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

摘要

We investigated a group of silicified volcanic rocks from the similar to 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).