Poldice

Poldice is an abandoned mining site in ex-industrial Southwest Cornwall. The area has a mining history spanning over 500 years and has seen the exploitation and processing of a number of ores including tin (Sn), copper (Cu) and arsenic (As). A soil geochemistry, EM conductivity, electrical resistivity and 2D mapping study has been undertaken with the aim of elucidating the spatial controls on soil contamination and the resource potential of the site. The results illustrate a strong positive correlation between old mining infrastructure and anomalously high concentration of various elements in the soils. Furthermore, analysis of other data suggests some potential for undiscovered mineral deposits of a nature not invested in during the mines operating years – something that may be of future research interest.

The Poldice Legacy

During the 16^th and 17^th century, Poldice is known to have been a major producer of Sn (CMWH, 2017a; Dines, 1956). By 1792, Cu production had taken over as the primary product, accounting for over £150,000 worth of sales (CMWH, 2017a) and by 1807, the mine reached peak production (Hines, 1956). Between 1815 and 1849 recorded output of Cu ore was 108,698t @ 6.25% Cu; 1525t of black Sn; 873t of As soot; 1822t of mispickel; and minor quantities of pyrite, ochre, zinc (Zn) (12t) and lead (Pb) (Hines, 1956).

Sn and Cu ore during the 18^th century was typically dressed by hand, becoming more mechanised from the early 19^th century (CMWH, 2017b). The dressing of Sn ore took place at the so-called ‘dressing floors’. Here, labourers would manually crush the ore, which was then passed on to be spalled (CMWH, 2017b). Once ore rock had been crushed, it would be fed to the stamps to be ground to a suitable fineness and the resulting ‘pulp’ would be taken away for separation. Gravity separation in buddles was the most common way to separate the ore mineral – cassiterite – from the gangue minerals (CMWH, 2017b; Hines, 1956). Often, the Sn streams (ore dressing floor waste piles) would be reworked to improve recovery (CMWH, 2017b). Cu was processed slightly differently to Sn, however. The concept of crushing a grinding remained the same, but separation was done by hand and the ore dressing floors were not used (CMWH, 2017b). Sn and Cu ore was associated with the undesirable accessory minor arsenopyrite. The pre-smelting process involved the ore being roasted in a ‘calciner’ in order to burn off As and S impurities that would otherwise render the cast Sn hard and brittle (CMWH, 2017b; Potts et al., 2002).

Poldice Mine ceased operation and closed in 1866 (Hines, 1956). Operation were resumed in the 1870s however, for the production of As (section 3.2.).

During the late 19^th and early 20^th century, West Cornwall witnessed a boom in As production in order to meet an increasing demand (Barton, 1971). In particular, As was utilised for its pigmental and toxic properties, seeing use as pesticides in farming, cosmetics, paints etc. (Allen, 2008). During the latter part of the 19^th century, SW England is believed to have supplied over 50% of global As demand (Hines, 1956). This revolution saw the revival of the Poldice mine operations where As was exploited as a major ore (Barton, 1971; CMWH, 2017a).

At Poldice, to process ore, stamps within the main buildings were used to crush and grind the rocks, presumably after the Sn processing stamps had been sold off and the dressing floor abandoned (Potts et al., 2002). The resultant As sands were then passed onto the shaking tables where dense arsenopyrite grains were sent for calcining, and the tailings dumped in the sand piles at the NE/W of the site (CMWH, 2017b). Brunton calciners were paired with a chimney stack, connected via the lambreth and flue (Barton, 1971). The lambreth was an elongate zig-zag structure composed of interconnected chambers through which the roasting fumes were passed through (Barton, 1971). As the toxic fumes cooled, As-oxide minerals would condense and crystallise on the walls of the lambreth (Acton, 1990). Residual fumes were drawn out of the lambreth through a sub-surface flue and expelled into the atmosphere from the chimney stack.

The mine was closed permanently in 1929 due to a decline in global As demand (CMWH, 2017a).

