Magmatic–Hydrothermal Origin of Fe-Mn Deposits in the Lesser Khingan Range (Russian Far East): Petrographic, Mineralogical and Geochemical Evidence

Abstract

Iron and iron–manganese deposits form three closely spaced clusters within the Lesser Khingan Range of the Russian Far East. Fe-Mn mineralization is hosted in Vendian–Cambrian carbonates and composed of magnetite, hematite, braunite, haussmanite, rhodochrosite and pyrolusite. The iron–manganese ores are closely associated with explosive intermediate–felsic breccias, magnetite-rich lavas, dolerites and mineralized lithocrystalloclastic tuffs. Magmatic rocks display both concordant and discordant relationships with Fe-Mn mineralization and contain abundant xenoliths of host carbonates. Both magmatic rocks (with the exception of Nb-enriched dolerites) and Fe-Mn ores are characterized by variable enrichments in large-ion lithophile and light rare earth elements and strong depletions in high-field strength elements compatible with the broad subduction setting for explosive volcanism and associated hydrothermal Fe-Mn ore mineralization. Nd-Sr isotope systematics suggest contamination by both ancient and juvenile continental crust and the involvement of recycled pelagic sediment in the formation of Fe-Mn deposits in the Lesser Khingan Range of the Russian Far East.

1. Introduction

Iron and iron–manganese deposits in orogenic belts of different ages contain substantial resources of these important industrial metals and include different genetic types such as iron oxide–copper–gold (IOCG), Kiruna-type iron oxide–apatite (IOA; apatite–magnetite), sedimentary exhalative, magmatic–hydrothermal, volcanogenic (Cuban-type) and skarn-type deposits, as well as many other subtypes of Fe-Mn mineralization [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. These “orogenic” deposits experienced variable scale contributions from a wide range of magmatic, hydrothermal, sedimentary and biological sources and processes [9,14,15,17,25,29,31,32,33,34,35,36,37,38,39,40,41,42,43]. A separate group of skarn-style magnetite mineralization (Cornwall-type deposits) has been documented in Pennsylvania and North China cratons in relation to interactions between basaltic magma, ore-forming fluids and host shale, sandstone and carbonate [44,45,46]. Many individual Fe- and Fe-Mn deposits and large Fe-Mn metallogenic belts are related to mafic-to-silicic explosive volcanism or contain detectable (substantial) volcanic components, as average global oceanic fluxes in dissolved iron and manganese from volcanic sources are estimated to be between 50 and 500 (median 180) and 0.6 and 3.2 (median 1.3) Gmol/year Fe and Mn, respectively [42]. Iron–manganese mineralization frequently occurs in young volcanic arcs (Aegean, Izu–Bonin–Mariana, Tonga–Kermadec, Vanuatu, New Britain, Kurile, Aleutian) and Cenozoic back-arc basins (Tyrrhenian Sea, Lau Basin, North Fiji Basin, Mariana Trough, Okinawa Trough; Sea of Japan, Sea of Okhotsk), [47,48,49,50,51,52,53,54,55,56,57,58,59] suggesting causal links between active volcanism (frequently explosive), hydrothermal activity (hydrothermal mounds, hot springs, etc.) and iron–manganese mineralization in modern and ancient subduction zones [6,10,11,12].

The Lesser Khingan Range (LKR) forms a part of the Bureya–Jiamusi–Khanka (BJK) superterrane located at the southeastern end of the Central Asian Orogenic Belt (CAOB) [60,61,62]. The southern part of the LKR includes a cluster of Fe-Mn deposits, which form a 40 km wide mineralized belt extending from north to south across the state border into China [63]. Traditionally, these deposits were viewed as sedimentary or volcanogenic-sedimentary due to the stratiform appearance of constituting ore bodies and close association with Vendian–Cambrian sedimentary formations [64,65,66]. Iron resources in this Fe-Mn belt were estimated at 3 billion tons of iron ore [66], which makes them potentially comparable to the largest iron–manganese provinces in the Russian Federation [67]. An alternative view was pioneered by Berdnikov et al. [68], who proposed hydrothermal (probably with some hydrogenic component) origin for Fe-Mn mineralization in the LKR, possibly within a paleo-subduction magmatic–hydrothermal context. The magmatic-hydrothermal origin was further supported by the recognition of a close spatial and possibly genetic association between the Fe-Mn ores and explosive breccia (“fluidolites”), as the latter exhibit clear evidence of volcanic origin (ignimbrite-like textures, presence of altered volcanic glass and igneous minerals, etc.) along with the presence of platinum-group elements (PGE) and gold in both Fe-Mn ores and associated explosive breccia [69,70,71,72,73,74,75]. Platinum particles from the Poperechny Fe-Mn deposit (one of the largest ore bodies in the LKR cluster) were dated at 125 ± 21 Ma, indicating a Mesozoic volcanic overprint [72,73]. Explosive breccias in the Poperechny, Kaylan and Kostenga deposits are characterized by the large-ion lithophile element (LILE) and light rare earth element (LREE) enrichment, coupled with high-field-strength element (HFSE) depletion, suggesting that this Fe-Mn-PGE-Au overprint occurred in the paleo-subduction zone environment [72,74,75].

In this paper, we present new data on the mineralogy, petrology and geochemistry (including Sr-Nd isotopes) of Fe and Fe-Mn ores and associated volcanic rocks from three principal sites in the LKR metallogenic province, namely Poperechny, Kaylan and Kostenga deposits. We will then attempt to achieve the following goals using the data set specified above: (1) document occurrences of igneous rocks in association with iron–manganese mineralization (including their structural and textural relationships) and evaluate the possible role of magmatic (especially explosive) processes in the formation of the LKR Fe and Fe-Mn deposits; (2) determine the composition and model isotopic ages of mantle and crustal sources involved in the formation of these deposits in the LKR using new Nd-Sr data; (3) evaluate the relative roles of magmatic and hydrothermal processes during the formation of the LKR Fe and Fe-Mn deposits within the broad geodynamic setting of paleo-subduction of the Izanagi–Pacific plate beneath the Northeast Asian continent.

2. Geologic Background

The Bureya terrane forms a part of the complex Bureya–Jiamusi–Khanka superterrane at the southeast end of the CAOB (Figure 1).

This is usually viewed as a relic of Precambrian sub-continental blocks (typically together with the Khanka terrane) that were thermally, metamorphically and magmatically reworked during Paleozoic and Mesozoic within the tectonically evolving CAOB structure [76,77,78,79,80], or as a part of the Bureya–Jiamusi–Khanka superterrane, and is linked to the break-up of the Gondwana supercontinent and later-stage accretion of these continental fragments to the southeastern edge of Eurasia [81,82]. The Bureya terrane is composed of Neoproterozoic, Paleozoic and Mesozoic terrigeneous and carbonate metasediments, along with various igneous rocks including gabbro, diverse granitoids, adakites and minor ultramafic rocks [75,76,77,78,79,80,83,84,85]. In essence, the Bureya terrane represents a large fragment of the Proterozoic accretionary prism metamorphosed under amphibolite–facies conditions during the Ordovician period [68,73,86].

