QUENCHED BLACK-SMOKER FLUIDS: EVIDENCE FROM BORNITE SCALES PRECIPITATED FROM SEAWATER-DOMINATED GEOTHERMAL FLUIDS ON THE REYKJANES PENINSULA, ICELAND

The Reykjanes geothermal system, on the onshore extension of the Mid-Atlantic Ridge (MAR) in Iceland, is dominated by 1-2 km-deep 270 to 315°C liquids with salinity similar to seawater. The composition and mineralogy of sulfide scales that precipitate from these fluids in wells are strikingly similar to hydrothermal chimneys at MAR submarine vents. Scales were sampled from five locations at Reykjanes: i) at the wellhead, where fluids arrive at surface, ii) in pipes upstream of the pressure control plate, iii) at this orifice plate and on valves at the depressurization point, where P and T decrease sharply by 10-20 bars and from 267 to 215°C, iv) downstream of the orifice plate, and v) and further downstream at the separation station. Scales at the wellhead consist of intergrown chalcopyrite and sphalerite, with minor galena, bornite and up to 150 ppm gold. Upstream of the orifice plate scales consist of skeletal intergrowths of chalcopyrite and sphalerite ± wurtzite and smectite. The largest drop in pressure (20 bars) occurs downstream of the orifice valve/plate, which is encrusted by 1-2 cm thick scales of bornite and digenite in graphic intergrowth (about 60 vol%), with granular sphalerite (20%), galena (15%) and minor silver (1%). This grades outward to a band of chalcopyrite (4%), intergrown with pyrrhotite-pyrite, and a rim of covellite with minor amorphous silica, possibly formed during cooling caused by well closure. The bornite-digenite represents a quenched product of the deep geothermal fluids, consistent with the composition of high-temperature fluids in MOR systems, which are close to saturation with bornite prior to the ubiquitous entrainment of seawater. However, bornite-dominated assemblages are rare in black smoker chimneys, where mixing with a small amount of seawater and conductive cooling occurs prior to deposition of mainly chalcopyrite, pyrrhotite, pyrite, and sphalerite. Downstream of the orifice plate, sulfide scales consist of sphalerite, chalcopyrite, pyrrhotite and only minor bornite, plus amorphous silica, similar to black smoker chimneys. Further downstream, amorphous silica increases enormously, with less chalcopyrite. The relatively cool fluid at the separation station, 200°C, precipitates mostly amorphous silica with minor sphalerite, pyrite and traces of galena.