A multi-isotope approach towards constraining the origin of large-scale Paleoproterozoic B-(Fe) mineralization in NE China
A multi-isotope approach towards constraining the origin of large-scale Paleoproterozoic B-(Fe) mineralization in NE China
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限制中国东北地区古元古代大规模 B-(Fe) 矿化起源的多同位素方法
DOI：
10.1016/j.precamres.2017.01.030
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发表时间：
2017-05
影响因子：
3.8
通讯作者：
Chao Tang
中科院分区：
文献类型：
--
作者：
Brian Kendall;Shizhen Li;Yue Wang;Chao Tang
硼酸盐矿床主要产于古生代蒸发沉积环境中，而在前寒武纪地层中则很少。然而，在中国东北地区的古元古代地层中，存在着大量的块状富B和镁硼矿床。此外，这些硼酸盐沉积物中的几种以Fe为主（例如，翁泉沟存款中Fe_2 O_3含量>60%。为了确定这些异常矿床的成因，我们获得了中国东北富镁的后仙峪硼酸盐存款和富铁的翁泉沟硼酸盐存款的围岩和矿石的B、Fe和Mg同位素数据。硼矿床的δ 11 B值（10.7 ± 4.4‰，n = 15）高于绝大多数火成岩和非蒸发沉积岩，表明B为蒸发沉积成因。硼矿床δ 56 Fe值在0‰附近（0.05 ± 0.18‰，n = 24），与岩浆岩相似，属岩浆成因。矿石及伴生岩石的δ 26 Mg值（−0.52 ± 0.34‰，n = 24）介于富镁碳酸盐岩和火成岩之间，表明镁的成因为沉积-岩浆混合成因。区域变质作用和强烈的变形作用通过促进火成岩和蒸发岩矿物的交代蚀变而显著改变了矿床，从而导致岩浆源和沉积蒸发源的混合以及沉积硼酸盐前体重结晶为变质硼酸盐矿物。因此，B、Fe、Mg同位素数据和地质矿物学观察表明，东北地区古元古代B-（Fe）矿床的形成经历了蒸发沉积富集B、Mg、火山作用引入Fe和Mg、区域变质/交代作用改造3个阶段。
Borate ore deposits occur predominantly in Phanerozoic evaporative sedimentary environments but are scarce in Precambrian strata. However, massive B- and Mg-rich borate deposits are abundant in the Paleoproterozoic strata of Northeast (NE) China. In addition, several of these borate deposits are dominated by Fe (e.g., >60% Fe2O3content in the Wengquangou deposit). To constrain the origin of these unusual deposits, we obtained B, Fe, and Mg isotope data on the wall rocks and ores of the Mg-rich Houxianyu borate deposit and the Fe-rich Wengquangou borate deposit in NE China. The δ11B values of the borate deposits (10.7 ± 4.4‰, n = 15) are higher than most types of igneous and non-evaporative sedimentary rocks, suggesting that B is of evaporative sedimentary origin. However, the borate deposits have a limited range of δ56Fe values near 0‰ (0.05 ± 0.18‰, n = 24), which is similar to igneous rocks and thus points to a magmatic origin for the Fe. The δ26Mg values of the ores and associated rocks (−0.52 ± 0.34‰, n = 24) are intermediate between Mg-rich carbonate rocks and igneous rocks, suggesting a mixed sedimentary and magmatic origin for the Mg. Regional metamorphism and intense deformation modified the deposits significantly by promoting metasomatic alteration of the igneous rocks and evaporite minerals, thus resulting in mixing of magmatic and sedimentary-evaporative sources as well as recrystallization of the sedimentary borate precursors to metamorphic borate minerals. Hence, the B, Fe, and Mg isotope data together with geological and mineralogical observations indicate that formation of the Paleoproterozoic B-(Fe) deposits in NE China involved a three-stage process: enrichment of B and Mg by evaporative sedimentation, introduction of Fe and additional Mg by volcanism, and modification by regional metamorphism/metasomatism.
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