U th dating

The uranium mineralization at Koongarra is associated with graphitic horizons within chloritized quartz-mica (±feldspar ±garnet) schists overlying the basal dolomite in the lower member.A 150 Ma period of weathering and erosion followed metamorphism.Radiometric dating has now been used for almost 50 years to establish “beyond doubt” the earth’s multibillion year geological column.

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.

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).

Of the various methods, uranium-thorium-lead (U-Th-Pb) was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated.

But the method does not always give the “expected” results, leading to fundamental questions about its validity. some of the basic assumptions of the conventional Rb-Sr isochron method have to be modified and an observed isochron does not certainly define a valid age information for a geological system, even if a goodness of fit of the experimental data points is obtained in plotting Sr ratios, even an excellent line of best fit between ratios obtained from good cogenetic samples, and yet the resultant isochron and derived “age” have no distinct geological meaning.

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.

A thick sequence of essentially flat-lying sandstones (the Middle Proterozoic Kombolgie Formation) was then deposited unconformably on the Archean- Lower Proterozoic basement and metasediments.

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