Setting, age, and hydrothermal footprint of the emerging Meliadine gold district, Nunavut

Abstract

The Meliadine gold district comprises a combination of orogenic greenstone- and BIF-hosted gold mineralization. The largest gold deposits (Tiriganiaq, Wesmeg, Normeg, F Zone, Pump, Discovery, and Wolf) are cospatial with the northwest-trending Pyke Fault and its inferred east-trending splays, which cut amphibolite- to greenschist-facies rocks within the Rankin Inlet greenstone belt. New U-Pb detrital zircon ages suggest that turbiditic and mafic volcanic host rocks at Tiriganiaq were deposited ?2.66 Ga. In contrast, a polymictic conglomerate south of the Pyke Fault, which is not known to host gold, yielded Paleoproterozoic U-Pb detrital zircons and was deposited at ?2.50 Ga. These new ages confirm that the Rankin Inlet greenstone belt comprises intercalated Archean to Paleoproterozoic supracrustal successions. Deformed Paleoproterozoic conglomerates suggest that the Archean to Proterozoic Rankin Inlet stratigraphy was reworked during the Trans-Hudson Orogeny (1.9-1.8 Ga). Gold in the district is associated with hydrothermally altered and sulphidized BIF and fault-fill quartz (} ankerite) veins. Hydrothermal alteration and pathfinder element enrichment (As-Te-Bi-Sb) can be mapped following a multivariate and probabilistic approach for 10s to 100s of meters beyond BIF-hosted replacement- style mineralization. Coarse-grained and idioblastic arsenopyrite crystals occur within and at the margins of folded quartz (} ankerite) veins and are also a good visual gold indicator. Gold is paragenetically late and occurs at arsenopyrite grain boundaries and/or as fracture fills. Clusters of gold inclusions coincide with recrystallized and sieve-textured arsenopyrite domains. These microtextures suggest that sulphide recrystallization liberated gold that was redistributed, at least locally, into low-strain microtextural sites along with other precious- and base-metals during a late fluid-assisted and deformation/metamorphic-driven remobilization. We propose that remobilization was concurrent with the growth of xenotime at ca. 1.86 Ga (new U-Pb ages), which postdates arsenopyrite and occurs together with gold in low-strain micro textural sites. New Re-Os arsenopyrite model ages range from 2.3 to 1.8 Ga and document a hitherto unrecognized pre-1.86 Ga hydrothermal and sulphide history. The range of Re-Os model ages tends to support partial open-system behaviour and/or mixing of disparate arsenopyrite generations that are evident from microtextures and in situ element mapping. Replicate analyses of the two most Re-rich and homogeneous arsenopyrite samples yield Re-Os model ages at ca. 2.27 and 1.90 Ga. We speculate that gold at the Meliadine gold district was initially introduced at 2.27 Ga and/or 1.90 Ga along with arsenopyrite and was subsequently remobilized, coupled with arsenopyrite recrystallization, during the Trans-Hudson Orogeny at 1.86-1.85 Ga.