Rare-earth element mobility during ore-forming hydrothermal alteration: A case study of Dongping gold deposit Hebei Province, China

Vol. 22 No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 2003

Rare-Earth Element Mobility During Ore- Forming Hydrothermal Alteration: A Case

Study of Dongping Gold Deposit, Hebei Province, China*

BAO ZHIWEI (,~d~,~=_~) AND ZHAO ZHENHUA ( ~ . ~ _ ~ ) ( Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China)

Abstract: REE mobility during hydrothermal ore-forming processes has been extensively investi- gated in recent years and the potential of REE to provide information about ore forming processes has commonly been recognized. The Dongping gold deposit, which is located in northwestern Hebei Province, China, occurring in the inner contact zone of the Shuiquangou syenite complex, is spatially, and probably genetically, related to the syenite. The deposit was formed under the moderate to high temperature (220~ to 320~ ) , weakly acidic to weakly alkaline, rather high

fo2 (lg, fo2 = - 30 ~ - 34) environment. The REE study of the host rocks, altered wall rocks,

ores and gangue minerals from the deposit suggests that the REEs have been mobilized and dif- ferentiated during K-feldspathization and silicification. The extremely altered syenite enveloping auriferous quartz vein shows positive Ce anomaly and larger LREE/HREE ratio than that of the unaltered syenite. The REE concentrations and patterns of the ores are determined by the ore types and mineral assemblages. LREE/HREE ratios in the gangue quartz and hydrothermal K- feldspars are relatively low. The most significant observation is that the gangne quartz shows sig- nificant positive Eu anomaly, whereas the hydrothermal K-feldspars show less significant or no positive Eu anomaly at all relative to the primary feldspar in the unaltered syenite.

It is evident that the REEs are mobile during K-feldspathization and silicification in the ore forming process. Weak to moderate K-feldspathization caused REE mobility without apparent differentiation with the exception of extreme K-feldspathization and silicification which resulted in significant depletion of HREE and Eu and relative enrichment of Ce. The REE, Y, U, Th and Au contents of the syenite decrease as the degrees of K-feldspathization and silicification of the rocks increase towards the auriferous quartz veins. As the ores were deposited under a rather oxidized environment, Ce 4 § predominated over Ce 3 +. The precipitation of the former in the form of CeO 2 or absorpted onto the secondary mineral assemblage resulted in the inconsistent removal of the REE and the relative Ce enrichment in the strongly altered rocks. In contrast, Eu was present mainly in a low valence state ( Eu 2+ ). The geochemical differences from the other REE 3 § and much less sites in the secondary minerals to accommodate the Eu released from the original minerals resulted in the enrichment of Eu in the fluids. The mobility and differentiation of REE and the coherent mobilities of Y, U, Th and Au also support the argument that the sye- nite is one of the source rocks for gold mineralization. The REE contents and patterns of the al- tered rocks enveloping the auriferous quartz vein could be used as a guide for locating ore veins in mineral exploration.

Key words: REE mobility; hydrothermal alteration; gold mineralization; syenite; Dong- ping

ISSN 1000-9426 * This research project was supported jointly by the "Eighth-Five Year Plan Period" National Climbing Project (KY-85-12-06-02,

B$5-34-05A-05) sponsored by the Chinese Academy of Sciences and the Headquarters of Armed Police for Gold Resources (95-Y- 25). * * Corresponding author, e-mail: Baozw@ gig. ac. cn, Fax: +86-204]5290084

46 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 22

1 Introduction

The Dongping gold deposit is located 14 km southeast of the Chongli Town, northwestern Hebei Province, People' s Republic of China, about 100 km south of the northern border of the North Chi- na Craton. It occurs within the inner contact zone of the Shuiquangou syenite complex. Like other alkali-type deposits, the Dongping gold deposit is characterized by enrichment of telluride, and a high ratio of Au/Ag in the ores. However, unlike other alkali-type deposits, such as the Mount Kare and Porgera, the Papua New Guinea (Richards and Kerrich, 1993; Richard and Ledlie, 1993 ) , the Emperor, Fiji (Anderson and Eaton, 1990) , and the Cripple Creek (Thompson et al. , 1985 ) , the Dongping deposit lacks associated volcanic rocks, roscoelite and fluorite in its ore mineral assem- blage, and thus is regarded as a new type of syenite-related gold mineralization ( Bao Zhiwei et al. , 1996a, b; Song Guorni and Zhao Zhenhua, 1996).

