1. Magmatic series and rock assemblage of ore-bearing rock masses related to ancient continental (marginal) cracking.
(1) Petrochemical composition and rock assemblage in Longshoushan rock area
Jinchuan rock mass is the largest rock mass in Longshoushan rock area, with a length of 6500m and a width of 527m m. The rock mass in this rock area is mainly dike-like, vein-like and lenticular, with a few lentils. The rock assemblage of the larger rock mass is: dunite-lherzolite-olivine lherzolite; The smaller rocks are peridotite-pyroxenite or peridotite-olivine pyroxenite, peridotite-olivine pyroxenite-gabbro, peridotite, pyroxenite, gabbro and metadiabase. Among these rock types, peridotite, olivine pyroxenite and pyroxenite are absolutely dominant, while gabbro and diabase only appear in three rock bodies. In Figure 4-2- 1, most of the samples in the Longshoushan rock area are projected in the subalkaline area. In Figure 4-2-2, all subalkaline samples are projected in the tholeiite series area. Therefore, the mafic-ultramafic rocks in Longshoushan rock area should belong to tholeiite series. In the silicate total analysis of 187 ultramafic rocks, there are 180 samples m/? The ratio is between 6.5 and 2.0, and most of them are between 5.87 and 3.15, indicating that ultramafic rocks in this area belong to iron ultrabasic rocks. There are four kinds of m/? It is 1.3 1 ~ 1.99, belonging to iron-rich ultrabasic rocks. These four samples and another 1 m/? Sample =2. 1 is projected on the left side of alkalinity and alkalinity boundary in the top area of Ol' in Figure 4-2- 1, which belongs to alkaline series. This may be due to the addition of excessive iron and a small amount of alkali metals in the process of strong serpentine. M/？ = 8.70 ~ 10.84, and their mg # = 0.9 ~ 0.92. These three samples may be mantle peridotite, or they may all be dunite formed by the accumulation of early crystalline olivine.
Fig. 4-2- 1 Ol'-Ne'-Q' illustration of Jinchuan ultramafic rocks
Ol′= Ol+3/4 Hy； Q′= Q+2/5 Ab+ 1/4 Hy； Ne′= Ne+3/5 Ab
Figure 4-2-2 AFM Diagram of Jinchuan Ultramafic Rock
The mg # of most iron ultrabasic rocks is between 0.86 and 0.76, and that of a few samples is between 0.73 and 0.74. According to the study of Green( 1985), the primary magma Mg # in balance with mantle peridotite is 0.63 ~ 0.73. In view of this, all iron ultrabasic rocks in Longshoushan rock area contain different amounts of accumulation facies. Iron-rich ultrabasic rocks and 1 m/? The Mg # = 2. 1 of iron ultrabasic rocks is 0.55 ~ 0.67, of which 2 samples are 0.66 and 0.67, belonging to the primary magma range, and the other 3 samples belong to the medium evolution magma range. However, these data may be the result of alteration and cannot represent the petrochemical composition before alteration.
M/？ It is 1.86, 1.99, 2.6 1, and still belongs to iron-basic rocks. Their Mg # values are 0.66, 0.67 and 0.73, respectively, which are all within the scope of primary magma. Whether the silicate data of gabbro can accurately represent the petrochemical composition before alteration and the primary magma in Longshoushan rock area is a subject worthy of study.
(2) Petrochemical composition and rock assemblage of mafic-ultramafic rock belt in the western margin of Yangtze plate.
The Ni-Cu-Pt-bearing rock bodies in this area are mostly in the form of rock plants, rock basins, bedrock, rock walls and dikes. Generally speaking, these rock bodies occur in the S-N belt on the western edge of the Yangtze plate and appear in groups on each section. The aspect ratio of rock mass is less than 5: 1, and the maximum exposed area is 58 km2. The basin-shaped, vein-shaped and dike-shaped rock masses are characterized by acid and alkali, and the accumulation bedding is developed. The rock mass in the form of rock strand is a compound rock mass with acid inside and outside the matrix, and different rock types are the products of magma pulsation of different intrusions.
