Precipitation of Precious and Base Metal-Bearing Scales at the Berlin Geothermal Field, El Salvador

摘要

Geothermal fluid and scale studies provide important opportunities to study ore-forming processes, and to assess the possible role of vapor as an agent of metal transport and deposition. Fourteen composite samples (scale, rock and corroded pipe fragments) were sampled at solid collectors and drains of two-phase, re-injection and vapor pipelines of the Berlin geothermal field, El Salvador. The samples consist mainly of: (1) sulfide- and electrum-bearing aluminum-rich amorphous silica that formed as scale in the wells; (2) sulfide- and electrum-bearing saponitic/vermiculitic clays that originated at depth close to the production horizon; and (3) altered metallic pipe fragments with As-S-bearing iron oxide-hydroxide grains formed at surface in vapor lines. Siliceous and clay-rich scales have Au and Ag concentrations averaging 60 and 739 ppm, respectively, and contain appreciable Cu, Pb and Zn (avg. 0.2 to 0.3 wt% each). Altered iron fragments contain an average 0.4 wt% As. Copper, lead and zinc occur primarily as chalcopyrite, galena and sphalerite, respectively. Gold and silver occur mainly in the form of electrum, which was deposited with base metal sulfides in the wells. Measured concentrations of gold in the vapor are ～4 times greater than that in water sampled at associated wellheads. This suggests that gold can be transported relatively efficiently in vapor, and that this process may be applicable to some epithermal systems.