VoL 8 ~r 4 | 1984

Geochemistry of Gold Deposits in West Hunan and East Guangxi, China

Lxu YINQJUN (~]~{~) , ZH^NO JINGRONG ( ~ ) , Q~AO ENGU^NG ( ~ ) , M^o HuIxIN ( ~ )

AND ZH^O MEIFANG ( / g ~ j ~ )

(Department of Geology, Nan]ing University)

A b s t r a c t

Six kinds of .dement association in the gold deposits in the studied region are distinguished: (1) tungsten-antimony-gold; (2) tungsten-gold; (3) antimonygold; (4) lead-zinc-silver- gold; (5) uranium-silver-gold; and (6) simple gold. The present paper deals with the dis- tribution, source, mineralization, migration and accumulation of gold. The results show that all ore-controlling strata or source beds related to the gold deposits have high background levels of gold; the gold and other ore-forming dements associated are obviously derived from the country rocks. The mineralization of gold is related chiefly to regional metamorphism or alkali-metaso- matism. In response to metamorphism of ore-controlling strata, gold was removed into solu- tions, and then transported and deposited in some parts of the ore-controlling strata. The main form of gold carried in the solutions was Na[Au(HS)2]. Native gold or dectrum were precipi- tated from the ore solutions and concentrated into ore deposits with the decrease of T, pH and fo2.

I n t r o d u c t i o n

W~thin the bou~.ds of western Hnnan and eastern Guangxi of south China gold de- posits are of extensive occurrence, with a variety of ore-forming elemant associations. The Xiangxi, Xi 'an, Mobing, Longshan, Tao-hua and Longshui gold mines are typical examples.

Based on the data from geological field investigations and laboratory s tudies the present paper discusses the distribution, source, transport and accumulation, existing forms and metallogenesis of gold. Samples were analyzed for trace gold mahfly by chemical preconcentration-emiss~on spectroscopy and part ly by colorimetry. The analy- tical results have shown th'at the relative average error is 18% for the chemical precon- eentration-emisslon spectroscopy, and the relative mean deviations with respect to re- peatability and reproducibility are 16.5% and 9.3%, respectively. In case 20 grams of sample are taken, the relative sensitivity can reach 0.5 ppb. Obviously this method can generally meet the needs in geochemical s tudy of gold.

R e g i o n a l G e o l o g y a n d C l a s s i f i c a t i o n o f Gold D e p o s i t s

The strata from the Proterozole Lengjaxi Group to the Quaternary system are weI] exposed in this region. However, almost all the primary gold deposits occur in the Cambrian or even older strafe. Their main characteristics are described in Table 1. In general, these strata tend to become younger and younger from northwest to southeast; lithologically volcanic rocks are relatively developed at the b~ttom.

308 GEOCHEMISTRY Vol. 3

Table I. Main features of pre-Ordovician strata in western Hunan and eastern Guangxi

Palaeozoic

Proterozoic

Stratigraphy

Post-Cambrian

Cambrian

Upper

Sinian

Sinian Z

Lower

Sinian

Wuqiangxi

Formation Banx i Ptb.

Group Madiyi

Formation

Zb

Z~

Ptbnw

Ptbnm :

Thickness ( m )

Maximum thickness: >7136

23--342

53--4914

250--4757

I00-->322C

Lengjaxi Group* >3000

Main rock type

Western Hunan: Dolomitic marl, carbonaceous shale, silicolite, etc.

Eastern Guangxi: feldspar-quartz sandstone (usually carbonaceous and locally tuffaceous) with some intercalated beds of (carbonaceous) shale, silico- lite, phgllite, limestone, etc.

Silicolite, dolomitic limestone, carbonaceous

shale, etc.

Chiefly consisting of sandy slate containing gravels, and drift psephyte, locally intercalated with sandstone, shale, tuffaceous rocks, intermediate-basic volcanic rocks, etc.

Weakly metamorphosed sandstone locally

intercalated with slate, tuffaceous sandstone or

slate, tuffite, etc.

Characterized by sericite-slate and calci-slate with intercalated beds of metamorphic sandstone, d01omitic limestone, tufts, basic volcanic rocks, etc.

Striped and phyllite slate(mostly carbonaceous), locally with intercalated beds of sandstone, tuffaceous sandstone or slate, basic volcanic rocks, etc.

* Corresponding to the Sipu Group.

There are four major structural belts in this region m. They are the jiangnan geoantielinal belt, the west Hunan post-Caledonian uplift, the Dayaoshan post-Caledo-

nian uplift and the tIercynian-Indosinian depression zone. The primary gold deposits all occur in the former three structural units, mainly controlled by secondary folds (especial- ly at the turning points). The shape of the gold depasits is, to a great extent, related to the faults.

Igneous recks distributed in the whole region include Proterozoic basic-ultrabasie dykes, intermediate-basic volcanic rocks and granitoid rocks of different ages m. In space the development of some gold deposits is closely related to the intermediate-basic rocks and Caledonian granitic rooks.

According to the associations of utilizable elements in the deposits, the gold depo- sits in this region can be divided into six types: W-Sb-Au, W-Au, Sb-Au, Pb-Zn-Ag-Au, U-Ag-Au and simple Au. The main characteristics of the representative deposits are given Jn Table 2.

Quartz veins coexisting in space or time with gold are found in these six types of gold deposit. With the exception of the U-Ag-Au type, quartz veins in the other types

No. 4 GEOCHEMISTRY 309

Association type

W-Sb~Au

W-Au

Sb-Au

Au

Pb-Zn-Ag-Au

U-Ag-Au

Table 2.

Sulfides abundance*

Rich

Very poor

Rich

Poor

Rich

Poor

Characteristics of the major types of gold deposit in western Hunan and eastern Guangxi

Occurrence of ore body

Predominantly stratoid

Intercrossed veins

Stratoid, intercrossed veins or occurring in contact z o n e

Veins in igneous rocks

Locality of representative deposit and its country rock

Western Hunan (Ptbnm)

Xi'an (Ptbnm)

Longshan (Zai)

Mobing (Ptb nra)

Taohua and Longshui

Zhanggongling(V63)

Lithological character of country rock

Sedimentary-metamorphic

rock series

Porphyritic granodiorite

* Sulfide content of deposits very poor in sulfides<l%, in sulfide-poor deposits 1--10%, and in sulfide-rich deposits> I0%.

are found in great numbers.

Geochemical Characteristics of Gold-bearing Formation

The data available indicates a possibility that there occur gold-bearing formations in this region, which provided ore-forming materials for gold mineralization. The main evidence is given in the following.

1. The gold deposits are restricted to the special horizons and controlled by li- thological characters, and mostly occttr as stratified or strato~d ore bodies (Figs. 1--2).

2. Gold abundance in all the ore-controlling strata is higher than the average value of the crust as well as that of the adjacen~t ore-barren strata (Fig. 3).

3. T~e associated utiIfizab]e elements in the deposits also have high enrichment coefficients in the country rocks (Table 3). In the Longshan and Longshui areas the enrichment coefficient of W in the strata is greater than unity, but the content of CaO is relatively low. So no scheelite mineralization, as in the Xiangxi mine, has yet been found in both the mines.

4. In the Xiangxi gold mine ramarkable positive correlations are noticed between Au and Sb in the ore-controlling strata, intensely altered country rocks near the ore bodies, and the ore bodies themselves (Table 4).

5. 63~S values for all the gold deposits vary over a wider range than those for meteorites, igneous rocks and volcanic emanation, but all fall in the ranges of 6s4S for metamorphic rocks and sulfides from sedimentary rocks (Fig. 4). The ore-controlling strata in the region are just designated to the sedimentary-metamorphic rock series.

6. No obvious anomaly of gold is noticed in the deep fault zone extencling across the Xiang"xi mining district (Fig. 5). I f gold had come from the deep levels of s crust rather than from the country rocks, such a large-scale fault system would have been an

810 GEOCHEMISTRY Vol.3

?4r ~

~ t bnw4 "' H Ptbnw2

j 4o o 40 80m

.llY l---fI'1 '~2 ~ 3

Fig. 1. Geological section of prospecting line 5 in the Mobing gold mine.

(after Geological Team No. 407, Hunan Geological Bureau) Ptbawx Member II of the Wuqiangxi Formation of the Banxi Group. 1. Stratig- raphical boundary line; 2. ore vein and vein number; 3. (tuffaceous) striped sandy slate.

Fig. 2.

215"

" ' x x X%X2~Ptbnm ~ x l Rd x ~ " \ QL3 {

Ptln '~-~t 'J Ptbnm \ \ u i I - " ' ~ \ ' ~ \ I

Geological section of prospecting line II-23 in the Xi'an gold mine.

(After the Geological-Survey Section of the Xiangxi Gold Mine) Ptl. Lengjaxi Group; Ptbn m Madiyi Formation of the Banxi Group; QL a - ore-bearing stratum and its number; Rd purplish-red slate; Gv greyish-green slate; F fault.

Locality Stratum

Taohua Eshi

Zbdn Zbd Lon@

shan Zan Zal Za r

Mobing Ptbnw

Ptbnw Xiangxi Ptbnm

Pt]n

4 Au (ppbl 8 12 , i r , , ,

I ! 12221 r-72 [Z]:

Fig. 3. Comparison of gold abundances in the selected stratigraphical units in western Hunan and eastern Guangxi. I. Ore-bearing strata; 2. ore-barren strata; 3. gold abundance line for the earth's crust.

ideal passage for t r anspor t of ore-forming materials. In fae,L however, there is no sign

of such a passage as indicated by the fact tha t there are almost no intrusive roeks in the mining disSriet.

No. 4 GEOCHEMISTRY 311

T a b l e 3. E n r i c h m e n t e x t e n t of o r e - f o r m i n g e l e m e n t s a n d t h e i r a s soc ia t ion t y p e s in t h e o r e - c o n t r o l l i n g s t ra ta

Locality

Xiangxi

Longshan

Longshui

Stratum

Ptbnm

Zai l

(~shl

Coefficient of enrichment

Au Sb

1.1 41.5

1.8 4.0

2.4 0.3

W

7.9

3.3

4.1

CaO(%)

1.95

0.38

0.74

Association type

W-Sb-Au

Sb-Au

Simple Au

T a b l e 4. C o r r e l a t i o n s b e t w e e n A u and Sb in V e i n No .4 and i ts c o u n t r y rock in

O c c t l f r e n c e

Strata

Altered rocks

adjacent to

i ore bodies

Madiyi Formation

of lhnxi Group

(Ptb am)

Vein No. 4

X i a n g x i g o l d m i n e

E[ement pair

Au-Sb

Au-Sb

Au -Sb

Coefficient of correlation ( r ]

+_0.6219

:hO. 6697

+0.7868

Critical value of correlation (ro.o~)

0.4973

0.5529

0.4555

21.00 .~0 .40 .60 .80 22.00 .20 .40 .60 .80 23.00 .20 .40 ,60 .80

[.~ ] d- Meteorite sulfur

I ~ ~ ] ~ ~ Biogenie H2S

I"~ Y] ~ ~ Sedimentary ~ ~ sulhde-r---

+58~0 +10.00 0.00 -33.91 -6638

Fig. 4. Sulfur isotopic characteristics of the selected strata-bound gold deposits in western

Hunan and eastern Guangxi.

The above facts show th, at the gold deposits occurring in the strata of this region are generally possessed of the features o~f China's strata-bound gold deposits", and thus should be designated to typical strata-bound deposits. Since ore-forming materials are mainly derived from the ore-controlling strata, the s~rat~ can be reasonably referred to as gold-bearing formations, or gold-source beds.

Gold in the gold-bearing form~ations is of non-homogeneous distribution, as indicated by a high coefficient of variation and positively skew distribution (Table 5). The

1) Wang Xiuzhang and Cheng Jingping: Types of Chinese Strata-bound Gold Deposits and Me- chanism of Their Formation (1981).

312 GEOCHEMISTRY VoI. 3

Fig. 5.

G~

Hongyanxi(1 :

Yuershan(2

Shiliuponggong( 21

Zhongwoxi(1) Shengtianao (2)

Guanzhu aug(3):

Luosiwan( 1 )

Gongiawan(2)

2:

Au (ppb)

Distribution of gold along the strike of a certain fault zone in Xiangxi.

(The numbers in the parentheses are the numbers of samples.)

Table 5. Distribution parameters of gold in the selected gold-bearing formations

Coefficient of Coefficient Critical value Locality Stratum variation (o~) of skew of skew (t0,0,)

Taohua

Longshui

Longshan

Mobing

Xiangxi

~ s h l

~ s h l

Za]l

Ptbnw

Ptbnm

103.0

144.0

102.0

76.0

101.0

+1.07

+1.54

+2.05

+0.57

+2.26

:1:I.63

:1:1.63

+O.55

-I-1.19

-I-0.73

~ Ptbn~ 51 --i

~ Pt ham

Au (ppbl Name of geologi cal sect i o n f ' ~ I~elative position [and distance

0 2 4 6 0 8 16 tn]eping Zaomuyuan Woxi

W 171~

5 l0 15 20 Qi aotan~Guanpingao I

- - E

Fig. 6. Variations in the content of gold in the selected strata of western Huaan.

resu l t s of the tests show t h a t gold in the go ld-bear ing format ions , excep t for the Mad iy i

F o r m a t i o n of the B a n x i Gorup and the J i a n g k o u F o r m a t i o n of the S in ian system, has

:No. 4 GEOOHEMtSTRY 318

a normal distribution. This implies that gold in the gold-bearing formations overlying the plane of unconformity exhibits an obviously skew distribution.

Gold at the bottom, of the tectonic sequences ctispl,ays an obvious tendency toward enrichment. For example, in the Qiaotang-Guanping'ao geological section of the Madiyi Formation of tire Lengjaxi Group at the bottom of the Xuefeng tectonic sequence, gold tends to increase from the top downwards, and reaches the maximum near the plane of unconformity (Fig. 6). In regard to the Anleping-Zaomuyuan geo~logical section, there is also a tendency for gold to increase from the top to ~the bottom in the Madiyi Forma- tion though the enrichment of gold is not so obvions at the bottom of the tectonic se- quence due ~o high levels of gold in the local strata of the Lengjaxi Group.

Moreover, in the gold-bearing formations the rocks rich in volcanic and carbonaceous materials have high gold contents, mostly higher than the background values of the cor- responding horizons (Table 6).

Table 6. Gold contents of some rocks in the se l ec ted gold-bearing formations

Locality

Xiangxi

Longshan

Mobing

Taohua

Loagshui

Stratum and gold abuodance(ppb)

Ptbnm (4.2)

Z.n(7.1)

Ptbnw

(9 .2)

(~shl

(11.7)

(9.7)

Sample No.

58

60

62

156

205-2

241

253

301

298

316

320

Rock type

Purplish-grey slate

Grey slate

Grey fine sandstone

Sandy slate with

gravel and tuff

Grey striped sandy slate with gravel

Black silicolite

with carbon

Carboilaceous

shale

Gold content (ppb)

14.0

5.9

12.5

6.5

16.7

470.0

20.6

30.5

13.1

23.0

42.1

Remarks

Containing

volcanic

materials*

Carbonaceous

* Based on thia-section examination and petrochemical restoratioa of the original rocks.

The origin of gold in the gold-bearing formations is closely related to volcanic ema- nation, chemical weathering and mechanical weathering. The creation of the most pri- mitive gold-rich strata may be explained as a consequence of intense voleazfic eruption during the early stage of eugeosyncline development in response ~ heterogeneous differ- entiation of the mantle in the early stage of evolution of the earth's crust. Oomposi- tionally, such primitive gold-bearing formations are characterized by abundant, volcanic materials. Ions, complex ions and c~lloids of gold resulting from chemical weathering can be concentrated through adsorption. For this reason, there exist positive correla- tions between Au, and A1208, Fe203 and organic carbon (Table 7). The occurrence of

314 GEOCHEMISTRY Vol. 3

Localiw

Mobing

Loogshan

Taohua

Langshui

Table 7.

Stratum

Ptbnw

Z~il

Cshl

(gshl

Contents of gold and some other components in some rocks of the selected gold-bearing

Rock type

Striped sandy slate with gravel

Striped metamorphic sandstone

Greyish-green sandy slate with gravel

Brownish-grey sandstone

Greyish-green sandstone

Carbonaceous shale

Carbonaceous feldspar- qu~rtz-seri2te sondstone

Au

(ppb)

20.6

3.0

15.4

7.4

7.0

5.9

23.0

2.0

0rmations

%

A1203 Fe20~

14.35 1.25

10.71 1.20

15.78 1.62

12.56 1.40

15.23 4.65

13.44 3.98

22.96 8.95

11.41 0.96

Organic

0.74

0.53

2.35

1.30

fragmental gold-bearing horizons in the Sinian strata is a clear indication that the enrichment of gold is due to mechanical weathering.

Geochemical Mechanism of Metallogenesis of Strata-bound Gold Deposits

Mobiliz~ion and migration of gold

How was gold m obilLzed, transported and concentrated into ore bodies in the gold- bearing formations ? We consider ttmt the process is related mainly to regional metamor- phism. The reasons are given as follows:

(1) All ore-controlling strata or rocks have undergone varying degrees of rework- ing by regional metamorphism, and thus contain large amoun,ts of sericite. (2) There is an obvious negative correlation between gold and sericite, i.e., the more sericite the rock con'tains, the lower the content of gold is (Fig. 7). (3) It is well known that the altera- tion of many minerals into sericite is mostly accompanied with the removal of SiO2 As- suming ttmt sericite formed in the process of alteration in one cubic kilameter of rock accounts for 10%, the amount of removed SiO~ is saffieier~'t ,to f~rm a qtmrtz vein measuring I m wide, 2 km deep and 17.7 km long. Moreover, the content of seriei.te in the gold-bearing formations averages more than 50%. Therefore, the amount of released SiO, is more than enough to form, so many quartz veins associated with gold in the gold deposits. (4) In camHarison .to its average content in sedimentary rocks, the content of H,O in the vaxions types of rock in this region is relatively low with the minimum dif- ference being 0.67% and the maximum 2.67%. If H,O was assumed to decrease at the rate of 1%, there would be 2.5X10' tons of water drained off from every one cubic kilo- meter of rock in response to regional metamorphism, producing enough aqueous solutions callable of extracting and transporting ore-forming materials. (5) As viewed from their mineraJogical ,and petrological charactoristies, the gold-bearing formations stiould be as-

No. 4 GEOCHEMISTRY 315

signed to the low-temperature metamorphic facies (after Dong Shenbao). The meta- morphic temperature is roughly close to the f~rmation temperature of the gold deposits restricted within the formations.

30

20

10

1 o

x \ x ~ x \ \ \

: \

\ \ \ \ x x ~ X x •

\ ~ xX • XXN\ x ~ ~ • NN

\ ~ x'X,x x

20 40 60 80 Sericite (%)

' 1 6 o

Fig. 7. Scatter diagram and straight line of regression of Au in sericite in the gold-source beds of the Madiyi Formation (Ptbam) at Xiangxi.

Therefore, it is considered that regional metaanorphism is a principal process re- sponsible for mobilization, migration, and Coneentra,tion of gold into ore deposits m.

Migration forms of gold

Up to now, several dozens of gold complexes, in the form of which gold can be transported in solutions, have been reported by many research workers both in China and abroad. But most research workers consider that sulfur, chlorine and hydroxyl are the importaat ligands in the gold complexes,and there is a relatively great probability of occurrence in geological environments.

Undoubtedly, a large amount of hydroxyl can be produced in the process of for~na- tion of ~ricite. In the studied region, the conten.t of sulfur tends to decrease with in-

100 15o "~ ..

!I" 100 "x\ x _ == -- U 50

" .$

50! 0 x \ 0 ' ' ' 7 ' 0 ' '10 30 50 0

Sericite(U) 20

X

D ~ [ I J I 1 I P

30 50 70 100 Sericit e(%)

Fig. 8. Scatter diagrams of S-sericite and Cl-sericite in some gold-source beds in western Hunan and eastern Guangxi,

316 G E O C H E M I S T R Y Vol. 3

creasing metamorp.l~ic grade and is reversely proportional to the content of serieite, while the opposite holds true for chlorine (Fig. 8). It is evident that the variation trend of sulfur is very similaz to that of gold during regional metamorphism.

Ore-forming solution analysis has revealed that there exist much closer positive correlations between Au and S, and Na than those between Au and C1, and K (Fig. 9). Moreover, in this region gold seems ~o have been transported in neutral-weakly alkaline solutions and deposited under weakly acidic conditions. It is clear that gold is tran- sported mainly in the form of sulfur-gold complex and subordinately in the form of chlorine-gold complex.

-I! 1 t t t : \ \ /

"r X /.',7

,o0 ' , / ; 02

\ \

/ ',7 ?,, 3c o'.l ' o:3 ' 0:~ ' 0.7 ' o : 9 ' : Na'/CNa'+K ' ) K'/(Na'+K'~ :~S/t~S+Cl )

CI-/ t~ +el')}

Fig. 9. Relations between the ratios of some ions in fluid inclusions in quartz and the

content of gold in quartz veins in the Xiangxi W-Sb-Au deposit.

1. Scheelite-native gold stage; 2. native gold-antimonite stage; 3. carbonate stage.

According to the diagram of stability fields of sulfur species in aqueous solutions TM, the measured Eh ( - 0 . 2 4 8 - - -0.312 V) and pH (mostly 7.02--8.5) values for metaznor- phism-formed aqueous solutions fall in the st'ability field of HS- in the diagram, i.e., sulfur exists main~]y in the form of HS- in the aqueous solutions of this field. Consider- ing Na + as a cation coordinated at the outside of the complex anion, it is reasonable that gold is transported in the soluble form of the gold complex Na[Au(HS)2] .

In the process of formation of those sulfide-poor gold deposits, gold seems mostli- kely .to have been transported in the form of hydroxyl complex.

Conditions of gold enrichment and farms of gold occurrence

Fluid inclusion studies of gold-beaxing quartz veins demonstrated that g~ld in ore solutions was deposited under moderat~-low temperature (150~176 weak acid (pYI =4.8--6.5) and low oxygen fugacity (log fo, = - 7 5 - - - - 7 8 . 6 ) conditions.

Tectonic faults paved the way for the emplacement of gold veins. On one hand they provided much space for ore deposition, and on the other hand the closed faults might serve as a natural fence for the concentration of ore-forming materials, on the flank

No. 4 GEOCHEMISTRY 317

of which gold woutd be deposited. In addition, carbonaceous layers or carbon-bearing horizons also contributed a great deal to the mineralization of gold because they might serve as a natural fence and were highly capable of adsorbing gold. The Taohua gold deposit is an example.

In the sulfide-bearing veins gold is selectively concentrated in sulfides, especially in arsenopyrite, pyrite and antiInonite (Table 8).

Table 8. Average contents of gold in major minerals from ~he strata-bound gold deposits (in ppm)

Element-association W-Sb-Au W-Au Sb-Au Simple Au Wpe

~ Deposit Xiangxi Xi'a/~ Longshan Mobing Taohua

Mineral

Antimonite 115.0 i I. 4

Pyrite 92 13.5 72.0 54.6 110.7

Tetrahedrite 2.8

Arsenopyrite 324.7 22.3 317.8

Galena 2.1 125.0 108.2

Scheelite 18.0 0.8

QuartZ 2.2 0.5 0. 047 0.2 11.5

Calcite 0. 004

In the gold-bearing quartz veins, extromely depleted in sulfides, gold prefers to be deposited in the location close to the country rocks (Figs. 10--11). This suggests that gold was derived from the country rocks, or it was reduction-precipitated because of easier acquirement of electrons from the country rocks.

s l 9 , 2o

Fig. 10. Native gold is Seen developed against the slate vein V2 (Qz) in the Mobing mine.

the country rock of quartz

In this type of gold deposit, gold occurs mostly in the form of native gold of high purity (Table 9). Generally, there are three types of associa~on of gold wi+h other minerals, i.e., inclusion gold (gold enclosed in quartz or sulfides), crevice gold (filling in the crevices of quartz scheelite or pyrite) and interstitial gold (interstitial to the

crystals of sulfides).

818 GEOCHEMISTRY Vol. 3

0

Ptbnw2 J

. .---"~1 ._: '~ a2 ~ a3

Ptbnw 2

, Sample

1

2

3

4

Au(ppm)

430

8

15

315

Fig. 1 i. Relations between sampling positions and gold contents in the samples from quartz vein Vz in Stope No. 11 of the Mobing mine.

(The solid triangles and the numbers standing by them represent the sampling positons and sample Nos., respectively.)

Table 9. Relat ive purity of nat ive gold in the se lec ted strata-bound ffold deposits in wes tern Hunan and eastern Guangxi

Deposit type

Representative deposit

^u (%~

Ag (~oo)

W-Sb-Au

Xiangxi

985.3

Sb-Au

Longshan

838.7--989.7

7.8

Mobing

986.5

Au

Taohua

991.9

5.8

Fac tors C o n t r o l l i n g M i n e r a l i z a t i o n of Gold Re la ted to Grani to id Rocks

All gold deposits related to granitoid rocks in the region are genetically connected with the Darting body. Both the Longshui and Zh~anggongling deposits are typical ex- amples. The former occurs in and adjacent to the contact zone between the rock body and the country rock and the latter lies in the rock body. All the ore bodies are control- led by the fault system.

The Da,Mng body is composed mainly of porphyritic granodiorite with restite (car- responding to biotite diorite in composition), and postdated biotite granite and vein rocks. The country rock of the gold ore bodies is the porphyritic granodiorite.

The eor~tent of gold tends to decrease in the granitic rocks from old to young in age (Table 10). This shows that the process of evolution of the rock body is one during which gold decreased , especially in the early stage of formation of the porphyritic gra- nodiorite.

Table 10. Gold contenta of granit ic rocks emplaced in the di f ferent stages of evolut ion of the Daning rock body

Rock type

Dark restite

Au(ppb)

9.0

Porphyritic granodiorite 3.7

Biotite granite 1.7

Granite-aplite I. 3

Remarks

Early

Late

No. 4 GEOCHEMISTRY 319

Varying degrees of chloritization and sericitization are commonly seen in the por- phyritic grunodiorite, in accompany with removal of gold and some other ore-f0~ming elemer~ts (Table 11). According to N. G. Banks (1973) m, in intermediate-acid igneous rocks chlorite contains much lower sulfur than biotite does, while phengite has the lowest content of sulfur (almost clo~e to zero). This implies that either chloritization or muscovitization of biotite could lead to mobilization of sulfur. Gold and sulfur re- leased in this way may form complex compounds with other related elements, so that they are transported together. Moreover, sulfur may be derived from decomposition of the primary sulfides.

Table II. Contents of ore-forming e lements in some rocks of the Darting body (ppm)

Rock type

Porphyritic granodiorite

Chloritized porphyritic granodiorite

Sericitized porphyritic granodiorite

Au

0.0037

0.0017

0.0012

Ag

0.20

n.d

0.12

Pb

30

I0

20

Zn

i00

<30

<30

U

8.8

7.7

7.3

Extensive crystallization of gold from the ore solution took p~aee at the time it mig- rated to an environment where the temperature, pH and fo~, as well as tho eo~tent of SiO~ in the solution were remarkably decreased (Figs. 12--13). The precivi~ted gola is usually present as native gold in the simple gold-type deposits, while ir ~akes the form of electrum in the d~posits assoedated with ~ilver. For instance, the gold present in the U-Ag-Au-type veins in the Zhanggongling mine occurs in Che form of electrum (gold 62.32%, Ag 37.56% on average).

50

40 e ~

< 30

2O

10 -79

D e c r e p i t a t i o n t e m p e r a t u r e of q u a r t z < ~ : > ;00 305 315 330 372

i i , i

"x..

t I h

-78 - + 7 ' -76 ' - 7 5

log~o2

Fig. 12. Diagram showing the relationship between the decrepitation temperature of fluid inclusions in quartz vein V, of the Longshui mine, fo2 of the ore solution and

the amount of removed gold.

(AAu = the difference between the gold content of quartz and that of the leacheate of fluid inclusions in the quartz)

I t should be pointed out that the ambient temperature in the stage of porphyritic feldspathization is generally high mad thus SiO2 is usually present as an inert compenent in such a metasomatic system. Therefore the porphyritic feldspathization is unfavoura-

820 GEOCHEMISTRY Vol. 3

90

Si02(1) r/g 130 170 210

50 �84

.~40

"~ 30

20

10

X N

4.'8 5)0 '5"2 5.4' 5:6 5.'8 6:0 6:2 pH

Fig. 13.

250

Diagram showing the relationship between the pH and SiO2 content of the ore solution and the amount of removed gold in the Longshui mine.

(z~Au .~ the difference between the gold content of quartz and that of the leacheate of fluid inclusions in the quartz)

ble for long-~stance transport of gold though i.t may have played a~ important role in removal of gold from the crystal lattices of the primary minerals. During chloritization and sericitization which took place at the final stage of or after the porphyritic feldspa- thization, the r slightly went down, but pH increased, resulting in mobiliza- tion of SiO~. As a result, gold ions or some gold complex comIraunds could be protected by Si02 and transported together with SiO~ in the solutions till SiO~ was reprecipitated in a suitable environment.

For these reaso~as, i,t .is considered that chlorit~zation and se~ricitizat~o.n are more closely connected with the concentrar and mineralization of gold than porphyritic feldspathization.

Conclusions

To sum up, the gold deposits in this region are conCr~lled by the specified gold- bearing strata or rocks; the ore-forming materials are precisely derived from ore-con- trolling country rocks themselves; the mineralization of gold is closely related to regional metaznorphism and alkali-metasomatism; in the process of mineralization gold is trans- ported mainly in the form of Na[Au(SH)~] ; in the environment where T, pH and fo2 are remarkably decreased, gold is concentrated into ores in the form of native gold or electrum.

Acknowledgements

Thanks are due to PLA 00535 Unit, PLA 00536 Unit, Geological Brigade No. 204 of Guangxi Provirmial Prospecting Co., the Geological Survey Sections of the mines where we attended our investigation-tours, and the Central Laboratory of the D~part- merit of Geology, Nanjing University for their close collaboration and great help in this work.

1~o. 4 GEOCHEMISTRY 321

References

[ 1 ] Department of Geology, Nanjing University, Journal of Nanjing University, Special Issues on Geo- logical Science (1980), 2--13.

[ 2 ] Moiseenko, V. G. et al., 24th Intern. Geol. Congr., Sect., 10, Geochemistry (1972), 159--165. [ 3 ] Barnes, H. L., Geochemistry of Hydrothermal Ore Deposits. Holt Rinehart and Winston, New

York, N. Y. (1967), 334--381. [ 4 ] Banks, N. G., Econ. Geol., 68, 5(1973), 697--703;