitioned into the residual fluids (see Figure 9). These fluids(water, halogens, metallic elements, boron, CO2, etc) eventually formed reservoirs under areas of positive relief in the pluton roof, beneath a carapace of already

sub-solidus granite.

Figure 9. The evolution of vapour pressure relative to confining pressure and declining temperature in a crystallising volatile-rich granite. The path B-C1-D1 is that followed by a plutonicbody in which the confining pressure remains greater than the evolved vapour pressure. Deuteric alteration will occur, pervasively altering the host rock, but the fluids will eventually be

dissipated by diffusion. The path B-C2-D2 is that taken when the granite body is emplaced at shallower levels and the fluids are released in pulses to form pneumatolytic veins of greisen

type (after Halls, 1987).

The chemical potential energy stored in the water-saturated magma is converted to PDV expansive work as the hydrous fluid exolves and fluid pressures in the rockincrease. The fluids, as they begin to collect and silicate minerals crystallise, exert an increasing vapour pressure on the system. If the granite body is emplaced at adepth where the confining pressure is never exceeded by the vapour pressure (or vapour pressure is low), then the fluids will eventually be dissipated by diffusion(Halls, 1987) into the surrounding rocks, giving rise to pervasive deuteric alteration (e.g. Cameron Quarry [SW701498], St Agnes; Hosking and Camm, 1985). If,however, the granite body is emplaced at shallower levels, then the vapour pressure may exceed the confining pressure (lowest principal stress) and the tensilestrength of the rock (Pint >= s3+T) giving rise to fracture propagation and fluid release.

This release of volatiles took place into swarms of autogenously generated tensile hydraulic fractures (Halls et al., 1999, 2000). The segregating magmato-hydrothermal fluids were saturated with the ore and gangue minerals that form the typical greisen (quartz-muscovite ± wolframite ± cassiterite ± stannite ± feldspar ±tourmaline, etc) veins of the province. These veins are characterised by their parallel arrangement (see Figure 10) and distinctive envelopes of alteration(pneumatolytic reactions, at temperatures between the granite solidus and 280°C, outside the field of feldspar stability, lead to the replacement of plagioclase andK-feldspar by characteristic muscovite-quartz ± tourmaline ± topaz assemblages).

Figure 10. The formation of parallel greisen veins where Pint >= s3+T and Pfluid wallrock < Pfluid fracture (after Halls, 1994).

Textural evidence (Halls, 1987) shows that most of these greisen veins opened in a tensile mode. The conditions necessary for the formation of fractures of this typeare that the least effective principal stress must equal the tensile strength of the rock (Halls et al., 1999, 2000) and that the differential stress must be small enough forGriffith extensional failure to occur. This can only take place (forces in the upper crust are typically compressive, with s1-s3 increasing with depth) where pore fluid

pressures, acting against the lithostatic stress, reduce the compressive stresses to a point where the failure point is reached and hydraulic fracturing occurs (see Figure11).


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Figure 11. The effect of increasing fluid pressure on the state of compressive stress in a rock. Two states of stress are depicted by Mohr circles (i) and (ii). In stress state (i) the differentialstress is large and the effect of increasing fluid pressure will bring it into contact with the failure envelope in the shear domain. In stress state (ii) the differential stress is small and

increasing fluid pressure will bring it into contact with the failure envelope in the tensile domain, resulting in the formation of extensional hydraulic fractures (from Halls et al., 2000).

When the fractures form, they will be aligned normal to s3 (the preferential direction of opening) and form a series of parallel veins, providing that the differential

stress >0. If the differential stress =0, the stress is hydrostatic and stress across all planes is equal; with no preferred direction of opening, a randomly-oriented set oftensile veins will result, which may give rise to a stockwork and/or brecciation (see Figure 12). If the differential stress increases to the point where a mohr circlewould intersect the Navier/Coulomb section of the failure envelope, then shear failure will occur. Some greisen veins show a hybrid extensional-shear mode ofopening (Halls et al., 2000) as this condition is gradually approached.

Figure 12. The formation of extensional fractures. The two stress states illustrated will give rise to parallel fractures, normal to s3 (a) where the differential stress is relatively large, or

randomly-oriented fractures (b) where the stress is hydrostatic (the Mohr circle is reduced to a point) and the differential stress is therefore zero, with no preferred direction of opening(from Halls et al., 2000).

In Cornish examples of greisen vein systems (e.g. Cligga Head, St Michael's Mount) it is not possible to see the full 3D extent of the vein system. Halls et al. (2000)report on Chinese exposures where this is possible. The individual veins in the array pinch out in all directions and take on the form of disc-shaped tensional fractures(within a zone bounded by points where stress conditions no longer allowed the fractures to propagate - in an isotropic rock the vein system would lie in a spheroidalor ellipsoidal zone near the roof of the intrusion) and it is therefore likely that the Cornish examples have a similar morphology.

