A 900 line km detailed aeromagnetic survey was carried out over the northern extremity of Spencer Gulf, in a search for possible Olympic Dam style mineralisation associated with a significant and complex magnetic high anomaly that is located almost entirely within the gulf. An earlier interpretation of the BMR's regional magnetic data had suggested that the source of this anomaly lay at > 650 m depth below sea level. However, the increased sensitivity of the presently reported heli-borne magnetic survey (60 E-W lines flown at 500 m line spacing, with a nominal 10 m detector height above water, compared with the BMR's 150 m terrain clearance) has resolved the original anomaly into a large number of individual magnetic highs and lows. It is suggested that the whole anomalous area is the result of an uplift of the magnetic bedrock, with its upturned edge sharply terminated under the centre of the gulf by strong downfaulting to the west, which has later been cross-cut by additional faults having a NW-SE strike. Importantly, the likely depth of overburden above the magnetic sources has been revised to be generally less than 300 m, making it comparable with that found at Olympic Dam.