C–O isotope geochemistry of the Dashiqiao magnesite belt, North China Craton: implications for the Great Oxidation Event and ore genesis

Abstract

The worldwide 2.33−2.06 Ga unique positive δ13Ccarb excursion has been correlated with the Great Oxidation Event (GOE). The Dashiqiao Formation in the Liaohe Group of the northeastern North China Craton formed at 2.2−2.174 Ga and hosts one of the world‐class magnesite deposits. Here we present major element and C and O isotope analyses of 22 samples from the Dashiqiao Formation and use the data to evaluate the impact of the GOE in the North China Craton, as well as the genesis of the Dashiqiao giant magnesite deposits. Six dolomitic marble samples from a ~600 m thick interval with 1.10 ± 0.04 of MgO/CaO (mol) ratios show higher δ13CPDB values of 0.6−1.4‰ (average 1.2 ± 0.3‰) than those of normal marine carbonates over the globe. However, they display lower δ18OSMOW of 16.4−19.5‰ (average 18.2 ± 1.1‰) as compared to their contemporaneous counterparts, suggesting that the primary carbonates in the Dashiqiao Formation should possess a positive δ13C anomaly (possibly 4.2‰) reflecting the impact of the GOE, and that the δ13C and δ18O values have been depleted in post‐sedimentation diagenesis and/or regional metamorphism. The >550 m thick magnesite layer in the studied section has MgO/CaO ratios ranging from 4.45−200.00. These rocks show δ13C and δ18O values of 0.1−0.9‰ and 9.2−16.9‰, with average values of 0.4 ± 0.2‰ and 13.3 ± 2.5‰, respectively, obviously lower than those of the underlying dolomites. The depletions of 13C and 18O in magnesites relative to dolomitic marbles are interpreted to be the result of hydrothermal alteration related to regional metamorphism leading to rock recrystallization and mass exchange. This interpretation is further confirmed from the hanging‐wall dolomitic marble and the veinlet‐filled magnesite from the ore layer. The former contains mega‐crystals of cylindrical talc and has δ13C of −2.6‰ and δ18O of 14.1‰, indicating that a local fluid–rock interaction between (argillaceous) dolomite and (siliceous) hydrothermal fluids poor in 13C and 18O resulted in the formation of talc and further depletion both in δ13C and in δ18O. The veinlet‐filled magnesite yields δ13C and δ18O values of −2.7‰ and 16.2‰, respectively, showing lower δ13C but higher δ18O than those of massive magnesite in the adjacent strata. Our observation thus strongly supports the interpretation that the massive magnesite interacted with low‐δ13C fluids which were possibly sourced from meteoric water at low temperature during post‐ore time. Thus, the formation of the Dashiqiao magnesite deposits involved primary sedimentation, diagenesis, regional metamorphism, hydrothermal replacement and local post‐ore fluid–rock interaction. Copyright © 2013 John Wiley & Sons, Ltd.

Ore geology, fluid inclusion and isotope geochemistry of the Xunyang Hg–Sb orefield, Qinling Orogen, Central China

“…In Figure , the δ 13 C and δ 18 O values of the carbonates from the Xunyang orefield show an obviously positive correlation ( r = 0.85). There are three possible mechanisms for this phenomenon (Zheng, , ; Peng and Hu, ): (1) fluid mixing; however, such mechanism is rare in nature (Zheng, ); (2) CO 2 degassing or decarbonation, which has a significant impact on δ 13 C, but little on δ 18 O (Zheng, ; Tang et al ., , , , b); and (3) water–rock interaction, characterized by carbonate dissolution. We prefer that the last interpretation at a high fluid/rock ratio (Tang et al ., , , ) resulted in a positive correlation between δ 13 C and δ 18 O in the orefield and list the evidences as below: (i) both the δ 13 C and δ 18 O ratios show a decreasing trend from carbonate strata through M‐stage hydrothermal calcites to L‐stage calcite veins (Table ; Fig.…”

“…There are three possible mechanisms for this phenomenon (Zheng, , ; Peng and Hu, ): (1) fluid mixing; however, such mechanism is rare in nature (Zheng, ); (2) CO 2 degassing or decarbonation, which has a significant impact on δ 13 C, but little on δ 18 O (Zheng, ; Tang et al ., , , , b); and (3) water–rock interaction, characterized by carbonate dissolution. We prefer that the last interpretation at a high fluid/rock ratio (Tang et al ., , , ) resulted in a positive correlation between δ 13 C and δ 18 O in the orefield and list the evidences as below: (i) both the δ 13 C and δ 18 O ratios show a decreasing trend from carbonate strata through M‐stage hydrothermal calcites to L‐stage calcite veins (Table ; Fig. ), which indicates that carbonate minerals in the Xunyang orefield were subjected to a remarkably hydrothermal alteration, especially, by meteoric water (Tang et al ., , ); (ii) during the ore‐forming process, the carbonate strata in the Xunyang orefield show a remarkable δ 18 O decrease by >10‰ and a narrow δ 13 C variation of −1.6‰ to 2.0‰ (Table ; Fig.…”

“…We prefer that the last interpretation at a high fluid/rock ratio (Tang et al ., , , ) resulted in a positive correlation between δ 13 C and δ 18 O in the orefield and list the evidences as below: (i) both the δ 13 C and δ 18 O ratios show a decreasing trend from carbonate strata through M‐stage hydrothermal calcites to L‐stage calcite veins (Table ; Fig. ), which indicates that carbonate minerals in the Xunyang orefield were subjected to a remarkably hydrothermal alteration, especially, by meteoric water (Tang et al ., , ); (ii) during the ore‐forming process, the carbonate strata in the Xunyang orefield show a remarkable δ 18 O decrease by >10‰ and a narrow δ 13 C variation of −1.6‰ to 2.0‰ (Table ; Fig. ), which was not merely caused by decarbonation that usually has a significant impact on δ 13 C, but little on δ 18 O (Zheng, ; Tang et al ., , b); and (iii) the δ 13 C values gradually turned negative in L‐stage, which could have resulted from a decrease of temperature (Zheng and Chen, ; Tang et al ., , ).…”

“…Considering that ore‐hosting rocks of the Xunyang Hg–Sb orefield and the Neoproterozoic–Triassic sequences in Southern Qinling are composed of carbonates, carbonaceous phyllites, carbonaceous slates, siltstones and siliceous rocks, which fairly match up with the source lithologies of the fluids, it can be inferred that the ore‐forming fluids were sourced from the Neoproterozoic–Triassic sequences. As already known, dissolution of carbonate strata could liberate 13 C‐enriched CO 2 into fluids (Tang et al ., , and references therein), whereas epizonogenic‐to‐metamorphic decomposition of organic matter would release 13 C‐poor CO 2 or CH 4 into fluids. Consequently, the δ 13 C values of −6.1‰ to −1.7‰ of the fluids can be sourced from the stratigraphic carbonates, mixing with those from reservoirs, such as organic matter or atmospheric CO 2 .…”

Geochronology and geochemistry of the Nanfen iron deposit in the Anshan-Benxi area, North China Craton: Implications for ∼2.55Ga crustal growth and the genesis of high-grade iron ores

“…The data for the Nanfen BIF and Nanfen high-grade iron ore follow Li et al (2008) and Li et al (2012). of submarine high-T hydrothermal fluids (Bau and Dulski, 1999;Douville et al, 1999;Tang et al, 2009Tang et al, , 2013a. The SiO 2 -Al 2 O 3 and (Co + Ni + Cu) − REY diagrams indicate that the Nanfen BIF has a genetic link with submarine hydrothermal activity ( Fig.…”