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Page 1: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

Vol.10 No. 3 CHINESE JOURNAL OF G E O C H E M I S T R Y 1991

REE Geochemical Characteristics of Volcanic Rocks in Zhejiang and Jiangxi Provinces and

Their Geological Significance

W A N G JIANFENG ( _ ~ l J ~ )

( Chengdu College of Geology, Chengdu )

Abstract

Based on the data of 64 samples, the REE geochemical characteristics of volcanic rocks in northern Zhejiang and eastern Jiangxi provinces are discussed in this paper. The REE distribution patterns in acid and intermediate-acid volcanic rocks in these areas display some s'tmilaxities, as indicated by rightward-inclined V-shaped curves with negative Eu anomalies, which are parallel to each other. In addi- tion, their REE parameters (ZREE, Z L R E E / Z H R E E , 6Eu ,Ce / Yb, L a / Sm, L a / Yb,ete. ) also va- ry over a narrow range with small deviations. HREE are particularly concentrated in the volcanic rocks as- sociated with uranium mineralization. The initial STSr / s6 Sr ratio in the volcanic rocks is a b o u t 0.7056 - 0.7139. All these features in conjunction with strontium isotopic data indicate that the rock-forming materials come from the sialic crust. The REE distribution patterns and REE geochemical parameters of the volcanic rocks, as well a s i a / Sm-La and Ce/ Yb - E u / Yb diagrams may be applied to the sources of rock-forming and ore-forming materials.

Developed in the Zhejiang-Jiangxi volcanic zone is a suite of Middle JUrassic-Late Cretaceous volcanic rocks which are characterized by extensive distribution and great thickness. The rock types include basic, intermediate, intermediate-acid and acid vol- canic rocks, of which the Late Jurassic acid and intcrmadiatc-acid volcanic rocks are of most extensive occurrence. Many years of investigations have been made on the stratigraphy and petrography of volcanic rocks and related mineralizations of U, M o and Ag in tcn volcanic basins and depressions of the Zhcjiang-Jiangxi volcanic zone. Meanwhile, about seventy volcanic rock samples wcre analyzed for their REE contents and six sets of volcanic rock samples for their Sr isotopic composition. ~) Generalized in this paper arc the REE geochemical characteristics of volcanic rocks in the area studied and also discussed is the significance of REE in the study of the genetic origin of volcanic rocks and associated uranium mincralization.

REE Geochemistry of Volcanic Rocks

In recent years we have invcstigated the Mesozoic acid, intermediate-acid and ba- sic volcanic rocks in ten volcanic depressions and basins at Xiangshan and Xinjiang, castern Jiangxi and at Shouchang, Shidamen, Shegaiwu, Xindun, Chun 'an , Baizhangling, Jinzijian and Tianmushan, northern Zhcjiang to shed light on the REE distribution. The results of REE analyses for the representative samples collected from the locations mentioned above are given in Table 1. At the same time, some of the samples were analyzed by using NAA techniques for comparison. The analytical re- sults are of good consistency.

1 )Wang Jianfeng et al. (1988)A study on the formation mechanism of Mesozoic volcanic rock-type U deposits in northwestern Zhcjiang.

Page 2: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

278 CHINESE J O U R N A L O F G E O C H E M I S T R Y Vol. 10

From the REE data of Table 1, we may come to the following conclusions : 1. Volcanic rocks in the various volcanic depressions and basins show slight varia-

tions in IEREE from 154.7 to 435.15 p p m . T h e ~ R E E of mineralized volcanic rocks may be so high as to be 7 2 5 . 3 - 2048.8 ppm. If Y is not taken into consideration in our calculation, the average IEREE contents of volcanic rocks from the eight volcanic depressions and basins in northern Zhcjiang are within the range of 177.61 - 3 5 2 . 1 ppm, with a maximum value of 352. 1 ppm for two samples of rhyolite from the Shidamen volcanic depression (on average)and a minimum value of 177.61ppm for two samples of volcanic rock from the Xindun volcanic basin. The ~REE con- tents of volcanic rocks in the various locations are all higher than those of the crust (L i Tong gave a value of 165.35 ppm, 1976)(Liu Yingjun, 1984). The average

REE content of three mortar rhyolite samples from the Xiangshan volcanic depres- sion is 199.6 ppm, and that of five sub-granite-porphyry samples is 172.09 ppm, both also being higher than that of the crust. However, the IE REE content of basic volcanic rock samples is relatively low, generally lower than, or close to, the crustal value. For example, the IEREE content of diabase-porphyrite at Xiazizhou, Jiande is 166. 18 ppm.

