Age and geological setting of the Rankin Inlet greenstone belt and its relationship to the gold endowment of the Meliadine gold district, Nunavut, Canada

Abstract

Archean greenstone belts host a significant proportion of the world's gold, typically in deposits that formed late during greenstone belt formation and cratonization. However, this is not always the case and, in the multiply reworked western Churchill Province (wCP), orogenic lode gold deposition post-dates greenstone belt formation by nearly one billion years. The spatial link between Proterozoic gold and Archean greenstone belts in the wCP is thus particularly striking although its significance is still not fully understood. The Meliadine gold district (2.8Moz contained Au in reserves, plus an indicated and inferred resources of 5.8Moz Au) represents an important example of this deposit style and is hosted within the Rankin Inlet greenstone belt (RIGB), which occupies a critical, but controversial position along the largely inferred boundary between the Hearne craton and the Chesterfield block.RIGB felsic volcanic rocks (ca. 2.66Ga) are structurally intercalated, and broadly coeval, with mafic volcanic and volcaniclastic rocks (2.66–2.64Ga), turbidite (≤2.66–2.64Ga), argillite, auriferous banded iron formation successions and syn-volcanic granodioritic to tonalitic intrusions (2.67–2.64Ga). Neoarchean basaltic to andesitic volcanic rocks possess calc-alkaline to primitive arc-like tholeiitic magmatic affinities along with lesser MORB-like basaltic compositions. Geochemically evolved lavas yield depleted 144Nd/143Nd ratios (ɛNd2.66Ga=−1.1 to +1.6) that reflect variable interaction with an evolved and hitherto undocumented Meso- to Neoarchean basement underlying the RIGB, whereas transitional, arc-like primitive tholeiitic and MORB-like basaltic samples overlap with the Nd isotopic composition of depleted mantle at ca. 2.66Ga (ɛNd2.66Ga=+1.6 to +2.7). These Neoarchean volcano-sedimentary panels represent the main auriferous rock package within the Meliadine gold district and are intercalated with deformed Paleoproterozoic conglomerate (≤2.50 and ≤2.155Ga). The latter are, in turn, unconformably overlain by a geochemically distinct pillowed-basalt sequence and a unique carbonate-siliciclastic package that presumably represent the remnants of Paleoproterozoic basins and are not known to host gold. The geological setting of gold deposits thus likely reflects this favourable Neoarchean lithostratigraphy in addition to metamorphism and fluid focusing along the reactivated faults during the collision of the Hearne and combined Chesterfield block-Rae craton at 1.90–1.85Ga.