Tectonic evolution and crustal setting of the middle Proterozoic Leichhardt River fault trough, Mount Isa region, northwestern Queensland

The tectonic evolution of the fault-bounded volcanic-sedimentary Leichhardt River fault trough is reinterpreted in the light of detailed mapping of a part of this structure. Two major unconformities are indicated, the first above the Kalkadoon-Leichhardt acid igneous complex, and the second at the base of the ore-bearing Mount Isa Group and its equivalents - the Mingera and Surprise Creek Beds. A complex succession of movements is indicated, involving (1) early faulting of the Kalkadoon-Leichhardt acid igneous complex; (2) long-acting penecontemporaneous faulting; (3) folding and uplift accompanying or post-dating emplacement of Sybella Granite, and (4) a younger major faulting phase. During phase (2) syndepositional block movements resulted in differential accumulation of volcanic flows and of sediments, reflected by abrupt thickness changes across faults. In other instances the observed thickness variations can be explained by strike faulting of lenticular sedimentary units. Basic dyke swarms which intrude the volcanic- sedimentary sequence of the Leichhardt River fault trough have postdated the folding and predated the major faulting event defined as phase (4). The Sybella Granite, thought by some workers to be younger than the Mount Isa Group on the basis of structure and lead isotope ratios, is considered by us to be older than this unit. The ore lead in the Mount Isa deposit could be related to erosion or leaching of lead from older rocks such as the Kalkadoon-Leichhardt basement complex and the Eastern Creek Volcanics. An age of between I500-1600 m.y. is indicated for the syngenetic lead deposit at Mount Isa. The Eastern Creek Volcanics show very high Pb relative to average basalt, and high Cu, Zn and Pb levels relative to other basic volcanic rocks in this region, and may have a genetic relation to the Mount Isa Cu and Ag-Pb-Zn ore deposits. Geochemical features of basalts of the Eastern Creek Volcanics are consistent with those of continental flood tholeiites, showing high FeO, FeO/ MgO ratios, TiO2, K2O, P2O5, Ba and Rb and low Al2O3. Structural, sedimentological and geochemical changes between the Mount Isa area and the CIoncurry area to the east - where ocean floor type tholeiites occur - can be interpreted in terms of a transition into a continental margin environment in this direction.