One size does not fit all: Refining zircon provenance interpretations via integrated grain shape, geochronology, and Hf isotope analysis

Abstract

Sediment provenance studies commonly utilize isotopic signatures to resolve detrital mineral sources and routing. However, non-unique ages and geochemical characteristics across geographically distinct crystalline source regions can lead to significant ambiguities in mineral provenance interpretations. Such ambiguity is apparent in southern Australia’s Cenozoic Eucla Basin, which hosts world-class heavy mineral sand resources. Here, new Hf isotope data are provided from four heavy mineral prospects (N = 8, n = 844 [N = samples, n = grains]). Zircon grain shape data are also presented for a suite of detrital Eucla Basin samples (N = 22, n = 35,604) and the basin’s underlying basement, the Coompana Province (N = 13, n = 824). The data are integrated with published detrital and non-detrital primary zircon data to investigate the efficacy of grain shape analysis to better resolve the basin’s mineral provenance. Zircon Hf isotope compositions indicate a primary Mesoproterozoic juvenile source for zircon melts (∼1250–1000 Ma, −2.5 < ɛ_Hf > ∼+5) with additional contributions from a range of juvenile to evolved late Archean to Phanerozoic-aged zircon bearing magmas (−28.0 < ɛ_Hf > +11). U–Pb geochronology and Hf isotopes are incapable of differentiating Mesoproterozoic-aged source rocks bounding the region for the majority of heavy mineral deposits analyzed as potential sources express overlapping crystallization ages and similarities in Hf-isotope characteristics. However, distinct zircon grain shapes (i.e., perimeter, major axis and circularity) facilitate improved differentiation across these Mesoproterozoic sources. Filtering of U–Pb age, Hf isotope and shape data implicate the underlying Madura and Coompana provinces as dominant sediment sources for Eucla Basin detritus aged ∼1400–1000 Ma. The lack of direct sediment pathways between the underlying basement provinces and placer sediments analyzed demonstrates the significance of zircon reworking from intermediate sedimentary basins in the formation of the economically significant Eucla Basin beach placers. Zircon grain shape represents a cheaply acquired and readily incorporated grain characteristic that can enhance provenance investigations.