Geology of the Cheechoo gold property, Eeyou Istchee Baie-James, Superior Province, northern Quebec
Authors: Fontaine A , Dubé B , Malo M , Turcotte J , Doucet D 
Published: May 2018

The Cheechoo property (Sirios Resources) is located at 15 km southeast of the Éléonore world-class gold mine (reserves and resources of 7.1 Moz Au). The property is mainly underlain by paragneissic wacke and migmatites belonging to the Opinaca
Subprovince, local banded iron formations and mafic dykes of the La Grande Subprovince, and the Cheechoo intrusion. The Cheechoo intrusion is dated at 2612 Ma, which is penecontemporaneous with the 2618 Ma Vieux Comptoir suite and the 2620-2600 Ma leucogranitic dykes and veins that are injected
within the supracrustal rocks of the La Grande Subprovince during amphibolite facies metamorphism. Locally, the Cheechoo intrusion shows a progressive transition and consanguineous relationships with quartz-feldspar pegmatite dykes. The later dykes are concentrated at the margins of the Cheechoo
intrusion and spatially associated with part of the gold mineralization. These dykes have a high variation of mineral assemblages and miarolitic cavities suggesting crystallization near or at the magmatic-hydrothermal transition. The Cheechoo intrusion has variable textures from massive, porphyritic
with feldspar phenocrysts to gneissic and sacharoidal with biotite porphyroblasts, suggesting a complex magmatic and metamorphic history. Lithogeochemical data indicates that the intrusion has granodioritic, trondhjemitic, metaluminous to peraluminous compositions, reduced nature (Fe2O3/FeO
&amp;lt; 0.2) and REE patterns, similar to some leucogranites of the Opinaca Subprovince. The high Na2O content (&amp;gt; 6 wt %) of the Cheechoo intrusion can be attributed to an early-Au sodic metasomatic event. Detailed mapping of stripped outcrops indicate that gold-bearing zones are
mainly hosted by the Cheechoo intrusion and to a lesser extent within surrounding metasedimentary rocks, pegmatite dykes and mafic schists. Gold mineralization is characterized by a network of sheeted quartz±(feldspar) veins with variable amounts of phlogopite, actinolite, diopside and
scheelite that are related to calc-silicate and potassic alteration halos, mainly developed in vein selvages. High-grade gold veins and their selvages generally contain less than 5 % of disseminated sulphides (arsenopyrite, pyrrhotite ± pyrite) and have a coarse pegmatitic texture with
local visible gold (similar to the Moni prospect). Crosscutting relationships between gold-bearing sets of veins indicates an early V1 veins network composed of quartz ± (feldspar, actinolite, scheelite, pyrite, pyrrhotite, arsenopyrite), followed by sheeted, extensional to en-echelon V2 veins
network (60-70% of the veins) characterized by quartz ± (feldspar, phlogopite, pyrrhotite and arsenopyrite). The V2 veins are cut by V3 extensional veins, composed of quartz ± actinolite. Gold mineralization has a Bi-W-As ± (Te, Se, Pb) metallic signature in association with Mg-K
enrichments associated with narrow alteration zones in vein selvages. In-situ delta-34S analyses of gold-associated sulphides are similar to those of typical Archean orogenic gold deposits and to values of sulphides from the Éléonore gold mine, thus suggesting fluid(s) of similar
compositions. The age of the host Cheechoo intrusion, 2612 Ma, represents the maximum age of gold mineralization. The structural characteristics of the auriferous system are best explained by a model of syn-tectonic (late-D2) emplacement of the Cheechoo intrusion and related leucogranitic dykes and
gold-bearing veins networks. The mineralogy of the auriferous veins, the hydrothermal alteration assemblages and the spatial association with the reduced intrusion, pegmatite dykes and mafic schists bare similarities with those of reduced intrusion-related gold systems. The Cheechoo property
represents a new style of gold mineralization in the Eeyou Istchee Baie-James municipality.