Geochemical signatures of gold-bearing shear zones in the Variscan basement, France
Abstract
The geochemical signature of Au mineralization is a function of its sulphide parageneses and associated wall-rock alteration. This signature can be used to classify a prospect as a function of the anomalies it causes (in rocks or, if conditions are favourable, in soil) in a given geologic environment.
The geochemical signature is made of three types of elements:
- 1.
(1) The major elements initially present in the country rocks but whose concentrations can be modified on introduction of the mineralization. A convention the authors have defined denotes the elements of interest in square brackets [ ], preceded by + or −, respectively, indicating enrichment or depletion.
- 2.
(2) Tracer elements, introduced by the mineralizing solutions, characteristic of the mineralized structure on a semi-regional scale. By convention they are written after those in square brackets.
- 3.
(3) Pathfinder elements specific to mineralized zones, reflecting remobilization and concentration phenomena, constitute tracers on a detailed scale. These are denoted in parentheses ( ) at the end of the signature.
Most of the Au-bearing shear zones in France have the following signature: The semi-regional tracer elements, As and Sb, appear to reflect mostly regional geological characteristics.
Arsenic is always present in Au-bearing shear zones in the Variscan basement of France. This is similar to structures that cut volcano-sedimentary rocks of the Cadillac basin in Quebec, Canada. By contrast, As is usually scarce in shear zones cutting the mafic and ultramafic rocks of the Abitibi greenstone belt. In this case As can be replaced by Cu.
Antimony, as a semi-regional tracer, appears characteristic of shear zones in France whereas Pb, as a detail pathfinder, is found in all geologic environments along the shear zone and is closely related to Au anomalies. Such as close correlation with Au limits its usefulness.