Stratigraphic geochemistry of Upper-Middle Proterozoic Suberathem in northern Guangxi, China

Vol.14 No.3 CHINESE JOURNAL OF GEOCHEMISTRY 1995

Stratigraphic Geochemistry of Upper-Middle Proterozoic Suberathem in Northern

Guangxi, China*

VAN M I N G ( [ ~ ] ~ ) , LIU Y I N G J U N ( ~ ] ~ ) AND M A D O N G S H E N G ( ~ f ~ )

(Department of Earth Sciences, Nanjing University, Nanfing, ~10008)

Abstract: The Upper-Middle Proterozoic epimetaxnorphic rock series in nor thern Guangxi is a suite of volcanic-terrigenous elastic formations transforming in facies from mobile to stable, which is similar to Eparchean normal argillo-arenaceous sediments bo th

in petrochemistry and in REE geochemistry. The Upper-Middle Proterozoic Suberathem in nor thern Guangxi is characterized by primordial enrichment of As and Sb, significant depletion in Sr and Hg, bimodal or polymodal distr ibution of Au and other ore-forming elements, and high variation coefficients and strong mobilities of Sn, Cu, Ni, Au, Ag, As and Sb. The trace element association is characterized by the predominance of siderophile and chalcophile elements occurring in the Middle Proterozoic and tha t of lithophile ele-

ments occurring in the Upper Proterozoic. It is suggested tha t in nor thern Guangxi is developed a series of ore-bearing formations with the Sibao Group as the protogenous basement and the Danzhou Group and Lower Sinian series as the derivative cap strata.

K e y w o r d s : P r o t e r o z o l c E r a t h e m ; s t r a t i g r a p h i c g e o c h e m i s t r y ; t r a c e e l e m e n t ; Guangxi

The Sibao Group, the Danzhou Group and the Sinian system of the Upper-Middle Pro-

terozoic Suberathem in northern Guangxi are distributed in the area from Jiuwandashan

to Yuechengling, which are composed mainly of argillo-arenaceous rocks interbedded with

ultrabasic-basic rocks, eruptive rocks and minor carbonate rocks and siliceous rocks with a

total thickness of over ten thousand meters. They are possessed of green schist facies and flysch

formation features. Stratigraphically geochemical investigations in Hunan, Jiang-xi, Zhejiang,

Guangxi and Guangdong (Liu Yingjun et al., 1987) showed that in South China Proterozoic-

Cambrian basement ore-bearing formations are dominant and cap-derived ore-bearing forma-

tions come next. This paper is concerned with the problem whether the Sibao Group, the

Danzhou Group and the Sinian system as the Proterozoic active-type volcanic-terrigenous elas-

tic sedimentary formations, like the Lengjiaxi Group, the Banxi Group, Shuangqiaoshan Group,

ISSN 1000-9426 * This project was granted by the National Natural Science Foundation of China (9488010) and the Ph.D.

Foundation of the State Educat ion Commission. Translated by the Hua Z i h a o ( ~ : : ~ )

232 CHINESE JOURNAL OF GEOCHEMISTRY Vol.14

Shuangxiwu Group, Chencai Group, etc., are the important ore-bearing formations in the region

of South China.

Geological Characteristics o f the Upper-Middle Proterozoic S u b e r a t h e m

The Sibao Group in northern Guangxi is distributed in the vicinity of Mt. Jinwandashan

and on both sides of Mt. Yuanbaoshan, which is composed of epimetamorphic argillo-arenaceous

rocks intercalated with basic lava, komatiite, pyroclastic rocks and stratiform or stratoid basic-

ultrabasic intrusive rocks, constituting pelagic turbidite flysch and basic-ultrabasic volcanic for-

mations. Such features of turbidites as horizontal bedding, gradational bedding, basal washing,

etc. are well developed. The rock series has experienced regional metamorphism to low-grade

greenschist facies with a total thickness greater than 3840 m (the basement not observed, the

Baotan area). As viewed from its lithological assemblage, the Sibao Group is divided into the

Wentong Formation and the Yuxi Formation from the bottom to the top. The lower part of

the Wentong Formation is composed of grey, greyish-green medium- to fine-grained metamor-

phic feldspathic silicarenite, coarse- to medium-grained metamorphic sandstone, metamorphic

siltstone, phyllite and slate intercalated with minor stratiform, stratoid basic-ultrabasic intru-

sive rocks with a total thickness of over 655 m; its upper part consists mainly of dark grey,

greyish-green fine-grained metamorphic sandstone, metamorphic siltstone, basic lava, tuff and

komatiite with a total thickness of 2514 m intercalated with quite a number of layers of ma-

rine volcanic rocks. The Yuxi Formation is made up of greyish-green, grey moderately thick

to thin stratiform metamorphic argillaceous siltstone, slate, sericite phy]lite and metamorphic

fine sandstone intercalated with intermediate-acid eruptive rocks in local places with a total

thickness of over 671 m.

The age of the Sibao Group has not been known. In terms of the isotopic age (1063 Ma)

of the Bendong granodiorite which shows an intrusive relationship with the Sibao Group and

is unconformably overlain by the Danzhou Group, many research workers have assigned the

Sibao Group to the Middle Proterozoic. As compared with the Fanjingshan Group in Guizhou

Province, the Lenjiaxi Group in Hunan Province, the Jiuling Group in Jiangxi Province and the

Shuangxiwu Group in Zhejiang Province, the Sibao Movement is considered to be the Jinning

Movement. Komatiitic basalt samples collected from the upper part of the Wentong Formation

gave a Rb-Sr age of 1667 Ma (Guangxi Regional Geological Survey Team, 1987). Eight mafic-

ultramalic volcanic rock samples from the upper part of the Wentong Formation of the Sibao

Group gave a Sm-Nd isochron age of 2219 Ma (Mao Jinwen, et al., 1990). It is suggested

that the Sibao Group is the product of the Early Proterozoic rather than that of the Middle

Proterozoic. From the above it can be seen that the rock-forming age of the Sibao Group needs

to be further studied.

The Danzhou Group is distributed along the area from Jiuwandashan to Yuechengling,

which is composed of metamorphic argillo-arenacebus rocks intercalated with minor carbonate

rocks. In the area from Longsheng to Sanmen there are intercalated with quite a number of

layers of spilitic keratophyre and pyroclastic rocks with numerous lense-shaped, stratoid basic-

No.3 CHINESE JOURNAL OF GEOCHEMISTRY 233

ultrabasic rocks intruding along the bedding, constituting a suite of unique ophiolite mictites.

With a total thickness of about 968-4780 m, the Danzhou Group is generally in angular-

unconformity contact with the Sibao Group only with the exception of the Yuanbaoshan district

where the two groups show a parallel unconformity relationship. According to its lithological

character, petrographical facies and indicator seams, the Danzhou Group can be divided into

the Baizhu Formation, the Hetong Formation and the Gongdong Formation from the bottom to

the top. A conformable contact relationship can be seen among the three formations. The lower

part of the Baizhu Formation is composed of conglomerate and argillo-arenaceous rocks while

the upper part consists of carbonate rocks intercalated with argillaceous rocks, constituting a

sedimentary cycle from the bottom to the top with well-developed horizontal bedding. The

Hetong Formation is composed largely of metamorphic argillaceous rocks intercalated with

arenaceous rocks. The argillaceous rocks in the upper part often contain carbonaceous material

and its dark grey and black colors make it different from the lower part. Moreover, flysch

cyclothems are obviously developed in the upper part of the Hetong Formation. The Gongdong

Formation is built up of grey, greyish-green sericite slate, sericite phyllite and sericite quartz

phyllite intercalated with metamorphic sandstone, metamorphic feldspathic silicarenite and

metamorphic siltstone with extensive presence of distinct bands or stripes, horizontal, oblique

and cross beddings, well-developed moldic structures and remarked flysch cyclothems.

The Danzhou Group is also characteristic of flysch or flyschoid sedimentary formations. In

the upper part of the Baizhu Formation are developed relatively stable calcareous layers and in

the upper part of the Hetong Formation are developed black carbonaceous phyllite, slate and

intermediate-basic volcanic rocks with the lithological assemblage bearing a strong resemblance

to that of the Madiyi Formation of the Banxi Group in Hunan Province and of the Jialu and

Wuye formations of the Xiajiang Group in southeastern Guizhou, indicating that the Danzhou,

Banxi and Xiajiang groups in Guangxi, Hunan and Guizhou are highly comparable. The zircon

U-Pb age of 837 Ma for the stratoid augite-peridotite-gabbro-diabase pluton occurring in the

Hetong Formation of the Danzhou Group led us to consider that the Danzhou Group was formed

during the Late Proterozoic (Guangxi Bureau of Geology and Mineral Resources, 1989).

The Sinian system is of extensive distribution and consists mainly of epimetamorphic

conglomerate-bearing argillo-arenaceous rocks, sandstone, argillo-arenaceous rocks, siliceous

rocks and minor dolomite, belonging to epeiric sea-bathyal facies sedimentation. The Sinian

system is in continuous conformable contact with the overlying Cambrian system and the under-

lying Danzhou Group with a total thickness of 36{}-4859 m. The Sinian system is divided into

the upper series and the lower series with the former further divided into the Changan Formation

(composed mainly of stratiform conglomerate-bearing argillaceous rocks of great thickness and

conglomerate- bearing arenaceous rocks intercalated with metamorphic fine-grained sandstone,

siltstone and mudstone), the Fulu Formation (consisting chiefly of arenaceous rocks and argilla-

ceous rocks locally intercalated with lense- shaped dolomite) and the Silikou Formation (made

up of stratiform and massive conglomerate-bearing clastic rocks of great thickness) and the

latter into the Doushantuo Formation (composed of dark-colored shales, carbonaceous shales,


siliceous shales, siliceous rocks locally intercalated with sandstone and lense-shaped dolomite)

and the Laobao Formation (made up of siliceous rocks with minor carbonaceous shales or

carbonaceous-siliceous shales in its upper and intermediate parts).

The Sinian system in northern Guangxi is comparable with that in Hunan and Guizhou

provinces. The lower age limit of the basement (the Changan Formation) of the Sinian system

in northern Guangxi is 800 Ma, the boundary age between the upper and lower series of the

Sinian system is 700 Ma and the top boundary age of the Sinian system is 615 Ma 1).

