METASOMATISM OF THE REVENUE GRANITE AND AUREOLE ROCKS, MT ISA-INLIER,QUEENSLAND - SYNDEFORMATIONAL FLUID-FLOW AND FLUID-ROCK INTERACTION

The Revenue Granite intruded as a sill into carbonate-rich, evaporitic sediments of the Mt Isa Inlier in the mid-Proterozoic. The granite wa s altered by high-temperature saline fluids shortly after emplacement. The alteration assemblages include albite, Cl-rich scapolite and diop side. Scapolitization and albitization both involve substantial losses of Fe, Mg and K from the host rock, and gains in Na and Ca. The miner al assemblages and mass-transfer calculations indicate that the metaso matizing fluids were Na-, Ca-, Cl-bearing brines. Formation of pyroxen e in veins relatively late in the metasomatic history was probably a c onsequence of saturation of the fluid in Fe and Mg due to relatively l ocal circulation (on the order of 100 m) of the fluids through overlyi ng dolerites. Stable isotope data suggest that the fluids were of domi nantly magmat ic origin. A > 1 parts per thousand difference in delta( 18)O values between the top and bottom of the Revenue Granite probably relates to the pattern of fluid circulation. Deformation was continuo us throughout metasomatism, and fluid flow was strongly structurally c ontrolled.