Toronto, Canada-headquartered Barrick Gold has maintained its gold production guidance for 2018 at between 4.5-million and 5-million ounces, but has slashed 40-million pounds off its copper production guidance, owing to operational challenges at its biggest copper mine – Lumwana, in Zambia.

The miner trimmed its copper production guidance from between 385-million and 450-million pounds, to a range of 345-million to 410-million pounds, at higher-than-initially-thought costs. The all-in sustaining cost (AISC) will increase to between $2.55/lb to 2.85/lb, from an initial guidance of $2.30/lb to $2.60/lb.

The revisions to the copper production and cost guidance primarily reflect operational challenges at Lumwana in the first half of the year, Barrick explained in its preliminary second quarter results announcement, released after the Canadian markets closed on Wednesday.

The miner said production at Lumwana should be higher in the second half of the year, driven by an improvement in grade and improved crusher reliability. In the first half, Lumwana produced 95-million pounds, accounting for most of the 168-million pounds of copper that Barrick produced in the six months.

Total second quarter copper output was 83-million pounds.

Meanwhile, Barrick’s gold operations performed in line with its expectations, with the operations producing 1.07-million ounces in the second quarter, taking half-year output to 2.12-million ounces.

“We expect gold production to be higher in the second half of the year following the completion of major planned maintenance shutdowns in the first half of 2018, along with reduced development and stripping in the second half of the year,” Barrick reported.

Gold cost of sales is forecast in the $810/oz to $850/oz range, cash costs in the $540/oz to $575/oz range and AISC is forecast to come in at between $765/oz to $815/oz.

The miner said increased production from the lower-cost operations at Barrick Nevada, in the US, and Pueblo Viejo, in the Dominican Republic, should reflect in the lower costs of the second half.

Full processing capacity has also been restored at the Porgera joint venture earlier than initial expectations, following the earthquake that struck Papua New Guinea on February.

Barrick will expand on its second quarter performance when it publishes its quarterly results on July 25.