DEBT - 2019 Convertible Debentures (Details) - 2019 Convertible Debentures - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
Jul. 10, 2024	Dec. 31, 2024	Jun. 30, 2024
DEBT
Interest rate			4.65%
Aggregate principal amount	$ 9,800	$ 9,649
Amount of borrowings redeemed in shares	$ 35,900
Conversion of shares	7,184,000
Conversion price	$ 5
Redemption of debt instrument	$ 400

Amount of cash outflow for interest to holders as part of redemption of debt instrument.

No definition available.

Name:	fsm_DebtInstrumentRedemptionPaymentOfAccruedInterest
Namespace Prefix:	fsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of borrowings redeemed in shares.

+ References

No definition available.

+ Details

Name:	fsm_DebtInstrumentRedemptionSettlementInShares
Namespace Prefix:	fsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The number of shares issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or payments in the period.

+ References

No definition available.

+ Details

Name:	fsm_IfrsDebtConversionConvertedInstrumentSharesIssued
Namespace Prefix:	fsm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

No definition available.

+ Details

Name:	fsm_IfrsDebtInstrumentConvertibleConversionPrice
Namespace Prefix:	fsm_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2025-01-01
-Paragraph 7
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2025-en-r&anchor=para_7&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_BorrowingsInterestRate
Namespace Prefix:	ifrs-full_
Data Type:	dtr-2022:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfDetailedInformationAboutBorrowingsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow to settle borrowings, classified as financing activities. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 7
-IssueDate 2025-01-01
-Paragraph 17
-Subparagraph d
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2025-en-r&anchor=para_17_d&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_RepaymentsOfBorrowingsClassifiedAsFinancingActivities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	ifrs-full_BorrowingsByNameAxis=fsm_DebenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: