The research data are 1:50000 aeromagnetic survey results and 1:500000 ground Bouguer gravity data. We use the reduction to the pole of aeromagnetic to study and analyze the deep–seated structure of the WQOB. The West Qinling is a large area of low magnetic anomaly quiet area. The amplitude of magnetic anomaly in the Ruoergai Basin on the south side of the WQOB is relatively high, and the amplitude of magnetic anomaly in the A’nyemaqen suture zone is slightly higher than that in the south (Figure 3A). In particularly, the southern edge of the WQOB has an obvious high magnetic banded anomaly in the NW direction. The NW high magnetic anomalies are mainly in the north edge of the West Qinling to the Qilian Orogenic Belt, and there are many high and low anomaly conversion areas. The scattered cluster high magnetic anomalies are consistent with the location of volcanic and intrusive rocks exposed on the surface. The magnetic anomaly amplitude of profile AB magnetic anomaly is—48.8 nT∼159.1 nT, with the highest point at the northernmost end of the survey line and the lowest value at the southern edge of the Qilian Orogenic Belt.

The QTP is the negative anomaly area with the lowest gravity anomaly value in China (Figure 3B). The northeast of the QTP is bounded by a huge gravity gradient belt. In the region, the gravity anomaly value increases gradually from southwest to northeast, and the gravity anomaly at profile AB also increases from south to north. There are obvious changes at the main faults and both sides, with the amplitude—386.2 mGal ∼−253.0 mGal. These gravity anomalies are related to widely exposed magmatic and metamorphic rocks.

The first vertical derivative of gravity anomalies and magnetic anomalies can enhance the gravity and magnetic effects of shallow near–surface geological bodies and suppress the influence of deep regional background field (Figures 4A, C). The first horizontal derivative of gravity anomaly and magnetic anomaly along the horizontal 45° direction focus on the shallow near–surface geological body, which has a good identification for the geological structure line perpendicular to this direction, and is also a sufficient basis for the division of secondary fault zone (Figures 4B, D). Generally, the total gradient mode of magnetic anomaly has a good correspondence with the shallow boundary (Figure 4E). The tilt derivative of magnetic anomaly and the horizontal gradient of tilt derivative of magnetic anomaly have the effect of enhancing boundary recognition (Figures 4F, G). There are still clear traces when the deep and large faults extend upward for 0.5–20 km (Figure 4H–L). Based on the above results, gravity edge detection lines reflect the boundary of density difference, while aeromagnetic edge detection lines reflect the boundary of geological body with magnetic difference. In order to better identify hidden faults, it is necessary to carry out comprehensive comparative analysis with known fault structures at different scales.

Through comparative experiments and practical applications, it is recognized that multi–scale edge detection method can more accurately extract model boundaries and is more suitable for structural information recognition and extraction (Hornby et al., 1999). Multi–scale edge detection applied to gravity and magnetic data is helpful to aid the interpretation of basement structure (Austin and Blenkinsop, 2008; Hackney et al., 2015). The detection results of different extension heights are superimposed, and different color line types are used to represent the boundaries of geological bodies with different depths or scales. The calculated results can also qualitatively indicate the development depth and dip characteristics of the structure. The denser the pencil of lines, the steeper the occurrence of the boundary structure development and the larger the dip angle. On the contrary, the sparser the pencil of lines, the slower the structural dip angle changes.

The results of multi–scale edge detection calculation using gravity and magnetic data can effectively identify a series of geological faults, including deep buried faults in the structural belt (Figure 5). In which, the boundary identification results of aeromagnetic data are consistent with the Shangdan ophiolite tectonic melange belt, namely, the West Qinling fault (Figure 5A). In particularly, according to the magnetic difference, the spatial position of Diebu–Sanhe fault is consistent with the multi–scale edge detection results, and it seems to indicate that this fault has the possibility of extending to the depth. On the other hand, the boundary recognition result of gravity data calculation shows that the density difference between the two sides of the Maqu–Tazang fault is large (Figure 5B), which is consistent with the division of Mianlue tectonic melange belt recognized by geology. The results of multi–scale edge detection also show the location of other secondary faults, such as Gahai North–Guanting fault, Hezuo–Min county–Dangchang fault.

