U th dating
In his conclusion in a recent paper exposing shortcomings and criticizing the validity of the popular rubidium-strontium (Rb-Sr) isochron method, Zheng wrote: . Zheng documented the copious reporting of this problem in the literature where various names had been given to these anomalous isochrons, such as apparent isochron, mantle isochron and pseudoisochron, secondary isochron, source isochron, erupted isochron, mixing line, and mixing isochron.Similar anomalous or false isochrons are commonly obtained from U-Th-Pb data, which is hardly surprising given the common open system behavior of the U-Th-Pb system.Owing to the isoclinal recumbent folding of metasedimentary units of the Cahill Formation, the typical rock sequence encountered at Koongarra is probably a tectono-stratigraphy (from youngest to oldest.) foliation of the schist sequence, which at Koongarra dips at 55° to the south-east.The dominant structural feature, however, is the reverse fault system that dips at about 60° to the south-east, sub-parallel to the dominant S foliation and lithological boundaries, just below the mineralized zone.
Superimposed on the primary prograde metamorphic mineral assemblages of the host schist units is a distinct and extensive primary alteration halo associated, and cogenetic, with the uranium mineralization.
Yet in the literature these problems are commonly glossed over or pushed aside, but their increasing occurrence from a variety of geological settings does seriously raise the question as to whether U-Th-Pb data ever yields any valid “age” information.
One such geological setting that yields these false U-Th-Pb isochrons is the Koongarra uranium deposit and the surrounding area (Northern Territory, Australia).
This secondary mineralization has been derived from decomposition and leaching of the primary mineralized zone, and forms a tongue-like fan of ore-grade material dispersed down-slope for about 80 m to the south-east.
The primary uranium mineralized zone in cross-section is a series of partially coalescing lenses, which together form an elongated wedge dipping at 55° to the south-east within the host quartz-chlorite schist unit, subparallel to the reverse fault.
Silicification has also occurred in fault planes and within the Kombolgie Formation sandstone beneath the mineralization, particularly adjacent to the reverse fault.