2. U - , Mo- and Ag-mineralized volcanic rocks of hydrothermal origin usually have higher IE REE than non-mineralized volcanic rocks of the same type. For exam- ple, the ~REE content of mineralized rhyolite samples from the Shidamen volcanic depression is 624. 8 ppm, that of mineralized mortar rhyolite samples from the Xiangshan volcanic depression is 2048.8 ppm, and that of mineralized breccia latite samples from the Xinjiang volcanic basin is 618.6ppm. This suggests that REE, espec- ially heavy REE, and ore-forming elements can be transported together in hydrothermal solutions.

300 200

200 % ~ .,....A..-' A"-....~.. 10 0

1 ~.~. ~.~ .... , . . . .

:, 50 �9 .'.. ...~" ,

3

2 , , J i L i i i i i i i i i I

LaCe Pr Nd PmSmEu Gd Tb Dy Ho Er TmYb Lu

i ;

~ / ~'11

2 L i , i i i i i i i i i I i

LaCe Pr NdPmSmEuGdTbDy H'o ErTmYb Lu

Fig �9 1. Chondrite-normalized REE distribution patterns in volcanic rocks from the Shouchang volcanic basin (curve Nos. are the same as sample Nos. in Table 1; the same below). 1. Rhyolitic crystal vitric tuff; 2. rhyolitic crystal tuff; 3. ball rhyolite; 4. rhyolite; 5. U-mineralized rhyolite;

6. U-mineralized rhyolite; 7. dacitic crystal tuff.

Fig. 2. Chondrite-norma5zed REE distribution patterns in volcanic rocks from the Shidamen and Shigaiwu volcanic depressions. 8. Magnophyric rhyolite; 9. magnophyric rh~ofite; I0. U-mineralized rhyolite; 1 I. dacitic ignimbrite.

Page 3: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

No.3 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 279

o

\

r,-

200

100

50

20

10

5

' , ~ 300

~. .~ ~ ...~ -,, 200 -.,.~-%. ~,

,..., " ~ - , ' - . ~ . ,, loo ~ %- :~x . x .~

\'~~-4",

ii is to

5

I l ~ F S d i I I , T~ I i I I , , 4 La Ce PmSmEuGd Dy Ho ErTmYb Lu 3

~

%'-~..

"" " ' \"~'~"~ ,~--, . 21

~ , ' 'N' ' ' ' & ~ > ' ' ' ' ' ' C_,ePr d PmSmEu Dy Ho Er Tm Yb Lu

Fig. 3. Chondrite-normalized REE distribution patterns in volcanic rocks from the Xindun, Chun 'an and Baizhangling volcanic basins. 12. Dacitic crystal tuff; 13. dacite-porphyrite; 14. dacitic ignimbrite; 15. dacific crystal tuff; 16. ball rhyolite; 17. rhyolitic dacitic ignimbrite.

Fig. 4. Chondrite - normalized REE distribution patterns in volcanic rocks from the Jinzijian and Tianmushan volcanic basins. 18. Rhyolitic vitric tuff; 19. dacitic rhyolitic ignimbrite; 20. dacitic'Fnyolitic ignimbrite; 21. rhyolitic ignimbrite; 22. dacitic rhyolitic ignimbrite.

o

\

1500

500

2OO

100

50

2O

10

5

, fi-y'~,~ 26 2 O0

/ too /

/ " ~ 50

/ 20 , ~ . c \ / . _ ~ . . ~ \

. . . . . . . . .25

V 27 3

Ce Nd Sm Gd Dy Er Yb 2 i , i i , i i i I , i i I i

La 1% Pin Eu Tb Ho Tm I~u

30

j y , . : . ;~ . . , a2

"~" "'-~ 31

La

Ce Nd Sm Gd Dy E r Yb

Pm Eu Ho Tm

Fig. 5 Chondrite-normalized REE distribution patterns in volcanic rocks from the Xiangshan volcanic pression. 23,24, and 25. Mortar rhyolite; 26. mineralized mortar rhyolite; 27. granite-porphyry; 28. mineralized granite-porphyry.

Fig. 6. Chondrite-normalized REE distribution patterns in volcanic rocks from the Xinjiang volcanic basin. 29,30. U-mineralized brecciat~! trachyandesite; 31,32. brecciated trachyandesite; 33. trachyte; 34. brecciated trachyandesite.

3. The chondrite-normalized REE distribution patterns in volcanic rocks from the various volcanic depressions and basins (Figs. 1 - 6 )show that : (1)Thevolcanic rocks studied are of LREE-enrichment type and the REE distribution patterns in various types of volcanic rock (including basic, intermediate and acid ) are represented by the rightward-inclined curves. ( 2 ) T h e left part of the chondrite-normalized curves rises leftwards and are relatively steep while the fight part basically keeps a horizontal exten-

Page 4: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

280 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol.10

Sequence Sample Sample

No. No. locality

8 9 10

11

12

13

14

15

16

17

18

CZ- 583 Shouchang volcanic

CZ- 585 basin

CZ- 590

CZ- 603 CZ- 530

CZ- 270

CZ- 597

CZ- 553 CZ- 492 Shidamen CZ- 295 volcanic

depres- sion

CZ- 6 Shigaiwu volcanic depression

CZ-319 ~(mdun volcanic

CZ- 321 basin

CZ- 346 Chun 'an volcanic

CZ- 329 basin

CZ- 251 Baizhang- ling

CZ- 369 volcanic basin

CZ- 386 Jinzijian volcanic basin

R~k type

Table 1. REE contents of some volcank rocks in northern 7hej inng

Chrono- REE content (ppm)

