Financial instruments and financial risk management - Market risk, Interest rate risk (Details) - Interest rate risk - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2025	Dec. 31, 2024
Financial instruments and financial risk management
Percentage of reasonably possible increase (decrease) in risk assumption	1.00%
Increase (decrease) in interest expense due to reasonably possible increase (decrease) in designated risk component	$ 563	$ 851

The increase (decrease) in interest expense as a result of an increase (decrease) in a designated risk component that was reasonably possible at the end of the reporting period. Excludes defined benefit plan obligations.

No definition available.

Name:	mtal_IncreaseDecreaseInInterestExpenseDueToReasonablyPossibleIncreaseDecreaseInDesignatedRiskComponent
Namespace Prefix:	mtal_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration