Working up an Apatite: Enigmatic Mesoarchean Hydrothermal Cu-Co-Au Mineralization in the Pilbara Craton

Abstract

Globally, significant examples of hydrothermal Cu-Co mineralization are rare within Archean greenstone belts, especially relative to the endowment of these terranes with other world-class hydrothermal ore deposits, particularly Au deposits. Using U-Pb geochronology of hydrothermal apatite, this study provides the first absolute age constraints on the timing of mineralization for the Carlow Castle Cu-Co-Au deposit. Carlow Castle is a complex, shear zone-hosted, veined Cu-Co-Au mineral system situated within the Paleo-Mesoarchean Roebourne greenstone belt of the Pilbara craton of northwestern Western Australia. Although U-Pb geochronology of this deposit is challenging due to low levels of radiogenic Pb in synmineralization apatite, mineralization is best estimated at 2957 ± 67 Ma (n = 61). Additionally, analysis of alteration phases associated with Carlow Castle mineralization suggests that it is dominated by a propylitic assemblage that is characteristic of alkaline fluid chemistry and peak temperatures >300°C. Within proximal portions of the northwest Pilbara craton, the period of Carlow Castle’s formation constrained here is associated with significant base-metal volcanogenic massive sulfide mineralization and magmatic activity related to back-arc rifting. This rifting and associated magmatic activity are the most likely source of Carlow Castle’s unique Cu-Co-Au mineralization. Carlow Castle’s Meso-archean mineralization age makes it among the oldest discovered Cu-Co-Au deposits globally, and unique in the broader context of hydrothermal Cu-Co-Au deposits. Globally, hydrothermal Cu-Co mineralization occurs almost exclusively as Proterozoic and Phanerozoic stratiform sediment-hosted Cu-Co deposits due to the necessity of meteorically derived oxidized ore fluids in their formation. This research therefore has implications for exploration for atypical Cu-Co deposits and Cu-Co metallogenesis through recognition of comparably uncommon magmatic-hydrothermal Cu-Co-Au ore-forming processes and, consequently, the potential for analogous Cu-Co-Au mineralization in other Archean greenstone belts.

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

“…Additionally, the intensity of such oxidative weathering processes in Archean settings would have been relatively limited in comparison to modern supergene processes and the transient nature of such conditions would preclude the multiple redox cycles necessary to generate significant heavy Cu isotope fractionation in crustal reservoirs (Johnson et al., 2021; Mathur & Fantle, 2015). Similarly, given the physicochemical conditions (minimal H + metasomatism, ∼300°C) of ore formation at Carlow Castle (Fox et al., 2021), it is unlikely that redox changes (oxidation of Cu + to Cu 2+ ) driven by hydrothermal ore‐forming processes would have induced significant heavy Cu isotope fractionation as CuCl 2 − is the predominant stable Cu‐Cl species in solution around 300°C, even under oxidizing conditions (Brugger et al., 2016; W. Liu et al., 2002; Maher et al., 2011; Syverson et al., 2021; Zhu et al., 2002). As such, the monovalent state (Cu + ) dominates Cu speciation both within solution and precipitated sulfide phases under these conditions (Pearce et al., 2006).…”

“…These findings suggest that Cu isotope analysis also has potential utility in understanding genetic processes in Archean ore‐forming systems and Cu transport in Archean Earth systems more broadly. Carlow Castle represents a relatively unique Archean structurally‐hosted hydrothermal Cu‐Co‐Au deposit (Fox et al., 2019, 2021). This underlines the importance of expanding the existing literature applying Cu isotopes to understand Archean geological processes.…”

“…Paragenetically, an increase in sulfur fugacity is observed through both of these ore mineral assemblages, with paragenetically early tellurides, selenides, and sulfosalts (e.g., hessite, clausthalite, cobaltite) and paragenetically late sulfides (e.g., pyrite, chalcopyrite, chalcocite) (Fox et al., 2019). Based on analyses of its characteristically propylitic hydrothermal alteration assemblage, it has been established that the Carlow Castle hydrothermal ore fluid was most likely neutral to slightly alkaline in pH (∼5.5 at 300°C), reaching peak temperatures of ∼300°C (Fox et al., 2021). However, a gradient in the temperature of this propylitic alteration assemblage is evident and defined by a decrease in actinolite and epidote relative to chlorite.…”

“…In this instance, the derivation from an unfractionated and homogeneous igneous source may account for the relatively limited fractionation and broadly normally distributed Cu isotope signature evident in the Carlow Castle Cu sulfide mineralization. However, there is no evidence of large-scale magmatic activity in the West Pilbara Superterrane that was both coeval with the formation of Carlow Castle (c. 2.95 Ga) and proximal to the deposit to provide an igneous driver of ore formation and source of metal (Fox et al, 2021). Nonetheless, an igneous metal source would be consistent with previous hypotheses, as it has been suggested that the greenstone belts that form much of the surrounding terranes could provide a potential source of metalliferous mafic rocks from which ore metals could be liberated (Fox et al, 2019(Fox et al, , 2021.…”

“…However, there is no evidence of large-scale magmatic activity in the West Pilbara Superterrane that was both coeval with the formation of Carlow Castle (c. 2.95 Ga) and proximal to the deposit to provide an igneous driver of ore formation and source of metal (Fox et al, 2021). Nonetheless, an igneous metal source would be consistent with previous hypotheses, as it has been suggested that the greenstone belts that form much of the surrounding terranes could provide a potential source of metalliferous mafic rocks from which ore metals could be liberated (Fox et al, 2019(Fox et al, , 2021. This is congruent with the established association between mafic igneous rocks and significant hydrothermal Co deposits, including non-magmatic hydrothermal deposits (Hitzman et al, 2017).…”