The Hongqiling magmatic Cu–Ni sulfide deposit is one of the most important deposits in the easternmost segment of the Central Asian Metallogenic Belt, northeast China. However, the existence or non-existence of magmatic chambers is still not been determined, which is the key to decipher the formation of Hongqiling Cu–Ni deposit. Therefore, this study proposes to adopt long-period magnetotellurics method to image the deep-seated metallogenic system in Hongqiling Cu–Ni deposit. Two-dimensional (2D) nonlinear conjugate gradient inversion technology allows us to describe entire magma plumbing system, including the mantle-derived magma channels (banded low-resistivity anomalies), deep separated chamber (low-resistivity anomaly C2), and shallow magma conduits (low-resistivity anomaly C1). These results indicated that the mantle-derived primitive magma gave rise to the parental magma for the mafic–ultramafic intrusions in Hongqiling Cu–Ni deposit and triggered the segregation of Cu–Ni sulfides in the shallow chamber. By combining the experimental simulation, geochemistry, geochronology, and geotectonics data in the study area, we suggest that the partial melting processes which produced the large magma plumbing system probably have been triggered by lithospheric delamination.
As an ultimate case of small intrusions that produced large to giant magmatic deposits, Hongqiling magmatic Cu–Ni sulfide deposit has been concerned and researched widely by geologists [1,2,3,4]. Extensive geochemical and geochronological data have been previously reported and have established some key hypotheses for the mineralization and tectono-magmatic evolution including:
The differentiation of a single mantle-derived magma ;
Multi-stage sulfide segregation in different magma conduits ;
A crust–mantle interaction model ; and
An asthenosphere underplating lithospheric mantle model .
In these models, it is generally accepted that the primitive for these ore-fertile magmas originated from partial melting of the upper mantle and then experienced complex evolution including multi-stage sulfide segregations in deep magma chambers. However, the existence or non-existence of magmatic chambers, and the size and morphology of them at depth, is still not been determined, which brings challenge to the applicability of existing metallogenic models to Hongqiling listed above. Long-period magnetotellurics (MT) has been used in depicting the deep magma plumbing system and structures in the lithospheric domain [9,10]. Based on the long-period MT results and previous studies, we propose a metallogenic model to illustrate the magmatic evolution and the formation of Hongqiling Cu–Ni deposit.
2 Geological setting
The Hongqiling mafic–ultramafic intrusions in the easternmost segment of the Central Asian Orogenic Belt (CAOB) are distributed north side of the northeast Dunhua-Mishan fault  (Figure 1a). The CAOB is bounded in the north by the Siberian Block and in the south by the Tarim and North China blocks and has traditionally been referred to as a series of orogenic or fold belts [11,12]. During the period from the Permian to the Triassic, the final closure of the Paleo-Asian Ocean occurred, and extensive ore-bearing mafic–ultramafic intrusions were subsequently extensively emplaced from the west to the east (e.g., Kalatongke, Poyi, Huangshandong, Huangshanxi, Huangshannan, Tianyu, Erbutu, Chajianling, Hongqiling, and Piaohechuan) [13,14,15,16,17,18] (Figure 1a). The formation ages of these intrusions indicate the diachronous closure of the Paleo-Asian Ocean from the west to the east [19,20].
3 Long-period MT method
3.1 Data acquisition and processing
Long-period MT data were collected along a north–south-trending section (251 km) from the North China Craton (NCC) to Song Liao Basin (SLB), which vertically cut across the east–west-directed accretion of the East Asian Pre-Pacific Province (Figure 2). This included 22 MT sites at intervals between 3 and 15 km.
Long-period MT data were acquired using an Aether 200 instrument (Crystal Earth Co., Ltd., USA) with a collection time of no more less than 10 h and a period range of 0.001–1,000 s. Then, this time series are processed by a statistically robust algorithm . The data from each MT site include five components: two horizontal electric field components (E x and E y ), two horizontal magnetic components (H x and H y ), and one vertical magnetic component (H z ). Information regarding lithospheric geoelectric structure can be embodied by the apparent resistivities (ρ a) and phases (φ) . Figure 3 displays the apparent resistivity (R xy or R yx ) and impedance phase (φ xy and φ yx ) curves of partial sites and the error bars are corresponding to the measurement errors of the observed data. The MT skin depth is the depth at which the MT field attenuates to (1/e) of its amplitude at the surface of the Earth  and can be calculated by equation (1):
where δ, ρ, μ, ω, and f represent the skin depth, bulk resistivity, magnetic permeability, angular frequency, and frequency, respectively. In geological terrains with a high resistivity, the MT signal has a larger penetration depth [24,25]. In this study, the bulk resistivity was estimated to be about 300 Ω m, and the lowest frequency was estimated to be 0.003 Hz. The skin depth and effective detection depth were nearly 160 and 100 km, respectively.
3.2 Analysis and inversion
After data processing, the phase-impedance tensor decomposition is used to estimate the regional dimensions . The pseudosection map of resistivity inversion in Figure 4 shows that the 2D deviations vary with frequency. The value of the 2D deviations is generally less than 0.3 over different frequencies . Only a few sites in low frequencies exhibit large 2D deviation values. This result indicates that the long-period MT section is generally compliance with 2D inversion .
The geoelectric strike can be identified by the rose diagram . Data in the frequency bands 100–1 Hz showed geoelectric strikes in the range N50°E–N60°E and S30°E–S40°E. Data in the frequency bands 0.1–0.001 Hz showed geoelectric strikes in the range N60°E–N80°E and S10°E–S30°E (Figure 5). The NE-NNE regional geological strike directions correlate well with the geoelectric strike. Moreover, the distribution of Carboniferous to Permian subduction-related igneous rocks have been found to be generally related to the north–south-directed subduction that accompanied the east–west closure of the Paleo-Asian Ocean . Thus, the inherent 90° ambiguity could be resolved well and the data were rotated into a S20°E direction .
The inversion was consistent with the 2D nonlinear conjugate gradient (NLCG) inversion in the TE and TM modes . The inversion models were discretized by a grid size of 46 × 90 in the X and Y directions with an initial uniform half-space resistivity of 100 Ω m. To ascertain the optimum value for the root mean square (RMS) misfit and the roughness for the 2D NLCG inversion, a number of inversions were performed . We finally choose the parameter τ = 10 for the regularization factor . After 200 iterations, the model responses are well fitted to the observed data, and the preferred model is obtained with an average normalized RMS of 2.12 for all sites.
3.3 Model test
To verify that the preferred model was properly inversed, the long-period MT result was test by some forward modeling (Figure 6). First, the C1 and C2 anomalies are displaced by 100 and 300 Ω m in forward model 1 and model 2. Then, forward calculation was adopted to assess the impact. The average normalized RMS misfits increase from 2.32 to 3.24 in forward model 1, and this trend is more sharply in forward model 2.
Figure 7 shows the results of 12 and 13 sites data fitting. The preferred inversion fitting curve (the purple) fits well with the observed data, while the modified model response curve (the green and red) cannot fit well under the period of 10 s. The forward model test results indicate that reliability of C1 and C1 anomalies in preferred inversion.
3.4 Interpretation of the MT profile result
The high-resolution lithospheric electrical properties were inversed using the 2D NLCG method and long-period MT data (Figure 8). The lithospheric geoelectric structure was found to be generally consistent with the major surface geological features. Based on the locations of regional faults and the similarities and differences in the inversion model, the long-period MT section was laterally divided into three segments: the NCC, ZGCL, and SLB  (Figure 8).