These rocks are composed of dunite, monoclinic (and lherzolite) peridotite, pyroxenite, gabbro, gabbro and gabbro. Rock mass, especially ore-bearing rock mass, is strongly altered. The common alteration types are serpentine, hypoamphibolite, chloritization, epidote, slump and carbonation. Eight rock assemblages can be divided: ① dunite-peridotite-gabbro-gabbro; ② peridotite-peridotite-peridotite-olivine gabbro type; ③ monoclinic gabbro type; ④ Monopyroxene-pyroxene-gabbro type; ⑤ monoclinic pyroxenite type; ⑥ Monocline pyroxene-gabbro type; ⑦ Subdiabase type; ⑧ Gabbro-diabase type. Among them, the rock mass with platinum as the main mineral often contains no or less mafic rocks, such as Jinbaoshan rock mass and Yangliuping rock mass. The rock mass with nickel ore as the main mineral often contains a certain amount of mafic rocks, such as Baimazhai rock mass and Lima River rock mass. This difference in mineralization specificity reflects the difference in alkalinity of ore-bearing magma. The magma alkalinity of the rock mass dominated by platinum ore is high, while that of the rock mass dominated by nickel ore is low.
Figure 4-2-3 Mafic-ultramafic rocks in the western margin of Yangtze Plate
Ol'-ne'-q' graph, ol'= ol+3/4hy, q'= q+2/5ab+ 1/4hy, ne'= ne+3/5ab.
Figure 4-2-4 AFM diagram of mafic-ultramafic rocks in the western margin of Yangtze plate
In Figure 4-2-3, only a few samples are projected in the alkaline region, and the vast majority of samples fall in the sub-alkaline region. In Figure 4-2-4, most subalkaline rocks are projected in tholeiite series area. It can be seen that the rocks containing nickel, copper and platinum in the western margin of Yangtze plate mainly belong to tholeiite series. A few alkaline series samples may be related to alteration metasomatism. In particular, strong slip fossilization will obviously change the chemical composition of rocks. There are 10 samples in Baimazhai rock mass, which belong to calc-alkaline series and may be related to assimilation and contamination. Zircon U-Pb dating shows that there are Proterozoic and early Paleozoic zircons in Baimazhai pluton. Because the intrusive rock belt containing nickel, copper and platinum minerals in the western margin of the Yangtze plate was formed from Carboniferous to Early Permian, these Proterozoic and Early Paleozoic zircons can only come from surrounding rocks. M/ ultramafic rocks? Most of the ratios are between 2.32 and 4.95, 1 sample is 5.72, and other 1 samples are 2.65,438+08, all of which belong to iron ultrabasic rocks. Mafic rocks? The ratio is mostly between 0.56 and 1.86; The other two are 2.06 and 2.27, which are slightly higher. Most of these mafic rocks belong to mafic basic rocks. The range of mg # of most ultramafic rocks in this area is between 0.70 and 0.83. The mg # of the other two samples is between 0.85 and 0.86, and that of the four samples is between 0.60 and 0.69. It can be seen that most ultramafic rocks contain a certain number of accumulation phases. The samples with Mg # between 0.70 and 0.83 are mainly pyroxenite. Whether these samples can approximate the composition of primary magma is a noteworthy problem. A few obviously low Mg # samples are strongly altered rocks, mainly serpentine and chloritization. The decrease of Mg # should be related to the introduction of iron in the alteration process. Mg # of mafic rocks ranges from 0.37 to 0.68, with a large span. Individual ore-bearing gabbro whose Mg # exceeds 0.70 should not be regarded as the original Mg # value of gabbro. The Mg # of most gabbro is lower than 0.63, which belongs to medium evolution magma. The Mg # of a few gabbro is between 0.66 and 0.68. Whether this part of gabbro can represent the primary magma in this area deserves further study.
(3) Petrochemical composition and rock assemblage of Xingdi Ⅱ rock mass.
