Vol. 2 No.3 G~OCH~MISTRY 1983

Gold Abundance in Granitoids in Southern China

Lw YINGJUN (~J~~), ZHANG ]INGRONG (5*Jl:~),

QlAO ENGUANG (1¥=.w. J't ) , WANG MANYUN (£~~)

AND MAO HUIXIN (=§:f;~)

(Department of' Geology, Nanjing University)

Abstract

Gold abundance in granitic rocks of different geological periods in southern China hasbeen estimated. A review of the quantitative data available indicates that unaltered graniticrocks have a rather restricted range in gold content, rarely exceeding 4 ppb and generallyranging from 1.4 to 3.3 ppb, The mean gold content tends to decrease from basic to acidicgranitoids. This tendency suggests that gold isnot concentrated in the residual silicate melt­during the formation of granitic rocks. It is necessary to establish the background valuesof gold for various rock types although it seems that gold abundance data for the graniticrocks of southern China can necessarily provide any geochemical clues or guides to areas fa­vorable or unfavorable for gold mineralization.

Introduction

Trace-element geochemistry of granitoids in southern China has been well documented,and considerable amounts of data are availablel'J. Nevertheless, no quantitative data on thedistribution of gold in the granitic rocks in this region are available due to difficulties encou­ntered in detecting the nanogram quantities of gold in rocks (gold in unaltered igneous rocksoccurs in several ppb concentration range.)

In this work the determination of gold was effected by a combined chemical concentra­tion (activated charcoal-adsorption) -ernission spectrometry, which has the advantage of de­tection sensitivity over other techniques available in China!".

In the case of 10-9 g sample, the sensitivity can reach 1 ppb, and of 20 g sample, upto 0.5 ppb, thus being completely satisfied for the determination of gold abundance in gra­nitic rocks.

42 representative samples, taken from unaltered and unweathered fresh rocks, havebeen studied in detail in respect to their geology, petrology, geochronology and origin.

Gold Abundance in Granitic Rocks of Various Geological Ages

Gold abundance data for granitic rocks of various ages are summarized in Table 1 andFig. 1. By comparing the results obtained by several different data-processing methods it canbe seen that all the data are generally consistent to one another with the exception of arith­metic means (x) which are found slight higher.

Gold abundance data for granitic rocks of various ages in southern China show that itscontent varies within a considerably narrow range, regardless of rock type or provenance.

Tab

le1.

Th

ed

iatr

ibu

tion

of..o

ldin

..ra

nit

icro

cka

ofva

riou

aa

,ea

inao

uth

ern

Ch

ina

Age

Do

ng

'an

Xu

efeu

gE

arl

yL

ate

Her

cyn

ian-

Earl

yL

ate

(Sip

u)

Ca

led

on

lan

Cal

edo

nia

nIn

do

smia

nY

ensh

ania

nY

ensh

ania

n

No.

ofro

ckbo

dies

22

34

612

13

No.

of

sam

ple

s6

68

711

3436

Ari

thm

etic

mea

n(x

)2.

22.

22.

13.

61

.44

.84.

1

Au

,G

eom

etri

cm

ean

(xg

)1

.9~.O

2.0

3.5

1.4

4.1

2.9

Mod

e(M

o)

1.9

l.fi

1.5

2.4

1.3

'J.61

.4pp

bM

edia

n(M

e)1

.91

.81

.93

.51

.33

.92.

H

Lu

mp

edp

aram

eter

mea

n2.

01

.91

.9B

.il

1.4

4.1

2.7

Mea

nsq

uar

ed

evia

tio

n1

.61.

52.

74.

10.

54

.13

.,

Co

effi

cien

tof

var

iati

on

(Cv%

)80

7914

212

436

100

119

Ben

do

ng

'Xue

unW

ugou

gsha

nS

hang

you

Gui

dong

To

ng

luZ

ho

ng

ten

g

F'a

nji

ng

shan

Mo

tian

lin

gC

izhu

Dan

ing

Gup

osha

nD

amao

shau

Ku

iqi

Roc

kbo

dyin

ves

tig

ate

dN

apen

gT

iesh

anlo

ng

Yu

nx

iao

Sh

izh

uy

uan

Qia

nm

uti

an

Den

gp

ux

ian

Xia

ng

hu

apu

Tab

leZ

.G

old

con

ten

t.of

ao~e

inte

rmed

iate

-baa

icro

cka

inao

uth

ern

Ch

ina

Roc

kty

pe

Au

(pp

b)

No.

ofsa

mp

les

Lo

cali

ty

Bio

tite

dio

rite

9.0

3D

aniu

g,

Gua

ngxi

Bas

icv

ein

rock

inm

etam

orp

hic

rock

s42

.44

'I'a

ohua

,G

uang

xi

Web

ster

ite

gab

bro

92.5

1L

elg

on

gx

jan

,H

un

an

Lam

pro

ph

yre

315

1X

iush

ao,

Hu

nan

Basa

lt·

100.

