Intergrowths of bismuth sulphosalts from the Ocna de Fier Fe-skarn deposit, Banat, Southwest Romania

Abstract

The Ocna de Fier skarn deposit is typified by texturally and compositionally specific intergrowths of Bi sulphosalts. Bismuthinite derivatives, makovickyite, lillianite homologues and galenobismutite have been documented by microanalysis. Among the bismuthinite derivatives, two new homogeneous phases in the series, “Phase 70” CuPbBi 7 S 12 and “Phase 88.6” Cu 0.33 Pb 0.33 Bi 7.67 S 12 , are reported; both had earlier been predicted as possible stable superstructures. Intergrowth textures occur within assemblages from the entire metasomatic evolution of the deposit and reflect relative stabilities of individual phases in the context of local equilibrium gradients. Primary features are exemplified by skeletal intergrowths in the gladite-krupkaite compositional range. These have been preserved during cooling and are distinct from other intergrowths resulting from the breakdown of earlier-formed minerals. The latter are typified by vermiform intergrowths in the range gladite-“Phase 88.6” and by lamellar and symplectitic lillianite-galena intergrowths containing remnants of initial heyrovskýite. Microanalytical data shows high levels of substitution in the sulphosalt structures, notably Cu in makovickyite and lillianite homologues. Bulk analyses of fine symplectites, together with some of the heyrovskýite-type remnants clearly show that small chemical differences existed within the initial crystals prior to decomposition. Despite the complex intergrown character, textural analysis and microanalytical data suggest that these assemblages could be related to crystal-growth processes evolving in a looping manner within the skarn-forming environment, parallel to oscillatory zoning patterns in garnet. Consequently, some of these intergrowths, e.g. gladite-krupkaite, characterised by order-disorder phenomena within a specific compositional range in the assemblage, could be subject for analysis of stacking sequences in terms of oscillatory growth processes.