Organic geochemistry of paleokarst-hosted uranium deposits, South China

Journal of Geochemical Exploration 68 (2000) 211–229 www.elsevier.nl/locate/jgeoexp

Organic geochemistry of paleokarst-hosted uranium deposits, South China M.-Z. Min a,b,*, Z.-W. Meng a,b, G.-Y. Sheng c, Y.-S. Min c, X. Liu d b

a Department of Earth Sciences, Nanjing University, Nanjing 210093, People’s Republic of China The State Key Laboratory of Mineral Deposit Research, Nanjing University, Nanjing 210093, People’s Republic of China c The State Key Laboratory of Organic Geochemistry, Guangzhou 510640, People’s Republic of China d Uranium Geology and Exploration Bureau, Changsha 410011, People’s Republic of China

Received 22 May 1998; received in revised form 22 November 1999; accepted 29 November 1999

Abstract The paleokarst-hosted uranium deposits in organic-matter, clay-rich Devonian–Carboniferous carbonates are an economically important, new type of uranium deposit in China. The organic matter intimately associated with the uranium mineralization in this type of deposit has been characterized by petrographic, isotopic, gas chromatographic, pyrolysis-gas chromatographic, infrared spectroscopic and elemental geochemical methods. Comparing genetic types of the organic matter in unmineralized and mineralized samples indicates that no fundamental differences are found. The organic matter is chiefly of marine origin and contains a minor terrestrial component. The immature nature of the indigenous organic matter in the unmineralized samples shows generally a low-temperature history ( ⱕ max. 65⬚C), and geologic data show a shallow maximal burial depth. By combining the organic geochemistry with the geological data, U–Pb dating and temperature determinations, an overall formation process for this type of uranium deposit is deduced. The formation of the paleokarst-hosted uranium deposits in South China is the result of: (1) repeated paleokarstifications of the Devonian and Carboniferous organic, clay-rich carbonate along the faults and unconformities between different strata because of the Hercynian and Yanshanian regional tectonism, and extensive formation of solution-collapse, solution-fault breccias; (2) accumulation of organic matter and clays in the paleocaverns and matrix of the breccias, fixation and adsorption of uranium by the organic matter and clays from the paleokarst waterflows that leached metals from the uranium-bearing host carbonates during their passage towards the karst zones; (3) reduction of uranium by the organic matter and formation of protore and low-grade ore; (4) circulation of heated formational waters and deep circulating, uraniferous meteoric waters by tectonic pumping, reworking the uranium-rich, paleocave-fillings, protore and low-grade ore, reduction and formation of primary uranium minerals (uraninite and coffinite) because of the reducing environment resulting from organic matter and sulfide. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: uranium deposit; paleokarstification; organic geochemistry; metallogeny; China

1. Introduction The association of organic matter with ore deposits has been extensively studied, for example, in connection with the genesis of Carlin gold deposits (Hausen * Corresponding author.

and Kerr, 1968; Ilchik et al., 1986), the Witswatersrand gold–uranium deposit (Zumberge et al., 1978), uranium and copper deposits (Jung and Knitzschke, 1976; Anderson et al., 1985; Kribek, 1989; Meunier et al., 1989; Hansley and Spirakis, 1992; Mauk and Hieshima, 1992; Nagy et al., 1993) and Mississippi Valley-type Pb–Zn deposits (Barton, 1967; Estep et

0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(99)00085-0

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Fig. 1. Geological sketch map of the Saqisan uranium deposit (A; modified after Qilinyi Mine, 1986) and cross sections of the Sanqilinyi (B; modified after Salinqi Party, 1980), Sanbaqi uranium deposit (C; modified after Salinsan Party, 1991). 1: Quaternary, 2: limestone, 3: dolomitic limestone, 4: fossiliferous limestone, 5: fossiliferous dolomite, 6: sandstone, 7: slate, 8: granite, 9: solution-collapse breccia, 10, 11: uranium orebody, 12: drill hole, 13: fault, 14: ore-bearing solution-fault breccia, 15: shear zone, 16: sampling point, 17: strike and dip of sedimentary rock.

al., 1980; Giordano and Barnes, 1981; Macqueen and Powell, 1983; Disnar et al., 1985; Gize and Barnes, 1987).This association has produced much speculation on the genetic relationship between organic

matter and mineralization. Also much organic matter has commonly been found in associated with the paleokarst-hosted uranium deposits in South China. Detailed petrologic and geochemical studies of

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organic matter associated with the uranium deposits are few. The processes that led to this organic and ore association are not clearly understood. It is generally considered that the organic matter associated with uranium ore may have played two roles in the formation of the deposits: in the preconcentration of uranium from solution, and in reducing U to the ⫹4 state during ore precipitation reactions (Meunier et al., 1989). Isotopic study of sulfidic sulfur from the uranium deposits has demonstrated that biogenic processes may play an important role in the origin of these deposits (Min, 1995). It is unclear, however, how and to what degree organic matter participates in the formation of these uranium deposits. The paleokarst-hosted uranium deposits in South China, thus, offer an opportunity to apply the approaches and techniques of organic geochemistry toward resolving some of these problems. The objective of this paper is to present analyses of organic matter and to consider the implications for the paleokarst-hosted uranium deposits represented by Sanqilinyi, Sanbaqi and Saqisan.

