Different geological backgrounds control the formation and spatial distribution of different types of igneous rock assemblages and gold deposits. Therefore, it is of great theoretical and practical significance to summarize the characteristics of various metallogenic geological backgrounds for deposit research and prospecting.

(I) Metallogenic geological background of magmatic arc of orogenic belt

The metallogenic volcanic geological background of China orogenic belt mainly includes ophiolite belt at plate junction, island arc and volcanic belt at active continental margin, intra-arc basin, back-arc basin and micro-landmass (intermediate block) on orogenic belt, and its characteristics are described as follows.

1. Ophiolite belt at plate junction

Ophiolite belt is the symbol of plate junction. A complete set of ophiolite suite should be composed of magnesium ultramafic rocks (harzburgite, harzburgite, etc.), accumulation complexes (olivine accumulation rocks, monoharzburgite and plagioclase accumulation rocks), basic sheet rock walls (diabase and gabbro porphyrite), pillow-shaped and massive lava (basalt) and radiolarian silicalite, carbonate rocks and fine clastic rock series. Ophiolites and their geological bodies are often cut, dismembered and mixed due to orogenic compression and shearing. Therefore, the outcrop of ophiolite suite in different areas is not complete, but the magnesian ultramafic rocks in it often exist, and retain traces of deformation (such as brittle-ductile shear, oblique thrust-translation of fault blocks, etc.) and low-grade metamorphism in different degrees (such as serpentine, slip petrochemical and magnesite mineralization, etc.).

Basalt associated with ophiolite can be formed in the mid-ocean ridge (oceanic ridge type) or in the transition from continental margin rift ocean basin to shallow ocean basin (quasi-oceanic ridge type), and the latter is more closely related to the regional distribution of gold deposits than the former, such as Jinsha River-Ailaoshan ophiolite belt in southwest China. Ridge basalt is mainly oceanic olivine tholeiite, with the content of SiO2(wB) of 42.42% ~ 49.46%, low K2O (.7% ~ .71%), and the value of k (k is w (K2O) × 1/[w (Al2O3)+w (TiO2) × 1 ]×. TiO2 _ 2 is moderately high (.64% ~ 3.2%), the world ridge type is 1% ~ 1.5%, the MgO content is relatively high and varies widely (6.6% ~ 13.47%), the total amount of rare earth elements varies from (42.28 ~ 186.77) × 1-6, and the ratio of light and heavy rare earth elements is .69. Besides olivine tholeiite, there are also Shi Ying tholeiite, basaltic andesite and rhyolite in quasi-oceanic ridge basalt. Basalt contains 44.75% ~ 5.25% of SiO _ 2 (WB), low Al2O3 (13.67% ~ 15.3%), high K2O (.23% ~ 1.39%), 12% ~ 2% of K, and 1.57% ~ 3.25% of TiO _ 2. .3%), rich in light rare earth elements, and the ratio of w(La)/w(Sm) is greater than 1.5. Therefore, the basalt characteristics associated with ophiolite are a good sign to distinguish the tectonic environment.

volcanic sedimentary rock series associated with ophiolite are usually basic tuff, tuff, siliceous rock, carbonaceous and argillaceous siltstone, which are part of host rock series. Among them, the content of SiO2 in tuff is 4.18% ~ 68.66% (average 54.42%), Al2O3 is 12.73% ~ 16.6% (average 14.4%), Na2O is .24% ~ 6.28% (average 3.26%) and K2O is .38% ~ .94. The background value of gold in metamorphic volcanic rocks is high.

In some ophiolite distribution areas, older basement metamorphic rock series are also exposed. The main lithology is amphibolite, granitic migmatite, mica schist and marble, and the background value of gold is (.71 ~ 1.7) × 1-9.

The gold deposits distributed in magnesite, basalt and volcanic-sedimentary rocks in the ophiolite assemblage in the combination zone are mainly epithermal gold deposits, and the ore bodies are often controlled by the mylonite system in the shear zone in space, which is related to the later hydrothermal activity of magmatic rocks. This kind of gold deposit occurs in gold-bearing quartz veins or altered rocks, and sometimes occurs with mercury ore and antimony ore. The gold deposits in western Junggar and Ailaoshan areas of China are produced in this kind of geological background.

