Banded iron formation-hosted gold mineralization in the Geraldton area, northwestern Ontario: structural setting, mineralogical characteristics and geochronology

Abstract

Renewed exploration activities in the Beardmore-Geraldton belt in the Archean Superior Province, northwestern Ontario, produced large stripped outcrops and new drill cores that revealed a wealth of new information on the structural and lithological settings of gold mineralization in this past-producing mining camp. The aims of this project are to establish the key geological parameters that controlled the genesis and distribution of gold mineralization in the area and to define geological and exploration models that incorporate the revised knowledge of the geological and structural setting, relative timing, mineralogical characteristics and geochemical footprints of the gold mineralization to fill in knowledge gaps about the gold distribution and geometry in the belt. The Beardmore-Geraldton belt consists of 0.15 to 10 km-wide panels of Archean metasedimentary rocks alternating with 1 to 5 km-wide panels of metavolcanic rocks. The older, ca. 2725 Ma metavolcanic panels are in fault contact with the younger metasedimentary panels. The deposition of the sedimentary rocks started at <2700 Ma, as indicated by the youngest detrital zircons dated from the Central and Southern Metasedimentary units and was complete by 2694.0 } 1.0 Ma, the crystallization age of the crosscutting quartz-feldspar porphyry. The Beardmore-Geraldton belt underwent four deformation events. During D1 deformation, the metavolcanic-metasedimentary sequences and the quartz-feldspar porphyry were thrust-imbricated and folded by F1 folds. The D1 event occurred between 2694 Ma, the age of the quartz-feldspar porphyry dyke in Geraldton, and 2690 } 1 Ma, the age of the post-D1 Croll Lake stock. During D2 deformation, south-tonorth shortening, regional-scale, west-plunging F2 folds and axial-planar, east-trending, steeply dipping S2 foliation were formed. The S2 foliation has been folded by S-shaped F3 folds that are associated with an east-trending, spaced axial-planar S3 cleavage, indicating a previously unrecognized D3 sinistral shear event in the belt. These three deformation events were overprinted by a D4 dextral transpression event. In the Beardmore-Geraldton belt, gold mineralization is typically hosted by mudstone, sandstone, banded iron formation and quartz-feldspar porphyry. Gold mineralization is commonly associated with locally auriferous quartz-carbonate veins. The mineralized quartz-carbonate}tourmaline vein selvages are characterized by semi-massive sulphide-sericite-carbonate replacement alteration halos where hosted in banded iron formation. A similar alteration halo is present in veins that are hosted in mudstone, sandstone and quartz-feldspar porphyry, although the sulphides are less abundant than when the veins are hosted in banded iron formation. At least two gold-mineralizing events, including possible remobilization, took place during the tectonic evolution of the belt. Gold-rich quartz-carbonate}tourmaline veins and the associated sericite-carbonatesulphide alteration halos are folded by F1 folds, suggesting that the first gold-bearing event is related to the early phases of the D1 deformation. East-northeast- to east-trending, locally auriferous quartz-carbonatetourmaline- sulphide veins cut F2 fold hinges but are folded by S-shaped F3 folds, suggesting a second, early D3 auriferous episode in the district. Northwest-trending sulphide-rich veins, which cut across early D3 tourmaline- rich veins and are folded by gentle Z-shaped F4 folds, may also have carried or remobilized some gold mineralization during D4 dextral transpression. An increase in gold grade is associated with elevated As, Te, Sb, and W concentrations and sericitization index. It is hoped that the new data and interpretation generated as part of this project will contribute to further mineral exploration success by defining new structural targets and establishing geochemical footprint vectors.