Distribution of rare earth elements in alkali-rich porphyry

As an indicator of geochemical action, rare earth elements have been widely used in all aspects of geochemistry. The study of rare earth elements is of great significance for understanding the evolution mechanism, origin, crystallization history and source rock materials of magma. Therefore, all the important alkali-rich porphyries in this area were sampled in this study, and a complete rare earth analysis was entrusted to the experimental center of Hubei Geological Bureau. The analysis results and some parameters are listed in table 3- 12 and table 3- 13. In order to explain the problem comprehensively, tables 3- 12 and 3- 13 list some previous analysis results.

I. Alkali-rich granite porphyry

1. Rare earth element content

1) The total amount of rare earth elements (σ REE), the ratio of light and heavy rare earth elements (σ lree/σ hree) and δEu values of alkali-rich granite porphyries in the western margin of Yangtze platform are basically similar, indicating that the alkali-rich granite porphyries in this area have the same material source, the same evolution characteristics and the same genesis.

2) The total amount of rare earth in alkali-rich granite porphyry is low, and the average σ REE is151.18×10-6 (between 90.19×10-6 and 293.42).

3) The alkali-rich granite porphyry in this area is highly enriched with light rare earths, and the ratio of total light rare earths (σ Ce) to total heavy rare earths (σ Y) is 3. 38 to 13.43, with an average of 8. 19, reflecting that alkali-rich granite porphyry is rich in cerium and rare earth elements.

4) In the La/SM-LA graphic projection (Figure 3-6), the projection points of alkali-rich granite porphyries in this area are all near the partial melting line, indicating that the alkali-rich granite porphyries are formed by magma that is partially melted and intruded in the deep (mantle).

Table 3- 12 Rare and Rich Element Contents of Soil Alkaline Granite Porphyry and Parameter Table 3- 12 Rare Earth Element Contents and Correlation Ratio of Intrinsic Rock of Alkaline Granite Porphyry

Note: The samples were determined by the Experimental Center of Hubei Geological Bureau.

Table 3- 13 Rare Earth Positive Element Content and Parameter Table 3- 13 Rare Earth Element Content and Correlation Ratio of Orthoporphyry Intrinsic Rock

Note: The samples were determined by the Experimental Center of Hubei Geological Bureau.

Fig. 3-6 la/sm-la content map of alkali-rich porphyry in Tu Tu 3.6.

2. Composition mode of rare earth elements

The composition pattern diagram of rare earth elements is a kind of direct view diagram drawn according to the average composition of chondrites composed of the earth's original materials. Therefore, studying the REE composition pattern of rocks is helpful to explore the genesis and material source of rocks. In this work, the representative rocks in this area were selected when studying alkali-rich granite porphyry. According to the analysis results of REE content in the whole rock, the normalized REE composition pattern diagram of chondrites (Figure 3-7) is compiled, in which the standard REE content of chondrites is determined by Helsman (1980) (the average content of 22 chondrites), and according to the following formula.

Metallogenic conditions and prospecting prospects of alkali-rich porphyry copper-gold polymetallic deposits in the western margin of Yangtze platform

[( Eu) r, (Sm) r and (Gd) r represent the ratio of their contents to chondrite contents, respectively]

The δEu values of each rock mass are calculated (Table 3- 12). As can be seen from Table 3- 12 and the rare earth composition pattern diagram:

1) The REE model curve of alkali-rich granite porphyry in the western margin of Yangtze platform is a smooth curve with steep dip to the right, with an average δEu of 0. 87, the range is 0. 74 ~ 0.95, which is very close to δEu value.

2) Eu is normal, and there is no positive or negative abnormality.

3) The REE composition patterns of alkali-rich granite porphyries are very similar, which reflects that they have the same genesis and evolution characteristics.

Second, syenite porphyry

1. Rare earth element content

1) The general characteristics of rare earth elements in syenite porphyry in the western margin of Yangtze platform are that the ratio of light and heavy rare earth elements is close to δEu value, reflecting the same material source, evolution characteristics and genesis.

2) A remarkable feature of syenite porphyry in the western margin of Yangtze platform is the enrichment of rare earth elements, with an average total amount of more than 400× 10-6 and a range of (380. 5 ~ 865.11)×10-6. Among them, the σ REE of the granite syenite porphyry in Laojiezi, Yao 'an is as high as 1833.63× 10-6 (not listed in the table). In addition to the high content of alkaline rocks, the enrichment of rare earth elements is mainly caused by chalcopyrite and other minerals, which are highly enriched in their secondary minerals.

3) The syenite porphyry is highly enriched with light rare earth elements, and the σ lree/σ hree ratio is close, ranging from 3. 88 rpm 18. 25, and most of them are above 10, which reflects that syenite porphyry in this area is rich in light rare earth elements.

In the LA/SM-LA diagram, the projection points are all near the partial melting line, indicating that syenite porphyry was formed by partial melting upwelling of deep magma.

Figure 3-7 Figure 3-7 Standard formula model of rare earth spherulite damaged rocks in alkali-rich porphyry main rock mass in the western margin of Yangtze platform Figure 3-7 Standardized chondrite flora of rare earth elements in alkali-rich poppy rocks in the western margin of Yangtze platform.

2. Composition mode of rare earth elements

The REE composition patterns of syenite porphyry in the western margin of Yangtze platform (Figure 3-7) are very similar, all of which are smooth curves with steep dip to the right, with δEu value of 1 and no europium anomaly.

The difference of REE composition model curves of rocks, especially the difference of δEu values, reflects the difference of genetic types and sources of magmatic materials. The right-leaning smooth REE composition pattern (Δ EU ≈1) reflects that magmatic materials may come from the upper mantle, and the REE composition patterns of alkali-rich porphyries (alkali-rich granite porphyries and syenite porphyries) in this area are very similar, indicating that their genesis and material sources are consistent.