P e t r o c h e m i s t r y and R EE G e o c h e m i s t r y

The petrochemistry and REE geochemistry data for the major types of rocks in t~e Upper- Middle Proterozoic Suberathem in northern Guangxi are listed in Tables 1 and 2.

T a b l e 1. C h e m i c a l c o m p o s i t i o n s a n d p a r a m e t e r s o f t h e m a j o r r o c k t y p e s in t h e U p p e r - M i d d l e P r o t e r o z o i c S u b e r a t h e m o f n o r t h e r n G u a n g x i

Strati- Chemical compost ion ( d r y , ~ ) P~r~meter grsphlc Rock type unit F ~ O / F e 3 + /

A l SiO 2 TiO 3 A 1 2 0 3 FeO F ' e 2 0 3 MnO MgO CaO Na20 K20 P205 (FoO-F (Fe3++

O H

M g O ) Fe 2 + )

0

Phyl l i te(1) 53 ,12 1 .01 23 .93 4 .88 3 .37 0 .14 5 .07 0 .13 0 .15 5 .69 0 . 0 2 0 .61 0 .38 0 .01 23 .7

Metamorphic 73 .45 0 .55 12 .80 4 .02 2 .36 0 .09 3 .07 0 .12 0 .60 2 .07 0 .06 0 .75 0 .35 0 .01 23 .3 sandstone(1) Metamorphic 6 7 . 5 8 0 .59 12.51 6 .49 3 .82 0 .15 1 .30 2 .12 3 .14 2 .20 0 .21 0 .88 0 .35 0 .14 21 .2

quartz dlorite(1) A i t e ~ d gahb~o 5 7 . 1 3 1 .18 14 ,63 9 . 6 0 4 . 3 8 0 . 1 8 3 .78 5 .26 1 .34 2 .43 0 .11 0 . 7 8 0 .29 0 , 3 6 12 .4

-diabase(3) Altered 5 3 . 8 2 0 .77 15.59 6 ,74 3 .76 0 .20 7 .26 5 .68 2 .74 1 .15 0 .06 0 .58 0 .33 0 .36 20 .3

basalt(2) Altered komati- 5 3 . 3 2 0 .74 14 .82 6 . 5 6 4 .05 0 .18 8 .60 7 .67 2 .54 1 .02 0 .07 0 .54 0 .36 0 .53 20 .0

litlc b ~ a l t ( 2 ) Altered augit ic 4 7 . 9 4 0 .81 9 ,86 10 .61 1.33 0 .17 2 1 . 1 7 7 .33 0 .30 0 .32 0 .11 0 .38 0 .10 0 .74 10.3

komati l te (3)

Metal;~orphlc 7 2 . 3 4 0 ,41 16 .91 1 .69 4 .86 0 . 0 7 0 .58 0 .13 0 , 1 5 2 .30 0 .02 0 .91 0 . 7 2 0 .01 4 1 . 2

sandstone(i) 0 r M e t a m o r p h i c 71 .53 0 .43 1 5 . 6 6 1.40 3 .73 0 . 1 4 1.61 0 .21 0 .93 3 .64 0 .11 0 .75 0 ,71 0 .01 3 6 . 4

~0 silt~ton, (1) ~ C - b e a r i n g 6 8 , 2 9 0 . 6 0 17 ,70 2 .22 3 .45 0 .05 1 .48 0 , 2 5 1 .50 4 .11 0 .02 0 .78 0 .72 0 .01 29 .5

phyll l te (1 )

Phynl te (1) 6 4 . 2 2 0 .48 2 1 . 9 0 1 .62 5 .94 0 ,11 1 .75 0 ,11 0 .29 3 ,48 0 .12 0 .80 0 . 7 7 0 .01 45.4.

Slate ( I ) 6 0 , 6 3 0 . 5 7 23 .71 1 ,67 3 .66 0 . 1 4 1 .49 0 . 1 0 0 .50 6 .26 0 .11 0 ,82 0 .75 0 .01 41 ,6

Sinlan Metsmorphlc 7 4 . 3 3 0 . 5 2 16 .00 1.31 2 ,87 0 .08 1 . I 0 0 .21 0 .09 3 .27 0 .09 0 . 7 8 0 . 6 6 0 .01 30 .8 sandstone (1 )

system Sl&te (1 ) 61 .20 0 .66 22 .03 2 .53 5 ,68 0 .14 2 .58 0 .11 0 .42 4 ,21 0 .13 0 .75 0 . 6 6 0.01 33 .4

An~lymed at the Experimental Center of Department of Earth Sciences; Nanjing University ; the numerals in the brackets &re the numbers

of samples,

(1) Differences in petrochemistry of the different types of rocks in the Upper-Middle Pro-

terozoic epimetamorphic rock series in northern Guangxi are closely related to the mineral

composition. The slates and phyllites have low SiO2 and high A1203 and K:O contents relative

to the metamorphic siltstones and metamorphic sandstones. All the samples show a petrochem-

ical trend of MgO> CaO, K20>NaO. By using the Niggli and Simonnan diagrams as well as

( a l - a l k ) - c diagram, we can see that all the samples fall within the range of normal sedimentary

1) Guangxi Bureau of Geology and Mineral Resources,1989, Paleogeography of the Sinian sedimentary facies in Guangxi and related mineral resources.

