HYDROTHERMAL ALTERATION AND THE CHEMISTRY OF ORE-FORMING FLUIDS IN ANUNCONFORMITY-TYPE URANIUM DEPOSIT

Compositions of hydrothermal chlorite and fine-grained white mica from the inner and outer alteration halos in the Koongarra U deposit were analyzed by electron microprobe and analytical electron microscopy. An alyses show that although chlorite and white mica compositions vary co nsiderably outside the main ore zone, they are uniform inside the ore zone. Ore-zone chlorite has a ratio of Fe/(Fe + Mg) of 0.25 and low oc tahedral occupancy (average 5.5 per formula unit), which may represent a mixture of di-and trioctahedral chlorite. White mica has a typical K + Na atomic content of 0.85 per formula unit. These compositions wer e used to calculate the activity ratios a(Fe)+2/a(H)(2)+, a(Mg)+2/a(H) (2)+, a(K)+/a(H)+, and a(Na)+/a(H)+ for the hydrothermal fluids associ ated with deposition of uraninite. Hydrothermal apatite analyses in co njunction with salinities suggested from fluid inclusion studies were used to calculate the pH of the fluids during the pre-ore alteration. The calculated pH values range from 4.8 to 6.0. Finally, the coexisten ce of chlorite with quartz and hematite was used to calculate oxygen f ugacities. The calculated values are about 2 log units higher than for the hematite-magnetite buffer at 200 degrees C. Consequently, the oxi dation state of the fluid lay in the hematite field and U was probably transported as uranyl complexes.