Wieland Boehme, PhD
W.H. Boehme1, S.M. Brueckner1, B. Lafrance1, M. Laverge1, J. Simmons1, D. Tinkham1, J. Belanger1, J.C. Ordóñez Calderón2
1Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada
2Kinross Gold Corporation, Toronto, Ontario, Canada
The Great Bear deposit is a new world-class gold deposit, located in the Red Lake greenstone belt of the Archean Superior province, NW Ontario. It has a resource estimate (measured + indicated) of 2.7 Moz Au at 2.81 g/t Au and an additional inferred resource of 3.9 Moz Au at 4.71 g/t Au. Gold is primarily hosted in highly strained felsic rocks of the LP deformation zone, a WNW-ESE trending deformation zone located 25 km S of Red Lake. Stratigraphic and petrographic analysis of representative drill core intervals, microprobe analysis of gold, and LA-ICP-MS U-Pb titanite geochronology were conducted to constrain host lithology, ore assemblage and textures, gold composition, and the timing of deformation and mineralization.
The Great Bear deposit is hosted by felsic volcanic rocks and metasedimentary rocks. The former consists of (1) a quartz-feldspar-porphyritic unit with a dark grey, aphanitic matrix and (2) an altered porphyritic unit with feldspar phenocrysts and an aphanitic matrix. The intercalated metasedimentary units consist of dark grey, fine-grained, massive to thinly bedded sediments. The ore assemblage comprises disseminated pyrite, arsenopyrite, and pyrrhotite, minor chalcopyrite, sphalerite, galena, and magnetite, trace Bi±Ag±Pb tellurides, scheelite, and native gold. A subtle zoning is characterized by gold enrichment and increased sulfide abundance in the central and southeastern LP deformation zone and is expressed by pyrite-arsenopyrite and pyrite-pyrrhotite dominated ore assemblages, respectively. Titanite from the gold-hosting quartz-feldspar porphyry yielded a U-Pb age of 2704 ± 13 Ma. Titanite occurs as equant to elongated grains aligned with the dominant foliation, locally showing biotite-filled pressure shadows and pyrite inclusions.
Native gold occurs disseminated in deformed quartz ± carbonate veins and host rocks. Locally, gold occurs as inclusions in coarse, recrystallized pyrite and foliation-parallel biotite. Gold fineness varies significantly, ranging from 830 to 968. Lower-fineness gold (830 – 891) is associated with abundant galena and Bi±Ag±Pb tellurides and occurs as irregularly shaped intergrowths, partially to fully surrounded by galena and Bi±Ag±Pb tellurides. Gold with a higher fineness (954 – 968) lacks this association. These preliminary results suggest multi-stage gold emplacement, possibly involving low-temperature polymetallic melts, and indicate that mineralization was emplaced prior to 2704 ± 13 Ma.