Fair value measurement - Summary of carrying values and fair values of financial instruments (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Trade and other payables
FINANCIAL LIABILITIES
Carrying value	$ 141.1	$ 124.3
Fair value	141.1	124.3
Long-term debt
FINANCIAL LIABILITIES
Carrying value	394.9	491.0
Fair value	355.0	530.8
Gold stream obligation
FINANCIAL LIABILITIES
Carrying value	174.7	194.0
Fair value	174.7	194.0
New Afton
FINANCIAL LIABILITIES
Carrying value	378.9	467.4
Fair value	378.9	467.4
Cash and cash equivalents
FINANCIAL ASSETS
Carrying value	200.8	481.5
Fair value	200.8	481.5
Trade and other receivables
FINANCIAL ASSETS
Carrying value	15.9	14.2
Fair value	15.9	14.2
Provisionally priced contracts
FINANCIAL ASSETS
Carrying value	2.3	1.2
Fair value	2.3	1.2
Gold and copper swap contracts
FINANCIAL ASSETS
Carrying value	(4.1)	(1.8)
Fair value	(4.1)	(1.8)
Foreign exchange forward contracts
FINANCIAL ASSETS
Carrying value	0.4	1.5
Fair value	0.4	1.5
Proceeds due from income tax refunds at Mesquite
FINANCIAL ASSETS
Carrying value	0.0	12.8
Fair value	0.0	12.8
Investments
FINANCIAL ASSETS
Carrying value	35.6	59.5
Fair value	$ 35.6	$ 59.5

The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

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