Application of lead isotopes and trace elements to mapping black shales around a base metal sulphide deposit

Abstract

Pb isotopic analyses have been used in southeastern New South Wales, Australia, to distinguish Ordovician black shales, which have no associated mineralization, from Silurian black shales in which mineralization is known to occur. The more radiogenic nature of the Ordovician Pb, as shown by analysis of the sulphide, whole rock, acid leach or residue, reflects a higher U/Pb environment compared with the Silurian which is due to the absence of volcanism in the Ordovician. V concentrations and Co/Ni ratios in sulphides may prove to be an important indicator to distinguish Silurian and Ordovician black shales but lithology, mineralogy, whole rock major and trace elements, and C and S isotopes give ambiguous answers. Co/Ni ratios in the Silurian black shale sulphides average ∼ 0.26 compared with 0.04 in the Ordovician sulphides. On the basis of Pb isotope and V concentration data, an unconformity between Middle-Upper Ordovician and Middle-Upper Silurian rocks and overturning of the strata is proposed for a drill hole previously mapped on palaeontological evidence as entirely Ordovician. No correlation between organic C and U concentration was observed in the whole rocks. An approximate negative correlation exists between 208Pb/ 206Pb and V and Ni contents; with higher Ni and V concentrations the sulphides have lower 208Pb/ 206Pb ratios but higher 206Pb/ 204Pb ratios. Shales distant from mineralization appear to have been wholly influenced by marine processes in comparison with samples close to the mineralization which are affected by volcanic conditions.