Genesis of Precambrian Strata‐bound Magnesite Deposit in NE China

minerals in the earth, and only two types of magnesite deposits present high economic signatures for the refractory industry (Pohl, 1990). One type of magnesite deposit is associated with ultrabasic rocks. The other is characteristic of giant strata-bound body, consisting of coarsely crystalline pure magnesite, which accounts for 90% of the economical magnesite deposits. Detail investigations have been carried out during last century, but the genesis of the strata-bound magnesite deposits is still in debate: epigenetic or sedimentary (Aharon, 1988; Pohl, 1990). The epigenetic genesis is suggested that the magnesite is formed by the enrich magnesium incursive fluid replacing the dolomite and calcite. On the contrary, the sedimentary view is suggested that the magnesite precipitates from the seawater and the formation of the magnesite occurres with precipitation from seawater in an evaporated environment or early diagenesis process. Generally, the magnesite could deposit in the modern evaporated environment such as sabkha and saline lakes (Melezhik et al., 2001). However, there are several significant differences (oxygen rations, the gypsum, the amount of magnesite deposit) between the magnesite in modern evaporated environment and the Precambrian period. Thus, the genesis of such huge and strata-bound magnesite deposit occurring in Precambrian era, especially in Protorozoic era, seems to be a problem (so called “magnesite problem”) (Aharon, 1988; Frank and Fielding, 2003). Here we use the stable isotopes (C, O, Mg) to study the genesis of the Huaziyu magnesite deposit. The Huaziyu magenesite deposit is one of the important mine in the magnesite deposit belt from Dashiqiao to Haicheng, NE China. The magneiste deposit is in the third section of the Dashiqiao Formation with the age at about 2.1 Ga (Zhang, 1988). The metamorphism and transformation event (ca. 1.9 Ga) and regional thermodynamic metamorphism (ca. 1.8 Ga) significantly affect the magnesite deposit and the metamorphic grade is from greenschist facies to amphibolite facies. The magnesite is the major mineral for the ore bed of the magnesite deposit, and Dolomite, quatze, pyrite, clinochlore are less presented. Most of the magnesite are recrystallized significantly with a seriate blastic texture and a larger grain size (about several to tens of millimeters), indicating an intensive recrystallization. The δCPDB value of the magnesite and surrounding dolomite range from −0.4 to 1.2‰, with an mean value about 0.6‰. These value are overlap with average δCPDB values of Phanerozoic marine limestones (−2.0 to 6.0‰) (Veizer and Hoefs, 1976), and are identical with the carbonate from the modern plaza or the sbhka. Moreover, the difference of the δCPDB between our results and the ultramafic-hosted magnesite on Margarita Island, Venezuela (δCPDB about −10‰) suggests that the carbon of our magnesite could be not from the decarboxylation of the organic carbon in the fluid metasomatism process (Abu-Jaber and Kimberley, 1992). The δOSMOW value of the magnesite and surrounding dolomite range from 6 to 18‰, with a mean value about 11.9‰. These δOSMOW value are consistent with the value of the sedimentary magnesite and carbonate, but differ from the value from the carbonate (δOSMOW is about 30‰ ) from the evaporation environment (Melezhik et al., 2001). All of these indicate that the formation of the magnesite may be related to the sedimentary process. Generally, the δMgDSM3 value of the modern seawater is about −0.83‰ and the value of the magmatite and the pyrolite present a heavier isotope composition (about −0.25‰). But, the value of the carbonate is significant different from seawater, with a board range (−5.3‰ to −1‰) and more negative value. A general order of the DONG Aiguo, ZHU Xiangkun, LI Shizhen, WANG Yue and GAO Zhaofu, 2014. Genesis of Precambrian Strata-bound Magnesite Deposit in NE China. Acta Geologica Sinica (English Edition), 88(supp. 2): 1559-1560.