Paleoenvironmental reconstruction of gold-bearing BIF from the Archean Cuiabá deposit based on petrographic and geochemical studies

• Published in: Journal of South American earth sciences Vol. 108; p. 103223
• Main Authors: Sena, Noreto Carvalhais, Figueiredo E Silva, Rosaline Cristina, Lobato, Lydia Maria, Duarte, Vinícius Nogueira, De Souza Martins, Breno
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2021
• Subjects: Banded iron formation (FFB); Cuiabá gold deposit; Paleoenvironment; Quadrilátero ferrífero; Banded iron formation (FFB); Cuiabá gold deposit; Quadrilátero ferrífero; Paleoenvironment
• ISSN: 0895-9811; 1873-0647
• DOI: 10.1016/j.jsames.2021.103223

In the world-class Cuiabá gold deposit, located in the Archean Rio das Velhas greenstone belt in Brazil, metamorphosed carbonate-dominated banded iron formations represent the main lithology hosting gold mineralization. The deposit's stratigraphy consists of a basal metavolcanic unit with carbonaceous metasedimentary rocks, followed by a layer of gold-bearing BIF and thin layer of carbonaceous metapelite, a upper metavolcanic unit, and turbiditic metapelites associated with metavolcanoclastic rocks. The BIF is essentially composed of fine-grained quartz and siderite, with variable contents of carbonaceous matter. It is assumed that these chemical sedimentary rocks, classified as Algoma-type BIF, have been formed in the deep-sea environment, relatively close to volcanic spreading centers. This hypothesis is supported by the BIF's geochemical signature, which in general presents a positive anomaly of La (La/La*paas = 0.64–3.43), Eu (Eu/Eu*paas = 1.27–4.05) and Y, reflecting characteristics inherited from seawater and high-temperature hydrothermal fluids (>250 °C). In addition, the commonly observed superchondritic Y/Ho ratio values reinforce the influence of seawater on the rock's geochemistry. The absence of Ce anomalies concerns the oxygenation conditions of seawater, indicating low oxygen availability. The low content of Al2O3 (<0,27%) and other immobile elements, typical of detrital sediments, corroborate the interpretation that these rocks have been deposited in a deep marine environment with low siliciclastic influence. Subordinately, ferruginous metachert, BIF with magnetite and BIF with silicates are also present in the Cuiabá gold deposit. This diversity may be related to specific changes in the basin during the deposition of ferruginous chemical sedimentary rocks - changes that may indicate, for example. variations in the detritic support or availability of carbonaceous matter. Furthermore, geochemical studies from the lower and upper mafic metavolcanic units of the Cuiabá gold deposit helped to constrain and provide a broader understanding of the depositional basin's geotectonic context. Consequently, a genetic model that includes the paleoenvironment of this sequence is proposed. Both rocks of the lower and upper units have very similar mineralogical composition, but register significant geochemical differences. The lower unit is a calc-alkaline basalt/andesite from a continental arc, interpreted as having been developed during the Rio das Velhas II event (2800-2760 Ma), while the upper unit is defined as basalt with a transitional affinity to tholeiitic, possibly corresponding to an ocean floor volcanism. •The REE + Y normalized to PAAS in BIFs and ferruginous metachert from the Cuiabá gold deposit reflect the influence exerted by the members seawater, hydrothermal fluid, and detrital contribution.•In the Cuiabá gold deposit occur four typologies of ferruginous chemical metasedimentary rocks, namely: carbonate-dominated BIF, ferruginous metachert, BIF with magnetite and BIF with silicates - the first one being the most common.•The BIF and ferruginous metachert of the Cuiabá gold deposit exhibit siderite as iron-rich primary mineral.•The REE + Y dataset suggests that the deposition of BIFs and ferruginous metachert occurred in a deep marine environment, with high-T hydrothermal fluids and low detrital contribution.•The metavolcanic rocks of the lower unit are classified as calc-alkaline basalts of continental arc, while those of the upper unit are characterized as transitional to tholeiitic basalts from ocean floor volcanism.