Potassium and rubidium metasomatism at the Julcani District, Peru

Dacite lavas in the central portion of the Julcani district are variably propylitized, more intensely so near veins. Along some veins the dacites are altered to mixtures of kaolin + quartz + or - alunite + or - pyrophyllite. Potassium and Rb contents increase with increasing alteration, Rb slightly more so than K; Sr decreases with alteration. Both in areal patterns and underground traverses, Rb and K contents mostly correlate positively with the locations of veins and regions of intense alterations. Inverse relations, both at and below surface, are noted in passing. Rb and K can be useful indicators of mineralization because they are commonly concentrated in rocks near orebodies; at least they delineate unaltered areas or call attention to areas that require closer inspection.