GIANT METASOMATIC SYSTEM FORMED DURING EXHUMATION OF MIDCRUSTAL PROTEROZOIC ROCKS IN THE VICINITY OF THE CLONCURRY FAULT, NORTHWEST QUEENSLAND

A giant alteration system including approximately 200 km(2) of rocks dominated by the secondary mineral assemblages lies near the prominent lineament formed by the Cloncurry Fault in the southeast of the Proterozoic Mt Isa Inlier. Alteration affected rocks that had previously undergone sillimanite zone metamorphism under pressures of at least 300 MPa. The main lineament was produced by postorogenic brittle transcurrent motion with limited displacement. It exploits older D-1 faults and occupies different structural positions within F-2 fold limbs that were reactivated as late/post-D-2 shear zones. Metasomatism and vein emplacement occurred under a range of retrograde conditions during the transition from a compressional regime in a ductile regional metamorphic environment, through late- to post-orogenic granitoid emplacement, and ultimately high-level brittle faulting. The dominant sodic-calcic alteration occurred in multiple overprinting fluid buffered systems at 400-500 degrees C with pressures initially above 200 MPa. Assemblages of albitic plagioclase + actinolite + titanite +/- quartz +/- magnetite + diopsidic clinopyroxene were formed by alteration of metabasites, calcsilicate rocks, metapelites and felsic igneous rocks. The alteration occurred in association with fluid overpressuring in retrograde brittle-ductile shear zones and also in purely brittle fracture and megabreccia systems. Younger K-feldspar + hematite alteration and silicification are highly localized by spaced dilational fractures and mostly formed at temperatures below 350 degrees C. Complex hypersaline H2O-NaCl-CaCl2-KCl-(?FeCl2) fluids with high Ca : Na ratios were present at all stages. The main controls on the nature of metasomatic products were probably temperature, pressure and the direction of fluid Row with respect to the geothermal gradient. The large volumes, high salinities and oxidized nature of the fluids responsible for alteration close to the Cloncurry Fault indicate that these could have played a significant role in the regional metallogeny through the mobilization of metals.