Abstract:

Algoma-type Banded Iron Formations (BIFs) are chemical sedimentary rocks comprised of alternating layers of iron-rich minerals and chert that are generally interstratified with bimodal submarine volcanic rocks in Archean greenstone belts. However, the geologic setting for Algoma-type BIF deposition remains equivocal due to the overprinting effects of post- depositional deformation and metamorphism and the absence of modern analogues for comparative studies. Recent studies suggest the abundance of REE+Y in chert bands may reflect the primary BIF geochemical signature and therefore may constrain geological settings favourable for BIF deposition. In this study, the results of LA-ICP-MS analysis of chert at three BIF-hosted gold deposits are presented to assess whether epigenetic gold mineralization is preferentially developed within a particular geochemical type of BIF. The three deposits studied are: 1) the Meadowbank deposit (Churchill Province); 2) the Meliadine gold district (Churchill Province); and 3) the Musselwhite deposit (Superior Province). The results of this study, which explore rare earth elements (REE) and yttrium as tracers of depositional processes for Algoma-type BIF, suggest that chert bands record: 1) interaction of seawater with Fe-oxyhydroxides, as suggested by heavy rare earth element enrichment coupled with La and Y enrichment; 2) high-temperature (>250 ºC) hydrothermal fluids, as suggested by positive Eu excursions; and 3) hydrogeneous contamination, which is suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio. Moreover, the pH conditions of the water column at the time of BIF deposition are evaluated using Ce/Ce* as a pH proxy, with acidic conditions associated with positive Ce/Ce* anomalies. This data set does not suggest there is a chemical preference of the studied BIF for epigenetic gold mineralization.