The rare-earth elements (REE) are widely used to model the petrogenesis and evolution of ig- neous, sedimentary and metamorphic rocks (e. g. see the summary by Haskin, 1984; Fleet, 1984). The REE are commonly regarded as being insensitive to all but the most intense hydrother- mal processes. However, literature supporting the mobility of REEs during hydrothermal processes has increased in recent years ( Nesbitt, 1979 ; Alderton et al. , 1980 ; Humpheris, 1984 ; Marsh, 1991; Bao Zhiwei, 1992; Gouveia et al . , 1993; Mongelli, 1993; Prndencio et al. , 1993; Van der Weijden and Van der Weijden, 1995 ). Recently, the potential of REE to provide information about ore forming processes through REE mobility during ore forming processes has commonly been recog- nized and detailed REE investigations have been performed on many metallic deposits (Taylor and Fryer, 1980, 1982; Campbell et al . , 1984; Giere, 1986; Whifford et al . , 1988; Lottermoser, 1992 ; Parr, 1992 ; Wood and Willams-Jones, 1994 ; Bierlein, 1995). However, there are surpris- ingly few studies of REE mobility on gold deposits, particularly, syenite-hosted gold deposits (Ker- rich and Fryer, 1979; Ludden et al. , 1984; Roslyakova et al. , 1988, Guilin and Trudel, 1990). The principal motivation of this study is to contribute to the understanding of REE behaviours in the hydrothermal processes of syenite-hosted gold mineralization and its potential application in the area of genesis modelling and mineral exploration.

2 Regional geology

Gold mineralization is widespread for more than 1500 km along the northern margin of the North China Craton, which is commonly referred to as the Sino-Korean Platform or Craton (Nie, 1997 ). This east-west trending belt of gold deposits in northeastern and north-central China contains about 25% of the known gold resources in the country. Three cycles of tectono-magmatic activities have been recognized in the area--Archean-Proterozoic, Hercynian-Indonian and Yanshanian. T h e North China Craton underwent Phanerozoic accretion and collision along both its northern and south- ern borders during the west-to-east closure of the Solonkar Ocean from Permain to Early Jurassic time. This resulted in the amalgamation of the North China Block with the Angara Craton ( Mueller et al. , 1991 ). A narrow belt of Phaneozoic terranes and subduction complexes, the Mongolian-Da Hinggan fold belt, was accreted to, and then subducted below, what is now the northern margin of the North China Craton during a Permain to Early Triassic event (Davis et al. , 1998). Within the craton margin, deformation took place mainly in Hercynian time and is characterized by major east- striking reverse fault zones. The Late Jurassic and Cretaceous Yanshanian deformation event affected the eastern part, which resulted in the emplacement of widely distributed Yanshanian granitoids.

No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 47

The gold-rich belt is immediately south of these Paleozoic accreted terranes, at the Archean-Protero- zoic margin, for a width of about 50 - 7 5 km in the west and or 200 - 3 0 0 km in the east. The Dongping gold deposit is located in the central part of the Mongolian-Da Hinggan fold belt, south of the Shangyi-Chicheng-Chongli fault which marks the boundary between the Inner Mongolian axis and the Yanshanian subsidence belt ( Fig. 1 ). The Shuiquangou syenite complex, which is probably ge- netically associated with gold mineralization, was emplaced during the period from Late Caledonian to Hercynian multi-phase thrusting of the fault.

Fig. 1. Geological sketch map showing the location and occurrence of the Dongping gold deposit in northwestern Hebei Province, China (modified from Song Guorui and Zhao Zhenhua, 1996).

3 Deposit geology

The Dongping gold deposit occurs in the inner contact zone of the Shuiquangou syenite complex. The Shuiquangou syenite complex is an east-west elongated pluton which is 55 km long and 5 to 8 km wide. Plunging generally to the south, it intruded the amphibole-plagioclase gneisses, plagioclase am- phibolites and pyroxene granulites of the Archean Jiangouhe Formation of the Sanggan Group. The Shuiquangou syenite complex consists mainly of augite hornblende syenite, augite hornblende alkali feldspar syenite and alkali feldspar syenite (Fig. 1 ). Some pyroxene syenite, hornblende monzonite, quartz monzonite and pyroxene diorite are locally distributed along the edge of the complex.

The alkali feldspar syenite which hosts the Dongping gold deposit exhibits medium- to coarse- grained inequigranular to equigranular textures, with minor potassium feldspar phenocrysts. It con- sists mainly of microcline, microperthite, albite and about 1% melanite (pyrenaite, Ti content ran-

48 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 22

ges from 0.87% to 2 .25% with an average of 1.4% ). The accessory mineral assemblage of the al- kali feldspar syenite consists of sphene, apatite, magnetite, diopside, minor aegirine and zircon. The Shuiquangou syenite complex was dated at 390 -386 Ma using the zircon SHRIMP U-Pb method and was believed to have been derived from a mixed source of lower crust and mantle ( Bao Zhiwei et al. , 1996a; Lou Zhenkuan et al. , 2001 ). Other intrusives including Yanshanian monzogranite and granite occur locally in the region.