Xingdi Ⅱ rock mass has an asymmetric wedge shape, with a length of 5 km from east to west and a width of 2 ~ 3 km from north to south, with an exposed area of about 16 km2. The cross section may be funnel-shaped. The rock mass consists of peridotite, pyroxenite and gabbro.
M/？ Most of them are between 3.66 and 5.71,belonging to iron ultrabasic rocks. M/？ They are 6.8, 7.39 and 7.44 respectively, belonging to magnesia ultrabasic rocks. The olivine content in these three peridotite samples has exceeded 75%, and pyroxene belongs to orthopyroxene. The M/ 1.68 ~ 3.6 1 of most mafic rocks are obviously rich in magnesium as a whole. M/？ Another 1 gabbro sample? It reached 7.39, indicating that MgO content was abnormally enriched. In Figure 4-2-5, except for 1 sample, all other samples are projected in the subalkaline region. In Figure 4-2-6, subalkaline samples are all projected in tholeiite series area. Based on this, it can be judged that the magma of this rock mass belongs to tholeiite series. The Mg # value of iron ultramafic rocks is 0.79 ~ 0.86, and they all contain a lot of accumulation phases. Mg # of three kinds of magnesia ultramafic rocks are 0.88 and 0.89 respectively, which are basically composed of olivine and orthopyroxene. The range of Mg # of mafic rocks is 0.78 ~ 0.63, which is obviously high, and the Mg # of gabbro is higher than gabbro. This may mean that hypersthene in gabbro is also a kind of accumulation.
Figure 4-2-5 Ol'-Ne'-Q' of Xingdi Ⅱ rock mass
Figure 4-2-6 AFM Diagram of Xingdi Ⅱ Rock Mass
2. Magmatic series and rock assemblage related to post-orogenic extension of fold belt.
Petrochemical composition and rock assemblage of intrusive rocks in (1) Karatongke metallogenic belt.
In the Karatongke metallogenic belt, 1 and No.2 rock bodies are large in scale, and the rock bodies are lenticular, irregular and veined. The surface shape of 1 rock mass is irregular spindle, the deep shape is wedge, and its long axis extends along 330. The surface is 700 meters long and 250 meters wide, with an area of about 0. 1 km2. No.2 rock mass is concealed below the surface 120 ~ 200 m, which is roughly a long flat vein with a steep dip to the northeast and a branch vein to the west. The main rock types are: phlogopite amphibole olivine syenite, Jinyun amphibole syenite, gabbro, gabbro, diorite and quartz diorite. The main petrological difference between this ore belt and other major metallogenic belts is that there are no ultramafic rocks, but there are a certain number of intermediate rocks.
On the OL ′-NE ′-Q ′ diagram (Figure 4-2-7), six silicate samples are fully analyzed and projected in alkaline rock area, and the remaining 55 samples are projected in subalkaline area. The latter has 16 samples projected on the AFM diagram in calc-alkaline rock area (Figure 4-2-8) and 39 samples projected in tholeiite series area (Figure 4-2-8). It can be seen that the Kalatongke rock mass is dominated by tholeiite series, supplemented by calc-alkaline series. A few samples projected in alkaline rock area should be related to alteration metasomatism. Kalatongke deposit is located in the Erqis fold belt and formed in the post-orogenic extension stage. Before these ore-bearing rocks were in place, B-type subduction occurred in this area. Some of these ore-bearing rocks belong to calc-alkaline series, which may indicate that their source areas are related to the cutting of the mantle wedge in the upper part of the plate, or the magma is influenced by it when it passes through the mantle wedge from the deep. The M/ boundary of most samples is between1.1~ 2.80, and the other two samples are 0.99 and 0.62, most of which are iron-basic rocks. The Mg # of various rocks ranges from 0.39 to 0.75, among which the Mg # of syenite is generally higher, while that of other rock types is generally lower than that of syenite. Judging from the degree of magma evolution reflected by Mg #, some syenites may contain a few accumulations, and the most likely accumulation is hypersthene. Mg # of syenite and gabbro are mostly in the range of primary magma, while diorite belongs to medium evolution magma.
