Diamond miner De Beers has lowered its production guidance for 2019 to between 31-million and 33-million carats.

The company, which on Thursday reported production of 35.3-million carats for the 2018 financial year, attributes the lower production guidance to its existing Venetia openpit, in South Africa, where the Venetia underground mine will become the principal source of ore from 2023.

“Associated with this, an increased proportion of production in 2019 is expected to come from De Beers Group’s joint venture (JV) partners, a proportion of which generates a trading margin, which is lower than the mining margin generated from own mined production,” the miner explained.

Meanwhile, De Beers’ 2018 production was in line with guidance of 35-million to 36-million carats and 6% higher than the 33.45-million carats produced in 2017.

The miner increased its rough diamond production by 12% to 9.1-million carats in the fourth quarter of 2018, with Debswana, in Botswana, having contributed 6.3-million carats.

The Orapa mine increased its output by 20% year-on-year to 3.6-million carats, driven by an expected favourable grade and higher plant use.

At the Jwaneng mine, production increased by 9% as a result of an increase in tonnes treated.

Further, Namdeb Holdings, in Namibia, increased its output by 3% year-on-year to 500 000 ct, as a result of the Mafuta crawler vessel, operated by Debmarine Namibia, having spent fewer days in port.

This was partly offset by the land operations following the transition of Elizabeth Bay to care and maintenance.

In South Africa, De Beers Consolidated Mines increased its output for the quarter by 7% to 1.2-million carats as a result of planned higher grade ore at Venetia.

De Beers’ diamond output in Canada was up by 5% to one-million carats for the quarter owing to higher grades at the Victor mine. This was partially offset by planned lower grades at the JV Gahcho Kué mine.

For the full year, Debswana produced 24.13-million carats – a 6% increase on the 22.68-million carats produced in 2017.

Namdeb increased its production by 11% year-on-year to two-million carats, but DBCM’s production decreased by 10% year-on-year to 4.68-million carats.

De Beers’ Canadian production increased by 19% year-on-year to 4.48-million carats.

Meanwhile, rough diamond sales volumes for the fourth quarter reached 9.9-million carats, compared with the 8.2-million carats sold in the fourth quarter of 2017.

For the full year, rough diamond sales volumes decreased by 4% to 33.7-million carats, compared with the 35.1-million carats sold in 2017.

The consolidated average realised price of $171/ct was 6% higher year-on-year, owing to a lower proportion of lower-value rough diamonds sold in 2018.