Tectonic framework and evolution of the Gawler craton, southern Australia

Abstract

The Gawler craton is the major crustal province in the southern Australian Proterozoic and is pivotal in models seeking to describe the evolution of Proterozoic Australia. The craton is host to the Olympic Dam iron oxide Cu-Au-U-REE deposit, as well as a number of other iron oxide copper-gold (IOCG), Au, and iron ore deposits. The evolution of the Gawler craton is dominated by two major phases of tectonic activity, Late Archean and late Paleoproterozoic-early Mesoproterozoic, in total spanning ca. 1 billion years. The Late Archean (2560-2500 Ma) basin development was coeval with arclike felsic magmatism and mafic-ultramafic magmas, including komatiites. Collisional deformation between 2480 and 2420 Ma led to ca. 400 m.y. of tectonic quiescence, conceivably within an early Paleoproterozoic continental interior. The second major phase of tectonic activity was in the middle to late Paleoproterozoic and early Mesoproterozoic (2000-1450 Ma). Paleoproterozoic tectonism was initially dominated by rift-related events, which produced a series of basins over the interval ca. 1900 to 1730 Ma, some of which contain significant iron ore reserves. Transient contractional deformation at ca. 1850 Ma led to batholith-scale granitic magmatism, in part derived from melting of the late Archean basement. Major basin development was finally terminated by the 1730 to 1690 Ma Kimban orogeny, the effects of which are widespread and include the formation of crustal-scale shear systems, granitic magmatism, and low- to high-grade metamorphism. The Kimban orogeny was followed by a renewed period of extension between ca. 1680 and 1640 Ma, leading to local magmatism and sedimentation which appears in part to have been coeval with the high-grade ca. 1650 Ma Ooldean event in the western Gawler craton. At ca. 1620 to 1615 Ma, the generation of the arc-related St. Peter Suite in the southern Gawler craton implies that craton components were located at an active plate margin. The arc environment was superseded by a transition to a continental interior regime, which coincided with the voluminous Gawler Range Volcanics (1595-1590 Ma) and Hiltaba Suite granitoids (1595-1575 Ma). The Hiltaba Suite is dominated by high T fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement. Emplacement of the Hiltaba Suite coincided with the major mineralizing interval in the Gawler craton. Regionally, two distinct mineral systems have been recognized: the Olympic IOCG province in the eastern part of the craton, and the gold-dominated systems within the central Gawler gold province. The spatial distribution of IOCG versus Au-dominated mineral systems appears to reflect regional variations in crustal composition and Hiltaba Suite petrogeneses. Hiltaba-aged granites in the Olympic IOCG province are isotopically more evolved, richer in U and Th, and oxidized compared to similar aged granites in the Au-dominated province. Modern-day heat flow in the IOCG province is significantly higher (90 ± 10 mWm-2) compared to the Au-dominated province (54 ± 5 mWm-2), suggesting there are important lithospheric compositional differences between the two metallogenic provinces, which may reflect an older phase of craton assembly. Widespread northwest-southeast contractional deformation coeval with the emplacement of the Hiltaba Suite is expressed by the formation and/or reactivation of shear zones ranging up to crustal scale. Within this regime, northwest-trending structures such as those in the vicinity of the Olympic Dam deposit are likely to have accommodated dilation associated with strike-slip movements. Intersection between these structures and northeast-trending contractional faults may have formed suitable structural traps for 1590 to 1580 Ma mineralization. The timing of syn-Hiltaba deformation overlaps with transpression and medium to high-grade metamorphism associated with the ca. 1570 to 1540 Ma Kararan orogeny which dominates the geophysical architecture of the central northern part of the craton. The youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca. 1470 and 1450 Ma and regional cooling. © 2007 Society of Economic Geologists, Inc.