A six-component chlorite solid solution model and the conditions of chlorite formation in hydrothermal and geothermal systems

A unique definition of the physico-chemical conditions of formation of hydrothermal ore deposits is commonly difficult to achieve and the chlorite solid solution model is being developed to assist in this task.The nonstoichiometry of chlorite has been represented by six thermodynamic components: Mg 6 Si 4 O 10 (OH) 8 (C1), Mg 5 Al 2 Si 3 O 10 (OH) 8 (C2), Fe (super +2) 5 Al 2 Si 3 O 10 (OH) 8 (C3), Fe (super +2) 5 Fe (super +3) 2 Si 3 -O 10 (OH) 8 (C4), Al 4 Si 4 O 10 (OH) 8 (C5), and Fe (super +2) 4 Fe (super +3) Al 2 Si 3 O 11 (OH) 7 (C6). Thermodynamic data for components 1, 3, 4, 5, and 6 were estimated making use of constraints imposed by the compositions of chlorites from the Salton Sea and Broadlands geothermal systems; the OH vein, Creede, Colorado; the Quaama granodiorite, New South Wales; and a metamorphic vein, Mt. Lyell, Tasmania. Activity-composition relations have been modeled assuming random mixing and equal interactions of atoms on energetically equivalent sites for components 1, 2, and 3. For components 4 and 5 the activity has been assumed equal to the mole fraction of the component and the activity of component 6 has been expressed as a 6 = gamma 6 X 6 where log gamma 6 = alpha (1 - X6) + beta /2(1 - X6) 2 and alpha and beta are temperature-dependent terms.Two geothermometers, 2Mg 5 Al 2 Si 3 O 10 (OH) 8 + 14/3SiO 2 + 8/3H 2 O (sub (1)) [harr] Al 4 Si 4 O 10 (OH) 8 + 10/6Mg 6 Si 4 O 10 (OH) 8 and Mg 5 Al 2 Si 3 O 10 (OH) 8 + 5/7Fe (super +2) 5 Fe (super +3) 2 Si 3 O 10 (OH) 8 + 3/7Fe (super +2) 5 -Al 2 Si 3 O 10 (OH) 8 + 25/21SiO 2 [harr] 10/7Fe (super +2) 4 Fe (super +3) Al 2 Si 3 O 11 (OH) 7 + 5/6Mg 6 Si 4 O 10 (OH) 8 + 5/21H 2 O (sub (l)) , have been calibrated and these together with the constraint imposed by the Gibbs-Duhem equation allow the temperature of formation, the ferric iron content, and the water content of chlorites coexisting with quartz and an aqueous phase at a known or assumed pressure to be calculated from an electron microprobe analysis of chlorite. This has permitted calculation of oxygen fugacities from Fe (super +2) 5 Al 2 Si 3 O 10 OH 8 + 1/40 (sub 2(g)) [harr] Fe (super +2) 4 Fe (super +3) Al 2 Si 3 -O 11 (OH) 7 + 1/2H 2 O (sub (l)) and sulfur fugacities where quartz and chlorite coexist with an iron sulfide enabling calculation of the H 2 S content of the fluid. Limited testing of the model suggests it has the capacity to elucidate different thermal and redox environments. At about 300 degrees C the calculated log f (sub O 2 ) and log f (sub S 2 ) values appear accurate to within an order of magnitude and calculated values of H 2 S concentrations to within a factor of 2. Other solution parameters, a (sub Fe (super +2) ) /(a (sub H (super +) ) ) 2 , a( (sub Mg (super +2) ) /(a (sub H (super +) ) ) 2 , and a (sub Al (super +3) ) /(a (sub H (super +) ) ) 3 may also be calculated once the temperature and redox state of the fluid have been established.