Source of zircon in world-class heavy mineral placer deposits of the Cenozoic Eucla Basin, southern Australia from LA-ICPMS U-Pb geochronology

摘要

The Eucla Basin of southern Australia is a newly recognised zircon-rich mineral sands province with mining operations underway at the world-class Jacinth-Ambrosia deposits. Detrital zircon U-Pb dating via laser ablation inductively coupled plasma mass spectrometry from samples distributed along the length of the Eucla Basin paleoshore system reveals the majority of zircons (c. 55%) are from c. 1300 to 1000 Ma source rocks. Many of the zircons of this age range have metamorphic textures indicating the source province(s) of these zircons underwent high-grade metamorphism during the Mesoproterozoic. A secondary provenance signature occurs at c. 1800-1600 Ma, with significant amounts of c. 1680-1600 Ma zircon. These two key age intervals, c. 1300-1000 Ma and c. 1800-1600 Ma match the major rock-forming intervals of the Albany Fraser Orogen and Musgrave Province, crystalline basement regions that crop out in the western and northern hinterland of the Eucla Basin and which are transected by extensive Early Cenozoic paleodrainage systems. The subordinant volume of Archean zircons in the paleoshoreline deposits are predominantly c. 2660 Ma or older and are most likely derived from the Yilgarn Craton. The primary source regions for the zircons thus lie to the west and north of the main sites of heavy mineral sand accumulation, located in the central and eastern portions of the paleoshoreline complex of the Eucla Basin. This is indicative of the dominant process of westerly longshore drift as the primary control on sediment movement and redistribution along much of the Eucla Basin coastline. Zircons of these ages were also likely derived from recycling sediments of Neoproterozoic and Phanerozoic intracontinental basins, in particular the Officer Basin, which contains dominantly c. 1350-1000 Ma zircons, and was widely incised by the Paleogene drainage network. Monazite U-Pb data from the Jacinth Mine records an abundance of c. 1350-1160 Ma grains, supporting source areas to the north and west of the Eucla Basin. Heavy mineral input from the more proximal Gawler Craton was minor. The zircon-rich mineral sands province of the Eucla Basin is thus a result of large-scale zircon erosion, transport, recycling and eventual concentration on beach shorelines where sand movement was controlled by dominant westerly longshore drift.