THE MOUNT BRUCE MEGASEQUENCE SET AND EASTERN YILGARN CRATON - EXAMPLES OF LATE ARCHEAN TO EARLY PROTEROZOIC DIVERGENT AND CONVERGENT CRATON MARGINS AND CONTROLS ON MINERALIZATION

The Late Archaean to Early Proterozoic Mount Bruce Mepsequence Set (formerly the Mount Bruce Supergroup) overlies and is restricted to the Pilbara Craton, and comprises two megasequences which comprise a total of six supersequences or supersequence packages that formed in different, but related, tectonic settings. The Chichester Range Megasequence, which is the older of the two megasequences, comprises two supersequences overlain by a supersequence package. The lower two supersequences are dominated by tholeiitic flood basalt and terrigenous clastic sedimentary rock, with only minor placer gold and uranium mineralization. These formed during the breakup of a Late Archaean craton between about 2.77 and 2.70 Ga. The overlying supersequence package is dominated by mudrock, carbonate sedimentary rock, iron-formation and was deposited in a divergent marginal setting bordering an open ocean between about 2.69 and 2.60 Ga. The Hamersley Range Megasequence comprises clastic sedimentary rock, iron-formation, carbonate sedimentary rock and felsic volcanic rock, was deposited between approximately 2.47 Ga and < 2.44 Ga in a convergent marginal setting. Between 2.69 Ga and < 2.44 Ga, iron-formation was deposited during periods of limited clastic sedimentation as the terrane evolved from a divergent marginal setting (shallow shelf or platform) to foredeep sedimentation. The Archaean-Proterozoic boundary is probably represented by a approximately 130 Ma lacuna that marks the boundary between the two megasequences. In the eastern Yilgarn Craton, the 2.7 to 2.6 Ga Norseman-Wiluna Belt is intensely mineralized with mesothermal Au and komatiite-associated Ni sulphide deposits. This belt, which is divided into tectonostratigraphic domains or terranes by a system of craton-scale faults and shear zones, is interpreted as a Late Archaean orogen (similar to those in the modem Pacific Rim), formed by the accretion of volcanic arcs and related basins to the proto-Yilgarn Craton. In contrast, greenstone belts in the western Yilgarn appear to be dominated by older > 2.9 Ga volcano-sedimentary assemblages. Gold mineralization occurred during the late stages of this orogeny which involved transfer of strain from the eastern Yilgarn, limited crustal thickening, metamorphism, and granitoid emplacement. Accretion into orogenic belts provides the best chance of preserving mineralization formed in deep-marine environments such as komatiite-associated Ni and volcanogenic massive sulphide mineralization, and explains the common close spatial association of these deposits with gold mineralization in greenstone belts.