Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa |
| |
| Authors: | Natalie Kositcin Neal J McNaughton Ian R Fletcher Birger Rasmussen |
| |
| Institution: | 1 Centre for Global Metallogeny, School of Earth and Geographic Sciences, University of Western Australia, Crawley WA 6009, Australia
2 Centre for Microscopy and Microanalysis, University of Western Australia, Crawley WA 6009, Australia |
| |
| Abstract: | Xenotime (YPO4) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
