Shengrong Li, Zhenmin Gao, Junfeng Shen. Diagenetic and Catagenetic Transference of Noble Metal Elements in Lower Cambrian Black Rock Series, Southwest China. Journal of Earth Science, 2004, 15(1): 84-90.
Citation:
Shengrong Li, Zhenmin Gao, Junfeng Shen. Diagenetic and Catagenetic Transference of Noble Metal Elements in Lower Cambrian Black Rock Series, Southwest China. Journal of Earth Science, 2004, 15(1): 84-90.
Shengrong Li, Zhenmin Gao, Junfeng Shen. Diagenetic and Catagenetic Transference of Noble Metal Elements in Lower Cambrian Black Rock Series, Southwest China. Journal of Earth Science, 2004, 15(1): 84-90.
Citation:
Shengrong Li, Zhenmin Gao, Junfeng Shen. Diagenetic and Catagenetic Transference of Noble Metal Elements in Lower Cambrian Black Rock Series, Southwest China. Journal of Earth Science, 2004, 15(1): 84-90.
Some extraditional types—black rock series types of platinum group element (PGE), gold and silver mineralization occurrences were found in the Lower Cambrian in Guizhou and Hunan provinces of southwest China where PGE concentration reaches more than 800×10-6. Sea floor hydrothermal fluid eruption was suggested to have been the main origin of the ore-forming materials. The whole process from the sedimentation to the redistribution of the ore-forming elements occurred on the conditions of intermediate to weak alkaline, weak reduction to weak oxidation. The temperature for the sedimentation and redistribution of the ore-forming elements was lower than 210 ℃. At such a low temperature, inert elements such as PGE, Au and Ag could quite easily be remobilized.
Platinum-group-element (PGE) ore deposits of magmatic liquation origin were identified to be the main type, or even the only type of PGE ore deposits. The ore bodies occur directly in the ultramafic-mafic igneous rocks or the contact zone between the igneous rocks and their wall-rocks. The forming temperature of the deposits was about 400-500 ℃ or even higher. For a long time, therefore, it had been considered that the enrichment, mineralization and transference of PGE could only occur in ultramafic-mafic igneous rocks at high temperature.
Some scholars, however, also made researches on the mobility of PGE at about 300 ℃. For instance, the modeling made by Mountain and Wood (1988) revealed that at about 300 ℃, PGE could migrate along with fluid as complex compounds of OH-, HS- and Cl-. The experiments made by Gammons et al. (1993, 1992) indicated that in the hydrothermal fluid with very high Eh and very low pH, the solubility of the complex compounds of Pt and Pd with Cl- could reach 1 000×10-6. On the condition of common Eh and pH, however, the solubility of the complex compounds of Pt and Pd with HS- was much higher than that with Cl-.
Plimer and Williams (Plimer, I. R., Williams P. A., Chemical Behavior of the Platinum Group Elements during Weathering (manuscript)) once systematically discussed the chemical behavior of the PGE during weathering and showed, through their equilibrium model calculations, that certain platinum group elements may be mobile in aqueous solutions on oxidizing conditions. They also pointed out that fewer data are available on the supergene geochemistry and mineralogy of the PGEs owing to their limited economic importance in surficial environments. On the basis of a few analyses, Li et al. (2002) made a primary study of the supergene mobility of noble metal elements in black rock series near Zhongnan Village, Zunyi area of Guizhou Province, and concluded that the noble metal elements in the black rock series might be mobilized and migrated under the weathering of "cold water" rich in [SO4]2- with pH value that was about 2.4 at the temperature below 50 ℃.
This paper focuses on the transference during diagenesis and catagenesis of platinum group elements, gold and silver in the Cambrian black rock series of Guizhou, Hunan provinces in southwest China. The result shows that these elements can be not only enriched at a relatively low temperature, but also remobilized and transferred in diagenetic and catagenetic periods.
GEOLOGY AND DIAGENETIC-CATAGENETIC EVOLUTION
In southern China is extensively distributed a Lower Cambrian black rock series along the NEE orientated Guizhou-Zhejiang region (Fig. 1). The geology of the black rock series has been described by Fan (1983), Zhang et al. (1987), Chen et al. (1990) and Coveney and Chen (1991). Paleogeographically, the region in the Cambrian period belongs to the Yangtze plate margin (Liu et al., 1996) metastable with both polycycle magmatism and a series of NE-trending deep-seated faults that had been acting since Proterozoic.
