(IDEX Online) - Rio Tinto reported a 37 per cent drop in Q3 production at its only diamond mine, Diavik, in Canada's Northwest Territories.It said the completion of an underground pipe, and an area of the open pit at the mine, 200 miles from the Arctic Circle, were to blame for the lower output.Rio Tinto said Q3 production for the three months to 30 September was 757,000 carats, down from 970,000 in Q2 and 954,000 in Q1.In February, it announced plans to invest $40m in underground mining that would prolong Diavik's life to early 2026.Rio Tinto, the world's second-largest metals and mining corporation, said underground mining below the existing A21 open pit operation should yield an additional 1.4m carats.Pic courtesy Rio Tinto of Diavik mine