No.3 CHINESE J O U R N A L OF G E O C H E M I S T R Y 235

argil lo-arenaceous rocks.

Table 2. R E E contents (ppm) and parameters o f the major rock types in the Upper-Middle Proterozoic Subera them of northern Guangxi

Stratigraphic unit Sibao Group DavLzhou Group Sinian system Metam- C-

Metamorphic orphic Altered Altered Altered Metamor- bearing Metamorphic Rock type Phyllite sandstone quartz gabbro- basalt komatiitic phic phyUite sandstone

diorite diabase basalt sandstone

REE

La 56.45 34.30 25.61 21.05 13.00 12.14 33.93 37.08 34.14 Ce 102.80 68.87 52.46 42.67 24.34 25.36 65.18 73.91 55.71 Pr 13.30 8.40 7.13 6.06 4.01 4.21 9.03 8.83 8.81 Nd 48.47 30.87 27.16 21.68 14.10 13.55 32.45 30.01 32.49 Sm 8.90 6.30 6.75 5.02 3.37 3.27 6.41 5.61 6.77 Eu 1.47 1.17 1.53 1.18 0.69 0.87 1.22 0.96 1.34 Gd 7.43 5.56 7.82 5.91 4.48 4.56 5.38 4.73 6.02 Tb 0.99 0.77 1.27 0.86 0.63 0.67 0.79 0.78 0.90 Dy 6.19 4.80 8.26 5.51 3.92 3.90 5.32 5.28 5.67 Ho 1.31 0.98 1.78 1.17 0.80 0.85 1.08 1.17 1.20 Er 3.61 2.60 5.03 3.27 2.40 2.41 2.90 3.39 3.35 Tm 0.57 0.40 0.78 0.50 0.37 0.37 0.46 0.58 0.53 Yb 3.93 2.'57 5.00 3.17 2.32 2.40 3.00 3.60 3.45 Lu 0.61 0.39 0.76 0.48 0.35 0.37 0.47 0.54 0.53 Y 29.58 24.95 44.38 29.04 21.87 20.58 26.27 29.72 29.78 EREE 285.61 192.93 195.72 147.57 96.65 95.51 193.80 206.19 190.69

ELREE 4.27 3.48 1.61 1.96 1.60 1.64 3.25 3.14 2.71 ~ H R E E SEu 0.54 0.59 0.65 0.67 0.55 0.69 0.62 0.56 0.63

Para- 6Ce 0.82 0.89 0.86 0.84 0.75 0.79 0.82 0.89 0.71

meter (L~L~a)N~ 9.29 8.64 3.31 4.30 3.63 3.27 7.32 6.66 6.40

Analyzed at the Experimental Center of Department of Earth Sciences, Nanjing University.

(2) Altered basal t , komat i i t ic basal t and augit ic komat i i te in the Sibao Group in no r the rn

Guangx i are character ized by low TIO2(<0.9%), N a 2 0 (<4%) and K 2 0 a nd high MgO (7.26-

21.17%). Studies have shown t h a t this volcanic series is des ignated to a tholei i te-komati i t ic

basa l t -komat i i t e format ion, which is character ized by F e O * / ( F e O * + M g O ) < 0 . 6 5 , Fe3+/ (Fe 3+

+Fe2+)<0 .4 , CaO/A1203 < 0.82 and A1203/TiO2 approximate to the rat io in chondri tes (20.4),

typical of mant le-source volcanic rocks and comparable with bas ic-ul t rabas ic volcanic rock

format ions in m a n y Archaean greenstone bel ts t h roughou t the world. The most significant

difference in chemical composi t ion between the Sibao Group basic intrusive rocks and al tered

gabbro and diabase on one hand and volcanic rocks on the other hand lies in the fact t h a t the

former is low in MGO(3.78%) and high in FeO* (13.44%). Still lower MgO a nd CaO contents are

produced in me tamorph ic quar tz diorite, bu t increased values are not iced for SiO2 and ALK.

(3) The Upper -Middle Proterozoic ep imetamorphic series in no r the rn Guangx i is charac-

terized by L R E E enr ichment , obvious signs of f ract ionat ion, remarked negat ive Eu anomal ies

and R E E d i s t r ibu t ion p a t t e r n typical of Eparchaean sed imenta ry rocks (Fig . l ) . As it is con-

236 CHINESE JOURNAL OF GEOCHEMISTRY Vo1.14

trolled by the grain size of sediments, the EREE of slate and phyllite is higher than that of

metamorphic siltstone and metamorphic sandstone. The REE distribution patterns in meta-

morphic quartz diorite, altered gabbro-diabase, altered basalt, altered komatiitic basalt in the

Sibao Group of northern Guangxi show a strong resemblance (Fig.2), and the rocks have very

close ELREE/EHREE ratios, ~Eu and 8Ce values and (La/Yb)N ratios except for their EREE

contents, reflecting that they are products of the evolution of a cognate magma.

200

100

0 e-"

e~ O

10

200 x\ "~x,.

,x --~ I00

�9 "~ '~ X.

k . k " - - _ I , o , . . . . . .

~'-:";r l r,.) ~ \ "x - ! "~.." -..-._.

