U-Pb dating of zircon and rutile from the gold-bearing White Hills Gravel, St Arnaud district, Victoria

Abstract

The White Hills Gravel is a widespread gold-bearing palaeontology in western and north-central Victoria. Its age is unconsidered by palaeontology but has been considered as Late Cretaceous to early Tertiary. Using coarse (> 1mm) concentrates of heavy minerals from an occurrence of the WHG near St Arnaud, we have obtained evidence for the age and provenance of the formation. U?Pb SHRIMP dating of two populations of zircons gave ages of 74.5 and 68.2 Ma, indicating a maximum age of latest Cretaceous for the WHG. The zircons are believed to have been sourced from basaltic volcaniclastic deposits since removed by erosion. These also provided coloured sapphires, spinel and possibly the sole diamond recovered from the concentrates. Such an assemblage is characteristic of eastern Australian gemfields associated with Cainozoic basaltic lavas. Other minerals in the concentrates include schorl, rutile, andalusite, anatase and metamorphic corundum. U?Pb LA-ICPMS dating of rutile gave an age of 393 Ma, within the age ange of Early Devonian granites in the region, which have intruded Cambro-Ordovician metasediments (St Arnaud Group). Sources within contact metamorphosed pelitic metasediments and associated pegmatites are indicated for the rutile, and probably for schorl, andalusite and natase. Corundum-bearing rocks are unknown in the region, so these may have a more distal and older source. Investigation of heavy mineral concentrates from other occurrences of the White Hills Gravel may enable regional correlation of sources and provide more evidence for particular depositional models.