At the Antamina deposit, Peru, accurate classification of exoskarns and endoskarns can be problematic when textures are mottled. In this study, we use whole-rock geochemical compositions (62 elements) of 221 samples to differentiate texturally similar endoskarns and exoskarns by comparing their compositions to least altered precursors (wall rocks and intrusive rocks). We present a simple method for discriminating these skarn types using immobile element bivariate plots. The most effective discriminators partition endoskarn and exoskarn into distinct domains defined by the composition of each precursor; these include Al2O3 versus heavy rare earth elements and some high field strength elements. Using these geochemical parameters, undifferentiated skarn samples can be more reliably classified as endoskarn or exoskarn. The effectiveness of these element pairs is attributed to their significantly different initial concentrations in wall rocks versus igneous precursors and their immobility during skarn formation. While immobile elements can differentiate the skarns, mobile element gains and losses (quantified using isocon analysis) provide insight on the bulk mineralogical and mass changes that take place during skarn formation.