Developing Geophysical Signatures To Constrain Geologic Mapping

Abstract

The Greens Creek ore deposit is one of the largest producing volcanogenic massive sulfide (VMS) deposits in southeast Alaska, and is found on north-central Admiralty Island in southeastern Alaska. This deposit and all other known VMS prospects in the region are found only in Triassic Hyd Group volcanic rocks. Future success of mineral exploration in the region is thus highly dependent on accurate geologic maps that accurately portray where these Hyd Group rocks are located in a region that is largely covered by dense vegetation, water, and tidal mud-flats. Because of this extensive cover, we have used ground and airborne geophysical information to constrain the geologic mapping. A large airborne geophysical survey, incorporating both electromagnetic (EM) and magnetic sensors was flown, supplemented by ground magnetic and VLF-EM resistivity profiles over key areas where geology could be unequivocally mapped. From these, we developed a matrix of geophysical signatures of key rock units that was then used in predictive geophysical models (essentially pseudo-geology maps) derived from the airborne data. Geologic ground control was then used to convert these models into a substantially revised geologic map of the region that now shows more than a 30% increase in Triassic Hyd Group rocks.