Crystallization sequences in matte and speiss from primary lead metallurgy
Résumé
The temperature evolution and crystallization sequences in matte and speiss from Pb metallurgy at Príbram (Czech Republic) were determined using a combination of microscopic observations (optical microscopy, SEM/EDS), chemical microprobe data (EPMA), X-ray diffraction (XRD) of original samples collected at the dumps, and samples studied by heating experiments (temperature steps 25, 400, 700, 900, 1100, 1200 C) and quenching experiments (400, 700 and 900°C). Immiscibility of sulphide and metallic melts was observed. Identification of the major phases (sulphides, metals and intermetallic compounds) helped in estimation of the temperature of the matte and speiss solidification. Microscopic observations indicate that the crystallization sequences begin with high-temperature phases, such as Ni3 Sn2 or NiSb (with melting temperatures of 1264°C and 1147°C, respectively) and, in sulphide melts, with crystallization of a high-temperature spinel phase (magnetite, Fe3 O4), Fe-rich wurtzite/sphalerite (ZnS) and pyrrhotite (Fe1 x S). The solidification sequence ends either with the formation of galena + Cu sulphide eutectic (500-600°C) within the sulphide melt (matte), or with low-temperature Pb + Sb myrmekitic assemblage formation (247°C) in the speiss. Possible geological implications related to the crystallization of sulphide ore bodies or their partial melting induced by regional metamorphic processes are discussed.