Sources of fluids in Archean hydrothermal stockwork-disseminated gold deposits of Abitibi, Canada: Insights from Duquesne, Dolodau, Lac Shortt and Canadian Malartic

Abstract

The Archean Eon was the most important period for the genesis of gold deposits with approximately 60% of the world’s gold depositing before 2500 Ma, and orogenic gold is the most common type of Archean gold deposit, accounting for 18% of the global gold production. However, genetic models for these deposits have remained controversial. After decades of investigation of field relationships, structural geology, igneous and metamorphic petrology, mineralogy, geochronology and major-, trace- and isotope geochemistry, most of the hypotheses that had been advocated for the origin of these deposits were negated but two have remained: the origin of the fluids which deposited the gold may be metamorphic or magmatic. The first argument for a magmatic origin of the fluids has been the spatial association of the gold deposits with intrusive rocks and the most efficient argument against the magmatic model has been the lack of documentation for the temporal association of the intrusions and the mineralization. On the other hand, the main evidence for a metamorphic origin of these deposits has been a lack of associated intrusive rocks, as the evidence that the ore fluid was metamorphic was usually supported by the failure of all other models. Four Archean gold deposits from the Wawa-Abitibi Subprovince of the Superior, Canada, which are spatially and temporally associated with magmatic rocks, are here investigated.Late-tectonic magmatic intrusions of intermediate compositions and high-K calc-alkaline to alkaline affinity are often associated with Archean gold deposits in the Superior and this association is often accompanied by dykes of calc-alkaline lamprophyres. Interestingly, some of these stocks were interpreted to have evolved from lamprophyric magmas, and lamprophyres, like syenites, commonly host hydrothermal and sometimes also magmatic carbonates. Duquesne and Canadian Malartic gold mines are examples of Archean gold deposits associated with syenitic/monzonitic intrusions and calc-alkaline lamprophyres. Furthermore, syenites are commonly found in association with carbonatites, and the association is known to host significant concentrations of gold at Lac Shortt gold mine and Dolodau gold deposit, Canada, which are examples of Archean gold deposits associated with carbonatites and syenites. To our knowledge, there is only one world occurrence of co-magmatic syenites, carbonatites, and lamprophyres with gold, at Lac Shortt, Canada. This study reports new trace elements concentrations, stable (δ13C, δ18O) and 87Sr/86Sr isotope ratios for mainly hydrothermal, but also some magmatic carbonates hosted in carbonatites, lamprophyres, syenites and the related mineralization from Duquesne, Dolodau, Lac Shortt and Canadian Malartic gold deposits.The characteristics of these four deposits which allows arguing that they are genetically related to magmatic-hydrothermal activity are presented and followed by geochemical results which show that the concentration of Sr in carbonates is inversely proportional to their 87Sr/86Sr ratios, and seen as three distinct populations on Sr/Mn vs Mn and Sr//Ba vs Ba plots. This suggests that the Archean sub-continental depleted but metasomatized mantle was the host of a regional-scale, high Sr fluid reservoir and that the metasomatized lower- and upper-continental crust hosted similar-sized fluid reservoirs of medium and low Sr concentrations, respectively. Carbonatite-syenite-related Lac Shortt and Dolodau gold deposits have carbonates with high Sr/Mn vs Mn and Sr/Ba vs Ba, as well as depleted mantle-like 87Sr/86Sr and high REE contents, suggesting that the magmatic-hydrothermal systems were rooted in a carbonatized, metasomatized, sub-continental depleted mantle. By contrast, syenite-lamprophyre and non-carbonatite-related Duquesne and Canadian Malartic gold deposits hosted carbonates with medium- to low Sr/Mn vs Mn and Sr/Ba vs Ba, and higher 87Sr/86Sr values, pointing toward the contribution of lower- and upper crustal fluids. At non-carbonatite-related syenite-hosted gold deposits, fluids recycled into the mantle would have triggered the partial melting of a metasomatized, non-carbonatized mantle and this melt would have evolved towards syenitic/monzonitic compositions and liberated alkali-bearing hydrothermal fluids. Carbon and oxygen isotopes importantly show that ore-related carbonates from the four intrusion-related gold deposits plot within a limited range of δ13C – δ18O values. An extensive compilation of carbonate δ13C and δ18O values does not infirm that carbonates from Archean intrusion-related gold deposits tend to plot within the ‘IRGD’ box, and suggests that those that plot outside the ‘IRGD’ box appear to originate from pure metamorphic orogenic gold deposits. Nevertheless, more work is required before this IRGD box can be confirmed, which should impact the prospectivity of Archean gold deposits.