An Empirical Model for the Formation of Archean Gold Deposits: Products of Final Cratonization of the Superior Province, Canada
A. C. Colvine, 1989. "An Empirical Model for the Formation of Archean Gold Deposits: Products of Final Cratonization of the Superior Province, Canada", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Superior province of Canada hosts hundreds of gold mines which have recorded production ranging from less than 1 metric ton up to 1,000 metric tons of gold. All of them occur within large-scale, transcurrent and oblique slip-shear deformation zones, which were active during the latest Archean. The deformation zones constitute a conjugate set to a north-northwest-directed compression of the Canadian Shield. Within these deformation zones gold camps are localized in extensional structures; many of these are pull-apart structures which were the loci of fluvial-alluvial sedimentation, a suite of felsic intrusions, and alkaline volcanism. The localization of gold at all scales, from individual veins up to camp scale, is attributed to the dilation zones produced by the shear deformation.
Although mineralization and attendant wall-rock alteration are the products of auriferous hydrothermal fluids of similar composition ascending along deformation zones, they are manifested in many styles in different deposits and even within a single deposit. Wall-rock alteration assemblages vary according to metamorphic grade of host rocks and reflect alteration mineral stabilities under elevated ambient pressure and temperature of regional metamorphism; metamorphism does not overprint alteration mineral assemblages. Thus mineralization formed during, not only late Archean regional shearing, but also regional metamorphism; wall-rock metamorphic grade is therefore an indicator of depth of formation of mineralization within deformation zones. Wall rocks range in metamoprhic grade from subgreenschist to amphibolite facies, indicating that gold deposition took place over a substantial range of depths, possibly in excess of 10 km.
The depositional model is based on observations of more than 30 deposits in various terrains in the Superior province. Distribution and style of mineralization and alteration are a function of fluid access to wall rocks through permeability generated by shearing; with increasing depth brittle, brittle-ductile, and ductile deformation corresponds to breccia-style, veining, and foliation-parallel mineralization, respectively. Alteration mineralogy also reflects depth of formation. The most prominent aspects of the mineralogy are the predominance of pyrrhotite over pyrite as the characteristic alteration sulfide and the absence of ankerite carbonatization in deposits which formed under higher pressure and temperature conditions. Physical and chemical characteristics of varied lithologies also affect response to shearing and alteration. The model therefore provides a degree of predictability to the distribution and styles of mineralization and alteration expected in any specific lithologic and metamorphic environment. The correspondence of the observations with the model suggests that Archean gold mineralization is one single deposit type.
At several widely separated locations, the mineralizing event took place around or after 2,680 Ma, at least 20 Ma after the cessation of greenstone volcanism. The mineralization is somewhat younger than the felsic intrusions, which are spatially associated with many gold deposits, and may be, temporally, more closely related to slightly younger, minor magmatism of a more alkaline or a lamprophyric affinity. In the Canadian Shield, the timing of the mineralization roughly corresponds to a late batholith emplacement and granulitization and accompanying magmatism in the lower crust. Thus, the mineralization, which involves heat and mass transfer, is a manifestation in the upper crust of the widespread cratonization processes in the very late Archean. The latest Archean was the most important metallogenic epoch for this style of mineralization.