Late-orogenic alpine-type (apatite)-quartz fissure vein mineralization in the Rheinisches Schiefergebirge, NW Germany: mineralogy, formation conditions and lateral-secretionary origin

Mineralogical, geochemical and fluid inclusion investigations of a representative suite of fissure vein mineralizations in the Rheinisches Schiefergebirge, NW Germany indicate a link to the latest stage of the Variscan orogenic evolution. Model P-T-conditions during initiation of fibrous fissure vein quartz growth are in the range 370-420 degrees C at 0.2-0.7 kbar. The dataset suggests significant fluid cooling during evolution of the vein systems. Minimum temperatures at the end of fibrous quartz growth lie in the range 140-190 degrees C, with conductive heat transfer and heat consumption during interaction with wallrock believed to be the main mechanisms responsible. Wallrock alteration is characterized by leaching and mobilization of most of the dominant vein components (quartz, albite, apatite), notably Si, Na and P. The principal stage of Vein formation is, on the basis of available data, believed to relate to a process of intra-formational redistribution or lateral secretion. However, part of those elements deposited both in wallrock and fissure veins were probably supplied directly by the external fluid. Rates of fissure opening and material deposition were in equilibrium during the principal growth stage of fibrous quartz. However, this situation evolved due to a slowing down of material supply and deposition coupled with an increased rate of fissure opening to produce open fissures and formation of idiomorphic quartz crystals within them. Deposition depths were in the range of 0.6-2.1 km, appreciably lower than estimations of overburden. We believe therefore that formation of the fissure vein systems took place along the retrograde late-orogenic exhumation path, in a transitional stage between Variscan collision and a late- to post-orogenic extensional regime. Fluid composition characteristics also strongly suggest a relationship to the latest stages of the Variscan mineralization cycle in which low-salinity brines dominate. Development of fissure vein systems during the latest stages of continental collision, identified here from the Variscan orogeny, can be considered analogous with similar phenomena in the Alpine orogenic belt.