In the ZGCL segment, at sites 7–10, a sudden change in the resistivity was observed, where the value decreased by one order magnitude from 700 to 60 Ω m. According to the previous research, this structure is understood to be loose, fractured, and filled with some low-resistivity filling materials  (Figure 8). Moreover, seismic imaging revealed a transition zone between high- and low-velocity anomalies, which is consistent with the site of the long-period MT survey. Thus, we infer that this structure, exposed near site 7, dipped steeply to the north is the Dunhua–Mishan fault . At sites 7–11 and sites 15–17, two low-resistivity anomalies (C1 and C3) were observed with the resistivity values of 101.8 and 101.6 Ω m in the upper crust (Figure 8). The petrophysical parameter of minerals such as the ore-bearing amphibolite pyroxenite, altered gabbro, and Cu–Ni sulfides are characterized by low-resistivity anomalies (Table 1). In addition, according to the deep seismic-reflection sections in the CAOB, the short and strong reflections represent multiple volcanisms . Thus, we infer that the low-resistivity anomalies C1 and C3 may represent the relics of ancient shallow magma chambers emplaced in the Triassic.
|Name||Number||Geological age||Specific resistivity (average)|
|log10 ρ (Ω m)|
|Skarn siliceous marble||20||Ordovician||3.44|
Vertically, the lithospheric geoelectric structure could be generally segmented into three layers. The first layer with an resistivity value ranging from 102.5 to 104.1 Ω m (except for conductor C1) represents crust at a depth from 28 to 40 km . The crustal structure beneath northeastern China as imaged by the Northeast China Extended Seismic (NECESS) Array receiver indicates that the Moho depth varies from 26.7 to 42.3 km from the NCC to the SLB . Thus, we argue that the first geoelectric layer represents the Moho depth (Figure 8). In Section 3.1, we have calculated the skin depth was nearly 160 km. Thus, the second layer with a value of 102.0 to 102.5 Ω m at depths from 52 to 100 km represents the lithospheric mantle, and the third layer with a value of 101.8 to 102.0 Ω m represents upwelling asthenosphere. Geomagnetic three-component depth sounding and terrestrial heat-flow data acquired for the region between Changchun and Tonghua had similar trends to that in our data . Furthermore, previous deep exploration investigations showed that the lithosphere–asthenosphere boundary (LAB) depths in this region are marked on the MT section  (Xiong et al., 2011, p. 31). In addition, considering the fact that the study area is located near the Solonker Suture and far away from the Jiayin–Mudanjiang Suture, we infer that its geoelectric structure should be closely related to the evolution of the Paleo-Asian Ocean rather than the evolution of the Paleo-Pacific Ocean. Moreover, Jurassic magmatism (185–158 Ma) in the Hongqiling deposit has been found to be dominated by granitoids with few mafic–ultramafic rocks, which indicates that the degree of upwelling of the asthenosphere and the reworking of the LAB were quite limited during the Late to Middle Mesozoic evolution .
4.1 Partial melting of the mantle
Ore-fertile magma formed by high degree of partial melting of the mantle generally has high Ni/Cu ratio and low Pd/Ir ratio . In geophysics, minute quantities of partial melting can have a tremendous influence on the overall electrical performances . Thus, resistivity can be used to estimate the volume of fluid/melt that is present in the mantle, although the geothermal model of the lithosphere requires simplification prior to calculating the degree of partial melting of the mantle. The geotemperature is 1,300 K at the depth of 50 km, and then, the geothermal gradient increment is 0.4 K/km  (Figure 9). The resistivity of olivine, which has a variable fluid content that ranges from 0 to 1,000 ppm, can be obtained using the Grades laboratory database . The Grades laboratory database states that the resistivity of dry olivine with a olivine fluid content of 350 ppm ranges from 60 and 120 Ω m (conductivity of 0.0083–0.0167 S/m) (Figure 9). However, at the same depth, the long-period MT section had a resistivity of ∼50 Ω m (conductivity of 0.02 S/m) between sites 10 and 13, which had lower resistivity than that the olivine fluid content of 350 ppm (Figure 9). The preferred explanation is that Hongqiling ore-bearing mafic-ultramafic intrusions are generally produced by the partially melting of mantle.
4.2 Mineralization model of Cu–Ni sulfide deposits
Long-period MT section successfully imaged the Dun-Mi structures which formed the magma conduit system for mantle-derived metalliferous magmas intrusion and fluids migration through the crust, as well as the shallow chamber (C1), the deep chamber chamber (C2), and the emplacement of the ore-bearing intrusions.
