Development of a mixed seawater-hydrothermal fluid geochemical signature during alteration of volcanic rocks in the Archean (∼2.7 Ga) Abitibi Greenstone Belt, Canada

We investigated a group of silicified volcanic rocks from the ∼2.72 Ga Hunter Mine Group (HMG), Abitibi Greenstone Belt, Canada, in order to document progressive compositional change associated with alteration in a subaqueous caldera system. Rocks of the HMG divide into three groups based on mineralogy and texture for petrographic and geochemical analyses. Volcanic features (phenocrysts, pseudomorphs after primary glass shards, lapilli, volcanic clasts) are preserved in all groups, despite changing mineralogy from primarily quartz, feldspar, chlorite (Groups 1 and 2), to quartz, hematite and carbonate (Groups 2 and 3). Compositionally, Group 1 rocks resemble volcanic rocks in the region, while Group 2 and 3 rocks show a change in mineralogy to iron, silica, and carbonate minerals, which is associated with depletion of many major and trace elements associated with volcanic rocks (Al₂O₃, Na₂O, K₂O, Zr). In addition, rare earth elements display a clear progression from volcanic signatures in Group 1 (Pr_SN/Yb_SN = 1.7–2.96, Eu_SN/Eu_SN^∗ = 0.84–1.72, Y/Ho = 25.20–27.41, La_SN/La_SN^∗ = 0.97–1.29, and Zr/Hf = 38.38–42.09) to transitional mixed volcanic, hydrothermal, and seawater signatures in Group 2 (Pr_SN/Yb_SN 1.33–2.89, Eu_SN/Eu_SN^∗ 1.33–2.5, Y/Ho = 23.94–30, La_SN/La_SN^∗ 0.93–1.34, and Zr/Hf = 40–70), to mixed hydrothermal and seawater signatures in Group 3 (Pr_SN/Yb_SN 0.62–2.88, Eu_SN/Eu_SN^∗ 1.30–7.15, La_SN/La_SN^∗ 1.02–1.86, Y/Ho = 25.56–55, and Zr/Hf = 35–50). We interpret that silicification of volcanic rocks (Group 1) produced transitional altered volcanic rocks (Group 2), and siliceous and jaspilitic rocks (Group 3), based on preservation of delicate volcanic features. Building on this explanation, we interpret that major, trace- and rare-earth element mobility occurred during the process of silicification, during which siliceous and jaspilitic rocks (Group 3) acquired aspects of the rare-earth element geochemical signatures of marine chemical precipitates. We conclude that seafloor silicification in hydrothermal depositional settings is capable of producing rocks that resemble marine chemical precipitates such as banded iron formation, and could be a process that is widespread in the Archean. Consequently, because silicified volcanic rocks from the HMG possess mixed seawater and hydrothermal rare-earth element characteristics similar to Archean iron formations and cherts, we suggest caution must be exercised when interpreting the geochemical information preserved in metamorphosed rocks where original genesis is unknown.