the geochemistry of lamprophyres in the laowangzhai gold...

Geochemical Journal, Vol. 36, pp. 91 to 112, 2002

91

*Corresponding author (e-mail: [email protected])

The geochemistry of lamprophyres in the Laowangzhai gold deposits,Yunnan Province, China:

Implications for its characteristics of source region

HUANG ZHILONG,1* LIU CHONGQIANG,1 YANG HAILING,2 XU CHENG,1 HAN RUNSHENG,1,2

XIAO YUNHUA,1 ZHANG BO2 and LI WENBO1

1The Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences,Guiyang 550002, China

2Department of Geology, Kunming University of Science and Technology, Kunmimg 650093, China

(Received July 10, 2000; Accepted December 20, 2001)

This paper analyzes the lamprophyres from the Laowangzhai gold deposits in Yunnan Province, China,which have close temporal-spatial relations with gold mineralization, for their major elements, trace ele-ments and rare earth elements (REE), as well as their Sr, Nd isotopic compositions and the contents offixed ammonia (NH4

+). The analytical results of major elements indicate that the lamprophyres in thisregion are potassic, calc-alkaline lamprophyres. Their M [Mg/(Mg+Fe2+)] and the contents of the transi-tion elements reflect that the rocks possess the characteristics of primary magmas. As compared with themid-ocean ridge basalts (MORB), the lamprophyres in this region are enriched in large ion lithophileelements (LILE) and high field strength elements (HFSE). Their REE distribution patterns are of the LREE-enrichment type and their trace element distribution patterns are of the strong incompatible element en-richment type, indicating that the rocks originated from a fertile mantle. As compared with the modernvalues of the primitive mantle, the lamprophyres in this region have relatively high 87Sr/86Sr(0.70644~0.70895) and relatively low 143Nd/144Nd (0.512436~0.512524). The contents of NH4

+ in therocks (74.34 ppm~468.7 ppm) are obviously higher than those of other types of mantle-source rocks (1ppm~27 ppm), but lower than those of the carbonaceous country rocks (799.6 ppm~1742 ppm) in the orefield. Various lines of evidence demonstrate that the lamprophyres which possess the above Sr, Nd iso-topic signatures and the contents NH4

+ of could not be derived from the magma with MORB characteris-tics during its ascending, or derived from the magma chamber, which had both been contaminated bycrustal materials with high 87Sr/86Sr, low 143Nd/144Nd and high contents of NH4

+, instead, the rocks are theresults of partial melting of the fertile mantle. It is the main mechanism of formation of the fertile mantlein this region that metasomatism of the mantle mixed with crustal materials by fluids resultant fromdewatering of crustal materials brought into the mantle during subduction and LREE-, LILE- and HFSE-rich fluids from the deep mantle (including the astrosphere).

Many typical cases to show the close temporal-spatial relations of lamprophyres with gold min-eralization have been reported from a large numberof large- and superlarge-sized gold deposits (Rockand Groves, 1988; Rock, 1990; Wyman andKerrich, 1989a, 1989b, 1993; Currie and Williams,1993; Taylor et al., 1994; Ashley et al., 1994;Duggan and Jaques, 1996). Studies of the rocks

INTRODUCTION

Lamprophyres, especially calc-alkalinelamprophyres, are one of the characteristic rocktypes in an orogenic belt (Turpin et al., 1988;Wyman and Kerrich, 1989a, b; Rock, 1990;Shappard and Taylor, 1992; Shand et al., 1994;Duggan and Jaques, 1996; Madhavan et al., 1998).

92 Z. Huang et al.

of this type are of great significance in exploringthe evolutionary history of mantle underneath theorogenic belt and getting a better understandingof the metallogenic regularities of related mineralresources. Among the genetic hypotheses oflamprophyres, the model of “continental crustcontamination of basic magmas” has been ac-cepted by more and more scholars (Nelson et al.,1986; Rock, 1990; Bernard-Griffiths et al., 1991).Their relatively high contents of incompatible el-ements, high field strength elements (HFSE) andrare earth elements (REE), and high (87Sr/86Sr)0

and low (143Nd/144Nd)0 (as compared with themodern values of the primitive mantle) are the

important grounds for the establishment of thismodel (Rock, 1990). In addition, Hall (1988) sys-tematically analyzed minettes of Groups I andGroups II (artificially classified by Hall (1988)),kimberlites, biotite carbonate rocks, and mantlexenoliths for their fixed ammonia (NH4

+) contents,and found that the contents of NH4

+ in Group Iare obviously higher than those of other mantle-source rocks but lower than those of crustal rocks.In combination with the fact that the (87Sr/86Sr)0

ratios in this group of minettes are generally high,it is considered that the rocks are derived frommantel-source magmas which had been contami-nated by crustal materials.

Fig. 1. Simplified geological map of the Laowangzhai deposits.

Lamprophyres in the Laowangzhai gold deposits, Yunnan Province, China 93

The Laowangzhai gold depostis in YunnanProvince, China is located in the northern segmentof the Ailaoshan fault zone, which is composedof several ore blocks such as the Donggualing andLaowangzhai etc. (Fig. 1). In the ore field arewidespread lamprophyres which intruded into allthe strata and other magmatic rocks in the ore field.Their Rb-Sr isotopic ages are 28.8 Ma~49.0 Ma(Hu et al., 1995) and fission track ages are 22.7Ma~27.2 Ma (Huang and Wang, 1997). The rocksare dominated by minettes, with a main mineralassemblage of pyroxene (20%±; volume percent,the same below) + mica (25%± ; mainlyphlogopite, minor Mg-rich biotite, the same be-low) + K-feldspar (40%±). A few kersantites arecharacterized by a main mineral assemblage ofpyroxene (20%±) + mica (25%±) + plagioclase(35%±). The rocks commonly show signs of al-teration, with carbonatization, sericitization,silicification, etc. being commonly seen. Somerocks have experienced mineralization, as re-flected by sulfide alterations (pyritization,stibnitization, arsenopyritization, etc.). Accordingto the extent of alteration and the signs of miner-alization, the lamprophyres in this region can bedivided into three categories, i.e., fresh (weaklyaltered), altered and mineralized.

