The Toronto-based miner produced 1.07 million ounces of gold and 83 million pounds of copper in the June quarter, "roughly in line" and slightly lower than the previous quarter respectively.
Barrick reduced its copper guidance from 385-450 million pounds to 345-410Mlb and also increased its all-in sustaining cost forecast to US$2.55-$2.85/lb from $2.30-$2.60/lb.
It expected a quarter-over-quarter increase of 11-13% on its C1 cash costs for June due to higher crusher repair costs.
"The revisions to our copper production and cost guidance primarily reflect operational challenges at Lumwana in the first half of the year," the company said.
"We expect higher production at Lumwana in the second half of 2018, driven by a steady improvement in grade and improved crusher reliability."
Lumwana contributed 47Mlb of copper to Barrick's June quarter production total of 83Mlb.
The company is expecting gold production to increase in the second half after major planned shutdowns were completed in the first half, along with reduced development and stripping in the second half.
It kept 2018 guidance at 4.5-5Moz, at an all-in sustaining cost of $765-$815/oz.
The outlook is likely to see the gold major fall behind US-based peer Newmont Mining in terms of gold production this year, with Newmont last year announcing improved 2018 guidance of 4.9-5.4Moz.
Barrick has forecast lower gold costs in the second half, with increased production slated from its lower-cost Barrick Nevada and 60%-owned Pueblo Viejo operations.
It also said full processing capacity had been restored earlier than expected at its Porgera joint venture in Papua New Guinea after a February earthquake.
Barrick's average market price was $1,306/oz of gold and $3.12/lb of copper during the quarter.
The company will report its quarterly results on July 25.
Barrick shares closed down 3.24% yesterday as both gold and copper prices fell.