Magnetic Studies in Mineral Resource Assessment: An Example from the Rocky Mountains, Montana and Idaho, USA

Geophysical studies form an integral part ot U.S. Geological Survey programs in mineral resource assessment; geophysical expertise for such assessment recently has been recognized as important also for environmental control and cleanup. Among the many geophysical methods used, the magnetic technique is especially valuable tor assessing a pervasively intruded geologic terrane, such as a 2ex2e region of the Rocky Mountains of Montana and Idaho selected here for special discussion. This region, which includes the worldfamous Butte mining district, contains large resources of gold and significant amounts of other metallic and non-metallic deposits, many of which formed during hydrothermal activity that occurred during and following the waning stages of maganism. The magnetic studies, coupled with measurements oo total magnetization and with analysis of gravity anomalies, serve to delineate (1) areas of chemical alteration and attendant mineralization, (2) occurrences of iron-formation and magnetite-rich skarn associated with mineralization, (3) thicknesses of intrusive rocks and basin fill, the latter relevant to deposits within the fill as well as deposits beneath the fill, (4) magnetically ''quiet'' zones having possible hydrocarbon potential and (5) subregions of relatively thick, steep-sided, subsurface intrusive rocks, identified on the basis of maximum horizontal gradients of pseudogravity anomalies and on the basis of bandpass- filtered second-vertical derivatives of reduced-to-pole anomalies. A map showing subsurface magnetic intrusive rocks based on filtered magnetic data may be compared with a geologic map showing exposed intrusive rocks; where exposed intrusive rocks are relatively nonmagnetic, they may be thin or intrinsically felsic or chemically altered, and thus possibly mineralized. The regional studies provide an important framework for commercial district-scale exploration.