Figure
1.
Locations of Lower Cambrian bone coal, vanadium & poly-element (Ni-Mo-As-Au-PGE and many others) deposits and biographical divisions (Liu et al., 1996). 1. North China type: A. North China division; B. Yangtze division; 2. transitional division; 3. Southeast China type: A. Jiangnan division; B. Zhujiang division; 4. Hetang period: A. bone coal; B. vanadium; C. poly-elements; 5. Xingji period: A. vanadium; B. poly-elements; 6. ancient land.
The Lower Cambrian black rock series of Hunan, Guizhou provinces (Niutitang Formation and the equivalent ones), part of the same rock series in South China, are divided into two sequences named respectively as Ni-Mo-PGE sequence and V-Cu-U sequence. The Ni-Mo-PGE sequence is emphasized in the present paper. The top of the sequence, marked by the gray shale of Palang Formation, is composed downwards of such five lithological members as thick-bedded black carbonaceous illite shale (> 30 m), thin-bedded black carbonaceous illite shale intercalated with carbonaceous-argillaceous chert and calcareous nodules (0-18 m), sulfide-rich black shale with phosphorous nodules (0.2-1.0 m), black argillaceous phosphorite (0.1-0.8 m), black carbonaceous-argillaceous chert (0-1.0 m). The underlying bedrock is the Upper Proterozoic dolomite.
The analyses of Au, Ag, PGE reveal that the noble metal elements (NME) are highly concentrated in the metal-rich shale (Table 1). The enrichment model of PGE in the black rock series of south China has been studied by Fan et al. (1984), Coveney et al. (1992a, b), Mourowchick et al. (1994), Li (1994) and Horan et al. (1994). The previous studies by the authors of this paper also revealed that the noble metal and other elements in the sulfide-rich bed were closely related with sea floor hydrothermal fluid eruption (Li et al., 2003b; Li and Gao, 2000).
Table
1.
Analyses of noble metal elements in black rock series (Li et al., 2003a)
The evolution and mineralization of the black rock series can be divided into four periods: syngenetic, diagenetic, catagenetic and hypergenetic. The syngenetic period is the most important for NME mineralization. The other three periods, however, are responsible for the further enrichment of NME.
Syngenetic Period
The sedimentation in this period is a mixed one of normal seawater sedimentation, organosedimentation and hydrothermal sedimentation, forming submarine ooze composed of clay minerals and silty fragments, phosphate and organism, framboidal and grained sulfide. The sulfide-rich ooze of the early sedimentation period was disturbed into strips and rubbles that were cemented by the late formation of the early coarse-grained sulfide and late fine-grained sulfide (Fig. 2a, b).
The eodiagenesis was characterized by the formation of phosphorous and calcareous nodules. The late diagenesis was characterized by the formation of microscopic sulfide-calcite or sulfide-hydroxylapatite veinlets (Fig. 2c, d).
Most of the phosphorous nodules occurred in the sulfide bed. The diameters of nodules, quite different, range mostly between 0.2 and 1 cm. The nodules bear the zoning structure composed of core, inner crust and outer crust. It is seen that nodules are cut into or encircled by laminated shale, and quite a few of them have no well-defined border with the shale.
Most of the calcareous nodules are found within the black carbonaceous illite shale made up predominantly of crystalline calcite with granularity decreasing outwards. The diameters range between 5 and 20 cm.
The microscopic veinlets, clearly seen only under a microscope, occurred within crevices of brittle phosphorous nodules and other materials. They are less than 0.2 mm in width and about 10 mm long. Their mineral compositions correspond to those of their host rocks.
Catagenetic Period
In this period formed veins, 1-2 cm wide and about 1 m long, filled predominantly with calcite and locally with quartz, anthraxolite and barite. Their mineral compositions do not rely directly on their host rocks.
Supergenetic Period
In this period, the sulfide minerals exposed to or near the surface were oxidized. The sedimentary geochemical facies, on the basis of mineralogical association of the black rock series (Li et al., 2003a; Li and Gao, 1996; Li, 1994), are known to be that pH≈7-8, Eh≈-0.3 V in syngenetic and diagenetic period with microquartz+pyrite+hydroxylapatite+organic matter association in the sulfide-rich bed, and that pH≈7.0-7.8, Eh≈0.0-0.1 V with microquartz+barite (+organic matter) association in the chert. In catagenetic period, the characteristic mineral association of quartz+calcite+barite+anthraxolite shows that pH≈7.8 and Eh≈0.1 V.
The forming temperature of the coarse-grained pyrite (syngenetic period) is determined to be 100-240 ℃, but mostly falls into the range between 115 and 185 ℃ observed with pyrite pyroelectricity thermometer. The peak temperature for the diagenesis is determined to be 154-210 ℃ observed with organic matter maturity thermometer. The forming temperature of the anthraxolite-quartz-calcite veins (catagenetic period) is determined to be 202-212 ℃ observed with organic matter maturity thermometer and 113-230 ℃, mostly between 130 and 150 ℃ observed with fluid inclusion thermometer.
TRANSFERENCE OF GOLD, SILVER AND PGE IN DIAGENETIC AND CATAGENETIC PERIODS
Transference in Diagenetic Period
The diagenesis reflecting the transference of noble metal elements is displayed by the development of the early phosphorous and calcareous nodules and the late microscopic sulfide calcite veinlets.
Transference during Formation of Phosphorous Nodules
Two phosphorous nodules were collected from the sulfide-rich bed respectively in Shancha, Zhangjiajie of Hunan Province (sample No. DS14, radius=4.5 cm) and in Tian'eshan, Zunyi of Guizhou Province (sample No. TZ05, radius=4 cm). The nodules were carefully separated from their host shale, and then divided into two parts: core and rim. The samples of core, rim and host shale were analyzed for noble metal elements.
Seven elements, Ru, Rh, Pd, Os, Ir, Pt and Ag, were all analyzed at China National Center of Geological Experiments and Analyses. The samples for PGE analyses were firstly dissolved with Na2O2. After distillation separation, the samples were ready for Os and Ru analyses. Enriched with thiourea, the samples were measured for Ir, Rh and Pt, and further extracted with D. D. O. Petroleum ether for Pd. The analyses of Os, Ru and Ir were conducted in Ce-As system with UV-120 spectrophotometer. Rh and Pt were analyzed with JP-2 polarograph and Pd with PE-3030 atomic absorption spectrometer. The samples for Ag analyses were firstly dissolved with HCl+HNO3 and then analyzed with PE-3030 atomic absorption spectrometer.
The analyses of Au were conducted at the Institute of High Energy Physics, Chinese Academy of Sciences with neutron activation method. The results of the analyses are shown in Table 2.
Table
2.
Noble metal element analyses of phosphorous nodules
The table shows that, of the eight noble metal elements, except Ru, Ir which were locally enriched in the nodule, the contents of the remaining three elements increase outwards from the core to the host shale, and the high content elements Pt, Pd, Au and Ag are more regular in this variation trend. In the nodule No. DS14, the variation of PGE also follows this regular pattern. In the nodule No. ZT05, elements Ru and Ir were enriched strongly in the rim, and, even so, the PGE in the host shale is still richer than that in the core.
The parameters Pd/Pt, Ag/Au and TD/SUHR show the same variation trend (Table 2). Previous literatures revealed that in hydrothermal fluid systems, Pd relative to Pt, Ag relative to Au, Pt and Pd relative to Os, Ru, Rh and Ir are more active (Li and Gao, 1994; Colodner et al., 1992; Rowell and Edgar, 1986; Keays, 1982). The superiority of relatively active elements over inert ones in the outer makes it clear that, in the nucleation of phosphorous nodules, part of the relatively active elements such as Pt, Pd and Ag were initiatively transferred outward, to a certain extent.
