Si02 PRECIPITATION ACCOMPANYING FLUID FLOW THROUGH GRANITE HELD I N A TEMPERATURE GRADIENT

Abs t rac t I n experiments s imula t ing the r i s e y d r o t h e r m a l f l u i d s f rom a geothermal rese rvo i r , water was f lowed a t a low r a t e down a temperature g r a d i e n t through f rac tu red and i n t a c t c y l i n d e r s o f Bar re and Wester ly Grani te. Temperatures ranged from 80 t o 105°C a t the ou te r edges o f the c y l i n d e r s t o 250 t o 300°C along a c e n t r a l borehole which housed the heat ing c o i l . As a r e s u l t o f minera l deposi t ion, p a r t i c u l a r l y Si02 , a t low temperatures i n the g r a n i t e samples, p e r m e a b i l i t i e s were reduced 10t o 100fo ld i n per iods o f 1 t o 3 weeks. Chemical analyses were made o f the lowtemperature f l u i d s discharged f rom the cy l i nde rs . Early-sampled f l u i d s were supersaturated w i t h respect t o several minera ls a t low temperatures i n t he g ran i tes . O f the oversatura ted species, Si02 showed the most r a p i d decrease w i t h time, and i n an experiment w i t h low i n i t i a l f l o w ra te , t he s o l u t i o n reached e q u i l i b r i u m wi th qua r t z a t the lowtemperature edge o f t he c y l i n d e r w i t h i n about 6 days. Increas ing the maximum temperature o f the g rad ien t a t constant con f i n ing pressure l e d t o h igher Si02 concent ra t ions i n the discharged f l u i d s . However, i nc reas ing c o n f i n i n g pressure along w i t h maximum temperature r e s u l t e d i n lower d i sso l ved Si02 contents, because o f enhanced r e a c t i o n a t the reduced f l o w r a t e s accompanying the pressure increase. The behavior o f Si02 cont ras ted w i t h most o the r d i sso l ved species, which were a f f e c t e d by changes i n temperature b u t n o t f l o w r a t e i n t he t ime o f the experiments.