Lead isotopes — a guide to major mineral deposits

Abstract Regional sampling and isotopic analysis of lead in surface gossans and ore minerals may indicate the potential for the discovery of a major mineral deposit and the most probable direction of the centre of mineralization. Interpretation hinges on the recognition of ordinary and anomalous leads , their genetic relationship, and their spatial distribution. Lead is unique among the elements in possessing the following properties: it has three radiogenic isotopes which cause natural variations in its isotopic composition; it has high atomic mass, making it insensitive to natural mass-dependent fractionation processes; and it has a natural affinity for sulphide minerals. These three properties make lead a sensitive indicator of the scale of natural ore-forming processes. Ordinary lead is commonly associated with major ore deposits. It is characterized by uniform isotopic composition (for a particular mineralization) closely resembling that of average terrestrial “ore-lead” at the time of mineralization. Anomalous leads are commonly observed in minor mineral occurrences. They are characterized by variable isotopic composition within a given district. Anomalous leads frequently result from radiogenic contamination of remobilized ordinary lead, in which case the least radiogenic lead samples may indicate the direction of the centre of mineralization.