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 t of ore at 4.17% Cu, 9.51% Zn, 4.56 g/t Au and 83.73 g/t Ag). It is hosted by the ∼2728 Ma 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 (~45% extrusive and ~55% intrusive) and a 600 to 700 m-thick upper part (~85% extrusive and ~15% 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 lobehyaloclastite 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.Beaudoin, for constructive comments on the manuscript. ReferencesAgrawal S, Guevara M, Verma SP (2008) Tectonic discrimination of basic and ultrabasic volcanic rocks through log-transformed ratios of immobile trace elements. Internat Geol Rev 50:1057-1079 Allard GO (1976) The Doré Lake Complex and its importance to Chibougamau geology and metallogeny. Ministère des richesses naturelles, report DP 368, 446 p.