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 FOUR – POLDARK MINE.

Poldark Mine, originally known as Wheal Roots [SW682315], is a small tin mine located adjacent to the hamlet of Trenear, 600 metres NE of Wendron, in the southwest of the Carnmenellis Granite (see Figure 14). The mine worked between 1720 and 1780, though few details of its history are known. The mine workings succeeded tin streaming in the adjacent valley, which can be dated back to prehistoric times. It appears that the lodes cut in the banks of the stream were originally worked by opencast methods (as were several other tin deposits across Carnmenellis), before underground mining began. The workings are fairly extensive and include the main adit level, a number of shafts and three stopes (the largest of which extends above and below adit level and is pumped out to allow access. Flooded workings continue below the main stope, to an unknown depth. The mine sett was taken over by the nearby Wendron Consols in the 1856, but there is no record of any further work being undertaken at Wheal Roots, apart from the use of the stamps and dressing floors for processing tin ore. The mine provides excellent exposure of main-stage blue peach lodes, as well as earlier and later phases of mineralisation.

Figure 14. A sketch map and plan of the location and workings of Wheal Roots. After Hamilton Jenkin, 1978.

The main lode/vein trend is ENE-WSW (see Figure 15). The earliest phase of mineralisation is in the form of millimetric (often anastomosing) schorl veins, which occur as fracture infills and as coatings on joint planes. These veins trend around ENE-WSW (050°-080°, dipping NW) and also (primarily as joint coatings) NW-SE (120°-122°, dipping NE). The more numerous ENE-WSW set form a ‘lode zone,’ several metres in width, within which the main-stage blue peach lodes were emplaced.

Figure 15. A rose diagram of lode and vein orientations at Wheal Roots.

Three worked lodes and a number of subsidiary structures (some of which were trialled) occur within the mine. The lodes have very different characteristics, although they are all tourmaline-dominated structures. The caunter lode worked in the adit (see Plate 5) trends 078°/60°/NW and reaches up to 0.30 m in width (pinching and swelling along both dip and strike). The lode is composed of blue peach, with a central quartz leader (2 cm in width); the lode margins are diffuse and irregular; on the footwall the lode passes into granite directly, on the hangingwall tourmalinised (schorl) granite is apparent on the margin before this passes into unaltered granite. It would appear that the main-stage mineralisation reactivated an earlier schorl-lined fracture (with associated tourmalinisation of the wallrocks) and overprinted the earlier assemblage (now present only as a relict structure on the hangingwall) while also replacing the granite at the lode margins. Later reactivation of the host fracture is evidenced by the late quartz (with minor haematite) leader. While being a strong structure in terms of its width and strike length, no visible cassiterite was seen in the lode. A spot sample, analysed for tin only, showed the grade to be 0.20% Sn over the whole width of the lode.

The Middle Lode and North Lode (the most extensively stoped and developed), trending parallel 060°/70°/NW, are even more diffuse in nature and consist largely of replaced granite (with obvious relict textures) which has been extensively tourmalinised by both black and , later, blue tourmaline. North Lode appears to consist of swarms of irregular anastomosing blue peach veins and veinlets (up to 1 cm in width), which can be seen on the walls of the stope, with occasional blue peach segments (some 30 cm in width) of short strike length that are exposed on the walls. These pass laterally into thin fractures with tourmalinised and sericitised margins, the alteration extending for a few cm on each side. Little of the lode remains in the main stope, which reaches well over 1 metre in width, so it is difficult to ascertain the true character of the lode.

Plate 5. A Blue peach lode, Wheal Roots. The lode has diffuse margins that overprint an earlier phase of schorl mineralisation. The quartz leader (centre) is evidence of later, low-temperature, reactivation. Rule (25 cm) for scale.

Middle lode is exposed within an ancient stope (see Plate 6), which appears to come close to the surface. The exposure in the old stope is curious in that the narrow lode (0.20 m) has been worked in a stope reaching well over a metre in width in places; such massive overbreaking of waste granite would have required the ore to be washed, cobbed (broken by hammer and all the granite removed from lode material) and hand-sorted prior to stamping. In addition the lode is very pale and weak (the tourmalinisation is not particularly pervasive and relict granite textures are readily visible); XRF analysis (see Table 3) showed it to be carrying only 91 ppm Sn, a fact that would not have gone unnoticed at the time, given the relative sophistication of vanning and assaying techniques practiced on the mine during the 18 th Century.

