Fracture-related hydrothermal alteration of metagranitic rock and associated changes in mineralogy, geochemistry and degree of oxidation: a case study at Forsmark, central Sweden
Red-staining of rocks due to fluid–rock interaction during hydrothermal circulation in fractures is a common feature in crystalline sequences. In this study, red-stained metagranitic rock adjacent to fractures in Forsmark, central Sweden, has been studied with emphasis on the mineral reactions and associated element mobility occurring during the alteration. The main mineral reactions associated with the hydrothermal alteration are an almost complete saussuritization of plagioclase accompanied by total chloritization of biotite. Magnetite has been partly replaced by hematite whereas quartz and K-feldspar were relatively unaffected by the hydrothermal alteration. We show that redistribution of elements on the whole rock scale was very limited and is mainly manifested by enrichment of Na2O and volatiles and depletion of CaO, FeO and SiO2 in the red-stained rock. However, on the microscale, element redistribution was more extensive, with both intragranular and intergranular migration of e.g. Ca, K, Na, Al, Si, Fe, Ba, Cs, Rb, Sr, Ti and REEs. The altered rock shows a shift towards higher total oxidation factors, but the change is smaller than 1σ and the red-staining of the rock is due to hematite dissemination rather than a significant oxidation of the rock. An increase in the connected porosity is also observed in the altered rock.