This study shows that aeromagnetic anomalies of less than 200 gammas are associated with topographic relief of exposed Precambrian granitic and volcanic rocks of the St. François Mountains. Anomalies resulting from hills coarsely crystalline granite are as high as 100 gammas in amplitude, whereas anomalies over comparable hills of fine-grained rocks, such as granophyre or devitrified volcanic rock, are as much as 200 gammas. Anomalies caused by normal faulting or shearing of igneous rocks have amplitudes of less than 100 gammas; they are observed best in profile. These anomalies are superposed on larger magnetic features related to pendants of volcanic rock in the roof of a granite batholith and are distinguished from large lateral variations in magnetic intensity by their low amplitude and small areal extent. Analyses of the compound anomalies yielded the subsurface configuration of isolated roof pendants of resistant extrusive rock in some areas. As many of these pendants have negligible remanent magnetization, induction theory was used in the analyses. Application of these results to the lead-mining areas shows that aeromagnetic patterns of low amplitude can guide mineral exploration in the region flanking the Ozark uplift, where isolated, buried hills of Precambrian igneous rocks controlled the development of some lead-bearing sedimentary structures in the overlying Cambrian carbonate strata.
