The Whirlwind prospect is located atop the southeast-trending spur of Stemwinder Mountain, 1.5 kilometres north-northwest of Hedley.

The region north of Hedley is underlain by a sequence of interbedded siltstone, argillite, limestone, quartzite and chert of the Upper Triassic Stemwinder Mountain Formation (Nicola Group), which is intruded by stocks, dykes and sills of diorite and gabbro of the Early Jurassic Hedley Intrusions. The Stemwinder, Aberdeen and Toronto stocks are three of the more prominent dioritic bodies intruding these sediments. Bedding generally strikes northeast and dips shallow northwest.

The underground workings are developed in limestone with some interbedded calcareous argillite along the southwest flank of the Stemwinder stock. The limestone progressively grades into coarse- grained garnet-pyroxene skarn, while the argillite changes to a greenish, siliceous fine-grained skarn to the northeast. Carbonate, quartz, scapolite, epidote, wollastonite, sericite, chlorite, orthoclase and prehnite are also present in the skarn. The sediments are intruded by various sills originating from the Stemwinder stock.

The diorite and flanking metasediments are cut by a fault zone dipping about 22 degrees to the northwest, generally following bedding. Two levels of underground workings have traced the zone over a strike length of 160 metres and a downdip distance of 73 metres.

The zone is comprised of brecciated wallrock cemented and partly replaced by dark-coloured cherty quartz. Mineralization consists of disseminations and stringers of arsenopyrite, pyrite, pyrrhotite and minor chalcopyrite in the cherty parts of the zone. This fault- controlled mineralization is discontinuous, and occurs over lengths of up to 67 metres, with widths of up to 1 metre. Sixteen channel samples, 23 to 107 centimetres long, taken over a distance of 37 metres in the No. 2 level assayed trace to 29 grams per tonne gold and up to 1.4 grams per tonne silver (Minister of Mines Annual Report 1947, page 150). A hole drilled in 1926 encountered 1.0 metre grading 11.3 grams per tonne gold (Minister of Mines Annual Report 1933, page 171). Grab samples of sulphide-rich material collected from the dump at the old workings gave maximum assay values of 29 grams of gold and 16 grams of silver per tonne, with 0.52 per cent copper and 2.5 per cent arsenic (Paper 1989-3, page 30).

Weaker mineralization also occurs in the enclosing garnet- pyroxene skarn. Arsenopyrite and traces of chalcopyrite, covellite and sphalerite are disseminated throughout the skarn.

Exploration of this prospect dates back to 1920. Most of the work was carried out by Hedley Amalgamated Gold Mines Ltd. between 1934 and 1949 and involved tunnelling, raising and surface and underground diamond drilling.