Origin of banded patterns in natural sphalerite

Mississippi Valley type (MVT) sphalerite is a zinc ore mineral (zinc sulfide) found in many sedimentary basins around the world. Its texture is described as a polycrystalline aggregate which results from the precipitation of metal-rich brines in a carbonate host rock. Typically, it exhibits a spatial pattern characterized by an alternation of colored bands with a length scale on the order of 0.1 mm. In our samples, the color of the bands correlates with the local iron composition. In order to understand the origin of banded patterns in MVT sphalerite, we propose here an extension of the competitive growth mechanism often used to model periodic precipitation patterns. In our model, precipitation from interacting brines, growth and dissolution of crystallite radius, and ripening are taken into account. As in all postnucleation models, the nucleation process is neglected. It is shown that our model may indeed generate patterns that are qualitatively compatible with the observed ones. This constitutes an example of self-organization in a geochemical system.