Insights into salty metamorphic fluid evolution from scapolite in the Trans-North China Orogen: Implication for ore genesis. (15th January 2021)

• Record Type: Journal Article
• Authors: Qiu, Zhengjie; Fan, Hong-Rui; Tomkins, Andrew; Brugger, Joël; Etschmann, Barbara; Liu, Xuan; Xing, Yanlu; Hu, Yi
• Is Part Of: Geochimica et cosmochimica acta. Volume 293(2021)
• Page Start: 256
• Page End: 276
• Publication Date: 2021-01-15
• Subjects: Scapolite -- Cl/Br ratios -- S-XANES analysis -- Deep fluid -- Syn-orogenic hydrothermal ore formation; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.10.030
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: Rock and fluid buffering scapolite formation conditions are identifiable. Br/Cl ratio of scapolite is a good tracer of deep fluid source and evolution. Oxidized, S-rich, salty metamorphic fluid buffered by scapolite-rich rocks. Evaporite-bearing orogenic belt facilitate for the base metal formation. Abstract: Scapolite is abundant in medium to high grade metamorphic rocks with evaporitic calc-silicate bulk compositions, and because it can be Cl − and SO4 2− rich, is able to provide a unique view into the evolution of deep brines during orogenic cycles. In the Zhongtiao Mountains, part of the Trans-North China Orogen (TNCO), scapolite occurs in two distinct mineralogical associations; scapolite-I in carbonate-bearing biotite schist and scapolite-II in amphibolites. Initially, metamorphic Cl-rich scapolite-I formed in the schist at low fluid-rock ratio and peak pressure (meionite content = 32–37%, Cl = 2.34–2.75 wt.%). This observation, together with high molar Cl/Br ratios (Cl/Br = 973–2236), implies a halite-bearing evaporitic protolith. During a second stage of scapolite formation in the calc-silicates, rims overgrew scapolite-I with distinct compositions (Me% = 31–32, Cl = 2.64–3.0 wt.%, Cl/Br = 408–892). The lower molar Cl/Br ratios in scapolite-I rims imply that the later fluid was distinctly different to the earlier metamorphic fluid, and may have evolved through a stage of halite saturation induced by phase separation. Halite saturation would drive Br enrichment in the remaining fluid because Cl is preferentially sequestered in halite. Also at this time, amphibolites were subjected to intense open system fluid-rock interactions resulting in formation of scapolite-II (Me% = 30–32, Cl = 2.74–2.97 wt.%, Cl/Br = 291–1426), similar to that of the scapolite-I. S 6+ /∑S ratios in scapolite obtained via synchrotron XANES indicate that equilibration between fluid and scapolite would buffer the fluid to oxidized conditions, with oxygen fugacity (log f O2 ) beyond −15. The scapolitic rocks in this region were thus able to play a key role in controlling the salinity and oxidation state of the deep metamorphic fluid, thereby creating enhanced potential for scavenging of a range of metals within the metamorphic terrane and promoting syn-orogenic hydrothermal ore formation.