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

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 ∼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.