Many mineralized veins have been observed in the syenite complex, but only several of them have been evaluated. The ore-hosting structures formed in three stages: (1) the first stage of nearly E-W trending tensional fractures; (2) the second stage of NNE-trending (the major ore-hosting) and NW- trending tensional-shear fractures, and (3) the third stage of NE-trending tensional-shear fractures. At the upper part of the deposit, veins are principally hosted by the stage-2 NNE-trending fractures, in which the ores occur as quartz veins with or without associated K-feldspar-rich envelopes. In the lower part of the deposit the ore bodies are mainly controlled by densely-developed NW-oriented joints, within K-feldspathized and silicified rocks. Individual ore bodies are shaped as en echelon veins and lenticles, with an average grade of 9 g/ t Au and an average thickness of 10.6 m (0.12 m in minimum and 36 m in maximum). The ore zones are tens to 1300 m in length and several to 800 m in plunge length. The metallic minerals include pyrite and minor chalcopyrite, galena, sphalerite, mag- netite, specularite, boruite, covellite, cerussite, native gold, and calaverite. The ores consist of less than 2% sulphides, with native gold as the most important gold-bearing mineral that accounts for a- bout 90% of the total gold. The gangue minerals are mainly K-feldspar and quartz.

The wall-rock alteration is dominated by K-feldspathization, silicification, pyritization, weak sericitization, and post-ore carbonatization. K-feldspathization resulted in coarse-gained to megacrys- talline barbierite replacing primary plagioclase and K-feldspar. Two types of K-feldspathization have been distinguished. One was developed along either side of the auriferous quartz veins with a width of several metres. Its intensity increases towards the quartz veins. The other type of K-feldspathization is closely associated with fracture offsets, occurring as zones of joints. Silicification occurs along either side of the auriferous quartz veins measuring tens of centimetres to 3 m in width as thin stringer and stockwork or finely disseminated quartz in the wall rocks. Pyritization is superimposed upon K-felspa- thization and silicification, with the pyrite occurring as disseminations or veinlets. The boundary be- tween the altered halo and the associated gold ore body is gradational and can only be determined by chemical analysis. Gold contents of the altered rocks are positively correlated with the intensity of si- licification and K-feldspathization. Sericitization occurs in the fractures of the K-feldspathized rocks, as a result of the replacement of microcline or albite by sericite around the rims.

Three types of ores have been recognized at the Dongping deposit : ( 1 ) pyrite-quartz vein type in the upper part ; (2) pyrite-quartz-K-feldspar type in the middle ; and (3) pyrite-K-feldspathized syenite type in the lower part. The pyrite-quartz vein type is the main ore type of the deposit, and consits of pyrite, native gold and quartz. The pyrite-quartz-K-feldspar type consists mainly of pyrite, quartz and K-feldspar, in which the quartz occurs as fine veins, veinlets or disseminations. The py- rite-K-feldspathized syenite type consists of pyrite, microcline, perthite and minor chalcopyrite, ga- lena, etc. Pyrite occurs as disseminations or thin veins.

Based on Pb, Si, S isotope and trace element investigations, it is believed that the syenite com- plex provided a principal gold source for the gold mineralization, although some of gold might also be derived from the Archean metamorphic rocks and Yanshanian granites. The ore-frorming fluid was basically heated meteoric water mixed with magmatic water from the Yanshanian granitoids. It is characterized by low salinity ( K § + Na § + Ca 2§ + Mg 2+ < 100 mg/L) with Na > K and C1 > F. The ores were formed at temperatures of 220 - 320~ pressures of (500 - 690) x 105 Pa, under weakly

No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 49

acidic ( pH = 5 - 7 ) , and rather oxidized ( lgfo2 = - 30 - - 34) conditions ( Song Guorui and Zhao

Zhenhua, 1996). The ore-forming age of the Dongping gold deposit was determined to be 157 - 177 Ma ( gangue

K-feldspar dated by the 39Ar-a~ method), e.g. Yanshanian orogeny. This suggests that there is a 150 Ma time gap between the intrusion of the Shuiquangou syenite complex and gold mineralization. It is suggested that the Dongping deposit resulted from the interaction of the circulating meteoric wa- ter heated by the Yanshanian granites which had activated, transferred, and redeposited the gold within the syenite complex (Song Guorui and Zhao Zhenhua, 1996).

4 Sampling protocol and analytical techniques

To investigate the REE behaviour during the hydrothermal alteration of gold mineralization, un- derground profile sampling across the auriferous quartz vein, altered host rocks and unaltered rocks was undertaken from the 1-70# vein group. The samples were collected from the wall of the adit, which is perpendicular to the NNE-trending drift at 1427 m elevation, approximately 100 m below the surface. The sampling interval between B1 and B2, B4 and B5 is 2 m, whereas the interval be- tween B2 and B4, B5 and B6 is 4 and 5 m, respectively (see Fig. 2) . B1 and B2 were sampled at the footwall of a O. 3 m thick auriferous quartz vein oriented 015~ ~ , and the distance of B2 to the vein is 0 .5 m. B4, B5, and B6 were sampled from the hanging wall of the quartz vein. B1 and B2 are pink, extensively K-feldspathized and silicified syenite with fine disseminated pyrite and limo- nite. B4 is light pink, moderately K-felspathized syenite with some disseminated pyrite. B5 is light grey, weakly K-feldspathized syenite. B6 is light grey, unaltered syenite. A few ore samples were collected from various veins at various levels in the Dongping deposit. Seven quartz samples and two hydrothermal feldspar samples, which occur as enclaves in the quartz veins, were selected from py- rite-quartz veins. One feldspar sample was selected from unaltered alkali-feldspar syenite. The sam- ples were separated by crushing them to 60 to 80 mesh firstly and then hand picked with the purity of over 99% under microscope.

