Distribution of trace Hg and its mode of occurrence over Fankou PbZn deposit, Guangdong, China

Journal of Geochemical Exploration, 33 (1989) 73-83

73

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Distribution of trace Hg and its mode of occurrence over Fankou Pb-Zn deposit, Guangdong, China ZHANG C H E N G L I A N G , WANG LIYIN, DONG YONG and P E N G S H E N G F E N G

Department of Geochemistry, Wuhan College of Geology, Wuhan Hubei Province, The People's Republic of China (Received July 2, 1987; revised and accepted January 12, 1988)

ABSTRACT Zhang, C., Wang, L., Dong, Y. and Peng, S., 1989. Distribution of trace Hg and its mode of occurrence over Fankou Pb-Zn deposit, Guangdong, China, In: X. Xie and S.E. Jenness (Editors), Geochemical exploration in China. J. Geochem. Explor., 33: 73-83. Blind ore bodies more than 200 m below the surface and ore-bearing faults are clearly indicated by soil-air Hg anomalies through an overburden 20-40 m thick over the Fankou Pb-Zn deposit. The background of soil-air mercury in this area is 50 ng m -:~ and the highest value encountered is 2000 ng m :% The maximum anomaly to background contrast is 40. The width of anomalies higher than 500 ng m -:~ can be greater than 600 m. The background Hg in soils of the area is 0.06 ppm and increases to 0.15 ppm where there are soil-air anomalies. The contrast of Hg in soils is only 2.5. Mercury in the unconsolidated overburden is in the form of free gas and is absorbed onto soil colloids such as clay minerals and Fe-Mn oxides. The primary sulphides contain 14-320 ppm Hg where Hg is incorporated isomorphically in sulphide minerals, which is confirmed by microprobe analysis. Based on a systematic analysis of the materials from surface and underground workings for the mode of occurrence of Hg, the authors suggest that the Hg-vapour anomalies in soil-air are the oxidation products of the ore.

INTRODUCTION

Qujiang tectonic basin, in which the deposit under consideration is located, is one of the important productive base-metal camps in China. Mineral exploration in this area, however, has been hindered by thick exotic overburden. In order to overcome this difficulty, gaseous Hg in the overburden is chosen as a pathfinder for concealed ores. The research included the distribution, mode of occurrence, and sources of the Hg. The effectiveness of Hg-vapour surveys over thick overburden is well-documented, and theoretical models of the formation of gas anomalies and the chemical processes of the origin of Hg-vapour anomalies over base-metal ore deposits have been discussed extensively. A selection

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© 1989 Elsevier Science Publishers B.V.

74

ZHANG CHENGLIANGET AL.

of recent papers dealing with these subjects are cited here (Wu, 1980; Geological Bureau of Shichung, 1981; Wu and Jin, 1981; Ruan et al., 1985). From observations on the mode of occurrence of Hg at different elevations, the authors have tried to explain the origin of the wide and intensive Hg-vapour anomalies over Fankou deposit. They also have advocated the use of Hg surveying as a prospecting method in Qujiang tectonic basin. o ;

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TRACE Hg IN FANKOU Pb-Zn DEPOSIT, CHINA

'13

GEOGRAPHY AND GEOLOGYOF THE STUDY AREA Fankou Pb-Zn deposit, located 20 km to the north of Shougua, is one of' the largest productive base-metal mines in southern China (Fig. 1 ). Structurally, it occurs in a submerged syncline at the north rim of Qujiang tectonic basin. The area is covered by 20-40 m of unconsolidated overburden. Bedrock geology, as revealed by drill-core study, is composed mainly of carbonates of Devonian-Carboniferous age, among which the middle Devonian Tianzhiling bioclastic limestone is the major ore-bearing strata. Controlled by northnortheast faults, the ore bodies are complex in shape and lie 100-700 m below the surface. The dominant ore minerals are pyrite, galena and sphalerite, with accessory siderite, chalcopyrite and stibnite. No Hg minerals have been found. RESEARCH METHOD Hg was determined by cold vapour atomic absorption with a gold wire trap as a pre-concentration medium. The behaviour of Hg in the near-surface conditions was studied through systematic analysis of Hg in different phases of the naturally occurring materials ranging from fresh rock, primary mineral, soil, water, to soil and atmospheric air. The modes of occurrence were estimated by means of mineralogical study, partial extraction, thermal release and electron microprobe profiling. DISTRIBUTION OF Hg IN DIFFERENT NATURALMATERIALS

Air The average concentration of Hg in air 1 m above the ground around the Fankou Pb-Zn deposit was found to be 8 ng m -:~, which is lower than the concentrations over other Pb-Zn deposits in China. The daily variation of Hg concentration in the air is from 2 to 12 ng m -:~.

