One hundred and twenty-two meters altitude aeromagnetics and ground data over the Cretaceous normal polarity extrusives of the Troodos ophiolite have allowed source modeling. These data have been digitized and recontoured in gridded and reduced to the pole (RTP) formats. The principal result is confirmation of large anomalies with amplitudes of several thousand nanoteslas and dimensions of 1--6 km. Since the extrusives are everywhere normally magnetized, anomalies must have an explanation other than polarity reversals. Strong magnetization of overlying little altered extrusives and dikes, and weak magnetization of underlying hydrothermally altered material, allows forward modeling in terms of source layer thickness. Detailed modeling was carried out for a ~50 km2 subarea containing two highs separated, with 3000 nT contrast, by a low for which ground information had been specifically acquired. The low coincides with a thick series of sheet flows containing former hydrothermal flow-ways (gossans). Modeling of a former reservoir of hydrothermal fluid beneath the low, with reduction of the magnetic source layer from 1000 to 250 m, is consistent with observations. For the extrusives as a whole, gossans are weakly associated with relatively low RTP field. Although individual gossans are too small to perturb the kilometer-scale anomalies, they do show several relationships with anomalies: Gossans are absent from areas of very high (>6500 nT) and very low (<1500 nT) RTP field and, within the intervening range, they are bimodally distributed. Reconsideration of the nature of gossans accounts for some of these features. ¿ 1998 American Geophysical Union