The iron and iron–manganese deposits and showings are localized within the sub-longitudinal zone of approximately 40 km wide, which stretches from the Amur river in the south to the Kimkan river over an approximate distance of 150 km [63,64,68]. Most of the individual sites are concentrated within three lateral clusters: (1) South Khingan with probably the best-explored Poperechny Fe-Mn deposit (9 million tons of ore at 21% Mn and 8%–34% Fe); (2) the Kimkan–Kostenga cluster with principal deposits of Kimkan, Kaylan and Kostenga (combined resources for the entire cluster are believed to be in excess of 600 million tons of ore [65,66]); (3) the Vostochny cluster, represented by a mid-size Bidzhan iron–manganese deposit (over 6 million tons of ore at 18.4% Mn and 13% Fe).

The Poperechny Fe-Mn deposit is hosted in the Vendian–Cambrian carbonates of the Murandavsky suite within the southern portion of the South Khingan cluster (Figure 1). Iron–manganese mineralization is represented by magnetite, hematite, braunite, hausmannite–braunite, braunite–hematite and rhodochrosite–hausmannite ores, along with their variably oxidized varieties [68,70,71,72]. Relatively unoxidized ore varieties typically contain around 20% Mn and 8% Fe, while oxidized and silicified ores may contain up to 35% of iron. Host Murandavsky suite limestones are partially dolomitized and intensely fractured coarse-grained sedimentary rocks with calcite, quartz and pyrite as typical fracture infill products. Murandavsky suite dolomites range from massive to locally sheared and are typically characterized by locally intense silicification and pyritization [68]. Volcanic rocks (tuffs, mineralized tuffs, explosive breccias) impregnate carbonate sediments and display both gradual transitions and discordant contacts with the Fe-Mn mineralization [70,72]. Abundant carbonate xenoliths in tuffs and explosive breccias, along with their discordant relationships with Vendian–Cambrian carbonates, are consistent with their later-stage formation, and probably contemporaneous with the iron-manganese ore [68,70,71,72].

The Kostenga Fe deposit is located in the central Kimkan–Kostenga cluster of iron–manganese mineralization. Similar to the Poperechny site, Kostenga mineralization is hosted by the Vendian–Cambrian Murandavsky carbonates, but Phanerozoic intrusive activity plays a significantly larger (both volumetrically and compositionally) role at Kostenga. Murandavsky dolomites at the southern terminus of the Kostenga deposit are intruded by the Ordovician granite, Middle to Late Carboniferous gabbro and Late Permian to Early Triassic granodiorite, while the ores and associated pyroclastics (“fluidolites”) are unconformably overlain by Cretaceous basalt–andesite–dacite–rhyolite volcanics [74]. Explosive volcanic rocks associated with Fe mineralization at Kostenga, and are represented by andesitic breccias with abundant angular fragments of host carbonates and quartz–magnetite (occasionally with stilpnomelane, hematite and rare braunite) mineralization, along with lithocrystalloclastic tuffs, which frequently have ignimbrite-like textures [75]. Some iron ores from the Kostenga deposit display (as in the case of the Poperechny site) textural and mineralogical characteristics similar to the “magnetite lava” samples from the El Laco iron deposit in Northern Chile [31,35,40,87,88,89].

The Kaylan Fe deposit, just 10 km to the NE from the Kostenga site, appears to occupy a structurally and stratigraphically higher position, as iron–oxide mineralization is developed at the contact between Vendian–Cambrian dolomites and Lower Cambrian terrigeneous–carbonaceous metasediments [68]. Magnetite ores seem to thin out towards the north, where the mineralization is overlain by Cretaceous felsic to intermediate lavas and tuffs. Magnetite–hematite ore lenses of from 0.2 to 2.4 m thick are intercalated with mineralized (predominantly magnetite) agglomerate tuffs, volcanic breccia, quartzite and massive hematite ore [75]. Similarly to the previously described deposits, phenocrysts and groundmass in some volcanic breccia are composed of magnetite, again resembling Fe-oxide volcanic rocks from the El Laco locality. Both explosive rocks and iron ore from the Kaylan deposit contain abundant fragments of the host carbonate, suggesting spatial and temporal proximity between the loci of volcanism and iron mineralization [75].

3. Analytical Methods

A total of more than 250 representative samples from the three key largest Fe and Fe-Mn deposits (Poperechny, Kostenga and Kaylan) in the LKR were analyzed in the course of this study by the various analytical methods summarized below. This study is based on an examination of 265 thin sections of sedimentary and volcanic rocks, along with Fe-Mn ores, XRF and ICP-MS determinations of major and trace elements in 212 samples (158 analyses were used to construct geochemical diagrams used in this study), and 148 photographs of hand specimens and polished sections of principal lithologies in the LKR Fe-Mn deposits (18 samples and polished sections used in this paper). Twenty-nine representative samples were analyzed in detail by the SEM-EDA.

Petrographic studies of Fe-Mn mineralization, along with igneous and sedimentary rocks, were carried out using an Imager A2m petrographic microscope (Carl Zeiss, Jena, Germany).

A detailed systematic study of mineral phases in ores and associated carbonates and volcanic rocks was completed using a VEGA 3 LMH TESCAN (Brno, Czech Republic) scanning electron microscope (SEM) with the Oxford X-Max 80 Gb energy dispersive spectrometer (EDS), with the following operating conditions: accelerating voltage of 20 kV, beam current of 530 nA and beam diameter of 0.2 µm. Reference samples including 37 natural and synthetic oxides, minerals and pure native metals (Oxford/108699 no. 6067) were used as standards. Co-standard Oxford/143100 no. 9864-15 was used for daily calibration of the SEM instrument. Accuracy of the EDS analyses was estimated to be ±0.1 wt.%. Special sample preparation protocols, reported in detail in [68] and designed to prevent contamination, were utilized to expose metallic phases in situ and determine their relationships with host silicate and oxide phases, as well as associated rock-forming and accessory minerals.

Major elements were measured on pressed pellets using a S4 Pioneer XRF spectrometer (Bruker, Leipzig, Germany). International LDI-3 (gabbro) and WMG-1a (mineralized gabbro) reference materials were used for calibration. The analytical accuracy of major elements in the course of this study was ±10%. Abundances of trace elements were determined with an ELAN 9000 ICP-MS (Perkin Elmer, Woodbridge, ON, Canada) after the acid digestion of a powdered sample. In addition to the standards listed above, geochemical reference samples BHVO-2 (USGS; Hawaiian basalt) and JB-3 (Geological Survey of Japan; Fuji basalt), along with Perkin Elmer standard solutions PE# N9300231-9300234 for internal calibration, were used to control the accuracy of analytical measurements. In the course of this study, the accuracy was ±5% for trace element abundances of >20 ppm and ±10% for chemical elements with abundances of <20 ppm. All the above-listed analytical procedures were carried out at the Khabarovsk Innovative-Analytical Center of the Institute of Tectonics and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, Khabarovsk, Russian Federation. Additional details of sample treatment and analytical features can be found in [70,75].

