Geoscience Reference
In-Depth Information
0,
2 μm
15
μm
c
d
py
u
py
20 μm
2
0 μ
m
e
f
Figure 7.1. sem images of detrital pyrites (
c
and
d
), uranitite (
f
), and chromite (
e
) from the
witwatersrand placer deposits of south Africa. Chromite (FeCr
2
O
4
) is also oxygen sensi-
tive, although chromite oxidation seems to require the influence of microbes and intermedi-
ate oxidants, such as manganese oxides. One finds detrital chromites in rivers today. Image
from Utter (1980); reproduced with permission.
was occasionally trapped among the cobbles and sands forming the
river bed.
3
This is very much like the river deposits mined 150 years ago
during the great California gold rush. Our tour guide has supplied us
with a Geiger counter, and tells us there is more to look for than gold.
We scan among the cobbles and occasionally hear the telltale chirp. We
look for the culprit with our hand lens and spot a spherical grain, now
cemented by surrounding quartz; but in character and shape, it looks
much like the other grains surrounding it (
ig. 7.1)
. We are told that this
is uraninite, a uranium oxide mineral (UO
2
). The fact that this piece of
uraninite has been worn into a spherical shape tells us that it was trans-
ported down river—hopping, bumping, and abrading away in the flow-
ing water, much like the other sands and cobbles we find in this ancient
river bed.
Uraninite, however, isn't quite like the other pieces of river sediment.
We don't find it in rivers today, and that's because it reacts easily with
oxygen, forming the water-soluble uranyl ion (UO
2
2+
). Holland recog-
nized this in his 1962 paper and used the evidence of the Witwatersrand
uraninites to argue that there was at most only “trace” amounts of oxy-
gen in the atmosphere when these ancient river sediments were depos-
ited. Subsequently, a raging debate has developed challenging the idea
