The relationship between Moho surface morphology and the distribution of rock gold deposits

The relationship between the Moho surface morphology and the distribution of rock gold deposits, as well as the relationship between space, time and genetic mechanism is relatively complex.

1. Crustal thickness and distribution of rock gold deposits

Crusts of different thicknesses accumulate different mineralizing elements and develop different metal deposits. It is generally believed that the thinner crust is enriched in Au and Cu and develops gold and copper deposits, the medium-thick crust is enriched in Pb and Zn and develops lead-zinc deposits, and the thicker crust is enriched in W and Sn and develops tungsten-tin deposits.

The crustal thickness in eastern my country is thinner than that in western China. From the current point of view, the gold content of the eastern crust is much stronger than that of the western crust. This is reflected in the fact that the eastern crust is distributed in Jiamusi, Jiaoliao, Baotou-Hohhot, Shanxi and Hebei, the middle and lower reaches of the Yangtze River, western Henan, Qinling, Ganchuan, and Yunnan Guizhou, Jiuling-Xuefengshan, western Guangdong, Fujian and Zhejiang are important gold mineralization areas; while gold mineralization in the western crust is poor, only better in northern Xinjiang, including the Altai gold mineralization area and the West Junggar gold mineralization area. The Qinghai-Tibet Plateau in the western crust has a crust thickness of 60-70km, and the distribution of gold deposits in this area is even rarer. It seems that the distribution of gold deposits in my country is consistent with the general conclusion of crustal thickness and element enrichment and mineralization mentioned above.

However, if we look at the relationship between the thickness of the earth's crust and the enrichment and mineralization of tungsten and tin elements, it is obvious that it is contrary to the above general conclusion. This is because the tungsten-tin mineralization in the Qinghai-Tibet region, where the crust is thick, is much inferior to the tungsten-tin mineralization in the Nanling community in southeastern China, where the crust is thin (Huang Ruihua, 1993).

What is the reason? We believe that it is related to the tectonic evolution and maturity of the earth's crust. The above general conclusions about crustal thickness and element enrichment mineralization only apply to the geosyncline area, because the crust is thinner in the first half of the development of the geosyncline area and the gold mineralization is stronger. However, in the second half of the development of the fold zone, the crust thickens and gold mineralization Mine weakens. After the development of the geosynclinal stage, eastern my country experienced the development of the platform stage, and was activated into the depression stage at the beginning of the Mesozoic Era, and reached the depression residual movement period, so the crust is highly mature. Due to the strong tectonic magma activation of the depression type, abundant gold and tungsten-tin elements were brought about, as well as the activation, transfer and re-enrichment of the originally existing Au, W and Sn elements. In addition, the residual movement of the depression The early rupture activities caused the continental crust to stretch and expand and become thinner. Therefore, it shows another scenario of crustal thickness and element-enriched mineralization characteristics, which is unique in that it is enriched in Au, W and Sn; and In the western crustal area, after experiencing the geosyncline stage, it briefly entered or even lost the platform stage, and then entered the development of the initial tectonic stage of the depression stage. The maturity of the crust was low, and due to the lack of tectonic magma activation in the intense stage of the depression stage, Therefore, the enrichment and mineralization of Au, W and Sn are poor.

It can be seen that due to the tectonic evolution and maturity of the earth's crust, the eastern crust of my country, which is thinner, is richer in both Au, W and Sn than the thicker western crust. , which is particularly reflected in the mineralization of Au, W and Sn in the Nanling area.

2. The relationship between crustal types and rock gold mineral distribution

The earth’s crust can be roughly divided into three types according to its composition and properties: silicate-magnesia crust, silicate-aluminum crust and others. transitional crust. It is generally believed that gold deposits are mainly related to the silico-magnesian crust, and gold deposits are also produced in other types of crust.