Huang and Wang (1997) reported the petrologi-cal and mineralogical characteristics oflamprophyres in this region. This paper presentsthe contents of major elements, trace elements andREE of lamprophyres in this region, as well asthe Sr, Nd isotopic compositions and the contentsof NH4

+. It is held that the lamprophyres in thisregion seem not to be the products of “basic mag-mas contaminated by continental crust materials”,but the results of partial melting of a fertile man-tle.

SAMPLES AND ANALYTICAL METHODS

This paper deals with relatively freshlamprophyre samples collected from the regionstudied. The criteria of sample selection are: be-ing weakly altered, grayish-black in color, exhib-iting primary magmatic texture, the rest minerals

(e.g., micas, feldspars, etc.) should possess theprimary optical characteristics with the exceptionof olivine and some pyroxenes in the rocks, whichhave been replaced by secondary minerals suchas serpentine, chlorite, carbonate, etc.

The major elements were analyzed with rou-tine wet chemistry at the Institute of Geochemistry,Chinese Academy of Sciences, and trace elementsand REE were analyzed with ICP-MS techniquesat Guangzhou Institute of Geochemistry, ChineseAcademy of Sciences and Institute ofGeochemistry, Chinese Academy of Sciences. Li(1997) and Qi et al. (2000) introduced the ana-lytical procedure of trace elements and REE withICP-MS at two Institute, respectively.

The Sr, Nd isotopic compositions wereanalyzed at the Isotope Laboratory of GuangzhouInstitute of Geochemistry, Chinese Academy ofSciences. First, lamprophyre samples were dis-solved with the mixed acid HF + HClO4, followedby the separation of Rb from Sr on the AG50 × 8(200 mesh~400 mesh) cation exchange column,and of Sm from Nd on the Dowex × 8 (200mesh~400 mesh) resin exchange column and P2O4

column, respectively. Then, 87Sr/86Sr and143Nd/144Nd isotopic ratios were measured on aMAT-61 multi-collector mass spectrometer pur-chased from the Finnigan Co. of West Germany.Chemical analysis was accomplished on thesuperpurified working platform in thesuperpurification laboratory, with the whole-pro-cedure blank Sr = 2 × 10–12 g/g and blank Nd =3 × 10–11 g/g. The analytical result of standardsample NBS-987 is 87Sr/86Sr = 0.710340 ± 20 andLa Jolla is 143Nd/144Nd = 0.511860 ± 10.

The contents of NH4+ were determined at the

Institute of Geochemistry, Chinese Academy ofSciences. The analytical procedure is: freshlamprophyre samples were roughly crushed andground, immersed with HF + HCl (1:1) for 30 minand then with benzene + methyl alcohol (1:1),followed by being washed with distilled water andbaked at 50°C till dryness. The resultant wasground as fine as –200 mesh and treated with H2O2

or NaOCl to remove organic matter, then bakedtill dryness, dissolved with HF+H2SO4, and dis-

94 Z. Huang et al.

t i l led on a nitrogen-fixing apparatus forcolorimetric analysis. A great deal of experimen-tal work has been done using this analytical pro-cedure (Luo and Gao, 1994) and the experimentalresults are generally consistent with the resultsfrom the standard BGS system, indicating that thisanalytical procedure can satisfy the requirementsfor the analysis of NH4

+ in rocks and ores.

GEOCHEMISTRY

Major elementsAs can be seen from Table 1, the contents of

CO2 in lamprophyres from this region are rela-tively high, ranging from 5.10 wt% to 10.51 wt%.At present, whether lamprophyres contain CO2 ornot is still an open question. In recent years a greatwealth of data (Roden, 1981; Allan andCarmichael, 1984; Bergman, 1987; Bergman et al.,1988; Shappard and Taylor, 1992; Taylor et al.,1994) are available, indicating that the contentsof CO2 in primary lamprophyres never over ex-ceed 0.5 wt%. According to the statistical data on116 primary olivine trachybasaltic lamprophyresamples (Rock, 1990), the geometric mean valueof CO2 is 0.42 wt%. Experimental petrologicalstudies (Esperança and Holloway, 1987; Foley,1989; Tatsumi and Koyaguchi, 1989) also indi-cated that Si- and alkali-rich magmas (for exam-ple, minette) could by no means be produced inan extremely CO2-rich environment in the uppermantle. Mineralogical studies (Huang et al., 1999)indicated that primary carbonates in thelamprophyres of this region (occurring as matrixand ocelli) was estimated to be less than 1%. Com-parative studies also demonstrated that the con-tents of MgO, CaO and Fe2O3 + FeO in thelamprophyres of this region are lower (or equiva-lent to) than those of the same rocks in other re-gions (Rock, 1990). Figure 2 shows that in thelamprophyres of this region there is no obviouscorrelation between CO2 and SiO2, Al2O3, CaO,MgO and Fe2O3+FeO, etc., implicating that car-bonization in the fresh lamprophyres of this re-gion should be attributed to external CO2, but thecomponents CaO, MgO (and FeO) were provided

by the lamprophyres themselves. So, the datashould be adjusted before a discussion is given tothe major element geochemical characteristics ofthe lamprophyres of this region. The adjustingmethod is described as follows: CO2 is deducted,the content of H2O is maintained in the primarydata (becauce the contents of H2O in lamprophyresfrom this region are among the range of those incalc-alkaline lamprophyres (Rock, 1990)), and thecontents of other oxides are converted to 100%-H2O. The results are presented in Table 1.