Plate 6. Middle Lode exposed on pillars within a stope at Wheal Roots. The lode, 20 cm in width, lies along the centre line of the pillars within the stope and consists of variably tourmalinised granite around a fracture (joint) which lies close to the footwall. The contacts vary from sharp to diffuse, the lode appearing to have formed largely by replacement of the host granite.

The lode characteristics suggest that high-temperature boron pneumatolysis played an important part in lode formation, overshadowing true fissure-infill as the dominant formation mechanism. Such a scenario supports the assertion that this area (and central Carnmenellis in general) records only the base of the tin zone and the roots of, now eroded, lode systems; as does the presence of high-temperature specularite in the ore, forming a similar lode assemblage to that seen in central Dartmoor at the base of the tin zone there.

Some of the main-stage structures carry rare slickenlines on internal surfaces. Those trending 060°-070° are characterised by very steep (>80°) oblique slip to true dip-slip slickenlines, while those trending 076°-086° (caunter orientation) are characterised by near-horizontal slickenlines, indicating that the structures belong to two structurally independent episodes.

The lodes and earlier schorl veins are cut by a series of quartz/chalcedony veins (with occasional sinistral offsets that may reach up to 0.35 m) trending NNE-SSW (010°-030°, with the majority around 020°), around N-S and (rarely) WNW-ESE. The quartz veins (normally 1-2 cm in width, but may reach 10 cm) are associated with extensive kaolinisation of the wallrocks and localised iron (haematite) staining. Within the kaolinised areas biotite can be seen to be breaking down, in-situ, to haematite. This material has been remobilised and lines thin fractures around the quartz veins and even within the blue peach lodes and later quartz leaders. These late, low-temperature, veins mark the crosscourse phase of mineralisation at this location. The late-stage argillic alteration of the granite post-dates the quartz mineralisation, by an unspecified time span.

XRF analysis (see Table 3) of two lode samples WR01 (blue peach/tourmalinised granite – North Lode, from within the main stope) and WR02 (tourmalinised granite – Middle Lode) show that both lodes are essentially barren (although Hamilton Jenkin (1978) records that samples of lode material taken during the 1970’s when the mine was being prepared for pubic opening, assayed at 17lb of black tin per ton – around 0.60% Sn, but even this value is below the level that most Cornish mines operated under), but this does not preclude the possibility that the lodes were sporadically rich enough in tin to warrant stoping (this can be the only logical explanation for the presence of the North Lode stope below adit level).

The analyses show low levels of tin, copper and all other metals (the majority well above background, but well below economic values), with silica being the most important component. In most respects these analyses are more typical of enriched granite rather than lode material, reflecting the low levels of economic mineralisation at the base of the tin zone.

Wheal Roots: Samples WR01 & WR02 XRF Analysis
Output Results including and taking account of LOF%.
	L.O.F.%	Fe2O3%	TiO2%	CaO%	K2O%	Al2O3%	MgO%
WR01	0.64	3.62	0.00	0.37	0.08	6.64	1.02
WR02	2.39	6.80	0.21	0.27	4.79	13.18	0.47
	Na2O%	MnO%	BaO%	S%	P2O5%	SiO2%
WR01	0.70	0.03	0.02	0.02	0.01	88.12
WR02	0.03	0.12	0.01	0.02	0.15	69.80
Traces ppm.
	Nb	Zr	Y	Sr	Rb	Th	Co
WR01	8	11	<10	42	<10	<10	10
WR02	19	85	71	125	677	<10	20
	V	La	Nd	Ce	Ga	Pb	Sn
WR01	3	7	21	53	12	6	521
WR02	12	24	22	68	31	12	91
	Cu	Zn	As	Ni
WR01	28	53	28	<10
WR02	146	108	151	7

Table 3. The results of XRF analysis of two samples of lode material from Wheal Roots.