The classification of my country's crustal composition types, if considered according to the characteristics of the mineralizing elements of the crust, the western crust can be classified as silico-magnesian crust because of the development of basic ultrabasic rock belts and marine basic. The volcanic rocks are widely distributed, and there are many Cr, Ni, Co, Pt, Fe and Ti and pyrite-type copper polymetallic minerals. However, as mentioned above, the mineralization and distribution of gold deposits are inferior to those in the eastern crust; the eastern crust can be classified as a siliceous-aluminum crust because of the development of acid intrusive rocks and acid volcanic rocks, and the development of rare earth, radioactive, Be, Li, and Rb , Cs, W, Sn, Bi, Mo, Cu, Pb, Zn, Ag, Hg, As, Sb and F (Huang Ruihua, 1978), but the gold mineralization is better than that in the western crust (see above).

It seems that the relationship between the distribution of rock gold deposits in my country and the types of crustal components is also different from the so-called "general rules", because it is precisely the gold mineralization of the "siliceous-aluminous crust" that strongly Gold mineralization in the "silico-magnetic crust".

What is the reason? We believe it is also related to the tectonic evolution of the crust and the maturity of the crust. We believe that the general relationship between the types of crustal components and the formation of gold deposits mentioned above is only applicable to the geosynclinal area, because the development history of the geosyncline area is a geotectonic process that evolved from oceanic crust to continental crust, that is, from silicon to magnesium. The process of transformation and evolution of geomorphic crust to siliceous-aluminous crust. In the early development stage of the geosyncline area, the earth's crust has strong silico-magnesia and strong gold mineralization, resulting in a series of gold deposits and associated gold deposits related to basic ultrabasic rocks, intermediate basic and intermediate acid volcanic rocks. In the later stages of development, During the fold belt period of this stage, the siliceous-aluminous nature of the crust was significant, but the gold mineralization was poor, so the relationship between the silico-aluminous crust and the distribution of gold deposits was poor. In my country, the eastern continental crust is basically a Cathaysian-type depression area with high crustal maturity, multiple crustal differentiations, a good degree of differentiation, strong tectonic magma activation, and strong gold mineralization. The western continental crust is mainly a Central Asia-type depression area, which has experienced a brief or even missing platform stage, and has not suffered or has just suffered from an intense period of depression tectonic magma activation. Therefore, the maturity of the crust is low, and the geochemical composition of the crust is low. The heterogeneity is poor, the activation and re-enrichment of gold are weak, and the investment in geological work is small, so the gold deposits show the characteristics of less distribution. This forms a new pattern of the relationship between crustal composition types and gold deposit distribution in China today. It can be seen that when discussing the relationship between crustal composition types and gold deposit distribution, the tectonic evolution history and crustal maturity of the local crustal area should also be considered.

It is generally believed that there is a good correspondence between the thickness of the crust and the type of crustal composition, that is, the thinner the crust, the stronger the silicon-magnesia, and the thicker the crust, the stronger the silicon-aluminum. However, , but this is not in line with the actual characteristics of my country's crust. This is due to the multi-stage evolution and development of crustal structure and the mutual evolution and transformation between oceanic crust and continental crust, which greatly complicates this problem.

3. Moho Slope Zone and Distribution of Rock Gold Minerals

As mentioned above, there is a north-northeast-trending mantle steep slope zone in eastern my country, with rock gold deposits in it. It is very developed and has various types of gold deposits. The western mantle steep slope zone is also a favorable gold mineralization prospect area, especially in its eastern ring section where many gold deposits have been detected. We know that the mantle steep slope zone is a weak zone in the crust and a reflection of the deep structure. It has a large gradient, strong changes in various physical and chemical conditions, and is a highly permeable zone, which is conducive to the occurrence of tectonic-magmatic activities and rock gold deposits. form.

The secondary mantle slope belt with relatively developed gold deposits is found in the northeastern and eastern mantle slope belt of the Junggar mantle ridge, where there is the famous Altai gold mineralization area; it is also found in Qaidam and the northern mantle trough of Tibet In the mantle slope zone between them, there are the Hoh Xil gold mineralization prospect area and the Bayan Har gold mineralization area.

