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INCOME TAX (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
CANADA
Income Taxes [line items]
Resource related expense accumulated with no deferred tax benefit recognized	$ 7.5	$ 8.2

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- Definition

Income Taxes [line items].

+ References

No definition available.

+ Details

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Namespace Prefix:	fsm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

Resource related expense accumulated with no deferred tax benefit recognized.

+ References

No definition available.

+ Details

Name:	fsm_ResourceRelatedExpenseAccumulatedWithNoDeferredTaxBenefitRecognized
Namespace Prefix:	fsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Details

Name:	ifrs-full_GeographicalAreasAxis=country_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: