The estimate is now 256 million ounces of silver equivalent at 789g/t, adding lead and using higher cut-off grades for a "more robust and reliable estimate".

Pure silver contained fell 19% from the 2009 estimate to 237Moz, although the average silver grade rose 25% to 731g/t and average vein width increased 15% to 2.2m.

Polymetal said the share of openpit resources comprised 46%, with the share of resource in the indicated category jumped from 50% to 61%, and the share of resources in the two largest veins increased from 73% to 80%.

The drilling results used data from 532 additional diamond drill holes covering 71,276m from 2017 and 2018.

Polymetal CEO Vitaly Nesis said the infill drilling had strengthened the belief that Prognoz may succeed Dukat as one of the largest primary silver deposits in the world.

The company aims to follow this up with 24.3km of diamond drilling in 2019 to upgrade the existing inferred resources to the indicated category, with a minimum 80% conversion rate expected.

It also plans to add at least 60Moz of contained silver by carrying out 15.7km of diamond drilling to establish new resources on extension of the two largest veins along the strike, as well as four other veins.

Polymetal will also do other work in 2019 necessary to complete the pre-feasibility study and estimate JORC-compliant reserves at Prognoz, including advanced metallurgical testing and detailed geo-metallurgical mapping of all indicated resources, geotechnical studies, and an analysis of potential construction sites. 

The company's shares ticked up 0.16% Wednesday to £6.08 (US$8).