Many Cornish greisen veins show textures indicative of single-pass infilling (each vein acting as a closed system in its own right), over time, with rate of mineralgrowth < rate of fracture opening, but others show repeated infilling due to 'autogenous hydraulic pulsation' (Halls, 1987, 1994). The original fracture forms in themanner outlined above, with fluid pressure in the fracture > pore fluid pressures in the wallrocks. Crystallisation crack-seals the fracture until internal fluid pressuresin the volatile reservoir again reach the point at which failure occurs. The weak point occupied by the original vein is reopened and the fracture is able to propagate toa higher structural level, still bound by tensile conditions (see Figure 13).


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Figure 13. The formation of extensional hydraulic fractures by autogenous pulsation (after Halls, 1994).

This process may be repeated a number of times until internal fluid pressures finally fall back or stress conditions change. This staged opening/reopening of veins canallow different parageneses to be present in adjacent veins if they formed at different times. Abrupt changes in the physical and chemical conditions prevailing in thefracture system are unlikely and this is reflected in the gradual changes in the mineralogy of individual veins, where zoning is present, which is in marked contrast tothe situation seen in the main-stage lodes.

Where the two occur together, the main-stage lodes post-date the greisen veins. The main-stage lodes acted as channels for more evolved Sn-B fluids from deeper inthe granite, than those seen in the greisen veins (Halls, 1994). They typically occupy faults and the early stages of paragenetic evolution appear to have beendominated by rapid fracture dilation related to seismic events. Another factor in their development is the large amount of boron in the residual fluids. With thecrystallisation of tourmaline, the sudden loss of water solubility saw a corresponding rise in vapour pressures that rapidly overcame confining pressures. Thepropagation of these fractures saw them tapping into areas of lower pressure (in the same way that dilated fault planes communicated up-dip with lower pressureregimes); the sudden catastrophic loss of pressure in the system lead to explosive decompression and hydraulic decrepitation (Halls and Allman-Ward, 1986;Allman-Ward et al., 1982) as pore fluid pressures in the wallrocks suddenly greatly exceeded internal fluid pressures, resulting in (if the tensile strength of the rock isexceeded) the spalling off of segments of the wallrocks along the fracture (see Figure 14).

Figure 14. The formation of hydrothermal breccias by explosive decompression (after Halls, 1994).

The sudden pressure loss and decrepitation lead to the production of much fine, comminuted debris and irregular clasts of wallrock and pre-existing lode infill. Ratherthan develop into a fluidised system, with rapid transport of material up the conduit, textures (microcrystalline intergrowths of tourmaline, quartz and cassiterite,which form the matrix for the hydraulic and cataclastic breccias) suggest very rapid adiabatic crystallisation, with the majority of clasts 'frozen' close to their point oforigin (Halls, 1994).

At South Crofty Mine such textures were very common in the tourmaline-dominated 'blue peach'-bearing lodes at depth (below 245 fm level). A number of lodesshowed the development of hydraulic breccias, with a quartz-cassiterite matrix cementing earlier fine-grained tourmalinite clasts. Occasionally, inherited clasts ofwolframite could be observed where a main-stage lode had reactivated the lode fracture previously occupied by an earlier quartz-feldspar-wolframite assemblage.Some lodes showed more than one episode of breccia formation, initiated by movements along the fault plane hosting the lode. Later, lower-energy, banded, dilationalinfillings were also common, especially on the hangingwall of the lodes, marking the shift to a much less energetic, less violent form of deposition over time.

The formation of hydrothermal breccias reaches its most extreme in the case of breccia pipes (Goode and Taylor, 1980; Halls and Allman-Ward, 1986; Allman-Wardet al., 1982; Taylor and Pollard, 1993). These features originate when a deep-seated reservoir of boron-rich fluid undergoes catastrophic decompression. At WhealRemfry, the sudden decompression (at depths of 2.6-3.8 km) lead to massive decrepitation of the wallrocks, fluidisation and transport of material from depth,implosion of material from above and near instantaneous crystallisation of the microcrystalline tourmaline matrix (Halls, 1994). Similar conditions probably gave riseto the breccia lodes of the Gwinear district and elsewhere (Goode and Taylor, 1980), with the sudden decompression of large, super-pressurised, deep-seated fluidreservoirs by fracture propagation or fault activation.

With the decline in fluid pressures over time, and at higher structural levels, the textures formed during hydraulic decompression were replaced by banded, dilationalinfilling (with occasional mylonites produced during faulting) and later open-space infilling (characterised by open, vuggy textures).


