Geochemical studies of the formation of gold deposits in the Shaoxing-Longquan Uplift Zone, Zhejiang...

Vol. 6 No.4 GEOCHEMISTRY 1987

Geochemical Studies of the Formation of Gold Deposits in the Shaoxing-Longquan Uplift Zone, Zhejiang Province

LIU YINGJUN ( ~ ' I ] ~ ) , SUN C h E N G Y U A N ( ~ ) AND SHA PENG (~1~ ~ )

(Department of Geology, Nanjing Unir~ersity)

Abstract This paper systematically deals with the geochemical features of major gold deposits in the Shaoxing-

Longquan Uplift Zone, Zhejiang Province, including the content and association of ore-forming elements and

trace elements, stable isotopic characteristics, the existing forms of gold, and the composition of ore fluids.

The authors consider that the or~bearing formations in this zone are a good supply of necessary elements and

ore fluids for the gold deposits in this area. It is also considered that some Au +-CI- and Au +-HS- or Au ÷-

CO 2 coordinated ions are the main transport forms of gold in ore fluids and the metallogenesis of gold involves

two stages: formation of pyrite and mineralization of Cu, Pb and Zn. In this paper is also presented a

comprehensive geochemical model for the tbrmation of gold deposits in this uplift zone.

In order to gain a deeper insight into the behaviour of gold in various geological environments

so as to provide more reliable basis for prospecting, increasing emphasis is being laid on

comprehensive studies of the geology and geochemistry of gold deposits on a regional, or even

global, scale. As an effort in this direction, further information has been obtained from the

systematic geochemical studies of some major gold deposits in the Shaoxing-Longquan Uplift Zone,

one of the most prominent noble metal metallogenic belt in East China.

The Chemistry of Gold Deposits in Relation with Gold-bearing Formations

As a sub-unit in the South China Caledonian Fold System, the Shaoxing-Longquan Uplift

Zone borders, along the Shaoxing-Jiangshan Fault Zone and the Yuyao-Lishui Fault Zone,

respectively, on the Jiangnan Ancient Island Arc and the Hercynian-Indosinian Fold Zone in

southeastern coastal area. The strata exposed in the Shaoxing-Longquan Uplift Zone are quite

simple, with most parts of the area covered by volcanic rocks of the Jurassic Moshishan Formation

within which are sparingly exposed metamorphic rocks of the Sinian-Cambrian Chencai Group and

low-grade metamorphic marine volcano-sedimentary rocks of the Proterozoic Shuang::iwu Group.

The Chencai Group and the Shuangxiwu Group, although comprising 1,5percent of the total outcrop

area, account for 90 percent in terms of either gold reserves or the number of gold prospects (Fig. 1).

As will be shown, many lines of evidence indicate that they are the source rocks of gold in the ore

deposits of this area.

Except those in which gold is of interest only as a byproduct, more Man ten gold prospects have

been already known in this area, which are significantly different in scale and type, and can be

grouped according to their elemental associations into three types, i.e., the Zhilingtou type, the

Huangshan type and the Babaoshan type. Studies show that these different types of deposits (or

prospects) may be related to the geochemical characters of their host strata (Table 1). For example,

in the northern part of the Uplift Zone, the Proterozoic Shuangxiwu strata and related high-grade

310 GEOCHEMISTRY Vo.6

Shuangxiwu Group

Chenca! Group

E_C he n c a i_."

Others Others Shuangxiwu M o s h i s h a n - - M o s h i s h a n ~ Group Formation 2 % Formation o~u [

Area Prospect Reserve

Fig. 1. The distribution of gold reserves and gold deposits in major stratigraphic units of the Shaoxing-Longquan Uplift Zone.

metamorphosed rocks have high Au and Te contents and small Ag/Au and As/Au ratios.

Consequently, gold deposits contained in these strata (such as those at Zhongao and Huangshan) are

characterized by Te-Au association (the Huangshan type), showing an apparent positive Te-Au

correlation (R/R0° o5 = 1.91), with native gold and calaverite as the principal gold minerals. While

relatively high Ag contents and large Ag/Au but small As/Au ratios are noticed in the metamorphic

rocks of the Chencai Group in the central and southern parts of the Uplift Zone, the Zhilingtou-type

gold deposits occurring in these rocks are characterized by the elemental association of Ag-Au. This

is demonstrated not only by a good positive correlation between these two elements (R/R°.os

= 1.83) but also by the common occurrence of a variety of silver minerals such as kustelite, native

silver and argentite. Similarly, comparatively high Ag/Au and As/Au ratios are characteristic of the

wide-spread Moshishan Group volcanic rocks in the southern part of the Uplift Zone, and this is

reflected by the presence of large amounts of arsenopyrite and native silver and by a close correlation

between As-Au and Ag-Au (R/R°.0s being 1.83 and 2.76, respectively) in the Babaoshan deposit.

Table 1. Alxmdltmees of ore metals in major stratiffraphie malts in the Shaozlng-Losqgqmm Uplift Zone

Metal

Ratio

Formation Shuangxiwu Group Chencai Group

Deposit type and

element association

Au(ppb)

Ag(ppb)

As(ppm)

Te(ppm)

Ag/Au

As/Au

36.6(25)

255(18)

2.21(7)

o.23(8)

7.0 0.06 x 10 3

Huangshan type

Te-Au

3.5(35)

106(35)

1.33(9)

n.d.

30.9

0.38 x 103

Zhilingtou type

Ag-Au

Moshishan Group

2.9(10)

533(10)*

lo4(8) n.d.

183.8

35.9 x 103

Babaoshan type

As-Ag-Au

* From Liang Zihao et al., 1965; Nos. of samples are given in the parentheses.

No.4 GEOCHEMISTRY 311

Minerals in the Ag-Au system are of great economic interest for all the gold deposits'in this area, and it has been noticed that the gold fineness in these minerals is a function of Au/Ag ratio in the surrounding strata (Table 2).

Table 2. Gold fineness as a function of Au/Ag ratio in the surrounding rocks

Deposit

Huangshan

Zhongao

Mali

Zhilingtou

Babaoshan

Gold fineness

950(7)

841(9)

591(12)

452(7)

288(9)

Au/Ag in the sur-

rounding rocks*

0.2o(5)

0.12(7)

0.053(4)

0.029(10) 0.OO6(8)

Schematic diagram

1 Au (~)s

O. 1 A ~ 6 ~ A~

O. Oll

I /,A GOld f i n e n e s s 0.001

200 400 600 800 1000

* Average of country rock samples collected sufficiently far away from the deposits; Nos. of samples are given in the

parentheses.

The close chemical relationship between host rocks and ore deposits mentioned above is further strengthened by trace element data from ore minerals, especially from pyrite. As illustrated in Table 3, a clear positive correlation exists between pyrite and country rocks in terms of Co/Ni, La/Y, K20 / Na20 and Hg / Sb ratios.

Table 3. Trace element abundances (in ppm) and some element ratine in pyrite from gold depodts and in surrounding rocks

Deposit

PY Co

CR

PY Ni

CR

PY La

CR

PY Y

CR

PY

Hg CR

PY As

CR

Huangshan

1035(8)

32.1(3)

64(8)

29.1(3)

1.18(8)

19.2(3)

1.6(8)

17.6(3)

3.7(8)

22.5 x 10- 3(3)

15.9(8)

2.6(3)

Zhongao

672(2)

15.6(4)

76(2)

14.6(4)

1.33(2) 15(4)

1.9(2)

19.1(4)

0.45(2) 9.3 x 10-3(4)

22(2)

1.9(4)

Mali

(prospect)

18(1)

23(4)

23(1)

34(4)

4.97(1)

27.8(4)

6.2(1)

17.2(4)

!.1(1) 10.6 x 10- 3(4)

>200(1)

1.53(4)

Zhilingtou

71(7)*

14(5)

92(7)* 22(5)

1.14(2)

35.4(5)

1.5(2) 23.8(5)

0.32(2) 6.5 x 10-3(3)

>2oo(2)

9.3(3)

Method of

determination

I.C.P.

I.C.P.

I.C.P.

I.C.P.

Pyrolysis-A.A.

Atomic fluorescence

spectrometry


~ " - ~ Deposit

Element ~ , ~

Sb

K20(%)

Na20(%)

Co/Ni

LaY

Hg/Sb

K20

Na20

PY

CR

PY

CR

PY

CR

PY

CR

PY

CR

PY

CR

PY

CR

Huangshan

8.0(8)

0.4(3)

0.oo78(8)

0.92(4)

0.47(8)

2.29(4)

16.2

1.10

0.73

1.09

0.46

56

0.017

0.40

Zhongao

20(2) 0.65(4)

0.014(2)

1.36(6)

0.95(2)

4.59(6)

8.8

1.07

0.70

0.79

0.02

14

0.015

0.30

Mall

(prospect)

22(1)

0.25(4)

o.o15(1)

2.14(1)

0.82(1) 3.42(1)

0.78

0.68

0.80

1.62

0.05

43

0.019

0.63

Zhilingtou

23(2)

0.5(3)

0.045(2)

4.15(7)

o.78(2)

1.81(7)

0.77

0.64

0.76

1.48

0.01

13

0.06

2.29

Method of

determination

Atomic fluorescence

spectrometry

I.C.P. and

chemical analysis

I.C.P. and

chemical analysis

Note: 1) PY: pyrite; CR: country rocks far away from the deposits; numbers in the parentheses denote the numbers of

samples.

2) * from Liaug Zihao et al., 19~5.

3) I.C.P. was conducted by the Central Laboratory of Jiangsu Bureau of Geology; Atomic fluorescence

spectrometry and pyrolysis-A.A, by the Laboratory of Team No. 814, East China Geological Exploration

Co., Ministry of Metallurgical Industry; Chemical analyses were made at the Central Laboratory of

Department of Geology,. Nanjing University.

Table 4. Comparison of the 5348 values (%0) of pyrites from gold deposits and surrounding rocks

Deposit Huangshan Zhilingtou

Pyrite from gold deposits 2.58(6) 5.38(44)

Pyrite from country rocks 2.67(1) 4.71(6)*

* from Hang Zihao et al., 1985.

Nos. of samples are given in the parentheses.


Pyrite is the most important sulfide in the various types of gold deposits in the Uplift Zone.

Sulfur isotope data show that the /5s4S values of pyrite from the Zhihngtou and Huangshan py deposits are reasonably consistent with those obtained from pyrite in the country rocks (Table 4),

suggesting that most of the sulfur in the deposits may have also come from the surrounding strata.

It is fairly clear from the above discussions that Au, Ag, Te, S and many trace elements in the

major gold deposits in this area have a close genetic relation with the host gold-bearing formations,

or, in other words, the surrounding rocks may have provided, through the action of hydrothermal

solutions, the necessary ore-forming components during the formation of these deposits.

Stable Isotopic Characteristics of Ore Deposits and Sources o f Ore-forming Fluids

The H - O isotopic compositions of fluid inclusions in quartz from major gold deposits in this

area are given in Table 5. As can be seen, 6 tSo varies widely from - 4 . 6 to +4.5%0 and shows, to

some extent, different features from deposit to deposit. In the Huangshan and Zhongao deposits, for

example, 6tsoH2 o is relatively large and is more or less typical of metamorphic connate water ttl.

With respect to the Zhilingtou deposit, ~ 1sort2 ° has a wide range of variation but is less in any case

than that of metamorphic water from the surrounding rocks (see sample AB7), being characteristic

of a mixture of metamorphic connate water with a small amount (10- -15%) of meteoric water as

seen in the ~1 so_g D diagram (Liang Zihao et al., 1985). And the ~1SOH:o of the Babaoshan deposit

f rom the Mesozoic volcanic rocks is very small ( - 4.6%0, only slightly higher than - 6.0%o, the g t 8 o

value of Mesozoic meteoric water in eastern China as given by Zhang Ligang), reflecting that the ore-

forming solutions are composed, for the most part, of meteoric waters heated by volcanic activity in

addition to a small portion of volcanic water.

Table 5. H-O isotopic compositions of major gold deposits

Sample

No.

PT 118

HB 54

AB 7

AB 8

AB 18

AB 22

ZT 1059

ZT 1077

LT051

Sample Homoge- Mineral nization 5IsOQ(%O) 5lson2 o

description T(°C) (%o)

Gold-bearing quartz vein 260 9.4 1.3

Gold-bearing quartz vein 350 9.4 4.5

Biotite-plagioclase gneiss 303 3.9

Gold-bearing quartz vein Quartz 345 3.1

Gold-bearing quartz vein 290 1.0

Gold-bearing quartz vein 245 -1.75

Quartz vein 250 8.9 --0.1

Quartz vein 250 8.2 - 0.8

Gold-bearing quartz vein 280 2.7 -4.6

~D(%O)

-60.4

-61.4

-58.6

-60.2

Deposit Ref.

Zhongao This study

Huangshan This study

Zhilingtou

Zhilingtou Liang Zihao

Zhilingtou et al., 1985

Zhilingtou

Zhilingtou This study

Zhilingtou This study

Ba.baoshan This study

Note: 81sOn2 o values were evaluated from 103Ln~=61SOo--~lsOAao=3.24x 106T-:--3.31 (Matsuhisa, 1979),

analyzed by Yichang Institute of Geology and Mineral Resources.

The sulfur isotopic composition of ore-forming solutions was calculated based on sulfur

the solution~ . The results (see Table 6) show that isotope data for the minerals in equilibrium with -[31


the ~34S values of the ore-forming solutions in Zhilingtou and Huangshan are very close to those of

the interstitial solutions in the surrounding metamorphic rocks, and that the c~3*S values of the

Zhongao deposit are also within the range of normal metamorphic hydrothermal solutions.

Table 6. Calculated 63"S values (%o) of ore--forming solutions and interstitial solutions in surrounding rocks

Deposit 334S value

Ore-forming solution

Interstitial solution

in surrounding rocks

Zhilingtou

4.21

4.19

Huangshan

1.64

2.47

Zhongao

6.80

n,d.

In addition, a remarkable positive correlation is seen between K + / ( K + + Na +) in the ore-

forming solutions as deduced from fluid inclusions and K 2 0 / ( K 2 0 + Na20) in the surrounding

strata (Fig.2a). Meanwhile, the homogenization temperaturesbf fluid inclusions in quartz collected

from orebodies may be related to the metamorphic temperature of the surrounding rocks based

either on phase-transition estimation or on fluid inclusion homogenization (Fig.2b), which is a

common feature as abserved in normal metamorphic-hydrothermal ore deposits 141.

0.8

. . . .0.6

Z 0.4

÷ ~ 0 . 2

400

,-300

/ ' 1

4oo

(b)

0 0.'2 0 4 0'. s 0.s' ,000 K~ O / ( KzO + Na tO ) T M ('C)

Fig.2. The properties of ore-forming fluids in relation to surrounding rocks. K + K20

a. - - in ore-forming fluids as a function of in surrounding rocks; K + + Na + K20 + Na20

b. formation temperature of ore deposit (TF) as a function of metamorphic temperature of country rocks (T~). 1. Zhongao; 2. Huangshan; 3. Zhilingtou; 4. Mali gold prospect; 5. Babaoshan.

As indicated by the above facts, the ore-forming solutions responsible for gold deposition in

the area are made up mainly of metamorphic connate waters from the surrounding strata plus some

meteoric waters heated during volcanism or metamorphism.

P h y s i c o c h e m i c a l Parameters o f O r e - f o r m l n g Solut ions and Transport Forms o f Gold

Systematic studies were conducted on fluid inclusions from quartz formed during gold

mineralization in major deposits in this area (Table 7). The compositions of gaseous and liquid


phases in fluid inclusions were analytical results obtained by the decrepitat ion-extract ion method

(ZS was measured as SO~-) . Pressure was evaluated from homogenization temperature, total

salinity and C02 concentration based on Clausius-Clapeyron 's equation and the Henry 's Law

coefficient for C02 in the H20-NaC1-CO 2 IS] system . f o 2 was calculated from the C H , - C O 2

equi l ibr ium and pH obtained from data on Fe 2 +, Ca 2 +, C02, ZS and f o 2 as given in Table 7 in

consideration of the presence of pyrite and the abscence of calcite.

Table 7. Composition of fluid inclusions in quartz contemporaneous with gold minermliution

Ore deposit Zhongao Huangshan Zhilingtou Babaoshan

Mineral analysed Quartz Quartz Quartz Quartz

Number of samples 2 8 7 3

C02

CH4

K +

Na +

Ca 2 +

Mg2 +

Fe2 +

Au +

Z

4.46

0.42

0.05

0,19

0.03

0.03

5.64

2.28

0.11

0.49

0.06

0.05

0.01

1.8x 10 -s

5.82

1.02

0.26

0.34

0.03

0.22

3.7 x 10- 6

1.29

0.65

1.02

1.29

0.20

0.52

0.01

n.d. 4x10 -6

0.35 0.72 0.85 3.04

0.09

0.35

2.91

0.18

0.52

3.t7

F -

CI-

ZS

Gas content

(mol / kgHzO )

Cation

concentration

(mol/kgH20)

Anion

concentration

(mol/kgHzO)

0.06

0.49

3.41

0.05

5.24

0.93

3.87 3.96 3.35 6.22

260 350 300 280 T(°C)

P(atm) 5OO 6OO 55O 27O

fOz (atm) 10- 34 10 - 35 10 - 3 s 10- 34

pH < 3.34 1.86---3.79 < 3.67 0.52--3.50

Note: Analysed by the Department of Geology, Nanjing University using the decrepitation-extraction method.

As can be seen from Table 7, the salinity and pressure of the ore-forming solutions are not very

high, therefore, the homogenit ion temperatures, al though no pressure correction has been made,

can be in large measure representative of the true temperatures at which the deposits were formed.

Fig.3 shows that gold deposits in this area have been formed over a comparatively wide temperature

range (10(0--500°C) through many stages of hydrothermal activity. However, major gold


30

20

mineralization is often restricted to a single stage whose temperature varies slightly from deposit to

deposit, for example: Huangshan 350':C, Zhilingtou 300~C, Bahaoshan 280c'C and Zhongao 260°C.

N ts~ ~

(a ~,

lot

N 12 15 N

10

0 200 40O 600

T ('C:

(b) (c) (d)

s j -

, 1

oqr! 240 320 400 240 320 t00 180 260 340

T ('c~ T t'C~ T (°C~

Fig.3. Histograms showing the homogenization temperatures of fluid inclusions in quartz from major gold deposits.

a. Huangshan; b. Zhilingtou; c. Babaoshan; d. Zhongao.

With the exception of Babaoshan, ore-forming solutions in this area share a common

compositional feature of H20 >> CO 2 > ~S > CI- -> F - and Na + > K + > Ca 2 + + Mg z + + Fe 2 +. In

view of the lowfo 2 and pH in the fluids, H2S must be the dominating sulfur species in the system.

40(]

,~300

200

100

A5 O N4

"3 E 3

~r

, t L . . - __

tt t2

3

tl t2

f t! w -38

-42 , I g / o = % ' 0 t~ t 2

Fig.4. Variations in the properties of ore-forming solutions in the early (tl) and late (t2) stages of mineralization in the Huangshan deposit.


Thus, the ore-forming fluids can be regarded as the NaCl-K-,S-type hydrothermal solutions rich in

CO, and H,S. In addition, as indicated by Fig.4, the content of C1- in the solutions tends to decrease

with Na + / K + ratio, fo-, and pH, while YS increases with decreasing temperature.

COz ( tool/ks H=O )

6X10_f I 2. 0 4. 0 6. 0 8.0 10. q

<ii i , . . . ,+ . + :

Au-CO= . .'." .. ~ 4XI0"' [ - ~ ' -"":" ' ->::i:i! = . . . . . . . . . : " ~

- !iiii!ii+i!iiii!ii+i ~ , 3Xl0--'a . : : ' - " i " : : :!::.; : : :

- ÷

2Xt0 -s

iXlO-S ~ *

i i i I i t [ , o12 o~4 o.e o.s .o

CI- ( m o l l k s ltzO)

COt( m o l / k g H=O )

2 .0 4.0 6.0 8.0 10.0 ' " ' ' . . . . . ' . - I t • • 1. sxlo -~ ( b ) ~ ^a +-'.: s

5XIO" s 1

++++ ~.0. 9 × 1 0 .+

o a

~= o. 6 x l o "s • . • = . . . . . . ' ~ . ' ~ ~ " i - " . " . . . " . . • • •

O. 3XIO "s := '" ' :"" ." " ; " : •

+0 . . . . . 0 2 0 6. 0 8.'0 10.

~S ( too l /k i lH20 )

Fig.5. Correlations between Au + and CI-, YS and CO 2 in ore-forming fluids responsible for the Huangshan (a) and

Zhilingtou (b) deposits.

Table 7 also shows that gold molalities range from 3 x 10- 6 to 2 x 10- s, equivalent to 0 .6- -4

ppm, in the ore-forming fluids, much higher than the average gold abundance in ordinary surface

waters, suggesting that it must occur as some kinds of complex in the solutions. Further

examinations revealed that there are positive correlations between the concentrations of Au + and

C1- as well as between Au + and CO2(Fig.5a ) but no close correlation between Au + - F - and Au +-S

with respect to the fluids from Huangshan. Meanwhile, as shown by Fig.5b, the variation in Au +

may be more closely related to ES or CO2 than to F - or CI - in Zhilingtou. On the basis of these data,

it is reasonable to expect that gold must have been transported as the complex Au+-CI - or Au + -

CO 2 in Huangshan but as + + the complex Au - H S - or Au -CO2 in Zhilingtou. The difference in gold

species between the two deposits may be ascribed to the fact that the ore-forming solutions in


Huangshan have higher temperature and Na + / K + but smaller ZS / C1- in comparison with those in

Zhilingtou.

Mode of Occurrence and Mineralization of Gold

As indicated by microscopic study and electron microprobe data as well as by the partition of gold among ore minerals 161, gold and silver occur mostly as independent minerals in fractures

(referred to as fracture gold) or in inclusions (referred to as inclusion gold) in host minerals such as

Table 8. Comparison of gold fineness between indnaion gold and fracture gold from major gold deposits

Deposit Inclusion gold Fracture gold

Zhongao

Huangshan

Zhilingtou

Mali(prospect)

843(5)

9,58(3)

469(t)

6o30)

839(4)

944(4)

499(6)

600(6)

Analysed by Electron Microprobe Lab, Guangdong Bureau of Geology and Mineral Resources. Nos. of samples are given in

the parentheses.

Table 9. Calculated temperatures for eqnilihrimn ~ in gold del~ite

Deposit

Mineral assemblage

Zhilingtou

Argentite + kustelite + sphalerite

+ pyrite

Equilibrium equation 4Ag* + S 2 =2AgzS

AG, as a function of T -43800+20.8 T(Barton, 1900)

1 Sphahrite geothermometer

Huangshan

Calaverite + native gold + altaite + galena

+ sphalerite + pyrite

2PbS + AuTe 2 = Au + 2PbTe + S z

48705- 38.7 T(MilIs, 1974)

lg X~h = 6.65-- 7340 T- 1 _~1$a,2 (Scott ~ d ~ n e s , 1971)

Gold fineness of Au-Ag minerals 520

Iron content of. sphalerite (wt%) 10.7

Equilibrium temperature

Temperature of formation of host

minerals

248°C

300°C

* Silver occurring as solid solution in kustelite.

950

3.3

255°C

350°C


pyrite, arsenopyrite (in Babaoshan exclusively) and quartz. Under the microscope, close association

between Au-Ag minerals and sulfides such as chalcopyrite, sphalerite and galena indicates that a

contemporaneous deposition can be observed especially in fractures. And there is convincing

evidence that these sulfides of Cu, Pb and Zn have been formed after pyrite and quartz, ~he principal

host minerals of gold.

Despite the existence of evidence suggesting that inclusion gold is earlier than fracture gold,

being formed at about the same time with enclosing minerals, there is little difference in gold

fineness between fracture gold and inclusion gold (Table 8). This indicates that they may have been

formed at the same stage, under similar temperature conditions. Temperature calculations based on

mineral associations in equilibrium with inclusion gold show that inclusion gold was formed at lower

temperature than the host minerals (Table 9).

The above results show that gold minerals (including both inclusion and fracture gold) in this

area were all formed later than the host minerals (mainly pyrite and arsenopyrite) and

contemporaneous with polymetallic sulfides.

In the light of correlation statistics with regard to Au, Ag and other trace elements (Table 10),

only Co and As bear a definite relation to the system of Au-Ag, while Cu, Pb and Zn appear to

behave independently. Because Co and As are representative of pyrite and arsenopyrite, the above

relationship reflects that the abundances of Au and Ag in the ore are, to a large extent controlled by

the amounts of pyrite or arsenopyrite and have nothing to do with polymetalhc sulfides.

Table 10. Correlated element assoeiatlons in major gold deposits

Deposit N

Zhongao Huangshan

Zhilingtou

Babaoshan

Au, Ag, Co

Au, Ag, Co Au, Ag

Au, Ag, As

Pb, Zn, Mn, Ga

Zn

Pb, Zn

Zn

Ca

Cu

Cu

Cu, Ni

La, Ce, Y, Yb, Sr

I.a, Ce, Yb

La, Ce, Nb, Yb,

Ti, Cr, Ni

V, Sc

Ti, Y, Sc, V

Co, Li, V, Sc, Y

Ti, V, Co, Pb

Note: Based on R-type group and R-type factor analyses.

From the above discussions it can be seen that gold grade in this area is dependent on the extent

of pyritization (or arsenopyritization) but that gold mineralization took place later than pyritization

and shows a close genetic relation with polymetallic sulfides. Therefore, the mineralization of gold

can be thought as involving two separated processes, i.e., first, the precipitation of gold and second,

the formation of gold minerals. In the first step, gold is expected to be precipitated as extremely fine

particles (on the order of colloidal range) and dispersed into crystallizing host pyrite in response to

the precipitation of a large amount of pyrite which strongly disturbs the stability of gold complexes

as the ore-forming solutions evolved to meso--e'pithermal stages. In the second step, as a result of the

formation of polymetallic sulfides, the dispersed gold particles would migrate into fractures,

crystallographic fissures and defects in the host minerals, giving rise to independent gold minerals.


The overall process of gold mineralization in this area can be summarized by a geochemical model as presented in Fig.6 which illustrates the controls of the three major systel.as, i.e., the gold- bearing source strata, the ore-forming fluids and the ore deposits, in the process of gold mineralization.

ore-forming.solution en t e r i ng in to an envlronmen¢ where the gold complexee become unetable

I u°(dtspersed ] c o l l o i d a l p a r t i c l e s )

I Au (d iepersed) J gold-bearing formation

leached I by metamorphic heated recycling

connate water meteoric water

IAu+-Cl - A~-RS- AB÷-C021 ore-forming f l u i d (NaOl-K2S-type hydrothormal tolut iom r i c h in O02~and H2S )

p r e c i p i t a t i o n of p y r i t e

f o r n a t i o n b u ° ( i n d e p e n d e n t m i n e r a l ] l p o l ~ t a l l l ~ c of economic I n t e r e s t S ]

e u l f i d e 8 pyrite, arsenopyrite and other "'

heat minerals F~.6. Genetic modcl~rgolddepositsintheShao~-Longqu~ Up~tZone.

Conclusions 1. The gold-bearing strata in the Shaoxing-Longquan Uplift Zone, Zhejiang, are major source

rocks of gold for the gold deposits in this area. The different geochemical characters of these gold deposits are ascribed to geochemical differences between the source strata in which the deposits are distributed.

2. Mixture of metamorphic connate waters in the country rocks with varying proportions of heated Mesozoic meteoric waters constitutes the bulk of the ore-forming fluids, which are chemically comparable with their surrounding rocks due to prolonged material exchange between them throughout the geological history.

3. The ore-forming fluids are weakly acid-reducing NaCI-K~S-type hydrothermal solutions. They are rich in C02 and HzS, containing 0.6--4 ppm gold. Gold is expected to be carried in the solutions as the complexes Au+-C1 -, Au+-HS - and Au+-C02.

4. The crystallization of pyrite (arsenopyrite) in the ore--forming solutions led to significant precipitation of gold as colloidal particles disseminated in ore minerals. Later, the disseminated gold was activated in response to the formation of polymetallie sulfides, resulting in economic

No.4 GEOCHEMISTRY ~1

concentrating of independent gold minerals.

5. A generalized genetic model has been proposed for the various gold deposits in this area.

These deposits are the natural outcome of the interaction of different geological processes over a

prolonged period of time.

Acknowledgements

This work has been benefited by the support from a number of geological and metallurgical

units in Zhejiang Province and from Engineer Ji Ronggui of the Central Laboratory of Guangdong

Bureau of Geology and Mineral Resources. To all of them the authors are greatly indebted.

References

[1] Taylor, H.P.Jr., Geochemistry of Hydrothermal Ore Deposits, 2nd ed., by H.L. Barnes, John Wiley and Sons, New York, 1979, 2,36--277.

[2] Liangzihao et al., Geological Review, 4(19~5), 330---339 (in Chinese). [3] Ohmoto, H., Econ. Geol., 76(1972), 551--578. [4] Belevtsev, R.N., Journal of Geology, 42, 2(1982), 1--17 (in Russian). [5] Ellis, A.J. and Golding, R.W., Am. J. Sci., 261(1963), 47----60. [6] Luo Zhenkuan et al., Acta Mineralogica Sinica, 1(1985), 48--56 (in Chinese).

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