As can be seen from Table 1, in thelamprophyres from the region studied SiO2 ac-counts for 47.92 wt%~56.64 wt%; Na2O + K2O,5.08 wt%~7.64 wt%; K2O/Na2O, 1.28~3.91. Asshown in the SiO2 - Na2O + K2O diagram (Fig.3), the data points of the lamprophyres all fallwithin the field of calc-alkaline lamprophyres(Rock, 1987). The K/Al (0.26~0.51) andK/(K+Na) (0.52~0.80, with the exception of onesample (YLW-41) whose K/(K+Na) is 0.46) ra-tios in the lamprophyres from this region fallwithin the field of potassic and K-richlamprophyres (K/Al < 0.80, K/(K+Na) > 0.5), asclassified by Lu et al. (1991). It can be seen clearlythat the lamprophyres in this region belong topotassic-K-rich calc-alkaline lamprophyres. Withthe exception of one sample (HQ-11) whose TiO2

accounts for 1.15 wt%, the contents of TiO2 in allthe lamprophyre samples vary between 0.63 wt%and 0.88 wt% and those of P2O5 between 0.46 wt%and 0.80 wt%, precisely lower than those of thesame rocks in other regions (Rock, 1990). Al-though the contents of other oxides vary over awider range, they can still be compared with thoseof the same rocks in other regions. Except forAl2O3, Na2O and K2O, the contents of other ox-ides in the minettes and kersantites of this regionare of no difference. The former rocks are rela-tively low in Al2O3, Na2O and high in K2O (Table1), just in consistency with the difference in ma-jor mineral assemblage between these two typesof rocks.

The M [Mg/(Mg+Fe2+)] of lamprophyres inthis region are relatively constant (71.02~85.61).With increasing M, the contents of SiO2, FeO and


Tabl

e 1.

M

ajor

ele

men

t of

lam

prop

hyre

s in

Lao

wan

gzha

i go

ld d

epos

its

(wt%

)

M =

Mg/

(Mg+

Fe2+

); L

ocat

ion:

D-D

ongg

uali

n, L

-Lao

wan

gzha

i, T

-Taq

iaoq

ing,

L1-

Lan

glit

ang,

K-K

udum

u; R

ock

type

: M

in-m

ine t

te,

Ke r

-ke r

sant

ite .

96 Z. Huang et al.

K2O in the lamprophyres of this region tend todecrease against an increase in Fe2O3, MgO andCaO, implying that the lamprophyres had experi-enced weak crystallization-differentiation duringpetrogenesis.

Trace elements and REETable 2 is the analytical results of trace ele-

ments and REE in lamprophyres fromLaowangzhai gold deposits. As can be seen frÏ›Fig. 4, there is not correlation relationship betweenthe contents of CO2 and trace elements and REEin lamprophyres from this region. This show the

analytical data can represent the contents of traceelements and REE of unaltered lamprophyres.

As can be seen from Table 2, there is no sig-nificant difference in contents of the transitionelements between minettes and kersantites andtheir average contents are close to those of calc-alkaline lamprophyres according to the statisticaldata of Rock (1987). In the rocks Sc: 18.9ppm~33.1 ppm, Cr: 292 ppm~523 ppm, Co: 24.9ppm~38.0 ppm, and Ni: 74.8 ppm~365 ppm, allfalling within the range of the primary magmaresponsible for lamprophyres (Sc: 15 ppm~30ppm, Cr: 200 ppm~500 ppm, Co: 25 ppm~80.0

Fig. 2. Plot of CO2 vs. major elements for lamprophyres. � is minettes, � is kersantites.


ppm, and Ni: 90 ppm~700 ppm), as shown by thestatistical data provided by Rock (1990). This in-dicates that the lamprophyres of this region pos-sess the characteristic features of primary mag-mas.

The large ion lithophile elements (LILE) con-tents of the lamprophyres in this region vary overa wide range and the HFSE contents are relativelyconstant (Table 2), both being higher than thoseof the primitive mantle, mid-ocean ridge basalts(MORB) and island-arc basalts (OIB), as reportedby Sun and McDoonough (1989). However, incomparison to the same rocks in other regions(Roden, 1981; Allan and Carmichael, 1984;Bergman, 1987; Bergman et al., 1988; Shappardand Taylor, 1992), the LILE and HFSE contentsof the lamprophyres in this region are rather low,with the average values being close to those ofcalc-alkaline lamprophyres (Rock, 1987). TheMORB-normalized trace element distribution pat-terns in the lamprophyres from this region are ofthe “camel-hump” type (Fig. 5), similar to the trace

element distribution patterns of calc-alkalinelamprophyres, which are characterized by the spe-cific negative “Ta-Nb-Ti” anomalies (Rock, 1990).As compared with MORB (Sun and McDoonough,1989), the lamprophyres are enriched in LILE andHFSE.

The REE contents of lamprophyres of differ-ent types in the region studied are (Table 2):∑REE: 136.40 ppm~255.95 ppm, LREE: 101.88ppm~220.43 ppm, and HREE: 12.64 ppm~17.77ppm, with LREE/HREE being 7.50~13.43, obvi-ously higher than those of the primitive mantleand MORB (Sun and McDoonough, 1989). Ascompared with the same rocks in other regions(Roden, 1981; Allan and Carmichael, 1984;Bergman, 1987; Bergman et al., 1988; Shappardand Taylor, 1992), the lamprophyres in this regionare relatively low in REE, especially LREE. Thechondrite-normalized REE distribution patterns inthe lamprophyres are represented by the right-in-clined LREE-enrichment-type curves (Fig. 6).Their (La/Yb)N = 10.8~42.53, and (Gd/Yb)N =

Fig. 3. Plot of SiO2 vs. (Na2O+K2O) for laprophyres (after Rock, 1987). CAL. Calc-alkaline lamprophyres, AL.alkaline lamprophyres, UML. Ultrabasic lamprophyres, LL. lamproites, ALK. Alkaline rocks, TH. tholeiites.

98 Z. Huang et al.

Tabl

e 2.

R

EE

and

tra

c e e

lem

e nt

of l

ampr

ophy

res

in L

aow

angz

hai

gold

de p

osit

s (p

pm)


Loc

atio

n an

d ro

c k t

y pe

are

sam

e as

Tab

le 1

.

100 Z. Huang et al.

2.21~5.28, with commonly observed weak Euanomalies (δEu: 0.68~0.91).

