The genesis and evolution of lode gold mineralization and mafic host lithologies in the late-Archean Norseman Terrane, Yilgarn Block, Western Australia
McCuaig, Thompson Campbell
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Six distinct types of mafic igneous rocks are present within the ca. 2714-2690 Ma Woolyeenyer Formation of the Norseman Terrane, East Central Yilgarn Superterrane (ECYS), Yilgarn Block, Western Australia. (1) Tholeiitic basalts and gabbros comprise the bulk of the formation, and are characterized by flat to slightly depleted mantle-normalized incompatible element patterns, and åNd values of +2 to +3. (2) High-MgO tholeiite dykes (HMT) with normalized REE and multielement patterns like those of tholeiites, albeit at slightly lower abundances. (3) Enriched high-MgO tholeiite dikes (EHMT) with MgO, Cr similar to HMT but with extreme Al-depletion, enrichment in incompatible elements and HREE depletion, and åNd values of -2.0 to -1.2. (4) High-MgO tholeiites with characteristics transitional to HMT and EHMT (THMT). Incompatible element enriched tholeiites transitional between HMT, HMT and tholeiite, subdivided into (5) ET1, characterized by enrichment in incompatibleelements and LREE, with relatively flat HREE patterns and ENd values ranging from -0.1 to 1.0, and (6) ET2, characterized by Al-depletion and pronounced HREE depletion. The HMT and tholeiites are comagmatic, and positive ENd values and comparable normalized trace element patterns suggest derivation of these rocks from a similar depleted source. Conversely, detailed modelling demonstrates that EHMT compositions cannot be generated by alteration or crustal contamination of HMT, Al-undepleted or Al-depleted komatiitic magmas, and require a source (1) that was enriched in incompatible elements by small volume partial melts prior to melting, and (2) where garnet was retained during melting. Generation of the above mafic lithologies require that incompatible element enriched and depleted sources existed at 2.7 Ga in the Norseman Terrane. The EHMT are the first report of such severely Al-depleted 2.7 Ga rocks from ECYS. The Archean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (northern, central, southern) are delineated on the basis of their spatial distribution, veining style, alteration mineralogy and metamorphic grade of host rocks. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, implying a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures, and there are progressively greater degrees of dynamically recovered textures in quartz. Initial Pb isotope compositions of galena and altaite from lode gold deposits of the Norseman Terrane yield the most variable initial Pb-isotopic compositions for these minerals of any Archean lode gold deposit. Interpreted initial Pb isotope compositions of galena and altaite are systematically more radiogenic with increasing proximity to felsic intrusive rocks and older crust.
DegreeDoctor of Philosophy (Ph.D.)
Copyright DateApril 1997