Interpretations and implications of LA ICP-MS analysis of chert for the origin of geochemical signatures in banded iron formations (BIFs) from the Meadowbank gold deposit, Western Churchill Province, Nunavut
Résumé
Among the many types of mineral deposits within Archean cratons, gold mineralization is an important economic commodity with over 20,000 metric tons of gold produced from greenstone belts in 2001. Of the Archean-early Paleoproterozoic gold deposits, several different types of mineralization are known, which includes the important Algoma-type banded iron formation (BIF) where gold is locally associated with sulfide-facies zones within regionally extensive oxide-facies. It is commonly accepted that the shale-normalized chemical signature of REE + Y of chert bands in Algoma-type BIFs may reflect one of the three processes, each of which may be relevant to the nature and origin of the gold mineralization: (1) direct seawater precipitation; (2) involvement of and contribution from hydrothermal fluids; and (3) replacement of precursor volcanic units due to silicification. An essential question in regard to the mineralization is, therefore, whether the gold mineralizing fluids have a preference for one geochemical type of iron formation versus another. In order to assess the relevance of these competing models, we report herein the results of a LA ICP-MS study of chert samples within different Algoma-type BIFs from the Meadowbank deposit (24.5 Mt proven/probable ore reserves grading 2.8 g/t (2011)) hosted in the Neoarchean Woodburn Lake Group of the Rae Domain of the western Churchill Province, Canada. This study used 39 carefully selected and characterized (i.e., petrography and SEM-EDS imaging) chert samples from the main deposit, the Central BIF, and four additional BIFs, the Far West, West, East and Grizzly zones, with data collected using line traverses along the chert bands. The geochemical data indicate that an ambient seawater signature (i.e., enrichment in HREE relative to LREE associated with positive La and Y anomalies) dominates the samples with a lesser hydrothermal component (characterized by a positive Eu anomaly) and the influence of detrital contamination can also be detected. These initial results indicate that the methodology and protocol employed provides a reliable means to assess and interpret the chemical signature of BIFs hosting gold mineralization. In the present case, the results for the Meadowbank deposit suggest that chert from mineralized BIF units does not record a typical chemical signature that may be used as a vector for potential gold mineralization.