Soil air carbon dioxide and oxygen measurements as a guide to oxidising sulphide have been assessed by several researchers and government geological surveys in the United States, Ireland, Africa and Saudi Arabia over the period 1983-1990. The rationale is that sulphide oxidation in the presence of water and oxygen will produce sulphuric acid, which in turn will attack any carbonates present to produce CO2. Oxygen is consumed in the process. Both CO2 and O2 are easily measured to an acceptable accuracy in the field with portable equipment. A South Australian soil air CO2/O2 project is presently being conducted jointly between mineral industry consultant Dr I.G. Watmuff and SADME. The aim of this project is to test the technique over areas of known mineralisation in a variety of geological and geographical settings. Previously reported tests (see progress reports 1, 2 and 3) have taken place since 1992 in the Mount Lofty Ranges, Moonta district, and the Mount Gunson area. This report details trials performed at the Western Mining Corp. Wirrda Well prospect, which lies 20 km SSE of Roxby Downs township. At Wirrda Well, Olympic Dam style IOCG-U mineralisation occurs within basement rocks underlying a cover sequence 355 m to 436 m thick of flat-lying, unmineralised Adelaidean Wilpena Group sediments. The subsurface mineralised zone extends from the basement top surface downwards for over 200 m, and is about 1 km wide and several km long. Published ore grade intercepts encountered by vertical diamond drillhole WRD9 aggregate to 215 m @ 0.8% Cu from 419 m to 634 m depth, which includes a highest grade zone with 14 m @ 2.1% Cu from 426 m to 440 m depth. In the soil air surveys previously run under this project, a response was sought from active sulphide oxidation. But the Wirrda Well mineralisation is far too deeply buried for active oxidation to occur, so here primary crustal CO2 rather than supergene CO2 was sought. A north-south traverse was sampled at 20 m intervals along a WMC grid line, over mineralised and non or weakly mineralised basement, to see if a corresponding anomalous response could be observed. An east-west traverse was similarly sampled along another grid line over a planar gravity gradient dipping steeply to the NE, to see if a response could be obtained defining a zone of deep fracturing. The gravity gradient had been interpreted to indicate a sharp contrast in cover rock thickness over basement, possibly caused by a downfaulted block of basement to the NE of the main mineralised zone.