2 ~10 5

Ce Nd Eu Tb Ho Tm L u Ce Nd Eu Tb Ho Tm Lu i i I i 1 I I I i I i 4 I i i . i , i i i , . . I I I

La Pr Sm Gd Dv Er Yb La Pr Sm Gd Dv Er Yb

Fig.1. REE distr ibution pat terns in the epimetamorphic series in the Upper-Middle Proterozoic

Suberathern of northern Guangxi.

Fig.2. R E E d i s t r ibu t ion p a t t e r n s in t h e Sibao

G r o u p m a g m a t i c rocks.

T r a c e E l e m e n t C o n t e n t s a n d T h e i r D i s t r i b u t i o n C h a r a c t e r i s t i c s

Contents and distribution characteristics of trace elements

Au: The content of Au is not so high in the Upper-Middle Proterozoic Suberathem (Table

3) and the average contents of Au in the Sibao Group, the Danzhou Group and the Lower

Sinian series are estimated at 1.2x 10 -9, 1.6• 10 -9 and 2.9x 10 -9, with the variation coefficients

being 100%, 75% and 54%, respectively. In the different rock assemblages of the Sibao Group

metasedimentary rocks such as metamorphic sandstone, metamorphic siltstone, slate, phyllite,

etc. are relatively high in Au contents (1.1x10 -9- 2.1x10 -9) and basic-ultrabasic rocks are

extremely low in Au contents (only 4 xl0-1~ resulting in a higher content of Au in the Yuxi

Formation dominated by metasedimentary rocks than in the Wentong Formation. The Upper-

Middle Proterozoic Wentong--~Falu formations in northern Guangxi show a tendency of Au

increasing. The distribution characteristics of Au are shown in Fig. 3.

Ag: The contents of Ag in the Sibao Group, the Danzhou Group and the Lower Sinian

series average 6• 10 -8, 5 x 10 -8 and 5 x 10 -8, respectively, approximate or equal to the average

value of the upper continental crust (S.R. Taylor, 1985), and their coefficients of variation are


as high as up to 133%, 80~ and 125%, respectively. In the Sibao Group metamorphic siltstone has the highest Ag content (9• -s) and a large variation coefficient of 166%. The Hetong

Formation of the Danzhou Group has the highest content of Ag (8• -s ) and the largest variation coefficient (75%). The ~l lu Formation of the Lower Sinian series is higher in Ag

content and larger in variation coefficient than the Changan Formation.

Table 3. Trace e lement contents and statist ical parameters of different stratigraphic units in the Upper-Middle Proterozoic

Suberathem of northern Guangxi

Strati- Statis- graphic t ical unit para- Ti V Cr Mn Co Ni W Sn Mo Bi As Sb Cu Pb Zn Au Ag Hg Sr Ba

meter Sibao �9 4097 157 126 1127 18 30 3.1 5.3 0.5 0.30 71.3 2.0 45 36 94 1.2 0.06 28 46 453

G r o u p ( 2 8 ) C V 20 46 83 37 61 83 32 204 20 67 263 190 89 28 35 100 133 14 96 45 F s 1.37 2.62 3.03 1.88 1.80 1.50 I . I I 0.96 0.38 1.50 47.53 I0 .00 1.80 1.80 1.32 0.67 1.20 0.35 0.13 0.82

Wentong �9 4171 187 152 1267 21 32 2.8 6.3 0.5 0.20 63.8 1.5 31 37 100 1.0 0.07 27 51 400

Ft .(17) C V 19 40 81 33 57 94 36 217 0 50 348 93 55 30 37 130 129 11 92 48

Yuxi �9 3982 I I 0 85 912 914 27 3.6 3.8 0.6 0 . 4 0 8 2 . 9 2.8 66 34 86 1.6 0.05 29 38 48 F r . ( l l ) C V 22 35 48 33 21 48 25 37 17 75 139 288 80 24 26 50 80 14 100 37

3998 I I i 61 863 17 34 3.4 4.3 1.4 0.34 12.5 1.3 33 27 103 1.6 0.05 29 40 794

Danzhou C V 23 34 43 50 47 35 35 28 114 94 76 92 58 74 34 75 80 21 58 50 G r o u p ( 3 3 ) F s 1.33 1.85 1.74 1.44 1.70 1.70 1.21 0.78 1.08 1.70 8.33 6.50 1.32 1.35 1.45 1.78 1.00 0.36 0.11 1.44

Baizhu �9 4623 138 82 900 .23 39 4.3 4.5 0.6 0.33 13.8 0.7 34 33 106 1.0 0.03 28 24 558

1%.(14) C V 21 25 24 30 22 18 33 20 33 58 78 86 38 73 20 100 33 14 21 46 Hetong �9 3524 100 56 738 14 32 3.0 3.9 2.8 0.45 15.2 2.5 40 26 79 2.0 0.08 32 39 1127

1%.(11) C V 18 31 34 71 43 50 10 10 79 109 60 56 60 69 53 35 75 22 38 39 G o n g d o n g ~ 3559 80 31 971 12 29 2.6 4.7 0.8 0.20 6.6 0.9 23 17 132 2.2 0.03 26 69 750

1%.(8) C V 12 10 23 49 67 34 8 43 25 0 39 56 57 29 11 64 0 4 35 18

3152 76 36 870 15 38 2.5 3.6 0.8 0.29 8.9 4.3 34 32 112 2.9 0.05 29 72 855 Lower C V 24 18 22 55 25 74 12 17 22 44 171 130 41 43 41 54 125 9 44 58

