The formation of epigenetic sediment-hosted ore deposits is often accompanied by hydrothermal illitization of the hosting sediment. These illites can be used to determine the age of the hydrothermal events. This paper deals on the investigation of hydrothermal illitization and related fluid inclusions in Triassic to Permian sediments of the Offenburg Trough in the Black Forest, Southern Germany.

The Black Forest is part of the Moldanubian zone of the Variscan Fold Belt of Central Europe. It is made up of a basement of Pre-Variscan gneisses and Variscan granites which is covered by Carboniferous to Permian sediments. The sedimentary cover was deposited in a number of NE-SW striking troughs formed during the Upper Carboniferous to Lower Permian. Sedimentation started locally in the Upper Carboniferous and held on until Jurassic. The sedimentary cover is mainly composed of Lower Permian and Lower Triassic consisting of arkoses, sand- and silt stones.

Sampling comprises the whole sedimentary cover rocks of the Offenburg Trough. Samples have been taken mostly from the basal series of the cover rocks since the hydrothermal overprint is recorded best in these series. Microscopic investigation of the arkoses, sand- and silt stones focused on feldspar alteration and hydrothermally grown illites resulting in three zones divided by their degree of feldspar alteration. In the central zone feldspar is completely replaced by an illite-quartz mixture. In the following zone of advanced alteration the feldspars are only partly altered along cleavage planes. Additionally, network like illites have grown on detrital grains. In an outer zone of minor alteration the feldspars exhibit initial illitization along cleavage planes and network like illites are found as well.

The age of this hydrothermal event was determined by K-Ar analysis on illites of the fractions < 2 µm and < 0.2 µm to be around 145 ± 4 Ma.

The fluid inclusion record reveals at least two different populations of aqueous inclusions in all samples. A third population can only be observed in the zone of complete alteration.

The first population are inclusions in detrital grains which have low salinities (T_m about 0°C) and give T_h of 150 to 300°C.

The second population in quartz overgrowth and also in detrital quartz grains show eutectic temperatures below -50°C and final ice melting of -30 to -20°C. Homogenisation temperatures are of 50 to 150°C. Some of these inclusions developed directly along the contact of detrital quartz grain and quartz cement while others are arranged isolated or along trails which partly cut the grain boundary of detrital quartz and overgrowth.

The third population is restricted to the illite-quartz pseudomorphoses after feldspar in the inner zone. These fluid inclusions have also low salinities with T_m around 0°C and homogenise at temperatures of about 130°C.

The recorded fluid inclusions are of different origin. The fluid inclusion population of the detrital grains are inherited being formed prior to sedimentation without any importance concerning the hydrothermal event. The first occurrence of hydrothermal alteration is documented by illitization of feldspar and additionally as network like grown illites on detrital grains. Fluid inclusions which document this event are rare and have only be found in the central part of the alteration aureole. Feldspar of this zone is completely replaced by a cogenetic illite-quartz mixture intimately grown together. Inclusions in quartz indicate that the illites of this stage have obviously grown from a low salinity fluid.

The second product of a hydrothermal event in the Offenburg Trough are the quartz overgrowth on detrital grains, which even overgrow illite and thus post-date illite growth. Moreover these quartz overgrowth result from a different fluid as indicated by the fluid inclusion record. The eutectic temperatures of these inclusions are below -50°C and point to Ca-Na-Cl brines as typical for post-Variscan mineralization of the Black Forest (Behr et al. 1987). The period between the illite growth and the quartz overgrowth can not be determined precisely since only the illites can be dated and therefore one can only make relative age relations in terms of quartz cementation is younger than illite networks.

The sedimentary cover rocks of the Offenburg Trough have been intensively hydrothermal altered as indicated by a widespread illitization which predates quartz cementation. Fluid inclusions indicate that illitization results from a low salinity fluid. Quartz cement, however, is precipitated from Ca-Na-Cl brines and its inclusions show no significant decrease in Th from the centre of hydrothermal alteration towards the outer zone so fluids must have spread very rapidly in the sediments. This advocates for an episodic fluid supply by seismic pumping (Sibson et al. 1975).

The age of the quartz cementation remains open since illite growth from low-salinity fluids can be precisely fixed by K-Ar dating in the sediments. It predates subsequent quartz cementation which results from Ca-Na-Cl brines.

Thus, in the sediments of the Offenburg Trough illite growth and quartz cementation is not cogenetic and at least some temporal difference between both processes is expected.

The illites are dated by K-Ar to be Jurassic and therefore these fluids must have been mobilised in an upper level of the crust and heated because of crustal thinning resulting from the break-up of the Atlantic ocean occurring at the same time (Mitchell & Halliday 1976).

We are highly indebted to Dr. U. F. Hein (IMMR, TU Clausthal) for assistance during the microthermometric analyses and for numerous stimulating discussions. We further thank Dr. N. Clauer (CNRS, Strasbourg) for K-Ar determination. Financial support was provided by the Deutsche Forschungsgemeinschaft.