Strontium and neodymium isotopes were measured using a Triton Tl (Thermo Finnigan, Bremen, Germany), multi-collector, solid-phase mass spectrometry at the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences, St. Petersburg, Russian Federation. Rb, Sr, Sm and Nd were extracted following the methods outlined in [90], and their elemental concentrations along with ⁸⁷Rb/⁸⁶Sr and ¹⁴⁷Sm/¹⁴⁴Nd isotope ratios were determined by isotopic dilution. Multiple runs of the BCR-1 reference sample were performed to determine analytical errors for Rb, Sr, Sm and Nd, which were estimated at ±0.5% for the course of this study. The total laboratory blank at the IPGG was 0.05 ng for Rb, 0.2 ng for Sr, 0.3 ng for Sm and 0.5 ng for Nd. Isotopic analysis reproducibility was controlled by the certified standards JNdi-1 (¹⁴³Nd/¹⁴⁴Nd = 0.512117) [91] and SRM-987 (⁸⁷Sr/⁸⁶Sr = 0.710240) [92]. The resulting ⁸⁷Sr/⁸⁶Sr value for SRM was 0.710275 ± 15 and the ¹⁴³Nd/¹⁴⁴Nd value for JNdi-1 was 0.512098 ± 9 for the period of this study. The Sr isotopic composition was normalized to ⁸⁸Sr/⁸⁶Sr = 8.37521 and the Nd composition was normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219.

4. Results

Individual deposits in the LKR cluster include various sedimentary (dolomite, limestone, siltstone, mudstone) and volcanic (explosive breccia, tuff, tuffaceous siltstone, magnetite-rich lava) rocks, along with the associated Fe- and Fe-Mn mineralization. The following section presents the results of a petrographic, mineralogical and geochemical study of principal rock lithologies at the Poperechny, Kostenga, and Kaylan sites.

4.1. Petrography and Mineralogy

Mineralization in the LKR deposits (Figure 2a–d) is represented by massive and laminated ore composed of (in highly variable modal proportions) magnetite, hematite, limonite, goessite, braunite, haussmanite and rhodochrosite with subordinate (minor) pyrolusite, quartz, sericite, chlorite, gedritic–gruneritic amphibole, stilpnomelane, apatite, monazite, xenotime, La-Ce-Pr-Nd-oxides, pyrite and other sulfides. Ore is frequently saturated with fragments of tuffaceous pyroclastic material and intensely limonitized (Figure 2b–d). Lithocrystalloclastic and lithoclastic tuffs are variably mineralized (magnetite, hematite, rhodochrosite) and contain ash- to lapilli-sized (typically 0.5–1 mm to 20 mm in size, e.g., Figure 2e) fragments of host carbonates and Fe-Mn mineralization, along with ash-sized particles composed of quartz, dolomite, chlorite, biotite, amphibole, magnetite and sericite. Tuffs also form thin layers (from 1 to 15 mm) in stratified rocks composed of variable combinations of explosive breccia and iron-manganese ore (Figure 3d,f). Tephra fragments in mineralized tuffs are commonly angular, beveled and asymmetrical, with larger fragments being elongated up to 15–20 mm in length (Figure 3f). Mineralized tuffs are also frequently variably limonitized (Figure 2f,g). Explosive breccia (Figure 2h,i) is composed of large (1 5, rarely 10–20 cm in size) clasts imbedded in a fine-grained matrix, which occasionally contains strongly altered fragments of felsic (?) volcanic glass [72]. Breccia clasts account for 40%–60% of the total rock volume and are composed of various carbonates (predominantly dolomite and marble), Fe-Mn mineralization and previous (pre-breccia emplacement) generations of intermediate to felsic pyroclastic rocks including ignimbrite-like tephra with fiamme-like textures [72]. The groundmass of most explosive breccia samples is also characterized by ignimbrite-type fluidal textures [68,72]. Both carbonate and magmatic clasts are replaced with variable amounts of hematite, limonite, goethite and rhodochrosite [68]. Other secondary minerals and features in the explosive breccia include sericite, chlorite, secondary sulfides, quartz–chlorite and carbonate veinlets, as well as locally intense carbonatization, pyritization and silicification.

The geologic relationships between explosive breccia, host Vendian–Cambrian carbonates of the Murandavsky suite and Fe-Mn mineralization are illustrated in Figure 3. Explosive breccia frequently forms an interlayered sequence with laminated Fe and Fe-Mn ores in all three LKR deposits (Kostenga, Kaylan, Poperechny) highlighted in the current study (Figure 3a). Locally, explosive breccia also displays clear discordant (extrusive) relationships with both the host carbonate (Figure 3b) and Fe-Mn mineralization (Figure 3c) suggesting later-stage injections of hot and fluidized felsic magma into the carbonate-Fe-Mn ore structural frame [68,72]. In most cases, however, locally explosive breccia has concordant contacts with the iron–manganese ore, which are sharp and devoid of any gradual transition features (Figure 3e). In some cases, explosive breccia forms laminated structures involving fine-grained tephra and Fe-Mn ore (Figure 3d), with pyroclastic rocks (both fine-grained mineralized tuff and explosive breccia) apparently filling the topographic irregularities (micro-relief lows) on the top of the fully consolidated Fe-Mn ore (Figure 3d). Also, locally, breccia-sized pyroclastic material alternates with ash-sized micro-layers (1–2 mm thick) within the massive-textured Fe-Mn mineralization (Figure 3f).

Microscopically, iron ores are represented by massive and laminated mineralization with crystalline to granular textures composed of magnetite, hematite, chlorite and quartz (Figure 4a). In addition, some feldspar (Figure 4a), monazite, apatite, sericite and amphibole (gedrite, ferrogedrite, grunerite) are observed in the Fe-ore from the Poperechny deposit [68,72]. Mineral abbreviations of all petrographic and mineralogical figures follow [93]. Explosive breccia typically contains angular fragments of dolomite and Fe-Mn mineralization, imbedded in a fine-grained matrix. The breccia matrix is characterized by an aphanitic to eutaxitic (Figure 4b) texture similar to the ones displayed by welded ignimbrite tuffs [94]. In addition to rock fragments, explosive breccia in the LKR Fe and Fe-Mn deposits contain variably shaped grains of quartz, amphibole, chlorite and sericite, along with unevenly distributed secondary Fe-oxides and hydroxides. The mineralized tuffs associated with Fe-Mn mineralization contain various lithic clasts and individual mineral grains in the fine-grained vitric matrix (Figure 4c,d). Some coarse-grained tuffs display a strong alignment of dolomitic clasts and magnetite grains in a non-welded ignimbrite-like matrix composed of magnetite, biotite and quartz (Figure 4c). Some crystalloclastic tuffs show variably deformed biotite laths (partially replaced with chlorite) imbedded in a magnetite–chlorite–feldspar–quartz matrix (Figure 4d). Magnetite-rich lava is represented by banded (Figure 4e) and massive (Figure 4f) porphyritic varieties with phenocrysts composed of equigranular magnetite, quartz and feldspar. Groundmass also occasionally contains biotite, amphibole, monazite, apatite and zircon, along with secondary chlorite, sericite, hematite and rhodochrosite. Banding, in some Fe-rich lavas, is emphasized by the strong alignment and orientation of magnetite aggregates, elongated carbonate fragments and light-colored feldspar-quartz bands (Figure 4e). Locally, magnetite-rich lavas are cross-cut by thin (<50 microns) secondary carbonate veinlets (Figure 4f). Basaltic intrusions associated with explosive breccia display typical ophitic (doleritic) textures composed of elongated prismatic clinopyroxene and plagioclase (Figure 4g). Dolerites also frequently contain large equant olivine phenocrysts (Figure 4h) and euhedral to subhedral late-stage amphibole grains (Figure 4g,h, respectively). Some host dolomites of the Vendian–Cambrian Murandavsky suite contain equant subhedral to anhedral olivine porphyroblasts (Figure 4i), most probably reflecting contact chemical changes due to the injection of hot intermediate to felsic magma into carbonate’s structural frame [68,72].

