Quartz veins with associated Sb-Pb-Ag±Au mineralization in the Schwarzwald, SW Germany: a record of metamorphic cooling, tectonic rifting, and element remobilization processes in the Variscan belt

Abstract

A combination of textural observations from 38 Permian Sb-Pb-Ag±Au-bearing quartz veins in the Schwarzwald (SW Germany) with isotopic, fluid inclusion, and geochemical data allows to refine genetic models for this common type of mineralization, to understand the origin of mineralogical diversity and the correlation with large scale tectonic events. Textures record four main mineralization stages: (I) Fe-As(-Sb±Au), (II) Pb-Zn-Cu, (III) Pb-Sb, and (IV) Ag-Sb. Stage I sulfides, dominated by pyrite and arsenopyrite, formed due to cooling of low-salinity metamorphic fluids during the Permian with maximum homogenization temperatures of 400 °C. Invisible gold in arsenopyrite, pyrite, marcasite, and stibnite was remobilized and locally precipitated as electrum at the end of stage I. Minerals of stages II and III comprise a rich diversity of Sb-bearing sulfosalts including, e.g., bournonite, zinkenite, and jamesonite. This assemblage formed during Jurassic times due to mixing of high-salinity, mid- and upper-crustal fluids (up to 27 wt% NaCl+CaCl2) with homogenization temperatures between 50 and 250 °C. Stage III is marked by a rich variety of Pb-Sb sulfosalts. Its local abundance is directly related to the presence of stage I mineralization and its extent of remobilization during a significant influx of Pb in the Jurassic-Cretaceous. The formation of the Upper Rhine Graben during the Tertiary reactivated especially NE-SW-oriented veins. Percolating Ag-rich fluids reacted with earlier stage III Pb-Sb sulfosalts forming Ag-rich minerals (stage IV) such as miargyrite, pyrargyrite, and stephanite. The transition from metamorphic fluids to basinal, saline (e.g., Pb-bearing) brines over hundreds of millions of years as is shown in this study and present in other Variscan occurrences indicates a possibly typical poly-stage characteristic of Sb deposits worldwide, which has, however, not been investigated in detail so far.