Vast underground deposits of metal ores yet to be mined at South Crofty in Cornwall could be worth in excess of £1.5 billion.
The unexploited tin, copper and zinc, along with cobalt, indium, wolfram and silver were valued in a report released yesterday.
Celeste Copper Corporation, which hopes to start commercial mining at the site between Redruth and Camborne within three years, confirmed that the estimated value was based on current world metal prices.
The report said tin deposits alone amounted to some 7.95 million tonnes.
Alan Shoesmith, who was appointed Celeste CEO earlier this year, said: "This is a report prepared by an independent expert who, under Canadian regulations, is deemed a 'qualified person'. Perhaps the most important element of the report is the quantification of exploration targets. They are a reappraisal of remaining mineralisation after historic mining activities."
The operation at South Crofty has attracted investment of £15m in the past 10 years, but up to £75m will be needed to enable the Canadian-owned company to move to full production.
Over the next 12 months, and in preparation for full production, the focus will be on pumping out shallow underground workings in the Dolcoath section of the mine. The company has said it is mainly interested in mining the areas of rock between existing flooded workings, but some parts of the huge network of shafts and levels will need to be pumped for safety reasons.
Crofty closed in 1998 after the price of tin hit an all-time low. But for the past six years international prices have been rising. In May this year Celeste announced it was making the Cornish mine its number one priority worldwide. It estimated a total investment of £100 million and the creation of more than 220 jobs.
Mr Shoesmith, who has more than 40 years of commercial and financial experience, said at the time: "Cornwall is rich and its resources should be exploited. The economic benefits this can bring to Cornwall are phenomenal."
A poly-metallic mining operation, the intention is to extract and process a range of metal ores. As well as the underground operation, a large semi-subterranean processing plant will be built. Ore will be fed directly into this "mill" and only the valuable concentrates brought to the surface for distribution.