U–Pb zircon dating endeavors on the host rocks of the Juomasuo gold deposit, Kuusamo supracrustal belt, eastern Finland

In situ U–Pb zircon dating was done for the host rocks of the Juomasuo gold deposit, located in the\nPaleoproterozoic Kuusamo supracrustal belt, eastern Finland. For dating purposes, five samples were\nchosen to represent the typical volcano-sedimentary rock types of the deposit in their mineralized and\nunmineralized form. The samples were studied petrologically and dated using the LA–MC–ICP/MS\ntechnique at the research laboratory of the Geological Survey of Finland and the Finland Geoscience\nLaboratory (SGL).\n\nThe U–Pb data revealed heterogenic detrital zircon populations even for the felsic volcanic rocks, and\nthus, all the samples were delineated as sedimentary/volcano-sedimentary sequences. The maximum\ndeposition ages fall in the 2.75–2.65 Ga window, and not a single Paleoproterozoic age was\nmeasured in the bulk of 159 measurements. The pervasively altered samples from the mineralized\nsequence of the Juomasuo deposit hosted a younger population of homogenized, BSE-pale/CL-dark\nzircons and domains within zircon grains, with 2.62–2.60 Ga ages. This age can be linked to the\nNeoarchean crustal growth event at 2.7–2.6 Ga, described from the Archean terrains all over the\nworld, including the Archean of the Karelia Province in Finland. Unfortunately, in the limits of this\nthesis, the homogenization age cannot be interpreted as the minimum depositional age of the studied\nmetasedimentary rocks, as it remains unclear, whether the homogenization of the zircons took place\nbefore or after the deposition.\n\nAlthough the majority of the precursors of the studied supracrustal rocks from Kuusamo were\npresumably deposited between 2.43 and 2.21 Ga, there are no corresponding ages of volcanism in\nthe area. The lack of U–Pb ages of that time frame is a global phenomenon, and the topic is thus\npertinent to modelling of Paleoproterozoic plate tectonics. It may be that the Kuusamo supracrustal\nbelt represents a deposition during a global slowdown in plate tectonics, thus recording an anomalous\nperiod in the history of planet Earth.