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Exploration in non-operating areas (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	$ 8,475	$ 11,879	$ 36,307
Personnel expenses [Member]
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	1,705	3,632	4,830
Services provided by third parties
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	3,326	3,611	22,764
Services provided by third parties
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	1,538	766	1,211
Lands [Member]
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	420	1,528	1,867
Short-term and low-value lease [Member]
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	327	415	1,524
Consumption of materials and supplies [Member]
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	201	328	1,420
Other expenses [Member]
Disclosure Of Exploration In Non-Operating Areas [Line Items]
Expense arising from exploration for and evaluation of non operating units	$ 958	$ 1,599	$ 2,691