The three–dimensional density and magnetic susceptibility physical property distribution of the region helps to infer the deep–seated structure of the WQOB. Firstly, the gravity and magnetic data is filtered by Butterworth band–pass filter (10 km–200 km) to calculate the residual anomaly, and then the three–dimensional inversion method under the regularization constraint is adopted (Li and Oldenburg, 1998; Li and Oldenburg, 1996). According to the research purpose, the inversion depth range is determined from the surface to the underground 60 km. The mesh generation scheme is 200×400×50, the horizontal grid spacing is 1 km, and the vertical grid spacing is increased from 0.1 km to 5 km. Finally, the 3D density distribution characteristics of ∼50 km (Figure 6) and the magnetic susceptibility distribution characteristics of ∼30 km (Figure 7) are calculated through iterative operation.

Bouguer gravity anomaly includes regional component reflecting deep crustal information and local residual component reflecting shallow crustal information. The general trend of Bouguer gravity anomaly is a reflection of deep crustal structure. The 3D gravity inversion results indicate that the deep density of the Ruoergai Basin is relatively high, but the density at the A’nyemaqen suture zone drops suddenly, and then gradually rises again after entering the QOB. There are low density areas in local areas. The northern margin of the WQOB is low density, and then it enters the Qilian Orogenic Belt and rapidly uplifts, and there is a sharp transformation of high and low density. The 3D magnetic inversion results show that the A’nyemaqen suture zone and the southern part of the WQOB have strong ferromagnetism, while the deep part of the northern edge of the WQOB has weak ferromagnetism. The main susceptibility features of the Qilian Orogenic Belt are local superimposed blocky and needle pattern anomalies. The magnetic susceptibility under the Qilian Orogenic Belt changes sharply, and the residual magnetic susceptibility can reach 2000 × 10⁻⁵ SI.

Here we apply an interactive inversion technique on gravity and magnetic anomalies to study the deep geological structure of the West Qinling along Tangke–linxia (profile AB in Figure 2). Interactive inversion technologies of gravity and magnetic anomalies along the undulating terrain profile is generally expressed in 2.5 D. The core process is that gravity and magnetic anomalies are calculated on the horizontal prism polygon model based on known geological structures, physical property data and semi–quantitative interpretation. After that, continue to adjust the model parameters until the calculated value of gravity and magnetic anomaly is consistent with the measured value of gravity and magnetic anomaly. Finally, we can obtain important information such as deep–seated structure, fault and metallogenic belt of the West Qinling Orogenic Belt (Figure 8).

The Curie depth in the results is calculated from aeromagnetic data. The Curie depth calculation method adopted in this article is the frequency domain high-order derivative power spectrum method (Spector and Grant, 1970), and is specifically implemented using the software GeoProbe 2.0. The depth and stratigraphic boundary of the Moho are mostly depended on the seismic reflection interface, with a depth of about 50 km. Below the Mianlue ophiolitic melange belt and the WQOB, the middle and lower crust are slightly uplifted. In addition, the overall density value of this region is relatively high. In the process of fitting the gravity and magnetic profile, the density of the upper mantle reaches 3.20 g/cm³. The density of the lower crust in the Ruoergai Basin is 2.94 g/cm³, and the density of the lower crust in the A’nyemaqen suture zone and the Linxia–Qinling transition zone is 2.9 g/cm³. The maximum density of the lower crust is located in the north of the section.

The upper crust includes magnetic basement and its overlying weak magnetic strata. There are strong magnetic ore–forming rocks, intermediate–acid volcanic rocks and intermediate–basic volcanic rocks in some areas. The magnetic susceptibility of the upper crust magnetic basement at fault F4 ∼ F5 is low, with a value of 20×10⁻⁵ SI. The magnetic susceptibility of the deep magnetic body at fault F5 ∼ F6 is the lowest, with a value of 10×10⁻⁵ SI. The magnetic susceptibility of deep magnetic body between Tangke and fault F1 is 1,200×10⁻⁵ SI. Ordovician intermediate–basic volcanic rocks exposed in the shallow part of the northernmost section, with a magnetic susceptibility value of 1,400×10⁻⁵ SI. The magnetic susceptibility of medium acid granodiorite between fault F5 and F6 is 40×10⁻⁵ SI ∼ 800×10⁻⁵ SI.

The initial geology model is built on the basis of cross sections (profile AB in Figure 2). Based on the range of physical properties for rocks and ores in the WQOB, we can get these physical parameters for modeling (Table 3). The parameters listed indicate that covers in the upper crustal non–magnetic or weakly magnetic. The magnetic distribution of the magnetic basement in the upper crust is uneven and varies greatly. The lower crust and mantle are nonmagnetic, while the magnetic rock mass in the upper crust is the main factor causing local magnetic anomalies, especially the ferromagnetic ore–forming rock mass near the surface. The density change of the crust from top to bottom is relatively continuous, from small to large. The overall change trend of gravity anomaly is mainly caused by the horizontal direction density change of the lower crust and the fluctuation of the upper and lower crust interface. The rapid decline of gravity anomaly in local areas is caused by fault zone. In particular, the strength and the center position of magnetic anomaly significantly also depend on the shapes of iron bodies. To obtain the best results, we corrected the models of the rock mass bodies constantly, until the residual anomaly between aeromagnetic fitting curve and the measured curve is the minimum. The relative theoretical parameters indicate that the strength and the center position of gravity anomaly also depend on shapes of the models. Similarly, we can get the least residual anomaly between gravity fitting curve and the measured curve by revising the shapes of every model constantly. Consequently, when both the gravity and aeromagnetic residual anomalies are the least, we can obtain the final model for West Qinling Orogenic Belt.

We processed the gravity and magnetic data, and the results revealed the fine lithosphere structure of the northern margin of the Songpan block and the West Qinling Orogenic Belt, South Qilian Orogenic Belt. The research results show that the depth of Curie depth gradually rises from south to north, rising from a depth of about 40 km in the Ruoergai Basin on the north side of the profile to the shallowest of 20 km in the middle Qinling. There is a significant difference in Curie depth between the north and south sides of the Mianlue suture. According to the simulation results of density and magnetic susceptibility, the crust of the WQOB is thick, and the magnetic susceptibility in the upper crust is weak. The structure of the WQOB obtained based on gravity and magnetic data is consistent with the deep seismic reflection results (Gao et al., 2014; Wang et al., 2014; Liu W. et al., 2021), which indicates that the West Qinling underwent intensive extension after collisional orogeny. Recent studies have confirmed that the magmatism in the West Qinling is interpreted as products related to ocean subduction and continental collision, and the addition of mantle–derived magmas to the continental crust ocean subduction process played an important role in crustal growth (Kong et al., 2019; Liu H. N. et al., 2021). More importantly, the distribution of large–scale deep fault tectonic framework can be inferred through cross inversion profile fitting of gravity and magnetic anomalies, such as Mianlue structure, Shangdan structure. The inversion results indicate that the Diebu–Sanhe fault may intersect with the Mianlue suture zone in the lower crust, with a depth of about 50 km or more at the intersection, and its correlation with the Mianlue structure is worth further research.

Based on the geophysical gravity and magnetic results of the deep–seated structure of the West Qinling, we focus on the mineralization processes related to the deposits, and describe the deep–seated tectonic hydrothermal mineral pipelining systems model of the West Qinling Orogenic belt (Figure 9A). The interactions between various interconnected magma reservoirs are widespread in West Qinling beneath the magmatic plumbing system, with multiple magmatic processes, and mineralization is likely genetically tied to magmatism (Song et al., 2023; Ma et al., 2024; Zhang et al., 2024). At present, three genetic models of Mesozoic gold mineralization have been proposed in the West Qinling Orogen (Guo et al., 2023). One model invokes a metal source from the metamorphism of crustal rocks during a regional thermal event resulting from the collision between the Yangtze and the North China cratons (Qiu et al., 2020; Yu et al., 2020b). The second model argues that the origin of gold deposits is intrusion–related and that the metals are derived from magmatic–hydrothermal fluids (Sui et al., 2020; Sui et al., 2017; Jin et al., 2017). The last model favors Carlin–like gold deposits and considers metal sources from magmatic and sedimentary rocks (Chen and Santosh, 2014; Liu et al., 2015a; Du et al., 2021b). This article takes some typical mineral deposits as the objects of analysis and discussion. The locations of these mineral deposits are distributed within the WQOB (Figure 9B). The Zaozigou and Dashui are considered Carlin–like gold deposits, and their formation is closely related to magmatic fluids. The ore–forming hydrothermal fluid of Carlin type gold deposits in the West Qinling is mainly magmatic water, with the participation of metamorphic water and atmospheric precipitation. The Zaozigou gold deposit is associated with multiple magmatic hydrothermal activities (Du et al., 2021b; Hu et al., 2021; Hu et al., 2022), and it can be considered as a shallow–seated, distal product of a reduced intrusion–related gold system. Isotopes indicate that the ore–forming fluids are likely a mixture of magmatic fluids exsolved from a reduced magma or a mixture of such a fluid with metamorphic–derived fluids devolatized from Paleozoic sedimentary sequence underlain by the ore–hosting Triassic strata (Sui et al., 2020). Laerma is considered as a Carlin type gold deposit, which is in the background of collision Orogenic belt compression structure. The formation of the Laerma deposit is closely related to the meteoric origin fluid, and some of it is also added by magmatic water and metamorphic water (Liu et al., 2015b; Deng and Wang, 2016). Synthesis of regional geologic and geochemical data on granitic intrusions and ore deposits suggests that the Laodou is a reduced intrusion–related Au deposit, and hydrothermal fluid components were largely derived from magmas represented by the quartz diorite (Jin et al., 2017). The Dewulu has been recognized as a sediment–and magmatic–hosted sheeted vein–type reduced Au–Cu skarn deposit, which is associated with reduced felsic to intermediate intrusions emplaced during early to middle Triassic (Sui et al., 2017).

In addition, the formation process of ore deposits related to Mesozoic fluid process are briefly summarized as follows. In the initial stage of mineralization, the late Paleozoic sedimentary mineralization superimposed shallow Metamorphism and hydrothermal process. Seafloor hydrothermal eruptions led to the formation of deposits containing gold. Following Indosinian to Yanshanian hydrothermal transformation, layer–controlled mineral deposits were ultimately formed. The superimposition of late magmatic intrusion or hydrothermal activation on marine volcanic sedimentary mineralization, resulting in reconstructed mineralization. In the second stage of mineralization, Mesozoic magmatic activity was associated with the widespread occurrence of iron, copper, gold, silver and other minerals in the Triassic system of West Qinling. Early to Middle Triassic marine volcanism–related deposits exhibit mineralization in marine volcanic sedimentary environments, and Late Triassic continental volcanism–related deposits are found in the western section of the Upper Triassic West Qinling. The iron, copper, gold and other mineral deposits are associated with the intrusion of acidic magma during the Triassic–Jurassic. As a result of the Indosinian movement in the Middle and Late Triassic, subduction and northward collision of the Qinling Plate occurred, leading to comprehensive uplift into landmasses. The magmatic activity persisted in localized regions until the early Yanshanian. The initial ore–forming fluid originated from magmatic water, while the later ore–forming fluid was a mixed solution of rock slurry water and atmospheric precipitation. After Triassic, intense intracontinental subduction occurred due to active intracontinental orogeny. The thermodynamic heating associated with tectonic and magmatic activity drives the convective circulation of atmospheric precipitation, causing ore–forming components in geological formations to migrate towards favorable tectonic settings for precipitation and mineralization (Lu et al., 2020; Xing et al., 2020; Du et al., 2021a; He et al., 2021; Yang X. Y. et al., 2022).

In summary, the geophysical inversion of deep–seated sturture in the West Qinling provide evidences for the opinion that metals originate from magmatic hydrothermal fluids. We support that the West Qinling structure represents important conduits for migrating magmatic–derived hydrothermal fluids. The gold, iron, and copper deposits in the study area are mainly distributed on both sides of inferred faults. The spatial distribution of gold deposits exhibits structural ore–control regularities, which are all controlled by structural systems and deep–seated fault structural belts of orders. At the end of Indosinian, with the closure of Mianlue Ocean, the tectonic setting of the West Qinling changed from oceanic subduction to continental collision. The lithosphere thermal anomaly caused by crustal thinning due to oceanic subduction is likely to provide heat source for hydrothermal activities in the WQOB. Accompanied by magmatic activity, it provides heat and material sources for mineralization. Regional magmatic activity also plays an important role in mineralization. The intrusion of granite can cause anomalies in local geothermal fields, providing a heat source for the mineralization of the intrusion–related gold deposits.