Element behaviour in myrmekite-producing sulphide-sulphosalt reactions

The behaviour of chemical components in three myrmekite-forming replacement reactions from siderite-hosted vein deposits, Siegerland, Germany, has been investigated. Compositional data on mineral phases involved in the reactions are combined with volume proportions in the myrmekite textures; atomic proportions of initial and final material are plotted on isocon diagrams. The replacement of Bi-meneghinite by an intergrowth of bournonite and galena is characterized by immobility of S and (Sb+Bi), whereas Pb and Cu are relatively mobile. Tetrahedrite inclusions in galena replaced by a myrmekite composed of bournonite, sphalerite and chalcopyrite show immobile behaviour of S and Sb. All other elements are extremely mobile; the Ag-content of tetrahedrite is completely removed during the reaction. Replacement of bournonite by a myrmekitic intergrowth of ullmannite, galena and chalcopyrite again shows a relative immobility of S and Sb. The immobility of S in all studied samples can be readily interpreted in terms of a simple cation diffusion model for sulphides (Birchenall 1974), whereas immobility of Sb or (Sb+Bi) can be related to the structural peculiarities of sulphosalts, especially to the energetic differences of various cation sites.