The Beardmore-Geraldton greenstone belt is located along the boundary between the Wabigoon and Quetico subprovinces in the Superior province of the Canadian Shield. Historical gold mines and current gold exploration camps are located throughout the Beardmore-Geraldton greenstone belt and adjacent Wabigoon subprovince in moderately to steeply dipping ductile to brittle-ductile shear zones in many types of lithologies, with various metamorphic grades and alteration styles. The shear zones typically strike east-west along the boundary of the Beardmore-Geraldton greenstone belt with the Wabigoon subprovince to the north as well as along the boundary with the Quetico subprovince to the south.
Field mapping and petrography, including microstructural analysis, were used to characterize the gold mineralization and investigate the control on gold mineralization in the Elmhirst, Castlewood, Pagwachuan, and Milestone properties in Greenstone, Ontario. Two property-scale and seven trench-scale maps were produced from field mapping and one hundred twenty-three transmitted and reflected light thin sections were prepared from samples collected from these properties. Lithology, structure, microstructure, metamorphic grade, alteration, and subprovince were considered as possible controlling factors on gold mineralization.
In the study area gold mineralization is hosted in relatively competent lithologies including schistose to mylonitic metamorphosed granodiorite, syenite, and pegmatite; chlorite-albite schist, schistose amphibolite, and garnet-hornblende-biotite schist. The regional-scale structures that host gold mineralization are ductile to brittle-ductile shear zones that have a penetrative, schistose foliation and rarely a mylonitic fabric. The property-scale structures that host gold mineralization are folded, mylonitized, and boudinaged quartz veins; shear-related early, late, and complex folds; folded fault breccia, and pressure shadows of rigid minerals within schistose to mylonitic fabric. Gold mineralization is located in areas of micro-folds, micro-boudins, micro-fault breccia, undulatory extinction, and grain size reduction along irregular grain boundaries, subgrain boundaries, folded healed fractures, and in pressure shadows within areas of strain heterogeneity. Gold mineralization is located in both areas of greenschist facies and amphibolite facies metamorphism in the study area. The styles of alteration also vary within the study area with sericite, hematite, ankerite, calcite, and unaltered samples all hosting gold mineralization. Both the Wabigoon and Quetico subprovinces host gold mineralization in similar structures and microstructures as the Beardmore-Geraldton greenstone belt although it is within different lithologies, metamorphic grades, and alteration styles.
Similar structure and microstructure are always present in areas of gold mineralization in the study area while lithology, alteration, metamorphic grade, and subprovince vary. Gold mineralization within stable prograde and retrograde metamorphic minerals indicates that gold mineralization was synchronous with ductile to brittle-ductile deformation and metamorphism. Further gold exploration in the study area should focus on structure and microstructure as indicators for gold mineralization.
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