The trace element concentrations of the rocks and ores were analysed by PE Elan 6000 ICP-MS at the Isotope Geochemical Laboratory of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The samples were dissolved with purified HF + HNO 3 in screw lid Teflon bombs for 10 days at 100~ The calibration solution was from JG-1. The limits of detection for the REEs range from 0.1 x 10 -~2 to 0.9 x 10 -12 and the relative standard deviation for REE is less than 5%. Quartz and K-feldspar samples were analyzed at the Institute of Nuclear Energy, Chinese Academy of Sci- ences, using the instrumental neutron activation analysis (INAA) technique. About 50 mg to 200 mg of K-feldspar and quartz samples were weighed and wrapped in aluminium foil and filter paper. Mixed chemical standards were used. About 40 mg of USGS standard rocks BCR-1, G-I, G-2 and NBS-SRM 1632A (coal) were weighed respectively, they were used as controls to check the accura- cy. The samples and standards were then wrapped together in aluminium foil and put in an alumin- ium can. Irradiation was carried out for 8 hours at the thermal neutron flux of about 6 x 1013 n �9

- 2 - 1 cm �9 s in the reactor at Chinese Institute of Atomic Energy. Upon cooling, the samples were transferred to fresh polyethylene vials. The gamma ray spectra of irradiated samples were measured on the 7 th , 15 th , and 30 th days after irradiation respectively, depending on the nuclear properties of individual radioactive isotopes. The gamma rays were detected by the computer-aided Ortee and Canberra HPGe detector system. The data were processed by the gamma spectral analysis program. The detect limitations for the REEs are as follows : 0.001 • 10-9 for Eu, 0.01 • 10 -9 for La and Sm, 0.1 • 10-9 for Yb and Lu, and 1 • 10-9 for Ce, Nd, Gd, and Th. The accuracy was checked

50 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 22

by analyzing USGS BCR-1, G-l, G-2, NBS-RSM 1632A, and the relative standard deviations are less than 10%.

Table 1. Major element ( % ) and trace element ( x 10-6 ) compositions of the samples from the profile traversing the auriferous quartz vein

LREE (La/Yb) N Sample No. La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ~REE Eu/Eu* Ce/Ce* ~ - ~

B-1 0.804 2.911 0.344 1.354 0.289 0.092 0.235 0.039 0.225 0.056 0.204 0,040 0.322 0.062 6.980 1.08 1.33 4.90 1.68

B-2 0.520 2.3180.204 0.714 0.115 0.020 0.083 0,009 0.036 0.007 0.025 0.004 0.028 0.005 4.090 0.62 1.71 19.8 12.5

B4 1.098 5.799 0.604 2,642 0.600 0.206 0.496 0,076 0.417 0.091 0.296 0.049 0.350 0.064 12.79 1.15 1.71 5.95 2.12 B-5 1.037 4.733 0.746 3.454 0.735 0.240 0.599 0.099 0.583 0.135 0.467 0.082 0.624 0.112 13.65 1.10 1.29 4.05 1.12

B45 2.002 5.574 0.877 3.367 0.706 0.225 0.552 0.091 0.518 0.123 0.419 0.079 0.630 0.118 15.28 1.08 1.01 5.04 2.14

DP92-273 0.325 0.850 0.120 0,476 0.129 0.046 0.130 0,023 0.130 0.030 0.090 0.012 0.079 0.012 2.450 1.10 1.03 3.85 2.77 DP92-227 1.533 3.242 0.452 1.629 0.316 0.086 0.222 0,033 0.169 0.036 0.113 0.018 0.117 0.020 7.990 0 .99 0.937 9.97 8.83

Dt92-230 0.6722.3800.3581.5610.4120.1430.5190.1000.6170.1380.4200.0640.4400.0777.90) 0.94 L17 2.33 1.03

DP92-251 0.6621.6350.2681,1030.2380.0610.1700,0280.1540.03400.1090.0180,1300.0254.640 0 .92 0.934 5,94 3.43

DP92-269 0.867 2.393 0.392 1.576 0.320 0.094 0.209 0.032 0.164 0.035 0.107 0.017 0.119 0.020 6.350 1 .10 0.988 8.03 4.91

Sample No. Zr Hf Nb Ta U Th SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 H20* Total

B-I 106 2.71 2.63 0.083 0.503 0.492 71.27 0.07 15.33 0 .85 0.31 0.02 0.33 0.70 5.31 5.01 0.30 99.52 B-2 83.6 1.72 5.12 0.151 0.3540.184 66.18 0 . 0 8 17.42 0.71 0.29 0.01 0.59 0.35 5.38 8.89 0.41 100.32

B-4 77.5 1.72 3.51 0.119 0.691 0.439 63.96 0.08 19.06 1.07 0.28 0.04 0.41 1.13 6.27 6.95 0.2 99.47 B-5 375 6.59 2.89 0. lll 1.47 0.782 64.93 0.08 17.84 1.25 0.37 0.05 0.42 1.17 6.49 6.63 0.23 99.48