....50

0A

nji

ang

,H

un

an

Bas

icre

eks"

300.

....60

0Y

uan

lin

g,

Hu

nan

*Geo

logi

cal

Tea

mN

o.23

7,H

un

anG

eolo

gica

lE

xp

lora

tio

nC

orp

ora

tio

n,

Min

istr

yo

fM

etal

lurg

ical

Ind

ust

ry.

~ Q t;:j o o t:I: t;:j

~ H r:JJ ~ ~ ~ I!:l

~45

--. 1

"J 2

(6 I 3

4

No.3 GEOCHEMISTRY

I

Dongan v I

(61 II

Xuefeng V I

(61 II

Early Caledonian v I

'8 ) II

Late Caledonian v'(7) I

I

Hercynian- Indos inian v II

1\\1 I

Early Yenshanian,V

(341I

Late Yenshanian I V( 36'

,Au (ppb) 0.6 2 3 5 10 20 30 60

Fig. 1. Diagram showing the distribution of gold in various types of granitic rockin southern China. The abscissa is the logarithmic one.

1. Varying range of gold content; 2. average content of gold; 3. number ofsamples; 4,. go!rl-abundanee line for the earth's crust and acidic rocks.

-14 26 66Xuef eng

-6\ 43 I ~7 --16 5·1 \5·1 (.'00-')

Early Cu ledonian I.ate. Calelloni<t"

//'

:~1~ 30'"

120J"'" 10

1)- 1....---"---"-'

-\6 III 36lIerrynlan- [ndosjruun

~l%

-·19 4\ 13\ 221

Early Yens hanian Late Ye nsh anian

Fig. 2. Histograms showing the frequeney distribution of logarl thrnic values for goldin the granitie rocks of southern China (the abseissa denoting the logarithmic values

of gold, in ppb).

Except for early Yenshanian granites whose gold abundance is generally close to the Clarke..value of the earth's crust and the average gold content of acidic rocks, the rest are all extremelylow in gold abundance (within the range of 1.4-3.3 ppb ) as compared with the Clarke valueand the average gold content of acidic rocks. The average gold abundance (3.3 ppb ) in lateCaledonian granites is slightly lower than that in early Yenshanian granites. Except for thegranites of these two geological periods ( i.e., early Yenshanian and late Caledonian) whosegold abundances are relatively high, granites of other geological ages are all characterized bylow gold abundance. Of them, the gold abundance in the Hercynian-Indosinian granitic rocks

246 GEOCHEMISTRY Vol. 2

is the lowest and so is the degree of gold dispersion; gold abundances in the Dong'an, Xue­feng and Early Caledonian granitic rocks are highly close to one other, although the latterhas a maximum coefficient of variation.

The data presented in Table 1 indicate that the gold distribution patterns for the Xue­feng, Caledonian and Yenshanian granitic rocks show different degrees of positive bias (Mo

< Me < X), while gold in the Dong'an and Hercynian-Indosinian granitic rocks displaysa normal distribution as indicated by its frequency distribution diagram (Fig. 2).

Gold Abundance in Various Types of Rocks

Granitic rocks in southern China are characterized by diverse type, complicated petro­logy, and close connection between the gold content and the petrographical composition.

Examinations of some intermediate and basic igneous rocks in southern China show thattheir gold contents are considerably high, some even more than 100 times higher the Clarkevalue of the earth's crust (Table 2), and still higher than those reported in literature [3,5,7].

This situation is particularly evident in the areas of Hunan and Guangxi.

In striking contrast to the intermediate and basic igneous rocks mentioned above,granitic rocks are precisely low in gold. Nevertheless, it is still obvious that the gold contentsof these granitic rocks tend to vary with rock type. As for the granitic rocks differing inpetrological characteristics, their gold contents tend to decrease in the order: intermediate­acidic - acidic- alkaline granites (Table 3).

Table 3. Average gold contenta of various type. of granitic rock. (Yenshanianin age) in southern China

Rock type Au(ppb) No. of samples Locality

Quartz monzodiorite 4.3 3 Tonglu, Zhejiang

Granodiorite-porphyry 4 ., 1 Fujiawu, Jiangxi

Biotite granite 3.1 8 Damaoshan, etc., Jiangxi

Alkali-granite 1.7 '3 Gushan, F'ujian

From the foregoing discussion, it can be seen that gold is distributed in such a mannerthat its average content decreases from basic, intermediate to acidic, alkaline rocks, indica­ting the siderophile nature of gold.

