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Proterozoic chemical sediments are depleted in 18O with respect to Phanerozoic analogs. The suggestion that this results entirely from weathering and low temperature exchange with groundwater is not tenable. Metamorphism also fails to explain observed low 18O values. Two hypotheses that fit the isotopic data are: 1) precipitation of chert and carbonate from a hot Proterozoic ocean (60° C for Precambrian X), or (2) precipitation of chert and carbonate from a Proterozoic ocean depleted in 18O. Both of these hypotheses have important implications.

Close association of diamictite with the Kuruman Iron Formation (Precambrian X) suggests that hypothesis...

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