Atlantic, which revealed an independent audit of its reserve base by Pennsylvania-based consultancy John T Boyd completed at the end of 2012, said it had 1.777 million tons at the Hazleton mine in Schuylkill County, rather than 1.375Mt.

Officials said the finding added about four years to Stockton’s lifespan, from 2020 to 2024, assuming last year’s production totals.

The company’s reassessment comes hot on the heels of its record production levels in 2012 of 161,659 tons. If that performance is taken into account, it represents an effective increase of more than 560,000t over the end of 2011 – or a 41% jump.

“We are pleased to announce this significant increase in Stockton's reserve base and subsequent expected mine life extension,” managing director Steve Best said.

“Stockton remains an important element of our strategy to become a major mid-tier anthracite producer.

“With an additional 400,000 tonnes (after deduction of the 2012 production of 161,659 tonnes) of high anthracite identified on site, and our recent acquisition of the Pott & Bannon site, I believe that we can look ahead with confidence over the coming months as we advance our production portfolio.”

Atlantic will confirm the revised estimate with drilling to determine the extent of prior bypassed coal on the block’s south wall, after which it will develop an updated mine plan for the recovery of remaining coal.

The UK-based producer is listed on AIM under the ticker ATC and on the OTCQX under ATCLY. Mid price at Monday market close was GBP0.165 ($US0.258), up about 10%.