A POST-DEFORMATIONAL, POSTPEAK METAMORPHIC TIMING FOR MINERALIZATION AT THE ARCHEAN BIG BELL GOLD DEPOSIT, WESTERN AUSTRALIA

Previous workers were unable to find any suitable criteria to time gold mineralization at Big Bell mine - an enigmatic. low-grade, large-tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the Murchison Province of the Archaean Yilgarn Craton, Western Australia. Earlier studies suggested that the gold mineralization occurred by hydrothermal alteration, either synchronous with volcanism or prior to peak metamorphism *. The following conclusions have been drawn from a detailed meso- and microstructural analysis of the mine sequence and greenstone belt hosting the orebody: (1) Prior to mineralizaton, the Big Bell area underwent five phases of deformation and M1 regional metamorphism to the staurolite-andalusite zone of the amphibolite facies for metasediments. (2) Granite emplacement post-dated D5 folding causing local, M2, thermal metamorphism. This peaked at the Ms + Qtz breakdown reaction for pelites, on or close to the And-Sil transition, but below the onset of partial melting, characterized by the narrow non-degenerate divariant field involving seven KFMASH phases: Qtz + Ms + Bt + Crd + Kfs + Als + H2O This hornfelsing consumed, cross-cut and post-dated S5 cleavage. Prograde Qtz + And + Sil veins replaced this hornfels assemblage (and S5 cleavage) and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post-deformational hornfelsing. (3 ) Retrograde M3 assemblages and two stages of gold-related mineralization, cross-cut and replaced all previous structures. The retrograde gold-bearing assemblages are both undeformed. Stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations: Py-Po-Ccp-Ilm-Mag-Au-Tur-Sch-Rt-Spn-Zrn-Ms (Bt) -Ep-Czo-Qtz The second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre-existing mineralogies, including quartz, are totally destroyed within sericitic domains. The stage 2 mineral associations are: Apy-Sbt-Sb-Mo-Sp-Au-Py-Po-Ilm-Mag-Ccp-sericite-Chl-carbonate (Cal)-Ep-Czo-Qtz (4) Gold mineralization at Big Bell was post-deformational and post-peak metamorphic. Unlike many Archaean gold deposits there are no obvious shear zones controlling the formation of the Big Bell gold deposit at the structural level exposed either on the surface or in drill core. Big Bell Mine was in a low-strain domain during the formation of late-stage sinistral sub-vertical D6 shear zones, that are typically concentrated at the granitoid-greenstone contacts. The retrograde character of the mineralization has been deduced from detailed meso- and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the Murchison Province.