Summary: The accessory minerals rutile and apatite have been studied in 77 known porphyry copper deposits. Their value as indicators has been well established on the basis of specific chemical and paragenetic variations which they show.
Rutile occurs as the only Ti-mineral in the quartz-sericite zone, is dominant in the biotite-orthoclase zone, and is generally found in the inner fringes of the chlorite-epidote zone. It forms in these zones mainly as a result of the destruction of sphene, but also from biotite and hornblende.
The length: width ratio of rutile crystals is 1·5:1 in the centre of a porphyry system, increasing gradually outward to 2:1. A characteristic red colour displayed in thin section is attributed to a high copper content ranging from 100 to 500 ppm. The ratio of Cr+V:Nb+Ta is also unusually high.
Apatite shows evidence of a complicated history of corrosion and redeposition accompanied by outward migration during the life of the porphyry system. The migration parallels that of copper and typically extends far into the host rocks. The apatite is enriched in chlorine. A plot of a versus c shows a clear separation of apatites of various genetic types, including tin and molybdenum porphyries.