B4 182 3.83 4.85 0.147 1.08 0.702 64.56 0.07 18.7 1.38 0.32 0.04 0.37 1.36 6.88 5.44 0.37 99.52

Note: Sample Nos. B1 - 1t6 come from a profile traversing an auriferous quartz vein ~rn 1-70# vein at 1427 level. DP92-273,227,230,251,269 are ore samples

from 1464, 1538, 1503, YI)6, YD3 adi/~, respectively; DP92- 273,227,269 are pyrite-quartz vein type ores; DP-92-230 is pyrite-K-feldsparthized syenite type ore; DP-92-251 is pyrite-quartz-K-feldspar type ore.

5 Results 100

~ [] o o The REE and major element compositions of

the samples from the profile traversing the aurifer-

~ g l ~ : ~ to . . . . . . . . . . . . . .

1 ~ "" ! |

0.1 ' ' ' ' 0.1 B-1 B-2 B-4 8-5 B-6 o B-I �9 ]~2 o

" ' '

~.~-~;:~; xxxxxx:,:x:,::,:x:,::,::,:.::.::,::.::.::-: fat r lht I~I SmBu Od "g$ Dy 1[~ 1~" 'rm~'b L~

,T-Z:::r-2:~-~:x~:xxx~cxxxxxxx:,:.: :.: :.: :.: :.: :-:

B~, t~ , - , - - , , ,~ I ;~K .. ,~-r---*..~,~*. I ~ K ~ I S m ~ L " ~ W ' , ' S " Fig. 3. The REE mobility and differentiation in a pro-

P I ,~ file traversing an auriferous quartz vein in 1-70# vein of

Fig. 2. The major e lement variations in the alteration the Dongping deposit (chondri te-normalized values giv-

profile traversing an auriferous quartz vein, in 1 -70# en by Boynton, 1984; for sample descriptions, see Ta-

vein of the Dongping deposit. B. Sample number, ble 1 and Fig. 2 ) .

No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 51

ous quartz vein are listed in Table 1. As shown in Table 1 and Fig. 2, Fe, Mn, Ca and Na contents of the samples decrease, while Si and K contents of the samples increase ( excluding B1, which con- tains some veinlet quartz ) towards the quartz vein as K-feldspathization and silicification are strengthened. The EREE amount of the host rocks decreases towards the auriferous quartz vein as K- feldspathization and silicification are intensified. The REE patterns of the weakly to moderately K- felspathized rocks are similar to those of the unaltered syenite except La, which seems to have been selectively leached, and Ce, which was less ready to remove than the other REEs ( Fig. 3 ). Howev- er, the extremely K-feldspathized and silicified syenite (B2) exhibits quite different REE patterns with negative Eu anomaly and apparent HREE depletion. The 8Ce (Ce /Ce*) increases as altera- tion is intensified from the fresh alkali-syenite (B6) through weakly and moderately K-feldspathized (B5, IM, B1 ) to extremely K-fekdspathized and silicified host rocks adjacent to the auriferous quartz vein (B2). Some high field strength elements (HFSE) , which were thought to be relatively immobile in aqueous fluids unless they are under high activities of F- (Pearce and Cann, 1971, 1973 ; Pearce, 1983 ) , are clearly mobilized and their contents decrease as the alteration degree in- creases. The Au contents were not analyzed in this profile, but two profiles traversing auriferous quartz veins analysed by Song and Zhao (1996) showed that the Au contents decrease in the weakly and moderately K-feldspathized syenite and then increase rapidly in the extremely K-feldspathized and silicified syenite adjacent to the auriferous quartz veins.

The REE contents of the ores are quite low, with slight LREE enrichment and positive Eu/ Eu *. The pyrite-K-feldspathized syenite type ore (DP92-230) shows flat REE patterns because it is primarily composed of hydrothermal K-feldspar ( Fig. 4) . The REE contents of the feldspars from the ores are slightly higher than those of the unaltered syenite. Moreover, their REE patterns are quite different ( Fig. 5, Table 2) . The REE patterns of feldspar from the unaltered syenite show LREE en- riched patterns with obvious positive Eu anomaly. In contrast, the REE patterns of hydrothermal feldspars from the ores show flat REE patterns without obvious Eu anomalies, weak negative Ce a- nomalies and lower ratios of LREE/HREE and (La/Yb) ~. The hydrothermal feldspar ( mainly of K- feldspar and albite) accommodates much less amount of Eu and more HREE relative to the primary feldspar.