2. magmatic arc of island arc and active continental margin

due to subduction and collision of plates, strong volcanism occurred, forming island arc and active continental margin volcanic arc. In the early stage of volcanism, basaltic, andesite volcanic rocks and a small amount of basalt were mainly formed, belonging to basaltic-andesite volcanic rock combination; In the later stage, basaltic-andesite-dacite-rhyolite volcanic rock assemblage was formed. Among them, pyroclastic rocks can reach more than 1/3 of the total thickness of the stratum, andesite volcanic rocks are mainly lithic tuff, and dacite and rhyolite volcanic rocks are mainly ignimbrite and tuff, forming a set of corresponding acidic and intermediate acidic intrusive rocks. The volcanic arc is close to the mainland, and the number of acidic volcanic rocks increases obviously.

island arc tholeiite contains 46% ~ 51% SiO _ 2 (WB), and the ratio of w(FeOT)/w(MgO) is low (< 1), Cr is 399×1-6, Ni is 16×1-6, which is close to the original magma composition, and TiO2 _ 2 is low (.85%). Rb, Ba, K and Sr are relatively high, while Zr, Hf, Nb and Ta are relatively low, which may be related to the metasomatism effect of subduction plate dehydration on wedge mantle.

the content of SiO _ 2 (WB) in island arc calc-alkaline andesite volcanic rocks is 52.8% ~ 6.8% (58.1% ~ 69.8% for dacite volcanic rocks), Al2O3 is higher (14.18% ~ 18.3%), and TiO2 _ 2 and w(FeOT)/w(MgO) are lower. The ratio of w(La)/w(Yb) is 7.46 ~ 13.16, which is relatively rich in Rb, Ba, K and Sr, but deficient in Zr, Hf, Th, U, Nb and Ta.

pyroclastic sedimentary rocks and carbonate rocks are also generated with island arc volcanic rocks, and the whole volcanic rock series is often alternating between land and sea. Generally, the early stage is dominated by marine facies, and in the late stage, there can be different amounts of continental sediments, and pillow structures can be found in marine basalt and basaltic andesite. In the continental and near-land coastal areas, the number of intermediate-acid ignimbrite increases, and the coastal tuff develops rhythmic bedding and oblique bedding, with common volcanic ash globule and lenticular tuff. The limestone interlayer in pyroclastic rocks and pyroclastic sedimentary rocks is mainly clastic limestone, which contains different amounts of cuttings, crystallites and bioclastic.

In the volcanic belt of island arc, epithermal gold deposits are mainly formed, which occur in the form of Shi Ying vein, hydrothermal breccia and hot spring deposits. Gold deposits and ore fields can be controlled by the fault system of volcanic dome, caldera or remote crater at the same time. Typical low-sulfur epithermal gold deposits, such as Axi gold deposit in western Tianshan, Xinjiang; High sulfur epithermal gold deposits, such as Jinguashi gold deposit in Taiwan Province.

3. Intra-arc graben basin

During the development of volcanic island arc, the intra-arc extensional basin (graben) was formed because the stress field changed from compression to tension in stages, which was located between the inner and outer arcs of volcanic arc in space. The early volcanic rock assemblage in the intra-arc basin is similar to that in the island arc area, mainly composed of calc-alkaline andesite, dacite and rhyolitic volcanic rocks, pyroclastic rocks, pyroclastic sedimentary rocks and normal sedimentary rocks. However, with the increasing tensile strength, basic magma erupts and basaltic volcanic rocks are formed. Under the condition that intermediate rock is not developed, a "bimodal" volcanic rock assemblage can appear.

The basalt in bimodal assemblage is obviously different from the island arc calc-alkaline basalt, showing the geochemical characteristics of transition from island arc basalt to oceanic basalt. Its SiO _ 2 (WB) content is 47% ~ 52.8%, K2O is low (.23% ~ .6%), w(FeOT)/w(MgO) is high, and FeOT content increases with the increase of SiO _ 2 content. TiO2 _ 2 is medium (1.1% ~ 1.7%), and light rare earth elements are weakly enriched, with the ratio of w(La)/w(Yb) of 4 ~ 5 and Yb of (2.3 ~ 3.) × 1-6, which is relatively richer in Ti, Zr, Hf, Ta and Nb than island arc basalt, but poorer in Sr, Ba and Rb. Taking dacite and rhyolite volcanic rocks in the peak combination, the SiO2 _ 2 is relatively concentrated between 7% and 76%, the Na2O+K2O content is 6% to 7.5% (K2O is similar to Na2O), and the light rare earth elements are enriched. The ratio of w(La)/w(Yb) is 9 ~ 12. There are often a certain number of andesite or Shi Ying andesite.

There are a certain number of carbonate rocks and fine clastic turbidite sedimentary rocks associated with volcanic rocks, and some areas (such as southwest China) have undergone slight metamorphism, forming slate, phyllite and marble. The background value of gold in the volcanic rock series in the whole inter-arc basin is (.95 ~ 7.9) × 1-9, with an average of 3.14×1-9, in which the horizons with volcanic exhalation-deposition or organic matter are more favorable for further prospecting.

under this background, large massive sulfide deposits are often formed, such as the Gacun lead, zinc, silver ore (associated gold) and Japanese black ore in the graben basin in the Yidun island arc in the northern section of the Three Rivers. Although the gold grade is not high, the reserves are large, so both base metals and precious metals have great economic value.