Structural Controls on Mineralisation.

The lodes of the orefield are not subject to a uniform set of controls. They have complex relationships with the granite contact, elvan dykes, caunter lodes,crosscourses and each other, which may be contradictory, even within very small areas (Phillips, 1814; Thomas, 1819; Carne, 1822; Henwood, 1843; Collins, 1912).


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The granite contact is important as a structural control in that it marks the boundary between the fracture/joint sets present in the granite and those present in theoverlying killas, which were exploited by the lodes during mineralisation. In the Camborne-Redruth District, the number of lodes that span this boundary are relativelyfew, and those that do are the 'major' long-strike/long-dip structures, such as Pryce's Lode or Dolcoath Main Lode. Most known lodes are only developed in the granite(tin-bearing) or in the killas (copper-bearing), though they share similar strike trends and dips. This is probably due to the differing rheological properties of the rocksin response to the regional stress field and internal thermal and fluid pressure stresses within the cooling granite (Jackson, 1979).

The granite contact was also seen as a thermal barrier by Davison (1921, 1927) and Dewey (1925), and while it is true that hypothermal W-Sn mineralisation isclosely associated with the granite (most production came from within 1 km of the contact; Jackson et al., 1989), Dines (1934, 1956) was able to show, at GeevorMine, that the tin zone dips at a shallower angle than the contact itself and payable extensions to lodes (Garnett, 1966b) in the granite pass out into the killas (Figure15).

Figure 15. The relationship between the granite/killas contact and the economic tin zone at Geevor Mine, Pendeen. The lodes passed through the contact without disturbance, loss of

grade or change in mineralogy, apart from cassiterite content (dependent on position relative to the boundaries of the tin zone). The seaward pitch of the tin zone saw several mines in theSt Just District follow lodes for considerable distances beneath the sea (after Dines, 1956).

A similar relationship between the granite contact and the economic tin zone was described in the Camborne-Redruth district (Llewellyn, 1946) and at South CroftyMine (LeBoutillier, 1995), with the tin zone pitching, at a shallow angle, to the north and west, suggesting that granite/killas temperatures, around the contact, hadlargely equilibrated by the onset of main-stage mineralisation.

Within the killas, the deposition of ore minerals was affected by the lithology of the host rocks (Collins, 1912; Alderton, 1993). Lode fractures that passed througharenaceous units rarely saw economic mineralisation deposited within them, while the same lode passing through pelitic host rocks was often mineralised. The classicexample of this type of behaviour is shown by the long section of Main Lode in the Chiverton Mines (see Figure 16), near Blackwater [SW734463].

Figure 16. Lithological control on mineralisation at the Chiverton Mines. The section is drawn in the plane of Main Lode; the lack of stoping in the sandstone units reflects the barren

nature of the lode within beds of this lithology (after Alderton, 1993).

The relationship between lodes and elvan dykes presents every possible permutation (Hosking, 1964, 1988; Henwood, 1843; Collins, 1912). Lodes may intersect (butnot fault) dykes, may fault dykes, may be faulted by dykes, may be deflected by dykes, or share a common hangingwall or footwall with a dyke (e.g. Wheal JaneMine, near Truro). Mineralisation may be diminished on intersecting a lode or may swell, become richer, or form ladder veins and stockworks (see Figure 17). Theclose spatial and temporal association of the lodes and elvans shows that mineralisation and elvan magmatism were broadly contemporaneous and diachronous.


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Figure 17 Relationships between main-stage lodes and elvan dykes (after Hosking, 1988).

At South Crofty Mine, in the North Pool section, an elvan dyke was found to cut reactivated Sn-W replacement bodies and a main-stage Sn lode (No:6N Lode). Inanother example an elvan dyke, refracted around the granite contact (Taylor, 1963) on the 260 fm level, was host to a series of mineralised stringers (an up-dipcontinuation of the No:9 Lode) that petered out on passing from the dyke into the killas above.

At Wheal Jane and Mount Wellington mines (Rayment et al., 1971; Kettaneh and Badham, 1978) near Truro, mineralisation is intimately associated with the footwallcontacts of elvan dykes. Much of the Sn-Cu-Zn mineralisation has been 'ponded' or trapped under the footwall of the 'B' Elvan and occupies a shear zone along themargin of the dyke (see Figure 18).

Figure 18. A section through the B Lode of Wheal Jane Mine. Polymetallic, multi-phase mineralisation has been impounded under the footwalls of the A and B elvan dykes, via feeder

lodes and footwall-parallel shear zones (after Rayment et al., 1971).

Both the mineralisation and dykes were viewed as infilling fractures generated by the intrusion of the granite batholith, but Cotton (1972) suggested that the hostfractures may have been pre-granite in origin and formed preferential sites for both dyke emplacement and mineralisation.

Pre-existing fault systems have played a major role in the tectonic history of the region (Andrews et al., 1998), influencing the formation of sedimentary basins, sitesof granitic emplacement and mineralisation. The NNW-SSE trending wrench faults known as fluccans, trawns, crosscourses or guides (depending on area and type ofinfill) occur throughout the Cornubian Orefield. Although the low temperature chalcedony infill typical of many crosscourses (LeBoutillier, 1996) is late in thesequence of mineralisation in the orefield, the fractures themselves appear to have been active (with little or no infill) from the earliest phases of mineralisation.

Hosking (1974) considered that many of the Sn vein systems in Cornwall developed from tension fractures or second-order shears that developed between pairs of


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wrench faults, and that under shear conditions these fractures could be dilated to form a ladder-like vein system. The distance between the pair of wrench faultsdetermined the strike length of the lode, whilst the duration and degree of movement on the faults affected the structural complexity, mineralogy and payability of thelode.

The pairs of bounding faults can act as impounding structures, with metal grades significantly raised adjacent to the faults (Carne, 1822) and notable changes inmineralogy within the lode (Henwood, 1843, noted that Malkin Lode of Ding Dong Mine (near Morvah, Penwith) changed in mineralogy on approaching acrosscourse). When a number of wrench faults operate at the same time, the lode fractures develop independently within each pair; however, lithological controls maybe such that the lodes developed between one pair is very similar to that between neighbouring pairs.

This has sometimes lead to the pattern being interpreted as a series of lodes dislocated by a later set of transverse faults, when in fact it is the lodes which are the laterfeature. There are numerous examples (Collins, 1912; Henwood, 1843) where a lode is greatly enriched against a crosscourse, and (when mined through and theassumed extension of the lode is located) on the other side of the fault is low-grade or barren. Perhaps the best example of this phenomenon is shown by Wheal Vor[SW625302], near Breage (Garnett, 1961; Hosking, 1974; Taylor, 1979), close to the Tregonning-Godolphin Granite (see Figure 19).

Figure 19. The wrench fault-bound lode system of Wheal Vor, near Breage (after Garnett, 1961).

Garnett (1961, 1962) considers the mechanism important in the formation of the lodes of Geevor Mine and cites examples where lode widths and grades are elevatedadjacent to crosscourses and the lodes beyond are poor or barren (see Figure 20).


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Figure 20. Diagram showing the influence of crosscourse fracture sets on the formation of the main-stage lodes at Geevor Mine (after Garnett, 1961).

The lodes either side of the Great Crosscourse (a composite wrench fault 'zone', ~100 metres wide, marked on surface by the valley of the Red River [SW663404])near Camborne (see Figure 21) were considered to be continuations of the same structures (Collins, 1912; Taylor, 1965; Farmer, 1991), but the number of lodes oneither side is uneven (though some show similarities in width, texture and mineralogy) and a number of lodes show distinct 'ponding' of tin grades against thecrosscourse ,e.g. Dolcoath Main Lode and its subsidiary structures (the Little Crosscourse on the west and the Red River Crosscourse on the east side of the GreatCrosscourse) and No:8 lode of South Crofty Mine.


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Figure 21. A long section through South Crofty Mine, showing the relative position of the Great Crosscourse and the reserve areas adjacent to it.

Within South Crofty Mine the No:4 Lode and Roskear D lode were equated with each other, on either side of the crosscourse (Farmer, 1991). They share somemineralogical and structural characteristics, but while the No:4 Lode (east of the crosscourse) is highly enriched (with extensive mineralised wallrock) against thecrosscourse, the Roskear D lode (west of the crosscourse) carries much lower grades (South Crofty Mine, unpublished data). This suggests that the Great Crosscourse(see Figure 22) also pre-dated the lodes and may be of pre-granite origin.

Figure 22. A plan of the 400 fathom level, South Crofty Mine, showing the position of the Great Crosscourse and the lodes worked. Although the spacing of the lodes on each side and their

mineralogy appears similar, grade trends suggest that the crosscourse pre-dated the lodes and influenced their formation.

Elsewhere within South Crofty Mine (Taylor, 1965), a crosscourse can be seen to intersect both the No:3 and No:4 lodes (in Robinson's Section, east of Robinson'sShaft), but only the No:3 Lode is displaced; showing that the fault was active between the deposition of the No:3 Lode (associated with early W-As-bearingreplacement bodies and quartz floors) and the No:4 Lode, before being infilled with late-stage chalcedony.


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Mineralisation in Cornwall - Lode Development, Structure and Geometry