Iron–manganese mineralization includes some additional accessory mineral phases, as suggested by detailed SEM mineralogical studies. The quartz–hematite–magnetite ore contains anhedral grains and segregations of Fe-rich dolomite and Mg-rich siderite (Figure 5a). Magnetite–hematite–quartz, magnetite–hematite–quartz–chlorite and magnetite–hematite–quartz–chlorite–sericite varieties also frequently include euhedral to anhedral grains of apatite (Figure 5b) and monazite (Figure 5d) and xenotime (Figure 5e). Some hematite-rich iron ores contain precious metal minerals such as non-stoichiometric silver chloride (Figure 5c). One sample of hematite-quartz ore from the Poperechny deposit includes a single anhedral grain of Pb-Bi selenotelluride (Figure 5f). Selenotellurides of chalcophile metals are common ore mineral phases in the wide range of epithermal, porphyry, orogenic and intrusion-related gold (gold–telluride) deposits [95,96,97,98,99].

Magnetite-rich lavas from the Kostenga and Poperechny Fe-Mn deposits in the LKR carry euhedral to subhedral magnetite phenocrysts and microphenocrysts hosted in amphibole–biotite–quartz–monazite groundmass (Figure 6a,b). Some samples also contain metasomatic rhodochrosite (Figure 6a), and possibly some other secondary Fe-Mn mineral phases [68]. Relatively rare amphiboles in the LKR magnetite-rich lava are represented by minute ferrogedrite crystals (Figure 6a). Some Fe-rich lava samples from the Kostenga deposit display distinctive fragmentation, which locally turns into macro- to micro-scale layering or banding consisting of alternating magnetite, quartz–magnetite-apatite and quartz–biotite mineral bands (Figure 6c). Somewhat similar textural and mineral banding has previously been reported from the magnetite-rich El Laco lavas and some other Fe-rich volcanic localities in northern Chile [35,39,46,100]. Magnetite lavas at both Poperchny and Kostenga sites carry some silver (in addition to PGE and Au, described earlier in more detail [69,70,71,72,73,74,75]) mineralization represented by various Ag-bearing alloys, sulfides and halides (Figure 6d–f). Cupriferous silver, along with Cu-Ag-Au alloys [74,75,101], forms elongated euhedral particles in the rhodochrosite–quartz–biotite groundmass (Figure 6d), while anhedral acanthite is present in the chlorite–magnetite-dominated matrix (Figure 6e). Another rather remarkable silver mineral phase is represented by equant grains or aggregates of non-stoichiometric silver chloride associated with euhedral magnetite grains in a fine-grained chlorite–quartz–magnetite groundmass (Figure 6f). Non-stoichiometric silver chloride, along with acanthite and other silver-bearing sulfides, appears to be relatively common in Paleozoic black shale–volcanogenic formations, arc plutonic root complexes and Cenozoic volcanic rocks from a range of subduction- and non-subduction-related environments, where its origins are linked to the involvement of subduction-related hydrothermal fluids enriched in water, sulfur, and chlorine [99,102,103,104].

4.2. Geochemistry

The whole-rock geochemistry of explosive breccia and other mineralized pyroclastic rocks, along with various textural types of ore from the LKR deposits, was previously reported in [68,69,70,71,72,73,74,75]. Here, we list some representative whole-rock major and trace element analyses (

Table 1. Representative major and trace element compositions of principal rock lithologies associated with the Poperechny deposit.

Sample#	3-10	4-08	4-17	128-1	789	789-д	117
Rock Type	Eb1	Eb2	Eb3	FMo	Do1	Do2	Dl
SiO2 (wt.%)	59.49	61.79	55.44	39.20	51.83	50.22	12.29
TiO2	0.99	0.70	0.66	0.40	1.56	1.53	0.04
Al2O3	13.64	12.74	10.71	2.62	10.41	10.24	0.40
Fe2O3	7.86	3.70	5.71	24.21	10.66	11.32	1.58
MnO	0.18	0.05	0.14	14.95	0.13	0.13	0.15
MgO	4.75	4.21	6.09	6.38	10.88	11.46	17.95
CaO	2.11	4.46	6.58	2.08	4.70	4.59	31.87
Na2O	2.44	0.19	0.28	0.88	5.51	5.00	0.00
K2O	2.86	3.39	2.65	2.16	0.32	0.33	0.02
P2O5	0.12	0.13	0.13	0.09	0.14	0.14	0.00
LOI	5.42	8.96	11.86	5.97	3.38	4.38	45.18
Total	99.85	100.33	100.25	98.95	99.52	99.35	109.48
Cr (ppm)	43.32	88.93	88.85	21.54	356.63	346.74	7.35
Ni	276.33	46.20	158.40	429.91	166.39	160.01	11.95
Co	12.57	9.59	29.90	419.43	45.57	43.85	1.45
V	75.67	177.39	162.93	83.75	134.24	132.69	5.73
Sc	16.08	10.96	10.81	6.73	17.96	17.95	0.36
Cs	4.49	7.28	6.41	18.65	2.54	2.83	0.09
Rb	100.08	126.59	102.26	127.68	8.10	9.15	1.04
Ba	1350.92	555.10	464.0	784.19	308.44	323.03	33.60
Sr	50.62	62.48	82.70	79.39	208.9	198.73	54.24
Zr	74.47	69.21	88.31	52.13	57.49	61.03	24.27
Y	14.31	15.10	16.73	11.51	15.37	16.11	1.39
Nb	4.82	3.28	3.20	7.43	9.92	9.94	1.05
Ta	0.40	0.24	0.32	0.23	0.84	0.80	<0.001
Hf	2.10	2.00	2.34	1.39	1.80	1.82	0.82
Th	8.31	6.58	6.39	2.81	0.68	0.63	0.43
U	0.64	1.46	1.90	0.81	0.21	0.22	0.05
La	33.29	23.46	23.74	10.45	6.11	6.17	1.10
Ce	69.73	50.09	51.09	25.27	13.81	13.58	2.08
Pr	7.63	5.66	5.76	2.76	1.84	1.89	0.28
Nd	31.02	24.16	24.63	10.67	8.73	8.78	1.19
Sm	6.11	4.72	4.79	2.25	2.67	2.72	0.25
Eu	0.99	0.93	0.96	0.48	1.02	1.05	0.07
Gd	5.75	4.56	4.65	2.60	3.57	3.59	0.29
Tb	0.65	0.52	0.55	0.39	0.56	0.57	0.03
Dy	3.47	2.95	3.16	2.39	3.19	3.26	0.19
Ho	0.59	0.55	0.60	0.51	0.60	0.61	0.04
Er	1.84	1.76	1.93	1.70	1.59	1.61	0.12
Tm	0.24	0.25	0.27	0.28	0.21	0.21	0.02
Yb	1.70	1.78	1.98	2.14	1.28	1.27	0.12
Lu	0.23	0.26	0.29	0.34	0.18	0.18	0.02
Ag (ppm)	0.65	1.36	0.55	0.14	0.16	0.24	0.16
Cu	45.69	47.29	47.32	6.07	51.53	38.26	14.54
Zn	103.55	64.48	103.94	59.86	127.97	131.43	18.17
Sn	2.80	2.36	1.75	0.80	1.17	0.82	<0.001
Sb	N.A.	N.A.	N.A.	0.38	N.A.	N.A.	0.01
Pb	4.54	5.31	14.18	11.74	2.33	0.92	0.27
Bi	N.A.	N.A.	N.A.	<0.001	N.A.	N.A.	1.32
As	N.A.	N.A.	N.A.	16.12	N.A.	N.A.	0.24

Note: N.A.—not analyzed. Eb—explosive breccia, FMo—Fe-Mn ore, Do—dolerite, Dl—dolomite.