Table 2. REE contents of gangue quartz and K-feldspar from the Dongping gold deposit

Sample No. Mineral Occurrence La Ce Nd Sm Eu Gd Tb YU Lu Total Eu/Eu * (La/Yb) N

DP13 Quartz 58#vein 0.078 0,182 0.096 0.009 0.007 0.013 0.002 0,011 0,002 0.400 1.98 4.78

DP269 QLlart2 YD3 0,457 0,781 0.373 0.022 0.098 0,030 0.004 0,019 0.004 1.788 11.7 16.2

I)42 (1) Quartz 1584 level 0.033 0,069 0,031 0,006 0,010 0,007 0.002 0.008 0.002 0,168 4.65 2.84

I)61 (1) Quartz 1538 level 0.037 0.074 0,036 0.006 0.009 0,008 0.002 0,010 0,002 0. 184 3.76 2.59

D77 (1) Quartz 1503 level 0.044 0.098 0.056 0,008 0.007 0.012 0,003 0,012 0.002 0.242 2.22 2.51

Dl17 (1) Quartz 1464 level 0.024 0.057 0.024 0.004 0.009 0.006 0.001 0.007 0,002 0.134 5.63 2.23

D146 (1) Quam 1427 level 0.020 0.046 0,029 0.005 0,007 0.013 0,004 0.017 0.003 0.144 2.84 0.80

DP251 K-feldspar YD6 nan 0.721 1.63 1.05 0.287 0.098 0,369 0.059 0.231 0.037 4,482 0.92 2.02

DIr258 K-feldspar CD10 run 1 ,45 2.93 1,71 0,395 0,177 0,519 0,075 0,314 0,056 7,626 1.20 2,68

DPIll K-feldspar Changdi 1.07 2.02 0.657 0.087 0.108 0.127 0.021 0,072 0,010 4,172 3.14 10.1

Note: DIE51 is K-feldspar enclave in the quartz vein; DP258 is K-feldspar from K-feldspathization ore; DPIll is K-feldspar from the unal-

tered syenite. ( 1 ) From Mo Cehui, 1995. All samples were analyzed using the INAA method conducted at the Institute of High Energy Phys-

ics, Chinese Academy of Sciences.

Eleven gangue quartz and feldspar samples from the Dongping gold deposit were analyzed using the INAA method. The total REE contents for 9 of the samples range from 0. 134 x 10 -6 to 1.79 x 10 -6 ( Table 2) . Most of the quartz typically exhibits bowed curves due to the depletion of the mid-

52 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 22

10

0.1

o DP92-273 n DP92-227

DP92-230 ,7 DP92-251 § DP92-269

i i i ~ i i i i i i ~ h i i

L a Ce P r Nd S m E u G d T b Ely Ho E r T m Y b L u

10

I 1

0.I

,~ DP251 "DP2.q8 o D P l U

i , , i i i , i i i t i i t i i i

L a Ce P r lqd S m E u Gd T b Dy I-io E r T m Y b L u

Fig. 4. Chondrite-normalized REE patterns of ores from the Dongping gold deposit (DP92-273, -227, -269 are pyrite-quartz vein type ores; DP-92-230 is py- rite-K-feldsparthized syenite type ore; DP-92-251 is pyrite-quartz-K-feldspar type ore).

Fig. 5. Chondrite-normalized REE patterns of feldspar from the Dongping gold deposit ( DP111 feldspar in un- altered syenite; DIY251, 258 hydrothermal feldspar in altered syenite. The chondrite-normalized REE pat- terns show that the hydrothermal feldspar exhibits smooth curves with less remarkable Eu anomalies).

10

1

0 .1

0 .01

o D P I 3 * D P 2 6 9 oi:)42

AD61 AD77 v D l l 7 ~D 1 4 6

i i t i i i i t t i i i i t

L a Ce P r lqd S m E u G d T b Ely 14o E r T m Y b L u

dle REE such as Sm, Gd, and Tb, with positive Eu anomalies (Fig. 6 ) . When almost all of the fluid in the inclusions was extracted by means of decrepitation or grinding, the EREE contents of the relict quartz samples will be extremely low (n x 10 - 12 ). Hence, even if we could not know the

absolute REE contents of the fluids in the inclu- sions, their REE patterns should be similar to those of gangue quartz, which may reflect the gross composition of the mixture of both primary and secondary inclusions.

6 Discussion

Fig. 6. The chondrite-normalized REE patterns of gangue quartz from 1-70# vein group of the Dongping 6 . 1 SEE mobility during the ore-forming by- gold deposit, drothermal alterations

The REE contents and patterns of the al- tered syenite samples are different from those of the unaltered syenite. As shown in the profile inves- tigation, the weak to moderate K-feldspathization only led to a limited decrease in REE concentra- tions without obvious differentiation, except an increase in positive Ce anomaly and somewhat prefer- ential loss of La ( Fig. 3 ). This is because only feldspars ( oligoclase, albite, microcline, perthite) have been altered in weakly to moderately K-feldspathized samples. However, the REE contents of the extremely K-feldspathized and silicified samples decrease dramatically and their HREE were preferentially leached, which is probably due to the dissolution of accessory minerals such as apatite

No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 53

and sphene. It seems that the behavior of the HREE is more active than that of the LREE during al- teration. Shannon (1976) indicated that during rock-fluid interaction, the ionic valence, ionic radi- us and temperature are important factors controlling alteration. The higher the ionic valence and tem- perature, the more stable the REE complexes, while the larger the ionic radius, the less stable the REE complexes. Under hexagonal co-ordination the ionic radius of REE 3§ decreases from La 3 § (1 .032 • 10 -1~ m) to Lu 3§ (0. 861 • 10 -1~ m ) , thus, the HREE tend to form more stable comple-

xes. The REE can be transported in the form of fluoride and chloride complexes in nearly neutral pH fluids (Wood, 1990; Hass et al. , 1995; Van Middlesworth and Wood, 1998).

