Concealed Deposits Research Articles

Joint Gramian inversion of gravity, magnetic and gravity gradient tensor data for the modelling of Mesoproterozoic iron oxide deposits, southeast Missouri, USA

SUMMARY Within the iron metallogenic province of southeast Missouri, USA, there are several mines that contain not only economic iron resources, magnetite and/or hematite, but also contain rare earth elements, copper and gold. An area including three major deposits, Pea Ridge, Bourbon and Kratz Spring, was selected for detailed modelling for the upper crustal magnetic susceptibility and density structures. For the study area, ground gravity and high-resolution airborne magnetic and gravity gradiometry data sets are available. An efficient and novel joint inversion algorithm for the simultaneous inversion of these multiple data sets is presented. The Gramian coupling constraint is used to correlate the reconstructed density and magnetic susceptibility models. The implementation relies on the structures of the sensitivity matrices and an efficient minimization algorithm to achieve significant reductions in the memory requirements and computational costs. Consequently, it is feasible to use a laptop computer for the inversion of multiple data sets, each containing thousands of data points, for the recovery of models on the study area, each including approximately one million model parameters. This is the first time that these multiple data sets have been simultaneously inverted for this area. The L1-norm stabilizer is used to provide compact and focused images of the ore deposits. For contrast, independent inversions of each data set are also discussed. In general, our results provide new insights about the concealed ore deposits in the Mesoproterozoic basement rocks of southeast Missouri. Both short- and long-wavelength anomalies exist in the recovered models; these provide a high-resolution image of the subsurface. The geometry and physical properties of the known deposits are determined very well. Additionally, some unknown concealed deposits are revealed; these could be economically valuable and should be considered in future geophysical and geological investigations.

Widespread nearly or nanoscale natural amorphous particles in critical zones from ore deposits

The advancement of nanotechnology has drawn more attention on synthetic nanoscale particles, while naturally nanoscale particles are often overlooked, especially nearly or nanoscale natural amorphous particles (nearly or NNAPs). Here, we report nearly or NNAPs obtained from multiple media. The sample media, including groundwater, ascending gas, soil and plant tissue, were obtained from eight concealed deposits with different types of minerals and three blank control group areas in the critical zones in China. The morphology, elemental combination and structure of nearly or NNAPs in these samples were characterized via transmission electron microscopy (TEM). The results show that the nearly or NNAPs are widespread compounds. This study established the elemental combination relationship of nearly or NNAPs and defined the particles as a native metal, S–O-, P–O-, and Cl–(O)-bearing as well as oxide nearly or NNAPs. Nearly or NNAPs could carry high proportion of ore-related elements, such as W, Pb, and Co in oxide nearly or NNAPs; Cu in S–O-bearing nearly or NNAPs; and Zn in P–O-bearing nearly or NNAPs. Nearly or NNAPs that carry ore-related elements were obtained from the surface area of the concealed deposits but not in the blank control group areas. With strong geochemical features, nearly or NNAPs can carry ore-related elements from the concealed ore bodies to the surface. These phenomena indicate nearly or NNAPs can be prospecting concealed ore bodies while possessing the environmental risks with high proportion of metal elements.

40Ar/39Ar Geochronology of Magmatic-Steam Alunite from Alunite Ridge and Deer Trail Mountain, Marysvale Volcanic Field, Utah: Timing and Duration of Miocene Hydrothermal Activity Associated with Concealed Intrusions

Porphyry and epithermal deposits are important sources of base and precious metals. Most actively mined deposits have been exhumed such that ore bodies are relatively close to the surface and are therefore locatable and economic to extract. Identifying and characterizing concealed deposits, particularly more deeply buried porphyry deposits, represents a far greater challenge for mineral exploration, and will become progressively more important as near-surface resources are gradually exhausted over time. We report high-precision 40Ar/39Ar dates for coarsely crystalline alunite that precipitated from magmatic steam in open fractures in Oligocene dacitic volcanic rocks, and a SHRIMP 206Pb/238U zircon date for one of several rhyolite dikes present at Alunite Ridge and Deer Trail Mountain, Utah. Both the magmatic-steam alunite and rhyolite dikes are related to concealed intrusions. The rhyolite dike yielded an age of 30.72 ± 0.36 Ma, which is older than a commonly cited 27.1 Ma age estimate for the Three Creeks Tuff Member of the Bullion Canyon Volcanics that is cut by the dike. 40Ar/39Ar data for samples of magmatic-steam alunite and sericite from six mines and prospects provide evidence for at least two periods of episodic hydrothermal activity at ca. 15.7–15.1 Ma and ca. 14.7–13.8 Ma, with the older and younger pulses of activity recorded at the more eastern and western sites, respectively. These two periods of hydrothermal activity are consistent with previous interpretations that Alunite Ridge and Deer Trail Mountain are underlain by two concealed porphyry stocks. 40Ar/39Ar analyses of individual bands in a sample of massive, centimeter-scale banded vein alunite yield indistinguishable ages with a weighted mean of 13.98 ± 0.12 Ma, consistent with a short-lived (≲250 ka) magmatic event with episodic vapor discharge recurring on short timescales (≲36 ka). 40Ar/39Ar geochronology of magmatic-steam alunite is a valuable tool to constrain the timing and duration of magmatic hydrothermal activity associated with unexposed intrusions and potentially porphyry deposits, and therefore may be useful in exploration.

Application of the fine-grained soil prospecting method in a loess-covered area of central China

Loess-covered landscapes occupy a large area of the globe and have substantially high potential for mineral exploration; however, as mineral information is obscured by loess cover, suitable prospecting techniques need to be applied. The fine-grained soil prospecting method—a deep-penetrating geochemical technology that can be used to identify concealed deposits—has been proven effective in some cases. In this study, we applied this method for mineral exploration in the loess-covered Luoning Basin in Central China. Our results revealed strong geochemical anomalies (e.g., Au, Ag, Pb, Zn, and Cu) over related and concealed ore deposits. Furthermore, a regional geochemical survey in the Luoning Basin revealed the spatial distribution and suites of elements that were consistent with the mineralization type. Moreover, higher geochemical anomalies of Au, Ag, Pb, and Zn were spatially related to an Au ore deposit and Ag–Pb–Zn polymetallic deposit, respectively. The results of a detailed geochemical survey in the Zhonghedi Ag–Pb–Zn polymetallic deposit further revealed that the distribution of surface element anomalies was associated with tectonic fractures throughout the exploration region, which are closely related to the distribution of orebodies. These results imply that the fine-grained soil prospecting method is suitable for anomaly delineation during detailed geochemical surveys as well as regional-scale surveys. Based on our results, we suggest that geochemical anomalies in the loess-covered surface in our study area are presumably the result of geogas dispersion processes; we provide a schematic model explaining how ore-forming and associated materials are brought to the surface by geogas flowing upwards along faults and fractures and, ultimately, accumulating in fine-grained loess soils. Overall, our research demonstrates that fine-grained soil prospecting can effectively aid the search for concealed mineral deposits in loess-covered landscapes.

Multi-scale exploration of giant Qulong porphyry deposit in a collisional setting

With the increasing demand for copper resources, exploration methods based on the discovered porphyry Cu deposits (PCDs) to target concealed orebodies are of greater importance. Single parameter is no longer sufficiently effective against concealed deposit exploration, and different combinations of survey techniques are required. The Qulong porphyry Cu-Mo deposit is the largest collisional PCD found in China. Here we evaluated the relationship between variations in multiple parameters and alteration to define exploration footprints around the porphyry Cu centers. In order to improve the exploration efficiency and accuracy, we combined geophysical (magnetic susceptibility and resistivity) and popular geochemical exploration methods (i.e., short wavelength infrared spectroscopy- SWIR, portableX-ray fluorescence- pXRF, and indicator minerals) to the drill hole ZK005 of Qulong PCD in southern Tibet seeking for a firm interpretation. Four orebody indicators were established in study of Qulong deposit, which are: (i) the fluctuations of Ca/K, Mg/Fe, Sr/Ti, and Si/Al ratios, (ii) long-wavelength white mica (2207–2214 nm) and short-wavelength chlorite (2247–2253 nm), (iii) sub-high magnetic susceptibility and (iv) low resistivity. Multi-scale prospecting methods can improve prospecting efficiency and narrow down the target area. Based on the local environment, selecting an appropriate combination of methods can improve exploration efficiency in the brownfields at a lower cost.

Ore-forming elements and their distribution of nanoparticles in the updraft from the Sanshandao concealed deposit, China

The search for concealed ore deposits has increasingly gained attention, due to the gradual depletion of surface resources over time. It is necessary to develop non-traditional geochemical theories and methods to locate concealed deposits. In this study, the morphological characteristics and chemical compositions of particles carried by the updraft of the Sanshandao deposit are observed through transmission electron microscopy. Results presented direct evidence that the composition of nanoparticles in the updraft is affected by the geological background, concealed ore bodies, and oxidation. Cl and Na in the study area are found to be higher than those in other deposits owing to the influence of seawater intrusion. Elements associated with ore-forming are contained in the updraft in the form of nanoparticles, which usually have the same distribution pattern and element assemblage in their particles. Oxygen is found to widely exist in the nanoparticles and matched well with the particle distribution. These combinations of elements from orebodies are not observed in the nanoparticles in the background area. Thus, we considered that nanoparticles associated with the orebodies are formed in the underground orebodies and migrate upward to the surface. Mapping can be used to understand the elemental composition and distribution of the nanoparticles. Based on the composition and distribution of nanoparticles, the elemental characteristics of the underground orebody can be inferred, and the minerals and the distribution range of the concealed orebodies can be predicted.

Application of compositional data analysis in geochemical exploration for concealed deposits: A case study of Ashele copper-zinc deposit, Xinjiang, China

One of the top priorities of ore body exploration geochemistry is to effectively identifying “anomalies” and separating them from “background” in geochemical data analyses, during which an accurate understanding of geochemical distribution patterns of elements and anomaly separation has been the focus of ore body hunting. However, geochemical data are compositional data with closure effect and require necessary logratio transformation to open closed data for the following statistical analysis. The invested dataset of 13 elements derived from 2264 surface rock samples from the northwestern Ashele mining area was processed and respectively analyzed as the raw data, logarithmically-transformed data, and isometric logratio (ilr) transformed data with exploratory data analysis (EDA) technology. Of which, the ilr transformed data yields an ideal data structure and scale distribution. The data then was analyzed with principal component analysis (PCA) and robust principal component analysis (RPCA), along with the compositional biplots and the first principal component score maps to identify the spatial distribution of element assemblages. After that, a comparative study of the geochemical distribution patterns based on the raw data of mineralized elements and the principal component scores based on ilr transformation data is conducted to explore the connection with mineralization. As a result, four prospecting prediction targets of two types are delineated in the study area.

Development of scientific and technological support for subsurfaceassessment for ore minerals

The problems of scientific and technological support of the geological exploration industry are considered. The factors affecting the replenishment and expansion of the mineral base of Kazakhstan and some other countries are analyzed. The need for the scientific and technological development compliance with the global scientific trend (cross-disciplines, mathematization and digitalization areas) is emphasized. The main difficulties related to forecasting of concealed ore deposits as well as methodological, physical and geological aspects of these activities are considered. A basic model of ore formation is proposed and the mutual spatial arrangement of ore deposits is analyzed. The concept of constructing an information geological space model (GSM) is considered. The prospective directions of developing the technology for concealed deposits forecasting are shown.

Formation of nano- or near-nanoparticles via oxidation in the Dabaoshan concealed deposit, Guangdong Province

Natural particles have always been abundantly present since the Earth’s origin. Over the years, researchers have focused on natural particles coming from the Earth’s surface. However, in this paper, we establish, via transmission electron microscopy (TEM), that many nano- or near-nanoparticles also can be produced in a concealed deposit. Particles collected from the oxidation zone in the Dabaoshan polymetallic concealed deposit were 50–700 nm in size and contained elements among the most abundant in the Earth’s crust, such as O, Mg, K, Ca, Si, Al, Ti, Fe and P, and concealed ore body-related elements. Nearly all particles were found to have O and S, indicating that these nano- or near-nanoparticles were formed via oxidation of primary minerals in the concealed deposit. According to their crystal form and chemical compositions, these particles were grouped into three main categories: (i) particle with low O content (O O > 10 wt.%) and (iii) particle with high O content (O > 30 wt.%). The crystal form and chemical compositions of particles can represent the degree of oxidation of primary minerals in the concealed deposit. Combining the close relationship with orebodies, nano- or near-nanoparticles can be used to be a useful prospecting method for deep-seated orebodies. Additionally, strong acids are also formed during the oxidation process that can destroy the ecosystem of the oxidation zone in the concealed deposit.

Characteristics of particles in groundwater and their prospecting significance for the Shijiangshan Pb-Zn-Ag deposit, Inner Mongolia, China

The Shijiangshan concealed deposit in Inner Mongolia, China is located in a semi-arid region with a thick sediment layer on the surface. Samples of particles from deep and shallow groundwater in the deposit were analyzed by transmission electron microscopy (TEM). Corresponding fault gouge and oxide ore powder were collected and analyzed at the same time. These particles had sizes ranging from 5 to 400 nm and contained metallogenic elements, such as Pb, Zn, As, Cu, and Ag. Groundwater particles can reflect the properties of the deep concealed ores because the metal-bearing particles in the groundwater are similar in composition to the deep ores. In this study, particles with a carbon film covering their surface were discovered in groundwater. The carbon coating helped particles to remain in their primitive state, and thus, retain the more direct metallogenic information. Shallow groundwater particles can be used to predict the composition of deep concealed ore deposits. In thick overburdened areas, where local geochemical information on abnormal elemental levels is not obvious and geophysical prospecting methods may have multiple solutions, the accuracy of prospecting can be improved by using particles in groundwater that provide concealed orebody information, making such particles very important for prospecting.

Three-Dimensional Regularized Focusing Migration: A Case Study from the Yucheng Mining Area, Shandong, China

Gravity migration is a fast imaging technique based on the migration concept to obtain subsurface density distribution. For higher resolution of migration imaging results, we propose a 3D regularized focusing migration method that implements migration imaging of an entire gravity survey with a focusing stabilizer based on regularization theory. When determining the model parameters, the iterative direction is chosen as the conjugate migration direction, and the step size is selected on the basis of the Wolfe–Powell conditions. The model tests demonstrate that the proposed method can improve the resolution and precision of imaging results, especially for blocky structures. At the same time, the method has high computational efficiency, which allows rapid imaging for large-scale gravity data. It also has high stability in noisy conditions. The developed novel method is applied to interpret gravity data collected from the skarn-type iron deposits in Yucheng, Shandong province. Migration results show that the depth of the buried iron ore in this area is 750–1500 m, which is consistent with the drilling data. We also provide recommendations for further mineral exploration in the survey area. This method can be used to complete rapid global imaging of large mining areas and it provides important technical support for exploration of deep, concealed deposits.

The characteristic of microstructural deformation of gold bearing pyrite in Jiaodong: The links between nanoscale gold enrichment and crystal distortion

Auriferous pyrite can host a large proportion of gold resource at giant gold deposits all around the world so that these sulfides minerals become key factors to reveal the genesis of ore deposits. In this paper, the focused ion beam-scanning electron microscope (FIB-SEM) method were conducted to present the in-situ transmission electron microscopy (TEM) high resolution images for dislocation ultrastructure study in auriferous pyrite. The TEM diffraction images evidences were provided to prove the evidence that gold cluster at the nanoscale could be existed among crystal interface or even crystal lattice (0.22 nm ~ 0.54 nm). Finally, we have proposed a model that link these nanostructures with gold concentration, gold aggregate in pyrite. The model illustrates the formation pathway of the of discrete invisible gold in pyrite grains which were controlled by shape and ultrastructure of pyrite crystalline, with ion-by-ion rapid diffusion on the surface or lattice. It’s well interpreting for inhomogeneous distribution of nanogold in pyrite resulted from combining with many of these basic pyrite units. In addition, it also reveals unique in-situ measurement result about crystal-fluid interface dynamic that identify the various lattice spacing with possibility for nanoparticle incorporation into deformed pyrite. This study not only provides a new insight into the interpretation of gold distribution and aggregation in pyrite that has significant indication for gold deposit formation in the world, but also provides important evidences for geochemical exploration for concealed deposits based on nanoscale gold.

云南羊拉铜矿江边矿段三维地质建模及隐伏矿床(矿体)定位预测

云南羊拉铜矿江边矿段三维地质建模及隐伏矿床(矿体)定位预测

Conjugate utilization of Landsat-8 OLI, ground gravity and magnetic data for targeting mafic cumulates within anorthositic-layered complex of Sittampundi, India

In this article, we have processed Landsat-8 spectral bands, ground gravity, and magnetic data for delineating the mafic cumulates within the anorthositic-layered complex in Sittampundi, Tamilnadu, India. In this regard, surface anomaly map of mafic cumulates was derived by processing spectral bands of Operational Land Imager (OLI) sensor using matched filtering method. Residual gravity, magnetic anomaly maps are also derived. Spatial position of geophysical anomalies corroborate well with the spectral anomalies. A promising zone of chromitite occurrences is identified at the eastern fringe of the study area based on the concurrence of spectral and geophysical anomalies. Spatial profiles of geophysical and spectral anomalies around this promising zone are similar to the respective spatial profiles observed around the known chromite bearing mafic cumulates. Depth continuity of the mafic cumulates is confirmed by deriving of gravity and magnetic anomaly map at different depth using upward continuation filters to confirm concealed deposit.

An aeromagnetic investigation of the Dapai deposit in Fujian Province, South China: Structural and mining implications

The Dapai region is considered as one of the largest polymetallic reserves in Fujian Province, South China. As exploration advances, so does the demand for prospecting the deeper concealed deposits in this place. However, in the past few years, due to the rugged surface topography and dense vegetation, few high-precision and large-scale ground surveys have been carried out in this area. Therefore, the understanding of the overall structure and non-exposed deposits remains insufficient to meet the challenges involved in deeper ore prospecting. In the present study, we applied a cost-effective and high-precision aeromagnetic survey to investigate the structural trends prevailing in the study area and delimit the areas with potential for finding deposits. To achieve the aforementioned goals, the tilt angle derivative was conducted on the pre-processed magnetic anomalies to highlight any deeply seated structural features, based on which reliability of the geologically-inferred faults was analyzed, and seven major faults and 22 minor faults were identified for the first time. Most of these deduced faults show an NS trend in the center part and an NW/NE trend in other parts of the study area. In addition, 3D generalized physical property inversion of the obtained residual magnetic anomalies was implemented to delineate the potential ore bodies, based on which an integrated prospecting target map was established. Our inversion results indicate that there are several isolated magnetic sources in the subsurface extending from a depth of a few hundred meters to no more than 3 km. This presented study not only leads to a deeper understanding of the Dapai deposit, but also provides references for studying other similar deposits where large-scale ground studies are hard to be executed.

Joint MT and Gravity Inversion Using Structural Constraints: A Case Study from the Linjiang Copper Mining Area, Jilin, China

We present a joint 2D inversion approach for magnetotelluric (MT) and gravity data with elastic-net regularization and cross-gradient constraints. We describe the main features of the approach and verify the inversion results against a synthetic model. The results indicate that the best fit solution using the L2 is overly smooth, while the best fit solution for the L1 norm is too sparse. However, the elastic-net regularization method, a convex combination term of L2 norm and L1 norm, can not only enforce the stability to preserve local smoothness, but can also enforce the sparsity to preserve sharp boundaries. Cross-gradient constraints lead to models with close structural resemblance and improve the estimates of the resistivity and density of the synthetic dataset. We apply the novel approach to field datasets from a copper mining area in the northeast of China. Our results show that the method can generate much more detail and a sharper boundary as well as better depth resolution. Relative to the existing solution, the large area divergence phenomenon under the anomalous bodies is eliminated, and the fine anomalous bodies boundary appeared in the smooth region. This method can provide important technical support for detecting deep concealed deposits.

A TEM study of particles carried by ascending gas flows from the Bairendaba lead-zinc deposit, Inner Mongolia, China

In order to find deep-seated deposit, there is a need to develop non-traditional surface geochemistry. Geogas is of particular interest. Particles transported by geogas from the deeply concealed deposits potentially hold great information. This study investigates particles carried by ascending gas flows from the Bairendaba lead-zinc deposit. This deposit is covered by a thick exotic substance, which hinders probing the ore body directly. The samples were collected using a static sampling method and were analysed by transmission electron microscopy (TEM) to observe particle features (such as shape, component, size, structure, and form of polymerization); these particles contain information on buried deposits. Numerous Pb-, Zn-, and Cu-bearing particles, as well as particles containing other metal compounds, occur as individuals or aggregates in the ascending gas flows. Most particles have a large oxygen (O) content; this is likely caused by the course of gas flows ascending from the deep ore body to the ground surface. Sulphur (S) is commonly associated with Pb, Cu, As, Fe, and Sb and occurs as sulphate in this deposit. Most metallic elements occur as sulphate, oxide, and carbonate in the detected particles. In contrast to the samples collected in the peripheral area, samples collected from the concealed deposit include numerous particles containing metallogenic elements, although particles containing major elements can be found in both areas. Moreover, the trend of each Pb- or Zn-bearing particle along a unique sampling line is directly related to the distribution of the corresponding ore bodies; this indicates that the particles were generated at a certain depth, carried by the ascending gas flow and directly provide important information regarding the concealed ore bodies. The anomalous area can be used as a basis for further prospect analysis and improve the prospecting success rate.

Metallogeny, structural evolution, post-mineral cover distribution and exploration in concealed areas of the northern Chilean Andes

Mineral exploration of prospective areas concealed by extensive post-mineralization cover is growing, being very complex and expensive. The projection of rich and giant Paleocene to early Oligocene porphyry-Cu-Mo belts in northernmost Chilean Andes (17.5–19.5°S) has major exploration potential, but only a few minor deposits have been reported to date, due to the fact that the area is largely covered by post-mineral strata. We integrate the Cenozoic stratigraphic, structural and metallogenic evolution of this sector, in order to identify the most promising regions related to lesser post-mineral cover and the projection of different metallogenic belts. The Paleocene to early Eocene metallogenic belt extends along the Precordillera, with ca. 30km wide, and includes porphyry-Cu prospects and small Cu (±Mo-Au-Ag) vein and breccia-pipe deposits. Geochronological data indicate an age of 55.5Ma for an intrusion related to one deposit and ages from 69.5 to 54.5Ma for hydrothermal alteration in one porphyry-Cu prospect and largest known Cu deposits. The middle Eocene to early Oligocene porphyry belt, in the Western Cordillera farther east, is associated with 46–44Ma intrusions. It is estimated to be 40-km wide, but is largely concealed by thick post-mineral cover. The youngest Miocene to early Pliocene metallogenic belt, also in the Western Cordillera, is well-exposed and includes Au-Ag epithermal and polymetallic veins and manto-type deposits.The Oligocene-Holocene cover consists of a succession of continental sedimentary and volcanic rocks that overall increase in thickness from 0 to 5000m, from west to east. These strata are subhorizontal in the west and folded-faulted towards the east. Miocene gentle anticlines and monocline flexures extend along strike for 30–60km in the Precordillera and were generated by propagation of high-angle east-dipping blind reverse faults with at least 300–900m of Oligocene bedrock offset. The thickness of cover exceeds 2000m in the eastern Central Depression, whereas it is generally less than 1000m in the Precordillera along the Paleocene to early Eocene porphyry-Cu belt and it can reach locally up to 5000m in the Western Cordillera, above the middle Eocene to early Oligocene belt.In the studied Andean segment, the Miocene to early Pliocene metallogenic belt is superimposed on the Paleocene to Oligocene belts in a 40–50km wide zone. This overlap may be explained by an accentuated migration of the magmatic front, from east to west, since ca. 25Ma, as a consequence of subduction slab steepening after a period of magmatic lull and flat subduction from ca. 30–35 to 25Ma. The identified areas of lesser cover thickness are prone to exploration for concealed deposits, especially along the projection of major porphyry-Cu-Mo belts.

Geochronology, geochemistry and Hf–Sr–Nd isotopes of the ore-bearing syenite from the Shapinggou porphyry Mo deposit, East Qinling-Dabie orogenic belt

The Shapinggou Mo deposit is located in the western Dabie mountains, the eastern part of the Qinling-Dabie molybdenum orogenic belt. Shapinggou Mo deposit is a concealed deposit with the ore body mainly hosted by explosive breccia of Gaijing and the granite porphyry as well as the syenite of Shapinggou. Geochemistry study show that the SiO2 contents of Shapinggou syenite range from 61.74 to 69.93%, and the A/CNK from 0.95 to 1.06, classified as metaluminous to weak peraluminous, belonging to alkalic to shoshonitic series. The Mo deposits in Qinling Mo belt formed in two main periods, i.e., the first period occurred in to the Early Cretaceous (145–130 Ma), the second period in the late Early Cretaceous (130–110 Ma). Most of the Mo deposits in Dabie region formed in the second period. The results of zircon U–Pb show that the age of the Shapinggou syenite is 111.3 ± 1.2 Ma, which belongs to the second period. Proterozoic-Archean inherited zircons suggest that it may include some more ancient crustal material like Kongling group. The ɛHf(t) values of Shapinggou syenite range from −15.6 to −8.0, TDM2(Hf) from 1.7 to 2.16 Ga, respectively. The ɛNd(t) values of the Shapinggou syenite range from −12.29 to −11.76, TDM2(Nd) from 1.85 to 1.89 Ga, the 87Sr/86Sr from 0.709 to 0.710, respectively. Results of zircon Hf isotope and whole rock Sr–Nd isotope of Shapinggou syenite indicate that the Mo ore-forming materials were mainly generated from old Yangtze craton, e.g., gneiss from Dabie orogeny, mixed with some juvenal mantle materials. The geodynamics of the Shapinggou Mo deposit corresponded to an extension period in Eastern China, which caused by large scale lithospheric thinning. The delamination caused asthenosphere upwelling and crust-mantle interaction, which provided the ore-forming material and heat.

Magnetic and Gravity Gradiometry Framework for Mesoproterozoic Iron Oxide-Apatite and Iron Oxide-Copper-Gold Deposits, Southeast Missouri

High-resolution airborne magnetic and gravity gradiometry data provide the geophysical framework for evaluating the exploration potential of hidden iron oxide deposits in Mesoproterozoic basement rocks of southeast Missouri. The data are used to calculate mineral prospectivity for iron oxide-apatite (IOA) ± rare earth element (REE) and iron oxide-copper-gold (IOCG) deposits. Results delineate the geophysical footprints of all known iron oxide deposits and reveal several previously unrecognized prospective areas. The airborne data are also inverted to three-dimensional density and magnetic susceptibility models over four concealed deposits at Pea Ridge (IOA ± REE), Boss (IOCG), Kratz Spring (IOA), and Bourbon (IOCG). The Pea Ridge susceptibility model shows a magnetic source that is vertically extensive and traceable to a depth of greater than 2 km. A smaller density source, located within the shallow Precambrian basement, is partly coincident with the magnetic source at Pea Ridge. In contrast, the Boss models show a large (625-m-wide), vertically extensive, and coincident dense and magnetic stock with shallower adjacent lobes that extend more than 2,600 m across the shallow Precambrian paleosurface. The Kratz Spring deposit appears to be a smaller volume of iron oxides and is characterized by lower density and less magnetic rock compared to the other iron deposits. A prospective area identified south of the Kratz Spring deposit shows the largest volume of coincident dense and nonmagnetic rock in the subsurface, and is interpreted as prospective for a hematite-dominant lithology that extends from the top of the Precambrian to depths exceeding 2 km. The Bourbon deposit displays a large bowl-shaped volume of coincident high density and high-magnetic susceptibility rock, and a geometry that suggests the iron mineralization is vertically restricted to the upper parts of the Precambrian basement. In order to underpin the evaluation of the prospectivity and three-dimensional models, an extensive statistical summary of density and apparent magnetic susceptibility measurements is presented that includes data on several hundred samples taken from the deposits, altered wall rocks, and unaltered country rocks.