Basic Control of Metal Mineralization

      Metallogenic control is the foundation of metallogenic regularity analysis, and in many cases, the two express similar or even identical content. As a special geological body with economic significance, the formation of metallic deposits is inseparable from basic environments and conditions, commonly referred to as metallogenic geological environments or conditions. These fundamentally control the region, type, combination, and characteristics of metallic mineralization, determining the basic laws of mineralization in a given area and the mineralization enrichment patterns of the mining area. This is the first issue we should clarify when prospecting for minerals in a particular region and conducting metallogenic analysis. The basic framework is as follows: the regional tectonic evolution of a region during different geological tectonic periods forms different but regular metallogenic environments, called tectonic environments (Ge Liangsheng, 2008). In different metallogenic environments, different types of major geological events occur, including important tectonic-magmatic activities, sedimentation, metamorphism, etc. The products and effects of these events are manifested through the geological characteristics of different elements within the environment. These elements, individually or mutually, couple in a certain spatiotemporal domain, forming different types of deposits and their corresponding geological characteristics, ultimately achieving control over mineralization. Traditionally, discussions on mineralization control have focused on analyzing various aspects, including stratigraphy (lithology), tectonics, magmatic activity, metamorphism, geophysics, and geochemistry.

1.1 Multi-stage temporal control and spatial hierarchical control

      The control of geological tectonic environment over mineralization is multi-phased in time and hierarchical in space, determined by the regional tectonic evolution over a long geological history. It is crucial not to underestimate the significance of seemingly macroscopic and overly qualitative discussions of regional tectonic evolution in solving specific mineral exploration problems. In-depth analysis of the vast amounts of information obtained from regional geological surveys, including stratigraphic and lithological assemblages, tectonic phases, magma formation and evolution, and metamorphic deformation, ultimately aims to reconstruct the tectonic evolution history and processes of the corresponding region. The geological tectonic environment is formed precisely within this history and process. In the history of geology, important theories and viewpoints proposed in the early stages, such as the platform theory, the terrane theory, and the geomechanics theory, can to a certain extent provide scientific descriptions and reasonable explanations for complex geological phenomena in a certain region. The most popular theories at present, such as plate tectonics, orogenic belts, and supercontinental cycles, may explain more geological phenomena and their mechanisms. They have all effectively constructed the geotectonic evolution processes in the corresponding regions, such as platform-trough transformation, stress transformation, basin-mountain opening and closing, uplift and subsidence, and related regional erosion and sedimentation, magmatic activity, tectonic processes, metamorphism, and biological succession.

      The multi-stage nature of geological tectonic environments refers to the fact that the same region can form corresponding geological tectonic environments at different geological historical stages due to tectonic evolution. For example, plate tectonics theory explains the processes of plate rifting, subduction, collision, suture, extension, and intraplate (continental) formation in a region through geological records, reconstructing its tectonic evolution history and determining the geological tectonic environment at different stages, such as whether it is an island arc, trench, or back-arc basin during the subduction (convergence) stage. The supercontinental cycle theory effectively compensates for the shortcomings of plate tectonics in explaining Precambrian tectonic evolution, and also explains that in the history of geological tectonic evolution, the same region may have multiple spiral-evolving cycles of plate opening and closing processes. It should also be recognized that even within a single phase of a plate opening and closing process, there will be pulsations of varying speeds and intensities. For example, the collisional orogenic stage can be further divided into pre-collision, primary collision, late collision, and post-collision stages. Modern mineralization research shows that many large and super-large deposits formed during the transition from one tectonic environment to another, i.e., at the edge of time. For example, the transition from collision to post-collision extension is primarily due to significant changes in the regional tectonic dynamics during such periods, shifting from compression to extension, or conversely, favoring the initiation of major geological events leading to mineralization. This complex cyclical tectonic evolution results in the multi-stage temporal control of the geological tectonic environment over regional mineralization.

      Spatial hierarchy refers to the relatively orderly spatial configuration of geological tectonic environments formed in a region during the same stage of tectonic evolution, exhibiting different levels such as large-scale, regional, zone, and local. Mineralization not only occurs within an environment but may also occur at the edges of the environment (including different geological bodies within the environment), i.e., spatial edges. For example, during the subduction stage of plate tectonic evolution, plate subduction creates spatially ordered geological tectonic environments on a large scale, such as subducting plates, trenches, forearms, island arcs, interarcs, back-arcs, and continental margins. Simultaneously, within the island arc tectonic environment, variations in the state, intensity, and changes of tectonic-magmatic activity may lead to the formation of zones such as magmatic activity and supraarctic rift deposition. Furthermore, within magmatic zones, internal contact zones, inner contact zones, and outer contact zones can be distinguished, or local environments such as deep-seated, intermediate-seated, shallow-seated, and ultra-shallow-seated environments can be identified. Similarly, the structures that serve as boundaries between different geological structural environments, as well as the structures within them, also exhibit hierarchical characteristics such as deep and large faults, regional structures, general structures, and faults. The same structure also exhibits complex temporal and spatial variations such as shear, tension, compression, and torsion. These all form macro-environments, micro-environments, and micro-environments with different hierarchical characteristics that are closely related to mineralization.

1.2 Single-factor control and multi-factor coupling control

      Tectonic evolution and its associated geological tectonic environment are realized through various major geological tectonic events, collectively referred to as geological events, including erosion or deposition, magmatic intrusion or eruption, tectonic movement, and deformation and metamorphism. The scale, type, intensity, frequency, stress, range of influence, and duration of these geological events determine the diversity of structural elements within the geological tectonic environment and their interrelationships. Even within the same type of geological tectonic environment, its internal structure and state vary depending on time, location, and specific events. Numerous studies have shown that major and characteristic geological events occurring during regional tectonic evolution effectively determine the dominant geological tectonic environment and important mineralization processes in the corresponding region.

      Single-factor control manifests itself in the individual control of various factors on mineralization within different metallogenic geological environments, including sedimentary lithological assemblage and state, tectonic development and style, magmatic activity type and mode, intensity and type of deformation and metamorphism, deep crust-mantle structure and interaction, geophysical state, and geochemical structure. Multi-factor coupling control refers to the spatiotemporal configuration of various structural elements within a geological tectonic environment and their coupled control on mineralization. These are the comprehensive response and coupling effects of significant regional geological tectonic events in a specific area, determining the type and development of the metallogenic system, the intensity and potential of mineralization, and the assemblage and occurrence state of mineralized products within that environment. These two factors are complementary and closely related; they are fundamental issues that need to be thoroughly studied in geological prospecting and exploration in a region, and are the reason why it is essential to clearly describe the regional geological characteristics and mining area geological characteristics, as well as other metallogenic background information, in mineral deposit prospecting and exploration. Not all structural elements in a geological tectonic environment are necessarily complete. For example, the structural elements of a simple sedimentary basin environment are mainly tectonic-sedimentary formations, with little or no magmatic activity. When multiple elements exist, their importance is not equal; the focus should be on understanding their main structural elements and characteristics.

      From the perspective of mineral exploration, studying the internal structural elements and their coupling relationships within the geological tectonic environment can be approached from two angles. First, the narrow definition of the geological tectonic environment, i.e., the metallogenic geological background, includes the strata, structures, spatiotemporal distribution and state, type and occurrence of magmatic rocks in a specific region, as well as the characteristics of various deformation and metamorphic processes. This essentially provides the spatial basis for mineralization. Second, the geochemical environment, including the geochemical state, distribution, and variation patterns of hydrothermal fluids, main ore-forming elements (minerals) and associated elements, media, and other related elements. This essentially provides the material basis for mineralization. In-depth analysis and coupling of these two angles primarily aim to identify the sources, transport channels, precipitation and enrichment spaces, and factors controlling the change and preservation of ore-forming materials (thermal, water, and minerals) that play a crucial role in mineralization; and to clarify the specific control and constraints of strata, lithology, and magmatic rocks on mineralization. Simply put, it involves the three sources of "thermal water and minerals," the three fields of "accumulation, transport, and storage," the three controls of "lithological layers," the three dimensions of "spatiotemporal matter," and the three states of "stability, change, and preservation."

      Unlike previous geological prospecting work, which mainly focused on the surface or shallow crust (around 500 meters deep), modern mineral resource exploration is constantly venturing into deeper areas, reaching depths of 5000 meters or even greater. This has led to increased attention to the deep geological structural environment. In fact, the geological structural environment referred to here includes the deep environment itself. Research shows that a thorough understanding of the deep geological structural environment is extremely important for correctly analyzing mineralization issues and clarifying regional metallogenic controls—a point often overlooked by geological prospectors. The significance of the deep geological structural environment for metallogenic control lies in two aspects: First, exploration practice and scientific deep drilling have proven that even in the deep crust, there are still ore bodies of enormous economic value. This is evident not only in energy resources such as oil and gas but also in metallic minerals. Currently, it is customary to define the area shallower than 500 meters as the first exploration space, and shallower than 1000 meters as the second exploration space. Further depths may reveal third and fourth exploration spaces. Therefore, research on the structure and coupling of elements in the deep geological tectonic environment is essential for deep mineral exploration, especially in mature exploration areas with a high level of activity. On the other hand, many explorable mineral deposits and ore bodies, particularly endogenous metallic deposits, are products of deep geological processes. These deep geological processes may provide important driving forces, energy, materials, and pathways. For example, deformation and metamorphism occurring in the middle and lower crust, deep fault structures cutting into the crust and even the mantle, magmatic activity originating from crust-mantle interaction zones, the mantle, or even deeper layers, and even certain metals from the Earth's core may be involved. In-depth research into the deep tectonic environment and its metallogenic dynamics helps to understand the genesis, metallogenic mechanisms, and regional metallogenic controls and patterns at a deeper level.

1.3 Overall Regional Control and Environment-Specific Control

      Modern mineral exploration emphasizes comprehensive, systematic, and holistic exploration. The control of geological tectonic environments over mineralization is holistic. The same environment does not necessarily form only one type, one class, or one deposit; conversely, a single deposit is not necessarily the product of a single geological process. The theory of metallogenic systems (Zhai Yusheng, 1999) essentially describes the basic connotation and mechanism of the overall control of mineralization by geological tectonic environments; the theory of metallogenic series (Chen Yuchuan, 1994; Zhai Yusheng, 1996) fully expresses the internal connections and combination patterns of the products of metallogenic systems. Together, they constitute the theoretical foundation for the holistic analysis of mineralization control by geological tectonic environments and for comprehensive, systematic, and holistic exploration. Due to the multi-phase nature of geological tectonic environments, which themselves change with regional tectonic evolution and are not always in the same environment; even at a certain geological stage, the geological (metallogenic) processes within the environment are diverse, resulting in complex spatiotemporal composites, mutual influences, and superimposed transformations of the metallogenic systems developed within them, ultimately leading to similar characteristics in the resulting mineral products. The so-called environment-specific control refers to the metallogenic specificity of the geological environment in which metallogenic geology occurs. This means that within a specific geological tectonic environment, a specific mineral type or combination thereof, a specific deposit type or combination thereof, and characteristic geological features will be exclusively formed (Ge Liangsheng et al., 1996; Ge Liangsheng, 2008). The deposit type is determined by the geological processes and spatial structure of elements occurring within the environment, while the mineral type is determined by the crustal and mantle geochemical structure of the environment. This issue will be discussed in more detail in the section on fundamental laws of metallogenesis below.

      Due to the multi-stage geological tectonic environment, the compounding, superposition, and alteration of multiple phases and types of mineralization within the same region ultimately form a complex metallogenic landscape that seems to lack specificity. How can we discern patterns and regularities within such a complex metallogenic landscape? This requires us, much like regional tectonic analysis, to skillfully extract key information based on extensive field geological surveys, measurements, observations, and comprehensive laboratory research. From the complex geotectonic evolution, we must clarify the spatiotemporal boundaries of the geological tectonic environment formed during important geological historical stages, identify the significant geological tectonic events that occurred, and determine the corresponding types of mineralization. We must skillfully apply classic metallogenic models and characteristic metallogenic indicators, and based on the principle of the metallogenic specificity of geological tectonic environments, peel back layers, match stages, and meticulously analyze to find the corresponding mineralized products from the complex assemblage of alteration, mineralization, and ore deposits. Building upon this foundation, we will further explore the comprehensive effects of mineralization and the superposition, transformation, and remodeling of mineralization products under multi-stage evolutionary conditions. This will allow us to establish a spatial structure of mineralization products—a metallogenic series map—based on a two-dimensional coordinate system of geological tectonic environment and geological history for the corresponding region. In this way, the overall mineralization control and specificity of the geological tectonic environment in the corresponding region will become readily apparent.