Similarly, in some composite plutonic bodies of multiperiod-and multistage-origin, thegold contents still tend to decrease from older to younger varieties. The Daning rock bodyin Guangxi is an outstanding example (Table 4).

Even in the same rock body, the distribution pattern of gold also varies from one petro­graphical facies to another. As for some typical magmatic granites, the gold contents tend todecrease from the marginal to the central facies. Taking the Fujiawu rock body for exam­ple, we can see that its marginal facies contains more gold (12.5 ppb ) than the central facies(4.2 ppb). .

No.3 GEOCHEMISTRY

Table 4. Averqe gold contenb of rocks of various petroa'enetic staa'es in theDaning rock body

247

Rock type Au(ppb) Evolutionary stage of the rock body

Biotite diorite g.O Early

Porphyritic granodiorite 3.7

tBiotite granite 1.7

Granite-aplite 1.3 Late

All the data are sufficient to show that gold has no tendency to become concentrated inthe residual silicate melt during magmatic differentiation and crystallization. This indicatesthat the geochemical behaviour of gold is distinct from that of the typical lithophile ore- for­ming elements, W, Sn, Be, Nb, Ta, etc. in the granitic rocks of southern China!".

Gold Abundance in Granitoids of Different Origins

Granitic rocks in southern China are of multi-originl'J. In comparison to typical meta­somatic-reworked granitic rocks, typical palingenetic-remelted magmatic granites have moregold (Table 5). This difference is attributable, on one hand, to the difference in materialsource and petrogenetic mechanism and, on the other, to the difference in regional or localgeological setting.

Table 5. Averqe gold contents of the two broad types of granitic rocks differingin origin from southern China

Genetic type No. of rock No. of Au(ppb) E."'ramplebodies samples

Palingenetic reformed-remelted 8 27 5.0 Tonglu, Zhongteng,magmatic granitic rock Lianhuashan, etc.

Metasomatic-reworked 5 15 1.2 Wugongsb.an, Damaoshan,granitic rock I Napeng, etc.

In regard to the same rock body or the same region, gold content tends to decrease

Table 6. Variation of gold content during granitization

Locality Rock type Au(ppb)

Migmatized metamorphic rock g.OBanded migmatite 1.2

Wugongshan, JiangxiOcular migmatite 1.2Migma tic granite 1.1

Locality Rock type Au(ppb)

Biotite monzonitic migmatie gneiss 27.0Light-color folded migma.ti te 20.0

Jinjiang, FujianMetaporphyritic monzonitic granite 2.5Migmatic granite 1.7

248 GEOCHEMISTRY Vol. 2

with increasing extent of granirization (Table 6), and this demonstrates that granitizationwould make gold become depleted.

From this, it may be concluded that granitization (especially earlier stages of graniti­zation of older age) is usually considered as the source of gold for mineralization by itself.

Gold Contents of Major Rock-forming' Minerals in Granitic Rocks

The gold content of monominerals in the rocks varies over a wider range than that ofwhole-rock samples simply due to the influence of some factors, such as impurities in theanalyzed minerals. Examinations of some selected major rock-forming minerals show that.gold tends to be higher is ferromagnesian minerals than in salic minerals from the gra­nitic rocks in southern China. In the salic minerals the highest average content of gold isnoticed in quartz, and similar gold contents are found both in plagioclase and K-feldspar.Gold in biotite varies over a wide range, even reaching up to 8.6 ppb; on the contrary, goldin hornblende and plagioclase is relatively constant. This implies that gold in biotite ishighly non-uniformly distributed, whereas it appears to occupy certain positions in thecrystal lattices of hornblende and plagioclase. Given in Table 7 are the average gold contentsof the major rock-forming minerals from the main body of the Daning granite-·-a porphy­ritic granodiorite in Guangxi selected in our study in consideration of its variety in minera­logy and typification in petrological character.

Table 7. Averqe contents of pld in major rock-formi~ mineral. in the Dani~

lrI'anite in Guanpi

Mineral No. of samples Average (ppb)

Plagioclase 2 2.6

K-£eldspar 2 2.8

Quartz 2 3.7Hornblende 2 4.4Biotite 3 6.1

The fact that the gold content of ferromagnesian minerals is higher than that of salicminerals in major rock-forming minerals essentializes the tendency of magmatic evolutionand why the gold content of intermediate-basic rocks is higher than that of granitic rocks asdescribed above. The decrease of gold in the granitic rocks of late periods and stages seemsto keep pace, to a large extent, with the increase of salic minerals.

As to its mode of occurrence in the granitic rocks, gold seems most likely to exist in theform of atom or native gold in the lattice or .in the crystal defects, and tends to be preferen­tially concentrated in ferromagnesian minerals, such as biotite and hornblende on accountof its high ionization potential, electronegativiry and oxido-reduction potential. In addition,gold tends to occur in some other minerals (e.g, quartz) due to the fully-filled electro';

orbit of Au".

Relationship between· Rock Gold Abundance and Gold Mineralization

It is commonly held that the higher the abundance of ore-forming metals in granitic

No. :I GEOCHEMISTRY 249

"bodies, the greater the possibility for these metals to be further concentrated into ore-gradelevels. However, such a speciality of ore-genesis in respect to gold in granitic bodies has notyet been well documented. The available data on gold abundance indicate that there is nosimple dependence of primary gold abundance in granitic rocks on gold mineralization. Itis just the geochemically siderophile nature of gold that makes it display no remarkable fe­atures of inheritance-evolution in the development of granitic rocks of various ages. Morerecent studies have shown that gold mineralization in southern China is characterized bymulti-period and multi-stage, i.e., the reworking of source beds by emplacement and meta­morphism of granitic rocks of various periods may give rise to gold ore deposits. Gold mi­neralization is also recognized in some younger granites formed during later stages in gold­bearing-bed-distributed areas, although gold ore deposits are more closely related in genesisto the older Xuefeng and Caledonian granites [6] .

Studies of gold abundance in the granitic rocks of southern China may lead us to arriveat such an important notion that gold abundance in unaltered granitic rocks of variour agesis generally low relative to its Clarke value in the earth's crust, and high values relative tothe Clarke are considered as exceptions. All the data suggest that the concentration of goldnever reaches the level of economic importance during the petrogenetic process of graniticrocks in southern China and gold mineralization developed in the granite bodies are, with­out any exception, related to late alterations.

The analysis of gold on the samples of unaltered rock in the gold ore-bearing part ofthe Daning rock body shows that the gold content of the porphyritic granodiorite closelyrelated to mineralization obviously decreases during chloritization and sericitization, andincreases rapidly in the pyrite-sericite-quartzitized zone (Table 8).

Table 8. Variation of &'Old content in altered rock.

Rock type

Fresh porphyritic granodiorite

Chloritized porphyritic granodiorite

Serici tized porphyritic granodiorite

Pyrite-sericite-quartzitized porphyritic granodiorite

Pyri te-sericite-quartzi te

Pyritized mylonite

Au(ppb)

3.7

1·7

37.951.5

2100

From the above, it can be concluded that in a given member of a rock body or in a gi­ven rock type from a certain region obviously higher or lower gold abundances than backg­round levels' are the manifestation of local anomalies related to late superimposed alterationor remobilization. This primary conclusion is generally concordant with the conclusions ofprevious workers!":

On the basis of data from southern China, it seems that gold abundance data for unal­tered granitic rocks alone do not provide unique or reliable geochemical criteria for identi­fying regions favourable or unfavourable for gold deposits, and, however, it is necessary forone to get a deep understanding of the difference between gold and other elements suchas W, Sn, Be, etc. in respect to their respective geochemical behaviours.

Gold abundance data for granitic rocks widespread in southern China are of great im-

250 GEOCHEMISTRY Vol. 1

portance in establishing background gold values for various types of granitic rocks withinthe region and elucidating regional geochemical characteristics. At present, scarce data areavailable on the mode of occurrence of gold in rocks and minerals, the reworking of goldsource-beds by granite intrusion and the transport and concentration of gold. Therefore,these key problems need to be further studied to improve our understanding of gold mine­ralization.

Acknowledgements

The authors wish to thank Prof. Xu Keqin for his concern and instructions. Some of thesamples were collected by the authors themselves from the field, and the other samples weresupplied by their collegues.

References

[1] Institute of Geochemistry, Academia Sinica: Geochemistry of Granitic Rocks in South China,pp. 225-356 and 379-381 (1979) (in Chinese).

[ 2] Laboratory of Henan Provincial Geological Bureau: 1980, Geophysical and Geochemical Ex-ploration, No.2, pp. 19-26 (in Chinese).

[3] Seherbakov, Yu. G.: 1964, Geokhimiya, No.6, pp. 518-528 (in Russian).[4] Department of Geology, Nanjing University: 1974, Scientia Siniea, No. I, pp. 55-72.[5] Tilling R. 1. et al.: 1973, Econ. Geol., Vol. 68, pp. 168-186.[ 6 ] Department of Geology, Nanjing University: 1980, J. of Nanjing University, Special Issues on

Geological Science, pp. 4&-47 (1980).[7] DeGrazia, A. R. et al.: 1964, Geochimica et Cosmoehimica Acta, No.5, pp. 559~564.