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Iron deposits in relation to magmatism in China
Accepted Manuscript Iron deposits in relation to magmatism in China Zhaochong Zhang M. Santosh Jianwei Li PII: DOI: Reference:
S1367-9120(15)00361-2 http://dx.doi.org/10.1016/j.jseaes.2015.09.026 JAES 2524
To appear in:
Journal of Asian Earth Sciences
To appear in:
Journal of Asian Earth Sciences
To appear in:
Journal of Asian Earth Sciences
Please cite this article as: Zhang, Z., Iron deposits in relation to magmatism in China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.09.026 Please cite this article as: Zhang, Z., Iron deposits in relation to magmatism in China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.09.026 Please cite this article as: Zhang, Z., Iron deposits in relation to magmatism in China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.09.026
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Editorial
Iron deposits in relation to magmatism in China 1. Introduction China has a rich reserve of iron ores, and hosts most of the major types of iron deposits recognized over the world. However, most of these deposits are low-grade ores (<50% Fe), and the high-grade iron ores only account for ~1% of the total iron ore resources (Zhang et al., 2014a). During 50s to 70s of the last century, two major research and exploration programmes were implemented on national level in China, focusing on the high-grade iron ores of banded iron formation (BIF) deposits. However, apart from several small deposits, no large high-grade iron deposits under the BIF category were discovered. Thus, the exploration and scientific studies on iron deposits came to a dead-end during 1980’s to 2005. In the recent years, however, there has been an increasing demand for iron resources due to China’s rapid industrialization and economic development. Thus, a new surge of studies and prospecting of high-grade iron deposits started, which resulted in many advances in our understanding of the formation and exploration of iron deposits. The tectonic architecture of China continent is defined by three Precambrian cratons or blocks: the North China Craton (NCC), the South China Block (SCB, including the Cathaysian Block and the Yangtze craton) and the Tarim craton surrounded by a series of Phanerozoic fold belts incorporating several micro-
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continental blocks. All the three Precambrian cratons or blocks have been overprinted by differential reactivation and destruction, as well as accretion in response to the prolonged subduction of the Pacific and Indian plates and related geodynamic processes, which have resulted in multiple pulses of iron mineralization ranging in age from Neoarchean to Cenozoic. Most of the iron deposits in China are closely related to the various tectono-magmatic processes.
2. General outline of the iron deposits in relation to magmatism The iron ore-related magmatic suites in China exhibit a compositional spectrum from mafic-ultramafic to intermediate-felsic rock suites. Following the recent classification scheme by Dill (2010), these iron deposits are categorized as: (1) Ti– Fe–(V) deposits related to mafic intrusions (high Ti); (2) apatite-bearing Fe oxide deposits (low Ti); (3) apatite-bearing Fe oxide deposits related to alkaline igneous rocks that can be classified into titanomagnetite–magnetite–apatite deposits and rare earth element (REE)-apatite Fe deposit; (4) contact metasomatic Fe deposits (Fe skarn), (5) volcanic-hosted Fe (unmetamorphosed) deposits and (6) metamorphosed volcanic-hosted Fe deposits (Algoma-type). However, the most economically important types of iron deposits in China include the Algoma type, volcanic-hosted Fe (unmetamorphosed) deposits, apatite-bearing Fe oxide deposits (low Ti), skarn iron deposits and Ti–Fe–(V) deposits related to mafic intrusions (high Ti). The salient characteristics of the six types of iron deposits are summarized below.
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Ti–Fe–(V) deposits related to mafic intrusions (high Ti)：Based on the rock association, this group can be classified into two sub-types: those associated with layered gabbroic intrusions and those associated with anorthosite complex. The examples for these include those in the Panxi area and in the Chengde area. However, unlike the large mafic-ultramafic complexes elsewhere in the world, such as those in Bushveld and Stillwater, where iron oxide bodies are hosted in the upper zone, the major iron oxide orebodies in the layered gabbroic intrusions in China occur mostly in the lower and middle zones. Most iron ores are disseminated, whereas massive ores are rare, and only recognized in the lower zone. Titanomagnetite and ilmenite are the dominant ore minerals. The second type of iron deposits is associated with Proterozoic anorthosite complex, and is recognized only in the Chengde area. The complex consists predominantly of anorthosite (～85-90%) with norite and mangerite as well as minor troctolite and hornblendite. The ore bodies in this category consist of disseminated and massive ores. The disseminated ores are generally hosted in gabbro at the contact zone between gabbro and anorthosite, and have no distinct boundary with the gabbro, whereas the massive ores show sharp contact with anorthosite, and are hosted in the vertical fractures of the previously consolidated anorthosites or troctolite. The main ore minerals are titanomagnetite with minor ilmenite.
Apatite-bearing Fe oxide deposits (low Ti): This type of iron deposits, or Kirunatype deposits are typically characterized by apatite-bearing low-Ti magnetite ores. It is also named as porphyry-type iron deposits (Ningwu Research Group, 1978) or
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terrestrial volcanic type (Zhao et al., 2004; Li et al., 2012) by Chinese geologists. Most of these deposits occur in the Ningwu and Luzong Cretaceous terrigenous volcanic basins along Middle-Lower Yangtze River Valley. The iron metallogeny in this case is genetically related to the Early Cretaceous (ca. 130 Ma) subvolcanic dioritic intrusions. In contrast to their volcanic counterpart, these subvolcanic dioritic intrusions are characterized by enrichment in Na relative to K. The iron orebodies occur along the contact zones between the subvolcanic dioritic intrusions and volcano-sedimentary country rocks, or in the apical zones of the dioritic subvolcanic intrusions. Most of the ores are stockwork-disseminated. Ore minerals include magnetite, hematite, pyrite and rare chalcopyrite, whereas gangue minerals are represented by albite, diopside, actinolite, apatite, epidote, anhydrite, chlorite and sericite.
Apatite-bearing Fe oxide deposits related to alkaline igneous rocks: This type of iron deposits is rather rare in China, and can also be classified as titanomagnetite– magnetite–apatite deposits and rare earth element (REE)-apatite Fe deposit. The titanomagnetite–magnetite–apatite deposits are represented by the Fanshan apatite (iron) deposit in NCC. The Fanshan intrusion consists of three main lithological units that are concentrically zoned with syenite at the core (Unit 1), surrounded by layered ultramafic rocks (clinopyroxenite and biotite clinopyroxenite; Unit 2), and an outer rim of garnet-rich clinopyroxenite and orthoclase clinopyroxenite and syenite (Unit 3). The intrusive rocks are composed of variable amounts of Ca-rich augite, biotite,
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orthoclase, melanite, garnet, magnetite and apatite. Apatite-magnetite ores (nelsonite) exclusively occur in Unit 2. The rare earth element (REE)-apatite Fe deposits is represented by the Bayan Obo deposit in the north margin of NCC, which is the world's largest REE resource and is also characterized by large tonnage of iron ores. The deposit is hosted in the Paleoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The H8 dolomite marble hosts REE-Nb mineralization and many Fe ore bodies. The mineral paragenesis of the Bayan Obo ore is exceedingly complex with over 170 minerals including 20 REE minerals and 20 Nb minerals. The REE-Nb-Fe ores occur as large lenses hosted in fine-grained dolomite. The ores are typically banded, and consist of magnetite, hematite, fluorite, riebeckite, aegirine, phlogopite, apatite, barite, monazite, and bastnäsite.
Contact metasomatic Fe deposits (Fe skarn): This type of iron deposits is the most widespread in China, especially in East China, although the large tonnage of iron ores is rare. Most of these skarn iron deposits formed at ca. 130 Ma, coeval with apatite-bearing Fe oxide deposits (low-Ti). The ore-related intrusive rocks exhibit a compositional spectrum from mafic to intermediate-felsic rocks. Except for the felsic intrusions (SiO2>70wt%), most of orerelated
intermediate-felsic
intrusions
(SiO2<67%)
display
a
metaluminous
composition and enrichment in Na relative to K together with negative correlation between SiO2 and total FeO. They also display typical calc-alkaline trends with high
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water contents and fO2 through suppression of plagioclase fractionation and promotion of early magnetite and amphibole fractionation. Like other skarns worldwide, those related to iron ores in China can also be subdivided into magnesian and calcic skarn, on the basis of their characteristic elemental association. Usually, calcic skarn is the dominant variety, and only few iron deposits related to magnesian skarns have been identified. The calcic skarn minerals contain diopside- hedenbergite series, grossular- andradite series, wollastonite, scapolite and vesuvian garnet. In contrast, diopside, forsterite, spinel, phlogopite, serpentine, humite, and talc are typical of magnesian skarns. In addition, albite and sodalite alteration is common during the early stage prior to the formation of the skarn. All economic iron ores formed during the retrograde alteration stage.
Volcanic-hosted Fe (unmetamorphosed) deposits: This type of iron deposits is termed as submarine volcanic iron deposits by Chinese geologists. Recent exploration shows that this is one of the most important iron deposits hosting high-grade iron ores. Several large iron deposits and many medium-small iron deposits with a total ore reserve of >1000 million tons of ores at average grade of 40% (up to >60%) have been explored in the Awulale belt in the western Tianshan of the middle part of Central Asian Orogenic Belt. Almost all these iron deposits are located in western China, including those in Western Tianshan, Eastern Tianshan, Beishan, Altay, Kaladawan area at eastern part of the Altyn Tagh Mountain and southwestern margin of the South China Block. Recent studies suggest that this type of iron deposits may
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be the most potential for prospecting high-grade iron deposits in China in the future. The iron deposits are commonly associated with submarine volcanic-sedimentary sequences. The ore-related volcanic rocks exhibit a compositional spectra from basic to intermediate-basic, and to intermediate-acid rocks as well as the volcaniclastic equivalents, but most are intermediate-basic, and characterized by enrichment in Na. These volcanic rocks are typically sodic, and have Nb/Y ratios below 0.7, implying that the rocks are subalkaline. Combined with the regional geological features, it has been inferred that these rocks possibly formed within a continental marginal arc setting (e.g., Zhang et al., 2012). The volcanic-hosted iron deposits in China have broadly similar characteristics to those of apatite-bearing Fe oxide deposits (low Ti) in alteration and mineralization. However, apatite is rare, and skarns in many iron deposits are common. Unlike the classic skarns that are developed at the contact between intrusive plutons and carbonate rocks, no plutons have been recognized near the skarns in these iron deposits. In general, most iron orebodies occur in stratiform, lenticular, lensoidal and veined or complex veined shapes, with magnetite as the dominant ore mineral. In contrast, some iron orebodies are also hosted in pyroclastic-sedimentary rocks, where hematite is the dominant ore mineral with minor magnetite and/or rhodochrosite and hausmannite.
Metamorphosed volcanic-hosted Fe deposits (Algoma-type): The banded iron formation (BIF) deposits have been traditionally divided into Algoma type associated
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with volcanic rocks) and Superior-type stratabound in sedimentary sequences. The BIF type of iron deposits are dominantly distributed in the NCC, with only minor occurrences in the southern margin of the SCB, and the Qinling, Qilian and Kunlun orogenic belts. The Anshan-Benxi in Liaoning province, Eastern Hebei-Miyun in Beijing, Wutai and Lüliang in Shanxi province and Middle Inner Mongolia are the most important BIF ore clusters, where Algoma-type BIF deposits dominate. Available geochronological data suggest that the Algoma-type BIF deposits in China mainly formed at ca. 2.6-2.5 Ga (Li et al., 2014a). The major host rocks include amphibolites (or hornblende plagioclase gneiss), amphibolites, biotite leptynite, mica quartz schist, biotite quartz schist, sericite chlorite and magnetite quartzite. The most common ores are low grade, with 20-40% total Fe and 40-50% SiO2, and magnetite is the dominant ore mineral in almost all deposits. Quartz, chlorite, cummingtonite, almandine and carbonate are the most common gangue minerals. Large tonnage of high-grade ores (>10 Mt) are rare, only recognized in the Gongchangling iron deposits in Liaoning province. These high-grade iron ores consist predominantly of magnetite in contrast to hematite as the principal ore mineral in other cratons worldwide, and are controlled by faults and folds, and especially concentrated in the axis of folds.
3. Genesis of high-grade iron ores Although the genesis of high-grade iron ores in different types of iron deposits have been a matter of considerable debate, there is a general consensus that the
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formation of these iron ores can be correlated to multi-stages of geological processes that elevated the quality of iron ores. These different stages may be broadly related to one tectonic-magmatic cycle or distinct geological events. The high-grade iron ores in Ti–Fe–(V) deposits related to mafic intrusions (high Ti), apatite-bearing Fe oxide deposits (low Ti), contact metasomatic Fe deposits (Fe skarn) and volcanic-hosted Fe (unmetamorphosed) deposits are the examples. In contrast, some high-grade iron ores could have been produced by distinct events in space and time. Those in metamorphosed volcanic-hosted Fe deposits (Algoma-type) are examples. Previous studies suggest that the high-grade iron ores in Ti–Fe–(V) deposits related to mafic intrusions and some apatite-bearing Fe oxide deposits (low Ti) might have formed through a series of magmatic processes, including extensive fractional crystallization, and possible crustal contamination and liquid immiscibility as well as flow differentiation. For example, the high-grade iron ores in the Panzhihua (associated with layered intrusion) and Damiao (associated with anorthosite complex) were considered to be generated by multi-stages of magmatic processes including early stage of crystal fractionation and liquid immiscibility and subsequent crystallization accompanying flow differentiation (e.g., Song et al., 2013; Zhang et al., 2014b; Zhao et al., 2009). Minor ore-magma mineralization in the Gushan apatitebearing Fe oxide deposit was proposed to be the result of separation of the Fe-rich ore liquid in response to assimilation of P-rich strata of Fe-rich dioritic magma generated by fractionation of primary basaltic magma (Hou et al., 2010, 2011). In contrast, most of those in apatite-bearing Fe oxide deposits, skarn iron deposits and volcanic-hosted
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Fe deposits were produced by magmatic and subsequent hydrothermal stages. The large tonnage of high-grade ores in these types of iron deposits appear to be related to high crystal fractionation of magmas. The intrusions associated with skarn iron deposits generally display multiple pulses of magma emplacement, and they appear co-magmatic (e.g., Li et al., 2014b; Jin et al., 2015; Xie et al., 2015). In addition, all submarine volcanic rocks associated with volcanic-hosted Fe deposits have a compositional spectrum, and the petrological and geochemical characteristics suggest that they were produced by fractional crystallization of parent magma (Zhang et al., 2012). Experimental and fluid inclusion studies show that the volume and composition of exsolved fluids from melt are controlled by many factors during the magma-fluid transition (Webster, 2004). The fractional crystallization and assimilations of carbonate rocks and evaporites could lead to the formation of ore fluids and elevated Cl concentration of exsolution fluids, favoring the transportation of Fe as chloride complexes (Zhang et al., 2014b). The iron mineralization has also experienced multiple hydrothermal stages, which have been recorded in the textural and compositional data for magnetite from many skarn and volcanic-hosted iron deposits (e.g., Hu et al., 2015; Zhang et al., 2014c). The studied samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization, which suggest that the dissolutionreprecipitation process has been important in significantly removing trace elements from early-stage magnetite to form high-grade, high-quality iron ores in hydrothermal
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environments. The high-grade iron ores formed through distinct events in time are represented by the Algoma-type in Anshan-Benxi iron ore cluster. There is a consensus that the high-grade iron ores can be attributed to hydrothermal processes rather than primary sedimentation (Li et al., 2015a and therein). In summary, the formation of the large tonnage of high-grade iron ores appears to be the result of multiple stages of magmatic-hydrothermal processes.
