Its London-listed shares fell 5.4% by 1257 GMT, making it the worst performer in an index of its peers. <.FTNMX1770> Shares touched their highest since June 2018 in the previous session.
Antofagasta's mines have operated with about two-thirds of the usual workforce on site because of the COVID-19 pandemic.
Disruptions to mining operations in Chile, the world's largest producer of mined copper, have been minimal, but helped to delay mining projects.
Antofagasta said a six-month delay to its flagship Los Pelambres mine had cost it $50 million (38 million pounds). It said work on all projects had resumed in stages.
The redesigning of a desalination plant at Los Pelambres would cost an additional $150 million.
Other costs are lower, however, due to cheaper fuel and a weaker local currency, and Antofagasta reiterated its full-year targets for production, costs and spending.
Realised copper prices were 12.5% lower in the first half compared to a year ago, while sales were down 2.2%, Antofagasta said.
This pressured the miner's earnings before interest, tax, depreciation, and amortisation (EBITDA), down 22% to $1.01 billion for the six months ended June 30.
Prices of copper, used in power and construction, have rebounded nearly 50% from lows in March to around $6,600 per tonne, lifted by supply concerns and higher demand from top consumer China. [MET/L]
Antofagasta, which cut its 2019 final dividend by $70 million in May, declared an interim payout that was 42% lower at 6.2 cents per share compared to the same period last year.
This was still in line with its policy of paying a minimum of 35% of underlying net earnings and was slightly ahead of consensus.
"It (the interim dividend) does indicate that the Antofagasta team feels it and its operations have adjusted to the present situation and operations are running well," Peter Mallin-Jones, an analyst at Peel Hunt, said.
Antofagasta's bigger London-listed rivals BHP, Anglo American and Rio Tinto have also kept dividends, although Glencore has not.
Looking to the longer term, a final investment decision will be made in early 2022 on a second concentrator at the Centinela mine, whose cost is expected to be around $2 billion, versus the $2.7 billion previously estimated, CEO Iván Arriagada told Reuters.
(Reporting by Shanima A in Bengaluru and Zandi Shabalala in London; editing by Uttaresh.V, Aditya Soni and Barbara Lewis)
By Shanima A and Zandi Shabalala