Soil air The background of Hg in soil air is estimated to be 50 ng m -:~, with a standard variation of 18.1%; 100 ng m -~ was selected for the threshold value. As the maximum value encountered was 2000 ng m -'~, contrast can be as high as 40. The soil-air Hg vapour was observed along several traverses using a sampling interval of 50 m. Figure 2 shows the result of a 3.5-km traverse I - I ' . The pronounced anomalies can be divided into three parts. The middle part, which is evidently related to the known major orebody, is unusual in t h a t the anomalies higher t h a n 1000 ng m -:~ last continuously for 500 m. The eastern part has values t h a t are much

76

ZHANG CHENGLIANG ET AL.

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Fig. 2. Hg traverse I-I' (seeFig. 1 for locationof cross-section). 1=limestone; 2=silt; 3 = bioclastic limestone; 4 = sandstone, 5 = old silt; 6 = faults; 7= ore bodies; 8 = Hg anomaly; 9 = drill bole. lower and more erratic, and is therefore of less interest. No commercial orebodies have been found in this area, though the anomalies reflect the existence of faults. T h e anomalies in the western p art have a width of 400 m and a peak of 1800 ng m -3, but have not yet been drilled. T h e 3.3-km traverse I I - I I ' is shown in Fig. 3. It is at right angles to the strata whereas traverse I - I ' is perpendicular to the strike of the faults (Fig. 1). A major anomaly 1200 m wide with a peak value of 1600 ng m -3 can be identified in this traverse, which could be the actual ext ent of the ore zone. T h e n o r t h part is background and the anomalous values in the south over blind ore bodies are subdued, which can be at t r i but e d to the absence of faults and deep burial of the ore body (400 m). T h e wide distribution of rice fields may have a negative effect on the collection of soil gas samples. From these two examples, one can be certain t h a t the soil-air Hg anomalies have clearly disclosed the locations of the faults and blind ore bodies under thick overburden and have indicated the mineralized areas. Apart from the well-developed fault system and mineralization, the 20-40-m-thick loose ma-

'RACE

Hg IN FANKOU

Pb-Zn DEPOSIT.

77

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terial seems to contribute much to the preservation of wide and intensive soilair Hg anomalies.

Soil T h e average c o n t e n t of Hg in soils over the ore deposit is 0.15 ppm, which is higher t h a n the background (0.06 ppm ) by a factor of 2.5. Table 1 shows the partial extraction of Hg from highly anomalous soil samples by water, sodium citrate, diluted hydrochloric acid, and aqua regia. Between 50 and 90% of the total Hg was extracted by a mixture of 0.3 N sodium citrate and 20% hydroxylamine, which suggests t h a t Hg in the soil exists mainly in readily extractable and exchangeable form.

Rock According to the data from 138 samples of different ages from an area of 450 km 2 in the Fankou district, the average concent rat i on of Hg in the rocks is 0.026 ppm; the c o n c e n t r a t i o n is slightly higher in shales. If one considers only the samples from the Fa nkou deposit, the average concentration increases to TABLE1 Hg extraction from anomalous soil samples (ppm) Location

Water

Sodium citrate

3.6% HCI

Residual

Total

Aqua regia

Over ore Over fault Gossan Periphery Background

0.011 0.01 0.02 0.01 0.001

0.117 0.145 0.318 0.04 0.053

0.021 0.006 0.134 0.006 0.008

0.039 0.034 0.28 0.024 0.009

0.187 0.195 0.760 0.080 0.071

0.150 0.178 0.620 0.063 0.059

TABLE 2 Hg concentration in different rocks in the Fankou region Rock

Number

Range (ppm)

Average (ppm)

Silt Limestone Granular limestone Bioclastic limestone Dolomite Silty dolomite Total

2 4 9 16 10 6 47

0.21 -0.517 0.075-0.517 0.14 -1.05 0.049-1.6 0.051-1.7 0.39 - 1.0

0.367 0.19 0.418 1.0 0.136 0.462 0.46

79

TRACE Hg IN FANKOU Pb-Zn DEPOSIT, CHINA

0.3 ppm. The ore-bearing strata, Tianzhiling bioclastic limestone, contains Hg concentrations as high as 1.6 ppm {Table 2), of which 20% can be cold-extracted by 3.6% HC1. The thermal release curves of Hg from rocks are around 450 ° C, suggesting that the increased concentration could probably be due to disseminated pyrite, which is frequently observed in thin sections.

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80

ZHANG CHENGLIANG ET AL.

Ore minerals

The Hg concentrations in separated ore minerals such as galena, sphalerite, pyrite, chalcopyrite, stibnite, and siderite were determined. These concentrations range from a few ppm to 80 ppm (Fig. 4). Weak extraction by a mixture of 0.3 N sodium citrate and 20% hydroxylamine only dissolves a few parts per thousand of the total Hg, suggesting that little is absorbed on the crystal surfaces. The thermal release curves for different minerals are characterised by single peaks at the temperatures near the breakdown points of the host minerals. Some samples containing hundreds of parts per million of Hg have been closely examined under the microscope and electron microprobe for possible Hg minerals, but neither cinnabar nor Hg-concentrating spots were found (Figs. 5 and 6). All the above-mentioned facts lead the authors to believe that Hg may occur in the primary minerals as isomorphic substitutes in the sulphides, chiefly sphalerite.

Fig. 6. Area scanning for Hg in the same spots as the previous photo.

TRACE Hg IN FANKOUPb-Zn DEPOSIT, CHINA

81

DISCUSSION

Among the materials analysed from the Fankou deposit, sulphides are the most important Hg carrier and hence probably the source of the surface Hg vapour. In order to determine the vertical distribution of Hg, samples from different elevations, fault gouge in F4, fissure water, and air in unventilated underground workings have been analysed for the mode of Hg. The ore-controlling faults, F3 and F4, are associated with a 3-6-m-wide breccia zone, in which the fillings are somewhat mineralized, and the sulphides are oxidized above zero elevation. Table 3 shows the comparison of Eh, pH and Hg concentration between different materials. Although the geology changes at different elevations, the figures indicate the following: (1) The total Hg concentrations in the fault fillings range from 510 to 890 ppb and the vertical change is relatively small; only at the - 120 m level is the concentration a little high. (2) The change in weakly absorbed Hg ion increases noticeably towards the surface. (3) Just above the zero elevation, which corresponds to the oxidation zone of the deposit, Hg vapour in underground air increases drastically. Beneath the oxidation zone, however, no Hg vapour anomaly was detected in the unventilated workings no matter where the sampling point was in the ore body. (4) Hg in the mine water increases upwards towards the surface. ( 5 ) Eh and pH determinations of fault fillings also show a distinct jump at or above the zero elevation. From the above observations, the authors conclude that the soil-air Hg anomalies in the overburden are products of surficial oxidation, and a general model can be proposed (Fig. 7). Firstly, the original Hg was released as Hg '~+ from the primary sulphide minerals through their oxidation into groundwater and may migrate with the movement of the water. Secondly, ionic Hg can TABLE 3 pH, Eh, and Hg concentrations at different elevations Elevation

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(m)

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number

ngm

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700 630 510 890 720

440 340 120 140 110

540 270 360 400 290

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247 240 36 29 50

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2 2 5

350 106 175

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82

ZHANG CHENGLIANG ET AL.

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CONCLUSIONS

( 1 ) Soil-air Hg anomalies at the Fankou Pb-Zn deposit result from surficial geochemical processes. (2) An Hg-vapour survey is a useful tool in areas of thick overburden such as the study area, helping to locate hidden faults and mineralised zones. (3) In sphalerite and galena, several dozens to 300 ppm of Hg may well be in the form of isomorphical admixture.

TRACE Hg IN FANKOU Pb-Zn DEPOSIT, CHINA

83

ACKNOWLEDGEMENTS T h e a u t h o r s t h a n k P r o f e s s o r R u a n T i a n j i a n for his a s s i s t a n c e in w r i t i n g this m a n u s c r i p t . T h e a n a l y s t s in t h e X - r a y l a b o r a t o r y a n d t h e colleagues of W u h a n College of Geology are t h a n k e d for t h e i r help.

REFERENCES Geological Bureau of Shichung, 1981. The effectiveness of soil-gas Hg vapour survey for U ore deposits. J. Geophys. Geochem. Explor., 12:26-28 (in Chinese). Ruan, T., Hale, M. and Howarth, R.J., 1985. Numerical modelling experiments in vapour geochemistry, II. Vapour dispersion patterns and exploration implications. J. Geochem. Explor., 23: 265-280. Wu, G., 1980. Migration of Hg and formation of Hg anomaly. Geol. Explor., 12:55-60 (in Chinese). Wu, Z. and Jin, Y., 1981. A mercury vapour survey in an area of thick transported overburden near Shanghai, China. J. Geochem. Explor., 15: 77-92.