Replacement minerals formed during alteration of volcanic rocks in the 2.7 Ga Abitibi Greenstone Belt carry rare-earth element attributes of mixed hydrothermal fluids and seawater

Primary minerals in Precambrian cherts and iron formations chemically precipitate from seawater, allowing these rocks to preserve geochemical signatures linked to early life. However, in addition to complex post-depositional reactions, hydrothermal conditions can lead to element mobility, altering primary mineralogy and complicating genetic interpretations. To document progressive compositional change associated with syn-formational alteration in a subaqueous hydrothermal depositional system, we investigated silicified volcanic rocks and associated chemical sediments from the Hunter Mine Group (HMG), and Deloro Assemblage (DA) of the 2.7 Ga Abitibi Greenstone Belt, Canada. Regionally, the terrane comprises felsic-mafic volcanic rocks and associated sedimentary units metamorphosed to greenschist facies. Locally, post-depositional addition of silica phases (silicification) affects SiO₂ concentrations. Within the HMG, rocks divide into basic groups: volcanic rocks, altered volcanic rocks, and siliceous/jaspilitic rocks. Based on texture, mineralogy, and geochemistry, we interpret that silicification of volcanic rocks produced transitional altered volcanic rocks and associated siliceous and jaspilitic rocks. We document that major- and trace-element mobility occurred during this process, where the introduction of Si-rich hydrothermal-seawater fluids facilitated the replacement of protolith volcanic rocks by new minerals precipitated during the alteration process. Some elements display a clear progression from volcanic (Al₂O₃ = 7.83-14.92 wt.%, Zr = 88-254 ppm, Eu_SN/Eu_SN∗ = 0.84-1.72) to transitional (Al₂O₃ = 1.09 - 5.46 wt.%, Zr = 7-56 ppm, Pr_SN/Yb_SN 1.33-2.89, Eu_SN/Eu_SN∗ 1.33-2.5), to mixed hydrothermal and seawater signatures (Al₂O₃ = 0.30-0.71 wt.%, Zr = <1 - 30 ppm, Pr_SN/Yb_SN 0.62-2.88, Eu_SN/Eu_SN∗ 1.30-7.15). Critically, replacement minerals carry rare-earth element attributes of mixed hydrothermal fluids and seawater. As a result, silicified rocks of the HMG geochemically resemble chert and iron formations within the DA. We conclude that seafloor silicification in the active hydrothermal setting of a collapsing subaqeuous caldera system is capable of producing rocks that resemble chert and iron formation, and such a process could be a widespread in the Archean.