The lodes in the deeper workings
The lodes in the shallow workings
South Crofty Mine (underground) occupies a rectangle of ground stretching E-W from Trevenson Street in Camborne to Barncoose (a distance of nearly 3.5 km) and N-S, from the A30 approach road to the railway line (a little over 1 km).
Figure 1. A geological/location map of the area around South Crofty Mine (courtesy of Simon Camm).
The surface workings of South Crofty Mine are situated on a series of metamorphosed sediments (dominantly slates) belonging to the Mylor Slate Formation, of Upper Devonian (Famennian) age, collectively known locally as 'Killas' (see Figure 1). At depths of around 148 fathoms (271 m) below surface, these sediments give way to granite of the Carn Brea stock (see Figure 2), which outcrops a few hundred yards to the south, close to the line of the railway, and forms the prominent hills of Carn Brea, Carn Arthen and Carn Entral close to the mine site.
Figure 2. A long section through the workings around New Cooks Kitchen Shaft of South Crofty Mine.
The mine worked a series of around 32 sub-parallel lodes that trend ENE-WSW, dip sub-vertically (commonly around 70 degrees) and are of a complex discontinuous nature (see Figure 3). Formerly many of these lodes were worked in the Killas for copper and minor amounts of lead, zinc and iron. This type of mineralisation was replaced progressively, as the granite contact was approached, by higher temperature tin and tungsten mineralisation that persisted to the deepest point of the mine at 490 fathoms (almost 3000 feet). The Carn Brea granite is a satellite stock of the Carnmenellis granite, itself a cupola of the Cornubian Batholith and was emplaced around 293 m.y. Small bodies of pegmatite and aplite are associated with the granite and it is also intruded by a later series of quartz porphyry 'Elvan' dykes. To the north of New Cooks Kitchen Shaft, along the line of the main Camborne-Redruth road lies a large Greenstone dyke. Within the granite and the killas the lodes are developed. They infill faults and fractures formed during and after the Variscan mountain building episode, and during the intrusion and cooling of the granite. These fracture zones now host complex multi-phase lode structures, some of which can be traced for 2 km or more on surface and for over 600 m down, below it.
Figure 3. A plan of the workings on the 340 fathom level (courtesy of Robin Boon, Andy Staples, Andy Seager & Allan Reynolds).
The lodes in the deeper workings.
In the deeper, 20th Century, workings (260 - 445 fm) of South Crofty six main phases (see Figure 4) of mineralisation were identified :
Figure 4. A general paragenetic sequence for the deep lodes of South Crofty Mine.
A. Prior to the onset of the main phase of mineralisation, a very early group of quartz veins were generated. These stacked veins vary from a few cm to a metre thick. They occur in swarms, particularly around the No.2, 3 and 4 lodes between Cook's Shaft and Robinson's Shaft and also in the North Pool area. They are commonly discontinuous in extent and lie close to the horizontal in orientation, which has lead to them being called 'quartz floors'. Many contain only quartz, but wolframite, arsenopyrite and feldspar are common associates, along with minor chalcopyrite and stannite (copper, iron, tin sulphide). Some of the lodes at South Crofty also show a similar mineralogy and it may be that many lodes were originally of this type and were largely replaced or overprinted by later phases. Only on or close to the granite/killas contact is this style of mineralisation to be seen in the main lode structures (Care's Lode, Roskear Complex, etc).
1. An early black tourmaline (schorl) phase, with a network of thin stringers of schorl (1-3 mm in width) emplaced in a NE-SW to ENE-WSW (050-060 degrees) fracture system or 'lode zone', which may be tens of metres across.
2.a) A blue tourmaline ('blue peach') phase. With the onset of the main phase of mineralisation, the major lode zone fractures were host to boron-rich fluids, depositing fine blue/black tourmaline and quartz as the main gangue (mining term for uneconomic or waste minerals found with the ore) minerals. This phase carries the majority of the economic tin mineralisation in the form of microcrystalline cassiterite (tin oxide). Some of this is disseminated within the veinstone, but there is evidence from a number of lodes that, after the initial tourmaline deposition, a residual quartz/cassiterite phase was produced. This sub-phase is seen commonly as a fine matrix (cement) in brecciated (shattered) sections of the lode. The brecciated textures are evidence of explosive decompression, caused by sudden losses in pressure, with very rapid injection and crystallisation of new fluids to crack seal the lode, 'freezing' the broken fragments of the earlier minerals in place. Some lodes show several episodes of reopening and have complex textures.
2.b) A chlorite ('green peach') phase. This lower temperature phase is characterised by dark green chlorite as the main gangue mineral, associated with fluorite and quartz. Tourmaline, present in only minor amounts, is often not visible with the naked eye. This phase sharply cuts the previous phases, often forming a discreet layer on the hangingwall of the lode, or replacing sections of the lode entirely. It is often characterised by coarse cassiterite (crystals to 1cm have been found), which may produce well-formed crystals of sparable type. The crystal size is evidence of much slower cooling than in phase 2 a). In the upper levels of the mine (260 fm Level and above), most lodes carry only this type of veinstone, which shows the effect of temperature control on the deposition of minerals within the lodes. It also shows that significant amounts of chlorite were present in solution during phase 2 a), deposited above the tourmaline and that this became dominant in successive phases of mineralisation as boron became depleted in the hydrothermal fluids.
3. A tin-barren fluorite phase. This phase occupies sections of the lodes with 'caunter orientation' which became infilled with fluorite, earthy haematite (iron oxide) and chlorite (some of the haematite may have formed from the decomposition of chlorite) with kaolin (china clay) and occasional mineral pitch (tar). These caunter segments, or 'jogs', have an E-W strike (090 degrees), and are best developed on Roskear D Lode, where they represent low grade pillars between the normal lode segments. Very little cassiterite is present in these segments, though occasionally remobilised material has been found in the wall rocks.
4. The caunter lode phase. These lodes occupy fractures having the same orientation (090 degrees) as the caunter jogs. They fault the earlier lodes, where they cross them, and are typified by mesothermal/epithermal mineralisation. Some lodes of this type were found in the North Pool area of the mine, such as Reeves Lode, which is a major structure in the district, running from Camborne to Redruth. At depth, the lode consisted mostly of fluorite, with occasional quartz/haematite/chlorite and marcasite (iron sulphide). On 260 fm Level it was explored and developed for tin (as the No.7 Lode) for a short strike length, but is barren below this level. At shallower levels (surface to 180 fm), the lode was a major copper producer and also carried minor amounts of lead, zinc and iron.
5. The crosscourse phase. Crosscourses are infilled wrench faults. Most post-date phases 1-4, though faults of this type can be found across all phases. Many have a rough N-S orientation and carry an epithermal mineralogy of chalcedony with earthy chlorite, haematite and minor amounts of marcasite and occasional copper and bismuth sulphides. Displacements along crosscourses vary from a few centimetres (they are typically of the order of a metre) to over 100 m in the case of The Great Crosscourse. Many lodes also show internal fracturing related to this phase and carry the same minerals as infilling/replacements within the lode, e.g. No.4 Lode. The Great Crosscourse is a major wrench fault system that forms a recognisable surface feature for some 5 km running from North Cliffs back inland past Brea village. The Red River flows along part of its course. The fault system may pre-date the granite and it has been active over a very long period. It is a stacked series of faults forming a zone some 100 m wide. The ground between the individual faults is heavily kaolinised (the granite is broken down to china clay) and crossed with chalcedony infillings along larger planes. It effectively divides the mine into two sections and was used as a boundary by many former mines (see Figure 5).
Figure 5. A section through the mine showing the lode zones and Great Crosscourse.
The lodes in the shallow workings.
The lodes in the upper part of the mine were worked primarily in the 19th Century (some were reworked in the early 20th Century) or before, as far back as the the late 16th Century. Although the majority of these workings are now inaccessible, the workings on North Tincroft Lode, above Deep Adit Level (140 feet) can still be examined (see Figures 6 and 7).
Figure 6. A section through the workings on North Tincroft Lode showing the position of Deep Adit Level.
Figure 7. A plan the workings on North Tincroft Lode down to the 175 Fathom Level.
Stopes on the North Tincroft Lode (that eventually becomes South Crofty Mine's Main Lode) at Deep Adit Level retain a large number of pillars of lode material. These are left as supports in the shallow dipping (35 degrees) stopes, some of which reach over 30 feet (10 metres) in width. The pillars show the lode material to be dominated by a tourmaline (predominantly schorl, with minor dravite)-fluorite-feldspar-quartz-massive sulphide assemblage, which consists of (in order of abundance) sphalerite, arsenopyrite, chalcopyrite and galena with cassiterite, wolframite, bismuthinite and minor gustavite, wittitchenite & matildite (a suite of Cu/Bi and Pb, Bi, Ag sulphides). Zinc values are typically very high and may reach up to 35% (Cu varies from 0.7 - 4%, but is typically at the lower end of this range; while Sn is usually <1%), although South Crofty and its predecessor mines never produced that metal as a saleable product (in fact it's presence was unrecorded until this fieldwork was undertaken). As is also typically high, reaching values of up to 15%. The assemblage is texturally complex and must have rendered the ore difficult to process.
Emplaced along the hangingwall or footwall (and occasionally within the central section of the lode) of the lode a second chlorite-dominated phase has been identified. This consists of a chlorite/quartz gangue carrying sulphides of arsenic and copper. Ageneralised paragenesis for the North Tincroft Lode is shown in Figure 8.
Figure 8. A general paragenesis for the North Tincroft Lode.
When this very dense material was examined chemically, optically and by S.E.M, it was found to contain up to 12% tin, 12% arsenic and 2372 ppm Cerium, and to date has produced 29 mineral species including unusual lead/copper sulphides, thorium minerals and some of the finest monazite crystals (to 2mm) seen in Cornwall (see Plates 1 to 6, below, taken by Nigel Hoppe).
Plate 1. Anatase & cuprite on chlorite. Width of field ~1.5 mm.
Plate 2. Apatite on chlorite. Width of field ~1.5 mm
Plate 3. Cassiterite crystal of 'sparable' type. Width of field ~1.0 mm.
Plate 4. Monazite. Width of field ~1.5 mm.
Plate 5. Monazite. Width of field ~1.0 mm.
Plate 6. Pyrrhotite with cuprite. Width of field ~2.0 mm.
The material is composed primarily of chlorite (Ferroan clinochlore - ripidolite) with irregular masses of pale green fluorite. This gangue carries irregular masses of chalcopyrite & arsenopyrite. The material is very dense and, apart from the vuggy areas, compact. Under the S.E.M the chlorite can be seen to carry a range of minerals, some of which only obtain a grain size of 1 micron and some occur as inclusions in other species. The paragenesis can be broken down into an early anhedral oxide suite, composed of cassiterite, anatase, ilmenite (with pyrophanite & ecandrewsite) & ilmenorutile. Cassiterite is the major phase and Sn accounts for 11 - 12.5% of the material in wt%. Monazite also occurs at this point, but also crystallises throughout the paragentic sequence to a very late stage. Intergrown with with this early suite are a series of sulphides; chiefly arsenopyrite, but also chalcopyrite, sphalerite (with inclusions of galena, native bismuth and an unknown Fe,Cu,Pb sulphide), pyrite, bornite & chalcocite. The latest phases are chlorite, fluorite and quartz (with inclusions of thorite) and monazite, filling voids between chlorite plates. The various textures are demonstrated in the photographs (Plates 7 to 9) below, taken by Tony Ball at CSM.
Plate 7. Early anhedral monazite (Mnz) abutting chalcopyrite (Ccp) in chlorite (Chl) gangue. The lath-like crystals (bottom left & above the monazite crystal) are anatase & ilmenite.
Plate 8. Anhedral cassiterite (Cst) intergrown with arsenopyrite (Apy) with sphalerite (Sp) in chlorite (Chl) gangue. The sphalerite contains occasional inclusions of galena (Gn).
Plate 9. Late stage quartz (Qtz) and monazite (Mnz) filling voids between sets of chlorite (Chl) plates at the end of the paragenetic sequence.
The textures encountered in the lode material are consistent with very rapid, chaotic, crystallisation from hydrothermal fluids enriched in Sn, Cu, As, Zn, Ti and Ce. XRF analysis shows the samples to be enriched in a variety of REE's in levels well above background for Cornish granites. The partitioning of the elements above into the magmatic-hydrothermal fluids, the incorporation of chalcophile elements into these fluids (and therefore, by inference, into the granitic magma) suggests that assimilation of xenolithic material and fluids of magmatic departure played a major role in the formation of this assemblage/deposit. Similar suites have been found elsewhere in Cornwall, many highly zinc-rich with elevated REE contents. Such previously unrecorded assemblages challenge our understanding of the metallogenic makeup of the orefield and should be the subject of future research; this has implications not only for local mineralisation events, but also for the formation of the Cornubian Batholith itself.
Three pdf files are available for download here (right-click on the required document link and click 'save as' to capture the document).
Document 1 is the paper: Le Boutillier, N.G., Shail, R.K. & Jewson, C. 2000. The cassiterite-anatase-ilmenite-monazite-sulphide-chlorite assemblage of North Tincroft Lode. Geoscience in southwest England, 10, 050-057.It is 5.14mb in size and can be downloaded here:- LeBoutillier-et-al-2000.pdf
Document 2 is the paper: Le Boutillier, N.G., Shail, R.K. and Jewson, C. 2003. Monazite in polymetallic chlorite-(tourmaline)-quartz-(fluorite)-cassiterite sulphide lodes and its potential for constraining the chronology of magmatic hydrothermal mineralisation in Cornwall. Geoscience in south-west England, 10, 403-409.It is 657kb in size and can be downloaded here:- LeBoutillier-et-al-2003.pdf
Document 3 is the report: Le Boutillier, N.G. 2005. Zinc and Cadmium at South Crofty Mine. Unpublished company report for Baseresult Ltd, 83pp. It is 2804kb in size and can be downloaded here:- Zinc%20Report.pdf
This page last updated on 15/10/2009
