GE/SI RATIOS AS A TRACER FOR SILICA IN THE DALES GORGE MEMBER OF THE HAMERSLEY GROUP, WESTERN AUSTRALIA

Banded ironstone formations (BIFs) are the most abundant chemical sediments in the Precambrian and are defined as thinly bedded or laminated chemical sediments containing greater than 15 wt% iron and commonly containing layers of chert (James, 1954). Accumulation of evidence suggests that the iron component has a dominantly hydrothermal source (Morris, 1993). What is not known, however is if the silica component is similarly hydrothermal or continentally derived. Ge/Si ratios offer the potential to resolve this issue because the Ge/Si ratio preserved in ironstone silica should reflect the ratio of its source.

A combined approach is adopted here for the analysis of Ge/Si ratios within early Proterozoic BIF from the Dales Gorge Member of the Hamersley Group, Western Australia. Point analyses of Ge in chert microbands were carried out using LA-ICP-MS as well as whole rock analysis of microband couplets to determine Ge/Si ratios. Preliminary results of LA-ICP-MS analysis of silica show a Ge/Si ratio of 0.7x10^-6. These results clearly fall within the continental end member range. However when similar analyses were made on both the silica rich and iron rich microbands (possibly representing a seasonal varve) the results showed that the BIF had an average Ge/Si ratio of 14.5x10^-6. Meanwhile the Ge content in the iron rich bands were much greater than the amount associated with the chert, approximately 4mM and 42mM respectively.

This implies one of two operating factors (i) that the source of silica and iron were decoupled during the Precambrian with silica having a dominantly continental source and iron a hydrothermal source or (ii), that major repartitioning of Ge takes during diagenesis.