Isotopic and geochemical features of newly formed zircon rims as a criterion for identification of feeding sources of Ti-Zr placers

Abstract

Zircons were studied from the Ti-Zr placers of the Murray Basin (Mindarie and WIM-150 deposits) and metamorphic rocks of the adjacent Kanmantoo Belt and the Ballarat Trough in Southeast Australia, and from Russian basins with Ti-Zr placers: the Cenomanian-Turonian and Poltavian basins in the East European Platform and the Sarmatian basin in the northern Caucasus, Stavropolye, and Kalmykia. The study of the primary source → weathering mantle → intermediate reservoir rock → economic placer system includes (1) morphostructural and paleofacies reconstructions of the studied territories; (2) quantitative analysis of ore mineral distribution in each element of this system; and (3) study of zircon typomorphism from the feeding source to the basin, where Ti-Zr placers have been deposited. In all elements of the system studied, zircons were examined using optical and cathodoluminescence microscopy, an electron microprobe, mass spectrometry (laser ablation and SHRIMP II), including U-Pb dating, Lu and Hf isotopes, distribution of trace elements (REE, Y, P), and comparative analysis of indicative ratios: Th/U, HfO2/ZrO2, Y2O3/(Y2O3 + REE2O3), (La + Sm)/(Gd + Yb), etc. Newly formed rims replacing detrital zircon grains and marking the time of late geological events were identified. The rims differ from the cores in distribution of trace elements. Genetic typification of diverse newly formed rims is based on discrimination of them by internal structure, isotopic and geochemical characteristics, which are used as a criterion of relationships between Ti-Zr placers and their inferred feeding sources. Based on these data, a prospecting model of the buried Ti-Zr placers with estimation of their resource potential has been produced.