**1. Introduction**

As a universal currency, gold has anti-inflation and safe-haven functions. It has always played an important role in the global financial system, especially during financial crises, which highlights its safe-haven function. As such, gold is currently used as a reserve by most governments to maintain economic and material stability. In addition to the official gold reserves, gold is essential for industry, healthcare, and high-tech fields. For China, gold is a credit instrument related to foreign trade and economic cooperation [1]. The Jiaodong Peninsula is the most important gold-producing area in China and the third largest gold-mining area in the world, with proven gold resources exceeding 5000 t [2]. For a long time, researchers have carried out geological, geophysical, and geochemical works in the Jiaodong Gold Mine and accumulated abundant data, which provided an important basis for deep prospecting [2–6]. Since the implementation of the exploration breakthrough strategic action in 2011 [5–7], major exploration breakthroughs have been made in important gold belts such as Sanshandao, Jiaojia, and Zhaoping in Jiaodong [7]. By 2020,

**Citation:** Li, B.; Peng, Y.; Zhao, X.; Liu, X.; Wang, G.; Jiang, H.; Wang, H.; Yang, Z. Combining 3D Geological Modeling and 3D Spectral Modeling for Deep Mineral Exploration in the Zhaoxian Gold Deposit, Shandong Province, China. *Minerals* **2022**, *12*, 1272. https://doi.org/10.3390/ min12101272

Academic Editor: José António de Almeida

Received: 3 August 2022 Accepted: 4 October 2022 Published: 9 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the prospecting depth had expanded from 500 m to 4000 m, forming large gold deposits such as Sanshandao, Jiaojia, Linglong, and Denggezhuang. Because of the metallogenic background, the mineralization and production environment are significantly different from those of other types of gold deposits known in the world; this type of deposit is named the Jiaodong-type gold deposit [5]. According to the different mineralization patterns, the gold deposits in this area can be subdivided into Jiaojia-, Linglong-, and Pengjiakuang-type gold deposits [6]. The Zhaoxian gold deposit is located in the northwest of the Jiaojia Gold Mine and is currently the mining area with the largest average exploration depth in China. In-depth analysis of the ore-body geological characteristics of this large gold mine will be of great significance for promoting deep ore prospecting in the Jiaojia ore field [8].

Recently, short-wavelength infrared (SWIR) spectroscopy has been widely applied in mineral exploration. Its wavelength range is between 1300 nm and 2500 nm, which can effectively identify minerals containing hydroxyl groups, amino groups, partial carbonates, and sulfates according to the difference between the reflection absorption of SWIR light and characteristic absorption peaks of the functional groups [9–12]. This method can quickly identify deep minerals and alteration zones in actual exploration and has been successfully applied to porphyry deposits, epithermal deposits, volcanogenic massive sulfide deposits, and some iron oxide–copper–gold (IOCG) deposits [13–16]. With the introduction of computer three-dimensional (3D) modeling technology, geological prospecting prediction has gradually developed from the two-dimensional (2D) plane to 3D space prediction. Three-dimensional geological modeling is a technology that provides 3D visualization through the integration of multisource and multiscale geoscience data [17–19]. Three-dimensional geological modeling allows capturing the geometry of structures at regional and local scales, visualizing the subsurface architecture of deposits and intrusions, and understanding fluid transport processes [20]. Data-driven geographic information system (GIS) based methods for integrating multisource and multisource spatial geoscience datasets have been widely used in mineral-potential mapping for exploration targeting in a data-rich mining area [18].

With the deep mineral-exploration work, SWIR-spectrum 3D alteration mapping has also been applied. For example, Harraden et al. (2013) carried out drill-hole SWIR exploration on the pebble-porphyry Cu–Au–Mo deposit in eastern North America, divided the wall-rock alteration zones in detail on the 3D scale of the mining area, and established the 3D geology and alteration model of the pebble deposit [21]. Chen et al. (2019) [22] performed mineral alteration mapping on the Tonglushan copper–gold deposit and established 3D attribute modeling based on the parameters of chlorite Pos2250 and kaolinite Pos2170, thus establishing altered-mineral-exploration targets in this area. Predecessors have conducted significant research on the altered minerals in the Jiaodong area, but most of them were based on element geochemistry, and no study has combined the SWIR spectrum and 3D modeling in this area. Therefore, this work aimed to use the SWIR spectroscopy measurement and interpretation of the altered-mineral datasets (depth > 1000 m) of the Zhaoxian gold deposit to identify the altered-mineral assemblage and zonation characteristics in the deep part of the mineralization area. By combining the SWIR and 3D geological modeling, the 3D multiparameter model of the altered minerals and the ore body can be quickly established; therefore, it is possible to compare and study the characteristics of alteration-zone minerals in depth and the spatial distribution of the mineralization in one dimension (1D), 2D, and 3D, and then make a 3D comprehensive prediction to optimize the favorable finding area and provide a basis for drilling verification.

#### **2. Deposit Geology and Mineralization**

The Zhaoxian gold deposit is located in the Jiaojia ore field of the northwestern Jiaodong Peninsula in China, which is approximately 15 km northeast of Laizhou City (Figure 1) [23]. The gold deposits in the region are controlled by NNE-trending faults, and their gold ore bodies are mostly hosted in the altered rocks within the footwalls of NNE–NE-trending faults. The deep part (1400 m to 2400 m) of the mining zone is bounded

by the Jiaojia main fault surface [2,24]. The wall-rock lithology is Linglong granite and metamorphic rock of the Jiaodong Group, and the footwall is ore-bearing altered-rock zones and Linglong-series monzogranite (ca. 160 Ma) and Guojialing-series granodiorite (ca. 130 Ma) [18]. trending faults. The deep part (1400 m to 2400 m) of the mining zone is bounded by the Jiaojia main fault surface [2,24]. The wall-rock lithology is Linglong granite and metamorphic rock of the Jiaodong Group, and the footwall is ore-bearing altered-rock zones and Linglong-series monzogranite (ca. 160 Ma) and Guojialing-series granodiorite (ca. 130 Ma)[18].

The Zhaoxian gold deposit is located in the Jiaojia ore field of the northwestern Jiaodong Peninsula in China, which is approximately 15 km northeast of Laizhou City (Figure 1) [23]. The gold deposits in the region are controlled by NNE-trending faults, and their gold ore bodies are mostly hosted in the altered rocks within the footwalls of NNE–NE-

*Minerals* **2022**, *12*, 1272 3 of 27

**2. Deposit Geology and Mineralization** 

**Figure 1.** Simplified regional geologic map of the Jiaodong Peninsula (modified by [23]). **Figure 1.** Simplified regional geologic map of the Jiaodong Peninsula (modified by [23]).

The altered rocks in the study area are distributed in strips, which are characterized by the superposition of various alterations and obvious alteration zoning (Figure 2) [3,18]. The altered rocks can be divided into the components of the main regional Jiaojia fault according to the alteration type, degree, and mineral assemblage. The hanging wall of the Jiaojia fault has sericite-granitized cataclastic rock and sericite granite zones, and the footwall has pyrite–sericite–quartz granitized cataclastic rock and pyrite–sericite–quartz granite zones. The gold ore body is mainly in disseminated, veinlet-disseminated, and veinlet forms (Figure 2). Pyrite is the main gold-bearing mineral, and the shallow zone is mostly enriched in pyrite–sericite–quartz cataclastic rocks, whereas the deep zone is mostly enriched in pyrite–sericite–quartz cataclastic rocks and pyrite–sericite–quartz granitized cataclastic rocks. The alteration develops along the wall rock of the fault structures, including potassium feldspar, hematite mineralization (reddenization), pyrite–sericite–quartz alteration, and carbonation, as well as chloritization, kaolinization, etc. Pyrite–sericite–quartz The altered rocks in the study area are distributed in strips, which are characterized by the superposition of various alterations and obvious alteration zoning (Figure 2) [3,18]. The altered rocks can be divided into the components of the main regional Jiaojia fault according to the alteration type, degree, and mineral assemblage. The hanging wall of the Jiaojia fault has sericite-granitized cataclastic rock and sericite granite zones, and the footwall has pyrite–sericite–quartz granitized cataclastic rock and pyrite–sericite–quartz granite zones. The gold ore body is mainly in disseminated, veinlet-disseminated, and veinlet forms (Figure 2). Pyrite is the main gold-bearing mineral, and the shallow zone is mostly enriched in pyrite–sericite–quartz cataclastic rocks, whereas the deep zone is mostly enriched in pyrite–sericite–quartz cataclastic rocks and pyrite–sericite–quartz granitized cataclastic rocks. The alteration develops along the wall rock of the fault structures, including potassium feldspar, hematite mineralization (reddenization), pyrite–sericite–quartz alteration, and carbonation, as well as chloritization, kaolinization, etc. Pyrite–sericite– quartz alteration is a general term for sericitization, silicification, and pyritization [8]. Pyrite-sericite-quartz altered rock and potassium-altered (reddenized) rock are usually gold-enrichment zones. The study area has multiperiod and multistage hydrothermal processes, which show the evolution and metasomatism of hydrothermal functions, coupled with multistage tectonic activities. According to the metallogenic relationships among the ore-controlling fault structures, hydrothermal veins, and gold mineralization, previous researchers have divided the hydrothermal metallogenesis into four stages, which are summarized in Table 1 [7,8,25].

Quartz–carbonate

marized in Table 1 [7,8,25].

**Figure 2.** Pyrite-type hand samples and ore-body microscope photos show the occurrence state of transparent minerals and gold (modified by [23]). (**a**)—Type I pyrite in sericitized granites (PyⅠ); (**b**)—Type II pyrite in quartz pyrite veins (PyⅡ); (**c**)—Type III pyrite in quartz sulfide ores (PyⅢ); (**d**)—Type IV pyrite in pyrite calcite vein ores (PyⅣ); (**e**–**h**)—transparent mineral forms under polarized light microscopy; (**i**–**l**)—occurrence state of gold in different types of pyrite. Py—pyrite; Ser—sericite; Qtz—quartz; Cal—calcite; Pl—plagioclase; Ap—apatite; Ttn—sphene; Au—natural gold; Gn—galena. **Figure 2.** Pyrite-type hand samples and ore-body microscope photos show the occurrence state of transparent minerals and gold (modified by [23]). (**a**)—Type I pyrite in sericitized granites (PyI); (**b**)—Type II pyrite in quartz pyrite veins (PyII); (**c**)—Type III pyrite in quartz sulfide ores (PyIII); (**d**)—Type IV pyrite in pyrite calcite vein ores (PyIV); (**e**–**h**)—transparent mineral forms under polarized light microscopy; (**i**–**l**)—occurrence state of gold in different types of pyrite. Py—pyrite; Ser—sericite; Qtz—quartz; Cal—calcite; Pl—plagioclase; Ap—apatite; Ttn—sphene; Au—natural gold; Gn—galena.

alteration is a general term for sericitization, silicification, and pyritization [8]. Pyrite-sericite-quartz altered rock and potassium-altered (reddenized) rock are usually gold-enrichment zones. The study area has multiperiod and multistage hydrothermal processes, which show the evolution and metasomatism of hydrothermal functions, coupled with multistage tectonic activities. According to the metallogenic relationships among the orecontrolling fault structures, hydrothermal veins, and gold mineralization, previous researchers have divided the hydrothermal metallogenesis into four stages, which are sum-

**Table 1.** Characteristics of hydrothermal metallogenesis and pyrite in different stages. **Table 1.** Characteristics of hydrothermal metallogenesis and pyrite in different stages.

amount of pyrite


#### disseminated **3. Materials and Methods**