Fig. 4-2-7 Ol'-Ne'-Q' Kalatongke metallogenic belt
Figure 4-2-8 AFM Diagram of Kalatongke Metallogenic Belt
(2) Petrochemical composition and rock assemblage of intrusive rocks in Huangshan rock belt.
The rock mass in Huangshan rock belt has many shapes, such as long strip, lenticular, gourd, horseshoe and torch, which indicates that the rock mass was affected by ductile shear during its formation and after consolidation. The outline is funnel-shaped or rock-tubular. The main rock types are plagioclase mica amphibole peridotite, amphibole pyroxene, olivine gabbro, gabbro, pyroxene diorite, diorite and a small amount of peridotite, among which mafic rocks are the main ones. Rock usually suffers from intense compression, and mineral deformation and crushing are common.
M/？ The values are between 4.07 and 6.01,which belong to iron ultrabasic rocks. The M/ value of most mafic rocks is between 4.9 1 ~ 2. 15, which is obviously higher than that of similar rocks. M/？ The value is between 0.68 ~ 1.52, which belongs to iron-basic rocks. M/ diorite? The numerical value is between 0.7 1 ~ 1.66, which belongs to the numerical range of iron-basic rocks. In Figure 4-2-9, a single data is predicted in alkaline rock area. In Figure 4-2- 10, 17 subalkaline samples are projected in calc-alkaline series, and most samples are projected in tholeiite series. It can be seen that the intrusive rock zone is dominated by tholeiite series, followed by calc-alkaline series. The tectonic setting of Huangshan intrusive rock belt is the same as that of Kalatongke, and the formation reason of calc-alkaline magma may be the same as that of Kalatongke, so I won't go into details here. Individual samples projected in alkaline areas may be related to alteration metasomatism. The Mg # value of ultramafic rocks is between 0.85 and 0.80, which is mainly composed of accumulated mafic minerals. Most samples of mafic rocks have Mg # = 0.69 ~ 0.84, and there are also some accumulations. The other four samples have Mg # = 0.4 1 ~ 0.66, belonging to moderately evolved magma. Diorite's Mg # = 0.38 ~ 0.66, mainly belongs to evolutionary magma, but the degree of evolution is obviously different.
Figure 4-2-9 Ol'-Ne'-Q' Schematic Diagram of Huangshan Rock Belt
Figure 4-2- 10 AFM Diagram of Huangshan Rock Belt
(3) Petrochemical composition and rock assemblage of the Hongqiling intrusive rock belt.
The rock mass forms of the Hongqiling intrusive rock belt are mainly rock basins, bedrock and rock walls. The main rock types are peridotite, olivine pyroxene, pyroxene, gabbro, gabbro and gabbro. In terms of volume, ultramafic rocks are the main ones, and mafic rocks are the auxiliary ones. The rock assemblage of different rock masses can be divided into plagioclase pyroxenite, gabbro-peridotite, hornblende amphibolite pyroxenite, pyroxenite-peridotite and gabbro-peridotite.
In the comprehensive analysis data of silicate in the Hongqiling intrusive rock zone, only a few samples belong to alkaline series (Figure 4-2-11); In the subalkaline series, only 1 sample belongs to calc-alkaline series, and the rest belongs to tholeiite series (Figure 4-2- 12). It can be seen that the Hongqiling intrusive rock belt basically belongs to tholeiite series. M/ ultramafic rocks? = 5.93 ~ 2.02, belonging to iron ultrabasic rocks. Synenite and gabbro? = 3.92 ~ 1.72, which is obviously higher than similar rocks. Mg # values of ultramafic rocks are mostly between 0.7 1 ~ 0.86; The other two samples are 0.64 and 0.67, of which 1 is explicitly named as ore-bearing altered pyroxenite, and the decrease of Mg # should be related to mineralization. According to the numerical range of Mg #, it can be seen that ultramafic rocks contain different amounts of accumulation phases. The Mg # of syenite is 0.80 and 0.8 1, and it also contains a considerable number of accumulation phases, probably mainly hypersthene. The mg # of gabbro is 0.64 and 0.72, which are at the edge of the range of primary magma mg #, showing weak differentiation characteristics.
Fig. 4-2- 1 1 Ol'-Ne'-Q' schematic diagram of hongqiling rock belt
Fig. 4-2- 12 AFM diagram of hongqiling rock belt
Magmatic series and rock assemblage of sheet intrusive rocks related to continental overflow basalt
This type of intrusive rock belt has two origins, one is the mafic-ultramafic intrusive rock belt in Bao Si, northern Guangxi, and the other is the intrusive body in the same period as Emei Mountain basalt, represented by Binchuan-Yongsheng area in Yunnan. Because the latter area is discussed in detail in Chapter 6, only the northern part of Guangxi is discussed here.
The mafic-ultramafic complex in Bao Si, northern Guangxi, is divided into baotan rock belt in the south and forest rock belt in the north. Most of the rocks in these two rock belts are bedrock or sheet, and a few are basin or hat. According to the rock assemblage, the rocks in the forest rock belt can be divided into four types: pyroxenite-amphibolite-pyroxenite, pyroxenite-amphibolite, amphibolite-pyroxenite-pyroxenite and peridotite-pyroxenite-gabbro, with ultramafic rocks being the absolute dominant. The main rock mass types in Baotan rock belt are metamorphic peridotite or pyroxenite-gabbro type and metamorphic pyroxenite-gabbro-diorite type. Among them, gabbro and pyroxenite (olivine pyroxenite) account for about half. There is no complete silicate analysis data in the forest rock belt, so only the petrochemical composition of Baotan rock belt is discussed.
Fig. 4-2- 13 Ol'-Ne'-Q' Schematic Diagram of Baotan Rock Belt
Fig. 4-2- 14 AFM Diagram of Baotan Rock Belt
Of the total analysis data of 14 silicate samples, only 4 samples belong to subalkaline series, and the rest 10 samples belong to alkaline basalt series (Figure 4-2- 13 and Figure 4-2- 14). It can be seen that the Baotan rock belt is dominated by alkaline basalt series, followed by tholeiite series. M/ ultramafic rocks? = 4.88 ~ 2.97 m/mafic rocks? = 2.02 ~ 1. 13, belonging to iron ultrabasic rocks and iron basic rocks respectively. Mg # of ultramafic rocks is mostly between 0.84 and 0.72, and other 1 samples is 0.66. This shows that most samples contain different amounts of accumulated crystalline phases. The Mg # of mafic rocks is 0.68 ~ 0.55, and the Mg # of four samples is 0.68 ~ 0.66, which is in standard primary magma and probably represents the petrochemical composition of primary magma. The gabbro with low Mg # represents the chemical composition of the moderately evolved magma.
4. Magmatic series and rock assemblage of ore-bearing rocks related to ophiolite.
(1) Petrochemical composition and rock assemblage of Jianchaling pluton.
Jianchaling rock mass is a monoclinic rock mass. Rock alteration is intense, and primary rock-forming minerals have been completely replaced. Altered rocks include chrysotile, chrysotile, magnesite, timely magnesite, magnesite schist and a small amount of diopside and talc schist. In the total analysis data of 9 silicate samples from Jianchaling rock mass, except for 1 sample, the other 8 samples are tholeiite series (Figure 4-2- 15 and Figure 4-2- 16), which shows that the rock mass basically belongs to tholeiite series. m/？ This value can be divided into two groups. One group is 9.55 ~ 17.09, belonging to magnesia ultrabasic rocks. The other group is 5.4 1 ~ 2.6 1, belonging to iron ultrabasic rocks. Judging from altered minerals, the samples belonging to magnesium ultrabasic rocks are mainly serpentine, while the samples belonging to iron ultrabasic rocks have strong slip fossilization. Accordingly, magnesium ultrabasic rocks generally represent the petrochemical characteristics of original rocks, while iron ultrabasic rocks are caused by specific alteration and metasomatism. Mg # of magnesium ultrabasic rocks is 0.9 1 ~ 0.95, which proves that the original rocks are mainly composed of high magnesium olivine and orthopyroxene.
Fig. 4-2- 15 OL'-NE'-Q' diagram of Delny and jianchaling rock masses.
Figure 4-2- 16 AFM Diagram of Delny and Jianchaling Rock Mass
(2) Petrochemical composition and rock assemblage of Delny rock belt.
Delny rock mass is a long and narrow monoclinic rock mass. The rock mass is poorly differentiated, and the lithofacies is single, mainly harzburgite, with a small amount of carbonate breccia peridotite, pyroxenite and serpentine.
The silicate analysis data of Delny rock mass all belong to tholeiite series (Figure 4-2- 16). m/？ = 7.52 ~ 1 1.24, 1 sample is 2 1.76, all of which belong to magnesium ultrabasic rocks. Mg # = 0.90 ~ 0.92, mainly composed of forsterite and orthopyroxene.
Verb (abbreviation of verb) Comparison of rock zones (regions) with different tectonic backgrounds
As far as the shape of rock mass is concerned, intrusions related to overflow basalt are bedrock-like, sheet-like, basin-like and hat-like not only in northern Guangxi, but also in the discovered rock mass in Binchuan-Yongsheng area of Yunnan. Its morphological characteristics are obviously different from the other three tectonic environments. Ophiolite-type ore-bearing rocks are essentially structural rocks sandwiched in structural zones and in structural contact with surrounding rocks. Both Delny rock mass and Jianchaling rock mass are monoclinic thick plates. This is related to the appearance of tectonic belt to some extent. The intrusions related to continental cracking and orogenic belts have different shapes, including dykes, dikes, basins, rocks and funnels. Moreover, there is no obvious difference in the shape of rock mass formed in these two tectonic environments.
As far as rock assemblage is concerned, there are obvious differences in rock assemblage formed in different tectonic environments. After excluding the influence of alteration metasomatism, ophiolite-type rocks all belong to magnesium ultrabasic rocks. Among the four tectonic environments, ophiolite rock mass has the highest MgO content and the highest basic degree. The rock mass with the lowest alkalinity should belong to orogenic belt. In the Karatongke rock belt, it is basically limited to basic rocks; Huangshan rock belt is dominated by basic rocks; In the Hongqiling rock belt, basic rocks also occupy a certain proportion. The alkalinity of intrusions related to continental cracking and intrusions related to overflow basalt is generally between the above two. However, in different rock areas and zones, the basic degree will be obviously different and cannot be generalized (Table 4-2- 1).
Table 4-2- 1 Mg # and m/? correlation table
M/？ And mg # are the highest, among which some samples in jianchaling obviously lost some MgO due to alteration, so its m/? And Mg # are slightly lower than those of Birney. Judging from their strong plastic deformation and rich magnesium, these two rocks should be mantle peridotite or metamorphic peridotite, rather than magmatic intrusive rocks. In addition, according to m/? And Mg # are, from high to low, Xingdi Ⅱ rock mass, Huangshan, Jinchuan, Hongqiling, western margin of Yangtze plate and Baotan. The foundation degree of Huangshan rock belt is not high, but compared with similar rocks, the MgO content is obviously higher, and the Hongqiling rock belt has a similar situation, which is worth studying. Except for a few samples, the rocks in the other three tectonic environments are basically iron-basic rocks and ultrabasic rocks. Except that some samples in Baimazhai, Kalatongke and Huangshan rock belts belong to calc-alkaline series, and a few samples in some rock masses belong to alkaline basalt series, most samples belong to tholeiite series. It can be seen that the iron-based and ultrabasic rocks of tholeiite series are the most important ore-bearing rock types of Ni-Cu-Pt deposits.
The εNd(t) of the whole rock, olivine and pyroxene in Jinchuan rock mass is between-2.60 and-3.78, and the variation range is very small. The εSr(t) values of most samples are between 17.0 ~ 128.8, and the other two samples are -2.3 and -2. 1, which have a wide range. The change of εSr(t) value should be related to hydrothermal alteration. As we all know, Rb and Sr have great chemical activity and are abundant in many hydrothermal fluids. Therefore, hydrothermal alteration will change the εSr(t) value of rocks to varying degrees. Nevertheless, it can be seen that Jinchuan rock mass should have a high positive εSr(t) value. The negative εNd(t) value of this rock mass should not be regarded as the result of assimilating continental crustal materials. First of all, the assimilation of surrounding rocks after magma invades the crust will lead to the inhomogeneity of εNd(t) values in different parts of the rock mass, which is characterized by the uniformity of εNd(t) values. Secondly, olivine is mainly in heap phase, mainly crystallized in deep magma chamber and ascending channel. Therefore, the εNd(t) value of olivine should not be affected by continental crust materials. The consistency of εNd(t) values of olivine, pyroxene and whole rock objectively reflects the εNd(t) value of magma. Coincidentally, according to the research of Li (1998), among the five whole rock samples of Xingdi Ⅱ, the εNd(t) of four samples is between-3.20 and-3.51,which is completely consistent with Jinchuan rock mass. In addition, the sample of 1 is+16. 12, and the reason remains to be investigated. The εSr(t) of rock mass is between +6 1.3 ~+70.0, which is also a high positive value. The combination of negative εNd(t) value and high positive εSr(t) value is a sign of mantle enrichment. The lithospheric mantle in almost all craton distribution areas in the world belongs to enriched mantle, while asthenosphere mantle and oceanic lithospheric mantle belong to depleted mantle. Therefore, the magma of Jinchuan pluton and Xingdi Ⅱ pluton originated from the continental lithospheric mantle of the Early Cambrian.
According to the research of Li et al. (1998), the εSr(t) values of the whole rock and copper-nickel ore in Kalatongke mining area are between-5.3 and-8.9; εNd(t)=+5.07~+5.30. The ε Sr (t) of the whole rock and copper-nickel ore in Huangshan mining area is -3.20 ~- 12.20, and the other two samples are +0. 1 and+1.20. εNd(t)=+6.63~+7.84. The author thinks that the small dispersion of εSr(t) in Huangshan mining area may be polluted by less than 5% continental crust materials. The whole rocks and copper-nickel deposits in Kalatongke and Huangshan have the characteristics of positive εNd(t) and negative εSr(t), indicating that they originated from the depleted mantle (Figure 4-2- 17). This is also the characteristic of most magmatic rocks in northern Xinjiang. Because the εNd(t) of the two places is generally lower than the εNd(t) value of mid-ocean ridge basalt (+8 ~+ 12), there are two possibilities. One possibility is that these magmas originated from the asthenosphere and assimilated some continental crustal materials during and after emplacement. Another possibility is that their magma source is a mixture of asthenosphere and younger lithospheric mantle, or it comes from the younger lithospheric mantle modified by asthenosphere. What kind of situation needs the cooperation of various isotope methods is not enough to give a final conclusion at present.
Fig. 4-2- 17 εNd(t)-εSr(t) correlation diagram of some rocks and ores.
Chen Yuchuan and Mao Jingwen (1995) studied the emplacement age of mafic-ultramafic rocks in northern Guangxi by Sm-Nd isochron method. Since Wang Hongzhen et al. (1990) defined the Bao Si period as between 1.05 ~ 1.70 Ga, we took 1.4 Ga as the average age of these invaders, and calculated the εNd(t) value accordingly. εSr(t) of mafic-ultramafic rocks in northern Guangxi is concentrated in two ranges, one is+2.60 ~+7.27; Another range is -0.28 ~- 1.96. This leads to two possibilities. One possibility is that these magma come from depleted mantle and enriched mantle respectively. Another possibility is that they all originated from the depleted mantle, and the negative εNd(t) value is the result of assimilating continental crustal materials. What kind of situation, but also the cooperation of various isotope methods, is currently inconclusive. But in any case, 7 samples in 10 are positive, indicating that these rocks are mainly from the mantle with moderate loss.
