by Samuel Bolton, Exploration Geologist at Cornwall Resources Limited
Located in east Cornwall, UK, the Redmoor tin-tungsten-copper deposit has been the focus of recent and highly successful exploration by Cornwall Resources Limited (CRL) – a subsidiary of UK-listed Strategic Minerals PLC. Redmoor’s success demonstrates the potential for substantial discoveries in one of the world’s most heavily mined, yet least explored areas. Two very productive diamond drilling campaigns totalling over 7000 m (22 966 ft) each, combined with existing data, have led to the definition of a globally-significant polymetallic inferred resource of 11.70 Mt at 1.17 % tin equivalent¹, hosted in a sheeted vein system (SVS) orebody. Mine and processing studies help underwrite the project’s potential economic viability, which will now advance to the next stage.
 Tin equivalent calculation:SnEq% was derived using the formula Sn(Eq)% = Sn%*1 + WO3%*1.43 + Cu%*0.40. WO3Eq% was derived using the formula: WO3(Eq)%= Sn%*0.7+WO3%+Cu%*0.28. Commodity price assumptions: WO3 US 33 000/t, Sn US 22 000/t, Cu US 7000/t. Metallurgical recovery assumptions: WO3 recovery: 72%, Sn recovery: 68% and Cu recovery: 85% and payability assumptions of 81 %, 90 % and 90 % respectively. Recovery and payability assumptions are taken from preliminary studies.
Location and history
East Cornwall is a region familiar with mining, surrounded by pastures and low-density housing, with excellent road, power and communication infrastructure, and a supportive local council, eager for mining to recommence.
The legacy Redmoor mine was last in operation in 1943. The area is rich with what remains of the tin and copper operations. Tin has been worked since antiquity, initially by surface tin-streaming methods, naturally progressing to underground operations as technology improved, via rapid development and deepening, during the Industrial Revolution. Copper and arsenic sulphide zones were often encountered beneath existing surface tin workings, and east Cornwall and west Devon became renowned for the production of these commodities and tungsten. During the two world wars, tungsten’s popularity and demand grew with the advent of magnetic separation.
Between 1978 and 1986, Southwest Consolidated Minerals Limited (SWM) explored the area utilizing geochemistry, geophysical and drilling programs, including over 30 diamond drill holes. SWM’s exploration led to the definition of an initial low-grade, tin-tungsten resource and complete technical studies. In 1986, work ended due to tin’s global price collapse, however, SWM’s invaluable amassed information has remained available, presenting an ideal opportunity for CRL to become involved.
Redmoor’s targeted orebody is an exo-greisen SVS. The deposit is hosted in Devonian slates and shales, comprising a 90-meter (295 ft) thick zone of veins dipping steeply to the north-north-west, with a strike-extent of at least 950 m (3117 ft) and a dip extent of over 550 m (1804 ft). It remains open at depth and along-strike to the west. The system is constrained to the east by Kit Hill Granite – a small local stock, which may have been a driver for the development of adjacent mineralization at Redmoor. Within the described envelope, many 5-15 m (16-49 ft) thick, high-grade zones of focussed veining occur, and it is these zones which constitute the current resource. Outside the main SVS are a number of other exploration opportunities in the form of Cornish lode-style structures, some of which were exploited in the 18th and 19th centuries, and are yet to be fully evaluated by CRL.
Drilling was undertaken by two contractors: Energold UK Limited (2017) and Priority Drilling Limited (2018). Operations ran for 12 hours per day, 13 days per fortnight under a General Permitted Development Order (GPDO) – issued by Cornwall Council, allowing work to proceed via its regional authority. Land access agreements were upheld with all surface landowners, utilizing two diamond core rigs at each program to yield H- and N-sized core. In 2017, Energold utilized an Atlas Copco CS1000 and an EGD Portable Series 3 rig to drill 20 holes with an average length of 352 m (1154.86 ft).
In 2018, Priority used two Atlas Copco CS14 rigs to drill 12 holes with an average length of 614 m (2014.44 ft). Both programs generated excellent recoveries, typically within 90-100 %. All holes were surveyed regularly during drilling, allowing close monitoring of deviation in real-time.
Drilling need not be noisy work
Council permissions required noise be at <55 dB LA90 15 minutes at the nearest occupied receptor, which could be as close as 50 m (164.04 ft) from the drill site, and <50 dB on Sundays. To ensure compliance, CRL enforced a noise limit of 50 dB at the boundary of the nearest property with regular monitoring. Straw bale walls and noise-attenuating mats were utilized to reduce drilling process noise. Local farmers were engaged to erect straw bale walls, which led to a local contracting opportunity. Bales were stacked in a brick-wall pattern to a typical height of 4 m (16.40 ft) to encompass the drill rig and working area, with the added benefit of providing a visual screen, thereby improving appearance and aligning with the area’s agricultural character. Both teams regularly inspected the bale walls to uphold their condition, which also boosted morale, providing drill crews with shelter from the wind, and making the drill sites a more comfortable place to work.
Steered drill holes
Variable deviations were encountered during drilling, particularly during longer holes. Dip deviation was typical, involving increasing dip measurements with depth, due to the weight of the drill string pulling the holes downwards. Azimuth deviation was attributed to two factors. First, the alternating competency of the orebody, comprising a series of hard (quartz) veins hosted in soft (slate) material. Second, local housing distribution and accessibility limited viable drill site locations, thereby reducing the ability to address the mineralization at the ideal 90 ° strike angle. These two factors caused the drill holes to tend to swing in azimuth turning towards parallelism with vein orientation. This deviation was a notable challenge as the drill program was expected to deliver regularly spaced intersections through the ore body. Fortunately, CRL and the drill contractor teams cooperated and worked well together. One hole utilized a steered drilling intervention, as deviation was so great due to old mining voids encountered early in the hole’s development.