Mine Site Detective Work

As part of the project at Poldice, 2D mapping and other observations were undertaken in order to try map out the historical operations and use these to draw later conclusions. Historical maps from the 1880s and 1900s were sourced from EDINA Digimap, which allowed us to locate some old infrastructure. Furthermore, at the site itself, walking around, we were able to produce a story of how the mine operated.

Selection of observations made during the 2D mapping field phase: A – Californian stamp rebar reinforced concrete at the ore dressing area the concrete feature marked with a ‘?’ may be to hold the Fe pylons and support the aerial ropeway; B – tactical view of two calciners at the eastern side of the main buildings used to feed As-rich fumes (yellow) into the flue; Bi – plan view schematic illustration of B… F = flue, L = lambreth; C – tactical view of the lambreth interconnected chambers though which As-rich fumes travel, crystallising  As soot on the walls; D – View looking east of a convex (near) and concave (far) buddles with the most northerly calciner in the background. The red circle indicates an air feeding hole. Image A and D sourced from Powley (2016a; 2016b), respectively.

Interpreting Geophysics

At the South of the project site, in the vicinity of a number of shallow shafts, one of the electromagnetics grids produced appears to have divulged some interesting information.

Selection of quadrature and in-phase grids acquired for EMP400__132 The A-B transect has been used in cross section view, to highlight the possible geometry of a subsurface mineralised lode that (nearly?) intersects the surface. The interpretation is one of a shaft (red) that is targeting the lode (blue). The quadrature data invokes high conductivity of the soils/mineralised lode, suggesting that it has a significant metalliferous component. Polygons c represents the area interpreted to be abundant in waste piles. Circles d represent the anomalous shaft – strong negative anomaly in the quadrature data, strong positive anomaly in the in-phase data.

Mapping the Trace of the Flue

With the aid of EM geophysics and the use of old maps, it was made possible to map what is assumed to be a more accurate trace of the flue that delivered fumes from the calciners to the chimney stack.

Diagram illustrating the relationship between the best-estimated trace of the sub-surface lambreth and the concentration of As in the soil. “?A” concerns the confusion regarding the exact geometry of the lambreth in the area… different pieces of evidence provide alternate scenarios.  “B?” and “C?” concern the significant disparity between historic maps and observations made in the 2D mapping relating to the orientation and existence of lambreth architecture in these areas. Also important to consider is the most northerly calciner and the projection of the flue towards it (C?). This was registered on the 1900s map, however, in reality, it appears this may not be the case and there has been confusion in the construction or interpretation of the map. The yellow polygons represent the trace observed on the 1900s historical map, whilst the blue polygons represent the lambreth trace observed in satellite imagery. The KI is of As concentration in soil samples (red=high, blue=low) [B] is the EMP400__130_15kHz grid showing the lambreth as a –ve anomaly

Elemental Concentration Factors

Fieldwork was undertaken at the Poldice Mine Site using soil sampling to elucidate the nature and extent of soil contamination by a variety of elements. The results show, as expected, that there is a serious degree of concentration of various metals and contaminants relative to their average crustal abundances.

Table illustrating the concentration factors for the elements sampled relative to the average crustal abundance, Uk soil concentrations, and SW regional soil concentration.

The Relationship Between Soil Contamination and Mine Site Infrastructure

To supplement this report, a weighted sum raster spatial analysis has been performed in ArcMap to quantify the control of MI on elemental distribution. This has been achieved by normalising the absolute data acquired during the pXRF analysis and generating KI rasters using the normalised values.

Weighted raster analysis of various elemental relationships, with the mine site infrastructure overlain (C=chimney stack; F=Flue; L=Lambreth; S=Stamps; E=Engine Shaft; M=Main Buildings).

In the map plots, the trends clearly quantitatively indicate a strong level of control by mine site infrastructure on elemental distribution, with the dressing and calcining phases bearing the greatest influence. Furthermore, for the Cu-Sn-As-Fe overlay, their is a very sharp, distinct trend. This metal association is believed to represent the paragenetic association of the ore, and this raster reinforces this assumption, indicating that these elements were contemporaneously extracted and processed. On the other hand, the Cd-Zn plot is very blobby and non-correlative around the flue, indicating that they were not processed from the same ore, or were efficiently separated.

 

Final Mark Received – 80%