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Guilmette, Carl

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Université Laval. Département de géologie et de génie géologique
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Gold mobilization during metamorphic devolatilization of Archean and Paleoproterozoic metavolcanic rocks

2020-07-21, Patten, Clifford, Guilmette, Carl, Pitcairn, Iain, Beaudoin, Georges, Molnár, Ferenc, Kolb, Jochen, Peillod, Alexandre

Volcanic rocks in Archean and Paleoproterozoic greenstone belts are abundant and have been suggested as potential Au source for orogenic Au deposits. The behavior of Au during metamorphism of these rocks is however, poorly known. We present ultra-low detection limit Au analyses from a suite of variably metamorphosed rocks from the Archean La Grande subprovince, Canada and the Paleoproterozoic Central Lapland greenstone belt, Finland. Both areas are well endowed in Au and have great potential for discovery of new orogenic Au deposits. The metavolcanic rocks in these belts are grouped in tholeiite and calc-alkaline magmatic series, for which the protolith Au contents are calculated using Au-Zr/Y power law regressions from greenschist facies samples. In the tholeiitic rocks, Au is compatible during magmatic processes and decreases with differentiation, whereas in calc-alkaline rocks Au is incompatible and increases with differentiation. Mass variation calculations show that up to 77 % and 59 % of the initial Au content is lost during progressive metamorphism up to upper amphibolite facies conditions (>550 °C) in La Grande and Lapland respectively. This study highlights first that metavolcanic rocks release Au during metamorphism in Archean and Paleoproterozoic greenstone belts and are thus a good potential source rocks for orogenic Au deposits, second that the Au fertility of the metavolcanic rocks is controlled by their mantle source and magmatic evolution and third that the metamorphic devolatilization model can be applied to Archean and Paleoproterozoic orogenic Au deposits.