The mount bruce megasequence set and eastern yilgarn craton: examples of late archaean to early proterozoic divergent and convergent craton margins and controls on mineralization

Abstract

Abstract The Late Archaean to Early Proterozoic Mount Bruce Megasequence 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 ∼2.47 Ga and 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.