Variation of color, structure and morphology of fluorite and the origin of the hydrothermal F-Ba deposits at Nabburg-Woisendorf, SE Germany

摘要

The Nabburg-Wolsendorf mining district is located at the boundary between the crystalline basement and the Mesozoic-Cenozoic foreland basin in SE Germany. The hydrothermal mineralization is renowned for its great variety of colors of fluorite (blue, black to dark purple, green, white to colorless, yellow to brownish yellow) and its extraordinary diversity of crystal morphol-ogy of fluorite (three temperature-controlled sequences: (1) octahedron-rhombohedron-hexahedron, (2) octahedron-hexahedron, (3) tetrakishexahedron-hexahedron). These colorful and morphologically different fluorites are encountered in seven structural types of fluorite mineralization controlled in their outward appearance by the viscosity of the mineralizing fluids (I: cleft-and-fissure ore, II: brecciated vein ore, III: fitting-breccia ore, IV: rotational-breccia ore, V: cockade ore, VI: layered/ striated ore, VII: impregnation and cement ore). Fluorite mineralization resulted from two hydraulically different processes. Type I through type III — vein mineralization have been explained by the low-viscosity or aquatic fluid model. The structural types IV through VII can only be explained by a high-viscosity or drill-fluid model with a maximum of fluid rock interaction below the paleowater level related to the Late-Variscan unconformity. Bivalent (Ca) cations are gradually substituted for by trivalent (Fe, Y) and even tetravalent (REE) cations with striking consequences for the fluorite coloration and the viscosity of the mineralizing fluid (blue- black to dark purple fluorites are replaced by the green, white to colorless and yellow to brownish yellow fluorites along with an increase in viscosity and structural change towards cockade and stripped fluorite ore). The fetid fluorite is a blue fluorite which suffered from radiation damage and thereby acts as a marker of the radioactive hot spot in the mineralizing district. The crystal morphology and structural type of fluorite veins are controlled by the temperature (100 ℃ and 200 ℃), the depth relative to the unconformity and the position of fluorite mineralization within the paleohydraulic regime.