) for the samples that were analyzed for Sr and Nd isotopes. In addition, in this section, we present major and trace element compositions of magnetite-rich lavas from the Poperechny and Kostenga deposits, which appear to play an integral role in the formation of iron–manganese deposits in the LKR (

Table 2. Major and trace element compositions of magnetite-rich lavas from the LKR.

Deposit	Kostenga			Poperechny
Sample#	757	758-1	759	116	116-1
SiO2 (wt.%)	69.60	39.20	65.40	33.37	22.66
TiO2	0.22	0.49	0.11	0.26	0.25
Al2O3	2.30	7.06	1.87	2.36	1.45
Fe2O3	22.01	11.65	26.48	33.53	25.52
MnO	0.03	6.18	0.99	5.26	17.01
MgO	1.57	7.46	1.51	4.77	4.77
CaO	1.01	9.17	0.68	6.51	6.54
Na2O	0.12	0.19	0.10	0.08	0.10
K2O	0.32	1.37	0.37	0.05	0.04
P2O5	0.75	0.11	0.32	0.04	0.05
S	0.01	0.00	0.01	N.A.	N.A.
F	0.55	0.11	0.66	N.A.	N.A.
LOI	1.58	17.56	1.48	12.50	22.05
Total	100.05	100.57	99.98	100.02	100.45
Cr (ppm)	61.94	53.52	22.84	12.77	9.92
Ni	14.28	54.77	37.11	70.45	61.17
Co	13.87	67.87	22.97	144.69	242.97
V	59.55	62.34	47.78	73.54	11.35
Sc	6.16	10.03	3.48	5.50	4.25
Cs	9.45	3.91	18.91	0.94	1.48
Rb	21.59	54.01	31.42	2.05	1.61
Ba	144.84	830.1	822.8	48.35	161.47
Sr	67.60	265.25	41.88	29.96	125.96
Zr	11.40	45.22	6.44	33.55	25.43
Y	11.63	10.02	4.82	6.95	8.16
Nb	2.83	8.65	1.38	5.07	3.48
Ta	0.07	0.55	0.03	0.09	0.04
Hf	<0.001	1.41	<0.001	0.77	0.42
Th	1.63	5.63	0.77	1.56	1.12
U	0.26	0.60	0.27	0.24	0.24
La	9.48	20.15	2.31	4.76	7.20
Ce	20.86	40.81	4.83	12.58	14.92
Pr	2.81	4.98	0.68	1.37	1.72
Nd	12.18	18.78	2.96	5.65	6.70
Sm	2.81	3.73	0.71	1.25	1.38
Eu	0.74	0.75	0.19	0.27	0.35
Gd	3.51	3.70	0.87	1.50	1.66
Tb	0.51	0.46	0.13	0.21	0.25
Dy	2.84	2.17	0.78	1.34	1.59
Ho	0.54	0.42	0.16	0.29	0.34
Er	1.45	1.28	0.52	0.99	1.12
Tm	0.20	0.21	0.08	0.15	0.18
Yb	1.29	1.45	0.57	1.05	1.21
Lu	0.19	0.24	0.10	0.16	0.19
Ag (ppm)	1.45	1.12	2.44	0.07	0.01
Cu	11.82	17.99	<0.001	6.01	6.13
Zn	2.74	69.83	32.89	24.45	15.90
Sn	<0.001	1.40	<0.001	0.62	0.31
Sb	0.74	0.38	0.31	0.08	0.86
Pb	4.33	26.99	0.79	<0.001	5.30
Bi	0.05	0.23	0.04	<0.001	0.12
As	1.46	13.35	1.23	0.27	1.33
Ir (ppb)	<0.001	<0.001	<0.001	1.75	2.89
Ru	<0.001	0.30	<0.001	0.49	1.88
Rh	<0.001	<0.001	<0.001	0.17	1.68
Pt	4.34	5.91	5.60	2.77	2.95
Pd	<0.001	<0.001	<0.001	3.42	97.11
Au	1335.6	289.96	1351.61	301.45	38.85

Note: N.A.—not analyzed.

).

Major element variations in explosive breccia and mineralized tuffs from the LKR iron and iron–manganese deposits identify them as strongly differentiated volcanic rock series. Pyroclastic rocks plot into the fields of picrite, picrobasalt, basaltic andesite, andesite, dacite and rhyolite on the TAS diagram [105], with individual analyses plotting into basanite, trachybasalt, basaltic trachyandesite and even tephrite compositional fields (Figure 7a). This evident dichotomy regarding the alkalies is further emphasized by the K₂O-SiO₂ relationships (Figure 7b), where most of the Kaylan volcanic rocks plot into the low-K tholeiitic and calc-alkaline fields of Peccerillo and Taylor [106], while magmatic samples from the Poperechny, and especially Kostenga, deposits populate the high-K calc-alkaline and shoshonitic compositional fields (Figure 7b). Interestingly enough, a few individual samples from all three deposits plot into all compositional fields (tholeiitic, calc-alkaline, high-K calc-alkaline, shoshonitic) on the K₂O-SiO₂ diagram (Figure 7b), possibly suggesting that different magma batches with respect to potassium content participated in the formation of Fe-Mn deposits in the LKR.

Trace element variations in explosive breccia, mineralized tuffs and Fe-Mn ores from the LKR deposits normalized to primitive mantle [107] are summarized in Figure 8. Explosive breccia from all three deposits under consideration (Poperechny, Kostenga, Kaylan) is characterized by a general enrichment in large-ion lithophile elements (LILE). Ba and Rb contents in Kostenga and Kaylan deposits exhibit significant varaitions, with Ba being depleted in Kostenga and Rb in Kaylan explosive breccia, while Cs is uniformly enriched in all three LKR deposits. High-field strength elements (HFSE), such as Nb, Ta, Zr and Hf, are depleted in all explosive breccia samples from the LKR with respect to LILE and light rare-earth elements (LREE) (Figure 8a,d,g). Explosive breccia is also characterized by the moderate enrichment of LREE to the heavy rare-earth elements (HREE) typical of most subduction-related volcanic rocks [108,109]. All analyzed explosive breccia samples also exhibit prominent negative Sr anomalies. Mineralized tuffs from the Poperechny and Kaylan deposits contain highly variable amounts of LILE, coupled with alternating positive and negative Rb and Ba anomalies (Figure 8b,h), while Kostenga tuffs display negative Ba and prominent positive U anomalies that are almost identical to the U enrichments observed in explosive breccia from the Kostenga and Kaylan deposits (Figure 8d,g). All tuffs are characterized by distinct depletions in HFSE (Nb, Ta, Zr and Hf) on primitive mantle normalized trace element patterns, and weak to moderate LREE enrichments over HREE (Figure 8b,e,h), which are broadly similar to the LKR explosive breccia (Figure 8a,d,g). Iron and iron-manganese ore from the Poperechny and Kostenga deposits display a general enrichment of LILE and LREE with respect to HFSE and HREE (Figure 8c,f). Mineralization in Kostenga is also characterized by prominent positive U and negative Sr anomalies (Figure 8f) similar to those observed in many pyroclastic rocks from the LKR (Figure 8d,e,g,h). Ores from the Kaylan deposit are characterized by the most fractionated primitive-mantle normalized trace element compositions, with depletions in Cs, Rb, U, Ta, Hf and HREE (Yb and Lu), coupled with strong enrichments in Ba, Nb, Sr and Y. In general, with the exception of the Kaylan ore, volcanic rocks (explosive breccias and mineralized tuffs) appear to share trace element characteristics with the iron–manganese mineralization.