It is worthy of note that the minettes in the re-gion can be divided into two groups in accord-ance with their REE contents (Table 2). One group(four samples, sample Nos. YD-20, SXC-3, YT-6and YK-1) has high ∑REE concentrations rang-ing from 255.97 ppm to 376.31 ppm, LREE from220.43 ppm to 342.14 ppm, and HREE from 15.61ppm to 18.34 ppm, with LREE/HREE varyingbetween 12.21 and 21.92; and the other group ischaracterized by low ∑REE values (ten samples),with ∑REE, LREE, HREE and LREE/HREE be-ing 136.40 ppm~225.07 ppm, 101.88 ppm~175.95ppm, 13.46~19.77 ppm and 6.62~11.96, respec-tively. The REE distribution patterns in the ∑REE-

high group are inclined more steeply than thoseof the ∑REE-low group (Fig. 6). The former grouphas (La/Yb)N = 19.13~40.62, (La/Sm)N =3.23~5.31, (Gd/Yb)N = 3.06~5.28, δEu =0.71~0.91, and δCe = 0.90~0.96, while the latterhas (La/Yb)N = 6.62~16.09, (La/Sm)N =2.23~4.00, (Gd/Yb)N = 1.98~2.88, δEu =0.68~1.00, and δCe = 0.89~1.01. As seen in theLa-La/Sm and La-La/Yb diagrams (Figs. 7A andB), the two groups of rocks are distributed roughlyalong an oblique line, indicating that the twogroups of lamprophyres are the product of vary-ing-degree partial melting of the mantle.

Sr, Nd isotopesListed in Table 3 are the results of Sr, Nd iso-

Fig. 4. Plot of CO2 vs. trace elements for lamprophyres. � is minettes, � is kersantites.


topic compositions for 7 lamprophyres in this re-gion. From the table we can see the following char-acteristics: (1) In the minettes in the region stud-ied the 87Sr/86Sr (The differences between the cal-culated (87Sr/86Sr)0 and (143Nd/144Nd)0 and the

measured ratios are 0.00009~0.00041 and0.000025~0.000029, respectively. In the discus-sion there would be no difference of geologicalinterest when either initial ratios or modern ratiosare used) are within the range of0.707558~0.707960 and the 143Nd/144Nd are0.512493~0.512551; in the kersantites with theexception of one sample (Sample YLW-44) whose87Sr/86Sr is 0.709041, the rest samples have87Sr/86Sr ranging from 0.706665 to 0.706973 and143Nd/144Nd from 0.512463 to 0.512551. It can beseen that the 87Sr/86Sr and 143Nd/144Nd in the samegroup of lamprophyres in this region are relativelyconstant. In this region the minettes have relativelyhigh 87Sr/86Sr, with a difference of0.000585~0.001295 between minettes andkersantites (Sample YLW-44 is an exception). Thisdifference is of no geological meaning in explor-ing the genesis of the rocks. Moreover, the twotypes of rocks have generally consistent143Nd/144Nd. So it can be considered that the twotypes of lamprophyres were derived from the samesource. (2) The 87Sr/86Sr of lamprophyres in thisregion are higher than the modern values of theprimitive mantle (0.7045) and 143Nd/144Nd arelower than the modern values of the primitivemantle (0.512638). εSr > 0 (28.1~63.7) and εNd <0 (–3.07~–1.34), as calculated by taking t = 35Ma (the average forming age of lamprophyres inthe region studied). In the 87Sr/86Sr-143Nd/144Nddiagram (Fig. 8) the rocks fall within the fourthquadrant and among the HIUM, EM1 and EM2.

The contents of NH4+

Listed in Table 4 are the contents of NH4+ in

lamprophyres from the region studied. For theconvenience of comparison, one mantle xenolithtaken from the Hannuoba alkali basalt in Hebei,China and some country rock samples from theore field were also analyzed for their NH4

+. Withrespect to the contents of NH4

+ in thelamprophyres, the following characteristics can beseen: (1) The contents of NH4

+ in thelamprophyres from the ore field are much higherthan those of other mantle-source rocks. For ex-ample, the contents of NH4

+ in ultrabasic rocks

Fig. 5. MORB-normalized trace elements forlamprophyres. The numbers in plots are same as inTable 2. CAL is the average of calc-alkalinelamprophyres (Rock, 1990), MORB from Sun andMcDoonough (1989).

102 Z. Huang et al.

(14 samples) are within the range of 1 ppm~12ppm (7 ppm on average); 14 ppm~34 ppm (17 ppmon average) for gabbro (15 samples); 9 ppm~27ppm (21 ppm on average) for basalt (21 samples);4 ppm~12 ppm (7.7 ppm on average) forkimberlite (3 samples); and 1 ppm~10 ppm (6.5ppm on average) for mantle xenolith (8 samples),as counted by Luo and Gao (1995), also higherthan the contents of fixed ammonia in Group-IIminettes (2 ppm~31 ppm, averaging 18 ppm for12 samples) but generally approximate to thoseof Group-I minettes (21 ppm~248 ppm, averag-

ing 146 ppm for 7 samples), as reported by Hall(1988). (2) In the ore field the contents of NH4

+

in the minettes (120.0 ppm~468.7 ppm, averag-ing 269.5 ppm for 11 samples) are obviouslyhigher than those of kersantites (74.34 ppm~135.9ppm, averaging 98.39 ppm for 7 samples). (3) Thecontents of NH4

+ in lamprophyres in the ore fieldare significantly lower than those of carbonaceouscountry rocks (1199.54 ppm~1342.46 ppm, aver-aging 1271.00 ppm for 2 samples) but higher thanthose of the mantle xenolith from the Hannuobaalkali basalt in Hebei, China (2.090 ppm).

Fig. 6. Chondrite-normalized REE patterns for lamprophyres. The numbers in plots are same as in Table 2. In D1 is the average of high-∑REE group of minettes, 2 is the average of low-∑REE group of minettes, 3 is the averageof kersantites, 4 is the average of calc-alkaline lamprophyres (Rock, 1990). Chondrite from Boynton (1984).


DISCUSSION

Crust contamination and source crust mixingThe lamprophyres in Laowangzhai gold depos-

its are calc-alkaline lamprophyres and their ma-jor elements, trace elements and REE can all be

compared with those of the same rocks in otherregions (Rock, 1990). Their outstanding featuresare relatively high (87Sr/86Sr)0 and low(144Nd/143Nd)0, as compared with the modern val-ues of the primitive mantle. There may be threeexplanations for the origin of the rocks with high

Sample Location Rock type Rb Sr 87Rb/86Sr 87Sr/86Sr (87Sr/86Sr)0 εSr

YD-20 Donggualin Minette 158.7 1097 0.41711 0.70769 ± 2 0.70748 +42.9YD-53 Donggualin Minette 259.0 935.4 0.8019 0.707558 ± 12 0.70716 +38.4YD-60 Donggualin Minette 255.9 896.4 0.8267 0.707960 ± 12 0.70755 +43.9YLW-36 Laowangzhai Kersantite 179.3 748.8 0.6934 0.706973 ± 10 0.70663 +30.8YLW-41 Laowangzhai Kersantite 132.8 838.3 0.45663 0.70694 ± 2 0.70671 +32.0YLW-42 Laowangzhai Kersantite 137.0 862.9 0.4598 0.706665 ± 16 0.70644 +28.1YLW-44 Laowangzhai Kersantite 186.6 2785 0.1940 0.709041 ± 14 0.70895 +63.7

Sample Location Rock type Sm Nd 147Sm/144Nd 143Nd/144Nd (143Nd/144Nd)0 εNd

YD-20 Donggualin Minette 8.48 47.86 0.1071 0.512493 ± 19 0.512468 –2.44YD-53 Donggualin Minette 8.784 41.40 0.1283 0.512526 ± 21 0.512497 –1.88YD-60 Donggualin Minette 5.426 23.89 0.1373 0.512551 ± 28 0.512520 –1.43YLW-36 Laowangzhai Kersantite 6.069 29.75 0.1233 0.512523 ± 18 0.512495 –1.91YLW-41 Laowangzhai Kersantite 5.80 28.46 0.1232 0.512488 ± 15 0.512460 –2.59YLW-42 Laowangzhai Kersantite 5.746 29.09 0.1194 0.512463 ± 16 0.512436 –3.07YLW-44 Laowangzhai Kersantite 6.665 34.38 0.1172 0.512551 ± 19 0.512524 –1.34

Fig. 7. Plot of La vs. La/Sm (A) and La vs. La/Yb (B) for lamprophyres. � is high-∑REE group of minettes, � islow-∑REE group of minettes, � is kersantites.

Table 3. The Sr and Nd isotopic compositions of lamprophyres in Laowangzhai gold deposits

Rb, Sr, Sm and Nd in ppm. In the processes of calculation of (87Sr/86Sr)0, (143Nd/144Nd)0, εSr and εNd, the λRb = 1.41 × 10–11 Y–1,λSm = 6.54 × 10–12 Y–1, t = 35 Ma (the average age of lamprophyres in Laowangzhai gold deposits), (87Sr/86Sr)UR = 0.7045(Depaolo and Wasserburg, 1979), (87Rb/86Sr)UR = 0.0816 (Depaolo and Wasserburg, 1979), (143Nd/144Nd)CHUR = 0.512638(Jacohson and Wasserburg, 1980), and (147Sm/144Nd)CHUR = 0.1967 (Jacohson and Wasserburg, 1980).

104 Z. Huang et al.

(87Sr/86Sr)0 and low (144Nd/143Nd)0: (1) the prod-ucts of partial melting of crustal materials withhigh (87Sr/86Sr)0 and low (144Nd/143Nd)0; (2) origi-nating from the primitive or depleted mantle and

then being contaminated by crustal materials withhigh (87Sr/86Sr)0 and low (144Nd/143Nd)0 in themagma chamber or in the process of upward in-trusion (crust contamination); and (3) originating

Fig. 8. Plot of 87Sr/86Sr vs. 143Nd/144Nd for lamprophyres. � is minettes in Laowangzhai gold deposits, � iskersantites in Laowangzhai gold deposits, � is lamprophyres in the East Erhai region, Yunnan Province (Zhu etal., 1992).

Sample Location Rock type Content Sample Location Rock type Content

YD-A Donggualin minette 286.20 YLW-34 Laowangzhai kersantite 127.77YD-B-1 Donggualin minette 220.79 YLW-36 Laowangzhai kersantite 92.11YD-20 Donggualin minette 177.63 YLW-41 Laowangzhai kersantite 79.99YD-25 Donggualin minette 209.30 YLW-42 Laowangzhai kersantite 74.34YD-50 Donggualin minette 232.47 YLW-43 Laowangzhai kersantite 98.60YD-51 Donggualin minette 427.85 YLW-44 Laowangzhai kersantite 79.99YD-53 Donggualin minette 468.72 YLW-46 Laowangzhai kersantite 135.94YD-60 Donggualin minette 309.37 YD-28-2 Donggualin carbonaceous wall rock 1199.54YLW-CL Laowangzhai minette 119.97 YD-38 Donggualin carbonaceous wall rock 1342.46YLW-23 Laowangzhai minette 269.88 HBR-1 mantle xenolith 2.090YLW-26 Laowangzhai minette 242.12

Table 4. The contents of NH4+ for lamprophyres in Laowangzhai gold deposits (ppm)

Sample HBR-1 taken from the Hannuoba alkali basalt in Hebei Province, China.


from the mantle mixed with crustal materials withhigh (87Sr/86Sr)0 and low (144Nd/143Nd)0 (sourcecrust mixing). It is commonly accepted by broadmasses of the geologists that lamprophyric mag-mas originated from the mantle (Rock, 1990).Lamprophyres in the region studied are rich inphlogopites and pyroxenes and also contain a cer-tain amount of olivines. Their M and the contentsof transition elements such as Sc, Cr, Co and Nishowed that the rocks have characteristics ofprimitive mantle-derived magmas, hence exclud-ing the possibility that the rocks are the productof partial melting of crustal materials with high(87Sr/86Sr)0 and low (144Nd/143Nd)0. The Sr, Ndisotopic compositions and the contents of NH4

+

also evidenced that the lamprophyres in the re-gion studied seem not to be either the product ofbasic magmas originating from the primitive ordepleted mantle contaminated by crustal materi-als with high (87Sr/86Sr)0 and low (144Nd/143Nd)0

in the magma chamber or in the process of up-ward intrusion.

