Abstract
Given that gold (Au) mostly remained in the incipient Earth mantle until ca. 3.9–3.8 Ga, a “proto-source” of gold may have been present in the dominantly mafic crust precursor born through first-stage melting of the early Earth mantle. In south-westernmost Greenland, a fragment of the North Atlantic Craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks, and harzburgite cumulates that were emplaced at ca. <3.19–3.01 Ga (e.g., Tartoq greenstone belt). Here, combining detailed sulphide petrography with rhenium-osmium-sulphur (Re-Os-S) isotope geochemistry of individual mineral separates of arsenopyrite from gold-sulphide mineralised shear zones, we pinpoint the precipitation of ca. 3.18–3.13 Ga (Re-Os model ages) hydrothermal arsenopyrite associated and coeval with arc-related magmatism of the Tartoq Group. We consider sub-seafloor hydrothermal alteration of the oceanic crust and magmatic activity to have supplied arsenic (As), Re, and Au, to result in the precipitation of the ca. 3.18–3.13 Ga arsenopyrite with primary invisible gold. Additionally, in major shear zones in a rigid juvenile continental crust, retrograde greenschist-facies metamorphism overprinted the ca. >3.0 Ga prograde amphibolite-facies metamorphic assemblages and caused local dissolution of arsenopyrite. During this retrograde tectono-metamorphic stage, in gold-rich shear zones, the Re-Os geochronometer in arsenopyrite was reset to a Neoarchean age while invisible gold was liberated and deposited as free gold with 2.66 Ga pyrite (Re-Os isochron ages). The initial Os isotope ratios of Neoarchean arsenopyrite (187Os/188Osi = 0.13 ± 0.02) and gold-bearing pyrite (0.12 ± 0.02) overlap with the estimated 187Os/188Os ratio of the Mesoarchean mantle (0.11 ± 0.01) and preclude contribution of radiogenic crustal Os from evolved lithologies in the accretionary arc complex, but instead, favour a local contribution in Os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids. Thus, the ca. 2.66 Ga lode gold mineralisation identified in the North Atlantic Craton may illustrate a gold endowment in shear zones in Earth’s stabilizing continental crust at the time of the 2.75–2.55 Ga Global Gold Event, through metamorphic upgrading of bulk gold which had originally been extracted from the Mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced Mesoarchean ocean.
Highlights
IntroductionOn Earth, the abundance of a given element (e.g., gold, Au), for example in the continental crust, may be either (1) the result of its partitioning from the internal material as a result of the differentiation of Earth’s bulk composition and reorganization of its interior structure through the impact of core-mantle-plate geodynamics (Shahar et al., 2019), or, (2) the result of a preserved extra-terrestrial input supplied over a prolonged period of time (e.g., iridium enrichment in the cap carbonate sequences – an outcome from the demise of the Cryogenian Snowball Earth; Bodiselitsch et al, 2005)
On Earth, the abundance of a given element, for example in the continental crust, may be either (1) the result of its partitioning from the internal material as a result of the differentiation of Earth’s bulk composition and reorganization of its interior structure through the impact of core-mantle-plate geodynamics (Shahar et al., 2019), or, (2) the result of a preserved extra-terrestrial input supplied over a prolonged period of time
The ca. 2.66 Ga lode gold mineralisation identified in the North Atlantic Craton may illustrate a gold endowment in shear zones in Earth’s stabilizing continental crust at the time of the 2.75–2.55 Ga Global Gold Event, through metamorphic upgrading of bulk gold which had originally been extracted from the Mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced Mesoarchean ocean
Summary
On Earth, the abundance of a given element (e.g., gold, Au), for example in the continental crust, may be either (1) the result of its partitioning from the internal material as a result of the differentiation of Earth’s bulk composition and reorganization of its interior structure through the impact of core-mantle-plate geodynamics (Shahar et al., 2019), or, (2) the result of a preserved extra-terrestrial input supplied over a prolonged period of time (e.g., iridium enrichment in the cap carbonate sequences – an outcome from the demise of the Cryogenian Snowball Earth; Bodiselitsch et al, 2005). It is critical to understand how and when a “proto-source” of gold was extracted from the Earth mantle and made available into the mafic crust precursor through first-stage melting of the mantle in the Archean. Second-stage melting of this mafic precursor, which produced more felsic rocks, would re-distribute gold in a more mature crust comprising tonalite-trondhjemite-granodiorite (TTG) bodies and granite-greenstone belts (Shirey and Richardson, 2011; Reimink et al, 2016; O’Neil and Carlson, 2017; O’Neil et al, 2019; Johnson et al, 2019; Laurent et al, 2020). To explain the distribution of gold deposits in the Earth crust through geological times (Groves et al, 2005; Goldfarb et al, 2001, 2010; Frimmel, 2018), it was proposed that the ca. To explain the distribution of gold deposits in the Earth crust through geological times (Groves et al, 2005; Goldfarb et al, 2001, 2010; Frimmel, 2018), it was proposed that the ca. 2.9–2.7 Ga gold source in Earth’s crust was redistributed in an array of ore deposit styles that could only be generated by the repeated action of plate tectonics (possibly starting from at ca. 3.2 to 3.0 Ga; Shirey and Richardson, 2011; Næraa et al, 2012; Tang et al, 2016; Smit et al, 2019) and the establishment from ca. 2.7 Ga of a continental crust rigid enough to be able to record strong regional deformation fabrics (Hawkesworth et al, 2019)
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