Mineralization and magmatic origin of the 2.1 Ga Zhaoanzhuang magnetite-apatite deposit, North China Craton: Constraints on the formation of the Paleoproterozoic iron oxide-apatite deposits
Mineralization and magmatic origin of the 2.1 Ga Zhaoanzhuang magnetite-apatite deposit, North China Craton: Constraints on the formation of the Paleoproterozoic iron oxide-apatite deposits
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DOI：
10.1016/j.precamres.2022.106839
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发表时间：
2022-09
影响因子：
3.8
通讯作者：
Caiyun Lan;X. Long;T. Zhao;M. Zhai
中科院分区：
文献类型：
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作者：
Caiyun Lan;X. Long;T. Zhao;M. Zhai
氧化铁磷灰石矿床为世界范围内的钢铁工业提供了大量的铁，但其来源一直存在争议。虽然IOA矿床通常与中基性岩浆岩有关，但在超基性岩浆岩中很少发现IOA矿床。华北克拉通昭安庄铁矿以富含低钛磁铁矿、磷灰石和超镁铁性矿物（如橄榄石、正辉石）为特征，其成因一直存在争议。这些超镁铁质矿物主要以磁铁矿包裹的集合体形式赋存于铁矿石中，具有典型的堆积状结构和高Fo值，表明其岩浆成因。矿石中磁铁矿的δ56Fe值在0.22±0.03‰~ 0.32±0.03‰之间，δ18O值在2.6‰~ 4.5‰之间，与岩浆岩和IOA矿床相似，与变质ib矿床不同。超镁质集合体和铁矿中的磷灰石均含2.07 ~ 2.69 wt%的F，小于0.65 wt%的Cl，高达7900 ppm的稀土元素（REE）， Sr-Nd同位素组成变化很大，反映了两类磷灰石均为岩浆热液成因。所有磷灰石颗粒的地壳样εNd(t)值（-7.1 ~ - 5.2）表明它们的Sm-Nd体系发生了重置，这是由碳酸盐流体的参与造成的。超镁铁质团聚体和铁矿中磷灰石的原位Sm-Nd同位素分析结果显示，两者的Sm-Nd等时线年龄相近，分别为2100±81 Ma （MSWD = 0.84）和2096±240 Ma (MSWD = 0.85)，可以限定铁成矿的最小年龄。结合地层的沉积年龄，确定矿床成矿年龄为2.3 ~ 2.1 Ga。矿石中超镁铁质团聚体（-1.2 ~ - 1.0）、铁矿（-1.8 ~ - 1.5）和磁铁矿（-1.6 ~ - 0.8）的εNd(t)值均为负，表明成矿岩浆具有微富集特征。结合它们的εNd(t)值与MgO含量的线性关系，认为卷入的幔源超基性岩浆在上升过程中极有可能受到地壳物质的污染。因此，昭安庄矿床是古元古代与超镁铁质岩浆伴生的罕见IOA矿床的良好案例。
Iron oxide-apatite (IOA) deposits provide large amounts of iron for steel industry worldwide, but their origins are still in dispute. Although IOA deposits were commonly genetically related to intermediate-mafic igneous rocks, the occurrences of IOA deposits in ultramafic rocks are scarce. The Zhaoanzhuang iron deposit in the North China Craton is characterized by comprising abundant low-Ti magnetite, apatite and ultramafic minerals (e.g., olivine, orthopyroxene), but its origin has long been controversial. These ultramafic minerals mainly occur as aggregates surrounded by magnetite in the iron ores and display typical cumulate textures and high Fo values, indicative of magmatic origin. The δ56Fe and δ18O values of magnetite from the ores range from 0.22 ± 0.03‰ to 0.32± 0.03‰ and from 2.6‰ to 4.5‰, respectively, akin to those in magmatic rocks and IOA deposits but distinct from those of metamorphic BIFs. Both types of apatite occurring in the ultramafic aggregates and the iron ores contain 2.07 to 2.69 wt% F, less than 0.65 wt% Cl, up to 7900 ppm rare earth elements (REE) and greatly variable Sr-Nd isotopic compositions, reflecting that both types are of magmatic-hydrothermal origin. The crust-like εNd(t) values (-7.1 to −5.2) of all apatite grains record reset of their Sm-Nd system, which is resulted from the involvement of carbonate fluids based on their similar REE patterns. In situ Sm-Nd isotope analyses of apatite from the ultramafic aggregates and the iron ores have yielded similar Sm-Nd isochron ages at 2100 ± 81 Ma (MSWD = 0.84) and 2096 ± 240 Ma (MSWD = 0.85), respectively, which can constrain the minimum age for the Fe mineralization. Combined with sedimentary age of the strata, the ore-forming age of the deposit was 2.3–2.1 Ga. The negative εNd(t) values of the ultramafic aggregates (-1.2 to −1.0), the iron ores (-1.8 to −1.5) and magnetite (-1.6 to −0.8) from the ores indicate that the ore-forming magma has a slightly enriched signature. Combined with linear relationship between their εNd(t) values and MgO contents, an involved mantle-derived ultramafic magma is most likely to be contaminated by the crustal material during ascending. Therefore, the Zhaoanzhuang deposit is a good Paleoproterozoic case of the rare IOA deposits associated with ultramafic magmas.