This technique proved satisfactory and would otherwise have missed the intended target. Empirical testing of the degree of deviation led to greater understanding and more precise hole planning, including predicted deviation. The work was very successful, resulting in holes of over 500 m (1640.42 ft) that regularly arrived within 15 m (49.21 ft) of target position without further recourse to steering.
Reflex’ ACT tool was utilized for orientation marks on the majority of core drilled in both programs so that CRL geologists could then measure structures and vein orientations in the logging facility. In all, 1946 structures were measured, including bedding, intrusive contacts, joint sets, veins and faults. This data has been crucial in interpreting geological setting, orebody model, structural orientation and variability of vein sets, and geological continuity of the orebody at depth. It has also been instrumental in developing future drill program targeting, examining extensions to the SVS.
Community perception was a key issue, having the potential to adversely affect CRL’s social license to operate (SLO), possibly limiting drill programs and the development of a future mine. It was therefore important to proactively manage the relationship and conduct positive community liaison – a key pillar of CRL’s operating philosophy. Multiple engagement meetings were well-attended by community members and officials to outline and discuss the work, before and during the two drill campaigns. The meetings provided CRL with a platform to explain plans while identifying and mitigating concerns. A delegate was appointed as the dedicated contact to immediately address community concerns. The CRL team and its two separate drilling contractors are proud of the fact that there were no complaints. They intend to maintain this record by sharing timely and clear communications, operating with consideration, and responding rapidly to community queries.
CRL began exploration in January 2017, with an aim similar to SWM’s of targeting the lode extensions and exploring potential around the SVS. Dynamic drill planning yielded strong results that refocussed the target to the SVS. Over 7000 m (22 966 ft) of diamond core was drilled in 20 holes in the first year of drilling, with encouraging results that included an apparent 14 m (45.90 ft) at 1.14 % tin equivalent from 457.10 m (1500 ft), containing 6 m (19.70 ft) at 1.98 % from 465.10 m (1526 ft) at CRD019. All drill core was digitally logged for lithology, veining, mineralization, weathering, geotechnical characteristics, and structure. Mineralized core sections were halved using a diamond core saw and analyzed at ALS, Ireland by ICP-AES multi-element analysis, following sodium peroxide fusion. A robust QA/QC program identified at an early stage, the potential for the coarse nature of high-grade tungsten mineralization to cause a nugget effect more often seen in high-grade gold samples. As a result, a customized sample preparation protocol was applied so that a large proportion of the crushed sample was pulverised, achieving good repeatability of sample analyses. Another QC success was in identifying that the selected technique was underestimating tungsten composition at high grades. In instances where tungsten grades exceeded 0.5 %, an additional analysis was applied using XRF to improve high-grade accuracy. The geological and analytical data was created and maintained in a database using Micromine software. While working in this 3D environment, CRL geologists carried out ‘live’ geological modeling, supported by structural orientation data, keeping the drill program informed as it progressed. The model was then transferred to independent resource consultants for resource modeling once the drilling was completed.
The 2017 drill campaign resulted in a new geological model of the SVS. A JORC-compliant inferred resource was issued by SRK (UK) in March 2018 of 4.5 Mt at 1 % tin equivalent1, contained within the SVS. This resource, together with Exploration Target potential, defined by notably high-grade assays in deeper holes (not included in the 2018 resource due to data spacing), encouraged CRL to conduct a second drilling campaign.
The second drill program occurred during Q3 and Q4 2018, with the aim of expanding the new inferred resource by drilling exploration targets down-dip and along-strike. The second campaign was completed in December 2018 and totalled over 7300 m (23 950.13 ft) of diamond drilling. Every drilled hole hit mineralization, and in most cases, it was of a stronger tenor than the 2017 work, validating the previous model. This led to an updated inferred mineral resource estimate being issued in March 2019 of 11.7 Mt @ 1.17 % tin equivalent, using a 0.45 % tin equivalent cut-off grade (Geologica UK).
The increased size and grade of the updated mineral resource – a 200 % increase in contained metal – is due to 2018’s highly positive drill hole results, confirming the strength of CRL’s geological model in projecting interpreted high-grade zones along-strike and down-dip. As a result, a significant proportion of March 2018’s Exploration Target was converted to Mineral Resource in the 2019 work, and a further Exploration Target of 4-8 Mt at 1-1.4 % tin equivalent was then defined.
In less than three years, CRL have successfully produced a globally-significant tin-tungsten-copper inferred resource and have completed preliminary mining and metallurgical studies, suggesting good economic fundamentals for the project. This work was made possible due to CRL’s efficient and high-quality operations which included positive community engagement and maintenance of social license to operate (SLO) throughout the drilling and associated operations. The geological success should not be attributed to ‘joining the dots’ between good assays. Rather – it was a nimble and investigative geological approach – ready to modify and optimize plans quickly, and follow-up on good results; maximizing interactions with local academic and industry experts, and developing a profound depth of geological knowledge in a high-quality and professional technical team. The success of this project highlights the potential within the Cornwall and West Devon region. Abundant resources remain in the ground, and a mining resurgence is possible if modern exploration techniques are applied. As Redmoor suggests, new mines in the region have the potential to be exceptionally high-grade, with limited impact, providing a solution to evolving societal demands for clean and low emission metal production.
For more information visit: www.cornwallresources.com