TORONTO, (Reuters) – Canada’s Barrick Gold Corp lowered its full-year copper production forecast on Wednesday while increasing its cost estimates, saying the change reflected operational challenges and planned work at its Lumwana mine in Zambia. Barrick, which looks likely to lose its rank as the world’s biggest gold producer this year, maintained its 2018 output estimate of between 4.5 million and 5 million ounces of gold at all-in sustaining costs of $765 to $815 an ounce. Rival Newmont Mining Corp has said it expects to produce between 4.9 and 5.4 million ounces of gold in 2018. Toronto-based Barrick lowered its 2018 copper production forecast to between 345 million and 410 million pounds, from an April estimate of 385 million to 450 million pounds. Mill shutdowns and lower grades hurt Lumwana’s performance in the first quarter. All-in sustaining production costs, an industry benchmark, are forecast between $2.55 and $2.85 per pound of copper for 2018, above its previous estimate of $2.30 to $2.60 per pound. Barrick, which plans to report full financial results on July 25, said it produced 83 million pounds of copper in the second quarter, according to preliminary production data, a slight drop from the first quarter. The company said it expects higher Lumwana production in the second half of the year, reflecting improved grades and crusher reliability. Second-quarter gold production of 1.07 million ounces mirrored first-quarter output, but all-in sustaining costs were about 5 to 7 percent higher, due to planned maintenance at its Nevada roaster and stripping at its Pueblo Viejo mine in the Dominican Republic, Barrick said. Gold production is expected to be higher in the second half of 2018 and at lower costs in comparison to the first six months, as maintenance work is completed and higher grades are mined at Barrick Nevada and Pueblo Viejo, the company said. The average market price for its products in the second quarter was $1,306 an ounce of gold and $3.12 per pound of copper, Barrick said.