Ann: Trading Halt, page-93

From the Randgold 2017 Annual report:

Produced 70,000 oz, sold 67,800oz (Higher due Domba processing)
Sales $86.1m
Profit from mining activity 19.1m EBITDA
30% tax on that = $5.73m with 80% ownership = $4.584

So assuming no interest/depreciation the total tax bill would be US$4.584m

The above figures are audited, so would stand up in arbitration
They don't quote the breakdown of tax paid for Morila, I'm assuming it would be the $4.584m less any depreciation etc. Overall Randgold paid over $50m in taxes and only had a 40% interest in Morila.
Their attributable 28k oz at Morila was insignificant compared with the 1.315m oz they produced in 2017
I can't see the shortfall the tax office are claiming as being very big, the most it could be is $4.584m and that's assuming they paid no tax at all and had no depreciation.