Yichang Institute of Geology and Mineral Resources (1994) and Liu, the project team, conducted a comprehensive isotopic study of Jianchaling rock mass. Their findings are summarized as follows. 34 kinds of ore sulfides were determined, and δ 34s =+6.1‰ ~+11.57 ‰, with an average of 8.95‰. The δ 34s of 43 ore samples determined by Liu is +6. 1 ‰ ~+ 12.9 ‰, with an average of +9.3‰. δ 34s of pyrite in 15 altered ultramafic rocks is +7.8 ‰ ~+ 15.3 ‰, with an average value of+1 1.8‰. It can be seen from these data that the S isotopic composition of sulfide in Jianchaling rock mass has obviously deviated from the composition range of mantle S, but has the composition characteristics of crust S. Seven antigorite samples δ 18O = 8.0 1‰~ 9.95‰, and eight viscose serpentine samples δ 1 8o = 6.04 ‰ ~ 8.95 ‰. From these data, it can be seen that only a few cellophane serpentines have δ 18O values within the O isotopic composition of mantle peridotite, while the δ 18O values of most samples are equivalent to those of crustal granite and some sedimentary rocks. 10 nickel mine 206pb/204pb =16.7075 ~19.696, 207pb/204pb =15.379 ~15.720, 208 Pb/204 Pb = 36.831~ 39.9970 serpentine 206 Pb/204 Pb = 206 These lead isotopic compositions indicate that the evolution of lead in ore after treatment is inconsistent with that in rock. Once the lead in the ore is formed, its lead isotope is stable and changes gently. The lead isotope evolution history of rocks is complex, which belongs to abnormal lead or the mixture of two or more lead sources, indicating that foreign lead is mixed during diagenesis or radioactive lead accumulates after diagenesis. We use λ Rb =1.42×10-1,87Sr/86SrUR(O)=0.7045, 87Rb/86SrUR(O)=0.0827 to calculate Chen Haoshou's Nine. 1 1 sample is +2.8, 3 samples are -45.7 ~- 194.2, 1 sample is -7835. It is rare for εSr(t) to change sharply in such a large range, so we can only assume that these Sr isotopic data cannot represent the original isotopic composition of rock mass. We use143 nd/144 ndchur (o) = 0.512638 to calculate seven pieces of Sm-Nd isotope data made by Chen Haoshou and others, ε nd (t) =-22.11~. From these data, the isotopic composition of neodymium is relatively stable. However, because the isotopic composition of S, O, Pb and Sr in Jianchaling pluton cannot reflect the isotopic composition of mantle rocks, it is still difficult to judge whether its nd isotope can represent its original Nd isotopic composition. If a positive judgment is made, the Jianchaling pluton should come from the continental lithospheric mantle enriched in the early Precambrian. This is contrary to the view that the rock mass belongs to ophiolite suite, because the oceanic lithospheric mantle belongs to deficit type. If a negative judgment is made, it can only show that the hydrothermal solution of the rock mass comes from the continental crust and is consistent with other isotopic systems.
To sum up, Jinchuan pluton aNd Xingdi Ⅱ pluton have negative εNd(t) value and positive εSr(t) value, which clearly indicates that they originated from the continental lithospheric mantle enriched in the early Precambrian, while the Nd and Sr isotopic compositions of rocks and ores in Kalatongke and Huangshan mining areas are opposite, indicating that they originated from the mantle with moderate deficit. Further exploration of the genesis of this moderately depleted mantle will help to understand the original source of ore-forming materials more accurately. Ultramafic-mafic rocks in northern Guangxi mainly come from moderately depleted mantle source areas, and whether some rocks come from enriched mantle source areas is still inconclusive. Further study on the magma source in this area will help to better understand the mantle dynamics background and the source of ore-forming materials during magma formation. The isotopic system of Jianchaling pluton is extremely complex. Except whether the Sm-Nd isotopic system can represent the original isotopic composition can not be finally determined, the S, O, Pb and Sr isotopic systems have completely changed and cannot represent their original isotopic composition. If we want to do further research on it, we should adopt isotope method which can better resist the influence of hydrothermal alteration.