4. Context of this special issue This special issue draws together related studies, and aims to highlight important recent discoveries related to the geologic setting, tectonic control, and petrogenesis of host rocks of iron mineralization and the metallogenic processes of iron ores (especially high-grade iron ores).
Salient highlights of the contributions to this
special issue are presented below.
Genesis of BIF deposits Precambrian banded iron formations (BIFs) are characterized by alternating chert and magnetite bands. However, The Tieshanmiaoe BIF deposit in the southern NCC has distinct features of ores, which comprise banded pyroxene-magnetite quartzite (BMQ) and disseminated magnetite pyroxenite (DMP). Whether the quartz-poor DMP represents metamorphosed iron-bearing ultramafic rocks or chemical sedimentary rocks is still unclear. The mineralogical and geochemical characteristics
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of the two types of ores show that the DMP and BMQ show similar major element contents, indicating a similar source through submarine chemical precipitation with little input from terrestrial or volcanic materials (Yao et al., 2015). The protoliths of the DMP and BMQ in the Tieshanmiao iron deposit are inferred to be quartzcarbonate iron-bearing formations which underwent subsequent metamorphism, but the DMP is rich in carbonate but relatively poor in silica. In most cases, the BIF-related high-grade iron deposits are mainly composed of hematite ores, and have been interpreted to be supergene in origin (e.g., Lascelles, 2012 and therein) or epigenetic hydrothermal enrichment (e.g., Thorne et al., 2004). However, those of BIF-related iron deposits in China are composed predominantly of magnetite in the Anshan-Benxi area in Liaoning Province. Based on the detailed comparison of the geology, geochemical and stable isotopic compositions of the highgrade iron ores and altered rocks from the mining area II of the Gongchangling deposit (G2) and Qidashan-Wangjiabuzi iron deposit (QW) in Liaoning Province, Li et al. (2015a) propose that desilicification process by hypogene alkaline-rich hydrothermal fluids was possibly responsible for the formation of high-grade iron ores in the G2 whereas iron activation-reprecipitation process generated the highgrade iron orebodies in QW. Based on the field relation, mineralogical and geochemical studies of the Macheng iron deposit in the eastern Hebei Province (the second largest BIF-related iron ore cluster), Wu et al. (2015) recognized two kinds of high-grade iron ores: massive oxidized high-grade ore that mainly consists of hematite with some magnetite,
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and massive high-grade magnetite ore composed predominantly of magnetite. They consider that the former may be correlated to supergene enrichment, caused by oxidation of magnetite and the leaching of gangue minerals from BIF to form highgrade ore, whereas the latter is probably related to intensive migmatization which produced high-grade ores by altering the primary iron ores. Constraining the age of the anatectic event related to migmatization could be key factor to the formation of high-grade iron ores in the eastern Hebei Province. Li et al. (2015b) present detailed textural relationship and internal structures of zircon grains from eight samples of migmatitic rocks representing the different Algoma-type BIFhosted iron deposits. Their studies show that the anatectic event lasted from 2511 to 2485 Ma, immediately following the BIF deposition, and is consistent with regional metamorphic events in the eastern part of the North China Craton. The Shilu Fe-Co-Cu ore district, located at Hainan Province of South China, is well known for high-grade hematite-rich Fe ores. However, depositional time of the host rocks has not been well constrained. Based on detrital zircon U-Pb dating, Wang et al. (2015a) considered ca. 1070-970 Ma as the maximum depositional time. In addition, Wang et al. (2015b) recognized three types of Co-Cu ores. Based on the textural and chemical composition of sulfides, they constructed a four-stage metallogenic model for the Cu-Co ores, involving ca. 1075-880 Ma deposition of the primary Co-Cu ore source beds, the syn-structural metamorphism, and two hydrothermal activities. Thus, the Shilu Co-Cu ores are considered to be a BIF origin, but greatly reworked and enriched by late structural deformation and hydrothermal
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activities. The Mesoproterozoic Jingtieshan BIF deposit is located in the Northern Qilian Orogenic Belt in NW China, and is composed essentially of specularite and jasper, with minor carbonate minerals and barite. Based on the detailed geochemical studies, Yang et al. (2015) conclude that the Jingtieshan BIFs share many characteristics with the three main types of global BIFs namely the Algoma-, Superior- and Rapitan-types, but they do not conform to all the features of any of these deposits. Thus, they proposed that the Jingtieshan BIFs represent a unique type, similar to that of sedimentary-exhalative mineralization.
Genesis of Ti–Fe–(V) deposits related to mafic intrusions (high Ti) The Panxi area in the Emeishan large igneous province, SW China is the largest Ti-Fe-V magnetite oxide cluster. Understanding fractionation processes of magma is a prerequisite to constrain the origin of Ti–Fe–V ores. To investigate the fractional crystallization history and origin of Ti–Fe–V deposit of the Taihe intrusion, She et al. (2015) measured the trace element compositions of magnetite and ilmenite by LAICP-MS method. Their results suggest that early crystallization of Fe-Ti oxides is the result of the high FeOt and TiO2 contents in the parental magmas, and the gravitational sorting and settling of magnetite and ilmenite result in formation of FeTi-V oxide ore layers. The Hongge mafic-ultramafic layered intrusion hosts the largest Fe-Ti-V ores plus interesting Cu-Ni-platinum-group element (PGE) mineralization which may have
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economic potential in the Panxi area. Liao et al. (2015) reports new whole-rock major and trace element compositions, PGE abundances and Sr-Nd isotopic data for selected cumulate rocks and basalts. They investigate the nature of parental magmas and the controls on its evolution processes from the source mantle en route to the surface. Their studies suggest that low-Ti and high-Ti basalts share a common parent magma with the Hongge intrusion, and they correlated the genesis of the massive Fe-Ti oxide layers in the middle zone of the Hongge intrusion to the event that multiple replenishment magma mixing with the residual Fe-Ti-rich liquid which heats the strata to attract H2O. The layered mafic-ultramafic intrusions generally represent an open-system process responsible for the magma generation, segregation, ascent, storage, mixing, crystallization and eruption. The traditional Assimilation-Fractional Crystallization (AFC) model has been developed to trace the geochemical paths of magmas (Depaolo, 1981). However, this model neglects the energy conservation and the assimilant compositional variations in the quantitative treatments of open-system processes. Thus, the Energy-Constrained Recharge, Assimilation Fractional Crystallization model has been developed (Bohrson and Spera, 2003). Based on this model, Li et al. (2015d) used published Sr-Nd isotopic data to simulate and evaluate the petrogenesis and mineralization of the Xinjie layered mafic-ultramafic intrusion in the Panxi area, hosting Fe-Ti-V oxide ore in the upper part and Ni-Cu-PGE sulfide deposits in the lower part. Their modeling is consistent with the observed scenarios, and shows that the processes are more complex than the previously thought.
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The ca. 1.7 Ga Damiao anorthosite complex in the NCC is a unique example hosting abundant titanomagnetite-dominated ore deposits in China. Both the Fe-Ti-Prich silicate rocks and massive Fe-Ti-(P) ores occur as discordant late-stage dikes cross-cutting early-stage anorthosites with irregular but sharp boundaries. Some latestage dikes comprise well-developed alternating late-stage anorthosites and Fe-Ti-Prich pyroxenites defining rhythmic layers, and the genesis of these ores remains poorly understood. Based on the field and petrographic observation and compositional variations, Li et al. (2015c) propose a sequence of crystallization and interpret the mechanism of the rhythmic layers as a result of slow and extensive fractional crystallization of the interstitial melt. They consider that high solubility of phosphorous probably plays an important role in the formation of rhythmic layering, whereas the massive Fe-Ti-(P) ores may represent the most evolved products of the residual magma.
Genesis of skarn iron deposits The Fushan iron deposit associated with dioritic intrusion is one of the typical Handan-Xintai type of skarn deposits in NCC. However, the iron source remains equivocal. Shen et al. (2015) present new zircon U-Pb age data from the ore-related dioritic rocks, the gneissic xenoliths in the dioritic intrusion and the Neoarchean basement rocks surrounding the deposit. The results show that the ore-related dioritic intrusion was emplaced at 128.8±1.9 Ma. Zircon grains from the basement rocks and xenoliths have the similar ages (Neoarchean to Paleoproterozoic) to inherited zircon
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in the dioritic intrusion. These results provide new evidence that the continental lower crust may be the major source of iron ores. Based on the studies of the Jinling skarn-type iron deposit in the Central NCC, Jin et al. (2015) also arrive at similar conclusions that the primary magma witnessed the interaction between the partial melts of relatively oxidized delaminated ancient crust and mantle peridotite. Furthermore, they proposed a two-stage model of Feenrichment process involving fractional crystallization of the primary magma giving rise to high Cl and Fe contents in the magmatic-hydrothermal fluid and later Feleaching process, accounts for the high-grade ore bodies. Unlike the major skarn-type iron deposits in the NCC which are generally associated with dioritic plutons, the Zhangmatun deposit is genetically related to gabbro. Based on the zircon U-Pb, mineralogical and geochemical studies, Xie et al. (2015) propose that the gabbroic magma were derived from amphibole-bearing spinel lherzolite in the enriched EMI-type upper mantle which was modified by incorporation of lower crust material, and they correlate the high-grade skarn iron deposit to the exsolution of Fe-rich magmatic fluids and subsequently leaching from the solidified gabbro at the hydrous skarn stage. Furthermore, the relatively high oxygen fugacity of the melt was also considered to be one of the controlling factors that led to the formation of the iron deposit. Based on the comparison of the ore-related Kuangshancun intrusive complex and
coeval barren Hongshan complex, Sun et al. (2015) propose that although both complexes were produced by partial melting of an enriched lithospheric mantle
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contaminated by lower continental crust components, the Kuangshancun complex was characterized by higher oxygen fugacity as compared to the Hongshan complex. Thus, they conclude that the high oxidation states and high water contents are considered as key factors that led to the iron mineralization in the Handan-Xingtai district. The relationship between evaporites and skarn Fe deposit remains poorly constrained. Zhu et al. (2015) determined the sulfur isotopic composition of pyrite, as well as the composition of halogen-rich minerals (scapolite and amphibole) in the Jinshandian skarn Fe deposit in SCB. The δ34S values for pyrite from the early and late retrograde stage are significantly heavier than those of sulfides from magmatichydrothermal fluid, indicating that sulfur in the Jinshandian ore-forming system was mostly derived from evaporites. This inference is further reinforced by the data of Cl contents of the amphibole and the scapolite. They infer that the hydrothermal system of the Jinshandian skarn Fe deposit probably experienced significant incursion of evaporites before or during the late prograde stage.
Genesis of apatite-bearing magnetite (low-Ti) Based on detailed petrographic observations and fluid inclusion studies on the Hemushan apatite-magnetite deposit in the southern part of the Ningwu iron ore district of the Middle-Lower Yangtze River Metallogenic Belt, Luo et al. (2015) propose that the ore-forming fluid was of magmatic origin at the early phase of the middle metallogenic stage. Progressive reaction of the ore fluids with the host rocks and boiling of the ore-forming fluid may be the potential mechanism of the deposition
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and enrichment of iron ore.
Genesis of volcanic-hosted Fe deposits Volcanic-hosted Fe deposits are one of the most important iron ores in China. However, the nature of primary magma and petrogenesis associated with these iron ores is poorly understood. Li et al. (2015e) identified some iron-rich fragments (IRF) from the Yamansu iron deposit in Eastern Tianshan Mountains, NW China. These fragments display typical volcanic textures and structures, such as porphyritic texture, and vesicle-amygdale structure. The magnetite is distributed in between the feldspar laths in ground mass. The IRF has an average total Fe of 26 wt%. The magnetite is mostly titanium–vanadium magnetite, with the TiO2 content ranging up to 4.86 wt.% and V2O3 content up to 3.20 wt.%. Thus, IRF has been considered to be produced by iron-rich melts and represents the products of the Fenner magma evolution from a basaltic magma.
5. Future prospects This special issue compiles the recent trends in research on the genesis of iron deposits in China. Although several important advances have been made in specialized disciplines, some issues that remain to be elucidated include: (1) the tectonic settings of volcanic-hosted Fe deposits; (2) the role of liquid immiscibility in the genesis of iron deposits; (3) the iron sources of skarn iron deposits: whether orerelated intrusions or the Fe-rich strata; (4) the relationship between magmatic nature
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and generation of iron-rich fluids; and 5) the role of evaporites on the formation of skarn and apatite-bearing magnetite ores. We hope that the papers presented in this special issue would encourage further work in these lines.
Acknowledgements The guest editors thank the all authors for their scholarly contributions to this special issue. We also thank the referees who provided their valuable time and constructive comments and suggestions. We acknowledge the valuable patronage and support from Editor-in-Chief, Prof. Bor-ming Jahn during the organization and preparation of this special issue. This special issue is financially supported by 973 Program (No. 2012CB416800) from the Ministry of Science and Technology of China.
Zhaochong Zhang* State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, PR China Corresponding author. Tel. : +86 10 82322195; fax: +86 10 82323419. E-mail address: [email protected].
M. Santosh State Key Laboratory of Geological Process and Mineral Resources,
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China University of Geosciences, Beijing 100083, PR China Department of Earth Sciences, University of Adelaide, SA 5005, Australia
Jianwei Li State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, PR China
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Origin and evolution of ore-forming fluids in the Hemushan magnetite-apatite deposit, Anhui Province, Eastern China, and metallogenic significance. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2014.08.018 Ningwu Research Group, 1978. Magnetite Porphyry Deposits in Ningwu Area. Geological Publishing House, Beijing, pp. 1–196 (in Chinese). She, Y.W., Song, X.Y., Yu, S.Y., He, H.L., 2015. Variations of trace element concentration of magnetite and ilmenite from the Taihe layered intrusion, Emeishan large igneous province, SW China: Implications for the magmatic fractionation and origin of Fe-Ti-V oxide ore deposits. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2015.03.029 Shen, J. F., Li, S. R., Santosh, M., Dong, G. C., Wang, Y. J., Liu, H. M., Peng, Z. D., Zhang, Z. Y., 2015. Zircon U-Pb geochronology of the basement rocks and dioritic intrusion associated with the Fushan skarn iron deposit, southern Taihe Mountains, China. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2015.01.009 Song, X. Y., Qi, H. W., Hu, R. Z., Chen, L. M., Yu, S. Y., Zhang, J. F., 2013. Formation of thick stratiform Fe-Ti oxide layers in layered intrusion and frequent replenishment of fractionated mafic magma: Evidence from the Panzhihua intrusion, SW China, Geochem. Geophys. Geosyst. 14, 712–732. Sun, Y., Xiao, L., Zhan, Q. Y., Wu, J. X., Zhu, D., Huang, W., Bai, M., Zhang, Y. H., 2015. Petrogenesis of the Kuangshancun and Hongshan intrusive complexes from the Handan–Xingtai district: Implications for iron mineralization associated with Mesozoic magmatism in the North China Craton. J. Asian Earth Sci. (this issue),
