Geology, structure and geochemistry of gold mineralization in the Geraldton Area, Northwestern Ontario

Gold mineralization in the Beardmore-Geraldton area occurs in a major, long-lived deformation zone, spatially coincident with the boundary between the Wabigoon subprovince to the north and the Quetico subprovince to the south. -- Regionally, the deposits are linearly distributed (east-west) parallel to the strata of the southern margin of the Wabigoon belt. Virtually all rock types host gold mineralization, except for the youngest intrusions, diabase and lamprophyre dykes. -- Gold mineralization in the Geraldton area occurs; 1) in quartz and quartz-carbonate veins in fractured greywacke, felsic and mafic intrusions and mafic volcanic rocks, 2) in disseminated sulphide-bearing, highly fractured, zones in greywacke and quartz-feldspar porphyry, and 3) in massive quartz-sulphide lenses in oxide iron formation. -- There is no primary sulphide or carbonate facies iron formation in the belt. All sulphide and carbonate minerals in iron formation are secondary alteration features associated with gold mineralization. The alteration is characterized by addition of CO₂, S, As, K, Ba and H₂O, and loss of Sr. There is no primary enrichment of gold in any of the rocks, background gold content of all rock types is less than 10 ppb. -- Gold-bearing veins occur in relatively young structures that crosscut all local Archean rock, and overprint several earlier phases of deformation. The major control on emplacement of veins is cleavage- and foliation-parallel fractures and dilations, and zones of intense deformation localized at the contacts of rocks of differing ductility. -- The U/Pb aircon age of felsic porphyry intrusions in the Geraldton area is 2691 +3/-2 Ma, significantly younger than volcanic rocks in the Onaman Lake area to the north, which are 2769 +6/-5 Ma. -- The age of gold mineralization is 2556 +72/-76 Ma as determined by a lead isochron on pyrite from mineralized felsic porphyries. This age is approximately 130 Ma younger than the age of the felsic porphyritic rocks and therefore there does not appear to be a direct magmatic- hydrothermal link between these intrusive rocks and the gold mineralization