Geochemistry of BIF/chert and mafic rocks from the Archean Lamego gold deposit: Constraints on depositional model, metallogenetic and geotectonic implications

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Metamorphosed ferruginous-carbonaceous chert and banded iron formation host gold mineralization in the Lamego orogenic gold deposit inserted in the Neoarchean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil. These chemical sedimentary rocks are classified as Algoma-type BIF and occur in a typical association of deep oceanic rocks. Their dominant iron mineral is primary fine-grained siderite, ankerite is subordinate, both hydrothermal and primary, whereas magnetite and stilplomelane occur locally and are secondary. Their geochemistry and rare-earth elements (REE) patterns indicate sedimentation in a deep marine environment with hydrogenous, high temperature hydrothermal fluids and detrital contributions. Among the main characteristics of the PAAS normalized patterns are positive La, Eu and Ce anomalies; heavy REE enrichment relative to the light and intermediate REE; and chondritic to superchondritic values for Y/Ho ratios. The volcanic rocks from the base of the Lamego stratigraphic sequence vary from basalt to andesitic basalt, with a calc-alkaline signature defined by the La/Sm ratio > 3. This composition of the Lamego deposit is similar to that of the basal volcanic rocks at the nearby, world-class Cuiabá deposit, where these constitute the host to mineralized, shear-related quartz veins. The Nb and Ta negative anomalies indicate that they are closely associated with continental arc basalts. The geochemical results suggest that the Lamego rocks are associated with a marine environment under the influence of a continental arc, probably related to the Rio das Velhas II magmatic event (2800 - 2760 Ma). Alternatively, some authors argue in favor of BIF deposition in restricted basins. In the case of Lamego, the latter hypothesis is supported by samples with Y/Ho ratios below chondritic values. In addition, these results extend the gold exploration horizon in the Rio das Velhas greenstone, since they further support the strong epigenetic character of the hydrothermal mineralizing event, which apparently overlapped the entire volcano-sedimentary rock package.