The remarkable positive Ce anomalies in the altered rocks may be caused by the oxidation of Ce 3 § to Ce 4 + in response to rock-fluid interaction. This means that the ore-forming fluids were rather oxidized, as is evidenced by the ore mineral assemblage and the results from fluid inclusion studies by Song Guorui and Zhao Zhenhua (1996) . Under oxidized and neutral to moderately acidic condi- tions, unlike other trivalent REE ions, Ce 3+ c a n be readily oxidized to Ce 4+ ( G o l d b e r g et al. ,

1963 ; German et al. , 1995 ) , and then precipitated in the form of CeO 2 or absorpted onto the sur-

face, and/or into the structure of secondary minerals. The differentiation of Ce with the other REE suggests that Ce was less mobile during hydrothermal alteration, which would account for the positive Ce anomalies in altered rocks and thus negative Ce anomalies in the ores.

Intensive K-feldspathization and silicification of syenite resulted in preferential leaching of HREE and Eu out of the rock. Eu could exist in divalent or trivalent state, depending on the redox condition. In hydrothermal solution, Eu 2§ is more stable under reduced condition and elevated tem- perature ( > 250~ ) , whereas dominance of Eu 3 § points to a more oxidized condition and lower tem- perature (Ban, 1991 ). The strong differentiation of Eu suggests that Eu is present mainly in diva- lent state in the hydtohermal system. Theoretic studies by Sverjensky (1984) have shown that in a- queous solution, Eu should be divalent and more soluble under most hydrothermal and metamorphic conditions at temperatures greater than -2500(:. Based on their theoretic calculations, Haas et al. (1995) further suggested that in submarine hydrothermal fluids Eu 2§ predominates over Eu 3§ at ap-

propriate oxygen fugacities. The Eu mobility is strong during ore-forming alteration, due to the re- moval of plagioclase-bounded Eu during the alteration of plagioclase to K-feldspar and albite as K- feldspar and albite hold much less lattice sites for Eu 2§ . Similar Eu mobility during ore-forming al-

teration has also been reported in VMS deposit (Whifford et al. , 1988). Eu 2§ behaved differently from the other REE 3 § and remained in the solution when the hydrothermal minerals ( e. g. K-feld-

spar) were precipitated under reduced and moderately oxidized conditions. The correlation coefficients of the REE and HFSE listed in Table 3 show that the HFSE are

poorly correlated with each other, which indicates that they were mobilized during hydrothermal al- teration. On the other hand, the rare-earth elements exhibit remarkable correlations due to their sim- ilar geochemical behaviors, and the LREE and HREE subgroups, in particular, are highly correla- ted with each other, which is in accordance with the differentiation of the REEs revealed by the pro-

file investigation.

6.2 REE characteristics of the gangue quartz

The solubility of REEs in natural solutions has been intensively debated in the geochemical and petrological literature in recent years. Experimental measurements of the complexation constants of REEs with inorganic ligands at 25~C and 1 x l0 s Pa (e . g. Cantrell and Byrne, 1987; Lee and Byrne, 1992,1993) show a strong tendency that the REEs form aqueous complexes at room temper-

ature. The extent of REE-complex formation is likely to increase at higher temperatures, strong com- plexation in aqueous fluids at elevated temperatures and pressures is likely to enhance the solubility

54 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 22

of the REEs in a manner similar to that demonstrated for other metals ( Sverjensky, 1987 ; Hemley et al. , 1992). The REE concentrations in submarine hydrothermal-vent solutions can be several thou- sand times higher those in ambient sea water. The REE concentrations in continental geothermal flu- ids are significantly higher than those in the average ground water ( Michard et al. , 1983 ; Michard, 1989). However, it is technically difficult to detect directly the REE compositions of ore-forming hydrothermal solution, therefore, REE characteristics of the ore-forming fluids can only be discussed indirectly through the investigation of the REE behavors in ores and gangue minerals.

