Special Mining Lease 491 covers an area of poorly exposed Precambrian rocks in the north-western sector of the Gawler Platform of western South Australia. General reconnaissance in late 1969 showed the geology of the area to be dominated by sediments metamorphosed to granulite facies, which have been complexly folded, faulted and intruded by granite and acid volcanics. Nickeliferous rock types were noted to occur adjacent to an iron formation at Blackfellow Hill, and land tenure was subsequently secured to assess the nickel and other base metal potential of this granulite environment, as well as carry out basic mapping in conjunction with the exploration, to gain an understanding of the geology of this previously uninvestigated area. Mapping of the 1250 square mile lease and an evaluation of some basic/ultrabasic bodies by auger and percussion drilling has now been completed. Most of the lease covers Carpentarian granite intrusives and some late Carpentarian acid volcanics. A large anticlinal remnant of granulite occurs in the western section, and other small remnants, surrounded by granite, occur with decreasing frequency eastwards. Basic and ultrabasic rocks appear to be of three types, occurring as normal dykes, plug-like intrusives with kimberlite affinities, and skarns of indeterminate origin. Later phase(?) granite intrusives in two areas were shown to carry anomalous copper contents. Under a joint venture agreement signed in early 1971 Otter Exploration NL undertook to separately carry out iron exploration within SML 491, by investigating a number of known banded iron formation (BIF) occurrences and by partly funding an aeromagnetic survey intended to disclose other occurrences concealed under Cainozoic cover. The aeromagnetic data so obtained indicate a generally NE-SW strike direction for the BIF units. Their dips appears to be steep, but their inferred depth extents and probable lateral limits are unreliable because of conflicting results from ground magnetic surveys run over several aeromagnetic anomalies. Examination of BIF outcrops with a strong aeromagnetic signature at Mount Christie and Fingerpost Hill suggests that they have undergone tight folding, possibly of similar type; the fold axes pitch to the north-east. Fingerpost Hill appears to be an antiformal structure. Lack of BIF outcrop in the West Well aeromagnetic anomaly area precludes any structural data being available, apart from that inferred from the magnetics.