Aeromagnetic and gravity studies of Payette National Forest, Idaho

Abstract

Aeromagnetic and gravity anomaly maps were compiled and interpreted to help define buried structure and igneous intrusions that might influence localization of undiscovered mineral deposits of the Payette National Forest. Three geologic terranes span the forest and exhibit distinctive aeromagnetic signatures. The Idaho batholith granitic terrane along the western edge of the North American continental craton exhibits a flat featureless field, except for pronounced highs associated with diorite and tonalite intrusive rocks along the western edge, 2) the terrane of the Salmon River suture zone reflects distinctive linear magnetic highs over mafic intrusions common in the zone and, 3) the allochthonous island-arc terrane of oceanic origin, consisting of metamorphosed plutonic, and volcanogenic, clastic, and carbonate rocks, coupled with effects of high magnetic susceptibility Columbia River basalts, resulted in numerous discrete aeromagnetic highs and lows with sources that are often fault controlled.Granitic rocks of the Tertiary (Eocene) epizonal plutons, located in the eastern part of the forest, were observed to have a higher magnetic susceptibility (180 x 10"5 cgs units) than granodiorite and two-mica granite rocks of the Cretaceous Idaho batholith interior (50 x 10"5 cgs units). Residual aeromagnetic anomaly maps, in shaded relief, were prepared by wave-length filtering (50 km high pass) the total-intensity data that had been reduced-to-the-pole. Pseudogravity gradient maxima were plotted on the map to help delineate magnetization boundaries that might signify geologic contacts or linear geologic features, such as faults or dikes.To supplement the aeromagnetic interpretations, a high pass isostatic residual gravity anomaly map was prepared to help distinguish subtle anomalies due to local sources in the upper part of the crust. The gravity anomaly data, in some cases, provided constraints on the interpretations of the aeromagnetic anomaly data and the geology, in applications to the mineral resource assessment. In particular, low amplitude gravity lows were observed to result from subtle density differences between most of the Cretaceous (2.60 g/cm3) and Tertiary granites (2.57 g/cm3).The USGS Payette Forest minerals assessment team prepared permissive tract maps of mineral resource deposit models that might be expected to occur in Payette National Forest. To demonstrate applications of aeromagnetic anomaly data in the assessment process, permissive tracts for four of the mineral deposit models are plotted on the residual aeromagnetic anomaly map of the forest.

Geology of the Payette National Forest and vicinity, west-central Idaho

“…No specific faults of this age are presently recognized in the forest. However, the general west-northwest trend of Neoproterozoic strata across the forest and a parallel large aeromagnetic anomaly (McCafferty, 1992;Kleinkopf, 1998) are herein interpreted to relate to a Neoproterozoic rift zone. The west-northwest orientation of Neoproterozoic plutons is thought to be directly related to Neoproterozoic normal fault control.…”

“…The northwesterly aligned Neoproterozoic plutons are probably in the hanging wall where the thrust fault may have ramped up the Neoproterozoic to early Paleozoic rift margin normal faults. Major aeromagnetic anomalies (McCafferty, 1992;Kleinkopf, 1998;McCafferty and others, 1999), which lie parallel to the southern margins of the plutons, give further evidence of preexistence of an important compressional structure.…”