Positive εNd(t) value (3–5) of Hongqiling mafic-ultramafic intrusions indicated that . The long-period MT section imaged the partial melting magma intruded the middle crust and formed a shallow chamber (C1) (Figure 10a). Wu et al. reported that the extensive distribution of Late Triassic A-type granites was the result of the direct partial melting of the juvenile crust . We note that upwelling in the asthenosphere can realize such high temperatures and that mafic–ultramafic intrusions are generally coeval with the A-type granites in this region . Thus, we suggest that the deep, separated chamber may have assimilated the SiO2-rich A2-type granites that were formed by underplated mantle-derived magma and that this triggered the PGE sulfide segregation prior to emplacement. The PGE-depleted magma subsequently intruded into the shallow chamber (C1) and then underwent variable degrees of crustal contamination before generating the second-stage sulfide segregation  (Figure 10b). In summary, the mineralization process involved voluminous, undersaturated, sulfide-mafic magma passing through the large magma plumbing system, where it reached sulfur saturation due to the addition of additional crust materials and triggered sulfide segregation . The lithospheric-scale Dun–Mi structure and its secondary structures control the magma conduit system in the study area (Figure 10c).
The top surface of the low-resistivity body in the upper mantle was consisent with the LAB. The trend of gradual thinning in the first layer indicates that a rapid uplift event occurred in this region. The depth variation in the third layer reflects the various degrees of asthenospheric upwelling, which suggests an extensional geodynamic regime that was probably induced by delamination. The volume of mafic–ultramafic magma in the shallow and deep chambers (C1 and C2) was found to be much larger than the volume of surface ore-related intrusions. This indicates that the strong extensional geodynamics regime in the study area is associated with intensive mantle-derived magmatism.
Three potential geodynamic processes could have formed such a large volume of mafic–ultramafic magma and the associated huge magmatic Cu–Ni–PGE deposits: (i) a mantle plume ; (ii) slab breakoff ; and (iii) lithospheric delamination . First, it is impossible that a mantle plume existed during the Triassic when the CAOB was strongly associated with the evolution of the Paleo-Asian Ocean. Second, unlike delamination, slab breakoff cannot trigger a significant thermal perturbation in the overriding lithosphere; thus, slab breakoff cannot account for the intensive magmatism or additional diverse nature of the magmatic observations [41,42,43]. Therefore, the generation of intensive mafic–ultramafic magmatism was triggered by the intensive delamination of the lithosphere. Besides, the long-period MT section from the south Inner Mongolia to the Bainaimiao provided an image of the residual lithosphere in the asthenosphere . Thus, we inferred that the genesis of the Hongqiling Cu–Ni deposits is related to lithospheric delamination.
The long-period MT section allowed the inversion of entire magma plumbing system, including the mantle-derived magma channels, deep separated chamber, and shallow magma conduits;
By combining the experimental simulation with findings of previous studies, we inclined that the primary magma of Hongqiling was formed by partial melting of the mantle, which are triggered by lithospheric delamination.
This research has been funded by the Engineering Research Center of Geothermal Resources Development Technology and Equipment, Ministry of Education, Jilin University and Science and Technology Plan Project, Langfang Heibei province.
Funding information: This research benefited from the support of Science and Technology Research Project in Higher Learning Institutions of Hebei Province (ZC2022106); Science and Technology Plan Project, Langfang (2022013081); Engineering Research Center of Geothermal Resources Development Technology and Equipment, Ministry of Education (22006), Jilin University, Changchun, 130026, China; and Shandong Provincial Engineering Laboratory of Application and Development of Big Data for Deep Gold Exploration Funds (SDK202221).
Author contributions: ZHX designed the surveys and carried them out. GWG performed the geophysical data processing. JYJ, FDL, and XGN draw relevant diagrams and prepared the article with contributions from all co-authors.
Conflict of interest: We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work; there is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Metallogenic model of the Hongqiling Cu–Ni sulfide intrusions, Central Asian Orogenic Belt: Insight from long period magnetotellurics”.
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