Evidence from Sr, Nd isotopic compositions(1) It was reported in previous studies that the

lamprophyres in this region have higher 87Sr/86Sr.87Sr/86Sr are 0.70649 and 0.70703 respectively fortwo minette samples from the Laowangzhai oreblock, 0.70714 for one lamprophyre sample fromthe Kudumu ore block (No. 3 Geological Party ofYunnan Province, unpublished); 87Sr/86Sr are0.707029 and 0.708041 respectively for twominette samples from the region studied (YunnanBGMR, 1990). 87Sr/86Sr and 143Nd/144Nd in thelamprophyres of the region studied are approxi-mate to those (0.706324~0.707119,0.512283~0.512580, respectively) oflamprophyres in the East Erhai region which alsobelongs to the Ailaoshan fault zone, as reportedby Zhu et al. (1992). No significant difference isnoticed in εSr and εNb values between thelamprophyres from this region and those from theEast Erhai region. As is shown in the 87Sr/86Sr-143Nd/144Nd diagram (Fig. 8), they all fall withinthe areas of East Erhai Tertiary ultrapotassic vol-canic rocks (Zhu et al., 1992), Tengchong Terti-ary volcanic rocks (Zhu and Mao, 1983) and al-

kali-rich intrusive rocks in the Ailaoshan faultzone (Zhang and Xie, 1995, 1997). From this itcan be seen that high 87Sr/86Sr and low 143Nd/144Ndare the common features of lamprophyres andother K-rich and alkali-rich intrusive rocks in theAilaoshan fault zone. According to Zhu and Mao(1983), relatively high 87Sr/86Sr and low143Nd/144Nd in the Tengchong Tertiary volcanicrocks could not be attributed to the contamina-tion of magmas by crustal materials, but to thenature of mantle source region itself. In their stud-ies of Sr, Nd isotopic compositions of East ErhaiTertiary ultrapotassic volcanic rocks, Zhu et al.(1992) drew a similar conclusion. In their studiesof Cenozoic volcanic rocks with relatively high87Sr/86Sr and low 143Nd/144Nd in Kansulak,Xinjiang, Hongliuxia, Yumen, Gansu, andTengchong, Yunnan, Xie et al. (1992) also pointedout that Cenozoic volcanic rocks in the peripheryof the Qinghai-Tibet Plateau mostly show no signof having been contaminated by crustal materi-als. After studying the Sr, Nd isotopic composi-tions of alkali-rich intrusive rocks in the Ailaoshanfault zone, Zhang and Xie (1997) suggested thatthe rocks with high 87Sr/86Sr and low 143Nd/144Ndwere not the products of crust contamination.

(2) The strata into which the lamprophyres in-truded in this region include calcareous slate,quartz greywacke and sericite slate. 87Sr/86Sr inthese rocks are 0.70847 (1 sample), 0.71285 (1sample) and 0.73040 (6 samples on average), re-spectively. If the 87Sr/86Sr in mantle primarymagma is taken to be 0.7038 (White and Hofmann,1982), in accordance with simple binary mixingestimation, the proportions of sample YD-20(87Sr/86Sr = 0.70769) which had been contami-nated by the aforementioned rocks will be 83%,43% and 15%, respectively, and those of sampleYW-41 (87Sr/86Sr = 0.70694) will be 67%, 35%and 12%, respectively. As can be seen from theestimated results, first, the contamination of pri-mary magma by crustal materials in such a bigproportion must lead to variations in chemicalcomposition of the rocks, which is precisely inconflict with the precious conclusion that the con-tents of Cr, Ni and Co in the lamprophyres of this

106 Z. Huang et al.

ultrabasic rocks, gabbro, basalt, kimberlite andmantle xenoliths), but lower than those ofcarbonaceous country rocks in the ore field. Suchan explanation seems unreasonable that the rocksin the ore field are of crust contamination origin.

(1) The lamprophyres themselves are enrichedin NH4

+. The NH4+ is similar to K+ both in

electrovalence and in ionic radium and it is presentmainly in minerals by replacing K+. So naturally-occurring K-rich minerals are also NH4

+-rich min-erals. Measurements (Wlotzka, 1961) indicatedthat in silicate rock-forming minerals, micas andfeldspars are the major NH4

+-hosted minerals andthe partitioning sequence of NH4

+ in these miner-als follows the order of biotite, orthoclase andplagioclase. According to the statistical data ofLuo and Gao (1995), the contents of NH4

+ in 110biotite samples are within the range of 5ppm~1880 ppm (averaging 300 ppm), 6 ppm~604ppm (averaging 164 ppm) in 25 orthoclase sam-ples, and 2 ppm~58 ppm (averaging 8 ppm) in 15plagioclase samples, also providing strong evi-dence for the above conclusion. In addition, min-erals derived from alteration of mica and feldsparsuch as sericite, muscovite, i l l i te andbuddingtonite are all the important NH4

+-hostedminerals. In lamprophyres in the Laowangzhaigold deposits, as in the same rocks in other re-gion, the major rock-forming minerals mica,orthoclase and plagioclase are all the NH4

+-hostedminerals. For this reason, the rocks are possibleto be enriched in NH4

+. In the Laowangzhai golddeposits the contents of NH4

+ in minettes are ob-viously higher than those of kersanites, just in con-sistence with the fact that the former’s light-colored minerals are predominated by orthoclase(relatively high in NH4

+) while the latter’s dark-colored minerals are predominated by plagioclase(relatively low in NH4

+). Early alterations in therelatively fresh lamprophyres in the ore field aredominated by chloritization, sericitization,carbonatization and silicification. Correlationsbetween NH4

+ and other components indicate thatin fresh lamprophyres from the ore field NH4

+ ispositively correlated with K2O, but shows no cor-relation with other major elements and volatile

region are close to those of mantle rocks and thoseof SiO2, Al2O3 and CaO are approximate to thoseof the same rocks in other region, and, second,the samples for Sr, Nd isotopic analysis are coun-try rocks differing in petrological character. If thesamples were contaminated by crustal materialsin a big proportion, they would show a significantdifference in Sr isotopic composition. Such a sig-nificant difference in Sr isotopic composition isalso in conflict with the conclusion that thelamprophyres of this region are relatively constantin isotopic composition. All this goes to show thatthere would be a little possibility that thelamprophyres of this region have been contami-nated by crustal materials in a relatively big pro-portion.