2. Geologic setting The Devonian–Carboniferous carbonate rocks in South China host several uranium deposits, which are characterized by their occurrence in solution collapse breccias and by the mineral association of uraninite, coffinite, carbonates and Fe, Cu, Pb, Zn, Ni sulfides. The deposits, represented by Sanqilinyi, Sanbaqi and Saqisan (Fig. 1), are an economically important, new type of uranium deposit in China. The three uranium deposits are located in the western part of the geologic region known as the South-China post-Caledonian geosyncline, and within the postCaledonian uplift (Sanqilinyi deposit) and the Hercynian–Indosinian depression (Sanbaqi and Saqisan deposit). The oldest rocks exposed in the three mine districts are metamorphosed Cambrian quartzose slate, which is overlain unconformably by slightly metamorphosed Ordovician sandstone. The unmetamorphosed Devonian carbonate with a total thickness of more than 700 m rests unconformably on the Ordovician sandstone and is overlain conformably by the unmetamorphosed Carboniferous carbonates. The deposits are hosted by carbonaceous pelitic limestone,

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fossiliferous micrite and dolomitic limestone of the middle Devonian (Sanqilinyi and Saqisan deposit) and late Carboniferous age (Sanbaqi deposit). The deposits of this type are strata bound and are not related to igneous intrusions except the Sanqilinyi deposit, which is close to a Hercynian granitic batholith with outcropped area of 238 km 2 and a Rb–Sr whole-rock isochron age of 283 Ma (Min and Gong, 1995) (Fig. 1). The orebodies in the three deposits are located in solution-collapse and solution-fault breccias along the faults and unconformities between different strata, and within the cores of the anticline or syncline where faulting and karsting were developed. The ore-bearing breccias are a composite solution-fault product in genesis. The breccias occurred mainly along fault zones and unconformities between different strata which permitted access to groundwater, leading to karst formation and eventually to the formation of solution-collapse breccia. The ore-bearing breccias were within the fault zones, or were distributed along a direction paralleling to the trend of the fault zones and the unconformities (Fig. 1). The breccia always has a corroded border and surface.The matrix of the breccia is composed of organic matter, clays and clay- to pebble-sized limestone fragments. The orebodies vary in morphology, and include lenticular, hopper-shaped, nested, prismatic and irregular bodies. The wall-rock alteration is generally weak and includes silicification, chloritization, dolomitization, pyritization, carbonatization and decoloration. The dominant primary minerals in the ores of the Sanqilinyi and Sanbaqi deposit are carbonates (calcite, dolomite), uraninite, coffinite, pyrite and marcasite, accompanied by minor amounts of pentlandite, millerite, ullmannite, molybdenite, niccolite, sphalerite, galena, chalcopyrite, antimonselite and quartz. The uraninite, the most important primary uranium mineral, occurs as veinlets ranging from 0.1 to 5 mm in width and disseminated grains from 0.01 to 0.5 mm in diameter. A small proportion of uranium was adsorbed by the clays and organic matter in the matrix of the breccias.

3. Methods All samples examined in this study were collected

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from drill cores taken from depths of more than 50 m and freshly blasted mine faces, in order to minimize anthropogenic contamination. Each sample was wrapped in aluminum foil. Plastic bags and printed newspapers were not used to avoid organic contamination. The locations of the drill holes and mine faces sampled are presented in Fig. 1. Organic analyses were done on 10 samples from 10 locations. Inorganic analyses were done on samples from 20 locations. It is believed that these samples can give a valid picture of the organic geochemistry for this type of uranium deposit. Part of each sample was crushed and milled to powders in a steel disc mill for organic and inorganic analyses, and representative pieces were retained for petrographic, mineralogical and geochemical studies. Major and trace elements were determined with a Philips PW-1480 sequential X-ray spectrometer at the State Key Laboratory of Organic Geochemistry, Guangzhou, following the procedures described by Franzini et al. (1972), with a relative accuracy of 1% for major elements and 10% for trace elements. The distribution and mineralogical occurrence of uranium was studied in polished section using standard microscopy under reflected light and an electron microprobe (JXA-8800M model, JEOL) equipped with an energy dispersive spectrometer. The operating conditions of microprobe were 20 kV accelerating voltage, 15 nA beam current and 2–5 mm beam diameter. Powdered rock samples were combusted in a Leco 320C/S instrument for determination of total carbon and sulfur contents. Organic carbon was determined by reaction of rock powder with 1 N HCl overnight at the State Key Laboratory of Organic Geochemistry, Guangzhou, following the procedure recommended by Mauk and Hieshima (1992). After decanting the solution, the sample was treated with hot 1 N HCl to remove refractory carbonate phases. Residual solids were collected on glass fiber filters, neutralized with distilled water, dried and combusted. Samples of powdered rock were extracted in a Soxhlet apparatus (48–60 h) using dichloromethane as a solvent. Soluble organic matter (bitumen) was concentrated using a Bucchi rotovapor and brought to a volume of 10 ml. An aliquot of bitumen was transferred to a glass vial, dried at room temperature under a stream of N2 gas, and weighed to determine yield. A second aliquot of

bitumen was deasphaltened in cold hexane and then separated into saturated hydrocarbons, aromatic hydrocarbons and polar compounds of silica gel (activated at 200⬚C) sequential elution with hexane, benzene and benzene–methanol (1:1). Seep oil samples were collected by Soxhlet extraction of oilstained rock chips and fractionated following the procedure recommended by Mauk and Hieshima (1992). Carbon isotope ratios of kerogen were measured on a Finngan MAT-252 model isotope mass spectrometer at the Beijing Petroleum Institute, following the technique of Deines and Wickman (1973). The overall reproducibility was in the range of 0.1– 0.2‰ for d 13C. The results are reported relative to the PDB standard. C, H, O, N concentrations of kerogen were measured on a CHN–O Rapid Organic Elemental Analyzer at the State Key Laboratory of Organic Geochemistry, Guangzhou, with a relative accuracy of ^0.3%. Organic matter types were determined on a RockEval V-model Oil Show Analyzer at the State Key Laboratory of Organic Geochemistry, Guangzhou, following the procedure recommended by Leventhal (1982) and Peters (1986). Gas chromatography of the saturated hydrocarbon fraction was carried out on a HP5890 II-model gas chromatograph at the State Key Laboratory of Organic Geochemistry, Guangzhou, following the procedure recommended by Mauk and Hieshima (1992). The oven of the gas chromatograph was programmed from 60 to 130⬚C at 10⬚C min ⫺1, from 130 to 300⬚C at 4⬚C min ⫺1, and held at 300⬚C for 15 min. Pyrolysis-gas chromatography of kerogens was measured on a CDS-PE9000 pyrolysis-gas chromatograph at the Beijing Petroleum Institute, following the procedure recommended by Coveney et al. (1987). Finely powdered sample (1–10 mg) is placed in a 2.5 cm by 2 mm quartz tube, which is then put in spiral heating coil of a pyroprobe pyrolysis device. The pyroprobe is inserted in the injection port of a gas chromatograph (GC) in helium carrier gas ( ⬇ 12 cm 3 min ⫺1) and heated for 10 s at 250⬚C. After the trap has warmed to room temperature, the pyrolysis products are temperature programmed at 6⬚C min ⫺1 from 50 to 280⬚C. The pyrolysis procedure is repeated at 400, 500, 600 and 700⬚C. Petrographic observations were made with a Zeiss II photomicroscope, at the State Key Laboratory of

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Table 1 Rock-Eval analyses of the host carbonates and ores (abbreviations: SB ˆ Sanbaqi deposit, SQ ˆ Sanqilinyi deposit, SS ˆ Saqisan deposit. Locations of drill cores and mine outcrops sampled are shown in Fig. 1) Sample no.

