Mineral deposits in the Rotokawa geothermal pipelines, New Zealand

At the point of flashing, at 224°C and 25 bar absolute, black silica scales enriched in Cu (5.6%), Ag (4.0%), Te (3.0%) Zn (1.5%), Pb (0.6%) and Au (0.2%) form in the wellhead of production well RK-9. Silver, Pb, Cu and Au are deposited as tellurides; Cu, Pb and Zn as sulfides; and Au as a native element. These elements decrease rapidly downstream of the wellhead to other parts of the surface geothermal pipeline. Mercury, B, As, Se, Sb and reduced S are partially transported in the gas phase and largely deposited at a distance from the wellhead. Total S and B occur primarily in the steam line where B is stabilised by silica deposited from carry-over brine and S by sulfide mineralisation. Arsenic and Se are mainly found in scales from the steam and mixing/reinjection lines; Hg in the condensate and mixing/reinjection lines; and Sb in the mixing/reinjection line where 80–105°C low-salinity condensate mixes with 140–170°C brine. A decrease in the solubility of minerals in solution is the primary cause for deposition of opal–A and increased concentrations of base and precious metal concentrations in scales. Scale deposition is thus induced by flashing at the wellhead of the production wells attached to the ORMAT binary plant, phase separation, dilution, cooling and mixing of cooled brine and condensate. The reaction of steel with circulating solutions and gases results in a corrosion mineral assemblage reflecting local oxidation potentials and variations in H2S partial pressures along the pipeline. Corrosion of steel introduces and augments trace elements such as P, Cr, Mn, Ni and Mo in the pipeline solutions. Selective incorporation or sorption of elements, onto opal–A and opal–CT or corrosion products such as goethite and pyrite, may increase concentrations of Li, B, Cl and P in opal; Cr, Mn, Ni, Cu, Zn and Mo in goethite and Cu, Zn, As, Hg, Sb and Se in pyrite. Wherever water circulates in the pipeline system, silica precipitates initially from solution as a gel, which later solidifies to opal–A and subsequently crystallises to opal–CT then quartz aggregates. The transformation of opal–A to more ordered and stable silica polymorphs in the pipelines is a kinetically-induced dissolution–reprecipitation–crystal growth process that is accelerated at temperatures 200–224°C.