Abstract:

The Côté Gold intrusion-related Au (-Cu) deposit, in the Archean Swayze greenstone belt of the Abitibi Subprovince, is a large-tonnage, low-grade deposit hosted by ca. 2741 Ma tonalite and diorite of the Chester intrusive complex which formed in a high-level sub-volcanic setting. The deposit contains a resource (indicated + inferred) of 8.3 M oz Au that is centered on a magmatic-hydrothermal breccia. Mineralisation occurs as disseminations or is associated with quartz ± carbonate vein arrays, and is broadly coincident with biotite, silica-sodic and sericite alteration zones. Lithogeochemical and petrographic/SEM-EDS studies (>600 samples) across the deposit are used to assess the petrogenesis of the magmatic host rocks, distinguish between magmatic versus hydrothermal breccia, differentiate alteration types, determine the precursor rock where alteration is most intense, and chemically fingerprint the deposit. Preliminary observations indicate the following: (1) the tonalite suite is characterised by unfractionated CN REE patterns and reflects melt generation in a low-P regime; (2) protolith rocks are best discriminated using CaO, TiO2 and Zr/TiO2; (3) the magmatic-hydrothermal breccia is characterised by an Au-Cu-F-Te-Zn association; (4) biotite alteration, often with intergrowth of chalcopyrite, is characterised by Ba and LREE enrichment often accompanied by the presence of allanite-euxenite-bastnaesite; (5) zones of intense sodic alteration, up to 40 m thick and approaching 10 wt% Na2O are characterised by a red-pink colour and a vuggy texture due to quartz dissolution. In thin section albite is pitted, contains abundant fluid inclusions and is mantled by clear albite. These rocks are considered to be episyenites; (6) sericite alteration zones, up to 150 m and cutting silica-sodic alteration, are characterised by an F ± Au ± Cu association; and (7) the chemistry of biotite in the breccia unit shows a reversal in Mg# after fractionation from diorite to tonalite (i.e., decreasing Mg#) which may indicate either injection of a new primitive melt or an oxidation-sensitive event coincident with formation of the breccia. Many of the features of the Côté Gold deposit suggest derivation of mineralisation and alteration by the flux of high-temperature fluids enriched in K, Fe, F, LREE, CO2, and Ba, in addition to Au, Te and Cu. The association of Au-Cu mineralisation with a magmatic-hydrothermal breccia and a chemical signature that is atypical for Archean orogenic-type gold deposits indicates a possible affinity with a spatially and temporally related intrusive centre.