Deposit

Sampling location

Corganic (%)

Tmax a

S1 (mg/g)

HI b (mg/g)

OI c (mg/g)

Description

G-1 G-2

SB SB

216k, 115 m 205k, 205 m

0.35 1.17

419 448

0.02 0.03

138 122

38 40

G-3

SB

206k, 181 m

1.58

428

0.05

110

51

G-4 G-5 G-6 G-7 G-8 G-9 G-10

SQ SQ SQ SS SS SS SS

408k, 78 m 408k, 113 m 402k, 137 m 381k, 106 m d D2 D1 D3

0.37 0.38 1.13 0.3 0.47 0.13 2.27

423 441 418 471 431 421 415

0.01 0.03 0.02 0.02 0.03 0.03 0.07

131 115 125 120 128 119 125

34 42 39 43 36 44 28

Unminelized host carbonate Low-grade ore, dark gray, clayrich High-grade ore, dark gray, clayrich Unmineralized host carbonate Low-grade ore, brecciated High-grade ore, brecciated Unmineralized host carbonate Low-grade ore, brecciated Low-grade ore, brecciated High-grade ore, black, clay-rich

a b c d

Temperature at maximum of pyrolysis peak. Hydrogen index (mg pyrolyzable hydrocarbons/wt% organic carbon). Oxygen index. Drill hole 318k is 0.5 km from the Saqisan deposit.

Organic Geochemistry, Guangzhou, using 40 × oil immersion objectives and Cargill oil of refractive index 1.515. Reflectance was measured at 546 nm on randomly oriented organic particles under unpolarized reflected light. Total error associated with each measurement is less than R% oil ˆ ^0:05; which is less than the natural variation among particles of each organic type.

4. Results and discussion 4.1. Total organic carbon One hundred and twenty-three whole-rock samples from the three deposits were analyzed for total organic carbon (TOC). The TOC results are as follows: at Sanbaqi …n ˆ 91†; 0.42–6.57% (mean of 2.04%); at Sanqilinyi …n ˆ 17†; 0.02–1.13% (mean of 0.75%); and at Saqisan …n ˆ 15†; 0.13–3.54% (mean of 1.74%). According to Tissot and Welte (1978), 0.2% TOC is average in nonreservoir carbonate rocks; 0.67% TOC is average in carbonate source rocks; and calcareous shale source rocks contain, on average, 1.9% TOC. Thus the Sanbaqi and Saqisan are above average, and the Sanqilinyi is below average in average organic content among nonreservoir and nonsource

carbonate rocks. In particular, the Sanbaqi is above average for nonsource carbonate rocks because of its organic-rich black shale bands.

4.2. Organic matter type Two distinct types of organic matter have been recognized in both unmineralized and mineralized carbonate samples from the deposits: kerogen and bitumen. The predominant type is amorphous kerogen, which is dispersed throughout the breccia matrix but is primarily associated with the clays in the matrix. The bitumen fills fractures in the rocks occurring as veinlets and irregular grains. It is commonly associated with late primary uranium minerals, (i.e. uraninite and coffinite) in the mineralized carbonate. Five analytical techniques have been used to characterize the organic matter (Rock-Eval pyrolysis, gas chromatography, pyrolysis-gas chromatography, infrared spectroscopy and carbon isotopes of kerogen). The Rock-Eval and elemental analyses for 10 carbonate and kerogen samples are presented in Tables 1 and 2. The Rock-Eval analyses show slight differences in hydrogen indices and temperature indices (Tmax) among the samples and are of little use as indicators of the organic matter type. Leventhal (1982) and Peters (1986) have also commented on the

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Table 2 Elemental and isotopic analyses of kerogens (abbreviations: SB ˆ Sanbaqi deposit, SQ ˆ Sanqilinyi deposit, SS ˆ Saqisan deposit. Features of the samples are described in Table 1. H/C, O/C and N/C are atomic ratios) Sample no. Deposit

Corganic (wt%)

C (wt%)

H (wt%)

O (wt%)

N (wt%)

Ash ⫹ S (wt%)

H/C

O/C

N/C

d 13CPDS (‰)

