Lithogeochemistry of the Shurlovaara Banded Iron Formation, Kostomuksha Greenstone Belt, Karelian Craton, Russia

A sample of a Mesoarchean banded iron formation (BIF) from the Shurlovaara Formation was studied. It consists of three types of layers: an iron-enriched (magnetite quartzite) type, a silica-enriched type (quartzites), and a type consisting of chlorite-amphibole schist. The BIF displays a sedimentary cyclicity: a cycle begins with schists, which are followed by magnetite quartzites, and then by quartzites. The mineral and chemical composition of minibeds of each type has its own distinctive features. The quartzites consist of quartz with accessory magnetite, chlorite, carbonate, pyrite, apatite and amphibole impurities. They are depleted in all minor and trace elements, except for Ba, Rb and As. Rare-earth elements (REEs) are characterized by depletion in light REEs (LREEs) and the presence of a poorly defined positive Eu-anomaly. The magnetite quartzites consist mainly of magnetite, quartz and chlorite with amphibole, apatite and pyrite impurities. The rocks of these minibeds are a major concentrator of V and Ga. They are considerably depleted in LREEs and clearly show a positive Eu-anomaly. The chlorite-amphibole schists are made up of quartz, chlorite and amphibole; pyrite, magnetite, titanomagnetite, apatite, and zircon are present as secondary and accessory elements. The schists are richer in practically all minor and trace elements than other rocks of the minibeds. REEs in the schists are enriched in heavy REEs, and their positive Eu-anomaly is the most defined. The chlorite-amphibole schists are modelled as a mixture of the destruction products of Kontokki komatiite-basaltic and felsic volcanics with addition of iron-silica substrate.