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doi: 10.1016/j.jseaes.2015.08.003 Thorne, W., Hagemann, S., Barley, M., 2004. Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit, Mt Tom Price, Western Australia. Miner. Deposita 39, 766–783. Wang, Z. L., Xu, D. R., Hu, G.C., Yu, L. L., Wu, C. J., Zhang, Z. C., Cai, J. X., Shan, Q., Hou, M. Z., Chen, H. Y., 2015a. Detrital zircon U–Pb ages of the Proterozoic metaclastic-sedimentary rocks in Hainan Province of South China: New constraints on the depositional time, source area, and tectonic setting of the Shilu Fe–Co–Cu ore district. J. Asian Earth Sci. (this issue), doi: 10.1016/j.jseaes.2015.04.014 Wang, Z. L., Xu, D. R., Zhang, Z. C., Zou, F. H., Wang, L., Yu, L. L., Hu, M. Y., 2015b. Mineralogy and trace element geochemistry of the Co- and Cu-bearing sulfides from the Shilu Fe-Co-Cu ore district in Hainan Province of South China. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2015.08.019 Webster, J.D, 2004.The exsolution of magmatic hydrosaline chloride liquids. Chem. Geol. 210, 33– 48 Wu, H. Y., Niu, X. L., Zhang, L. C., Pirajno, F., Luo, H. B., Qin, F., Cui, M. N., Wang, C. L., Qi, M., 2015. Geology and geochemistry of the Macheng Algomatype banded iron-formation, North China Craton: Constraints on mineralization events and genesis of high-grade iron ores. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2015.05.024 Xie, Q. H., Zhang, Z. C., Hou, T., Santosh, M., Han, L., Cheng, Z. G., 2015. Petrogenesis of the Zhangmatun gabbro in the Ji’nan complex, North China Craton:
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Implications for skarn-type iron mineralization. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2015.03.040 Yang, X. Q., Zhang, Z. H., Duan, S. G., Zhao, X. M., 2015. Petrological and geochemical features of the Jingtieshan banded iron formation (BIF): A unique type of BIF from the Northern Qilian Orogenic Belt, NW China. J. Asian Earth Sci. (this issue), doi:10.1016/j.jseaes.2015.03.024 Yao, T., Li, H. M., Li, W. J., Li, L. X., Zhao, C., 2015. Origin of the disseminated magnetite pyroxenite in the Tieshanmiao-type iron deposits in the Wuyang region of
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Miner. Deposits 31, 941–964 (in Chinese with English abstract). Zhao, T. P., Chen, W. Zhou, M. F., 2009. Geochemical and Nd-Hf isotopic constraints on the origin of the ~1.74-Ga Damiao anorthosite complex, North China Craton. Lithos 113, 673–690. Zhao, Y. M., Wu, L. S., Bai, G., Yuan, Z. X., Ye, Q. T., Huang, M. Z., Rui, Z. Y., Sheng, J. F., Lin, W. W., Deng, S. P., Mao, J. W., Bi, C. S., Dang, Z. F., Wang, L. F., Zhang, Z. H., Chen, W. S., 2004. Metallogeny of the major metallic ore deposits in China. Geological Press, Beijing (in Chinese). Zhu, Q. Q., Xie, G. Q., Mao, J. W., Li, W., Li, Y. H., Wang, J., Zhang, P., 2015. Mineralogical and sulfur isotopic evidence for the incursion of evaporites in the Jinshandian skarn Fe deposit, Edong district, Easter China. J. Asian Earth Sci. doi:10.1016/j.jseaes.2015.05.022
S1367-9120(15)00361-2 http://dx.doi.org/10.1016/j.jseaes.2015.09.026 JAES 2524
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Please cite this article as: Zhang, Z., Iron deposits in relation to magmatism in China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.09.026 Please cite this article as: Zhang, Z., Iron deposits in relation to magmatism in China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.09.026 Please cite this article as: Zhang, Z., Iron deposits in relation to magmatism in China, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2015.09.026
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Editorial
Iron deposits in relation to magmatism in China 1. Introduction China has a rich reserve of iron ores, and hosts most of the major types of iron deposits recognized over the world. However, most of these deposits are low-grade ores (<50% Fe), and the high-grade iron ores only account for ~1% of the total iron ore resources (Zhang et al., 2014a). During 50s to 70s of the last century, two major research and exploration programmes were implemented on national level in China, focusing on the high-grade iron ores of banded iron formation (BIF) deposits. However, apart from several small deposits, no large high-grade iron deposits under the BIF category were discovered. Thus, the exploration and scientific studies on iron deposits came to a dead-end during 1980’s to 2005. In the recent years, however, there has been an increasing demand for iron resources due to China’s rapid industrialization and economic development. Thus, a new surge of studies and prospecting of high-grade iron deposits started, which resulted in many advances in our understanding of the formation and exploration of iron deposits. The tectonic architecture of China continent is defined by three Precambrian cratons or blocks: the North China Craton (NCC), the South China Block (SCB, including the Cathaysian Block and the Yangtze craton) and the Tarim craton surrounded by a series of Phanerozoic fold belts incorporating several micro-
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continental blocks. All the three Precambrian cratons or blocks have been overprinted by differential reactivation and destruction, as well as accretion in response to the prolonged subduction of the Pacific and Indian plates and related geodynamic processes, which have resulted in multiple pulses of iron mineralization ranging in age from Neoarchean to Cenozoic. Most of the iron deposits in China are closely related to the various tectono-magmatic processes.
2. General outline of the iron deposits in relation to magmatism The iron ore-related magmatic suites in China exhibit a compositional spectrum from mafic-ultramafic to intermediate-felsic rock suites. Following the recent classification scheme by Dill (2010), these iron deposits are categorized as: (1) Ti– Fe–(V) deposits related to mafic intrusions (high Ti); (2) apatite-bearing Fe oxide deposits (low Ti); (3) apatite-bearing Fe oxide deposits related to alkaline igneous rocks that can be classified into titanomagnetite–magnetite–apatite deposits and rare earth element (REE)-apatite Fe deposit; (4) contact metasomatic Fe deposits (Fe skarn), (5) volcanic-hosted Fe (unmetamorphosed) deposits and (6) metamorphosed volcanic-hosted Fe deposits (Algoma-type). However, the most economically important types of iron deposits in China include the Algoma type, volcanic-hosted Fe (unmetamorphosed) deposits, apatite-bearing Fe oxide deposits (low Ti), skarn iron deposits and Ti–Fe–(V) deposits related to mafic intrusions (high Ti). The salient characteristics of the six types of iron deposits are summarized below.
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Ti–Fe–(V) deposits related to mafic intrusions (high Ti)：Based on the rock association, this group can be classified into two sub-types: those associated with layered gabbroic intrusions and those associated with anorthosite complex. The examples for these include those in the Panxi area and in the Chengde area. However, unlike the large mafic-ultramafic complexes elsewhere in the world, such as those in Bushveld and Stillwater, where iron oxide bodies are hosted in the upper zone, the major iron oxide orebodies in the layered gabbroic intrusions in China occur mostly in the lower and middle zones. Most iron ores are disseminated, whereas massive ores are rare, and only recognized in the lower zone. Titanomagnetite and ilmenite are the dominant ore minerals. The second type of iron deposits is associated with Proterozoic anorthosite complex, and is recognized only in the Chengde area. The complex consists predominantly of anorthosite (～85-90%) with norite and mangerite as well as minor troctolite and hornblendite. The ore bodies in this category consist of disseminated and massive ores. The disseminated ores are generally hosted in gabbro at the contact zone between gabbro and anorthosite, and have no distinct boundary with the gabbro, whereas the massive ores show sharp contact with anorthosite, and are hosted in the vertical fractures of the previously consolidated anorthosites or troctolite. The main ore minerals are titanomagnetite with minor ilmenite.
Apatite-bearing Fe oxide deposits (low Ti): This type of iron deposits, or Kirunatype deposits are typically characterized by apatite-bearing low-Ti magnetite ores. It is also named as porphyry-type iron deposits (Ningwu Research Group, 1978) or
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terrestrial volcanic type (Zhao et al., 2004; Li et al., 2012) by Chinese geologists. Most of these deposits occur in the Ningwu and Luzong Cretaceous terrigenous volcanic basins along Middle-Lower Yangtze River Valley. The iron metallogeny in this case is genetically related to the Early Cretaceous (ca. 130 Ma) subvolcanic dioritic intrusions. In contrast to their volcanic counterpart, these subvolcanic dioritic intrusions are characterized by enrichment in Na relative to K. The iron orebodies occur along the contact zones between the subvolcanic dioritic intrusions and volcano-sedimentary country rocks, or in the apical zones of the dioritic subvolcanic intrusions. Most of the ores are stockwork-disseminated. Ore minerals include magnetite, hematite, pyrite and rare chalcopyrite, whereas gangue minerals are represented by albite, diopside, actinolite, apatite, epidote, anhydrite, chlorite and sericite.
