DETERMINATION OF EASILY ACCESSIBLE METAL FRACTIONS IN ROCKS BY BATCH LEACHING WITH ACID CATION-EXCHANGE RESIN

Abstract

The easily accessible fraction of metals in “rocks” is derived from leaching experiments. Aliquots of rock powders are mixed with ion-exchange resin and distilled water. The pH self adjusts to a value of ~ 3. After batch leaching times of 2–48 hr at 70°C the resin is quantitatively separated from the rock powder, and eluted with 4 N HNO3. The metal contents of the leachates are determined and related to the respective contents of the whole rock. The leaching kinetics of each element is related to that of a reference element. Aluminium is most commonly chosen as a reference element because it is considered to represent the dissolution behaviour of the main rock-forming minerals. The easily accessible trace-element fraction (EAT) is given as the “percentage of leached element at 1% Al dissolution”. These EAT values are reproducible and independent from grain size and sample/resin ratio.The procedure is tested on basaltic glass and variably altered gneisses. The glass shows congruent release of all metals. A slightly crystalline glass of the same chemical composition displays heterogeneous leaching behaviour. The leaching kinetics of variably altered gneisses provide insight into the mineralogical siting of elements and their accessibility to fluids. Consequences for Rb-Sr dating of rocks are discussed.