The geology of the Beardmore-Geraldton belt, Ontario, Canada: geochronology, tectonic evolution and gold mineralization

Field: Earth and Planetary Sciences (Physical Sciences)

The Beardmore-Geraldton belt (BGB) is a greenstone belt straddling the boundary between the \neastern Wabigoon subprovince and the Quetico metasedimentary subprovince. It consists of shearbounded panels of metavolcanic and metasedimentary rocks which were imbricated during an \nearly thrusting event and regionally folded and sheared during subsequent deformation events. It \nhosts several historic gold mines with a combined past production exceeding 4 Moz gold. New \ngold resources were delineated within the last ca. 10 years at the Hardrock deposit in the eastern \npart of the belt which highlighted the need for a better understanding of the controls on gold \nmineralization in the belt. \nThe BGB comprises three panels of metasedimentary rocks, representing a southward transition \nfrom fluvial to deltaic to deep oceanic basin plain environments, overlying three panels of older, \nca. 2725 Ma, metavolcanic rocks, representing back arc, island arc, and oceanic crust. Detrital \nzircon geochronology of the BGB and adjacent northern Quetico metasedimentary rocks suggests \nthat these rocks formed from sediments derived by the erosion of ca. 2700 Ma to 2900 Ma source \nrocks and older >3200 Ma Mesoarchean craton of the eastern Wabigoon subprovince. The \nemplacement of crosscutting 2694±1 Ma feldspar-quartz porphyry (FQP) dikes, marks the end of \nsedimentation in the belt. The panels were subsequently imbricated during an early thrusting event \n(D1) which ended with the emplacement of the stitching 2690±1 Ma Croll Lake stock. The \ncomposition of the FQP dikes and Croll Lake stock (i.e. SiO2, ferromagnesian oxide and transition \nelement contents, [La/Yb]N ratio) indicate a shift from tonalite-trondhjemite-granodiorite (TTG) \nsuite to sanukitoid suite magmatism over a 4 Ma period. The formation of the sanukitoid melts, \nwhich involves the addition of a mantle melt component, is consistent with their generation during \ndelamination or slab break-off as the BGB metavolcanic and metasedimentary panels were thrustimbricated and accreted to the Wabigoon subprovince during closure of the Quetico basin. \nD1 structures are expressed by early, isoclinal, F1 folds with an axial planar S1 cleavage. They are \nfolded by regional F2 folds with an axial planar, regional, S2 cleavage, which becomes more \npronounced along belt-parallel, sinistral, high strain zones characterized by asymmetrical, Sshaped F2 folds and NE-striking, en echelon, quartz-carbonate veins. The association of regional \nfolds and cleavage with sinistral high strain zones suggest that these structures formed during \nregional sinistral transpression (D2) across the belt. During subsequent dextral D3 transpression, \nthe regional F2 folds and S2 cleavage were overprinted by Z-shaped F3 folds and by a second \nregional cleavage (S3), and the D2 high-strain zones were reactivated as dextral shear zones. Within \nthe reactivated high strain zones, F1, F2 and F3 fold axes are parallel to a strong, shallowlyplunging, stretching lineation that formed during D2 and was accentuated and rotated towards the \norientation of the fabric attractor during D3. The fabric attractor is oblique to the intersection \nlineation between the reactivated S2 cleavage and dextral shear bands, suggesting that the \ndeformation during D3 had triclinic symmetry. \nThe Hardrock gold deposit is one of several deposits hosted by the Bankfield-Tombill deformation \nzone near Geraldton in the eastern part of the BGB. Gold was emplaced during early D1 thrusting \nand D2 sinistral transpression in association with strongly folded, early D1, quartz-carbonate veins \n(V1) and E- to NE-striking, syn-D2, tourmaline-rich (V2) and quartz-carbonate veins (V3) veins. \nGold is present in association with inclusion-rich pyrite with elevated Au, Ag, As, Bi, Co, Ni, Pb, \nSb, Te concentrations and along fractures and as inclusions in the veins and sulfides. The deposit \nis surrounded by a broad (≤250 m) alteration envelope of carbonate and sericite with elevated \npathfinder S, Te, As, W, and Bi concentrations. Early introduction of gold in the belt during \nthrusting and sinistral transpression differs from previous studies which favored late tectonic \nemplacement of gold in D3 shear zones and fold hinges.