Authigenic magnetites from the Belden Formation at Bassam Park, Colorado, were analyzed for their 18O/16O composition using secondary ion mass spectrometry. The Δ18O values of magnetite-rimmed pyrite (MagPy) grains averaged -8.5? (versus VSMOW). Small (25--50 μm) iron oxide grains had a range of Δ18O values from -9.7? to -5.5?, consistent with a variably hematized magnetite population having a Δ18O value of around -9? for magnetite. The Δ18O results suggest that the magnetite rims and grains formed in contact with a water having Δ18O of near 0? or less, implying a meteoric or connate source rather than a highly evolved orogenic or basinal fluid. This supports paleomagnetic results that suggest the formation of authigenic magnetites under Mesozoic burial conditions at some Belden Formation localities, rather than in response to orogenic fluid migration driven by Early Tertiary tectonic uplift. The Δ18O results also support the observation that chemical remanent magnetizations related to authigenic magnetite formation cannot be universally applied to the dating of fluid migration events associated with orogenic deformation. ¿ 1998 American Geophysical Union