G-1 G-2 G-3 G-4 G-5 G-6 G-7 G-8 G-9 G-10

0.35 1.17 1.58 0.37 0.38 1.13 0.37 0.47 0.13 2.72

68.713 53.624 55.659 55.743 65.757 59.883 52.197 62.074 64.140 72.720

7.574 4.934 3.272 6.521 4.396 3.400 6.610 6.367 4.721 3.887

9.337 4.929 4.078 5.346 8.760 5.501 4.728 5.871 7.433 7.266

1.799 0.507 1.269 0.847 1.149 0.713 1.236 0.501 1.239 1.276

12.577 36.006 35.722 31.543 19.938 30.503 35.229 25.187 22.467 14.851

1.324 1.105 0.706 1.405 0.803 0.682 1.521 1.232 0.884 0.462

0.102 0.069 0.055 0.072 0.100 0.069 0.068 0.071 0.087 0.075

0.022 0.008 0.020 0.013 0.015 0.010 0.020 0.007 0.017 0.015

⫺26.7 ⫺26.1 ⫺25.3 ⫺31.2 ⫺20.0 ⫺24.8 ⫺28.0 ⫺25.4 ⫺22.6 ⫺19.8

SB SB SB SQ SQ SQ SS SS SS SS

limitations of the Rock-Eval technique for characterizing organic matter in whole-rock samples. The gas chromatograms of saturated hydrocarbons for eight bitumen samples from the three deposits are given in Fig. 2. All eight samples show bimodal distributions in the total saturated hydrocarbon. All bitumens from the unmineralized samples (G-1, G-4, and G-7) have a high ratio of pristane and phytane relative to normal alkanes (n-alkanes) and low saturate to aromatic ratios (0.30–0.65). The normal alkanes range from C15 to C37 with an odd carbon number preference; maxima concentrate around C17–19 and C27–29. Such features are typical of samples from the immature stage or very early stages of hydrocarbon generation in conventional marine source rocks deposited under reducing conditions (Tissot and Welte, 1978), and are in keeping with the range of vitrinite reflectances of less than 0.5% Ro of a suite of host carbonate samples from the Sanbaqi ore district (Min et al., 1996). Han and Calvin (1969) analyzed saturated hydrocarbon distributions in a series of algae and bacteria and reported that C15– 21 was the predominant n-alkane of all the photosynthetic organisms examined. However, terrestrial plants possess a waxy cuticle which consists partly of high molecular weight normal alkanes in the range C21–37 (Eglinton and Hamilton, 1967).

Therefore, most of the normal alkanes present in the unmineralized carbonate samples, in the C15–21 range, were probably derived from algae, while the high molecular weight normal alkanes (C21 ⫹ ) present in the samples were possibly derived from terrestrial plants. The higher plant macerates were observed during petrographic studies. The terrestrial plant fragments may have partly existed in marine sediments during the Carboniferous time, and part of them may have been brought by the paleo-waterflows into the paleokarst solution network which controlled the location and formation of the deposits. The pyrolysis-gas chromatograms of five fraction kerogens from the deposits are given in Fig. 3. It is indicated that the pyrograms for the unmineralized samples (G-1, G-4 and G-7) from the deposits (Table 1) contain chiefly marine-type organic matter which yields mainly a series of regularly spaced peaks representing n-alkanes (C5–C26) (Coveney et al., 1987). In addition, these pyrograms also show an irregular pattern of pyrolysis peaks originating from lignin degradation between the regular pyrolysis peaks. The latter is typical of the pattern shown by organic matter that formed in a terrestrial environment. Thus, the pyrograms show a mixture of major marine-type and minor terrestrial-type organic matter, coinciding with the data which result from the gas

Fig. 2. Gas chromatograms of total saturated hydrocarbons from the three ore districts,showing the effect of mineralization. Normal C15 and C17 are indicated by 15 and 17, pristane by Pr, and phytane by Ph. (a), (c) and (e) are from the unmineralized host carbonates of Sanbaqi, Sanqilinyi and Saqisan, respectively; (d) and (f) are from the low-grade mineralized equivalents of Sanqilinyi, Sanbaqi and Saqisan, respectively; (b), (g) and (h) are from the high-grade mineralized equivalents of Sanbaqi, Sanqilinyi and Saqisan, respectively. Features of the samples are described in Tables 1 and 2.

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Fig. 3. Pyrolysis-gas chromatograms (700⬚C is the temperature of maximum yield) of kerogens isolated from the five samples of the three uranium deposits. Sample features are shown in Tables 1 and 2.

chromatography above. The pyrograms for mineralized samples (G-2 and G-8) also show mainly a series of regularly spaced peaks representing n-alkanes that result from pyrolysis of marine-type organic matter, and an irregular pattern of pyrolysis peaks representing terrestrial-type organic matter between the regular

peaks with respect to the pyrograms of unmineralized samples. The pyrograms of mineralized samples contain more irregular patterns of pyrolysis peaks originating from lignin degradation, indicating more terrestrial-type organic matter in the mineralized samples than in the unmineralized samples.

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Fig. 4. Infrared spectra of kerogens isolated from the six samples for three uranium deposits. Features of the samples are described in Tables 1 and 2.

The infrared spectra of kerogens isolated from unmineralized and mineralized carbonate samples are shown in Fig. 4. Spectral interpretations were based on the work of Rouxhet et al. (1980). The strongly aliphatic nature of the kerogens isolated from unmineralized samples G-1, G-4 and G-7 is indicated by prominent bands at 2920 and 2850 cm ⫺1, resulting from methylene groups. A portion of the broad band centered around 1720 cm ⫺1 is presumably those by CyO of carbonyl and carboxyl groups. The diffuse band centered around 1100 cm ⫺1 may indicate some aromaticity. The major component to the 1100 cm ⫺1 band is presumed to be from aliphatic structures. A small shoulder at 1420 cm ⫺1 is suggestive of carbonyl groups. The infrared spectra of kerogens isolated from the mineralized samples G-3, G-6

and G-10 are featureless. This is consistent with their low H/C less than 0.7 and high maturity levels (Ro ⬎ 1.0%) as shown in Fig. 5. The 10 kerogen samples from the host rocks and ores gave d 13C values of: ⫺26.7, ⫺26.1 and ⫺25.3‰ for the Sanbaqi deposit: ⫺31.2, ⫺20.0 and ⫺24.8‰ for the Sanqilinyi deposit: and ⫺28.0, ⫺25.4, ⫺22.6 and ⫺19.8‰ for the Saqisan deposit (Table 2). The d 13C values of the kerogen isolates vary from ⫺31.2 to ⫺19.8‰. These values are mostly in the range reported for ancient (Paleozoic or younger) marine type-II kerogen (Hoefs, 1980). The differences in the isotopic composition of organic carbon in the carbon-rich formations can be explained either by different origins of organic matter or by different water depths of organic matter accumulation (Kribek,

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Fig. 5. Atomic ratios of element H, C, O of kerogens isolated from Sanqilinyi ( × ), Sanbaqi (⫹) and Saqisan (W) uranium deposit plotted on a van Krevelen diagram. The maturation pathways indicated are for the three kerogen types (Tissot and Welte, 1978).

