TY - JOUR
T1 - Insights into salty metamorphic fluid evolution from scapolite in the Trans-North China Orogen
T2 - Implication for ore genesis
AU - Qiu, Zhengjie
AU - Fan, Hong Rui
AU - Tomkins, Andrew G.
AU - Brugger, Joël
AU - Etschmann, Barbara
AU - Liu, Xuan
AU - Xing, Yanlu
AU - Hu, Yi
PY - 2021/1/15
Y1 - 2021/1/15
N2 - Scapolite is abundant in medium to high grade metamorphic rocks with evaporitic calc-silicate bulk compositions, and because it can be Cl− and SO42− 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. S6+/∑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 fO2) 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.
AB - Scapolite is abundant in medium to high grade metamorphic rocks with evaporitic calc-silicate bulk compositions, and because it can be Cl− and SO42− 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. S6+/∑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 fO2) 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.
KW - Cl/Br ratios
KW - Deep fluid
KW - S-XANES analysis
KW - Scapolite
KW - Syn-orogenic hydrothermal ore formation
UR - http://www.scopus.com/inward/record.url?scp=85096721078&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2020.10.030
DO - 10.1016/j.gca.2020.10.030
M3 - Article
AN - SCOPUS:85096721078
SN - 0016-7037
VL - 293
SP - 256
EP - 276
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -