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Vol. 18 No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 1999
Sphalerite Chemistry, Niujiaotang Cd-Rich Zinc Deposit, Guizhou, Southwest China*
YE IAN (~h ~" ~ ) AND LIU TIEGENG ( ~ ] ~ ) ~ ) ( Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry,
Chinese Academy of Sciences, Guiyang, 550002)
Abstract Sphalerite in the Niujiaotang CA-rich zinc deposit, Duyun, Guizhou is characteristi-
cally light-yellow in color with significant enrichment of cadmium which ranges from 0 . 8 3 % to
1.97 % (averaging 1 .38 % ) in concentration in the mineral, corresponding to an enrichment
coefficient as high as 30 .47 to 72.96. In comparison with other major Pb-Zn deposits in the
world ( the Mississippi Valley deposits, and the Fankou and Jinding deposits in China) the Niu-
jiaotang deposit is n - n • 10 times richer in cadmium. Sphalerite in the deposit is also rich in
Ga and Ge, but poor in In, Mn and Fe, suggesting some special mechanisms that govern the
geochemical behavior of these trace elements. Except for a minor amount of independent min-
erals like greenockite, cadmium occurs mainly as isomorphous impurity in the crystal lattice of
sphalerite. During weathering and leaching under supergene condition, cadmium was separated
from Zn, resulting in some secondary minerals of CA, including oxides and otavite.
Key words: Niujiaotang Cd-rich zinc deposit; sphalerite; Cd
Geological Setting
The deposit is located between the "N-S Running Tectonic Deformation Area" in the South Guizhou Platform Depression (a part of the Yangzi Platform) and the South China Early Paleozoic Fold Zone. It constitutes the south end of the Pb-Zn mineralization zone in eastern Guizhou which extends northwards through Kaili, Zhenyuan, Tongren to Songtao (Cheng Guoyong et al., 1992). The Zaolou fault system, the major structure in the area, runs NE through the field, cutting across the Wangsi Anticline. Outcrops in the area range in age from Upper Sinian to Cambrian ( the Loushanguan Group) , among which the Wuxun Formation (Upper Sinian-Lower Cambrian) is composed of carbonate and fine clastic rocks, carbonaceous shales, mudstones and siliceous rocks. The orebody is hosted in algal limestone of the Qingxu Formation overlying the Wuxun Formation. The age of the deposit has been assigned to Late Caledonian ( 4 7 5 - 499 Ma) based on lead isotopic determinations (Wang Huayun, 1993).
The Niujiaotang deposit consists of four sectors (Wangjiashan, Zuowantian, Mapo and Sizidong, see Fig. 1 ) with a total reserve (in C + D + E grade) of over 0 .35 million tons of zinc and 5299.14 tons of cadmium. The orebodies are lenticular, dipping at 5* - 25 ~ and measuring 1.10 - 2 . 0 3 m in thickness, and are more or less conformable with the surrounding strata (Fig. 2) . The ores contain 5 .85% - 24 .48% Zn. Major ore minerals are sphalerite, smith- sonite, pyrite and small amounts of galena, hemimorphite, limonite and some oxides of cadmi- um. Gangue minerals are mainly dolomite in addition to subordinate calcite and quartz. Ore
ISSN 1000-9426 * This project was jointly supported by the National Natural Science Foundation of China (No. 496-33110) and the Laboratory Foundation of ()re Deposit Geochemistry of Chinese Academy of Sciences.
No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 63
texture and structure are variable from place to place, described by previous authors by a variety
of terms such as fine anhedral mosaic and euhedral crystalline, spherical, fractured, replace-
ment remnant, disseminated, massive, veinlet and so on. Dolomitization is the major type of
alteration around the orebodies and pyritization and silicification are of less importance.
F5 12
E2S I
13 i ~ i r exq 2
j J
~lq ;t
etp2
6 O / 21
Fno
l~2S I x
ip 2
0 200 4?Ore e~P2 V
'55 ,~lq2
[- 21 9 17 3 ~ 4 ~7111
[ " ~ 6 [iTY-"] 1 3
Fig. 1. Geologic sketch map of the Niujiaotang zinc deposit, Duyun, Guizhou. 1. Middle Cambrian Shilengshui Formation, section II ; 2. Middle Cambrian Shilengshui Formation, section I ; 3. Middle Cambrian Gaotai Formation; 4. Lower Cambrian Qingxudong For- mation, section 1] ; 5. Lower Cambrian Qingxudong Formation, section I ; 6. Lower Cambrian Balang Formation, section I1 ; 7. normal fault; 8. reverse fault; 9. fault of un- known nature; 10. axis of fold; 11. stratigraphic boundary; 12. orebody and No. ; 13. stratigraphic attitude.
Sphalerite Chemistry
Sphalerite in the deposit is poorly crystallized showing subhedral to anhedral forms and is
mostly light yellow in color. Brownish, reddish brown and black sphalerites are found occasion-
ally. Chemical analyses of pure sphalerite samples (containing less than 2 % impurities as exam-
ined by X-ray diffraction by Gong Guohong of the Institute of Geochemistry, Chinese Aeademy
64 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 18
tl t / / / - _ _ - - ,
3160-as' ._...]/ / ~ " ~ - - ~ - - - " ~ 0 40 80m
i,03 F,
t ~ y;q- InNg Iizl9 l;;zll0 7311 1 12 I -q13 F~ Fm
Fig. 2. Geological cross-section, Niujiaotang zinc deposit, Duyun, Guizhou. 1. Oolitic dolomite of great or intermediate thickness; 2. shales; 3. thick pisolitic dolomite; 4. thick dolomite; 5. dolomite of great or intermediate thickne.~; 6. argillaceous dolomite of great or intermediate thickness; 7. thick and thin limestones; 8. orelxxly and No. ; 9. reserve fault; 10. normal fault ; 11. stratigraphic boundary, reconstructed; 12. stratigraphic sym- bol; 13. direction of hydrothermal fluid movement.
of Sciences) of different colors are listed in Table 1. It is striking that the sphalerite is highly
enriched in Cd but poor in Fe. The Z n / C d ratios are between 3 1 . 2 7 and 75.25, averaging
46.12, with concentration coefficients as high as 30.47 - 72.96 (averageing 51 .10) . In addi-
tion, microprobe analy~s, by the authors and Geological Team No. 104 of Guizhou Province, also show that Ga and Ge are present in significant amounts in the sphalerite. Apparent differ-
ences can be recognized in chemical composition among sphalerites of different colors. The
black sphalerite is relatively rich in Fe but poor in Cd, S and Zn (S 3 1 . 1 6 % , Zn 6 3 . 0 0 % , Fe 2 . 3 5 % , Cd 1 .15% ) with a Zn/Cd ratio of 57 .01 and the concentration coefficient of Cd of
42.52. The brown sphalerite is intermediate in Fe, Cd, S and Zn concentrations (S 31.48 %,
Zn 63.49 %, Fe 0 .90 %, Cd 1.37 % on average), with a Zn/Cd ratio of 45.42 and CA concen-
tration coefficient of 50 .85. The light yellow sphalerite is impoverished in Fe and is rich in Cd, S and Zn (S 32.38 %, Zn 6 4 . 6 6 % , Fe 0 . 8 2 %, Cd 1 . 6 2 % ) , with a Zn/Cd ratio of 36 .04
and Cd concentration coefficient of 59.94. No apparent correlation can be established with re-
spect to Pb, Cu, Ag and Mn among the sphalerites of different colors. It is noticed that cadmium is positively correlated with zinc, with a correlation coefficient
of 0 .46 . Meanwhile, it is to some extent negatively correlated ( - 0 . 1 8 ) with iron but shows
no relation with sulfur.
No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 65
Table 1. Chemical compositions of sphalerite, Niujiaotang deposit ( x 10 -6)
Sequmce ~ S(%) Zn(%) Fe(%) Cd(%) Pb Ag Ca Ga Ge h Mn Zn/Cd Cd,,~nnatim No. ccdfzi~t
1 ~ 32.52 62.47 2.41 0.87 682 14.49 0 71.80 32.32
2 Black 33.86 62.46 2.99 0.82 379 12.62 0 75.25 30.74
3 Black 33.21 60.55 2.98 0.03 758 10.36 0 72.95 30.47
4 Black 30.72 63.74 2.09 1.52 552 5.64 89 0 39.55 56.30
5 Iladt 30.72 63.92 0.78 1.45 431 18.08 73 0 42.51 53.70
6 llaek 29.64 64.69 1.14 1.52 605 11.30 64 0 40.77 56.30
7 Ilaek 27.82 63.19 4.80 1.02 500 12.71 69 0 56.23 37.78
A~nge Elaek 31.16 63.00 2.35 1.15 558 12.18 74 0 57.01 42.52
8 ~ 30.96 62.32 0.86 1.31 631 12.43 80 0 45.88 48.58
9 ~ 30.41 63.21 0.97 1.50 479 12.42 60 0 40.47 55.56
10 lh~na 32.46 63.73 0.82 1.12 645 15.54 67 0 55.11 41.48
11 K'~ca 32.08 64.70 0.95 1.56 750 12.71 74 0 39.81 57.78
Av~'g,e Bn~wn 31.48 63.49 0.9{) 1.37 601 13.28 70 0 45.32 50.85
12 Light yd~ 34.17 65.91 0.77 1.50 152 51.45 0 43.94 55.55
13 Light ydk~ 32.16 66.36 0.80 1.54 833 53.13 0 43.09 57.04
14 Light ydlow 33.61 64.09 0.71 1.46 909 42.8~ 0 43.90 54.07
15 Light~:nv 30.66 65.11 0.35 1.06 289 1.40 41 0 58.22 39.06
16 Lightydk~ 30.21 63.17 0.98 1.64 526 59.32 177 0 36.89 60.74
17 Lightydk~ 31.86 64.64 1.01 1.97 658 9.89 54 0 31.27 72.96
18 Lightydlmv 31.60 63.85 1.11 1.76 539 36.72 106 0 34.61 65.19
19 Lightydl~ 33.85 64.30 1.53 1.96 605 73.45 165 0 30.82 72.96
20" IAght ydbw 32.81 64.62 0.45 1.92 2300 0 0 33.66 71.11
21" Light ydl~ 32.85 64.57 0.50 1.37 2000 0 0 47.13 50.74
A~,~ge Lightydl~ 32.38 64.66 0.82 1.62 564 41.02 107 2150 0 36.04 59.94
22 Light ydl~ 54.93 0.57 1500 2900 1 96.36 21.11
23" Light ydlow 33.17 65.63 0.51 100 160 128.38 18.89
Analyst: Li S t n ~ f m m I r ~ t u t e d ~ , ~AndmydS:ieaoes. Sml~markdby * azt ~ malyses. ~mdsmt&Nas. 22 and23 are fmm'Retxrt d detaihdia'a~s~ationdthe Maposect~ Niuiixxang ck~t, l),r/un" (No.104 Team, C,6n~lhmau d C, n i ~ aKI btan- al Resan~ 1994); CA mnomtntim a:dfi6ent = smpte oantmt/240g/t.
Mode of Occurrence, Source and Enr ichment of Cadmium
Mode o f occurrence
A s is wel l k n o w n , c a d m i u m is s i m i l a r to z inc in g e o c h e m i c a l b e h a v i o r o w i n g m a i n l y to
t h e i r s i m i l a r i t y in t h e t e t r a h e d r a l cova l en t b o n d a n d o t h e r c ry s t a l s t r u c t u r e s . T h i s a c c o u n t s for
t h e o b s e r v a t i o n t h a t c a d m i u m in n a t u r e o c c u r s m o s t l y in s p h a l e r i t e as i s o m o r p h o u s i m p u r i t y .
T h i s is a lso t r ue in t h e N i u j i a o t a n g depos i t . M i c r o p r o b e a n a l y s e s s h o w t h a t c a d m i u m is p r e s e n t
in v e r y h i g h c o n c e n t r a t i o n s ( T a b l e 1) a n d is d i s t r i b u t e d u n i f o r m l y in s p h a l e r i t e , i n d i c a t i n g n o
p r e s e n c e of a n y i n d e p e n d e n t c a d m i u m m i n e r a l . H o w e v e r , c a d m i u m w o u l d u n d e r g o s e p a r a t i o n
f r o m z inc , p a r t i c u l a r l y u n d e r s u p e r g e n e c o n d i t i o n s , to r e s u l t in i n d e p e n d e n t m i n e r a l s of c a d m i -
u m . A s w e no t i ced , for e x a m p l e , ( 1 ) T i n y c r y s t a l s ( + 5/~m ac ros s ) of g r e e n o c k i t e ( C d S ) a re
f o u n d a r o u n d p r i m a r y s p h a l e r i t e in some places in t h e d e p o s i t . T h e g r e e n o c k i t e in t h i s case m a y
a lso be of p r i m a r y o r i g i n . ( 2 ) S o m e b r o w n i s h e a r t h y - l i k e m i n e r a l s , p r o b a b l y o x i d e s of c a d m i u m
or g r e e n o c k i t e , a re c o m m o n l y r e c o g n i z e d as e n c r u s t a t i o n o n z inc o x i d e o res . ( 3 ) O t a v i t e (CA-
66 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 18
CO3) grains ( + 8~m in diameter) are found in smithsonite in the oxidation zone. These inde-
pendent cadmium minerals, except for the primary greenockite, are mainly present in the oxi- dation zone of the deposit and are most likely of secondary origin. Cadmium may have been sep- arated from sphalerite during oxidation as CdSO4 which may in turn react with existing spha-
lerite to form greenockite and, on the other hand, otavite may have resulted from the interac- tion between cadmium oxides and CO2 according to the following reactions:
CdSO4 + ZnS--~CdS + ZnSO4
o r
C d O + C O - 2 - ~ C d C O 3
Source and enrichment of cadmium
It is noticed that cadmium is ( 1 0 - 2 0 ) • 10 -6 in dolomite in the area and is up to (40 - 340) • 10 -6 in the dolomite wall rocks within the deposit. It is ten to thousand times higher than the background abundance of cadmium of about 0 .2 • 10- 6 in the crust ( Liu Yingjun et al . , 1984). Obviously, the area is unique in its abnormally high cadmium background.
During the Longwangmiao period of Early Cambrian, the area was a shallow-sea environ- ment marginal to a platform (Cheng Guoyong et al. , 1992). Zinc and cadmium in seawater were concentrated in abundant algae via some biological mechanisms resulting in large amounts of algal ooliths which were rich in gn and Cd. Then, the zinc and Cd in the sediments were ac- tivated during dolomitization in the process of diagenesis, giving rise to sphalerite being di~em- inated throughout the strata. These sphalerite-bearing bodies are in conformity with the strata, forming zones of mineralization or, in favorable conditions, low-grade orelx)dies. These zinc and cadmium, particularly the Qingxudong Formation, were further activated by hot brines that rose along the gaolou Fault System during Middle and Late Caledonian and superimposed upon pre-existing mineralizations, resulting in the zinc orebodies as we see today.
Table 2. Comparison of sphalerite compositions among Pb-Zn deposits of different types ( x 10 -4)
s~und ~ t l~=/t t~e
in e~tktm hm
Fe{%} Za(%) Cd{~}
Ag Ga
C,e
In
Mn
~Cd CA cmcestntm
Pdyma~ de~t d ~an~hyd~d~nal
delmt d sub
~kan~h~a~aenml
acn
Upper Niuim0taag Fank~ Xim6eshan Xit ie~ Caijiayiag Yiz~mn
4.11 11.68
64.43 55.37
0.80-4.80 5.01 1.41
60.55-65.11 59.90 65.09
5.96 4.20
62.94
0.83-1.97 0.19 0.23
1.40 - 73.45 195 21
160 - 2300 345 95
160 - 2900 95 102
+I 7
tm~ 125 49
0,31
202
20
<1
38
0.20 0.29 0.22
18 27 1112
9 2 50
0.2 7 4
431 68 142
1750 688
03df_2wlmt
31,27-75.25 323.81 203.60
30.47-72.96 7.04 8.52
207.84 276.85
11.52 7.41 10.74
286.9
8.15
DetadI~iactaag m tn:~ ~ m t l ~ thosedCaiiiaying m bxn P~n J~yong et al. {1993}, ~ edzrs ~ from~mg Qi~n (1987).
I~hgma.c hydmthamal sed2caa~tm
, ~ u i k ~ ~lish Ishrds
3.81
56.30 60.00
0.25 0.39
118 30
55 96
3 5
47 2
2770 67
225.20 153.85
9.26 14.44
No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 67
2.0
1.5
1.C
0.5
Fe=5% N
\ " o e o \
\
k o e \
I tx x \ ~o \
\
�9 �9 Irl \\
I I
! \
Fe(%)
d ~
1 / / / /
/ /
/ ! I I
l l0 -
CdxlO
F MnxlO
Fig. 3. Cd-Fe diagram of sphalerite, x Black spha-
lerite; A brown sphalerite; �9 light yellow spha-
lerite. (for legends, see Fig. 5).
Fig. 4. Fe- (Mn • 10)-Cd diagram of sphalerite
(for legends, see Fig. 5).
10,
8
6
/ /
f f
J / ' /
/ / ol
\ \ ,~Glgln>l
I / / Ca/ln=l
m / / / / *5 / /
2 . " *" / " 1"1 ", c <1 ,/ N / / /olO / / \
/ 11/ / I I I / o8// I
o 2 4 6 s hln
Fig. 5. lnln-lnGa diagram of spha]erite. I . Magrnatic hy-
drothermal deposit; II . volcanic hydrothermal deposit; III .
sedimentary reworked deposit. 1. Niujiaotang; 2. Missis-
sippi; 3. Central Asia; 4. Pb-Zn deposits in Hunan; 5. Fankou; 6. Jinding; 7. Xitieshan; 8. Xiaotieshan; 9.
Huangshaping; 10. Dabaoshan; 11. Qibaoshan.
D i s c u s s i o n s
As is generally held (Liu Yingjun et al., 1984), sphalerite of light colors is formed at lower temperatures and contains higher Cd, Ge and Ca but is poor in Fe and In. Sphalerite in the Niujiaotang de- posit is mainly bright yellow colored and is extremely high in Cd (0.83 % - 1.54 %, mostly over 1.10%, with a maximum of 1.97% detected), i. e. , several tens of times higher than in other Pb-Zn deposits such as Jinding, Fankou and Dongsheng- miao. The sphalerite is also rich in Ge and Ga but relatively poor in Ag, Fe, Mn and In. As shown in Table 2, these chemical characters are similar to those of sphalerite in strata bound Pb-Zn deposits in carbon- ate hosts but differ from those of hy- drothermal Pb-Zn mineralizations. Also, Cd in the sphalerite is several times higher
than in sphalerite in Pb-Zn deposits of syngenetic origin. The characteristic high level of CA in the deposit reflects a characteristically unique behavior of cadmium during the formation of the deposit that merits special attention.
Based on his studies of trace elements in sphalerite from 36 major typical Pb-Zn deposits throughout the world, Zhang Qian (1987) noticed that sphalerite of different origins can be readily distinguished in lnIn-lnGa, Fe-(Mn • 10)-Cd and Cd-Fe diagrams (Figs. 3, 4 and 5).
68 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 18
In these diagrams, as can be seen, samples of the Niuj iaotang deposit are all p lo t ted in the field
of sed imen ta ry - reworked deposits .
In summary , the Niuj iao tang deposit should belong to a s t ra ta bound deposit in carbonate
rocks formed at low t empera tu re s through sed imenta t ion and rework ing . Cadmium is enriched
in the sphaleri te to a s ignif icant extent , wi th a reserve of over 5000 tons, const i tut ing a large
deposit of this e lement . Ga and Ge are also found in high levels and are of economic interest as
byproducts . Fe, In and Mn are relat ively low in the deposit . Ca dmium and o ther trace ele-
men t s occur most ly as i somorphous impur i t ies in sphaler i te lat t ice. Greenocki te is present in
small amounts . Some secon.dary minerals of cadmium were formed in response to weather ing
and leaching.
References
Cheng Guoyong, Zhong Yitian, and Huang Genshen, 1992, Geological characters and conditions of mineraliza- tion of the Niujiaotang zinc deposit, Duyun, Guizhou: Geology of Guizhou, v. 9, n. 3, p. 203 - 211 (in Chinese).
Liu Yingjun, Cao Liming, I.i Zhaolin et al., 1984, Geochemistry of individual elements: Beijing, Science Press, p. 372 - 377 (in Chinese).
Pan Jiayong, Zhang Qian, and Ri Zhongyao, 1993, Trace elements in major ore minerals, Caijiaying lead-zinc dep~it, Hebei: Journal of Guilin Metallurgical and Geological Institute, v. 13, n. 4, p. 386- 393 (in Chi- ne~se ) .
Wang Huayun, 1993, Geochemical characteristics of lead-zinc deposits in Guizhou: Geology of Guizhou, n. 4, p. 274 - 289 (in Chinese).
Wang Huayun, 1996, Regularities and mode of genesis of lead-zinc mineralizations in eastern Guizhou: Geology of Guizhou, v. 6, n. 1, p. 7 - 289 (in Chinese).
Zhang Bizhi, Qin Ming, and Li Mingdao, 1994, Mineralization conditions and genesis, Niujiaotang zinc ore- field, Duyun: Geology of Guizhou, v. 11, n.4, p .287-293 (in Chinese).
Zhang Qian, 1987, Trace elements in galena and sphalerite and their geochemical significance in distinguishing the genetic types of Pb-Zn ore deposits: Chinese Journal of Geochemistry, v. 6, n. 2, p. 176 - 190.