logical symbol La Ca Pr Nd Sm Eu

Rhyolitic crystal vitric tuff Rhyolitic crystal tuff Ball rhyolite Rhyolite

U-mineralized rhyolite U-mineralized rhyolite

Dacitic crystal tuff

Magnophyric rhyolite

U-mineralize d rhyolite

Dacitic ignimbrite

Dacitic crystal tuff Dacite- porphyfite

Dacitic ignimbrite

Dacitic crystal tuff

Ball rhyolite

Rhyolitic dacitic ignimbrite

Rhyolitic vitric tuff

J3S 68.00

J3h 78.00

J3h 33.80

J31 67.80 J31 14.10

130.00 16.10 57.40 12.30 0.67

151.00 18.60 66.10 13.80 1.19

77.10 9.72 33.60 8.36 0.32

124.00 14.80 47.10 10.10 0.51 128.0 7.01 22.60 l i .10 0.35

J3S 24.00 39.00 5.56 20.40 5.08 0.23

J31 26.20 60.20 7.44 26.40 5.89 0.23

J3S 83.40 165.0 20.2 65.8 12.6 0.96 J3S 67.60 133.0 16.0 54.4 12.3 0.54

J3S 79.5 155.0 18.3 64.8 14.1 0.85

J31 49.29 101.6 8.11 41.93 9.92 0.35

J3h 38.5 68.5

J3h 36.4 68.7

I

8.75 !31.5 6.59 0.81

8.53 31.7 6.62 8.86

J31 47.2 89.3 11.3 42.1 8.71 1.22

J31 50.0 95.3 11.1 40.5 8.23 0.70

J3h 21.2

J31 75.8

51.1 5.83 18.7 4.23 0.22

115.0 1 6.9 59.6 11.4 1.31

J31 50.5 89.6 11.7 41.9 8.52 0.87

Page 5: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

No.3 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 281

and eastern Jiaagxi *

Gd Tb Dy Ho

9.94 1.86 8.85 1.81

10.50 1.89 8.80 1.77

6.51 1.33 6.51 1.22

7.20 1.43 6.41 1.17

Er Tm Yb Lu

4.63 0.76 4.27 0.59

4.68 0.80 4.67 0.67

3.00 0.49 2.79 0.39

2.60 0.43 2.20 0.28 17.50 6.18 52.90 12.40 37.90 5.01 33.30 4.37

5.40 1.19 9.79 2.08 7.14 1.06 7.24 1.05

4.89 1.04 5.76 1.18 3.23 0.50 3.84 0.40

Calculated parameter U

Z LREE 5ZREE _ _ 6Eu (ppm)

HREE

367.38 3.43 0.18 6

413.27 3.89 0.29 4

218.03 2.95 0.13 5

312.63 5.05 0.18 19 725.3 0.34 0.08 25

187.82 1.01 0.14 412.2

184.98 2.16 0.13 5

9.18 1.84 8.85 1.95 5.34 0.92 5.23 0.78 435.2 10.3 2.10 10.6 2.07 5.62 1.00 5.95 0.87 381.4 15.2 3.03 23.7 5.19 16.8 2.32 15.0 2.02 624.8

3.99 0.26 54 2.91 0.14 220 1.70 0.18 3925

5.21 0.77 5.31 1.05 2.71 0.44 4.39 0.26 269.7 4.25 0.14 4.7

5.88 0.95 6.4 1.23

5.92 1.00 6.46 1.28

3.82 0.56 4.02 0.62 216.2

4.08 0.60 4.28 0.65 215.0

2.51 0.39 13

2.46 0.42 0.8

8.02 1.38 8.83 1.68

6.85 l.lO 6.64 1.23

5.14 0.73 5.07 0.76 278.9

3.47 0.49 3.27 0.50 263.9

2.53 0.44 0.8

3.55 0.28 3

3.76 0.95 5.66 1.31

8.96 1.43 8.54 1.61

4.06 0.75 4.68 0.65 163. l

4.76 0.67 5.18 0.71 360.1

1.64 0.17 2

3.49 0.39 0.7

6.92 1.13 7.95 1.53 5.01 0.72 6.29 0.80 286.9 2.41 0.34 �9 - -

Page 6: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

282 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol .10

Sequence

No.

19

20

21

22

23 24 25 26

27

28

29

30

31 32

33 34

Sample

No.

CZ- 415

C Z- 433

C Z- 436

CZ- 444

Z-4

Z-5 Z-7 Z-3

S ' 2

S-4

JY- 1

JY- 5

JY '6 JY" 2

JY- 3 JY" 8

Sample

locality

Tianmushan volcanic basin

~angshan volcanic depression

Xinjiang volcanic basin

Rock type

Dacitic rhyolitic ignimbfite

Rhyolitic ignimbrit e Dacitic rhyolitic ignimbrite

Mortar rhyolite

Mineralized mortar rhyolite Granite- porphyry Mineralized granite porphyry

Mineralized brecciated trachyandesite

Brecciated trachyandesite

Trachyte Brecciated trachyandesite

Chrono- REE content (ppm)

logical I symbol La Ca Pr Nd Sm Eu

J3h 44.6 81.6 10.1 36.2 7.67 1.31

J31 70.1 131.0 15.8 52.1 9.72 1.04

J31 59.3 115.0 14.0 49.9 10.1 1.37

J31 52.0 102.0 12.6 45.9 9.43 1.21

J31 36.1 73.6 8.79 32.1 6.74 0.45 J31 30.8 59.3 6.65 23.0 4.88 0.46 J31 32.5 58.1 7.94 29.1 6.44 0.48 J31 63~5 100.0 13.7 40.8 10.4 1.91

J31 32.7 63.2 7.45 25.7 5.68 0.66

J31 33.1 64.0 7.98 27.3 6.24 0.99

J3e 56.9 110.0 13.6 48.9 9.64 2.20

Jae 59.9 101.0 10.5 36.9 6.98 2.26

J3e 47.4 81.5 8.86 30.7 5.12 1.06 J3e 31.2 54.5 6.82 24.5 4.41 0.56

Jse 46.4 93,2 10.7 40.2 8.32 0.72 J3e 42.9 82,6 10.1 37.9 7.03 1.64

* REE were analyzed by plasma emission spectrometer at the Central Lab of the Bureau of Geology and College of Geology. Note: The results of REE analyses show that the contents of Y are : 50.20, 50.80, 32.90, 28.60,372.00,

31.4,41.0,228.0,112.0, 15.7, 19.0,32.1 and 37.9.

sion or slightly inclines rightwards, indicating that in the process of formation of vol- canic rocks LREE experienced intense fractionation relative to HREE. ( 3 ) T h e REE distribution curves of volcanic rocks in different strata or formations of the Mesozoic volcanic complexes in various locations are characterized as being parallel to one an- other (e. g. Curve Nos. 1, 2 and 4 which represent the Shouchang, Huangjian and Laocun formations, respectively ). This shows that the REE contents of volcanic rocks of various volcanic formations are generally synchronously variable and that volcanic rocks of the different volcanic units are products of different phases of comagmatic eruption. ( 4 )As compared with the adjacent rare earth-elements Sm and Gd, Eu dis-

Page 7: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

No.3 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 283

Gd Tb Dy Ho Er Tm Yb Lu

Calculated parameter U

Y.LREE ~: REE t$ Eu

Y- HREE (ppm)

6.44 1.18 6.44 1.32 3.87 0.62 4.18 0.61

7.27 1.36 7.33 1.47 4.22 0.73 5.18 0.72

8.44 1.62 9.37 1.99 6.22 0.95 6.88 0.92

7.42 1.32 7.28 1.45 4.21 0.68 4.93 0.69

237.8 2.74 0.56 2.5

354.5 3.74 0.37 - -

350.3 2.48 0.45 - -

294.8 3.11 0.43 - -

5.71 1.08 6.34 1.17 3.84 0.60 3.93 0.59 215.5 2.73 0.22 4.60 0.94 5.83 1.22 4.67 0.79 5.68 0.96 187.4 2.01 0.30 " - - 5.70 1.07 6.44 1.26 4.30 0.69 4.69 0.74 195.9 2.20 0.24 - -

17.5 8.90 88.7 31.7 159.0 32.2 287.0 43.5 2048.8 0.13 0.44 980

5.17 0.96 5.53 1.06 3.41 0.49 3.28 0.49 187.2 2.61 0.37

5.65 1.19 7.53 1.47 5.46 0.87 6.17 0.95 209.9 1.99 0.51 - -

9.72 2.48 23.3 7.10 35.9 7.33 54.6 8.97 618.6

7.04 1.62 13.7 3.64 15.4 2.86 22.0 0.53 399.3

3.80 0.63 3.06 0.60 1.67 0.26 1.63 0.23 202.2 3.43 0.64 3.45 0.70 2.16 0.37 2.50 0.37 154.7

6.50 1.18 6.22 1.18 3.37 0.54 3.23 0.47 254.3 5.91 1.03 6.11 1.27 4.39 0.75 5.35 0.81 245.7

0.64 0.71 700

1.20 0.99 540

6.33 0.71 8.49 3.74 0.43 10.1

3.65 0.29 5.44 2.87 0.76

Mineral Resources, Sichuan Province and U was analyzed at the Lab of Chemical Analysis,Department No.3,Chcngdu

58.60, 36.90, 53.10, 59.0,209, 0, 37.72, 78.1,37.9, 47.5,34.5,40.0, 48.2, 53.5,41.7.46.5,64.3,43.7,34.5,37.6, 36.4,1150.0,

plays a remarkable depletion, i. e., a negative Eu anomaly ($ Eu < 1 ). As a result, a valley can be seen at Eu in each of the REE distribution patterns. As for the 34 sam- ples listed in Table 1, the t~Eu values are all less than 0 . 7 , and for half of the samples the ~Eu values are less than 0.33. It is evidenced that in the area studied acid and inter- mediate-acid volcanic rocks resulted from magatic eruption in response to partial melting of upper crustal material. The REE distribution patterns of basic volcanic rocks in various locations generally exhibit no Eu anomaly, with $ Eu > 1. For exam- ple, the diabase-porphyrite in Xiazizhou, Jiande has a c~ Eu value of 1.012, indicating a deep-seated material source.