Apatite-bearing Fe oxide deposits related to alkaline igneous rocks: This type of iron deposits is rather rare in China, and can also be classified as titanomagnetite– magnetite–apatite deposits and rare earth element (REE)-apatite Fe deposit. The titanomagnetite–magnetite–apatite deposits are represented by the Fanshan apatite (iron) deposit in NCC. The Fanshan intrusion consists of three main lithological units that are concentrically zoned with syenite at the core (Unit 1), surrounded by layered ultramafic rocks (clinopyroxenite and biotite clinopyroxenite; Unit 2), and an outer rim of garnet-rich clinopyroxenite and orthoclase clinopyroxenite and syenite (Unit 3). The intrusive rocks are composed of variable amounts of Ca-rich augite, biotite,
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orthoclase, melanite, garnet, magnetite and apatite. Apatite-magnetite ores (nelsonite) exclusively occur in Unit 2. The rare earth element (REE)-apatite Fe deposits is represented by the Bayan Obo deposit in the north margin of NCC, which is the world's largest REE resource and is also characterized by large tonnage of iron ores. The deposit is hosted in the Paleoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The H8 dolomite marble hosts REE-Nb mineralization and many Fe ore bodies. The mineral paragenesis of the Bayan Obo ore is exceedingly complex with over 170 minerals including 20 REE minerals and 20 Nb minerals. The REE-Nb-Fe ores occur as large lenses hosted in fine-grained dolomite. The ores are typically banded, and consist of magnetite, hematite, fluorite, riebeckite, aegirine, phlogopite, apatite, barite, monazite, and bastnäsite.
Contact metasomatic Fe deposits (Fe skarn): This type of iron deposits is the most widespread in China, especially in East China, although the large tonnage of iron ores is rare. Most of these skarn iron deposits formed at ca. 130 Ma, coeval with apatite-bearing Fe oxide deposits (low-Ti). The ore-related intrusive rocks exhibit a compositional spectrum from mafic to intermediate-felsic rocks. Except for the felsic intrusions (SiO2>70wt%), most of orerelated
intermediate-felsic
intrusions
(SiO2<67%)
display
a
metaluminous
composition and enrichment in Na relative to K together with negative correlation between SiO2 and total FeO. They also display typical calc-alkaline trends with high
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water contents and fO2 through suppression of plagioclase fractionation and promotion of early magnetite and amphibole fractionation. Like other skarns worldwide, those related to iron ores in China can also be subdivided into magnesian and calcic skarn, on the basis of their characteristic elemental association. Usually, calcic skarn is the dominant variety, and only few iron deposits related to magnesian skarns have been identified. The calcic skarn minerals contain diopside- hedenbergite series, grossular- andradite series, wollastonite, scapolite and vesuvian garnet. In contrast, diopside, forsterite, spinel, phlogopite, serpentine, humite, and talc are typical of magnesian skarns. In addition, albite and sodalite alteration is common during the early stage prior to the formation of the skarn. All economic iron ores formed during the retrograde alteration stage.
Volcanic-hosted Fe (unmetamorphosed) deposits: This type of iron deposits is termed as submarine volcanic iron deposits by Chinese geologists. Recent exploration shows that this is one of the most important iron deposits hosting high-grade iron ores. Several large iron deposits and many medium-small iron deposits with a total ore reserve of >1000 million tons of ores at average grade of 40% (up to >60%) have been explored in the Awulale belt in the western Tianshan of the middle part of Central Asian Orogenic Belt. Almost all these iron deposits are located in western China, including those in Western Tianshan, Eastern Tianshan, Beishan, Altay, Kaladawan area at eastern part of the Altyn Tagh Mountain and southwestern margin of the South China Block. Recent studies suggest that this type of iron deposits may
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be the most potential for prospecting high-grade iron deposits in China in the future. The iron deposits are commonly associated with submarine volcanic-sedimentary sequences. The ore-related volcanic rocks exhibit a compositional spectra from basic to intermediate-basic, and to intermediate-acid rocks as well as the volcaniclastic equivalents, but most are intermediate-basic, and characterized by enrichment in Na. These volcanic rocks are typically sodic, and have Nb/Y ratios below 0.7, implying that the rocks are subalkaline. Combined with the regional geological features, it has been inferred that these rocks possibly formed within a continental marginal arc setting (e.g., Zhang et al., 2012). The volcanic-hosted iron deposits in China have broadly similar characteristics to those of apatite-bearing Fe oxide deposits (low Ti) in alteration and mineralization. However, apatite is rare, and skarns in many iron deposits are common. Unlike the classic skarns that are developed at the contact between intrusive plutons and carbonate rocks, no plutons have been recognized near the skarns in these iron deposits. In general, most iron orebodies occur in stratiform, lenticular, lensoidal and veined or complex veined shapes, with magnetite as the dominant ore mineral. In contrast, some iron orebodies are also hosted in pyroclastic-sedimentary rocks, where hematite is the dominant ore mineral with minor magnetite and/or rhodochrosite and hausmannite.
Metamorphosed volcanic-hosted Fe deposits (Algoma-type): The banded iron formation (BIF) deposits have been traditionally divided into Algoma type associated
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with volcanic rocks) and Superior-type stratabound in sedimentary sequences. The BIF type of iron deposits are dominantly distributed in the NCC, with only minor occurrences in the southern margin of the SCB, and the Qinling, Qilian and Kunlun orogenic belts. The Anshan-Benxi in Liaoning province, Eastern Hebei-Miyun in Beijing, Wutai and Lüliang in Shanxi province and Middle Inner Mongolia are the most important BIF ore clusters, where Algoma-type BIF deposits dominate. Available geochronological data suggest that the Algoma-type BIF deposits in China mainly formed at ca. 2.6-2.5 Ga (Li et al., 2014a). The major host rocks include amphibolites (or hornblende plagioclase gneiss), amphibolites, biotite leptynite, mica quartz schist, biotite quartz schist, sericite chlorite and magnetite quartzite. The most common ores are low grade, with 20-40% total Fe and 40-50% SiO2, and magnetite is the dominant ore mineral in almost all deposits. Quartz, chlorite, cummingtonite, almandine and carbonate are the most common gangue minerals. Large tonnage of high-grade ores (>10 Mt) are rare, only recognized in the Gongchangling iron deposits in Liaoning province. These high-grade iron ores consist predominantly of magnetite in contrast to hematite as the principal ore mineral in other cratons worldwide, and are controlled by faults and folds, and especially concentrated in the axis of folds.
