Volcanology, chemo-stratigraphy, geochronology, hydrothermal alteration and VMS potential of the Lemoine Member of the Waconichi Formation, Chibougamau district, Abitibi greenstone belt, Québec

Abstract

The Archean Lemoine volcanogenic massive sulfide (VMS) deposit in the Chibougamau mining district (Abitibi greenstone belt) is one of the richest to have been mined in the world (758,070\xa0t of ore at 4.17% Cu, 9.51% Zn, 4.56\xa0g/t Au, and 83.73\xa0g/t Ag). It is hosted by the ~\u20092728\xa0Ma Lemoine Member of the Waconichi Formation. Field mapping, core logging, petrography, lithogeochemistry, and U-Pb zircon geochronology helped define the stratigraphy, volcanic architecture, and hydrothermal history of the Lemoine Member east of the former mine, and document the complex relationships between effusive, intrusive, and hydrothermal activity. There, the Lemoine Member is informally divided into a 700 to 900-m-thick lower part (~\u200945% extrusive and ~\u200955% intrusive) and a 600 to 700-m-thick upper part (~\u200985% extrusive and ~\u200915% intrusive). Extrusive units in the lower part have a tholeiitic to transitional magmatic affinity and are dominantly felsic, whereas those of the upper part have a transitional to calc-alkaline affinity and are dominantly mafic. The effusive felsic units are mostly interpreted as one or more lobe-hyaloclastite flow(s) per unit. Possible volcanic vent sites for all effusive units were identified. Three types of hydrothermal alteration are present in the lower part of the Lemoine Member in the study area, from stratigraphic base to top: chlorite-sericite, sericite-chlorite, and sericite-chlorite-carbonate±epidote. Within this broad pattern, three paleo-hydrothermal upflow zones are proposed, and some correspond with known base and precious metal anomalies and/or proposed volcanic vent locations.