Gold distribution in the Archean Tanzanian Craton: Evaluating the effects of intracrustal differentiation

Abstract

This study evaluates the vertical distribution of gold in the continental crust. Implementing a recently published method by Pitcairn et al. (2006a) for the chromatographic separation of gold from acid-digested rocks using diisobutyl ketone (DIBK), followed by analysis using standard addition inductively coupled plasma mass spectrometry (ICP-MS), high- and low-grade metamorphic rocks of the Tanzanian Craton, representative of the lower and upper crust, respectively, are analyzed to determine the distribution of gold in the crust. Greenstone belt basalts have the highest gold concentrations (ave.=60 (+193/-19) ng/g), followed by greenstone belt andesites (ave.=1.4 (+3.6/-0.6) ng/g). The lowest concentrations are observed in granulite-facies lower-crustal xenoliths (ave.=0.4 (+1.0/-0.1) ng/g). Gold is incompatible in silicates and can partition into hydrothermal and/or magmatic fluid during high-grade metamorphic dehydration reactions or partial melting, particularly if sulfides break down during these processes. Rise of buoyant mobile phases may explain the depletion of gold in the lower crust.