3. Genesis of high-grade iron ores Although the genesis of high-grade iron ores in different types of iron deposits have been a matter of considerable debate, there is a general consensus that the
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formation of these iron ores can be correlated to multi-stages of geological processes that elevated the quality of iron ores. These different stages may be broadly related to one tectonic-magmatic cycle or distinct geological events. The high-grade iron ores in Ti–Fe–(V) deposits related to mafic intrusions (high Ti), apatite-bearing Fe oxide deposits (low Ti), contact metasomatic Fe deposits (Fe skarn) and volcanic-hosted Fe (unmetamorphosed) deposits are the examples. In contrast, some high-grade iron ores could have been produced by distinct events in space and time. Those in metamorphosed volcanic-hosted Fe deposits (Algoma-type) are examples. Previous studies suggest that the high-grade iron ores in Ti–Fe–(V) deposits related to mafic intrusions and some apatite-bearing Fe oxide deposits (low Ti) might have formed through a series of magmatic processes, including extensive fractional crystallization, and possible crustal contamination and liquid immiscibility as well as flow differentiation. For example, the high-grade iron ores in the Panzhihua (associated with layered intrusion) and Damiao (associated with anorthosite complex) were considered to be generated by multi-stages of magmatic processes including early stage of crystal fractionation and liquid immiscibility and subsequent crystallization accompanying flow differentiation (e.g., Song et al., 2013; Zhang et al., 2014b; Zhao et al., 2009). Minor ore-magma mineralization in the Gushan apatitebearing Fe oxide deposit was proposed to be the result of separation of the Fe-rich ore liquid in response to assimilation of P-rich strata of Fe-rich dioritic magma generated by fractionation of primary basaltic magma (Hou et al., 2010, 2011). In contrast, most of those in apatite-bearing Fe oxide deposits, skarn iron deposits and volcanic-hosted
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Fe deposits were produced by magmatic and subsequent hydrothermal stages. The large tonnage of high-grade ores in these types of iron deposits appear to be related to high crystal fractionation of magmas. The intrusions associated with skarn iron deposits generally display multiple pulses of magma emplacement, and they appear co-magmatic (e.g., Li et al., 2014b; Jin et al., 2015; Xie et al., 2015). In addition, all submarine volcanic rocks associated with volcanic-hosted Fe deposits have a compositional spectrum, and the petrological and geochemical characteristics suggest that they were produced by fractional crystallization of parent magma (Zhang et al., 2012). Experimental and fluid inclusion studies show that the volume and composition of exsolved fluids from melt are controlled by many factors during the magma-fluid transition (Webster, 2004). The fractional crystallization and assimilations of carbonate rocks and evaporites could lead to the formation of ore fluids and elevated Cl concentration of exsolution fluids, favoring the transportation of Fe as chloride complexes (Zhang et al., 2014b). The iron mineralization has also experienced multiple hydrothermal stages, which have been recorded in the textural and compositional data for magnetite from many skarn and volcanic-hosted iron deposits (e.g., Hu et al., 2015; Zhang et al., 2014c). The studied samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization, which suggest that the dissolutionreprecipitation process has been important in significantly removing trace elements from early-stage magnetite to form high-grade, high-quality iron ores in hydrothermal
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environments. The high-grade iron ores formed through distinct events in time are represented by the Algoma-type in Anshan-Benxi iron ore cluster. There is a consensus that the high-grade iron ores can be attributed to hydrothermal processes rather than primary sedimentation (Li et al., 2015a and therein). In summary, the formation of the large tonnage of high-grade iron ores appears to be the result of multiple stages of magmatic-hydrothermal processes.
4. Context of this special issue This special issue draws together related studies, and aims to highlight important recent discoveries related to the geologic setting, tectonic control, and petrogenesis of host rocks of iron mineralization and the metallogenic processes of iron ores (especially high-grade iron ores).
Salient highlights of the contributions to this
special issue are presented below.
Genesis of BIF deposits Precambrian banded iron formations (BIFs) are characterized by alternating chert and magnetite bands. However, The Tieshanmiaoe BIF deposit in the southern NCC has distinct features of ores, which comprise banded pyroxene-magnetite quartzite (BMQ) and disseminated magnetite pyroxenite (DMP). Whether the quartz-poor DMP represents metamorphosed iron-bearing ultramafic rocks or chemical sedimentary rocks is still unclear. The mineralogical and geochemical characteristics
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of the two types of ores show that the DMP and BMQ show similar major element contents, indicating a similar source through submarine chemical precipitation with little input from terrestrial or volcanic materials (Yao et al., 2015). The protoliths of the DMP and BMQ in the Tieshanmiao iron deposit are inferred to be quartzcarbonate iron-bearing formations which underwent subsequent metamorphism, but the DMP is rich in carbonate but relatively poor in silica. In most cases, the BIF-related high-grade iron deposits are mainly composed of hematite ores, and have been interpreted to be supergene in origin (e.g., Lascelles, 2012 and therein) or epigenetic hydrothermal enrichment (e.g., Thorne et al., 2004). However, those of BIF-related iron deposits in China are composed predominantly of magnetite in the Anshan-Benxi area in Liaoning Province. Based on the detailed comparison of the geology, geochemical and stable isotopic compositions of the highgrade iron ores and altered rocks from the mining area II of the Gongchangling deposit (G2) and Qidashan-Wangjiabuzi iron deposit (QW) in Liaoning Province, Li et al. (2015a) propose that desilicification process by hypogene alkaline-rich hydrothermal fluids was possibly responsible for the formation of high-grade iron ores in the G2 whereas iron activation-reprecipitation process generated the highgrade iron orebodies in QW. Based on the field relation, mineralogical and geochemical studies of the Macheng iron deposit in the eastern Hebei Province (the second largest BIF-related iron ore cluster), Wu et al. (2015) recognized two kinds of high-grade iron ores: massive oxidized high-grade ore that mainly consists of hematite with some magnetite,
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and massive high-grade magnetite ore composed predominantly of magnetite. They consider that the former may be correlated to supergene enrichment, caused by oxidation of magnetite and the leaching of gangue minerals from BIF to form highgrade ore, whereas the latter is probably related to intensive migmatization which produced high-grade ores by altering the primary iron ores. Constraining the age of the anatectic event related to migmatization could be key factor to the formation of high-grade iron ores in the eastern Hebei Province. Li et al. (2015b) present detailed textural relationship and internal structures of zircon grains from eight samples of migmatitic rocks representing the different Algoma-type BIFhosted iron deposits. Their studies show that the anatectic event lasted from 2511 to 2485 Ma, immediately following the BIF deposition, and is consistent with regional metamorphic events in the eastern part of the North China Craton. The Shilu Fe-Co-Cu ore district, located at Hainan Province of South China, is well known for high-grade hematite-rich Fe ores. However, depositional time of the host rocks has not been well constrained. Based on detrital zircon U-Pb dating, Wang et al. (2015a) considered ca. 1070-970 Ma as the maximum depositional time. In addition, Wang et al. (2015b) recognized three types of Co-Cu ores. Based on the textural and chemical composition of sulfides, they constructed a four-stage metallogenic model for the Cu-Co ores, involving ca. 1075-880 Ma deposition of the primary Co-Cu ore source beds, the syn-structural metamorphism, and two hydrothermal activities. Thus, the Shilu Co-Cu ores are considered to be a BIF origin, but greatly reworked and enriched by late structural deformation and hydrothermal
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activities. The Mesoproterozoic Jingtieshan BIF deposit is located in the Northern Qilian Orogenic Belt in NW China, and is composed essentially of specularite and jasper, with minor carbonate minerals and barite. Based on the detailed geochemical studies, Yang et al. (2015) conclude that the Jingtieshan BIFs share many characteristics with the three main types of global BIFs namely the Algoma-, Superior- and Rapitan-types, but they do not conform to all the features of any of these deposits. Thus, they proposed that the Jingtieshan BIFs represent a unique type, similar to that of sedimentary-exhalative mineralization.
Genesis of Ti–Fe–(V) deposits related to mafic intrusions (high Ti) The Panxi area in the Emeishan large igneous province, SW China is the largest Ti-Fe-V magnetite oxide cluster. Understanding fractionation processes of magma is a prerequisite to constrain the origin of Ti–Fe–V ores. To investigate the fractional crystallization history and origin of Ti–Fe–V deposit of the Taihe intrusion, She et al. (2015) measured the trace element compositions of magnetite and ilmenite by LAICP-MS method. Their results suggest that early crystallization of Fe-Ti oxides is the result of the high FeOt and TiO2 contents in the parental magmas, and the gravitational sorting and settling of magnetite and ilmenite result in formation of FeTi-V oxide ore layers. The Hongge mafic-ultramafic layered intrusion hosts the largest Fe-Ti-V ores plus interesting Cu-Ni-platinum-group element (PGE) mineralization which may have
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economic potential in the Panxi area. Liao et al. (2015) reports new whole-rock major and trace element compositions, PGE abundances and Sr-Nd isotopic data for selected cumulate rocks and basalts. They investigate the nature of parental magmas and the controls on its evolution processes from the source mantle en route to the surface. Their studies suggest that low-Ti and high-Ti basalts share a common parent magma with the Hongge intrusion, and they correlated the genesis of the massive Fe-Ti oxide layers in the middle zone of the Hongge intrusion to the event that multiple replenishment magma mixing with the residual Fe-Ti-rich liquid which heats the strata to attract H2O. The layered mafic-ultramafic intrusions generally represent an open-system process responsible for the magma generation, segregation, ascent, storage, mixing, crystallization and eruption. The traditional Assimilation-Fractional Crystallization (AFC) model has been developed to trace the geochemical paths of magmas (Depaolo, 1981). However, this model neglects the energy conservation and the assimilant compositional variations in the quantitative treatments of open-system processes. Thus, the Energy-Constrained Recharge, Assimilation Fractional Crystallization model has been developed (Bohrson and Spera, 2003). Based on this model, Li et al. (2015d) used published Sr-Nd isotopic data to simulate and evaluate the petrogenesis and mineralization of the Xinjie layered mafic-ultramafic intrusion in the Panxi area, hosting Fe-Ti-V oxide ore in the upper part and Ni-Cu-PGE sulfide deposits in the lower part. Their modeling is consistent with the observed scenarios, and shows that the processes are more complex than the previously thought.
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The ca. 1.7 Ga Damiao anorthosite complex in the NCC is a unique example hosting abundant titanomagnetite-dominated ore deposits in China. Both the Fe-Ti-Prich silicate rocks and massive Fe-Ti-(P) ores occur as discordant late-stage dikes cross-cutting early-stage anorthosites with irregular but sharp boundaries. Some latestage dikes comprise well-developed alternating late-stage anorthosites and Fe-Ti-Prich pyroxenites defining rhythmic layers, and the genesis of these ores remains poorly understood. Based on the field and petrographic observation and compositional variations, Li et al. (2015c) propose a sequence of crystallization and interpret the mechanism of the rhythmic layers as a result of slow and extensive fractional crystallization of the interstitial melt. They consider that high solubility of phosphorous probably plays an important role in the formation of rhythmic layering, whereas the massive Fe-Ti-(P) ores may represent the most evolved products of the residual magma.
Genesis of skarn iron deposits The Fushan iron deposit associated with dioritic intrusion is one of the typical Handan-Xintai type of skarn deposits in NCC. However, the iron source remains equivocal. Shen et al. (2015) present new zircon U-Pb age data from the ore-related dioritic rocks, the gneissic xenoliths in the dioritic intrusion and the Neoarchean basement rocks surrounding the deposit. The results show that the ore-related dioritic intrusion was emplaced at 128.8±1.9 Ma. Zircon grains from the basement rocks and xenoliths have the similar ages (Neoarchean to Paleoproterozoic) to inherited zircon
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in the dioritic intrusion. These results provide new evidence that the continental lower crust may be the major source of iron ores. Based on the studies of the Jinling skarn-type iron deposit in the Central NCC, Jin et al. (2015) also arrive at similar conclusions that the primary magma witnessed the interaction between the partial melts of relatively oxidized delaminated ancient crust and mantle peridotite. Furthermore, they proposed a two-stage model of Feenrichment process involving fractional crystallization of the primary magma giving rise to high Cl and Fe contents in the magmatic-hydrothermal fluid and later Feleaching process, accounts for the high-grade ore bodies. Unlike the major skarn-type iron deposits in the NCC which are generally associated with dioritic plutons, the Zhangmatun deposit is genetically related to gabbro. Based on the zircon U-Pb, mineralogical and geochemical studies, Xie et al. (2015) propose that the gabbroic magma were derived from amphibole-bearing spinel lherzolite in the enriched EMI-type upper mantle which was modified by incorporation of lower crust material, and they correlate the high-grade skarn iron deposit to the exsolution of Fe-rich magmatic fluids and subsequently leaching from the solidified gabbro at the hydrous skarn stage. Furthermore, the relatively high oxygen fugacity of the melt was also considered to be one of the controlling factors that led to the formation of the iron deposit. Based on the comparison of the ore-related Kuangshancun intrusive complex and
coeval barren Hongshan complex, Sun et al. (2015) propose that although both complexes were produced by partial melting of an enriched lithospheric mantle
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contaminated by lower continental crust components, the Kuangshancun complex was characterized by higher oxygen fugacity as compared to the Hongshan complex. Thus, they conclude that the high oxidation states and high water contents are considered as key factors that led to the iron mineralization in the Handan-Xingtai district. The relationship between evaporites and skarn Fe deposit remains poorly constrained. Zhu et al. (2015) determined the sulfur isotopic composition of pyrite, as well as the composition of halogen-rich minerals (scapolite and amphibole) in the Jinshandian skarn Fe deposit in SCB. The δ34S values for pyrite from the early and late retrograde stage are significantly heavier than those of sulfides from magmatichydrothermal fluid, indicating that sulfur in the Jinshandian ore-forming system was mostly derived from evaporites. This inference is further reinforced by the data of Cl contents of the amphibole and the scapolite. They infer that the hydrothermal system of the Jinshandian skarn Fe deposit probably experienced significant incursion of evaporites before or during the late prograde stage.
Genesis of apatite-bearing magnetite (low-Ti) Based on detailed petrographic observations and fluid inclusion studies on the Hemushan apatite-magnetite deposit in the southern part of the Ningwu iron ore district of the Middle-Lower Yangtze River Metallogenic Belt, Luo et al. (2015) propose that the ore-forming fluid was of magmatic origin at the early phase of the middle metallogenic stage. Progressive reaction of the ore fluids with the host rocks and boiling of the ore-forming fluid may be the potential mechanism of the deposition
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and enrichment of iron ore.
Genesis of volcanic-hosted Fe deposits Volcanic-hosted Fe deposits are one of the most important iron ores in China. However, the nature of primary magma and petrogenesis associated with these iron ores is poorly understood. Li et al. (2015e) identified some iron-rich fragments (IRF) from the Yamansu iron deposit in Eastern Tianshan Mountains, NW China. These fragments display typical volcanic textures and structures, such as porphyritic texture, and vesicle-amygdale structure. The magnetite is distributed in between the feldspar laths in ground mass. The IRF has an average total Fe of 26 wt%. The magnetite is mostly titanium–vanadium magnetite, with the TiO2 content ranging up to 4.86 wt.% and V2O3 content up to 3.20 wt.%. Thus, IRF has been considered to be produced by iron-rich melts and represents the products of the Fenner magma evolution from a basaltic magma.
5. Future prospects This special issue compiles the recent trends in research on the genesis of iron deposits in China. Although several important advances have been made in specialized disciplines, some issues that remain to be elucidated include: (1) the tectonic settings of volcanic-hosted Fe deposits; (2) the role of liquid immiscibility in the genesis of iron deposits; (3) the iron sources of skarn iron deposits: whether orerelated intrusions or the Fe-rich strata; (4) the relationship between magmatic nature
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and generation of iron-rich fluids; and 5) the role of evaporites on the formation of skarn and apatite-bearing magnetite ores. We hope that the papers presented in this special issue would encourage further work in these lines.
Acknowledgements The guest editors thank the all authors for their scholarly contributions to this special issue. We also thank the referees who provided their valuable time and constructive comments and suggestions. We acknowledge the valuable patronage and support from Editor-in-Chief, Prof. Bor-ming Jahn during the organization and preparation of this special issue. This special issue is financially supported by 973 Program (No. 2012CB416800) from the Ministry of Science and Technology of China.
Zhaochong Zhang* State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, PR China Corresponding author. Tel. : +86 10 82322195; fax: +86 10 82323419. E-mail address: [email protected].
M. Santosh State Key Laboratory of Geological Process and Mineral Resources,
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China University of Geosciences, Beijing 100083, PR China Department of Earth Sciences, University of Adelaide, SA 5005, Australia
Jianwei Li State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, PR China
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