(3) Sample YLW-44 has an abnormally high87Sr/86Sr (0.709041), which seems not to be theresult of crust contamination, because this sam-ple has an extremely high 87Sr/86Sr and its143Nd/144Nd (0.512551) is also slightly higher thanthat of the same rock in the ore field, just in con-flict with the fact that the rocks contaminated bycrustal materials are characterized by high87Sr/86Sr but low 143Nd/144Nd ratios. Moreover, ascan be seen from its geochemical composition(Table 4), this sample shows no significant dif-ference in major and trace elements from, thoughits Sr content is obviously higher than that of thesame rock in the ore field, implying that the rockis probably not the product of crustal contamina-tion. The authors hold that the fact that sampleYLW-44 has an abnormally high 87Sr/86Sr ratiomay be attributed to the involvement of externalSr (Nd and other elements) in the process of al-teration of the rock (the selected sample displayssigns of weak alteration), as is evidenced by theabnormally high Sr (2785 ppm) and relatively highNd (34.38 ppm) contents, but relatively low87Rb/86Sr (0.1940) and 143Nd/144Nd (0.1172) ra-tios (relative to the same rocks in the ore field).

Evidence from NH4+ The contents of NH4

+ inlamprophyres in the Laowangzhai gold depositsare generally close to those of Group-I minettes,as reported by Hall (1988), obviously higher thanthose of other types of mantle-source rocks (e.g.,


components. That is to say, early alterations havelittle influence on the contents of NH4

+ in thelamprophyres. The high contents of NH4

+ in therocks may be an outstanding feature of their own.

(2) The high contents of NH4+ in lamprophyres

appear not to be the result of crustal contamina-tion. Studies have demonstrated that the most out-standing feature of crustal material-contaminatedbasic magmas is the slightly higher 87Sr/86Sr inthe rocks (Nelson et al., 1986; Rock, 1990;Bernard-Griffiths et al., 1991). 87Sr/86Sr inlamprophyres in the Laowangzhai gold deposits(Table 3) are actually higher than the modern val-ues of the primitive mantle. However, as discussedpreviously in a number of respects, high 87Sr/86Srin the lamprophyres of this region seem not to bethe result of crust contamination, but are attrib-uted to the nature of the rocks themselves. As canbe seen from the NH4

+-87Sr/86Sr diagram (Fig. 9),there is no correlation between NH4

+ and 87Sr/86Srfor the minettes and kersantites in this region, in-dicating that both of them are not the result of crustcontamination. If the average content of NH4

+ inbasic rocks is taken to be 21 ppm (previous pa-per), the average content of NH4

+ in thecarbonaceous rocks of this region will be 1271.00ppm (this paper). According to simple binary mix-ing estimation, the lamprophyres (the average con-tent of NH4

+ = 269.50 ppm) in this region con-taminated by country rocks account for 20%. Asstated previously, the contamination of primarymagmas by upper crust materials in such a bigproportion will surely lead to variations in thecontents of major and trace elements. However,such variations are just in inconsistence with thegeochemical characteristics of lamprophyres inthis region. This also demonstrates that the highcontents of NH4

+ in the lamprophyres seem not tobe the result of crust contamination.

The above two possibilities have been ruledout. We consider that the high 87Sr/86Sr and low143Nd/144Nd for the lamprophyres in the regionstudied are attributed to the source crust mixing.The source crust mixing is directly related to platesubduction. As viewed from the geotectonic de-

velopment, the Ailaoshan fault zone once experi-enced subduction and this viewpoint has been ac-cepted by most geological workers (Duan andZhao, 1981; Li, 1988; Luo, 1990; Cong et al.,1993; Zhang and Xie, 1997). This viewpoint canalso been provided by the lamprophyres exposedin the Ailaoshan fault zone (including thelamprophyres in Laowangzhai gold deposits) aresimilar to Tengchong Tertiary volcanic rocks andalkali-rich intrusive rocks that have subductionenvironment and source crust mixing (Zhu andMao, 1983; Zhang and Xie, 1997) in high87Sr/86Sr and low 143Nd/144Nd. Another piece ofevidence for the source crust mixing due to sub-duction is that the incompatible element distribu-tion patterns show negative Ta-Nb-Ti anomalies(Fig. 5). As Rock (1990) stated that the negativeTa-Nb-Ti anomalies of lamprophyres are not nec-essarily one-to-one correlated with the subductionenvironment, however, there are not youngerlamprophyres showing negative Ta-Nb-Ti anoma-lies beyond the subduction environment. The rock-forming ages of lamprophyres in the region stud-ied are relatively young (Himalayan) and thisshows that it is reliable to use the negative Ta-Nb-Ti anomalies of lamprophyres as the index todistinguish the subduction environment.

Fig. 9. Plot of NH4+ vs. 87Sr/86Sr for lamprophyres.

The numbers in plots are same as in Table 3.

