Neoarchean orogenic, magmatic and hydrothermal events in the Kalgoorlie-Kambalda area, Western Australia: constraints on gold mineralization in the Boulder Lefroy-Golden Mile fault system
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The Boulder Lefroy-Golden Mile (BLF-GMF) fault system is the most intensely mineralized structure (>2150 t Au to 2015) in the Archean Yilgarn Craton, Western Australia. The fault system links the Kalgoorlie and Kambalda mining districts in the Eastern Goldfields Province, a continental-margin orogen subdivided into the western Kalgoorlie ensialic rift and the eastern Kurnalpi volcanic arc. After rifting, the 2.73–2.66 Ga greenstone-greywacke succession in the Kalgoorlie-Kambalda area underwent five phases of orogenic deformation, predominantly during ENE-WSW shortening: D1 upright folding at ca. 2680 Ma, D2 sinistral strike-slip faulting at 2678–2663 Ma, D3 folding of late conglomerate-turbidite successions at 2665–2655 Ma, D4 dextral strike-slip faulting at 2655–2640 Ma and D5 east-northeast-striking normal faulting. Regional prehnite-pumpellyite to greenschist facies burial metamorphism took place during D1 and D3 crustal thickening, and amphibolite facies aureoles formed around granite batholiths during and after D3 at 400 ± 100 MPa pressure. The D2 BLF offsets D1 folds by 12 km SW-side south and contains a porphyry dyke (2676 ± 7 Ma) boudinaged by transtensional oblique-slip along a line pitching 21° southeast. The BLF is linked by transverse D2 thrusts to other sinistral faults recording strike-slip until 2663 ± 7 Ma. Late D2 strike-slip movement alternated with early D3 shortening. D3 thrusts accommodated strain in fault blocks of rigid mafic-ultramafic volcanic rocks consolidated during D1, while the sedimentary rocks in D3 synclines were foliated at high strain.Biotite-sericite alteration and gold-pyrite mineralization in the BLF-GMF system took place at 11 ± 4 km burial depth in faults active during D2 and D3. The Golden Mile (1708 t Au) and other deposits are associated with stocks and dykes of high-Mg monzodiorite-tonalite porphyry, part of a late-orogenic (2665–2645 Ma) mantle-derived suite of adakitic affinity. Hornblende and apatite compositions indicate that these intrusions are characterized by high water contents (5–6 wt% H2O in melt), by high oxidation states (dNNO +1.0 to +2.4 log units) and by igneous anhydrite. Some stocks contain pervasive anhydrite-pyrite mineralization of low gold grade (0.4 g/t). Biotite-sericite-pyrite ore bodies such as those at Kanowna Belle (140 t Au) also replace faulted metamorphic rocks above batholith domes cored by plutons of the monzodiorite suite. The D4 strike-slip faults are barren at Kambalda but control gold quartz-vein ore at Kalgoorlie (2651 ± 9 Ma), and Au-Ag breccia ore at Black Flag (<2648 ± 6 Ma).
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