In-situ trace element and Fe-isotope studies on magnetite of the volcanic-hosted Zhibo and Chagangnuoer iron ore deposits in the Western Tianshan, NW China
In-situ trace element and Fe-isotope studies on magnetite of the volcanic-hosted Zhibo and Chagangnuoer iron ore deposits in the Western Tianshan, NW China
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DOI：
10.1016/j.chemgeo.2017.02.001
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发表时间：
2017-03
期刊：
影响因子：
3.9
通讯作者：
T. Günther;R. Klemd;X. Zhang;I. Horn;S. Weyer
中科院分区：
文献类型：
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作者：
T. Günther;R. Klemd;X. Zhang;I. Horn;S. Weyer
石炭系支博铁矿床和查岗诺尔铁矿床分别位于西天山造山带阿乌拉勒铁成矿带的火山口中心和同一火山构造的侧翼。在（粗质）安山岩到流纹岩寄主岩中，有几个10 ~ 100米大、板状到透镜状的磁铁矿为主的层状矿体。磁铁矿成矿主要以块状铁矿（部分呈柱状网状或流状）和浸染状磁铁矿为主。不同矿石类型的微量元素和同位素研究揭示了两大类磁铁矿：一类以两个矿床的块状部分角化矿石为代表，富集Ti、V、Ni和HFSE（如Y），其浓度与铁氧化铜金（IOCG）矿石相似；δ56Fe值（高达0.4‰）支持正岩浆成因，对应于~ 800°C的同位素源计算。I族磁铁矿中fetotall和δ56Fe（+ 0.4‰~ - 0.1‰）与不相容的微量元素（Si、Al、Nb、Ti、Y）呈正相关，解释为ralehi型分选过程的结果。V、Ni、Mn值的减小反映了成矿时fo2条件的变化。与ⅰ组相比，ⅱ组以查岗诺尔浸染型矿石为代表，其Ti、V、Ni、Y等元素含量相对较低，δ56Fe的主导范围约为0‰~ - 0.5‰。这些结构化学特征与石榴石—放线石—透辉石—绿帘石—碳酸盐—钾长石共生特征与热液铁矽卡岩矿相一致。研究样品中磁铁矿的多元素分布模式相似，且具有相同块状矿型的δ56Fe比值重叠，且两者矿床距离较近，表明不同类型铁矿具有共同的富铁来源。与正岩浆ⅰ群磁铁矿相反，浸染型矿石中微量元素δ56Fe比值从0‰下降至- 0.5‰，不能简单地用简单的罗利分选或蚀变过程来解释。因此，第二组磁铁矿形成模式为双峰形成模式，包括近端正岩浆矿体的铁的部分再活化和随后的远端再沉淀。这些过程是由晚期热液流体驱动的，这些热液流体起源于邻近的深部花岗/花岗闪长岩岩体。
The Carboniferous Zhibo and Chagangnuoer iron deposits are situated within a caldera centre and along the flank of the same volcanic edifice, respectively, in the Awulale Iron Metallogenic Belt of the Western Tianshan orogen. Several stratiform 10 to 100 m large, tabular to lenticular shaped magnetite-dominated ore-bodies occur in (trachy-) andesitic to rhyolitic host rocks. The magnetite mineralization mainly occurs as massive iron ores, partly with columnar-network or flow textures, and as disseminated magnetite ores. Trace element and isotope investigations of the different ore types reveal two major groups of magnetite: Group I, represented by the massive, partly brecciated ores from both deposits, is enriched in Ti, V, Ni, and HFSE such as Y, with concentrations similar to Iron Oxide-Copper-Gold (IOCG) ores. The δ56Fe values (up to 0.4‰) support an ortho-magmatic origin corresponding with an isotopic source calculation at ~ 800 °C. Positive correlations between Fetotaland δ56Fe (from + 0.4‰ to − 0.1‰) and incompatible trace element contents (e.g. Si, Al, Nb, Ti and Y) in Group I magnetite are interpreted to be the consequence of a Raleigh-type fractionation process. Decreasing V, Ni and Mn values indicate changingfO2conditions at the time of ore genesis. Group II, which is represented by the disseminated ores from Chagangnuoer, is - compared to Group I - relatively depleted in elements like Ti, V, Ni and Y and further spans a dominant δ56Fe range from about 0‰ to − 0.5‰. These textural and chemical characteristics and the garnet-actinolite-diopside-epidote-carbonate-K-feldspar paragenesis are in accordance with hydrothermal Fe-skarn ores.The similar multi-element patterns of magnetite from all investigated samples, the overlapping δ56Fe ratios of the same massive ore-type from Zhibo and Chagangnuoer and the close proximity of both deposits indicate a common source of Fe-enrichment for the different iron ore types. In contrast to the ortho-magmatic Group I magnetite, reverse trace element trends with decreasing δ56Fe ratios (from 0‰ to − 0.5‰) among the disseminated ores cannot simply be explained by a straightforward Raleigh fractionation or alteration processes. Therefore, a bimodal formation model is suggested for the Group II magnetite formation, including a partial remobilization of iron from the proximal, ortho-magmatic ore bodies and a subsequent distal re-precipitation. These processes were driven by late-stage hydrothermal fluids, which originated from deeper- seated granitic/granodioritic intrusions in the immediate vicinity.