INTEGRATED GEOPHYSICS FOR MINERAL EXPLORATION IN DRIFT-COVERED VOLCANIC TERRAINS - EXAMPLES FROM NORTHERN VANCOUVER-ISLAND, CANADA

Known mineral occurrences in northern Vancouver Island are typically h osted in volcanic units of the Bonanza Group. At a local scale, the mi neralization is associated with advanced argillic bedrock alteration a nd is often intimately related to porphyry intrusions. On a larger sca le, faults are thought to exert the most significant control on the di stribution of mineralized host rocks. Poor exposures and a complex gla cial history limit the use of traditional methods of geological mappin g and mineral exploration in this region and to date geophysical metho ds have been under-utilized. Here we present findings from four standa rd geophysical (gravity, magnetics, electromagnetics and seismic refra ction) methods, recently deployed here to elucidate the subsurface geo logy, as well as to identify new targets for base metal exploration. R esults at two different sites show that the integrated interpretation of geophysical data, constrained by physical rock property measurement s, yields detailed images of the subsurface at a fraction of the cost of drilling. At one site, east of Rupert Inlet, the final subsurface m odel shows that the Bonanza Group is not nearly as extensive as previo usly presumed. An extension of the Holberg Fault is identified some 50 km east of the visibly mapped outcrop and an extensive zone of altera tion around the fault is recognized. Furthermore, a number of the meth ods provide support for the existence of a porphyry dike at this site. At the second site, north of Rupert Inlet, magnetic and electromagnet ic data prove effective at mapping alteration and locating shear zones beneath a relatively thin drift cover. Together, these results help o utline a strategy for exploration in drift-covered terrains and show t hat a redirection of exploration effort is warranted in the case of no rthern Vancouver Island.