24 diamond drill core samples of primary copper ore from the Ukaparinga mine, which is located about 4 km south of Williamstown, plus 3 hand specimen samples of oxidised ore taken from a near surface adit of this mine, have been investigated by petrographic, mineragraphic and electron microprobe methods to try to determine possible mineralogical causes for Amdel's previously demonstrated abnormally low copper extraction from certain samples of ground oxidised ore, when they were subjected to sulphuric acid leaching. In the current study both laboratory leached and un-leached ground versions of the above samples were examined. It was found that the low copper extraction is due to the presence in the ore of biotite containing a significant solid solution proportion (1-1.5%) of copper. The primary ore is essentially a quartz-biotite-muscovite schist which contains up to a few percent of chalcopyrite and pyrite. Textural and stratigraphic evidence suggest that the sulphides are syngenetic, and in this respect the Ukaparinga mine ore resembles that at Kanmantoo, although being of different age. The biotite in the primary ore contains no detectable copper, and thus it appears that it is only under near-surface oxidising conditions that copper (released by oxidation of chalcopyrite) enters the biotite crystal lattice, whereupon it can become inaccessible to artificial leach reagents.