Sinian F s 1.05 1.27 1.03 1.45 1.50 1.90 0.89 0.65 0.62 1.45 5.93 21.5 1.36 1.60 1.58 1.61 1.00 0.36 0.21 1.55

series(14) Changan �9 3364 78 37 821 13 22 2.5 3.7 0.8 0.27 3.9 4.6 28 26 86 2.3 0.04 29 89 792 1%.(9) C'V 17 7 22 25 17 21 13 17 17 31 76 144 21 45 9 58 19 9 27 30

Fulu ~ 2771 73 34 957 17 67 2.5 3.4 0.8 0.34 18.0 3.6 45 43 158 4.2 0.08 30 42 969

1%.(5) C V 31 29 21 77 25 42 10 15 29 51 126 65 40 19 31 30 123 9 47 77

Analyzed at the Lab. of Team 814 under the East China Non-ferrous Geological Explorat ion Bureau. The numberals

in the brackets are the numbers of samples , ~ - m e a n value; C V - v a r i a t i o n coefficient; F s - d e g r e e of enrichment relative

to the average value of the upper continental crust (S. R. Taylor , 1985). F r . - F r o m a t i o n . Au and Hg in ppb and the

other e lements in ppm.

As, Sb: The enrichment degrees of As and Sb in the Sibao Group (relative to the average

values of the upper continental crust) are 47.53 and 10.00 and the variation coefficients are 263~ and 190%, respectively; those of As and Sb in the Danzhou Group are 8.33 and 6.50 and

the variation coefficients are 76% and 92%, respectively; those of As and Sb in the Lower Sinian

series are 5.93 and 21.50 and the variation coefficients are 171~ and i30%, respectively. All this goes to show that As and Sb are the elements inherent in the Upper- Middle Proterozoic

Snberathem in northern Guangxi. As compared with other rock assemblages, the Sibao Group

metamorphic sandstone and metamorphic siltstone have the highest contents of As and Sb and


the largest variation coefficients. The highest As and Sb contents are reported in the Hetong

Formation of the Danzhou Group. As compared with the Changan Formation, the content of

As is relatively high in the Fulu Formation, with a larger variation coefficient. However, in the

same formation Sb shows an opposite trend.

25 - 25

20 - / [~ (a) 20 1 (b) ~r~ (c)

15~ II I~ o

10 ~. 10 20

, I M I I H h 0

I l , l I I , , i I I I I i , I I

-0. 58-0. 34-0. 100. 140. 38 -0. 58-0. 34-0. 100. 140. 380. 620. 86 0. 020. 260. 500. 74 lgAu lgAu lgAu

Fig.3. The log frequency distribution histograms of gold contents in the Upper-Middle Proterozoic Suberathem of northern Guangxi, a. Sibao Group; b. Danzhou Group; c. Lower Sinian series.

Sn, Cu, Ni: The enrichment degrees of Sn, Cu and Ni in the Sibao Group (relative to the

values of the upper continental crust) are 0.96,1.80 and 1.50, with the variation coefficients

being 204%, 89% and 83%, respectively. In the Sibao Group basic-ultrabasic rocks the contents

of Sn, Cu and Ni are highest and the variation coefficients are largest, just in coincidence with

the extensive occurrence of Sn-polymetallic mineralization in the Sibao Group basic-ultrabasic

rocks in northern Guangxi. A slight decrease is found in the enrichment degree of Sn, Cu and

Ni in the Danzhou Group and the Lower Sinian series. In addition, the distribution range of

their contents is relatively constant and the variation coefficients are relatively small.

Hg, Sr: The enrichment degrees of Hg and Sr in the Sibao Group (relative to the values of

the upper continental crust) are 0.35 and 0.13 and their variation coefficients are 14% and 96%,

respectively; those of Hg and Sr in the Danzhou Group are 0.36 and 0.11 and the variation

coefficients are 21% and 58%, respectively; those of Hg and Sr in the Lower Sinian series are

0.36 and 0.21 and the variation coefficients are 9% and 44%, respectively. This indicates that

Hg and Sr are of obvious depletion in the Upper-Middle Proterozoic Suberathem in northern

Guangxi. The elements Hg and Sr are of strong activity in groundwater so that these two

mobile elements can migrate on a large scale in the horizontal and vertical directions under the

action of geothermal water (Ma Dongsheng and Liu Yingjun, 1991). The remarked depletion

of Hg and Sr in the Upper-Middle Proterozoic Suberathem in northern Guangxi suggests that

it experienced strong reworking by geothermal water after its formation and such reworking

process is precisely connected with mineraliT.ation. In view of the high variation coefficients

of Au, Ag, As, Sb, Sn, Cu and Ni in the Upper-Middle Proterozoic Suberathem in northern

Guangxi, it can be considered that these elements were affected by later reworking processes

and hence they belong to the strongly mobile elements, implying that northern Guangxi is not

only a Sn-polymetallic metaUogenic belt, but also probably a Au-Ag-As-Sb metallogenic belt.