Magnetite-rich lavas from the Kostenga and Poperechny Fe-Mn deposits are characterized by primitive mantle-normalized patterns with clear LILE enrichment compared to HFSE and HREE, emphasized by the pronounced negative anomalies of HFS elements such as Nb, Ta and Zr (Figure 9a). Some Fe-rich lava samples also share negative Sr anomalies with the LKR pyroclastic rocks (Figure 8 and Figure 9); in other magnetite lava samples, the Sr anomaly is virtually absent. Rare earth element distribution in magnetite-rich lava is also comparable to the explosive volcanic rocks from Poperechny and Kostenga, displaying moderate LREE enrichment in reference to the HREE and a very weak, but still detectable, negative Eu anomaly (Figure 7b). In general, the trace element geochemistry of magnetite-rich lava from the Poperechny and Kostenga sites is broadly similar to other volcanic rocks from the same deposits in the LKR.

Finally, the dolerite intrusions associated with explosive breccia at Poperechny exhibit trace element compositions that are enriched in HFSE (Nb = 9.92 and 9.94 ppm; Ta = 0.80 and 0.84 ppm; Table 1) and only slightly enriched in LREE compared to HREE (La/Sm = 2.28 and 2.26, La/Yb = 4.77 and 4.86, respectively, in samples Do1 and Do2; Table 1).

4.3. Sr and Nd Isotopes

The results of Sr and Nd isotope measurements in explosive breccia, host dolomite, Fe-Mn mineralization and dolerite from the Poperechny iron–manganese deposits are summarized in

Table 3. Sr and Nd isotope data for principal rock lithologies in the Poperechny Fe-Mn deposit.

Sample#	3-10	4-08	4-17	128-1	789	789-д	117
Rock Type	Eb1	Eb2	Eb3	FMo	Do1	Do2	Dl
Rb (ppm)	0.88	178.2	1.18	149.6	2.71	9.1	0.6
Sr (ppm)	3.59	88	4.29	163	9.28	200	71
87Rb/86Sr	0.7124	5.8887	0.7925	2.6633	0.8450	0.1325	0.0240
87Sr/86Sr	0.713988	0.752422	0.715581	0.724809	0.709951	0.705883	0.707933
±2 SD	5	4	8	5	5	4	5
(87Sr/86Sr)i	0.71277	0.74238	0.71423	0.72027	0.70851	0.70566	0.70789
Sm (ppm)	1.62	2.66	2.07	0.58	2.28	2.76	1.04
Nd (ppm)	11.59	19.09	12.08	2.19	7.48	8.75	5.15
147Sm/144Nd	0.0846	0.0841	0.1038	0.1599	0.1842	0.1905	0.1224
143Nd/144Nd	0.512017	0.511975	0.512109	0.512408	0.512728	0.512737	0.512127
±2 SD	3	4	4	10	5	8	4
εNd(t)	−10.40	−11.21	−8.90	−3.92	1.95	2.03	−8.83

Note: Eb—explosive breccia, FMo—Fe-Mn ore, Do—dolerite dikes, Dl—dolomite.

. The explosive breccia displays extremely variable initial ⁸⁷Sr/⁸⁶Sr ratios that range from 0.71277 to 0.74238, and uniformly low negative ε_Nd(t) values that range from −8.90 to −11.21 (Table 3). These Nd-Sr isotopic characteristics in the explosive breccia are coupled with La/Sm and La/Yb enrichments (4.95–5.44 and 12–19.58, respectively; Table 1). Host dolomite from the Murandavsky suite has a similar Nd-isotope composition to the volcanic breccia (ε_Nd(t) of −8.83) and a lower initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70789 (Table 3). The Fe-Mn ore displays an initial ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.72027, which is within the range of Sr-isotope values in the volcanic explosive breccia, and a more radiogenic isotope composition of neodymium ε_Nd(t)= −3.92 (Table 3). Finally, later-stage dolerite bodies, which cut both host dolomite and Fe-Mn mineralization, have a slightly depleted Nd-isotopic composition above CHUR values (ε_Nd(t) of 1.95 and 2.03), and relatively low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.70851 and 0.70566, respectively (Table 3), suggesting that Nb-enriched dolerites (Nb = 9.92 and 9.94 ppm, respectively; Table 1) were derived from a heterogeneous magmatic source.

5. Discussion

Iron–manganese mineralization in the Lesser Khingan Range (LKR) of the Russian Far East (Kostenga, Kaylan and Poperechny deposits) is hosted in Vendian–Cambrian carbonates and closely associated with a variety of explosive volcanic rocks. The latter include intermediate to felsic volcanic breccia, variably mineralized lithocrystalloclastic tuffs and magnetite-rich lavas [68,69,70,71,72,73,74,75]. Iron and iron–manganese mineralization is represented by massive and finely laminated ore (Figure 2a–d and Figure 3a,c–f) composed of magnetite, hematite, braunite, haussmanite, rhodochrosite and pyrolusite with minor quartz, chlorite, serecite, gedritic-gruneritic amphibole, monazite and apatite [68,72,75]. Both Fe- and Fe-Mn mineralization and host Neoproterozoic carbonates are intruded by explosive andesitic to rhyolitic breccia (discordant contacts in Figure 3b,c), indicating its magmatic origin and post-Precambrian emplacement. Locally, finely laminated ore is interbedded with explosive volcanic breccia and mineralized tuff (Figure 3a,c–e) or contains numerous thin tephra-rich horizons (Figure 3f). Many samples of explosive volcanic rocks are intensely ferruginized and partially replaced with hydrothermal Fe-Mn mineralization. These observations suggest that at least some part of the Fe-Mn ore formation is contemporaneous with and/or post-dates the intermediate to felsic explosive volcanism. The presence of magnetite-rich lava (Figure 4e,f and Figure 6a–c; Table 2) that is compositionally similar to the El Laco magnetite flows in northern Chile [110] at Kostenga and Poperechny sites further attests to the importance of Fe-rich melts in the formation of the LKR iron- and iron–manganese deposits [68,69,70,71,72,73,74,75]. The entire carbonate-hosted, volcanic–hydrothermal package is later overprinted with low-temperature metasomatic alterations including albite, chlorite, sericite and quartz.

