Genesis of a giant Paleoproterozoic strata-bound magnesite deposit: Constraints from Mg isotopes

Abstract

Giant strata-bound magnesite deposits are absent in modern and most Phanerozoic sedimentary environments but occur predominantly in Precambrian strata. These deposits may have formed directly through precipitation of evolved Mg-rich seawater in an evaporative shallow-marine setting or, alternatively, by epigenetic-hydrothermal replacement of the Mg-rich carbonate precursor. To test these hypotheses, we obtained the first Mg isotope data from the world's largest strata-bound magnesite deposit belt, hosted by the ca. 2.1 Ga Dashiqiao Formation in Northeast China. The Mg isotope compositions (δ 26 Mg) of most magnesite ores in the Huaziyu deposit are heavier (-0.75 ± 0.26‰) than most Proterozoic sedimentary dolomite. The Mg isotope compositions and 2 major and trace element data indicate that the magnesites are probably not of hydrothermal origin. Instead, a Mg-rich carbonate precursor precipitated from evaporating seawater in a semi-closed system. Diagenetic brines altered the Mg-rich carbonate precursor to magnesite. Subsequently, recrystallization during regional metamorphism produced coarsely crystalline and saddle magnesite. These interpretations are consistent with the geological features and other geochemical data (element concentrations and C and O isotopes) for the magnesite ores. Hence, we interpret the formation of the Huaziyu magnesite deposit to be dominated by evaporative sedimentation and brine diagenesis.

Stable Isotope (S, Mg, B) Constraints on the Origin of the Early Precambrian Zhaoanzhuang Serpentine-Magnetite Deposit, Southern North China Craton

“…The marlstone data are from [121]; the limestone data are from [50,121], the normal marine dolostone data are from [50,119,120]; and all of the other dolostone data are from [119]. The Huaziyu magnesite and dolomitic marble data are from [145]. The igneous rock data are from [60,61,141,143].…”

Petrological and Geochemical Constraints on the Protoliths of Serpentine‐Magnetite Ores in the Zhaoanzhuang Iron Deposit, Southern North China Craton

“…The other low-Ti picritic or basaltic magma could also evolve into low-Ti iron oxide deposit via crystallized differentiation or liquid immiscibility, such as the low-Ti iron oxide deposits in the Emeishan large igneous province (ELIP) related to low-Ti basalts and grabbroic intrusions . High-Mg sediments on the earth are often associated with seawater carbonate precipitation, such as Mg-rich borate deposits in the Lieryu Formation of China, which may have formed directly through precipitation of Mg-rich seawater or diagenesis via Mg-rich brine in an evaporative sedimentary environment (Chen Congxi et al, 2003;Hu Guyue et al, 2014Dong et al, 2016Dong et al, , 2017 and magnesite deposit in other countries (Pohl, 1990;Melezhik et al, 2001;Frank and Fielding, 2003;Henjes-Kunst et al, 2014 (Arora et al, 1995;Xu et al, 2014). And furthermore, the low-Ti iron ore deposit derived from this low-Ti basaltic magma was considered to be associated with liquid immiscibility of low-Ti Fe-rich melts, generally forming apatite-magnetite ores , rather than Mg-rich magnetite ores from Zhaoanzhuang iron deposit.…”

“…Similar Mg-rich and Ti-poor iron deposits also occur in other areas of the North China Craton, including the Lilaozhuang magnesite-magnetite deposit hosted in the Neoarchean-Paleoproterozoic Huoqiu Group (Huang Hua et al, 2013), and the Lieryu boron-magnesite-magnetite deposit (Feng Benzhi et al, 1995;Sun Houjiang and Wu Chunlin, 1996;Dong Aiguo, 2016) hosted in the Paleoproterozoic Liaohe Group (Fig. And Lieryu boron-magnesite-magnetite deposit located in the northern margin of the NCC, has been considered to be derived from Mg-rich marine carbonate rock that had experienced evaporative process (Hu Guyue et al, 2014Dong et al, 2016Dong et al, , 2017. The origin of the Lilaozhuang magnesite-magnetite deposit is debated, with recent studies assigning a similarity with BIF (generally pronounced positive Eu anomalies and depletion of light REE and enrichment of heavy REE), which was formed in a confined oceanic basin along continental margin (Huang et al, 2017).…”

“…Therefore, the high-Mg/low-Ti nature of the Zhaoanazhuang iron ore is inconsistent with maficultramafic intrusion-associated iron ores, Fe-Mg-Si-rich sedimentary sequence as a protolith can well fit the features above. High-Mg sediments on the earth are often associated with seawater carbonate precipitation, such as Mg-rich borate deposits in the Lieryu Formation of China, which may have formed directly through precipitation of Mg-rich seawater or diagenesis via Mg-rich brine in an evaporative sedimentary environment (Chen Congxi et al, 2003;Hu Guyue et al, 2014Dong et al, 2016Dong et al, , 2017 and magnesite deposit in other countries (Pohl, 1990;Melezhik et al, 2001;Frank and Fielding, 2003;Henjes-Kunst et al, 2014). The primitive mantlenormalized trace element spider diagrams (Figs.…”

“…The origin of the Lilaozhuang magnesite-magnetite deposit is debated, with recent studies assigning a similarity with BIF (generally pronounced positive Eu anomalies and depletion of light REE and enrichment of heavy REE), which was formed in a confined oceanic basin along continental margin (Huang et al, 2017). And Lieryu boron-magnesite-magnetite deposit located in the northern margin of the NCC, has been considered to be derived from Mg-rich marine carbonate rock that had experienced evaporative process (Hu Guyue et al, 2014Dong et al, 2016Dong et al, , 2017. Tectonically, the Zhaoanzhuang iron deposit and Lilaozhuang magnesite-magnetite deposit belong to the same metallogenic belt located in the southern part of the NCC and the ore-bearing strata are also comparable , possibly revealing their similar protolith.…”