(IDEX Online) - Lucara says the underground expansion plan at its Karowe diamond mine, in Botswana, will take longer and cost more because of technical problems.In an update issued yesterday, the Canadian miner said it now expects the project to cost $683m, an increase of 25 per cent on the original $547m estimate.Production from the underground project is now expected to start in the first half of 2028, rather than the second half of 2026.The underground expansion should extend the life of Karowe until at least 2040 and deliver around $4bn of additional revenue.Eira Thomas, CEO said: "Lucara has made tremendous progress on the Karowe underground expansion project, despite many challenges over the last year as we transitioned into the main sink phase of the underground development. "Schedule delays due to longer than anticipated grouting activity have impacted the project timeline, however the grouting methodology selected has proven to be effective in controlling water inflows."Open pit operations, which began at the mine in 2012, are expected to end in 2026.Karowe is one of the world's biggest producers of large, high quality, Type IIA diamonds. It has recovered three +1,000-cts stones, including the 1,758-cts Sewelo and the 1,109-cts Lesedi La Rona.Pic courtesy Lucara shows the Karowe mine