From the above discussions it is evidenced that the REE distributions of volcanic

Page 8: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

284 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 10

rocks bear abundant information on rock-forming material source, magmatic evolu- tion and minerogenesis. For this reason, REE can be used as indicators of the genesis of volcanic rocks and the material source pertaining to petrogenesis and minerogenesis.

Application of REE in Identification of Geneses of Volcanic Rocks

As the REE distribution of volcanic rocks bears abundant information on rock-forming material source, magmatic evolution and minerogenesis, in conjunction with the peculiar geochemical behavior of REE in the process of magmatic evolution it is considered that REE, in combination with field observations as well as with mine - ral identification,chemical analysis and isotopic studies at laboratory, can be used to effectively identify the rock-forming materials and the geneses of volcanic rocks.

1 .TheREE distribution patterns and geochemical parameters mentioned above show that the intermediate and acid volcanic rocks in the area studied were derived from the sial of the upper crust with respect to their rock-forming material source. The available data indicate that in the area studied acid and intermediate-acid volcanic rocks are gene- rally similar to the basement felsic metamorphic rocks in REE distribution patterns with the exception of tSEu values which show significant differences between the two types of rocks (Wang Jianfeng and Liu Feng, 1987 ). Both types are characterized by LREE enrichment as indicated by the rightward-inclined curves. From this it can be deduced that in regard to their rock-forming material source the intermediate and acid volcanic rocks are thought to have been derived entirely or partly from the remelting of the Prepaleozoic basement felsic metamorphic rocks, while the basic volcanic rocks such as lamprophyre and diabase-porphyrite are of deeper derivation, i. e., from the sima or the upper mantle, although their REE distribution patterns and geochemical parameters arve similar to those of the intermediate and acid volcanic rocks.

2. Y. LREE / Y. HREE ratios in the volcanic rocks studied ( except for those mineralized volcanic rocks ) are mostly greater than 2, with C e / Y b ranging from 4 to 40 and L a / S m varying between 3 and 9. Except for the basic volcanic rocks, all the volcanic rocks have a 3 Eu value of less than 0.70 and 50% of the samples have a 3 Eu value of less than 0.33 (Table 1 ). These geochemical parameters show that during magma evolution L R E E / HREE, especially LREE experienced a certain degree of fractionation prior to the eruption of magma onto the surface, or that LREE enrichment and Eu depletion are inherent in the original rocks. Volcanic rocks of this type can only be formed from remelting and reworking of sialic material derived from the continental crust (Tu Guangchi, 1984 ).

3. In the L a / Sm-La diagram volcanic rock samples from various locations are plotted on the oblique lines, as can be seen in Fig. 7. This situation is similar to that of the continental crust reworking-type granites in the Nanling region, but different from that of the syntectic granite plutons (Fig. 8 ).

If C e / Yb and E u / Yb are taken as ordinate and abscossa respectively on a plot, the data points of intermediate and acid volcanic rock samples from various loca- tions would be of steeply-inclined linear distribution near the Ce /Yb coordinate. This diagram is also similar to the C e / Y b - Eu / Yb diagram of the continental crust reworking-type granites in the Nanling region.

REE geochemical characteristics described above provide evidence suggesting that

Page 9: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

No.3 CHINESE JOURNAL OF GEOCHEMISTRY 285

7

\ 5

4- ~ . '

~ 0 2 A 3

�9 §

I I I i I | I i l

10 20 ~ 40 50 60 70 80 90

La (~m)

Fig. 7. La/ Sm-La diagram of volcanic rocks ~om Jiangxi and Zhejiang provinces. 1. Volcanic rock from the Shouchang volcanic basin; 2. volcanic rock l~om the Shigaiwu and Shidamen volcanic depressions; 3. volcanic rock from the Baizhangling volcanic basin; 4. volcanic rock from the ~tangshan volcanic depression.

