P,T,X conditions of crystallization of Imperial Topaz from ouro Preto (Minas Gerais, Brazil): Fluid inclusions, oxygen isotope thermometry, and phase relations

The intense orange-yellow to cherry-red topaz found in quartz, hematite, dolomite, euclase veins in the area of Ouro Preto (Minas Gerais, Brazil) is known in trade as "Imperial Topaz". Only pink topaz from the Mardan District in Pakistan has similar gemmological properties and paragenesis. Fluid inclusion data, oxygen isotope geothermometry and considerations on the stability of kaolinite, pyrophyllite and carbonates found as inclusions in the Imperial Topaz from Ouro Preto indicate that topaz crystallized at about 360°C and 3.5 kbar. A high CO₂ activity precluded flourination of the carbonates and stabilized topaz relative to fluorite. Flourine and beryllium, needed to form topaz and euclase, as well as CO₂ were presumably provided by metamorphic fluids produced by the Brasiliano tectonothermal event at about 600 Ma.