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 (>250ºC) 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 δ 18 O of primary amorphous silica (+27‰) was modified to values of around +8 to +20‰ during diagenesis at temperatures >100°C with a fluid having δ 18 O H2O = 0 to 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.

(-2703 Ma), qui provoque la superposition de la schistosité régionale et de paragenèses du pic métamorphique sur les empreintes hydrothermales préexistantes ainsi que la réactivation des failles longitudinales en zones de cisaillement à mouvement inverse. En somme, cette étude démontre que la région de Chibougamau expose un énorme système minéralisé volcano-plutonique archéen atypique, avec des dépôts Cu-Au magmatiques-hydrothermaux proximaux (camp minier du lac Doré) et des dépôts épithermaux polymétalliques distaux centrés sur des systèmes plutoniques satellites s'inscrivant dans la même lignée pétrogénétique. L'étude contribue par le fait même à mettre à l'avant-plan des processus tels l'activité volcano-plutonique calco-alcaline, la formation de caldeiras et l'implication de l'eau de mer en tant qu'agent confinant (profondeur bathymétrique) et agent chimique comme responsables de minéralisations polymétalliques atypiques en contexte d'arc. Dans l'ensemble, cela supporte l'idée que le potentiel minéral des arcs volcano-plutoniques calco-alcalins anciens a pu être sous-estimé dans le passé, surtout lorsqu'on prend conscience de l'immense richesse de leurs équivalents récents.

This project is designed to establish if there is a distinctive geochemical signature for the types of banded iron-formations (BIF) that contain gold mineralization and whether a hydrothermal footprint for the mineralization can be detected. Herein are reported the preliminary geochemistry results of a LA ICP-MS study of 39 chert samples for BIFs from the Meadowbank deposit in the Rae Domain of western Churchill Province where gold mineralization is associated with several Algoma-type BIFs within the Neoarchean Woodburn Lake Group. The main deposit is located in the Central BIF, which has been in production since 2010 with 24.5 Mt proven/probable ore reserves grading 2.8 g/t (2011). Recently, mineralization has also been identified associated with BIFs in the Far West, West, East and Grizzly zones. The geochemistry of the cherts from these five BIFs, as determined from line traverses of chert using the in situ LA ICP-MS method, has identified an ambient seawater signature (characterized by enrichment in HREE relative to LREE, positive La, Gd, and Y anomalies) and a hydrothermal signature (characterized by a positive Eu anomaly), with some influence of crustal contamination.

The Meadowbank banded iron-formation-hosted world-class gold deposit is comprised in the polydeformed and metamorphosed 2711 Ma Pipedream-Third Portage sequence of the Woodburn Lake Group, which also comprises mafic and ultramafic rocks, and quartzite. At least four phases of regional Trans- Hudsonian (Proterozoic) deformation are documented in the Meadowbank deposit area: 1) isoclinal FP1 folds and DP1 fault zones, strongly overprinted by younger deformation; 2) north-trending isoclinal FP2 folds and associated DP2 fault zones cutting the stratigraphy and mineralization; and two set of younger folds, 3) open to closed southwest-plunging FP3 folds, and 4) shallowly to moderately inclined, open to tight, chevron-style mesoscopic FP4 folds. The bulk of the gold at Meadowbank is hosted in banded iron-formation and occurs at or near the contact with sheared ultramafic rocks where it is associated with pyrrhotite±pyrite and traces of chalcopyrite and arsenopyrite. Gold-rich quartz-pyrrhotite±pyrite veins are locally present in intermediate to felsic volcaniclastic rocks interlayered with banded iron-formation units. The ore-associated mineral assemblages include grunerite and/or cummingtonite and chlorite in banded iron-formation layers, whereas sericite±chlorite dominate in altered volcaniclastic rocks. Geochemical analyses of the major rock types are essential to further discriminate the host units, thus increasing the knowledge of the stratigraphic and structural setting of the deposit. Crosscutting relationships indicate that the bulk of the gold was introduced prior to DP2, preferentially near or along fault zones developed at contacts between banded iron-formation units and ultramafic rocks.

No abstract