Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

Abstract

Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial melting of the asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether similar metallogenic processes also operated in the Precambrian remains obscure. Here we address the issue by investigating the origin, fO2, and S contents of calc-alkaline plutonic rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that the ca. 1886–1881 Ma ore-forming magmas, originated from a mantle-dominated source with minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalite-magnetite-quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized, sulfur-rich arc magma associated with porphyry Cu mineralization already existed in the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments from the subducted oceanic lithosphere at that time.

Mélange Signatures and Low Oxygen Fugacity in Eclogite Xenoliths From the Crust‐Mantle Transition Below a Mesoproterozoic Collision Belt

“…This precludes a role for the transfer of sulfate from the subduction-modified oceanic crust in oxidizing the overlying lithosphere, at least during formation of the 1.2-1.0 Ga NNFB, and is consistent with findings that the paucity of oxidized magmas prior to Neoproterozoic deep ocean oxygenation reflects the reduced nature of slab-derived components (Evans & Tomkins, 2011). 2022)), range of estimates for modern oceanic crust (MORB) bracketed by Frost and McCammon (2008) and Nicklas et al (2019), range of modern arc magmas (Evans et al, 2012), intra-oceanic arc crust at 1 GPa (Bucholz & Kelemen, 2019), Neoproterozoic arc crust (Zhang et al, 2017), Paleoproterozoic arc crust (Meng et al, 2021) and orogenic eclogite under prograde metamorphic conditions (Tao et al, 2020). Average fully propagated 1σ uncertainties on Fe 3+ /ΣFe and ∆logƒO 2 (FMQ) from Table 1 and on depth assuming a 100°C uncertainty in temperature along a conductive geotherm are shown as error bars in (c) and (d).…”

“…Fraction of oxidized sulfur (S 6+ /ΣS) in melts at 1.5 GPa and 950°C, similar to conditions recorded by deep crustal eclogites, from Matjuschkin et al (2016). Shown for comparison are Kaapvaal eclogite xenoliths from the literature (database in Aulbach et al (2022)), range of estimates for modern oceanic crust (MORB) bracketed byFrost and McCammon (2008) andNicklas et al (2019), range of modern arc magmas(Evans et al, 2012), intra-oceanic arc crust at 1 GPa(Bucholz & Kelemen, 2019), Neoproterozoic arc crust(Zhang et al, 2017), Paleoproterozoic arc crust(Meng et al, 2021) and orogenic eclogite under prograde metamorphic conditions(Tao et al, 2020). Average fully propagated 1σ uncertainties on Fe 3+ /ΣFe and ∆logƒO 2 (FMQ) from Table1and on depth assuming a 100°C uncertainty in temperature along a conductive geotherm are shown as error bars in (c) and (d).…”

Linking the Jehol Biota Evolution to the Early Cretaceous Volcanism During the North China Craton Destruction: Insights From F, Cl, S, and P

“…To calculate melt S contents, we used the formula $${\mathrm{S}}_{ap}\,(\text{wt. }\%)\,=\,{D}_{S}^{ap-m}\left({f\,\mathrm{O}}_{2}\right)\,\times \,{\mathrm{S}}_{melt}\,\times \,\left[{D}_{S}^{ap-m}(\text{AST})/{D}_{S}^{ap-m}(1,000{\degree}\mathrm{C})\right]$$ (Meng et al., 2021), in which $${D}_{S}^{ap-m}\left({f\,\mathrm{O}}_{2}\right)$$, $${D}_{S}^{ap-m}(\text{AST})$$, and $${D}_{S}^{ap-m}(1,000{\degree}\mathrm{C})$$ can be acquired from experimental data of Konecke et al. (2019) and Parat and Holtz (2004).…”

“…Due to the absence of well‐preserved melt inclusions in our ancient (>100 Myr) continental volcanic samples, it is not appropriate to use the first two methods to estimate their original volatile contents. In this paper, we quantified the pre‐eruptive S, Cl, and F contents in melts by combining the measured volatile concentrations in clinopyroxene (cpx) phenocrysts and apatite inclusions in phenocrysts with experimentally determined partition/exchange coefficients between minerals and melts (Baker et al., 2022; Callegaro et al., 2014; Dalou et al., 2014; H. J. Li & Hermann, 2017; Meng et al., 2021). The advantages of this method are the following: (a) Cpx and apatite are widely developed and crystallized early in the samples; (b) apatite inclusions are not susceptible to volatile diffusion, crystallization, or bubble growth after entrapment, and can effectively record the pre‐eruptive volatile contents of magmas (Stock et al., 2016); (c) the analytical methods to determine F, Cl, and S contents in cpx and apatite are robust.…”

“…To calculate melt S contents, we used the formula (1, 000°C) can be acquired from experimental data of Konecke et al (2019) and Parat and Holtz (2004). This method was chosen because it considers the effects of both temperature and oxygen fugacity, and deviations between the measured and calculated results are limited (Meng et al, 2021). Because of the non-Nernstian partitioning behavior of F and Cl between apatite and melt (Boyce et al, 2014), we employed the exchange partition coefficients of F, Cl, and OH between apatite and melt to quantify the volatile contents in the equilibrated melts.…”