Nd isotopic, petrologic and geochemical investigation of the Tulawaka East gold deposit, Tanzanian Craton
Cloutier, J and Stevenson, RK and Bardoux, M, Nd isotopic, petrologic and geochemical investigation of the Tulawaka East gold deposit, Tanzanian Craton, Precambrian Research, 139, (3-4) pp. 147-163. ISSN 0301-9268 (2005) [Refereed Article]
New isotopic (Sm–Nd) and geochemical data are presented for volcano-sedimentary and granitic lithotypes of the Tulawaka East gold deposit in the Late Archean Sukumaland Greenstone Belt of the Tanzanian Craton. The data help to characterize the geological environment of the greenstone belt/gold deposit and the crustal evolution of the Tanzanian Shield. The volcano-sedimentary units were intruded by leucogranites and aplitic dykes that were deformed and metamorphosed to amphibolite facies grades. The Au mineralization occurs in thrust faults at the contact between the aplite dykes and the volcano-sedimentary units. The metavolcanic rocks are largely basaltic to intermediate in composition with negative Nb anomalies and flat rare earth element (REE) profiles varying from 10 to 100× chondritic values. Initial ɛNd 2.8 Ga values vary from +0.9 to +5.3 and Nd model ages (Tdm) range from 2.8 to 3.0 Ga. The metasedimentary rocks are generally enriched in light REE (100× chondrite) with weak negative Eu anomalies and ɛNd 2.8 Ga values vary from 0.0 to +2.6. The leucogranites are garnet-rich and peraluminous with strong negative anomalies in Eu and Ti and with flat REE profiles (10–100× chondrite). The aplite dyke is also peraluminous with similar negative Eu and Ti anomalies and a flat light REE, but is depleted in heavy REE. The similarity in composition with the leucogranite suggests that the two are cogenetic and formed by fractionation of a more mafic parental magma. The combination of the flat REE element patterns and negative Nb anomalies among the mafic volcanic rocks suggests formation as an immature (island?) arc at ca. 2.8 Ga. However, Nd isotope signatures in the metasedimentary rocks provide evidence of an older crustal sedimentary provenance (ca. 3.0–3.1 Ga) and proximity of a continental influence. The age of the leucogranite and aplite dyke are constrained by a Sm–Nd garnet-whole-rock age of ca. 2.5 Ga, indicating that the shear zone-hosted gold mineralization is younger than 2.5 Ga. The range of isotopic compositions found in the Tanzanian Shield overlap with those of the Zimbabwe Craton to the south.