108 Z. Huang et al.

Mantle metasomatismAs stated previously, the M and contents of the

transition elements (Sc, Cr, Co, Ni) in thelamprophyres of this region indicated that therocks possess the characteristics of primary mag-mas. The rocks are more enriched in LILE andHFSE than the MORB (Sun and McDoonough,1989), their REE distribution patterns are of theLREE enrichment type and the incompatible ele-ment distribution patterns are of the strong incom-patible element enrichment type. All this goes toreflect that the lamprophyres in this region origi-nated from a fertile mantle. Huang (1996) con-ducted modeling calculations of the REE contentsof the mantle in the source region of lamprophyresin the region studied by taking sample YD-20(∑REE-high group of lamprophyres) and sampleYLW-21 (∑REE-low group of lamprophyres) forexample and the results showed that the mantle inthe source region of lamprophyres in the regionstudied is obviously enriched in REE, especiallyLREE (Fig. 10). The two samples represent the

products of 7.5% and 13% partial melting of REE-enriched mantle, respectively. This provides fur-ther evidence suggesting that the lamprophyres inthe region studied are the products of varying–degree partial melting of the fertile mantle.

Although the source crust mixing can interpretwhy lamprophyres in the region studied are rela-tively high 87Sr/86Sr and low 143Nd/144Nd, it can-not lead to the formation of the LREE-, LILE- andHFSE-rich fertile mantle. So it is believed that themantle (source region) of lamprophyres has un-dergone metasomatism by fluids relatively en-riched in LREE, LILE and HFSE. The fluids thatmetasomatized the mantle in the subduction envi-ronment mainly include: (1) fluids resultant fromdewatering of crustal materials brought into themantle in the process of subduction (Holm andMunksgaard, 1982; Ujike, 1988; Fabries et al.,1989; Maury et al., 1992); (2) ascending fluidsfrom the deep mantle (including the astrosphere)(Green and Wallace, 1988; Meen et al., 1989).Source crust mixing of lamprophyres in this re-

Fig. 10. Chondrite-normalized REE patterns of modelled source region for lamprophyres. The M and the contentsof the transition elements of Sample YD-20 (represent the high-∑REE group of lamprophyres) and YLW-21 (rep-resent the low-∑REE group of lamprophyres) reflect that the rocks possess the characteristics of primary mag-mas. The modelled results (Huang, 1996) show that two samples were the products of 7.5% and 13% partialmelting of mantle rocks, respectively. Base on this results, Huang (1996) calculated the REE contents of sourcerocks for lamprophyres. This diagram is Chondrite-normalized REE patterns of calculating results. 1 is the mod-elled result of YD-20, 2 is the modelled of YLW-21, 3 is the REE pattern of pyrolites two times that of chondrites(Frey, 1984). Chondrite from Boynton (1984).


gion indicate that the mantle has undergonemetasomatism by fluids resultant from dewateringof crustal materials, but whether this kind of flu-ids is enriched in LREE, LILE and HFSE or not isstill an open question. In contrast, it has been con-firmed by many research results that fluids fromthe deep mantle (including the astrosphere) areenriched in LREE, LILE and HFSE (Green andWallace, 1988; Meen et al., 1989; Navon et al.,1988; Schrauder and Navon, 1994; Schrauder etal., 1996; Liu et al., 2001). This kind of fluidsalso contains a certain amount of CO2, and themagma derived from partial melting of a fertilemantle formed as a result of metasomatism by thefluids usually contains a certain amount of car-bonates. The mineral assemblage in lamprophyresfrom the Laowangzhai gold deposits contains asmall amount of matrix carbonate and ocelli car-bonates (<1%, of which the δ13CPDB are –4.13‰~–4.71‰ and –5.10‰~–6.17‰, respectively(Huang et al., 1999, 2002), the ocelli carbonatesare enriched in Sr (3124 ppm~3337 ppm), Ba(5763 ppm~5860 ppm) and LREE (860.45ppm~894.91 ppm) (Huang et al., 2002), indicat-ing the geochemical characteristics ofcarbonatites. All these characteristics show thatthe mantle (source region) of lamprophyres in theregion studied have experienced metasomatism byfluids from the deep mantle (including the astro-sphere).

CONCLUSIONS

In the Laowangzhai gold deposits thelamprophyres which are closely related with goldmineralization both in time and in space areHimalayan calc-alkaline lamprophyres. The M andthe contents of the transition elements reflect thatthe rocks possess the characteristics of primarymagmas. The rocks are more enriched in LILE andHFSE than the MORB (Sun and McDoonough,1989). Their REE distribution patterns are of theLREE-enrichment type and their incompatible el-ement distribution patterns are of the strong in-compatible element-enrichment type. As com-pared with the modern values of the primitive

mantle, the lamprophyres have relatively high87Sr/86Sr and low 143Nd/144Nd. The contents ofNH4

+ in the rocks are obviously higher than thoseof other types of mantle-source rocks. Many linesof evidence show that the lamprophyres could notbe products originating from the primitive or de-pleted mantle and then being contaminated bycrustal materials with high (87Sr/86Sr)0, low(144Nd/143Nd)0 and high contents of NH4

+ in themagma chamber or in the process of upward in-trusion, instead, the rocks are the results of par-tial melting of the fertile mantle. It is the mainmechanism of formation of the fertile mantle inthis region that metasomatism of the mantle mixedwith crustal materials by fluids resultant fromdewatering of crustal materials brought into themantle during subduction and LREE-, LILE- andHFSE-rich fluids from the deep mantle (includ-ing the astrosphere).

Acknowledgments—This research project was finan-cially supported jointly by the Major State Basic Re-search Program of People’s Republic of China (No.G1999043203), the Pre-selected Projects under theState Climbing Program (No. 95-Pre-39), the ChineseAcademy of Sciences Innovational Program (No.KZCX2-101), and the National Natural Science Foun-dation of China (No. 49602026). We thank Li Sunrong,Qi Liang and Luo Taiyi in Institute of Geochemistry,Chinese Academy of Sciences and Li Xianhua inGuangzhou Institute of Geochemistry, Chinese Acad-emy of Sciences for measuring major elements, traceelements, rare earth elements, isotopic compositionsand fixed ammonia. We further thank Profs. WangLiankui, Xie Guanghong and Li Xianhua in GuangzhouInstitute of Geochemistry, Chinese Academy of Sci-ences and Profs. Hu Ruizhong and Qiu Yuzhou in In-stitute of Geochemistry, Chinese Academy of Scienceswhose reviews helped improve this manuscript.

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the geochemistry of lamprophyres in the laowangzhai gold...

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