Total systems analysis of the Mount Isa eastern succession

The lower crustal basement of the Eastern Succession of the Mt Isa inlier is divided into a western block and an eastern block, the latter with a distinctly younger crustal residence age. The overlying metasedimentary and meta-volcanic rocks were deposited in extensional basins and comprise two major Cover Sequences (CS2 and CS3). CS2, consisting of mafic and felsic volcanic rocks overlain by carbonates, was deposited from ca 1780 to 1690 Ma diachronously from west to east, possibly in a back-arc environment. CS3 (quartzites, pelites, carbonates and volcanic rocks), was deposited from ca 1655 to 1600 Ma, with important lateral facies changes from west to east. Major thickness changes in both sequences are localised over NS trending structures, which were presumably basin margins. Low pressure metamorphism and albitisation occurred at the time of EW extensional basin formation. \ud \ud The Isan Orogeny from ca 1600 Ma to 1500 Ma was dominated by EW compression that lasted at least episodically until ca 1500 Ma. From ca 1550 to 1500 Ma, voluminous mafic and felsic potassic magmatism was emplaced at mid-crustal levels. Despite having A-type geochemical signatures, the granites are syn-tectonic. P-T-t paths were complex, showing an initial anticlockwise segment, followed by a clockwise segment and then heating events corresponding to granite intrusion. Metamorphism was caused by advective heating in the lower-middle crust, due to melt migration, and conductive heating in the upper crust. The major crustal structures produced during the orogeny were km scale upright folds and steep faults. The coincidence of these major structures with thickness changes in the cover sequences suggests that positive inversion was a dominant process during the orogeny. A preliminary 3D geological model has been constructed which identifies some key relationships of the basement and cover sequences, and provides important pointers for further research in resolving the architecture of the region.\ud \ud Iron Oxide Copper Gold (IOCG) deposits in the Eastern Succession have strong spatial associations with the tectonic CS2/CS3 contact, with faults and fault bends and with mafic intrusions. The role of mafic rocks in IOCG genesis appears to be important in many cases: metals may have been transported and deposited from magmatic-hydrothermal fluids on crystallisation of the mafic rocks, or from metamorphic fluids that subsequently leached them. In the largest IOCG deposit, Ernst Henry, fluids from highly fractionated Williams Batholith rocks may have mixed with primitive mantle or gabbro-derived fluids. In all cases strong structural controls in the form of NS and NE-SW faults, bends and intersections are apparent. The importance of structural controls on fluid pathways is demonstrated by numerical modelling, which predicts known deposits and corresponds closely with prospectivity results. The prospectivity studies highlight a number of targets under cover, based on geophysical data. \ud \ud The base metal endowment of the Eastern Succession can be seen as a product of several geological processes in a volume of crust that has experienced high fluid fluxes and prolonged deformation and thermal anomalism. Reactivation of structures and lithological diversity are also ingredients of a highly distinctive geological history that has lead to extensive mid-Proterozoic mineralization. \u