Copper Isotope Fractionation in Archean Hydrothermal Systems: Evidence From the Mesoarchean Carlow Castle Cu-Co-Au Deposit

Copper isotope analysis has emerged as a promising tool for understanding genetic processes in Cu ore deposits. However, applications of this analytical technique to Archean Cu deposits have been extremely limited, even though Archean terranes are among the most economically endowed on Earth. As such, this study presents the first Cu isotope analysis of an Archean Cu deposit, the Mesoarchean Carlow Castle hydrothermal Cu-Co-Au deposit. Archean primary Cu sulfide ore samples and Cenozoic supergene Cu ore samples were analyzed. Primary ore samples are isotopically light, with & delta;65Cu values ranging between -0.80 & PLUSMN; 0.02 & PTSTHOUSND; and 0.00 & PLUSMN; 0.007 & PTSTHOUSND;, whilst supergene samples are isotopically heavier and range between -0.50 & PLUSMN; 0.01 & PTSTHOUSND; and 0.62 & PLUSMN; 0.005 & PTSTHOUSND;. In primary ore samples, a relationship is observed between the Cu isotope signature, ore grade, and alteration assemblage that records the isotopic and physicochemical evolution of the Carlow Castle deposit's hydrothermal ore-forming system. A mafic igneous source is suggested as a metal source in the Carlow Castle Cu-Co-Au deposit. The limited heavy isotopic fractionation of supergene Cu ore samples in this study is interpreted to reflect limited redox cycling of Cu due to in situ oxidative weathering of vein-hosted Cu sulfides in the overlying Cenozoic supergene system. This differs from previously studied deposits where significant Cu transport and multiple stages of isotopic enrichment are often evident in supergene Cu enrichment layers. The results of this study suggest that Cu isotope analysis could be valuable in understanding genetic processes in hydrothermal Cu deposits, including Archean ore deposits and terranes. The two stable isotopes of copper can be distinguished from small differences in their atomic masses. Analysis of the relative abundances of these isotopes in minerals from copper enriched mineral deposits has emerged as a powerful new tool to understand the processes that give rise to these economically important copper concentrations in Earth's crust. However, virtually all previous studies utilizing copper isotope analysis to understand the ore formation have focused on relatively young deposits. This is primarily because geological processes influenced by Earth's oxygenated surface environment leave the clearest copper isotopic signature. In this study, we present the first application of copper isotope analysis to an ore deposit that formed before Earth's atmosphere was oxygenated: the three-billion-year-old Carlow Castle copper-cobalt-gold deposit in Western Australia. Perhaps unexpectedly, we observed clear evidence of the geological processes that formed the Carlow Castle deposit through the lens of copper isotopes. These results suggest that copper isotope analysis could be a novel tool to understand the geological processes of ancient Earth and the formation of ancient copper deposits. These findings may help us explore for new mineral deposits to satisfy the rapidly growing copper demand of the future green economy. Archean hydrothermal processes induce Cu isotope fractionation outside expected Bulk Silicate Earth valuesCopper isotopes may provide a novel record of the physicochemical evolution of Archean hydrothermal systemsCopper isotope analysis could have practical exploration applications in structurally complex Archean terranes

Automated summary

Copper isotope analysis is a promising tool for understanding genetic processes in Cu ore deposits, but its applications to Archean Cu deposits have been limited. This study presents the first Cu isotope analysis of an Archean Cu deposit, revealing the relationship between the Cu isotope signature, ore grade, and alteration assemblage. These findings highlight the importance of copper isotope analysis in understanding the genetic processes of hydrothermal Cu deposits.