Transference during Formation of Calcareous Nodules
The lenticular calcareous nodules from the thick-bedded black carbonaceous illite shale of Tian'eshan Mountain near Zunyi were analyzed on the redistribution and transference of the noble metal elements during the nucleation. The sample procession and analysis method were the same as those described in above section. The lens (No. ZT10), 14 cm×8 cm×5 cm in size, is composed mainly of black crystalline calcite. The core of the lens is about 6 cm in diameter and the calcite here is fine-grained (0.3-2 mm). The rim of the lens is about 4 cm thick with medium- to coarse-grained (3-5 mm) calcite. The size of the calcite increases gradually from the core to the rim. Fine-grained sulfide and clay mineral are observed in the marginal portion of the lens.
Noble metal element analyses were done respectively on the core, rim and host shale of the calcareous nodule No. ZT10, as shown in Table 3.
Table
3.
Noble metal element analyses of calcareous nodule No. ZT10
Within the nodule, no striking differences arise between the contents of relatively active elements Au, Pt, Pd and inert elements Os, Ru, Rh and Ir. On the contrary, in the host shale, active elements such as Au, Ag, Pt and Pd were strongly enriched, whereas inert elements such as Os, Ru, Rh and Ir are near or even lower than those within the lens in contents. It can be observed from Table 2 that both Pd/Pt and TD/SUHR reach their maximum values in the host shale, concordant with the results of the phosphorous nodules.
Transference during Formation of Microscopic Sulfide-Calcite Veinlets
Electronic microprobe technique was employed in the study. The analyses were done in China University of Geosciences (Beijing) with JCXA-733 main engine and Link 860Ⅱenergy spectrometer. The diameter of beam was 10 μm, the accelerating voltage was 15 kV and the electric current of beam was 2×10-8 mA.
In order to estimate the transference intensity of elements, both diagenetic sulfide, calcite and hydroxylapatite, and syngenetic quartz, coarse- and fine-grained sulfide were analyzed.
In view of the characteristics of EMPA, a concept picking up frequency (fpu) is introduced to the study: fpu =N1/N, where N1 is the number of elements great than 0 in content, and N is the number of elements totally analyzed.
The statistical results are listed in Tables 4 and 5. It is noticed that the noble metal elements in syngenetic minerals are much higher than those in diagenetic equivalent minerals, and that pyrite, the main sulfide in the rock series, is the major enrichment body of noble metal elements of different periods, and also the major carrier. Even so, the comprehensive fpu values of 8 elements in diagenetic period reach as high as those listed in Table 4, illustrating that a great quantity of noble metal elements were remobilized by diagenetic process and transferred in certain spatial range.
Table
4.fpu values of different elements in minerals
A sample from catagenetic calcite vein (No. ZT07) is studied. The vein occurs in the black shale over the sulfide-rich bed in Tian'eshan Mountain near Zunyi. The vein, composed mainly (80%-90%) of calcite and a little quartz, extends vertically for nearly 3 m with diverging and converging forms, with the branches about 2 cm wide individually, about 10 cm on the whole.
The method for the analyses of Au, Ag, PGE of the calcite vein and its host shale was described in above section. The results are shown in Table 6. It is noticed that the PGE in the calcite vein is about half of that in the host shale, whereas the values of Pd/Pt and Ag/Au for calcite vein are greater than those for the host shale, indicating that the relatively active elements were easier to transfer from the host shale to the Ca-CO2-H2O fluid during the catagenetic period.
Table
6.
Noble metal element analyses of calcite vein and its host shale
The investigation into the transference of elements on the basis of subjective understanding of the nucleation mechanism of various nodules is a new attempt. This method involves some problems that should be solved: How were the elements originally distributed before nucleation? What was the exact nucleation mechanism? In the present paper, the noble metal elements were supposed to have been distributed homogeneously before the nucleation of the nodules, and the nucleation was supposed to have occurred in the diagenetic period starting from the core composed perhaps largely of organic materials. It is possible that the homogeneous distribution of elements might not exactly reflect the real situation in the syngenetic period, but this model might be approximate easily to the reality.
The elements in the microscopic veinlets are difficult to be measured. Microprobe technique is the only way we can take in this case. Due to the large disparities between different microanalyses, the average values of different microanalyses are unreliable to be taken as the contents of elements. For this reason, the fpu values used above might be good for the correlation.
According to the data supplied in this paper, it is clear that at a low temperature such inert elements as PGE, Au and Ag could quite easily be remobilized. The embryonic distribution forms of Au, Ag and PGE in the black rock series were settled in as early as the syngenetic period. Later, mainly in diagenetic and catagenetic periods, the remobilization changed their distribution patterns and, to a certain extent, enhanced their enrichment in the sulfide-rich bed. The conclusion about the behavior of PGE at a low temperature is somewhat similar to that obtained by Colodner et al. (1992).
ACKNOWLEDGMENTS:
This work was funded by the National Natural Science Foundation of China (Nos., 40073012 and 49873013), the National Key Project for Basic Research (G1998040800) and China Geological Survey (199910200264, 2001020223023). Thanks are also offered to Professors Chen Nansheng, Fan Delian, Zhang Aiyun and Academicians Tu Guangci and Ouyang Ziyuan for their helps in this study.
Chen, N., Yang, X., Liu, D., et al., 1990. Lower Cambrian Black Rock Series and Associated Stratiform Deposits in Southern China. Chinese Journal of Geochemistry, 9(3): 244-255 doi: 10.1007/BF02837688
Colodner, D., Boyle, E. A., Edmond, J. M., et al., 1992. Post Depositional Mobility of Platinum, Iridium, and Rhenium in Marine Sediments. Nature, 358: 402-404 doi: 10.1038/358402a0
Coveney, Jr. R. M., Chen, N., 1991. Ni-Mo-PGE-Au-RichOres in Chinese Black Shales and Speculations on Possible Analogues in the United States. Mineral. Deposita, 26: 83-88 doi: 10.1007/BF00195253
Coveney, Jr. R. M., Mourowchick, J. B., Grauch, R. I., et al., 1992a. Field Relations, Origins, and Resource Implications for Platiniferous Molybdenum-Nickel Ores in Black Shales of South China. Explor. Mining Geol., 1(1): 21-28
Coveney, Jr. R. M., Mourowchick, J. B., Grauch, R. I., et al., 1992b. Gold and Platinum in Shales with Evidence against Extraterrestrial Sources of Metals. Chemical Geology, 99: 101-114 doi: 10.1016/0009-2541(92)90033-2
Fan, D., 1983. Polyelements in the Lower Cambrian Black Shale Series in Southern China. In: Augustithis, S. S., ed., The Significance of Trace Elements in Solving Petrogenetic Problems and Controversies. Theophrastus Publications S A, Greece. 447-471
Fan, D., Yang, R., Huang, Z., 1984. The Lower Cambrian Black Shale Series and the Iridium Anomaly in South China. In: Academia Sinica, ed., Developments in Geosciences, Contribution to 27th International Geological Congress, Moscow. Science Press, Beijing. 215-224
Gammons, C. H., Bloom, M. S., Yu, Y. M., 1993. Experimental Investigation of the Hydrothermal Geochemistry of Platinum and Palladium: 3. The Solubility of Ag-Pd Alloy+ AgCl in NaCl/HCl Solutions at 300℃. Geochim. et Cosmochim. Acta, 57: 2469 doi: 10.1016/0016-7037(93)90410-X
Gammons, C. H., Bloom, M. S., Yu, Y. M., 1992. Experimental Investigation of the Hydrothermal Geochemistry of Platinum and Palladium: 1. Solubility of Platinum and Palladium Sulfide Minerals in NaCl/H2SO4 Solutions at 300 ℃. Geochim. et Cosmochim. Acta, 56: 3881-3894
Horan, M. F., Morgan, J. W., Grauch, R. I., et al., 1994. Rhenium and Osmium Isotopes in Black Shales and Ni-Mo-PGE-Rich Sulfide Layers, Yukou Territory, Canada, and Hunan and Guizhou Provinces, China. Geochim. et Cosmochim. Acta, 58: 257-265 doi: 10.1016/0016-7037(94)90463-4
Keays, R. R., 1982. Iridium and Palladium as Discriminants of Volcanic-Exhalative, Hydrothermal, and Magmatic Nickel Sulfide Mineralization. Eco. Geo., 77(6): 1535-1547 doi: 10.2113/gsecongeo.77.6.1535
Li, S. R., 1994. Geochemistry of Au-Ag-PGE in Lower Cambrian Black Rock Series of Hunan and Guizhou Provinces: [Dissertation]. Institute of Geochemistry, Chinese Academy of Sciences, Beijing (in Chinese with English Abstract)
Li, S. R., Gao, Z. M., 1994. Some Typical Partition and Distribution Patterns of Platinum Group Elements. Mineralogical Magazine, 58(A): 825-826
Li, S. R., Gao, Z. M., 1996. The Thermal Evolution Condition of Lower Cambrian Black Rock Series in Guizhou-Hunan Region. Geology Geochemistry, (4): 30-34(in Chinese with English Abstract)
Li, S. R., Gao, Z. M., 2000. Source Tracing of Noble Metal Elements in Lower Cambrian Black Rock Series of Guizhou-Hunan Provinces, China. Science in China (Series D), 43(6): 625-632 doi: 10.1007/BF02879506
Li, S. R., Shen, J. F., Xu, H., et al., 2003a. Reflectance and Carbon Isotopes of Kerogen in Lower Cambrian Black Shales of Zunyi and Zhangjiajie, SouthwestChina: Indicators to the Source of Au-Ag-PGE. Proceedings: International Symposium of the Kanazawa University 21st Century COE Program, 1: 229-234
Li, S. R., Xiao, Q. Y., Shen, J. F., et al., 2003b. Rhenium-Osmium Isotope Constraints on the Age and Source of the Platinum Mineralization in the Lower Cambrian Black Rock Series of Hunan-Guizhou Provinces, China. Science in China (Series D), 46(9): 919-927
Li, S. R., Xiao, Q. Y., Shen, J. F., et al., 2002. Supergene Mobility of Noble Metal Elements in Black RockSeries. Progress in Natural Science, 12(6): 449-453
Liu, B., Quan, Q., Feng, Q., et al., 1996. Textbook of Geohistory. Geological Publishing House, Beijing(in Chinese)
Mountain, B. W., Wood, S. A., 1988. Chemical Controls on the Solubility, Transport and Deposition of Platinum and Palladium in Hydrothermal Solutions: A Thermodynamic Approach. Economic Geology, 83: 492-510 doi: 10.2113/gsecongeo.83.3.492
Mourowchick, J. B., Coveney, Jr. R. M., Grauch, R. I., et al., 1994. Cyclic Variations of Sulfur Isotopes in Cambrian Stratabound Ni-Mo-(PGE-Au) Ores of Southern China. Geochimica et Cosmochimica Acta, 58(7): 1813-1823 doi: 10.1016/0016-7037(94)90538-X
Rowell, W. F., Edgar, A. D., 1986. Platinum-Group Element Mineralization in a Hydrothermal Cu-Ni Sulfide Occurrence, Rathbun Lake, Northeastern Ontario. Eco. Geo., 81(5): 1272-1277
Zhang, A., Wu, D., Guo, L., et al., 1987. Geochemistry and Metallogenic Significance of the Marine Black Shale Formation. Science Press, Beijing(in Chinese)
Shengrong Li, Zhenmin Gao, Junfeng Shen. Diagenetic and Catagenetic Transference of Noble Metal Elements in Lower Cambrian Black Rock Series, Southwest China. Journal of Earth Science, 2004, 15(1): 84-90.
Shengrong Li, Zhenmin Gao, Junfeng Shen. Diagenetic and Catagenetic Transference of Noble Metal Elements in Lower Cambrian Black Rock Series, Southwest China. Journal of Earth Science, 2004, 15(1): 84-90.
Figure 1. Locations of Lower Cambrian bone coal, vanadium & poly-element (Ni-Mo-As-Au-PGE and many others) deposits and biographical divisions (Liu et al., 1996). 1. North China type: A. North China division; B. Yangtze division; 2. transitional division; 3. Southeast China type: A. Jiangnan division; B. Zhujiang division; 4. Hetang period: A. bone coal; B. vanadium; C. poly-elements; 5. Xingji period: A. vanadium; B. poly-elements; 6. ancient land.
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