About Covas Project
The past producing Covas Tungsten Mine is located about 100 kilometres north of Porto, Portugal's second largest city. Historic records show that past production from the mine between 1951 and 1974 totaled 366,000 tonnes at 0.61 per cent WO3
. Ore was mined from an open pit and shallow underground workings, and processed using gravity and flotation methods, producing separate wolframite and scheelite mineral concentrates. Subsequent exploration by Union Carbide and others, from 1974 to 1980, included 329 diamond drill holes. Recently, Blackheath Resources has drilled a further 52 drill holes resulting in the determination of the current NI 43-101 Resource Report
, which is one of the highest-grade reported open-pit tungsten project in Europe. Mineralization remains open in multiple directions including high-grade potential areas that will be tested during the next drill campaign focusing on resource growth and grade increase.
The Covas property was awarded a three year Experimental Mining Licence, renewable for a further two years before the grant of a full Mining Licence, (August 20, 2013). Blackheath holds the property under an option from Avrupa Minerals Ltd. under a previously announced amendment to the Covas Joint Venture earn-in agreement (see news release of May 12, 2014). To date Blackheath has earned an 75% interest in Covas.
The Covas Tungsten Project, which consists of seven separate deposits in close proximity, is estimated to contain:
- 449,800 MTUs WO3 indicated mineral resource based on 1,081,000 tonnes at an average grade of 0.42% WO3
- 767,100 MTUs WO3 inferred mineral resource based on 2,211,000 tonnes at an average grade of 0.35% WO3
The current indicated and inferred mineral resources are stated using a cut-off grade of 0.10%. 19 total drill holes had interval grades capped to a lower average grade, allowing for a more conservative average grade. The price of tungsten is currently approximately $270 per metric tonne unit (MTU) (10kg) of contained tungsten trioxide (WO3).
Resource Estimate Highlights:
- Substantial increase in MTUs over previous historical estimates
- Shallow depth. Over 80% of the resource is located within 60 metres of surface
- Mineralization remains open in multiple directions including high-grade potential areas that will be tested during the next drill campaign focussing on resource growth and grade increase
- Higher grade resource using a 0.25% WO3 cut-off
- 375,200 MTUs WO3 indicated based on 686,000 tonnes at an average grade of 0.55% WO3 and
- 584,900 MTUs WO3 inferred based on 1,281,000 tonnes at an average grade of 0.46% WO3
Past production at Covas is well documented. The mineralized material was produced in small open pits in two ore deposits (Fervença and Cerdeirinha) and from shallow underground workings at Valdarcas, mined using the shrinkage mining method. Two Portuguese companies (Geomina Lda. and Gaudêncio, Valente e Faria Lda.) exploited some of the depositsand the recorded production from mjines of both companies during the period 1951 to 1974 was 366,000 tonnes averaging 0.61% WO3
The two economic tungsten minerals (wolframite and scheelite) are crystallized in fine or medium grain size. The main gangue minerals are silicates and apatite, together with abundant sulphides, mainly pyrrhotite, arsenopyrite and pyrite. In the late 70's, the same tailings were retreated for scheelite recovery by flotation.
Conventional ore dressing (80% liberation at 48 mesh) produced a concentrate with an average wolframite/scheelite ratio of 1.6:1. The capacity of the beneficiation plant at Covas was 50 tonnes/8 hours. The ore processing flow sheet included a vibrating grizzly to split the ore size in two fractions; +25 mm and - 25 mm.
- The +25 mm material was crushed in a jaw crusher in closed circuit with the grizzly. The -25 mm was crushed in a roll crusher and sent to a rod mill working in close circuit with vibrating screens.
- The - 1mm ground material was then sent to flotation cells for sulphides flotation. The floated materials (sulphides) of this circuit were final tailings (reverse concentration) and sent to the tailings pond.
The (un-floated) concentrates were then classified in a hydrosizer in two fractions; coarse and fines. Both products were concentrated by gravity in vibrating shaking tables. The pre-concentrates were briefly conditioned with sulphuric acid and "gasoil" prior to passing over vibrating tables again. The table concentrates were dried and screened to prepare two size lots. Each size fraction was treated separately using magnetic separators.
Two types of concentrate were then produced; a wolframite concentrate and a scheelite concentrate. Both were subjected to a leaching process using hydrochloric acid (HCl) to remove some phosphate material to produce the final products. The wolframite grade concentrates were typically 70% WO3
and the scheelite grade concentrates were typically 75% WO3
During the period 1970 to 1972, a Czech Republic Company (Metalimex) carried out exploration work, including trench sampling of the outcrop skarn bodies and a diamond drilling campaign, mainly at the Lapa Grande and Muito Seco areas. The results of that work are not well recorded. At the same time, the Portuguese Governmental Organization for Exploration (Serviço do Fomento Mineiro - SFM), at the request of Geomina Lda, undertook an extensive magnetic survey of a large area of 4 square kilometers.
Union Carbide Exploration
In 1974, Union Carbide Geotécnica Portuguesa - Assistência Mineira Lda, a 100% Portuguese subsidiary of Union Carbide Corporation of New York, decided to invest in exploration for additional reserves and resources of WO3. Union Carbide signed option agreements with both of the previous operating companies - (Geomina Lda. and Gaudêncio, Valente e Faria Lda.) and the Portuguese Government. Starting in 1974 and until the end of 1979, this new Company undertook a complete comprehensive exploration programme including:
- Geologic mapping at scales of 1:5000; 1:2000 and 1:500. (Blackheath is in possession of basic geological maps for the area and a number of plans and geological drill sections. There is a very large exploration database held by INETI which is also available.)
- Geophysical prospecting using magnetometer surveys, induced polarization, resistivity and VLF-EM. Geochemical prospecting including soil sampling, stream sediments and rock analysis of diverse samples of outcrops of skarn.
- Diamond drilling with a total of 329 drill holes totalling 26,403 metres. (There is a very large amount of data available for the drilling and Blackheath has considerable but not all useful drill data.)
- One of the diamond drill programs carried by Union Carbide Corporation in 1979 was to obtain samples for metallurgical testing.
Over the life of exploration of the Covas tungsten project, it is estimated that Union Carbide and other companies expended millions of dollars. As an index of partial expenditures, the 329 drill holes totaling approximately 26,000 meters, would cost, in terms of present drilling costs at Can $150 per meter of core, approximately Can$ 3.9 million. This does not include geological, geophysical or geochemical studies, metallurgical work and other engineering costs. The mines included at least five open pits (Cerdeirinha, Fervenca and Valdarcas, (Geomina Limitada), Mina de Fraga and Lapa Grande (Gaudencio Valente and Faria Lda.), and underground workings (Valdarcas). Numerous geological reports and detailed geological plans and sections which have described much of the geological value of the work expended are available in files belonging to INETI and other government departments. Blackheath has access to a large database of technical information.