Earlier publications proposed the Late Precambrian (Late Riphean–Vendian) age and volcanogenic–sedimentary origin of the Fe-Mn mineralization in the LKR, primarily on the basis of their concordant geologic relationships with host dolomite and limestone of the Neoproterozoic Murandavsky suite [63,64,65,66]. Modern U-Pb age determinations of detrital zircons yielded younger ages of 535 ± 6 Ma, linking the formation of Murandavsky carbonates to the Neoproterozoic–Lower Cambrian boundary [111]. Recently, based on the geology, mineralogy and geochemistry of Fe-Mn mineralization and its close spatial and temporal association with explosive volcanism (“fluidolites”), Berdnikov and co-authors primarily suggested hydrothermal Fe- and Fe-Mn ore formation in the LKR on the basis of geochemical information [68]. For example, LKR ores plot into the field of hydrothermal Fe-Mn formations in (Ce/Ce*)_N − (Y/Ho)_N, Si-Al, Fe-Mn-(Co + Ni + Cu) × 10 and (Co + Ni) − (Zn + Cu + V + Pb) discrimination diagrams for Fe-Mn mineralization [68]. Various geochemical indicator ratios in most iron–manganese ores from the LKR such as (Mn + Fe)/Ti (109–193), Ba/Ti (>0.15), Y/Ho (21–35), (Y/Y*)_N (>1), (Eu/Eu*)_N (>1), along with relatively low total REE + Y (<100 ppm) and Th (0.46–7.93 ppm), are compatible with the hydrothermal origin of iron and manganese minerals in the LKR. In particular, the positive Eu anomaly and the lack of a negative Ce anomaly in the majority of ore samples from the LKR deposits argue against any substantial role of seawater precipitation in the formation of Fe-Mn oxides and hydroxides [112,113,114,115,116]. However, in individual samples from the Poperechny deposit, some of these indicator parameters are compatible with the hydrogenic Fe-Mn mineral formation [117,118,119] suggesting that, at least locally in the LKR, a limited hydrogenic (seawater) component was also involved in the ore formation process [68].

Age constraints on the hydrothermal Fe-Mn ore formation and explosive volcanism in the LKR are quite limited and primarily based on the documented intrusive relationships between intermediate to felsic breccia, Vendian–Cambrian carbonates and iron–manganese ores (Figure 3b,c), along with the new model Sm-Nd ages reported in this study (Table 3). Model Sm-Nd ages of from 1338 to 1864 Ma suggest the presence of a Proterozoic crustal component. In addition, voluminous rhyolite volcanism in the LKR, which is texturally and compositionally similar to the dacitic–rhyolitic tuffs and explosive breccia associated with Poperechny, Kostenga and Kaylan Fe-Mn deposits, was dated by SHRIMP-II at 102.98 ± 0.57 Ma and 103.58 ± 0.35 Ma [120]. Interestingly, Cretaceous magmatism (granite and dioirite porphyry, andesites, dacites, rhyolites) within the Chinese portion of the Lesser Khingan Range is dated at 107–124 Ma, and is associated with the prolific Au mineralization throughout the Chinese LKR [121,122,123]. It is important to emphasize that all Fe-Mn deposits within the Russian part of the LKR contain substantial amounts of native Au, gold-bearing alloys and other precious metal minerals (Figure 5c and Figure 6d–f), and are locally associated with economic precious metal grades in excess of 13 g/t of combined Au + PGE [68,69,70,71,72,73,74,75]. It has been proposed that Cretaceous magmatism and associated ore-forming processes (Fe-Mn, Au-Ag-PGE, etc.) in the LKR are potentially linked to the complex tectonic processes related to the subduction of the Paleo–Pacific (Izanagi) plate under the edge of the Eurasian continent [62,85,124,125,126,127,128].

The geochemical characteristics of Fe-Mn mineralization and associated igneous rocks suggest a broad subduction-related origin. Pyroclastic rocks and magnetite-rich lavas from the LKR iron–manganese deposits form strongly differentiated (picrite to rhyolite; Figure 7a) volcanic suites primarily belonging to low-K and high-K types (Figure 7b), not unlike the differentiated low-K tholeiitic, calc-alkaline and shoshonitic magma types typical of the Circum–Pacific convergent margins [129,130,131,132,133,134,135,136]. Trace element distribution, specifically the relatively high LILE/HFSE, LILE/LREE and LILE/HREE ratios coupled with distinctive negative HFSE (Nb, Ta, Hf and Zr; Figure 8) anomalies in volcanic explosive rocks and magnetite-rich lavas, suggest their derivation from subduction-related sources. Their presence among volcanic breccia samples with high MgO contents (8–11 wt.%; [68,72,73,74,75]) is supportive of the differentiation of mantle-derived magma as a primary petrogenetic process [72,73,74,75]. This is also consistent with the unusually high Ni contents (276–558 ppm) in some basaltic andesites and andesites from the Poperechny deposit indicative of primary, mantle-derived mafic and intermediate melts for the LKR volcanics [72]. It is also possible that some dacitic to rhyolitic volcanic rocks with very low MgO, Cr and Ni contents and high Ba/Nb, Ba/La, Rb/Y and La/Ta ratios could be direct melting products of the mafic lower crust beneath the NE Asian convergent margin [72,78]. The iron–manganese ore in the LKR is also characterized by prominent HFSE depletions (especially, Ta) in primitive mantle-normalized patterns and, with the exception of the Kaylan deposit, general enrichment in LILE with respect to the rare earth elements (Figure 8c,f,i). Uranium enrichments in the Fe-Mn mineralization at the Kostenga and Poperechny deposits (Figure 8c,f) are also indicative of their predominantly hydrothermal origin due to the uptake of uranium by phosphates (monazite, xenotime) and Fe-Mn oxides (for example, hematite) under hydrothermal conditions [137,138,139,140]. Monazite, xenotime, and hematite are common mineral phases in the LKR iron–manganese ores (Figure 5a,b,d,e). Dolerite intrusions associated with the LKR mineralization display elevated HFSE concentrations (~9 ppm Nb; Table 2), which are comparable to the high-Nb basalt (HNB) magmas in subduction zones [141,142]. The HNB melts in modern and ancient magmatic arcs are believed to be products of adakite–mantle wedge interactions and, in case of the LKR dolerites, may indicate the possible involvement of the downgoing or stagnant Paleo–Pacific slab beneath the NE Asian convergent margin in the Mesozoic magmatism within the LKR.

Strontium and neodymium isotopes provide some further insights into the formation of Fe-Mn deposits in the LKR. Explosive volcanic breccia exhibits highly radiogenic ⁸⁷Sr/⁸⁶Sr isotope ratios (up to 0.752422) and low radiogenic Nd isotope composition (ε_Nd(t) of from −8.90 to −11.21), can be plotted in the field of marine sediments [143] and is close to the characteristic values of Phanerozoic granites within the Central Asian Orogenic Belt [144,145] and ancient upper continental crust on the ε_Nd(t) – ⁸⁷Sr/⁸⁶Sr diagram (Figure 10).

Host dolomite shares a radiogenic Nd-isotopic signature (ε_Nd(t) of −8.83; Table 3) with the LKR explosive breccia but displays a less radiogenic Sr-isotope composition. The host dolomite is still plotted in the marine sediment field of [109] (Figure 10). The iron–manganese ore has a less radiogenic isotopic composition of neodymium and highly radiogenic Sr-isotope ratios, suggesting the involvement of a juvenile crust in their formation at the NE Asian convergent margin. However, both volcanic explosive breccia and associated Fe-Mn mineralization experienced protracted contamination with both ancient and juvenile crustal components. Nd-isotope variations suggest that the Nd signature may have been introduced by the assimilation of Neoproterozoic carbonates, as volcanic breccia contains abundant dolomite clasts and xenoliths (Figure 2, Figure 3 and Figure 4). However, the extremely radiogenic Sr-isotope compositions of some explosive breccia indicate the involvement of an ancient upper continental crust, probably not unlike some Precambrian clastic sediments. Such clastic metasediments are known within the Jiamusi terrane in NE China and their isotopic characteristics may be a suitable contaminant component for the Mesozoic volcanism throughout the eastern part of the CAOB [146]. Both ancient and juvenile crust were involved in the formation and evolution of the CAOB, as indicated by abundant geologic, geochronological and isotopic data [147,148,149,150,151,152], and both upper crustal types, based on the Nd-Sr signature (Figure 10), may have participated in the formation of volcanic rocks and the Fe-Mn mineralization in the LKR. Later-stage dolerite intrusions display less radiogenic Nd-Sr isotope characteristics (Figure 10) and are broadly similar to arc-related volcanic rocks, possibly representing the radiogenic isotope signature of the mantle beneath the LKR and easternmost segment of the CAOB.

Carbon isotopes are currently successfully used to trace deep carbon recycling and processing through various parts of the subduction and collision zones [153,154,155,156,157]. δ¹³C variations in explosive volcanic breccia (from −24.5 to −28.4‰), iron–manganese ore (from −25.2 to −29.4‰) and host Vendian–Cambrian carbonates (from −22.0 to −28.3‰) are similar to the isotopic δ¹³C signature of biogenic carbon, which average −25‰ over more than 3.8 Ga of the Earth’s planetary evolution [158,159,160]. These values are comparable to the very light carbon isotope signature of some asthenospheric and lithospheric diamonds (δ¹³C from −10‰ to −40‰; [155,161,162]) and moissanites (δ¹³C from −18‰ to −35‰; [163,164,165]) brought to the surface in kimberlite and lamproite-like magmas, and tectonically exhumed high-pressure chromitites that appear to record the deep subduction zone recycling of biogenic carbon into the planetary mantle [154]. Pore fluids generated from the accretionary prism of the Cascadia active continental margin have a δ¹³C of −35‰ to −63‰ [166], while carbon isotope composition in calcite veins from the Shimanto accretionary complex in southwest Japan is estimated to range from −10‰ to −19‰ [167]. The carbon isotope composition of volcanic gases emitted from arc volcanoes along the Andean, Aleutian–Alaskan and Costa Rican convergent margins is more variable and somewhat heavier, at a δ¹³C ranging from +3‰ to −21‰, which may reflect the small-scale mixing of carbon sources and fluid–fluid/fluid–solid interactions that occur during outgassing within these subduction zone environments [168,169,170]. Hydrothermal fluids at convergent margins are characterized by predominantly negative and highly variable C-isotope values, which are commonly interpreted as the result of the subduction zone recycling of both biogenic matter and abiogenic (carbonates) sediments, followed by carbon outgassing through arc volcanoes, accretionary prisms and submarine hydrothermal vents [154,171,172,173]. At least in one case of the submarine Brothers volcano in the Kermadec arc, δ¹³C-depleted hydrothermal fluids are associated with elevated Fe and Mn fluxes in the typical subduction zone tectonic setting [174]. Berdnikov and his co-authors have previously interpreted a highly negative carbon isotope signature in the iron–manganese ore and explosive breccia from the LKR to be the result of biogenic carbon recycling through the paleo-subduction zone at the edge of the Asian continental mass [68].

The formation of such unusual iron and iron–manganese hydrothermal mineralization in association with explosive Fe-rich magmatism (Fe-rich lavas and tuffs) is still insufficiently studied and poorly understood [25,32,35,36,37,38,41,110]. However, further research into deposits of this type appears to be quite important, as a better understanding of magmatic–hydrothermal iron deposits may provide a solid foundation for adequate genetic models and guide successful exploration activities in various prospective regions worldwide. One of the possible models of the formation of Fe-Mn mineralization in the Lesser Khingan Region is presented in Figure 11.

We propose that LKR Fe- and Fe-Mn mineralization was formed as a result of the complex interplay between the processes of marine sedimentation coupled with multi-stage subduction (Figure 11). Earlier, Golozubov and Khanchuk [175] suggested that during the Late Jurassic–Early Cretaceous period, the fossil Izanagi plate experienced protracted flat subduction beneath the Jiamusi–Bureya–Khanka super-terrane (Figure 11a). This flat subduction was accompanied by the formation of compressional structures and off-scraping frontal accretion, involving rocks enriched in sedimentary manganese and iron [18,54,70]. At the end of the Early Cretaceous, the deep-sea trench shifted to the east [176]. Devolatilization and partial melting of the new slab generated an upwelling flow composed of Fe-rich melts and fluids, which scavenged additional ore components during their ascent through the previously formed mineralized accretionary complexes. It is also possible that the generation of these metal-rich magmas and fluids and their emplacement in the shallow crust was further facilitated by the formation of slab window environments in the flattened oceanic slab [175]. The contamination of upwelling ore-forming magmas and fluids by the underlying ancient continental crust resulted in the strongly radiogenic Nd isotope signature observed in volcanic rocks associated with Fe-Mn mineralization in the Lesser Khingan Range of the Russian Far East.

6. Conclusions

• Iron and iron–manganese deposits and showings form several closely spaced clusters in the Lesser Khingan Range of the Russian Far East. Mineralization is composed of magnetite, hematite, braunite, hausmannite, rhodochrosite and pyrolusite, hosted in Vendian–Cambrian carbonates. Fe- and Fe-Mn ores are intruded by explosive breccia, tuffacerous pyroclastic formations and “magnetite lava”, and are occasionally interbedded with the latter. Locally, hydrothermal Fe-Mn mineralization is developed over igneous material.
• The geochemical characteristics of volcanic rocks and Fe- and Fe-Mn mineralization in the LKR suggest that they were produced from subduction-related mantle and crustal sources, most probably within the active continental margin of NE Asia.
• Textural, mineralogical, and geochemical features of the Fe- and Fe-Mn ores support their formation under hydrothermal conditions (possibly with some minor input from hydrogenic processes) in association with active explosive basaltic to rhyolitic volcanism and prolific submarine hydrothermal vents. Carbon isotopes also suggest the involvement of the recycled pelagic component in the formation of iron–manganese ores and associated volcanic rocks.
• Sr and Nd isotope variations in products of explosive volcanism and Fe-Mn ore indicate the pervasive contamination of the ore-forming volcanic–hydrothermal system with ancient and juvenile continental crusts. Later-stage dolerite magmas represent primary mafic melts derived from the subduction-modified mantle domains beneath the northeastern edge of the Asian continental mass.