THE EVOLUTION OF THE DEEP FLOW REGIME AT SOULTZ-SOUS-FORETS, RHINE-GRABEN, EASTERN FRANCE - EVIDENCE FROM A COMPOSITE QUARTZ VEIN

Drilling at Soultz-sous-Forets, France, conducted as part of the Europ ean Hot Dry Rock Project, intersected a fracture lined with vein quart z and actively producing hot (150 degrees C) water at 2174 m depth in the granite basement to the Rhine Graben. At least seven generations o f quartz are present within the vein, but fluid inclusion homogenizati on temperatures show that temperatures remained within 15 degrees C of the present field temperature throughout its development, apart from a few rare pulses of hotter water, in contrast, freezing measurements indicate two distinct fluids. The more saline one, ranging from 10 to 15 wt % NaCl eq, dominated the early stages of vein fill, while the se cond, ranging from 0 to 8 wt % NaCl eq, become more prominent in later stages. Oxygen isotope analyses of different quartz generations also show two populations in addition to magmatic relicts. In the early sta ges a population with delta(18)O from 14 parts per thousand to 18 part s per thousand predominates and is consistent with growth from a sedim entary basin brine. Later quartz shows lighter values (12 parts per th ousand -13 parts per thousand), resulting from precipitation from a fl uid isotopically identical to the modern fluid, which is in equilibriu m with granite feldspar at a temperature similar to 25 degrees C highe r than the present vein temperature. The change in delta(18)O does not , however, correlate with that in fluid salinity, reflecting the diffe rent effects of fluid-rock interaction on the two parameters. Changes in fluid salinity, possibly resulting from meteoric inputs into the de ep formation waters of the Rhine Graben, occurred early in the flow pa th of the Soultz fluids because there is no decrease in fluid temperat ure associated with even the most dilute inclusion fluids. With time t he path by which these fluids from deep in the Mesozoic sediments of t he graben have made their way to the Soultz site has changed, resultin g in greater interaction with granite, as recorded in the oxygen signa ture.