(magnification 210X; scale bar 50 microns) Plate 8.Globular pitchblende mass (black shape just to the left of centre) being altered marginally to sklodowskite (grey concentric sheath). Kasolite (light grey) and sklodowskite (dark grey) replacing a former pitchblende vein. Supergene colloform banded pitchblende (grey) deposited in what was originally a void.
Located 225 km (140 miles) east of Darwin in the Northern Territory (Fig.
1), the Koongarra uranium deposit is one of four large high grade deposits now being developed in the area, known as the Alligator Rivers Province.
The deposit lies beneath a gently sloping erosion surface just below the sandstone cliffs of the Mount Brockman Mesa, an outlier from a regionally extensive sandstone plateau (Fig. This thick, flat-lying sandstone unconformably overlies the ore-hosting schists, and in the recent past served as a cover rock for the Koongarra orebodies.
The PB in the galena is supposed to have migrated from the pitchblende where it was supposedly produced by radioactive decay. Kasolite (white) and uranophane (grey) replacing a former pitchblende vein.
Note that the former vein shape, even the sub-grains, have essentially been preserved.
There are 4 main lines of evidence that the system at Koongarra has been open (4,5,6,7) and the implications of these are that the attempts to date Koongarra using U/Pb isotopes are invalid. 6 as several generations of pitchblende and galena.
Mineralogical and textural studies of the ore under both optical and electron microscopes indicate that there have been at least three remobilisations of the uranium during the history of the ore. Plates 1–6 illustrate examples of the ore textures as seen under the microscopes, the descriptions accompanying the plates indicate how the textures have been interpreted. Electron microprobe analyses of pitchblendes, that is, micro-analyses of volumes of pitchblende between 5 and 10 cubic microns (Table 2), reveal that pitchblende compositions, particularly U, Pb and Ca contents, change not only from grain to grain within any one sample regardless of which generation of pitchblende it is, but even at the microscopic level within pitchblende grains themselves. 8 illustrates how Pb and Ca are both substituting for U in the UO cubic lattice.Because this so-called “common” Pb correction has to be applied to the raw data before calculation of the U/Th/Pb ages.This, of course, is again another way of saying that not all the Pb is derived by radioactive decay, but that it was with the U and Th “in the beginning.” The standard used to correct the data in Table 1 was the Mt Isa Pb* standard with an isotopic composition: Pb age is taken as the standard isotopic Pb clock (3), the consensus radiometric age of the Koongarra uranium deposit is 0 yrs, since this is the only isotopic Pb date supported directly by a majority of samples.A logical extension of these data & conclusions is to suggest as others already have (9,10).that U/Pb ratios may have nothing to do with the age of a mineral.The banding is produced by a time sequence of pitchblende deposition. A sklodowskite (white) vein composed of radiating aggregates of needle-shaped crystals.