Mineralization associated with scale and altered rock and pipe fragments from the Berlin geothermal field, El Salvador; implications for metal transport in natural systems

Composite fragments sampled at solid collectors and drains of two-phase, re-injection, and vapour pipelines of the Berlin geothermal field, El Salvador, consist mainly of sulphide- and electrum-bearing aluminium-rich amorphous silica scale, sulphide- and electrum-bearing saponitic/vermiculitic clay from the reservoir, and altered metallic pipe linings containing As-S-bearing iron oxide-oxyhydroxide grains. Siliceous and clay-rich precipitates contain concentrations of gold and silver in excess of 180 and 8000 ppm, respectively, and appreciable concentrations of copper, lead, zinc, and antimony. Altered iron fragments contain substantial arsenic. Copper, lead, and zinc occur mainly as chalcopyrite, galena, and sphalerite, respectively, in amorphous silica and clay; near the surface, chalcopyrite transported from depth alters to bornite. Gold and silver occur mainly as electrum, which deposited with base metal sulphides in the clay precipitates, and amorphous silica at higher levels in the well. Electrum precipitates in the wells due to the rapid drop in temperature and loss Of H2S associated with boiling. The concentration of gold in vapour is similar to 4 times greater than that in water from associated wellheads. This suggests that gold can be transported efficiently by vapour, and implies that such transport may be important in the formation of some hydrothermal ore deposits. (c) 2005 Elsevier B.V. All rights reserved.