Mineral redox buffer in ore forming processes – insights from scapolite

Hamisi, Jonathan1,2; Etschmann, Barbara1; Micklethwaite, Steven 1; Tomkins, Andrew 1; Pitcairn, Iain 2; Wlodek, Adam 3; Morrissey, Laura4; Brugger, Joël1

1School of Earth, Atmosphere & Environment, Monash University, Melbourne, Australia, 2Department of Geological Sciences, Stockholm University, Stockholm 106-91, Sweden, 3Department of Mineralogy, Petrography and Geochemistry, AGH-University of Science and Technology, Kraków 30-059, Poland, 4School of Natural and Built Environments, University of South Australia, Adelaide, Australia

Metal transports in ore forming fluids is highly dependent on pH, ligands species (S, Cl, F) and redox conditions. The scapolite group is a family of tetragonal aluminosilicate consisting of meionite (Me; Ca4Al6Si6O24CO3), marialite (Ma; Na4Al3Si9O24Cl) and silvialite (Si; (Ca,Na)4Al6Si6O24(SO4,CO3), respectively rich in [CO3]2-, Cl, and [SO4]2-. During fluid/rock interaction occurring during mineralising process of scapolite bearing terranes, scapolite breakdown  or crystallization releases into the fluids its volatile components. We analysed  a set of 17 scapolite samples sourced from various geological context (metamorphic terranes hosting iron-oxide copper and gold deposits (IOCG), skarns deposits, and scapolite placers). Scanning Electron Microscope and Electron Probe Micro Analyser results show that our sample set contains S as SO3 up to 1.29 wt% (n=215) and Cl up to 3.68 wt% (n=215). In Ca-rich pelite scapolite coexists with graphite and in lesser extent traces of pyrite and has typically a low S concentration. Scapolite hosted in calcsilicates rocks has typically higher S concentration and coexists with pyrite minor chalcopyrite, hematite and/or magnetite and little to no graphite. Sulfur K-edge (2472 eV) X-ray Absorption Near Edge Structure (XANES) spectra collected on the samples provided evidence of the coexistence of several form of S species in scapolite in the form of oxidized S (S6+, S4+) and reduced S (S2-, S, as well as polysulfides). Using the spectra intensity, we evaluate qualitatively the ratio ΣS oxidized/Σ total S (ΣS oxidized + ΣS reduced). Our results show that variation of the ΣS oxidized/Σ total S ratio can be used to trace redox conditions prevailing during scapolite breakdown or crystallisation. Ca-rich graphite bearing-pelite have a lower ΣS oxidized/Σ total S ratio compared to calcsilicates rocks providing evidence that Ca-rich graphite bearing-pelite will typically produce a reduced fluid during devolatilization while calcsilicates rocks will produced a more oxidized fluid as it contains a higher content of oxidized S. As scapolite crystalizes (sink for S) or devolatilizes (source for S) depending on whether it contains more reduced S than oxidised S or vice versa, the volatile released to the fluids will buffer the redox conditions of the fluids, in the case of mineralising brines, this will result in changes of the fluids chemistry to ideal composition for metal transport, given that the fluids will be very reactive. Therefore, scapolite may act as a buffer for redox conditions of the fluids. 


Jonathan Hamisi is a PhD student at Monash University. His current research focuses on scapolite as a sources for ligands in ore forming process and albitisation as mechanism for mobilizing metals in IOCG systems.

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