As for trace element ratios, Sr/Ba, Co/Ni, and Au/Ag ratios in the Sibao Group are 0.10,

0.60 and 0.02; those in the Danzhou Group, 0.05, 0.50 and 0.03; those in the Lower Sinian

series, 0.08, 0.39 and 0.06, respectively. As compared with normal marine strata, Sr/Ba ratios

in the Upper-Middle Proterozoic Suberathem in northern Guangxi are biased toward lower

values, reflecting the loss of Sr due to leaching from the strata during late-stage superimposed

reworking processes. Co/Ni ratios show a tendency of decreasing and Au/ Ag ratios show a

tendency of increasing in the order of the Sibao Group --* the Danzhou Group --* the Lower

Sinian series.

Trace element correlations

The results of the R-type cluster analysis of twenty trace elements in the Upper-Middle Pro-

terozoic Suberathem are shown in Fig.4. Within the range of critical correlation coefficients,the

following three trace element associations are recognized in the Sibao Group: Au-W-Ba, Sb-

Bi-Pb-Mo and Sr-Cr-Co-V-Mn-Ni. The correlation system of Au can reflect to some extent

the existing forms of Au in a sedimentary formation. In the Sibao Group Au shows no corre-

lation with all elements except for W and Ba, reflecting that Au exists mainly in the form of

irregularly distributed independent Au minerals. This form of Au is chemically active and is

easy to remobilize and transport during metamorphism. Sr can, together with the siderophile

elements Cr, Co, V, Mn and Ni, form a correlation group, demonstrating that a considerable

portion of Sr is present in the Sibao Group basic-ultrabasic rocks and there has occurred strong

isomorphic replacement between Sr 2+ and Ca 2+.

1.=0 0.8 0.6 0.4 0.2 0 1~0 0.8 0.6 0.4 0.2 0 1 0 0.8 0.6 0.4 0.2 0 R ,' ~ ' ,l , ~ R , , i~ ~ "='" R ~ ' I [ =' '

I I Au I Ti , I A u

A g I ....

I t g ~ ~-r _.J

Sb ~ Ni Bi ' " ~ I L Co------'-'--~ ~ I Pb - ~---~ u

z . - - - - .

~nC~ P , . j ~ . ~ k ! I ,~g~s �9 I ~I ~Sb 1

Sr t Hg n=28 Ro.os=0. 37 n=33 R o . o s = 0 . 35 n=14 . R o . o s = 0 . 5 3

(a) (b) (c)

Fig.4. R-type cluster analyses of trace elements in the Upper-Middle Proterozoic

Subera them of nor thern Guangxi .

a. Sibao Group; b. Danzhou Group; c. Lower Sinian series.

No correlation is found between Au and any other element in the Danzhou Group and

Au is a strongly independent element. This aspect is similar to the situation encountered in

the Sibao Group. As can be seen from Fig.4, there are three major correlation groups in the

Danzhou Group: Ti-V-Cr-W-Co, Mn-Ni-Zn and Hg-Mo-Ag-Sb-As. The first correlation group

is an association of lithophile elements, the second is lithophile, siderophile and chalcophile


in nature and the third is a typical association of chalcophile elements. The recognition of a

lithophile element association in the trace element correlation groups is an important feature

which makes the Danzhou Group differ from the Sibao Group.

As compared with the Sibao and Danzhou groups, there are more remarked correlations

between Au and other elements, especially chalcophile elements in the Lower Sinian series of

northern Guangxi. As, Bi, Cu, Zn and Ni are involved in the critical correlation coefficients.

As can be seen from Fig.4, there are two major trace element correlation groups, i.e., Au-Bi-Pb

and Ag-Mn-Co-Zn-Ni-Cu, the former being a chalcophile element association and the latter

a siderophile-chalcophile element association. The Lower Sinian series is the most important

Au-bearing horizon in the region of northern Guangxi. The major ore-host formation is the

Changan Formation. Correlation analysis shows that in the Changan Formation Au is correlated

with the chalcophile elements Ag, As, Bi, Sn and Pb and the trace element correlation groups

are Au-Pb-Ag-Bi-As-Sn and Zn-Cr-V; in the Fulu Formation Au is correlated only with Ni

and the trace element correlation groups are Au-Ni, As-Cr, Sb-Pb and Ag-Mn-Co, which are

relatively scattered.

Ore-bearing formations

As a complex suite of volcano-sedimentary formations, the Sibao Group has the following

features: (1) Although the average abundance of Au in the Sibao Group is low, the possibility

could not be ruled out that there exist some Au- host horizons. For example, clastic sediments

interstitial to lavas contain as much Au as 5.9• 10 -9 and part of the altered gabbro-diabase and

komatiitic basalt contains as much Au as 7.0• 10-9-1.6x 10 -s. Gold of this kind is the strongly

mobile "visible gold" which would contribute a lot to the formation of meta-hydrothermal gold

deposits. Like other Au-bearing formations, Au in the Sibao Group is of polymodal distribution.

