Petrographic and Geochemical Characterization of the Moncou Zinc Mineralization: An Example of a Polyphase Zn Remobilization in the Central Grenville Province (Lac Saint-Jean, Quebec)

Abstract The carbonate-dominated supracrustal sequences of the western Grenville Province formed between ∼1200 and ∼1300 Ma and are well-known for high-grade metamorphosed SEDEX and VMS deposits. However, there is limited information on base metal mineralization hosted in the clastic-dominated supracrustal sequences in the Allochthonous Medium to Low Pressure Belt of the central Grenville Province. The Moucou zinc occurrence (>1.3 wt.% Zn), located 50 km NW of Lac Saint-Jean, Quebec, provides an example of such mineralization. This zinc occurrence is hosted in metavolcanic rocks interbedded with quartzite layers of the Barrois Complex, a clastic-dominated supracrustal sequence, and metamorphosed under granulite facies. We present geochemical data, titanite geochronology, and petrographic observations for the Moncou zinc occurrence. The metavolcanic rocks have a transitional signature, Nb and Ta negative anomalies, and Ba and Th enrichments suggesting a back-arc setting comparable to the well-mineralized carbonate-dominated supracrustal sequences of the western Grenville Province. Zinc mineralization consists of (i) a first sphalerite generation (Sp1) systematically surrounded by a thin layer of plagioclase+quartz (Pl+Qz); (ii) deformed Zn-rich biotite crosscut by the titanite grains dated at ∼1000 Ma, indicating the minimum age for the first zinc remobilization; and (iii) Zn-rich chlorite spatially associated with a second sphalerite generation (Sp2). These observations suggest that Zn mineralization occurred following this sequence: a pre-metamorphic event crystallizing Sp1 as it is surrounded by anatectic melt (Pl+Qz) and shows typical high-temperature crystallization textures, then a syn- to post-metamorphic event characterized by the Zn-rich biotite, and finally a low-temperature hydrothermal event crystallizing the Zn-rich chlorite and the second sphalerite generation. This study reveals the potential for Zn mineralization of the clastic-dominated supracrustal sequences of the Medium to Low Pressure Belt such as the Barrois Complex.