The Henkel Petrophysical Plot: Mineralogy and Lithology From Physical Properties

Abstract The Henkel plot (logarithm of magnetic susceptibility versus density of rock samples) reveals that most rocks fall on either a “magnetite trend” or a “paramagnetic trend.” Interpretation of gravity and magnetic surveys is improved when the mineralogical and lithological basis of these trends is understood. We present a quantitative mineralogical mixing model, involving the components QFC (quartz‐feldspar‐calcite), FM (ferromagnesian silicates), and M (magnetite) and discuss the geological processes which produce or modify these mixtures. Igneous rocks mostly plot on the magnetite trend, where the FM/M ratio is about 10. The density‐susceptibility mineralogical mixing model is compatible with the CIPW mineral calculation for igneous classification from chemical analyses. Sedimentary and metamorphic processes usually involve oxidation, reduction, and/or iron loss, all which are magnetite‐destructive and lead to petrophysical measurements along the paramagnetic trend where FM/M > 1,000. Mineralization, with the introduction of sulfides and oxides, leads to dense rocks which do not plot along the magnetite nor paramagnetic trends. This quantitative analysis provides a method to integrate geological processes in the interpretation of geophysical surveys. Plain Language Summary The Henkel plot (logarithm of magnetic susceptibility versus density of rock samples) is useful for linking geophysical data and geological interpretation. Our study merges thousands of rock physical properties measurements with their corresponding rock types and minerals. Given this globally applicable database, we calibrated a model reducing these many parameters to three basic groups of minerals and their physical properties. This model permits users of remote sensing data to come up with equivalent rock and mineral types and a spatial view of geological processes. It also allows geochemical data on igneous rocks to predict their physical behavior and control on regional geophysical mapping. Key Points The Henkel plot (logarithm of magnetic susceptibility versus density of rock samples) helps integrate geological and geophysical analysis We present a quantitative mineralogical mixing model involving three components quartz‐feldspar‐calcite, ferromagnesian silicates, and magnetite Major geological processes leading to petrophysical properties on different regions of the Henkel plot are easily explained