Petrogenetic interpretation from gamma-ray spectrometry and geological data: the Arch Lake zoned peraluminous granite intrusion, Western Canadian Shield

Airborne gamma-ray spectrometry data acquired over the Arch Lake granite batholith in the Western Canadian Shield display compositional patterns not previously identified by geological field mapping. An integrated analysis of gamma-ray spectrometric, geochemical and mineralogical data was conducted to map and analyse these patterns, and to obtain insight in their petrogenetic significance. Enhancement and cluster analysis of the K, eTh and eU grids reveal that the Arch Lake granite is zoned in a complex manner with a Th-rich centre and Th-rich, locally U-rich outer margins. The Th-rich centre broadly corresponds to highs in interpolated patterns of Zr and TiO2, reflecting a regional mineralogical association between biotite, monazite, zircon and Fe-oxides. The negative trend of this Th-rich mineralogical association with SiO2 is not consistent with normally-zoned plutons formed by differentiation of a single magmatic pulse. Instead, two spectrally distinct K-eTh zones suggest that the Arch Lake batholith is a composite intrusion. Gamma-ray spectrometry surveys can be considered a useful tool for mapping subtle compositional variations in granites. Integrated analysis on the basis of geochemical data and mineralogical observations, however, is required to constrain the possible petrogenetic interpretations.