CLASSIC CORNISH FIELDWORK LOCATIONS
NANJIZAL
Access & location:
Nanjizal or Mill Bay (SW357236) lies 2.4 km NW of Porthgwarra and 2 km SE of Land's End. It can be reached only by the coast path from either location, or via the footpath from the hamlet of Trevilley (SW35782451), which is just off the B3315, 1.6 km ESE of Land's End.
Pros & cons:
The walk to Nanjizal along the coast is breathtaking (especially in fine weather), with views of the high castellated granite cliffs. From Land's End the views over the Armed Knight and Enys Dodnan towards the Longships Lighthouse are particularly spectacular and well worth photgraphing. Nanjizal itself can be visited at all states of the tide (the old mine workings are just off the coast path), though excellent exposures of pegmatite, a granite sill, faults and pneumatolytic/hydrothermal veining can also be seen on the beach at low tide. If going into the underground workings (access is very easy) take care if hammering - a helmet is advisible, as are goggles. The tourmaline/cassiterite ore is extremely hard and liable to produce razor-sharp splinters quite capable of cutting the skin - unprotected eyes would fare very badly if struck!!
Geology:
Nanjizal is a large bay, cut out of the coarse-grained megacrystic granite of the St Buryan Lobe (Salmon & Powell, 1998), with Quaternary head deposits occupying the cliff tops and alluvial deposits covering the narrow valley floor of the Nanjizal Stream.
A sketch map of Nanjizal, showing the location of the mine workings and other features.
The valley of the stream runs along a curved fault system, trending 050° to 135°, which crosses the beach and passes out to sea. The onshore exposure, just below the lowest point of the cliffs, shows the fault to be made up of a large number of discrete fractures in a zone some 4 metres across (the main fracture is infilled with ferroan jasper, quartz and limonite). Within this zone the granite is heavily kaolinised and intensely veined. These veins range from anastomosing millimetric fracture infills to polyphase (quartz-dominated) infills up to 0.75 metres across. Several phases of veining can be identified by cross-cutting relationships and mineralogy ranging from early tourmaline/quartz veins (050°-070°) to later haematite ± quartz veins (300°-315°).
On the beach a fine-grained granite sheet sheet, up to 1.0 m thick, outcrops at the foot of the cliffs in the north side of the bay. The sheet has somewhat diffuse contacts with the host CGMG and dips gently at 10° to the ENE. Schorl is concentrated along the upper contact in some areas along strike. The host granite is well-jointed with prominent sets trending NNW-SSE and ENE-WSW; it is also heavily veined by several generations of quartz-tourmaline and quartz-haematite veins; feldspars are often horizontally-aligned.
Within the bay, three major vein sets can be distinguished on the basis of cross-cutting relationships, morphology and paragenetic associations. These veins outcrop throughout the cliffs and are well exposed, particularly in the area around the beach. The earliest vein set is a vertical/sub-vertical, apparently largely tensile, series of veins trending predominantly between 050° and 070° (with the largest proportion trending 060°). These veins are infilled by schorl with central leaders of quartz and reach up to 10 mm in width. The schorl shows typical bridging textures (Halls et al., 1999) that are cleanly cut by the later quartz infill that has 'crack sealed' the reactivated fractures. The second major vein set comprises swarms of vertical/sub-vertical millimetric (1-3 mm across on average) schorl veins, describing a conjugate set, trending predominantly along 340° and 280°. These veins, often closely spaced, also appear to be of largely tensile origin (NE-SW extension; sigma1 vertical, sigma 2 at 310° and sigma 3 at 220°) and have remarkably uniform widths and orientations over dip heights of several metres. Some veins show anastomosing textures and slightly thicker veins appear to have more than one stage of infilling (though the majority appear to be single phase).
A subset of this vein sequence occurs as swarms of veins on the foreshore and cliffs on the north side of the bay, where they appear to be largely joint-controlled. The economic mineralisation exploited at Nanjizal is associated with these veins. They are marked by variable widths along strike (though most vary between 1-10 mm) and dips that range from vertical to 70°; although essentially composed of schorl, larger veins also carry minor quartz and feldspar. The veins are also marked by alteration haloes (extending a few cm either side of the vein) in the host granite which stand out on weathered surfaces. This alteration appears to be largely due to silicification rather than boron metasomatism. A number of the 340° trending veins were observed to cut millimetric 280° trending veinlets and thus the 'economic' veins may represent a late 'pulse' at the end of this particular sequence.
The third, and latest, major vein set comprises a quartz-dominated assemblage that trends predominantly between 280° and 325°, but with some veins trending 345° and 045°. This vein set is also partially joint-controlled, but a number of veins (which range from a few cm to ~0.70 metres in width) occupy shear fractures that occasionally show sinistral oblique dip-slip slickenlines. Vein-parallel fractures show similar slickenline orientations; they often occur in closely spaced swarms and carry minor amounts of haematite on polished surfaces; the surrounding granite is haematised in part and there is also evidence of kaolinisation around these fractures. The quartz veins are typically steep-dipping or vertical and the larger veins show a banded texture. The quartz infill is supplemented by limonite-rich bands (which becomes almost jasper-like in places) and some chalcedonic silica. The largest vein of this set, on the beach, trends 090/75/S and is heavily brecciated and recemented, with multiple banding. The vein lies in the roof of a large sea cave, and it may have been stoped in the past, but the evidence is inconclusive. This vein set cuts all the other tourmaline veins on the beach and surrounding cliffs; the relationship between these veins and the 'economic veins' was not demonstrated in outcrop, but it is likely that the quartz veins post-date all the tourmaline-dominated assemblages.
The workings at Nanjizal are poorly documented (LeBoutillier et al., 2002) and only one primary source is known (Carne, 1845). The workings are inferred to have commenced in the 1840's on the site of much older trials, on a series of stanniferous schorl veins termed 'floors' by the miners. The 'floors' were followed northwards into the hillside from the cliff edge where several small schorl veins were observed. The 'floors' were described as discontinuous, isolated, lenses of schorl, with subordinate quartz, ranging from one to sixteen feet in width and a maximum dip-height of 20 feet. These masses were cut by further, narrow, schorl (& quartz?) veins that showed cassiterite enrichment at the intersection points. Carne considered the schorl lenses to be contemporaneous with the host granite and the cross-cutting schorl veins to have been feeder pathways for the tin mineralisation. The ore was transported down the cliff path (now the coastal footpath) to the stamps (the original wheel pit is still preserved) for processing.
The economic mineralisation at Nanjizal is related to two veins swarms (trending 320°-346°) on the north side of the bay, which can be seen cutting the foreshore and cliff line. These swarms in themselves do not appear to have been worked below the cliff tops, but at the elevation of the coast path a drive [SW3538123813] has been mined on a ~10 cm wide vein that appears to be the result of the merging of smaller structures. This drive falls away rapidly from the portal and at least one further level has been driven off it northwards into the hillside. Also at the level of the cliff path, an openwork [SW3571423733] has been excavated on an elliptical body of stanniferous tourmalinite, some two metres across. It seems possible that the pod may have formed, at least partially, by replacement of the host granite, but if this is the case, no relict granite structure now remains. The contacts are very sharp, and are also polished; the host granite is extensively kaolinised and is cut by a series of (similarly polished) vertical shears the run parallel with the margins of the pod (310°), suggesting that the whole structure lies within a fault zone (with the tourmalinite acting as a durable, rigid block, within the much softer, sheared, altered granite). At the western end (lower left) of the pod, a network of bridging veins (schorl and quartz, 1 mm to 2 cm in thickness, dipping 320°/45°/SW) connect it to a similar elliptical structure just appearing in the floor of the openwork. An alternative explanation for their origin may be that these features represent tension gash infillings within a fault; if this is so, they describe a top sense of shear to the left, which would conform to the pattern of sinistral shear seen in some of the other veins at this location.
The entrance to the underground workings of Nanjizal Mine.
Between the adit drive and the openwork [SW3552723797] a much larger excavation can be seen extending NW from the coast path (see photograph above). This initially takes the form of an openwork, extending 15 metres from the path, before passing into the hillside as a drive, some 3 metres high by 2 metres wide. Just prior to this point what appears to be an exploration crosscut has been driven approximately 20 metres towards 050°, partially within head. The surface has been breached at what appears to be the end of the crosscut. The main drive extends a further 23 metres into the hillside; there is an extensive (8 metres) breach of the surface to leave an open gunnis only 3 metres from the portal and a second small breach 2 metres beyond that.
The 'main lode' of Nanjizal Mine, exposed adjacent to an open gunnis. The schorl/quartz vein is ~0.50 m in width and can be seen to pinch and swell down dip. This section of the vein was evidently poor and has been left in place by the former miners.
The final 9 metres of the workings is within a stope measuring some 7 metres long by 6 metres (maximum) high, with a 2 metre stub at the NW end. The stope varies from 2-3 metres in width as the miners took some of the satellite veins during extraction. The walls of the stope and gunnis show that several veins run into the workings and merge to from larger structures; of these larger veins several combine to form the main 'lode' or 'floor' that has been mined. This structure measures ~1 metre wide at its greatest extent and trends generally 302/70/SW. In overall appearance the structure has a lensoid form with multiple incoming and outgoing splays along strike and several subparallel veins within the heavily reddened granite. Some vein surfaces show slightly oblique (80°, sinistral) dip-slip slickenlines, while within the stope a prominent surface records true dip-slip movement, with downthrow to the SW (212°). Sections of the main 'lode' and satellite veins remain on the walls and back of the stope, with thin E-W schorl veins cut by later NW-SE trending veins.
The main overhand stope at Nanjizal Mine. Looking NW along the strike of the main vein, a number of subsidiary structures can be seen on both the footwall and hangingwall of the stope. The host granite is partially kaolinised and reddened by haematite staining, but this is likely to post-date the mineralisation by some considerable time. The lode in the back and breast of the stope is still strong, and it is not clear if falling tin values or economics lead to the cessation of mining here. Width of stope ~1.5 metres.
In hand specimen the lode material appears to be largely composed of coarse-grained black schorl (in radiating acicular masses 1-3 cm long) with minor amounts of quartz and feldspar. Cassiterite, also being dark, cannot be made out with the naked eye, but the lode 'stuff' is relatively dense and heavy in the hand. Under the microscope, however, euhedral to subhedral honey brown cassiterite crystals interlocked in the matrix were clearly visible. Free-standing 'sparable' crystals within small vugs were also found, in the range of 0.15-0.30 mm in length; exceptional crystals reached a little over 0.50 mm. Optical examination of the material under plane polarised light showed that three generations of tourmaline are present. The earliest (light-mid brown under PPL) occurs in agglomerations of subhedral zoned crystals that individually reach up to 100 µm across. EDS analysis shows this tourmaline to be the Mg-rich member dravite (NaMg3Al6(BO3)3Si6O18(OH)4); which was also detected during XRD analysis. Dravite is overgrown by the much more abundant Fe-rich tourmaline schorl (NaFe3Al6(BO3)3Si6O18(OH)4) which varies from pale lilac to deep blue in PPL and occurs in two generations of crystals. During EDS analysis some of this schorl was seen to carry significant amounts of Mg, though the majority appears very close to the end-member composition, indicating something of a transition in fluid chemistry over time. The schorl crystals occur in two morphologies: as large euhedral zoned crystals, up to 200 µm across, that occur in agglomerated masses as well as singly; and as fine 'felt' overgrowths on the larger zoned crystals and radiating groups or single euhedral acicular crystals (often <50 µm in length) scattered as inclusions in other phases. Other phases present include apatite (Ca5(PO4)3F), in euhedral to subhedral clear crystals to 400 µm in size, sometimes intergrown with, or nucleating on, the large zoned schorl; rare subhedral zircon (ZrSiO4) to 40 µm in size; orthoclase in anhedral to subhedral crystals reaching several mm in size (clouded in PPL); quartz in irregular anhedral crystals reaching up to 10 mm across; and cassiterite in zoned euhedral crystals, up to 600 µm in size, occurring as radiating groups or as single euhedral crystals. The zoning in some of the cassiterite crystals is very pronounced, with brown (Fe-rich) cores passing outwards, via yellow-brown, to almost transparent rims. Several crystals can be seen to have nucleated on the large zoned schorl crystals, while being overgrown or acting as nucleation sites for the finer 'felt' schorl. Rutile (TiO2) was detected by XRD, but not XRF, nor was it seen during SEM investigation of the ore sample.
The final phase of mineralisation at Nanjizal is that of the low temperature quartz-chalcedony-limonite veins. These are associated with much fine-scale shearing and fracturing of the host rocks and localised haematisation and kaolinisation. The fractures appear largely extensional, but there is a small sinistral shear component. Some protracted movement is evidenced by recemented breccias in some of the veins. It is likely that the faulting, within the tourmaline vein system, in the stope (dip-slip normal faulting and slightly oblique sinistral shear) also dates from this time and, possibly, the early phases of movement on the Nanjizal Fault which cuts the tourmaline mineralisation and runs parallel (at the beach) to the fracture-hosted quartz veins.
This style of mineralisation is unusual in Cornwall (although more sites are becoming known in west Penwith, such as Boscawen Point and Carn Galver), where tin mineralisation is normally associated with later blue tourmaline (blue peach); however, similar assemblages are known from Dartmoor (Scrivener, 1982), where lodes of this style appear at the base of the tin zone. It is therefore possible that the mineralisation at Nanjizal Mine marks the base of a localised emanative centre (Dines, 1934), since removed by erosion.
This page last updated 22/09/2003