The Qinling and Dabashan areas in the middle of the central mantle platform area are areas where two mantle steep slopes are connected to the east and west. If you cut a section along the east-west direction of this section, it can be seen that this section is actually a long-span section. Large mantle slope zone. In this mantle slope area, gold deposits are extremely developed, and the Xiao Qinling gold mining area and Qinling gold mineralization area are located in this area.

By the way, I would like to point out that the relationship between my country's gold deposits related to alkaline rocks and the mantle slope belt is relatively close. Because this type of gold deposit is found in northern Hebei, southern Shanxi, and western Henan in the eastern mantle slope belt, the Altun Mountains in the western mantle slope belt, the mid-Yunnan-Dayao area, the Fenghuoshan area in the secondary Hoh Xil mantle slope belt, and the Himalayas Manpo area is Mamupu, Wusula and other places. The internal reason is obvious, that is, when the two mantle slope belts in the east and west are in the depression stage, they are highly active and the differential movement is very strong, which leads to the rupture and dislocation of the mantle rock shell, making the mantle material creep active and tectonic. The development of geochemical processes triggers the intrusion of mantle-derived materials. Alkaline magma generally comes from mantle sources. Therefore, gold mineralization related to alkaline rocks is closely related to the mantle slope zone.

4. Distribution of Moho uplift, depression and rock gold deposits

As can be seen from Figure 9-1, the more typical Moho uplift areas are the Sichuan Basin mantle uplift area and Tarim mantle ridge.

The Sichuan Basin is a mantle uplift area that is subducting in all directions. The depth of the Moho at the top is nearly 39km. Rock gold deposits are not very developed in this uplift area, and some rock gold deposits are only found in the west. However, the gold mines in the area around the uplift are very developed. To the north is the Shaanxi-Gansu-Sichuan Golden Triangle Area and the Xiaoqinling Gold Mineral Area, to the south is the Yunnan-Guizhou-Guangxi Golden Triangle Area, to the west is the Western Sichuan Gold Mineralization Area, and to the east is the Hunan-Guizhou Gold Mineralization Belt. The Tarim mantle ridge is composed of three secondary mantle ridges, and their axes are connected in an arc shape that is slightly convex to the south. At present, the distribution of rock gold deposits is small.

As can be seen from Figure 9-1, the more typical mantle depressions are found in the trough-shaped depressions in the Tianshan Mountains, Chaduo Gangri area in northern Tibet, Gangdise, Dunhua and Nanling communities.

The gold mineralization in the northern mantle trough of Chaduo Gangri and the southern mantle trough of Gangdise is poor. At present, rock gold deposits are not developed. However, the Tianshan Man'ao gold mineralization is good, such as the Axi gold deposit (see above). The Dunhua mantle depression gold deposit has good mineralization and is distributed in the Nancha-Binghugou gold mineralization belt and the Yalujiang gold mineralization belt. The trough-arc mantle depression in Nanling Community mainly contains some gold deposits in southern Hunan and southern Jiangxi.

From the above, it can be seen that the distribution of control rock gold deposits in the mantle slope belt is the most obvious, followed by the mantle depression. As for the mantle rise, attention should be paid to the distribution of gold deposits around it. Therefore, in the future, gold prospecting work should be strengthened in the eastern mantle steep slope zone and the eastern ring section of the western mantle steep slope zone, the surrounding areas of the Sichuan mantle rise, Jiaoliao, Nanling Community, Altai, Tianshan and other areas.

It can be seen from Figure 9-1 that the Kunlun-Qinling-Dabie Mountains structural belt seems to be an intermittent mantle slope belt, or an oblique mantle slope belt. This belt is an important deep structural belt for gold mineralization, especially the intersection area with the two mantle steep slope belts in the east and west. It is also a favorable target area for finding super-large gold deposits.