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MANAGEMENT OF FINANCIAL RISK (Disclosure of Nature and Extent Arising from Financial Instruments - Commodity Price Risk) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Increase (decrease) in sales due to price changes of commodity | $ 700 | $ (1,500)
Liquidity amount available | 213,100 |
Silver Commodity Type [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 433 |
Change percentage, commodity | 10.00% |
Gold Commodity Type [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 246 |
Change percentage, commodity | 10.00% |
Lead Commodity [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 488 |
Change percentage, commodity | 10.00% |
Zinc Commodity [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 449 |
Change percentage, commodity | 10.00% |

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- Definition Increase (decrease) in sales due to price changes of commodity. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Liquidity amount available. + References No definition available. + Details
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Namespace Prefix: | fsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Percentage change on commodity price percentage. + References No definition available. + Details
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- 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
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- Details
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Namespace Prefix: |
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Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |