Abstract: The Rhenohercynian thrust- and fold-belt of Central Europe hosts a syn- to post-kinematic Variscan vein mineralisation, which is restricted to the Rhenish Massif, Germany. It is formed during four major stages and siderite is the principal ore mineral of the ‘main stage’. The latter can be traced throughout the massif, but is mainly developed in the Siegerland district as synkinematic lodes which are hosted by low-grade metamorphic pelites of Lower Devonian age.

Fluid inclusion studies prove large-scale homogeneous ore forming fluids of low salinity (≤5 wt.% NaCl equiv.) and Na(-K > -Fe ≫ Mg)-Cl composition, which are CO₂ undersaturated (X_co2 = 0.003–0.1). Siderite precipitation at 220−≤320°C and 0.7–1.4 kbar can be deduced by microthermometry, chlorite themometry, = 0.003–0.1). Siderite precipitation at 220−≤320°C and 0.7–1.4 kbar can be deduced by microthermometry, chlorite themometry, REE fractionation and experimental data. Thereby the maximum formation temperature comes close to or even exceeds the peak metamorphic temperature. From Mn contents and δ¹³C-δ¹⁸O variation of siderite a trend is apparent in which formation temperatures gradually decrease from S.E. to N.W. across the belt. This trend correlates with decreasing degree of host-rock deformation and decreasing metamorphic grade. Fluid composition as well as stable isotopes and REE fractionation of siderite point to ore-forming solutions, which were generated and equilibrated at depth during prograde metamorphism.

Structural characteristics of the lodes, age determinations, and P-T estimations prove fluid ascent and siderite precipitation during and/or immediately after peak metamorphism predating the postkinematic magmatism of the Rhenohercynian belt. Main(-siderite)-stage mineralisation of the Rhenish Massif is classified as metamorphogeneous.