The invention concerns a method and apparatus for producing silica concentrates from geothermal fluids containing at least 300 ppm silica, by passing the fluid at a temperature above 80° C. and at a pH reduced to between 4.0 and 7.5 through a reverse osmosis membrane. In the diagram, geothermal fluid (I) is passed to a separator (2) to be flashed to produce steam (3) and separated geothermal water (SGW) (4). The SGW (4) is passed to a heat exchanger (5) then inlet pump (7). Acid is introduced to the geothermal fluid flow at a dosing means (6) to reduce the pH and an anti-sealant may also be introduced. The geothermal fluid is then passed to a reverse osmosis unit (8) to produce a concentrate (9) and a permeate (10). Following reverse osmosis, the concentrate and permeate may be treated with other processes to produce the desired product and concentration. For example, if precipitated silica is produced, the concentrate is passed to a curing tank (11) and to a thickener (12). The precipitated silica is collected (13) while the retained fluid is removed (14).