ASSEMBLAGES OF Ag-Bi SULFOSALTS AND CONDITIONS OF THEIR FORMATION: THE TYPE LOCALITY OF SCHAPBACHITE (Ag0.4Pb0.2Bi0.4S) AND NEIGHBORING MINES IN THE SCHWARZWALD ORE DISTRICT, SOUTHERN GERMANY

Abstract

Several examples of hydrothermal vein-type mineralization with matildite, schapbachite, ourayite, eskimoite, berryite, benjaminite, miharaite, and gustavite occur along the Northern Kinzigtal Fault (NKF) in the Schwarzwald ore district, in southwestern Germany. The complex history of naming of the sulfosalts schapbachite and matildite is shortly revisited here. The sequence of mineralization starts with gustavite, ourayite and eskimoite, followed by native bismuth and bismuthinite. These assemblages are overgrown or replaced by a cubic solid-solution of the Ag–Bi–Pb sulfosalt, schapbachite, which later exsolved into galena and matildite. Subsequently, this intergrowth was again overgrown by berryite, benjaminite, chalcopyrite and tennantite. The temperature of formation of the sulfosalt assemblages straddles the conversion of schapbachite (cubic AgBiS 2 ) to matildite (hexagonal AgBiS 2 ). The upper limit based on untwinned acanthite is 179°C, whereas recalculated compositions of schapbachite–galena intergrowths from various localities indicate a formation temperature above 144°C. Calculated fields of stability of matildite show that it is stabilized by higher temperatures or by its solid solution with PbS in favor of an assemblage of Bi- and Ag-sulfide, and by a high sulfur activity, low pH and an f (O 2 ) around the hematite–magnetite buffer in favor of the more common assemblage of native bismuth and native silver.