1989). The isotopic composition of organic carbon is also related to biodegradation and thermal maturation. The more negative d 13C values (⫺28.0 to ⫺31.2‰) of the Devonian carbon-rich formations (samples G-4 and G-7) can be explained by decomposition of organic matter in a shallow water environment, while the d 13C values (⫺26.7‰) of the Carboniferous carbon-rich formation (sample G-1) are controlled mostly by the isotopic composition of the carbon of the continental macroflora, which varies in a range from ⫺20 to ⫺27‰ (Galimov, 1980). From this standpoint, the organic matter in the ores and host rocks was decomposed in a shallow water environment and may be incorporated with the carbon of the continental macroflora for the Carboniferous carbonates and carbon-rich sediments in the paleokarst caverns. 4.3. Thermal maturity The gas chromatography, petrography, infrared spectra and elemental analyses of the organic components can be used to deduce the thermal maturity of the samples. For interpretation in terms of maturity and organic

type, the atomic ratios H/C and O/C of the 10 kerogen samples have been plotted on a van Krevelen diagram in Fig. 5. It is indicated that elemental compositions of the 10 kerogen samples isolated from the three uranium deposits fall into the fields of kerogen I and II, respectively. Only the kerogen isolated from the unmineralized host carbonates (samples G-4 and G-7) has type I composition. Algae-rich kerogen have type I and II composition (Tissot and Welte, 1978). Type III kerogen, which consists of polyaromatic compounds such as lignin, is usually derived from the tissues of higher land plants (Henry et al., 1992). Thus, the organic matter in the three ore districts was derived mainly from marine algae on the basis of elemental compositions of the kerogen. The decrease of O/C atomic ratios of kerogens and increase of solid bitumen reflectances toward the high-grade ores are, therefore, a reflection of increasing thermal maturity (Fig. 5). The elemental ratios of kerogens isolated from the low-grade ore samples imply that the kerogens had been heated to a maturity equivalent to the main stage of petroleum generation (catagenesis), and those from the high-grade ore samples have a thermal maturity at the onset of metagenesis. The organic matter in the unmineralized host carbonate is immature.

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Table 3 Elemental analyses of the ores and host rocks from the paleokarst-hosted uranium deposits (sampling location and description of the samples are shown in Table 1; n.a. ˆ no analysis) Sample no. Major (wt%) SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Loss Corg Total Trace (ppm) U Ba Be Co Cr Cu Mo Re Ni Pb V Zn

G-1

6.33 0.06 2.52 0.72 0.22 0.01 8.42 40.67 0.20 0.50 0.02 39.93 0.35 99.95 3.6 13.4 2.3 3.3 12.1 36.7 4.5 n.a. 17.6 28.3 18.2 33.6

G-2

4.52 0.28 5.97 1.11 0.20 0.05 7.07 36.42 0.15 1.29 0.02 41.70 1.17 99.95

267.1 3.5 17.8 130.6 240 7.3 n.a. 53.4 1051 160 783

G-3

6.65 0.12 3.03 0.96 0.19 0.01 7.46 38.46 0.25 0.67 0.06 40.19 1.58 99.63

345.1 7.2 24.5 204.5 251 11.4 n.a. 125 1240 172 846

G-4

6.99 0.06 2.75 1.38 0.18 0.07 0.78 48.01 0.33 0.78 0.03 38.19 0.37 99.92 5.0 58.4 0.5 3.4 11.6 30 3.4 n.a. 10.3 20.1 12.1 60.2

G-5

8.90 0.15 4.46 0.38 0.49 0.03 2.24 44.70 0.33 1.75 0.04 35.70 0.38 99.55

1383 2.9 11.3 71.2 330 55.3 n.a. 45.2 300 47.2 100

These are supported by the kerogen color, infrared spectra and the marked odd carbon-numbered predominance in the n-alkanes (Figs. 2 and 4). The HI values of the 10 samples are nearly the same, whereas the H/C values vary (Tables 1 and 2). It is most likely that the organic matter was subjected to radioactive oxidation of uranium ore. Solid bitumen reflectance is a thermal maturity indicator on the assumption that it is nearly equivalent to vitrinite reflectance values in the range of 0.5–to 1.5R% oil (Robert, 1980; Nagy et al., 1993). Twentyeight solid bitumen samples from the three deposits were measured on polished section for reflectance at the Institute of Geochemistry, Academia Sinica, following standard procedures (Bustin et al., 1983). At least three types of bitumens can be recognized on the basis of their reflectance (Min et al., 1996). Those at some distance from the orebody (⬎200 m), and not

G-6

11.92 0.22 5.77 0.87 0.84 0.13 1.40 38.29 0.27 2.22 0.05 36.40 1.13 99.51

1100 1.9 10.5 63.3 1420 500 n.a. 50.1 270 100 320

G-7

1.17 0.01 0.29 0.19 0.06 0.01 0.71 54.68 0.13 0.49 0.06 41.71 0.37 99.88 3.1 31.7 0.1 3.7 6.6 9.7 0.01% 1.5 13.7 17.2 12.3 5.1

G-8

G-9

G-10

5.25 0.15 1.35 2.18 0.17 0.02 8.15 42.99 0.13 0.49 0.03 38.01 0.47 99.39

5.59 0.08 2.58 0.78 0.27 0.12 20.55 27.27 0.13 0.51 0.04 41.78 0.13 99.83

50.84 0.66 17.43 1.69 2.56 0.11 2.43 8.36 0.64 4.68 0.15 7.32 2.27 99.14

59.9 0.3 11.8 25.8 18.4 0.32% 6.4 143 22.2 87.6 160

128.8 3.9 19.9 85.4 35.5 0.14% 4.3 256 35.5 354 184

474.9 10.3 44.7 110.8 90 0.51% 9.3 1100 81.4 812 238

associated with any mineralization have a low reflectance (Ro ⬍ 0.5%). The second type of bitumen occurred in the low-grade ore has intermediate reflectance (Rorandom of 0.5–1.0%). The bitumens associated with the high-grade ore comprise the third type, and have a high reflectance (Rorandom of 1.0–2.0%). It is apparent from these data that the organic matter is thermally more mature in the ores than in the unmineralized host rocks.

