The origins of multiple magnetizations of the Archean Stillwater Complex have been investigated through scanning electron microscopy and scanning transmission electron microscopy observations of mineralogical relations, using representative samples from nine sites in mafic Banded series rocks. On the basis of directional grouping and demagnetization behavior, three magnetizations (here labeled A, B, and C) have been recognized. The natural remanent magnetization (NRM) is typically dominated by only one of these magnetizations and multicomponent behavior in individual specimens is rare. The A remanence resides in magnetic grains of high median destructive fields and high, discrete laboratory unblocking temperatures and is inferred to be a primary thermoremanent magnetization, of circa 2.71 Ga age. The B and C magnetizations, of lower median destructive fields and more distributed unblocking temperatures, are inferred to be secondary and related to alteration, including serpentinization, involving limited, moderate to low-temperature fluid interaction, perhaps in response to thermotectonic events (e.g., mafic dike emplacement). Samples with NRM dominated by the A magnetization contain titanium-free magnetite needles (width<1 μm and maximum dimension between about 2 and 50 μm) with preferred orientations in cumulus plagioclase crystals; these oxides are interpreted to have formed during initial crystallization of cumulus phases. In samples containing only B or C magnetizations, such needles are absent in cumulus plagioclase, whereas secondary Fe-bearing silicates, particularly zoisite and chlorite, are conspicuous. The samples dominated by the B or C magnetizations contain anhedral low-chromian magnetite (FeCr2-xFexO4, 1.5) along fractures in both cumulus plagioclase and pyroxene and in zones of more pervasive alteration of pyroxenes. The paragenesis of these oxides is consistent with an origin related to hydrothermal alteration. Hysteresis data show that plagioclase crystals containing magnetite needles have higher remanent coercivities (Hcr=60 to 100 mT), whereas plagioclase crystals with anhedral chromian magnetite have remanent coercivities less than 60 mT, consistent with their alternating field response. Because oxygen isotope data show no indication of appreciable alteration of host plagioclase, we infer that the absence of magnetite needles and their associated A magnetization are caused by selective alteration of the magnetite needles to nonmagnetic phases in the remagnetized gabbros. In this case, rock magnetic and paleomagnetic data appear to be higher resolution indicators of fluid/rock interaction than the stable isotope data. Anhedral chromian magnetite precipitated during hydrothermal alteration and therefore carries secondary magnetizations.¿ 1997 American Geophysical Union