ALBERT

Notes: Abstract The vein-type gold deposits of middle to late Archean age constitute one of the major sources of gold production in the world. These deposits are highly diverse in character, but are invariably found in proximity to mafic and ultramafic volcanic rocks, and closely associated with tectonically disturbed zones, or ‘breaks’, in the ancient crust. Using the Larder Lake ‘break’ of northern Ontario and Quebec as a model, a general theory of genesis for such deposits is proposed. The Larder Lake ‘break’ is of linear configuration, up to four miles wide, and extending over a distance of approximately 150 miles. Many rich and extensive vein-type gold deposits are associated with it. This ‘break’ is recognized as a highly folded and faulted, dominantly sedimentary stratigraphic unit, formed as part of the development of the Blake River geosyncline of the Abitibi greenstone belt. Faulting is pervasive within the ‘break’ but it is non-continuous. Under an oxygen-poor, somewhat acidic weathering regime believed extant in Archean time, gold and associated constituents, derived as weathering products from mafic and ultramafic volcanics, appear to have been concentrated as solutions and sols in muddy surficial accumulations along a paleo-continental margin. On transfer to a shallow marine environment, the gold-bearing solutions became concentrated mainly as cherty and pyritic evaporitic residues. These residues, infolded and variably metamorphosed and remobilized during ensuing geosynclinal development, formed the numerous and apparently heterogeneous gold deposits of the Larder Lake ‘break’. Mode of redistribution of gold-bearing residues and metamorphism of their host rocks varies widely. In mild form, little vein development is present and original sedimentary features of the host rock are often preserved. In more extreme cases, true hydrothermal veining is evident and complete recrystallization of the host rock has taken place, sometimes to the extent of formation of a melt with intrusive capabilities. In any case, the gold and related vein constituents remain closely associated with the primary host material and alteration effects are largely derived from the recrystallization of original host rock constituents. The suggested mode of genesis clarifies the relationship between various types of ‘break’-related, vein-type gold deposits and offers an explanation of the complex stratigraphy and structure of the ‘break’ environment. It also suggests a penecontemporaneous relationship between the Larder Lake ‘break’ and other auriferous ‘breaks’, of varying configuration, within the Superior Province of the Canadian Shield. The concept can be used advantageously in exploration for new ‘breaks’ and new deposits.