Pb pb isochron dating

The Koongarra uranium deposit occurs in a metamorphic terrain that has an Archean basement consisting of domes of granitoids and granitic gneisses (the Nanambu Complex), the nearest outcrop being 5 km to the north.

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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.

Multiple isoclinal recumbent folding accompanied metamorphism. The Lower Proterozoic Cahill Formation flanking the Nanambu Complex has been divided into two members.

The lower member is dominated by a thick basal dolomite and passes transitionally upwards into the psammitic upper member, which is largely feldspathic schist and quartzite.

Even a claimed near-concordant U-Pb ‘age’ of 862 Ma on one uraninite grain is identical to a false Pb-Pb isochron “age,” but neither can be connected to any geological event.

Open system behavior of the U-Th-Pb system is clearly the norm, as is the resultant mixing of radiogenic Pb with common or background Pb, even in soils in the surrounding region.This alteration extends for up to 1.5 km from the ore in a direction perpendicular to the host quartz-chlorite schist unit, because the mineralization is essentially stratabound.The outer zone of the alteration halo is most extensively developed in the semi-pelitic schists, and is manifested by the pseudomorphous replacement of biotite by chlorite, rutile and quartz, and feldspar by sericite.Silicification has also occurred in fault planes and within the Kombolgie Formation sandstone beneath the mineralization, particularly adjacent to the reverse fault.Association of this outer halo alteration with the mineralization is demonstrated by the apparent symmetrical distribution of this alteration about the orebody.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.

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