7 6 5

M 3

1

�9 I �9

, 1 0 2 |

, i , l , i 6'0 I I0 20 30 40 50 70

La (ppm)

!

8O

Fig. 8. La/Sm-La diagram of granite plut ons in the Nanling region (Zhang Bangtong, 1983 ). I. Syntectic granite pluton; ll.Sontinental crust rcworking-type granite pluton. I. Granite pluton HM; 2. granite pluton M; 3. granite pluton ZG.

intermediate-acid and acid volcanic rocks in eastern Jiangxi and northern Zhejiang were formed from the remelting and eruption of continental-crust-derived sialic materi- al. This conclusion is in good agreement with the results of isotopic studies (see Table 2 ). As can be seen from Table 2, the initial STSr/S6Sr ratios in some volcanic rock samples from the area studied are within the range of 0 .7056- 0.7139, indicating that the volcanic rocks were formed as ~a result of shallow intrusion or eruption of the magma produced from remelting of sialic material. Therefore, the material sources and geneses of volcanic rocks of different types can be distinguished in terms of R E E

distribution patterns, various geochemical parameters and related diagrams.

Table L Initial sTgr / ~ ratios and isotopic sges (Ma)of volc~ir rocks

;r Sample Sample Rock type (87Sr / 86Sr ) Isotopic age or

No. No. locality epoch

1 CZ-216 P ujiang Porphyritic 0.707867 117.55 rhyolite

2 CZ-217 Tonglu Porphyritic 0.713411 94.99 rhyolite

3 CZ-226 Tonglu Rhyolite 0.713919 103.58

4* - - Jiande Volcanic 0.7121 +0.0027 Leat Jurassic rock

5* - - Jinhua Rhyolite 0.7089+0.00021 Late Jurassic

6* - - ~angshan Mortar 0.713 Late Jurassic rhyolite

7 CZ- 204 Shigaiwu Granite- 0.705618 149.30

porphyry

* Cited from the literature; the rest presented by the Central Lab of Chengdu College of G-eology.

Page 10: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

286 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol. 10

Application of REE in the Study of Uranium Deposits

The following regularities and characters can be generalized by comparing the REE geochemical characteristics of mineralized volcanic rocks with the non-mineralized equivalents :

1. In comparison with non-mineralized volcanic rocks, the mineralized ones are characterized by high EREE and low ~ ;LREE/ E H R E E . For example, in the Shouchang volcanic basin non-mineralized volcanic rocks have ~ REE values ranging from 184.98 to 413.27 ppm and E L R E E / E H R E E ratios varying from 2.15 to 5.054 while mineralized rhyolite has a E REE value as high as up to 725.3 ppm and a

L R E E / E HREE ratio of 0.34 ; in the Shimen volcanic depression non-mineralized magnophyric rhyolite has EREE values of 381.4 and 435. 15 ppm and E L R E E / E H R E E ratios of 2.91 and 3.991 while mineralized porphyritic rhyolite has a EREE value of 6 2 4 . 8 p p m and a E L R E E / E H R E E ratio of 1.70; in the Xiangshan volcanic depression non-mineralized mortar rhyolite has I2 REE values standing between 187 .40-215 .5 ppm and ~ L R E E / E H R E E ratios ranging from 2.01 to 2.73 while mineralized mortar rhyolite has ~REE values ranging from 209.9 to 2048.8 ppm and E L R E E / EHREE ratios reduced to 0 . 1 3 - 1.99 (Table 1 ).

2. In comparison with non-mineralized volcanic rocks, mineralized volcanic rocks are characterized by HREE enrichment so that their REE distribution patterns are ob- viously V- shaped ( Figs. 1, 2, 5and 6). As can be observed in the I2 La-Nd-E Sin- Ho- EEr- Lu diagram the data points of non-mineralized volcanic rocks are alllocated at the join ELa-Nd-~Sm-Ho and are concentrated at the apex ~La-Nd while those of mineralized volcanic rocks are plotted at the join ~La-Nd-I2Er-Lu. The stronger the degree of uranium mineralization is, the closer to the apex E Er-Lu the data points of the host rocks will be (Fig. 9 ).

