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Título

On the search for the hydrothermal fingerprint: zircon geochemistry and hydrothermal alteration, the Carajás IOCG’s case

Texto do resumo

The demand for critical metals to supply the “green energy” transition has led to several new developments in the application of isotope geochemistry and geochronology for mineral exploration. Minerals associated with metasomatic alteration caused by infiltration of hydrothermal fluids, and ores, develop characteristic geochemical signatures; an expanding field of research using these data is growing. Zircon is among the main minerals used in these geochemical studies, and the characterization of hydrothermal vs magmatic zircon is still a challenge, due to the great variation in chemistry, mostly dependent on the hydrothermal fluids physico-chemical properties. Hydrothermally altered rocks that host world-class Archean iron oxide-copper-gold (IOCG) deposits in the Carajás Mineral Province (CMP) in Brazil have zircon with diverse geochemical signatures for U, Th, Hf and REE from those of non-altered igneous rocks. U-Pb geochronology of zircon from the Furnas and Paulo Afonso IOCG deposits indicate: i) the ~2.75 Ga zircon grains from the basement and adjacent host rock have pristine magmatic morphology and internal structures and geochemical composition with no indication of metasomatic alteration (e.g., sieve and/or bread-crust texture, U, and LREE enrichment); ii) the ~2.55 Ga zircon crystals from the same rocks exhibit metasomatic alteration internal textures and compositions; and iii) the ~1.9 Ga zircon grains have anomalously high U concentrations, but typical magmatic morphology and internal structures and REE compositions. These two deposits are within the same regional trend as the giant Salobo deposit, the Cinzento shear zone, and have comparable type of metasomatically altered zircon (geochemistry and features) that record the ca. 2.55 Ga episode, the main IOCG mineralization period.
We argue this type of zircon (ca. 2.55 Ga) occur generally associated with the regional shear zones and are mostly associated with well-known IOCG deposits at the CMP. These zircon crystals are interpreted as having crystallized or been altered under metasomatic conditions caused by infiltrating hydrothermal fluids with similar geochemical compositions, in syn-tectonic metasomatic activity, rather than a single magmatic event. The zircon complicated U-Pb geochronology, geochemical compositions, and morphology and internal structures are indicative of metasomatic participation and the interaction with ore-endowed fluids that formed the nearby IOCG deposits. Accordingly, this zircon type is suggested as a pathfinder of syn-mineralization hydrothermal episodes and, consequently as a guide for mineral exploration elsewhere for IOCG.

Palavras Chave

Carajás; IOCG; hydrothermal zircon; Salobo