Depositional setting of Algoma-type banded iron formation

摘要

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 degrees 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 approximate to 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 delta O-18 of primary amorphous silica (+27 parts per thousand) was modified to values of around +8 parts per thousand to +20 parts per thousand during diagenesis at temperatures >100 degrees C with a fluid having delta O-18(H2O) = 0-5 parts per thousand. 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. (C) 2016 Elsevier B.V. All rights reserved.