Abstract:

Gold deposits hosted by Archean banded iron formation represent a key exploration target in Canada. As part of the Targeted Geoscience Initiative-4 Lode Gold project, the goal of the project is to characterize the structural controls on gold mineralization and their footprints. The study area, the Beardmore-Geraldton Belt (BGB), is located along the boundary between the granite-greenstone Wabigoon and the sedimentary Quetico subprovinces This area was chosen because of the on-going advanced exploration that provides access to large stripped outcrops showing the relationships between mineralization and structures as well as recent drill holes that provide access to new information on the nature, distribution and chronology of the gold-bearing hydrothermal alteration, allowing to further increase our understanding of mineralizing processes in BIF-bearing Precambrian successions. During the summer of 2013 stripped outcrops were mapped in detail and drill cores intersecting mineralized zones in different host rocks have been logged and sampled for petrography and lithogeochemistry. <br/>All the three mapped strippings are located along the north limb of the regional F2 Hard Rock anticline. Tight F1 folds are refolded by gentle to tight, S-shaped, F2 folds that are parasitic folds to the Hard Rock anticline. S-shaped F3 folding of S2 at the Headframe and OPP stripped outcrop suggests, local sinistral shearing prior to dextral shearing during D4. D4 is a late, dextral transcurrent shearing event that produced multiple Z-shaped drag folds with a strong axial planar foliation and dextral shear bands.<br/>Gold mineralization occurs as semi-massive sulphide replacement associated with quartz-Fe carbonate veins at the OPP and Headframe East stripped outcrops. It is folded by S-shaped F2 folds suggesting that mineralization was emplaced either pre- or early-D2. Fe-carbonate alteration, is folded by F1 folds. However, the mineralization exposed of the stripped outcrops does not show evidence of deformation by F1 folds. If this iron-carbonate alteration is associated with gold mineralization this would suggest the possibility of a pre-D2 presumably syn-D1 mineralization. Alternatively, this may suggest that there are several stages of iron-carbonate alteration. The stripped outcrops clearly expose the complexity of the structural setting of the gold deposits in the Geraldton district with several generations of foliations and folds. Better defining the chronology of deformation in relationship with the emplacement of the bulk of the gold mineralization could impact on the understanding of the distribution and geometry of the auriferous lodes and also help better define the key parameters responsible for their formation.