Recent research on the iron ore deposits hosted by Archean BIF in the Yilgarn Craton of Western Australia has shown that they are the product of multiple superimposed hypogene and supergene alteration events. The absolute timing of these hypogene alteration events is poorly constrained. This research project will examine the timing of the hypogene alteration events affecting BIF at the Weld Range greenstone belt and the Koolyanobbing and Windarling Fe ore camps in the Yilgarn Craton. Representative examples of the different hypogene ore zones will be inspected using the SEM and EPMA with the intention of finding hypogene alteration minerals that are useful for absolute dating.
A key outcome of the project has been the identification of hydrothermal phosphate minerals (apatite, monazite and xenotime) associated with most hypogene alteration mineral assemblages in the studied deposits. Hydrothermal apatite grains are most abundant and coarse-grained (>100 microns), but are low in U, Pb and Th compared with monazite and xenotime. Hydrothermal monazite is the next most abundant phosphate mineral, with xenotime only observed in a few samples. Monazite and xenotime grains are up to 30 microns in diameter, mostly subhedral to anhedral, and are present as single grains or in clusters. Monazite grains are compositionally homogeneous, whereas the xenotime grains display well-defined zonation, with discrete cores and rims.
A suite of monazite grains from the different deposits and multiple hypogene alteration events has been collected and are presently being prepared for in situ dating by SHRIMP analysis.
Watch this space in the upcoming months for the results!