What lies beneath? A reflection on the porphyry copper exploration model

SummaryElectrical methods have been applied to the search for porphyry copper and IOCG deposits since the early 1950s. While there is a generally accepted model of disseminated sulfides giving rise to a chargeability response, no clear association has been attached to what EM surveys may be responding to. Work in the early 1990s (Nickson 1993) showed the well-developed supergene blankets over a porphyry copper could be conductive; this observation was however, never applied formally to generally accepted porphyry targeting models. The presence of other conductive zones associated with porphyry copper deposits is even less well studied. On the geological side, while there is a vast body of literature describing porphyry copper deposits and how to discover them, in very few cases do these studies even speculate if anomalous concentrations of sulfides could be conductive. On the geophysical side, observations of unexpected conductivity associated with porphyry systems is sometimes noted but these observations typically stop short of suggesting that there could be a more general observation made that a new class of geophysical feature should be defined. The present study is felt to have gathered a sufficient number of case studies which show that a significant number of porphyry copper deposits posse a mineralogical character which can be identified with EM techniques. This thesis can have significant implications as to how porphyry copper are explored for, especially those at depths >500 m, a generally accepted cut-off for IP techniques.