The Lemoine Auriferous Volcanogenic Massive Sulfide Deposit, Chibougamau Camp, Abitibi Greenstone Belt, Quebec, Canada: Geology and Genesis

Abstract

The Lemoine auriferous volcanogenic massive sulfide deposit (0.76 Mt at 4.6 g/t Au, 4.2 wt % Cu, and 9.5 wt % Zn) is part of the Chibougamau camp located in the northeastern part of the Abitibi greenstone belt. The deposit is hosted by a steeply dipping, S-facing homoclinal volcanic succession (~2729−2726 Ma Waconichi Formation, Lemoine Member) composed of effusive and intrusive tholeiitic rhyolites and andesites cut by comagmatic diorite and gabbro dikes and overlain by transitional to mildly calc-alkaline basalts, andesites, and rhyolites.Seven predominant synvolcanic alteration assemblages, now intensely deformed and metamorphosed to upper greenschist facies, were defined based on their mineralogy and position relative to ore. Albite-quartz, sericite-carbonate, sericite-chlorite, sericite-chlorite-(Zn), and chlorite-sericite-epidote-carbonate assemblages define semiconcordant zones that are stacked from the paleoseafloor to the deep footwall. In contrast, chlorite and chlorite-sericite-chloritoid assemblages overprint the other alteration zones and form subconcordant to now transposed discordant zones in the deposit footwall. Most alteration assemblages are characterized by relative SiO2, CaO, and Na2O mass losses and relative FeO, MgO, K2O, and CO2 mass gains. Whole-rock oxygen isotopes indicate that the temperatures of alteration ranged from ~100° to 150°C (sericite-carbonate assemblage) to ≥350°C (sericite-chlorite, chlorite-sericite-chloritoid, and chlorite assemblages). The chloritesericite-chloritoid assemblage, and to some extent the sericite-chlorite assemblage, are associated with strong to near total depletion of light rare earth elements possibly due to reaction with Cl-bearing, mildly acidic fluids at depth in the ore-forming hydrothermal system.The sulfide lens was formed in part on the seafloor and in part by subseafloor replacement. The massive sulfides are particularly rich in Bi, suggesting a possible magmatic input into the Lemoine ore-forming hydrothermal system. High Bi concentrations in the mineralizing system are likely to have enhanced Au precipitation by scavenging the precious metal from the hydrothermal fluids. The Au-rich nature of the Lemoine auriferous volcanogenic massive sulfide deposit can be explained by a combination of very efficient metal transport, highly effective capture of metals at or near the seafloor, and a possible magmatic contribution to the hydrothermal

Targeted Geoscience Initiative 4: Contributions to the understanding of volcanogenic massive sulphide deposit genesis and exploration methods development

“…The evidence for a direct magmatic input of Au can be circumstantial, but the involvement of magmatic fluids can be readily inferred in many ancient Au-rich VMS systems. The presence of extensive zones of aluminous alteration (metamorphosed advanced argillic alteration) at Bousquet 2-Dumagami and LaRonde Penna, for example, has been interpreted as evidence for a magmatic input into the hydrothermal system , and similar inferences have been made for Lemoine (Mercier-Langevin et al, 2014b), Boliden (Mercier-Langevin et al, 2013) and Nurukawa (Ishiyama et al, 2001). Such aluminous alteration zones develop in response to condensation of H 2 S vapour or disproportionation of magmatically derived SO 2 that produce very low-pH, acidic, and oxidizing fluids, which leach most elements in the rock except for Al and Si.…”

“…At Lemoine, intense leaching of light rare earth elements in the high-temperature footwall alteration zones is considered to indicate acidic conditions in the presence of both CO 2 and HCl-bearing fluids sourced from the underlying Doré Lake synvolcanic intrusive complex. The presence of high Bi contents in the Au-rich ore is also considered to be evidence of a magmatic input (Mercier-Langevin et al, 2014b), which may have been facilitated by the relative abundance of shallow felsic intrusive rocks in the deposit host succession (Boulerice et al, 2015).…”

Deep Into the Chibougamau Area, Abitibi Greenstone Belt: Structure of a Neoarchean Crust Revealed by Seismic Reflection Profiling

“…Seismic data show no major difference between the structure of the crust of the Chibougamau and Matagami areas. The Chibougamau area is not, however, renowned for its VMS deposits, the Lemoine deposit excepted (Mercier‐Langevin et al, 2014). There may be significant differences in the extent, efficiency, and duration of both hydrothermal systems that cannot be explained by geodynamic context differences and that should be investigated by dedicated studies.…”

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

“…Lemoine was one of the richest VMS deposits in terms of total net smelter return of Au, Cu, Zn and Ag in Canada and 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; Riverin 2003;Lafrance and Brisson 2006;Mercier-Langevin et al 2014). The gold grade is high for a VMS deposit, meaning that Lemoine classifies in the "auriferous" category (Mercier-Langevin et al 2011).…”

“…The stratigraphy and hydrothermal alteration in the area adjacent to the former Lemoine mine is well understood (Lafrance and Brisson 2006;Mercier-Langevin et al 2014;Ross et al 2016). However, further east in the Lemoine Member of the Waconichi Formation, there are knowledge gaps that limit our understanding of the overall context in which the exceptionally precious and base metal-rich Lemoine VMS deposit formed.…”