Table 3. REE and HFS element correlations in the altered and unaltered syenite from the Dongping gold deposit

R La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Nb Zr Y

La 1

Ce 0.87 1

Pr 1.00 0.89 1

Nd 0.99 0.88 1.00 1

Sm 0.99 0.86 1.00 1.00 1

Eu 0.99 0.83 0.99 0.99 1.00 1

Gd 0.98 0.82 0.98 0.99 0.99 1.00 1

Tb 0.98 0.80 0.97 0.98 0.99 0.99 1.00 1

Dy 0.96 0.79 0.96 0.96 0.97 0.98 0.98 0.99 1

Ho 0.94 0.79 0.93 0.94 0.95 0.96 0.96 0.98 0.99 1

Er 0,90 0.78 0.90 0.90 0,91 0.91 0.92 0.94 0.97 0.99 1

Tm 0.82 0.73 0.81 0.81 0.82 0.83 0.83 0.86 0.91 0.95 0.98 1

Yb 0.73 0.67 0.73 0.73 0.74 0.74 0.75 0.78 0.84 0.89 0.94 0.99 1

Lu 0.64 0.59 0.64 0.64 0.65 0.66 0.66 0.70 0.76 0.82 0.88 0.95 0.99 1

Hf 0.64 0.68 0.67 0.67 0.67 0.67 0.66 0.66 0.68 0.70 0.73 0.76 0.78 0.78 1

Ta 0.90 0.93 0.93 0.92 0.91 0.89 0.88 0.87 0.85 0.84 0.82 0.77 0.70 0.63 0.78 1

Th 0.77 0.89 0.81 0.80 0.77 0.73 0.72 0.69 0.65 0.63 0.60 0.54 0.47 0.40 0.62 0.86 1

U 0.10 0.21 0.12 0.11 0.10 0.09 0.08 0.08 0.10 0.13 0.18 0.25 0.30 0.36 0.34 0.18 0.28 1

Nb 0.88 0.90 0.90 0.89 0.88 0.86 0.85 0.84 0.83 0.82 0.81 0.77 0.71 0.65 0.78 0.98 0.86 0,20 1

Zr 0.47 0,51 0.50 0.51 0.51 0.51 0.51 0.52 0.54 0.57 0.61 0.66 0.69 0.70 0.96 0.60 0.46 0.33 0.61 1

Y 0.92 0.77 0.91 0.92 0.93 0.94 0.95 0.97 0.99 1.00 0.99 0.95 0.90 0.84 0.71 0.82 0.61 0,14 0.80 0.59

The REE patterns of primary and secondary (hydrothermal) feldspars are quite different. The REE patterns of hydrothermal feldspars are quite smooth with slight or no positive Eu anomaly and HREE are enriched as compared with the whole rock (Fig. 5 ) . Thus, in comparison to the unal- tered syenite, the hydrothermal fluids should have more smooth REE patterns with more remarkable positive Eu anomalies. As stated above, REE concentrations in quartz lattice are extremely low. The REEs are mainly distributed in their fluid inclusions in quartz samples. There is a small proportion of secondary fluid inclusions in gangue quartz. So the REE patterns of gangue quartz should be able to represent those of ore-forming fluids ( Fig. 6 ) . The REE patterns of ore-forming fluids from the Dongping deposit are quite smooth with remarkable positive Eu anomalies. This is somewhat similar to those of geothermal waters (Miehard, 1989 ; Hopf, 1991, 1993 ; Haas et al. , 1995). The REE contents of the ores are usually lower than those of unaltered syenite because the ores are composed of either K-feldspathized and silieified syenite or quartz-K-feldspar veins. The REE patterns of the ores vary with various mineral assemblages. For instance, the ore sample DP92-230, which is py- rite-K-feldspathized syenite ore and is composed mainly of hydrothermal K-feldspar, shows much flatter REE patterns than other ore samples.

The coherent mobility of REE, Au (Song Guorui and Zhao Zhenhua, 1996) and other trace el- ements, such as Th, U, indicate that the syenite contributes to Au mineralization. A large portion of

No. 1 CHINESE JOURNAL OF GEOCHEMISTRY 55

gold for the mineralization was mobilized from syenite during alteration. This is in accordance with the results of isotopic studies (Song Guorui and Zhao Zhenhua, 1996). The fact that the REE con- tents are decreased and the REEs differentiatied in the altered rock towards the auriferous quartz vein could be used as an exploration tool to locate the orebody.

7 Conclusions

The analytical results of REE in rocks across a section of unaltered syenite, weakly to moder- ately, and extremely K-feldspathized and silicified syenite demonstrate that the REE were signifi- cantly mobilized by hydrothermal alteration during gold mineralization of the Dongping deposit. The variation and differentiation of REE in various altered syenites cannot be simply ascribed to the dilu- tion of secondary minerals. The REE patterns of gangue quartz represent those of the hydrothermal solution characterised by flat curves and high positive Eu anomalies. The rather oxidized nature of the fluid resulted in the positive Ce anomalies on the REE patterns of the altered wall rocks, while Eu was relatively depleted due to much less sites in the secondary minerals to accommodate the Eu released from the original minerals which resulted in less remarkable Eu anomalies in the hydrother- mal K-feldspar and positive Eu anomalies in the fluid phase. The coherent mobility of REE, Th, U and Au in the altered rocks supports the argument that the ore-forming materials were derived mainly from the syenite.

Acknowledgements: The authors wish to thank Prof. J. Guha and Prof. Lu Huanzhang at the Uni- versity of Quebec at Chicoutimi for their constructive comments and suggestions. The Dongping Gold Corporation is thanked for the access of the deposit and the assistance during field sampling.

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