THE EFFECT OF GEOCHEMICAL COMPOSITION ON THE MAGNETIC RESPONSE OF IGNEOUS INTRUSIONS NEAR BUTTE, MONTANA: USING OPEN-SOURCE GEOCHEMISTRY TO INTERPRET EARTH MRI DATA

The historic Butte mining district in southwest Montana includes the world-class Butte porphyry Cu-Mo deposit and additional porphyry, skarn, and epithermal systems that are related to Late Mesozoic to Paleogene, subduction-related I-type stocks and plutons of the Boulder batholith. Previous geochemical studies classify the intrusions as calcic to alkali-calcic, metaluminous to strongly peraluminous, magnesian granitoids, in which the geochemical composition can influence the type of metalliferous deposit. As part of the Earth Mapping Resources Initiative (Earth MRI), the USGS collected high resolution airborne magnetic and radiometric data to better understand the mineral systems associated with the Boulder batholith. This study uses open-source geochemical data to evaluate how chemical differences between intrusions may affect the magnetic response. Geochemical data are compiled from Cretaceous intrusions and paired with the reduced-to-pole anomalous magnetic field (AMF) grid and published geologic maps. The magnetic response and geochemistry of each intrusion was determined using median AMF and averaged geochemical values. Results reveal that median AMF values negatively correlate with aluminum saturation, alkalinity, and Fe index and positively correlate with combined iron content and oxidation state. Median AMF values are also highest for the less evolved rocks that have granodiorite, diorite, and gabbro compositions and corresponds to negative correlations with silica content and Rb/Sr values. These trends can be used to improve interpretation of buried magnetic features, where sedimentary country rock and basin fill have lower AMF values. The trends may also be useful for assessing metal potential when geochemical data are not available since oxidation state and the degree of magma differentiation can control metal composition and zoning in granitoid-related mineral systems.