(2) The trace element associations in the Sibao Group are characterized by siderophile and

chalcophile elements. Au is weakly correlated with other elements. (3) The elements Sn, Cu,

Ni, Au, Ag, As and Sb in the Sibao Group have relatively large variation coefficients, belonging

to strongly mobile elements, of which Sn, Cu and Ni are related with the basic-ultrabasic

rocks and Au, Ag, As and Sb are correlated mainly with the Sibao Group meta~sedimentary

rocks. (4) In northern Gu~ngxi is extensively developed mineralization of Sn, Cu and Ni, as is

observed in Jiumao, Yidong, Shaping, Honggang, etc. The Dapoling Cu-Ni deposit is the only

one formed from a komatiitic magma in China. (5) In recent years, with strengthening work

on the geology of gold deposits, some gold prospects or occurrences have been discovered in

the Wentong Formation of the Sibao Group 1), such as those in Diwu, Wentong, Tongmakou,

etc. Genetically, the gold deposits belong to the meta-hydrothermal sulfide-quartz vein gold

deposits.

From the above discussions it is considered that the Sibao Group in northern Guangxi is

a primary ore-bearing formation characterized by a multi-element association of Sn-Cu-Ni-Au-

1) Guangxi Regional Surveying Team, 1990, A preliminary study on the auriferous potential of the Precambrian system in northern Guangxi.


Ag-As-Sb. It provided a material source not only for the formation of Au and Sn deposits,

but also for the Danzhou Group and Lower Sinian derivative ore-bearing formations. Previous

studies on the ore-bearing formations showed that they are often associated with sedimentary

formations, of which the volcano-sedimentary and terrigenous clastic sedimentary formations

are most important (Liu Yingjun and Ma Dongsheng, 1991). A variety of ore-bearing formations

would be formed within the same area. They possess obvious inheritance both in temporal-

spacial relationship and in composition, thus constituting an evolution series associated with

one or more derivative ore-bearing formations with the oldest protogenous ore-bearing forma-

tions as the background. So long as the Upper-Middle Proterozoic Suberathem in northern

Guangxi is concerned, the Sibao Group should belong to the active-type volcano-sedimentary

primary ore-bearing formations and the Danzhou Group and the Lower Sinian series should

belong to the active-type and stable-type terrigenous clastic sedimentary derivative ore-bearing

formations. The inheritance among the Upper-Middle Proterozoic ore-bearing formations in

northern Guangxi is reflected in such common element geochemical characters as the primary

enrichment of As and Sb, the obvious depletion of Sr and Hg, the bimodal or polymodal distri-

bution of Au and other ore-forming elements and the relative large variation coefficients of Au,

Ag, As and Sb. In addition to the inheritance from the Sibao Group -* the Danzhou Group

--* the Lower Sinian series, there is still an evolutionary trend, as is reflected by a decrease in

contents of the siderophile and chalcophile elements Ti, V, Cr, Mn, Co, Sn, As, etc. and an

increase in contents of the chalcophile and oxyophile elements Sb, Au, Sr, Ba, etc.

The Au-Ag-As-Sb derivative ore-bearing formations in the Danzhou Group and the Lower

Sinian series are also supported by the data concerning rock assemblage, elemental geochemistry

and mineralization characters. (1) Developed in the Danzhou Group are calcareous phyllite,

black carbonaceous slate or phyllite and carbonate interbeds while in the Lower Sinian se-

ries are developed conglomerate-bearing argiUo-arenaceous rocks intercalated with lense-shaped

carbonate rocks. These strata often contain high ore-forming elements and hence serve as the

important ore-host strata in the Danzhou Group and the Lower Sinian series. (2) The Danzhou

Group and the Lower Sinian series are strongly enriched in As and Sb. Although the average

content of Au is not too high, there are Au-bearing horizons in local places (the highest content

of Au is up to 5.2• -8) and Au contents are of bimodal distribution as can be seen in the

log frequency distribution histogram. In regard to trace element associations, there are not

only chalcophile but also lithophile element associations. (3) An increasing number of Au and

Sb mineralization prospects have been discovered in the Danzhou Group and the Lower Sinian

series, as exemplified by those occurring in the Gongdong Formation (including the Pingdeng

Au deposit and the Gangnan Au prospect at Longsheng, the Heigang Sb prospect at Luocheng

and the Dilin Sb prospect at Longsheng) as well as in the Changan Formation (including the

Gaobang and Tongmushan Au prospects at Sanjiang, the Banhe, Paotian, Laozhai, Fenshuiao,

Yangtouping and Maling Au prospects at Longsheng and the Jiujiangcun Sb prospects at Lu-

ochenghuanqun).

242 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.14

References

Guangxi Bureau of Geology and Mineral Resources,1985, Regional Geology of Guangxi: Beljing, Geological Press, p.18-20 (in Chinese).

Liu Yingjun and Ma Dongsheng, 1987, Geochemical characteristics of the Au-bearing formations in South China: Contributions to Geology and Mineral Resources Research, v.2, p.1-4 (in Chinese).

Liu Yingjun and Ma Dongsheng, 1991~ Geochemistry of gold: Beijing, Science Press, p.195-206 (in Chinese). Ma Dongsheng and Lin Yingjun, 1991~ Geochemical characteristics and genesis of strata-bound gold deposits in

the Jiangnan gold metallogenic belt: Scientia Sinica (edition B), v.4, p.424--433 (in Chinese). Mao Jingwen, Zhang Zongqing and Dong Baolin~ 1990, Sm-Nd isotopic chronology of the Sibao Group at the

south margin of the Jiangaan Old Land: Geological Review, v.36, p.264-268 (in Chinese).

Documents

Stratigraphic geochemistry of Upper-Middle Proterozoic Suberathem in northern Guangxi, China