Friday, June 10, 2016

Depositional setting of Algoma-type banded iron formation

Depositional setting of Algoma-type banded iron formation


Gourcerol et al


Algoma-type banded iron formations (BIF) are chemical sedimentary rocks characterized by alternating layers of iron-rich minerals and chert that are generally interstratified with bimodal submarine volcanic rocks and/or sedimentary sequences in Archean greenstone belts. However, the geological setting for Algoma-type BIF deposition remains equivocal due to the effects of post-depositional deformation and metamorphism, and absence of modern analogues for comparative studies. It is commonly accepted that the abundance of rare earth element and yttrium (REE + Y) in chert bands may retain a primary geochemical signature and therefore constrain their geological setting. In order to explore the latter, a geochemical study using the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) methodology was done using cherts from four Canadian BIF-hosted gold deposits. These results suggest that chert bands record: (1) interaction of seawater with Fe-oxyhydroxides, as suggested by their heavy REE enrichment coupled with La and Y enrichments; (2) contributions from high-temperature (>250 °C) hydrothermal fluids, as suggested by positive Eu excursions; and (3) detrital contamination, which is suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio (i.e., Y/Ho ≈ 27). Water-column pH conditions at the time of BIF deposition are evaluated using Ce/Ce∗: a positive Ce/Ce∗ anomaly suggests relatively acidic conditions (i.e., pH ⩽ 5) for most of the chert samples, but more alkaline conditions (i.e., pH ⩾ 5) for samples showing Fe-oxyhydroxide precipitation within chert bands. Finally, in situ using secondary ion mass spectrometry (SIMS) analysis (n = 73) of chert from Meliadine show the δ18O of primary amorphous silica (+27‰) was modified to values of around +8‰ to +20‰ during diagenesis at temperatures >100 °C with a fluid having δ18OH2O=0–5‰δ18OH2O=0–5‰. Thus, whereas there has been O isotopic exchange during diagenesis, the REEs and trace elements are not modified in the chert due to the low concentrations of these elements in the reacting fluid of sea water origin.

No comments: