Accessory minerals of fluorite and their implication regarding the environment of formation (Nabburg–Wölsendorf fluorite district, SE Germany), with special reference to fetid fluorite (“Stinkspat”)

The Nabburg–Wölsendorf vein-type fluorites in SE Germany which are characterized by a pronounced timebound variation in mineral color (blue → black → green → white/colorless → yellow → brown) were investigated for their accessory minerals included in or associated with these different fluorite types. Nabburg–Wölsendorf is not a unidirectional fluorite mineralization but a polyphase mineralizing process with several repetitions of the various color types during the vein mineralization event. Accessory minerals of the fluorite mineralization include silicates, sulfates, oxide/hydroxides, phosphates, sulfides and carbonates. These minerals are intimately intergrown with fluorite in the vein-type mineralization and are efficient tools to broaden knowledge on the depositional environment of fluorite that is otherwise poor in marker minerals for redox conditions, temperature, alkalinity/acidity and fluid viscosity. Pyrite, galena and cinnabar, are indicative of hypogene processes, while uranyl phosphates and APS minerals furnish evidence for supergene mineralizing processes. Some minor constituents may be used as marker minerals of redox conditions (hematite, galena, and goethite) or held to be characteristic for the changing pH regime (kaolinite and smectite). Oxidizing conditions prevailed during fluorite mineralization, excluding some phases when fetid and white fluorite precipitation ran through a maximum. Yellow-brown honey spar marks the oxidizing physical–chemical conditions by its brown mineral color at the end of fluorite mineralization. Oxidizing conditions prevailed also during supergene mineralization.The pH of the mineralizing fluids of the hypogene stages was greater than pH 7. During the supergene stage, Eh fluctuated, but was mainly > 0; the pH of fluids ranged between 7 and 4. The black color of fetid fluorite and its pungent odor resulted from the U present primarily in its tetravalent state. Incomplete haloes recognized in some fetid fluorites reflect “frozen” uranium disequilibria and an early phase of supergene alteration. In this respect, the fetid fluorite (“Stinkspat”) plays an important part and held to be a marker for the radioactive “hot spot” as well as “ghost” uranium mineralization. Fetid fluorite is defined as to its mineral color, morphology and characteristic spectral features so as to allow for a precise distinction from other dark blue or green fluorite varieties by its outward appearance and easy-to-handle methods. The accessory minerals under study are indicative of the depocentre of the fluids which lies close to an unconformity but does not, however, deliver information on the source of the mineralizing fluids. The nearness to the potential source rock of the fluids can only concluded from the presence of cerite-(Ce) which formed prior to fluorite. This REE silicate is cast as a marker of the root zone of the fluorite veins which fade out into late Variscan granites. Cerium is assumed to have been derived from monazite an accessory mineral largely found outside the fluorite veins, proper, in the late Variscan granites. In the waning stages of fluorite mineralization different types of REE-bearing phosphates, belonging to the rhabdophane solid solution series, and some APS minerals (aluminum–phosphate–sulfate solid solution series) evolved.