4. Back-arc basin and continental margin basin

In the late stage of plate subduction, back-arc spreading basin or continental margin ocean basin can be formed. Among them, it is characterized by the development of volcanic-sedimentary rock series. Volcanic rocks are mainly basalt, and sometimes acidic volcanic rocks appear in a "bimodal" combination. Sedimentary rocks are mainly carbonate rocks and turbidites, often containing different amounts of organic matter.

in the well-developed back-arc basin, the basalt is close to the characteristics of oceanic tholeiite or ridge basalt. The content of SiO _ 2 (WB) is 47% ~ 49%, MgO is 6.5% ~ 9.5%, and it is alkali-poor, and (Na2O+K2O) is 2.7 ~ 3.2. Often only at the initial stage of extension, alkaline basalt with high alkalinity appears, or bimodal volcanic rocks can appear in the extension basin developed on the basis of submarine continental crust.

The bimodal volcanic rocks in the back-arc basin are basaltic-rhyolite assemblage. Among them, the content of SiO _ 2 (WB) in basalt is 48% ~ 49%, TiO2 _ 2 is low (1.1% ~ 1.7%), and P2O5, Na2O+K2O and Al2O3 are high, which are .5% ~ .8%, 7.% ~ 7.2% and 17.7% ~ 18.1% respectively. The content of SiO2 _ 2 in acidic volcanic rocks can reach 76%, Na2O+K2O is 7.1% (K2O is 3.8% ~ 4.6%), light rare earth elements are enriched, and the ratio of w(La)/w(Yb) is 37.8 ~ 41.5, and K, Rb, Sr and Ba are enriched, so it belongs to rhyolite with high alkalinity.

The black fine clastic sedimentary rocks associated with the back-arc volcanic rocks are mainly siliceous rock series and clastic rock series. The former is composed of siliceous rocks, slate and carbonate rocks, while the latter is composed of feldspar-Shi Ying complex sandstone, lithic complex sandstone and a small amount of black slate, phyllite and sericite mother rocks with different grain sizes. Siliceous rocks are mostly blocky, breccia, porous, banded, layered and spherical (algal) in shape. The content of SiO _ 2 (WB) is 81.42% ~ 98.13%, generally 94.21% (due to pyrite, barite, argillaceous and carbonaceous impurities), and the background value of gold is (2 ~ 3) × Slates are mainly carbonaceous slate, siliceous slate and a small amount of silty slate, with an organic carbon content of 2% ~ 5% and a background value of gold of (2 ~ 3) × 1-9, in which the organic carbon content of carbonaceous slate is as high as 22% and the background value of gold is as high as 5 × 1-9. Carbonate rocks are mostly lenticular, mostly dolomite, calcareous dolomite and argillaceous dolomite, and their chemical composition is W (Cao) >: W(MgO), 3% ~ 4% CO2 and 1% ~ 2% MgO.

from the above, it can be seen that this set of black sedimentary rock series is rich in organic matter, which determines that a certain amount of gold was absorbed by submarine microorganisms during their formation, and volcanic activity heated seawater to form a submarine hydrothermal system, which continuously extracted gold from volcanic rocks and sedimentary rock series in the convection cycle, resulting in favorable conditions for the formation of volcanic-sedimentary gold deposits and some fine-grained disseminated gold deposits. Some gold deposits in the western margin of the Yangtze platform occur in the environment of back arc or continental margin basin.

5. Intermediate block (micro-block)

In the large composite orogenic belt, there are often some micro-blocks. During the continental activation period, with the activity of large strike-slip fault system, a series of intermediate-acid intrusive rocks with shallow or subvolcanic facies developed on the edge of the micro-block, and the rock masses are all small rocks, branches and dykes.

the intrusive rocks on the micro-block are composed of calc-alkaline diorite-granodiorite and calc-alkaline adamellite-Shi Ying adamellite-adamellite intrusive rocks, and the main lithology is granodiorite porphyry and adamellite porphyry, with the content of SiO2(wB) being 66.1% ~ 71.1%, Al2O3 being 14.86% ~ 17.6%. The total amount of rare earth elements is (139.67 ~ 28.93) × 1-6, the initial ratio of 87Sr/86Sr is .75 ~ .77, and the ratios of 26 Pb/24Pb, 27Pb/24Pb and 28Pb/24Pb are 18.6 ~ 18.89 and 15.488 respectively.

The surrounding rocks of the rock mass are volcanic-sedimentary rock series of orogenic belt. Besides intermediate-basic and intermediate-acid volcanic rocks, there are pyroclastic sedimentary rocks, turbidites and carbonate rocks, and some of them also contain some organic matter.

in this background, porphyry copper-molybdenum ore and copper (gold) ore are often formed near porphyry bodies, while epithermal gold deposits are often formed in sedimentary rock series far away from porphyry bodies, and skarn copper (molybdenum) deposits can be formed when carbonate rocks meet.

(II) Metallogenic geological background of volcanic rocks and intrusive rocks in continental activation zone

Tectonic uplift, depression, large strike-slip fault zone and extensional rift zone are distributed in continental activation zone, and the geological background of magmatic activity related to gold mineralization mainly includes