Abstract

Seawater-dominated fluids discharge from the subaerial Reykjanes geothermal system, Iceland. There is a sharp pressure decrease in surface pipes at an orifice (throttle point), and Cu-rich scales deposit at this orifice that consist largely of bornite and digenite, along with sphalerite and other sulfides. The bornite and digenite form complex intergrowths with sphalerite and galena, accompanied by high concentrations of gold and silver (up to 590 ppm and 2.3 wt %, respectively). The precipitates form in response to rapid boiling (flashing) of the hydrothermal fluids due to the sharp decrease in pressure (~37–22 bars) at the orifice, downstream of the well-head. The boiling, and concomitant separation of the vapor phase, results in a temperature decrease from 252° to 220°C across this throttle over distances of centimeters. There is a nearly quantitative deposition of metals from solution compared to concentrations in reservoir liquids, resulting in formation of the metastable Cu-FeS solid solution, rich in silver and gold, plus sphalerite and galena. A high degree of supersaturation of metals in the quenched liquid is indicated by the very fine grain size of the sulfides and dendritic textures. Postdepositional cooling likely caused exsolution of bornite and digenite from the high-temperature solid solution; in addition, silver was expelled from the Cu-Fe-S solid solution and remobilized into late fractures in the scales, whereas the gold appears to have remained in solid solution or as submicroscopic inclusions in the bornite and digenite.

The observed mineral assemblage at Reykjanes contrasts with that of sea-floor black smokers, despite similar fluid compositions. The sea-floor hydrothermal vents characteristically have Cu-rich linings composed predominantly of chalcopyrite, which is precipitated by a combination of conductive cooling of high-temperature (~350°C) hydrothermal fluids and mixing with seawater. The difference in sulfide mineralogy between the Reykjanes orifice scales, with Cu sulfides dominated by bornite and digenite, and many sea-floor vents may provide an indication of the subsea-floor mineralization where fluids boil before discharging at sea-floor vents.

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