A feasibility study of the application of the Pb-Pb isotope step-leaching technique to ore minerals

Abstract

Pb-Pb step-leaching (PbSL) is a digestion technique based on sequential acid treatment of a mineral resulting in the selective recovery of radiogenic and common Pb components from the crystal lattice, making single-phase Pb-Pb dating possible. Developed originally by R. Frei and colleagues at Bern, the technique has had mixed success. Initial studies produced reliable dates for silicates and mixed sulphides, however there is some conflict surrounding the validity of the PbSL isochrons, with some authors claiming that isochrons might be the product of initial Pb heterogeneity and/or Pb mixing due to post crystallisation U or Pb introduction. This study builds on previous observations and tests the reliability of PbSL in directly dating ore minerals such as sulphides (chalcopyrite, pyrite, pyrrhotite/pentlandite) and oxides (magnetite), in an attempt to obtain the timing of mineralisation from a phase that is unambiguously linked to the ore-forming process. This is based on the often surprisingly high U contents (relative to what might be anticipated), yielding rather higher than expected initial U/Pb ratios in several types of low-Pb sulphides. The technique is applied to well-constrained samples from a number of predominately Australian mineral deposits covering a range of mineralization types.