4.4. Possible relationships between the ore and organic matter Possible relationships between the ore and organic matter may be recognized from elemental compositions, mineralogy and sulfur isotopes of the ores and host rocks, pyrolysis-gas chromatograms of the kerogen

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Fig. 6. Scatter plots of (a) uranium vs. organic carbon; (b) sulfur vs. organic carbon; and (c) uranium vs. sulfur for the samples from Sanqilinyi (X; SQ), Sanbaqi (O; SB) and Saqisan ( × ; SS) uranium deposit.

and gas chromatograms of the saturated hydrocarbon fractions for this type of uranium deposits. The chemical compositions of the selected ore and host carbonate samples from the deposits were analyzed and are presented in Table 3. The carbonates enriched in organic carbon are generally enriched in trace elements, especially molybdenum, vanadium, lead, copper, zinc, cobalt and nickel. In addition to uranium that has been mined, molybdenum and rhenium have been recovered as by-products at the Saqisan deposit, and copper, lead and zinc at the Sanqilinyi deposit. The concentrations of metals and organic carbon vary widely. Chemical analyses of 123 samples from the three deposits for uranium, organic

carbon and total sulfur are plotted in Fig. 6. It is shown that a weak positive correlation exists between uranium, organic carbon and total sulfur concentrations. However, uranium vs. organic carbon data indicate that uranium content seem to be independent of the organic carbon content in the rocks. Correlation coefficients of 0.52 for uranium vs. organic carbon, and 0.44 for uranium vs. total sulfur, were calculated. In Fig. 6a and b, all points cluster into two groups: samples from the Sanbaqi deposit have uniformly high C concentrations and varying U, S concentrations, while the samples from other two deposits have low C concentrations and varying U, S concentrations. Fission-track and microprobe analyses

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223

Fig. 7. Photomicrographs of mineralized carbonate at the paleokarst-hosted uranium deposits. (a) Carbonaceous clays in matrix (in black) of the breccias at Sanbaqi deposit. (b) X-ray emission image of the area in photomicrograph (a) for U (microprobe analysis). (c) Microsphere of uraninite (in white) in organic matter (in black) at Saqisan deposit. (d) X-ray scanning image crossing the uraninite microsphere in photomicrograph (c) for U. Uranium is also present in the organic matter (microprobe analysis).

indicate that uranium is finely disseminated in the clay matrix and is concentrated preferentially in and adjacent to organic carbon (Fig. 7). Uranium minerals and organic carbon are often associated with each other in space. Uraninite and coffinite were detected as original uranium minerals by microscope, microprobe and chemical method (Fig. 7; Table 4). Both uranium minerals occur mainly along fractures, interstitially in granular material and in matrix of the breccias. In particular, there are numerous submicroscopic uraninite and coffinite grains, less than 1–5 mm in diameter, within or adjacent to the organic carbon and have a colloidal texture composed of microspheres (Fig. 7). The uraninite and coffinite are commonly intergrown and most likely formed during the same episode.

The weak correlation between uranium content and organic matter type is shown in Fig. 3, where U values are seen to increase as a function of rising values for terrestrial organic matter indicators (non-n-alkane peaks). Pyragrams of five samples show at least two types of organic matter are present. One type shows a predominance of n-alkane and -alkane pyrolysis products and is represented by the unmineralized carbonate samples G-1, G-4 and G-7 with uranium content of 3.6, 5.0 and 3.1 ppm (Table 3), respectively. The other shows more aromatic components and is represented by the mineralized samples G-2 and G-8 with uranium content of 520 and 630 ppm, respectively. These pyrograms show a variation in types of organic matter which can be related to uranium content (Leventhal, 1981; Coveney et al., 1987).

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Table 4 Chemical and electron microprobe analyses of uraninite and coffinite from the three uranium deposits (n.a.: no analysis) Deposit

Sanbaqi

Sample no. Mineral

C122 a,b Uraninite

R12 a,c Coffinite

UO2 UO3 SiO2 TiO2 Al2O3 Fe2O3 e CaO MgO Na2O K2O P2O5 PbO ThO2 Loss Total

52.34 24.45 5.81 0.34 0.10 0.11 6.20 0.60 0.04 0.22 0.26 1.54 0.03 6.39 98.42

71.22 (U3O8)

a b c d

Sanqilinyi

22.53 n.a. 1.93 0.31 3.21 n.a. n.a. 0.15 n.a. 0.05 0.11 n.a. 99.51

R10 c Coffinite 69.42 (U3O8) 24.11 n.a. 2.21 n.a. 4.09 n.a. n.a. 0.13 n.a. 0.04 0.12 n.a. 100.12

Saqisan

MI b,d Uraninite

R02 c,d Coffinite

R05 c,d Coffinite

41.21 22.83 10.64 0.31 2.11 1.24 6.99 1.25 0.40 0.18 0.32. 1.30 0.02 n.a. 99.41

72.31 (U3O8)

70.43 (U3O8)

21.58 n.a. 2.64 0.43 1.14 n.a. n.a. 0.09 n.a. 1.28 0.12 10.61 99.59

21.92 n.a. 2.41 0.78 3.02 n.a. n.a. 0.18 n.a. 0.85 0.36 n.a. 99.95

G01 c Uraninite 84.08 (U3O8) 1.98 n.a. 0.10 0.25 10.81 n.a. n.a. 0.02 n.a. 2.77 0.01 n.a. 100.02

G11 c Coffinite 70.68 (U3O8) 20.45 n.a. 2.50 1.25 2.44 n.a. n.a. 0.25 n.a. 1.34 0.40 n.a. 99.31

Data from the Salinsan Party (1991). By wet chemical analysis. By microprobe analysis. Data from the Salinqi Party (1980).

Comparison of gas chromatograms for the mineralized and unmineralized samples of the same facies from the three deposits shows that the mineralized samples partly remove n-alkanes and isoprenoid alkanes in the gas chromatograms of their saturated hydrocarbon fractions (Fig. 2). The degrees of biodegradation for all the samples shown in Fig. 2 are very light, although unresolved complex mixtures, socalled UCM, in the samples G-4–G-7 suggest slight bacterial alteration. The biodegradation of organic matter can occur at any time, even in the laboratory during sample storage. UCM is present not only in immature samples (G-4 and G-7), but in comparatively mature samples (G-5 and G-6), suggesting that biodegradation is not related to uranium mineralization. The uranium mineralization occurs in the dark gray matrix of the breccias and paleocave-fillings where organic matter and clays are abundant. There, the organic matter may play the following important role in the ore formation: (1) During the formation of paleokarst-hosted uranium deposits in the carbon-rich sedimentary rocks, the organic matter of host rocks became

oxidized by the action of oxygen dissolved in orebearing solutions. This process gave rise to organic polymers similar to humic acids, which reacted with UO22⫹ to form insoluble uranium organic compounds (Kribek, 1989). (2) Strong adsorption and pre-concentration on uranium. This is partly supported by the data in Figs. 6 and 7, and very good statistical correlation between uranium and organic carbon in sandstone and shale of USA, France and other places reported by Swanson (1961), Leventhal (1981) and Meunier et al. (1989). In the present examples, the organic matter and clays, which accumulated in the matrix of the breccias formed by repeated paleokarstification and movement along faults and shear zones, strongly adsorbed uranium from the paleokarst-waters. Consequently, protore and low-grade ore containing as much as 200–600 ppm U were locally derived (Fig. 7). The organic matter- and clay-rich paleocave-fill sediments became a reservoir of uranium and other metallic elements. The high-grade ore resulted from hydrothermal reworking the uranium-rich sediments, including the protore and low-grade ore. This is supported by the fact that the high-grade ore zones

M.-Z. Min et al. / Journal Geochemical Exploration 68 (2000) 211–229 Table 5 Average fluid inclusion homogenization temperatures for quartz and calcite from the ores Sample no.

Mineral

Sanbaqi deposit Q-1 Quartz Q-2 Quartz Q-3 Quartz Q-4 Quartz Q-5 Quartz Q-9 Quartz Q-15 Quartz Sanqilinyi deposit M-1 Calcite M-3 Calcite M-4 Quartz M-5 Calcite M-6 Calcite Saqisan deposit D-8 Quartz D-15 Quartz D-23 Quartz D-13 Quartz D-14 Quartz

Mineralizing stage

Average homog. temp.

Early Early Early Middle Middle Middle Late

181 178 180 172 168 155 150

Early Middle Late Late Late

281 200 197 175 113

Early Early Middle Late Late

251 210 191 150 110

225

paleocaverns by paleosurface waterflows during the Jurassic to Cretaceous time when there was a dense forest in the land of South China. (4) Indirectly reducing hexavalent uranium and promoting precipitation of quadravalent uranium mineral by reduction of sulfate to sulfide (Mauk and Hieshima, 1992): ⫺ ⫹ SO2⫺ 2CH2 O ! 2HCO⫺ 3 ⫹ HS ⫹ H2 4

6HS⫺ ⫹ UO2 …CO3 †4⫺ 3 ! UO2

…Uraninite†

⫹ 3H2 O

…3†

⫹ 6S ⫹ 3CO2 …4†

The process in reaction (3) can also be mediated by bacteria at temperatures below 90⬚C, or can occur abiogenically at higher temperatures. A high sulfur content (⬎10 wt% SO3) and presence of biogenic sulfur and framboidal pyrite in the ores give evidence for bacterial activity which produces H2S capable of reducing uranium. 4.5. Other related research data

are commonly surrounded by the low-grade ore zones, and occur in the shear and fault zones of the low-grade ore. (3) Directly reducing hexavalent uranium and promoting precipitation of quadravalent uranium minerals by reactions such as that shown in the following equations (e.g. Gustafson and Curtis, 1983; Leventhal, 1986b): 2UO2 …CO3 †4⫺ 3 ⫹ 2CH2 O ⫹ 2H2 O ! …1†

2UO2 ⫹ 8CO2 ⫹ 4H2 O ⫹ O2

…Uraninite†

or ⫹ 2UO2 …CO3 †4⫺ 3 ⫹ 8H ⫹

C

…Organic†

2UO2 ⫹ 7CO2 ⫹ 4H2 O

! …2†

…Uraninite†

Oxidation of one molecule of CH2O or one atom of C to CO2 can lead to reduction of 1–2 molecules of UO2(CO3)34⫺ to UO2. At the paleokarst-hosted three uranium deposits, some land-derived organic matter in the ores may have been brought into underground

As host strata of the paleokarst-hosted uranium deposits, carbonaceous pelitic limestone, fossiliferous micrite and dolomitic limestone with interbedded shale of the middle Devonian to early Carboniferous age are important regional source rocks of uranium for formation of uranium deposits in South China (Min, 1993). The best evidence is that the carbonaceous– siliceous–pelitic rock type uranium deposits in South China are partially hosted by the rocks (Min, 1995). These rocks are dark gray to black in color and rich in uranium (3.5–55 ppm U), organic matter, clays and pyrite. Uranium probably formed organo-uranyl compounds which indicates that adsorption or ion exchange (Szalay, 1964; Doi et al., 1975) were critical factors for uranium concentration in the rocks. Uranium associated with clays either was concentrated along the edges of clayey layers, or occurred as discrete U-bearing minerals less than 0.1 mm in diameter (Meunier et al., 1989). Fluid inclusion-filling temperatures from calcite and quartz in the high-grade ore range from 281 to 113⬚C for the Sanqilinyi deposit, from 181 to 150⬚C for the Sanbaqi deposit and from 251 to 110⬚C for the Saqisan deposit (Table 5). The data of temperature

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determination show that the high-grade mineralization of the deposits has a hydrothermal origin. However, there are no vein minerals suitable for temperature determination in the protore and lowgrade ore. It is thus speculated that the protore and low-grade ore may be formed at ordinary to low temperatures and have an infiltration origin (Min, 1987). The formation time of the uranium deposits can be constrained by geological evidence and U–Pb isotopic ages of the uraninite from the ores. The orebodies of the three deposits are all hosted by the paleokarst breccias which formed primarily during two episodes of paleokarsting. The timing of the first paleokarst episode is indicated by the presence, in the clay-rich breccia matrix, of sporopollens such as atatrisporites, conoarisporptes and ricoisporites, which existed during the late Triassic and early Jurassic age in South China. The timing of the second episode of paleokarstification is indicated by the presence, in the cavity-fillings, of the sporopollens (ceyebyopollenites, carlqlensis, callialesporites and dampiri) which existed during the early Cretaceous age in South China. The second episode of paleokarstification is the most important for the uranium mineralization. It is thus apparent that the uranium mineralization in the three deposits took place at least during or after the first paleokarst episode, viz. in the early Jurassic age. Eleven uraninite samples were determined for U–Pb isotopic age in the present study. Three uraninite samples from the Sanqilinyi deposit yielded U–Pb ages of 65.0, 59.2 and 30.7 Ma; 135.1, 131.3, 119.2, 65.2 and 30.1 Ma for five uraninite samples from the Sanbaqi deposit; and 140.0, 120.0 and 60.0 Ma for three uraninite samples from the Saqisan deposit (Min, 1987, 1993, 1995). These U–Pb isotopic ages lie within the time range of the early Yanshanian (early Cretaceous, viz. approximately 144–98 Ma) and late Yanshanian (early Tertiary, viz. approximately 65–25 Ma) regional tectonism, respectively. This regional tectonism affected the vast region of South China. The regional tectonic events are, therefore, the most likely driving mechanism for large-scale fluid focusing (Plumlee et al., 1994), leading to formation of not only the paleokarst-hosted uranium deposits discussed here, but also other types of granite-, sandstone- and volcanichosted uranium deposits in South China (Zhang, 1991).

The indigenous organic matter in unmineralized host carbonates is thermally immature, as indicated above, and therefore maximum burial temperatures of the host carbonates may not exceed 65⬚C (Leventhal, 1986a). It is highly unlikely that the mineralizing fluids derived from a relatively distant source could maintain higher temperatures than those of the surrounding rocks. There is no evidence of igneous activity in the Sanbaqi and Saqisan district at the time of mineralization. In the Sanqilinyi district, the Hercynian granitic batholith intruded long before formation of the uranium deposit. Thus, a magmatic source of heat for the hydrothermal mineralization of the three deposits is highly speculative. It is suggested that the mineralizing fluids were heated and driven by regional tectonic low-temperature thermal events. This conclusion is supported by the fact that the main stages of mineralization at the three deposits are essentially coeval with the early and late Yanshanian regional tectonism in South China. Also, the shears and faults provided the plumbing system that eventually formed the paleokarst zones favorable for the mineralization (Wallace, 1989).

5. Summary and conclusions The following conclusions can be drawn from the present study: (1) Organic geochemical and petrographic studies of the organic matter show that the organic matter at the three uranium deposits is chiefly of marine origin and contains only a minor terrestrial component. The original substance of the organic matter is of sapropelic–humic type in origin. There are no fundamental differences between the genetic type of organic matter in background samples and those samples spatially associated with uranium mineralization. The terrestrial-type organic matter in the ores may be brought into the underground paleocaverns by the paleosurface waterflows during the paleokarst episodes, viz. during the Jurassic to Cretaceous age when there was a dense forest in South China. (2) The organic matter in unmineralized host carbonate is immature. The kerogens isolated from the low-grade mineralized samples have a maturity equivalent to the main stage of petroleum generation

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227

Fig. 8. Diagrammatic figure illustrating the proposed model for geochemical and hydrological processes responsible for mineralization of the paleokarst-hosted U deposits in South China.

(catagenesis) and those from the high-grade ore samples have a thermal maturity at the onset of metagenesis, which results from action of the hydrothermal solutions. The late high-grade mineralization events represent thermal anomalies with respect to the host rocks. (3) Organic matter and uranium mineralization are intimately associated at the paleokarst-hosted uranium deposits, South China. The organic matter had both direct and indirect important roles in preconcentration of uranium and other ore-forming elements in the source rocks and paleocave-fillings, and in subsequent precipitation of primary uranium minerals in the ores. (4) The paleokarstification by groundwater played an important role in localization and formation of the deposits. It not only provided a channel for the mineralizing solutions and favorable sites of ore deposition, but also created protore, containing as much as 200–600 ppm U, by adsorption and reduction of uranium by organic matter and clays, which were accumulated in some parts of the extensive solution breccia along faults and in reworked breccias formed by repeated karstification. (5) The formation of the paleokarst-hosted uranium

deposits in South China is the result of: (1) repeated paleokarstifications of the Devonian and Carboniferous clay-, organic matter-rich carbonate along the faults and unconformities between different strata because of the Hercynian and Yanshanian regional tectonism, and extensive formation of solutioncollapse, solution-fault breccias (Fig. 8); (2) accumulation of organic matter and clays in the paleocaverns and matrix of the breccias, fixation and adsorption of uranium by the organic matter and clays from the paleokarst waterflows that leached metals from the uranium-bearing host carbonates during their passage towards the karst zones; (3) reduction of uranium by the organic matter and formation of protore and lowgrade ore; (4) circulation of heated formational waters and deep circulating, uraniferous meteoric waters by tectonic pumping, reworking the uranium-rich, paleocave-fillings, protore and low-grade ore, reduction and formation of primary uranium minerals (uraninite and coffinite) because of the reducing environment resulting from organic matter and sulfide.

Acknowledgements The authors wish to acknowledge the State Key

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Laboratory of Organic Geochemistry, Guangzhou, who provided financial support and assistance for present research. The authors are also grateful to the Uranium Geology and Exploration Bureau of CentralSouth China for its help in providing research samples and in field geological work. Permission to incorporate here some of the unpublished data from 303, 305 307 Geologic Parties and 701 Uranium Mine on the three U deposits is also acknowledged by the authors. Reviews of the manuscript by Drs. B. Spiro and J.A. Lermthal, and one anonymous reviewer for Journal of Geochemical Exploration greatly improved the communication of the scientific information.

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