Lease liabilities - Sale and leaseback (Details) $ in Thousands	6 Months Ended
Jun. 30, 2023 USD ($)
Lease liabilities.
Total proceeds from sale and leaseback arrangement	$ 16,564
ROU asset recognized as part of a sale and leaseback arrangement	15,733
Financial liability recognised as total proceeds from the sale of the equipment exceeded the fair value of the equipment at the time of sale	$ 609

The amount of financial liability recognised as total proceeds from the sale of the equipment exceeded the fair value of the equipment at the time of sale.

No definition available.

Name:	mtal_FinancialLiabilityRecognisedAsTotalProceedsFromSaleOfEquipmentExceededFairValueOfEquipmentAtTimeOfSale
Namespace Prefix:	mtal_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant