SCIENCE IN CHINA (Series D)
VOI. 41 ~ 0 . 4
~ u g u s 1998
t
Helium and argon isotopic geochemistry of
Jinding superlarge Pb-Zn deposit *
HU Ruizhong (
)
', G. ~ u r n e ?, P . G. Burnard2, ZHONG Hong ( @
%)l ,
YE Zaojun ( D t B T ) ' and BI Xianwu (+@&)I
( 1. Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China; 2 . Department
of Earth Sciences, University of Manchester, Manchester M I 3 9PL, UK)
Received November 14, 1997
Abstract
The study results of He and Ar isotopes from fluid inclusions in pyrites formed during mineralization stage
of Jinding superlarge Pb-Zn deposit in west Yunnan, China are reported. The data show that the 40Ar/36Arand 3He/
4He ratios of fluid inclusions are respectively in the range of 301.7-385.7 and 0.03-0.06Ra,
suggesting the oreforming fluid is a kind of air saturated meteoric groundwater. On the basis of research on coupled relationships among
He, Ar, S and Pb isotopes, the evolution history of ore-forming fluid of the deposit can be summarized as (i) air saturated meteogenic groundwater infiltrated down and was heated+ (ii) leached S, C and radiogenic He, Ar from the
(iii) leached Pb and Zn from mantle-derived igneous rocks located in the bottom of the basin+ (iv)
basinal strata
ore-forming fluid ascended and formed the deposit. Due to this process, the isotope signatures of crustal radiogenic He,
atmospheric Ar (with partial radiogenic 40Ar), crustal S and mantle-derived Pb remained in the ore-forming fluid.
+
Keywords: Jinding superlarge Pb-Zn deposit, fluid inclusion, ore-forming fluid, He and Ar isotope, air saturated
meteoric water.
Jinding Pb-Zn deposit, which was found in the 1960s, is the largest Pb-Zn deposit in China,
also one of the 17 superlarge Pb-Zn deposits with Pb + Zn> 10 000 000 t all over the world. Many
researches have been made on the ore-forming process since it was found. According to the fact
that lead isotopic compositions of ore minerals in the deposit are of mantle origin signature, it was
suggested that the formation of the deposit was directly related to the mantle.
Noble gases especially He and Ar, possessing distinct isotopic compositions between the
crustal and mantle reserviors, can be used as an extremely sensitive tracer to display the process of
crust-mantle interaction. Because of the specific property, they have been extensively employed to
trace the sources and water-rock interaction histories of contemporary crustal fluids since the
1 9 6 0 s " ' ~ ~It
. has been realized that fluid inclusions in pyrites can well retain noble gases since the
end of the 1980s and their diffusive loss from the fluid inclusions is not significant within 100 Ma,
even for the most easily diffusive ~ e ' " . Based on the result, the He and Ar isotopic compositions
of some ancient ore-forming fluid were investigated, depending on the analytical data from fluid
inclusions of minerals in the deposits, thus extending the use of noble gas isotopes in the study of
ancient ore-forming f l ~ i d [ ~ - ~AS
] . one of a few solid deposits which have been studied on He and
Ar isotopes so far, this paper reports the results of He and Ar isotope from fluid inclusions in
pyrites formed during mineralization stage of Jinding superlarge Pb-Zn deposit, and through the
* Project supported
by A30 Project of the National Climb~ngProgram of China and University of Manchester.
No. 4
He & Ar ISOTOPIC GEOCHEMISTRY OF JINDING SUPERLARGE Pb-Zn DEPOSIT
443
combined consideration on information about S and Pb isotopes of the deposit provided by previous
researches, the evolution history of ore-forming fluid is further discussed.
1
Geological setting
Jinding superlarge Pb-Zn deposit, located in Lanping Country, western Yunnan Province,
occurs at the northern edge of Lanping Mesozoic-Cenozoic Basin controlled by Lancangjiang large
fault and Honghe-Ailaoshan large fault. The deposit is situated in a Cenozoic arch structure within
the basin, from north to east, surrounded by six ore blocks, such as Beichang, Paomaping, Jiayanshan, Xipo, Baichaoping and Fengzishan, with the total area of less than 10 km2. The orebody is characterized by bedded form, then layered form, lens and veins, etc. The major ore minerals are pyrite, galena and sphalerite, with ore-forming temperature ranging from about 200 to
250°C . Beichang is the main ore block of the deposit with reserve more than 85 % of the total[91.
The basinal strata are composed of the Triassic to Tertiary system, with the deposit hosted in
the Tertiary clastic rocks. Six salt-bearing formations occurred in the basin (located in T3,J2, K2
and
repectively), accompanied by more than 120 salt deposits and ore spots (sylvite, halite
and gypsum), in response to abundant C1 and S in the basinal strataLg1.Additionally, background
Zn 93 x
values of Pb and Zn in basinal volcanic rock strata are also very high: Pb 113 x
in TZPvolcanic rock, Zn 104 x
in TZcvolcanic rock. The same trend is also shown in
Zn 110 x
in trachyte,
Cenozoic igneous rocks lying to the east of the basin: Pb 62 x
in sinaite, much higher than the crustal clark value of corresponding rocks, while
Pb 119 x
and Zn near
the average Pb value of the other sedimentary rocks in the basin is about 35 x
the crustal abundance of western Yunnan and also the crustal clark value[g1.
2 Analytical methods and results
The samples of this study are pyrites formed during the ore-forming stage, which are taken
from Beichang, Jiayanshan and Xipo ore blocks. An all-metal mass spectrometer (Mass Analyser
Products 215), with a resolving power of > 700, was used for helium and argon isotope measurements of fluid inclusions in pyrites at Universi~yof Manchester. The analytical methods were
described in detail by Stuart et al. '4,81. The concrete procedure is: ( i ) load approximately 50500 mg pyrite of 0.5-1.5 mm grains, which has been washed in actton ultrasonic bath and then
baked, into screw-type crushers; ( ii) heat the sample, which has been loaded into the crew-type
crushers and onlined with the gas extraction system, at < 150°C for 24 h to remove adsorbed atmospheric gases before crushing, meanwhile pump the gas extraction system to a high vacuum;
(iii) crush the samples in the system of high vacuum( about
mbar), and extract the gases
hosted in fluid inclusions of samples, and then purify the extracted gases; (iv) transfer the purified helium into mass spectrometer for He isotope measurement; ( v ) transfer the purified argon
into mass spectrometer for Ar isotope measurement.
Analytical results of He and Ar isotopic compositions from fluid inclusions of the deposit are
shown in table 1 , with analytical uncertainties within f 10 % . 4 ~ abundances
e
are 5.4-17.4 x
~ o - ' c ~ ~ s Tand
P / 4~ 0 ~1.6-8.7
r
X 1 0 - 7 c m 3 ~ ~,~indicating
/g
a minorvariation on He, Ar
abundances of fluid inclusions from various samples. These samples are characterized by ( i ) all
the pyrites analyzed have fine crystal forms and have not been modified since they are formed,
6 . 9 3 1 10-'"2.335~
~
lo-'
336.97f9.55
0.0748 1 . 9 4 4 ~ 1 0 - ' 5~ . 1 0 6 ~ 1 0 - ~7 . 4 1 6 ~ 1 0 - " 2 . 6 2 9 ~ 1 0 - '
0 . 1 3 3 0 2 . 5 9 2 ~ 1 0 - ' ~7 . 1 7 3 ~ 1 0 - ' 2.401x10-'0 7 . 7 1 5 ~ 1 0 - '
354.48i11.65
653.95f18.58
0.3519 2 . 5 9 2 ~ 1 0 - l 46 . 1 3 0 ~ 1 0 - ' 8 . 8 4 4 ~ 1 0 - ' ~ . 0 7 3 ~ 1 0347.51k9.85
-~
8 . 4 2 4 ~ 1 0 - l 52 . 2 6 9 ~ 1 0 - ' 1 . 9 1 3 X 1 0 - ' ~ . 3 7 8 X 1 0 ~ 385.71f11.20
~
1 . 7 5 0 ~10-14 3 . 8 6 1 ~
1 . 2 9 3 ~ 1 0 - '4~. 0 4 8 ~ 1 0 - ~313.20f 10.25
3He
Rc/Ra
0.04i0.00
-
-
-
-
4He
"Ar
/ c ~ ~ S T P . ~/ c- ~' ' s T P . ~ - '
0.03f0.00
6 . 8 2 6 ~ 1 0 .3~. 5 1 4 ~ 1 0 - ~
fluid inclusions. It is only a rough standard of their actual concentration in fluid inclusions.
sphere; sample weights are the fraction which passed through a 100 pm sieve after crushing; the "concentration" of 4 ~and
e 40Aris relative to the amount of hosted mineral instead of the
in excess of that derived from the atmo-
2 . 6 2 ~ 1 0 - ~1 f. 4 8 ~ 1 0 0.03k0.00
-~
1 . 0 8 x 1 0 ~ 5 i 5 . 8 2 X 1 0 ~ 7 0 . 0 3 ~ 05. .0309 4 x 1 0 ~ 7 5 . 8 0 0 ~ 1 0 ~ 7
2 . 9 3 ~ 1 0 ~ ~ f 2 . 6 10.03k0.00
~ 1 0 ~ ~ 1 . 7 4 2 ~ 1 0 -8 ~. 7 3 3 ~ 1 0 - '
4.4X10-5f7.18X10-6 0 . 0 3 f 0 . 0 0
2 . 5 2 1~ 0 - 5 f 2 . 3 1x
8 . 7 7 ~ 1 0 - ~ i 4 . 9 4 ~ 10.05f
0 - ~ 0.00 0 . 6 6 0 ~lo-' 1 . 5 8 7 ~ 1 0 - ~
0.00011k6.08X10~6
6 . 9 9 ~ 1 0 - 4~. i2 3 ~ 1 0 -0.06f
~
0.00
/'%
Measured at University of Manchester; 3He/4He ratlos (Rc) are expressed relative to the air ratio (Ra) 1.39 x 10 - 6 ; 'OAr expresses the
1
JD17 pyrite
total
2
1
pynte
1
total 0.255 1 1 . 1 3 4 ~ 1 0 -1~. ~
699~
JD15 pyrite
JDll
40Ar
/36Ar
6 . 3 0 0 ~ 1 0 - ' ~l . l O O ~ l 0 - ~
5 . 7 1 1 ~ 1 0 - " 1 . 8 7 2 ~ 1 0 - ~327.70i 10.12
36Ar
@Ar
/C~~STP /C~~STP
2
4He
/cm3sTP
5 . 0 4 0 ~ 1 0 - ' 5~ . 9 8 8 ~ 1 0 - 7~ . 2 1 4 ~ 1 0 - " 2 . 1 7 7 ~ 1 0 - ~301.71211.98
3He
/cm3sTP
1
Hosted
Sample
Crush
mineral
wet./g
JD14 pyrite
Sample
Table 1 He and Ar isotope compositions of fluid inclusions in pyrites of Jinding deposit
No. 4
He & Ar ISOTOPIC GEOCHEMISTRY OF JINDING SUPERLARGE Pb-Zn DEPOSIT
445
suggesting the primary inclusions are dominant in pyrites, which is consistent with the fact that
fluid inclusions from transparent minerals associated with the pyrites are dominated by primary
ones; (ii) the analytical samples are all pyrites less than 65Ma, so the loss of He trapped in inclusions can be neglected13]; (iii) even if using a U abundance of 3 x 1 0 - ~ ( m abe~much higher than
( T h is hardly dissolved in hydrothermal fluid),
the actual value of fluid inclusions), Th/U-0
and the mineralization age of 65Ma (the deposit is hosted in Tertiary clastic rocks and 65Ma is the
e the
upper limit of Tertiary) as the boundary conditions to subtract in situ radiogenic 4 ~ since
~ e being subtracted and
fluid inclusions were trapped['01, the difference between the 3 ~ e / 4 after
the measured ratio is only within the analytical uncertainty; (iv) K abundance is very low in
r
by K decay cannot be very high. On the basis of the fact above, the
pyrite, the 4 0 ~production
analytical results of He and Ar isotopic compositions shown in table 1 can roughly represent the
initial value of primary inclusions or ore-forming fluid.
Due to the very low atmospheric He abundance relative to other primordial volatiles, the
problem of contamination on crustal fluid by atmospheric He is generally unimportantt8'"]. Cosmogenic helium produced by spallation reactions is also considered an unlikely source of He as all
samples for the study were taken from
underground workingL4' . Therefore, it may be concluded that there are
only two possible He sources in fluid inclusions analysed, the crust and the
mantle.
. 0.2
It can be found that the He and Ar
o . 1s
isotopic compositions of ore-forming
fluid are not characterized by mixing of
- - --- - - - -- -- -- ------ --different isotopic reservoirs and no inCrust
formation of mantle-derived noble gases
can be identified from the 40
Ar
200
300
400
500
600
700
'@~r/~~Ar
vs. Rc/Ra and 3 ~ e / 3 6VS.
~ r4 0 ~ r / 3 6 ~ r
~ 1 shows
b
l Fig.~1. The plot of 40Ar/36Arvs. Rc/Ra of fluid inclusions. Rc expresses
plots ( figs 1 and 2 ) . ~
that the 3He/4He value of samples and Ra is the 3 ~ e / 4 value
~ e of air.
that 3 ~ e / 4and
~ e40Ar/36Arof ore-form700ing fluid are 0.02-0.06 Ra and 301.7385.7 ( except JD15 ) respectively, with
/
600 ,
Mantle
Ar isotopic composition matching air satu500 rated water (40Ar/36~-295.5)[41and He
isotopic composition mainly corresponding
to crustal radiogenic He ( 3 ~ e / 4 ~ e
1-0.01--0. 05Ra, Ra is atmospheric 3 ~ e /
saturated water
4 ~ r ae t i ~ ) ~ ~ .Obviously,
'~].
the ore-form200
ing fluid is a kind of air saturated water
1001
(meteoric water or connate water) having
,.
*
-1
I
obtained
S
radiogenic He from crust. On the
0.00004
0.00008
0.00012
o
I
3He/36~r
Fig. 2. The plot of 3He/36ArVS. 40Ar/36~r
of fluid inclusions.
other hand, the 4 0 ~ r / 3 6ratios
~ r of all the
samples are a little higher than that of
SCIENCE IN CHINA (Series D)
446
Vol. 41
atmosphere (295.5), indicating that there exist some excess 4 0 ~inr the samples. The facts imply
that the temperature of the air saturated water is relatively high (higher than Ar closure temperature of general K-minerals, e. g . 200°C ) . Therefore, during the interaction between fluid and
rocks that the fluid flowed through, the fluid obtained not only the crustal radiogenic He but also
partial radiogenic 4 0 ~ r especially
,
for sample JD15. It is consistent with the fact that the oreforming temperature of the deposit is generally higher than 200°C .
3
Discussion and conclusion
The large dataset[13] indicates that Pb isotopic composition of the deposit ( 2 0 6 ~ b / 2 0 4=~ b
18.150-18.558,
2 0 7 ~ b / 2Pb
0 4= 15.252-15.640,
~
b Pb =
/ 37.80-38.66)
~
~
~ is characterized by mantle-derived Pb, thus the formation of the deposit is once considered to be related to the
deep process. In fact, the mineralization of the deposit may not have the relationship to mantle
fluid. Main evidence is shown as follows.
3 . 1 No mantle-derived He in ore-forming fluid
Helium has two kinds of isotopes, 3 ~ and
e 4 ~ eGenerally
.
there are four possible sources for
He in the crustal fluid, mantle, crust, atmosphere and cosmogenic He. The He abundance in the
atmosphere is very low. The atmospheric He does not significantly affect He abundance and isotopic compositions of crustal fluid[8,111.For the samples taken from underground or places exposed to the earth's surface for less than 100 years (can rule out the possibility of existing cosmogenic 3 ~ inethe ~ a m ~ l e s ' ),
~ , only
~ . ~ the
] crust and the mantle are the possible source areas for
He. This is the case of samples from Jinding deposit. Mantle-derived He is the remnant of primordial He of the earth, and rich in 3 ~ e compared
,
with crustal He, and 3 ~ e / 4 about
~ e 69Ra. He in continental crust is radiogenic, which is the product of U and T h decay and consequent neutron recations (mainly with Li) . Although its 3 ~ e / 4value
~ e is dependent on U, T h and
Li contents in the crust, the typical value is about 0.01-0.05 ~ a [ ~The
. factor
~ ~ of
~ about
.
1000
difference between the helium isotope ratios of the mantle and the crust makes He isotope method
more effective than the other methods in determining whether there exist mantle-derived components involved in crustal fluid or not. Even if a little mantle-derived He is added to crustal fluid, it
can be easily identified by He i s ~ t o ~ e [ ~l4-l6]
- ~ ' ~. On
' the basis of research above, however, there
is no mantle-derived He involved in the ore-forming fluid of Jinding deposit.
3.2
Crustal source S in ore-forming fluid
According to previous study on Jinding deposit[91, P4svalues of sulfides show a wide range
from - 32%0 to 16%0, generally - 109!-22%0 . The S isotopic composition obviously indicates
that the S in ore-forming fluid must be crustal instead of magmatic source. With the combined
consideration that many salt-bearing formations and salt deposits, enriched in S and C1, exist in
the Lanping Basin, it is not difficult to conclude that the mineralizers in ore-forming fluid, such as
S and C1, are mainly provided by S- and C1-rich salt strata within the basin under the condition of
bacterium reduction ( in such a condition, when the sulfur of sulfate was transferred to sulfur,
the
3.3
P4Sextreme difference between s@-and H2S could be up to 6 0 % 0 ) [ ' ~ ] .
Time for Pb adding to ore-forming fluid
As a sulphophile element, Pb is dominantly transported in the form of S and/or C1 complexes
No. 4
He & Ar ISOTOPIC GEOCHEMISTRY O F JINDING SUPERLARGE Pb-Zn DEPOSIT
447
in hydrothermal fluid, it should be difficult for S and/or C1 depleted fluid to transport a great
amount of Pb. Therefore, a lot of Pb does not enter fluid until S and/or C is enriched in the fluid. The above study indicates that He is radiogenic, and S, C1 are mainly derived from basinal
strata. It is unimaginable to assume that crustal fluid infiltrated down and leached Pb from the
mantle, then returned to the shallow crust and deposited, at the same time the information about
crustal signatures of He and S isotope was reserved in the fluid.
3 . 4 Pb isotope and the evolution history of ore-forming fluid
It is well known that the variation of Pb isotopic composition is mainly controlled by radiogenic decay, and during various chemical processes (such as dissolution and chemical reactions)
isotopic fractionation does not exist due to the little relative mass difference between its isotopes.
It is believed that, therefore, the Pb transported into fluid through water-rock interaction and
then deposited from the fluid will basically have the same isotopic composition as the Pb in original
rocks"" .
It is shown that the Pb isotopic composition of sulfides in Jinding deposit is very consistent
with that of mantle-derived Triassic volcanic rocks at the bottom of the basinLg1.As stated above,
the background values of Pb and Zn in the igneous rocks are much higher than crustal clark value,
and also much higher than that of the other strata within the Lanping Basin. Therefore, these
rocks are most possibly a source of Pb and Zn of Jinding deposit. Obviously, the reason why the
sulfides of Jinding deposit are characterized by mantle-derived Pb is mainly that they inherited the
Pb isotopic characteristics of mantle-derived rocks providing Pb and Zn. This does not imply that
the Pb in ore-forming fluid is directly derived from the mantle.
On the basis of the above results, we can simply summarize the forming and developing process of ore-forming fluid of Jinding deposit as follows: (i) Air saturated surface water infiltrated
down and heated+(ii) leached S, C1 and radiogenic He, Ar from the basinal strata by water-rock
interactions- (iii) leached Pb and Zn from mantle-derived igneous rocks located at the bottom of
the basin-+ (iv) ore-forming fluid ascended and formed the deposit. Due to the process, the iso,
S
topic signatures of crustal radiogenic He, atmospheric Ar (with partial radiogenic 4 0 ~ r )crustal
and mantle-derived Pb remained in the ore-forming fluid.
Acknowledgement The field work was supported by Geological Section of Jinding Mine. The laboratory work was
supported by Mr. D. Blagburn. The research was guided by Academician T u Guangzhi. We would like to express our
sincere thanks to all the units and those who helped us in this work.
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