Tectonics-mineralisation relationships within weak continental lithospheres: a new structural framework for Precambrian cratons

Abstract

Many Archaean and Paleoproterozoic cratons show deformation patterns that differ from those observed in modern orogens. On the other hand, they constitute an important part of present-day emerged continents and contain a large part of continental mineral resources known to date. On the basis of a summary of structural data from some typical field examples and of results of analogue modelling, we emphasize that pop-down tectonics marked by vertical burial of supracrustals within an underlying hot and weak crust may be the most suitable model to account for deformation patterns of many ancient deformation zones. An overview of relationships between structural patterns and mineralisation in several ancient deformation zones further emphasizes that pop-down tectonics provides a very promising structural framework for mining exploration in Precambrian cratons.

The Rosebel gold mining district (Trans-Amazonian belt, Suriname), a new structural framework

“…A similar feature is also observed in western Africa (Gapais, 2018) that was linked to the Trans-Amazonian belt before the opening of the Atlantic Ocean. In a recent review paper, Gapais (2018) proposed that a common deformation mode of hot Paleoproterozoic belts was driven by vertical pop-downing of upper crustal deposits within underlying weak ductile crustal layers, with associated implications for ore concentrations.…”

Eburnean deformation pattern of Burkina Faso and the tectonic significance of shear zones in the West African craton

“…The first tectonic process could explain early HP-LT apparent geotherms recorded by greenstones (Ganne et al, 2012(Ganne et al, , 2014Block et al, 2015). But transpressional burial of supracrustals by pop-down tectonics (Gapais et al, 2014;Gapais, 2018) and subsequent upward extrusion (Thompson et al, 1997) might as well and would be consistent with early and long-lasting transpression leading to the sWAC finite structural pattern. If PTt paths of Birimian supracrustals may be "reminiscent of modern collisional orogens" (McFarlane et al, 2019), the pervasive transpressive structural pattern of the Eburnean orogen does not compare at all the thrust or gravitational collapse patterns of Variscan or Alpine collision belts.…”

“…The Eburnean orogen of the sWAC affects the so-called Birimian crust, which consists of tholeitic to calc-alkalcaline mafic volcanics (preserved in greenstone belts that also comprise clastic sediments) and broadly coeval and younger TTG-like complexes and granitoid plutons (Milési et al, 1989(Milési et al, , 1992Block et al, 2016a;Parra-Avila et al, 2017, 2018Wane et al, 2018;Grenholm et al, 2019). Birimian mafic and felsic magmatism and Eburnean deformation and metamorphism took place essentially between 2.27 and 1.96 Ga (Grenholm et al, 2019).…”

“…Such a coexistence of brittle and ductile structures during progressive deformation tends to be ignored in the phase approach. The second reason to challenge the phase structural approach is the ubiquitous three-dimensional strain partitioning between strike-slip shearing, reverse shearing and planar and linear fabrics that coexist in transpressional contexts (Hudleston et al, 1988;Tikoff and Teyssier, 1994;Gapais et al, 2005, Gapais, 2018Harris and Bédard, 2014). The third reason is that by favoring a local phase approach, one tends not to consider the spatial kinematic diversity of localities as part of a progressive/bulk deformation pattern that would be coherent on a regional scale (examples of regional kinematic integration are provided, for instance, by Martelat et al, 2000;Gapais et al, 2005;Harris and Bédard, 2014;Cochelin et al, 2017).…”

“…In particular, whether they record large displacement or could be tight to a far-field kinematic framework or to a kind of plate kinematics is not straightforward (Treloar and Blenkinsop, 1995;Hanmer et al, 1997;Gapais et al, 2005;Chardon et al, 2008;Harris and Bédard, 2014). Those shear zones also tend to control the location of endogenous mineral deposits as typified by orogenic gold (e.g., Bedeaux et al, 2017;Gapais, 2018). Characterization of shear zones geometries and kinematics at various scales should therefore be useful to link local deformation histories with orogen-scale processes and mineral systems.…”