First Quantum Minerals refuted claims from the Zambia Revenue Authority (ZRA) that the company had an outstanding tax bill of roughly US$8 billion.

The claim originates from US$540 million of equipment brought in by First Quantum between 2012 and 2017 for its Sentinel copper mine that the ZRA says were miscoded, resulting in underpaid import taxes. The majority of the bill comes from a resulting US$5.7 billion in interest and US$2.1 billion in penalties.

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In a statement released by First Quantum on Tuesday, the Vancouver-based miner said they “unequivocally refute the assessment, which does not appear to have any discernable basis of calculation.”

In a conference call with investors on Wednesday, CEO Philip Pascall stated that First Quantum will assign a team to thoroughly review the more than 23,000 relevant bills of entry, a process he estimated would take four to six months.

Pascal acknowledged there may have been miscoded imports but was confounded by the size of the bill, saying: “The penalty is not reflective of what the penalty might be had someone completed a full review and ended up with a difference of any description.” Pascal added that the ZRA already approves the assessment and duty payable on every imported item.

First Quantum’s share price dropped 12 per cent on the Toronto Stock Exchange to close at $16.97 on Tuesday but had partly recovered as of Thursday with the price returning above$18 per share. The company’s Zambia operations –the Sentinel mine and Kansanshi mine, the largest copper mine in Africa – account for 84 per cent of its total revenue.