Abstract

The Miesbrunn pegmatite–aplite swarm (MPAS), part of the Hagendorf–Pleystein pegmatite province, southeastern Germany, was emplaced syn- to postkinematically relative to the Variscan tectonometamorphic processes (stage I). The MPAS developed along the contact between the autochthonous Moldanubicum and the nappes of the Bohemicum. Supercritical mobilization of REE, Zr, Mg, Zn, Li, Be and B during stage II led to a silicate–phosphate mineral association between 500 and 600°C. Below 475°C and a pressure of 3.8 kbar, contact-metasomatic reactions caused Mg phosphates to appear. Subsequently to the emplacement of the felsic intrusives bodies, the shear structure was reactivated and used as a conduit for fluids bearing S, As, Bi, Sb, Hg and U, from which sulfides and arsenides precipitated at 350°C. Hypogene alteration during stages III and IV affected the primary phosphates at temperatures below 100°C, with fluctuating redox conditions when the MPAS was brought closer to the paleosurface as a result of strong uplift of the crystalline basement. Phosphates of Fe2+ formed during this epithermal mineralization in the pH range 5 to 10, whereas the stability field of Fe3+ phosphates extended toward lower values, below a pH of 5. The mineral assemblages of the MPAS are on one hand representative of mineralization at the deepest parts of the pegmatite, close to its root zone, and on the other, within the uppermost parts of pegmatites as they were pervasively altered by near-surface fluids. Sulfur isotopes of sulfides found in the MPAS and its wallrocks show a significant trend toward more strongly negative values away from the pegmatites and aplites. A survey of sulfides using their S isotopes may well be a useful exploration tool for “blind” pegmatites (i.e., not exposed).

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