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Fe Isotope Geochemistry of Hydrothermal Fe Exhalites
Author(s) -
SUN Jian,
ZHU Xiangkun,
LI Zhihong
Publication year - 2017
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.13173
Subject(s) - geology , hematite , hydrothermal circulation , geochemistry , sedimentary rock , mineralogy , basalt , seismology
The sediments atop the sequence of ophiolite usually contain Fe(‐Mn‐Si) exhalites, chemical sediments that are mainly composed of amorphous Fe‐Mn oxy‐hydroxides and chert/jasper. They were precipitated from hydrothermal fluids produced by deep leaching of basalt particularly during volcano activity or seafloor spreading. These hydrothermal Fe exhalites provide a good record for the depositional environment and the ocean environment as well. A well‐preserved Phanerozoic Fe deposit, Motuosala Fe‐Mn deposit, resulted from hydrothermal exhalation, was investigated for its trace element and Fe isotope geochemistry. The deposit is located in Xinjiang province, China and is hosted in a suit of Carboniferous volcano‐sedimentary clastic rocks. The Fe deposit is mainly composed of massive hematite Fe oreand banded hematite‐jasper ore. The hematite ore/band and jasper band were subjected to be analyzed. They are both composed mainly of Fe 2 O 3 and SiO 2 , with very low contents of Al 2 O 3 and TiO 2 (>1%), indicating they were chemical precipitates with little detrital contamination. They both show slightly LREE depleted or near flat PAAS‐normalised REE patterns, with positive Eu anomalies and Y anomolies, indicating that they were sourced from a mixture of high‐temperature fluids and seawater. Compared to the hematite Fe ore/band, the jasper band shows higher Eu SN /Eu SN * but lower Y/Ho values. δ 56 Fe values for the hematite Fe ores are clustered around −0.3‰, similar to those for high‐temperature fluids. The jasper samples show heavier Fe isotope compositions varying from −0.1‰ to 0.5‰, indicating that they were resulted from partial Fe precipitation. For all samples, δ 56 Fe values are related to Y/Ho and Eu SN /Eu SN * values. The results indicate that the hematite Fe ore and jasper were deposited in different environments. The jasper was deposited in a more anoxic condition with higher hydrothermal fluids/seawater ratio, probably when the hydrothermal activity was more intense; while the hematite Fe ore was deposited in a more oxic condition with lower hydrothermal fluids/seawater ratio, probably when the hydrothermal activity was weaker.