~ S m - Ho

�9 1

+ 2

L a - Nd ~ E r - Lu

Fig. 9. ~ La- N d - ~z Sin- Ho - ~ E r- Lu triangular diagram of some volcanic and intrusive rocks fi'om Zhejiang and Jiangxi provinces (5, 6,10, 26, 29 and 30 represent the data points of U-mineralized volcanic rocks). 1. Zhejiang volcanic rock; 2. Jiangxi volcanic rock; 3. intrusive rock within the volcanic rock- distributed area. I. Projection field of Zhejiang volcanic rocks; II. Projection field of Jiangxi volcanic rocks and intrusive rocks from both provinces; III. Projection field of mineralized volcanic rocks.

Page 11: REE geochemical characteristics of volcanic rocks in Zhejiang and Jiangxi provinces and their geological significance

No.3 CHINE SE J O U R N A L O F G E O C H E M I S T R Y 287

Co-enrichment of uranium and heavy rare-earth elements in ore-bearing volcan- ic rocks is obviously related with their similar geochemical ehaviors. For instance, both of them can be dissolved in carbonic acid solutions and transported as carbonate complexes and the decomposition of these complexes may provide favour- able conditions" for their precipitation.

The data developed from uranium deposit Nos. 6122, 6124, 52, 9301 and 9302 show that the ~REE content of mineralized volcanic rocks is 1 . 5 - 2 times higher than that of non-mineralized ones. The EHREE content would be much higher. Simi- lar cases are also reported in other parts of the world. As pointed out by R. P. Taylor (1979), in the process of epithermal uranium mineralization the dissociation of uranium from carbonate complexes and its precipitation were accompanied by strong HREE (especially Tb and Dy) enrichment and intense LREE depletion. In their study of volcanic rock- associated uranium deposits R.P .Taylor and B. L. Fryer (1982) found that within the ore zone the Y REE content of mineralized volcanic rocks is 2.5 - 3.5 times higher than that of non-mineralized rhyolitic volcanic rocks in the surroundings (except for Eu) , and also pointed out that LREE and HREE are relatively enriched relative to M REE (Taylor , R. P. and Fryer, B. L. , 1985 ). REE addition during hydrothermal and metallogenic processes may account for the increase of EREE in uranium ores of hydrothermal origin. Therefore, the geochemical characteristics of rare-earth elements may shed light on the metallogeny of uranium.

Conclusions

From the above discussions we can come to the following conclusions: 1. The geochemical characteristics of rare-earth elements and strontium isotopic

composition show that acid and intermediate-acid volcanic rocks in the area studied were derived from the sial with respect to their material source and seem to have been formed from eruption and consolidation of the magma produced as a result of remelting of basement felsic metamorphic rocks under T-P rising conditions. There- fore, the rocks are high in Y:REE but low in 6Eu.

2. Significant differences are noticed between U-mineralized volcanic rocks and normal non-mineralized volcanic rocks in REE distribution patterns. The former is intensely enriched in HREE because of REE fractionation during magmatic and hydrothermal activities as well as of HREE addition.

3. The geochemical characteristics of rare-earth elements in volcanic rocks bear much information about petrogenesis, magmatic evolution and ore-forming material source, so they can be used as indicators to investigate the aspects concerned.

References Liu Y'ml~iun et al. (1984)ttement Geochemistry.Science Press, Bering, 194 - 216 (in Chinese ). Taylor, R. P. and Fryer, B.L. (1985) Explanation of the formation of hydrothermal deposits with the help of REE

geochemistry (translated by Nie Fengiun). Geology-Geochemistry, (5), 20-23(in Chinese). Tu G uangchi et al. (1984)Geochemistry,Science Press, Shanghai, 243 - 266(in Chinese ). Wang Jianfeng and Liu Feng (1987)Bulletin of Chengdu College of Geology, 14 (1), 1 - 8 (in Chinese).