We report MC-ICP-MS Si-isotopic measurements ( δ29Si recalculated as δ30Si) on micro-subsamples (at 500 μm scale) from several major rock types of the ∼3.8-Ga-old Isua Greenstone Belt (IGB, southern West Greenland) and surrounding Eoarchaean terrains. With a large overall range of variations (- 2.80‰ < δ30Si < + 0.68‰), they demonstrate strong involvement of surface fluids enriched with dissolved Si. The resistance of Si-isotopes to metamorphic resettings and metasomatic overprints is also established. Metabasaltic pillows and metasediments display similar 30Si-enriched signatures, suggesting that emergent surfaces of the Eoarchaean protocrust were composed of slightly weathered, hydrothermally altered, mafic-ultramafic bodies. Isua magnetite-quartz Banded Iron Formation (BIF) is strongly depleted in 30Si relative to all coeval rocks. This depletion supports Rayleigh-controlled precipitation from seafloor-vented hydrothermal fluids. In contrast, banded quartz-pyroxene rocks (from Akilia Island, some 150 km southwest of the IGB), which some authors have identified as BIF-related, yield quartz with Si-isotopic composition ( δ30Si = - 0.36‰) similar to metamorphic-derived quartz ( δ30Si = - 0.50‰). This supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers.