In glaciated terrains, geochemical maps portray bedrock provenance and the integrated effects of glacial processes affecting the distribution, concentration, and partitioning of minerals in sediments. In those maps, indicators of economic mineralization can be difficult to distinguish in the context of natural background, especially at low trace metal concentrations. Sample mineralogy, which can be inferred from lithophile elements, provides a key basis for interpreting sediment provenance and controls on background variation in metal concentrations. In non-carbonate terrain, the principal rock-forming minerals digested by aqua regia are Mg-bearing phyllosilicates, including trioctohedral chlorite, biotite, and phlogopite. Hence, Mg analyses directly reflect the concentrations of those minerals. In a regional geochemical survey of till in Nunavut, strong linear correlations (r>0.840, n=1842, p<0.0001) for Cu and Cr with Mg concentrations indicate Mg-bearing phyllosilicates are the principal metal hosts, and that the metals are bound in mineral lattice structures in direct proportion to Mg. Thus, metal:Mg ratios express geochemical properties of the mineral(s) hosting the metal, and are independent of mineral partitioning among size fractions that results from either glacial or postglacial processes. Ratio maps can be used to establish till provenance and infer aspects of bedrock composition not evident in single-element geochemical maps. Ratio anomalies could indicate metals derived from economic indicators such as sulphide minerals, and metal-rich particulate from anthropogenic sources.