Lead isotope geochemistry and paragenetic study of inheritance phenomena in metallogenesis; examples from base metal sulfide deposits in France

Abstract

A comparative study of French hydrothermal sulfide deposits shows how paragenetic studies and Pb isotope geochemistry are complementary tools in the understanding of inheritance and recycling phenomena in metallogenesis. The sulfide deposits are a major result of interaction between earlier deposits and later remobilizing hydrothermal fluids. These interactions have a different expression depending on whether the earlier deposits or the remobilizing fluids, or both, were Pb bearing or not. The type of the minerals and the shape of the Pb isotope pattern of a mineral deposit, which is based on the distinct isotopic signatures of the various Pb-bearing events, are governed by these different interactions.On the scale of an deposit consisting of several successive parageneses, detailed paragenetic studies serve to distinguish the various parageneses, each of which is usually the result of a specific metallogenic process with its own Pb isotope composition. Pb isotope analyses then establish any relationships between these successive parageneses. When all parageneses have Pb-bearing minerals, one can estimate the degree of contamination of a later stage paragenesis (newly introduced Pb) by lead that was remobilized or recycled from earlier parageneses. Thus, the Pb isotope heterogeneity of an deposit generally results from a polyphase history with associated inheritance phenomena that calls on several sources. When lead is present throughout a complex mineral deposit but its isotopic-lead composition is homogeneous, then the later parageneses have simply recycled the earlier Pb. In some cases of layered epithermal barite-galena veins, such homogeneity means that the successive parageneses belonged to a single metallogenic event, usually without any tectonic break between the various parageneses.When considering metallogeny on a regional scale, this complementary approach of comparing Pb isotope geochemistry and parageneses can reveal regeneration phenomena, as is well represented by the hybrid Pb-Sb veins of the French Massif Central, where Pb, Sb, and S were inherited from older deposits.Pb isotope analyses on the scale of individual mineral grains, carried out on galena as well as on other Pb-bearing minerals and on berthierite, all of which have well-defined places in the paragenetic sequence, not only can quantify remobilization phenomena but also can demonstrate genetic relationships between deposits of different age in the same district.This, in turn, leads to an estimate of the weight of the recycled metals of a mineral deposit (the ore source), relative to that of metals newly extracted from rocks by hydrothermal fluids (the lithologic source). This method, which has proven to be valid for most of the sulfide deposits in France, can be applied to any other mineral deposit or mineral district with a complex metallogenic history.